Adsorption of Viologen-Based Polyelectrolytes on Carboxylate

Langmuir 1996, 12, 5087-5092

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Adsorption of Viologen-Based Polyelectrolytes on Carboxylate-Terminated Self-Assembled Monolayers Luis A. Godı´nez, Rene´ Castro, and Angel E. Kaifer* Chemistry Department, University of Miami, Coral Gables, Florida 33124-0431 Received May 16, 1996. In Final Form: July 23, 1996X The adsorption of viologen-containing polyelectrolytes on the surface of carboxylate-terminated, selfassembled monolayers was investigated by voltammetric methods. The two polyelectrolytes used had either butyl (VC4) or undecyl (VC11) linkages between the repeating viologen (4,4′-bipyridinium) subunits. Both polyelectrolytes were found to adsorb at neutral pH on monolayers prepared by the self-assembly of either 3-mercaptopropionic (HSC2COOH) or 8-mercaptooctanoic (HSC7COOH) acids on gold. However, the adsorption was optimized by using the more hydrophobic polyelectrolyte (VC11) and the longer thiol (HSC7COOH). The gradual deprotonation of the HSC7COOH monolayer in the pH range 6-11 enhances the adsorption of the VC11 polyelectrolyte. From these data, an apparent pKa of ∼8 for the monolayer -COOH groups was obtained. The overall degree of packing of the viologen subunits at the monolayersolution interface was found to be rather poor. The combined monolayer-polyelectrolyte interfacial structures did not block effectively the reduction of dissolved Ru(NH3)63+ ions on the underlying gold electrode.

Introduction Self-assembled monolayers (SAMs) prepared by the spontaneous chemisorption of thiolates on gold can be used to create surfaces of controlled composition.1-6 Functionalization of the ω-terminus of monomeric thiol molecules leads, after their self-assembly on the gold surface, to the positioning of the selected functional groups on the exposed monolayer surface.7-11 In this regard, carboxylate-terminated SAMs have been used by several groups because of the synthetic accessibility of the ω-mercaptocarboxylic acids and the polarity and reactivity of the -COOH groups. These monolayers are increasingly used as templates for the adsorption of cationic polyelectrolytes, metal ions, and other species.12-15 An interesting property of carboxylate-terminated SAMs is that their surface charge density can be controlled by the pH of the contacting solution. The pKa of the monolayer -COOH groups has been determined to be several units higher * To whom correspondence should be addressed: tel, (305) 2843468; fax, (305) 662-4007; e-mail, [email protected]. X Abstract published in Advance ACS Abstracts, October 1, 1996. (1) Rojas, M. T.; Ko¨niger, R.; Stoddart, J. F.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 336. (2) Huisman, B.-H.; Rudkevich, D. M.; Veggel, F. C. J. M.; Reinhouldt, D. N. J. Am. Chem. Soc. 1996, 118, 3523. (3) Lu, T.; Zhang, L.; Gokel, G. W.; Kaifer, A. E. J. Am. Chem. Soc. 1993, 115, 2542. (4) Davis, F.; Stirling, C. J. M. J. Am. Chem. Soc. 1995, 117, 1192. (5) Collard, D. M.; Fox, M. A. Langmuir 1991, 7, 1192. (6) Zhang, L.; Godı´nez, L. A.; Lu, T.; Gokel, G. W.; Kaifer, A. E. Angew. Chem., Int. Ed. Engl. 1995, 34, 235. (7) Caldwell, W. B.; Chen, K.; Mirkin, C. A.; Babinec, S. J. Langmuir 1993, 9, 1945. (8) Willner, I.; Lion-Dagan, M.; Marx-Tibbon, S.; Katz, E. J. Am. Chem. Soc. 1995, 117, 6581. (9) Delamarache, E.; Sundarababu, G.; Biebuyick, H.; Michel, B.; Gerber, C.; Sigrist, H.; Wolf, H.; Ringsdorf, H.; Xanthopoulos, N.; Mathieu, H. J. Langmuir 1996, 12, 1997. (10) Zhang, L.; Lu, T.; Gokel, G. W.; Kaifer, A. E. Langmuir 1993, 9, 786. (11) Mrksich, M.; Grunwell, J. R.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 12009. (12) Li, J.; Liang, K. S.; Scoles, G.; Ulman, A. Langmuir 1995, 11, 4418. (13) Bharathi, S.; Yegnaraman, V.; Rao, G. P. Langmuir 1995, 11, 666. (14) Collinson, M.; Bowden, E. F. Langmuir 1992, 8, 1247. (15) Zhou, Y.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M.; Wells, M. J. Am. Chem. Soc. 1996, 118, 3773.

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than that exhibited by similar carboxylic acids in solution.16-18

In this paper we report the results of our work with SAMs formed by two different ω-mercaptocarboxylic acids acting as templates for the adsorption from solution of two different viologen-based polyelectrolytes (see structures). The results indicate that the carboxylateterminated monolayer enhances the adsorption of the electroactive, cationic polyelectrolyte. However, the relative hydrophobic character of the viologen polyelectrolyte constitutes an even more important component of the overall adsorption driving force. Experimental Section Materials. 3-Mercaptopropionic acid was supplied by Aldrich. 8-Mercaptooctanoic acid was prepared according to a published method.17 The viologen polyelectrolytes were synthesized by refluxing an equimolar mixture of 4,4′-bipyridine and the corresponding dibromoalkane in acetonitrile.19 The resulting precipitate was collected by filtration and washed with dichloromethane to remove unreacted starting materials and dried under vacuum. This procedure gave VC11 with an average degree of polymerization (n) of 16 viologen units and VC4 with n ) 3.4. In order to obtain longer VC4 chains, we repeated the reaction in DMF at 80 °C. Under these conditions the isolated VC4 polyelectrolyte was found to have n ) 9.7. This was the sample of VC4 polyelectrolyte utilized throughout this work. 1H NMR spectroscopy was utilized to assess the purity and average degree (16) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370. (17) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (18) Tarlov, M. J.; Bowden, E. F. J. Am. Chem. Soc. 1991, 113, 1847. (19) Factor, A.; Heinsohn, G. E. Polym. Lett. 1971, 9, 289.

© 1996 American Chemical Society

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Godı´nez et al.

Figure 1. 400 MHz 1H NMR spectrum of the VC11 polyelectrolyte in D2O. of polymerization of the isolated bromide salts of the viologen polyelectrolytes. Figure 1 shows the 400 MHz 1H NMR spectrum of VC11 in D2O. The degree of polymerization was obtained from the ratio of the integrated proton resonances from fully quaternized bipyridinium units (observed at 8.42 and 9.02 ppm) to those from the terminal monoquaternized bipyridinium units (at 7.83, 8.32, 8.64, and 8.83 ppm). Terminal diquaternized bipyridinium subunits with pendant -(CH2)10-CH2Br chains were also considered. However, they were rare (representing only about 10% of all polyelectrolyte terminal groups) as evidenced by the small intensity of the -CH2Br triplet at 3.38 ppm (see Figure 1). Hexaamineruthenium(III) chloride was purchased from Strem Chemicals. Gold (99.999%) was purchased from Johnson Matthey. All other chemicals were of the best quality commercially available. Deionized water was further purified (to a final resistivity g18 MΩ‚cm) by passage through a pressurized Barnstead Nanopure four-cartridge system. Equipment. The electrochemical equipment has been described in detail elsewhere.20 Procedures. The gold bead working electrodes were made by annealing the tip of a gold wire (99.999%, 0.5 mm diameter) in a gas-oxygen flame. The roughness factors of these electrodes were typically in the range 1.1-1.2.21 However, the viologen surface coverages reported here are not corrected for surface roughness. The geometric areas of the gold electrodes were calculated from the slopes of the linear plots of cathodic peak current versus (scan rate)1/2 obtained for the diffusion-controlled reduction of Ru(NH3)63+. For this purpose, we utilized the previously measured diffusion coefficient of this electroactive ion (7.5 × 10-6 cm2/s at 25 °C in 0.1 M NaCl).22 The preparation of SAMs on the gold bead electrodes was done by immersing the electrodes overnight in a 1.0 mM solution of the mercaptocarboxylic acid in ethanol. The electrochemical behavior of the modified electrode was characterized in the range +0.7 to -0.65 V vs SSCE (sodium chloride saturated calomel electrode) and compared to the previously recorded response of the clean gold bead electrode. Flat background current responses were observed in this potential range for both the naked and the SAM-covered gold electrodes. Commonly, monolayer deposition leads to diminished background currents in this potential range due to decreased electrode capacitance brought about by the presence of the organic monolayer. Electrodes which did not behave in this way were discarded.

Results and Discussion The surface that -SC2COOH and -SC7COOH monolayers project to the contacting solution is expected to be (20) Bernardo, A. R.; Stoddart, J. F.; Kaifer, A. E. J. Am. Chem. Soc. 1992, 114, 10624. (21) Rodrı´guez, J. F.; Mebrahtu, T.; Soriaga, M. P. J. Electroanal. Chem. 1987, 233, 283. (22) Gomez, M.; Li, J.; Kaifer, A. E. Langmuir 1991, 7, 1797.

Figure 2. Cyclic voltammetric response of solutions containing 20 µM polyelectrolyte in pH 7 phosphate buffer (I ) 0.1): (a) VC11 on a Au/SC7COOH electrode (A ) 0.013 cm2); (b) VC4 on a Au/SC2COOH electrode (A ) 0.014 cm2). Scan rate ) 0.2 V/s.

negatively charged at neutral pH due to the partial deprotonation of the exposed -COOH groups. Corn and co-workers have demonstrated that, at similar pH values, carboxylate-terminated SAMs act as effective templates for the irreversible adsorption of poly-L-lysine.23 Therefore, we anticipated that similar interactions would drive the adsorption of the viologen polyelectrolytes VC4 and VC11. Figure 2a shows the cathodic voltammetric response exhibited by a Au/SC7COOH electrode immersed in a 20 µM solution of VC11 (all viologen concentrations are calculated and reported using the molecular mass of the monomeric unit) buffered at pH 7 (phosphate buffer, I ) 0.1). The cyclic voltammogram clearly shows a reversible redox couple centered around -0.53 V vs SSCE, consistent with the reversible monoelectronic reduction of the viologen subunits. At these viologen concentration levels and due to the small diffusion coefficients expected for the polyelectrolyte chains, it is safe to assume that the observed faradaic current response arises from viologen subunits held in the vicinity of the electrode surface. The small separation observed between the anodic and cathodic peak potentials (see data in Table 1) is also consistent with the voltammetric behavior expected from surfaceconfined electroactive species.24 Therefore, the voltammogram confirms the adsorption of the VC11 polyelectrolyte on the Au/SC7COOH electrode surface. Figure 2b shows the result of a similar cyclic voltammetric experiment performed with a Au/SC2COOH electrode immersed in a 20 µM solution of the more hydrophilic polyelectrolyte VC4. Undoubtedly, the level of faradaic current associated (23) Jordan, C. E.; Frey, B. L.; Kornguth, S.; Corn, R. M. Langmuir 1994, 10, 3642. (24) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons: New York, 1980.

Polyelectrolyte Adsorption on Monolayers

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Table 1. Voltammetric Data for the First Viologen Reduction Obtained upon Adsorption of Viologen-Based Polyelectrolytes on Several Surfacesa surface

polyelectrolyte

E°′, V vs SSCE

∆Ep, mV

Γ, mol‚cm-2

bare Au Au/SC2COOH Au/SC7COOH bare Au Au/SC2COOH Au/SC7COOH

VC4 VC4 VC4 VC11 VC11 VC11

-0.58 -0.59 -0.55 -0.52 -0.52 -0.53

10 20 25 20 24 30

3.5 × 10-11 3.2 × 10-11 5.9 × 10-11 4.6 × 10-11 5.0 × 10-11 7.2 × 10-11

a Data obtained at pH 7 (phosphate buffer, I ) 0.1). The concentration of monomeric viologen is 20 µM in all experiments. Scan rate was 0.100 V‚s-1.

with the monoelectronic reduction of the viologen subunits is clearly smaller in this case, indicating that the length of the mercaptocarboxylic acid used to prepare the SAMs and/or the length of the aliphatic linkages between neighboring viologen subunits in the polyelectrolyte chains play an important role in determining the extent of adsorption at the monolayer-solution interface. In order to assess the relative importance of these two structural factors in the overall adsorption phenomena, we performed a series of comparative experiments, surveying all the possible SAM/polyelectrolyte combinations. In addition to Au electrodes derivatized with the two types of SAMs, we also investigated the adsorption of the polyelectrolytes on bare Au. The observed surface coverages (Γ) in all cases are given in Table 1. These Γ values were obtained from the voltammetric data and, strictly speaking, measure the coverage of electroactive viologen subunits. Electroinactive viologen subunits, if present, will not be detected by our measurements. The largest surface coverage was observed on Au/SC7COOH electrodes with the VC11 polyelectrolyte. The Γ values observed with the VC11 polyelectrolyte were always larger than those obtained with VC4. This reveals the importance of the polyelectrolyte’s hydrophobic character on its adsorption characteristics. The more hydrophilic polyelectrolyte (VC4) is better solvated by the water molecules in the bulk solution and, thus, has a lower tendency to aggregate at the monolayersolution interface. The effect of the chain length of the mercaptocarboxylic acid is not so marked. The longer thiol (HSC7COOH) seems to more effectively enhance the adsorption of either polyelectrolyte, probably due to its more efficient self-assembly at the Au surface. In general, the observed surface coverages tend to increase from bare Au to Au/SC2COOH to Au/SC7COOH, but the effect is not as strong as that caused by varying the length of the aliphatic linkages of the polyelectrolyte. In this regard, it must be noted that both polyelectrolytes adsorb appreciably on bare Au surfaces. This particular finding may reflect the negative charge density on the surface of the Au electrode which is anticipated at the negative potential values used to record the voltammetric response of the system.25 From the results of Table 1, we decided to focus our effort on the most efficient adsorption process, i.e., that of VC11 on Au/SC7COOH electrodes. To further verify that the voltammetric response shown in Figure 2a arises from surface-confined viologen groups, we investigated the scan rate dependence of the cathodic peak current for viologen reduction.24 The results are plotted in Figure 3 (squares). The linearity of the plot is again consistent with the confinement of the viologen groups on the surface (25) Schmid, G. M.; Curley-Fiorino, M. E. In Encyclopedia of the Electrochemistry of the Elements; Bard, A. J., Ed.; Marcel Dekker: New York, 1975; Vol. 4, Chapter 3.

Figure 3. Scan rate dependence of the cathodic peak current observed on a Au/SC7COOH electrode immersed in (9) a 20 µM VC11 solution buffered at pH 7 (phosphate buffer, I ) 0.1). The circles (b) represent measurements with the same electrode after transferring it to a polyelectrolyte-free solution (pH 7, phosphate buffer, I ) 0.1).

of the COOH-terminated monolayer, but are the polyelectrolyte chains irreversibly adsorbed on the monolayer surface? To answer this question we extracted the electrode utilized to obtain the data of Figure 3 from the polyelectrolyte solution, rinsed it with pure water, and immersed it in pure buffer solution. The voltammetric response was almost identical to that observed in the polyelectrolyte solution. Figure 3 also shows peak current data (circles) obtained in the polyelectrolyte-free solution. Throughout the range of scan rates surveyed the observed current decrease is less than 15%. Therefore, a large fraction of the polyelectrolyte adsorbs irreversibly on the Au/SC7COOH surface. This finding is in excellent agreement with the report of Corn and co-workers who found irreversible adsorption of poly-L-lysine on a similar COOHterminated SAM.23 We also investigated the dependence of the measured surface coverage on the viologen concentration in the contacting solution. The results of these experiments are presented in Figure 4. The surface coverage values increase with the viologen concentration following approximately the shape of a typical adsorption isotherm until a limiting saturation value of ∼1 × 10-10 mol/cm2 is reached. The fact that the system reaches a limiting Γ value is consistent with the voltammetric response at these low concentrations arising from viologen units held at the electrode surface. Substantial current contributions from solution viologen units (diffusional behavior) would result in linear increases of the measured response with the viologen concentration.24 Such behavior is only observed at higher viologen concentrations (c > 0.1 mM). The limiting Γ value in the plot of Figure 4 corresponds to a molecular surface area of ∼170 Å2 per viologen subunit. We modeled several possible conformations of the viologencontaining monomers in the polyelectrolyte chains in an attempt to associate a monomer conformation to the experimentally determined surface coverage. For instance, Figure 5 provides two possible monomer conformations. The so-called “flat” conformation requires a substantial amount of interfacial surface per monomer unit, while the “folded” conformation makes better use of the interfacial surface and leads to a much better packed structure. However, both conformations would result in

5090 Langmuir, Vol. 12, No. 21, 1996

Figure 4. Dependence of the measured viologen surface coverage on the concentration of monomeric viologen subunits in the solution. The data were obtained with a Au/SC7COOH electrode immersed in a pH 7 phosphate buffer solution containing variable VC11 concentrations.

Figure 5. Schematic representation of possible VC11 polyelectrolyte packing modes on the surface of carboxylateterminated SAMs.

more efficient interfacial packing of the polyelectrolyte chains than supported by the experimental evidence. From these simple considerations, we conclude that the degree of packing of the viologen-based polyelectrolytes at the monolayer-solution interface is rather poor. Does the degree of adsorption of VC11 at the monolayersolution interface depend on the state of protonation of the monolayer -COOH groups? To address this question we immersed a Au/SC7COOH electrode in a solution containing VC11 (20 µM in viologen monomer) and recorded the voltammetric reduction of the surface-confined viologen groups as the pH of the solution was titrated from 6 to 11. The results are given in Figure 6. In this set of experiments we monitored the relative amount of viologen subunits adsorbed at the interface by measuring the cathodic peak current for their reversible monoelectronic reduction. The integration of the cathodic wave to yield the cathodic charge and the corresponding viologen surface coverage was seriously hindered by the proximity of the hydrogen evolution reaction. The position of this wave is

Godı´nez et al.

Figure 6. pH dependence of the cathodic peak current observed on a Au/SC7COOH electrode immersed in a 20 µM VC11 solution also containing 0.1 M NaCl. The pH was adjusted by additions of HCl or NaOH.

obviously highly pH dependent, a factor that decreased substantially the precision of the integrated charge for viologen reduction. Therefore, we elected to measure the cathodic peak current as the best experimental parameter to estimate the degree of polyelectrolyte accumulation on the monolayer surface. The sigmoidal shape of the plot strongly suggests that the adsorption of viologen polyelectrolyte at the monolayer-solution interface follows the charge density developing at the interface due to the deprotonation of the -COOH groups. As a larger fraction of the interfacial -COOH groups ionizes, the surface charge density becomes more negative and the interface attracts a greater number of positively charged viologen subunits, thus enhancing polyelectrolyte adsorption. To ensure that the observed pH-induced changes in the voltammetric behavior were indeed due to the ionization of the terminal -COOH groups of the monolayer, we performed control experiments with bare Au electrodes. In this case, the changes observed in the cathodic peak current as the solution pH was titrated from 6 to 11 were within the error margin of the measurements (