onto the Surface of 3-Mercaptopropionic Acid Monolayers As

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Potential Driven Deposition of Poly(diallyldimethylammonium chloride) onto the Surface of 3-Mercaptopropionic Acid Monolayers Assembled on Gold Wesley Sanders Department of Chemistry, Virginia Polytechnic Institute and State UniVersity, Blacksburg, Virginia 24060-0212

Mark R. Anderson* UniVersity of Colorado DenVer, Department of Chemistry, Campus Box 194, P.O. Box 173364, DenVer, Colorado 80217-3364 ReceiVed May 16, 2008. ReVised Manuscript ReceiVed August 1, 2008 Electrochemical impedance spectroscopy (EIS) and quartz crystal microbalance (QCM) measurements are used to examine the ability of applied potential to drive the ionic self-assembly of poly(diallyldimethylammonium) chloride (PDDA) onto a substrate modified with a monolayer of 3-mercaptopropionic acid (3-MPA). The potential of zero charge (PZC) of the gold electrode modified with a monolayer of 3-MPA was found by differential capacitance measurements to be -0.12 ((0.01) V versus Ag-AgCl. Changing the substrate potential to values positive (-0.01 V vs Ag-AgCl) of the PZC induces interfacial conditions that are favorable for the electrostatic deposition of cationic polymers onto the surface of 3-MPA monolayers. This result is also consistent with experimental observations obtained when the 3-MPA-modified substrate is exposed to 0.10 mol L-1 NaOH solutions. When potentials equal or negative to the PZC are applied to the substrate, no significant accumulation of the PDDA is found by either QCM or EIS measurement. This result is consistent with results obtained when the 3-MPA modified substrate is exposed to 0.10 mol L-1 HCl solutions where no PDDA adsorption is expected because the monolayer is neutral under these conditions. Changes in the impedance and quartz crystal frequency obtained after potential is applied to the substrate are interpreted in terms of the applied potential creating interfacial conditions that are favorable for the deprotonation of the terminal carboxylic acid groups and the subsequent electrostatic assembly of the polycation onto the negatively charged monolayer.

Introduction Integrated chemical systems are described as devices with many chemical components on the same substrate each with key individual functions.1 One of the most common integrated chemical systems makes use of the immobilization of enzymes on the surface of modified electrodes. These molecular sensors make it possible for the electrochemical detection of biological analytes.2 In many cases, the enzymes used to detect physiological species are immobilized onto modified electrode surfaces utilizing ionic self-assembly.1 For example, Hodak and co-workers report the use of a 3-mercapto-1-propanesulfonate monolayer as a negatively charged template for the electrostatic assembly of a poly(allylamine)-ferrocene/glucose oxidase bilayer for the detection of glucose.1 Wu and co-workers report that ionic selfassembly was used to immobilize a polyallylamine-urease bilayer on the surface of an 11-mercaptoundecanoic acid modified electrode for the detection of urea.3 When using an ω-functionalized thiol as an anchor for the electrostatic assembly of polyelectrolytes, the solution pH is generally manipulated to create the conditions required for electrostatic assembly.1,4,5 * To whom correspondence should be sent. E-mail: mark.anderson@ cudenver.edu. (1) Hodak, J.; Etchenique, R.; Calvo, E. J. Langmuir 1997, 13, 2708–2716. (2) Shervedani, R. K.; Mehrjardi, A. H.; Zamiri, N. Bioelectrochemistry 2006, 69, 201–208. (3) Wu, Z.; Guan, L.; Shen, G.; Yu, R. Analyst 2002, 127, 391–395. (4) Jordan, C. E.; Frey, B. L.; Kornguth, S.; Corn, R. M. Langmuir 1994, 10, 3642–3648. (5) Mizutani, F.; Sato, Y.; Yabuki, S.; Hirata, Y. Chem. Lett. 1996, 251–252.

Manipulation of the surface charge of a functionalized SAM with solution pH is an effective way to create conditions favorable to immobilize polyelectrolytes onto substrates; however, care has to be exercised in the selection of the modified interface and the experimental conditions used during polyelectrolyte deposition. The surface pKa of ionizable monolayers and the isoelectric point of biological polyelectrolytes to be deposited have to be considered carefully so that an appropriate bulk pH can be chosen to induce the ionic self-assembly of polyelectrolyte onto the substrate. To address the possibility of incompatible monolayer pKa values and polyelectrolyte isoelectric points, a method that uses potential to induce the deposition of polyelectrolytes on bare gold surfaces was developed.6-9 Electric-field-directed layerby-layer assembly (EFDLA) uses an applied direct current (DC) voltage to control the adsorption of polyelectrolyes to metal surfaces.8 Gao and co-workers report using this technique to assemble arrays of CdTe nanoparticle/PDDA bilayers onto both gold substrates8 and indium tin oxide (ITO) substrates.7 Shi and co-workers used this technique to immobilize glucose oxidase/ PDDA and hydrogen peroxide catalase/PDDA bilayers to ITO substrates for glucose detection.6 In this manner, they are able to use the polarity of the potential to either induce or prevent the assembly of the polyelectrolyte onto the substrate. Potential (6) Shi, L.; Lu, Y.; Sun, J.; Zhang, J.; Sun, C.; Liu, J.; Shen, J. Biomacromolecules 2003, 4, 1161–1167. (7) Gao, M.; Sun, J.; Dulkeith, E.; Gaponik, N.; Lemmer, U.; Feldmann, J. Langmuir 2002, 18, 4098–4102. (8) Sun, J.; Gao, M.; Zhu, M.; Feldman, J.; Mohwald, H. J. Mater. Chem. 2002, 12, 1775–1778. (9) Sun, J.; Gao, M.; Feldmann, J. J. Nanosci. Nanotechnol. 2001, 1, 133–136.

10.1021/la801512g CCC: $40.75  2008 American Chemical Society Published on Web 10/24/2008

Deposition of PDDA onto 3-MPA Monolayers on Au

control in these applications is used as a stimulus to drive the assembly. Others have investigated the role of potential on the ionization of monolayers containing terminal carboxylic acid groups. 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.10,11 Fawcett subsequently extended this treatment to include the Stern layer and discreteness of charge effects.12,13 Cao also investigated the influence that applied potential has on the ionization of ω-functionalized monolayers using surface-enhanced Raman spectroscopy.14 These studies show that a positive shift in potential applied to the substrate shifts the pK1/2 of the confined acid/base group negative to the bulk pKa. In contrast, Sugihara et al. find that application of negative potentials to the substrate shift the surface pK1/2 of a monolayer of 16-mercaptohexadecanoic acid in a negative direction.15 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.16 Lennox et al. extend Smith and White’s thermodynamic treatment to include the kinetics of the electric-fielddriven protonation and deprotonation of the terminal carboxylic acid group.17 These previous results offer clear evidence that substrate potential can impact the ionization of monolayers containing acid or base functionality.10,11 The role of potential on this ionization and possible applications of substrate potential on the self-assembly process, however, have not been clearly defined. Here, we investigate the use of applied potential to drive ionic self-assembly of a polyelectrolyte onto a carboxylic acidterminated self-assembled monolayer in a solution buffered at a pH where the carboxylic acid group of the monolayer would otherwise be neutral. Unlike the EFDLA process described by Gao et al., relatively small potential changes about the potential of zero charge (PZC) of the interface are used to influence the ionization of the terminal carboxylic acid functionality of the 3-mercaptopropionic acid (3-MPA) monolayer. As demonstrated by Anderson,18 Chang et al.,19 and more recently by Sanders et al.,20 the substrate potential can be used to influence the pH local to the interface. This would have the same net effect on the ionization of a mercaptoalkanoic acid monolayer as the negative shift in the pK1/2 described by White and by Fawcett. Altering the pH at the interface with applied potential may create local conditions that favor ionization of the monolayer and drive the subsequent electrostatic assembly of polyions. Electrochemical impedance and quartz crystal microbalance (QCM) measurements are used to follow the potential driven adsorption of poly(diallyldimethyl ammonium chloride) (PDDA) onto the surface of 3-MPA monolayers assembled on gold.

Experimental Chemicals. 3-MPA and PDDA were purchased from the Aldrich Chemical Co. (Milwaukee, WI). Potassium hexacyanoferrate (II) trihydrate and potassium hexacyanoferrate (III) were purchased from (10) Smith, C. P.; White, H. S. Anal. Chem. 1992, 64, 2398–2405. (11) Smith, C. P.; White, H. S. Langmuir 1993, 9, 1–3. (12) Andreu, R.; Fawcett, W. R. J. Phys. Chem. 1994, 98, 12753–12758. (13) Fawcett, W. R.; Fedurco, M.; Kovacova, Z. Langmuir 1994, 10, 2403– 2408. (14) Cao, X.-W. J. Raman Spectrosc. 2005, 36, 250–256. (15) Sugihara, K.; Shimazu, K.; Uosaki, K. Langmuir 2000, 16, 7101–7105. (16) Bryant, M. A.; Crooks, R. M. Langmuir 1993, 9, 385–387. (17) Burgess, I.; Seivewright, B.; Lennox, R. B. Langmuir 2006, 22, 4420– 4428. (18) Anderson, M. R.; Evans, D. H. J. Am. Chem. Soc. 1988, 110, 612–617. (19) Dorain, P.; Vonraben, K.; Chang, R. Surf. Sci. 1984, 148, 439–452. (20) Sanders, W.; Vargas, R.; Anderson, M. Langmuir 2008, 24, 6133–6139.

Langmuir, Vol. 24, No. 22, 2008 12767 Fischer Scientific company. PDDA was low molecular weight, 20% in water. All chemicals were analytical grade and were used without further purification. All other solutions were prepared with water deionized with an 18 MΩ Milli-Q ion exchange filter from Millipore Incorporated. 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 rinsing the electrode with deionized water, it was electrochemically cleaned by cycling the potential from -0.8 to 2 V versus Ag/AgCl at 0.1 mV s-1 in 0.5 M sulfuric acid. The monolayer was prepared by immersing the gold disk working electrode in 5 mL of ethanol containing 0.005 mol L-1 3-MPA for at least 15 min. Reductive desorption measurements conducted with gold substrates modified with 3-MPA monolayers indicate that a maximum surface coverage of 2.5 ((0.2) × 10-9 mol/cm2 is obtained after only 10 min of adsorption time. Additional adsorption time does not increase the monolayer coverage, nor does it alter the behavior of the monolayer in our measurements. Following adsorption of the mercaptan, the modified electrode was rinsed with ethanol and deionized water. 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 three electrode 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 containing 0.10 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 (0.20 V vs Ag/AgCl). The frequency range used for these measurements extended from 1.0 × 105 Hz to 0.1 Hz. Quantitative estimates of the equivalent circuit parameters were obtained by fitting the experimental data to the Randles equivalent circuit 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).21-23 For polyelectrolyte deposition when the substrate was at the opencircuit potential, 3-MPA modified electrodes were placed in either 0.10 mol L-1 NaOH/1.8 × 10-4 mole L-1 PDDA or 0.10 mol L-1 HCl/1.8 × 10-4 mole-L-1 PDDA for 1 min. Following exposure to these conditions, the substrate was removed from the solution, rinsed with deionized water, and placed into a pH 5 solution containing of 0.005 mol L-1 Fe(CN)6-3/-4 and 0.10 mol L-1 KCl for the impedance measurements. For the polyelectrolyte deposition with applied potential, the 3-MPA modified gold substrates were placed into pH 4 phosphate buffer electrolyte solutions containing 1.8 × 10-4 mole L-1 PDDA, and the substrate potential was then held either at the PZC or potentials more positive or more negative than the PZC for 1 min. After 1 min, the substrate potential was returned to open circuit, and the substrate was removed from the solution, rinsed with deionized water, and then placed in the pH 7 solution containing 0.005 mol L-1 Fe(CN)6-3/-4 and 0.10 mol L-1 KCl for the impedance measurement. QCM measurements were performed with an in-laboratory constructed QCM oscillator connected to an HP model 5334B frequency analyzer.24 Quartz crystals (5 MHz resonant frequency) from International Crystal Manufacturing (Oklahoma City, OK) (21) Gyepi-Garbrah, S. H.; Silerova, R. Phys. Chem. Chem. Phys. 2001, 3, 2117–2123. (22) Janek, R. P.; Fawcett, W. R.; Ulman, A. Langmuir 1998, 14, 3011–3018. (23) Bjoerefors, F.; Petoral, R. M., Jr.; Uvdal, K. Anal. Chem. 2007, 79, 8391– 8398. (24) Buttry, D. A.; Ward, M. D. Chem. ReV. 1992, 92, 1355–1379.

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Figure 1. Complex impedance plot for a gold substrate modified with 3-MPA when exposed to different pH solutions.

having a 0.13 cm diameter gold electrode were used as the substrate. Prior to 3-MPA immobilization onto the crystal’s gold electrodes, the quartz crystals were immersed in piranha solution (3:1 concentrated H2SO4/30% H2O2) for less than 1 min to clean the surface. They were then rinsed with water, dried in a stream of N2, and immersed in an ethanol solution of 0.005 mol L-1 3-MPA overnight. For the applied potential and the open-circuit potential experiments, the 3-MPA modified quartz crystals were subjected to the same experimental conditions as the electrodes used in the impedance experiments. Following the application of potential, the substrate potential was returned to open-circuit, and the modified quartz crystals were removed from the solution, rinsed with deionized water, and then dried. Frequency measurements were made in air (the frequency was measured for ∼15 min, with discrete sampling of the frequency every 1 min) before and after the monolayer modified crystals were exposed to the different experimental conditions. The frequency changes reported are an average of at least three separate experiments.

Results and Discussion The charge density of the carboxylic acid-terminated monolayers can be altered by adjusting the pH of the adjacent solution.25-27 This is experimentally monitored by electrochemical impedance spectroscopy (EIS) measurements with the 3-MPAmodified interfaces using Fe(CN)6-3/-4 as a redox probe (Figure 1). As shown in Figure 1, the impedance of the interface toward the Fe(CN)-3/-4 redox probe increases with increasing solution pH. This is consistent with experiments conducted by Kim et al.25 and by Sanders et al.20 Kim et al. assert that the fully protonated 3-MPA monolayer surface allows a close approach of the Fe(CN)6-3/-4 redox probe, producing relatively low chargetransfer resistances.25 In contrast, at basic pH, the monolayer deprotonates, and this introduces repulsive electrostatic interactions between the anionic carboxylate groups confined to the surface and the anionic redox couple, resulting in larger measured impedance. The impedance is measured at different bulk solution pH values, as reported by Kim et al.25 and Sanders et al.,20 and the charge transfer is extracted from a fit of the data to an equivalent circuit to construct a titration curve for the 3-MPA modified electrodes.20,25 A surface pK1/2 of approximately 5.7 ( 0.2 is obtained for the 3-MPA monolayer. This value is consistent with the pK1/2 value of 5.2 by an electrochemical (25) Kim, K.; Kwak, J. J. Electroanal. Chem. 2001, 512, 83–91. (26) Pedrosa, V. A.; Paixao, T. R. L. C.; Freire, R. S.; Bertotti, M. J. Electroanal. Chem. 2007, 602, 149–155. (27) Mendes, R. K.; Freire, R. S.; Fonseca, C. P.; Neves, S.; Kubota, L. T. J. Braz. Chem. Soc. 2004, 15, 849–855.

Figure 2. Complex impedance plot for a 3-MPA monolayer before (line) and after (diamonds) exposure to the cationic polymer PDDA in 0.10 mol L-1 HCl. The EIS measurements were conducted in a buffered pH 5 solution containing 0.005 mol L-1 Fe(CN)6-4 after the modified interface was removed from the acidic PDDA solution.

method or 5.7 by a contact angle titration for 3-MPA monolayers reported elsewhere.28 As shown in Figure 1, by altering the solution pH to more basic values, the impedance increases. This is due to the charge density of the 3-MPA monolayer increasing at more basic pH. When the interface is deprotonated, the negatively charged monolayer can be a substrate for the electrostatic deposition of cationic polyelectrolytes.29,30 Deposition of the polyelectrolyte can also be monitored by EIS measurements. Figure 2 shows impedance data for a 3-MPA-modified electrode measured in a pH 5 solution before and after exposure to a solution containing 1.8 × 10-4 mole L-1 PDDA in 0.10 mol L-1 HCl. At pH 5, the 3-MPA monolayer will be partially deprotonated. After exposure of the monolayer to the cationic PDDA in the acidic solution, the impedance measured at pH 5 does not significantly change from that observed prior to exposure to the PDDA, suggesting that little or no adsorption of the polymer to the interface occurs. This result is consistent with the expectation that the monolayer will be neutral under these experimental conditions and there will not be a strong electrostatic driving force for polyelectrolyte assembly. Results obtained after exposure of the 3-MPA-modified interface to acidic solution conditions should be contrasted with the data shown in Figure 3 where the impedance is measured for a 3-MPA modified electrode in a pH 5 solution before and after exposure to solutions containing 1.8 × 10-4 mole L-1 PDDA in 0.10 mol L-1 NaOH. As shown in Figure 2, if the interface does not change, then the impedance should remain the same. After exposure to the basic conditions, however, we find that the impedance decreases significantly. The decrease in the impedance when the 3-MPA modified interface is exposed to basic solutions containing PDDA is representative of the adsorption of the PDDA to the deprotonated carboxylate groups at the interface. After the (28) Zhao, J.; Luo, L.; Yang, X.; Wang, E.; Dong, S. Electroanalysis 1999, 11, 1108–1111. (29) Hammond, P. T.; Whitesides, G. M. Macromolecules 1995, 28, 7569– 7571. (30) Clark, S. L.; Montague, M.; Hammond, P. T. Supramol. Sci. 1997, 4, 141–146.

Deposition of PDDA onto 3-MPA Monolayers on Au

Figure 3. Complex impedance plot for a 3-MPA before (line) and after (rectangles) exposure to the cationic polymer PDDA in 0.10 mol L-1 NaOH. The EIS measurements were conducted in a buffered pH 5 solution containing 0.005 mol L-1 Fe(CN)6-4 after the modified interface was removed from the basic PDDA solution.

cationic PDDA has adsorbed at the interface, the interfacial attraction between the positively charged polymer confined to the interface and the anionic redox probe in the solution lowers the measured interfacial impedance. These results are consistent with literature reports, and show that impedance spectroscopy measurements are sensitive to the electrostatic interaction between the ionic redox probe and the excess charge present at the interface.20,25 To support this interpretation, QCM measurements were conducted to monitor the frequency/mass change when the modified interface is exposed to different solution pH. After exposure to PDDA in acidic solutions, the frequency decreases by 4 ( 1 Hz compared to the frequency of a quartz crystal modified with 3-MPA prior to exposure to these conditions. In contrast, after exposure of the crystal to PDDA in basic solutions, the frequency decreases by 13 ( 2 Hz. This frequency change is consistent with values previously reported for the electrostatic deposition of polyelectrolytes.8 There is a clear difference in the frequency change associated with the 3-MPA monolayer on exposure of the modified substrate to the PDDA polymer under conditions where the interface is expected to be neutral (acidic solutions) compared to when the interface is expected to be anionic (basic solutions). This result is consistent with the electrostatic deposition of the PDDA polycation onto the negatively charged surface. For these initial measurements, the substrate was at open circuit potential during exposure to the PDDA; consequently, the bulk solution pH establishes the experimental conditions that dictate whether the 3-MPA monolayer will be neutral or ionized, and whether the PDDA will deposit by electrostatic interactions with the modified interface. As shown previously, applying potential to the substrate causes the accumulation of ionic charge on the solution side of the interface, and, in an aqueous solution, this may impact the pH of the solution adjacent to the substrate to create conditions favorable for the deprotonation of the monolayer.18-20 Anderson also shows that the interfacial pH change can alter the percent protonation of ionizable groups confined to the interface.18 This suggests that relatively small values of the applied potential can impact the pH local to the interface enough to alter the protonation of the terminal carboxylic acid groups confined at the interface and induce electrostatic deposition of polycations from solution.

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Figure 4. Complex impedance plot for a substrate modified with 3-MPA before (diamonds) and after (squares) exposure to PDDA while the substrate potential is held at -0.01 V vs Ag/AgCl. The EIS measurements were conducted in a buffered pH 7 solution containing 0.005 mol L-1 Fe(CN)6-3/-4 before and after potential is applied to the substrate.

If the potential is held at the PZC, then no organization of the double layer occurs, and the ionization of the 3-MPA monolayer is established by the bulk solution pH. Differential capacitance measurements are used to experimentally estimate the PZC for the 3-MPA modified interface.16 A pH 4 phosphate buffer was used as the supporting electrolyte as impedance measurements were taken from -0.6 to 0.6 V versus Ag/AgCl in 0.1 V increments. From our experimental data, the PZC for a 3-MPA monolayer is found to be approximately -0.12 ((0.01) V versus Ag/AgCl. This value is consistent with that reported by Becka and Miller (-0.11 V vs SCE) for a 14-mercapto-tetradecanol monolayer.31 When the substrate potential is adjusted to a value positive to the PZC, conditions are established that result in the pH local to the interface being more basic than the bulk value.14,18,19 Under these conditions, the 3-MPA monolayer may be deprotonated, generating interfacial conditions that are favorable for the electrostatic deposition of the polyelectrolyte. Figure 4 shows the impedance spectroscopy measurement before and after a potential more positive than the PZC (-0.01 V vs Ag/AgCl) is applied to the 3-MPA modified substrate in the presence of PDDA and a pH 4 bulk solution. As indicated by the bulk solution EIS measurements (Figure 1), at pH 4, the 3-MPA monolayer is in its protonated neutral form and will not favor adsorption of the polycation. Although the monolayer should be protonated at pH 4 and conditions are not favorable for the electrostatic deposition of the PDDA polycation, the interfacial impedance decreases after exposure to the PDDA (the charge-transfer resistance is 1880 Ω before exposure to PDDA and 740 Ω after exposure) and a substrate potential held at values positive to the PZC (compared to the impedance measured with the same sample prior to exposure to PDDA and positive applied potentials). This EIS result is analogous to the behavior observed when the modified substrate is exposed to the 0.10 mol L-1 NaOH/1.8 × 10-4 mole L-1 PDDA solution, and is consistent with the deposition of the cationic PDDA onto a charged, deprotonated monolayer. Quantitative estimates of the charge-transfer resistance of the interface subjected to the positive potential yield a value of the (31) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1993, 97, 6233–6239.

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measured when the modified crystal is exposed to the 0.10 molL-1 NaOH solution with that found when the modified substrate is exposed to solution buffered at pH 4 and subject to an applied potential positive to the PZC suggests that the positive applied potential similarly deprotonates the 3-MPA monolayer, allowing the PDDA to electrostatically deposit onto the monolayermodified substrate. Both the QCM and EIS measurements suggest that application of a potential positive to the PZC can induce ionization of the 3-MPA monolayer to create conditions that are favorable for the electrostatic assembly of the PDDA polycation. If the substrate potential, however, is held at the PZC or slightly negative to the PZC, no evidence of appreciable PDDA deposition is observed. This is consistent with the monolayer remaining protonated under these experimental conditions.

Summary

Figure 5. Complex impedance plot before (line) and after (squares) the 3-MPA-modified substrate is exposed to PDDA solution while the substrate potential is held at -0.13 V vs Ag/AgCl. The EIS measurements were conducted in a buffered pH 7 solution containing 0.005 mol L-1 Fe(CN)6-3/-4 before and after potential is applied to the substrate.

charge-transfer resistance (740 Ω) which is similar to the value obtained when the PDDA is deposited under open-circuit conditions with basic solution pH (680 Ω). These results suggest that potentials positive to the PZC establish interfacial conditions that favor deprotonation of the 3-MPA monolayer and the subsequent electrostatic deposition of the PDDA polycation. Figure 5 shows the impedance behavior of the 3-MPA-modified interface before and after the substrate potential is held at the PZC or slightly negative to the PZC (-0.13 V vs Ag-AgCl) while being exposed to PDDA in a pH 4 solution. Under these conditions, the impedance behavior does not change significantly from that observed prior to PDDA exposure. This result is consistent with the results found when the 3-MPA-modified interface is exposed to the acidic PDDA solution, suggesting that applied potential at or slightly negative to the PZC does not create conditions that favor the adsorption of the PDDA. QCM measurements were also conducted to monitor the frequency change of the modified substrate after the potential is altered in the presence of the PDDA. After a potential positive to the PZC is applied to the 3-MPA-modified quartz crystal in the presence of a pH 4 buffer solution containing the PDDA polymer, the frequency of the quartz crystal decreases by 26 ( 2 Hz (relative to the reference frequency measured under the same experimental conditions with the potential maintained at open-circuit measured in air). This frequency change is compared to a frequency decrease of 31 Hz measured after the 3-MPAmodified quartz crystal is exposed to a 0.10 mol L-1 solution of NaOH containing PDDA. If the substrate potential is held at or slightly negative to the PZC using the same solution conditions, a frequency change of only 3 ( 1 Hz is measured. This result is compared to a 4 Hz frequency change measured when the potential is at open-circuit in the presence of a 0.10 mol L-1 HCl solution. When exposed to the basic solution, the 3-MPA monolayer is deprotonated, and the QCM frequency change is attributed to the electrostatic assembly of the cationic PDDA onto the monolayer. The correspondence of the frequency changes

Impedance and QCM measurements show that applied potential can be used to drive the adsorption of cationic polymers onto ω-functionalized monolayers, even if the bulk solution conditions favor the terminal carboxylic acid groups of the monolayers being in their protonated form. This result is attributed to the potential-induced ionization of the terminal carboxylic acid groups in the 3-MPA monolayer. Impedance measurements show that charge-transfer resistance of 3-MPA/PDDA bilayers formed when an applied potential more positive than the PZC is considerably lower than the charge-transfer resistance of 3-MPA monolayers before exposure to PDDA. The change in the charge-transfer resistance brought about by the potential driven assembly is similar to that observed after PDDA is adsorbed to the 3-MPA monolayer from 0.10 mol L-1 NaOH solutions. QCM measurements also show that there is an increase in mass after a potential more positive than the PZC is applied to the 3-MPA-modified electrode in the presence of PDDA. This mass increase is consistent to that measured when PDDA is adsorbed from basic solutions. After the substrate potential is held at the PZC or slightly negative to the PZC, both EIS and QCM measurements do not have evidence of significant change in the monolayer behavior, suggesting that PDDA adsorption does not occur under these experimental conditions. The work described in this manuscript suggests that applied potential can impact the adsorption of polyelectrolytes by altering the ionization of the terminal carboxylic acid group. Adsorption of a cationic polyelectrolyte to the surface of a carboxylic acidterminated monolayer when exposed to bulk conditions where the surface is expected to be neutral is interpreted as being due to the local pH at the interface differing from bulk pH under applied potential conditions. Because the self-assembly phenomena depends on the intermolecular interactions that exist among molecules, this work shows that the substrate potential provides a relatively simple mechanism for influencing those interactions and for controlling the self-assembly process in situ. Acknowledgment. Partial support of this research was provided by the Jeffress Memorial Trust. Supporting Information Available: Titration curve obtained by plotting the charge-transfer resistance from the impedance spectroscopy of 3-MPA at different pH values. Raw frequency data of a 3-MPA monolayer before and after exposure to 0.10 mol L-1 HCL and NaOH solutions containing 1.8 × 10-4 mol L-1 PDDA. This material is available free of charge via the Internet at http://pubs.acs.org. LA801512G