Langmuir 2008, 24, 6133-6139
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Characterization of Carboxylic Acid-Terminated Self-Assembled Monolayers by Electrochemical Impedance Spectroscopy and Scanning Electrochemical Microscopy Wesley Sanders and Ricardo Vargas Department of Chemistry, Virginia Polytechnic Institute and State UniVersity, Blacksburg, Virginia 24060-0212
Mark R. Anderson* Department of Chemistry, UniVersity of Colorado DenVer, Campus Box 194, P.O. Box 173364, DenVer, Colorado 80217-3364 ReceiVed December 28, 2007. ReVised Manuscript ReceiVed March 11, 2008 Electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) are used to monitor changes in the ionization of monolayers of 11-mercaptoundecanoic acid. When using an anionic redox probe, Fe(CN)6-4, the charge-transfer resistance of the 11-mercaptoundecanoic acid monolayer-modified interface increases in a sigmoidal fashion as the solution is made basic. The opposite effect is observed when using a cationic redox probe. The inflection points of these two titration curves, however, differ when using the different redox probes. This result is taken as being characteristic of the influence that applied potential has on the ionization of the monolayer. The role of substrate potential on the ionization of the monolayer is further investigated by SECM. The SECM measurement monitors the concentration of Ru(NH3)6+3 as the potential of the substrate is varied about the potential of zero charge. For monolayers of 11-mercaptoundecanoic acid in solutions buffered near the pKa of the terminal carboxylic acid, potential excursions positive of the PZC cause an increase in the concentration of Ru(NH3)6+3 local to the interface, and potential excursions negative of the PZC cause a decrease in the local concentration of Ru(NH3)6+3. Similar experiments conducted with an interface modified with 11-undecanethiol had no impact on the local concentration of Ru(NH3)6+3. These results are interpreted in terms of the influence that applied potential has on the pH of the solution local to the interface and the impact that this has on the ionization of the monolayer.
Introduction Preparation of interfacial structures by molecular self-assembly has been actively studied for many years.1–6 These well-defined modified interfaces can be used to study fundamental interfacial properties or can be used in different practical applications. To broaden the utility of interfaces prepared by molecular selfassembly, many have investigated the properties of monolayers containing terminal functionality.7–13 These studies show that the structural and physical properties of the functionalized monolayers can be controlled by both the identity of the functionalization as well as the ensemble structure of the modified interface. These results further suggest that the chemical and * To whom correspondence should be addressed. E-mail: mark.anderson@ cudenver.edu. (1) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932–950. (2) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 42, 365–385. (3) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312– 1319. (4) Decher, G.; Hong, J. D. Ber. Bunsen-Ges. Phys. Chem. Chem. Phys. 1991, 95, 1430–1434. (5) Allara, D. L. Polym. Mater. Sci. Eng. 1994, 71, 772. (6) Stranick, S. J.; Parikh, A. N.; Tao, Y. T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636–7646. (7) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682–691. (8) Taylor, C. D.; Anderson, M. R. Langmuir 2002, 18, 120–126. (9) Cavadas, F.; Anderson, M. R. Langmuir 2003, 19, 9724–9729. (10) Cavadas, F.; Anderson, M. R. J. Colloid Interface Sci. 2004, 274, 370. (11) Willner, I.; Blonder, R.; Dagan, A. J. Am. Chem. Soc. 1994, 116, 9365– 9366. (12) Lion-Dagan, M.; Katz, E.; Willner, I. J. Am. Chem. Soc. 1994, 116, 7913– 7914. (13) Duevel, R. V.; Corn, R. M. Anal. Chem. 1992, 64, 337–342.
physical properties of an interface can be designed and/or controlled to some extent by altering the structural details of the molecules used to generate the interface. Carboxylic acid groups are frequently used as terminal functionality in monolayer-modified interfaces because they (i) introduce hydrogen-bonding interactions among the molecules present along the interface, (ii) offer a reactive group for subsequent chemical reaction so that three-dimensional interfacial structures can be created,13 or (iii) can be used to change the polarity of the interface.7 Because of these factors, the carboxylic acid group also introduces interesting structural and chemical properties to the interfacial ensemble that can be leveraged either during the self-assembly process or after self-assembly once the monolayer has been formed. For example, the acid group may prevent efficient organization of the alkyl chains due to the hydrogen-bonding interactions among the head groups dominating the lateral interactions within the assembly. Several research groups show that this feature may contribute to phase separation during the adsorption of binary mixtures of mercaptans.14–17 Similarly, ionization of the acid group establishes electrostatic interactions that impact the structure of the interfacial layer, and the acid/base chemistry of the monolayer can be used to experimentally modulate the interfacial physical properties. Ward (14) Ichii, T.; Fukuma, T.; Kobayashi, K.; Yamada, H.; Matsushige, K. Appl. Surf. Sci. 2003, 210, 99–104. (15) Hobara, D.; Ota, M.; Imabayashi, S.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1998, 444, 113–119. (16) Imabayashi, S.; Gon, N.; Sasaki, T.; Hobara, D.; Kakiuchi, T. Langmuir 1998, 14, 2348–2351. (17) Imabayashi, S. i.; Hobara, D.; Kakiuchi, T.; Knoll, W. Langmuir 1997, 13, 4502–4504.
10.1021/la704059q CCC: $40.75 2008 American Chemical Society Published on Web 05/22/2008
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et al., for example, demonstrate using quartz crystal microbalance measurements that deprotonation of a ω-mercaptoalkanoic acid monolayer is accompanied by a large change in the viscoelastic properties of the interface.18 This was attributed to structural changes within the monolayer that result from the deprotonation of the acid group. In these experiments, the interfacial layer is titrated and an estimate of the interfacial pK1/2 (the pH at which one-half of the interfacial carboxylic acid groups are deprotonated) determined on the basis of the frequency change of the quartz crystal. Protonation and deprotonation of the terminal acid-base groups can potentially be used to experimentally modulate the properties of the interface. Several research groups have proposed monolayers in which the structure and properties can be experimentally switched between two different conditions. Rao et al. report using applied potential to “open” and “close” a monolayer of 2-mercaptobenzthiazole.19 At high positive potentials, they proposed that the 2-mercaptobenzthiazole reorients at the interface creating channels for materials to diffuse to the Au substrate surface where they undergo a heterogeneous electrochemical reaction. Takehara et al. report using complexation reactions to “open” and “close” channels within a self-assembled monolayer.20 The reversibility of this process is a function of the formation constant for the complex and the ability to remove the lanthanide from the interfacial complex. Willner et al. generate a selfassembled monolayer in which redox processes are turned on and off based on a structural change of the molecules that make up the monolayer.11,12,21–24 In these systems, the structural change is induced by either photochemical or thermal isomerization of a spiropyran molecule that has been covalently attached to a confined cystamine monolayer. The spiropyran can be isomerized either thermally or photochemically between the spiropyran and the protonated merocyanine forms. Unlike Rao et al. and Takehara et al., Willner’s system has a clear stimulus that both opens and closes the gate. Experimental modulation of the interfacial properties is an attractive concept as interfaces with switchable properties could be used for (i) directed, in situ assembly of three-dimensional interfacial structure, or (ii) experimentally selectable selectivity for analysis. In this work, we use electrochemical impedance spectroscopy (EIS) and the scanning electrochemical microscope (SECM) to monitor the charge density of the modified interface as it is modulated by changes in solution pH and changes to the potential applied to the substrate. Using applied potential, we show that the interfacial charge density can be modulated in situ and the ability to modulate the interfacial charge density impacts how the interface interacts with species in the adjacent solution.
Experimental Section Chemicals. 11-Mercaptoundecanoic acid was purchased from the Aldrich Chemical Co. (Milwaukee, WI). Potassium hexacyanoferrate(II) trihydrate, potassium hexacyanoferrate(III), and hexamine ruthenium(III) chloride were purchased from Fischer Scientific (18) Wang, J.; Frostman, L. M.; Ward, M. D. J. Phys. Chem. 1992, 96, 5224– 5228. (19) Bharathi, S.; Yegnaraman, V.; Rao, G. P. Langmuir 1993, 9, 1614–1617. (20) Takehara, K.; Aihara, M.; Ueda, N. Electroanalysis 1994, 6, 1083–1086. (21) Katz, E.; Lion-Dagan, M.; Willner, I. J. Electroanal. Chem. 1995, 382, 25–31. (22) Katz, E.; Willner, I. Electroanalysis 1995, 7, 417–419. (23) Doron, A.; Portnoy, M.; LionDagan, M.; Katz, E.; Willner, I. J. Am. Chem. Soc. 1996, 118, 8937–8944. (24) Katz, E.; Shipway, A. N.; Willner, I. The electrochemical and photochemical activation of redox-enzymes. In Electron Transfer in Chemistry. Vol. 4: Heterogeneous Systems, Solid State Systems, Gas Phase Systems. Section 1: Catalysis of Electron Transfer; Balzani, P. P. M. A. J. R., Ed.; Wiley-VCH: Weinheim, Germany, 2001; pp 127-201.
Sanders et al. Co. All chemicals were analytical grade and were used without further purification. 11-Mercaptoundecanoic acid solutions were 0.001 M in absolute ethanol. All other solutions were prepared with water deionized with an 18 MΩ Milli-Q ion exchange filter from Millipore Inc. Electrochemical Measurements. Impedance measurements were performed using a model 604B electrochemical analyzer from CH Instruments (Austin, TX) interfaced to a personal computer. A gold disk electrode purchased from Bioanalytical Systems (West Lafayette, IN) having a geometric area of 0.048 cm2 was used as the working electrode. Electrochemical experiments were performed in a threeelectrode cell at room temperature. A Ag/AgCl electrode was used as the reference electrode, and a platinum wire served as the auxiliary electrode. Electrochemical impedance measurements of the modified gold electrode were performed in 7.5 mL solutions of 0.005 mol L-1 Fe(CN)6-3/-4 or 0.005 mol L-1 Ru(NH3)6+3 containing 0.1 mol L-1 KCl in phosphate buffer solutions. The phosphate buffer solutions were prepared with a total phosphate concentration of 0.050 mol L-1. Impedance data were obtained by applying a sinusoidal potential modulation with a 5 mV amplitude symmetrically about the formal potential of the redox probe. The frequency range used for these measurements extended from 1.0 × 105 to 0.1 Hz. Quantitative estimates of the equivalent circuit parameters were obtained by fitting the experimental data using the nonlinear least-squares fitting routines of the software package LEVM 7.0 (available from Solartron, www.solartronanalytical.com, written by James Ross Macdonald). Data are fit to the Randles equivalent circuit to extract estimates of the interfacial capacitance and the charge-transfer resistance.25–28 Each impedance titration is conducted by measuring the impedance response at each solution pH with a single modified substrate. Scanning Electrochemical Microscopy. The SECM instrumentation was constructed locally using a Thorlabs Inc. (Newton, NH) XYZ platform equipped with Z600 series actuators operated in a closed loop configuration.29 The initial X, Y, and Z positions of the microelectrode relative to the monolayer-modified substrate are established manually under observation of a Leica GZ6E stereomicroscope. The actuators are controlled during the scanning electrochemical microscopy with locally written software. Step resolution for this instrument is 40 nm. Once positioned manually, program control positions the SECM microelectrode probe vertically above the substrate by measuring an approach curve. The electrode position is fixed when the current measured by the microelectrode during the approach curve has changed ∼50% from the bulk value measured after manual positioning. Monolayer Preparation. The gold disk electrode was pretreated using the following procedure prior to monolayer deposition. The electrode was polished with 0.05 µm alumina grit followed by sonication in deionized water. After the electrode was rinsed with deionized water, it was electrochemically cleaned by cycling the potential from -0.8 to 2 V at 0.1 mV s-1 in 0.5 M sulfuric acid. Following this preparation, the electrode is found to have a surface roughness of 1.2 ((0.05).30 The monolayer was prepared using an electrochemical deposition method similar to procedures found in the literature.31–33 The gold disk working electrode was placed in 5 mL of ethanol that contains 0.005 mol L-1 11-mercaptoundecanoic acid solution and 0.1 mol L-1 lithium perchlorate. Several 0.1 V s-1 linear potential sweeps (25) Gyepi-Garbrah, S. H.; Silerova, R. Phys. Chem. Chem. Phys. 2001, 3, 2117–2123. (26) Janek, R. P.; Fawcett, W. R.; Ulman, A. Langmuir 1998, 14, 3011–3018. (27) Bjoerefors, F.; Petoral, R. M., Jr.; Uvdal, K. Anal. Chem. 2007, 79, 8391– 8398. (28) Cohen-Atiya, M.; Nelson, A.; Mandler, D. J. Electroanal. Chem. 2006, 593, 227–240. (29) Roach, D. M.; Hooper, S. E.; Anderson, M. R. Electroanalysis 2005, 17, 2254–2259. (30) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559–3568. (31) Brett, C. M. A.; Kresak, S.; Hianik, T.; Brett, A. M. O. Electroanalysis 2003, 15, 557–565. (32) Sumi, T.; Uosaki, K. J. Phys. Chem. B 2004, 108, 6422–6428. (33) Ron, H.; Rubinstein, I. J. Am. Chem. Soc. 1998, 120, 13444–13452.
Characterization of Carboxylic Acid-Terminated SAMs
Figure 1. EIS data obtained at pH 3 and pH 8 for monolayers prepared with (A) undecanethiol and (B) 11-mercaptoundecanoic acid.
were performed between -1.0 to 0.0 V vs Ag–AgCl until the resulting i versus E curves are superimposable. Following electrochemical deposition, the electrode was rinsed with ethanol and deionized water.
Results and Discussion In the EIS experiment, a small amplitude AC potential is applied to the system and the current response is measured. Because of the resistance and capacitance associated with the electrochemical system, the AC current response is phase-shifted relative to the excitation potential program. By fitting the experimental data to the theoretical equivalent circuit, estimates of the system impedance parameters can be obtained. For a monolayer-modified interface, the interfacial capacitance is influenced by the monolayer dielectric and by the ability of the monolayer to limit the distance of closest approach of the redox probe. The chargetransfer resistance of the system is related to the influence that the monolayer has over the heterogeneous kinetics of the redox reaction.34 For an ionizable monolayer, both of these parameters should be influenced by the percent ionization of the interfacial layer. This is illustrated in Figure 1, which shows the impedance behavior of two different monolayers prepared with either n-undecanethiol or 11-mercaptoundecanoic acid measured in solutions containing the redox couple Fe(CN)6-3/-4 that are buffered at pH 3 and 8. For the n-undecanethiol monolayer, the impedance behavior is independent of the electrolyte pH. This result is expected as the interfacial structure and properties of this monolayer are independent of the pH of the adjacent solution. With the 11-mercaptoundecanoic acid monolayer system, however, the impedance behavior is a strong function of the electrolyte pH. At pH 3, the 11-mercaptoundecanoic acid monolayer is neutral, and the magnitude of the impedance is similar to that observed with the undecanethiol monolayer. At pH 8, however, the magnitude of the impedance found for the 11-mercaptoundecanoic monolayer increases by a factor of nearly (34) Zhao, J. W.; Luo, L. Q.; Yang, X. R.; Wang, E. K.; Dong, S. J. Electroanalysis 1999, 11, 1108–1111.
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Figure 2. Titration curve generated by plotting the charge-transfer resistance obtained from EIS measurements when using Fe(CN)6-3/-4 as a redox probe vs solution pH for monolayers of 11-mercaptoundecanoic acid. The titration data shown are obtained at each pH value using a single substrate. Error bars represent the standard deviation of the chargetransfer resistance estimates from the fit to the equivalent circuit.
10 as compared to the behavior of the n-undecanethiol monolayer and to that at pH 3 for the 11-mercaptoundecanoic acid monolayer. This behavior is thought to be due to the deprotonation of the interfacial carboxylic acid groups at the more basic pH and the subsequent electrostatic repulsion between the negatively charged interface and the anionic redox probe.34 Under the basic conditions, the electrostatic repulsion between the redox probe and the interfacial charge prevents the probe from efficiently approaching the interface, and this impacts the system impedance. This interpretation suggests that the charge density of the interface may impact the magnitude of the experimentally observed chargetransfer resistance.34–36 By varying the pH of the electrolyte solution, the 11mercaptoundecanoic acid monolayer can be titrated and the charge density of the interface fractionally changed;34,36 consequently, the impedance behavior should vary as the solution pH changes.36 This behavior has been demonstrated previously36 and is illustrated in Figure 2. To construct this titration curve, impedance measurements are conducted with a single gold substrate modified with an 11-mercaptoundecanoic acid monolayer, and the chargetransfer resistance at each pH value is extracted by fitting the EIS data to the equivalent circuit. As can be seen, the charge-transfer resistance for the 11-mercaptoundecanoic acid monolayermodified electrode generally follows the sigmoidal response that is characteristic of a typical titration curve. The general impedance behavior as a function of solution pH is reproducible with multiple different instances of the gold substrate modified with 11mercaptoundecanoic acid. Under acidic conditions, the chargetransfer resistance is constant, characteristic of the interfacial monolayer having the same charge density at each of these pH values because it remains neutral. Under very basic conditions, the charge-transfer resistance also is constant but attains a limiting value that is greater than the charge-transfer resistance found under acidic pH conditions. The larger value for the chargetransfer resistance is taken as being representative of the (35) Quan, C.; Brajtertoth, A. Anal. Chem. 1992, 64, 1998–2000. (36) Komura, T.; Yamaguchi, T.; Shimatani, H.; Okushio, R. Electrochim. Acta 2004, 49, 597–606.
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Figure 3. Titration curve obtained by plotting the charge-transfer resistance obtained from EIS measurements using Ru(NH3)6+2/+3 as the redox probe vs solution pH for monolayers prepared with 11mercaptoundecanoic acid. The titration data shown are obtained at each pH value using a single substrate. Error bars represent the standard deviation of the charge-transfer resistance estimates from the fit to the equivalent circuit.
monolayer attaining its highest charge density and the interfacial charge density again remaining constant at these basic pH values. At intermediate pH values, there is a transition between the low charge-transfer resistance of the neutral interface and the high charge-transfer resistance of the fully charged interface, consistent with the expectation that the value of the charge-transfer resistance for this system is dependent on the charge density of the interface. The pH value at the point along the curve that is halfway between the low charge-transfer resistance limit and the high chargetransfer limit is taken as the pK1/2. For these experimental conditions, this value is ∼5.5.18,37 To probe the role that an electrostatic interaction between the redox probe and the ionizable monolayer has on the experimental impedance response, a second set of experiments were conducted that use cationic Ru(NH3)6+3 as the redox probe. In these experiments, the charge-transfer resistance decreases as the pH of the electrolyte solution becomes more basic (Figure 3). Given our electrostatic explanation of the impedance behavior with the Fe(CN)6-3/-4 redox probe, the system response to the cationic Ru(NH3)6+3 redox probe is consistent with an interfacial electrostatic interaction. When the monolayer is deprotonated, attraction between the negatively charged interface and the cationic redox probe should draw the redox probe closer to the interface and the charge-transfer resistance should correspondingly decrease. This is the behavior that is observed. The apparent pK1/2, however, is shifted to more basic pH (∼7.5) when using Ru(NH3)6+3 as the redox probe. The shift of the apparent pK1/2 when using the different redox probes for the impedance measurements is intriguing. The pK1/2 values reported previously for similar monolayers range between ∼5 and ∼10 when using a variety of different experimental measurement techniques.18,37–40 These different behaviors of the acid/base properties of monolayer-modified electrodes reported previously may be related to the different experimental details used when making the measurements. For our measurements, the only difference in the two impedance titrations is the identity of the redox probe used. With the impedance measurements, the potential of the working electrode is sinusoidally oscillated symmetrically about the standard potential for the redox probe being used in that measurement. For Fe(CN)6-3/-4 the standard potential is approximately 0.20 V vs a Ag–AgCl reference (37) Creager, S. E.; Clarke, J. Langmuir 1994, 10, 3675–3683. (38) Kakiuchi, T.; Iida, M.; Imabayashi, S.; Niki, K. Langmuir 2000, 16, 5397–5401. (39) Sugihara, K.; Shimazu, K.; Uosaki, K. Langmuir 2000, 16, 7101–7105. (40) Hu, K.; Bard, A. J. Langmuir 1997, 13, 5114–5119.
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electrode, and for Ru(NH3)6+3 the standard potential is approximately -0.30 V vs a Ag–AgCl reference electrode. The different potentials used when making measurements with these two redox probes may influence the experimentally observed pK1/2.41–45 The potential of zero charge (PZC) for gold electrodes modified by self-assembly with a monolayer of 11-mercaptoundecanoic acid is found by differential capacitance measurements to be -0.13 ((0.05) V vs Ag–AgCl. When the impedance measurements are made with the Fe(CN)6-3/-4 redox probe, therefore, the potential of the substrate is held at a potential that is positive of the PZC. Based on the Gouy-Chapman–Stern model of the structure of the electrochemical double layer, at potentials positive of the PZC, anionic components of the electrolyte should be in greater concentration close to the interface.46 For buffered aqueous solutions, this suggests that the pH adjacent to the interface should be more basic than the bulk value when the impedance measurements are made with the Fe(CN)6-3/-4 redox couple. When using the Ru(NH3)6+3 as the redox probe in the impedance measurements, the applied potential is negative of the PZC. Under these conditions, cationic components of the electrolyte should be in greater concentration close to the interface, and the interface will be more acidic than the bulk pH. The influence of electrode potential on the interfacial pH has been experimentally demonstrated by Dorain et al., Anderson, and Cao.47–49 Given the influence that the applied potential can have on the pH local to the interface, the pH that the interface experiences during the experiment is not accurately reflected by the bulk value during the impedance measurements. For our EIS measurements, therefore, the actual pK1/2 for the 11-mercaptoundecanoic monolayer, therefore, probably lies between the two experimental values determined with the different redox probes. The apparent pK1/2 shift when using the two different redox probes suggests that both the solution pH and the potential applied to the substrate contribute to the charge density of the monolayer. The influence of the substrate potential on the ionization of the terminal carboxylic acid group is experimentally probed using SECM measurements. In these measurements, the SECM probe is brought into close proximity of the monolayer-modified interface and is rastered back-and-forth along a 30 µm × 30 µm grid. Initially, the substrate potential is held at the PZC (-0.13 V vs Ag–AgCl) of the 11-mercaptoundecanoic acid monolayermodified surface, and the microelectrode probe is maintained at a potential where the Ru(NH3)6+3 is reduced at its mass-transport limit (-0.5 V vs Ag–AgCl). At the PZC, there is no excess charge on the substrate and the solution pH adjacent to the interface is representative of the bulk value; consequently, the percent ionization of the interface is dictated by the solution pH. For these initial measurements, the bulk solution is buffered at pH 6, a value close to what appears from our measurements to be the apparent pK1/2 of the interface. Under these conditions, the monolayer should be approximately 50% deprotonated, and any small change in the pH local to the interface, for example, caused (41) Smith, C. P.; White, H. S. Langmuir 1993, 9, 1–3. (42) Smith, C. P.; White, H. S. Anal. Chem. 1992, 64, 2398–2405. (43) Andreu, R.; Fawcett, W. R. J. Phys. Chem. 1994, 98, 12753–12758. (44) Fawcett, W. R.; Fedurco, M.; Kovacova, Z. Langmuir 1994, 10, 2403– 2408. (45) Burgess, I.; Seivewright, B.; Lennox, R. B. Langmuir 2006, 22, 4420– 4428. (46) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; 2nd ed.; John Wiley & Sons, Inc.: New York, 2001. (47) Dorain, P. B.; Vonraben, K. U.; Chang, R. K. Surf. Sci. 1984, 148, 439– 452. (48) Anderson, M. R.; Evans, D. H. J. Am. Chem. Soc. 1988, 110, 6612–6617. (49) Cao, X. W. J. Raman Spectrosc. 2005, 36, 250–256.
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Figure 4. SECM images monitoring the concentration of Ru(NH3)6+3 near the surface of an electrode modified with 11-mercaptoundecanoic acid as the potential of the substrate is varied from the PZC (-0.13 V vs Ag–AgCl) to (A) -0.05 V vs Ag–AgCl, a value positive of the PZC, and (B) -0.17 V vs Ag–AgCl, a value negative of the PZC. The arrows indicate where in the SECM scan the potential was altered from the PZC (i) and back to the PZC (ii). The lower portion of the figure shows cross-sections obtained at 15 µm.
by a change in the substrate potential, should result in a change in the charge density of the monolayer. Initially, while the substrate potential is maintained at the PZC, the current measured by the SECM probe is relatively constant, characteristic of constant effective concentration of the Ru(NH3)6+3. After ∼10 min, the potential of the substrate is stepped to a value more positive than the PZC (Estep ) -0.05 V vs Ag–AgCl). At this substrate potential, the pH of the solution adjacent to the electrode should become more basic, according to the GCS model of the electrochemical interface, and the monolayer-modified interface should take on a greater charge density. The presence of greater negative charge at the interface attracts a higher concentration of the cationic redox probe, and the current measured by the SECM probe increases (Figure 4A). After ∼3-4 min at the step potential, the substrate potential is returned back to the PZC, and the current measured by the SECM probe is found to return to the initial value. An analogous experiment using the same solution conditions is conducted; this time, however, the substrate potential is stepped to values negative of the PZC (Estep ) -0.17 V vs Ag–AgCl). Under these experimental conditions, the pH of the electrolyte adjacent to the interface should become more acidic at the step potential, and the charge density of the monolayer will decrease as the monolayer is protonated. With a decrease in the charge density of the modified interface, the local concentration of the Ru(NH3)6+3 decreases and the current measured by the SECM probe also decreases (Figure 4B). Control experiments were conducted using a substrate modified with 11-mercaptoundecanoic acid that is exposed to an electrolyte solution buffered at pH 8. This pH is just basic enough that the 11-mercaptoundecanoic monolayer is fully deprotonated. Under these experimental conditions, potential excursions to values more positive than the PZC will make the local pH more basic; however, this change in the system will not impact the charge
density of the monolayer as it is deprotonated to the extent possible by the bulk solution pH, nor should it influence the concentration of the cationic redox probe in the vicinity of the interface. This is confirmed experimentally and illustrated in Figure 5A, where there is no observed change in the SECM probe current when potentials more positive of the PZC are applied to the substrate. Altering the substrate potential to values more negative of the PZC should, however, make the interfacial pH more acidic than the bulk value and cause a decrease in the charge density of the interface. The decreased interfacial charge density will lower the local concentration of the cationic probe, and the SECM probe current is observed to correspondingly decrease (Figure 5B). For these measurements, both the initial potential (EPZC ) -0.13 V vs Ag–AgCl) and the step potential (Estep ) -0.17 V vs Ag–AgCl) are well positive of the formal potential of the Ru(NH3)6+3/+2 redox couple (-0.30 V vs Ag–AgCl). It is possible, therefore, that changes in the substrate potential would alter the probe current due to changes in the SECM feedback. Both the initial and the step potentials, however, are more than 150 mV positive of the formal potential, and both potentials should oxidize the Ru(NH3)6+3 at the mass-transport limited rate. We anticipate that any contribution to the probe current, therefore, due to changes in the SECM feedback with the potential change would be transient before decaying back to the mass-transport limited value. Because the current change persists and is not transient, we believe that changes in the SECM feedback do not adequately explain the observation. In addition, changes in the probe current due to SECM feedback when altering the substrate potential should be independent of the solution pH. The observation that the probe current response to potential changes is different in the pH 8 solution as compared to the pH 6 solution again suggests that the SECM feedback alone does not adequately explain the experimentally observed behavior.
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Figure 5. SECM image of the concentration profile of Ru(NH3)6+3 above a substrate modified with 11-mercaptoundecanoic acid in a solution buffered at pH 8 while the substrate potential is altered from the PZC (PZC of the neutral interface, -0.13 V vs Ag–AgCl) to (A) -0.05 V vs Ag–AgCl, a value more positive of the PZC, and (B) -0.17 V vs Ag–AgCl, a value more negative of the PZC. The arrows indicate where in the SECM scan the potential was altered from the PZC (i) and back to the PZC (ii). The lower portion of the figure shows cross-sections obtained at 15 µm.
Figure 6. SECM images of the concentration of Ru(NH3)6+3 in close proximity of a substrate modified with undecanethiol as the substrate potential is altered from the PZC (-0.48 V vs Ag–AgCl) to (A) -0.41 V vs Ag–AgCl, a value more positive of the PZC, and (B) -0.53 V vs Ag–AgCl, a value more negative of the PZC. The arrows indicate where in the SECM scan the potential was altered from the PZC (i) and back to the PZC (ii).
A second set of control experiments were conducted using an electrode modified with a monolayer of n-undecanethiol. The PZC of this alkanethiol monolayer is found to be -0.48 V vs Ag–AgCl, a value consistent with PZC values reported by Sondeghuehorst.50 As can be seen in Figure 6, there is no obvious change in the magnitude of the probe current when the substrate potential is altered to potentials either positive or negative of the PZC. This behavior is consistent with our interpretation of the experimental behavior found with 11-mercaptoundecanoic acid monolayers. With the alkanethiol monolayer, local changes in the interfacial pH brought about by altering the substrate potential will not influence the charge density along the interface and have no impact on the interfacial concentration of the redox probe. These sets of experiments show that substrate potential can influence the ionization of ω-functionalized monolayers and that applied potential impacts the distribution of ionic species near (50) Sondaghuethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1992, 8, 2560– 2566.
the monolayer-modified interface. The influence of substrate potential on the ionization of carboxylic acid- or amine-terminated monolayers has been previously studied. Smith and White developed a theoretical treatment that discussed the ionization of the terminal functional group in terms of the electric field strength at the interface.41,42 Fawcett subsequently extended this treatment to include Stern layer effects.43,44 Using White’s theory, Crooks explains the capacitance behavior of monolayers prepared with 4-mercaptopyridine and 4-aminothiophenol as a function of solution pH and substrate potential.51 Lennox et al. extend Smith and White’s thermodynamic treatment to include the kinetics of the electric field driven protonation and deprotonation of the terminal carboxylic acid group.45 As in our work, Cao investigates the influence that applied potential has on the ionization of ω-functionalized monolayers.49 Using surface-enhanced Raman spectroscopy, Cao monitors the concentration of ClO4- ions at the interface and finds that the (51) Bryant, M. A.; Crooks, R. M. Langmuir 1993, 9, 385–387.
Characterization of Carboxylic Acid-Terminated SAMs
surface pKa of 2-aminoethanethiol shifts to different values as the potential applied to the substrate changes. This result is consistent with our EIS data and with previous SERS studies that show the influence that applied potential has on interfacial pH.47,48 Cao attributes the observed pKa shift of the confined molecules to changes in the pH local to the interface that are brought about by the change in potential. Lennox et al. call this shift in the interfacial pKa an apparent change because it involves solution pH changes that are isolated to the interface.45 In this case, the role of the substrate potential is to influence the composition of the electrochemical double layer to create conditions favorable for deprotonation of the terminal carboxylic acid. We believe that a similar mechanism is responsible for the SECM results and for the apparent shift in the pK1/2 measurments.
Summary As suggested by Lennox, our results indicate that many different factors impact the ionization of the confined acid-base functionality at the interface. As has been previously shown, the pH of the adjacent solution clearly influences the protonation/ deprotonation of the terminal acid group. Our results, along with those from White, Lennox, and Cao, show that the potential applied to the substrate also can have a significant influence on the ionization of the terminal group.41,42,45,49 This result is evident from the SECM results. We interpret this influence as being characteristic of local changes to the interfacial pH created by accumulation of ions on the solution side of the interface in response to changes in the net charge on the substrate introduced by the potential changes. The changes to the percent ionization of the monolayer are most evident when the pH of the adjacent solution is near the pKa of the confined acid/base group. Because the distribution of ions on the solution side of the interface in response to the charge associated with the substrate can influence the local pH, it is likely that ionization of the monolayer itself may also impact the local pH. As the 11-mercaptoundecanoic acid monolayer is deprotonated, a second
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contribution to the total interfacial charge is introduced to the system, for example, that from the ionized monolayer itself. The contribution to the total interfacial charge made by the ionization of the monolayer acid group has been neglected in this analysis. Neglecting this contribution is reasonable for this analysis, as our interest is in modulating the ionization of the monolayer by altering the potential applied to the substrate. Were our interest in determining the actual ionization of the interface, then the synergistic influence that substrate potential and monolayer ionization have on the pH local to the interface would need to be determined. When titrating a monolayer that contains ionizable functionality, the pH of the solution adjacent to the interface may be different from the bulk value, especially near the pKa when the interface begins to have substantial ionization. Quantitatively evaluating the pK1/2 of a confined monolayer that contains an acid/base functional group by titration, therefore, requires that interfacial double-layer effects be taken into consideration. Our results show that potential applied to the substrate can impact the charge density of the ionizable functionality of the monolayer. We believe this occurs, at least in part, because changes to the potential applied to the substrate alter the pH local to the interface. For this reason, the influence of the applied potential is greatest when the pH of the adjacent solution is near the pKa of the ionizable group. The ability to modulate the charge of the surface in situ by altering the substrate potential has several potential applications. We are currently investigating using potential to selectively deposit polyelectrolytes onto the monolayer-modified substrates. Acknowledgment. This work was supported in part by the Jeffress Memorial Trust. We also acknowledge the National Science Foundation for supporting R.V. as a summer REU student by CHE-0244068. W.S. and M.R.A. also thank Professor John Morris for helpful discussions. LA704059Q
