pH- and Thermosensitive Polyaniline Colloidal Particles Prepared by

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Langmuir 2007, 23, 8-12

Articles pH- and Thermosensitive Polyaniline Colloidal Particles Prepared by Enzymatic Polymerization† Rodolfo Cruz-Silva* Centro de InVestigacio´ n en Ingenierı´a y Ciencias Aplicadas (CIICAp), UAEM. AV. UniVersidad 1001, Col. Chamilpa, CP 62210 CuernaVaca, Morelos, Me´ xico

Layza Arizmendi, Mayela Del-Angel, and Jorge Romero-Garcia Departamento de Materiales AVanzados, Centro de InVestigacio´ n en Quı´mica Aplicada (CIQA) BlVd. Enrique Reyna, CP 25100 Saltillo, Coah, Me´ xico ReceiVed June 26, 2006. In Final Form: NoVember 1, 2006 Polyaniline colloids were prepared by enzymatic polymerization using chitosan and poly(N-isopropylacrylamide) as steric stabilizers. The resulting nanoparticles undergo flocculation by changing the pH or temperature of the aqueous dispersions. The environmentally responsive behavior of these colloids contrasts with that of polyaniline colloids synthesized using poly(vinyl alcohol) as the steric stabilizer. The colloid size was a function of the steric stabilizers and ranged from approximately 50 nm for polyaniline particles prepared in the presence of chitosan and partially hydrolyzed poly(vinyl alcohol) up to 350 nm for the particles synthesized using poly(N-isopropylacrylamide). UVvisible and Fourier transform infrared spectroscopic studies indicate that polyaniline colloids are spectroscopically similar to those obtained by traditional dispersion polymerization of aniline by chemical oxidation. These polyaniline colloids have potential applications in thermochromic windows and smart fluids.

1. Introduction Polyaniline colloids have been widely studied because of their potential applications in electrorheological fluids,1,2 sensors,3 electrostatic discharge,4 and anticorrosion coatings.5 Waterdispersible colloids also provide an easy way to overcome problems of polyaniline processability, such as its low solubility in most organic solvents, because they can be cast as films or blended with other water-soluble polymers. The synthesis of polyaniline colloidal particles is commonly carried out by either chemical6,7 or electrochemical8 oxidation of aniline in the presence of a steric stabilizer. During the formation of the polymer, the stabilizer is adsorbed on the surface of the growing polymer particles, avoiding their aggregation and macroscopic precipitation. There are several studies concerning the influence of the oxidation rate,6 addition of organic solvents,7 and different stabilizers9 on the morphology of the polyaniline particles. † Part of the Stimuli-Responsive Materials: Polymers, Colloids, and Multicomponent Systems special issue. * Corresponding author. E-mail: [email protected]. Tel: (+52)-777329-7900 ext 6241. Fax: (+52)-777-329-7984.

(1) Cho, M. S.; Cho, Y. H.; Choi, H. J.; Jhon, M. S. Langmuir 2003, 19, 5875-5881. (2) Aoki, K.; Chen, J.; Ke, Q.; Armes, S. P.; Randall, D.P. Langmuir 2003, 19, 5511-5516. (3) Huang, J.; Virji, S.; Weiller, B. H.; Kaner, R. B. J. Am. Chem. Soc. 2003, 125, 314-315. (4) Kulkarni, V. G. Synth. Met. 1995, 71, 2129-2131. (5) Wessling, B.; Posdorfer, J. Synth. Met. 1999, 102, 1400-1401. (6) Stejskal, J.; Spirkova, M.; Riede, A.; Helmstedt, M.; Mokreva, P.; Prokes, J. Polymer 1999, 40, 2487-2492. (7) Chattopadhyay, D.; Mandal, B. M. Langmuir 1996, 12, 1585-1588. (8) Innis, P. C.; Norris, I. D.; Kane-Maguire, L. A. P.; Wallace, G. G. Macromolecules 1998, 31, 6521-6528. (9) Riede, A.; Helmstedt, M.; Riede, V.; Stejskal, J. Langmuir 1998, 4, 67676771.

Recently, we studied the enzymatic polymerization of aniline in dispersed media as an environmentally friendly alternative to preparing sterically stabilized polyaniline colloids.10 The peroxidase/hydrogen peroxide system has some advantages over traditional oxidizing agents, such as better control of the oxidation rate and the reduction of oxidation byproducts to water.11 This method has been used in template-assisted aniline polymerization to obtain water-soluble polyaniline12 and polyaniline colloids.13,14 In addition, peroxidases are a widely available group of enzymes obtained from renewable resources, which can be considered to be nontoxic and biodegradable. Continuous advances in biotechnology as well as the growing use of green synthetic routes are important factors that point to an increase in chemical processes such as enzymatic polymerizations.15 However, environmentally sensitive polymers have received a great deal of attention in recent years, and their synthesis and applications have become a major field in polymer science. Colloidal particles whose flocculation is triggered by external stimuli are regarded as smart materials. Two of the most interesting environmentally sensitive polymers are poly(N-isopropylacrylamide) (PNIPAM) and chitosan. The former has been extensively (10) Cruz-Silva, R.; Ruiz-Flores, C.; Arizmendi, L.; Romero-Garcia, J.; AriasMarin, E.; Moggio, I.; Castillon, F. F.; Farias, M. H. Polymer 2006, 47,15631568. (11) Cruz-Silva, R.; Romero-Garcı´a, J.; Angulo-Sa´nchez, J L.; Ledezma-Pe´rez, A.; Arias-Marı´n, E.; Moggio, I.; Flores-Loyola, E. Eur. Polym. J. 2005, 41,1129-1135. (12) Liu, W.; Kumar, J.; Tripathy, S. K.; Senecal, K. J.; Samuelson, L. J. Am. Chem. Soc. 1999, 121, 71-78. (13) Thiyagarajan, M.; Samuelson, L. A.; Kumar, J.; Cholli, A. L. J. Am. Chem. Soc. 2003, 125, 11502-11503. (14) Cholli, A. L.; Thiyagarajan, M.; Kumar, J.; Parma, V. C. Pure Appl. Chem. 2005, 77, 339-344. (15) Gross, R. A.; Kumar, A.; Kalra, B. Chem. ReV. 2001, 101, 2097-2124.

10.1021/la0618418 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/23/2006

pH-/ThermosensitiVe Polyaniline Colloidal Particles

studied because its lower critical solution temperature (LCST) is close to room temperature. When a PNIPAM solution is heated above its LCST, the polymer undergoes a coil-to-globule transition and becomes insoluble. Pelton16 first prepared PNIPAM-stabilized particles by polymerizing N-isopropylacrylamide in the presence of polystyrene and polystyrene-butadiene latexes. Recently, several types of polymer colloids,17,18 silica,19 gold nanoparticles,20 and carbon nanotubes21 have been surface modified with PNIPAM in order to show a thermal response in an aqueous dispersion. Chitosan, however, is a cationic polysaccharide that consists of glucosamine and acetylglucosamine linked units. This polymer is produced on a large scale by the deacetylation of chitin, one of the most abundant naturally occurring polymers, which is commonly obtained from crustacean shells. Chitosan combines unique properties, such as biocompatibility, pH sensitivity, and good mechanical properties, which have fostered its application in catalytic supports,22 enzyme immobilization,23 heavy-metal recovery,24 and tissue engineering.25 Polyaniline is an interesting material by itself for the development of smart devices and has been used to prepare artificial muscles and actuators.26 However, the combination of this electrically conducting polymer with environmentally responsive polymers such as chitosan results in a composite that combines a mechanical response under pH changes27,28 and electrical stimuli.29 PNIPAM derivatives have also been combined with electrically conductive polymers, such as polypyrrole, to achieve a composite with thermosensitive swelling behavior in water.30 The aim of this work was to synthesize polyaniline colloidal particles by enzymatic polymerization using environmentally sensitive polymers as steric stabilizers. The use of a biocatalytic polymerization pathway results in well-defined colloidal particles, whereas the adsorption of PNIPAM and chitosan confers pH and temperature sensitivity to the polyaniline particles. The morphology of the colloids was studied by transmission electron microscopy whereas their chemical characterization was done by Fourier transform infrared and UVvis spectroscopic techniques. 2. Experimental Section 2.1. Materials. Aniline (Aldrich) was distilled at reduced pressure and stored at low temperature in the dark prior to use. Partially hydrolyzed (87-89%) poly(vinyl alcohol) (Mw ) 13 000-23 000), fully hydrolyzed poly(vinyl alcohol) (Mw ) 70 000-100 000), N-isopropylacrylamide, p-toluenesulfonic acid, N-methyl-2-pyrro(16) Pelton, R. H. J. Polym. Sci. Part, A, Polym. Chem. 1988, 26, 9-18. (17) Tsuji, S.; Kawaguchi, H. Langmuir 2004, 20, 2449-2455. (18) Senff, H.; Richtering, W.; Norhausen, Ch.; Weiss, A.; Ballauff M Langmuir 1999, 15, 102-106. (19) Li, D.; Jones, G. L.; Dunlap, J. R.; Hua, F.; Zhao, B. Langmuir 2006, 22, 3344-3351. (20) Zhu, M. Q.; Wang, L. Q.; Exarhos, G. J.; Li, A. D.Q. J. Am. Chem. Soc. 2004, 126, 2656-2657. (21) Xu, G.; Wu, W. T.; Wang, Y.; Pang, W.; Wang, P.; Zhu, Q.; Lu, F. Nanotechnology 2006, 17, 2458-2465. (22) Macquarrie, D. J.; Hardy, J. J.E. Ind. Eng. Chem. Res. 2005, 44, 84998520. (23) Vazquez-Duhalt, R.; Tinoco, R.; D’Antonio, P.; Topoleski, L. D. T.; Payne, G. F. Bioconjugate Chem. 2001, 12, 301-306. (24) Navarro, R.; Guzma´n, J.; Saucedo, I.; Revilla, J.; Guibal, E.; Macromol. Biosci. 2003, 3, 552-561. (25) Li, J.; Yun, H.; Gong, Y.; Zhao, N.; Zhang, X. Biomacromolecules 2006, 7, 1112-1123. (26) Gao, J.; Sansinena, J. M.; Wang, H. L. Chem. Mater. 2003, 15, 24112418. (27) Shin, S. R.; Park, S. J.; Yoon, S. G.; Spinks, G. M.; Kim, S. I.; Kim, S. J. Synth. Met. 2005, 154, 213-216. (28) Kim, S. J.; Shin, S. R.; Spinks, G. M.; Kim, I. Y.; Kim, S. I. J. Appl. Polym. Sci. 2005, 96, 867-873. (29) Kim, S. J.; Kim, M. S.; Shin, S. R.; Kim, I. Y.; Kim, S. I.; Lee, S. H.; Lee, T. S.; Spinks, G. M.; Smart Mater. Struct. 2005, 14, 889-894. (30) Han, J. S.; Lee, J. Y.; Lee, D. S. Synth. Met. 2003, 124, 301-306.

Langmuir, Vol. 23, No. 1, 2007 9 lidinone, and hydrogen peroxide were reagent grade and purchased from Aldrich. Soybean peroxidase (RZ ) 1.3, activity 56 U/mg) and horseradish peroxidase (type II, activity 240 U/mg, RZ ) 1.9) were acquired from Sigma Chemical Co. High-molecular-weight chitosan was purchased from Carbomer, the degree of deacetylation (calculated by FTIR spectroscopy31) was 87%, and the viscosimetric molecular weight (Mv ) 9.70 × 106) was determined by the Mark-Howinkz equation according to the method reported by Rinaudo et al.32 2.2. Synthesis of PNIPAM. PNIPAM was synthesized by free radical polymerization at 60 °C as follows: 6.79 g of Nisoproylacrylamide was added to a mixture of 60 mL of isopropanol and 40 mL of water. The reaction medium was carefully degassed and kept under a nitrogen purge. Then, 274 mg of ammonium persulfate was dissolved in 20 mL of degassed water and added to the reaction mixture. Finally, 15 µL of N,N,N′,N′-tetramethylethylenediamine was added as a catalyst. After the reaction, the polymer was purified by centrifugation at a temperature above its LCST, dissolved in water, and dialyzed for 1 week against deionized water. Finally, the polymer solution was freeze dried. 2.3. Enzymatic Synthesis of Polyaniline Colloids. Polyaniline colloids were synthesized in dispersed media using either poly(vinyl alcohol), PNIPAM, or chitosan as the steric stabilizer. Typically, 1.2 g of the steric stabilizer, 215 µL of aniline, and 456 mg of toluenesulfonic acid were dissolved in 20 mL of water. The procedure was slightly different when chitosan was used as the steric stabilizer. In this case, 240 mg of chitosan and 215 µL of aniline were added to 20 mL of deionized water, and toluenesulfonic acid was added slowly until a pH of 3.0 was reached. Once the reaction mixture containing the steric stabilizer, the aniline, and the toluenesulfonic acid was prepared, the solution was stirred for 6 h and cooled in a water/ice bath. Afterward, 2.0 mL of a 1.2 mg/mL enzyme solution was added to the reaction mixture with stirring. All reactions were carried out using horseradish peroxidase, except for those carried out in the presence of chitosan, where soybean peroxidase was used. To start the polymerization, 2 mL of a 3.75 wt % hydrogen peroxide solution was added dropwise using a peristaltic pump under vigorous magnetic stirring for a time lapse of 2 h. The polyaniline particles were separated from the steric stabilizer solution using several centrifugation-redispersion cycles. During the isolation of the particles, 0.1 N NH4OH solution was used for dedoping (i.e., removal of the toluenesulfonic acid and therefore conversion of the polyaniline to its electrically nonconductive form). In the case of reactions carried out using chitosan, a diluted aqueous toluenesulfonic acid solution (pH of 2.0-2.5) was used to isolate the polyaniline colloids. 2.4. Characterization. Transmission electron microscopy (TEM) images were obtained on a JEOL-2000EX microscope using an accelerating voltage of 80-100.0 kV. The samples were prepared by drying a diluted sample of the isolated dispersions on a copper grid. UV-visible (UV-vis) spectra were obtained on a Shimadzu2410 spectrophotometer. To follow the reaction progress, 50 µL samples were withdrawn and dispersed in 5.0 mL of 1.0 N HCl, stopping the reaction by irreversible enzyme inactivation. Transmittance FTIR measurements were conducted on a Nicolet Magna IR550 spectrophotometer using pressed pellets made of dedoped polyaniline samples mixed with KBr. The UV-vis spectra of the dispersions were measured using a 100 µL aliquot dispersed in 3.0 mL of a hydrochloric acid or an ammonium hydroxide solution (0.2 N).

3. Results and Discussion 3.1. Enzymatic Synthesis of Polyaniline Colloids and Morphology. During the synthesis of polyaniline colloids, the reaction mixture became blue and then purple, indicating the rapid formation of polyaniline oligomers after starting the addition of hydrogen peroxide.6 This stage is very short and lasts (31) Brugnerotto, J.; Lizardi, J.; Goycoolea, F. M.; Argu¨elles-Monal, W.; Desbrieres, J.; Rinaudo, M. Polymer 2001, 42, 3569-3580. (32) Rinaudo, M.; Milas, M.; Dung, P. L. Int. J. Biol. Macromol. 1993, 15, 281-285.

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Figure 1. UV-vis spectra evolution during the enzymatic polymerization of aniline using chitosan as a steric stabilizer. The spectra correspond (from bottom to top) to the samples withdrawn and diluted in hydrochloric acid at different polymerization times (min): 0 (before H2O2 addition), 3, 10, 15, 20, 30, 45, 90, 135, 180, and 240. The inset shows a plot of the absorbance at 400 nm vs time.

Figure 2. UV-vis spectra of polyaniline colloidal particles synthesized using partially hydrolyzed poly(vinyl alcohol) as a steric stabilizer dispersed in (a) 0.2 N ammonium hydroxide and (b) 0.2 N hydrochloric acid.

approximately 15 s, and the reaction mixtures were dark green from that point on. Figure 1 displays the UV-vis spectra of samples withdrawn at different times from the reaction mixture and placed in hydrochloric acid (1.0 N), indicating that the enzymatic polymerization proceeds without pernigraniline salt formation. After 5 min of polymerization, a peak at approximately 705 nm due to polaron emission was clear and shifted toward 725 nm 10 min later. This is most likely due to an increase in the electronic conjugation of the polyaniline as the molecular weight increases because in the first few minutes mainly polyaniline oligomers are expected to be formed. From this time on, no shifts were observed in this peak. The enzymatic oxidation mechanism is different from that of the chemical polymerization of aniline, which shows a characteristic blue color during the oxidation stage due to the presence of pernigraniline salt, the oxidized form of polyaniline.33 After polymerization, the polyaniline particles were isolated from the steric stabilizer solution by several centrifugation-redispersion cycles. Poly(vinyl alcohol)- and PNIPAM-stabilized polyaniline particles can be dedoped by treatment with aqueous ammonia and redispersed in deionized water. Figure 2 shows the spectrum of dedoped polyaniline particles synthesized using partially hy(33) Chakraborty, M.; Mukherjee, D. C.; Mandal, B. M. Langmuir 2000, 16, 2482-2488.

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drolyzed poly(vinyl alcohol). The sample shows two transitions at 330 and 600 nm due to the π-π* transition and the benzenoidto-quinoid transition,8,33 respectively. When this sample was placed in an acidic medium, the spectrum (Figure 2) is similar to that of chitosan-stabilized polyaniline particles in an acidic medium (Figure 1), showing the polaron bands at 400 and 750 nm.11 Sterically stabilized polyaniline particles prepared with fully hydrolyzed poly(vinyl alcohol) or PNIPAM showed similar spectroscopic features. In the case of chitosan-stabilized polyaniline particles, because of their colloidal instability in alkaline media, the spectrum was acquired in N-methyl-2-pyrrolidinone solution. However, a similar spectrum to that of poly(vinyl alcohol)-stabilized particles in an aqueous alkaline medium was obtained. These electronic transitions shown indicate the changes between the emeraldine base and emeraldine salt forms of the polyaniline.12 The enzymatically synthesized polyaniline colloids also showed electrochemical behavior (Figure S1, Supporting Information) similar to that of polyaniline colloids electrochemically synthesized in the presence of sulfonic acids.8 The two reversible oxidation waves during the anodic scan indicate the oxidation of polyaniline from leucoemeraldine to emeraldine at 185 mV versus Ag/AgCl and the subsequent oxidation from emeraldine to pernigraniline at 580 mV versus Ag/AgCl, in agreement with the UV-vis results. The morphology of the polyaniline colloidal particles was characterized by transmission electron microscopy (TEM) and is shown in Figure 3. By using partially hydrolyzed poly(vinyl alcohol), spherical particles of approximately 60 nm diameter (Figure 3a) were obtained, whereas fully hydrolyzed poly(vinyl alcohol) led to spherical particles of 150 nm average diameter (Figure 3b). The higher stabilizer efficiency of the partially hydrolyzed poly(vinyl alcohol) is ascribed to the carbonyl groups of this polymer, which are known to hydrogen bond to polyaniline by >CdO‚‚‚H-N< interactions,34 thus increasing their adsorption on the conductive polymer colloids. However, using PNIPAM as a steric stabilizer led to spherical particles with a diameter between 100 and 300 nm (Figure 3c), whereas chitosan produced oval-shaped particles ranging from 60 to 120 nm diameter, together with a few rodlike colloids (Figure 3d). The deviation from the spherical shape in polyaniline colloids prepared by dispersion polymerization has been attributed either to an excessively high particle growth rate or to a low steric stabilizer adsorption rate.6,7 In the case of the polymerization carried out using chitosan as a stabilizer (Figure 3d), the high viscosity of its solution limited the amount of chitosan in the reaction media to 1.0 wt %; therefore, the reduced availability of the stabilizer may have reduced its adsorption rate on the particles, resulting in the formation of non-spherical colloids. Nevertheless, the size differences of the samples with a spherical shape (Figure 3a-c) may be attributed to the different efficiency of the steric stabilizers because these reactions were carried out at the same steric stabilizer concentration (5.0 wt %) while all other experimental parameters were kept similar. In addition, it has been shown before that the aniline oxidation rate is a key parameter in controlling the shape of polyaniline colloids.6,35 In this regard, we must point out that the enzymatic polymerization of aniline shows a longer and smoother oxidation stage11 than chemical oxidation, which makes enzymatic oxidation a very suitable pathway for aniline dispersion polymerization. 3.2. Characterization of Colloids by Fourier Transform Infrared Spectroscopy. The Fourier transform infrared (FTIR) (34) Zheng, W.; Angelopoulos, M.; Epstein, A. J.; MacDiarmid, A. G. Macromolecules 1997, 30, 2953-2955. (35) Stejskal, J.; Sapurina, I. J. Colloid Interface Sci. 2004, 274, 489-495.

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Figure 3. TEM images of polyaniline colloidal particles synthesized using different polymers as steric stabilizers: (a) Partially hydrolyzed poly(vinyl alcohol), (b) fully hydrolyzed poly(vinyl alcohol), (c) PNIPAM, and (d) chitosan. The scale bar in (a), (b), and (c) is 200 nm, and in (d) is 500 nm.

Figure 4. FTIR spectra of isolated polyaniline colloidal particles after dedoping synthesized in the presence of (a) partially hydrolyzed poly(vinyl alcohol), (b) fully hydrolyzed poly(vinyl alcohol), (c) chitosan, and (d) PNIPAM.

spectra of the isolated polyaniline colloidal particles are shown in Figure 4. The characteristic peaks of polyaniline in its emeraldine base form were found for all samples: the peak at 1305 cm-1 corresponding to the C-N stretch, the peak at 1136 cm-1 due to the C-H in-plane bending mode, and the peak at 828 cm-1 due to the C-H out-of-plane bending mode for a 1-4 substituted aromatic ring.11 The peaks at 1506 and 1590 cm-1 are assigned to the C-C stretching modes of the benzenoid ring and the quinoid ring,11 respectively. These signals confirm a 1-4 substitution pattern of the aromatic ring and an emeraldine oxidation state of the polymer because no peaks due to ortho coupling or branching were observed, in agreement with our previous study.10 Although several redispersion-centrifugation cycles were carried out carefully to remove the soluble steric stabilizers from the particles, FTIR revealed that a very small amount of stabilizer remained attached to the polyaniline particles. For instance, the spectrum of the sample synthesized using partially hydrolyzed

poly(vinyl alcohol) (Figure 4a) shows a peak at 1735 cm-1 due to CdO stretching of the acetylated units in the stabilizer. In addition, the spectra of chitosan- (Figure 4c) and PNIPAMstabilized particles (Figure 4d) showed a broad peak at 3500 cm-1 due to -NH2 stretching. The spectrum of chitosan-stabilized particles also showed a very weak peak at 1095 cm-1 due to C-N stretching in the glucosamine units.28 Similarly, the spectra of the PNIPAM- (Figure 4d) and poly(vinyl alcohol)-stabilized particles (Figure 4a,b) showed small peaks between 2800 and 3000 cm-1 due to symmetric and asymmetric stretching vibrations in methyl and methylene units.21 Among the samples, the polyaniline colloids prepared with fully hydrolyzed poly(vinyl alcohol) (Figure 4b) showed the minor contribution from the stabilizers. The mechanism of attachment of steric stabilizers to the conductive polymer colloids has been widely studied and seems to proceed most likely by physical entanglements and noncovalent attractive forces, such as hydrogen bonds.6,35 3.3. Thermal- and pH-Induced Flocculation. Once isolated from the steric stabilizer solution, the poly(vinyl alcohol)stabilized polyaniline particles could be redispersed in water over a wide range of pH and temperature. In contrast, the dedoped PNIPAM-stabilized polyaniline particles could be redispersed in water only at temperatures between 0 and 25 °C and remained stable as a blue dispersion for several days (Figure 5a). However, after placing the vial in water at 60 °C, a temperature above the LCST of PNIPAM, the colloids flocculated and became visible as macroscopic aggregates within 5 min (Figure 5b). Approximately 10 min later, the dispersion appears as a clear liquid that contains small black agglomerates (Figure 5c). The flocculation was completely reversible by cooling the solution to a temperature below 20 °C under mild agitation. This process was repeated several times without an apparent change in the stability of the dispersion. Similarly, the chitosan-stabilized polyaniline particles were stable in 0.1 N hydrochloric acid as a green dispersion (Figure 6a), but when the colloidal particles were placed in 0.1 N ammonium hydroxide (Figure 6b), they

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Figure 5. PNIPAM-stabilized polyaniline particles dispersed in deionized water: (a) at 25 °C and after placing the vial in water at 60 °C for (b) 5 min and (c) after 10 min.

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by the chitosan, which is soluble only in an acidic medium. In fact, the approximate pH at which chitosan-stabilized polyaniline colloids become unstable can be determined by pH titration. Figure S2 (Supporting Information) shows that flocculation of the isolated polyaniline particles rapidly occurred when the pH of the dispersion became higher than the pKa of the chitosan, approximately 6.3.36 Above this pH value, chitosan undergoes a coil-to-globule transition,37 losing its cationic charge and solubility. Polyaniline particles stabilized with fully and partially hydrolyzed poly(vinyl alcohol), both nonionic polymers, exhibited good colloidal stability in acidic or alkaline media over a wide range of temperature. Nevertheless, when an adequate amount of acetone was added to the mixture, the samples rapidly flocculated, possibly because of the lack of poly(vinyl alcohol) solubility in acetone-water mixtures. These experiments confirmed that the steric stabilizers provide colloidal stability to the polyaniline particles when they are in a medium where they can adopt an extended coil conformation, a condition needed for steric stabilization. In a medium were a coil-to-globule transition of the stabilizer occurred, the colloids underwent rapid flocculation because of the loss of steric stabilization. One interesting aspect of this method is its simplicity. In previous work, the adsorption of environmentally sensitive polymers on the surface of several types of colloids has been done by performing additional functionalization steps.18,19 In the case of these polyaniline colloids, the adsorption occurred spontaneously during the synthesis, most likely through hydrogen bonding, resulting in an easy way to prepare smart polyaniline colloids.

4. Conclusions In summary, we have demonstrated a simple and environmentally friendly route for synthesizing polyaniline colloidal particles with smart behavior. By using chitosan and PNIPAM as steric stabilizers, pH- and temperature-dependent colloidal stability can be imparted to the polyaniline colloids. Both the electrochemical behavior and the spectroscopic features of the enzymatically synthesized polyaniline colloids are similar to those prepared by chemical or electrochemical oxidation. These colloids have potential applications in smart devices such as thermochromic windows, temperature-responsive electrorheological fluids, actuators, and colloids for separation technologies. Acknowledgment. We acknowledge Jorge Sepulveda (IFCUNAM) for TEM images. Alondra Escamilla and Gabriela Padron are acknowledged for experimental support. This project was partially funded by CONACYT and PROMEP thorough the SEP2004-CO1-46046 and UAEMOR-PTC151 projects.

Figure 6. Chitosan-stabilized polyaniline particles dispersed in (a) 0.1 N hydrochloric acid and (b) 0.1 N ammonium hydroxide solutions.

Supporting Information Available: Cyclic voltammetry of polyaniline colloids stabilized with chitosan in 0.1 N HCl. pH-dependent colloidal stability of the polyaniline dispersion synthesized using toluenesulfonic acid as the doping agent and chitosan as the steric stabilizer. This material is available free of charge via the Internet at http://pubs.acs.org. LA0618418

immediately flocculated. Again, the polyaniline colloids could be redispersed by adding hydrochloric acid in excess of the stoichiometric amount to the alkaline solution. This behavior suggests that these polyaniline particles are sterically stabilized

(36) Sorlier, P.; Denuziere, A.; Viton, C.; Domard, A. Biomacromolecules 2001, 2, 765-772. (37) Bodnar, M.; Hartmann, J. F.; Borbely, J. Biomacromolecules 2005, 6, 2521-2527.