Langmuir 2007, 23, 9083-9091
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Diffuse Layer Properties of Thiol-Modified Gold Electrodes Probed by Direct Force Measurements Samuel Rentsch,† Hans Siegenthaler,‡ and Georg Papastavrou*,† Laboratory of Colloid and Surface Chemistry, Department of Inorganic, Analytical and Applied Chemistry, UniVersity of GeneVa, Switzerland, and Department of Chemistry and Biochemistry, UniVersity of Berne, Switzerland ReceiVed April 5, 2007. In Final Form: June 5, 2007 The diffuse layer properties of modified gold electrodes under potentiostatic control have been determined by direct force measurements. These measurements have been performed with a colloidal probe consisting of a silica particle attached to the cantilever of an atomic force microscope. The gold electrodes were modified by self-assembled monolayers (SAMs) of different thickness. Additionally, the terminating functional groups of the monolayer have been varied. The interaction force profiles have been fit to the full solutions of the nonlinear Poisson-Boltzmann equation. An accurate quantitative description of the force profiles has been obtained by taking charge regulation between the surfaces into account. The diffuse layer potentials obtained from these fits were studied in dependence of the potential applied to the gold electrode. The capacitance of the SAM and the potential of zero charge (pzc) have been determined for various SAMs of different thickness and surface termination. The values obtained by our direct force measurements are in agreement with the ones reported by classical electrochemical techniques. The capacitance of the SAM depends primarily on the thickness of the monolayer and its crystalline structure. Pronounced differences in the pzc for the different functional groups have been found. These changes are related to the dipole moment of the functional group terminating the SAM. Our data are in agreement with ion adsorption, but this effect seems to be less pronounced than for bare gold electrodes.
Introduction The surface modification of metal electrodes is of central importance for many applications of electrochemistry, such as corrosion protection, interfacing electrodes to biological samples, or tuning electrode properties for analytical purposes. The latter approach has received much attention in recent years since electrochemical detection schemes allow for high sensitivity, whereas the direct electronic response allows for miniaturization.1 The diffuse layer properties of such a modified electrode determine not only the transport of chemical species to the electrode but also their adsorption to the surface. The diffuse layer of an electrode depends not only on the applied potential but also on the nature of the surface modification. With increasing miniaturization of electrodes, for example, in microfluidic applications or combinatory lab-on-a-chip approaches, the possibility of a laterally resolved measurement of electrode properties by local probe techniques becomes increasingly important. Here we demonstrate how two electrode properties, the potential of zero charge (pzc) and the total capacitance can be measured by atomic force microscopy (AFM). This technique does not only allow for high lateral resolution but also provides a complementary approach to many classical electrochemical methods. A very versatile approach to achieve defined and tunable surface modifications of electrodes is based on self-assembled monolayers (SAMs) from thiols.2,3 Thiols adsorb readily on gold and other metals or metal oxides such as silver or mercury. The structure and properties of such thiol-SAMs have been determined by * To whom correspondence
[email protected]. † University of Geneva. ‡ University of Berne.
should
be
addressed.
E-mail:
(1) Drummond, T. G.; Hill, M. G.; Barton, J. K. Nat. Biotechnol. 2003, 21, 1192-1199. (2) Ulman, A. Chem. ReV. 1996, 96, 1533-1554. (3) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103-1169.
various analytical approaches such as ellipsometry, reflection IR spectroscopy, contact angle measurements, and electrochemical methods.4-6 Such SAMs are of crystalline order as long as the end group of the thiol is not significantly larger than the diameter of the alkyl chain and the chain is sufficiently long.7 The choice of the groups terminating the SAM toward the solution not only determines the surface properties in terms of reactivity and free interfacial energy2 but also influences the electrochemical response of the thereby modified electrode. The electrochemical behavior of SAM-modified electrodes can be distinguished between nonelectroactive and electroactive monolayers. Many electroactive SAMs enable electron-transfer reactions, which can be directly detected by suitable electrochemical techniques, for example, for SAMs terminating in hexacyanoferrate groups.6,8 Nonelectroactive SAMs are most often used to block the adsorption of species to the surface of an electrode, to study electron tunneling, or to protect the surface of the electrode for example from corrosion processes.6 With respect to this study, SAMs provide a highly defined model system to experimentally verify basic assumptions about the applicability of the fundamental relations in electrochemistry, such as the Gouy-ChapmanStern model.9 The dependence of the diffuse layer properties of SAM-coated electrodes on the applied potential have been studied by various (4) Sondaghuethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1995, 11, 22372241. (5) Sondaghuethorst, J. A. M.; Fokkink, L. G. J. J. Electroanal. Chem. 1994, 367, 49-57. (6) Finklea, H. O. Electrochemistry of organized monolayers of thiols and related molecules on electrodes. In Electroanalytical Chemistry.: A Series Of AdVances; Dekker: New York, 1996; Vol. 19, pp 109-335. (7) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (8) Alleman, K. S.; Weber, K.; Creager, S. E. J. Phys. Chem. 1996, 100, 17050-17058. (9) Hu, K.; Chai, Z.; Whitesell, J. K.; Bard, A. J. Langmuir 1999, 15, 33433347.
10.1021/la700987u CCC: $37.00 © 2007 American Chemical Society Published on Web 07/12/2007
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techniques. The total differential capacitance can be determined from the hysteresis in cyclic voltammetric experiments7,10,11 or by impedance spectroscopy.12,13 A complementary technique is the measurement of the wetting properties by the Wilhelmyplate technique.4,5,14 A rather novel approach to determine the diffuse layer of a solid interface is the measurement of interaction forces by AFM or the surface force apparatus (SFA).15-17 Such direct force measurements have been widely applied to the measurement of diffuse layer properties of oxide surfaces,18 SAMs,19,20 or polymer covered surfaces,21 and a number of comprehensive reviews for this technique are available.15-17 Nevertheless, until now, there have been much fewer studies on the diffuse layer properties of electrodes under potentiostatic control by AFM and SFA.9,22-30 Long-range interaction forces can be measured by AFM by the so-called colloidal probe technique. This technique consists of the attachment of a µm-sized colloidal particle to the end of an AFM cantilever.31,32 By this means, the interaction geometry is well defined in comparison with a normal AFM tip, and the Derjaguin approximation can be utilized.33 Generally charges originate from the dissociation of surface groups or specific ion adsorption at surfaces without applied potentials.33,34 Most direct force measurements on potential-controlled surfaces are restricted to bare electrodes, in particular those concentrating on the longrange interaction forces.22-24,28,29 Very few organic electrode materials or electrode modifications have been investigated by this technique.26,35 The potential-dependent adsorption of pyridine on gold surfaces36 as well as an electro-addressable SAM as a function of the applied potential have been examined.9 Nonelectroactive monolayers of alkanethiols have been examined by Kwon and Gewirth,30 primarily concentrating on the influence of the potential-induced desorption on the interaction forces, in particular at high ionic strengths.
The diffuse layer properties of an electrode under potentiostatic control are most often probed by direct force measurements in an asymmetric system, where the probe is a silica particle and the sample is a flat electrode.22-24,28,29 In this case, it is necessary to first determine the properties of the probe surface in order to extract the surface properties of the sample from the force versus distance profiles. The colloidal probe technique is not restricted to the sphere-plane geometry but can also be used to probe the interaction between two colloidal particles.37,38 Such an approach is especially advantageous since the surface properties can be determined in a completely symmetric system. In this study, we determine the diffuse layer properties of SAM-modified gold electrodes by direct force measurements with the colloidal probe technique. The measurement and quantitative analysis of the diffuse layer of such nonelectroactive SAM-modified gold electrodes at low ionic strengths in an asymmetric system has not been previously reported to the best of our knowledge, whereas the adhesion behavior at high ionic strengths30 or the long range forces for electroactive monolayers9 have been reported previously in detail. We concentrate here on two different parameters: the thickness of the SAM and the influence of the functional groups terminating the SAM. We restrict this study to surface terminations with nonionizable functional groups (-OH, -CH3, and -CF3). Direct force measurements provide a complementary approach to classical electrochemical methods, and in the past large differences between the electrochemical charge and the effective surface charge as obtained by direct force measurements have been reported.9,27 These differences have been attributed to ion adsorption. In particular, we want to address the question if such discrepancies can also be found for nonionizable electrodes coated by an organic monolayer.
(10) Miller, C.; Cuendet, P.; Gratzel, M. J. Phys. Chem. 1991, 95, 877-886. (11) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1993, 97, 6233-6239. (12) Finklea, H. O.; Snider, D. A.; Fedyk, J.; Sabatani, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660-3667. (13) Fawcett, W. R.; Fedurco, M.; Kovacova, Z. Langmuir 1994, 10, 24032408. (14) Iwami, Y.; Hobara, D.; Yamamoto, M.; Kakiuchi, T. J. Electroanal. Chem. 2004, 564, 77-83. (15) Senden, T. J. Curr. Opin. Colloid Interface Sci. 2001, 6, 95-101. (16) Butt, H. J. Analyzing electric double layers with the atomic force microscope. In Encyclopedia of Electrochemistry; Bard, A. J., Stratmann, M., Eds.; Wiley-VCH: Weinheim, Germany, 2003; Vol. 1, pp 225-252. (17) Butt, H. J.; Cappella, B.; Kappl, M. Surf. Sci. Rep. 2005, 59, 1-152. (18) Hartley, P. G.; Larson, I.; Scales, P. J. Langmuir 1997, 13, 2207-2214. (19) Kane, V.; Mulvaney, P. Langmuir 1998, 14, 3303-3311. (20) Warszynski, P.; Papastavrou, G.; Wantke, K. D.; Mohwald, H. Colloid Surf. A 2003, 214, 61-75. (21) Hartley, P. G.; Scales, P. J. Langmuir 1998, 14, 6948-6955. (22) Hieda, H.; Ishino, T.; Tanaka, K.; Gemma, N. Jpn. J. Appl. Phys. I 1995, 34, 595-599. (23) Hillier, A. C.; Kim, S.; Bard, A. J. J. Phys. Chem. 1996, 100, 1880818817. (24) Raiteri, R.; Grattarola, M.; Butt, H. J. J. Phys. Chem. 1996, 100, 1670016705. (25) Hu, K.; Fan, F. R. F.; Bard, A. J.; Hillier, A. C. J. Phys. Chem. B. 1997, 101, 8298-8303. (26) Doppenschmidt, A.; Butt, H. J. Colloid Surf. A. 1999, 149, 145-150. (27) Wang, J.; Bard, A. J. J. Phys. Chem. B. 2001, 105, 5217-5222. (28) Frechette, J.; Vanderlick, T. K. Langmuir 2001, 17, 7620-7627. (29) Barten, D.; Kleijn, J. M.; Duval, J.; van Leeuwen, H. P.; Lyklema, J.; Cohen, Stuart, M. A. Langmuir 2003, 19, 1133-1139. (30) Kwon, H. C.; Gewirth, A. A. J. Phys. Chem. B. 2005, 109, 10213-10222. (31) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239241. (32) Butt, H. J. Biophys. J. 1991, 60, 1438-1444. (33) Israelachvili, J. Intermolecular and Surface Forces; Academic Press: London, 1992. (34) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980. (35) Wang, J.; Feldberg, S. W.; Bard, A. J. J. Phys. Chem. B 2002, 106, 10440-10446. (36) Frechette, J.; Vanderlick, T. K. J. Phys. Chem. B 2005, 109, 4007-4013.
Materials. 1-Dodecanethiol, 1-hexadecanethiol, 6-mercapto-1hexanol, and 11-mercapto-1-undecanol were purchased from SigmaAldrich and were used without further purification. 16-Mercapto1-hexadecanol was purchased from Frontier Scientific Inc., and 1H,1H,2H,2H-perfluorodecanethiol was purchased from ABCR. Both thiols were used as received. Solutions of these thiols were prepared from ethanol of analytical grade (Sigma-Aldrich), with the exception of the solution of the perfluorodecanethiol, which has been prepared with heptane of analytical grade. All aqueous solutions were prepared from deionized water of Milli-Q grade (Millipore). The pH and ionic strength of the aqueous solutions were adjusted by addition of HCl and KCl, respectively. Prior to pH adjusting, the 0.1 mM KCl solution was degassed with nitrogen for at least 30 min. Sample Preparation. The modified gold electrodes were prepared from commercial gold covered glass slides (Arrandee, Schro¨er Metallhandel), which were cleaned in hot Piranha solution (mixture of concentrated sulfuric acid and hydrogen peroxide 30% in a ratio of 4:1 v/v, Caution: piranha solutions are very aggressive, corrosive solutions, and appropriate safety precautions should be utilized including the use of acid resistant gloves and adequate shielding) for 20 min and rinsed with copious amounts of water. Before first use, these gold electrodes were flame annealed with a butane gas flame. Flame annealing was carried out carefully and in short intervals to prevent breakthrough of chromium through the gold layer. Since in the subsequent preparation a thiol layer is adsorbed to the gold surface, eventual traces of chromium are not in contact with the solution and have no influence on the electrochemical behavior of the modified electrode. Thiol adsorption was carried out
Experimental Section
(37) Li, Y. Q.; Tao, N. J.; Pan, J.; Garcia, A. A.; Lindsay, S. M. Langmuir 1993, 9, 637-641. (38) Rentsch, S.; Pericet-Camara, R.; Papastavrou, G.; Borkovec, M. Phys. Chem. Chem. Phys. 2006, 8, 2531-2538.
Thiol-Modified Gold Electrodes from 1 mM ethanolic solutions for at least 12 h. In the case of the fluorothiol, the adsorption was carried out from a 10 mM solution in heptane for at least 3 days. Directly before the measurement the SAM-modified electrodes were removed from the thiol solution and rinsed subsequently with ethanol and MilliQ water before being placed in the fluid cell. Colloidal probes were prepared from tipless AFM cantilevers (CSC12, µ-masch, Lithuania) with a nominal force constant of 0.03 N/m. Colloidal silica particles with 6.8 µm diameter (Bangs laboratories) were attached to these cantilevers with UV-curable glue (Optical Adhesive 63, Norland Products) under an optical microscope by means of a micromanipulator. Colloidal particles of the same batch as used for the preparation of the colloidal probes were attached by an analogous procedure to glass slides in order to allow for the measurement of interaction forces in the sphere-sphere geometry. The glass slides (MenzelGla¨ser, Germany) were cleaned before particle attachment by the RCA method.39 Colloidal probes and substrates with attached particles were cleaned directly prior to measurement for 90 s in air plasma at 6.8 W (PDC 32G, Harrick Scientific, NY). AFM Imaging. The surface topography of the gold electrodes has been determined by tapping mode AFM (Multimode Nanoscope IIIa, Veeco, CA) with standard tapping cantilevers (OMCLAC160TS, Olympus). The cantilevers were pre-selected for a tip radius smaller than 15 nm by measuring a calibration sample (Aurora Nanodevices). The surface roughness of the thiol-covered gold electrodes and of the colloidal silica particles was obtained from scans with a size of 1 × 1 µm at different positions of the sample or different particles, respectively. In general, flat terraces of low roughness dominate the topography of the flame annealed gold electrodes, which is different than the hemisphere shaped surface topography of untreated gold substrates. A root-mean-square (rms) roughness of 4 nm was calculated after a second-order flattening algorithm was applied to the raw image data. The surface roughness of the colloidal silica particles was found to be on the order of 1.8 nm (rms roughness).38 Electrochemical Characterization. The thiol-modified gold electrode was potentiostatically controlled by a home-built potentiostat based on a design previously used for electrochemical tunneling microscopy.40 For the here-presented electrochemical measurements in combination with the AFM, an open fluid cell has been constructed (cf. Figure 1). The gold-coated sample, which acts as working electrode, is located in the center of the fluid cell and a bare gold wire of 100 mm length is arranged in a circular manner around this electrode and acts as a counter electrode. The reference electrode is a pseudo Ag/AgCl electrode consisting of silver chloride electrodeposited on a silver wire, which is mounted directly inside the liquid cell. The dissolution of silver ions from this electrode leads to concentrations smaller than 2 × 10-6 M in the solution and is considered to have negligible influence on the measurements. The potential difference between this pseudo Ag/AgCl electrode and a commercial Ag/AgCl (3 M KCl) electrode (Metrohm, Switzerland) was +130 mV under identical solution conditions (pH 4.7 and 0.1 mM KCl) as used in the force experiments. All potentials applied to the modified gold electrode are given versus a standard calomel electrode (SCE) to facilitate comparison with previous literature in this field. The performance of the electrochemical AFM cell and the potentiostat was verified by acquiring cyclic voltammograms of hexacyanoferrate solutions. The state of the SAM-modified electrode was controlled directly before the measurements by acquiring a number of cyclic voltammograms in KCl electrolyte solution under identical conditions as for the measurement of the interaction forces. The potential range applicable without thiol desorption has been determined in an independent set of measurements based on cyclic voltammetry. Thiol desorption is detectable by a sudden increase in current. Within the potential range without thiol desorption, the cyclic voltammograms have been repeated (>10) without detectable change. (39) Kern, W. J. Electrochem. Soc. 1990, 137, 1887-1892. (40) Ammann, E.; Beuret, C.; Indermuhle, P. F.; Kotz, R.; de Rooij, N. F.; Siegenthaler, H. Electrochim. Acta 2001, 47, 327-334.
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Figure 1. Schematic representation of the setup with which the diffuse layer properties of modified gold electrodes were probed by AFM. The electrode modification is obtained by self-assembled monolayers (SAMs) of different thickness h and functional groups (-OH, CH3). The potential applied to the gold electrode is controlled by a potentiostat and maintained in the capacitive regime. Force Measurements. The interaction forces were measured on an AFM equipped with closed loop control for all three axes (MFP3D, Asylum Research, CA) and a homemade electrochemical cell. Immediately before the force measurements, the fluid cell was extensively rinsed with MilliQ water and the gold counter-electrode was carefully annealed in a butane gas flame. For each applied potential, at least 80 approach and retraction cycles with an approximate velocity of 0.8 µm/s were acquired. The force profiles of the series did not show significant differences between the first and the last curve. The figures as well as the fits are based always on averages over the whole series. The measured deflection versus piezo displacement curves were converted to force versus distance profiles by standard procedures.15,17 The force constants were determined individually for each cantilever by the added mass method.41 The separation distance D was obtained from a linear fit of the constant compliance region, and has an accuracy of about 0.5-1.0 nm for the colloidal probe/sample combinations examined. Interaction forces were determined by averaging at least 40 approach and retraction cycles. The force curves were acquired with a frequency of 0.3 Hz, corresponding to an approach velocity of ca. 0.8 µm s-1. The averaged normalized force profiles F/Reff are compared to the solution of the Poisson-Boltzmann equation for two infinite symmetric plates by means of a recently presented algorithm.42 Further details concerning the quantitative analysis of the interaction force profiles can be found elsewhere.43 The measurement of the interaction forces between two silica particles in the sphere-sphere geometry has been performed in open Petri dishes under the same conditions (0.1 mM KCl, pH 4.7) as the measurements of the SAM-modified electrodes. The two particles, one attached to the probe and one attached to the substrate, were first coarsely aligned by optical microscopy; the following fine alignment was achieved by a procedure similar to force volume plots. Further details are given elsewhere.38 Interaction forces were determined by averaging at least 80 approach and retraction cycles, which have been acquired with a frequency of 0.3 Hz. The data analysis was performed analogously to the one for the modified electrodes. (41) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. ReV. Sci. Instrum. 1993, 64, 403-405. (42) Pericet-Camara, R.; Papastavrou, G.; Behrens, S. H.; Borkovec, M. J. Phys. Chem. B 2004, 108, 19467-19475. (43) Pericet-Camara, R.; Papastavrou, G.; Behrens, S. H.; Helm, C. A.; Borkovec, M. J. Colloid Interface Sci. 2006, 296, 496-506.
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extended layer of silicidic acid, which leads to additional steric forces dominating the interaction at short separations.44 The interaction forces between the two silica particles have been fitted to the solutions of the nonlinear Poisson-Boltzmann equation for two identical surfaces. Besides the classical boundary conditions of constant potential (CP) and constant charge (CC), charge regulation between the surfaces has also been taken into account by the so-called constant regulation (CR) approximation.42 The fits are based on separation distances larger than half of the nominal Debye length, which corresponds to separation distances larger than approximately 15 nm. At these distances, contributions to the interaction forces by van der Waals forces or steric forces do not affect the diffuse layer potentials obtained from the fits.43 The Debye length is defined as Figure 2. Long range interaction forces upon approach between two silica spheres at pH 4.7 and 0.1 mM KCl. The inset in semilogarithmic representation demonstrates the exponential decay at large separation distances. The solid and dashed curves are fits to the nonlinear Poisson-Boltzmann equation for a symmetric system applying various boundary conditions. The dashed lines indicate fits to the classical boundary conditions of constant charge (CC) and constant potential (CP), whereas the solid line indicates a fit to the constant regulation approximation (CR), which takes into account charge regulation.
Results and Discussion The diffuse layer properties of the gold electrodes modified by a self-assembled monolayer (SAM) were studied with the setup depicted in Figure 1. The interaction forces between the silica particle mounted to an AFM cantilever and the electrode were measured as a function of the separation distance. The resulting force versus distance profiles were fitted to the solutions of the nonlinear Poisson-Boltzmann equation in order to determine the diffuse layer properties of the SAM-modified gold electrode. The ionic strength of the solution was always 0.1 mM KCl and pH 4.7. The data reported here are restricted to SAMs with nonionizable surface groups. We studied SAMs with three different types of functional groups: hydroxyl (-OH), methyl (-CH3), and fluoromethyl (-CF3) groups. In addition, for the former OH- and CH3-terminated SAMs, the thickness of the monolayer was varied by thiols of different length. Determining the Surface Properties of the Colloidal Probe. In order to allow for a quantitative determination of the interaction forces, the surface properties of the colloidal silica particle were first determined. By measuring the forces between two colloidal particles in the sphere-sphere geometry the interaction between identical surfaces can be probed. These measurements were also performed under exactly the same conditions in terms of pH and added salt against a flat SAM-modified electrode; the results are presented further down. Figure 2 shows the interaction force between two laterally aligned silica spheres as a function of their separation distance at pH 4.7 and 0.1 mM KCl. The interaction force profile is repulsive over the full separation range. The repulsive forces at large separation distances are due to the overlap of the two diffuse layers originating from the charged silica surfaces. The characteristic exponential dependence of the interaction force on the separation distance is illustrated by the semilogarithmic representation of the same force profile in the inset of Figure 2. An attractive force at small separation distances due to van-der Waals forces is absent for the silica-silica interaction. This finding is in accordance with previously reported direct force measurements between silica surfaces and is attributed to the presence of an
κ-1 )
x
0kT
2NAe2I
(1)
where 0 is the total permittivity of water, kT is the thermal energy, NA is Avogadro’s number, e is the elementary charge, and I is the ionic strength of the solution in mol/L. The decay length of the force profile in Figure 2 is approximately 25 nm, which is close to the theoretical value for the Debye-length of ∼27 nm expected for a solution of 0.1 mM KCl at pH 4.7. The small difference can be attributed to dissolved CO2 in the open fluid cell. We obtain an average diffuse layer potential of approximately -55 mV for the silica surface for various sphere-sphere combinations. This potential is to a large extent independent of the boundary condition chosen for the fit. This potential is in good accordance with previously reported values obtained at similar conditions18,45 but is lower than the theoretical value calculated on base of the surface chemistry of silica.42 The same discrepancy is also observed for electrophoretic mobilities of silica particles.46 The sign of the potential cannot be determined directly from the force measurements alone, but the negative charge of silica at this pH has been reported by various other techniques.18 In addition to the diffuse layer potential, the regulation parameter p of the constant regulation approximation (CR) can be determined from the fits. This parameter summarizes the charge regulation between the two surfaces upon approach and replaces the microscopic description of the surface chemistry on the basis of the number of surface groups, their pK, and the Stern layer capacitance.42 The regulation parameter p is defined as
p)
CD CI + CD
(2)
where CD is the diffuse layer capacity and CI is the inner layer capacity.42 The diffuse layer capacitance CD is given by34
CD ) 0κ cosh(2eψD/kT)
(3)
where ψD is the diffuse layer potential of the surfaces at infinite separation.42 Charge regulation becomes increasingly important at reduced separation distances between the surfaces. At separation distances below approximately one Debye length, neither the constant charge nor the constant potential boundary conditions provide an adequate description of the interaction forces. Instead, excellent (44) Vigil, G.; Xu, Z. H.; Steinberg, S.; Israelachvili, J. J. Colloid Interface Sci. 1994, 165, 367-385. (45) Considine, R. F.; Drummond, C. J. Langmuir 2001, 17, 7777-7783. (46) Kobayashi, M.; Skarba, M.; Galletto, P.; Cakara, D.; Borkovec, M. J. Colloid Interface Sci. 2005, 292, 139-147.
Thiol-Modified Gold Electrodes
Figure 3. Interaction force profiles upon approach between a colloidal probe (silica particle) and a gold electrode modified by (a) 11-mercapto-1-undecanol and (b) 1-hexadecanethiol. The interaction forces have been measured at 0.1 mM KCl and pH 4.7. For each electrode, two force profiles, which were acquired at different potentials applied to the gold electrode, are shown. The lines indicate fits to different boundary conditions (CC, constant charge, CP constant potential, CR constant regulation approximation for both surfaces, and CR0, constant regulation for the silica surface and constant potential for the modified electrode).
agreement between the constant regulation approximation and the experimental data has been found for regulation parameters in the range of p ) 0.6-0.8. The upper limit for p is in good agreement to the one calculated by the basic Stern model with commonly accepted parameters for the silica surface and the comparable solution conditions.42 We use this value for the regulation parameter in the following analysis of the diffuse layer properties of SAM-modified electrodes. Diffuse Layer of a Modified Gold Electrode Probed by a Colloidal Silica Probe. Figure 3 shows some typical force profiles between modified gold electrodes and a silica particle as colloidal probe. The data in Figure 3a originate from measurements versus a gold electrode coated by 11-mercapto-1-undecanol and the ones in Figure 3b from an electrode coated by 1-hexadecanethiol. During the acquisition of each force profile, a potentiostat (cf. Figure 1) maintains the potential applied to the SAM-modified gold electrodes constant. For each applied potential, a series of approximately 80 force curves was recorded while the same potential is constantly applied to the electrode during the series. Figure 3, panels a and b, represents force profiles at different potentials applied to the gold electrode. The set of force profiles for each surface modification were obtained with the same colloidal probe and in the same electrolyte solution, thus differences in the force profile have to be attributed solely to the
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potentials applied to the electrodes. The potentials applied during the force measurements fall in the previously determined potential range where no thiol desorption occurs. In the following paragraphs, we will discuss the force profiles of the asymmetric system consisting of the gold electrode under potentiostatic control and the silica particle. In particular, we will examine the extent to which the charge regulation between the surfaces must be taken into account in order to describe the measured force profiles. For the hydroxyl-terminated electrode in Figure 3a, we find a highly repulsive interaction at applied potentials of -0.22 V. Conversely at potentials of +0.28 V, the forces are attractive over the full range of separation distances and lead to an instability of the cantilever at approximately 30 nm. At this point, the cantilever jumps to the surface. The general character of the interaction profile is an indication of the change of sign of the surface charge of the electrode due to the potential applied. In the case of the methyl-terminated surface the interaction is only initially repulsive at a potential of -0.52 V but switches into an attractive interaction at smaller separation distances. Indeed, for applied potentials of -0.12 V, the interaction is attractive over the whole interaction range for the same modified electrode. Nevertheless, the transition between repulsive and attractive interaction occurs at different applied potentials for the different thiols, which indicates the influence of the SAM on the diffuse layer properties of the electrode. The force profiles can be quantitatively evaluated by fits to the solutions of the nonlinear Poisson-Boltzmann equation. For the asymmetric system of silica particle and electrode the parameters for the former are fixed to the previously determined values of diffuse layer potential and regulation parameter. The coincidence of the decay length with the value expected on the basis of the Debye-Hu¨ckel approximation has been verified by fits at large separation distances and is found to agree with an ionic strength of 0.15 mM in most cases. This ionic strength is treated as constant throughout the fitting procedure for a series of force curves. Therefore the only free parameters in the fits are the diffuse layer potential and the regulation parameter of the electrode surface. The interaction forces were only fit down to separation distances of 15 nm or to the onset of cantilever instability. By this means, the influence of van der Waals interactions on the fitting process is reduced. Let us start first with the simplest case of identical, classical boundary conditions for both surfaces, thus both electrode and colloidal probe are either exclusively CC or CP. In this case, only the diffuse layer potential of the modified electrode enters as free parameter in the fits. The broken lines in Figure 3 indicate the corresponding solutions of the nonlinear Poisson-Boltzmann equation for these boundary conditions. Large deviations from the experimental data are already obvious at intermediate distances for both boundary conditions and the general shape of the interaction cannot be reproduced in many cases (cf. Figure 3b, φ ) -0.52 V). Despite their widespread use for asymmetric systems, the boundary conditions of CC or CP applied to both surfaces are failing to describe the interaction force profiles, in particular at low applied potentials. Let us consider the case that the electrode, which is connected to a potentiostat, remains at a constant potential (p ) 0) and only the silica particle charge is regulated upon approach. In the framework of the constant regulation approximation, this corresponds to the case pSi ) 0.8 and pel ) 0. These boundary conditions lead to much better results (cf. thin solid traces marked by CR0) and, in particular, provide a satisfactory accordance with the experimental data at sufficiently large potentials. Nevertheless, in many cases, the
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Figure 4. Interaction force profiles upon approach between a colloidal probe and a gold electrode modified by 11-mercapto-1undecanol. By variation of the applied potential the interaction forces can be tuned from repulsive to attractive. The solid lines indicate fits to the constant regulation approximation and provide the diffuse layer potential and the regulation parameter for the modified electrode as a function of the applied potential.
introduction of charge regulation for the electrode surface also provides a better accordance with the experimental data, in particular at low applied potentials and for OH-terminated SAMs. Charge regulation represents an important contribution to the overall interaction forces in this asymmetric combination of surfaces, as predicted previously on theoretical grounds.42 The regulation parameter pel for the electrode is found to be near that of constant potential (CP) but not necessarily identical to CP, ranging from 0 to 0.6. The regulation parameter pel obtained from the fits depends in particular on pSi, which has been determined previously in an independent set of measurements. Thus small, inevitable variations in the preparation of the silica might lead to noticeable differences for the fitted regulation parameter of the electrode. The choice of the boundary conditions does not only influence the shape of the interaction profile but also, to a certain extent, the diffuse layer potentials obtained from the fits. In the case of 11-mercapto-1-undecanol, we find a diffuse layer potential of the electrode of ψD ) -47 mV instead of ψD ) -36 mV for boundary conditions of constant potential for the electrode and for charge regulation, respectively. In the following fitting process, the regulation parameter for the electrode has been introduced into the fits in the interval from p ) 0 (i.e., CP) to p ) 1 (i.e., CC) in order to provide the best correspondence to the experimental data. A detailed discussion of how such a regulation parameter for the electrode can be interpreted is given at the end of the paper. Figure 4 summarizes a typical series of measurements for an electrode modified by 11-mercapto-1-undecanol. The force profiles were acquired at various different potentials applied to the electrode. Starting from an applied potential of φ ) -0.22 V, the potential was varied over a range of 0.7 V. It should be mentioned that the potential range applicable to a modified electrode is limited by thiol desorption.47,48 The dependence of the transition from a completely repulsive interaction to an attractive interaction is monotonic and does not depend on the order at which the potentials are applied to the modified gold electrode. Deviations from this general behavior have been observed sometimes and were interpreted as alterations in the (47) Boubour, E.; Lennox, R. B. J. Phys. Chem. B 2000, 104, 9004-9010. (48) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335-359.
Rentsch et al.
structure of the monolayer due to the externally applied potential, for example, due to partial thiol desorption. The solid lines indicate the fits to the full solution of the PB equation with charge regulation for both surfaces as discussed above. We find that the monotonic increase of the diffuse layer potential depends on the externally applied potential as reported previously for bare gold electrodes.23,28,29 For a 11-undecanol monolayer, we obtain a diffuse layer potential of ψD ) -36 mV for a minimum applied potential of φ ) -0.22 V and ψD ) 63 mV for φ ) +0.48 V. The former applied potential leads to a completely repulsive interaction and the latter to a completely attractive interaction. At an applied potential of approximately φ ) +0.03 V the diffuse layer charge for this modified electrode practically vanishes (ψD ∼ -1 mV) and the resulting interaction force can be considered negligible at all separation distances. This pzc represents an important property of the electrode and depends on the composition of the monolayer terminating the electrode surface.4,6,49 In comparison to the point of zero charge, which refers by common usage to external solution parameters such as ionic strength and pH, the pzc is given only in reference to the externally applied potential under certain, defined solution conditions. In the following, we will discuss a simple model, which allows a quantitative analysis of the diffuse layer potentials in dependence of the applied potential. In particular, we are interested in how the structure, the thickness, and the functional groups terminating the monolayer influence the diffuse layer properties of the modified electrodes. Influence of Electrode Modification on Diffuse Layer Properties. Figure 5 shows the diffuse layer potentials for different SAMs as a function of the potential applied to the modified electrodes. Figure 5 represents in the upper row (a-c) hydroxyl-terminated SAMs and in the lower row methylterminated SAMs (d,e). Additionally, the data for a partially fluorinated thiol (f) are shown. The diffuse layer potentials represented in Figure 5 were obtained from fits analogous to the ones shown in Figures 3 and 4. For each potential φ applied to the electrode, a diffuse layer potential ψD has been obtained from the fits. The different symbols in the graphs represent completely independent experiments in terms of electrode and colloidal probe. The measured diffuse layer potentials follow for all SAMs (with the exception of the fluoro-thiol) the general trend expected: the diffuse layer potential increases monotonically with increasing applied potential. Nevertheless, clear differences exist between the various SAMs in terms of the increase of diffuse layer potential with applied potential and the pzc. The simple model on which the following quantitative analysis is based is depicted in Figure 6. The relation between applied potential and resulting diffuse layer potential is described in this model in a first approximation by two capacitances in series.7 The first capacitance leads to the potential drop between the metal and the outer limit of the SAM, whereas the second capacitance results from the extended counterion distribution of the diffuse layer. The capacitance CL of the monolayer is given by
1 h ) L C 0
(4)
where h is the thickness of the SAM, is its average dielectric constant, and 0 is the vacuum permittivity. The diffuse layer capacitance CD is given by eq 3. Therefore, the inverse of the SAM capacitance increases in a linear fashion with the thickness (49) Sinniah, K.; Cheng, J.; Terrettaz, S.; Reuttrobey, J. E.; Miller, C. J. J. Phys. Chem. 1995, 99, 14500-14505.
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Figure 5. Diffuse layer potentials as obtained from the fits versus the applied potential for different self-assembled monolayers (SAMs) on the gold electrode. The lines indicate fits to the two capacitor model (cf. Figure 6) with literature values7,10,50 for the SAM capacitance CL and the pzc φpzc as the only free parameter (broken lines). The solid lines indicate fits with CL and φpzc as free parameters. Table 1. Capacitance CL of Thiol-SAMs on a Gold Electrode thiol
CL from fit [µF/cm2]
CL from literature [µF/cm2]
SH-(CH2)11-OH SH-(CH2)16-OH SH-(CH2)6-OH SH-(CH2)11-CH3 SH-(CH2)15-CH3 SH-(CH2)2-(CH2)7-CF3
0.8 1.1 1.0 1.2 0.8 +1.0
0.0a
a
Literature data from Becka et al.11
b
-0.36a +1.05b
Literature data from Iwami et
al.14
both free parameters (Figure 5, solid lines) are found. The deviations between both fits fall mostly within the scatter of the data with the exception of the special cases of the very short hydroxyl-terminated thiol and the partially fluorinated thiol. First we discuss the SAM-capacitances compiled in Figure 7a. Commonly dielectric constants of 2.3-2.6 are assumed for thiol-based SAMs,51 whereby the influence of the solution conditions and the terminal groups on the dielectric constants cannot be excluded.6 The numerical values obtained from the fits show a satisfactory agreement of our data with the literature values for the resulting capacitances of hydrocarbon thiols. The linear increase of the inverse SAM capacitance with the thickness of the SAM is shown for the literature data in Figure 7a. The higher capacitances found here for the two shorter OH-terminated thiols can be attributed to the formation of imperfect, noncrystalline monolayers, which facilitate the penetration of water into the film. Such monolayers have a reduced capacitance, as demonstrated recently.52 In the case of the fluorinated thiol, we observe practically no functional dependence of the diffuse layer potential on the applied potential, which is not in agreement with the behavior expected on the basis of SAM capacitances reported by other techniques.50 We attribute this deviation to pronounced ion adsorption due to the large dipole moment of the fluoromethyl group.53 In this case, adsorbed ions affect the diffuse layer properties more strongly than the externally applied potential. In agreement with our measurements, a large negative diffuse layer potential is reported for those films by direct force measurements in open circuit configuration.54 Figure 7b and Table 2 summarize the pzc for the different thiols. The pzc is significantly different for the hydroxyl- and methyl-terminated SAMs. The length of the thiol does not have a large influence on the pzc, which is in accordance with previous wetting studies.5,14 The only exception is the short hydroxylthiol, which forms noncrystalline films, and thus, it is likely that the underlying gold surface contributes to the pzc. The values for the pzc determined here are in the range of those obtained by different, classical electrochemical techniques4,11 The low dependence of the pzc on the length of the thiol has been described previously.11 The shift of the pzc for the longer thiols with different functional groups results most likely from differences in the dipole moment of the functional groups at the film-liquid interface.14,49 This interpretation is particularly supported by the large pzc of the fluorinated thiol, which has by far the largest dipole moment, and that its pzc is on the order of 1.0 V as previously reported from wetting measurements.14 The influence of the dipole moment of the functional groups might be amplified (50) Naud, C.; Calas, P.; Commeyras, A. Langmuir 2001, 17, 4851-4857. (51) Rampi, M. A.; Schueller, O. J. A.; Whitesides, G. M. Appl. Phys. Lett. 1998, 72, 1781-1783. (52) Berron, B.; Jennings, G. K. Langmuir 2006, 22, 7235-7240. (53) Colorado, R.; Lee, T. R. J. Phys. Org. Chem. 2000, 13, 796-807. (54) Ederth, T.; Tamada, K.; Claesson, P. M.; Valiokas, R.; Colorado, R.; Graupe, M.; Shmakova, O. E.; Lee, T. R. J. Colloid Interface Sci. 2001, 235, 391-397.
Figure 7. Inverse SAM-capacitance (a) and pzc (b) versus the number of carbon atoms in the SAM. The filled symbols represent the values obtained from fitting SAM capacitance and potential of zero-charge simultaneously, whereas the open symbols represent capacitances for the SAMs from Porter et al. and Miller et al., respectively.7,10 For comparison further literature values for SAM capacitance and pzc are compiled in the graphs.5,11,14,49 Round symbols represent OH-terminated SAMs and square symbols CH3-terminated SAMs.
additionally by the presence of a layer of hydrophobically bound water molecules as suggested recently.55 Water molecules tend to adopt a preferred orientation at the border of the aqueous phase and the hydrophobic surface. The preferential adsorption of hydroxyl ions to the surface of a fluorinated monolayer leads to a highly negative charge, which has been previously reported for fluorinated SAMs.54 Diffuse Layer Properties and Ion Adsorption. Direct force measurements on bare gold electrodes report large differences between the effective charge as determined from the diffuse layer properties and the electronic charge as determined by conventional electrochemical techniques.27 Similar findings have been reported for electroactive monolayers on gold9 or charged thiol-monolayers under open circuit conditions.19 The observed discrepancies are attributed to ion adsorption at the electrode surface.19,27,56 In the following discussion, we want to address the question of how important the process of ion adsorption is for electrodes modified by nonionizable SAMs. Ion adsorption to the surface of these monolayers corresponds to a layer of nonhydrated ions between the functional groups terminating the SAM and the diffuse layer. This additional compact inner layer or Stern layer, which was originally proposed by Gouy, is not included in the sketch in Figure 6. Such a layer would lead to an additional potential drop and thus represents an additional capacitance CI in series with CL and CD. The overall capacitance would be reduced according to eq 5 due to the additional term 1/CI on the right side. Comparing the fitted values for the SAM capacitance with the ones obtained from other techniques (cf. Table 1), we can estimate a lower limit of the Stern-capacitance to be in the order of 1.4-8.5 µF/cm2. A major assumption of this (55) Schweiss, R.; Welzel, P.; Knoll, W.; Werner, C. Chem. Commun. 2005, 256-258. (56) Attard, P. J. Phys. Chem. 1995, 99, 14174-14181.
Thiol-Modified Gold Electrodes
Figure 8. Regulation parameter p as a function of the applied potential for two different SAMs, 11-mercapto-1-undecanol (open symbols) and 16-hexadecanethiol (closed symbols). The curves indicate an upper and a lower estimation for the regulation parameter. The upper curves are calculated by assuming that the inner layer capacitance is given by the SAM capacitance. The lower curves are calculated assuming that the inner layer capacitance originates only from potential independent Stern layer with a constant capacity of 40 µF/cm2. An infinite inner layer capacitance would correspond to a constant regulation parameter of zero.
estimation is that the Stern capacitance would be independent of the applied potential. In the light of the potential dependent adsorption of ions reported for bare electrodes29 or the orientation of water on bare electrodes as a function of the applied potential,57 this assumption is most likely overly simplistic. The Stern capacitances obtained by the above estimation are relatively small; thus, further contributions or alternative explanations for the capacitances obtained by different techniques (cf. Figure 7) can be related to the surface roughness of the gold electrode. However, ion adsorption, if present for the here-examined monolayers, is supposed to have particularly an influence on the regulation parameter. The functional dependence of the regulation parameter on the inner capacitance CI is given by eq 3. Figure 8 shows the experimental data for the regulation parameter as a function of the applied potential applied for two typical SAMs. The upper limit of the inner layer capacitance corresponds to the SAM capacitance from the literature values in Table 1. The resulting overestimation confirms that for the charge regulation only the inner layer capacitance resulting from ions adsorbed at the outer border of the SAM is contributing. Such a model has been proposed previously to describe double layer effects at SAMs containing acid/base groups.13 The second pair of curves is calculated on the basis of such an inner layer capacitance of 40 µF/Cm.2 The large number of data points with p ) 0 would correspond to even higher inner layer capacitances. Nevertheless, the asymmetric distribution of regulation parameters for the OHterminated SAM suggests the possibility of potential dependent ion adsorption. Despite the relatively good agreement of the values obtained here for the SAM capacitance and pzc with data reported by classical electrochemical techniques, a number of questions (57) Schultz, Z. D.; Shaw, S. K.; Gewirth, A. A. J. Am. Chem. Soc. 2005, 127, 15916-15922.
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remain open at the present stage. In particular, the regulation parameter and the total capacitance CT do not provide a coherent description as to which extent ions adsorb to the SAM. The approach of modifying a gold electrode by different SAMs is well suited to probe the validity of Gouy-Chapman theory for the interaction between surfaces, but the scattering of the data must be reduced. This scattering is mostly related to the inherent uncertainties originating from an asymmetric system of colloidal probe and electrode and can thus be substantially improved by gold-coated colloidal probes with identical SAM modification as for the flat electrode. By means of a bipotentiostat, both surfaces, flat electrode and colloidal probe, can be controlled independently of each other. Such a setup would provide the possibility to study with unprecedented accuracy not only the charge regulation between surfaces during approach42 but also the potential dependent adsorption of ions on the interaction force and thus the effective charge of modified electrodes. Such studies are particularly interesting in respect of recently proposed amphifunctional models, which explicitly account for ion adsorption.58
Conclusions The forces between a colloidal silica particle and a SAMmodified electrode under potentiostatic control were determined by AFM. In particular, we studied electrodes modified by thiol monolayers terminating in nonionizable end groups. The interaction forces upon approach were quantitatively evaluated by fits to the solutions of the nonlinear Poisson-Boltzmann equation for different boundary conditions. By this approach the diffuse layer properties of the modified electrode can be determined in a complementary way to classical electrochemical techniques. The interaction forces between the colloidal probe and the modified electrode can be described quantitatively over the whole distance and potential range, only if charge regulation is taken into account. A series of force measurements at different applied potentials allowed the measurement of the capacitance of the SAM and the determination of the pzc. The values for the capacitance of the SAM are in agreement with crystalline monolayers for the longer thiols. The pzc is dominated by the dipole moment of the terminating functional group, as demonstrated by the measurement of a partially fluorinated thiol. The values obtained for SAM capacitance and the pzc are in agreement with the ones by classical electrochemical techniques. Ion adsorption to the modified electrode is less pronounced than reported for bare gold electrodes and the hydroxyl and methylterminated electrodes with crystalline order behave as nearly ideally polarizable electrodes. Acknowledgment. We thank S. Jeanneret (University of Geneva) and R. Schraner (University of Berne) for the construction of the potentiostat for this study and F. Bujard (University of Geneva) for the construction of the fluid cell. We thank F. Campana and N. Eichenberger (both University of Berne) for their help in testing and designing the equipment in an initial phase of the project. In particular, we thank M. Borkovec for his support and helpful discussions. The Swiss National Science Foundation provided financial support for this research. LA700987U (58) Duval, J.; Lyklema, J.; Kleijn, J. M.; van Leeuwen, H. P. Langmuir 2001, 17, 7573-7581.
