Electropolymerized Films Formed from the Amphiphilic Decyl Esters of

Biomacromolecules 2005, 6, 1698-1706

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Electropolymerized Films Formed from the Amphiphilic Decyl Esters of D- and L-Tyrosine Compared To L-Tyrosine Using the Electrochemical Quartz Crystal Microbalance Kenneth A. Marx,* Tiean Zhou, and Dang Long Center for Intelligent Biomaterials,Department of Chemistry, University of Massachusetts, Lowell, Massachusetts 01854 Received December 22, 2004; Revised Manuscript Received February 14, 2005

Using the electrochemical quartz crystal microbalance (EQCM), we compared thin films formed on Pt by electropolymerization of L-tyrosine to that of the amphiphilic monomers, decyl esters of D- and L-tyrosine (DEDT and DELT). Mass build-up and film properties were determined as a function of monomer concentration via frequency, f, motional resistance, R, and charge passage, Q, measurements. Films were found to occur by a combination of monomer electropolymerization and adsorption for DEDT and DELT, but only by electropolymerization for L-tyrosine. This difference in film formation process for the monomers is reflected in the net mass build-up for each film, as represented by calculated df/dQ values. For the adsorbing monomers DEDT and DELT, films possessed concentration dependent df/dQ values, more than 100-fold greater than that for L-tyrosine film formation under equivalent electropolymerization conditions. During the entire film growth process, all three films exhibited no significant energy dissipation properties (∆R invariant). Concentration dependent adsorption of significant levels of unpolymerized but self-assembled DEDT and DELT monomers account for the subsequent time dependent mass loss observed from the films maintained in buffer in the absence of monomer. Contact angle measurements demonstrated a pH dependent increase in the surface hydrophilicity of films electropolymerized from the DEDT, DELT, and L-tyrosine monomers but not films formed from phenol and 3-nitrophenol monomers. This behavior is consistent with the monomers’ known changes in titration/charge state properties with increasing pH. This study provided insight into the film formation, stability, and surface hydrophilicity resulting from electropolymerization of these related tyrosine based monomers. This information is critical to assessing the utility these films may have in the development of new biomaterials and as biological macromolecule or cell immobilization strategies in biosensors. Introduction Thin polymeric films can be formed via electropolymerization of monomers on electrode surfaces in aqueous solution. These systems are an attractive area of study because the synthesis is under electrochemical control and these thin films can incorporate biological recognition or signal transduction elements to form, for example, enzyme electrode biosensors.1,2 A significant focus of thin film electropolymerization research has centered on forming and studying highly conjugated polymers. Examples include both conducting polymers3,4 as well as nonconducting polymer films formed from monomers such as phenol and its derivatives (refs 1, 2, 5-8: review). In the case of films derived from electropolymerization of phenol and its derivatives, these have been determined to be thin (10-100 nm) due to the selflimiting properties of the films on the electrode surface.9,10 We have previously carried out experiments to electrochemically polymerize: phenol, phenol derivatives, the phenolic side chain amino acid tyrosine, and the amphiphilic decyl * Corresponding author. Phone: (978) 934-3658. Fax: (978) 934-3013. E-mail: [email protected].

ester derivatives of D- and L-tyrosine (DEDT and DELT), with and without physically entrapping the enzyme horseradish peroxidase.1 These films possessed sensitivities to H2O2 detection that varied only slightly from one another whether determined via the direct electrode or enzyme mediated oxidation of this species. The amphiphilic compounds DEDT and DELT selfassemble spontaneously in aqueous solution into long rodlike or tubular aggregates a few microns wide and hundreds of microns in length.11,12 This self-assembly process is characterized by a pH dependent critical micelle concentration (cmc) in the sub mM range. The formation of aggregates and their binding to a gold coated quartz surface in the quartz crystal microbalance (QCM) was observed via frequency, f, decrease (mass binding increase) and no appreciable motional resistance, R, increase (increasing viscoelastic, energy dissipation behavior) to be pH dependent.13 The pH dependence of these parameters is due to the aliphatic decyl chain, the driving force for aggregation, modulated by the amino acid amine groups’ protonation-deprotonation equilibrium in the pH range of 6-8.14 In another study of the electropolymerization of DEDT compared to tyrosine, we observed diffusion limited electropolymerization behavior for tyrosine and an

10.1021/bm049189n CCC: $30.25 © 2005 American Chemical Society Published on Web 04/14/2005

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Figure 1. Electrochemical quartz crystal microbalance schematic of the 3-electrode setup used for carrying out the electropolymerization/ adsorption studies.

absence of adsorption behavior. In contrast, a significant adsorption of DEDT was observed qualitatively on the Pt electrode, accompanying, but independent of DEDT monomer electropolymerization.7 The difference in behavior of these two compounds is due to the aliphatic decyl chain of DEDT that gives this compound its amphiphilic character. Our goals in this study were to assess the relative contributions of monomer adsorption versus electropolymerization to the kinetics of film formation on Pt electrodes comparing L-tyrosine and the tyrosine based amphiphilic DEDT and DELT monomers. To carry out this comparative study of film formation, we utilized the electrochemical quartz crystal microbalance (EQCM), a valuable tool for forming and characterizing films by both adsorption and electropolymerization (ref 15: review). We determined the relative contributions to film formation of both adsorption and electropolymerization as a function of varying concentrations of the monomers, above and below their cmc. Once formed, the film stability with time was also measured. Finally, contact angle measurements of the electropolymerized films at varying pHs demonstrated differences in relative surface hydrophilicity consistent with the known differences in monomer titration/charge state behavior. This study provided a significant qualitative understanding of film formation, stability, and surface hydrophilicity in these related tyrosine based monomers. Such information is necessary to understand the role these films could play in the development of new biomaterials and as biological macromolecule or cell immobilization strategies for use in biosensors. Materials and Methods We carried out the EQCM experiments in this report as previously described using a 3-electrode setup schematically indicated in Figure 1 .7 A 9 MHz AT-cut quartz crystal, covered with upper and lower Pt (0.3 µm thick; Seiko, EG&G) electrodes was used in a Teflon well-holder with Viton O-rings to seal the crystal. The exposed upper Pt surface was used as the working electrode (WE) and the lower electrode was not exposed to the electrochemical solution. A Pt coil counter electrode (CE) and a Ag/AgCl/3 M NaCl reference electrode (RE) were used in the electro-

chemical film formation measurements with a 350 µL working electrochemical solution volume. Resonant frequency, f, and admittance, A, (from which motional resistance, R, was calculated) of the quartz crystal were monitored with a QCA 917 quartz crystal analyzer (Seiko EG&G) and the values transferred to a PC using WINWEDGE 32, version 3.0 software (TAL Tech.). Current and potential were recorded separately from f and A by the system previously described.1 The current passed at the electrode during individual electropolymerization experiments was used to calculate the charge Q passed during experiments. The entire range of the linear f vs Q plots was then used to calculate the slope, df/dQ. These slopes all had linear fits with R2 values >0.97. Before carrying out electropolymerization experiments, the Pt surface of the QCM was cleaned with acetone, ethanol, and then water. Constant potential (+0.75 V) electropolymerizations were performed at 25 °C in 0.44 M Na phosphate buffer pH 6.5 with the monomer concentrations varied in the mM range. Temperatures were measured to vary not more than about 0.5° C during the 3 h electropolymerization period. At the 3 mM L-tyrosine condition only, cyclic voltammetry was used to electropolymerize the film under the condition of 20 mV/s sweep potential cycled between 0.2 and 1.1 V vs Ag/AgCl. A total of 66 cycles over ∼2 h were carried out. DEDT and DELT stock solutions were freshly prepared as previously described. L-tyrosine was tissue culture grade (Fisher Scientific). For the contact angle measurements, optically transparent indium tin oxide (ITO), (40 ohm/square, 25 mm × 7 mm, Delta Technologies, Stillwater, MN) slides were cleaned with acetone and ethanol. A Pt wire (0.1 mm) was connected to the ITO glass slide with conducting H-20 E silver epoxy (Epoxy Technology, Billerica, MA), and the connecting epoxy portion was covered with 2-Ton Devcon clear epoxy. Before electropolymerization, the ITO surface was polished and cleaned with a 1 µm diamond slurry followed by a 0.05 µm alumina suspension (PK-4 polishing kit, Bioanalytical Systems, West Lafayette, IN). The ITO slide was then washed with 95% ethanol for 10 min followed by sterile PBS. As described previously, films were then electropolymerized upon the ITO electrode with stirring (300 rpm) via cyclic potential scanning from -0.2 V to +1.0 V until the oxidative current became constant.1 Monomer concentrations

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Figure 2. Schematic of the tyrosine and phenolic monomers studied, DELT and DEDT self-assembly and the adsorption versus electropolymerization processes occurring at the Pt electrode surface. For simplicity, we have not attempted to indicate the -O- ether linkage products formed via the phenolic -OH that may also represent a fraction of the linkages found in the electropolymerized phenolic film products.

were 0.1 mM for L-tyrosine, DEDT, and DELT and 5 mM for phenol and 3-nitrophenol. Then the films were washed in PBS followed by distilled water. The contact angles of the different surfaces were measured by the sessile drop method using a self-designed goniometer. A model 3222HPC HeNe laser (Hughes Aircraft) of 5 mW maximum power output was used as the light source. A 10 µL water droplet of varying adjusted pH values was carefully deposited onto each surface with a micro Electropipet (Matrix Technologies, Lowell, MA). Then the left and right contact angles were measured multiple times and an average value and standard deviation were calculated. The initial pH value of the water droplet was confirmed at the end of each contact angle measurement. Results and Discussion We have studied the electropolymerization of a number of tyrosine and phenolic based monomers that are shown schematically in Figure 2. Of these, only the amphiphilic compounds DEDT (R1D) and DELT (R1L), a subset of all the Ri phenolic derivatives we studied, self-assemble in aqueous solution as we illustrate in the upper left of Figure 2. We previously measured the cmc values of these compounds optically using Raman scattering.12 At a given pH, both isomers were found to have identical cmc values; at pH 5.5, cmc ) 0.23 mM, whereas at pH 6, cmc ) 0.17 mM. Given that we are investigating these monomers at an electrode interface in the present study and at a higher K+ and phosphate molarity than in the previous study, we decided to estimate the cmc electrochemically at pH 6.5, the pH value we used in this study. The electrochemical cmc measurement is most relevant to the present study since DEDT has been shown previously to adsorb to a Pt surface.7 We measured the current for varying concentrations of DEDT at + 0.75 V (data not shown) and found that the current saturates at around the cmc ) 0.15 mM DEDT condition.

The current saturates due to this being the condition where the maximum free DEDT monomer concentration exists. Figure 2 illustrates both the Pt electrode adsorption process (A) of self-assembled DEDT and DELT as well as the general oxidative free radical based electropolymerization process (E), characteristic of all of the monomers. Both processes may contribute to the mass of the films formed on the Pt electrode that we investigate in the experiments presented below. In Figure 2, we have shown the electropolymerization product to be either ortho- or meta-carboncarbon linked phenol rings, capable of retaining conjugation levels across adjacent rings. This is probably not the only product formed but is likely to be the predominant product formed under these conditions. Unsubstituted phenol forms a para-ether linked polymer called poly(phenylene oxide) as a result of electropolymerization.8 For para-substituted phenolics, such as the DELT, DEDT, and L-tyrosine in this study, para-linked polymers are not a possibility. For parasubstituted phenols, it has been shown that acidic solutions promote predominantly the ortho-carbon-carbon linked product, whereas basic solutions form predominantly the -O- ether linkage between adjacent rings involving the phenolic oxygen.16-18 Since mildly acidic conditions are what we use for electropolymerization in this study, the orthocarbon-carbon linked ring-ring product shown is likely to be the predominant product species in our films. One brief report of L-tyrosine electropolymerization has shown somewhat similar properties to that of the phenolic compounds.19 Film Formation by Electropolymerization vs Adsorption. To begin to understand the relative contributions of monomer adsorption versus electropolymerization to film deposition at the Pt electrode, we carried out a series of EQCM experiments. As a function of varying concentrations of DEDT, DELT, and L-tyrosine (R2L), we measured the quartz crystal frequency shift, ∆f, and motional resistance shift, ∆R, due to film formation. At the beginning of the experiment, the f and R values measured for the bare Pt



Figure 3. Film formation measured via ∆f and ∆R shift values during the 3 h synthesis phase for DEDT, DELT, and L-tyrosine at +0.75 V in 0.44 M phosphate buffer pH 6.5 at the following concentrations: (A) 0.035 mM DEDT; (B) 0.104 mM DEDT; (C) 0.31 mM DEDT; (D) 0.11 mM DELT; (E) 0.326 mM L-tyrosine; (F) 3 mM L-tyrosine.

electrode were used to normalize all subsequent measured f and R values. Therefore, the bare Pt electrode had both a ∆f and ∆R equal to 0. Film formation was followed at +0.75 V, which is close to the monomer oxidation peak potential that we previously measured1 for all of the phenolic and tyrosine based monomers used in our present study. In Figure 3, we display the time course of the ∆f and ∆R shifts from EQCM experiments for a number of monomers at different concentrations. For all of the film data, the ∆f and ∆R values were continuous, without any abrupt changes. Thus, there is no evidence for film slippage on the gold surface of the oscillating quartz crystal. In panel A, 0.035 mM DEDT was studied, a concentration well below the cmc. During the 3 h time course, there was a continuous ∆f drop to a final value 370 Hz below the starting f value. During the same time period, there was no net change in ∆R at the Pt-solution

interface, indicating that all of the ∆f change was due to mass deposition on the electrode surface. Therefore, the film formed is not dissipating any more energy than the original Pt surface and the film formed exhibits largely elastic mass behavior.7,13,15 With increasing DEDT concentration, in panels B (0.104 mM, just below cmc) and C (0.31 mM, well above cmc), significantly greater film masses grew on the surface over the 3 h time period than at 0.035 mM. Similar ∆f shifts were observed for 0.104 mM close to 1000 Hz and for 0.31 mM close to 1200 Hz. This behavior is expected for the cmc being around 0.15 mM, where the maximum solution monomer concentration is achieved. For the 0.31 mM condition, no significant increase in film electropolymerization would be expected above that observed for 0.15 mM. Again, for both films, there was little if any change in ∆R, indicating no

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Figure 4. Dependence of the films' final measured ∆f shifts on the monomer concentration during +0.75 V electropolymerizations for DEDT, DELT, and L-tyrosine films. The fit lines are linear for the L-tyrosine and sigmoid curves for the DEDT and DELT data. The fit lines are included to simply indicate the data trends.

Figure 5. Dependence of df/dQ calculated from individual electropolymerization experiments on the monomer concentration during film synthesis for DEDT, DELT and L-tyrosine films. A linear fit was used for the L-tyrosine point and a hyperbolic curve was fit to the DEDT and DELT data to indicate the data trends.

increase in energy dissipation by the newly formed films compared to the original Pt surface. The film formation behavior of the DELT isomer is indistinguishable from that of DEDT. In panel D, we present the film formed using 0.11 mM DELT. The total ∆f shift and invariant ∆R value across the 3 h time course resembles very much that of DEDT at a similar concentration. This behavior is in agreement with an earlier QCM study of DEDT in which electropolymerization was not studied but increasing mass binding to a gold surface was followed via a ∆f drop of around 1000 Hz with little initial ∆R change until after a 1 h lag period.13 Also, Pt adsorption of DEDT, but not L-tyrosine, has been observed in the absence of electropolymerization.7 A real difference in EQCM measured film formation behavior results from using the L-tyrosine monomer. In panels E and F, we present film formation results for 0.326 mM and 3 mM L-tyrosine. At the 0.326 mM condition, there is only a small decrease in ∆f of 170 Hz, due to a significantly smaller film mass bound to the Pt than was the case for either DEDT or DELT. Again, there was no change in ∆R, indicating no change in energy dissipation relative to the original Pt surface as a result of the film formation. At the nearly 10-fold higher 3 mM concentration, where we carried out this electropolymerization via cyclic voltammetry, not quite a 600 Hz decrease in shift value occurred due to film formation. Aside from the larger magnitude of the f shift from the higher monomer concentration, no significant differences were noted between the film behavior in this experiment and that of the film in the constant +0.75 V potential at 0.326 mM. Therefore, it would seem that either electropolymerization of L-tyrosine is slow or that the polymeric product formed does not readily adsorb to the Pt electrode surface. In Figure 4, we present a plot of all the measured ∆f shifts for electropolymerized films at 3 h, relative to the value for the starting monomer concentrations. For DEDT and DELT, there is a rise in bound film mass that appears to quickly reach a maximum around 0.1 mM and then plateaus. The sigmoid equation shown as the fit line is not meant to suggest mechanistic behavior but simply provides a good fit to the data. The monomer concentration where the ∆f plateau

region occurs is close to the electrochemically measured cmc, as well as what the optically measured cmc would be for DEDT and DELT at this pH, based upon our earlier measured values at slightly lower pH values.12 Only a small ∆f shift was observed for L-tyrosine at the 0.33 mM monomer concentration. This is because a far lower level of Pt adsorption occurs for the electropolymerized L-tyrosine product than what we observed for the electropolymerized amphiphilic DEDT and DELT. During monomer electropolymerization, in addition to the measured ∆f, we calculated the charge, Q, passed at the electrode surface. The ∆f shift is representative of the total mass being deposited on the Pt surface from monomer adsorption as well as electropolymerization. However, the total Q represents primarily that contribution due to electropolymerization of the monomers in each experiment. Here we are assuming that the charging current, after a brief initial approach to reach steady-state behavior, is invariant for the 3 h electropolymerization period for all of the different monomer concentrations. Making this assumption, the charging current can be neglected. Therefore, we calculated the slopes of ∆f vs Q curves from each of the different experiments where monomer concentrations were varied (data not shown). For each monomer, these df/dQ values reflect the relative amounts of total mass deposited versus the total amount of charge transfer via electropolymerization only. We plot these df/dQ values vs starting monomer concentration in Figure 5. It is clear that both DEDT and DELT possess similar film formation behavior that is significantly different than that of L-tyrosine. The relative monomer df/dQ values at 0.33 mM are 1.50 for DEDT and DELT and 0.0144 for L-tyrosine. This indicates a 104-fold greater mass deposition for DEDT and DELT/electron passing at the electrode than for L-tyrosine at equivalent monomer concentrations. This large difference is far greater than that expected for the ratio of df/dQ values (a ratio of 1.7) based upon Faraday’s law, and the ratio of the masses of the two monomers (DELT/DEDT ) 307: L-Tyr ) 182) assuming all electropolymerized product bound to the electrode. That the measured ∆f values of the films do not


reflect the monomer masses due to a solely electropolymerization process was supported by our calculation of the monomer molecular weights from Faraday’s law. We obtained values well below the actual values. For example, we calculated masses between 100 and 130 g/mol from most of the DEDT and DELT experiments, and 1.53 g/mol for L-tyrosine, compared to the actual monomer values of 307 g/mol and 182 g/mol. We believe that these large differences observed between monomer masses or df/dQ ratios are due to a combination of factors. Self-assembly and surface adsorption occurs for electropolymerized DEDT and DELT but not L-tyrosine. This accounts for a large portion of the difference in monomer mass or df/dQ values. Also, the L-tyrosine monomer has been studied previously by electropolymerization and was found to have poor film formation properties at neutral pH.19 Apparently, the L-tyrosine product is far more soluble than electropolymerized DELT and DEDT. This lowers its df/dQ value below that expected for the electropolymerized products binding to the electrode surface. That L-tyrosine does not adsorb well to the Pt electrode in the absence of the applied electropolymerization potential is also in agreement with our previous study.7 In a prior study of DEDT and DELT monomers, we presented SEM evidence demonstrating that above their measured cmc values, these monomers self-assemble to form long rodlike/plate like structures of ∼2-3 µm average diameter that possess improved mechanical stability following enzymatic polymerization.12 Furthermore, in a related study, self-assembled DEDT bound to the gold surface of a QCM device in the absence of electropolymerization,13 behaving similarly to our measurements in the present study. These prior observations and the current comparisons to L-tyrosine support the idea presented here that the rapid Pt electrode adsorption due to binding of self-assembled DEDT and DELT, produces larger binding levels than that of the L-tyrosine electropolymerization product. Our observations are also consistent with experiments where we used X-ray photoelectron spectroscopy and AFM to measure complete gold surface coverage by enzymatically polymerized DELT and DELT:tyrosineamide (1:1) comonomer mixtures below their cmc concentrations.20,21 Enzymatically polymerized products are similar to those formed here electrochemically, with O-ring to ring linkages predominating in the products.22,23 In these experiments, the DEDT and DELT polymer products adsorbed to gold very similarly, reaching 100% coverage asymptotically by 24 h and producing films estimated to be greater than 10 nm.20 At 3 h, the electropolymerization time condition studied here, glass substrates with deposited gold films exhibited 10.5:14). These changes result in polymer film conformation changes that are presently unknown, but that produce the overall relative hydrophilicity increase exhibited by both films. It is of interest to note that in the case of all three tyrosine monomer based films, going from the R-NH3+ state at low pH to the anion state at high pH produces a trend to more hydrophilic film surfaces. In particular, the contact angle decreases for both DELT and DEDT films were of significantly greater magnitude than for L-tyrosine going from pH 6.1 to 9.0. This suggests that the electropolymerization of DELT and DEDT resulted in a film possessing a much higher level of O-ring to ring linkages and retention of titratable phenolic groups than was the case for the L-tyrosine electropolymerization product.


Conclusions Films electropolymerized from L-tyrosine produced significantly lower mass build-up compared to DELT and DEDT films. The latter two monomers exhibited a substantial monomer synthesis concentration dependent increase in their observed film masses. This behavior agrees with the solution self-assembly properties and the Pt and gold surface binding behaviors of these monomers previously described by our laboratory.7,12-14 Both DEDT and DELT self-assembled above their cmc values and bound Pt surfaces, whereas L-tyrosine did not self-assemble and exhibited little Pt binding in the absence of an applied oxidative potential. The rates of formation of all of the films increased with monomer concentration. Self-assembly adsorption rates of DEDT and DELT monomers dominated their electropolymerization rates, even below the cmc. All three films exhibited little to no change in energy dissipation behavior during their synthesis or upon storage. A 15-20 h film stability experiment demonstrated that the DEDT and DELT films lost increasing mass percentages from their surfaces as a function of increasing monomer concentrations during their synthesis. Overall the films formed from these two monomers were unstable and of poor quality. The film data are consistent with the concept of a significant fraction of unpolymerized adsorbed monomer present in these films at or near their cmc values. Finally, film surface hydrophilicities studied via measured contact angles at different pH values showed properties consistent with the charge state of the different films. The two phenolic monomer based films we examined had little change in their surface properties with increasing pH. By contrast, all of the tyrosine based monomer films demonstrated increasing surface hydrophilicity as the pH was increased from 3.8 to 9.0. Furthermore, at all pH values, DELT films exhibited a consistently greater surface hydrophilicity compared to films formed from the DEDT isomer. This suggested a novel but uncharacterized difference in the stereochemistry of the two films’ surface functional groups. Acknowledgment. The authors acknowledge support from a TURI Grant from the Commonwealth of Massachusetts, an Office of Collaborative Research Grant from UML, NSF EEC-0425826 and NIH Grant R21 GM58583. References and Notes (1) Long, D. D.; Marx, K. A.; Zhou, T. J. Electroanal. Chem. 2001, 501, 107-113. (2) Bartlett, P. N.; Tebbutt, P.; Tyrrell, C. H. Anal. Chem. 1992, 64, 138-142. (3) Forteir, G.; Brassard, E.; Belanger, D. Biosens. Bioelectron. 1990, 5, 473-490. (4) Garguilo, M. G.; Huynh, N.; Proctor, A.; Michael, A. C. Anal. Chem. 1993, 65, 523-528. (5) Eddy, S.; Warriner, K.; Christie, I.; Ashworth, D.; Purkiss, C.; Vadgama, P. Biosens. Bioelectron. 1995, 10, 831-839. (6) Finklea, H. O.; Snider, D. A.; Fedyk, J. Langmuir 1990, 6, 371376. (7) Marx, K. A.; Zhou, T. J. Electroanal. Chem. 2002, 521, 53-60. (8) Yuquing, M.; Kianrong, C.; Xiaohua, W. Trends Biotechnol. 2004, 22, 227-231. (9) Babai, M.; Gottesfeld, S. Surf. Sci. 1980, 96, 461-475. (10) Bartlett, P. N.; Cooper, J. M. J. Electroanal. Chem. 1992, 362, 1-12.

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(11) Sarma, R.; Alva, K. S.; Marx, K. A.; Tripathy, S. K.; Akkara, J. A.; Kaplan, D. L. Mater. Sci. Eng. C 1996, 4, 189-192. (12) Marx, K. A.; Alva, K. S.; Sarma, R. Mater. Sci. Eng. C 2000, 11, 155-163. (13) Marx, K. A.; Zhou, T.; Sarma, R. Biotechnol. Prog. 1999, 15, 522528. (14) Angelopoulos, A.; Marx, K. A.; Oh, K. S. unpublished experiments. (15) Marx, K. A. Biomacromolecules 2003, 4, 1099-1120. (16) Gattrell, M.; Kirk, D. W. J. Electrochem. Soc. 1992, 139, 27362744. (17) Glarum, S. H.; Marshall, J. H.; Hellman, M. Y.; Taylor, G. N. J. Electrochem. Soc. 1987, 134, 81. (18) Lapuente, R.; Cases, F.; Garces, P.; Morallon, E.; Vazquez, J. L. J. Electroanal. Chem. 1998, 451, 163-171. (19) Malfoy, B.; Reynaud, J. A. J. Electroanal. Chem. 1980, 114, 213223.

Marx et al. (20) Angelopoulus, A. P.; Marx, K. A.; Oh, K. S., Proceedings MRS: Polymer Interfaces and Thin Films; Materials Research Society: Pittsburgh, PA, 2002; Vol. 710, pp 207-212. (21) Marx, K. A.; Lee, J. S.; Sung, C. Biomacromolecules 2004, 5, 18691876. (22) Dordick, J. S.; Marletta, M. A.; Klibanov, A. M. Biotechnol. Bioeng. 1987, 30, 31-36. (23) Colonna, S.; Gaggero, N.; Richelmi, C.; Pasta, P. Trends Biotechnol. 1999, 17, 163-169. (24) Solomons, T. W. G. Organic Chemistry; J. Wiley & Sons: New York, 1976. (25) McCarley, R. L.; Thomas, R. E.; Irene, E. A.; Murray, R. W. J. Electroanal. Chem. 1990, 290, 79-92.

BM049189N

Electropolymerized Films Formed from the Amphiphilic Decyl Esters of

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