Detecting a transition-metal ammine at tailored surfaces Sayeedha Iqbal,a Felix J. B. Kremer,b Jon A. Preece,*a Helmut Ringsdorf,b Martin Steinbeck,b J. Fraser Stoddart,b Jie Shenc and Nigel D. Tinkerd aSchool of Chemistry, University of Birmingham, Edgbaston, Birmingham, UK B15 2T T bInstitut fu� r Organische Chemie, Johannes Gutenberg-Universita�t, Becher Weg 18–20, 55099Mainz, Germany cDepartment of Applied Physics, De Montford University, T he Gateway, L eicester, UK L E1 9BH dBNFL , Springfields Works, Salwick, Preston, L ancashire, UK PR4 0XJ The fabrication of surfaces by forming Langmuir films, which incorporate amphiphiles containing hydrophilic 18-crown-6 (18C6) derivatives, at a gas/water interface is described.These Langmuir films can be transferred to a hydrophobised quartz crystal microbalance (QCM), using the Langmuir–Blodgett technique. The QCM response has been measured in aqueous solution as a function of the concentration of the transition metal complex [Co(NH3)6]Cl3 which was injected into a vial in which the filmcoated QCM had been immersed.By comparing various surfaces covered with hydrophilic polyether and hydroxy functions and hydrophobic methyl groups, and by varying the composition of the films so as to increase the separation between the 18C6 macrocycles, it has been demonstrated that surfaces can be tailored that will enhance the binding of the [Co(NH3)6]3+ trications.In 1959, Sauerbrey1 showed that, when a quartz crystal comes ammine complexes depend significantly on the nature of the solvent molecules. In the times of Werner, these non-covalent into contact with a gas, the change in frequency DF of the quartz crystal, sandwiched between two excitation electrodes, bonding interactions were not well understood at a molecular level. However today, with the advent of various spectroscopic of natural resonant frequency F0 (s-1), density r (2.648 g cm-3), and shear modulus m (2.947×1011 dyn cm-2 techniques and X-ray crystallography, intermolecular interactions are becoming more fully understood.Indeed, new areas for AT-cut quartz) is related to the adsorbed mass Dm by the relationship: of science are emerging from the study of molecular interactions: they include crystal engineering,27 host–guest28 and supramolecular chemistry.29 They are all disciplines which rely DF=-2Fo2 A(rm) Dm 1/2 (1) upon the natural concepts of self-assembly30 and self-organiswhere A is the exposed surface area (m2) of the quartz.This pioneering activity led to the development of the quartz crystal microbalance (QCM). The QCM has proved to be a highly versatile instrument for the determination of the amount of material deposited from the gas phase2 on to a solid surface.Applications include thickness monitors in metal evaporation and deposition processes, and the detection of gas-phase analytes, such as hydrocarbons, water vapour and other volatile compounds. A more demanding, but potentially more important, area in which the QCMis being employed currently, is in liquid media,3 where it has been used to measure interfacial processes at electrode surfaces.4 It has also been employed as an immunosensor5 at the nanogram level to monitor antibodies, 6 bacterial growth,7 cells,8 proteins,9 and microbes,10 as well as to detect surfactants,11 anaesthetics,12 antibiotics,13 bitter and odorous substances,14 DNA hybridisation,15 pH changes,16 enzyme reactions,17 liposomes,18 chiral recognition, 19 intercalation,20 and even cell growth.21 Many of these experiments involve the molecular recognition of a substrate from the subphase to a biological receptor which has been deposited on the QCM surface.There are, however, very few examples in the literature where the QCM has been utilised to detect recognition events involving totally synthetic systems.22 The research reported in this paper relates to detecting molecular recognition events within a wholly synthetic system.It is known from many crystal structures23 that transitionmetal (ammine) complexes (TMCs) hydrogen bond24 via the hydrogen atoms of their ammine ligands to the oxygen atoms of crown ethers, e.g. 18-crown-6 (Fig. 1). This type of molecular Fig. 1 Second-sphere coordination in the solid state: (a) illustrating recognition is an example of second-sphere coordination,25 a the first- and second-sphere ligands, and (b) illustrating the supramol- concept first discussed by Alfred Werner26 in 1912 when he ecular 151 polymer formed between [Cu(NH3 )4(H2O)]2+ and 18C6 (hydrogen atoms are omitted for clarity) noted that the optical properties of chiral transition-metal J.Mater.Chem., 1997, 7(7), 1147–1154 1147ation31 to construct arrays of molecules held together by non- Systems I and III form poorly expanded monolayers with extremely low collapse pressures of approximately 32 and 28 covalent bonding interactions.32 Here, we report the chemical modification of the 18-crown- mNm-1, respectively (Fig. 3). Presumably, the poor stability of these films is a result of (i) the larger area requirement of 6 (18C6) macrocycle in order to facilitate its incorporation33 into Langmuir films which can be deposited on to a QCM by the polyether moieties relative to the alkyl chains (especially in the case of the crown ether lipid), (ii) the electronic repulsion the vertical dipping method, such that the hydrophilic head groups are exposed to the aqueous environment.An aqueous between electron lone pairs on the oxygen atoms, and (iii) the high solvation of the polyether moieties by the water molecules. solution of [Co(NH3)6]Cl3 can then be injected into the aqueous environment in which the LB film-coated QCM is Conversely, the isotherm of the octadecanol 3 monolayer is very stable, collapsing at just below 60 mN m-1, forming a immersed, and the frequency response can be measured as a function of the concentration of [Co(NH3)6]Cl3 to yield solid analogous phase.Thus, a compromise is required between the poorly expanded films of 1 and 2 and the well condensed information about the kinetics and thermodynamics of the recognition process.34 film of 3. This compromise was achieved by cospreading the two ether compounds 1 and 2 with octadecanol 3.Fig. 3 shows the isotherms of the monolayers formed from systems II (1 Results and Discussion with 3) and IV (2 with 3) in which one molar equivalent of octadecanol 3 is cospread with the ether amphipiles 1 and 2, General remarks respectively. The isotherms of systems II and IV are less The compounds, which were used in the present research, are expanded, relative to systems I and III, and these two compo- listed in Fig. 2. Compound 1 is a chemically modified 18C6 nent films are stable until around 45 mN m-1. It should be derivative in which one of the methylene hydrogen atoms has noted that, at pressures between 30 and 45 mN m-1, the been replaced by an oxymethylene octadecanoate chain to isotherms have a short phase change from a liquid analogous make it amphiphilic in nature.Compound 2 is a linear phase to a close-packed phase. Isotherms of cospread mixtures polyether analogue of 1 in which the polyether is bonded with molar ratios of the ether compounds to octadecanol 3 of covalently to the aliphatic chain by an ester linkage. 152 and 154 (and 158 in the case 1) were recorded and showed Compounds 3 and 4 are simply the commercially available the general trend that, as the amount of octadecanol was octadecanol and thiooctadecanol, respectively.Compound 5 is increased, the film became less expanded. This point can be the kinetically inert transition-metal ammine complex appreciated from inspection of data recorded in Table 1 where [Co(NH3)6]Cl3. Compounds 1–4 were chosen for several the area per molecule at 25 mN m-1 (the pressure at which reasons.Firstly, 1 and 2 were utilised to establish if a macro- the films were transferred to theQCM for all systems) decreases cyclic eect was operating in addition to purely non-specific as the proportion of octadecanol 3 increases. binding. The hydroxy compound 3 was employed to establish if the transition-metal ammine complex had any anity for a QCM studies hydrophilic surface.Conversely, the thiowas utilised to establish if the [Co(NH3 )6]Cl3 5 had an anity for a hydro- The monolayers at the gas/water interface transferred with good transfer ratios (0.85–0.95) on to hydrophobised quartz phobic surface. Additionally, the hydroxy amphiphile 3 was employed to increase the separation of the polyether head- supports by the vertical dipping mode into the aqueous subphase, thus achieving X-type deposition.groups in cospread monolayers incorporating 1 and 2. This incorporation of 3 into monolayers formed from ether contain- The QCM was covered with a monolayer of the various systems I–V by the same vertical dipping technique, after the ing compounds 1 and 2 allows control over the intermolecular separation between the ether moieties in these monolayers, QCM surface was hydrophobised with a polymer solution, leading to an X-type deposition which is illustrated in Fig. 4. something which is crucial for the transition-metal ammine complex to become inserted into the monolayer. Thus, various The QCM is held on to a Teflon case by a vacuum and remains immersed in a vial which contains 3 ml of water and systems were investigated in which the molar ratio of the octadecanol to the ether amphiphiles was increased systemati- a stirrer bar.Several 50 ml aliquots of a 0.1 mM aqueous [Co(NH3)6]Cl3 were injected into the vial and the change in cally. These systems are illustrated in Fig. 2. frequency of the QCM was monitored as each injection was performed.Monolayer formation The QCM responses for the 152 ratios for systems II and A few points are noteworthy about the Langmuir film forming IV, together with system V, the one component octadecanol 3 ability of the single and mixed component systems. Firstly, system, are shown as a function of time in Fig. 5. It can be consider the single component systems, systems I, III and V.observed that initially the QCM frequency drops rapidly after each injection and then reaches a plateau at equilibrium between the [Co(NH3)6]3+ trications being adsorbed and not Fig. 2 Lipid compounds and transition-metal ammine complexes used in this research, showing the orientation of the molecules when adsorbed on to the QCM. The tabulated information describes the Fig. 3 Isotherms of systems I–VI constitutions of systems I–VI. 1148 J. Mater. Chem., 1997, 7(7), 1147–1154Table 1 Thermodynamic data for systems II, IV, V, VI adsorbed on to the QCM upon injections of 50 ml of 0.1 mM aqueous [Co(NH3)6]Cl3 solution into 3 ml of water in which the QCM was immersed relative system binding (ratio) Aa/nm2 n×10-14b DFc/Hz indexd Ka/dm3 mol-1 e Ka/dm3 mol-1 f II (151)A 0.318 1.62 1234 20 373 297 II (151)B 0.318 1.62 1308 22 354 363 II (152)A 0.289 1.78 4399 66 121 131 II (152)B 0.289 1.78 4452 67 128 141 II (154)A 0.253 2.04 3508 46 150 163 II (154)B 0.253 2.04 3796 49 131 143 II (158)A 0.201 2.56 1765 18 395 448 II (158)B 0.201 2.56 1900 20 319 339 IV (151)A 0.299 1.71 2422 22 262 283 IV (151)B 0.299 1.71 2259 35 253 278 IV (152)A 0.281 1.82 1568 23 343 383 IV (152)B 0.281 1.82 1960 19 250 282 IV (154)A 0.254 2.02 1778 26 361 328 IV (154)B 0.254 2.02 2063 27 272 289 VA 0.189 2.73 1735 17 339 375 VB 0.189 2.73 2213 22 224 244 VI —g 2.73 102 1 87 97 aArea per molecule when film is transferred to the QCM.bNumber of amphiphiles transferred to the QCM; calculated from the area per molecule at 25 mN m-1 for the Langmuir films of systems II and VI and 50 mN m-1 for system V multiplied by the area of the gold electrode on to which the films were transferred (area=0.513 cm2).cChange in frequency at infinite [Co(NH3 )6]Cl3; calculated from the Lineweaver Burke plot of the reciprocal of concentration of TMC against the reciprocal of the change in frequency.dCalculated from normalising the change in frequency at infinite [Co(NH3)6]Cl3 concentration. Normalisation achieved by accounting for the fact that dierent numbers of amphiphiles were transferred to the QCM for the various systems. eAssociation constant (k1/k-1). fAssociation constant calculated from the quotient of the gradient and the value of the intercept on the y-axis of the straight line obtained from the Lineweaver Burke plots (LWB).gChemisorbed from solution. Fig. 4 Diagrammatic representation of the QCM experimental set-up with a magnification of a cartoon representation of the recognition event on the QCM to detect complexation on a receptor-derivatised QCM surface in contact with a solution containing complementary guest molecules adsorbed on to the surface.The three curves demonstrate that, the self-assembled monolayer of thioocatedecanol 4. The first point to note is that system V, which presents a purely after each successive injection of the [Co(NH3)6]Cl3 aqueous solution, (i) the change in frequency is less, (ii) the crown ether hydrophobic surface to the aqueous environment, has a very small response to the TMC as subsequent injections are made.containing film, system II, has a considerably greater response than its linear counterpart, namely system IV, and (iii) the Indeed, the same response was recorded when pure water was injected. Thus, the small changes in frequency are not a result one-component octadecanol 3 system, which presents a purely hydrophilic surface to the aqueous environment, has a similar of the TMC having an anity for this hydrophobic surface, but merely a result of the physical changes experienced by the response to system IV.This data is more clearly presented in a concentration QCM as the water level rises which, in turn, exerts a slightly greater pressure on the QCM.35 Contrast this result with the dependent titration curve for the QCM response.Fig. 6 shows the QCM response for system IV, the octadecanol 3 and the hydrophilic surfaces of systems IV and V. In these cases, the QCM response is much larger than for system VI, or when amphiphilic polyether 2, in the molar ratios 151, 152 and 154, together with system V, pure octadecanol 3, and system VI, pure water is injected into the vial containing the QCM with J.Mater. Chem., 1997, 7(7), 1147–1154 1149Fig. 7 QCM concentration-dependent titration curves for the hydrophilic system II upon injection of 50 ml aliquots of 0.1 mM aqueous [Co(NH3)6]Cl3, illustrating the significantly dierent responses for the various ratios of 153 and the significantly greater response for the Fig. 5 Comparison of the time-dependent titration curves for system II 152 ratio system II, relative to the hydrophilic surfaces of system IV (152) and system IV (152) as 50 ml aliquots of a 0.1 mM [Co(NH3)6]Cl3 and V are injected into the vial containing the QCM DFt , DFeq, and DFmax are the changes in frequency after 10 s from the first injection, at equilibrium for the first injection and at infinite concentration of the TMC, i.e.when all the surface recognition sites are filled, obtained from the y-intercept of the Lineweaver Burke plot of the reciprocal of concentration against the reciprocal of change in frequency.The results from Table 2 are depicted graphically in Fig. 8. The ratio of the molar equivalents of octadecanol 3 against the ether amphiphiles 1 and 2 is plotted on the x-axis and the rate of complexation (rate on, k1) plotted on the left hand side y-axis and the rate of decomplexation (rate o, k-1) plotted on the right hand side y-axis.First of all, consider the rates of complexation and decomplexation for the linear polyether lipid in system IV. Here, it is evident that there is very little variation of these two parameters, illustrating once again the very non-specific nature of the complexation event involving the surfaces containing Fig. 6 QCM concentration-dependent titration curves for the hydro- the linear polyether and octadecanol 3. However, when one philic systems IV and V, and the hydrophobic system V, upon considers the crown ether lipid containing films of system II, injection of 50 ml aliquots of 0.1 mM aqueous [Co(NH3)6]Cl3, it is evident that, at the 152 to 154 ratio of crown ether lipid illustrating the non-specificity of all the hydrophilic surfaces to octadecanol 3, maxima result for both the rate on (k1) and rate o (k-1).However, it is slightly surprising that, on closer inspection of Fig. 8, the dierence between the complexation these hydrophilic surfaces. However, for all of these hydrophilic surfaces, the responses are very similar.This result indicates and decomplexation rate is at its smallest at the 152 ratio of system II. This result means that the binding of the TMC is that the complexation event on the surfaces formed from system IV and V is not a result of a complementary molecular apparently weakest for this ratio, even although it has been established that the QCM response is largest for system II recognition event, i.e., it is a result of non-specific non-covalent bonding interactions.(152). This apparently anomalous behaviour will be discussed later in the paper. The non-discriminatory nature of the molecular recognition event between the [Co(NH3)6]3+ trications and systems IV The thermodynamic data37 for the binding events of these systems are shown in Table 1.This data is summarised graphi- and V, illustrated in Fig. 6, contrasts extremely well with the amphiphilic crown ether containing films of system II (Fig. 7). cally in Fig. 9 and 10. Fig. 9(a) illustrates the maximum38 QCM responses obtained from the Lineweaver Burke plots, Initially, when only one equivalent of octadecanol 3 is present in the film with the amphiphilic crown ether, an intermediate derived from the graphs illustrated in Fig. 6 and 7. It is clearly evident that, for the linear polyether lipid systems (system IV), QCM response is obtained between that of the purely hydrophobic surface of system VI and the non-specific hydrophilic the response is essentially independent of the composition of the film. This result is in contrast with system II, the crown surfaces of system IV and V.However, when two molar equivalents of octadecanol 3 are introduced into the film, the ether lipid containing films, where the largest response by the QCM is with the 152 molar ratio film. Fig. 9(b) represents a response of the QCM is much greater than those of the nonspecific hydrophilic surfaces of systems IV and V. When four normalisation of the data in Fig. 9(a), to compensate for the slightly greater number of amphiphilic molecules which are equivalents of octadecanol 3 are introduced into the films of system II, the response of the QCM is then intermediate transferred to the QCM at 25 mN m-1 as the molar proportion of the octadecanol increases in the film. Thus, one could expect between the non-specific hydrophilic surfaces and the film formed from 1.0 molar equivalent of 1 and 2.0 molar equival- greater QCM responses for films with more molecules per unit area.However, upon inspection, this expected increase in ents of 3. Finally, when 8.0 molar equivalents of octadecanol 3 are incorporated into the film with the crown ether amphiph- response is clearly not observed. System IV surfaces still have no distinct feature, and the maximum of system II is still at ile, the QCM response is very similar to the non-specific hydrophilic surfaces of systems IV and V.the 152 ratio. Thus, the 152 ratio film of system II complexes to more [Co(NH3)6]3+ trications than any of the other films The kinetic data for these systems is shown in Table 2, where 1150 J.Mater. Chem., 1997, 7(7), 1147–1154Table 2 Kinetic dataa for systems II, IV and V adsorbed on to the QCM upon injections of 50 ml of 0.1 mM aqueous [Co(NH3)6]Cl3 solution into 3 ml of water in which the QCM was immersed system DFt=10sb/s-1 DFeqc/s-1 DFmaxd/s-1 k1 k-1 II(151)A 222 474 1234 14.51 0.039 II(151)B 200 487 1308 13.17 0.037 II(152)A 679 738 4399 25.50 0.211 II(152)B 682 785 4452 21.43 0.167 II(154)A 626 702 3508 26.76 0.178 II(154)B 598 684 3796 22.36 0.169 II(158)A 319 700 1765 14.53 0.037 II(158)B 403 659 1900 19.57 0.061 IV(151)A 315 672 2259 11.22 0.044 IV(151)B 457 733 2422 17.58 0.067 IV(152)A 296 572 1568 15.94 0.046 IV(152)B 287 577 1960 12.17 0.049 IV(154)A 370 670 1778 18.09 0.050 IV(154)B 290 636 2063 11.05 0.040 VA 277 632 1735 12.58 0.037 VB 284 603 2213 10.44 0.046 aY.Ebara and Y. Okahata, J. Am. Chem. Soc., 1994, 116, 1209. bChange in frequency 10 s after the first injection of the TMC. cEquilibrium change in frequency after first injection of TMC. dMaximum change in frequency at infinite concentration of TMC extrapolated from the Lineweaver Burke plot of reciprocal of change in frequency against reciprocal of concentration of TMC.Fig. 8 Plot of the rate of complexation (rate on, &) and rate of decomplexation (rate o, $) as the amount of octadecanol increases in systems II (—) and IV (A) Fig. 10 Plot of the binding constants obtained both kinetically ($) and thermodynamically (&) for (a) system II and (b) system V as a function of the amount of octadecanol in the monolayer studied in this paper, while all system IV (and system V) surfaces complex only approximately one-third of the number of trications complexed by system II (152 ratio) and are independent of surface composition.Fig. 10 depicts the variation of the binding constants, calculated both kinetically (k1 /k-1 ) and thermodynamically (Lineweaver Burke plots of data in Fig. 6 and 7) of the films towards the TMC as a function of the ratio of the ether lipids 1 and 2 and octadecanol 3.These graphs illustrate the extremely good agreement between the kinetically established binding constants and the thermodynamically established ones. Again, it is evident that system IV and V surfaces, containing the linear polyether lipid, complex to the [Co(NH3)6]3+ trications independent of the surface composition, such that they all have a Ka of approximately 300 dm3 mol-1.In contrast, it is evident Fig. 9 Plots of (a) the maximum change in frequency at infinite once again that system II, containing the amphiphilic crown concentration of [Co(NH3)6]Cl3 (obtained from Lineweaver Burke ether 1 and octadecanol 3, has a minimum at the 152 ratio. plot), and (b) the normalised change in frequency on taking into account the dierent number of amphiphiles transferred to the QCM However, this minimum is actually illustrating that this surface J.Mater. Chem., 1997, 7(7), 1147–1154 1151has the lowest binding constant towards the [Co(NH3)6]3+ has the greatest QCM response, as a result of the 151 (trication5crown ether) stoichiometry, rather than the 15several with a value for Ka of approximately 125 dm3 mol-1.This behaviour is apparently anomalous, since it has already been (trication5polyether) stoichiometry for the system IV films. Furthermore, the low Ka value for system II (152) is a result established that this molar ratio of system II equates with the largest QCM response, i.e. it complexes to the largest amount of the fine balance between the correct steric fit of the TMC in the surface cavities and the weak second-sphere hydrogen- of the [Co(NH3)6]Cl3, as illustrated graphically in Fig. 7 and 9. In order to explain this anomalous behaviour, the following bonding interactions enabling the [Co(NH3)6]3+ trications to slip in and out of the surface cavities easily, such that the rate two models are proposed.Firstly, consider system IV, the linear polyether 2 and the octadecanol 3 containing films. o is not so dierent, relative, to the rate on, leading to a low Ka value for system II. These systems have a relatively large binding constant relative to system II (152) but bind substantially less [Co(NH3)6]Cl3. This behaviour can be explained by inspection of the model Conclusions in Fig. 11 where we considerthat each [Co(NH3)6]3+ trication, when it reaches the hydrophilic surfaces is ‘captured’ by several The observation in the solid state of the so-called secondsphere coordination between transition-metal ammine com- polyether arms. The net eect is that, in order to break free from the surface, several polyether arms have to unravel plexes and 18C6 ligands, has prompted the chemical modifi- cation of 18C6 to make it amphiphilic in nature, such that it simultaneously from the [Co(NH3)6]3+ trications.Thus, the rate of decomplexation is slow relative to the rate of com- could be incorporated into Langmuir–Blodgett films on solid supports. By utilising a QCM as the solid support, it has plexation, resulting in a relatively large binding constant, Ka.Now consider system II (the 152 ratio variation). Here, the proved possible to detect this second-sphere coordination upon introduction of the transition-metal ammine complex largest QCM response is observed (Fig. 9) compared with all the systems studied, yet this system has the lowest binding [Co(NH3)6]Cl3 into a solution in which the film-coated QCM was immersed, and has enabled the kinetic and thermodynamic constant (Fig. 10). Consider the model in Fig. 12. Here, the crown ether moiety is spaced out at just the correct distance characterisation of these very weak molecular recognition events, something which was not possible by other techniques. to allow the [Co(NH3)6]3+ trication to slip easily into the surface cavity created by two neighbouring crown ethers.It turned out that the complexation was critically dependent on the composition of the film. The evaluation of the kinetic Additionally, the spacing is such that each crown ether binds to two [Co(NH3)6]3+ trications such that the overall stoichi- and thermodynamic data has enabled a model of the surface recognition event to be formulated. This model highlights that, ometry of the film is 151 with respect to the trication and 18C6 head groups.This model then establishes why this film although these very weak NMH,O hydrogen-bonding interactions are competing with the competitive aqueous environment, the recognition still occurs. By tailoring the film, the 18C6 moieties are preorganised36 in the film such that they create many surface recognition sites which are stereoelectronically compatible with the [Co(NH3)6]3+. However, the binding of these trications is weak, relative to the linear polyether containing surfaces which bind many fewer trications but in a significantly stronger manner.The recognition in natural systems atinterfaces by hydrogenbonding interactions is of utmost importance, since it is responsible for immuneresponses,37 amongst other biologically important signals.It follows that the study of simpler synthetic systems38 is of considerable value in shedding light on the more complex biological recognition events as well as for the development of new sensors.4 Additionally, we have demonstrated that the QCM oers Fig. 11 Model of the complexation event between the system IV yet another valuable tool to the research worker who is surfaces and the [Co(NH3)6]3+ trications, illustrating that several studying unnatural supramolecular systems, where the recog- polyether arms complex with the [Co(NH3)6 ]3+, leading to a small nition event is between relatively small molecules (Mr#300 u), decomplexation rate, relative to the system II (152) model (depicted in Fig. 12) in contrast with the majority of studies carried out to date which have been concerned with the detection of naturally occurring macromolecules5–21 (Mr>30000 u). Experimental Solvents were dried using literature methods where necessary or used directly as obtained from the suppliers. Thin layer chromatography (TLC) was performed on aluminium sheets coated with Merck 5554 Kieselgel 60F. Developed plates were scrutinized in an iodine chamber.Column chromatography was performed using Kieselgel 60 (0.040–0.063 mm mesh, Merck 9385). Melting points were determined with an Electrothermal 9200 melting point apparatus and are uncorrected. Microanalyses were performed by the University of Birmingham and the University of Sheeld Microanalytical Services.Low-resolution mass spectra were obtained from a Fig. 12 Model of the complexation event between the system II Kratos Profile mass spectrometer, operating at 4 kV and using surfaces and the [Co(NH3 )6]3+ trications, illustrating the 151 molar 70 eV for electron impact mass spectrometry (EIMS). Proton ratio between the crown ether head groups and the [Co(NH3)6]3+, nuclear magnetic resonance (NMR) spectra were recorded on resulting in a high [Co(NH3)6]3+ uptake coupled with the small binding constant, relative to system IV (Fig. 11) Bruker AC 300 (300 MHz) spectrometer, using the deuteriated 1152 J. Mater. Chem., 1997, 7(7), 1147–1154solvents as the lock. 13C NMR spectra were recorded on the were removed in vacuo and the residue was taken up in CH2Cl2 (50 ml) and washed with aqueous Na2CO3 (50 ml) and H2O Bruker AC 300 (75 MHz) spectrometer.The isotherm measurements were all recorded on a self- (2×50 ml). The organic layer was then dried (MgSO4) and the solvents were removed to aord a clear liquid which was made trough with a Wilhelmy pressure pick up system. The spreading solutions consisted of CHCl3 and the lipids 1–3, and purified by silica gel column chromatography (eluent: CH2Cl2–MeOH) to aord 7 as a clear oil.Yield 9.6 g (42%); mixtures thereof, in the concentration range 0.5–1.0 mg ml-1, of which between 25 and 50 ml were spread from a syringe on 1H NMR (300 MHz, CDCl3) d 3.70–3.50 (14H, m, OCH2CH2O), 3.45 (3H, s, OCH3), 2.81–2.78 (2H, t, to an aqueous subphase or an aqueous subphase with the [Co(NH3)6]Cl3 dissolved in it.Each isotherm was carried out CH2OCH3); 13C NMR (75 MHz, CDCl3 ) d 72.5, 71.8, 70.5, 70.5, 70.5, 70.2, 70.2, 61.6, 58.9. EIMS: C9H20O5 requires m/z over a 20 min period. The [Co(NH3)6]Cl3 was of analytical quality as defined by the commercial supplier and was used 208 [M+]. Found: m/z 209 [M+H]+. without further purification. [Co(NH3)6]Cl3 subphases were prepared freshly each day from Milli Q water (resistivity ca.Lipid ether 2. A solution of monomethoxytetraethyleneglycol 7 (2.0 g, 9.6 mmol) and NEt3 (1.2 g, 11.5 mmol) dissolved in 18 V cm-1). The quartz crystal microbalance consisted of a quartz crystal (9 MHz, AT-cut, d=8 mm) covered with gold dry toluene (25 ml) was added dropwise to a stirred solution of octadecanoyl chloride (3.5 g, 11.5 mmol) in dry toluene electrodes (area=0.53 cm2) obtained from Quartzkeramik GmbH.The quartz crystal was mounted on a Teflon dipstick (10 ml) maintaining the temperature below 10°C. The reaction mixture was then allowed to warm to room temperature and (Fig. 4). In order to ensure no short circuits between the two gold electrodes, a silicon sealing ring was placed between the H2O (20 ml) was added.The solution was then concentrated in vacuo to give a waxy o-white solid which was dissolved Teflon holder and the crystal. The QCM was held in place by a vacuum on the non-covered face. The QCM was hydro- CH2Cl2 (50 ml) and washed with H2O (50 ml×2). The organic layer was dried (MgSO4), filtered, and the filtrate concentrated phobised with a ‘silicon solution’ obtained from Serva.The quartz crystal was driven by an in-house oscillator (15 V, in vacuo and purified by silica gel column chromatography (eluent: EtOAc–Me2CO, 352) to yield the acyclic polyether 2 100 mA), the oscillation shape controlled by a Hameg (HM604) oscilloscope. The frequency change was recorded with an as a white waxy solid. Yield 4.4 g (96%); 1H NMR (300 MHz, CDCl3) d 4.24–4.19 (2H, m, CH2CO2), 3.73–3.52 (14H, m, Iwatsu universal counter (SC7201).Transfer of the Langmuir films of systems I–IV to the QCM was achieved on a computer- OCH2CH2O), 3.46 (3H, s, OCH3), 2.32–2.28 (2H, t, CH2OCH3), 1.65–1.55 (2H, m, CH2CH2CO2 ), 1.33–1.20 (28, controlled trough from KSV Instruments (KSV5000) utilising a vertical dipping method at 20°C and a film pressure of 25 s, CH2CH2), 0.89–0.84 (3H, s, CH2CH3); 13C NMR (75 MHz, CDCl3) d 173.9, 72.6, 71.9, 70.6, 70.3, 69.2, 63.4, 61.7, 59.0, 34.2, mN m-1 for systems I–IV and 50 mN m-1 for system V, with a dipping speed of 2 mm min-1. The films were held at 25 or 31.9, 29.5, 29.4, 29.1, 24.9, 22.7, 14.1.EIMS: C27H54O6 requires m/z 474 [M+]. Found: m/z 475 [M+H]+. Anal. Calc.: C, 50 mN m-1 for 20 min before deposition to ensure they were stable and were compressed with a barrier rate of 5 mm min-1. 68.31; H, 11.46. Found: C, 68.38; H, 11.61%. System VI was chemisorbed on to the QCM clean gold surface from a solution (0.1 mM) of thiooctadecanol 4 in CHCl3 . The Financial support by the Royal Society (J.A.P.) and by BNFL (S.I.) is gratefully acknowledged. gold surface was cleaned with MeOH and CHCl3 .Synthesis References 2-Oxymethyl-18-crown-6-octadecanoate 1. A solution of 2- 1 G. Z. Sauerbrey, Phys., 1959, 155, 206. hydroxymethyl-18-crown-6 5 (0.50 g, 1.70 mmol) and NEt3 2 Applications of Piezoelectric Quartz Crystal Microbalances, ed. C. Lu, Elsevier, New York, 1984, vol. 7; J. F. Alder and (0.260 g, 2.60 mmol) was dissolved in dry toluene (25 ml) and J.J. McCallum, Analyst, 1983, 108, 1169; K. Bodenho�fer, it was added dropwise to a stirred solution of octadecanoyl A. Hierlemann, G. Noetzel, U.Weimar and W. Go�pel, Anal. Chem., chloride (0.64 g, 2.10 mmol) in dry toluene (10 ml) whilst 1996, 68, 2210. maintaining the temperature at 10°C. The reaction mixture 3 (a) R. Schumacher, Angew. Chem., Int. 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