摘要:

J. Chem. SOC. Faraday Trans I 1986,82 1033-1050 Electrochemical Sensors Theory and Experiment W. J. Albery," P. N. Bartlett,? A. E. G. Cass D. H. Craston and B. G. D. Haggett Department of Chemistry and Centre for Biotechnology Imperial College of Science and Technology South Kensington London S W7 2A Y A comparison is made of potentiometric and amperometric sensors. For amperometric sensors there are advantages in using the wall-jet system with ring-disc or packed-bed electrodes. Particular applications to the determi- nation of proteins root death total iron and the concentration of NO; are described. Enzyme electrodes using organic salts are capable of the direct oxidation of the enzyme itself. A theoretical description is presented. Experiments with glucose oxidase show that the transport of glucose through the membrane is the rate-limiting step.By contrast with a sensor for choline using choline oxidase we find that the transport of substrate through the membrane and the unsaturated enzyme kinetics are each partially rate- limiting. Electrodes made of conducting organic salts also oxidise NADH. For an ethanol sensor using ethanol dehydrogenase we find that the rate-limiting steps are those involving homogeneous enzyme kinetics. There are two main methods for constructing electrochemical sensors potentiometry and amperometry. In the first local equilibrium is set up at the sensor interface and the electrode or membrane potential is measured. In the second the electrode potential is used to drive an electrode reaction and the current resulting from that reaction is measured.The features of these two alternative approaches are compared in table 1. Potentiometric sensors require rapid electrode kinetics while with amperometric sensors sluggish electrochemical reactions can be switched on by the electrode potential ; typically the electrochemical rate constant increases by a power of 10 for every 120 mV increase in electrode potential. Potentiometric sensors whether they are the classical ion-selective electrodes or integrated devices such as ISFETs and CHEMFETs suffer from another disadvantage. The potentiometric signal can be corrupted by mV of noise. Percentage errors are summarised in table 2. On the other hand because there is no net consumption of analyte a potentiometric sensor does not perturb the analyte concentration; mass transport is unimportant.This is not true for the amperometric sensor where if the mass transport is not controlled then the current i will be given by the general form of the Albery equation i = f[N L(dUGC/dt)] where N is the number of windows open in the laboratory L is the number of lorries (trucks) passing outside and dUGC/dt is a complicated periodic function describing the visitations of the University Grants Committee. Controlled Hydrodynamic Voltammetry One way of avoiding the complexities of solving the Albery equation is to use an electrode with controlled hydrodynamics. The most widely used electrode of this type is the rotating-disc electrode. While this electrode is convenient for the investigation of f- Now at the Department of Chemistry and Molecular Sciences University of Warwick Coventry CV4 7AL.35-2 1033 1034 Electrochemical Sensors Theory and Experiment Table 1. Comparison of potentiometric and amperometric sensors method of operation electrode kinetics response mass transport sensitivity Table 2. Errors in concentration for potentiometric sensors AE/mV error (%) n = l ca. amperometric po ten tiome tric measure potential a t i = O must be fast measure transport- limited current electrode potential can drive reaction c is linear function of current giving c is exponential function of EF/RT giving good dynamic range but making c sensitive to errors in measurement of E normal dynamic range and normal response to errors in measure- ment of i must be controlled unimportant mol dm-3 mol dm-3 ca.2 5 electrode mechanism it is not so suitable for application to flow-through sensors. ChanneP3 or wall-jet electrode^^-^ are to be preferred. The wall-jet system is superior to the channel electrode for the following two reasons. First a higher percentage of the analyte reaches the electrode (ca. 7% compared with ca. 0.5% ). Secondly there is a smaller dead space since there is no need for a lead-in length to establish the correct hydrodynamics. A further advantage is that a packed-bed electrode can be easily inserted just upstream of the jet to give a double electrode system.The wall-jet system is illustrated in fig. 1. In some applications it is desirable to have a double electrode system. The central disc electrode is surrounded by a concentric ring electrode to make a ring-disc electrode. The rotating ring-disc has been developed over a number of years. Recently we have ShownlO that by simple transformations involving the geometry of the electrode the theory developed for the rotating system9 may be extended to the channel and wall-jet regimes. The collection efficiency and titration curves depend on two geometric parameters a and B which in their turn depend on the radii of the ring-disc electrode i t . rl the radius of the disc rz the inner radius of the ring and r3 the outer radius of the ring. Definitions of a and /3 in terms of the three radii for the three different hydrodynamic regimes are collected in table 3.40 n = 2 AE/mV error (%) 7 19 2 5 10 10 9 3 19 1035 Ag/AgCI reference elect rod e P (r3/r2I3 - (r2/r1I3 W. J. Albery et al. Fig. 1. A wall-jet electrode. The inset shows the flow pattern. Table 3. Definitions of the ring-disc geometric parameters a and P a @2/rA3 - 1 (r2/r1)9/8 - 1 rotating ring-disc wall-jet ring-disc double channela (r3/r1)gls - (r2/r,)9/s (12 /wl ('3/'2)-('2/'1) a In this case measuring from the upstream edge of the upstream electrode the gap lies between 1 and 1 and the downstream edge of the collecting electrode is at 13. Bromine Microtitrations We have shownll l2 that the wall-jet ring-disc electrode may be used to determine the concentrations of proteins by microtitration with bromine.The detector is designed to be used with an h.p.1.c. column where the column is being used to separate a mixture of proteins. We have found that a typical protein molecule P will react rapidly with several hundred molecules of bromine upstream disc electrode 2Br- + Br + 2e solution P + nBr + PBr, downstream ring electrode Br + 2e + 2Br-. The unreacted bromine is measured on the ring electrode. Fig. 2 shows the response of the ring current to increase bromine generation on the disc. The insets show the concentration patterns in the vicinity of the ring-disc electrode. The displacement of the titration curve to higher disc currents is proportional to the concentration of protein in the solution.Typical results for cytochrome c are shown in fig. 3. We have shown that 1036 0 Electrochemical Sensors Theory and Experiment Fig. 2. Ring current us. disc current curves for a bromine microtitration. The insets show the bromine zone spreading from the disc electrode. The displacement of the curve from the collection efficiency line observed in the absence of protein is proportional to the concentration of protein P. 8 12 i&A Fig. 3. Typical titration curves obtained after the addition of successive aliquots of cytochrome c. many proteins can be determined in this way.12 Furthermore by carrying out the titration at different pH one can obtain information that is characteristic of the particular protein.Another application of the bromine titration technique arises in the monitoring of nutrient solutions used to grow crops such as tomatoes by hydroponics. Here if root death occurs there is a release of organic material into the nutrient film that is flowing over the roots of the plants. This release can be measured as an increased consumption of bromine. Typical results for the murder of a busy lizzy plant are shown in fig. 4. 13.5 Fig. 4. Titration curves obtained during the demise of a busy lizzy. iD/CtA 180 5 4 9.0 ’% 4.5 solution in - Besides wishing to have an early indication of root death it is desirable for the horticulturalist to be able to monitor the principle nutrients in the nutrient film.An important trace element is iron which is present as the EDTA complexes of both FeII and FeIII. Using the packed-bed wall-jet electrode shown in fig. 5 where the packed Packed-bed Wall-Jet Electrodes W. J. Albery et al. 1037 135 315 225 solution out t packed bed counter compartment 270 counter electrode wall j e t disc electrode c 2 I ( Ag /Ag CI Fig. 5. The packed-bed wall-jet electrode (PBWJE). 1038 12 10 2 ’ 8 1 6 4 2 Electrochemical Sensors Theory and Experiment 0’ 0’ 8 1.5 6 0 4 6 2 4 2 Fig. 6. Typical results for the determination of total iron in hydroponic solutions using a PBWJE. 0.5 [ Fe] /mmol dm-3 5 0’ @/ 1.0 15 10 [ NO;]/mmol dm-3 0 Fig.7. Typical results for the amperometric determination of NO; using a PBWJE. bed is made of reticulated vitreous carbon we can reduce all the FeIII to FeII on the packed bed and then use the wall-jet electrode to determine the total iron in the s01ution.l~ Typical results are shown in fig. 6 . The most important nutrient is NO;. Pletcher and Poorabedi14 have shown that NO; can be determined amperometrically on a copper electrode. We have found that good limiting currents can be measured on a fresh copper electrode but that after several minutes the electrode is poisoned. This problem can be overcome15 by using a packed bed of copper chips and a platinum wall-jet electrode. A fresh copper disc is plated from W. J . Albery et al. 1039 Cu2+ generated on the bed electrode.The concentration of NO; is measured from the current on the fresh copper electrode. The copper is stripped off the wall-jet electrode and the cycle is then repeated. Typical results are shown in fig. 7. This method of renewing the surfacq of a solid electrode whenever it is required confers on solid-state electrochemistry the singular advantage so long enjoyed by mercury-drop polarography. Me TCNQ NMP’ Enzyme Electrodes Greater selectivity for compounds of biochemical interest can be obtained by using an enzyme to recognise the target species. An enzyme electrode is usually an amperometric sensor where the rate OF an enzyme-catalysed reaction is measured electrochemically. Many enzymes involved in oxidation and reduction reactions contain redox groups such as iron copper or quinone centres.However these centres are surrounded by a coat of protein and this coat prevents efficient electron transfer to ordinary electrodes. For this reason the first generation of enzyme electrodes used electrochemistry to detect the product of the natural enzyme reaction. The classic example is the glucose sensor using glucose oxidase. The reaction scheme is as follows solution glucose + FAD -+ gluconolactone + FADH FADH + 0 -+ FAD + H,O electrode H,O + 0 + 2H+ + 2e. The device is a fairly complicated one with two membranes one to keep the enzyme in place and one to protect the electrode from being poisoned by the enzyme. Since 0 is involved in the reaction scheme the response of the system is sensitive to the ambient 0 concentration.These disadvantages can be overcome by eliminating the 0 reaction and using instead an electron-transfer mediator. These second-generation enzyme electrodes have been developed for instance by Hill and Higgins.16-18 In their glucose oxidase electrode they use ferrocene/ferrocinium [Fe(Cp),/Fe(Cp)l] as a mediator solution glucose + FAD -+ gluconolactone + FADH FADH + 2Fe(Cp)i + FAD + 2Fe(Cp) + 2H+ electrode 2Fe(Cp) -+ 2Fe(Cp)i + 2e. Our own work has been concerned with the development of third-generation devices in which the enzyme reacts directly on the electrode itself. We have found that conducting organic salts like NMP+TCNQ- (1) are particularly good electrode materials for the direct transfer of electrons to and from enzymes.Similar conclusions have been reached by Kulys et al.199,0 The reaction scheme is as simple as it can be NC Ncm: solution glucose + FAD -+ gluconolactone + FADH NMP+TCNQ- electrode FADH -+ FAD + 2H+ + 2e. Many of these materials were first prepared by Melby21 and their electrochemistry has been investigated by Jaeger and Bard.22’ 23 We have presented elsewhere a general theory for this type of unmediated enzyme electr~de,,~ and we will now summarise the main conclusions of our theoretical approach. Electrochemical Sensors Theory and Experiment 2e 1 Fig. 8. The enzyme electrode. P (p-1 external m ediu m 1040 and also constants25 and kk where The Model Fig. 8 illustrates the enzyme electrode and the kinetic scheme.As regards the enzyme kinetics we assume the following model for a one substrate - one product enzyme which converts substrate S into product P and which in the course of this conversion is itself converted from E into E’ k k-2 k-1 S+E @ ES f k E’P e E’+P. For each step in the above scheme we write K = k,/k- KTD = Kl K2 K3 where KTD describes the overall equilibrium between S + E and P + E’. The transport of S and P through the membrane is described by the mass-transfer rate X is either S or P Dx is the diffusion coefficient and Kx the,partition coefficient of X in the membrane and LM is the thickness of the membrane. The electrode reaction described by k is assumed to be irreversible. All the primed rate constants are heterogeneous rate constants usually measured in cm s-l.Lower-case letters are used to denote the concentrations of the different species and for S and P the subscripts GO and 0 refer to the concentrations outside and inside the membrane respectively. We assume that the electrolyte layer behind the membrane is so thin (approximately a few pm) that there is no concentration polarisation in this layer. k3 k-3 1041 W. J. Albery et al. 1 I # ' I # 3 $ 1 I- f 2 E'+P 3 - - - - IG E+S - - Fig. 9. Schematic free-energy profile illustrating the free-energy differences associated with each of the 10 possible rate-limiting kinetic terms in eqn (1) to (3) The three terms that make up k,, in eqn (2) are circled and the three terms that make up kcat/& in eqn (3) are boxed.The four terms in the bottom row where the reactants are E+S make up the s term in eqn (1). The rest of the terms are found in the first term of eqn (1). !a The Steady-state Equation In the steady state we find24 that the flux j (usually measured in mol cm-2 s-l) is given by and ex is the total concentration of the enzyme. The expressions for kcat and KM/kcat have been discussed by Albery and Knowles.2s The free-energy diagram in fig. 9 shows how each term in eqn (1)-(3) corresponds to a possible rate-limiting free-energy difference in the enzyme kinetics. The advantage of the reciprocal expressions in eqn (1)-(3) is that the different possible rate-limiting processes are separated in this way.27 We now discuss the various terms in eqn (1).1042 First there are two terms which include L. These terms can only be dominant if the enzyme kinetics are rate-limiting; the first of these terms with kcat corresponds to the saturated enzyme and the second term with KM/kC,; to the unsaturated enzyme. If the flux j becomes close to the limit imposed by transport through the membrane j x kss, then the concentration polarisation means that the enzyme inside the membrane is less saturated than one would expect from the external concentration s,. This effect is described by the first bracket which reduces the significance of the kcat term. Electrochemical Sensors Theory and Experiment Secondly the simple term k'-l will be dominant if the electrode kinetics are rate limiting and if nearly all the enzyme is converted into E'; these conditions arise when the electrode kinetics are slow and there is no product inhibition.The rate constant k occurs in the same bracket as Lk,, since in either case the rate-limiting step involves turnover of the enzyme. Thirdly the other two terms involving k' are also cases where the electrode kinetics are rate-limiting. In the first bracket most of the enzyme is present as either ES or E'P while in the second bracket most of the enzyme is present as E and therefore requires S to be converted into E'. These terms are larger the larger the concentration of P behind the membrane whether this is because of the external concentration (p,) or because of slow transport of the generated P across the membrane ( j/rp).This product inhibition arises because in going from E ES or E'P to E' and thence to the rate-limiting transition state on the electrode P has to be released. This does not apply if E' is the dominant enzyme species when one obtains the simple k-l term. Finally we have the last term on the right of eqn (1). This term will dominate if the transport of S through the membrane is rate-limiting. Under these conditions j does not depend on the enzyme concentration; the kinetics of both the enzyme and the electrode are fast enough to consume S as soon as it passes through the membrane. No Product Inhibition Eqn (1) is a cubic in j but in our view little insight can be obtained by solving the cubic. It is however unlikely that for any system all the terms will be significant.The important application of the analysis is the identification of the rate-limiting process. For this purpose we have considered24 a number of special cases of eqn (1). In particular we take the case where there is no product inhibition. We rearrange eqn (1) into a form which is similar to a Hanes plot2* for the analysis of Michaelis-Menten kinetics In this equation we have introduced the effective electrochemical rate constant for the enzyme electrode at low substrate concentrations k; E where ( 5 ) lkME = KM/(eZ Lkcat) + lk&* We have introduced a similar parameter in our treatment of modified electrodesz9* 30 and indeed the KM term corresponds to the layer case of that treatment. Secondly we have introduced the equivalent of the Michaelis constant for the enzyme electrode KME where The significance of KME is similar to that of the Michaelis constant in homogeneous enzyme kinetics.For concentrations smaller than KME the system is unsaturated the current is proportional to the concentration of substrate and is governed by the rate constant kkE. For concentrations greater than KME the system becomes saturated and the flux reaches a maximum value. This flux can be characterised by the equivalent of kcat x m I - Q L 1 Y Fig. 10. (A) typical plots of flux against substrate concentration for different values of kdE/kk for the case where there is no product inhibition. For these curves (B) and (C) show the corresponding Hanes plots and plots of eqn (17) respectively. The values of kd/kk are as follows:- (-) 0.00.s,. Because k& describes a flux per unit area it has the usual dimensions (cm s-l) of an electrochemical rate constant. From eqn (5)-(7) we find For an enzyme electrode under unsaturated conditions this equation relates the kinetic description used by enzyme kineticists (kcat/KM) to the electrochemical rate constant (k’) used by electrochemists. The first stage of the analysis is to find kdE by making a Hanes plot of s,/j against Eqn (4) shows that this may be a curve but the limiting value as s + 0 gives [s,/j10 = (kkE)+. Next for values of s,/j significantly greater than [s,/’Jo we calculate values of p where 1.00 (-.-) 0.80 (- - .) 0.50 and (--) 1.0 W. J . Albery et al. I I 0 .o (8) 5 .O 4.0 3.0 2 .o 2.0 I 2.0 4.0 I 1 .o 0 .o 4 .O 1.0 P P = ( .i / s ) / ( j / s ~ ) o G 1. I 1 8.0 I 8.0 6 .O 1 6 .O S IKME s IK*E 2.0 1043 (9) 1044 f T 50 0 Electrochemical Sensors Theory and Experiment 150 5 > 3 E 100 20 10 [glucose] /mmol dm-3 Fig. 11. Variation of current with concentration of glucose for glucose electrodes made of three different salts of TCNQ- m TTF+; 0 NMP+; 0 Q'. Substitution in eqn (4) gives .Y=-=- 1 p-'-1 S A plot of y against p will then determine from the intercept at p = 0 the value of KME and from the intercept at y = 0 the ratio k M E / k $ . The relative importance of the transport and enzyme kinetic terms in eqn (5) for kdE is given by this ratio.If the ratio is unity then the transport of S across the membrane is rate-limiting. If on the other hand the ratio is much less than unity then the unsaturated enzyme kinetics are rate-limiting. Hence this is a valuable diagnostic plot. In the development of these electrodes it is vital to identify the rate-limiting step in the overall performance of the device so that research can be concentrated on improving the membrane the enzyme or the electrode kinetics. Fig. 10 shows typical j us. s curves Hanes plots and plots of eqn (10) for different values of kkE/kk. It is interesting that for the case where transport across the membrane is cleanly rate-limiting we obtain a sharp dog-leg plot of flux against concentration of substrate.This arises because under these conditions neither of the two rate-limiting processes transport or enzyme turnover under saturated conditions depends on the internal substrate concentration so; hence the flux is simply limited by the slower of the two processes. KME 30 100 80 10 I v) 1 .Y - 5 I v - 8 60 W - Y 40 20 0 The results in fig. 11 are analysed by the procedure presented above. The first stage of the analysis is to plot [glucose]/j = 2AF[glucose]/i against [glucose] as given by eqn (4). To compensate for the different areas (A) of the electrodes we carry out the analysis in terms of the current densities ( i / A ) . These plots are shown in fig. 12. From the intercepts I at zero concentration we find the electrochemical rate constant kLE where H Fig.12. Hanes plots28 of the data in fig. 11 for the measurement of glucose using three different salts of TCNQ- 0 TTF+; 0 NMP+; 0 Q+. and TTF Q' We have found that the enzyme glucose oxidase can be oxidised on six different conducting organic salts3' Results for three of the salts showing the variation of current with concentration of glucose are shown in fig. 11. In these salts the anion is TCNQO-; the cations are NMP+ TTF" (2) and Q+ (3). values of kkE are collected in table 4. kk = KsDs/LM. Glucose Electrodes W. J. AIbery et al. 1045 0 0 0 0 0 m o m a 0 10 30 20 [glucose]/mmol dm-3 1046 K M Eb kMEa kist EC 9 x 10-2 8 x 20 22 11 1.4 x 80 60 4.7 x 10-5 1.3 x 10-5 Calculated from eqn (1 3).a Calculated from eqn (1 1). Calculated from eqn (6). P 0.5 p = Ij/[glucose]. - - I 2 E -. a A 40 20 0 Next the parameter p [eqn (9)] is calculated where From eqn (10) y is plotted against p where 1 (1 -x). Electrochemical Sensors Theory and Experiment Table 4. Results for membrane electrodes /cm s-' /mmol dm-3 electrode material /cm s-' 3.0 x 10-5 TTF+TCNQ- NMP+TCNQ- Q+TCNQ- 1 .o Fig. 13. Plots of the data in fig. 10 according to eqn (13) for the measurement of glucose using three different salts of TCNQ- 0 TTF+; 0 NMP'; 0 Q'. ' = [glucose] p-1- 1 - - K~~ Pk', Plots of eqn (13) for the three electrodes are shown in fig. (13). In each case good straight lines are obtained showing the success of the analysis.The fact that in each case 1047 W. J . Albery et al. 4 I 8 I 2 1 6 1 [choline]/mmol dm-3 Fig. 14. Variation of current with concentration of choline for a choline electrode made of TTF+TCNQ-. Fig. 15. Plot of the data in fig. 14 according to eqn (10). y = 0 when p = 1 means that from eqn (13) we can conclude that kLE = k; and that in eqn (1 1) (14) KM/(eZ Lkcat) Q (kk)-l* This means that at low substrate concentrations the rate-limiting step is the diffusion of glucose through the membrane. The subsequent enzyme and electrode steps are so fast that they are not rate-limiting. This is the most desirable condition for a reliable sensor since the enzyme and electrochemical kinetics do not affect the response of the sensor.As long as this condition is maintained any decay in the enzyme or electrode activity has no effect. The results in fig. 11 show that glucose concentrations can be determined in the range 50 pmol dm-3 to 10 mmol dm-3. The fact that the transport of the glucose through the membrane is rate-limiting explains why the values of khE in table 4 are all so similar and do not depend on the electrode material. Returning to fig. 13 from the intercepts and eqn (13) we can calculate values of KME. Results are collected in table 4. In eqn (6) for K M E we have already shown that the k; 1048 Electrochemical Sensors Theory and Experiment Table 5. NAD-NADH enzyme systems analyte application enzyme alcohol dehydrogenase lactate dehydrogenase malate dehydrogenase alcohol lactate malate glutamate glutamate dehydrogenase glucose glucose dehydrogenase fermentation dairy industry fermentation fermentation food industry fermentation glycerol bile acids clinical fermentation clinical nitrate glycerol dehydrogenase 1 1 hydroxysteroid dehydrogenase nitrate reductase oestradiol oestradiol 17 dehydrogenase agriculture water industry agriculture food industry clinical fermentation amino acids amino acid dehydrogenase clinical food industry transport term dominates the numerator.Taking a value for k, of 800 s-l 32 and a value of L of several pm we find that the denominator in eqn (6) is dominated by the k electrode-kinetic term.Hence we can calculate values of the electrochemical rate constant k' for the oxidation of the enzyme; these values are reported in table 4. It is satisfactory that the three materials in table 4 are indeed excellent electrocatalysts for the direct oxidation of glucose oxidase with electrochemical rate constants k' which are all greater than cm s-l. We have also investigated the stability of the electrode. An electrode was run continuously for 28 days. During that time the response declined by only 20%. At the end of the period of the rate-limiting process was still transport of the glucose through the membrane. The enzyme and electrode kinetics were both still sufficiently rapid to handle the substrate that arrived.These results are very encouraging and show that the system is both stable and robust. Choline Oxidase Another enzyme which can be directly oxidised on TTF+TCNQ- is choline oxidase. Typical results for the variation of current with concentration of choline are shown in fig. 14. The same analysis is applied and fig. 15 shows the p plot. In this case the straight line passes through 2 on the x axis. This shows that in eqn (5) the two terms are equal. The transport through the membrane and the unsaturated enzyme kinetics are equally ra te-limi ting. NADH Systems We have also shown that these materials make good electrodes for the oxidation of NADH.33 Over 250 enzymes use NAD+ as a cofactor and hence we can design sensors using a wide variety of different solution dehydrogenases SH + NAD+ for dehydrogenase - a typical substrate S + NADH SH, + H+ NMP+TCNQ- electrode NADH ___+ NAD+ + H+ + 2e.0.5 0 -4 0 . 3 2 3 0.2 0.1 3 0 Fig. 16. Variation of current with concentration of ethanol for an ethanol electrode made of 0 A . 0.3 - - \ -0 c( - I i 6 0.1 - W. J . Albery et al. 6 [ ethanol]/mmol dm-3 0 I NMP+TCNQ-. fi 0.4 0 " - 0 I 0.2 P m 0.2 1 0 Fig. 17. Plot of the data in fig. 16 according to eqn (10). In table 5 we collect together some examples of the possible application of this type of sensor. In fig. 16 we show results obtained for an ethanol sensor using ethanol dehydroogenase. Application of the analysis gives the p plot shown in fig.17. In this case a horizontal straight line is obtained. This means that the unsaturated enzyme kinetics are rate-limiting. Hence in the three cases described here we have shown how the p plot can discriminate between the glucose electrode where transport is rate-limiting (fig. 13) the ethanol electrode where enzyme kinetics are rate-limiting (fig. 17) and the choline electrode where both processes are partially rate limiting (fig. 15). We believe that such analysis leading to the identification of the rate-limiting step is essential for a proper understanding of these devices. We are grateful to the following colleagues who carried out some of the experimental work reported in this paper Dr L. R. Svanberg on protein titrations Mrs M.M. P. Net0 and Mr C. P. Jones on the determination of iron and NO, respectively Mr B. J. Driscoll 1049 9 12 0 0.6 Electrochemical Sensors Theory and Experiment on choline oxidase and Mr K. W. Sim on ethanol dehydrogenase. We thank the S.E.R.C. the A.F.R.C. the Gulbenkian Foundation Pharmacia B.P. and Genetics International for financial support. 1050 References 1 W. J. Blaedel C. L. Olson and L. R. Sharma Anal. Chem. 1963 35 2100. 2 W. J. Blaedel and L. N. Klatt Anal. Chem. 1968 40 512. 3 R. Braun J. Electroanal. Chem. 1968 19 23. 4 J. Yamada and H. Matsuda J. Electroanal. Chem. 1973 44 189. 5 B. Fleet and C. J. Little J. Chromatogr. Sci. 1974 12 747. 6 M. Varadi and E. Pungor Anal. Chim. Acta 1977 94 351.7 A. N. Frumkin and L. I. Nekrasov Dokl. Akad. Nauk SSSR 1959,126 115. 8 W. J. Albery and S. Bruckenstein Trans. Faraday Soc. 1966 62 1920. 9 W. J. Albery and M. L. Hitchman Ring-Disc Electrodes (Clarendon Press Oxford 1971). 10 W. J. Albery and C. M. A. Brett J. Electroanal. Chem. 1983 148 201. 11 W. J. Albery L. R. Svanberg and P. Wood J. Electroanal. Chem. 1984 162 29. 12 W. J. Albery L. R. Svanberg and P. Wood J. Electroanal. Chem. 1984 162,45. 13 W. J. Albery and M. M. P. M. Neto Portugaliae Electrochimica Acta 1985 3 67. 14 D. Pletcher and Z. Poorabedi Electrochim. Acta 1979 24 1253. 15 W. J. Albery B. G. D. Haggett C. P. Jones M. J. Pritchard and L. R. Svanberg J. Electroanal. Chem. 28 C. S. Hanes Biochem. J. 1932 26 1406. 1985,188,257. 16 A.E. G. Cass G. Davis G. D. Francis H. A. 0. Hill W. J. Aston I. J. Higgins E. V. Plotkin L. D. L. Scott and A. P. F. Turner Anal. Chem. 1984 56 667. 17 A. E. G. Cass G. Davis H. A. 0. Hill I. J. Higgins E. V. Plotkin A. P. F. Turner and W. J. Aston in Charge and Field Eflects in Biosystems ed. M. J. Allen and P. N. R. Usherwood (UK Abacus Press London 1984) p. 475. 18 A. E. G. Cass G. Davis H. A. 0. Hill and D. J. Nancarrow Biochim. Biophys. Acta to be published. 19 J. J. Kulys A. S. Samalius and G. J. S. Svirmickas FEBS Lett. 1980 114 7. 20 J. J. Kulys and A. S. Samalius Bioelectrochemistry and Bioenergetics 1983 10 385. 21 L. R. Melby Can. J. Chem. 1965 3 1448. 22 C. D. Jaeger and A. J. Bard J. Am. Chem. Soc. 1979 101 1690. 23 C. D. Jaeger and A. J. Bard J. Am. Chem. Soc. 1980 102 5435. 24 W. J. Albery and P. N. Bartlett J. Electroanal. Chem. 1985 194 211. 25 W. J. Albery Electrode Kinetics (Clarendon Press Oxford 1975) p. 58. 26 W. J. Albery and J. R. Knowles Biochemistry 1976 15 5631. 27 W. J. Albery and J. R. Knowles Biochemistry 1976 15 5588. 29 W. J. Albery and A. R. Hillman Annu. Rep. Progr. Chem. Sect. C 1981 377. 30 W. J. Albery and A. R. Hillman J. Electroanal. Chem. 1984 170,27. 31 W. J. Albery P. N. Bartlett and D. H. Craston J. Electroanal. Chem. 1985 194 223. 32 M. K. Weibel and H. J. Bright J. Biol. Chem. 1971 246,2734. 33 W. J. Albery and P. N. Bartlett J. Chem. Soc. Chem. Commun. 1984 234. Paper 511880; Received 21st October 1985

ISSN:0300-9599    

DOI:10.1039/F19868201033

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J . Chem. Soc. Faraday Trans. I 1986,82 1051-1070 Sensors from Polymer Modified Electrodes Mark W. Espenscheid Amiya R. Ghatak-Roy Robert B. Moore 111 Reginald M. Penner Marilyn N. Szentirmay and Charles R. Martin" Department of Chemistry Texas A and A4 University College Station Texas 77843 U.S.A. The construction and operation of ionomer-film modified electrodes are described including the fundamentals of the ion-exchange processes. Ion- exchange selectivity coefficients for a series of alkylsubstituted pyridinium cations in Nafion have been measured and the large values of these coefficients are attributed to the hydrophobic effect; a free-energy relation- ship relating the coefficient to the size of the ion has been established. The use of Nafioncoated electrodes for the in uiuo measurement of neurotrans- mitters and for the preconcentration of analytes present in low concentra- tion is discussed Experiments on the dynamics of the response of ionomer modified electrddes are reported and very high ionic transport rates can be achieved for novel ionically conductive composite polymer membranes.The covalent attachment of redox groups such as ferrocene allows one to control the release of counterions by switching the redox state of the ferr ocene . This paper will deal primarily with ionomer-film modified electrodes. An ionomer is a linear or branched organic polymer which contains covalently attached ionizable groups. Ionomers generally contain rather low quantities of ionizable groups (i.e. less than ca.10 mol% ionizable monomers) and are not covaIently cross-linked. Thus ionomers are chemically and morphologically distinct from conventional organic ion-exchange materials (e.g. styrenesulphonate-based resins). The structures for the two ionomers to be discussed in this paper are shown in fig. 1. Ionomer-film modified electrodes were prepared by coating ca. 1 pm thick ionomer films onto vitreous carbon or Pt surfaces using conventional solution-casting techniques2 Ionomers are suitable for a variety of electrochemical processes and devices including energy conversion and storage systems3 and industrial electrolytic pro~esses.~ Nearly all of these applications rely on the ability of the ionomer to exchange and transport counterions and reject co-ions (ions of the same charge as the fixed site).It seems likely that these properties could be of use in an electroanalytical system and we are investigating the possibility of using ionomer-film modified electrodes as electrochemical sensors. This paper will focus on fundamental aspects of ionomer-modified electrode sensor response. Topics to be discussed include (1) the thermodynamics of ionomer- counterion interactions (2) the dynamics of ionomer modified electrode response and ( 3 ) in situ film regeneration in ionomer modified electrodes. Thermodynamics of Ionomer-Counterion Interactions Fundamentals of the Ion-exchange Process in Perfluorosulphonate Ionomers Our interest in this subject stems from observations made in Prof. Freiser's laboratory concerning the selectivity of Nafion (see fig.1) membrane-based ion-selective electrodes.6 These electrodes showed remarkable affinities for large hydrophobic cations (e.g. alkylammonium ions) relative to simple monovalent or divalent inorganic cations.6 This unusual ion-exchange selectivity was later echoed in voltammetric studies on Nafion-film 1051 1052 0 I 0 I SO3H Polymer Modified Electrode Sensors C I FZ CF-CF3 0 I I C F2- C F2 - SO H ( a ) C I F2 c I F ( b ) Fig. 1. Structures of perfluorosulphonate ionomers. (a) DuPont Nafion (b) Dow PFSA. modified electrodes conducted in Prof. Bard's laboratory.' From a qualitative point of view these studies suggest that if Ogn+ represents a hypothetical hydrophobic organic cation the position of equilibrium in the ion-exchange reaction shown in eqn (1) would lie very far to the right Ogn+(aq) + nNa+(film) -+ Ogn+(film) + nNa+(aq).Ogn+(ag) + Ogn+(film). (1) If this is the case the ionomer film could be used to extract and preconcentrate hydrophobic organic cations from a contacting aqueous phase. By coating the film on an electrode surface a subsequent electrochemical experiment could be used to quantitate the film entrapped cation (vide infra). The extent of the preconcentration effect depends on the magnitude of the ion-exchange selectivity coefficient. Since no ion-exchange data for hydrophobic cations in Nafion had been reported in the literature we set out to determine selectivity coefficients for a variety of organic cations in Nafion.An electrochemical ion-exchange isotherm methods and a chromatographic method9 were used to obtain these ion-exchange data. In the isotherm method a Nafion film-modified electrode (initially in the Na+ form) was exposed to a very dilute (ca. lo+ mol dm-3) solution of the electroactive cation whose selectivity coefficient was to be measured (the solution was also 0.1 mol dm-3 in supporting electrolyte NaClO,). The film was then equilibrated with the solution by rapid rotation of the electrode. Depending on the ion and its concentration this process could take as long as 25 h.8 When film-solution equilibrium was achieved the quantity of electroactive ion exchanged into the film was determined coulometrically.8 This procedure was then repeated for increasingly higher concentrations of the electroactive ion.The extent of the exchange of the electroactive cation (Ogn+) for Na+ in the Nafion film can be expressed in terms of both a distribution coefficient k, and an ion-exchange selectivity coefficient at. kD is the equilibrium coefficient for the hypothetical (vide infra) partition reaction ( 2 ) More importantly kD is the slope of the partition isotherm for the electroactive counterion (see fig. 2) and as such gives a measure of the extent of the ion-exchange reaction. is the equilibrium coefficient for the ion-exchange reaction shown in eqn (1) and is given bylo where the a are activities in the solution phase and the x are the equivalent ionic fractions of -SO; sites occupied by each ion in the film phase.Details of this method and of the calculations involved are presented in ref. (8). Fig. 3 shows voltammograms for Nafion-coated electrodes after equilibration with 1 M. W. Espenscheid et al. I . . . . I . . . . 1 . . . 1 . 4 2.5 0 solution concentration/ 1 0-8 mol dm-3 10 5 solution concentration/ 1 0-8 mol dm-3 2 3 solution concentration/ 1 O-' rnol dm-3 1.5 0.5 1 2 solution concentration/ lo-' mol dm-3 Fig. 2. Ion-exchange isotherms for (a) Ru(bpy)g+ (bpy = 2,2-bipyridine) (b) Ru(NH,)i+ (c) methyl viol ogen and (d) ferrocen ylme thy1 trime t h ylammoni um . very dilute solutions of the various electroactive counterions. These voltammograms are the first indicators of the magnitudes of the ion-exchange selectivity coefficients for these electroactive counterions.No signals above background currents would be detected at uncoated electrodes in these extremely dilute (< 5 x mol dm-3) solutions. Clearly as suggested above the Nafion film preconcentrates these cations and the equilibrium film concentrations are orders of magnitudes higher than the solution concentrations. Ion-exchange reactions should be described by an equilibrium-coefficient expression like that shown in eqn (3) which takes into account the solution and film concentrations of both the exchanging and exchanged ions. However when the concentration of the exchanged ion (Na+) in the aqueous phase is high and the concentration of the exchanging ion (Ogn+) in the film is very low the aNa and zNa terms in eqn (3) are constant and the reaction may be regarded as a partition reaction [eqn (2)].119 Simplifying the ion-exchange reaction to a partition reaction is attractive because the extent of the reaction may be conveniently visualized by plotting a partition isotherm ([Ogn+],il us.[Ogn+Iw)3 l4 Fig. 2 shows partition isotherms for the various electroactive cations studied here. The linear portions of these isotherms show the (low) concentration regions over which it is appropriate to use eqn (2) to describe the ion-exchange process. The slopes of these 0 0 1053 l2 1054 I I I I 1.2 0.8 -0.2 I Polymer Modified Electrode Sensors u I I 1 I 0.2 I I -0.4 0.0 0.8 I I I -0.6 I -0.8 (c) 4.36 x 0.4 EIV us.SCE Fig. 3. Cyclic voltammograms recorded at a scan rate of 0.1 V s-l for Nafion-coated electrodes in equilibrium with (a) 1.51 x mol dm-3 Ru(bpy)$+ (6) 2.78 x mol dm-3 MV2+ ( d ) 3.40 x 0.100 mol dmP3 NaClO,. mol dm-3 Ru(NH3):+ mol dm-3 FA+. Supporting electrolyte I I 1 0 . 4 0.0 0.0 1055 cation Ni3+ kD M. W. Espenscheid et al. Table 1. Ion-exchange distribution coefficients kD and selectivity coefficients Kg:? K M ~ + 7.9 x 105 2.5 x lo6 3.7 (k0.5) x lo4 2.1 x lo7 5.7 (+ 1.1) x lo6 1.1 x 106 2.6 x 104 1.5 (k0.2) x 104 7.3 (k0.9) x 104 740d MV2+ a FA+ Ru(NH3):+ Ru(bPY):+ R~(NH,);+ c a Methyl viologen. Ferrocenylmethyltri- methylammonium. Calculated from Ee. shift. Assuming zNa+ = 0.90.isotherms k, are shown in column 2 of table 1. The immenseness of these slopes clearly shows that Nafion greatly prefers these electroactive counterions over Na+. Furthermore these k are essentially Nafion-water preconcentration factors and their immense sizes suggest that Nafion would indeed be a useful preconcentration material for these or similar ions. While the partition isotherm allows for a convenient visualization of the extent of the ion-exchange reaction if ion-exchange data obtained here are to be compared with existing data for this or other ion-exchange systems and if the data outside of the linear isotherm region are to be treated ion-exchange selectivity coefficients [eqn (3)] should be calculated. Ion-exchange selectivity coefficients for the variouselectroactive counterions are shown in column 3 of table 1.These coefficients were calculated from points on both the linear and non-linear portions of the isotherms. While as indicated by the standard deviations there is some scatter in these coefficients no trends in PNi with xos were observed. The magnitudes of these ion-exchange selectivity coefficients are truly remarkable. Yeager and Steck studied exchange reactions of alkali-metal and alkaline-earth ions in Nafion.loV l5 (While these authors used H+ as the exchanged ion and we used Na+ they have shown that K p z 1 ;lo this allows for rough comparisons of their coefficients with ours.) The largest selectivity coefficient for a monovalent ion observed by Yeager and Steck was SHS which has a value of 9.1 ;lo the largest for a divalent ion was K p which had a value of ca.30.15 Selectivity coefficients for the ions studied here are from 3 to 6 orders of magnitude larger than these values; clearly Nafion shows tremendous preference for the ions studied here over the simple inorganic ions studied by Yeager and Steck. Conventional cation-exchange materials (i.e. sulphonated styrenedivinylbenzene resins) show ion-exchange selectivity coefficients for the alkali-metal and alkaline-earth ions of the same order of magnitude as those for Nafion (i.e. l-1O).l6 However since most of the ions studied here are hydrophobic organic cations it is of interest to compare the ion-exchange data obtained here with selectivity coefficients for organic ions on conventional ion-exchange resins.Gregor and Bregman studied ion-exchange reactions of a variety of alkyl- and phenylalkyl-ammonium ions on resins containing ca. 1-35% di~iny1benzene.l~ While selectivity coefficients as large as ca. 15 were observed,17 none of the resins showed the remarkably large ion-exchange selectivity coefficients exhibited by Nafion. Because most of the ions in table 1 are hydrophobic these data corroborate the conclusion reached by Martin and Freiser (from ion-selective electrode experiments) that Nafion shows tremendous affinity for hydrophobic cations6 This conclusion is also corroborated by our luminescence studies of Nafion.l89 l9 However because the ions in table 1 have different structures and charges it is impossible to assess quantitatively the 1056 Polymer Modified Electrode Sensors Table 2.Ion-exchange selectivity coefficients on ODs-Nafion columna pyridinium ion 9.2 x lo2 2.3 x 103 6.8 x lo3 2.2 x 104 ethylp yridinium prop ylpyridinium butylpyridinium pentylpyridinium a Mobile phase = 0.95 mol dmP3 aqueous TMABr flow rate = 1 cm3 min-l low3 cm3 sam- ples of mol dm-3 aqueous pyridinium solu- tion injected. contribution of the hydrophobic interaction to the magnitude of the selectivity coefficient. The contribution of the hydrophobic effect could be assessed by evaluating selectivity coefficients for an homologous series of organic counterions.20 Because the electrochemical isotherm approach is tedious and time consuming we decided to study selectivity for such an homologous series using a high-performance liquid chromatographic method (HPLC).21 Commercially available octyldecyl-derived silica (ODS) particles (5 pm average diameter) were coated with Nafion by exposing the particles to a solution of Nafion and then evaporating the solvent.21 After sizing these Nafion-coated particles were packed into stainless-steel HPLC columns using conventional slurry packing techniques.21 An homologous series of N-alkylsubstituted pyridinium cations was prepared by reacting pyridine with the appropriate alkylbromide.21 These ions were injected onto the Nafion-based columns and eluted with aqueous tetramethylammonium bromide. The ion-exchange selectivity coefficients for the alkylpyridinium ions were calculated from the measured retention volumes using standard HPLC procedures.21 Ion-exchange selectivity coefficients for the various alkylpyridinium ions are shown in table 2.Note again that the selectivity coefficients for these hydrophobic organic cations are orders of magnitude greater than selectivity coefficients obtained by Yeager and Steck for inorganic cations.1° Furthermore the selectivity coefficients shown in table 1 are measured us. tetramethylammonium (TMA+) whereas Yeager’slO are measured us. H+. Because inorganic ions will not elute the larger pyridinium ions from the Nafion values of KgY if they could be measured directly would be even larger than the values (m. TMA+) reported here. The role of hydrophobic interactions in the binding of counterions to Nafion is more dramatically demonstrated and placed on more quantitative terms through a plot of log KgLA us.number of carbon atoms in the pyridinium ion. This ‘free-energy plot’20 (fig. 4) is linear (slope = 0.380 correlation coefficient = 0.996) demonstrating that each increment of molecular bulk (i.e. hydrophobicity) adds an inwement of thermodynamic driving force to the ion-exchange reaction. From the slope of this line it can be calculated that each additional carbon contributes an additional -0.518 kcal mol-l to the standard free-energy change for the ion-exchange reaction of an alkylpyridinium ion. Fig. 4 shows quantitatively that the hydrophobic effect is responsible for the enormity of ion-exchange selectivity coefficients for organic cations in Nafion.As noted above conventional ion-exchange materials do not show this marked preference by hydrophobic counterions.17 We believe that the difference between the exchange characteristics of Nafion and conventional resins can be explained by considering the structural differences between these polymers. First conventional ion-exchange resins are covalently cross- linked while Nafion is not. The Gibbs-Donnan equation (the fundamental thermo- 1057 M . W. Espenscheid et al. 51 1 6 11 8 7 9 no. of C atoms 10 Fig. 4. Plots of log K;GA vs. number of carbon atoms in the pyridinium cation for (a) Nafion (1 100 E.W.) and (b) Dow PFS/(1065 E.W.) (based on HPLC columns). dynamic equatiotl for ion-exchange reactions)22 accounts for the effect of cross-linking through inclusion of a PV term.22 This term discriminates against ions of large size (large V).179 22 Since Nafion is not cross-linked there should be no PV-based discrimina- tion against large ions and therefore large ions may be partitioned into Nafion without thermodynamic penalty.The second important structural difference between Nafion and conventional ion- exchange resins is that in Nafion only ca. 1 in every 8 monomer units is sulphonated while conventional resins are close to 100% sulphonated. We believe that the large segments of uncharged chain material allow for a greater extent of interaction with the non-polar parts of the exchanging ion. Sensor Development Because a Nafion film can preconcentrate a hydrophobic organic cation at a substrate electrode surface Nafion-coated electrodes show promise for development as sensitive and selective sensors for electroactive organic cations.Indeed we propose that Nafion or other ionomer-coated electrodes could be used in a trace analytical technique which we have called ion-exchange voltammetry. Ion-exchange voltammetry (i.e.v.) is qualitatively similar to anodic stripping voltammetry (a.s.v.) ; a comparison of these two techniques is shown in fig. 5. In a.s.v. a metal ion is preconcentrated at a small-volume Hg electrode by reduction of the ion to the amalgam. After this cathodic preconcentration the amalgamated metal is oxidatively stripped from the Hg electrode and the resulting anodic current is recorded and related to the concentration of the metal ion in the solution.1.e.v. also uses a preconcentration step to achieve high sensitivity; in this case however the inherent thermodynamic driving force of the ion-exchange reaction is used to preconcentrate an organic cation into a small-volume ionomer film on the electrode surface. The film-entrapped ion is then either oxidized or reduced and the resulting current is recorded and related to the concentration of ion in the aqueous solution (fig. 5). We have recently shown that ion-exchange voltammetric determination of the 1058 Polymer Modijied Electrode Sensors (a) a.s.v. (1 ) preconcentrate electrochemically M2+ + Hg + 2e- + Hg(M) (2) reoxidize-record current Hg(M) + Hg + Mz+ + 2e- Fig. 5.Schematic representations of anodic-stripping voltammetry and ion-exchange voltammetry. DA DHB NE 5-HT og2+ + e- - o’g. Fig. 6. Structures of neurotransmitters dopamine (DA) norepinephrine (NE) and 5-hydroxytriptamine (5-HT) and of dihydroxybenzylamine (DHB). dication methyl viologen at a Nafion-film-coated electrode yields an improvement of three orders of magnitude in detection limit relative to an uncoated electrode.* In a collaborative effort with Prof. Adams of the University of Kansas we are currently assessing the feasibility of using ion-exchange voltammetry at Nafion-coated microelectrodes for in vivo determinations of dopamine and other neurotransmitters (see fig. 6 for structures) in rat brain.23 Because these transmitters are amines they are protonated at physiological pH values and thus can be ion exchanged into Nafion.On the other hand the biogenic amine metabolites and the important brain substance ascorbic acid are anions at physiological pH values and thus should be rejected by the Nafion film. A recent report from Professor Adams’ laboratory shows that Nafion-coated carbon microelectrodes do indeed show very little anion interferen~e.~~ This differen- I Y ( b ) i.e.v. ( 1 ) preconcentrate via ion exchange Og” + 2(P-S03H) + (P-S03)20g+2HS (2) reduce film-entrapped ion- record current OH 1059 M. W. Espenscheid et al. Table 3. Ion-exchange selectivity coefficients for protonated forms of the neurotransmitters* no. ofOH KNT+ TMA+ no. of molar mass/ OH DOUPS groups molar mass/g ion 170 140 154 56.7 70.0 77.0 K ; s 2.5 x lo2 1.5 x 7.5 x 102 4.5 x 103 8.3 x lo2 5.0 x 1.6 x 104b norepinephrine 3,4-dihydroxybenzylamine dopamine serotonin 2 3 2 1 177 2.7 x 103 177 a 3,4-Dihydroxybenzylamine included for comparison.Calculated from KgZ and conversion factor obtained by eluting 3,4-dihydroxybenzylamine with both TMABr and NaBr. tiation of the neurotransmitters (NT) from metabolites and ascorbate has been sorely needed for in viuo electro~hemistry.~~9 24 In addition to the selectivity advantage since NT are organic cations they should be preferentially partitioned into a Nafion film at an electrode surface; thus the coated electrode should show better detection limits than an analogous naked electrode.Since hydroxy substitution renders a molecule more hydrophilic this preferential partitioning should be partially mollified by the hydroxy groups present on these ions and we anticipated seeing a mass (hydrophobicity increases ion-exchange selectivity coefficient) us. hydroxy substitution (hydrophilicity decreases selectivity coefficient) tradeoff. This hydrophobic-hydrophilic tradeoff is evident in the aqueous-Nafion ion-exchange data for NT (table 3). The largest ion-exchange selectivity coefficient is observed for 5-HT+ (see fig. 6) which has the highest molecular weight and the fewest hydroxy groups; the smallest coefficient is observed for NE+ which while not having the lowest molecular weight has the largest number of hydroxy groups.In fact a plot of KzE us. the empirical parameter obtained by dividing the molar mass by the number of hydroxy groups (table 3) is quite linear (slope = 114 correlation coefficient = 0.995). While this empiri- cal parameter seems to be devoid of thermodynamic significance it is apparently useful for predicting ion-exchange selectivity coefficients at least within the limited class of compounds studied here. The adherence of the ion-exchange data for NT+ to the predicted trends reinforces our earlier conclusions regarding the importance of hydro- phobic interactions to counterion binding in N a f i ~ n . ~ - ~ Since all of the selectivity coefficients in table 3 are quite large a Nafion film should preconcentrate these ions and ion-exchange voltammetry at a Nafion-coated electrode should result in improved detection limits.This is demonstrated by the data shown in fig. 7. A Nafion-coated electrode was equilibrated for 3 min (with stirring) in a solution lob7 mol dm-3 in DA+ and 0.01 mol dm-3 in Na+ (phosphate buffer pH 7.0). After the equilibration period a cyclic voltammogram was obtained and the process was repeated for successively higher concentrations of DA+. Fig. 7 [curve (a)] shows a plot of anodic peak current (background corrected) us. concentration of DA+ at a Nafion-modified electrode. Note that because of the preconcentration effect the detection limit (d.1.) for DA+ is ca. mol dm-3. This d.1. is two orders of magnitude lower than the corresponding d.1. at a naked electrode.While curve (a) in fig. 7 clearly shows that significantly improved d.1. can be obtained at Nafion-coated electrodes such electrodes might suffer from interference not observed at naked electrodes. Organic cations present in the analyte solution which are not electrochemically active could (if their concentrations or selectivity coefficients were large enough) partition into the film and occupy a fraction of the Nafion -SO; sites. Since this would decrease the number of sulphonate sites available for the analyte cation the sensitivity for the analyte cation could be decreased. This possibility was explored by 1060 9 - 8 - 7 - c g a .i 5 - 6 - -. 4 - 3 - 2 - 0 1 2 3 4 5 6 7 8 DA' concentration/ 1 0-6 mol dm-3 Fig.7. Peak current for oxidation of DA+ (background corrected scan rate = 100 mV s-l) at a Nafion-modified electrode (8.9 x mol SO; sites) 0s. [DA+]. Supporting electrolyte was 0.004 mol dm-3 Na,HP0,/0.002 mol dm-3 NaH,PO, pH = 7.0. Concentration of phenylethyl- amine (a) 0 (b) 2 x and (c) 2 x obtaining DA+ calibration curves in the presence of phenylethylamine which is protonated at pH 7.0 but not electrochemically active. As indicated in curves (b) and (c) of fig. 7 reduced sensitivities to DA+ are seen when phenylethylamine is present. From a practical point of view this means that analyses with Nafion-modified electrodes will require some knowledge of the composition of the analyte solution. An identical situation is encountered in analyses with ion-selective electrode~.~~ Nafion-coated microelectrodes (100-300 pm) were checked for such inter- ferences by calibrating in brain homogenates at pH 7.4.23 The calibration curves obtained showed no significant decrease in slope when compared to buffer-only calibration curves.23 Dynamics of Ionomer Modified Electrode Response and New Fast Ion-transporting Membranes The maximal preconcentration advantage for an ionomer film-modified electrode will be obtained when ionomer film-analyte solution equilibrium is achieved.The time required for establishment of equilibrium depends on the size charge structure and concentration of the analyte counterion and on the ionic strength of the analyte solution. We have shown that relatively small counterions (e.g.DA+) at the lO-'mol dm-3 concentration level reach equilibrium with a 1 pm thick Nafion film in less than 3 mhZ3 In contrast larger counterions [e.g. Ru(bpy):+] at the mol dm-3 concentration level may require as long as one day to reach equilibrium,8 which from an analytical viewpoint is an unacceptably long time. One can of course decide to wait only a small fraction of the total equilibration time before assaying the quantity of analyte partitioned into the film but the maximal preconcentration advantage will not be realized.8 Polymer Modijied Electrode Sensors mol dm-3. 1061 M . W. Espenscheid et al. ionomer impregnated pores t ineit PTFE porous membrane Fig. 8. The ionically conductive composite polymer membrane concept. In general the rate of film-solution equilibration is limited by the rate of ionic diffusion in the film.8 Thus membranes with higher ion-transport rates would produce modified electrode sensors with faster response times; we are currently attempting to develop such fast ion-transporting membranes.We have recently described a new series of ionically conductive composite polymer membranes.26 The concept behind these membranes is illustrated schematically in fig. 8. A porous inert host material which is initially neither wetted by water nor ionically conductive is impregnated with an ionomer which renders the membrane water swollen and counterion conductive. We reasoned that because the counterion moves through a water-filled pore ionic diffusion in these membranes should be fast.Prototype membranes of this type were prepared by impregnating Gore-tex (a commercially available porous polytetrafl~oroethylene)~~ with Nafion. A Gore-tex membrane (0.2 pm mean pore diameter) was stretched over the face of a wax-impregnated graphite (WIG) electrode and held into place with a collar of heat-shrinkable Teflon tubing (fig. 9). Nafion was loaded into the Gore-tex by immersion of the membrane into an ethanolic Nafion solution (0.7-2.5 w/v %).26 The electrode was then removed from the impregnating solution and the solvent was allowed to evaporate (room tempera- ture 5 h). Fig. 10 curve (a) shows a ‘cyclic voltammogram’ for a native Gore-tex membrane- modified WIG electrode in aqueous supporting electrolyte. Because the Gore-tex membrane is not wetted by water the electrode surface is completely insulated from the solution phase and no background currents are observed.However Nafion-impregnated Gore-tex (NIGT) is wetted and ionically conductive and background voltammograms at NIGT-modified WIG [fig. 10 curve (b)] and naked WIG electrodes are essentially identical. The ionic conductivity of NIGT is even more dramatically illustrated by the voltammograms shown in fig. 11 ; a NIGT-modified WIG electrode was immersed into a solution which was 3 mmol dm- in Ru(NH,):+ and the potential was scanned over the Rurrl/I1 redox wave. An Ru(NH,)i+ ‘loading voltammogram’ very similar to that observed at a Nafion-film modified electrode is obtained.2 Ru(NH,)g+ is entering the NIGT membrane diffusing through the membrane to the electrode surface and undergoing reduction and reoxidation at the electrode surface.When Ru(NH,)i+ loaded NIGT-modified WIG electrodes are transferred to supporting electrolyte solutions a fraction of the complex is retained by the membrane (fig. 12); this is again analogous to the results obtained with Nafion film-coated electrodes.2 As shown in fig. 12 the differences in potential between the anodic and cathodic peaks for 1062 Polymer Modijied Electrode Sensors wire 6 m m glass tube heat shrinkable Teflon sleeve 9 mm glass tube mercury - wax impregnated graphite h e a t shrinkable' Tef Ion sleeve 10 p A I t I I Nafion impregnated Gortex membrane Fig. 9. The Nafion-impregnated Gore-tex film-modified wax-impregnated graphite electrode.( b ) I I I T I I I U.b (a) U n u .4 U .L -0.2 -0.4 -0.6 -0.8 EIV vs. SCE Fig. 10. Background voltammograms (200 mV s-l) in 0.1 mol dm-3 NaClO,. (a) Native Gore-tex membrane-modified WIG electrode (b) Nafion-impregnated (14.6 w/w % ) Gore-tex membrane- modified WIG electrode. the Ru(NH3):+I2+ waves decrease as the quantity of Nafion incorporated increases. This suggests as might be expected that the ionic conductivity of NIGT increases with the quantity of Nafion incorporated. As noted above we initiated this research in the hope of developing fast ion- transporting membranes. To obtain a quantitative measure of the facility of ion transport chronocoulometric experiments2 were used to determine diffusion coefficients for Ru(NH,)g+ and Ru(NH,)f+ in NIGT membranes.As shown in table 4 these diffusion coefficients are ca. three orders of magnitude larger than the analogous diffusion coefficients in Nafion. These data clearly show that ionically conductive composite membranes can produce very high ion-transport rates. A multitude of questions remain to be answered before the practical utility of these membranes can be assessed; these include how do pore size of the host and quantity of the ionomer incorporated affect the rate of ion transport how do pore-size distribution and chemical properties of the host affect the rate of ion transport how chemically and thermally stable are these composites and can permselective membranes be prepared using this approach? We are currently addressing these questions.1063 n M. W. Espenscheid et al. SCE Fig. 1 1. Loading voltammograms for incorporation of Ru(NH,)i+ into a NIGT membrane at a WIG electrode surface. Solution was 3 mmol dm- in Ru(NH,);+ and 0. 1 mol dm- in NaClO (every fifth scan recorded). I n Situ Film Regeneration Ionomer-modified electrodes are examples of chemical sensors in which a membrane is placed between the analyte solution and the detection system; the purpose of the membrane in such a sensor is to provide selective chemistry such that the detection system ‘sees’ or has opportunity to respond to only the desired analyte species. We have discussed chemical selectivity in ionomeric systems in this paper. In fact ionomers provide a very rudimentary form of chemical selectivity (see Conclusions section) yet ionomer-based electrochemical sensors can be quite useful for certain types of analyses (vide supra).In addition to ‘building’ chemical selectivity into the membrane one must also ‘ build in ’ facile mass-transport ; if not the sensor’s response time will be prohibitively long. We have presented a general scheme for providing facile mass-transport in polymer membranes. There is another factor which relates both to selectivity and to mass transport which must be considered when designing a sensing membrane. Consider a well stirred solution containing the analyte species which is in contact with a sensor membrane. Furthermore assume that the membrane has been appropriately designed so that (1) there is a large FAR 1 36 1064 Polymer Mod$ed Electrode Sensors 0.2 0.0 A/ Fig.12. Ru(NH3),3+-loaded NIGT-WIG electrodes. Voltammetry in 0.1 mol dm-3 NaClO solution. Nafion content of membranes (a) 7.33 (b) 10.9 and (c) 14.7%. Table 4. Comparison of diffusion coefficients in Nafion (1 100 EW) and Nailon-impregnated Gore-tex (0.2 pm mean pore diameter Gore-tex; 9 w/w % Nafion) diffusion coefficients / c m 2 s-1 NIGT Nafion diffusing ion 2.3 x 10-9 3.4 x 10-9 1.9 x 10-6 4.1 x 10-6 Ru(NH,);+ Ru(NH&+ thermodynamic driving force for the partitioning of the analyte species into the membrane and (2) mass transport within the membrane is facile. Because the solution is well stirred solution mass-transport is also facile and the combination of these three factors will insure that a large quantity of the analyte will be partitioned very rapidly into the membrane phase.This is clearly the ideal situation for detection of the analyte but it presents a problem after the detection process is complete. Unless this (electrochemical) detection step drastically alters the thermodynamics of the partition reaction the analyte will be preferentially retained by the film after analysis rendering the film essentially useless for further analyses. The bottom line is we have designed a membrane which from a thermodynamic point of view loves the analyte and from a dynamic point of view can rapidly satisfy that love by quickly incorporating this analyte. The question then becomes since one has 1065 X i ...TO s polymer film M . W. Espenscheid et al. I(cH~;HHcH~- electrochemical ox idat io n 1 2 support1 ng electrolyte s u P P ~ rti ng electrolyte Fig. 13. Proposed model for electrorelease of counterions from hypothetical electroactive ion exchange polymer film-modified electrode. stacked the deck in the opposite direction how does one then remove this analyte when the analysis is complete? One possible route for purging the membrane is to remove the sensor from the analyte solution and treat it chemically (e.g. with a very high ionic strength solution) to remove the analyte. This chemical treatment could however introduce additional problems and would in any case be unacceptable if continuous monitoring (e.g.in vivo or process-stream analysis) was required. We have recently described a new series of electroactive ion-exchange polymers or electroactive ionomers which offer an elegant alternative to the film purging problem.28 Electroactive ionomers are polymers which contain both electroactive and ion-exchange functionalities. We prepared a prototype series based on styrene vinylferrocene (the electroactive functionality) and styrenesulphonate (the ion-exchange monomer). These terpolymers were prepared from the commercially available monomers using conventional free-radical initiator techniques2* When coated onto an electrode surface and exposed to an electrolyte solution the ferrocene group in these terpolymers can be reversibly oxidized and rereduced ; when the ferrocene group is in the reduced form the styrene sulphonate group needs a counterion (cation) and in analogy to ion-exchange voltammetry at a Nafion-modified electrode this cation could be an analyte species (e.g.DA+). When oxidized however the ferricinium group created might function as the counterion for the styrenesulphonate (see fig. 13) resulting in the expulsion of the analyte counterion from the film. Thus we reasoned that an electroactive ionomer film should be able to electrorelease an incorporated analyte counterion. To test this electrorelease hypothesis terpolymers having the following approximate mole percentages of styrene (STY) styrenesulphonate (SS-) and vinylferrocene (VFc) were prepared :2a high-SO; = (STY),,-(VFc),,-(SS-) ; mid-SO; = (STY),,-(VFc),,- (SS-) ; low-SO; = (STY),,-(VFc),,-(SS-),.The electrochemical characteristics of these polymers were studied using polymer film-modified glassy carbon electrodes in conjunc- tion with aqueous supporting electrolyte solutions. Cyclic voltammetric experiments at these modified electrodes produced the rather unusual voltammograms shown in fig. 14. Note that polymers containing high mole percentages of SS- show two distinct Fc+/O redox waves one with an E" similar to E" for Fc+/O in water and one with a significantly more positive E". These data suggest that there are two chemically distinct classes of Fc in these polymer films. We believe that the low E" form corresponds to Fc in an aqueous-like microenvironment (note that the quantity of this form scales with the quantity of SS- and therefore the quantity of water in the film) and that the high E" form results from Fc in a drier chemical microenvironment.We are further investigating the genesis of this interesting electrochemical behaviour. 36-2 1066 Polymer Modijied Electrode Sensors E/V vs. SCE (bl E/V us. SCE Fig. 14. Steady-state cyclic voltammograms for (STYHVFcHSS-) terpolymer film-modified electrodes. (a) (STY),,-(VFc),,-(SS-), high -SO; ; (b) (STY),,-(VFc),,-(SS-), mid -SO; ; (c) (STY),,-(VFc),,-(SS-), low -SO;. Supporting electrolyte = 0.1 mol dm-3 Na,SO,. Scan rate = 200 mV s-l. The modified electrodes in fig. 14 were exposed to 1 mmol dm- methyl viologen solution rinsed and placed back in a solution containing only supporting electrolyte.By analogy with Nafion these polymers incorporate methyl viologen via ion exchange as evidenced by the new voltammetric wave at ca. -0.7 V (fig. 15). Note that as would be expected the quantity of methyl viologen incorporated increases with the mole percentages of SS- in the polymer. Fig. 16 shows that these polymers can electrorelease counterions. Voltammogram (a) in fig. 16 was obtained after exposing a terpolymer modified electrode to a solution containing Ru(NH,)i+. A prominent voltammetric wave for electrostatically incorporated RulI1 is seen. The electrode was then held at +0.8 V for 1 min. According to the scheme shown in fig. 13 this should cause Ru(NH,)i+ to be expelled from the film; the diminution in the RulI1/I1 voltammetric wave [curve (b) fig.161 shows that Ru (NH,)i+ has indeed been expelled. When the potential is held for another minute at +0.8 V the RuIII’II wave is barely preceptable [curve (c) fig. 161 indicating that nearly all of the Ru(NH,)i+ has been discharged. This process is perfectly reversible; Ru(NH,)i+ can be reincorporated 1067 I 1 - I / E/Vvs. SCE M . W. Espenscheid et al. I 1 I I T \ 1 f"" o.L o ' 2 / '/-=.a' T EIVus. SC CE I Fig. 15. Steady state voltammograms for terpolymer film-modified electrodes after exposure to 1 .O mmol dm-3 methyl viologen solution. Terpolymer compositions and scan conditions are given in the caption to fig. 14. via re-exposure to the Ru(NH,)i+ solution and discharged again by holding the potential positive of the Fc+/O wave.It is in principle possible that the loss of Ru(NH,)i+ from the film (fig. 16) resulted from ion exchange of Ru(NH,)i+ with Na+ from the supporting electrolyte. To test this possibility the experiment described above was repeated but the potential of the electrode was held at 0 V (where Fc remains in the reduced form) rather than at +0.8 V. While some loss of Ru(NH,)Z+ is observed (fig. 17) note that even after 9 min at 0 V most of the complex has remained in the film. Clearly leaching can not account for the dramatic loss of Ru(NH,)i+ shown in fig. 16. The analytical use of these electroreleasing polymers is demonstrated in fig. 18. Curve (a) in fig. 18 is an ion-exchange voltammetry calibration curve (vide supra) for methyl viologen at a terpolymer film-modified electrode.Note that the detection limit at this electrode is < lo-' mol drn-, over two orders of magnitude better than at a naked electrode. These data show that the ion-exchange voltammetry preconcentration advan- tage is observed at these terpolymer-modified electrodes. After exposure to the most concentrated calibration solution a large quantity of methyl viologen has been partitioned into the film and a prominent redox wave is seen [curve (a) fig. 191. The potential of 1068 Polymer Modijied Electrode Sensors 5bA .I Fig. 16. Cyclic voltammograms for a (STY),,-(VFc),,-(SS-), film-modified electrode immersed in 0.1 mol dm-3 Na,SO, 200 mV s-l. (a) After exposure to 1 .O mmol dm-3 Ru(NH)i+; (b) as in (a) but after electrode had been potentiostatted for 1 min at +0.8 V and then potentiostatted for 1 min at 0.0 V; (c) after repeating sequence described in (b) (see text).Fig. 17. Cyclic voltammograms for polymer film-modified electrode described in fig. 5 immersed in 0.1 mol dm-3 Na,SO, 200 mV 0. (a) After reloading in Ru(NH3);+ solution; (b) (c) and (d) after potentiostatting at 0 V for 3 6 and 9 min respectively. the electrode was then held at +0.8 V for 30 min which [as indicated by the loss of the methyl viologen redox wave curve (b) in fig. 191 resulted in the expulsion of the methyl viologen from the film. The calibration process can then be repeated [curve (b) fig. 181 producing a calibration curve essentially identical to the first. Conclusions We have shown that ionomer-modified electrodes are potentially useful electroanalytical sensors.Because of their unusual ion-exchange selectivity perfluorosulphonate ionomer- modified electrodes can preconcentrate electroactive organic counterions resulting in dramatically improved detection limits for these ions relative to the naked electrode. We M. W. Espenscheid et al. [ MVz']/ 1 0-5 mol dm-3 Fig. 18. Ion-exchange voltammetry calibration curves for methyl Gologen at a (STYXSS-WFc) film-modified electrode. (a) First calibration curve ; (b) after electrorelease of methyl viologen introduced during first calibration. Supporting electrolyte = 0.1 mol dm-3 Na,SO,. Scan rate = 200 mV 0. EIV us. Ag I AgCl Fig. 19. Voltammograms for electrode described in fig.18. (a) Before electrorelease of methyl viologen ; (b) after electrorelease of methyl viologen. call this preconcentration-based electroanalytical scheme ion-exchange voltammetry. Ion-exchange voltammetry has been shown to be quite useful for in vivo analyses of neurotransmitters. Further studies of this technique particularly of the effect of potential-scan waveform on detection limits are in progress. We have also briefly examined the dynamics of ionomer-modified electrode response. At very low concentrations of exchanging (analyte) ion the rate of film-solution equilibration can be prohibitively slow. Since film mass-transport is in general the rate- determining step there is clearly a need for ionomer membranes with higher ionic diffusivities.We have shown that the ionically conductive composite polymer membranes developed in our laboratory can have very high ionic diffusivities. Thus these composite 1069 Polymer Modified Electrode Sensors membranes show promise for use in electrochemical sensors and in a variety of other electrochemical processes and devices. Further fundamental research will be required before the practical utility of these membranes can be assessed. Finally we have discussed film regeneration in electroanalysis with ionomer-modified electrodes. We have shown that a new class of electroactive ionomers can accomplish film regeneration by electroreleasing the incorporated analyte counterion. Films of these electroactive ionomers also preconcentrate the analyte counterion ; thus ion-exchange voltammetry at electroactive ionomer-modified electrodes produces detection limits superior to those obtained at a naked electrode.In closing it is important to point out that while ionomer-modified electrodes have greater selectivity than a naked electrode ionomers provide only a very rudimentary form of selectivity (charge type and mass or hydrophobicity selectivity). The challenge to the electrochemist (or better still to the electrochemist in collaboration with a synthetic- polymer chemist a biochemist etc.) is to ' build' better and more selective chemistry into membranes so that sensors with higher chemical specificity can be obtained. One obvious approach is to incorporate chemically selective biochemical species into membranes; indeed potentiometric sensors based on this concept are now commercially a ~ a i l a b l e .~ ~ There are also other approaches to building chemically selective membranes; we hope to be able to report on some of our attempts at developing such membranes soon. This work was supported by the Office of Naval Research and the Robert A. Welch Foundation. 1070 References 1 Ions in Polymers Advances in Chemistry Series 187 ed. A. Eisenberg (American Chemical Society Washington 1980). 2 C. R. Martin and K. A. Dollard J. Electroanal. Chem. 1983 159 127. 3 F. G. Will J . Electrochem. Soc. 1979 126 36. 4 R. S. Yeo and D-T. Chin J. Electrochem. Soc. 1980 127 549. 5 W. Grot Chem.-Ing.-Tech. 1978 50 299. 6 C. R. Martin and H. Freiser Anal. Chem. 1981 53 902.7 C. R. Martin I. Rubinstein and A. J. Bard J. Am. Chem. Soc. 1982 104 4817. 8 M. N. Szentirmay and C. R. Martin Anal. Chem. 1984 56 1898. 9 R. B. Moore 111 J. E. Wilkerson and C. R. Martin Anal. Chem. 1984 56 2572. 10 H. L. Yeager and A. Steck Anal. Chem. 1979 51 862. 11 0. Samuelson Ion Exchange in Analytical Chemistry (John Wiley New York 1953) p. 37. 12 E. R. Tompkins and S. W. Mayer J. Am. Chem. Soc. 1947 69 2859. 13 H. S. White J. Leddy and A. J. Bard J. Am. Chem. Soc. 1982 104 481 1. 14 J. R. Schneider and R. W. Murray Anal. Chem. 1982 54 1508. 15 A. Steck and H. L. Yeager Anal. Chem. 1980 52 121 5. 16 J. A. Marinsky J. Phys. Chem. 1967 71 1572. 17 H. P. Gregor and J. I. Bregman J. Colloid Sci. 1951 6 323. 18 N. E. Prieto and C. R. Martin J. Electrochem. Soc. 1984 131 751. 19 M. N. Szentirmay N. E. Prieto and C. R. Martin J. Phys. Chem. 1985,89 3017. 20 J. A. Marinsky and Y. Marcus Ion Exchange and Solvent Extraction (Marcel Dekker New York 1974) vol. 6 p. 5. 21 Details of this method are described in R. B. Moore 111 J. E. Wilkerson and C. R. Martin Anal. Chem. 1984 56 2572. 22 J. Fietelson in Zon Exchange ed. J. A. Marinsky (Marcel Dekker New York 1969) vol. 2 chap. 4. 23 G. Nagy G. A. Gerhardt A. F. Oke M. E. Rice R. N. Adams R. B. Moore 111 M. N. Szentirmay and C. R. Martin J. Electroanal. Chem. 1985 188 85. 24 G. A. Gerhardt A. F. Oke G. Nagy B. Mughaddam and R. N. Adams Brain Res. 1983 290 390. 25 C. R. Martin Trends Anal. Chem. 1982 1 175. 26 R. M. Penner and C. R. Martin J. Electrochem. Soc. 1985 132 514. 27 Gore-tex is a registered trademark of W. L. Gore and Associates. 28 M. W. Espenscheid and C. R. Martin J. Electroanal. Chem. 1985 188 73. 29 R. P. Buck J. Chem. Soc. Faraday Trans. I 1986,82 1169. Paper 5/ 1956; Received 6th November 1985

ISSN:0300-9599    

DOI:10.1039/F19868201051

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC. Faraday Trans. I 1986,82 1071-1079 Trace-metal Analysis in Hydroponic Solutions Christopher M. A. Brett" Departamento de Quimica Universidade de Coimbra 3000 Coimbra Portugal Maria M. P. M. Net0 CECUL Calqada Bento Rocha Cabral 1200 Lisboa Portugal A wall-jet ring-disc electrode detector has been developed for the direct on-line monitoring of the trace metals manganese zinc and copper at the micro- and submicro-molar level in hydroponic nutrient solutions. The technique employed is of anodic or cathodic stripping voltammetry with collection (a.s.v.w.c. or c.s.v.w.c.) at ringdisc electrodes. All three elements may be quantitatively determined by s1.s.v.w.c. using in situ-plated thin- mercury-film electrodes. Interference effects and intermetallic compound formation are discussed.It is shown that manganese may be quantified alternatively by C.S.V.W.C. via its deposition as MnO,. Results in test solutions and in nutrient solution agree well with theoretical predictions and the reproducibility is good. A method for determining the toxic trace-metal nickel by a.s.v.w.c. is also developed. The quantitative estimation of trace metals at the submicromolar level is very important in many applications involving biological environmental or industrial samples. This usually relies on spectrometric techniques the analysis often taking place somewhere removed from the sample collection site. An ideal sensor would be an on-line monitor and would produce results quickly and frequently with the minimum of assistance.Flow-through amperometric electrochemical sensors are able to satisfy these criteria. As they are based on solid electrodes many of the difficulties experienced by non- electrochemists with the more classical dropping mercury electrode disappear. They can be placed anywhere in a flow system and computer automation is relatively easy to accomplish. Here we describe the development of a flow-through sensor based on the wall-jet principle to determine trace-metal ions in hydroponic solutions based on the nutrient film technique. The plants are grown in a shallow stream of flowing nutrient solution in sloping troughs within glasshouses the solution being continuously recirculated and slowly rep1enished.l The levels of beneficial or toxic trace metals can have a marked effect on plant growth and will vary according to the crop.Table 1 shows a typical set of concentrated nutrient solutions which would be added to the circulating solution as necessary. In table 2 are recommended levels of trace elements for a tomato crop (which are not necessarily the optimum). We shall be concerned with the first three of these trace elements namely manganese zinc and copper. We will also consider the measurement of nickel a particularly toxic trace metal which can appear by for example corrosion of monel heaters used to warm the nutrient solution in winter. Electrochemical Detector There are several different types of electrochemical flow-cell available including thin- layer,2 tubular3 and wall-jet4 types. The last of these has advantages with respect to the other configurations owing to its higher sensitivity ease in use and ease of maintenance and well defined hydrodynamics.1071 1072 Trace-metal Analysis in Hydroponic Solutions Table 1. A typical formulation for concentrated hydroponic nutrient stock solutions solution I/dm-3 100 g Ca(N03) hydrate solution III/dm- 100 g HNO 42 g HaPo element Table 2. Typical trace-element levels in nutrient solutions@ (p.p.m.) 100 g MgSO 10 g Fe EDTA complex 0.25 g H3B03 1.2 g MnSO 0.08 g cuso 0.1 g ZnSO 0.02 g (NH,),MoO concentration /mol dm- Mn Zn c u B Mo solution II/dm- 160 g KNO 1 .o 0.08 0.05 0.15 a By dilution from table 1 formulation. A fine jet issuing from a circular nozzle impinges on the centre of a disc electrode perpendicular to the jet the solution then spreads out radially.It has been shown5 that under laminar flow any species which reaches the disc electrode and subsequently leaves the diffusion layer cannot react again. This is very useful in practice since it means that the volume of the wall-jet cell is not important for its efficient use. The wall-jet ring-disc electrode cell analogous to the rotating ring-disc electrode allows extra possibilities in studies of electrochemical reactions. We have used a wall-jet ring-disc electrode (w.j.r.d.e.) for this work. 0.03 Voltammetric Method In order to increase the measured signal and to improve the signal-to-noise ratio we generally use a preconcentration step to deposit the electroactive species on the electrode followed by a stripping step for the actual measurement.Table 3 shows four methods for achieving this the most widely used being stripping voltammetry (s.v.) and potentiometric stripping analysis (p.s.a.). P.s.a. has the advantage in some cases that the oxidant in the stripping step can be oxygen obviating the need for deoxygenation.1° The range of application of S.V. is large and therefore we have used this technique. In stripping voltammetry there are a number of ways of carrying out the stripping step the current for which contains Faradaic and non-Faradaic (mainly capacitative) contributions. The simplest is a linear scan which for oxidation of a deposited metal will be in the anodic direction; the limit on sensitivity is the capacitative contribution which means high scan rates cannot be employed.To obviate this several more sophisticated waveforms have been invented such as differential pulse,ll staircase,12 phase selective a.c.13 and subtractive stripping v~ltarnmetry.~*$ l5 ~~ ~~ 1.8 x 10-5 1.2 x 10-6 7.9 x 10-7 1.4 x 10-7 3.1 x 10-7 - method stripping voltammetrya adsorptive stripping voltammetryb po tentiome tric stripping analysisC stripping chronopotentiometryd C . M . A . Brett and M . M . P. M . Net0 Table 3. Principles of preconcentration methods for electrochemical detection of trace species deposition (preconcentration stripping step step) potential control potential control potential control adsorption (no applied potential) potential control reaction with oxidant or reductant in solution (no applied potential) current control potential control a Ref.(6). Ref. (7). Ref. (8). Ref. (9). r iD,L = 1.38 nFD%v-AVj adc rf We have used a method first described by Johnson and Allen,16 namely anodic stripping voltammetry with collection (a.s.v.w.c.). It can be applied to any double electrode system and has been used at the r.r.d.e.,16 at the double tube electrodel' and at the wall-jet ring-disc electrode.18 The ring potential is held constant in such a way that it collects the species stripped from the disc. We may write where the integration is carried out over the whole transient. iR,L and iD,+ are the limiting currents at ring and disc respectively qR and qD the respective charges involved and No is the steady-state collection efficiency.The diffusion-limited current at a wall-jet electrode is given by4 i us. t i us. t E us. t E us. t where D is the diffusion current of the electroactive species v the kinematic viscosity of the solution vf the volume flow rate a the nozzle diameter c the bulk concentration and rl is the disc electrode radius. Thus it is simple to check the accuracy of the technique in test solutions containing known quantities of trace metals by use of eqn (3). Another important aspect is that whatever the type of stripping waveform used at the disc electrode the ring response integrated over the whole current transient should be unaffected here we use potential pulse.In general because of the rather negative deposition potentials involved it is necessary to use a thin-film mercury electrode on a glassy carbon electrode substrate which we prepared by in situ deposition. A.s.v.w.c. with a linear scan stripping step using an r.r.d.e. with mercury films on both the glassy carbon disc and glassy carbon ring have been ~ep0rted.l~ (4) Experiment a1 The wall-jet ring-disc electrode cell (Oxford Electrodes) was of the Fleet and Little design20 and has been described previously.18 The electrodes used were glassy carbon- 1073 measurement Trace-metal Analysis in Hydroponic Solutions 1074 platinum and platinum-platinum disc-ring combinations in a Kel-F sheath.A glassy carbon-glassy carbon r.r.d.e. was also employed. After initial polishing with 6 and 3 pm diamond lapping compound the electrodes were then hand polished with 1 and 0.3 pm alumina made into a slurry with triply distilled water and finally washed thoroughly with triply distilled water. The final polishing step was repeated every day. In testing the procedures analytical grade reagents were used and made into solution with triply distilled water. Nutrient test solutions were made up from laboratory grade reagents in order to conform with the commercial solutions. After mixing of the test solution and supporting electrolyte deoxygenation was carried out using oxygen-free nitrogen (Air Liquide) presaturated in the supporting electrolyte. It has been shown that further purification of the oxygen-free nitrogen is unnecessary.21 The solution was pulled through the wall-jet detector by means of a Pharmacia P3 peristaltic pump placed downstream.To minimise pump pulsations a long length of Teflon tubing and a hollow glass ball between detector and pump provided damping. Flow rates were calibrated volumetrically every day. The normal bipotentiostat was modified to enable integration of the ring transients. These were recorded either on a Gould Advance digital storage oscilloscope OS4020 with subsequent readout or on a Philips PM8120 XYt recorder. Results and Discussion The trace metals manganese zinc and copper can all be determined by a.s.v. Relative to an Ag I AgCl reference electrode (wall-jet cell) we found the deposition potentials necessary to be - 1.8 - 1.2 and -0.4 V respectively on an in situ-deposited mercury film electrode (m.f.e.) with glassy carbon substrate.Owing to the large negative deposition potential for manganese we also investigated its deposition as MnO and cathodic stripping with collection (c.s.v.w.c.). An important parameter in this type of collection technique is the transit time between disc and ring which will affect the shape of the ring-current transient. At the wall-jet (unlike rotating electrodes) the radial velocity decreases with radius. In order to have a reasonable transit time (ca. 0.3 s) the ring-disc electrode dimensions need to be quite small. Our electrodes have the approximate dimensions disc radius rl = 0.17 cm; inner ring radius r2 = 0.18 cm; outer ring radius r = 0.19 cm.In this way we were able to perform the a.s.v.w.c. and C.S.V.W.C. experiments with high reproducibility. The experi- mental value of No was 0.160 independent of flow rate and was reproducible; the fact that it is slightly higher than the theoretical value has been ascribed to edge effects in the cell.18 Laser and ArieP in their work on a.s.v.w.c. at the r.r.d.e. have noted that a minimum mercury plating time is necessary on the ring electrode in order to get the full response predicted by eqn (3). There are two reasons for this one is the shielding factor caused by the disc electrode and the other is that a relatively thick film of mercury has to be deposited to accommodate the large quantity of metal ions reaching it from the disc stripping pulse.However with judicious care and several minutes’ plating time at the ring good results are obtained. For the a.s.v.w.c. experiments the supporting electrolyte employed was NaClO, added in sufficient quantity to give a 0.1 mol dm-3 solution. M.f.e. were formed in situ from HgII in the test solution added to give 5 x mol dm-3. We now consider results obtained for the individual trace elements in ‘ideal’ conditions and in the nutrient solution. The a.s.v.w.c. (as opposed to a.s.v. with just a disc electrode) of Mn and Zn has not yet been carried out at the wall-jet ring-disc electrode as an electrode with a glassy carbon ring and glassy carbon disc is still under development. Although they will not be shown here good results were obtained for all collection-type experiments at an analogous rotating ring-disc electrode.We will focus on the more interesting aspects of what we found. c u + Zn \ \ 1075 0.4 E/V vs. Ag I AgCl cu 1 E/V 11s. Ag 1 AgCl - C. M . A . Brett and M . M . P . M . Net0 -1.6 (b 1 l -0.8 Fig. 1. The effect of Cu-Zn intermetallic compound formation shown by linear-scan stripping at the wall-jet m.f.e. Solution mol dm-3 Zn" lop6 mol dm-3 Cu'I 0.1 mol dm-3 NaClO, 5 x loF5 rnol dm-3 HgII. Deposition period 90 s; flow rate 0.040 cm3 s-l stripping scan rate 100 mV 0. (a) Deposition of Cu alone at -0.6 V (b) deposition of Cu and Zn at - 1.5 V. Copper The technique of a.s.v.w.c. at the wall-jet ring-disc electrode has already been demon- strated for copper at the mol dm-3 leve1,18 the detection limit probably being rather lower.It functions equally well at glassy carbon and platinum electrodes and also at the m.f.e. Zinc In order to determine zinc applying a deposition potential of - 1.2 V us. Ag IAgCl deposits both copper and zinc. Cu-Zn intermetallic compound formation in mercury electrodes is well known and several papers have been devoted to the resolution of the problem which arises owing to the copper and copper-zinc stripping peaks being virtually coincident. These have involved the use of a twin-electrode optimising the potential stripping waveform and preconcentration time23 and addition of a third element which has a significantly higher formation constant for intermetallic compound formation with copper such as gallium to mask it totally.24 We propose a simple procedure for resolving this problem it involves a two-step experiment (i) deposition of Cu alone at -0.4 V followed by its stripping; (ii) deposition of Zn and Cu at - 1.2 V-first strip the Zn alone (at say -0.5 V) and immediately afterwards the Cu-Zn and Cu.Simple subtraction gives the correct Cu and Zn concentrations. If (ii) is carried out quickly and the deposition times are not very short then there is minimal error. This is illustrated in fig. 1 in a linear scan experiment for clarity. In linear scan stripping at the m.f.e. the peak height is proportional to the concentration of the species being stripped.25 To ensure good reproducibility we strip the mercury film after each of these experiments.Manganese Manganese@) may be quantified by deposition on an m.f.e. at - 1.8 V and subsequent anodic stripping. There is only the odd example26 with reduced precision (owing to the very negative deposition potential and the fact that manganese tends to form a dilute amalgam). We have investigated the deposition of MnO and its subsequent stripping. C.S.V. of MnII has been described at the rotating disc electr~de.~' There is obviously a pH 1076 (a) 1.5 Y p CY 1 .o \ 0.5 0 .O 60 Trace-metal Analysis in Hydroponic Solutions /A 120 deposition time/s Fig. 2. Manganese analysis via electrode preconcentration as MnO,. Conditions Pt-Pt w.j.r.d.e.flow rate 0.04 cm3 s-l 0.1 mol dm-3 KC1 with borate buffer @H 7.2). (a) Cyclic voltammogram at scan rate 5 mV s-l. (b) Typical C.S.V.W.C. results. Deposition potential +0.75 V; stripping potential 0.0 V. Lines correspond to addition of (2 4 and 6) x lo-’ mol dm-3 MnL1 to blank. dependence in the deposition and stripping steps; we found in accord with previous work that a borate buffer with a pH slightly above neutrality worked most efficiently. The cyclic voltammogram in fig. 2(a) exhibits two peaks on the cathodic scan. The first of these is due to a change in oxide composition (MnO is a non-stoichiometric oxide) and the second to reduction to Mn*1.28 Nevertheless if deposition is at +0.75 V and stripping at 0.0 V we find good agreement between experiment and theory [fig.2(b)] so the oxygen manganese ratio at + 0.75 V must be very close to two. I’ 180 1077 0.4 C. M . A . Brett and M. M . P . M . Neto Mn EIV vs. Ag I AgCl Fig. 3. Nutrient solution linear scan stripping at wall-jet m.f.e. Electrolyte added to solution to give 0.1 mol dm-3 NaClO, 5 x mol dm-3 HgII. Flow rate 0.040 cm3 s-l. Deposition time 120 s; stripping scan rate 100 mV s-l. Edep = - 1.8 V. 120 deposition time/s 180 mol dm-3 Hg'l. Deposition potential -0.4 V; stripping poten- is the theoretical prediction from eqn (3). 1.5 0.5 60 0 -0 0 Fig. 4. Copper a.s.v.w.c. in nutrient solution at glassy carbon disc Pt ring w.j.r.d.e. with 0.1 mol dm-3 NaC10 and 5 x tial 0.0 V. ( x ) are experimental points and (-) Nutrient Solution A linear stripping scan for the nutrient solution obtained at the wall-jet m.f.e.is shown in fig. 3. It shows clearly the high manganese concentration and the effect of the Cu-Zn intermetallic compound formation as discussed above. A typical response for copper is given in fig. 4. In mixing the nutrient solution with supporting electrolyte there is of course a dilution effect. Since the nutrient solution trace-metal levels are well above the detection limits this presents no problems. No interferences were found either for a.s.v.w.c. or for C.S.V.W.C. of Mn. 0.0 2 1078 -- n a - cr b P CZ 0.01 -. 0.0 Fig. 5. Nickel results at a Pt-Pt w.j.r.d.e. in 0.1 mol dm-3 KSCN and 0.1 mol dm-3 KC1. Flow a.s.v.w.c.experiments. Deposition potential - 0.9 V; stripping potential + 0.3 V. Deposition times rate 0.040cm3 s-l. (a) Cyclic voltammogram at scan rate 100mVs-l. (b) Typical plot from Nickel The very toxic trace-element nickel is relatively difficult to estimate quantitatively by stripping voltammetry owing to the slow electrode kinetics of the NiII aq(Nio couple. It has been shown that the electrode kinetics can be improved by the presence of complexing species :29 applications are in a.~.v.~O and in adsorptive stripping vo1tarnmet1-y.~~ Following the earlier a.s.v. paper we add 0.1 mol dm-3 KSCN and 2 .o 0 Trace-metal Analysis in Hydroponic Solutions mol dm-3 1 .o [ NiII] added/ varied between 20 and 210 s. 1079 C . M . A . Brett and M . M . P .M. Net0 0.1 mol dm-3 KCl to the test solution. Under such conditions there is > 50% Ni(SCN)+ and little free aqueous ion. Quantitative estimation down to lo-' mol dm-3 was possible with short deposition times (see fig. 5). We hope by optimising the solution composition deposition and stripping potentials to lower the detection limit further and eliminate the non-zero intercept corresponding to a blank determination probably linked with electrolyte impurities or with adsorption processes occurring on the electrode surface. Because the electrolyte and other experimental conditions are different from those used in the other trace metal determinations there is no noticeable interference effect in analyses for nickel using nutrient solution. This masking is a very important advantage.Conclusion We have demonstrated the successful application of new electrochemical sensors to the monitoring of trace-metal levels in hydroponic solutions. In parallel with the glassy carbon-glassy carbon wall-jet ring-disc electrode to enable total on-line monitoring automation is also being investigated. An approach to this in the laboratory has recently been described.32 With computer automation operator assistance can be minimised. One can imagine a larger number of independent nutrient solution concentrates than in table 1 which could be dosed according to data obtained by the computer. The stored data could also be used for more fundamental studies of the mechanism of nutrient uptake by plants. References 1 Commercial Applications of NFT (Grower Books London 1982).2 F. E. Woodward and C. N. Reilley Comprehensive Treatise of Electrochemistry (Plenum Press New York 1984) vol. 9 p. 353. 3 W. J. Blaedel and S. L. Boyer Anal. Chem. 1971,43 1538. 4 J. Yamada and H. Matsuda J. Electroanal. Chem. 1973 44 189. 5 W. J. Albery and C. M. A. Brett J. Electroanal. Chem. 1983 148 201. 6 See for example F. Vydra K. Stulik and E. Julakova. Electrochemical Stripping Analysis (Ellis Horwood Chichester 1976). 7 R. Kalvoda Anal. Chim. Acta 1982 139 11 and references therein. 8 D. Jagner Analyst (London) 1982 107 593 and references therein. 9 L. Luong and F. Vydra Coll. Czech. Chem. Commun. 1975,40 1490; 2961. 10 T. M. Florence J. Electroanal. Chem. 1984 168 207. 11 T. R. Copeland J.H. Christie R. A. Osteryoung and R. K. Skogerboe Anal. Chem. 1973 45 2171. 12 U. Eisner J. A. Turner and R. A. Osteryoung Anal. Chem. 1976 48 1608. 13 W. L. Underkofler and I. Shain Anal. Chem. 1965 37 218. 14 W. Kemula Pure Appl. Chem. 1967 15 283. 15 J. Wang and M. Ariel Anal. Chim. Acta 1981 128 147. 16 D. C. Johnson and R. E. Allen Talanta 1973 20 305. 17 G. W. Schieffer and W. J. Blaedel Anal. Chem. 1978 50 99. 18 W. J. Albery and C. M. A. Brett J. Electroanal. Chem. 1983 148 21 1. 19 D. Laser and M. Ariel J. Electroanal. Chem. 1974 49 123. 20 B. Fleet and C. J. Little J. Chromatogr. Sci. 1974 12 747. 21 A. Hamelin personal communication. 22 D. A. Roston E. E. Brooks and W. R. Heineman Anal. Chem. 1979,51 1728. 23 J. A. Wise D. A. Roston and W. R. Heineman Anal. Chim. Acta 1983 154 95. 24 T. R. Copeland R. A. Osteryoung and R. K. Skogerboe Anal. Chem. 1974,46,2093. 25 W. T. de Vries and E. van Dalen J. Electroanal. Chem. 1967 14 315. 26 See for example A. I. Zebreva R. N. Matakova and R. B. Zholdybaeva Zh. Anal. Khim. 1983 38 1325. 27 E. Hrabankova J. Doleial and V. MaSin J. Electroanal. Chem. 1969 22 195. 28 E. Kostikova and P. Beran CON. Czech. Chem. Commun. 1982,47 1216. 29 D. R. Crow and M. E. Rose Electrochim. Acta 1979 2441. 30 M. M. Nicholson Anal. Chem. 1960,32 1058. 31 H. Braun and M. Metzger Fresenius 2. Anal. Chem. 1984 318 321. 32 H. Gunasingham K. P. Ang B. Fleet C. C. Ngo and P. C. Thiak J. Electroanal. Chem. 1985,186,51. Paper 511881; Received 10th October 1985

ISSN:0300-9599    

DOI:10.1039/F19868201071

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC. Faraday Trans. I 1986,82 1081-1098 Anodic Detection in Flow-through Cells Dennis C. Johnson,* John A. Polta Theresa Z. Polta Glen G. Neuburger Jodi Johnson Andrew P-C. Tang In-Hyeong Yeo and John Baur Department of Chemistry and Arnes Laboratory of the U.S.D.O.E. Iowa State University Ames Iowa 50011 U.S.A. Virtually all organic compounds and numerous inorganic compounds are predicted from thermodynamics to be oxidized at potentials accessible at commonly used solid electrodes in aqueous media by reactions which require transfer of oxygen from H,O to the oxidation products. These anodic reactions are usually kinetically inhibited because of their complexity. Progress is summarized to develop electrocatalytic processes for anodic detection in flowing aqueous solutions.Pulsed amperometric detection at Pt electrodes is applicable for the sensitive detection of the HCOH group in all alcohols polyalcohols and carbohydrates the nitrogen in amines amino acids and aminoglycosides and sulphur in most inorganic and organic compounds. Pure and doped lead dioxide electrodes are described for amperometric detection at constant potential. The voltammetric response of electrocatalysed oxygen-transfer reactions is dictated by the potential dependence of the process transferring oxygen from H,O to the electrode surface so that attempts at voltammetric resolution of mixtures are not generally effective. The most useful analytical application of the electrocatalysed processes for mixtures is in the form of amperometric detection in liquid-chromatographic systems.Electrochemical (EC) detection in miniature flow-through cells applied in conjunction with high-performance liquid-chromatographic (LC) separations joins together the analytical strengths of both component techniques. EC detection is highly sensitive at moderately low cost whereas LC in its numerous forms is far superior to electroanalytical voltammetry in resolving power for analysis of complex mixtures. It is the goal of research summarized here to achieve maximum electrocatalytic activity for anodic EC detection even at the sacrifice of voltammetric selectivity. None of these reactions has been found to produce a persistent Why anodic detection? The major emphasis of EC detection in flowing solutions continues to be for anodic (oxidative) detecti0n.l Cathodic (reductive) detection must contend inevitably with the cathodic response of dissolved 0 in samples and the mobile Several procedures have been applied successfully for minimizing the level of dissolved 0 in LC/EC however these procedures are accompanied inevitably by some inconveniences.Virtually all organic compounds and numerous inorganic species are predicted thermodynamically to be oxidized in aqueous solutions at potentials available at commonly used solid electrode materials (e.g. Pt Au and C). These reactions predomi- nantly require transfer of oxygen from H,O to the oxidation products and are generally kinetically inhibited because of their complexity. Oxidation of simple alcohols and other hydrocarbons has been studied extensively for possible use in fuel 1081 1082 Anodic Detection in Flow-through Cells reversible potential and the degree of irreversibility increases with extended exposure of the electrodes to the organic compound.The analytical utility of irreversible reactions can be satisfactory even if a substantial overpotential is necessary to drive the reaction at a significant rate. However to obtain acceptable analytical precision reaction kinetics at the applied overpotential must (i) be fast enough to result in mass-transport controlled signals or (ii) be time invariant if heterogeneous kinetic rates are less than the rate of mass transport. The Problem of Fouled Electrode Surfaces The most common observation reported for anodic detection at solid electrodes especially Pt is rapid loss of electrode resp~nse.~-~ This is usually attributed to strong adsorption of reactants and/or free-radical reaction products.In some instances free radicals at high concentration in the diffusion layer undergo polymerization reactions with subsequent coverage of the electrodes by adherent polymeric films.7 Even in purified solvents the presence of organic compounds at trace levels is blamed for observed losses in electrode a~tivity.~l The tendency for Pt to adsorb free radicals and other species is easily understood from the electronic structure of the metal designated as [Xe]s2p6d9. Hence a partially filled d orbital exists at the Pt surface which facilitates adsorption of free radicals and compounds with non-bonded electron pairs.Molecular H is dissociately adsorbed as are unsaturated hydrocarbons presumably with formation of the diradical. As a consequence Pt is well known as an effective hydrogenation ~atalyst.~ Metals with saturated surface d orbitals e.g. Au are relatively ineffective as hydrogenation catalysts. A hydrogenation catalyst is also a dehydrogenation catalyst in the absence of a large partial pressure for H,(g) since the position of equilibrium is not altered by a catalyst. Accordingly even saturated hydrocarbons can be adsorbed at oxide-free Pt electrodes and anodic current results from oxidation of the dissociated Ha when E > E&+,H2. Amperometric detection of organic compounds based on this scheme can be very sensitive but with some interference by simultaneous cathodic detection of dissolved 0,.The free-radical products of the surface-catalysed organic dehydrogenation are strongly adsorbed and useful analytical signals from the reaction H' -+ H+ + e are transient. We observe that loss of activity for Pt electrodes at constant potential is more rapid in flow systems constructed from plastic components (e.g. Teflon tubing and Kel-F detector cells) than at rapidly rotated electrodes in all glass cells. We attribute this to adsorption of plasticizers which continually ' bleed' from the plastics into the aqueous mobile phase. Potentiodynamic Cleaning of Electrodes All useful publications related to electroanalytical voltammetry at solid electrodes carefully report the procedural details for electrode activation (preconditioning) to obtain reproducible results.These procedures at noble metal electrodes usually begin with mechanical polishing to a mirror-like finish followed by alternate anodic and cathodic polarization of the ele~trode.~-~ A large positive potential at noble-metal electrodes in conjunction with the concomitant formation of surface oxides is highly effective for oxidative desorption of all carbonaceous and inorganic adsorbates. However the continued oxidations of most of these compounds from solution are usually not observed at these same electrodes at constant potential when the surface oxide has reached its final stable form. Gilman8 has reviewed the necessity of oxidative removal of adsorbed impurities in conjunction with quantitative studies of adsorption of oxygen and hydrogen at Pt electrodes.Clark et ul.l0 reported greater precision for anodic detection of ethylene at Pt electrodes when positve potential pulses were applied to clean the electrode. D. C. Johnson et al. 1083 MacDonald and Duke1' reported similar results for anodic detection of p-aminophenol. Stulik and Hora12 described the improved precision for cathodic detection of Fe3+ and Cu2+ at Pt in acidic media when the constant detection potential was interrupted by brief anodic cleaning pulses. Fleet and Little1 reported improved analytical precision at carbon electrodes when cleaning pulses were applied and Fleet has received a patent for use of cleaning pulses at wall-jet electr~des.'~ Specific examples given by Fleet were for anodic detection of methyl-4-hydroxybenzoate n-propylhydroxybenzoate 4-methoxyphenol 4-amino- phenol 4-nitrophenol and 4-chlorophenol.Detection was at 1 . 1 V us. SCE with cathodic cleaning pulses to -0.2 V at a frequency of 5 s-l. Berger who was involved in the original work with Fleet,15 has continued to study detection with cleaning pulses at carbon e1ectrodes.l6 Tenygll' has recommended also application of cleaning pulses at carbon electrodes. Application of a voltage pulse train (0.5 s-l) for 5 min was demonstrated by van Rooijen and Poppels to restore the response of glassy carbon electrodes which have become deactivated by exposure to organic compounds. Anodic Electrocatalytic Pulsed Amperometric Detection (PAD) at Platinum Electrodes The electroanalytical technique of pulsed amperometric detection (PAD) applied for anodic detection at Pt electrodes is based on the premise that the Faradaic signal for oxidative desorption of organic compounds and free radicals is applicable for quantitative detection of all organic compounds which can adsorb at a Pt surface.Few oxidation reactions coupled to oxygen-transfer processes have found analytical use at traditional anodic materials at constant or slowly varied potentials. Obviously the problem has kinetic rather than thermodynamic origins. Even the simple oxidations HSO; + HSO, NO; + NO; As(OH) + OAs(OH) and HCHO + HC0,H are kinetically slow. We conclude the obvious that the oxygen of H,O is not in an activated state as would be appropriate for 0-transfer coupled with e-transfer at electrode surfaces.Both reactant and H20 must be activated simultaneously in the same region of space i.e. the electrode surface for the reaction probability to be significant. For an electrode material to be useful as a 'universal' anode it must electrocatalytically promote both e-transfer and 0-transfer processes. The appropriate electrocatalyst is expected to offer a modest stabilization energy for oxygen produced by anodic discharge of H20. A large stabilization energy for the adsorbed oxygen results in an inactive surface oxygen described more accurately as a 'surface oxide'. This highly simplified concept for successful electrocatalytic 0-transfer is illustrated below where S is the electrode surface [O] is semi-stable surface oxygen R is the reactant and RO is the oxidation product net R + H 2 0 - RO+2H++2e 4 S[O]+ R - S + RO Platinum Oxide and 0-Transfer Reactions The literature on the anodic formation of surface oxide at Pt electrodes has been reviewed by W00ds.l~ It is commonly concluded that the first step corresponds to the reversible generation of adsorbed hydroxy r a d i c a l ~ ~ ~ - ~ ~ by the reaction Pt + H 2 0 -+ PtOH + H+ +e.(4) 1084 A place exchange between the adsorbed 'OH and Pt atoms follows rapidly to produce the observed voltammetric hysteresis :23-27 PtOH -+ OHPt. A further oxidation step generates the oxide phase Anodic Detection in Flow-through Cells PtOH and OHPt + PtO + H+ +e.Experimental Basis of PAD Response28 Pulsed amperometric detection is achieved with triple-step potential waveforms as illustrated in fig. 1 (a) and (b). In both cases sampling of the electrode current occurs near the end of the detection period (1,) at potential El. In waveform (a) E2 > El in order to accelerate oxidative removal of all remaining adsorbate; E3 is large and negative in order to reduce the oxidized electrode surface to the clean metal quickly. Analyte is adsorbed during period t to be detected following the subsequent application of El. In waveform (b) analyte is adsorbed at E2. The application of E3 > El prior to electronic sampling of electrode current at El is useful to initiate slow anodic detection reactions.Furthermore the baseline signal is always smaller for waveform (b) and therefore the signal-to-noise ratio is enhanced. The current for oxide formation at a Pt electrode following a positive step in applied potential at a Pt electrode is given empirically by -iox = q / t ( 5 ) (6) The anodic generation of PtO is very effective in achieving oxidative desorptioti of adsorbed organic and inorganic specie~.~-~9 l9 An electrocatalytic link between PtO formation and 0-transfer reactions has been implicated by numerous studies; see ref. (28). The strongest evidence perhaps comes from studies of the oxidation of Br- and I- at rotated Pt ring-disc electrodes in acidic media. Adsorbed Br- is oxidatively desorbed as HOBr29 and adsorbed I- (existing as 1°),* is converted into 10;319 32 simultaneously with oxide formation.We conclude that oxygen from adsorbed 'OH is transferred to adsorbed Br- and I" simultaneously with anodic e-transfer to produce HOBr and 10;. (7) where c is a rate constant proportional to electrode area r is the applied overpotential for oxide formation (E-E"') and t is time., The i against t response is shown in fig. 2(a) for the absence of adsorbate (0 = 0). For the presence of an electroactive adsorbate (0 > 0) i is suppressed at short t after the application of the potential step. For t $ 0 adsorbate is oxidatively desorbed and oxide forms at surface sites previously occupied by the adsorbate. The total current is therefore larger at t $ 0 for the presence of adsorbate (0 > 0) than for the absence of adsorbate (8 = 0).The response patterns28 observed for PAD in FI (flow injection) and LC systems are easily predicted from the i against t response in fig. 2(a). Detection peaks are illustrated in fig. 2(b) for two values of delay time in the pulse waveform prior to sampling of electrode current (i.e. 1,). The baseline response in fig. 2(b) is determined by the value of iox at 8 = 0 given by eqn (7) for the value chosen for t,. The detection peak height corresponding to a peak concentration producing the value 8 > 0 represented in fig. 2 (a) can be 'negative' for short t, 'zero' for intermediate t or positive for longer t,. The signal-to-baseline ratio invariably is greatest for longer t (ca. 150-300 ms). Optimum sensitivity and signal-to-noise ratio must be determined for each analyte; however compounds having the same electroactive functional group have very similar response patterns.1085 J + 1 'detection ' €1 ( t J Eapplied 'adsorption' €2 t t 2 ) 'activation ' € 3 ( t 3 ) (baseline 1 -i D. C. Johnson et al. 'adsorption ' Fig. 1. Triple-step potential waveforms for pulsed amperometric detection (PAD). (a) indicates measurement of current. €3 ( r 3 ) ( b ) 'negative detection' ' pos i i i ve de tec f ion ' tls I Fig. 2. Basis for anodic electrocatalytic pulsed amperometric detection at a Pt electrode in flow-injection and liquid-chromatographc systems. (a) Transient i us. t response following a positive potential step from E3 to El in waveform (a) (fig.1). (b) Negative and positive detections for PAD in FI and LC systems depending on value of t in waveform (a) (fig. 1). 1086 1 I d 2 I 74.6 103.7 132.8 Fig. 3. Voltammetric response for glucose and sucrose at a rotated Pt disc electrode under waveform (a) (fig. 1). Conditions 0.8 mm2 disc area 500 rev min-l rotation rate 0,-free 0.20 mol dm-3 NaOH. Waveform El(tl) = varied (200 ms) E2(t2) = 0.800 V (200 ms) E3(t3) = - 0.900 V (600 ms). Curves (a) residual (b) 1 .O mol dmP3 sucrose and (c) 1 .O mmol dm-3 Alcohols and carbohydrate^^^-^^ Amines37,28,38 Anodic Detection in Flow-through Cells 0.066 0.333 glucose. The HC-OH group in alcohols polyalcohols and carbohydrates is adsorbed at Pt electrodes and efficiently detected by PAD.Highest sensitivity is observed under alkaline conditions which is consistent with the production of H+ in the anodic reaction mechanism up to and including the rate-controlling step. The voltammetric response of glucose (a ‘ reducing ’ monosaccharide) and sucrose (a ‘ non-reducing ’ disaccharide) are shown in fig. 3 obtained by PAD using an incremental change of El with each repetitive application of waveform (a) in fig. 1. These curves are representative of all simple alcohols polyalcohols and carbohydrates. Detection with PAD at Pt electrodes has been applied successfully for chromatographic analysis of carbohydrate mixtures using a cation-exchange column (in the Ca2+ form) with pure H,O as the mobile 36 and an anion-exchange column using an alkaline mobile phase.28 Samples included beer wine coconut milk and a carbonated beverage.Aromatic amines are successfully detected at constant potential on solid electrode^.^ Detection by PAD at Pt electrodes in alkaline media is successful for aliphatic amines (primary secondary and tertiary) amino acids (primary and secondary) and amino- glycosides antibiotic^).^^^ 38 The voltametric response for urea and glycine by PAD is shown in fig. 4. Application to chromatographic separations has used anion-exchange columns for amino acids28 and underivatized columns for aminoglyc~sides.~~ EIV us. SCE ........ (C) . -0.200 -0.467 -0.734 -1.000 .* *. . . . . I I I I I . . . .... . . 1087 -1.000 I I I 1 I I I I I I I I I D.C. Johnson et al. EIV us. SCE ........... 0.333 0.066 -0.200 -0.467 -0.734 -I (hi ........................... ...... ............ \" J ......... Fig. 4. Voltammetric response for urea and glycine at a rotated Pt disc electrode under waveform (a) (fig. 1). Conditions and waveform as in fig. 3. Curves (a) residual (6) 1.0 mmol dmP3 urea and (c) 1.0 mmol dm-3 glycine. EIV us. SCE Fig. 5. Voltammetric response for urea and thiourea at a rotated Pt disc electrode under waveform (a) (fig. 1). Conditions and waveform as in fig. 3. Curves (a) residual (6) 1.0 mmol dm-3 urea and (c) 1.0 mmol dm-3 thiourea. Sulphur PAD at Pt electrodes in alkaline media is sensitive to all sulphur compounds (inorganic and organic) for which a non-bonded pair of electrons resides on the S to facilitate adsorption to the electrode surface.Hence sulphate and sulphonic acid derivatives of organic compounds are not detected. The voltametric response of virtually all sulphur compounds to the PAD waveform is adequately illustrated by the response for thiourea 1088 Anodic Detection in Flow-through Cells (4 T (c) 30.0 and ( d ) 70.0. -i Fig. 6. Flow injection (FI) peaks for thiourea with detection by PAD. Conditions 0.25 mol dm-3 NaOH at 0.59 cm3 min-l 39 mm3 samples. Waveform E,(t,) = 0.500 V (300 ms) E,(t,) = 0.800 V (10 ms) E3(t3) = -0.750 V (500 ms). Concentration of thiourea bmol dmP3) (a) 10.0 (b) 15.0 I shown in fig. 5; the response of urea is shown for comparison. Flow injection peaks for thiourea are shown in fig.6 and separations of mixtures of sulphur compounds with detection by PAD are illustrated in fig. 7. Detection of sulphur compounds is more sensitive than detection of carbohydrates and amines. This probably results from a larger value of n for the electrocatalytic oxidation. The detection limit for thiourea is ca. 1 ng in a 50 mm3 sample. Electroinactive Adsorbatesz8 42 In principle any adsorbate will cause a change in the constant cy in eqn (7) and therefore can be detected by PAD using waveform (a) (fig. 1). Polta and demonstrated the detection of CN- and C1- by PAD at detection potentials at which these adsorbed ions are not electroactive. A prerequisite for successful applications of PAD for electroinactive adsorbates is that desorption of the adsorbate occur efficiently at E2 in the waveform.We have not yet found an adsorbate not easily removed at a potential 100-300 mV beyond the value for onset of significant anodic breakdown of H20. Sensitivity and Calibrationz8* 39-42 PAD at Pt electrodes responds primarily to adsorbed analytes. Hence the detection peak signal in FI and LC systems is expected to be a function of adsorption kinetics the adsorption isotherm and the adsorption period (lads) in the potential waveform [e.g. t in waveform (a) fig. 11. For a short tads and rapid adsorption kinetics the net rate of -i .-.1 .- C I T Fig. 7. Chromatographic separations of sulphur compounds with detection by PAD. (A) Inorganic mixture. Conditions 0.10 mol dm-3 NaOH-50 mmol dm-3 Na,SO at 1 .O cm3 min-l Dionex AS-6 anion exchange column 50 mm3 sample.Peaks (a) 5.0 mmol dm-3 Na,S03 (b) 0.15 mmol dm-3 Na,S and (c) 1.5 mmol dm-3 Na,S,O,. (B) Organic mixture. Conditions 0.10 mol dmP3 NaOH at 0.60cm3minP1 Dionex AS-6 anion exchange column 50mm3 sample. Peaks (a) 5.0 mmol dm-3 urea (b) 0.30 mmol dm-3 thiourea (c) 0.50 mmol dm-3 ethylene thiourea and (d) adsorption is limited by the rate of mass transport (k,,). Therefore the peak fractional coverage by adsorbate (S,) is given by Plots of - ip against fads and log (- ip) against log (tad& are shown in fig. 8 for thiourea. The rate of mass transport at short lads is limited by diffusion as given by the Cottrell equation even in the presence of a significant stream velocity.Consequently the log-log plot is linear with a slope of 0.5 for tads < 1000 ms. For 1000 < tads/ms < 3000 the rate of mass transport is limited by convection in the fluid stream and ip increases linearly with tads. For fads -+ 10 s 8 approaches the equilibrium value and ip is independent of tads. TO D/6 = (D/zt)i 1089 D. C. Johnson et al. d I 0.50 mmol dm-3 thioacetamide. (8) O,(t) = 2 1 t (dC/dx),, dt (9) = 5 To 0 Iot k, dt. (10) 1090 0.75 - 40 0 2.0 I ' I 1.25 - v - 1 .;p 1.00 - I I 0.50 - I 4.0 Anodic Detection in Flow-through Cells 2.5 DD I 1% (t,dslmS) I 3.0 I I t a d s 6.0 E3(t3) = 0.700 V (50 ms). 3.5 I 8.0 2.0 o(t) = (ip/ip max)t 0.0 Fig.8. Variation of peak response for thiourea in a FI system with detection by PAD with variation in adsorption time. Conditions 0.25 mol dmV3 NaOH injections of 0.10 mmol dmP3 thiourea in 0.25 mol dmP3 NaOH. Waveform E,(t,) = 0.600 V (350 ms) E2(t2) = - 1.00 V (varied) For long tads Bp(t) approaches the limit for the prevailing adsorption isotherm. In the case of the Langmuir isotherm Bp(co) is given by where K is the equilibrium constant for adsorption and Cp is the peak concentration of analyte. Although a fully satisfying theoretical basis has not yet been formulated the peak signal for PAD in FI and LC systems is assumed proportional to Bp as given by where ip,max is the value of ip at infinite C, i.e. Bp(t) + 1. For the case of mass-transport-limited adsorption Bp(t) is a linear function of C,.For the isotherm-limited response plots of l/i against l/Cp are linear with a non-zero intercept according to 1 l i p = 1 / ip max + ( 1 / K ) ( 1 /ip,max) ( 1 /Cp>* The value of Cp in eqn (1 3) represents the peak concentration for FI or LC experiments and is related to the analytical concentration (Co) of the sample injected by the dispersion constant for the system. To summarize linearity of ip vs. Co plots is assured for dilute samples with small tads. Alternatively at higher concentrations l/ip us. 1/C plots are useful; however two standard samples are necessary to define the calibration curve. All calibration data obtained to date for PAD are in support of this predicted response.I I I I 10.0 (12) (13) D. C. Johnson et al. 1091 Baseline Response43 The baseline for applications of PAD at Pt electrodes in FI and LC systems is given by eqn (7) and represented in fig. 2(b). Since the baseline signal is never zero for PAD in electrocatalytic detections at Pt electrodes attention must be given to the control of factors causing high-frequency fluctuation and drift of the baseline. A cause of high-frequency fluctuation can be the variation of the applied detection potential for consecutive applications of the potential waveform resulting from voltage error in the potentiostatic control amplifier. Since q in eqn (7) is also dependent on the value of IF” for the formation of surface oxide [eqn (l)] fluctuation in solution pH causes large changes in iox:43 Hence it is important that the solution passing through the detector be adequately buffered.Use of pH gradients for LC/PAD will be highly problematic. The greatest cause of baseline drift is the slow increase in ‘ true’ electrode area resulting from electrode roughening which occurs with alternate anodic and cathodic polarizations.lg* 441 45 New Pt electrodes (i.e. freshly polished) require 1 h or more to reach a stable state of surface roughness. Platinum electrodes which have stabilized have the distinct appearance of being coated with ‘platinum black’. They should not be repolished in applications for PAD.46 Future Prospects for Electrocatalytic PAD Present application of PAD in our laboratory for detection of the HC-OH group at Au electrodes is highly succe~sfu1,~~ and sensitivities are larger than for detection at Pt electrodes.This is in support of the recommendation by Dionex Corp. for the use of PAD at a Au electrode for chromatographic determination of carbohydrates.47* 48 The response of amines and amino acids at Au is inferior to detection at Pt.46 Detection of sulphur compounds is also more sensitive at Au electrode~.~~? 49 Future applications of PAD are expected to include detections at carbon electrodes. The observation of benefit from cleaning pulses has been noted.13-19 Although still of great analytical use in LC/EC many of the organic oxidations do not occur at transport-limited rates at carbon electrode~.~~ Numerous reports have described the increased electrocatalytic activity of carbon electrodes resulting from application of large positive potential p ~ l s e s .~ ~ - ~ ~ This treatment results in increased surface roughness57* s8 and an increase in surface-bound oxygen which can catalyse some electrochemical reactions.57 Electrocatalysed Anodic Detection at Oxide Electrodes The literature of anodic organic electrochemistry is extensive and only a few comprehensive reviews are ~ i t e d . ~ ~ - ~ ~ Seldom do anodic electrosynthetic reactions occur at transport- limited rates and their direct applicability for electrochemical detection in FI and LC systems should not be presumed. Many electrosynthetic procedures specify non-aqueous conditions to avoid ‘ undesirable ’ products resulting from nucleophilic attack of H,O on electrogenerated intermediate products.As a basis for the general anodic detection of organic compounds where an electrical current is the only product of interest the presence of H,O is desirable because it provides for low-energy oxidation routes. Carbon in its various forms has received extensive attention for anodic detection in LC systems.l? 63-66 Whereas numerous organic compounds of interest are detected carbon cannot be characterized as a ‘universal’ anode with electrocatalytic activity for 0-transfer reactions. The observation that certain oxide electrodes are good anodic analyte Table 1. Evaluation of n (equiv mol-l) from exhaustive controlled potential coulometrya Anodic Detection in Flow-through Cells nlequiv m o P PbO PbO - BiO,.l.O+O.I 1 .o fO.1 2.1 f O . 1 2.0 f 0.1 1 .o fO.1 5.9 f 0 . 1 5.0fO.l 8.1 fO.1 6.0 f 0.1 5.5 f0.8 5.8f0.1 6.1 fO.l 7.1 fO.l 2.0 f 0.1 ’ 1.OfO.1 6.1 kO.1 5.0 fO.l 7.9 f 0.1 6.0 f 0.1 6.8 k 0.1 7.9 f 0.1 5.9f0.1 6.3k0.1 1.7 + O . 1 6.6f0.1 6.0 f 0.1 2.4 & 0.2 10.0 f 0.1 2.9 f 0.1 10.2k0.1 10.5 f0.2 no rxn. 6.7 fO.1 6.0 f 0.1 2.0 f 0.2 no rxn. no rxn. no rxn. 2.1 fO.1 4.2 f 0.1 7.4 + 0.1 2.8 f 0.2 2.1 kO.1 4.0 k 0.1 9.3 & 0.2 4.6k 0.1 ~ Fe3+ OAs(OH) OSBr 1092 Fe2+ As(OH) Br- I- Mn2+ s,o;- SCN- H,N-C(S)-NH H,C-C( S)-NH H,C(SH)-CH,-S0,H C,H,( S)-CH,-CO,H C,H,-sO,H H,C( SH)-CH(NH,)-CO,H H,C-s-c,H,-CO,H EDTA HO-C(CH,-CO,H),-CO,H HO,C( HC-OH),CO,H H,C(OH)-CO,H HCO,H HCHO HO-C,H,-OH C,H,-oH HO-C,H,(CO,H)-SO,H C6H806 0-transfer reactions.59 ??? a Conditions 0.1 mol dm- HC10,.Uncertainty is standard deviation from triplicate results. Products estimated from n; SO:- identified by Ba2+ test. electrocatalysts is not new [see e.g. ref. (59) and (67)-(72)]. Our interest in lead dioxide as an analytical anode results from its successful applications for some electrosynthetic ~ 10 MnO; 2so:- SO:- + CN- SO:- + H,H-C(0)-NH SO:- + H,C-C(0)-NH H0,S-CH,-CH,-S0,H ??? ??? C,H,-sO,H H03S-CHz-CH(NH2)- CO,H H3C-s0,-c6H,-c0,H 2C0 + 4HC0,H 2C0 + 2HC0,H OHC-CO,H - (0)C6H4(0) (0)C6H4(0) Lead Dioxide Electrodes So-called ‘lead dioxide’ (PbO,) is a non-stoichiometric oxide described by the approxi- mate formula Pb01.98,739 74 which has the desirable anodic electrode properties of high electronic conductivity and large overpotential for 0 evolution in aqueous media.Some effort has been given to investigate PbO as an analytical anode e.g. ref. (70) and (75)-(80) but such use has not been widespread. Because of the limited potential range at PbO electrodes (ca. 500mV) the material is not considered to be useful for voltammetric applications. In anticipated amperometric detection in liquid chromato- graphy voltammetric resolution is of lowest priority; electrocatalytic activity is of highest priority. Lead dioxide electrodes can be prepared by deposition on many solid conducting s u b s t r a t e ~ .~ ~ - ~ ~ We use Pt and Au as substrates and observe no difference in the amperometric response of PbO electrodes prepared at the two noble metals. An electrocatalytic role for PbO in anodic reactions has apparently been overlooked by many.73 We find many oxidations requiring extensive 0-transfer especially involving sulphur which occur rapidly at constant potential on PbO electrodes. Most of these productC soq- + ?? ~~~~~ ~~ ~~~~ 7 -i D. C. Johnson et al. 1 2 0 u A . . Fig. 9. Flow injection peaks for thiourea with detection at a PbO electrode. Conditions 0.10 mol dmP3 HC10 at 1.0 cm3 min-l 193 mm3 samples 1.52 V us. SCE detection potential. Concentration of thiourea (mmol dmP3) (a) 0.500 (b) 0.200 (c) 0.100 and ( d ) 0.050.same reactions cannot be maintained at significant rates on Pt Au or C electrodes at constant potential. Values of n for several reactions at PbO electrodes are represented in table 1. The reaction products indicated are estimated from n ; in the case of sulphur compounds production of SO:- was verified using Ba2+. The applicability of PbO electrodes for anodic detection in flowing solutions is illustrated by FI peaks for thiourea in fig. 9. For an experiment of limited duration peak heights are reproducible and a linear function of thiourea concentration. Over longer periods anodic sensitivity was observed to drift for numerous analytes studied.84 We conclude that pure electrodeposited PbO is not sufficiently active for routine anodic applications.Our effort to increase the electrocatalytic activity for 0-transfer reactions is based on doping of PbO with other metal oxides. Doped Lead Dioxide Electrodes By analogy with n-type electronic semiconductors based on doping the Group IV element Si with As from Group V we are attempting to prepare ‘n-type’ 0-transfer electrolysts by doping PbO with Group V oxides at various concentrations (see table 2) to form materials designated by the general formula PbO; MO2.,.* The ‘p-type’ mixed oxides of the type PbO;MO,., prepared by doping with Group I11 oxides are not expected to have electrocatalytic activity for anodic 0-transfer reactions. Values of n (in equiv mol-l) for several oxidation reactions at PbO * BiO,.electrodes are given also in table 1 in comparison to PbO,. This electrode was prepared by deposition from 0.1 mol dm-3 HClO containing equal concentrations of Pb2+ and BPI1. X-Ray-fluorescent analysis of the electrode indicated approximately equal amounts of the two metals in the mixed oxide. In general reaction products are the same for the two electrodes but reaction rates are increased significantly at the mixed oxide electrode. A dramatic example of the increased activity of the doped electrode is the case of citric 1093 1094 Anodic Detection in Flow-through Cells Table 2. Mixed oxide electrode materials based on the PbO matrix IVA VA - IIIA - Si Ge Ga (Ga203) In Sn Pb T1 ' p-type' oxides ' n-type' oxides PbO2'MO,.PbO,'MO2. acid. The oxidation is rapid at the mixed oxide with n x 10 whereas the reaction is so slow at the PbO electrode as to make integration of the i us. t curve impractical. 1 = As Sb (Sb205) Bi * . nFADCb 6 + ( D / k ) where 6 = 1.62Db-hi and k is the rate constant. Values of k can be obtained from the intercepts of the linear plots of l / i against l / d - 1 i - - knFACb+0.6nFADb-! 1 1 Cb (i) Values of k calculated from plots of l / i against I / & for the oxidation of Mn2+ are given in table 3 for various electrode^.^^ The oxidation of Mn2+ was chosen as the test reaction because of the large number of oxygens transferred Mn2+ + 4H,O -+ MnOg + 8H+ + 5e. Electrodes of the type PbO,.MO,. are more reactive than PbO, and PbO;MO,.electrodes are less active. The electrocatalytic activity of PbO - BiO,. was determined as a function of the concentration of BPI in the plating solution. The reaction rate increased to the mass-transport-limited value for [Bi1I1]/[PbI1] z 1 . Values of k for oxidation of several compounds are given in table 4 for the PbO and PbO * BiO,. electrodes. The electrocatalytic benefit of doping with BiO,. is concluded to be general. (16) Kinetic Studies The current in a rotated-disc electrode under coupled transport-kinetic control is given empirically by Mechanism The oxidizing power of the highest oxidation states of bismuth and arsenic differ substantially. Hence the observation of enhanced electrocatalytic activities for PbO - BiO,.and PbO -AsO,. appears unrelated to the thermodynamic properties of the redox couples of doping metal oxides. The anodic voltammetric response of Mn2+ and AsIII at rotated PbO and PbO * BiO,. disc electrodes are shown in fig. 10. Whereas the observed Ei for Mn2+ is virtually the (15) - TI1" In111 GalI1 4.1 x 10-3 2.3 x 10-3 3 . 4 ~ 10-3 3.5 x 10-3 2.0 x 10-2 5.7 x 10-2 6.9 x lo- BiIII BiIII > 1 x 10-1" PbO - BiO,. > 1 x 10-lC AsV AsV a Electrodes deposited from 1 .O mmol dm-3 PbII- 0.1 mmol dm-3 HClO,. Kinetic measurements 1 .O mmol dm-3 in Mn2+-0. 1 mmol dm-3 HC10,. Effectively mass-transport-controlled. Table 4. Rate constants for oxidations at rotated PbO and PbO - BiO,. disc electrodesa compound manganese@) sulphosalicylic acid phenol hydroquinone cystine thiophenacetic acid D.C. Johnson et al. Table 3. Rate constants for oxidation of Mn2+ at rotated doped PbO disc electrodes Mn2+ + 4H,O + MnO; + 8H+ + 5e concentration doping ion /mmol dm-3a k/cm s-lb - 0.10 1 .o 1 .o 0.10 0.10 1 .o 1 .o PbO d (4 (5) 4.1 fO.1 x > 1 x 10-lc (7) 4.5f0.9 x 1.1 f0.2 x (4) (2) 9 . 7 f 0 . 4 ~ 2 . 9 f 1 . 2 ~ lo- (6) 9 . 0 k 0 . 4 ~ loF3 1 . 5 f 0 . 4 ~ lo- (7) 1.4k0.3 x 2.2f 1.2 x lo- a PbO deposited from 20 mmol dmP3 PbII-O.1 mmol dm-3 HC10,; PbO BiO,. deposited from 20 mmol dm-3 PbII- 20 mmol dm-3 Bi1I1/O.1 mmol dm-3 HClO,. Uncertainty given for 90% confidence interval.Measurements done for 1 .O mmol dmV3 analyte in 0.1 mmol dm-3 HClO,. Effectively transport-limited. No reaction. same at the two electrodes the transport-limited current is obtained only at PbO - BiO,.,. We conclude that the rate controlling step i.e. 0-transfer occurs without charge transfer. The Ei for AslI1 at PbO . BiO,. is nearly the same as for Mn2+ even though the reversible potential is ca. 0.31 V for the AsV/AslI1 couple. We conclude the observed E+ is characteristic of the discharge of H,O to produce the active adsorbed oxygen at the electrode surface i.e. reaction (2). Summary Results have been summarized for anodic electrocatalysed detections at Pt electrodes under pulsed potential waveforms and at pure and doped PbO electrodes at constant 37 1095 FAR 1 1096 (A) Anodic Detection in Flow-through Cells (B) t - I m A EIV us.SCE Fig. 10. Voltammetric response of Mn2+ and As'" at rotated Pb0,s and PbO,-BiO,. disc electrodes. Electrodes 0.5 cm2 (-) PbO deposited from 20 mmol dm-3 Pb'I-1 .O mol dm-3 HClO,,(--)PbO - BiO,~,depositedfrom20 mmol dm-3Pb11-20 mmol dm-3Bi111-l.0 mol dm-3 HClO, 900 rev min-l. Solutions (A) 1.0 mmol dm-3 Mn2+-0. 1 mol drn-3 HClO, (B) 2.5 mmol dm-3 As1I1-O.1 mol dm-3 HClO,. Curves (a) residual (b) analyte present. potential. The oxidative detection reactions are concluded to occur with transfer of adsorbed oxygen from the electrode surfaces to the oxidation products. The adsorbed oxygen is regenerated by oxidation of H,O. Application of the detection procedure at Pt is restricted largely to alkaline media whereas detection at PbO electrodes is applicable for many compounds in acidic media.The voltammetric response for the electrocatalytic processes are controlled generally by the anodic transfer of oxygen from H,O to the electrode surface. Hence analytical applications for mixtures requires prior separation and the most advantageous applica- tion is for anodic detection in chromatographic effluents. Portions of this research were funded by the Office of Basic Energy Sciences of the U.S. Department of Energy (contract no. W-7405-ENG-82) National Science Foundation (grant no. CHE-8312032) and Dionex Corp. Sunnyvale CA U.S.A. 1 D. C. Johnson S. B. Weber A. M. Bond R. M. Wightman R. E. Shoup and 1.S. Krull Anal. References Chim. Acta in press. 2 K. Bratin and P. T. Kissinger J. Liq. Chromatogr. 1981 4 suppl. 2 321. 3 J. Debowski K. Duszczyk W. Kutner D. Sybilska and W. Kemula J. Chromatogr. 1982 241 141. 4 M. 0. Funk M. B. Keller and B. Levison Anal. Chem. 1980 52 771. 5 M. W. Breiter Electrochim. Acta 1963 8 973. 6 J. Giner Electrochim. Acta 1964 9 63. 7 R. N. Adams Electrochemistry at Solid Electrodes (Marcel Dekker New York and Basel 1969). 8 S. Gilman in Electroanalytical Chemistry ed. A. J. Bard (Marcel Dekker New York 1967) vol. 2. 9 0. J. Johnson Res. Inst. Catal. Kokkaido Univ. Paper V 1972; cited by H. B. W. Patterson Hydro- genation of Fats and Oils (Applied Science Publishers London 1983) chap. 5. 10 D. Clark M. Fleischmann and D.Pletcher J. Electroanal. Chem. 1972 36 137. 11 A. MacDonald and P. D. Duke J. Chromatogr. 1973 83 331. 12 K. Stulik and V. Hora J. Electroanal. Chem. 1976 70 253. 13 B. Fleet and C. J. Little J. Chromatogr. Sci. 1974 12 747. 14 B. Fleet (EDT Supplies Ltd. London England) US. Patent 4059406; Nov. 22 1977. 15 T. A. Berger personal correspondence June 1985. 16 T. A. Berger (Hewlett-Packard Corp. Avondale Pennsylvania) US. Patent 4496454; Jan. 29 1985. 1097 17 J. Tenygl in Electrochemical Detectors ed. T. H. Ryan (Plenum Press New York 1984) pp. 89-103. 18 H. W. van Rooijen and H. Poppe Anal. Chim. Acta 1981,130,9. 19 R. Woods in Electroanalytical Chemistry ed. A. J. Bard (Marcel Dekker New York 1976) vol. 9. D. C. Johnson et al. 20 M. W. Breiter Electrochim.Acta 1963 8 925. 21 S. Gilman Electrochim. Acta 1964 9 1025. 22 M. W. Breiter J. Electroanal. Chem. 1964 7 38. 23 B. E. Conway and S. Gottesfeld J . Chem. Soc. Faraday Trans. I 1973 69 1090. 24 D. Gilroy and B. E. Conway Can. J. Chem. 1968,46 875. 25 P. Stonehart H. A. Kozlowska and B. E. Conway Proc. R. SOC. London Ser. A 1969,310 541. 26 H. Angerstein-Kolowska B. E. Conway and W. B. A. Sharp J. Electroanal. Chem. 1973 43 9. 27 B. V. Tilak B. E. Conway and H. Angerstein-Kozlowska J. Electroanal. Chem. 1973 48 1. 28 D. S. Austin J. A. Polta T. Z. Polta A. P-C. Tang T. D. Cabelka and D. C. Johnson J. Electroanal. 31 D. C. Johnson J. Electrochem. SOC. 1972 119 331. Chem. 1984 168 227. 29 D. C. Johnson and S. Bruckenstein J . Electrochem. SOC.1970 117,460. 30 R. F. Lane and A. T. Hubbard J. Phys. Chem. 1975 79 808. 32 D. S. Austin D. C. Johnson T. G. Hines and E. T. Berti Anal. Chem. 1983 55 2222. 33 D. Gilroy J. Electroanal. Chem. 1977 83 329. 34 S. Hughes P. L. Meschi and D. C. Johnson Anal. Chim. Acta 1981,132 1. 35 S. Hughes and D C. Johnson Anal. Chim. Acta 1981 132 1 1 ; 1983 149 1. 36 S. Hughes and D. C. Johnson J. Agric. Food Chem. 1982,30 712. 37 J. A. Polta and D. C. Johnson J . Liq. Chromatogr. 1983 6 1727. 38 J. A. Polta D. C. Johnson and K. E. Merkel J. Chromatogr. 1985 324 407. 39 T. Z. Polta and D. C. Johnscn J . Electroanal. Chem. submitted. 40 T. Z. Polta G. R. Luecke and D. C. Johnson J. Electroanal. Chem. submitted. 41 T. Z. Polta J. Johnson and D. C. Johnson unpublished results.42 J. A. Polta and D. C. Johnson Anal. Chem. 1985 57 1373. 43 J. A. Polta I-H. Yeo and D. C. Johnson Anal. Chem. 1985 57 563. 44 D. C. Johnson D. T. Napp and S. Bruckenstein Electrochim. Acta 1970 15 1493. 45 A. C. Chialvo W. E. Triaca and A. J. Arvia J. Electroanal. Chem. 1983 146 93. 46 G. G. Neuburger and D. C. Johnson unpublished results. 47 P. Edwards and K. K. Haak Am. Lab. 1983,4 78. 48 R. D. Rocklin and C. A. Pohl J. Liq. Chromatogr. 1983 6 1577. 49 A. Ngoviwatchai and D. C. Johnson unpublished results. 50 R. M. Wightman M. R. Deakin P. M. Kovach W. G. Kuhr and K. J. Stutts J. Electrochem. Soc. 1984 131 1528. 51 A. G. Ewing M. A. Dayton and R. M. Wightman Anal. Chem. 1981,53 1842. 52 J. F. Evans and T. Kuwana Anal. Chem. 1977 49 1632.53 R. M. Wightman E. C. Palk S. Borman and M. A. Dayton Anal. Chem. 1978 50 1410. 54 F. G. Gonan C. M. Fombarlet M. J. Buda and J. F. Pujol Anal. Chem. 1981,53 1386. 55 R. C. Engstrom Anal. Chem. 1982,54,2310. 56 R. Takamura S. Inoue and F. Kusu Bunseki Kagaku 1984 33 198. 57 P. M. Kovach and R. M. Wightman Abstract No. 8 International Electroanalytical Symposium May 30 1985 Chicago IL U.S.A. 58 J. Wang and L. D. Hutchins Anal. Chim. Acta 1985 167 325. 59 N. L. Weinberg and H. P. Weinberg Chem. Rev. 1968 68 449. 60 Organic Electrochemistry ed. M. M. Baizer (Marcel Dekker New York 1973). 61 Encyclopedia of Electrochemistry of the Elements Organic Section ed. A. J. Bard and H. Lund (Marcel Dekker New York 1978-84) vol. XI-XV. 62 T. Shono Electroorganic Chemistry as a New Tool in Organic Synthesis (Springer-Verlag Berlin 1984).63 P. T. Kissinger Anal. Chem. l977,49,447A. 64 K. Stulik and V. Pakakova J. Electroanal. Chem. 1981 129 1. 65 D. A. Roston R. E. Shoup and P. T. Kissinger Anal. Chem. 1982,54 1417A. 66 R. E. Shoup Recent Reports on Liquid ChrornatographylElectrochemistry (BAS Press W. Lafayette IN 1982). 67 M. Fleischmann K. Korinek and D. Pletcher J. Electroanal. Chem. 1971,31 39. 68 C. 0. Huber T. A. Berger and R. E. Reim Electrocatalysis on Nonmetallic Surfaces Natl. Bur. Standards ( U S ) Spec. Publ. 1976 455 183. 69 P. Griindler and T. Bottger Chemia Analityczna 1980 25 1089. 70 T. N. Morrison K. G. Schick and C. 0. Huber Anal. Chim. Acta 1980 120 75; B. S. Hui and C. 0. Huber Anal. Chim. Acta 1982 134 211; J. B. Kafil and C. 0. Huber Anal. Chim. Acta 1982 139 347. 71 S. Trasatti J. Electroanal. Chem. 1980 111 125. 72 G. Filardo F. DiQuarto S. Gambino and G. Silvestri J. Appl. Electrochem. 1982 12 127. 73 J. P. Can and N. A. Hampson Chem. Reu. 1972,72,679. 37-2 1098 Anodic Detection in Flow-through Cells 74 J. P. Pohl and H. Rickert in Electrodes of Conductive Metallic Oxides ed. S. Trasatti (Elsevier Amsterdam 1980) pp. 183-220. 78 G. Bllanger Anal. Chem. 1974,46 1576. 75 M. T. Becke Chem. Analyst 1960 50 14. 76 D. R. Tallant and C. 0. Huber J . Electroanal. Chem. 1968 18 413. 77 C. 0. Huber K. Dahnke and F. Hinz Anal. Chem. 1971,43 152. 79 G. Kainz and G. Sontag Z . Anal. Chem. 1974 269 267. 80 P. Grunther Anal. Lett. 1981 14 163. 81 A. C. Ramamurthy and T. Kuwana J . Electroanal. Chem. 1982 135 243. 82 R. G. Barradas and A. Q. Contractor J. Electroanal. Chem. 1982 138 425. 83 C. Efron and M. Ariel Anal. Chim. Acta 1979 108 395. 84 A. P-C. Tang J. Baur and D. C. Johnson unpublished results. 85 I-H. Yeo and D. C. Johnson unpublished results. Paper 5 / 1882; Received 26th September 1985

ISSN:0300-9599    

DOI:10.1039/F19868201081

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC. Faraday Trans. I 1986,82 1099-1104 Convolution of Voltammograms as a Method of Chemical Analysis Keith B. Oldham Trent University Peterborough Ontario Canada K9J 7B8 Voltammetry is extensively employed for chemical analysis but in all existing methods the analyte concentration is calculated from some feature (usually a peak or plateau) of the voltammogram so that most of the experimental data remain unused. Here a concept is proposed that permits measurement of the bulk analyte concentration from every point on the voltammogram. The principle exploited is to apply a suitable convolution procedure to the Faradaic current thereby generating the concentration excursions of the electroactive reactant and product at the electrode surface. These excursions are then combined with the Nernst equation to measure the analyte concentration.The concept is tested experimentally and is shown to have promise as a means of enhancing analytical precision and rejecting interferences. Voltammetric techniques occupy a secure position in the repertoire of the analytical chemist they constitute the preferred method for the assay of a large number of organic and inorganic species in solution. By a ' voltammetric technique ' is meant one of the many methods1 that share the following characteristics. In voltammetry an electroactive species reaches a stationary electrode by diffusion alone. A programmed potential waveform is imposed on the electrode previously at equilibrium as a result of which a Faradaic current flows.Often the readout of the voltammetric technique is a graph a so-called voltammogram displaying the current as a function of potential. Alternatively the current may be processed in some way (differentiated differenced semi-integrated etc.) prior to display versus potential. The large number of voltammetric techniques arises from the diversity of potential programmes from differences in current processing and from the variety of possible cell geometries. Except in the experimental example that concludes this article no particular voltammetric technique will be assumed here so that the discussion will apply generally to all such techniques. When voltammetry is used as an electroanalytical method it is of course the bulk concentration of the electroactive analyte that is sought.The standard methods of determining this concentration are based on some feature of the voltammogram usually the height of a voltammetric peak or wave. Thus these analytical procedures use the current data only from the immediate vicinity of the feature whereas at least in principle information about the bulk analyte concentration is present in the current values at all potentials. Greater analytical precision is to be expected from a procedure that uses all the current data. This is the philosophy that motivates the present article. Concentration Excursions If 0 represents the analyte then c$ will denote the sought bulk concentration. Let us assume that the voltammetric reaction is a reduction O(so1n) + ne + R(so1n) (1) 1099 Convolution of Voltammograms c& = cg exp [nF(E- P ) / R T ] (2) must apply where E is the instantaneous electrode potential and Ee is the standard electrode potential F R and T being the Faraday constant the gas constant and temperature respectively.Let t = 0 be the instant at which the voltammetric experiment commences; then the concentration of 0 at the electrode surface is c& prior to this instant. At any time during the experiment the difference c&-c& represents the change that has occurred in the concentration of 0 at the electrode surface. This difference plays a salient role in what follows and we shall call it the ‘concentration excursion’ of 0. The corresponding quantity for the reduction product R would generally be ck-cg but in experimental practice the bulk product concentration cg is usually zero and it is convenient to assume that this is always the case.Hence the concentration excursion of R is simply ck. Notice that the signs in the definitions of the concentration excursions have been selected such that each will normally be positive. (3) (4) 1100 to a product species R. The symbols c6 and cg will be used to denote the concentrations of species 0 and R at the surface of the working electrode (more exactly at the outer Helmholtz plane2 adjacent to the working electrode). These surface concentrations will each change in value as the voltammetric experiment proceeds but thermodynamic constraints prevent their varying independently. If reaction (1) is reversible the Nernst relationship and Convolution Procedures In the last two decades a number of procedures have been discovered that provide a direct method of calculating the concentration excursions ck-c& and cg (or cg-cg if the solution is not initially devoid of R) from measurements of the Faradaic current i.These procedures a catalogue of which has been p~blished,~ have the following features in common (a) all are independent of the way in which potential varies with time; (b) all are independent of the rate and mechanism of the electron-transfer process provided that no significant accumulation of intermediates occurs ; (c) knowledge of the electrode area A and of the diffusion coefficient Do or D of the appropriate species is required; ( d ) in order to generate surface excursion data corresponding to the time instant t current values i(z) are needed over the time interval 0 < z < t from the beginning of the experiment or z) up to time s,’ i(z)g(t t ; (e) - all dz involve equivalently integrals of the s form i(t - z) g(z) dz where g is an appropriate function.Mathematically the operation in eqn (3) is known as a ‘convolution’ and therefore the procedures of deriving concentration excursions from current values are termed convolution procedures. The different varieties of convolution procedure arise from differences in electrode geometry and from whether or not the species 0 or R are involved in homogeneous reaction with components of the electrolyte solution.The archetypal convolution procedure is semi-integration; this is the appropriate procedure when the electrode is planar the diffusion field is semi-infinite and there are no homogeneous reactions. The convolution function g(z) is then simply (nz)-f and the expressions c&- co-- s - nAFD$ m - nAF(nD,); 1 [ ( t - z ) f i(z) dz The convolution function g(z) in this case has (m)+ exp (- kz) replacing the (m)+ term that is adequate for semi-integration. Whereas simple semi-integration yields the concentration excursions at the surface of a planar electrode more complicated convolution functions are needed when the electrode is spherical. These have been discussed recently by Myland et aL8 These authors define functions fs and fa that represent the appropriate convolution functions when 0 is the cation of an amalgamable metal and the voltammetric reaction is (6) occurring at a mercury sphere of radius r.They demonstrated that the convolution procedures (7) K. B. Oldham hold m (sometimes denoted I ) being the Faradaic semi-integral. Under the names ' semi-integral electroanalysis ' and ' neopolarography ' this particular convolution procedure as well as related derivative techniques has been used extensively for electr~analysis,~ while under the name 'convolution voltammetry' it has been employed by Saveant and coworkers5 as a tool in mechanistic studies. More recently Bond et a1.6 have used semi-integration innovatively to measure electrode kinetic parameters. Another convolution procedure known as ' kinetic convolution ',7 is appropriate when the product of an electron-transfer process at a planar electrode is involved in a subsequent homogeneous reaction O+ne + R + P.k O(so1n) + ne -+ R(ama1) nAFr and cb,-cf) = - i(t - z) fs(Do z / r 2 ) dz ck = - nAFr 1 1 - I, P t i(t - z) fa(D z / r 2 ) dz which they term ' spherical convolutions ' generate the concentration excursions in these circumstances. Yet other convolution procedures are appropriate in other circumstance^,^ but the above three examples suffice to demonstrate the generality of the concept. [See ref. (3) for information on how in practice one may implement the convolution procedures.] Several applications of convolution procedures may be envisaged not all of which have yet been exploited.Thus these procedures may be applied to studies of electrode reversibility to measure standard electrode potentials to determine the kinetic parameters of electron-transfer processes to elucidate the mechanisms of homogeneous reactions and measure their rate constants to determine diffusion coefficients to estimate the thickness of films on modified electrodes and to perform chemical analysis both by classical feature-reliant methods and the ' featureless' concept that is the subject of this article. Implementation of the New Concept Once the two concentration excursions cb,-c& and cg are determined it is trivially simple for a reversible electrode reaction to combine their values with the Nernst term [see eqn (2)] so as to give the bulk analyte concentration (c8 - c&) + ck exp [nF(E- P ) / R T ] = c;.Such a calculation can be carried out for each instant during the experiment so that an array of bulk analyte concentrations becomes available. Such values should in theory all be equal. In practice errors will contaminate each datum especially those cb values corresponding to the more positive potentials (which are especially prone to errors arising (10) 1101 (8) (9) 1102 I t Table 1. Format of the experimental data and quantities derived therefrom I1 IV I11 I cg-c; I Convolution of Voltammograms V cg -0.50 I 0 t = A t = 2A t = 3A tj = j A -0.40 20 10 I 2 0 .................. .J “ I .-10 - - -20 2 0 from small discrepancies in the E - Ee term). A weighted mean will represent the ‘ best’ bulk concentration. Nowadays electrochemical instrumentation is mostly digital so that current data are usually available as a set of i values at equally spaced time instants. The potential programme provides E values at the same instants so that the results of a voltammetric experiment take the form of the first three columns in table 1. Column IV the concentration excursion of species 0 is computed by an appropriate convolution algorithm [that given in ref. (3) for example] that uses all the preceding current data; i.e. (cB-cS~)~ is based upon i,, i, i, ... as well as ij. Similarly the appropriate convolution procedure is used to generate values for column V of table 1.The final step in the calculation of (~5)~ simply applies eqn (10) to values drawn from the j row of columns 11 IV and V. Constants # Do and D are required in order to perform the convolution procedures as well as information about the electrode geometry. Literature values of the constants may be employed or better they may be obtained from the analysis of similar experiments carried out on solutions of known composition. Fig. 1. Experimental cyclic voltammogram. E i ’ . .. VI 43 -0.41 10 EO El E E Ej -0.50 I 1 EIV us. Ag I AgCl -0.60 I -0.60 I I . . . . I 8 4 6 ti S 1103 10 8 8 2 4 I K. B. Oldham EIV us. Agl AgCl 6 I tl s Fig. 2. Concentration excursions (a) cg - c& (b) cg and (c) the calculated bulk analyte concentration cg.It cannot be emphasized too strongly that any potential-time programme may be employed with this concept. Moreover the potentials encountered during the experiment need not span the entire range of the voltammetric wave a valuable consideration when chemical interferences are present. The theory of the concept is insensitive to whether the potential is changed continuously (linear or non-linear ramps triangles) or discon- tinuously (steps pulses) but the discontinuous current that results from the latter perturbations is more difficult to convolve accurately. An Example Fig. 1 shows the cyclic voltammogram obtained when a triangular potential waveform was imposed on a hanging mercury drop immersed in a solution containing cadmium ions so that (1 1) Cd2+(aq) + 2e -+ Cd(ama1) was the electrode reaction.Other experimental conditions were as described by Myland.8 The convolution procedures indicated by eqn (4) and (5) were applied to the current data shown in fig. 1 and produced the concentration excursions shown as the two lower curves in fig. 2. The shapes of these curves are interesting. Note that during the backward potential scan the concentration excursion c t - c6 actually becomes negative. This reflects the fact that the surface Cd2+ concentration then exceeds its bulk value as a result of the concentrating of Cd that occurred in the mercury drop during the time when its potential was negative. This build-up of Cd within the drop is evidenced by the steady climb of cg apparent in fig.2 during the period of negative polarization. The topmost curve in fig. 2 was calculated via eqn (10) and should be a horizontal line corresponding to the actual analyte concentration of 5.00 mmol dm-3. At all times in the 1.7 < t / s < 6.7 range the measured cb concentration lies within the range 4.9-5.2 mmol dm-3 acceptably close to the true value. The potentials to which these measurements correspond range from -0.486 V to the reversal potential of -0.633 V and back to -0.536 V (ie. 60 -+ -87 -+ 10 mV with respect to P). Outside this range c becomes inconstant much too large and is evidently erroneous. Perhaps the most remarkable aspect of these experimental results is that accurate 1104 Convolution of Voltammograms analyte concentrations are calculable as early as 2.0 s into the experiment.A glance at fig. 1 shows that at this time the cyclic voltammogram had barely departed from the base line! Thus Cd2+ could have been accurately assayed even in the presence of other electroactive species provided these do not reduce at potentials more positive than -0.50 V versus AgIAgCl. Summary Each corresponding to a specific geometric and kinetic circumstance there exist a variety of convolution procedures that may be applied to voltammetric currents. These procedures which are independent of the details of the potential programme and of the electron-transfer process generate the concentration excursions of the electroactive reactant and product.The two concentration excursions may be combined with Nernst’s equation to produce a measurement of the analyte concentration. Such a measurement is possible at each and every point in the voltammogram and not merely at its features. This new concept offers the possibility of increased analytical precision as well as an opportunity to alleviate interference. The generous financial support of the National Science and Engineering Research Council of Canada is acknowledged with gratitude. Portions of this work were described at the 68th Canadian Chemical Congress (Kingston June 1985) and at the 30th Congress of the International Union of Pure and Applied Chemistry (Manchester September 1985). References 1 A. J. Bard and L. R. Faulkner Electrochemical Methods (John Wiley Chichester 1980) chap. 5 and 6. J. Chem. SOC. Dalton Trans. 1985 1213. 8 J. C. Myland K. B. Oldham and C. G. Zoski J. Electroanal. Chem. 1985 193 3. 2 Laboratory Techniques in Electroanalytical Chemistry ed. P. T. Kissinger and W. R. Heineman (Marcel Dekker New York 1984) chap. 1. 3 K. B. Oldham Anal. Chem. submitted. 4 M. Grenness and K. B. Oldham Anal. Chem. 1972 44 1121; F. C. Soong and J. T. Maloy J. Electroanal. Chem. 1983 153 29; M. Goto and D. Ishii J. Electroanal. Chem. 1975 61 361; G. Zhu and E. Wang Acta Chim. Sinica 1982 40 897. 5 J. C. Imbeaux and J. M. Saveant J. Electroanal. Chem. 1973 44 169; L. Nadjo J. M. Saveant and D. Tessier J. Electroanal. Chem. 1974 52 403; J. M. Saveant and D. Tessier J. Electroanal. Chem. 1975 61 251; 1975 65 57. 6 A. M. Bond T. L. E. Henderson and K. B. Oldham J. Electroanal. Chem. 1985 191 75. 7 F. E. Woodward R. D. Goodin and P. J. Kinlen Anal. Chem. 1984 56 1920; A. Blagg S . W. Carr G. R. Cooper I. D. Dobson J. B. Gill D. C. Goodall B. L. Shaw N. Taylor and T. Boddington Paper 511883; Received 10th October 1985

ISSN:0300-9599    

DOI:10.1039/F19868201099

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC. Faraday Trans. 1 1986,82 1105-1116 Analytical Applications of Gas Membrane Electrodes Stanley Backenstein* and James S. Symanski Department of Chemistry State University of New York at Buffalo Buffalo New York 14214 U.S.A. Porous and permeable membrane electrode structures are now widely exploited in sensors for species in aqueous and gas phases. Potentiometry and amperometry have beenapplied frequently in conjunction withmembrane electrode methodology. Only recently has the classic technique of conduc- tometry been used in tandem with membrane electrodes. Ambient-level sulphur dioxide and carbon dioxide conductance sensors based on structures involving porous membranes and thin water layers are described and the principles governing their response are discussed.An electrode separated from an analyte-containing phase by a membrane through which the analyte must pass to reach it is known as a membrane electrode. The electrode may be separated from the membrane by a thin layer of aqueous solution. Alternatively it may be deposited on or partially into the membrane. On reaching the electrode the analyte is determined by a suitable electrochemical technique. Membrane electrodes are widely used as sensors in gases and in aqueous solutions. Gas membrane electrodes are particularly convenient to determine the concentration of species present in the gas phase. They are readily incorporated into small hand-held monitors that meet critical design requirements. These requirements include low power consumption read-on-demand display fast response analytical selectivity response compensation over a wide range of ambient temperatures and use by non-technical personnel.Various kinds of structures for analytical gas membrane electrodes have been designed. The membranes are conveniently classified into two types permeable or porous. Gas membrane electrodes based on these membranes have electrocatalysts (metals) deposited on the membrane side not contacting the gas phase. Also traditional fuel-cell structures may be used. Regardless of the electrode structure a variety of electrochemical techniques have been employed including potentiometry amperometry and conductometry . Membranes A permeable membrane behaves as a homogeneous phase so that the transport of a species into through and out of it can be described readily.This description includes the distribution equilibria involving the membrane and the species in the two phases contacting the membrane the transport of species from one face to the other and the mass-transport situation existing in the boundary layer adjacent to the membrane faces. Classical diffusion and boundary-layer theory are usually sufficient in modelling such an electrode structure. A porous membrane behaves as a two-phase structure. Those used in gas membrane electrodes are hydrophobic and frequently are made from Teflon which also provides the mechanical structure of the membrane. One common type of membrane can be visualized as being a sintered mat of Teflon particles with tortuous interconnected channels filled with gas between the two faces.There are two unique problems in describing transport of species through a porous membrane. The first involves the area 1105 1106 Gas Membrane Electrodes of the membrane that is effective in the transport process. It is normally considered to be the gas-phase area in the membrane surface. The second problem is the uncertainty in the effective diffusional path length between the two membrane faces. If a gas phase contains the analyte the pores within the membrane are filled with this gas and the analyte. Thus no partitioning of analyte occurs at the membrane face in contact with the gas phase. However if the analyte-containing phase is a liquid selective partitioning into the gas-filled membrane pores may occur.The differences between permeable and porous membranes have important consequences. First it is possible to obtain selectivity for the analyte with respect to other species by using a permeable membrane. This selectivity results from differences in partition coefficients of the various species and the membrane phase as is the case of sulphur dioxide with respect to both nitric oxide and nitrogen dioxide at a polyethylene membrane.l However the diffusion coefficient of the analyte in a permeable membrane is orders of magnitude smaller than it is in the gas pores of a porous membrane. Thus usually it is necessary to use very much thinner and more fragile permeable membranes to approach the fluxes of analyte found using porous membranes.On the other hand a large value of the analyte’s partition coefficient can increase the flux through the permeable membrane and minimize this difficulty. Secondly a hydrophobic porous membrane shows no selectivity for the analyte with respect to other species present in a gas phase. However it is selective with respect to species present in an aqueous phase. This selectivity is governed by the partial pressures of the dissolved species. Hence selectivity in gas-phase analysis problems can only be obtained with porous membrane-based gas membrane electrodes by using selective electrochemical techniques and/or chemical procedures. The simplest of the chemical techniques frequently are satisfactory. For example a porous mat filled with sodium bicarbonate interposed between the gas phase and the membrane prevents a wide variety of acid and basic gases from reaching the membrane.Porous Electrodes Porous electrodes may be deposited on a membrane face in several ways. The main techniques are vacuum evaporation sputtering and thermal decomposition of dissolved metal compounds usually from organic phases.2 The latter technique is convenient for laboratory-scale studies as it is simple and requires no expensive equipment. Vacuum evaporation and sputtering are more versatile where a wide variety of metals are needed.3 Thicker metal deposits are most conveniently prepared by the first technique which makes use of commercially available ‘metal inks’ and other materials widely used in the electronics and ceramics industries.? Fuel-cell-type electrodes are made from a mixture of the electrocatalyst and Teflon particles by applying heat and pressure.A wide variety of such structures have been fabricated and the details of their fabrication are usually not disclosed by the manufacturer. Elect roc hemical Techniques The most frequently used membrane-based electrochemical techniques are potentiometry and amperometry. It is only recently that conductometry has been combined with membrane methodology. This paper deals primarily with application of membranes in conductometry but the distinctive characteristics of all the techniques will be summarized briefly. 7 Engelhard Industries East Newark New Jersey U.S.A. manufactures a wide variety of such materials.S. Bruckenstein and J. S. Symanski 1107 Potentiometry The major analytical field of ion-selective electrode methodology is founded on the response of selective ‘membranes’ to dissolved ionic species. These membranes can be solid-state or liquid and exhibit ion specificity. Existing ion-selective electrodes have been combined with permeable or porous membranes to produce a unique class of aqueous sensors. The principle involves separating an external aqueous phase from the ion-selective electrode by a membrane. In one variation the membrane is affixed to the electrode so that a very thin layer of electrolyte solution separates the membrane and the electrode. The analyte in the external aqueous phase diffuses through the membrane reacts with something present in the electrolyte solution to produce a species that is then detected and quantitated by the electrode.Ordinarily the potential of the electrode is logarithmically related to the analyte’s concentration in the external aqueous phase. Some recent examples of this approach are potentiometric sensors for CO which involve the measurement of P H . ~ ~ Transport to the membrane is usually ill-defined and depends on natural convection in the external aqueous phase. Transport through the membrane or the thin electrolyte film or rate of reaction within the electrolyte film is usually rate-limiting in determining the potentiometric response of the electrode. It is important to note that the electrode is not a sink for the diffusing species rather the chemical or biochemical reaction within the thin electrolyte layer causes the layer to function as a sink.High sensitivity and selectivity is possible with this technology when appropriate reaction chemistry can be coupled to an ion-selective electrode. The analyte can be caused to react within the membrane if a reagent is incorporated into the membrane. Enzymes are used in membranes in this way and a variety of molecules of biochemical interest are now determined in this way. Examples of this approach coupled to integrated-circuit technology are summarized elsewhere.6 Amperometry Amperometry is widely used to determine volatile species present in aqueous and gas phases. The Clark electrode for dissolved oxygen is a classic example of the former while there exist many illustrations of the latter.Among the latter are sensors for hydrogen s~lphide,~? hydrogen ~yanide,~q carbon mon~xide,~? lo nitric oxide’ll nitrogen dioxide1’ and chlorine.12 In all sensors the indicator electrode potential is held at a potential which results in the electrolysis of the analyte species. The resultant current is proportional to the analyte’s concentration in the phase contacting the membrane. Various schemes have been used to establish the indicator electrode’s potential. A three-electrode potentiostat is the most versatile since it places the least electrochemical restraints on the choice of reference electrode. Two-electrode circuits using large-area reference electrodes are common. An artificial distinction has been made between two-electrode cells in which a voltage is applied between the reference and the indicator electrode to establish the desired potential (electrolysis cell) and those in which the reference electrode is shorted to the indicator electrode through a current-measuring device (galvanic cell).There are a large number of publications in this area and patents make up a substantial portion of this literature. Conduc tometry One concept involved in these sensors is based on the transport of a species through the membrane to a pure water solution followed by measurement of the conductivity. Thus any species which reacts with water to produce ions may be determined with high sensitivity. However this conductometric method does not provide selectivity and 1108 A 6 C Fig.1. Schematic representation of the sensing region of a conductometric cell (A) mixed-bed ion exchanger (B) cell body (Plexiglass) (C) thin-layer chamber for deionized water (D) conductance electrodes; (E) porous Teflon membrane (F) filter for removing interferents (activated G Gas Membrane Electrodes carbon). H2O reservoir 1' I 1 i filter gas I Fig. 2. Diagram of thin-layer sensor. The arrows show the direction of water flow when the pulse pump is actuated manually. The one-way check valves A and B control the direction of water flow. additional techniques must be combined with conductometry if a selectivity problem exists. Van Kempen and Kreuzer constructed a conductometric sensor from a double lumen catheter whose tip was covered with a membrane permeable to carbon di0~ide.l~ A set of conductance electrodes situated in each lumen measured the conductances of the solution in each lumen.Water flowed from one lumen to the other passing over the membrane. The conductance difference between the water in the two lumens yielded the carbon dioxide concentration in the gas phase contacting the permeable membrane. Control of water flow rate is necessary to assure reproducible results in this sensor as it is in all non-equilibrium sensors based on steady-state hydrodynamic conditions. 1109 26 27 dioxide 165 5 1 10 S. Bruckenstein and J. S. Symanski Table 1. Response time (in s) plotted against gas flow rate and partial pressure for carbon dioxide gas flow rate/dm3 min-l 0.5 0.2 0.05 0.5 24 25 Table 2.Response time (in s) plotted against gas flow rate and partial pressure for sulphur gas flow rate/dm3 min-l 300 210 270 145 120 40 120 120 110 40 50 55 Himpler et aZ.14 described another carbon dioxide sensor in which carbon dioxide diffused through a dialysis membrane into a thin water layer whose conductance change was determined. Their design provided no convenient way to replace the water in the thin layer and the sensor exhibited a slow increase in conductance with time. Martinchek concurrently described a sensor using a porous Teflon membrane in a thin water layer geometry that included an integral mixed-bed ion-exchange column and a simple means for replacing the water behind the membrane.2* l5 This approach provided a sensor with long time zero stability.Further details about this kind of methodology as implemented in a portable sensor are given below. Portable Conductometric Gas Membrane Electrode for Carbon Dioxide and Sulphur Dioxide Fig. 1 and 2 illustrate one way we have applied membrane methods to conductometric determination of gaseous species. The manually operated pulse pump in fig. 1 forces water from the reservoir through the mixed-bed ion-exchange column into the thin water layer C of fig. 2. Water initially present in this layer is collected in the water reservoir for re-use. An analyte present iri the gas phase diffuses through the filter F (designed to remove interferents) and the porous Teflon membrane into the thin water layer.Conductance electrodes deposited on the water side of the membrane monitor the conductance. After a sufficient time has passed the conductance becomes constant. The equilibration time required varies with the analyte gas flow rate and the cell geometry. The thin layer cell described previously2v l5 for the determination of atmospheric carbon dioxide was tested from ambient levels to high partial pressures of carbon dioxide. A steady-state conductance was attained in ca. 25 s independent of gas flow rate as seen in table l.29 l5 However times of ca. 5 min were required to attain conductance steady state for ppm levels of sulphur dioxide.16 Therefore the thin-layer cell was modified to 1110 shorten this time response.The modified cell was capable of determining 20 ppb SO with a response time of 110 s as seen in table 2. Even this cell showed a marked dependence of response time on the gas flow rate and Pso2. The reasons for the differences in the behaviour between carbon dioxide and sulphur dioxide are discussed below. Gas Membrane Electrodes Ka (2) XO,(aq)+H,O,H+(aq)+HXO;(aq). Reactions (1) and (2) describe the equilibrium of gaseous carbon dioxide and sulphur dioxide with dissolved CO and SO,. Raman,17 infrared1* and ultravioletlg absorption studies indicate that aqueous solutions of sulphur dioxide consist almost exclusively of uncombined SO molecules. No evidence has been found to support the presence of the H,SO molecule in solution. Also the primary undissociated species in aqueous carbon dioxide solutions is CO,(aq) although a small amount of H,CO exist at equilibrium.The rates of both reactions (1) and ( 2 ) O are fast compared to the timescale of mass-transport-controlled equilibration of the thin water layer for both carbon dioxide and sulphur dioxide. Thus the sum of these reactions the overall equilibrium between gaseous XO and the ions formed in the water layer can be used to discuss the mass-transport-controlled transient conductance response of both species. The overall equilibrium constant K (= Kp K,) describes the equilibrium K Theory The conductometric sensor’s response to both CO and SO is described using the following equilibria where X represents either C or S At equilibrium (3) where K is the ‘apparent’ dissociation constant of XO,(aq) and Kp is the Henry’s law constant describing the equilibrium between XO in the gas phase and XO,(aq).(4) (6) In eqn ( 5 ) and (6) square brackets signify equilibrium concentrations,f-terms are activity coefficients and A-terms are equivalent ionic conductances of the designated species. The equilibrium concentrations of XOi- (and HS,OT)~~ are negligible because the first ‘ apparent’ dissociation constant of XO,(aq) is many orders of magnitude larger than the second., Also for the range of studied partial pressures the ionic strength is so small that the activity coefficients of H+ and HXO; approach unity and the equivalent ionic conductances do not vary. Substituting eqn (4) and (5) into eqn (6) yields the expression for the specific conductance of the water layer at equilibrium (8) XO,(g)+H,OeH+(aq)+HXO;(aq) [H+] = [HXO,] and the specific conductance IC is given by IC = 10-3(AH+[H+]+;1HSO;[HX0~]).The experimental cell conductance S is proportional to IC. Therefore (Aw++;lrrxo;). S2 = (A/O)’ KPxo2 where 8 is the cell constant (cm-l) and A is equal to 1111 s2,,/p,as KP species I 1.20 1.29 x lo- 42 I 395 27 1 so2 co2 NH3 H2S 3.39 x 10-2 54.9 0.1 4.45 x 10-7 1.75 x 10-5 1.26 x 10-7 8 . 6 ~ 2.6 x lo- 6.6 x lo-' 378 a Constants from ref. (25H31). Eqn (8) applies to sulphur dioxide and carbon dioxide. Martinchek has verified this expression for CO for partial pressures near ambient to 100% carbon dioxide and Symanski has confirmed it for SO,l6 from 0.02 to 50 ppm.Eqn (8) also holds for any gas that dissolves in water to form an acid or base which is weakly dissociated. For example NH also obeys eqn (8).16 The ratio of the square of the equilibrium conductance for another gaseous species divided by it partial pressure with respect to the same quantity for SO is given by Table 3 presents the calculated results for various gases obtained by substituting the appropriate thermodynamic constants into eqn (9). The fourth column of the table compares the analytical sensitivity of these gases relative to SO,. For routine determi- nations of SO in the range of 0.1-5 ppm both hydrogen sulphide and carbon dioxide will contribute negligibly to the SO response at their ambient levels.Ambient CO (400 ppm) yields a response equivalent to ca. 3 x lo-* ppm SO,. Ammonia which is unlikely to be encountered in SO determinations would yield a false SO response of 0.03 ppm ifpNH3 was 1 ppm. These calculations show that an atmospheric gas membrane conductance sensor for sulphur dioxide does not need a filter for ambient levels of interferents. On the other hand an atmospheric carbon dioxide sensor needs an appropriate filter15 to remove interfering levels of sulphur dioxide and other gases that dissolve to produce relatively strongly dissociated acids or bases. S. Bruckenstein and J. S. Symanski /mol dme3 atm-l l2-l cm2 K,/mol dm-3 Table 3. Thermodynamic constants for SO and various gases comparison of sensitivities us.s 0 a s2,s/Pso2 (10) Discussion The transient conductance responses for carbon dioxide and sulphur dioxide are markedly different. Carbon dioxide transients are virtually independent of partial pressure and of gas flow rate whereas the opposite behaviour is found for sulphur dioxide.16 Conductance equilibration in the water layer is controlled by diffusion of XO through it. For carbon dioxide the diffusion process is not perturbed significantly by the dissociation step reaction (3). This simple diffusion model has been treated by Crank,, and the time required to reach 98 % of the equilibrium uptake of XO after a water pulse is t = 1.50 L2/D where L is the thickness of the thin water layer and D is the diffusion coefficient of the dissolved species in water.Using experimental values of L = 0.004-0.005 in.? and t 1 in. = 2.54 x lop2 m. 1112 Gas Membrane Electrodes 0.2 0.5 Table 4. Values of 2 for sulphur dioxide and carbon dioxide at different partial pressures 0.05 0.966 0.897 0.873 9.66 6.13 4.34 1 .o 0.858 0.1 0.25 0.5 1 .o 5.0 3.08 1.39 0.44 5.5 cm2 s-l yields t = 15-24 s; this result agrees satisfactorily with the data for carbon dioxide but not for the traces for sulphur dioxide. A gas stream containing 1 ppm sulphur dioxide requires ca. 2.5 min to equilibrate the water layer at a flow rate of 0.5 dm3 min-l.16 At the same flow rate only 40 s are required for equilibration with 55 ppm sulphur dioxide.ls This phenomena was also observed by Terraglio and Manganelli.24 They reported that the rate of solution of sulphur dioxide into water over a range of atmospheric concentrations of 0.31-3.3 ppm was a function of the partial pressure of the gas with saturation being reached more rapidly at higher concentrations.D = The shorter response time for sulphur dioxide as its partial pressure increases is a consequence of the very substantial dissociation of sulphurous acid occurring at these low partial pressures. In general the total solubility of XO depends on the sum of all the aqueous undissociated forms and the ionic forms produced by dissociation. Taking the relevant equilibria into account yields where the first term on the right-hand side arises from the undissociated form and the second term from HXO,.Table 4 summarizes solubility calculations based on eqn (1 1) for relevant partial pressures of sulphur dioxide and carbon dioxide. The parameter 2 [ = S/[xO,] (g)] compares the molar concentrations of the gas in the water layer and in the gas phase at equilibrium. It measures the accumulation of XO in the aqueous phase. Zso2 and Zco2 are calculated to be 1.39 x lo3 and 0.858 respectively for the partial pressures listed. Sulphur dioxide is concentrated more than a thousand-fold in the thin water layer from the gas phase. Therefore the slow equilibration time of SO results from the longer time necessary to deliver enough gaseous sulphur dioxide to the thin water layer through the membrane.In the case of carbon dioxide no such accumulation occurs. Furthermore the decrease of 2 as the partial pressure of sulphur dioxide increases explains the accompanying faster response time. The dependence of equilibration time on concentration is the result of the significantly large value of the ‘apparent’ dissociation of SO,(aq) (K = 1.3 x which controls the total solubility of sulphur dioxide in water at ppm levels. When the thin water layer is in equilibrium with 5 ppm sulphur dioxide ca. 98% of the dissolved species is in the form of HSO,. The second term of eqn (1 1) is significantly larger than the first one for sulphur dioxide. This is not true for carbon dioxide. Table 4 shows that over a twenty-fold range of Pco2 2 varies by no more than 13%.In contrast for a twenty-fold range in Pso (0.25-5.0 ppm) 2 decreases by a factor of 4.4. intercept (s.d.) /n-2 species s 0 a 1113 0.11 (0.078) x lo-" -0.03 (0.03) x 10-l' 0.09 (0.08) x s 0 a c 0 a S. Bruckenstein and J . S. Symanski Table 5. Least-squares calibration data for carbon dioxide and sulphur dioxide pressure (pprn) 0.0-1 .oo 0.00.20 &10000 slope (s.d.) /Q-2/(PPm)-1 6.39 (0.337) x 10-l' 6.75 (0.219) x lo-" 10.6 (0.15) x a Strip cell 120 s data gas flow rate = 0.5 dm3 min-l. Circular cell 16 s data natural diffusion. Analytical Consequences The sensor described previous1yl5 functioned by capturing the conductance of the thin water layer precisely 16 s after the pulse pump in fig. 3 refilled the thin layer with fresh ion-exchanged water.The conductances obtained this way obeyed eqn (8) from ambient carbon dioxide levels to pure carbon dioxide. However even after 5 min the conductances obtained for flowing gas streams (0.5 dm3 min-l) containing sulphur dioxide in the range 1-10 ppm did not agree with theory. Therefore although the thin-layer conductometric cell used for carbon dioxide has considerable sensitivity when used with low levels of sulphur dioxide its slow response time and deviation from equilibrium theory are drawbacks. Geometry of Membrane Cell The membrane in the cell used for the carbon dioxide sensor was circular and placed in the cell so that mass transport could only occur normal to both of the membrane's faces. A new cell was designed in which the thin layer was a very narrow and shallow channel milled into a Plexiglas block.ls The porous membrane with electrodes on the water side was placed across this channel.This geometry provided gas transport to one membrane face over an angle of 180" rather than just normal to the membrane. Transport through and into the thin water layer was still constrained to be normal to the membrane face. As a consequence of the increased sulphur dioxide flux to the membrane the flux of sulphur dioxide into the thin water layer increased and the equilibration time decreased considerably for the new cell design the strip cell. Strip Cell Behaviour A comparison of calibration curves obtained using the circular cell design with carbon dioxide and the strip cell design with sulphur dioxide is given in table 5.The flow rate of gas to the cell was 0.5 dm3 min-l for sulphur dioxide a velocity which was high enough to make the results independent of the gas flow rate. The time required to reach 95% of the equilibrium conductance for various values of Pso2 ranged from 60 s (1.0 ppm) to I10 s (0.02 ppm). These response times are a marked improvement over those listed in table 2 obtained in the circular cell geometry for much higher concentrations of sulphur dioxide. The signal equivalent of the N blank at 120 s is ca. 3 x lo-* ppm sulphur dioxide. The difference in the slopes of the two sets of sulphur dioxide data is only 6% well within the experimental error introduced by the gas-proportioning system we used.1114 L Gas Membrane Electrodes Fig. 3. Continuous conductometric sensor (exploded view) (A) water (B) glass or plastic tube (C) conductivity electrodes on porous Teflon membrane (D) gas phase containing carbon dioxide (E) mixed bed of ion-exchange beads. Conclusions A thin water layer conductance cell that has a gas-porous membrane wall affords a simple accurate and convenient sensor for the determination of atmospheric SO for concentrations as low as 20 ppb. The relatively large value of the aqueous partition coefficient of sulphur dioxide in water and its acid dissociation constant produce a significant increase in sensitivity as compared with other gases which dissolve in water to form ions and which are likely to be found under ambient conditions.Continuous Carbon Dioxide Sensor The sensors described above operate in a read-on-demand mode. They provide values for carbon dioxide or sulphur dioxide concentration on manually operating a water pulse pump. Fig. 3 illustrates a design for a continuous conductometric analyser which we evaluated for carbon dioxide determinations. It consists of a tube B with porous Teflon membrane dividing the tube into two parts. The gas phase containing carbon dioxide D contacts one side of the membrane and the other side contacts water A. This side of the membrane has deposited conductance electrodes C separated from a mixed bed ion of ion-exchange beads E by a thin fine screen (not shown). If the carbon dioxide concentration in the gas phase is stepped from zero to some finite value a transient change in aqueous carbon dioxide concentration occurs.This transient change in carbon dioxide concentration inside the water phase in the region of the screen is shown in fig. 4. The concentration of carbon dioxide is constant as a good approximation from the bulk of the gas phase through the membrane to the boundary between the membrane and the water. There is a decrease in concentration of carbon dioxide on the water side of the membrane because of the carbon dioxide's partition coefficient. Its gradient is initially very high on the aqueous side of the membrane and the gradient rapidly becomes constant because the ion-exchange beads act as a sink for the diffusing aqueous carbon dioxide. This situation is analogous to the constant 1115 A S.Bruckenstein and J. S. Symanski water-layer screen B ,-> -distance Fig. 4. Transient linearized concentration profiles (A-B) gas phase (B-C) inside membrane pores ; (El E and Em) gradients for carbon monoxide in order of increasing time after a step change in carbon dioxide concentration at the gas phase-membrane boundary. potential generation of a species at a rotating-disc electrode or any other electrode with a steady-state diffusion layer thickness. As the gradient becomes constant lines El E and E in order of time after a step chany in pco2 so does the conductivity. Under these conditions the conductivity varies as P&, (ambient to 1%) just as it does for the equilibrium read-on-demand sensors. The response time for such a sensor was of the order of 95 s for 1% carbon dioxide at a sample gas flow rate of 0.5 dm3 min-l.The sensitivity of this device is larger than the read-on-demand sensor because the effective thickness of the thin water layer is larger and is related to the size of the ion-exchange beads. This sensor configuration has wide application and is readily miniaturized. This work was supported by the United States Air Force Office of Scientific Research under Grant no. AFOSR 83-0004. 3 I. Bergmann Amperometric Gas Monitors. A New Generation paper given at the 30th International References 1 J. W. Harrison D. L. Gilbert P. A. Lawless and J. H. White Development Strategy for Pollutant Dosimetry (1975) p. 74. 2 G. Martinchek Ph.D. Thesis (State University of New York at Buffalo N.Y.1983). Congress of Pure and Applied Chemistry Section 8.B.5 New Electrochemical Sensors September 10 1985). 4 R. K. Kobos S. J. Parks and M. E. Meyerhoff Anal. Chem. 1982,54 1976. 5 M. A. Jensen and G. A. Rechnitz Anal. Chem. 1979 51 1972. 6 T. Morizzumi and Y. Miyahara Transducers '85 1985 International Conference on Solid-state Sensors and Actuators. Digest of Technical Papers. IEEE Catalog no. 85CH2127-9 p. 148. 7 F. Opekar and S. Bruckenstein Anal. Chem. 1984 56 1206. 8 F. Opekar and S. Bruckenstein Anal. Chim. Acta in press. 9 G. L. Holleck J. L. Bradspies S. B. Brummer and L. L. Nelsen Final Report contract no. NAS8-2903 1 December 1973. 10 S. Bruckenstein and W. Sherwood US. Patent 4052478 (1977); U.S.Patent 4166775 (1979). 1 1 J. Kosek Ph.D. Thesis (State University of New York at Buffalo N.Y. 1980). 12 H. Tataria A. A. Schneider and L. E. Martin U.S. Patent 4,184,937 (1980). 13 L. H. Van Kempen and F. Kreuzer Respir. Physiol. 1975 24 89. 14 H. A. Himpler S. F. Brand and M. J. D. Brand Anal. Chem. 1978 50 1627. 15 J. S. Symanski G. A. Martinchek and S. Bruckenstein Anal. Chem. 1983,55 1152. 1116 16 J. S. Symanski Ph.D. Thesis (State University of New York at Buffalo N.Y. 1985). 17 A. Simon and A. Pisctschan Z. Anorg. Alfgem. Chem. 1961 313 281. 18 L. H. Jones and McLaren E. J. Chem. Phys. 1958,28,995. 19 P. A. D. J. DeMaine Chem. Phys. 1957 26 1036. 20 D. M. Kein J. Chem. Educ. 1960 37 141. 21 D. J. Spedding and P. Brimblecombe Atmos. Environ. 1974 8 1063. 22 I. M. Kolthoff Treatise on Analytical Chemistry ed. P. J. Elving and E. B. Sandell (Wiley Interscience New York 1959) part I vol. I p. 432. 23 J. Crank Mathematics of Diffusion (Oxford University Press London 1st edn 1956) chap. 4. 24 R. M. Terraglio and F. P. Manganelli J. Air. Pollution Control 1967 403. 25 International Critical Tables (McGraw-Hill New York 1st edn 1928) vol. 111 p. 259. 26 International Critical Tables (McGraw-Hill New York 1st edn 1929) vol. VI pp. 260-261. 27 A. E. Rabe and J. F. Harris J. Chem. Eng. Data 1963,8 334. 28 H. S. Harned and B. B. Owen Physical Chemistry of Electrolyte Solutions (Reinhold New York 2nd edn 1950) pp. 617 and 589-591. 29 R. A. Robinson and R. H. Stokes Electrolyte Solutions (Butterworths London 2nd edn 1959) p. 463. 30 T. Shedlovsky and D. A. MacInnes J. Am. Chem. Soc. 1935,57 1705. 31 W. Stumm and J. J. Morgan Aquatic Chemistry (Wiley Interscience New York 1970) p. 102. Gas Membrane Electrodes Paper 512010; Received 15th November 1985

ISSN:0300-9599    

DOI:10.1039/F19868201105

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. Soc. Faraday Trans. I 1986,82 11 17-1 126 Conducting Polymer Gas Sensors Jan J. Miasik,? Alan Hooper and Bruce C. Tofield* Materials Development Division AERE Harwell Oxfordshire OX1 1 ORA Recent results with solid-state semiconductor gas sensors based on organic sensor elements are reviewed. Devices based on metal phthalocyanines show useful responses to NO,. Lead phthalocyanine combines the highest conductivity with the maximum sensitivity to NO,. A thin-film lead phthalocyanine sensor has successfully been used to monitor NO produced by shot-firing in coal mines. To obtain reasonable conductance and speed of response and recovery phthalocyanine sensors have been operated at 170 "C. Conducting polymer materials and particularly chemically doped polypyrrole show responses to toxic gases at ambient temperature.Initial work using polypyrrole black impregnated filter paper showed a response to ammonia. More recently using polypyrrole films electrochemically deposited over electrode arrays responses to nitrogen dioxide and hydrogen sulphide have also been obtained. Organic-semiconductor gas sensors may have advantages compared to metal-oxide devices in their sensitivity to toxic gases and in their ability to operate at or near room temperature. However the mechanisms of device function are not yet well understood. Increasing concern with environmental and personal protection together with widespread requirements for more accurate process control has created a need for new or improved sensors for measuring both physical and chemical parameters.This need for better sensors is strongly influenced by the increasing use of intelligent microelectronics for monitoring and control. Prominent among the requirements are sensors for a wide range of gases from organic and inorganic pollutants which must be measured at the parts per million level or lower through to flammable gases such as methane and hydrogen or gases such as water vapour and CO, monitored for control of environmental comfort which must be measured at the level of 1 % or more. While many methods of gas sensing have been developed in the laboratory environment and some have found commercial application there remains over a wide range of both applications and gases to be measured a continuing requirement for specific and sensitive gas sensors which combine durability and stability of response with cheapness.From life-support systems through food processing industrial drying and monitoring of chemical plant to vehicle exhaust pollution control the frequent lack of cheap and reliable gas sensors inhibits the installation and use of the most cost-effective control and monitoring systems. The semiconducting gas sensor where the resistance of a porous pellet or thin film of a metal oxide such as SnO or ZnO can be sensitive to the presence of combustible or toxic gases in air has emerged in recent years as a particularly economical solution to certain gas monitoring app1ications.l While some applications for sensing of toxic gases have been proposed for metal oxide semiconducting gas sensors the principal applications at present are in the detection of flammable gases such as H, CO and CH, at relatively high concentrations to protect against fire or explosion.A large market has t Also at Department of Chemistry University of Kent Canterbury. 1117 1118 Conducting Polymer Gas Sensors been developed in Japan in particular where a high proportion of homes are equipped with flammable gas detectors based on SnO,. The principles and properties of such devices have recently been reviewed. Although such devices are cheap and straightforward in application current designs which are based mainly on SnO, suffer from certain shortcomings which militate against their wider application. Prominent among these are their relative lack of sensitivity to toxic gases at low concentrations their sensitivity to ambient moisture and their inherent lack of specificity; this last disadvantage can be overcome to some extent by careful control of microstructure and use of incorporated catalysts.Finally a serious dis- advantage is the requirement in most applications for operation at 300 "C or above. The power drain required entails that battery operation is only practical for intermittent use. However because of the sensor drift which is observed in practice on heating and cooling such on-off operation is not generally practicable. In the majority of applications therefore it is necessary to power devices continuously from the mains. A wide range of applications including portable personal monitors and all uses where mains power is not available are therefore inaccessible with present technology.The development of semiconducting gas-sensing materials which could be sensitive to a wider range of gases and particularly to toxic gases which could show significant sensitivity at low concentrations and could also operate closer to ambient temperature would permit consideration of this type of device for a much wider range of applications than is satisfied at present. In this paper we describe results on semiconducting polymeric organic materials which show some promise for the detection of toxic gases close to room temperature. Phthalocyanine Sensors Several organic materials have been shown to exhibit resistivity changes when exposed to various gases.The class which has been investigated most fully in recent years is the metal phthalocyanines. Although conducting polymers of the type discussed below can be synthesised from phthalocyanine (Pc) prec~rsors,~ gas-sensing studies have concen- trated mainly on unpolymerised metal phthalocyanines which can be vacuum sublimed on to substrates to form thick-film sensor structures. In air hydrogen and metal phthalocyanines are poorly conducting p-type semiconductors. On exposure to highly electrophilic gases such as NO, significant conductance increases are observed for devices operated above 100 "C. Hydrogen and several metal phthalocyanines are sensitive to NO, but lead phthalo- cyanine combines the highest sensitivity with the highest conductance permitting measurement of NO down to 1 ppb in air.4 H,Pc shows a similar sensitivity to NO changes but has an order of magnitude lower conductance making measurements at low NO partial pressures more difficult.PbPc gas sensor construction has been described4 and the response of different metal phthalocyanines has been detem~ined.~ The responses of several different metal phthalocyanines to changes in NO partial pressures at 170 "C are shown in fig. 1. PbPc and ZnPc are more sensitive than the transition-metal phthalocyanines tested. MgPc (not shown) is very insensitive. Neither the mechanism of gas response nor the understanding of such variations in sensitivity in terms of the electronic structure of the solid and the gas-solid interactions are presently well understood.It is believed that the NO molecules replace at least some of the adsorbed 0 molecules present on the Pc surface in air. Some of the complexities of the absorption process revealed through conductivity changes have been reported.6 While it seems fairly clear that the effects result from surface absorption rather than bulk equilibration with NO, much more must be done before there is a clear understanding of the nature of the active sites and the effect of surface structure on the response. Nevertheless while a lack of such detailed understanding of sensor response is clearly 1119 J. J. Miasik A . Hooper and B. C. Tofield 3 x 10-9 COPC A NiPc 100 / 10 concent rat ion NO2 / ppb Fig. 1. The variation of conductance with NO concentration for metal phthalocyanines at 170 "C [after ref.(5)]. a barrier to ultimate device optimisation it has never been a bar to the development of gas-sensor technology and an industrial application of PbPc NO sensors has recently been described. In mines it is necessary to identify fires or indicators of fires at the earliest opportunity. This can be achieved by using a products of combustion (POC) sensor based for example on a metal-oxide semiconductor. However there is a possibility of interference from gases produced either from explosives used in shot-firing or from diesel engine exhausts. The higher temperatures characterising the latter events however result in enhanced NO production compared to that in fires and an NO sensor in combination with a POC sensor may be used to distinguish the routine event from the fire.Such an application has been demonstrated' in a U.K. coal mine using a tube bundle system and an array of sensors to sense inflammables POC NO and NO,. While the NO sensor was not sensitive to shot-firing events because of the strong absorption of NO on the walls of the tube bundle system the NO sensor which consisted of a PbPc sensor immediately behind a CrO oxidiser (which converted NO to NO immediately in front of the device) was clearly sensitive to shot-firing operations while being totally unaffected by the presence of products from a fire. The PbPc sensors were reported to retain both stability and sensitivity over a period of more than six months of continuous operation.Phthalocyanine gas detectors can indeed monitor inorganic pollutants such as NO,. They are sensitive only to highly electrophilic gases such as Cl, F and BF,. NO sensors are therefore insensitive to the other gases which may be present in a typical sensing environment for example flammable gases products of combustion or molecules such as HCl SO and H,S. Other advantages of phthalocyanine sensors are their ease of preparation and their fairly long-term stability. Disadvantages are their low conductance long recovery times even for operation around 170 "C and the memory and history effects which can be observed. The latter may be ameliorated if a better understanding of the device function is obtained. High-temperature operation is necessary both to obtain sufficient conductance and acceptable speed of response and recovery for devices as conventionally prepared.Although phthalocyanines are stable to temperatures well above 170 "C the appreciable volatility observed above this temperature limits their use at higher temperatures. In any case as observed above operation above 100 "C is undesirable in many applications in 1120 polyacetylene (PA) Conducting Polymer Gas Sensors polyphenylene sulphide ( P W poly t hiop hene (PT) H Fig. 2. Backbone repeat units of some bond-alternant polymers. High conductivity may be induce by negative-ion doping to create positive defects on the polymer backbone. Some polymers ca also be doped by positive ions. view of the power-drain required for sensor heating and the inability to operatt continuously via battery operation.More rapid response and recovery has been demonstrated8 for NO sensing at roon temperature using asymmetrically substituted CuPc films laid down by Langmuir- Blodgett techniques. No demonstrations of the viability of such very thin film device$ over extended periods of operation have however yet been reported. Conducting Polymer Sensors Since the discovery of a relatively straightforward synthetic route towards thin films of polyacetyleneg and the discoverylO that semiconducting polyacetylene could be converted into a very highly conducting form by chemical doping with iodine or other ionic dopants there has been considerable and growing interest in such doped conducting organic polymers.While original interest was directed particularly at the possible application of such materials as reversible electrodes in alkali-metal batteries,ll several other potential applications may be envisaged. These include electromagnetic interference shielding gas sensors displays conducting plastics capacitors junction devices and metals replacement. The lack of processability of most of these materials in contrast to conventional plastics is presently a significant barrier to exploitation. Many organic monomers may be polymerised to yield a linear carbon-carbon bond backbone with alternating single and double bonds. Many such materials have now been demonstrated to show large conductivity enhancements on chemical doping although most work to date has been carried out with relatively simple repeat units based on easily available monomer precursors (fig.2). The work on gas sensing reported in this paper has been conducted using polypyrrole. Doped conducting polypyrrole is one of the most stable conducting polymers in ambient environments. The semiconducting un-doped polymers can be synthesised in a number of ways for example by catalytic or chemical oxidation of the monomer or by elimination from a polymeric precursor. Doped material may also be prepared directly in the conducting state for example by reaction of the monomer with NOPF or analogous salts (and elimination of NO). Plasma polymerisation techniques may also be used. The chemical doping of the polymers may be achieved by direct reaction with oxidising species to produce an insertion material containing negative ions.Charge compensation is achieved Plate 1 J . Chem. Soc. Faraday Trans. 1 Vol. 82 part 4 Plate 1. Sensor structure consisting of doped polypyrrole electrochemically synthesised on to interdigitated gold electrodes screen-printed on an alumina substrate. J J. MIASIK A. HOOPER AND B. C. TOFIELD (Facing p . 1 12 1) 1121 doped copper aluminium graphite trans PA J . J . Miasik A . Hooper and B. C . Tofield si I icon conductive resins (Al-flake-filled resins) semi-insulating resins e.g. carbon-fi lled plastics) I comparison materials ’filled‘ conductive polymers PPP pps undoped conducting polymers Fig. 3.Comparison of the resistivities of doped and undoped conducting polymers with reference materials and with ‘filled ’ conductive polymers such as Al-flake-filled resins and carbon-loaded plastics. by the creation of positively charged defects on the polymer backbone leading to a large increase in electrical conductivity. Examples of such reactions are iAsF + PPS -+ [PPS+AsF,] + AsF,. Some materials e.g. polyacetylene or polyparaphenylene may also be negatively doped to a highly conducting state by insertion of positive ions such as Li+ or Na+. The doping levels generally achieved are much higher than those used in inorganic semiconductors such as silicon. Dopant ion to monomer repeat ratios can vary typically from 1 100 to 1 3 depending on the material under study and the preparative conditions.Study of the details of the mechanisms of conductivity enhancement is still a topic of active research.12 Note that while high values of conductivity up to lo3 S cm-l have been observed in doped polyacetylene typical values observed in materials such as doped polypyrrole often fall in the range 1-100 S cm-l. The conductivities observed are therefore more comparable to those of ‘ filled’ conductive polymers such as Al-flake-filled resins rather than the much higher values of simple metals such as copper and aluminium (fig. 3). It is unlikely that doped conducting polymers which may be characterised as being very good semiconductors have sufficiently high conductivities to find many applications as direct replacements for metals.For their evaluation as gas sensing elements it is convenient to study materials in thin-film form laid down over interdigitated electrodes. In this case direct electrochemical synthesis of the doped conducting polymer is most convenient. Electrochemical oxidation of the polymer directly on to the electrode substrate permits good control of growth rate and film thickness facile incorporation of a range of counter ions and permits the stoichiometry of the doped material to be modified in a relatively straightforward fashion. We have generally synthesised polypyrrole from aqueous solution although other materials such as polythiophene must be synthesised from organic solvents such as acetonitrile. In sensor work BF; introduced from LiBF in solution during synthesis is a suitable counter ion.A black polypyrrole layer laid down on an interdigitated gold electrode screen-printed on an alumina substrate is shown in plate 1. The fairly close electrode spacing permits growth of the polymer film across the inter-electrode gap so producing a sensor 1122 Conducting Polymer Gas Sensors structure. The conductance of materials such as BFT-doped polypyrrole in thin-film sensor form is sufficient at ambient temperature to make conductance monitoring of gas sensor response quite straightforward. While the general principles covering the formation and properties of conducting polymers are understood the materials as synthesised are often rather poorly character- ised. Chain lengths and molecular weights are often quite difficult to establish as is the degree of cross-linking which is generally thought to be present in almost all materials.Most materials particularly in the doped form are amorphous so that characterisation of the molecular structure is difficult. Many materials and particularly those synthesised electrochemically or catalytically are fairly porous but the nature and extent of the porosity and microstructure is generally not well known neither is the homogeneity of materials at the macromolecular level. With such poor understanding of the chemical compositional and structural characteristics of most materials it is not surprising to find that understanding of the surface chemistry particularly in terms of conductance mechanisms and gas absorption sites is more or less non-existent.The gas-sensing properties of polypyrrole were demonstrated by Nylander et al. l3 using a sensing element consisting of pyrrole black impregnated filter paper. Pyrrole is readily oxidised even in air at ambient temperature and a conducting polymeric form may be readily prepared by chemical oxidation. Nylander et al. used acidified hydrogen peroxide to precipitate pyrrole black onto the filter paper substrate. Although known for many years pyrrole black is poorly characterised but in preparations from HC1 presumably contains an oxidised polypyrrole backbone with Cl- counter ions. It was discovered that the resistance of the pyrrole black containing filter paper was sensitive to the presence of ammonia either in argon or in oxygen-argon mixtures.Sensor operation could be obtained at room temperature and at high ammonia concentrations (0.5-5% ) an almost linear response was obtained with typically 30% resistance change per one percent of ammonia concentration. Response times of < 1 min were observed at these highconcentrations but this increased significantly at lower ammoniaconcentrations in the ppm range although the sensitivity was higher at the lower concentration range. Several other gases including H, CO CO, CH and several aromatics yielded no response. The device was sensitive to other amines giving rise to large reversible responses as with ammonia. A certain sensitivity to moisture possibly resulting from NH,OH formation from reaction between NH and the N-hydrogen was observed.Both pyrrole black and the metal phthalocyanines are p-type semiconductors. The exposure of electrophilic gases such as NO to the phthalocyanines acts to withdraw electron density from the Pc network so increasing the conductance. Electron-donating gases such as ammonia if adsorbed have an opposite effect in that the carrier density will be reduced. Hence there is a resistance increase of conducting polypyrrole on exposure to ammonia. Such an explanation of the response is necessarily highly qualitative at the present stage of understanding. The work with pyrrole black and in particular the interesting responses which could be achieved at room temperature have encouraged us to pursue the characterisation of polypyrrole gas sensor response using electrochemically prepared films on interdigitated electrodes (plate 1).Preliminary results are reported here. We show that such sensors are indeed responsive to ammonia in line with the earlier work but that responses to other gases can also be obtained under suitable conditions. We demonstrate response both to NO and H,S. All work so far has been conducted at ambient temperature using films prepared from aqueous solution. The qualitative response of a device sensitive both to ammonia and NO is shown in fig. 4 where it will be seen that the resistance changes are as expected increasing with exposure to ammonia and decreasing in NO,. Depending on the preparative conditions devices may be made sensitive to a particular gas and insensitive to others although a detailed understanding of the factors affecting selectivity is not yet to hand.1123 5 Fig. 4. Typical qualitative response characteristic at room temperature of a conducting polypyrrole sensor to pulses of (a) NH and (b) NO in air. J . J. Miasik A . Hooper and B. C. Tofield 4 NH3 on 60 75 105 30 15 90 15 120 0 135 45 10 timelmin time/min Fig. 5. Resistance changes at room temperature for a conducting polypyrrole sensor to 15 min pulses of 0.1 % NH in air. For the screening of gas responses we have found it convenient to operate in a mode where gas pulses are supplied at the 0.1 % level in air for periods of 15 min on and 15 min off. Gas mixing and data collection are under computer control. Examples of responses obtained for one particular sensor treated to be sensitive to ammonia H,S or NO are shown in fig.5-7. Response is very rapid in all cases although equilibrium values seem to be achieved only in the case of H,S within this time period. Although there is some evolution in baseline resistance experiments over longer periods of up to a day show good reversibility of sensor response (fig. 8). 1124 620 61 0 g 600 \ 5 590 Y 1- 580 570 1 90 560 I 0 Conducting Polymer Gas Sensors I I 60 time/min 30 Fig. 6. Resistance changes at room temperature for a conducting polypyrrole sensor to 15 min pulses of 0.1 % NO in air. Fig. 7. Resistance changes at room temperature for a conducting polypyrrole sensor to 15 min pulses of 0.1 % H,S in air.The response curve as a function of gas concentration has not yet been completely determined for any of the gases studied but responses at three different ammonia concentrations (fig. 9) show as expected an increasing response as the concentration is raised from 0.01 % through 0.1 to 1 % in air. 1125 1.5 I I I I 20 0 ' 0 2 4 18 22 J. J. Miasik A . Hooper and B. C . Tofield I I I I I 6 16 14 12 1 8 10 time/h Fig. 8. Resistance changes at room temperature for a conducting polypyrrole sensor to 15 min pulses of 0.1 % NH in air over a period of 20 h. Conclusions The recent work at Harwell outlined in this paper confirms the sensitivity of conducting polypyrrole to ammonia and demonstrates also that the material can be sensitive to other gases.Results have been presented for NO and H,S. A thin-film device structure on a conventional interdigitated electrode array printed on a ceramic substrate has been demonstrated. 1126 The sign of the resistance change observed in the presence of ammonia and NO is consistent with the p-type behaviour of polypyrrole. The response observed with H,S is the same sign as that with NO,. Although the resistance of polypyrrole as currently prepared does increase somewhat over extended periods in air initial results to date indicate good reproducibility of sensor results over periods of several hours. The material is also very stable against chemical attack and sensor response is maintained after immersion for example in strong acid or strong alkali.References Conducting Polymer Gas Sensors Along with phthalocyanine detectors for NO sensing the results to date on polypyrrole clearly indicate that doped conducting polymers have interesting potential for sensing toxic gases such as ammonia NO and H,S. Of particular attraction is the lack of interference by flammable gases such as H, CO and CH, or by CO, and the fact that devices can be operated at room temperature. Nevertheless our understanding of device function is extremely primitive compared to the situation with metal-oxide devices such as those based on SnO,. To provide a better understanding which may be necessary to assist development of commercial devices it will be necessary to characterise more fully not only device structure but also the properties of the polypyrrole which may affect its gas sensitivity.These include most of the areas of generaluncertainty in our understanding of conducting polymers mentioned above as well as the nature of the gas-polymer interaction. The effect of moisture mentioned in the earlier work must also be clarified. The analogous requirements for phthalocyanine sensors have been discussed above. If progress in these areas can be made there is no doubt that both phthalocyanine and conducting polymer based devices offer much promise as resistance modulating gas sensing elements for toxic gases. They are thus very complementary to metal oxide devices which are most useful in the detection of flammable gases. Conducting polymer devices have the added attraction of being able to operate at or close to ambient temperature.We gratefully acknowledge the support received for work on conducting polymers from the Department of Trade and Industry. The assistance of P. Taylor with the experimental work on conducting polymer gas sensors is also gratefully acknowledged. 1 J. Watson and R. A. Yates Electron. Eng. 1985 May 47. 2 P. T. Moseley and B. C. Tofield Muter. Sci. Technol. 1985 1 505. 3 T. J. Marks Science 1985 227 881. 4 B. Bott and T. A. Jones Sensors and Actuators 1984 5 43. 5 T. A. Jones and B. Bott in Transducers '85 (1985 International Conference on Solid-state Sensors and Actuators) Digest of Technical Papers (IEEE 1985) pp. 414-417 6 J. D. Wright A. T. Chadwick B. Meadows and J. J. Miasik Mol. Cryst. Liq. Cryst. 1983 93 315. 7 B. Bott and T. A. Jones ref. (9 pp. 128-131. 8 S. Baker G. G. Roberts and M. C. Petty ZEE P r x . 1983,130,260. 9 H. Shirakawa and S. Ikeda Polym. J. 1971 2 231. 10 P. J. Nigrey A. G. MacDiarmid and A. J. Heeger J. Chem. Soc. Chem. Commun. 1979 594. 11 D. MacInnes Jr M. A. Druy P. J. Nigrey D. P. Nairns A. G. MacDiarmid and A. J. Heeger J. Chem. Soc. Chem. Commun. 1981 317. 12 A. J. Heeger Philos. Trans. R. Soc. London Ser. A 1985,314 17. 13 C. Nylander M. Armgarth and I. Lundstrom Proc. Zntl. Meeting on Chemical Sensors Fukuoka (1983) ed. T. Seiyama K. Fueki J. Shiokawa and S. Suzuki (Elsevier Amsterdam 1983) pp. 203-207. Paper 5 / 1884; Received 2 1st October 1985

ISSN:0300-9599    

DOI:10.1039/F19868201117

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC. Faraday Trans. I 1986,82 1127-1133 Semiconducting Tetrapyrrole Pigment Gas Sensors C. L. Honeybourne,* J. D. Houghton R. J. Ewen and C. A. S. Hill Bristol Polytechnic Frenchay Bristol BS16 I Q Y A process of molecular tuning of the physicochemical properties of organic semiconductors is described with special reference being made to natural tetrapyrrole pigments. The use of such organic semiconductors in the form of thin films is covered with emphasis being placed upon relative performance characteristics. The view is expressed that chemical discrimination is possible. The work described in this paper entails the tuning of a chemical system to give a required response to a given stimulus. The stimulus in this case is the ingress of electron acceptors such as N,O or dinitrotoluene.The response that is required is an increase in dark d.c. surface-current across a thin film. The prerequisites for detecting electron acceptors by an electrical method are (i) low ionisation potential; (ii) good intermolecular overlap; and (iii) a tendency towards pseudo-linearity. These prerequisites clearly point to extended heteroatomic conjugated molecules. The generic term ‘molecular sensor element’ can be used to embrace the whole family of possible molecules that one might choose. Ideally one should be able to tune the properties of a molecular sensor element by facile derivativisation at its periphery and/or metal ion insertion into a coordination hole. A further aspect of tuning is the ‘sensor matrix’ in which or to which the molecular sensor element is bound.One can think of a sensor element in a pure form dispersed in a permeable polymer matrix or directly bound to a polymer spine. In addition physical tuning can take place in the selection of the ‘sensor form’; e.g. amorphous pure film microcrystalline pure film crystal face or an ordered Langmuir-Blodgett film with or without stearic acid. Finally one can select the design of ‘sensor head’ which may exhibit different performance characteristics (e.g. surface cell or sandwich cell). If one has a highly ordered cofacial array of superimposed stacked conjugated rings as in poly(siloxyphthalocyanine) then one might think in terms of solid-state band theory applied to the organic system. Recent calculations by Honeybourne have shown on the basis of estimated band widths and charge carrier velocities that the tight-binding version of band theory is a tenable approximation for interplanar spacings of < 0.35 nm in columnar stacks.l An Arrhenius-like expression for the conductivity 0 is then appropriate.0 = oo exp [ - ~ / 2 k T ] in which E is the band gap. However in the case of surface dark d.c. measurements in the presence of gases there are sound reasons for assuming that band theory is inappropriate (i) we are dealing with surface states not bulk states; (ii) the surface is chemically dirty owing to the presence of sorbed gas; (iii) intermolecular spacings are often well in excess of 0.35 nm; and (iv) the surface is amorphous i.e. physically dirty.In such cases Mott-hopping is a more likely candidate for the mechanisms of charge propagation ; therefore 0 = A exp(-B/Ti) in which B is a complex function of intermolecular overlap and the density of states at the Fermi Level (NF) and A is a linear function of NF intermolecular spacing and the FAR 1 38 I127 1128 ( d ) (a 1 Tetrapyrrole Pigment Gas Sensors ( b ) (c) ( d ) dihydrodibenzotetra-aza-[ 141-annulene (TAA). Fig. 1. (a) Hemiporphyrazine. (b) and (c) macrocyclised bis-1,lO-phenanthroline (X = N or C). binding energy l e v Fig. 2. X-ray photoelectron spectrum in the N(1s) region of TAA doped with N,O,. (a) TAA; (b)TAA+NO,. characteristic phonon frequency.2 Owing to the temperature-dependent dynamics of the ingress and egress of electron acceptors it has not been possible to establish the mechanism of charge propagation in our systems by studying current versus temperature data.However in argon-ion implanted systems we find that Mott-hopping is the mechanism of charge propagation. In our extensive studies of gas detection using organic semiconducting films we elected to use tetra-aza tetradentate macrocyclic conjugated ligands as the molecular sensor elements and to use pure amorphous films as the sensor f01-m.~ The dark d.c. surface current was measured in a blackened test-chamber containing numerous electrical feed-through. This test-chamber can be evacuated or filled with low levels of electron- acceptor gas in dry air. The organic material was deposited across a network of platinum C.L. Honeybourne et al. 1129 R Q \ ' 0 ( d ) (e ) Fig. 3. (a) 7t-System of bacteriochlorophyll. (b) Tetra-meso-substituted porphin (R = aryl). (c) n-System of phaeoporphin. ( d ) n-System of chlorophyll. (e) Phthalocyanine. I I I I I 1 750 650 550 4 50 3 50 190 250 Fig. 4. U.v.-visible spectrum in transmission through a 0.1 pm film of meso-tetraphenyl porphin (TPP) (a) before doping with N,O,; (b) doped with N,O,; and (c) after heating (b). electrodes either by vacuum sublimation (in the cases of synthetic porphins and porphin models) or by solution deposition (in the case of natural porphyrins). The first class of molecules selected was hemiporphyrazine [fig. 1 (a)] and its first-row transition-metal complexes. However only after very extended periods of exposure to high levels of electron acceptors is any measurable increase in current observed.The two types of bis-phenanthroline macrocycles [fig. 1 (b) and (c)] first synthesised by Gotoh and coworkers,4 degrade rapidly in the presence of N,O and C1, give poor responses and are photolabile compounds. For these reasons we have eliminated the foregoing three classes of compound from our work programme. The tetra-azadibenzo-[ 141-annulenes [TAA fig. 1 (d)] have been studied3 very thoroughly and offer considerable promise for 38-2 Tetrapyrrole Pigment Gas Sensors 1130 EVAC 10- lo-' lo-' v Pm 50 Fig. 5. Changes in dark d.c. surface current of a 1 pm film of rneso-tetra-p-methoxyphenyl- porphinatoplatinum(n) upon exposure to N20 in dry air.reversible detection of N204 at low temperatures (0-50 "C). It is possible to detect the chemisorbed N204 by X-ray photoelectron spectroscopy in the N(1s) region (fig. 2). Latterly we have turned our attention to the tetrapyrroles all of which contain a delocalised 18 z-electron system as exhibited by the molecular structure of bacterio- chlorophyll [fig. 3 (a)]. Owing to difficult syntheses or difficult extraction procedures in the bulk natural porphyrins have not been studied to any great extent as gas sensors. However the rneso-substituted tetra-aryl porphins are readily made [fig. 3 (b)]. The effect of N204 on for example tetraphenylporphin (TPP) is to generate the radical cation (fig. 4). The new visible band disappears upon gentle heating and the original characteristic four-band pattern in the visible region reappears.The N204 has extracted an electron from the organic species generating a hole in the valence level. This is a high-level doping analogue of the low-level p-doping in inorganic semiconductor technology. The effect of N204 on the dark d.c. surface current of a film of the platinum complex of tetra-p-methoxyphenylporphin (PtMeOTPP) is shown in fig. 5. Attention is drawn to the reversibility of the effect which is brought about simply by pumping the dopant gas away. The X.p. spectrum in the N(1s) region of tetraphenyl porphin (fig. 6) when clean and after low and high levels of exposure to N204 clearly shows the ingress of N204 into the film and the generation of a surface-bound species.Upon attempting EVAV V \ 'T' 100 NO - tlmin C. L. Honeybourne et al. i i I ij I I I 4 00 4\ 0 I ! 1 1 1 1 1 1 1 1 1 1 1 1 405 binding energylev Fig. 6. X-ray photoelectron spectrum in the N( 1s) region of tetraphenylporphin exposed to N,O,. (a) Clean (b) brief exposure and (c) long exposure. to clean the gas-doped film by brief argon-ion etching severe damage of the organic species occurred and argon ions were more or less permanently trapped in a what had become a highly conducting film. The retention of argon over a period of several months is proved by the X.p. spectrum in the argon (2p) region (fig. 7 ) . We have recently extended our work to include naturally occurring tetrapyrrole pigments containing either the phaeoporphyrin [fig.3 (c)] or chlorophyll [fig. 3 (d)] bonding framework. These pigments were obtained biosynthetically from wild-type and mutant strains of the photosynthetic bacterium Rhodopseudomonas sphaeroides and from higher green plants (see table 1). By far the largest response to N,O was given by protochlorophyllide (P-lide). This response albeit to quite high levels of gas was also rapid. The rise time was 48 s to 90% of maximum and reversed to 10% of maximum in only 10 s. This material is therefore 1131 I 1 1132 Tetrapyrrole Pigment Gas Sensors 1 I I I I I I I 240 24 5 binding energy/eV Fig. 7. X-ray photoelectron spectrum in the Ar(2p) region of argon-ion implanted tetra-p- methoxyphenylporphin after prolonged exposure to laboratory air.Table 1. Sources of natural tetrapyrrole pigments studied as gas-sensing films compound phaeoporphyrins protochlorophyllide source etiolated leaves of Hordeum vulgare Rh. sphaeroides (wild type 2.4.1) Rh. sphaeroides (mutant V,) chlorophylls 2-desvinyl-2-acetylphaeoporphyrin as Mg 2,4-divinylphaeoporphyrin as chlorophyll a and b 2-desvinyl-2-hydroxyethyl chlorophyllide a fresh lettuce Rh. sphaeroides (mutant 0,) chlorophyllide a Ailanthus altissima (lipid extract) the most rapidly reversible molecular sensor element we have yet found at room temperature. All other natural pigments in this study proved quite unsuitable. In contrast the tetra-azatetrabenzotetrapyrroles known as the phthalocyanins (Pc) [fig.3 (e)] are the least reversible and most sensitive molecular sensor elements for the detection of electron acceptor^.^ These have been studied extensively by Jones and coworkers6 and by Wright and coworker^.^ Langmuir-Blodgett films of a tri-tertiary butyl derivative of copper phthalocyanine is an effective sensor for low levels of N,04.8 Certain naturally occurring porphyrins derived from mammalian blood such as mesoporphyrin IX give reasonable quality Langmuir-Blodgett films which are being studied for their efficacy as gas sensors by Tredgold et al? We are currently synthesising a range of twelve new hexa-alkylporphin dicarboxylic acids in an effort to improve film quality by changes in the relative positions of the acid groups and in the bulk of the hydrophobic region.The appeal of the Langmuir-Blodgett technique is its potential compatibility with microcircuit technology and the possibility of constructing a gas-triggered chemical-field effect transistor.* Apart from noting that the foregoing work programme progressed from the pure to the applied involved molecular architecture and encountered a plethora of beautiful 1133 C. L. Honeybourne et al. colours a pragmatic summary of relative sensitivities and reversibilities of various molecular sensor elements constitutes a suitable conclusion. The abbreviations have been used in the running text of the paper. Relative magnitudes of responses to low levels of gas at 25 "C N,O, Pc > ZnMeOTPP > CuTAA > CuMeOTPP > NiMe,TAA (21, CuTAA > NiTAA p NiMe,TAA > CuMeOTPP > ZnMeOTPP HCl NiMe,TAA B NiTAA > CuMeOTPP > ZnMeOTPP > MeOTPP.Relative reversibility of response to N,O at 25 "C P-lide p TAA > MeOTPP > CuMeOTPP > NiTAA + Pc. It is clear that the prospects for a multi-head device for gas-sensing and gas-discrimination are bright. References 1 C. L. Honeybourne Mol. Phys. 1983 50 1045. Oxford 1971) p. 39. 8 G. G. Roberts Contemp. Phys. 1984,25 109. 9 R. H. Tredgold R. Jones and A. Hoorfar Thin Solid Films 1984 113 115; M. Young Ph.D. Thesis (Lancaster 1985). 2 N. F. Mott and E. A. Davis Theory of Electrons in a Non-Crystalline Medium (Clarendon Press 3 C. L. Honeybourne R. J. Ewen and C. A. S. Hill J . Chem. Soc. Faraday Trans. I 1984,80 851. 4 S. Ogawa T. Yamaguchi and N. Gotoh J. Chem. Soc. Perkin Trans. 2 1974,976. 5 C. L. Honeybourne and R. J. Ewen J. Phys. Chem. Solids 1983,44 215. 6 B. Bott and T. A. Jones Organic Semiconductor Gas Sensor for NOX (Health and Safety Executive Sheffield 1982). 7 R. L. van Ewyk A. V. Chadwick and J. D. Wright J. Chem. Soc. Faraday Trans. I 1980,76,2194. Paper 5/1885; Received 21st October 1985

ISSN:0300-9599    

DOI:10.1039/F19868201127

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC. Faraday Trans. 1 1986,82 1135-1143 Aspects of the Optimization of Poly(viny1 chloride) Matrix Membrane Ion-selec tive Electrodes J. D. R. Thomas Department of Applied Chemistry Redwood Building U WIST P.O. Box 13 CardijTCFl 3XF PVC matrix membrane electrodes have been improved for dealing with an extended range of ions by the design of new electroactive components with appropriate plasticising solvent mediators. New approaches to calibrating the electrodes with ion buffers increases confidence in their use. Ion permeation studies in membranes demonstrate ion-selectivity and form a basis for electrochemical response mechanism of PVC matrix membranes containing trapped liquid ion-exchangers and neutral carrier complexes respectively. Immobilization of active sites on the membrane matrix has led to only limited improvements in ISE design.Rather more successful is the use of alternative solvent mediators for overcoming sample matrix interferences as in the case of samples containing anionic surfactants and certain biochemical components. PVC matrix membrane electrodes can be designed for dealing with various measuring situations such as small sample sizes flowing samples and muscle fluids. Poly(viny1 chloride) (PVC) matrix membranes have boosted the development of liquid ion-exchanger and neutral carrier ionophore ion-selective electrodes (ISE).l? These essentially liquid-based membranes may be easily prepared by the simple expedient of dissolving PVC in a solution of liquid ion-exchanger or neutral carrier ionophore systems in tetrahydrofuran and slowly evaporating at room temperat~re.l-~ The PVC matrix membrane ISE possess similar response behaviour to their liquid membrane counterparts but the much simpler design the considerable economy in sensor materials and sometimes longer lifetimes are additional desirable features that have led to their more general use.The PVC-based membranes consist of the electroactive component (ion-exchanger or ionophore) and plasticising solvent mediator trapped in the PVC matrix. Early work6 showed that ca. 30% content of PVC was appropriate since smaller proportions led to fragile membranes while larger amounts led to sluggish electrochemical response. The plasticising solvent mediator should preferably be of high viscosity for longer functional lifetimes of ele~frodes,~-~ and the nature of the solvent mediator itself can have a significant influence on the selectivity of the electroactive component.1° Choice of Electroactive Components and Calibration The single most important factor in optimizing ISE concerns the choice and development of selective electroactive components.Thus the origins of the successful commercial liquid ion-exchanger ISE lie in the use of materials with proved selectivity in solvent extraction work such as dialkylphosphates for calcium ISE and long aliphatic chain quaternary ammonium salts for nitrate and perchlorate ISE. There is a close relation between electrode selectivity coefficients kTtg and competitive ion-pair extraction constants (EQB/EQA) for Q extractantll (table 1).This and earlier studies on naturally 1135 1136 Br- k!%3.B I- 3.01 17 PVC Matrix Membrane ISE Table 1. Ratios of ion-pair extraction constants (EQ,) for the tetrabutylammonium ion compared with selectivity coefficients kp&B of a PVC matrix membrane nitrate ISE with tetradocylhexa- decylammonium nitrate sensora counter ion B NO; 1.39 log (extraction constant) (log EQx) 1 .o 1 c1- -0.11 0.005 0.032 a Data from ref. (1 1). Value for liquid membrane nitrate ISE. c10 3.48 800 1 20 1.29 O.Olb 0.79 EQB/EQN03 ~~ ~ ~ 42 occurring ionophores e.g. valinomycin for potassium ISE has been followed by the synthesis of improved materials and of organic ionophores with highly selective complexing properties for certain ions.Investigations of the nitrate ISE based on tetra-alkylammonium nitrate in PVC have shown that the linear calibration range is extended when the alkyl chain is C or longer.12 This is significant since the measurement of nitrate is of considerable practical interest and the interfering perchlorate iodide and certain other anions are purely the concern of research laboratories. However precautions must be taken against excessive amounts of chloride in samples. Among the organophosphate calcium ion sensors the calcium bis-dialkylphosphate systems have been superseded by calcium bis-di(4-octylpheny1)phosphate used in conjunction with dioctylphenylph~sphonate~~-~~ or with certain trialkylphosphates (usually tripentyl- or trioctyl-phosphate).16 It matters but little in terms of selective calcium ISE response whether the octyl group is the n-octyl chain or the isomeric 1 ,I ,3,3-tetramethylb~tyl.~9 1 5 9 l6 This is important since the starting material in the synthesis of the sensor is 4-alkylphenol and the 1,1,3,3-tetramethylbutyI isomer is much more ac~essible.~~ 1 7 9 la The neutral carrier sensor N,N-di[( 1 1 -ethoxycarbonyl)-undecyl]N,N-4,5-tetramethyl- 3,6-dioxaoctane diamide,1g-21 is also a very effective calcium ion sensor.Despite its more difficult synthesis it frequently possesses an edge such as by rather better selectivity for calcium over magnesium.22 This neutral carrier is in a short list23 of five of the most attractive synthetic carrier molecules for sensing metal cations the other four being selective for Li+ Na+ and Ba2+ as appropriate.There is a recent review24 on neutral carrier based ISE and new ones continue to be added such as a macrocyclic polyether amide with two polyether rings for calcium ionsz5? 26 and a lipophilic crown-4-derivative for lithium.27 Apart from improved selectivity the new calcium ISE systems behave well during calibration and use with ion-buffers when calibrations are possible down to ca. lo- mol dmd3 [Caz+]. The use of ion-buffer systems for calibrating ISE stems from solutions containing various proportions of copper@) ions to EDTA or NTA [or calibrating copper electrodes.28 This has been extended to calcium ISEf2 and adopted and tested by others.139 21y 29-33 It is significant that the calcium ion-buffer calibration extrapolates to that for serially diluted calcium ion standards.Hitherto methods of obtaining calcium ion standards down to mol dm-3 have been based either on the serial dilution of solutions containing the metal ion with a single complexing ligand such as NTA EDTAl31 1 4 9 21y 34 or EGTA34 35-38 or on the variation of pH in a solution of fixed metal to ligand concentration^.^^ 1 3 9 31 Such calibrations are laborious and the conditions are not applicable to those found in uiuo since both the pH and the concentration of calcium buffering ligands vary only slightly in biological 1137 J. D. R. Thomas fluids and cells. In viuo conditions are more closely approached by a method39 where free calcium ion concentrations are adjusted by adding increments of total calcium to a mol dm-3 EGTA solution at pH 7.4.This method is essentially a titration of EGTA by Ca2+ ions and therefore lacks precision in the unbuffered region near to the equivalence point. To overcome the above shortcomings a method has recently been developed32* 33 whereby standard free calcium ion concentrations from 10-8-10-3 mol dm-3 may be obtained by means of a single calibrating titration with a calcium solution of a mixture of calcium-buffering ligands namely ethylene glycol bisv-aminoethyl ester)-N,N,N’ N’-tetra-acetic acid (EGTA) N-(2-hydroxyethyl)ethylenethiaminetetra-acetic acid (HEEDTA) and nitrilotriacetic acid (NTA). The effectiveness of the calibration procedure at pH 7.4 (Tris buffer adjusted with hydrochloric acid) has been demonstrated on PVC matrix membrane calcium ISE based on calcium bis-di-4-( 1,1,3,3-tetramethylbutyl) phenylphosphate electroactive material and a commercial membrane containing a neutral carrier electroactive Thus the neutral carrier calcium ISE may be used down to lob8 mol dm-3 [Ca2+] in the presence of millimolar levels of magnesium.33 The same work showed that a PVC matrix membrane organophosphate based electrode with a solvent mediator mixture of dioctylphenylphosphonate/decan-1-01 in the ratio of 1 10 (v/v) can be used to determine these higher levels of rnagne~ium.~~ So far there is no magnesium ISE.The best approaches for the direct potentiometric measurement of magnesium therefore fall on the divalent e l e c t r ~ d e ~ ~ ~ ~ ~ or on an electrode based on a neutral carrier i o n ~ p h o r e .~ ~ However the neutral carrier system still shows preference of calcium over magnesium by a factor of ca. 20 while the divalent electrode can fail to give accurate analysis in standard analyte~.~O Attempts to obtain electroactive sensors for magnesium from complexes of the metal with polyalkoxylates have 42 for example the extra methyl in polypropoxylates over polyethoxylates was sufficient in its steric influence to yield just another calcium ISE41 instead of the barium ISE of polyethoxylate-metal ion complexes.8 An interesting feature of the polypropoxylate-based calcium ISE is its high interference from lithium ions,41 which has been exploited in the development of an ISE for lithium ions.43 Of the barium and lithium polyalkoxylates investigated as electroactive components for lithium the tetraphenylborate of the barium complex with PPG- 1025 polypropoxylate used in conjunction with dioctylphenylphosphonate in PVC gave the best lithium ISE.43 Although this is not as selective as lithium electrodes previously described with electroactive components of N,N,N’,N’-tetraisobutylcyclohexane-cis- 1,2-dicarboxylic diamide44 and dodecylmethyl- 14-cr0wn-4,~~ respectively the response towards lithium ions is of sufficiently high quality for it to be a realistic ISE.Furthermore there is the added advantage that the polypropoxylate sensor is much more easily prepared in the laboratory than are the other neutral carrier sensors.Tests with lithium-containing serum samples obtained from patients under treatment for manic depression show the polypropoxylate-based lithium ISE to be more effective in the microconduit flow-injection analysis measurements studied than either of the other two ISE.4s However there is still scope for considerable improvement in lithium ion sensors. Selectivity of PVC Matrix ISE Membranes Both potentiometric and ion transport studies indicate that PVC-based ISE membranes tend to be permselective to counter ions and accordingly their electrical properties are not significantly influenced by hydrophilic co-ions. Experimental evidence for cation permselectivity has been obtained for all analytically relevant neutral carrier based membrane electrodes and a cation transference number close to 1.0 has been found in electrodialysis experiment^.^^ Likewise cation permselectivity has been proved for organophosphate-based PVC ISE membrane^,^^ i.e.the membrane extracts and 1138 PVC Matrix Membrane ISE Table 2. d(C"/C')/dt data for solution/membrane systems [data from ref. (47)]. Membranes calcium from Orion 92-20-02 liquid ion-exchanger in PVC (exchanger batch A) [d(C"/C)/dt]/10-7 s-* series solution B(externa1) / mol dm-3 19 2.5 18 2.2 solution A(interna1) / 1 0-3 mol dm-3 solution 45CaC1 CaCI 45CaCl BeC1 45CaCl MgCI CaCl 45~a~15 BeC1 45CaCl MgCI 45CaC1 8.0 8.4 8.5 9.1 45CaCI SrCl 45CaCI s~cI 45CaCI BaCl 45CaC1 2 1 4 3 6 5 7 8 9 10 BaC1 in ISE.8.4 7.5 permeates cations from the sample solution but only negligibly small amounts of hydrophilic anions. The extent of ion permeation is related to selectivity of the ISE.479 48 Thus the data of table 2 for a calcium ISE membrane show that calcium permeates more than other cations as measured by d( C"/C)/dt data for those conditions where radiotracer flux between the initially inactive (tracer concentration C") and active (tracer concentration C ) solutions increases linearly with time. The unexpectedly low d(C"/C')/dt value for 45Ca on the active side and with beryllium chloride solution on the other side of the membrane (table 2) (on the premise that beryllium ions interfere strongly with calcium ISE response) is attributed to the high affinity of the ion-exchanger in the membrane for beryllium.This is evidenced in separate experiments by the increased concentration of beryllium in the membrane.48 Another interesting feature of ion permeation in liquid ion-exchanger based PVC matrix membranes concerns the effect of solvent mediator (table 3). Thus membranes with calcium ion-sensor and solvent mediators which give good calcium ISE properties give high values for d(C"/C)/dt data.49 Also the equal sensing of calcium and magnesium ions by the divalent ion electrode based on Orion 92-20-32 divalent exchanger (dialkyl- phosphate plus decan-1-01 solvent mediator) is reflected in the value of 32 x s-l for d(C"/C)/dt for permeation of 45Ca from calcium chloride through a membrane of the exchanger in PVC into counter solutions of calcium chloride and magnesium chloride respe~tively.~~ As mentioned above tetraphenylborates (TPB) of barium complexes with certain polyethoxylates e.g.Antarox C0880 (a nonylphenoxypolyethaneoxy unit) are good sensors for barium ISE.8 Relevant to the calcium ion permeation in the alkyl- and alkylphenyl-phosphate membranes discussed above there is very little permeation of barium ions from initially radioactive barium chloride solutions through the barium-form membranes into initially inactive barium chloride Nevertheless in counts of the membranes there is a continued uptake of 133Ba with time (table 4). This trend corresponds to observations by other that only a small proportion of available ligands in carrier type membranes participate in complex formation with cations.However in this case it is more likely to be the nature of the polyalkoxylate-metal complexation that is responsible for the small extent of permeation. Thus the confor- mation of the polyalkoxylate chain around the barium ions gives a stable entity with only small lability but sufficient for some ion transfer to produce the potentiometric response The conformation of the barium-polyalkoxylate complex is destroyed when a solvent mediator batch D of Orion 92-20-02 liquid ion-exchanger (for reference) dioctylphenylphosphonate tributylphosphate tripen tylphosphate trioctylphosphate Table 4. 133Ba uptake by eight membranes containing Ba,.- Antarox C0880 - TPB (0.04 g) + 2-nitrophenylphenyl ether (0.36 g) in PVC (0.17 g)a uptake of 133Ba time/h C"/C' Cact/Cinact by membrane (%) 9.6 5.2 12 6.0 6.5 a Data from ref. (49). Table 3. d(C"/C')/dt data for solution/membrane systems with membranes containing calcium bis-di(4- 1,1,3,3-tetramethylbutylphenyl)pho~phate and various solvent mediators in PVC mol dmd3 calcium chloride on each side of membrane one side active with 45Caa J. D. R. Thomas proportion of detection radiotracer limit of in membrane corresponding ion-selective ion-selective at end of d(C"/C') experiment /d t 18 (% ) 4.0 2.4 tri( 1,1,3,3-tetramethylb~tyl)phosphate membrane no. 4 2 - 3.2 0.01 24 48 0.01 6 - 3.1 2.1 0.03 0.02 72 0.01 144 0.01 503 a Data from ref.(51). Active solution Inactive solution potential is applied to electrodes placed in solutions on each side of the membrane.52 Thus the membranes were unable to maintain stable current flows for applied potentials of 2 V for the currents of 15 pA decayed to 7 pA in 4 h recovered on reversal of the polarity and fell to 5 pA in the next 4 h with no further evidence of recovery of current upon further potential reversals.52 Organophosphate-based membranes on the other hand are characterized by stable current flows over prolonged periods and over successive polarity reversals.52 Immobilization of Electroactive Sites Functional lifetimes of ISE based on polymer matrix membranes with trapped liquid ion-exchanger electroactive components are normally considerably shorter than for solid-state membrane electrodes.Among the reasons for this is the leaching of active components from the membranes especially at the membrane-solution interface. mol dmP3 133BaC12. mol dmV3 BaCl,. Not counted. electrode electrode /mol dm-3 Ca2+ lifetime 32 2.6 59 37 25 7.5 x 3.0 x 1.2 x 6.0 x 1.1 x 6.0 x 1 .o 1.8 2.4 4.0 1.6 1.4 1.8 1.4 5.6 6.6 b - 6.6 1139 3 months 3 months 1 week 3 months 4 weeks 3 months 1140 PVC Matrix Membrane ISE Various attempts have been made to lessen this detrimental effect by the covalent bonding immobilization of active components to the polymer matrix membrane upp port.^^-^* For anionic surfactant ISE a tertiary amine was bound to the ends of PVC chains and converted into a positively charged quaternary ammonium site by reaction with an alkyl Electrode membranes cast from tetrahydrofuran solution were conditioned in aqueous solutions of sodium dodecylsulphate in order to exchange the bromide ions with dodecylsulphate.Grafted cationic surfactant-sensitive electrode membranes were made by the low- temperature polymerization of vinyl chloride using the SO; radical anion.53 This gave a polymer of relative molecular mass ~ 7 7 0 0 0 in which ca. one-third of the polymer chains terminated with sulphonated end groups. Membranes were conditioned by exchanging the associated hydrogen ions for the desired surfactant cation.53 Among the approaches to immobilizing the ion-exchange site of calcium ISE is the binding of the styrene-b-butadiene-b-styrene (SBS) triblock e l a ~ t o m e r .~ ~ - ~ ~ Thus membranes were produced by firstly cross-linking SBS with triallyl phosphate followed by alkaline hydrolysis of the resulting covalently bound trialkylphosphate grouping to yield a pendant dialkylphosphate capable of acting as calcium ion sensor.54 Good calcium ISE of lifetimes in excess of 6 months were obtained although of limited selectivity towards calcium over important ions such as sodium and magnesium. Use of triundec- 10-enyl phosphate and of diallyl phenylphosphonate instead of triallylphosphate gave robust cross-linked membranes but without significant improvements in ~electivity.~~ An alternative approach for immobilizing the organophosphate sensor is to condense monodecyl dihydrogen phosphate with hydroxy groups of a partially hydrolysed vinyl chloride-vinyl acetate copolymer (copolymer VAGH).57 The resultant product when fabricated into membranes incorporating dioctylphenylphosphonate yielded good calcium ISE but without the expected advantage of extended lifetime over the PVC matrix membrane electrodes with a physically trapped sensor.57 Grafting of mono- octylphenylphosphonate to copolymer VAGH in PVC with calcium ion-sensor led to only a slightly lengthened lifetime while phosphonated polystyrene by Friedel-Crafts and free-radical processes was rather less succe~sfu1.~~ Membrane Modifications to Meet Sample Matrix Interferences Incorporation of a phenyl group between alkyl and the phosphorus oxygen of the dialkylphosphate calcium ion-sensors leads to less interference from pH.13-15 However freedom from pH interference at pH < 5 is best for a calcium ISE based on the macrocyclic siloxane tetracosamethylcyclododecasiloxane.59 A significant interference of calcium ISE response is that by anionic surfactants e.g.the addition of such a small amount as 2 x mol dm-3 sodium dodecylsulphate (SDS) lowers the e.m.f. of calcium ISE in many circumstances.60 This observation also applies to electrodes made from a commercial (Philips IS 561/SP) PVC matrix membrane for calciums1 (table 5). Replacement of some of the dioctylphenylphosphonate (DOPP) plasticising solvent mediator by decan- 1-01 in the organophosphate membrane electrodes reduces the interference by dodecylsulphate but at the expense of some loss of calcium ion-selectivity.60 Mechanistic studies by X-ray fluorescence and chromatography on calcium ISE membranes of bis-di[4-( 1,1,3,3-tetramethylb~tyl)-phenyl]phosphate sensors and DOPP solvent mediator in PVC show that SDS is an effective agent for leaching membrane components especially DOPP.61 Electrodes of membranes with the same sensor but with trioctyl phosphate as plasticising solvent mediator instead of DOPP are much less susceptibles1 to interference by SDS (table 5).The leaching of lipophilic membrane components raises concern for the clinical/bio- chemical field where natural surfactants are frequently present in samples.This concern 1141 J. D. R. Thomas Table 5. AE Caused by mol dm-3 SDS on calcium PVC ISE responsea - 3 CaC12/mV 1 0-2 mol dmP3 1 0-4 mol dm-3 solvent mediator component CaCl,/mV - 68 - 70 - 61 -2 - 65 - 70 -113 dioct ylphen ylphosp hona te-PVC trioctylphosphate-PVC tripen t ylp hosp hate-PVC Philips IS 56 1 /SP Ca2+ (PVC) membrane a Data from ref. (61). All electrodes based on calcium bis-di[4- (1,1,3,3-tetramethylbutyl)phenyl] phosphate except for Philips IS 561/SP Ca2- membrane. is justified since there has been much debate on which measuring technique provides the most accurate values of ion concentration for clinical use.s2 Related to these are studies on the effect of protein concentration on ISE measurements of ionized calcium.63 To complement such observations and studies on the interferences of calcium ISE by anionic surfactants several biochemical components in calcium-containing solutions have been studied for their effect on various forms of calcium ISEs4 (table 6).Here also the PVC electrodes with trioctylphosphate solvent mediator and calcium bis-di[4-( 1,1,3,3- tetramethylbutyl)phenyl]phosphate sensor are the most resistant.s4 However in this case contrary to the above observations for added anionic surfactant the commercial Philips IS 56 1 /SP membrane is reassuringly relatively resistant to the effect of added biochemical component (table 6). Architectural Optimization PVC matrix membrane electrodes can be modified to suit various measuring situations.For example samples down to 1 mm3 can be handled by sandwiching between a specially prepared flat surface of a reference electrode fabricated from a glass cone and socket and a PVC membrane electrode of conventional design.65 Flow injection analysis samples can be analysed by suitably positioned conventional ISE or by ISE based on tubular membranes.6s Fluids inside biological cells can be measured with specially designed microelectrodes with PVC matrix membranes and conventional inner filling ~ o l u t i o n s . ~ ~ ~ 24 The above architectural forms are either too fragile or otherwise unsuitable for plunging into muscle flesh for measurements of ions in muscle fluids. For this and other applications there is the design of all-solid-state PVC matrix membrane micro-electrodes using glass and Perspex capillaries back-filled with silver-conducting epoxy surfaced with a PVC matrix membrane for interfacing with the test fluides7 Conclusion Ion-selective electrodes have tremendous scope in terms of practical applications.However their use as in-dwelling sensors imposes harsh criteria for dealing with samples of very diverse character. PVC matrix membrane electrodes can be optimized for meeting many of these demands but there continues to be the need for further investigations in order to recognize and elucidate problems so that the ISE can be improved for use in new applications. ~~ Philips membrane electrode /mol dm-3 1142 component (0.05 mol dm-3) - 1.6 -6.8 - 0.6 - 10.5 1.7 0.2 -1.0 -6.5 0.8 - 2.8 0.2 -0.1 PVC Matrix Membrane ISE Table 6.AE Caused by biochemical components in calcium chloride in 0.15 mol dm-3 sodium chloride solutions on calcium ISE responsea AE caused by added components to solutions/mV TOPd /mol dm-3 TPPC /mol dm-3 DOPPb /mol dm-3 - 1.1 ~~ - 3.5 - 9.8 - 7.0 -0.1 - 11.3 - 85.7 - 40.0 - 15.0 - 9.7 -41.5 - 24.0 - 17.4 2.5 - 6.5 ~ -2.8 -7.2 -1.5 -3.8 0.5 1.9 0.2 0.2 2.8 -0.2 0.2 -0.3 -0.1 ~ ~ ~ 0.4 - 1.8 - 26.5 - 25.1 - 39.8 - 59.8 ~~ 3.3 1.7 - 0.8 -0.5 ~ _ _ 0.4 ~ 0.0 0.4 5.7 -0.1 - 0.5 ~~ ~~ lo- SDS (for comparison) -85 DBSS (for comparison) - 12.5 cholic acid cholesterol lecithin vitamin D urea glucose - 13.1 4.1 - 5.6 - 3.0 -0.9 -0.2 a Data from ref.(64). All electrodes based on calcium bis-di[C( 1,1,3,3-tetramethylb~tyl)phenyl]- phosphate except for Philips IS 561 /SP Ca2+ membrane in PVC with solvent mediator indicated. Dioctyl phenylphosphonate. Tripentyl phosphate. Trioctyl phosphate. The author thanks the S.E.R.C. for supporting this work by grants and studentships through the CASE scheme in association with Unilever Research (Port Sunlight Laboratory) and the Central Electricity Research Laboratories. Thanks are also extended to the British Council for Visiting Fellowships and the University of Wales UWIST the University of Technology Baghdad and the Foundation of Technical Institutes Baghdad for studentships.Also the many coworkers are thanked for their dedication. References 1 G. J. Moody R. B. Oke and J. D. R. Thomas Analyst (London) 1970,95,910. 2 G. J. Moody and J. D. R. Thomas in Ion-Selective Electrodes in Analytical Chemistry ed. H. Freiser (Plenum Press New York 1978) vol. 1 p. 287. 3 G. J. Moody and J. D. R. Thomas in Ion-Selective Electrode Methodology ed. A. K. Covington (CRC Press Boca Raton Florida 1979) vol. 1 p. 11 1. 4 A. Craggs G. J. Moody and J. D. R. Thomas J. Chem. Educ. 1974,51,451. 5 G. J. Moody and J. D. R. Thomas Ion-Selective Electrode Reviews 1979 1 3. 6 G. H. Griffiths G. J. Moody and J. D. R. Thomas Analyst (London) 1972,97,420.7 J. E. W. Davies G. J. Moody and J. D. R. Thomas Analyst (London) 1972,97 87. 8 A. M. Y. Jaber G. J. Moody and J. D. R. Thomas Analyst (London) 1976,101 179. 9 A. Hulanicki M. Maj-Zurawaska and R. Lewandowski Anal. Chim. Acta 1978 98 151. 10 A. Craggs L. Keil G. J. Moody and J. D. R. Thomas Talanta 1975 22,907. 11 G. J. Moody and J. D. R. Thomas Chem. Ind. (London) 1974,644. 12 H. J. Neilsen and E. H. Jansen Anal. Chim. Acta 1976 86 1. 13 J. Ruzicka E. H. Hansen and J. C. Tjell Anal. Chim. Acta 1973,67 155. 14 H. M. Brown J. P. Pemberton and J. D. Owen Anal. Chim. Acta 1976,85 261. 15 L. Keil G. J. Moody and J. D. R. Thomas Anal. Chim. Acta 1978,% 171. 16 G. J. Moody N. S. Nassory and J. D. R. Thomas Analyst (London) 1978 103 68. 17 J.D. R. Thomas Lab. Pract. 1978 27 857. 18 A. Craggs P. G. Delduca L. Keil B. J. Key G. J. Moody and J. D. R. Thomas J. Inorg. Nucl. Chem. 1978,40 1483. 19 D. Ammann E. Pretsch and W. Simon Helv. Chim. Acta 1973,56 1780. 20 D. Ammann R. Bissig M. Guggi E. Pretsch W. Simon I. J. Borowitzand L. Weiss Helv. Chim. Acta 1975 58 1535. 1143 J. D. R. Thomas 21 D. Ammann M. Guggi E. Pretsch and W. Simon Anal. Lett. 1975 8 709. 22 T. S. Tsieng personal communication. 23 W. E. Morf and W. Simon Hung. Sci. Instrum. 1977 41 1. 24 D. Ammann W. E. Morf P. Anker P. C. Meier E. Pretsch and W. Simon Ion-Selective Electrode Revs. 1983 5 3. 25 K. Kimura K. Kumani S. Kitazawa and T. Shono J. Chem. Soc. Chem. Commun. 1984,442. 26 K. Kimura K. Kumami S. Kitazawa and T.Shono Anal. Chem. 1984,56 2369. 27 S. Kitawawa K. Kimura H. Yano and T. Shono J. Am. Chem. Soc. 1984 106,6978. 28 R. Blum and H. M. Fog J. Electroanal. Chem. 1972,34 485. 29 A. Craggs G. J. Moody and J. D. R. Thomas Analyst (London) 1979 104,412. 30 A. Craggs G. J. Moody and J. D. R. Thomas Analyst (London) 1979 104 961. 31 0. Scharff Anal. Chim. Acta 1979 109 291. 32 M. Otto and J. D. R. Thomas Anal. Proc. 1984,21 369. 33 M. Otto P. M. May K. Murray and J. D. R. Thomas Anal. Chem. 1985,57 151 1. 34 Y. S. Kim and G. M. Padilla Anal. Biochim. 1978 89 521. 35 A. Coray C. H. Fry P. Hess J. A. S. McGuigan and R. Weingart J. Physiol. 1980 305 60P. 36 C. 0. Lee D. Y. Uhm and K. Dresdner Science 1980,209 699. 37 E. Marban T. S. Rink R. W. Tsien and R. Y.Tsien Nature (London) 1980 286 845. 38 B. A. Bulas and B. Sacktor Anal. Biochem. 1979,95 62. 39 D. M. Bers Am. J. Physiol. 1982 242 C404. 40 S. K. A. G. Hassan G. J. Moody and J. D. R. Thomas Analyst (London) 1980 105 147. 41 A. M. Y. Jaber G. J. Moody and J. D. R. Thomas Analyst (London) 1977,102,943. 42 P. H. V. Alexander G. J. Moody and J. D. R. Thomas to be published. 43 V. P. Y. Gadzekpo G. J. Moody and J. D. R. Thomas Analyst (London) 1985 110 1381. 44 A. F. Zhukov D. Erne D. Ammann M. Guggi E. Pretsch and W. Simon Anal. Chim. Acta 1981 131 117. 45 S. Kitzazwa K. Kimura H. Yano and T. Shono Analyst (London) 1985,110,295. 46 V. P. Y. Gadzekpo G. J. Moody and J. D. R. Thomas Analyst (London) in press. 47 A. Craggs G. J. Moody J. D. R. Thomas and Anne Willcox Talanta 1976 23 799.48 A. M. Y. Jaber G. J. Moody J. D. R. Thomas and Anne Willcox Talanta 1977,24,655. 49 G. J. Moody and J. D. R. Thomas J. Power Sources 1983,9 137. 50 A. Craggs B. Doyle S . K. A. G. Hassan G. J. Moody and J. D. R. Thomas Talanta 1980 27 277. 51 B. Doyle G. J. Moody and J. D. R. Thomas Talanta 1982 29 257. 52 B. Doyle G. J. Moody and J. D. R. Thomas Talanta 1982,29,608. 62 J. H. Ladenson Anal. Proc. 1983 20 554. 63 R. B. Payne RSC International Symposium on Electroanalysis in Biomedical Environmental and Industrial Sciences UWIST Cardiff 5-8 April 1983 Paper 13. 64 S. A. H. Khalil G. J. Moody and J. D. R. Thomas Analyst (London) 1985 110 353. 65 J. D. R. Thomas in Ion-Selective Electrodes ed. E. Pungor (Akademiai Kiad6 Budapest 1978) p. 175. 66 A. J. Frend G. J. Moody J. D. R. Thomas and B. J. Birch Analyst (London) 1983,108 1357. 67 S. A. H. Khalil G. J. Moody J. D. R. Thomas and J. L. F. C. Lima Analyst (London) in press. 53 S . G. Cutler and P. Meares J. Electroanal. Chem. 1977 85 145. 54 L. Ebdon A. T. Ellis and G. C. Corfield Analyst (London) 1979 104 730. 55 G. C. Corfield L. Ebdon and A. T. Ellis Anal. Proc. 1981 18 1 12. 56 L. Ebdon A. T. Ellis and G. C. Corfield Analyst (London) 1982 107 288. 57 L. Keil G. J. Moody and J. D. R. Thomas Analyst (London) 1977 102 274. 58 P. C. Hobby G. J. Moody and J. D. R. Thomas Analyst (London) 1983,108 581. 59 C. J. Olliff and G. R. Pickering British Patent 1558553 (C1 GOlN27/30) 03 January 1980. 60 A. Craggs G. J. Moody J. D. R. Thomas and B. J. Birch Analyst (London) 1980 105,426. 61 A. J. Frend G. J. Moody J. D. R. Thomas and B. J. Birch Analyst (London) 1983 108 1072. Paper 511886; Received 16th September 1985

ISSN:0300-9599    

DOI:10.1039/F19868201135

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chew. Soc. Faraday Trans. I 1986,82 1145-1 159 Ion Recognition by Macrocyclic Hosts Ian 0. Sutherland Department of Organic Chemistry The Robert Robinson Laboratories University of Liverpool P.O. Box 147 Liverpool L69 3BX Two general approaches described are for enhancing the selectivity of the complexation of alkylammonium cations by crown ethers. The first involves the introduction of steric barriers in a monocyclic system and the second involves the use of bridges and macropolycyclic systems. The extent of molecular recognition by these host molecules and the effects of structural rigidity and flexibility are discussed. The recognition of small and large guest molecules by host macromolecules usually proteins is an essential feature of very many biological processes.This recognition involves the formation of a complex between the guest and host molecules in such a manner that the association constant for complex formation between the host and the selected (recognised) guest molecule or molecules is much greater than for complex formation with other molecules which may be present in the system. Such behaviour has been noted for many natural host molecules including enzymes1 and drug receptors,2 and also for smaller natural products such as cyclodextrin~~ and antibiotics of the vancomycin group.* Since the selection of the guest molecule is based upon non-covalent interactions between the two components of the complex it requires complementary charge distribution and space filling by the guest and host as shown diagramatically in fig.1 which depicts selective complexation of a guest dipeptide in an appropriate host cavity. Such selective complexation of ions accompanied by transport of the complexed ion across a lipophilic membrane provides a basis5 for the ion-selective electrodes which have been developed for simple spherical metal cations and a number of other ionic species. The host molecules used in these devices are structurally simpler than proteins and include the natural ionophoric antibiotics as well as a large number of synthetic host (carrier) molecules. However although the development of synthetic host molecules that select or recognise spherical metal cations has proved to be a successful area of research for the synthetic chemist,6 the design and synthesis of host molecules that will select more complex organic ions have been less well developed.The major advances in the latter area have come from the discovery’ that crown ethers such as 18-crown-6 (l) form + complexes with primary alkylammonium cations RNH, in addition to a range of metal cations. In this paper an approach to the selective complexation of alkylammonium cations will be described and some general conclusions will be drawn regarding structural 1145 1146 Ion Recognition by Macrocyclic Hosts Fig. 1. Recognition by shape and charge distribution of a dipeptide by a synthetic receptor. Fig. 2. Charge distribution in the complex 18-crown-6. ButNH,. The filled circles represent positive charges and the open circles negative charges; in each case the diameter of the circle is proportional + to the magnitude of the charge.requirements for selective host molecules. It will also be shown that in some circumstances guest recognition by synthetic host molecules may be similar to that shown by the most selective biological host molecules. The selective formation of a guest-host complex in solution8 requires the organisation of non-covalent binding sites in the cavity of the larger host molecule that are complementary to binding sites in the guest species (cf. fig. 1). For complexation in aqueous solution it is possible to include hydrophobic interactions in host design,? but for non-polar solvents or lipophilic membranes binding sites are generally charged or polar groups and guest discrimination is usually based upon the introduction of steric + barriers as described below.This is well illustrated by the complexes 18-crown-6. RNH, formed most readily in non-polar solvents such as chloroform and dichloromethane which are held together in solution by the electrostatic attraction between the complex array of positive and negative charges in the two components of the complex illustrated diagramatically in fig. 2. Although it is convenient to think of complexes (2) of simple cations in this way it is also useful to regard the alkylammonium cation complex (3) as an example of specific hydrogen bonding. t Considerable progress has been made recently in the design of such hosts,e in most cases water-soluble cyclophanes and similar complexation by non-protein natural products such as the cyclodextrins,3 has been extensively studied.Although of considerable general interest such studies are less directly relevant to the design of carrier molecules for ion-selective electrodes than investigations of complex formation in non-polar solvents. I. 0. Sutherland 1147 ( 5 ) s -guest ( 9 1 A - twist ( 8 ) ( a ) Fig. 3. Recognition enhancement by the incorporation of (a) barriers and (b) bridges into synthetic macrocyclic receptors . The recognition of the group R by the 18-crown-6 system in the complexes (3) is rather limitedlo because non-bonded interactions other than electrostatic interactions between the guest and host molecules do not change very much as the group R is changed for example in the series R = H Me Et Pri But.Recognition enhancement may be achieved either by the introduction of steric barriers into monocyclic hosts or by the use of bridged (polycyclic) hosts as shown in fig. 3. The use of steric barriers has been imaginatively developed by Cram and coworkers,l' particularly in their studies12 of enantioselective complexation by optically active hosts such as (5) based upon the axial chirality of the 2,2'-dihydroxy- 1,l '-binaphthyl system. The chiral barriers in these hosts shown diagramatically in (4) are rather similar to those found (6) in the optically active diaza- 18-crown-6 derivatives (7) which we have studied,13 and both hosts selectively complex the (S)-phenylethylammonium cation rather than the (R)-enantiomer.The enantioselective complexation of chiral primary alkylammonium cations by hosts of this type may be generalised and predicted by considering the number of chiral barriers in the macrocycle their distribution in the three sectors of the macrocycle corresponding 1148 Ion Recognition by Macrocyclic Hosts a n t i anti R 12,15,18 syn syn - R ~ ia syn anti- Fig. 4. Stereoselectivity in complexation of alkylammonium cations by diaza crown ethers. The circles represent the crown ether macrocycle and the numbers refer to ring sizes where complexation occurs in the direction indicated by the arrow. 18 t N U N R + to the three substituents in a chiral guest cation LMSCNH,,? and the direction (clockwise or anticlockwise) of the increasing steric interactions in the macrocycle.Thus hosts (4) and (7) may be represented diagramatically by (8) in which the two chiral barriers are necessarily located in two of the three sectors of the macrocycle. The representation of these barriers by the rectangles indicates the direction of increasing steric barrier (barrier closer to ring) and it can be seen in (8) that both barriers increase in an anticlockwise (A) direction. These hosts may therefore be described as [2,2] A hostst and on the basis of available experimental evidence hosts of this type complex preferentially with the enantiomer of a chiral ammonium cation in which the sequence of groups L M S is in a clockwise direction ( C ) when then guest is viewed down the C-N bond as shown in (9). It has been possible to classify1* other optically active 18-crown-6 analogues in a similar way and rules may be derived relating enantioselectivity + are located.+ to macrocycle classification. The distinction between the enantiomers of a chiral cation such as PhCHMeNH, involves recognition by the host molecule of the difference between a hydrogen and a methyl substituent. Such a distinction which is not uncommon in biological examples of molecular recognition,l implies that it should be possible to make synthetic host molecules which will distinguish between the members of the series X-H X-Me X-Et X-Pri X-Bd or between the homologues of straight-chain compounds X[CH,],Y. Although barriers such as those in (5) and (7) might be used to effect such distinctions we have investigated the alternative approach to recognition enhancement by the introduction of bridges (see fig.3) and we have inve~tigatedl~ bicyclic hosts (lo) analogous to the cryptands and also tricyclic hosts (11). Whereas the two faces of the receptor macrocycle are identical in hosts such as (5) and (7) this is clearly not the case in (10) and (ll) and the arrows in these structures indicate that the receptor macrocycles should complex preferentially in the indicated direction so that an inclusion complex is obtained. This particular problem may be solved by using the 12- or 15-membered diaza-crown ethers (12) and (13) as the macrocycles since these were shown in our early workla to form only syn,syn-complexes whereas diaza- 18-crown-6-derivatives (14) or (15) form syn,syn- syn,anti- and anti-anti-complexes (fig.4). The bicyclic hosts (16a-c) were synthesised15 as hosts having the correct directed complexation [see arrow in (17)] and these compounds were investigated for selective Substituents are labelled (L) large (M) medium and (S) small according to relative steric bulk. 1 The numbers refer to the number of steric barriers and the minimum number of sectors in which they 1149 RN ( c ) + complexation of the primary alkylammonium cations RNH,. lH n.m.r. spectroscopy was selected as the method for investigating the complexes because protons in guest molecules located in the cavity as shown in (18) will lie in the shielding zone of the aromatic ring in the bridge and hence give n.m.r.signals which are shifted upfield as compared with the signals of the free cation. T F extent of these high-field shifts is summarised for all three hosts and a variety of guest cations in fig. 5. The results shown in fig. 5 indicate that high-field chemical shifts tend to be larger + I . 0. Sutherland H k_.dR + for H,NC(a)H than for H,NC(a)-C@)H and for small groups R rather than large groups. Thus there is some selectivity for the location of guest protons in the shielding zone of the aromatic system but on the basis of competition experiments selectivity in guest complexation is rather limited. The single rather flexible bridge in the hosts (16) is evidently not adequate to give the degree of recognition that is sought and these bicyclic hosts require considerable structural modification if they are to be used as selective cation carriers.(17) Ion Recognition by Macrocyclic Hosts Gly Et PhCH,GlyGly Pr' fa Me Et GlY Me But hi Et Pr' Me Pr' Et 1150 (164 (16a) I PhCH Gly Gly Et I a P But Pr'Et J 0 0.5 A6 ( P P ~ + + 1 .o + + RNH, as in the complex (20) or for a single bis-ammonium cation as in the complex (21). In particular the series of bis-cations (22) in which the length I varies according to the value of n would be a particularly appropriate series of guests for testing the selectivity of hosts of this type through systematic variation of the distance din the host molecule (21). A series of hosts of this type (23) was prepared1'? l8 by the reaction of diazacrown ethers with a bis-benzyl halide (scheme 2).The three aromatic systems (24a-c) were chosen for the bridges because in all cases the direction of the Ar-C bonds are parallel and the distance d in (21) varies as shown below each formula. The selectivity of each of these hosts (23) in complexing dications (22) was examined using the spectroscopic methods described below. + Fig. 5. Induced upfield shifts of (CH-NH,) and (CH-C-NH,) protons of guest alkylammonium cations RNH in complexes with hosts (16a-c). For each host the upper line refers to a-protons and the lower line to Q-protons the entries Me Et etc. refer to the group R in RNH,. Tricyclic crown ether derivatives (ll) based upon two connected diaza crown ether units as in (19) are potentially host molecules for two simple alkylammonium cations d 1 T + + The lH n.m.r.spectra of the complexes formed between (23) and NH,(CH,) NH in CD,Cl,-CD,OD (9 l) showed the expected high-field shifts of the guest CH groups as compared with the free guest cation and this effect is exemplified by the 220 MHz n.m.r. spectrum of a 2 1 (guest:host) mixture of host (25) and the guest salt I. 0. Sutherland 1151 3 Ar = a--*- 1 0 2 6 (PPm) + + Fig. 6.220 MHz lH n.m.r. spectrum of a 1 2 mixture of host (25) and guest H,N(CH,),NH,. 2NCS- complexed guest cation. (This convention is also used in fig. 7 and 8). + in CD,Cl at (a) -60 and (b) 25 "C. The labels 7a 7Q etc. refer to the indicated CH groups of the guest; entries in parentheses ( ) refer to the free guest cation while other entries refer to the + NH,(CH,),NH 2NCS-.At 25 "C [fig. 6 (b)] the spectra of the free and complexed guest are time-a~eraged'~ by rapid exchange of the free and complexed species but at low temperatures (- 60 "C) this process becomes slow on the n.m.r. timescale and a set of high-field signals is assignable to the CH groups of the guest dication with a further set of signals assignable to the CH groups of the free dication [fig. 6(a)]. The shift to high field of ca. 2 ppm for the signals of the complexed guest dication shows very clearly that it lies in the host cavity and is flanked by the biphenyl groups of the bridges as shown diagramatically in (26). In general similar results are obtained for all the hosts (23) and guest dications that fit into the host cavity.Because guest exchange is slow at low temperatures it is possible 1152 + H 3 N - p - NH3 t 5Y 0 I 3 1 4 I Ion Recognition by Macrocyclic Hosts + ga 5P 1 2 I 6 (PPm) + + + + H,N(CH,),NH .2NCS- and H,N(CH,),NH .2NCS- in CD,Cl at - 60 "C. Fig. 7.220 MHz 'H m.m.r. spectrum of a 1 1 1 mixture of host (25) and guest to study competitive complexation of a pair of dications in a very direct manner (fig. 7). The lH n.m.r. spectrum of a 1 1 1 mixture of host (25) and the two guest salts + + + NH,(CH,),NH . 2NCS- and NH,(CH,),NH,. 2NCS- at - 60 "C shows signals for the shorter dication in the normal position for the free dication whereas those of the longer dication show the characteristic lugh-field shift of a complexed dication and are observable in the region 6 = - 1 to 1 ppm.The results of a series of competition experiments using this host (25) are summarised in table 1; in particular the guest dications (22 n = 5 and 6) are complexed almost equally readily (fig. 8) and each of them excludest the competing dications (22 n = 4 and 7) from complexation. Similar competition experiments were carried out using other hosts (23) and the same series of + + guest salts NH,(CH,) NH * 2NCS- and the results of these studies are also reported in table 1. Other tricyclic hosts based upon diaza-l8-crown-6 (14) have been studied by Lehn and coworkers20 using methods similar to those outlined above and other techniques.In all cases the tricyclic hosts show high selectivity and distinguish between guests differing by just a single CH group. Examination of the results in table 1 shows that guest selection depends upon the length 7 The spectrum of the host and the competing guests in a 1 1 1 ratio shows only signals for a single complexed species and a single free species suggesting a preference of 3 10 1 for the complexed species. ,. . A l l I I I. 0. Sutherland 1 I H,N(CH,),NH,. 2NCS- in CD,Cl at - 50 "C in (a) a 1 1 1 ratio and (b) a 1 0.5 0.5 ratio. + d/Ab selectivity 4 J + host (23a n = 1) (23a n = 2) (27 a) (23b n = 2) (27 b) (23c n = 1) (23c n = 2) - i h I 2 + Table 1. Selectivitya in complex formation between hosts (23) and (27) and the biocations NH,(CH,),NH 6 (ppm) Fig.8. 220 MHz lH n.m.r. spectrum of host (25) and guest H3N(CH,),NH3.2NCS- and bridge 3 I crown sizes 12,12 15 15 15 18 18 18 18 18 18 18 :: ::> a In non-polar solvents. The numbers n refer to the guest dication NH,(CH,),NH and the inequality signs indicate which dication of a competing pair is complexed preferentially. The length d is defined in formula (21). The length I is defined in formula (22) and refers to the dication which is complexed preferentially in the selectivity sequence or the average length of two dications in cases where a pair of dications is selected. Data taken from ref. (20). + + + 6a 67 H3N-W 6B 1153 L I 0 + -1 l/AC 3.7 3.7 4.9 4.9d 6.2 6.9 7.4d 8.1 8.7 10.Od + 1154 Ar h = o .a A d of the CH,ArCH bridge and also upon the size of the diaza crown macrocycles. Thus hosts based upon 18-membered rings tend to select the longer guest dication + + guest salt (22) and the distance din the host (21) shows that each NH group of the guest dication approaches closer to the receptor diaza crown macrocycle as the size of the macrocycle increases from 12- through 1 5 to 18-membered as summarised diagrama- tically in formulae (28a-c). Thus selectivity of complexation of tricyclic hosts may be adjusted coarsely by changing the distance d by selection of the aryl group in the bridge and a finer adjustment may be achieved by changing the size of one or both of the macrocycles.These two structural adjustments permit the synthesis of a selective host tricycle for each of the dications in the series H,N(CH,),NH, n = 2-7. The apparently rigid tricyclic hosts (23) and (27) are each able to adopt a number of different conformations. The diaza-crown macrocycles can adopt chiral conformations for example the [3 3 3 31 conformation (29) of the diaza-12-crown-4 system2 which can adopt the two enantiomeric conformations (29a) and (29b). Hence a tricycle containing two diaza crown macrocycles can exist as two diastereoisomers interconverted by + u h = 1 0 A + + AN + + NH,(CH,),,,NH rather than the shorter guest dication NH,(CH,) NH selected by the analogous host based upon 12-membered rings with the same aromatic residues in the bridges.This effect was studiedz1 in more detail by examining the selectivity of the asymmetrical tricyclic hosts (27) synthesised by the route outlined in scheme 3. In particular the tricycles (27) based upon the 15- and 18-membered diaza crown ethers (13) and (14) show high selectivity for complexation with the dications NH,(CH,) NH,. This selectivity is also included in table 1. Comparison of the length 1 of the preferred Ion Recognition by Macrocyclic Hosts 'N Ar + + 1155 + + I. 0. Sutherland conformational inversion of one of the crown conformations [cf. (29a) (29 b) for diaza- 12-crown-41. Furthermore there are many conformational possibilities for macro cycle^^^ and it is not possible to predict with certainty which particular conformation will be favoured either in the free state or in a complex.In addition to this uncertainty regarding macrocycle conformation for tricycles (23) containing two 1 5-membered diaza crown systems the macrocycles may be arranged with the shorter CH2CH20CH2CH2 bridges in a syn (Ma) or anti (30b) relationship. Finally for tricyclic systems including two 2,6-disubstituted naphthalene units in the bridges the naphthalene units can be arranged in a crossed (31 a) or parallel (31 b) fashion. Examination of the n.m.r. spectra of complexes of host (23) and (27) over a wide temperature range shows that in all cases where the guest dication is strongly bound the complex involves to a major extent a single conformation of the tricycle; in some cases no second conformation can be detected and in other cases the presence of small amounts of a second conformation can be deduced from the n.m.r.spectra. Thus complexation by these rigid tricyclic receptors of the dications H,N(CH,) NH tends to be a highly stereoselective process but it does R L (31a) 1156 t d' 5.8 T J. 5.8 1 t (6) n = 2 r 3 > 1 ~ 5 Ion Recognition by Mucrocyclic Hosts (a) n = 2 ~ 3 ~ 4 5 + + Fig. 9. Selectivity in complexation of NH,(CH,) NH by. the flexible macrotricyclic hosts (32) and (33). The indicated distances refer to the rigid p-xylyl bndges [cf. distance din (21)]; the distance d can vary according to the conformation adopted by the C-C bonds indicated by thickened + + lines.not necessarily involve the conformation adopted by the free host. In particular a host analogous to (23 b) has been shown2* by X-ray crystallography to adopt a conformation in the crystalline free host that differs significantly from that adopted in the crystalline complex with H,N(CH,),NH and the host (23a n = 1) has been shown by lH and 13C n.m.r. spectroscopy to adopt a conformation in solution in the free state which differs + + + + from that adopted in a solution of the complex with H,N(CH,),NH3. The hosts (23) and (27) although rigid are apparently not preorganised for complexation but this does not prevent them acting as highly selective hosts. In view of the high selectivity in complexation shown by the rigid tricyclic hosts (23) and (27) it was of interest to examine less rigid hosts.Two flexible systems were examined;l8V 21 the first (32) contained a relatively rigid 15-membered macrocycle and a rather flexible 24-membered macrocycle and the second (33) contained a rigid p-xylyl bridge and a more flexible 3,6-dioxaoctane bridge. The first host (32) was found to complex each of the three dications H,N(CH,) NH, n = 2 - 4 equally readily to give the selectivity sequence shown in fig. 9(a) and hence the range of possible degrees of + penetration of the NH group into the 24-membered macrocycle indicated by the range of values of h in (34). The second host (33) complexed each member of the pairs of + + dications NH,(CH,),NH, n = 2 3 and n = 4 5 equally readily with the first pair complexed more strongly than the second pair as indicated by the selectivity sequence shown in fig.9(b). This result suggests that the host (33) can adopt two conformations in which the separation of the two crown rings differ; these conformations are probably distinguished by the torsion angles adopted by the thickened bands in fig. 9(b) but it is not possible to decide which two of the many possible conformations are involved. ( 341 receptors can show very high recognition for a single guest dication NH,(CH,) NH, and in favourable circumstances relatively small synthetic host molecules can show selectivity in complexation comparable with that shown by the natural protein-based systems. (ii) Selectivity for dications is a function of the separation of the two receptor sites and the extent to which the functional group of the guest penetrates the receptor group of the host.(iii) Synthetic receptors can be highly selective even if they are not completely ‘ pre-organised ’ but selective complexation is a conformationally selective process. (iv) + The studies outlined in this paper lead to a number of conclusions. (i) Rigid tricyclic Conclusions OR’ I Me0 YI1 t The labels I-VII and A-F identify and relate the aromatic rings and binding functional groups in structures (35) and (36). 1157 I. 0. Sutherland h = 1 . 3 to - 1 . 2 A + Ar 0 t (36) Ion Recognition by Macrocyclic Hosts 1158 Finally flexible receptors can select a range of guest dications rather than a single dication.The selectivity that has been achieved using macrotricyclic host's (23) and (27) is based upon a ditopic receptor showing recognition through the fixed separation of two receptor groups. Similar strategies might be used to achieve comparable results with dianions25 and zwitterions.2s The more complex problem of general guest recognition as outlined in fig. 1 has yet to be solved. An encouraging indication of what may be possible is provided by the antibiotics of the vancomycin group.4 Thus the receptor site of these polycyclic natural products such as ristocetin (35) binds acylderivatives of d-ala-d-ala in a selective fashion through a combination of hydrophobic forces and the specific electrostatic interactions summarised in (36). A combination of the rigidity of a polycyclic cyclophane framework and the correct disposition of functional groups is successful in these natural products as well as in the rather simpler synthetic host molecules described in this paper.This may well provide a basis for a significant increase in the range of guest molecules that can be distinguished by synthetic receptors. It is a pleasure to thank my coworkers who are named in the references for their important contributions to the work described in this review. I also acknowledge with thanks our fruitful collaboration with Dr R. F. Newton of Glaxo Research and Dr P. Camilleri of Shell Research and the support given by their companies for our work. Finally I wish to record my appreciation for the many excellent 400 MHz n.m.r.spectra run by the S.E.R.C. n.m.r. service at Sheffield. References 1 The Chemistry of Enzyme Action ed. M. I. Page (Elsevier Amsterdam 1984); Enzyme Chemistry ed. C. J. Suckling (Chapman and Hall London 1984). 2 Cellular Receptors ed. D. Schulster and A. Levitzki (Wiley Chichester 1980). 3 M. L. Bender and M. Komiyana Cyclodextrin Chemistry (Springer Berlin 1977); G. L. Trainor and R. Breslow J. Am. Chem. Soc. 1980,103 154; I. Tabushi T. Nabeshima and K. Yamamura J. Am. Chem. SOC. 1982 104 2017; Y. Matsui T. Nishioka and T. Fujita in Biomimetic and Bioorganic Chemistry ed. F. L. Boschke (Springer Berlin 1985) p. 61. 4 J. R. Kalman and D. H. Williams J. Am. Chem. Soc. 1980 102 897; 906; C. M. Harris and T. M. Harris J. Am. Chem. Soc. 1982,104,363 ; D.H. Williams M. P. Williamson D. W. Butcher and S. J. Hammond J. Am. Chem. Soc. 1983 105 1332; M. P. Williamson D. H. Williams and S. J. Hammond Tetrahedron 1984 40 569; M. P. Williamson and D. H. Williams Eur. J. Biochem. 1984 138 345; M. P. Williamson and D. H. Williams J. Chem. Soc. Perkin Trans. I 1984 949; J. C. J. Barna D. H. Williams and M. P. Williamson J. Chem. Soc. Chem. Commun. 1985 254. 5 W. E. Morf D. Ammann R. Bissig E. Pretsch and W. Simon in Progress in Macrocyclic Chemistry ed. R. M. Izatt and J. J. Christensen (Wiley New York 1979) p. 1 ; T. S. Ma and S. S. M. Hassan in Organic Analysis using Zon Selective Electrodes (Academic Press London 1982 vol. 1 and 2; P. C. Meier D. Ammann W. E. Morf and W. Simon in Medical and Biological Applications of Electrochemical Devices ed.J. Koryta (Wiley Chichester 1980) p. 13. 6 For some recent reviews see M. Okahara and Y. Nakatsuji in Biomimetic and Bio-organic Chemistry ed. F. L. Boschke (Springer Berlin 1985) p. 37; M. Takagi and K. Ueno in Host Guest Complex Chemistry IIZ ed. F. Vogtle and E. Weber (Springer Berlin 1984) p. 39; G. W. Gokel and S. H. Korzeniowski Macrocyclic Polyether Syntheses (Springer Berlin 1982); H. G. Lohr and F. Vogtle 7 C. J. Pedersen J. Am. Chem. Soc. 1967 89 2495; 7017; 1970 92 386; 391; C. J. Pedersen J. Org. Acc. Chem. Res. 1985 18 65. Chem. 1971,36 254; 1690; C. J. Pedersen and H. K. Frensdorff Angew. Chem. Znt. Ed. Engl. 1972 11 16. 8 P. Kollman Ace. Chem. Res. 1985 18 105; S. Weiner P. Kollman D. Case C. Ghio G.Alagona S. Profeta and P. Weiner J. Am. Chem. SOC. 1984 106 785. 9 K. Odashima A. Itai Y. Iitake and K. Koga J. Am. Chem. Soc. 1980 102 2504; 1. Tabushi Y. Kimura and K. Yamamura J. Am. Chem. Soc. 1978 100 1304; 1981 103 6486; K. Odashima T. Soga and K. Koga Tetrahedron Lett. 1981 531 1 ; I. Tabushi and K. Yamamura in Cyclophanes Z ed. F. Vogtle (Springer Berlin 1983) p. 145; K. Odashima and K. Koga in Cyclophanes ZZ ed. P. M. Keehn and S. M. Rosenfield (Academic Press New York 1983) p. 629; M. Dhaenens L. Lacome J. M. Lehn and J. P. Vigneron J. Chem. Soc. Chem. Commun. 1984,1097; F. Diederich Nachr. Chem. 1159 I. 0. Sutherland Tech. Lab. 1984 32 787; H. J. Schneider K. Philippi and J. Pohlmann Angew. Chem. Int. Ed. Engl. 1984,23,908; F. Diederich and K.Dick J. Am. Chem. SOC. 1984,106 8024; 8037. 10 F. de Jong and D. N. Reinhoudt Stability and Reactivity of Crown Ether Complexes (Academic Press London 198 1). 11 D. J. Cram and K. N. Trueblood in Host-Guest Complex Chemistry 1 ed. F. Vogtle (Springer Berlin 1981) p. 43; G. M. Lein and D. J. Cram J. Am. Chem. SOC. 1985 107,448; and earlier papers in the series ' Host-Guest Complexation '. 12 D. J. Cram and J. M. Cram Ace. Chem. Res. 1978,11,8; J. M. Timko R. C. Helgeson and D. J. Cram J. Am. Chem. SOC. 1978 100 2828; S. C. Peacock L. A. Domeier F. C. A. Gaeta R. C. Helgeson J. M. Timko and D. J. Cram J. Am. Chem. SOC. 1978 100 8190; S. C. Peacock D. M. Walba F. C. A. Gaeta R. C. Helgeson and D. J. Cram J. Am. Chem. SOC. 1980 102 2043. 13 D. J. Chadwick I.A. Cliffe I. 0. Sutherland and R. F. Newton J. Chem. SOC. Chem. Commun. 1981 992; D. J. Chadwick I. A. Cliffe I. 0. Sutherland and R. F. Newton J. Chem. SOC. Perkin Trans. 1 1529. 1984 1707. 14 A. B. Kyte Ph.D. Thesis (University of Liverpool 1985). 15 A. B. Kyte K. A. Owens I. 0. Sutherland and R. F. Newton to be published. 16 M. R. Johnson N. F. Jones I. 0. Sutherland and R. F. Newton J. Chem. SOC. Perkin Trans. 1 1985 1637 and earlier papers in the series ' Formation of Complexes between Aza Derivatives of Crown Ethers and Primary Alkylammonium Salts'. 17 M. R. Johnson I. 0. Sutherland and R. F. Newton J. Chem. SOC. Chem. Comrrzun. 1979 309; R. Mageswaran S. Mageswaran and I. 0. Sutherland J. Chem. SOC. Chem. Commun. 1979 722; I. 0. Sutherland Heterocycles 1984 21 235.18 N. F. Jones A. Kumar and I. 0. Sutherland J. Chem. SOC. Chem. Commun. 198 1 990. 19 G. Binsch Topics Stereochem. 1968 3 97; I. 0. Sutherland Annu. Rep. NMR Spectrosc. 1971 71; I. 0. Sutherland in Applications of NMR Spectroscopy to Problems in Stereochemistry and Conforma- tional Analysis ed. A. P. Marchand (VCH Publishers Florida to be published). 20 F. Kotzyba-Hibert J. M. Lehn and P. M. Vierling Tetrahedron Lett. 1980 941; J. P. Kintzinger F. Kotzyba-Hibert J. M. Lehn A. Pagelot and K. Saigo J. Chem. SOC. Chem. Commun. 1981 833; F. Kotzyba-Hibert J. M. Lehn and K. Saigo J. Am. Chem. SOC. 1981 103 4266. 21 A. Lumar R. K. Lewis S. Mageswaran and I. 0. Sutherland to be published. 22 J. C. Metcalfe J. F. Stoddart and G. Jones J. Am. Chem. SOC. 1979,99,8317; J. Krane and 0. Aune Acta Chem. Scand. Ser. B 1980 34 397; J. C. Metcalfe J. F. Stoddart G. Jones W. E. Hull A. Atkinson I. S. Kerr and D. J. Williams J. Chem. SOC. Chem. Commun. 1980 540. 23 M. J. Bovill D. J. Chadwick I. 0. Sutherland and D. %'atkin J. Chem. SOC. Perkin Trans. 2 1980 24 C. Pascard C. Riche M. Cesario F. Kotzyba-Hibert and J. M. Lehn J. Chem. SOC. Chem. Commun. 1982 357. 25 F. Vogtle H. Siger and W. M. Muller in Host-Guest Complex Chemistry I ed. F. Vogtle (Springer Berlin 1981) p. 143; B. Dietrich M. W. Hosseini J. M. Lehn and R. B. Sessions J. Am. Chem. SOC. 1981 103 1282; F. Peter M. Gross M. W. Hosseini J. M. Lehn and R. B. Sessions J. Chem. SOC. Chem. Commun. 1981 1067; M. W. Hosseini and J. M. Lehn J. Am. Chem. SOC. 1982,104 3525. 26 N. M. Richardson I. 0. Sutherland P. Camilleri and J. A. Page Tetrahedron Lett. 1985,26 3739. Paper 51 1887; Received 5th October 1985 39 FAR 1

ISSN:0300-9599    

DOI:10.1039/F19868201145

出版商:RSC    

年代:1986

数据来源: RSC