Enzymatic polymerization of aniline in the presence of different inorganic substrates

Materials Chemistry and Physics 105 (2007) 136–141

Enzymatic polymerization of aniline in the presence of different inorganic substrates E. Flores-Loyola a,b,∗ , R. Cruz-Silva c , J. Romero-Garc´ıa a , J.L. Angulo-S´anchez a, , F.F. Castillon d , M.H. Far´ıas d a Centro de Investigaci´ on en Qu´ımica Aplicada, Blvd. Enrique Reyna No. 140, CP 25100 Saltillo, Coah, Mexico Escuela de Ciencias Biol´ogicas, UA de C. Carr. Torre´on-Matamoros Km 7.5, Ciudad Universitaria, CP 27400 Torre´on, Coah., Mexico c Centro de Investigaci´ on en Ingenier´ıa y Ciencias Aplicadas, UAEM. Av. Universidad # 1001, Col. Chamilpa, CP 62210, Cuernavaca Mor., Mexico d Centro de Ciencias de la Materia Condensada de la UNAM, Apdo. Postal 2681, CP 22800 Ensenada, B.C., Mexico b

Received 29 May 2006; received in revised form 23 March 2007; accepted 8 April 2007

Abstract The effect of different inorganic substrates in the structure of polyaniline synthesized by enzymatic oxidation was studied. The polymer characterization was done by electronic absorption and X-ray photoelectron spectroscopy. The substrates studied were: controlled pore glass, mordenite, zeolite Y, zeolite MCM-41, Wollastonite, silica gel, fuming silica and short glass fibers type E. Polyaniline was synthesized in the presence of the substrates under acidic aqueous conditions, using hydrogen peroxide as oxidizer and HRP or SBP enzymes as catalyst. The composition of the substrates strongly affected the degree of electronic conjugation of the synthesized polyaniline, whereas the pore size and the enzyme type apparently had no effect. The chemical structure of polyaniline enzymatically synthesized was more sensitive to the substrate composition than that chemically synthesized. Apparently substrates containing alkaline ions, such as sodium and calcium, promoted the formation of the branched, non-conductive polyaniline form. The effect of the substrates on the polyaniline structure can be explained considering the local pH effect of the templates surface on the coupling reaction of aniline radicals. © 2007 Elsevier B.V. All rights reserved. Keywords: Polyaniline; Enzymatic synthesis; Porous materials; Peroxidases

1. Introduction Polyaniline (PANI) has attracted considerable attention due to its interesting electrochemical behavior [1], the reversible nature of its electrical conductivity as well as its environmental stability. Furthermore, its conductivity is not only controlled by the degree of chain oxidation but also by the protonation level [2,3]. All these properties make PANI attractive for applications as an electrically conductive material in the preparation of composites with new and specific properties. Dunsch [4] studied the aniline polymerization mechanism, and found that a linear structure is obtained only under acidic conditions due to 1,4 coupling of

∗ Corresponding author at: Centro de Investigaci´ on en Qu´ımica Aplicada, Blvd. Enrique Reyna No. 140, CP 25100 Saltillo, Coah, Mexico. Tel.: +52 871 757 1785; fax: +52 871 757 1795. E-mail address: [email protected] (E. Flores-Loyola).  Deceased on 28 October 2006.

0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.04.041

aniline radicals. Under near-neutral or alkaline pH, the aniline radicals may undergo substitution at the ortho and para positions, or even undergo multisubstitution, yielding a branched structure [5]. The polymerization mechanism for the enzymatic reaction in aqueous conditions at neutral pH [6,7] proceeds preferentially by 1,2 substitutions rather than by the 1,4 substitution observed under acid conditions. There is also the possibility to have multi-substitution in the rings yielding branched chains, which reduce mostly the optical and electric properties of PANI, restraining their potential applications. A different approach to control the structure during the synthesis of a polymer is by using a porous inorganic material as template. Mesoporous and nanoporous materials often provide environments within their pores where the polymerization reaction is carried out under different conditions to that of the reaction media. These materials may act as a template by promoting certain coupling reactions of the radicals. For these reasons, confined monomers usually polymerize in a more ordered and efficient way, and undesirable reactions, such as

E. Flores-Loyola et al. / Materials Chemistry and Physics 105 (2007) 136–141

branching or crosslinking, are avoided due to steric hindrances, resulting in a polymer with better properties compared to those synthesized in solution media [8]. Indeed, PANI has been synthesized by in situ chemical oxidation of aniline in the presence of different substrates and, in this case, the polymer is adsorbed both on the surface and within the pores of the substrate. These hybrid materials have potential applications in new technologies such as electrorheological fluids, sensors and catalysts [9,10]. PANI has been prepared using porous materials as templates, and some of these works are those reported by Wu and Bein [8], Bein and Enzel [11] and Enzel and Bein [12], who synthesized PANI by chemical oxidation within the channels of zeolite molecular sieves. They found that aniline in acidic zeolites can form intrazeolite polyaniline by oxidative polymerization reaction, in analogy to the oxidative coupling of aniline in acidic solution and that while the level of intrazeolite acidity has a strong influence on the polymerization reaction, the nature of the channel system controls the degree of polymer oxidation that affects its electronic properties. Besides, porous glass/polyaniline composites have been prepared by in situ oxidative polymerization of aniline [13,14]. These composites represent a new class of materials, where synthetic conductors are encapsulated in an insulating inorganic mesoporous host. This process has been used to modify a wide range of polymeric [15,16] and inorganic materials [12,17,18]. Mica [19], glass [13,14], silicon dioxide [20], and metallic oxides [21,22] have been successfully modified with polyaniline by in situ chemical polymerization. One of the most important potential applications of these materials is their use as electrically conductive fillers in advanced polymer composites. Recently, enzymatic polymerization of aniline has become very attractive because it is a clean and environmentally friendly process [23] that is carried out under milder conditions with high reaction yield. Although a wide range of peroxidases has been used to synthesize PANI, commercially available horseradish peroxidase (HRP) and soybean peroxidase (SBP) [24] have been the most studied. This biocatalytic approach has been applied mainly to synthesize PANI in the form of water–soluble complexes using different polyacids as templates [25–27]. These polyelectrolyte-templates provide a local acid pH and promote the head-to-tail coupling of aniline radicals. However, under acidic conditions, the template-free approach also yields electrically conductive PANI with a structure quite similar to that chemically synthesized [28]. Immobilization of peroxidase on polymeric substrates has been used to synthesize thin films of polyaniline [29], and recently, nanowires of PANI have been obtained by the enzymatic approach [30]. However, the usefulness of the enzymatic polymerization approach to synthesize PANI by in situ polymerization on inorganic substrates has not been examined. The aim of the present work was to study the PANI enzymatically synthesized in the surface and within the pores of several inorganic substrates. The electronic conjugation degree was studied by UV–vis and X-ray photoelectron spectroscopic techniques, whereas the thermal stability was studied by thermogravimetry. The effects of substrate composition and morphology, enzyme type, and monomer confining on PANI properties are also discussed.

137

2. Experimental part 2.1. Materials Hydrogen peroxide (29.9 wt%), Horseradish peroxidase (HRP) (238 U/mg), Soybean peroxidase (SBP) (53.4 U/mg), and p-toluensulfonic acid (99%) (TSA), were purchased from Sigma. Aniline and ammonium hydroxide were acquired from Qu´ımica Din´amica, Mex. N-Methyl-2-pyrrolidinone (NMP) was acquired from Aldrich. Aniline was distilled at reduced pressure and stored at −28 ◦ C prior to use, and all other reagents were used as received. The porous inorganic substrates studied were: controlled pore glass (CPG-LP), 72.9 nm average pore diameter (Electro-nucleonics), controlled pore glass (CPG-SP), 7.5 nm average pore diameter (Sigma), sodium zeolite Y (NaY) and silica gel (SIL) (mesh size 70–230, 500 m2 /g BET surface) were both acquired from Aldrich. Molecular sieve MCM-41 was synthesized following the procedure reported by Beck et al. [31] and characterized in a previous work [32]. Natural zeolite, mordenitetype from the San Luis Potosi State in Mexico, whose characterization has been reported [33], was used in two forms: as received (MOR) and acid treated (MORAT). Solid substrates were: Wollastonite NYAD grade F-1 was a gift from Nyco Minerals (WOL-F1), Glass Fiber Type E (SGF-TE) that was acquired from Grupo Vitro, Mex. and fuming silica (Cab-O-Sil, M5) was acquired from Cabot, Co (FSIL).

2.2. Polyaniline synthesis PANI synthesized by the chemical method reported by Wei and Hsueh [34] was used as reference for polymer structure. Acid-treated mordenite was prepared by stirring overnight the as-received mineral in HCl aqueous solution (1.0 N). The PANI was synthesized by enzymatic polymerization in the inorganic porous hosts as follows: aniline was absorbed into the different hosts from the liquid phase, at room temperature, for 7 days. The aniline saturated hosts were immersed in 60 ml of distilled water containing an equimolar amount of TSA with regard to aniline. Afterwards, HRP or SBP peroxidase was placed into the mixture reaction and the hydrogen peroxide added during the total reaction time. The rate of addition was adjusted to a maximum value of 3.6 ␮l/min in order to prevent denaturing of the enzyme. This mixture was kept at 0 ◦ C under continuous stirring for a time period depending on the aniline concentration, typically between 5 and 8 h. The product thus obtained was recovered from the polymerization vessel, filtered and washed thoroughly with water followed by methanol, to remove residual monomer and oligomers, and then lyophilized.

2.3. Characterization Pore size measurements in mordenite were performed by thermoporometry, using a modulated DSC TA Instrument Model 2920. The experimental details can be found elsewhere [35]. Alternatively, the pore size distribution was determined by nitrogen physisorption using the BJH model [36] on the desorption isotherm. The electronic absorption spectra of the PANI were acquired in NMP solutions, using a diode array spectrophotometer (Hewlett Packard HP 8452A). For these analyses, the solid samples were dedoped with NH4 OH 0.2 M, lyophilized and extracted with NMP under stirring. Monomer content and PANI thermal stability in the inorganic substrates was determined by thermogravimetric analysis (TA Instruments TGAQ500), using a 20 ◦ C/min heating rate under nitrogen atmosphere from room temperature to 600 ◦ C and under air from 600 to 800 ◦ C. X-ray scattering microanalysis was carried out using an energy-dispersive X-ray spectrometer (EDAX) coupled to a scanning electron microscope (TOPCON SM-510). The samples were coated with an Au-Pd alloy before analysis. The X-ray photoelectron spectroscopy (XPS) analysis was carried out on a modified laser ablation system, Riber LDM-32, using a Cameca Mac3 analyzer. The base pressure in the analysis chamber was in the low 10−10 Torr range, and about 10−9 Torr in the sample loading chamber. The X-ray Al K␣ line at 1486.6 eV was used for excitation. The binding energies were calibrated with reference to Cu 2p3/2 at 932.67 eV and Ag 3d5/2 at 368.26 eV, respectively. The resolution attained with this set-up is 1.1 eV measured on the C 1s signal of a graphite target. Spectra were collected by acquiring data every 0.2 eV and the energy resolution was 0.5 eV. The core-level spectra for C 1s, and N 1s, were obtained. Background subtraction was done using the Tougaard

138

E. Flores-Loyola et al. / Materials Chemistry and Physics 105 (2007) 136–141

method [37]. In addition, wide-scan spectra were gathered by acquiring data every 1.0 eV with an energy resolution of 3 eV. Infrared spectra were acquired using a Nicolet Magna 550 FT-IR working in transmission mode. The spectra were obtained from KBr pellets containing the PANI extracted from the inorganic hosts using NMP.

3. Results and discussion The UV–vis absorption spectrum of PANI synthesized by the chemical method (C-PANI), without host, was used as reference in subsequent analysis. The spectrum of this sample (Fig. 1) showed two bands, one at 330 nm (band 1) associated with a ␲ → ␲* transition of the conjugated-rings system, and other at 636 nm (band 2) corresponding to the quinoid excitonic transition [38,39]. These bands are typical of the reported spectrum for PANI in the emeraldine base form. The intensity ratio (I330 /I636 ) is indicative of the relative amount of benzenoid to quinoid units in PANI [40], and consequently has been used to determine the polymer oxidation state. This ratio was 1.33 for the chemically synthesized PANI, which is within the range reported for the emeraldine base form [40].

Table 1 Chemical composition of the substrates by energy dispersive X-ray spectroscopy (EDS) Sample

Si

O

Al

Na

Ca

Fe

C

MOR MOR-AT CPG-LP

27.51 29.23 57.76

38.29 46.24 42.28

5.22 5.2 –

1.17 0.49 –

3.05 0.76 –

1.99 1.88 –

22.02 14.86 –

The UV–vis spectrum of PANI enzymatically synthesized in presence of natural mordenite is showed in Fig. 1. There are two bands, one at 290 nm and other at 380 nm, the first one may be due to the presence of ␲–␲* transitions with low degree of conjugation [26] and that at 380 nm is related to the benzenoid rings. The blue shift with regard to C-PANI can be attributed to the branching in polymer and therefore to the loss of conjugation [41]. According to that reported by Dunsch [4], the PANI branched structure obtained by enzymatic synthesis in the natural zeolite is possibly due to local pH differences within the pores, caused by alkaline impurities in the mordenite. This assumption is in agreement with the pH change that undergoes the reaction medium from an initial value of 2.5 to 5.2 at the end of the reaction. The change of pH might be caused by impurities such as calcium carbonate or iron and aluminium

(Table 1), which react with the TSA shifting the pH to higher values. Acid-treated mordenite showed a decrease in carbon, calcium, and sodium as measured by EDS, although no changes were observed in iron or aluminium (Table 1) suggesting that they are part of the mordenite structure. The PANI synthesized showed an UV–vis spectra (Fig. 1) with a weak absorption at 650 nm, which indicates a low amount of quinoid moieties. This indicates that within the mordenite pores there was still basic local pH or ions, which favor the synthesis of non-conductive PANI. PANI was also synthesized in MOR-AT using SBP, which is less sensitive to irreversible denaturation at low pH [42]. However, the UV–vis spectrum of PANI obtained with this enzyme (Fig. 1) shows similar electronic structure than PANI synthesized with HRP. The composition effect of the inorganic substrate on the PANI structure was evaluated using substrates with different content of aluminium and calcium. The UV–vis spectra of the PANI synthesized using SBP as catalyst, in zeolite NaY with 8 wt% of Al, zeolite MCM-41 with less than 1% of Al, and wollastonite with no aluminium but with 24% of Ca are shown in Fig. 2. As it can be observed, as the aluminium content in the inorganic substrate diminishes, the band at 636 nm, corresponding to the quinoid excitonic transition, increases. These results confirm that the aluminium content in the host affects the PANI structure in an important way. However, the PANI synthesized in WOL-F1, which has no aluminium but has a high calcium content show a UV–vis spectrum which does not match that of emeraldine but rather one of lower content of quinoid moieties, as can be noted by the weak band at 567 nm. PANI structure

Fig. 1. UV–vis spectra comparison of PANI enzymatically synthesized in the presence of MOR and MOR-AT, using HRP or SBP as catalyst.

Fig. 2. UV–vis spectra of enzymatically synthesized PANI in the presence different inorganic substrates.

3.1. Synthesis within porous substrates

E. Flores-Loyola et al. / Materials Chemistry and Physics 105 (2007) 136–141

Fig. 3. Comparison of UV–vis spectra of enzymatically synthesized PANI in the presence of different substrates.

depends on the pH conditions during syntheses [4], thus the influence of aluminium and calcium on the PANI structure can be ascribed to the influence of the alkaline local pH on the surface of the hosts. On the other side, some authors have reported [43,44] that the presence of certain amounts of metallic ions, may block substrate interaction causing competitive inhibition on peroxidases, and that this effect is more marked at lower pH values, producing a decrease in peroxidase activity. However, in this work the amount of these ions in solution, where peroxidase is, is as low that their effect on the enzyme activity is minimal. To confirm our hypothesis, PANI was synthesized within non-ionic porous substrates, such as controlled pore glass with different pore size (CPG-LP and CPG-SP). The normalized UV–vis spectra of the enzymatically synthesized PANI in these hosts using SBP enzyme are shown in Fig. 3. Both spectra showed the two bands (322 and 640 nm) typical for the PANI emeraldine base

139

[38]. The I330 /I630 absorbance intensity ratio was 1.39 for CPGSP and 1.48 for CPG-LP, indicating that the partially oxidized form of polyaniline was obtained in presence of these substrates. No effect of the type of enzyme or pore size on the PANI chemical structure was observed. These results are in agreement with those of Gorgatti Zarbin et al. [14], who synthesized PANI within porous Vycor glass by in situ aniline oxidative polymerization reaction and found that polyaniline can be obtained inside the nanometric porous structure in the state of emeraldine salt. To analyze the high surface area and the confined environment on the PANI structure, the enzymatic synthesis was carried out in presence of solid substrates such as glass fiber (SGF-TE), fuming silica (FSIL), and silica gel (SIL). The UV–vis spectra of these samples (Fig. 3) showed the two signals characteristic of emeraldine base at 320 and 620 nm. In spite of the differences in the surface structure of the substrates, no effect of the templates shapes on the PANI conjugation was observed. X-ray photoelectron spectroscopy is a very sensitive and surface specific analytical technique widely used to characterize the electronic environment of PANI. In Fig. 4a and d, the widescan spectra of PANI synthesized in MOR-AT and MCM-41 using SBP as catalyst are shown. The peaks at 110, 162, 291, 404, and 540 eV, are due to Si 1s, Si 2s, C 1s, N 1s, and O 1s, respectively. Fig. 4c and f shows the high-resolution N 1s core level spectra of the MOR-AT and MCM-41, respectively. The peak fitting was done using the reported binding energies for the quinoid amine (–N ), benzenoid amine (–NH–) and the positively charged nitrogen (N+ ) at 398.2, 399.4 and slightly above 400 eV, respectively [45]. The quinoid/benzenoid ratio (–N /–NH–) for PANI synthesized in MCM-41 sample was of 0.45. This value indicates that nitrogen is mostly in benzenoid amine form and therefore in the oxidation state of emeraldine, in agreement with the UV–vis spectrum of this sample. But considering the signal around 400.8 eV associated with chain defects,

Fig. 4. Comparison of XPS spectra of undoped PANI synthesized in MCM-41: (a) wide scan, (b) C 1s core-level spectrum, (c) N 1s core-level spectrum; and PANI synthesized in MOR-AT (d) wide-scan spectrum, (e) C 1s core-level spectrum, (f) N 1s core-level spectrum.

140

E. Flores-Loyola et al. / Materials Chemistry and Physics 105 (2007) 136–141

Fig. 5. FT-IR spectra of the extracted enzymatically synthesized PANI in the presence of: (a) MCM-41 and (b) MOR-AT.

loss of 94% due to degradation between 400 and 650 ◦ C. On the other hand, for PANI synthesized in porous substrates a much less pronounced weight loss between 6 and 10% was observed at 400–600 ◦ C as a result of the polymer degradation. These results indicated that the encapsulated PANI in porous substrates is thermally more stable than bulk PANI. This is because the encapsulation of PANI chains in the pores of some material with higher thermal stability limits the release of gases product of the polymer thermal degradation. This is in agreement with the reported higher thermal stability of polyaniline/inorganic hybrid composites [51]. PANI synthesized in presence of non-porous solid substrates showed a similar degradation pattern than that synthesized in porous substrates, in both cases the initial weight loss was approximately 3 wt% at 150 ◦ C, attributed to the loss of water, and another weight loss between 400 and 700 ◦ C, related to polymer backbone decomposition. 4. Conclusions

we can suppose that the polymeric chains, which are in the substrate surface, are slightly branched while those inside the pores are mainly linear and type-emeraldine. For PANI synthesized in MOR-AT the ratio of –N /–NH = 0.1 (Fig. 4f), which could indicate the presence of high branching [46], which diminishes the amount of quinoid moieties by the rings multi-substitution. This is further supported by the two signals with binding energy of 400.5 and 402.1 eV, associated to chain defects such as tertiary nitrogen [47] indicative of crosslinking [48]. The C 1s core-level spectra of both samples (Fig. 4b and e) are consistent with those reported for chemically synthesized PANI [49]. To corroborate the XPS results, FT-IR analysis were carried out. In Fig. 5 the spectra of PANI synthesized in MCM-41 (Fig. 5a) and MOR-AT (Fig. 5b) using SBP as catalyst are shown. Characteristic signals of polyaniline can be noted in both spectra, [50] such as non-hydrogen bonded N–H stretching (3280 cm−1 ) and C–C stretching for benzenoid and quinoid groups (1510 and 1600 cm−1 , respectively). However, in Fig. 5b, that last signal is weak and appears as a shoulder because of the presence of the strong C O stretching absorption, at 1677 cm−1 , from the residual NMP used for PANI extraction, which also appears in Fig. 5a. The signals at 1301 cm−1 from C–N stretching, and 1176 cm−1 from C–H bending are also associated with polyaniline. The aromatic C–H bending in plane (1120 cm−1 ) and out of the plane (819 cm−1 ) indicates a 1,4 disubstituted aromatic ring for PANI synthesized in MCM-41 sample. The benzenoid/quinoid absorbance intensity ratio, A1510 /A1600 is higher for PANI synthesized in MOR-AT than for that synthesized in the presence of MCM-41. This means that a lower amount of quinoids moieties exists in the first sample, which is in agreement with the higher B/Q ratio obtained from the UV–vis results and with that obtained by XPS. The thermal stability of PANI samples was determined by thermogravimetric analysis (TGA). The degradation curves (not shown) of enzymatically synthesized PANI in all porous substrates, as well as C-PANI, presented an initial weight loss of approximately 5 wt% around 150 ◦ C attributed to water evaporation. Additionally, C-PANI showed a relatively quick weight

Polyaniline was enzymatically synthesized in the presence of different inorganic substrates under acidic conditions. The ratio of benzenoid to quinoid (B/Q) moieties in the PANI synthesized in substrates without metallic ions on their surface was between 1.3 and 1.5, well within the range defined for the chemical structure of emeraldine. The B/Q ratio was 6.0 for the PANI in MOR-AT whereas no absorption bands due to quinoid rings were observed in MOR. Accordingly, emeraldine base form of PANI was synthesized only in presence of substrates composed of SiO2 , whereas non-conjugated PANI was obtained in MOR, MOR-AT and WOL-F1. The effect of composition of the substrate on the polymer was attributed to the impurities or ions in its structure (Ca, Na, Al, and Fe), which generate an alkaline local pH that induces branching reactions during the polymer growth. In addition, electronic conjugation of the PANI synthesized was insensitive to pore size or surface area of the substrates, as well as to the enzyme type when using substrates of SiO2 base composition. Nevertheless the presence of alkaline ions on the substrates strongly affected the electronic conjugation of the synthesized polyaniline. Acknowledgments The authors thank to Antonio Diaz for his help with XPS analysis, Blanca Huerta for Thermal analyses and Oscar Ayala for physisorption experiments. This project was funded by CONACYT project SEP-2004-CO1-46046 and J-50313. References [1] H.L. Tassi, J.L. Schindler, C.R. Kannewurf, M.G. Kanatzidis, Chem. Mater. 9 (1997) 875. [2] W.-S. Huang, B.D. Humphrey, A.G. Macdiarmid, J. Chem. Soc., Faraday Trans. 182 (1986) 2385. [3] A.G. Macdiarmid, A.J. Epstein, Faraday Discuss., Chem. Soc. 88 (1989) 317. [4] L. Dunsch, Electroanal. Chem. Interf. Electrochem. 61 (1975) 61. [5] F. Lux, Polymer 35 (1994) 2915. [6] P.D. Nayak, Des. Monomers Polym. 1 (1998) 259.

E. Flores-Loyola et al. / Materials Chemistry and Physics 105 (2007) 136–141 [7] C.H. Lim, Y.J. Yoo, Process Biochem. 36 (2000) 233. [8] C.-G. Wu, T. Bein, Science 264 (1994) 1757. [9] I.S. Sim, J.W. Kim, H.J. Choi, C.A. Kim, M.S. Jhon, Chem. Mater. 13 (2001) 1243. [10] D. Xie, Y. Jiang, W. Pan, D. Li, Z. Wu, Y. Li., Sens. Actuators B 81 (2002) 158. [11] T. Bein, P. Enzel, Mol. Cryst. Liq. Cryst. 181 (1990) 315. [12] P. Enzel, T. Bein, J. Phys. Chem. 93 (1989) 6270. [13] P.T. Sotomayor, I.M. Raimundo Jr., A.J. Gorgatti Zarbin, J.J. Rohwedder, G. Oliveira-Neto, O.L. Alves, Sens. Actuators B 74 (2001) 157. [14] A.J. Gorgatti Zarbin, M.-A. De Paoli, O.L. Alves, Synth. Met. 84 (1997) 107. [15] W. Jia, R. Tchoudakov, E. Segal, R. Joseph, M. Narkis, A. Siegman, Synth. Met. 132 (2003) 269. [16] M.S. Cho, Y.H. Cho, H.J. Choi, M.S. Jhon, Langmuir 19 (2003) 5875. [17] M.M. Verghese, S.M. Kumaran Ramanathan, M.N. Ashraf, B.D. Kamalasanan, Malhotra, Chem. Mater. 8 (1996) 822. [18] B.-H. Kim, J.-H. Jung, S.-H. Hong, J. Joo, A.J. Epstein, K. Mizoguchi, J.W. Kim, H.J. Choi, Macromolecules 35 (2002) 1419. [19] W. Jia, R. Tchoudakov, E. Segal, R. Joseph, M. Narkis, A. Siegmann, Synth. Met. 132 (3) (2003) 269. [20] S.-C. Huang, C.-T. Huang, S.-Y. Lu, K.-S. Chou, J. Porous Mater. 6 (1999) 153. [21] Z. Zhang, M. Wan, Synth. Met. 132 (2003) 205. [22] T.K.H. Starke, G.S.V. Coles, Sens. Actuators B 88 (2003) 227. [23] D. Chinn, J. Dubow, M. Liess, M. Josowicz, J. Janata, Chem. Mater. 7 (1995) 1504. [24] E. Arias Marin, J. Romero, A. Ledezma-Perez, S. Kniajansky, Polym. Bull. 37 (1996) 581. [25] W. Liu, J. Kumar, S. Tripathy, K.J. Senecal, L. Samuelson, J. Am. Chem. Soc. 121 (1999) 71. [26] W. Liu, L. Ashok Cholli, R. Nagarajan, J. Kumar, S. Tripathy, F. Bruno, L. Samuelson, J. Am. Chem. Soc. 121 (1999) 11345. [27] R. Nagarajan, S. Tripathy, J. Kumar, F. Bruno, L. Samuelson, Macromolecules 33 (2000) 9542. [28] R. Cruz-Silva, J. Romero-Garc´ıa, J.L. Angulo-S´anchez, A. Ledezma-P´erez, E. Arias-Mar´ın, I. Moggio, E. Flores-Loyola, Eur. Polym. J. 41 (2005) 1129. [29] S. Alvarez, S. Manolache, F. Denes, J. Appl. Polym. Sci. 88 (2003) 369. [30] B.-K. Kim, Y.H. Kim, K. Won, H. Chang, Y. Choi, K.-J. Kong, B. Whan Rhyu, J. Kim, J.-O. Lee, Nanotechnology 16 (2005) 1177.

141

[31] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [32] E. Flores-Loyola, Enzymatic synthesis and physical-chemical characterization of polyaniline in nanoporous inorganic hosts, Doctoral Thesis, CIQA, M´exico, 2004, p. 59. [33] R. Hern´andez-Huesca, L. D´ıaz, G. Aguilar-Armenta, Sep. Purif. Technol. 15 (1999) 163. [34] Y. Wei, K.F. Hsueh, J. Polym. Sci. Part A: Polym. Chem. 27 (1989) 4351. [35] M. Iza, S. Woerly, C. Danumah, S. Kaliaguine, M. Bousmina, Polymer 41 (2000) 5885. [36] E.P. Barret, L.G. Joyner, P.P. Halenda, J. Am. Chem. Soc. 73 (1951) 373. [37] S. Tougaard, Surf. Interf. Anal. 11 (1988) 453. [38] A.J. Epstein, J.M. Ginder, F. Zuo, R.W. Bigelow, H.S. Woo, D.B. Tanner, A.F. Richter, W.-S. Huang, A.G. Macdiarmid, Synth. Met. 18 (1987) 303. [39] E.M. Conwell, C.B. Duke, A. Paton, S.J. Leyadev, Chem. Phys. 88 (1988) 3331. [40] Y. Wei, K.F. Hsueh, G.-W. Jang, Macromolecules 27 (1994) 518. [41] K. Shridhara Alva, J. Kumar, K.A. Marx, S.K. Tripathy, Macromolecules 30 (1997) 4024. [42] K. Chattopadhyay, S. Mazumdar, Biochemistry 39 (2000) 263. [43] A.Y. Louie, T.J. Meade, Chem. Rev. 99 (1999) 2711. [44] A.N.P. Hiner, L. Sidrach, S. Chazarra, R. Var´on, J. Tudela, F. Garc´ıaC´anovas, J.N. Rodr´ıguez L´opez, Biochimie 83 (2004) 667. [45] E.T. Kang, K.G. Neoh, K.L. Tan, H.S. Nalwa, Handbook of Organic Conductive Molecules and Polymers, vol. 3, Wiley, Chichester, England, 1997, p. 121. [46] Z.H. Ma, H.S. Han, K.L. Tan, E.T. Kang, K.G. Neoh, Int. J. Adhes. Adhes. 19 (1999) 359. [47] E.T. Kang, Z.H. Ma, K.L. Tan, O.N. Tretinnikov, Y. Uyama, Y. Ikada, Langmuir 15 (1999) 5389. [48] E.M. Scheer, A.G. Macdiarmid, S.K. Manohar, J.G. Masters, Y. Sun, X. Tang, M.A. Druy, P.J. Glatkowsky, V.B. Cajipe, J.E. Fischer, K.R. Cremack, M.E. Josefowicz, J.M. Ginder, R.P. Mccall, A.J. Epstein, Synth. Met. 41–43 (1993) 735. [49] Y. Chen, E.T. Kang, K.G. Neoh, W. Huang, Langmuir 17 (2001) 7425. [50] J.A. Akkara, D.L. Kaplan, H. Kurioka, H. Uyama, S. Kobayashi, J. Yue, J. Epstein, Macromolecules 24 (1991) 4441. [51] J. Prokes, I. Krivka, E. Tobolkov´a, J. Stejskal, Polym. Degrad. Stabil. 68 (2000) 261.