J. Chem. SOC., Faraday Trans. I, 1988, 84(9), 2979-2986 Preparation and Electrochemical Behaviour of a Methylene Blue-modified Electrode based on a Nafion Polymer Film Ziling Lu and Shaojun Dong* Changchun Institute of Applied Chemistry, Academia Sinica, Changchun, Jilin 130021, People's Republic of China A methylene blue (MB) chemically modified electrode has been prepared by incorporating MB molecules into a Nafion polymer film on a glassy carbon surface. The electrochemical behaviour of the MB-modified polymer film electrode is discussed in detail. The electrode reaction of MB bound to the polymer film shows a reversible, two-electron transfer process with good stability and reproducibility. The equation of half-wave potential E; us. pH was deduced theoretically and was proved to be reasonable experimentally by the effect of solution pH on the MB-modified polymer-film electrode.The influence of supporting electrolytes on the electrode is discussed. The chemically modified electrode (CME) has been rapidly developed in recent years.l-'O Potential new materials for modification and preparation of stable polymer-film electrodes have been widely studied. Research on dye-modified electrodes has centred on the development of electrochromic materials,' spectroelectrochemistry * and applications such as electrocatalysis and electroanalysis. Here we report the preparation and electrochemical behaviour of an MB-modified electrode based on a Nafion polymer. The redox process and some effects of MB-modified polymer-film electrodes are discussed. Experiment a1 Materials A Nafion 117 (DuPont) membrane was used and dissolved according to the procedure of Martin et al.l0 to obtain the polymer solution (ca.4 mg cmP3). Methylene blue (MB) was an indicator reagent, purified via chromatography on silica using methanol as eluent, the purity was checked using t.1.c. (silica, MeOH :acetic acid 9 : 1). There was no difference in electrochemical behaviour of the MB-modified electrodes prepared using purified and unpurified MB. MB was dissolved in water to obtain MB solution. All other reagents were of analytical-reagent grade. Doubly distilled water was used. Apparatus A conventional single-compartment cell equipped with a platinum-wire counter- electrode and saturated potassium calomel reference electrode (SCE) was employed. The working electrode was a home-made Teflon-shrouded glassy carbon disc electrode (geometric area 0.13 cm2) polished to a mirror with MgO powder.Electrochemical measurements were made with model CV 47 voltammograph (BAS) and recorded on a home-made model LZ3-204 X- Y recorder. Chronoamperometric response was recorded by a home-made model MS- 1650B digital memoryscope. Unless otherwise mentioned, 0.09 mol dmP3 H,SO, was used as a supporting electrolyte solution. All potential values are given us. SCE. 2979 98-22980 MB-modified Polymer Electrode ( b ) , -0.2 0.0 0.2 0.4 0.6 -0.2 0.0 0.2 0.4 0.6 EIV EIV Fig. 1. Cyclic voltammograms of the MB-modified electrodes made by (a) a one-step method; (b) a two-step method: lo-’ cm3 Nafion solution coated glassy carbon electrode in the blank solution after immersing in rnol dmV3 MB solution.Supporting electrolyte 0.09 mol dm+ H’SO,, scan rate = 20 mV s-l. Results and Discussion Preparation of MB-modified Electrodes based on Nafion Polymer MB-modified electrodes can be prepared by a one-step method, i.e. lo-’ cm3 Nafion and (5-10) x lop3 cm3 MB solutions are mixed and added to the surface of glassy carbon electrode, then allowed to evaporate. The uniformly and tightly attached polymer coatings are obtained by the association between MB cations and the SO, sites of Nafion. The film electrode is rinsed several times with doubly distilled water to remove unbound MB molecules. This electrode can be used for electrochemical studies, and the cyclic voltammogram is shown in fig.1 (a). We also tried a two-step preparation: glassy carbon was initially coated with Nafion solution, evaporated and then immersed in MB solution to obtain an MB-bound Nafion polymer-film electrode. Unfortunately, MB-modified electrodes prepared by the latter method present an irreversible or asymmetric cyclic voltammogram with poorly defined shape, shown in fig. 1 (b). This is attributed to a non-homogeneous distribution of bound dye within the polymer. Thus we used the one-step method to prepare MB-modified polymer film electrodes as they exhibit well defined redox peaks over the MB concentration range from 5 x to 1 x lop2 mol dm-3. The peak current (i,) of the polymer-film electrode increases with increasing concentration of MB solution and gradually reaches a constant value, showing the saturation of the association of MB cations with SO, sites of Nafion polymer film.El remains constant and does not relate to the concentration of MB; the slope of log i, us”. log ZI is ca. 0.8. Electrochemical Behaviour of MEmodified Polymer-film Electrodes From the cyclic voltammogram of MB-modified electrodes in 0.09 mol dm-3 H2S0, supporting electrolyte solution [fig. 1 (a)] it can be seen that E; = 0.18 V, ip,/ipc. = 1 and i, increases with scan rate v, and AE, = 35 mV at v = 20 mV s, which slightly increases with increasing v, resulting from the IR drop of the polymer-film electrode. This suggests that the electrode reaction of bound MB is a reversible redox process. Let us consider the structure of the MB bound Nafion polymer-film electrode: there exist both hydrophobic and hydrophilic domains in Nafion.The -S03H groups of Nafion in the hydrophilic domain can dissociate to form -SO, anions in aqueous solution and these can associate with large MB cations by ion-exchange and bind MB from solution onto the Nafion polymer film (scheme 1).Z . Lu and S. Dong 298 1 GC hydrophobic domain + MB+ 9ByT Scheme 1. GC Table 1. Comparison of cyclic voltammetric data of MB- modified electrode with those for MB solution bound MB 0.200 0.165 0.18 35 1 0.8 free MB 0.215 0.185 0.20 30 1 0.5 a a = 3 log ip/a log u. Supporting electrolyte 0.09 mol dmP3 H,SO, solution. Scan rate u = 20 mV s-I. We consider that the bound MB should be situated in the hydrophilic domain of the Nafion polymer film and that the redox processes of bound MB are controlled by the charge transfer in the polymer film.When the MB-modified electrode comes into contact with aqueous solution the properties of the Nafion polymer film and the redox processes of the MB-modified electrode should be greatly affected by conditions such as pH and supporting electrolyte etc. From the experimental data in table 1, it can be seen that the redox reaction of the MB-modified electrode is in agreement with that of MB in solution. Based on the redox reaction of MB in aqueous solution" we can infer the overall reaction occurring at MB in the modified polymer-film electrode : (film) + 2H+ + 2e Me2N N Me, ox H Red Supposing that the redox reaction of MB-modified Nafion polymer-film electrode occurs in the hydrophilic domain similarly to that in bulk aqueous solution, we can deduce the half-wave potential equation as : Ef = E;' + (RT/2F) In [H+]&[Ox],/[Red], c",x = [OXHI, + [OX], = [OX],(' + [H+la/&) Cfed = CRedH21, + [RedHIf + [Red], = [Red],(] + [H+Ia/Kr, + [H+I3Kr1 Kr2) (2) (3) (4) where aq represents aqueous solution and f the polymer film2982 0.4 MB-modiJied Polymer Electrode - (a1 1 3 5 7 9 11 PH 0.2 - 3 f 0.0 - ;u" rA > \ - 0 .2 * 0 2 4 6 8 10 Fig. 2. pH effect for MB-modified Nafion polymer-film electrode (a) E; us. pH, (6) iPa us. pH. Britton-Robinson buffer was used. PH where KO, K,, and Kr2 represent the dissociation constants'l of the oxidised and reduced states, respectively. Therefore El = constant + (RT/2F) In ([H+]z + K,,[H+]: + K,, Kr2[H+]:)/(K0 + [€-I+],).(6) Because KO is very small, EI = constant + (RT/2F) In ([H+]: + K,,[H+]: + K,, K,,[H+],). (7)2. Lu and S. Dong 2983 Table 2. Effect of the composition of the electrolyte ~~~ LiCl -0.11 -0.23 120 -0.17 NaCl -0.11 -0.20 100 -0.15 KCI -0.12 -0.23 110 -0.18 CaCl, -0.08 -0.24 160 -0.16 KBr -0.10 -0.20 100 -0.15 KI -0.11 -0.23 120 -0.17 Na,SO, -0.15 -0.24 90 -0.195 KReO," 0.035 -0.025 60 0.005 a 0.02 mol dm-3, others 0.1 mol dmT3. Scan rate = 50 mV s-l. These electrolytes are electro- inactive in the region of interest. If the redox reac ion of bound MB conducts in the hydrophilic d main of the polymer film, we must prove the validity of eqn (7) by experiment. It can be seen from fig. 2 that the experimental results conform very well with the half-wave potential equation (where E is in mV).When pH < 5.6, [H+I3 $ Kr,[H+]2+Kr,K,2[H+] 4 = constant + (RT/2F) In [H+I3 = constant - 90 pH. thus When pH > 5.6, K,, Kr2[H+] 9 [H+I3 + Kr1[H+l2 thus The slopes of EL us. pH in fig. 2(a) are -90 and -30 mV per pH unit, respectively, at pH < 5.6 and p h > 5.6. The relationship between i, and pH is quite similar to that between E; and pH. The above results reveal that the nature of the hydrophilic domain is close to that of bulk aqueous solution and the supposition is reasonable that the redox reaction of MB- modified Nafion polymer-film electrode takes place in the hydrophilic domain. In fact, there are two more interactions for .the bound MB sites: one is an electrostatic interaction between cation-bound MB and the anionic SO, site of the Nafion polymer, in which the higher the charge is, the stronger is the interaction.The other is a hydrophobic interaction between the large organic MB molecule and the hydrophobic domain of the Nafion polymer. These interactions make bound MB different from free MB in aqueous solution. It can be seen from table 1 that E5 1 o f the bound MB is more negative than that of free MB in aqueous solution in the same cases. Furthermore, both interactions make the MB-modified electrode more stable. = constant - 30 pH. Effects of Supporting Electrolytes The movement of counter-ions from or to the polymer film must be accompanied by the redox process of the polymer-film electrode to maintain charge equilibrium in the film.Thus the electrochemical behaviour of the modified electrode will be affected by the nature of counter-ions and their concentrations in solution. Table 2 gives the cyclic voltammetric data of MB-modified Nafion polymer-film electrode in different electrolyte solutions. It can be seen that the voltammetric data (&, i, and AE,,) are quite similar for Li+, Na+, K+ and Ca2+ with the same anion in the electrolyte. ALE, is a little large for2984 MB-modified Polymer Electrode 1 J -0.4 - 0 . 2 0.0 0.2 - 0.2 0.0 0.2 0. L Et v Fig. 3. Cyclic voltammograms of the MB-modified electrode. Supporting electrolyte (a) 0.1 mol dmP3 NaCI, (b) 0.02 mol dm-2 KReO,. Scan rate = 50 mV s-l. Table 3. Effect of the concentration of the electrolyte 0.0 1 0.05 0.5 1 .o 5.0 0.005 0.05 0.1 0.5 1 .o 0.002 0.005 0.02 0.05 - 0.080 -0.105 -0.100 -0.105 - 0.035 -0.135 -0.145 -0.150 -0.160 -0.185 - 0.005 0.020 0.035 0.03 NaCl -0.175 90 -0.180 75 -0.175 75 -0.185 80 -0.125 90 Na,SO, -0.220 85 -0.235 90 -0.240 90 -0.240 80 -0.275 90 KReO, - 0.09 85 -0.065 85 -0.025 60 -0.03 60 -0.13 -0.14 -0.14 - 0.145 - 0.08 -0.178 -0.19 -0.195 -0.20 - 0.23 - 0.048 - 0.023 0.005 0.00 2.1 2.3 2.8 2.8 2.3 2.0 2.2 2.2 2.6 2.65 3.0 3.2 4.05 6.1 Scan rate = 50 mV s-l .Ca2+, possibly because of strong ion association between Ca2+ and the SO; groups of Nafion. For different anions, voltammetric data are also similar for halogen ions (Cl-, Br- and I-), but different for some anions such as SO:-, ClO, and ReO;. The redox peaks of the MB-modified electrode are not well defined in LiC10, solution.E; shifts negatively in Na2S0, solution. It is interesting to see that in KReO, solution, ip apparently increases, E; shifts positively and AEp decreases with respect to other electrolytes. Fig. 3 shows the cyclic voltammograms of MB-modified electrode in NaClZ. Lu and S. Dong 2985 80 60 9 40 2 20 0 1 2 3 4 5 r-3 1s-3 Fig. 4. Current us. t-i for the oxidation of the MB-modified electrode in 0.09 mol dm-3 H,SO, solution. The potential was stepped from -0.2 to 0.5 V. and KReO, solutions. The results suggest that when the redox process of the MB- modified electrode takes place, the counter-ions involved are anions not cations. Furthermore, we conclude that increasing the radius of the counter-ion (anion) makes E; more positive (e.g.ReOJ and that counter-ions with higher charge numbers like SO:- make Ei more negative. This may result from bound MB Nafion polymer-film electrode having a good selectivity for ReO,. Table 3 shows the effects of concentrations of supporting electrolytes on the voltammetry of MB-modified electrode. It can be seen that i, for the MB-modified electrode increases with increasing concentration of electrolyte. However, i, can also decrease when the concentration is too high (e.g. 5 mol dm-3 NaCl), similar to the case of polyvinyl ferrocene film electrodes.12 For NaCl Ei changes slightly with the concentration; for Na2S0, E; shifts slightly to more negative values with increasing concentration, but for KReO, as electrolyte, E; shifts apparently to more positive values and ip is much higher than it is in other electrolyte solutions (although the concentration of KReO, is much lower owing to its solubility).The voltammogram of the MB-modified electrode can be restored to its original form when the electrode is taken out of the ReO; solution and rinsed with doubly distilled water. Stability of the MB-modified Electrode 'The MB-modified electrode is stable in air or solution. No change appears after it is dried in air for several days. i, of the electrode remains constant after immersion in ReO, electrolyte for one day, and only a small decrease appears after two days. In 0.09 mol dm-3 H,SO, the cyclic voltammograms are the same during consecutive scans, but ip decreases slightly when the electrode is immersed for a longer time, indicating the possible dissociation of MB molecules from the polymer film in aqueous solution.The good stability makes the MB-modified electrode useful in applications such as electrochromism and analysis etc. Related work is in progress.2986 MB-modijied Polymer Electrode Chronoamperometric behaviour of MB-modified Nafion Polymer-film Electrode Chronoamperometry can be used to investigate the diffusion processes of electroactive materials bound in polymer-film electrodes. When a potential step from -0.2 to 0.5 V was applied to the Nafion-MB/GC electrode in 0.09 mol dmP3 H,SO, for complete reductionl to complete oxidation, the current us. time transient was recorded. The plot of i us. 1-5 (fig. 4) shows a linear r5gion with zero iptercept (Cottrell region).Thelslope of the linear region yielded CD;pp (= slope x nu/nFA) = 9 x mol cm-2 s-5 and Dapp = 8 x lo-’ cm2 s-l (the quantity of MB attached is estimated from the charge, Q, consumed in complete oxidation or reduction of the film; the film thickness was calculated using 1.58 g cm-3 as the wet Nafion 117 film densities of the 1100 eq.wt polymer13 based on the Cottrell equation and assuming an average concen- tration of the electroactive MB molecules distributed uniformly throughout the film of the layer (these may yield some error in calculating Dapp). The deviations from Cottrell equation which appear at longer times are due to thin-layer effects. References 1 S. Dong, Fenxi Huanxue (Analytical Chemistry), 1985, 13, 870. 2 H. S. White, J. Leddy and A. J. Bard, J. Am. Chem. Soc., 1982, 104, 481 1. 3 C. R. Martin, 1. Rubinstein and A. J. Bard, J. Am. Chem. Soc., 1982, 104, 4817. 4 D. A. Buttry and F. C. Anson, J, Am. Chem. Soc., 1982, 104, 4824. 5 A. E. Keifer and A. J. Bard, J. Phys. Chem., 1986, 96, 868. 6 F. F. Fan and A. J. Bard, J. Electrochem. Soc., 1986, 133, 301. 7 S. Dong and F. Li, J. Electroanal. Chem., 1986, 210, 31; 1987, 217, 49. 8 R. Memming, Prog. Surf. Sci., 1984, 17, 7. 9 E. Yeager, Electrochim. Acta, 1984, 29, 1527. 10 C. R. Martin, T. A. Rhoades and J. A. Ferguson, Anal. Chem., 1982, 54, 1639. 11 Edmund Bishop, Indicators (Pergamon Press, Oxford, 1972), pp. 503-504. 12 G. Inzelt and L. Szabo, Electrochim. Acta, 1986, 31, 1381. 13 C. R. Martin and K. A. Dollard, J. Electroanal. Chem., 1983, 159, 127. Paper 7/1156; Received 29th June, 1987