Enzymatic Synthesis of Conducting Polyaniline in Micelle Solutions

Natick, Massachusetts 01760. Received July 15, 2002. In Final Form: October 1, 2002. Peroxidase-catalyzed template-guided polymerization of aniline ha...
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Langmuir 2002, 18, 9696-9704

Enzymatic Synthesis of Conducting Polyaniline in Micelle Solutions Wei Liu, Jayant Kumar,* and Sukant Tripathy† Center for Advanced Materials, Departments of Chemistry and Physics, University of Massachusetts Lowell, Lowell, Massachusetts 01854

Lynne A. Samuelson* Natick Soldier Center, U.S. Army Soldier & Biological Chemical Command, Natick, Massachusetts 01760 Received July 15, 2002. In Final Form: October 1, 2002 Peroxidase-catalyzed template-guided polymerization of aniline has been carried out in aqueous micellar solutions of surfactants, such as sodium dodecylbenzenesulfonate, (SDBS), hexadecyltrimethylammonium bromide (CTAB), and polyoxyethylene(10) isooctylphenyl ether (Triton X-100). The properties of these enzymatically synthesized polyanilines strongly depend on the structure of the surfactants used in the formation of micelles as shown by the UV-vis-near-IR absorption spectroscopy. The micelles formed by strong acid surfactants such as SDBS are suitable templates for the enzymatic synthesis of conducting polyaniline. In solution, the micelles may serve as nanoreactors for the aniline monomer prior to the reaction and provide the necessary low-pH local environment for the growth of conducting polyaniline. The formation of low-pH local environment is further confirmed by the pH change of the bulk solution after the loading of aniline onto the micelles. FTIR, NMR, and UV-vis-near-IR absorption spectroscopy as well as cyclic voltammetry are used in the characterization of the synthesized polyaniline. The enzymatically synthesized SDBS micellar polyanilines are electrically active and soluble in organic solvents such as DMSO and DMF in emeraldine base form.

Introduction Polyaniline is one of the most interesting conducting polymers1 due to its stability and interesting electrical and optical properties. Potential applications of polyaniline include organic lightweight batteries,2 microelectronics,3 electrochromic displays,4 electromagnetic shielding,5 and sensors.6 Typically, conducting polyaniline is synthesized chemically7 or electrochemically8 in acidic solutions and is not soluble in common organic solvents. Alternative methods have been designed to improve the solubility and processability of the synthesized polymers. These methods †

Deceased. * To whom all correspondence should be addressed.

(1) (a) MacDiarmid, A. G. Synth. Met. 1997, 84, 27. (b) MacDiarmid, A. G.; Chiang, J. C.; Richter, A. F.; Epstein, A. J. Synth. Met. 1987, 18, 285. (c) Chinn, D.; Dubow, J.; Liess, M.; Josowicz, M.; Janata, J. Chem. Mater. 1995, 7, 1504. (d) MacDiarmid, A. G.; Epstein, A. J. In Science and Applications of Conducting Polymers; Salaneck, W. R., Clark, D. T., Samuelsen, E. J., Eds.; Adam Hilger: Bristol, England, 1990. (e) Cao, Y.; Li, S.; Xue, Z.; Guo, D. Synth. Met. 1986, 16, 305. (2) (a) Genies, E. M.; Hany, P.; Santier, C. J. J. Appl. Electrochem. 1988, 18, 285. (b) Kaneko, M.; Nakamura, H. J. Chem. Soc., Chem. Commun. 1985, 346. (3) (a) Paul, E. W.; Rico, A. J.; Wrighton, M. S. J. Phys. Chem. 1985, 89, 1441. (b) Huang, W. S.; Lecorre, M. A.; Tissier, M. J. Vac. Sci. Technol. 1991, B9, 3428. (c) Chen, S.-A.; Fang, Y. Synth. Met. 1993, 60, 215. (4) (a) Kitani, A.; Yano, J.; Sasaki, K. J. Electroanal. Chem. 1986, 209, 227. (b) Jelle, B. P.; Hagen, G. J. Electrochem. Soc. 1993, 140, 3560. (5) (a) Wood, A. S. Mod. Plastic Int. 1991, Aug, 3. (b) Epstein, A. J.; Yue, J. US Patent No. 5237991, 1991. (6) (a) Svetlicic, V.; Schmidt, A. J.; Miller, L. L. Chem. Mater. 1998, 10, 3305. (b) Sukeerthi, S.; Contracor, A. Q. Anal. Chem. 1999, 71, 2231. (c) Sangodkar, H.; Sukeerthi, S.; Srinivasa, R. S.; Lai, R.; Contractor, A. Q. Anal. Chem. 1996, 68, 779. (d) Liu, C.-H.; Liao, K.-T.; Huang, H.-J. Anal. Chem. 2000, 72, 2925. (7) (a) Focke, W. W.; Wnek, G. E.; Wei, Y. J. Phys. Chem. 1987, 91, 5813. (b) Wu, C.-G.; Chen, J.-Y. Chem. Mater. 1997, 9, 399. (c) Liu, G.; Freund, M. S. Macromolecules 1997, 30, 5660.

included using ring or N-substituted aniline monomers,9,10 posttreatment of the synthesized polyaniline with fuming sulfuric acid,11 and dispersion polymerization of aniline in the presence of steric stabilizers.12,13 Recently, a biocatalytic approach has been developed to synthesize water-soluble conducting polyaniline at a pH around 4.0.14 In this biological approach, the aniline is coupled together by peroxidase-catalyzed oxidation in the presence of an anionic polyelectrolyte. The anionic polyelectrolyte serves as a template to electrostatically complex the monomer (8) (a) Diaz, A. F.; Logan, J. A. J. Electroanal. Chem. 1980, 111, 111. (b) Watanabe, A.; Mori, K.; Iwabuchi, A.; Iwasaki, Y.; Nakamura, Y.; Ito, O. Macromolecules 1989, 22, 3521. (c) Verghese, M. M.; Ramanathan, K.; Ashraf, S. M.; Kamalasanan, M. N.; Malhotra, B. D. Chem. Mater. 1996, 8, 822. (9) (a) Leclerc, M.; Guay, J.; Dao, L. H. Macromolecules 1989, 22, 649. (b) Wei, Y.; Hariharan, R.; Patel, S. A. Macromolecules 1990, 23, 758. (c) Nguyen, M. T.; Dao, L. H. J. Electroanal. Chem. 1990, 289, 37. (d) Nguyen, M. T.; Paynter, R.; Dao, L. H. Polymer 1992, 33, 214. (10) (a) Chevalier, J.-W.; Bergeron, J.-Y.; Dao, L. H. Macromolecules 1992, 25, 3325. (b) Bergeron, J.-Y.; Dao, L. H. Macromolecules 1992, 25, 3332. (c) DeArmitt, C.; Armes, S. P.; Winter, J.; Urbe, F. A.; Gottesfeld, S.; Mombourquette, C. Polymer 1993, 34, 158. (11) (a) Yue, J.; Epstein, A. J. J. Am. Chem. Soc. 1990, 112, 2800. (b) Yue, J.; Wang, Z. H.; Cromack, K. R.; Epstein, A. J.; MacDiarmid, A. G. J. Am. Chem. Soc. 1991, 113, 2665. (c) Chen, S.-A.; Hwang, G.-W. J. Am. Chem. Soc. 1994, 116, 7939. (d) Chen, S.-A.; Hwang, G.-W. J. Am. Chem. Soc. 1995, 117, 10055. (e) Chen, S.-A.; Hwang, G.-W. Macromolecules 1996, 29, 3950. (f) Wei, X.-L.; Wang, Y. Z.; Long, S. M.; Bobeczko, C.; Epstein, A. J. J. Am. Chem. Soc. 1996, 118, 2545. (12) (a) Stejskal, J.; Kratochvil, P. Langmuir 1996, 12, 3389. (b) Riede, A.; Helmstedt, M.; Riede, V.; Stejskal, J. Colloid Polym. Sci. 1997, 275, 814. (c) Riede, A.; Helmstedt, M.; Riede, V.; Zemek, J.; Stejskal, J. Langmuir 2000, 16, 6240. (13) (a) Zang, X.-X.; Sadighi, J. P.; Mackewitz, T. W.; Buchwald, S. L. J. Am. Chem. Soc. 2000, 122, 7606. (b) Hua, M.-Y.; Huang, G.-W.; Chuang, Y.-H.; Chen, S.-A.; Tsai, T.-Y. Macromolecules 2000, 33, 6235. (14) (a) Samuelson, L. A.; Anagnostopoulos, A.; Alva, K. S.; Kumar, J.; Tripathy, S. K. Macromolecules 1998, 31, 4376. (b) Liu, W.; Kumar, J.; Tripathy, S. K.; Senecal, K. J.; Samuelson, L. A. J. Am. Chem. Soc. 1999, 121, 71.

10.1021/la0206357 CCC: $22.00 © 2002 American Chemical Society Published on Web 11/12/2002

Conducting Polyaniline in Micelle Solutions

prior to the reaction and also as a dopant for the resulting polyaniline. Because of the good solubility of the polyelectrolyte template, the synthesized polyaniline and polyelectrolyte complex is water-soluble and can be processed into thin films and fibers15 for further applications. Most importantly, the presence of the template may inherently minimize the parasitic branching and promotes a more para-directed, head-to-tail coupling of aniline. So far, polyelectrolytes such as sulfonated polystyrene (SPS),14 poly(vinylphosphonic acid) (PVP),16a and DNA16b have been demonstrated as “good templates” for enzymatic synthesis of conducting polyaniline. Horseradish peroxidase is a member of the family of heme peroxidase,17 which is capable of catalyzing the polymerization of anilines and phenols in the presence of hydrogen peroxide.18 Usually, the peroxidase-catalyzed polymerization of aniline and phenols produces low molecular weight products in aqueous solutions. A variety of conditions including mixed aqueous-organic solvents,19 reverse micelles,20 water-air interface,21 and the presences of templates14 have been investigated to optimize these enzymatically catalyzed reactions. The polyanilines enzymatically synthesized in both organic and aqueous solutions without templates are not electrically active due to the low molecular weight and the branched structure. The use of the anionic polyelectrolytes as templates in peroxidase-catalyzed polymerization of aniline lead to the first enzymatic synthesis of conducting polyaniline, thus opening the door for the synthesis of electroactive conducting polymers using a biocatalytic approach. The role of template in promoting a more para-directed, head-to-tail coupling of aniline in this biological approach has been investigated recently by nuclear magnetic spectroscopy (NMR) studies.22 The local environment in the vicinity of the template moieties has a charge density and pH which are different from those of the bulk solution. This local environment is critical in anchoring and aligning the aniline monomers for reaction and ultimately controls the form of polyaniline (conducting or insulating) obtained during reaction. Strongly acidic polyelectrolytes, such as sulfonated polystyrene (SPS), are the most favorable templates because they provide a lower, local pH environment that serves as a type of nanoreactor to both charge and preferentially align the aniline monomers through electrostatic and hydrophobic interactions to promote the desired head-to-tail coupling. (15) Wang, X.; Schreuder-Gibson, H.; Downey, M.; Tripathy, S.; Samuelson, L. Synth. Met. 1999, 107, 117. (16) (a) Nagaragan, R.; Tripathy, S.; Kumar, J.; Bruno, F. F.; Samuelson, L. Macromolecules 2000, 33, 9542. (b) Nagarajan, R.; Liu, W.; Kumar, J.; Tripathy, S. K.; Bruno, F. F.; Samuelson, L. A. Macromolecules, in press. (17) Dunford, H. B. In Peroxidases in Chemistry and Biology; Everse, J., Everse, K. E., Grisham, M. B., Eds.; CRC Press: Boca Raton, FL, 1991; Vol. 2, pp 1-24. (18) Saunders, B. C.; Holmes-Siedle, A. G.; Stark, B. P. In Peroxidase; Butterworths: London, 1964. (19) (a) Dordick, J. S.; Marletta, M. A.; Klibanov, A. M. Biotechnol. Bioeng. 1987, 30, 31. (b) Akkara. J. A.; Senecal, K. J.; Kaplan, D. L. J. Polym. Sci., Part A: Polym. Chem. 1991, 29, 1561. (c) Akkara, J. A.; Salapu, P.; Kaplan, D. L. Indian J. Chem. 1992, 31B, 855. (d) Wang, P.; Dordick, J. S. Macromolecules 1998, 31, 941. (e) Ikeda, R.; Uyama, H.; Kobayashi, S. Macromolecules 1996, 29, 3053. (20) (a) Rao, A. M.; John, V. T.; Gonzalez, R. D.; Akkara, J. A.; Kaplan, D. L. Biotechnol. Bioeng. 1993, 41, 531. (b) Premachandran, R.; Banerjee, S.; John, V. T.; McPherson, G. L.; Akkara, J. A.; Kaplan, D. L. Chem. Mater. 1997, 9, 1342. (c) Premachandran, R. S.; Banerjee, S.; Wu, X.-K.; John, V. T.; McPherson, G. L.; Akkara, J. A.; Ayyagari, M.; Kaplan, D. L.; Macromolecules 1996, 29, 6452. (21) Bruno, F.; Akkara, J. A.; Samuelson, L. A.; Kaplan, D. L.; Marx, K. A.; Kumar, J.; Tripathy, S. K. Langmuir 1995, 11, 889. (22) Liu, W.; Cholli, A. L.; Nagarajan, R.; Kumar, J.; Tripathy, S. K.; Senecal, K. J.; Bruno, F. B.; Samuelson, L. A. J. Am. Chem. Soc. 1999, 121, 11345.

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Interestingly, it was found that micelles formed from aggregating strongly acidic surfactant molecules such as sodium dodecylbenzenesulfonate (SDBS) also provide a suitable local environment that leads to the formation of conducting polyaniline. The local environment created by this micellar aggregation of strongly acidic surfactant molecules is similar to that of strongly acidic polyelectrolytes. Each of these systems can form charge double layers around which counterions such as H+ may be condensed and can form hydrophobic pockets with which the monomers may associate.22 The ability to specifically control the formation and surface charge density of micelles and their use as templates in the enzymatic polymerization of anilines may offer new possibilities to tailor the synthesized conducting polyaniline. Acidic surfactants such as SDBS,23 SDS,24 and dinonylnaphthalenesulfonic acid (DNNSA)25 have been demonstrated as good dopants to manipulate the solubility and processability of chemically synthesized conducting polyaniline. In the present work, the peroxidase-catalyzed polymerization of aniline is expanded into aqueous micelle solutions of SDBS, Triton X-100, and CTAB. The details of the synthesis and characterization of the enzymatic micellar polyaniline are discussed. Experimental Section Materials. Horseradish peroxidase (HRP) (EC 1.11.1.7) (200 unit/mg) was purchased from Sigma Chemicals Co., St. Louis, MO, with RZ > 2.2. A stock solution of 10 mg/mL in pH 6.0, 0.1 M phosphate buffer was prepared. Aniline (99.5%) and each of the surfactantssdodecylbenzenesulfonic acid, sodium salt (99%, SDBS); polyoxyethylene(10) isooctylphenyl ether (99%, Triton X-100); and hexadecyltrimethylammonium bromide (CTAB)s were obtained from Aldrich Chemicals Co. Inc., Milwaukee, WI, and used as received. All other chemicals and solvents (analytical grade or better) were commercially available and used as received. Measurement of pH Change. A 0.1 M, SDBS micelle solution was prepared by dissolving 3.48 g of SDBS surfactant into 100 mL of distilled water. The pH of the solution was adjusted to 4.3. Similarly, a pH 4.3, 0.1 M, aniline solution was made by dissolving aniline into distilled water. By mixing different amounts of the two stock solutions together, solutions with the molar ratio of aniline to SDBS varied from 0.2 to 5 were prepared. Distilled water at pH 4.3 was used to adjust the volume of the final solution to 1 mL. The molar ratios (numbers in the front of the parentheses) of aniline to SDBS and the overall composition (numbers inside the parentheses with the format as aniline:SDBS:water (mL)) of these solutions were 0.2(0.1:0.5:0.4); 0.4(0.2:0.5:0.3); 0.6(0.3: 0.5:0.2); 0.8(0.4:0.5:0.1); 1(0.5:0.5:0.0); 1.25(0.5:0.4:0.1); 1.7(0.5: 0.3:0.1); 2.5(0.5:0.2:0.3); and 5(0.4:0.5:0.1), respectively. The bulk pH’s of these solutions were measured using an Orion model 520 pH meter. Enzymatic Polymerization in Micelle Solutions. A stock micelle solution was prepared by dissolving surfactants into a 0.1 M phosphate buffer to a concentration of 0.2 M with continuous stirring, followed by the adjustment of the pH to 4.3 with HCl. The micelles were formed spontaneously as the concentration of the surfactant in the solution was greater than the cmc (critical micellar concentration); the known cmc of SDBS is 1.6 mM.26 Similarly, a stock solution of 0.2 M aniline in a 0.1 M, pH 4.3, phosphate buffer was made. Usually, the reaction solutions were (23) (a) Rethi, M.; Ponrathnam, S.; Rajan, C. R. Macromol. Rapid Commun. 1998, 19, 119. (b) Haba, Y.; Segal, E.; Narkis, M.; Titelman, G. I.; Siegmann, A. Synth. Met. 2000, 110, 189. (c) Su, S.-J.; Kuramoto, N. Synth. Met. 2000, 108, 121. (24) Kim, B.-J.; Oh, S.-G.; Han, M.-G.; Im, S.-S. Langmuir 2000, 16, 5841. (25) (a) Xie, H.-Q.; Ma, Y.-M.; Feng, D.-S. Eur. Polym. J. 2000, 36, 2201. (b) Kinlen, P. J.; Liu, J.; Ding, Y.; Graham, C. R. Remsen, E. E. Macromolecules 1998, 31, 1735. (26) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975; p 55.

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prepared by mixing the aniline and SDBS stock solutions together into a phosphate buffer. To test the effects of the surfactant concentration, peroxidasecatalyzed polymerization of aniline was performed in the solutions with molar ratios of aniline to SDBS varied from 3 to 0.6. In each solution, the concentration of aniline was the same, and the concentration of SDBS varied. The molar ratios (the number in the front of the parentheses) of aniline to SDBS and composites (the number inside the parentheses with the format as aniline: SDBS:buffer(mL)) of these solutions were 3(30:10:80); 1.5(30: 20:70); 1(30:30:60); 0.75(30:40:50); and 0.6(30:50:40), respectively. Before the addition of HRP, the pH of each solution was checked and adjusted to around 4.3 with 1 N HCl. To each solution, 3 mL of HRP stock solution (10 mg/mL) was added. The reaction was then initiated by the addition of diluted H2O2 (0.2 M). After dropwise addition of a stoichiometric amount of H2O2 (regarding to the aniline monomer) under vigorous stirring for 2 h, the reaction was left to stir for at least an additional 1 h. Precipitates may be formed in solutions with high ratio of aniline to SDBS. After the reaction was completed, acetone was added to collapse the micelles and precipitate the synthesized polyaniline/SDBS complex from the solution. The synthesized polyaniline was collected by centrifugation and washed with 50% acetone/water mixture to remove the aniline monomer and the oligomer. The resulting precipitates were first dried in the hood at room temperature and then dried in oven under vacuum at 50 °C for 24 h for further characterization. The SDBS micellar polyaniline aqueous solution was synthesized under the conditions of low molar ratio of aniline to SDBS or added limited amount of H2O2, following a similar procedure. The synthesized solution was purified by ultrafiltration with a Spectrum MiniKros sampler system using a MiniKros Sampler UF module with 10 kDa molecular weight cutoff to remove the aniline monomer, oligomer, phosphate sodium salts, and excess SDBS surfactant. The enzymatically synthesized SDBS polyanilines complex can be dissolved in polar organic solvents such as DMF and DMSO in base form. To dissolve the SDBS polyaniline, the SDBS/ polyaniline was ground to very fine powder first. To 2 mL DMF or DMSO, 0.1 g of this powder sample was dispersed, and then 50 µL of 10% NH3‚H2O was added. The solution was ultrasonicated until all the powder was dissolved. The resulting solution was dark blue in color, indicating the polyaniline was in emeraldine base form. The thin film cast from this solution turns to green after drying, due to the presence of the SDBS in the solution. The SDBS surfactant in the synthesized micellar polyaniline can also be removed by dedoping following the typical procedure described in the literature.31 The enzymatically synthesized SDBS polyaniline powders, 0.2 g, was ground to very fine powder first. Then, 10 mL of 10% NH3‚H2O was added. After the solution has been stirred for 24 h, the polyaniline was collected by centrifugation and washed three times with deionized water to remove the excess NH3‚H2O. The precipitates were dried in a hood for further characterization. This dedoped polyaniline shows good solubility in organic solvents, such as DMF and DMSO. The enzymatic polymerization of aniline was also carried out in the mixed micellar solutions. The mixed micellar solutions were prepared by mixing 0.2 M SDBS and Triton X-100 stock (27) Fendler, J. H. Membrane Mimetic Chemistry: Characterizations and Applications of Micelles, Microemulsions, Monolayers, Bilayers, Vesicles, Host-Guest Systems, and Polyions; John Willey & Sons: New York, 1982; pp 6, 206. (28) (a) Stafstrom, S.; Bredas, J. L.; Epstein, A. J.; Woo, H. S.; Tanner, D. B.; Huang, W. S.; MacDiarmid, A. G. Phys. Rev. Lett. 1987, 59, 1464. (b) Ginder, J. M.; Epstein, A. J. Phys. Rev. B 1990, 41, 10674. (c) Wudl, F.; Angus, R. O.; Lu, F. L.; Allemand, P. M.; Vachon, D. J.; Nowak, M.; Liu, Z. X.; Heeger, A. J. J. Am. Chem. Soc. 1987, 109, 3677. (29) (a) Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1989, 32, 263. (b) D’Aprano, G.; Leclerc, M.; Zotti, G. Macromolecules 1992, 25, 2145. (30) (a) Angelopoulos, M.; Liao, Y.-H.; Furman, B.; Graham, T. Macromolecules 1996, 29, 3046. (b) Ayyagari, M. S.; Marx, K. A.; Tripathy, S. K.; Akkara, J. A.; Kaplan, D. L. Macromolecules 1995, 28, 5192. (31) Kababya, S.; Appel, M.; Haba, Y.; Titelman, G. I.; Schmidt, A. Macromolecules 1999, 32, 5357.

Liu et al. solution into a 0.1 M, pH 4.3 phosphate buffer. The molar ratios of SDBS to Triton X-100 in these micellar solutions varied from 1 to 0.25. In each case, the aniline and SDBS were kept at the same concentration, and only the concentrations of Triton X-100 varied in the solutions. The composition of the reaction solutions are shown as (aniline:SDBS:Triton X-100:buffer (mL)) (30:10: 10:70); (30:10:20:60); (30:10:30:50); and (30:10:40:40). The peroxidase-catalyzed polymerizations of aniline in these solutions were performed following a similar procedure as described above. GPC Molecular Weight Measurement. The molecular weight of the synthesized polyaniline was measured by gel permeation chromatography (GPC), using a Waters 150-C ALC/ GPC system, equipped with a refractive index detector, and two PLgel GPC columns connected together. Polystyrene standards with the molecular weight of 377,400, 96,000, 20,650, 5460, and 1300 were used as references in the molecular weight measurements. The sample solution was prepared by dissolving 15 mg of SDBS polyaniline in 3 mL of 0.5% LiBr DMF solution in base form and was filtrated through a 0.45 µm filter prior to the measurement. A 0.5 µL aliquot of this solution was automatically injected. DMF containing 0.5% LiBr was used as a carrier solvent in a flow rate of 1 mL/min. UV-vis-Near-IR Spectra. The UV-vis-near-IR spectra were recorded on a Perkin-Elmer Lambda-9 UV/vis/near-IR spectrophotometer. A reference cell containing distilled water was used in the measurement. To obtain UV-vis-near-IR absorption spectra of the polyanilines synthesized in various surfactant solutions, the peroxidase-catalyzed polymerizations were performed in a 3 mL, pH 4.3 phosphate buffer containing 5 mM of aniline and surfactant. To avoid the precipitation of the synthesized polyaniline in all these solutions, only 10% of the stoichiometric amount of H2O2 needed to complete the polymerization reaction was added into the solutions. After 20 min of the addition of the H2O2, the UV-vis-near-IR spectra of these solutions were recorded at a scan rate of 240 nm/min. (The solutions may need to be diluted to make the absorption value in a proper range.) The change of UV-vis-near-IR absorption spectra during the reversible doping and dedoping process of the micellar polyaniline solution was also recorded. A 3 mL aliquot of aqueous SDBS polyaniline solution with proper concentration (the absorption value at 320 nm around 1.5) was first adjusted to pH 4.0. A UV-vis-near-IR absorption spectrum of this solution was recorded first. Then this solution was titrated with 1 M NaOH. After each addition of 1 M NaOH, an absorption spectrum was recorded again. After the pH of the solution reached 11, the solution was titrated with 1 M HCl following a similar procedure back to pH 4. FTIR Spectra. The FTIR spectra of SDBS micellar polyaniline were recorded on a Perkin-Elmer 1720 FT-IR spectrometer from KBr pellet and thin films on AgCl crystal window. The KBr pellet was prepared by pressing the mixture of the polyaniline powder as synthesized and KBr under 1200 lb. The thin films were prepared by casting the DMF solutions of base form SDBS micellar polyaniline on AgCl crystal window. To dope the polyaniline, the thin film was usually exposed to HCl vapor for at least 20 min before recording of the FTIR spectrum. NMR Spectra. The 13C NMR spectrum was recorded on a Bruker ARX 500 MHz NMR spectrometer. The operating frequency on the ARX 500 instrument for performing 13C NMR was 125 MHz. To eliminate the interference of the aromatic carbons from SDBS molecules, the polyaniline was synthesized following the procedure described above using 13C-labeled aniline as monomer. 20 mg of the synthesized 13C-labeled SDBS micellar polyaniline was dissolved in 0.6 mL mixture of d6-DMSO and NH3‚H2O. The polyaniline was dedoped and showed a dark blue color in this solution. A 13C NMR spectrum of this solution was recorded. TGA Measurements. Thermogravimetric analysis (TGA) of the synthesized polyaniline complex was carried out on a Du Pont thermal analyzer, TGA 2950 (TA Instrument Inc.), under a N2 atmosphere in the temperature range 30-700 °C at a heating rate of 10 °C/min. All the polyaniline samples were ground to fine powder and dried in a vacuum oven at 50 °C for 24 h prior to the measurements. Usually, 4-8 mg samples were loaded for the TGA analysis.

Conducting Polyaniline in Micelle Solutions Cyclic Voltammetry. The electrochemical characterization of the SDBS polyaniline was carried out on an EG&G potentiostat/ galvanostat model 263. Cyclic voltammograms were recorded by using a three-electrode cell in 1 M HCl solution at room temperature. A platinum wire and Ag/AgCl standard electrode were used as counter and reference electrode, respectively. The thin films cast from DMF solution of the micellar polyanilines on a platinum plate was used as the working electrode. The potential scan range was set from -0.1 to 1.0 V. The cyclic voltammograms of the enzymatically synthesized polyaniline were recorded at the rates of 20, 40, 60, 80, and 100 mV/min. To test the electrochemical stability of the SDBS micellar polyaniline, 50 cycles of cyclic voltammograms were consecutively recorded at a rate of 50 mV/min with the potential between -0.1 and 1.0 V. Conductivity Measurement. The conductivity of the synthesized polyaniline was measured by a typical four-probe method. The polyanilines were ground to fine powder and dried in a vacuum oven at 50 °C for 24 h prior to the measurements. The disk pellets with the diameter of 1.27 cm of these dried polyanilines were prepared by pressing under 1200 lb. The conductivity measurements were carried out on the pressed pellets using a Cascade Microtech four-point probe. The obtained conductivity values are the average of several readings at different regions.

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Figure 1. Schematic representation of enzymatic polymerization of aniline in aqueous micelle solutions.

Results and Discussion Enzymatic Polymerization. The enzymatic polymerization of aniline in micellar solution was performed as shown schematically in Figure 1. First, a micellar solution is prepared by dissolving the surfactant in phosphate buffer with the concentration over its cmc (critical micellar concentration). It has been well established that surfactants will aggregate spontaneously to form micelles when the concentration is over its cmc.27 Because of the presence of the hydrophobic micellar core and hydrophilic interface, these micelles are capable of dissolving water-insoluble substances.27 A certain amount of aniline is added into the solution to be loaded onto the micelles. The loading of aniline onto the micelles is a process that is driven by both the hydrophobic and electrostatic forces.27 These micelles may serve as templates to orient and organize the aniline molecules in the solution. In this specific system, aniline monomer may be intercalated between the surfactants as components of the micelles22 or located in the Stern layer surrounding the surface of micelles. The orientation or organization of aniline molecules in the micelle solutions may affect the regioselectivity and kinetics of the peroxidase-catalyzed coupling reactions. The enzymatic polymerization of aniline in these systems is carried out under similar conditions as described in the polyelectrolyte systems. Conducting polyaniline is formed surrounding the hydrophilic region of the micelles. By adjusting the ratio of aniline to SDBS and the H2O2 added, the synthesized micellar polyaniline may be controlled to remain in aqueous solution or precipitate out of solution. Different types of surfactants with molecular structures shown in Scheme 1 are selected to form the micelle templates for enzymatic polymerization of aniline. Figure 2 shows the UV-vis-near-IR spectra of the polyaniline synthesized in various surfactant micelle solutions. In the following discussion, the absorption band at approximately 800-1200 nm due to the polaron transition28 is compared as a signature of the formation of conducting polyaniline. The UV-vis-near-IR absorption features of the polyaniline synthesized in micelle solutions of neutral surfactants, Triton X-100 (curve a), and cationic surfactants, CTAB (curve b), are quite similar. Very strong absorptions are observed from 400 to 600 nm, which may be due to either the branched or oligomeric polyaniline. The polyanilines with strong absorption in this region

Figure 2. UV-vis-near-IR absorption spectra of polyaniline synthesized in CTAB, Triton X-100, and SDBS micelle solutions. Scheme 1

usually are not conducting. The absorption in the region from 800 to 1200 nm due to the conducting form of polyaniline is very weak, and no apparent absorption peak is observed. Therefore, we may conclude that most of the polyaniline synthesized in neutral and cationic micelle solutions are not electrically conducting. In contrast, the polyaniline synthesized in SDBS micellar solutions gives strong polaron absorption at the region from 800 to 1200 nm as shown in Figure 2 (curve c). Meanwhile, no absorption peak is observed at the region from 450 to 600 nm. These absorption features suggested that conducting polyaniline was formed from peroxidasecatalyzed polymerization of aniline in SDBS micelle solutions. A shoulder peak is observed at around 720 nm, which may be due to the formed polyaniline intermediate.29 This peak disappears with the progress of the reaction, and the absorption peaks at around 1000 nm get stronger and extend to the near-IR region. The appearance of the strong absorption at near-IR region will be discussed further in a later section. Local Environment Formed by Micelles. The differences in the behavior of the enzymatic polymerization of aniline in various micelle solutions may be explained by the variation of the local environment formed by different surfactants. As we have mentioned in a previous

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Figure 3. Plot of bulk pH of the solution as a function of the molar ratio of aniline to SDBS. The solutions were prepared by mixing two stock solutions, pH 4.3, 0.2 M SDBS, and aniline at different molar ratios.

paper, a low-pH local environment is necessary in the enzymatic polymerization of aniline in order to synthesize conducting polyaniline.22 In all of these micelle solutions, similar hydrophobic cores are formed; however, the hydrophilic interfaces of the micelles are quite different due to the difference in the headgroup structure of the surfactants. As shown in Scheme 1, the headgroup of Triton X-100 is polyoxyethylene with 10 repeat units without any charge. Therefore, a neutral hydrophilic interface is formed surrounding the Triton X-100 micelles in the solution. No electrostatic interactions between the headgroup of Triton X-100 and charged aniline or other species are expected at the hydrophilic interface of the micelles. The pH around the local environment of the Triton X-100 micelle is similar to that of the bulk solution. On the other hand, if a positively charged hydrophilic region in the CTAB micelle solution is formed, anionic species such as OH- will be likely attracted in this region through electrostatic interaction. The condensation of OHspecies in the Stern layer will lead to a higher pH in the local environment of the micelles compared to the bulk solution. Meanwhile, the positively charged aniline monomer will be expelled away from this positively charged hydrophilic interface area of CTAB micelle. Obviously, the necessary local environment for the enzymatic synthesis of conducting polyaniline does not exist in either neutral or cationic micelle solutions. The micelles formed by strong acidic surfactants such as SDBS have negative charged interfaces in the solution. These micelles may serve as nanoreactors to complex the positive charged aniline through electrostatic and hydrophobic interaction. More importantly, the low-pH local environment, which is necessary for the chain growth of conducting polyaniline, exists in SDBS micelle solution due to the condensation of H+ surrounding the Stern layer as we reported previously.22 The presence of the lower pH local molecular environment in SDBS micelle solution was further confirmed by the pH change of the bulk solution after the loading aniline monomer on to the micelles (Figure 3). An interesting jump of the bulk pH from 4.3 to 5.1 is observed after the mixing of aniline and SDBS micelle stock solution. As we have mentioned above, the loading of aniline onto the micelle is a process driven by the electrostatic and hydrophobic interaction. The comicelles or complexes of aniline and SDBS are formed spontaneously when two stock solutions, anilines, and SDBS micelles were mixed together. No chemical reaction is involved in this mixing process. Thus, the bulk pH change is because of the inhomogeneous distribution of the protons in the media; some protons were trapped in the local molecular environments formed by SDBS micelles. Therefore, a necessary low-pH local environment for the growth of conducting polyaniline is provided by

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Figure 4. Plots of the conductivities of the enzymatically synthesized SDBS micellar polyaniline as a function of the feed molar ratio of aniline to SDBS.

aqueous micelles. The bulk pH returns with the increase in the molar ratio of aniline to SDBS may be due to the dominance of aniline solution in the system. These results demonstrate that SDBS micelles will be the suitable templates for enzymatic synthesis of conducting polyaniline. In the subsequent section, SDBS is employed as a model surfactant in the discussion for the enzymatic synthesis of conducting polyaniline in micelle solutions. Effects of Molar Ratio of Aniline/SDBS. The effect of the feed molar ratio of aniline to SDBS on the conductivity of the synthesized polyaniline was investigated by performing enzymatic polymerization of aniline in various SDBS micellar solutions. In all the reactions, aniline concentration was kept at 50 mM, and the concentrations of SDBS were varied from 17 to 67 mM. The synthesized polyanilines were purified following the same procedures as described in the Experimental Section. Figure 4 shows the relationship between the conductivity of the synthesized polyaniline and the molar ratios of aniline to SDBS in the solutions. With the increase of the molar ratio of aniline to SDBS in the reaction media, the conductivity of the synthesized polyaniline increased. When the molar ratio of aniline to SDBS increased from 0.5 to 3, the conductivity increased from ∼6 × 10-5 to ∼2 × 10-3. The SDBS micellar polyaniline with conductivity as high as ∼10-2 S/cm was enzymatically synthesized. As we have mentioned above, the SDBS micelles serve as nanoreactors to complex with the charged aniline monomers prior to the reaction. At high molar ratio of aniline to SDBS, the local concentration of aniline will be higher. The chain growth of the polyaniline will benefit from this high local aniline concentration due to the ease for the aniline monomers to find each other for coupling during the reaction. In contrast, at lower molar ratio of aniline to SDBS condition, the local aniline concentration of each micelle nanoreactor will be lower. The possibility for aniline monomers to couple together to form polymers will be significantly reduced during the reaction. On the other hand, it has been reported previously that the size and structure of the micelles are strongly dependent on the concentration of the surfactant and are also influenced by the solutes solubilized in the micelle solution.27 Different types of micellar aggregates may be formed when the surfactant concentration is above the cmc. It is considered that the change of the molar ratio of aniline to surfactant may alter the micellar shape or structure significantly, which in turn may influence the orientation and organization of aniline in the local environments of the micelles and lead to the synthesis of a different type polyaniline. The conductivities of these synthesized polyaniline are also measured after additional doping by

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exposing the pellets to HCl vapor for 20 min. However, no measurable difference in the conductivity of SDBS polyaniline was observed. The yield of the enzymatically synthesized micellar polyaniline is also increased with the molar ratio of aniline to SDBS (data not shown). Usually, the isolated yield for the reaction is ∼60% under the condition of molar ratio of aniline to SDBS at ∼2.0-3.0. When the molar ratio of aniline to SDBS decreases, the yield of the synthesized polyaniline is also decreased significantly. It should be noted here that the yield we mentioned is isolated yield. The conversion yield of aniline monomer may still be high, but some byproducts are formed during the reaction such as low molecular weight species and were removed by washing with 50% acetone at the purification stage. To obtain the molar ratio of aniline repeating unit to SDBS in the complex, these samples are further characterized by elemental analysis (performed by Quantitative Technologies Inc., Whitehouse, NJ). In fact, the molar ratio of aniline repeating unit to SDBS is equal to the molar ratio of N/S in the sample. Therefore, the molar ratio of aniline to SDBS can be obtained by analyzing the content of elements N and S. The content of SDBS varied significantly in the synthesized polyaniline complex with the molar ratio of aniline to SDBS fed in the enzymatic polymerization. A molar ratio 3.0 of N/S is obtained for the polyaniline synthesized in a solution with the feed ratio of aniline to SDBS 3:1. The content of SDBS in the synthesized polyaniline complex is decreased dramatically when the feed molar ratio of aniline to SDBS decreases. The elemental analysis results are consistent with the conductivity measurements. The polyaniline obtained in the low feed molar ratio of aniline to SDBS solution might be either low molecular weight or branch structured, which in turn is not able to complex with SDBS. Therefore, it is reasonable that lower conductivities are measured for these samples. Similar results are observed in the mixed micellar system (data not shown here). As expected, the conductivity of the polyaniline synthesized in the mixed micelles solution of SDBS and Triton X-100 decreases with increasing content of Triton X-100 in the system. This result agrees well with our previous observation that the polaron bands decrease when the content of Triton X-100 increases in the mixed micelles. As we have discussed previously, a low-pH local environment is necessary in the enzymatic synthesis of conducting polyaniline. The introduction of Triton X-100, a neutral surfactant, will dramatically reduce the charge density on the interface of the micelles, which may result in the increase of the pH in the local environment around the micelle. The molecular weight of the synthesized SDBS micellar polyaniline was measured by gel permeation chromatography (GPC). To eliminate the aggregation of polyaniline in the solvents,18 0.5% LiBr was added in the sample solution and carrier solvents.30 The SDBS/polyaniline complex synthesized in the solution with 3:1 molar ratio of aniline to SDBS gave the average molecular weight of 10 000 (Mw). The molecular weight of polyaniline obtained for the same molar ratio is not very sensitive to an increase in SDBS concentration in the range of concentrations used in our experiments. We believe that in this concentration range of the surfactant, the micelle size and shape do not significantly change and only the micelle concentration changes. For much larger concentrations of SDBS one can expect a change in the structure, shape, and size of the micelle which may affect the properties and molecular weight of the polyaniline.

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Figure 5. NMR spectrum of enzymatically synthesized micellar 13C-labeled polyaniline in d6-DMSO. The structure of the base form of polyaniline and the assignment of the resonance peaks are shown as insets. 13 C NMR. Taking advantage of the good solubility of the synthesized emeraldine base form of the micellar polyaniline in DMSO and DMF solutions, 13C NMR spectroscopy is used in characterizing the structure of the synthesized polyaniline. Solid-state NMR characterization of the chemically synthesized PANI-SDBS system by Kababya et al.31 reveals that the surfactant molecules as dopants strongly interacts with the imine groups on the polyaniline and cannot be totally removed by a standard dedoping procedure. To eliminate the interference from the aromatic carbons of SDBS molecules, 13C-labeled aniline monomer is used in the peroxidase-catalyzed polymerization of aniline. Figure 5 shows the 13C NMR spectrum of the enzymatically synthesized SDBS 13Clabeled polyaniline in base form in d6-DMSO solution. The molecular structure of polyaniline emeraldine base and a summary of chemical shift assignments are also shown as insets in Figure 5. The observed peaks broaden probably due to the distribution of the molecular weight. The assignment of the resonance peaks in this spectrum is obtained from previously published data from the chemically synthesized polyaniline.32 The resonance peaks in our NMR spectrum are the same resonance peaks as the linear base form of polyaniline. The peak at round 116.2 ppm is assigned to the protonated carbon 6 from the benzenoid ring. The protonated benzenoid carbons 2 and 3 gave peaks at the region from 121.6 to 124.8 ppm. The peak at around 137.6 ppm is assigned to the protonated carbon 8 from the quinoid ring. The resonance peak from the nonprotoned aromatic carbons 4 and 5 are observed at around 144.3 ppm. Another nonprotonated carbon 1 shows the resonance peak at 148.7 ppm due to electron withdrawing from the quinoid ring. The resonance peaks at the region from 155 to 158 ppm are from the nonprotonated carbon of the quinoid ring, C7. The presence of the quinoid carbon resonance peaks in the region from 155 to 159 ppm is a signature of the formation of conducting polyaniline. From the recorded NMR spectrum, we may conclude that the linear structure is dominant in this enzymatically synthesized SDBS micellar polyaniline, which is very crucial for the conductivity of the synthesized polymer. Solution NMR spectra of chemically synthesized polyaniline have been reported by Kenwright et al. which show a higher resolution than that available from solid-state techniques.33 They have demonstrated that the majority of the material present in the samples of leucoemeraldine

(32) (a) Kaplan, S.; Conwell, E. M.; Richter, A. F.; MacDiarmid, A. G. J. Am. Chem. Soc. 1988, 110, 7647. (b) Espe, M. P.; Mattes, B. R. Schaefer, J. Macromolecules 1997, 30, 6307. (33) Kenwrigtht, A. M.; Feast, W. J.; Adams, P.; Milton, A. J.; Monkman, A. P.; Say, B. J. Synth. Met. 1993, 55-57, 666.

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Figure 7. Evolution of UV-vis-near-IR spectra of SDBS micellar polyaniline during titrating the solution from pH 4 to 11 by 1 N NaOH.

Figure 6. FTIR spectra of SDBS micellar polyaniline: (a) KBr pellets of the SDBS/polyaniline as synthesized; (b) thin film cast from DMF solution of the SDBS/polyaniline complex in emeraldine base form; (c) thin film cast from DMF solution of the polyaniline after SDBS was removed; (d) thin film of (c) was redoped by exposure to HCl vapor for 20 min.

and emeraldine base have the postulated structures but found the presence of the defects in the postulated structures at concentrations up to 5 mol %. A similar “defect” structure may also be present in the enzymatically synthesized SDBS/polyaniline complex. Two resonance peaks at around 130 and 127 ppm (labeled with stars) are observed in Figure 5, which may not be from the linear polyaniline. It is suspected that these peaks may be due to some branched polyaniline, such as polyaniline formed from the coupling at the ortho position. The formation of branched polyaniline will lower the conductivity of the polyaniline complex as we mentioned in the Introduction. FTIR. Figure 6 shows the FTIR spectra of SDBS micellar polyaniline in different states in the region from 4000 to 500 cm-1. The bands at around 1600 and 1500 cm-1 are due to quinone and benzene ring deformation, and the band at 1310 cm-1 is assigned to C-N stretching of a secondary aromatic amine.34 Broad peaks appearing at around 3200 cm-1 are assigned to the N-H vibration. The two bands appearing at 1030 and 1000 cm-1 in Figure 1a,b are due to the asymmetric and symmetric stretching of SO3-, indicating the presence of SDBS in the sample.35 The presence of the SDBS as dopant in the synthesized polyaniline is further confirmed by the strong C-H vibration band appearing at around 3000 cm-1 in Figure 1b. The SDBS/polyaniline complex as synthesized in its conducting form is shown in Figure 1a with a relatively strong quinone band due to the presence of the dopant SDBS. It is interesting that the SDBS surfactant in the complex can also be removed by dedoping the polyaniline complex powder with NH3‚H2O as described in the Experimental Section. The dedoped polyaniline shows good solubility in polar organic solvents such as DMF and DMSO. Figure 6c,d is the FTIR spectra of thin films cast from the dedoped polyaniline DMF solution. Compared to Figure 6a,b, the vibrational band at ∼1000 cm-1 for the SO3- group and the band at around 3000 cm-1 for the alkyl chain have disappeared, indicating that most of the (34) (a) Boyer, M.-I.; Quillard, S.; Rebourt, E.; Louarn, G.; Buisson, J. P.; Monkman, A.; Lefrant, S. J. Phys. Chem. B 1998, 102, 73827392. (b) Quillard, S.; Louarn, G.; Lefrant, S.; Macdiarmid, A. Phys. Rev. B 1994, 50, 12496. (35) Chen, S.-A.; Hwang, G.-W. Polymer 1997, 38, 333.

SDBS was removed during the dedoping process. It is possible that there are some residues of SDBS due to the strong interaction between the dopants and polyaniline which may not be detectable in the IR spectrum. It is known that vibrational spectroscopy is extremely sensitive to the electronic structure change in polyaniline.34a By using infrared spectroscopy, one can easily register all interconversions between different states of polyaniline involving both oxidation and protonation processes. The relative intensities of the bands at around 1600 and 1500 cm-1 are strongly dependent on the form of the conducting polyaniline. The strong intensity of quinone vibration band in Figure 6a shows that the polyaniline is in a doped state. However, enhancement of the benzene ring vibration at around 1500 cm-1 can be observed in Figure 6b, suggesting that some polyaniline is dedoped in the thin film after it was cast from DMF solution. Similarly, one may observe in Figure 6c,d that the intensity of the band at 1600 cm-1, due to the quinone ring deformation, increased remarkably relative to the intensity of the band at 1500 cm-1 from the benzene ring by turning from emeraldine base to salt form, because of the increase of quinone units in the polymer chain after acid doping. Redox Reversibility. Figure 7 displays the evolution of absorption spectra of the SDBS micellar polyaniline in aqueous solution with increasing pH from 4 to 11 by titrating with 1 M NaOH. At pH 4, the micellar polyaniline is in the doped state as indicated by the presence of the polaron band transition at about 420 nm and the region from 800 to 1200 nm as well as the π-π* transition of the benzenoid rings at 310-320 nm. It is worthwhile to note that the absorption from 800 to 1200 nm is featureless and extends toward the near-IR region. The degree of the broadening of the near-IR absorption, which can be considered as a measure of polaron delocalization, depends strongly on the selection of an appropriate protonation agent-solvent couple. The strong broadening of the nearIR absorption was observed in several different solvent systems for protonating polyaniline and addressed by several authors.35 MacDiarmid et al.36 introduced the term “secondary doping” to describe the interactions of m-cresol and other phenols with protonated polyaniline. The reasons for the presence of the strong featureless broadening absorption at near-IR region for this SDBS micellar polyaniline is still not clear and will be one of the interesting parts of our future investigations. The synthesized micellar polyaniline shows reversible reduction/ oxidation behavior in the absorption spectra with varying pH, which is similar to that observed in the SPS polyaniline (36) Rannou, P.; Gawlicka, A.; Berner, D.; Pron, A.; Nechtschein, M.; Djurado, D.; Macromolecules 1998, 31, 3007. (37) MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1994, 65, 103. MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1995, 85.

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Figure 9. TGA curves of polyaniline synthesized in aqueous buffer solution (a) and the enzymatically synthesized SDBS micellar polyaniline (b). All these curves are recorded under a N2 atmosphere with a temperature increase rate of 10 °C/min.

Figure 8. Cyclic voltammograms of (a) SDBS micellar polyaniline complex and (b) dedoped polyaniline with the SDBS dopants removed recorded at scan rates of 20, 40, 60, 80, and 100 mV/s, respectively.

solution. As the pH of the solution is increased, the polaron bands at 420 nm and near-IR region gradually disappear, and a strong absorption due to exciton transition of the quinoid rings at 560-600 nm begins to emerge. At the same time, the absorption bands at 320 nm, which are due to π-π* transitions of the benzenoid rings in the polyaniline, increase with a pH increase. At pH 11, a blue micellar PANI solution is formed, indicating that the PANI has been fully dedoped to the emeraldine base form. The dedoped polyaniline can be redoped by titrating with 1 N HCl (the data are not shown). This pH induced redox reversibility demonstrates that the electroactive polyaniline was enzymatically synthesized in the aqueous micelle solutions. Cyclic Voltammetry. The electrochemical properties of the enzymatically synthesized SDBS micellar polyaniline are characterized by cyclic voltammetry. Figure 8 shows the cyclic voltammograms (CV) of the films of (a) SDBS micellar polyaniline complex and (b) dedoped polyaniline with the SDBS dopants removed. The CV curves were recorded in 1 M HCl solutions at the scan rates of 20, 40, 60, 80, and 100 mV/s, respectively. Only one set of broad redox peaks at E1/2 ) ∼0.50 V is dominant in Figure 8a. Another set of redox peaks is not very clear and shown as a shoulder with E1/2 at ∼0.3 V in the spectra. However, after the SDBS dopant in the micellar polyaniline complex is removed, the first set of redox peaks of the polyaniline become more clear as shown in Figure 8b. The presence of SDBS dopants in the synthesized poly-

aniline complex will influence the electrochemical properties. The details of the effects of these dopants are still under investigation. These recorded CV curves suggest that the enzymatically synthesized micellar polyaniline is electrochemically active. Focke et al.7a have reported the scan rate dependence of the reduction and oxidation peaks earlier. The peak potentials for the first redox process vary linearly with the square root of scan rate. Since the first redox peaks are not well resolved in the CV curves of the micellar polyaniline, this phenomenon may not be clearly verified in this case. However, the cell currents do increase with the scan rate during CV measurements, indicating that insertion/removal of the chlorate counterion into the polymer matrix may be a diffusion-controlled process.7a To test the electrochemical stability of the SDBS micellar polyaniline, the cyclic voltammograms were repeatedly recorded at the rate of 50 mV/min between the potential -0.1 and 1.0 V vs Ag/AgCl. The tested samples were still electrochemically active as observed in the CV curves after 50 scans, indicating good electrochemical stability of the synthesized micellar polyaniline. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) measurement of enzymatically synthesized SDBS micellar polyaniline was performed under a N2 atmosphere. TGA curves for the polyaniline synthesized in aqueous solution with and without SDBS micelle templates are compared in Figure 9. The initial mass loss between 40 and 100 °C is due to moisture content. No abrupt weight loss is found from the TGA curve of polyaniline synthesized in aqueous solution. This may indicate that the tested sample is a mixture of different molecular weights and structures of polyaniline. About 36% weight loss is observed in the temperature range of 30-280 °C for the polyaniline synthesized in aqueous solution, which may be attributed to the low molecular weight species in the sample. This result confirms that enzymatic polymerization of aniline in aqueous solution without template produces low molecular weight products. However, very little weight loss is observed for the enzymatically synthesized micellar polyaniline in the same temperature range, which suggests the absence of low molecular weight products in the synthesized SDBS micellar polyaniline complex. The weight loss of SDBS polyaniline complex is observed around 300 °C. Almost 65% weight loss is observed from 300 to 500 °C. It is

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difficult to clearly differentiate the weight loss associated with SDBS and polyaniline due to their complexation. Conclusions Polyanilines were enzymatically synthesized in aqueous micelle solutions. The properties of the synthesized polyaniline strongly depend on the structure of the surfactants as well as the concentration of the surfactant in the solution. The micelles formed by strong acid surfactants such as SDBS are suitable templates for the enzymatic synthesis of conducting polyaniline. These micelles serve as nanoreactors which complexes to the positively charged aniline monomers prior to the reaction and provide the necessary low-pH local environments for the chain growth of conducting polyaniline during the polymerization. The enzymatically synthesized SDBS

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polyanilines are soluble in the organic solvents such as DMF and DMSO in base form and are electrochemically active as shown by the CV, conductivity, and UV-visnear-IR measurements. The use of micelles formed by the surfactants as template in the enzymatic synthesis of conducting polyaniline may offer new possibilities for tailoring the chemical, electrochemical, and physical properties of polyaniline. Acknowledgment. We thank Dr. Ferdinando Bruno for the useful discussion and Dr. Nagarajan for the assistance in the conductivity measurements. This work was supported by US Army Natick Labs and the Office of Naval Research. LA0206357