Deposition of Polypyrrole Thin Film through the Molecular Interaction

pubs.acs.org/Langmuir © 2009 American Chemical Society

Deposition of Polypyrrole Thin Film through the Molecular Interaction with a Designer Peptide Juyoung Choi,† Inho Lee,†,‡ and Sang-Yup Lee*,†,‡ †

Department of Chemical and Biomolecular Engineering and ‡Specialized Graduate School of Hydrogen and Fuel Cell, Yonsei University 134 Shinchon-dong, Seodaemun-gu, Seoul 120-749 Korea Received April 15, 2009. Revised Manuscript Received August 14, 2009

A conductive chlorine-doped polypyrrole (PPyCl) thin film was prepared via chemical oxidation on a nonconductive silicon wafer where a designer peptide with specific affinity for PPyCl was immobilized. The physical and electrical properties of PPyCl films prepared in this way were characterized. The designer peptide greatly improved the adhesion of the PPyCl film to the substrate because of the specific interaction between the peptide residues and polypyrrole film. This improved adhesion force suggests the existence of additional molecular interaction as well as electrical attraction between the Asp side chain and the PPyCl backbone. In addition, the PPyCl film on the peptide-immobilized surface showed increased conductivity. Investigation on the molecular structure indicated that the PPyCl backbone structure was controlled during the polymerization by interaction with the designer peptide. This designer peptide could be useful for the preparation of biopolymer hybrid materials and at interfaces where a biocompatible linker is required.

Introduction Conductive polypyrrole (PPy) thin films have been used in the fabrication of many electrical devices such as electrodes and sensors.1-3 Recently, the applicability of the PPy film is expanding to bioelectrical system research in which chemical modification of PPy is required for the conjugation with various biomolecules.4-6 The pyrrole derivative molecules can provide a platform for conjugation with biomolecules while maintaining their original electrical conductivity. These biocompatibility and electrical conductivity make the PPy film promising for use in bioelectrical devices. However, only limited research has been carried out on the interaction between biomolecules and unmodified PPy. Considering the complexity of the chemical modification of pyrrole monomers, being able to use pyrrole without such modification would expand its versatility. Negatively charged biomolecules can interact with the positively charged PPy backbone. This raises the possibility that the physical properties of the PPy film could be controlled using peptides. This biomolecular interaction would be of importance specifically when the PPy film is directly assembled with peptides. In general, electrochemical polymerization method has been used when fabricating thin film on a solid substrate. High-quality PPy films were obtained through the electrochemical polymerization that can be controlled by physicochemical factors such *To whom correspondence should be addressed. Telephone: þ82-2-21235758. Fax: þ82-2-312-6401. E-mail: [email protected].

(1) Coche-Guerente, L.; Deronzier, A.; Galland, B.; Moutet, J.-C.; Labbe, P.; Reverdy, G.; Chevalier, Y.; Amhrar, J. Langmuir 1994, 10, 602–610. (2) Ramanathan, K.; Bangar, M. A.; Yun, M.; Chen, W.; Myung, N. V.; Mulchandani, A. J. Am. Chem. Soc. 2005, 127, 496–497. (3) Cosnier, S. Appl. Biochem. Biotechnol. 2000, 89, 127–138. (4) Abu-Rabeah, K.; Polyak, B.; Ionescu, R. E.; Cosnier, S.; Marks, R. S. Biomacromolecules 2005, 6, 3313–3318. (5) Shimomura, M.; Miyata, R.; Kuwahara, T.; Oshima, K.; Miyauchi, S. Eur. Polym. J. 2007, 43, 388–394. (6) Gomez, N.; Lee, J. Y.; Nickels, J. D.; Schmidt, C. E. Adv. Funct. Mater. 2007, 17, 1645–1653. (7) Guiseppi-Elie, A.; Wilson, A. M.; Tour, J. M.; Brockmann, T. W.; Zhang, P.; Allara, D. L. Langmuir 1995, 11, 1768–1776. (8) Lee, S.; Sung, H.; Han, S.; Paik, W. J. Phys. Chem. 1994, 98, 1250–1252. (9) Li, Y.; Yang, J. J. Appl. Polym. Sci. 1997, 65, 2739–2744.

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as surface functionality, electrolyte type, and dopant concentrations.7-9 Through this method, PPy films are often deposited on patterned conducting surfaces10,11 or metallic devices.12,13 However, electrochemical polymerization cannot be used universally; the substrate to be coated must be electrically conductive because it needs to work as an electrode. Generally, ITO glass, gold-plated glass (or wafers), and ion-doped conductive silicon wafers are only used in thin film deposition. In addition, the substrate can be damaged by oxidation during the polymerization process.14 Therefore, the electrochemical polymerization method is rarely used on low-conductive biomolecular surfaces or on insulating surfaces such as glass and undoped silicon wafers. The chemical oxidation polymerization method does not share these limitations. This method can be applied to various substrates with complex structures. After simple surface modification of the substrate, reliable PPy deposition was achieved through oxidation polymerization. In past studies, the surface has been modified with surfactant,15,16 self-assembled monolayers,17 or other charged polymers.18,19 One interesting phenomenon is that a dense PPy film coating has been achieved only on hydrophobic surfaces, while discrete, particle-like PPy deposition is observed on hydrophilic surfaces.20,21 Though hydrophilic surfaces show (10) Gorman, C. B.; Biebuyck, H. A.; Whitesides, G. Chem. Mater. 1995, 7, 526– 529. (11) Seo, I.; Pyo, M.; Cho, G. Langmuir 2002, 18, 7253–7257. (12) Weiss, Z.; Mandler, D.; Shustak, G.; Domb, A. J. J. Polym. Sci., Part A 2004, 42, 1658–1667. (13) Khan, W.; Kapoor, M.; Kumar, N. Acta Biomater. 2007, 3, 541–549. (14) Skotheim, T. A. Handbook of Conducting Polymers; Marcel Dekker: New York, 1986; Chapter 8, p 268. (15) Funkhouser, G. P.; Arevalo, M. P.; Glatzhofer, D. T.; O’Rear, E. A. Langmuir 1995, 11, 1443–1447. (16) Yuan, W.-L.; O’Rear, E. A.; Grady, B. P.; Glatzhofer, D. T. Langmuir 2002, 18, 3343–3351. (17) Huang, Z.; Wang, P.-C.; MacDiarmid, A. G.; Xia, Y.; Whitesides, G. Langmuir 1997, 13, 6480–6484. (18) Fou, A. C.; Rubner, M. F. Macromolecules 1995, 28, 7115–7120. (19) Onoda, M.; Tada, K.; Shinkuma, A. Thin Solid Films 2006, 49, 61–72. (20) Wang, P.-C.; Lakis, R. E.; MacDiarmid, A. G. Thin Solid Films 2008, 516, 2341–2345. (21) Wang, P.-C.; Huang, Z.; MacDiarmid, A. G. Synth. Metal. 1999, 101, 852– 853.

Published on Web 09/04/2009

DOI: 10.1021/la901326q

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Scheme 1. Scheme of the Surface Modification and Polypyrrole Deposition Process: (a) Silanization of the Silicon Wafer Surface with APTES; (b) Surface Modification with Glutaraldehyde; (c) Immobilization of T59 Peptide; (d) Deposition of PPy

stronger adhesion forces than hydrophobic surfaces, the sparse surface coverage limits the use of hydrophilic surfaces for PPy deposition. Therefore, a novel approach to surface treatment is required to obtain a dense PPy thin film using chemical oxidation. With this objective, we introduced a designer peptide as a hydrophilic binder molecule which has specific affinity to the chlorine-doped PPy (PPyCl). In this report, physical properties of a dense chlorine-doped polypyrrole (PPyCl) thin film deposited on a peptide-immobilized surface were investigated. Surfaces modified with the designer peptide were characterized and the effects of the peptide on the adhesion force and electrical conductivity of the PPyCl film were investigated. The designer peptide worked as a binding molecule at the interface between PPyCl and the silicon wafer, promoting electrical conductivity.

Experimental Section Materials. Pyrrole monomer (98%), 3-aminopropylethoxysilane (APTES, 99%), and ferric chloride (FeCl3, 97%) were purchased from Aldrich. Glutaraldehyde (GA, 25%) was purchased from Junsei chemical. Phosphate buffered saline (PBS, pH 7.4, 0.1 M) was purchased from Gibco. Double distilled water and absolute pure ethanol for reaction and washing were purchased from Tedia and Duksan chemicals, respectively. All chemicals were used as received without additional purification. A 12-mer designer peptide, T59 (Thr-His-Arg-Thr-Ser-Thr-Leu-Asp-Tyr-Phe-ValIle)22 and FITC-labeled T59 were purchased from Peptron (purity 95þ%). Chemical structure of T59 peptide is presented in Scheme 1. Undoped silicon (100) wafers were used as a substrate. (22) Sanghvi, A. B.; Miller, K. P.-H.; Belcher, A. M.; Schmidt, C. E. Nat. Mater. 2005, 4, 496–502.

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Peptide Immobilization on the Silicon Wafer. The silicon wafer was cut into a 1  1 cm square and cleaned for 1 h at 80 °C in a fresh piranha solution (a mixture of hydrogen peroxide (H2O2, 33%) and sulfuric acid (H2SO4, 98%) in 1:3 volume ratio). After the wafer was thoroughly rinsed with deionized water and pure ethanol in sequence, its surface was modified through reaction with APTES to introduce an amine group. The surface modification was carried out in a fresh ethanol solution containing 5% APTES for 2 h at room temperature, following curing on a hot plate at 125 °C for 2 h. To the amine-modified surface, a crosslinker of GA was introduced by reaction with 2.5% GA in PBS buffer solution. The reaction with GA was carried out for 1.5 h at room temperature and unreacted GA was removed by washing with PBS solution. Finally, peptide immobilization was performed by reacting 0.02 mg/mL peptide solution in PBS buffer (pH = 7.2) with the GA-modified surface for 30 h at room temperature. Subsequently, unbound peptides were washed away with PBS buffer solution and water. The amount of immobilized peptide was determined by BCA (bicinchoninic acid) method using UV visible spectrum at 562 nm after isolation of immobilized peptide using a commercial BCA reagent kit (Pierce). For the peptide isolation, a 2  2 cm2 wafer substrate with immobilized peptides were immersed in 6 M HCl for 48 h to detach the peptide from the substrate, and then the HCl solution was neutralized with 6 M NaOH. A hydrophilic surface with carboxylates was prepared for the control experiment. Succinic anhydride was reacted with amine groups on the ATPES forming carboxylates on the exposed surface. The schematic layout of the surface modification and peptide immobilization is presented in Scheme 1. Deposition of Polypyrrole on Substrate via Chemical Oxidation. A volume of 7.5 μL pyrrole monomer was mixed with 10 mL water and sonicated for 1 min to disperse the monomer. The surface-modified silicon wafer was immersed in Langmuir 2009, 25(19), 11495–11502

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Figure 1. FT-IR spectra of modified silicon wafer surfaces. Each spectrum represents after modification of (a) APTES, (b) GA, and (c) T59 peptide. Table 1. Water contact angle of each modified surface (measured by sessile drop method) surface modified with silicon wafer contact angle (deg)

spreading

APTES

GA

COOH

T59 peptide

71.8 ( 5.7

67.4 ( 3.5

30.6 ( 4.8

27.5 ( 5.5

the pyrrole monomer dispersion in pH 6.2, and then polymerization and deposition of PPy was performed by adding 0.5 mL FeCl3 solution (0.5 M). The polymerization was carried out for 12 h under mild shaking. Large PPy particles attached on the wafer substrate were removed by 1 min sonication in the ethanol and water mixture. After sonication, the PPyCl-coated surface was cleaned with water and ethanol again. The PPyCl-coated surfaces were dried in a vacuum oven at 25 °C for 6 h. Characterization. After each step of surface modification, the wafer surfaces were characterized using FT-IR spectroscopy (Excalibur Series FTIR, DIGIlab Co.) to confirm the modification. Water contact angles of the modified surfaces were measured by static sessile drop method. The optical and fluorescence images of peptide-immobilized surfaces were taken using a microscope equipped with an integrated color CCD camera (Axiovert 200, Zeiss). The surface morphology and the PPyCl film thickness were observed with a SEM (Model S-4200, Hitachi) and an AFM (Veeco, Nanoscope 3D) in tapping mode. After scratching a line with a razor on the PPy film to expose silicon wafer surface, the film thickness was measured from section analyses of the scratched area (See Supporting Information, Figure S1). The pull-off test of the deposited PPyCl film was performed using a UTM (Universal Testing Machine, LR10KPlus Series) to examine the adhesion force. PPyCl films on 1 cm2 silicon wafers were fixed using adhesive tape for the pull-off test. The electrical conductivity of the PPy film was measured using a 4-point probe (Napson Co. RT-70). The water contact angle, pull-off test, and conductivity measurements were performed more than three times each.

in Scheme 1, the success of the modification was confirmed by FTIR spectrometry. Figure 1 shows the FT-IR spectra of the modified substrate before PPyCl deposition. The APTES reaction introduced amine groups onto the surface whose characteristic amine peak is present at 1531 cm-1.23 Another peak at 1411 cm-1 indicates the Si-CH2 bond of APTES. Couple of peaks at 1006 and 1062 cm-1 represent symmetric and asymmetric vibration of Si-O bond, respectively. Glutaraldehyde, working as a crosslinker, reacted with these amine groups. After GA modification, three characteristic peaks at 1564 (ethylenic bond), 1654 (imine bond), and 1730 (free aldehydic bond) cm-1 were observed and the previous amine peak nearly disappeared.24 Finally, these GA peaks disappeared when T59 peptide was reacted with GA to be immobilized on the surface. Detailed peak assignments for the T59 peptide were not made because too many functional groups are in the peptide. However, weakened peaks at 1564 and 1654 cm-1 and new peaks at 1728, 1548, and 1364 cm-1, representing carboxylic acid groups and carboxylate, indicate that the peptide did bind to the surface.25 Properties of modified surfaces were confirmed by static sessile drop contact angle measurement with water. Contact angles after surface modification are present in Table 1. After the modification with APTES and GA, the surface of the wafer turned hydrophobic, with contact angles of 71.8° and 67.4°, respectively. The large contact angle of the APTES-modified surface suggests that the propyl chains of the APTES are exposed to the surface. Because of the short alkane chain length of ATPES, it would not

Results and Discussion Peptide Immobilization on the Substrate. The functional groups on the substrate surface had a significant impact on the deposition of PPyCl. After every surface modification presented

(23) Chiang, C. H.; Ishida, H.; Koenig, J. L. J. Colloid Interface Sci. 1980, 74, 396–404. (24) Beppu, M. M.; Vieira, R. S.; Aimoli, C. G.; Santana, C. C. J. Membr. Sci. 2007, 301, 126–130. (25) Si, S.; Mandal, T. K. Langmuir 2007, 23, 190–195.

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construct a densely packed monolayer and made hydrophobic surface.26 Hydrophobicity of the surface was reduced after the T59 peptide was immobilized on it. The water contact angle of the surface was reduced to 27.5° after immobilization of the peptide, which is comparable to the contact angle of a carboxylateterminated surface. The water contact angle results suggested that both carboxylate- and peptide- decorated surfaces have similar degree of hydrophilicity. In a microscopic view, the peptide immobilized surface is amphipathic due to the coexistence of polar (or charged) and hydrophobic residues in the peptide. The low water contact angle might be driven by the polar and charged residues and C-terminus of the surface-exposed peptide. Immobilization of peptide to the GA-modified substrate was also confirmed by fluorescence microscopy. The same peptide with a fluorescence probe was immobilized through the same procedure to visualize peptide immobilization. Figure 2 shows the optical and fluorescence microscopy images of the peptideimmobilized surface. The peptides were positioned on the substrate evenly, confirming the successful immobilization of designer peptide. In addition to the visualization, the amount of immobilized peptides was quantified. The concentration of the immobilized T59 peptide was determined to be 47.0 μg/cm2. Properties of PPyCl Films on Peptide-Immobilized Surfaces. To investigate the effect of the T59 peptide on PPyCl film formation, PPyCl deposition was performed after each surface modification. First, the surface morphologies of deposited films were observed. SEM images of deposited PPyCl films are present in Figure 3. Aggregated PPyCl particles were sparsely deposited on piranha-cleaned bare silicon substrate (Figure 3a) as expected based on previous results.20,21 However, once the surface was modified with GA or T59 peptide, dense PPyCl films were obtained. The SEM images of PPyCl films deposited on GA- or T59-immobilized substrate are shown in Figure 3, parts b and c, respectively. Considering that the GA-modified surface is hydrophobic, formation of a dense film agrees with the work of Wang et al.20,21 It is noteworthy that a dense film deposition was also achieved on the amphipathic T59 peptide-immobilized surface. A preferential adsorption of pyrrole monomer to the hydrophobic surface was supposed to form a PPyCl layer on the hydrophobic surface.21 In the same fashion, preferred adsorption of pyrrole monomer or oligomer on the hydrophobic residues is speculated on the T59 peptide-immobilized surface resulting in continuous and dense film. Because the T59 peptide has not only three hydrophobic amino acids but also negatively charged Asp side chain and C-terminus, they can attract the pyrrole monomer as well as positively charged oxidized monomer or oligomer, too. The carboxyl groups at C-terminus and Asp side chain were charged negatively before the initiation of PPy polymerization because the pH of suspending medium (containing Py monomer) was 6.2. When polymerization was initiated by FeCl3 solution, pH decreased finally to be ca. 2.0. However, at the early stage of the polymerization, pH change is slight such that positively charged oligomer would be attracted to. Thereby, the pyrrole monomer density would increase near to the surface and consequently a continuous PPyCl layer can be generated. These results indicate that the preference of monomer to the surface is another key factor determining density and morphology of deposited PPyCl as well as the hydrophilicity or hydrophobicity of a surface. To investigate the peptide’s effect on PPy film deposition, growth of the PPyCl film was monitored during deposition. In Figure 4, the PPyCl film thickness is plotted against deposition (26) Qin, M.; Hou, S.; Wang, L.; Feng, X.; Wang, R.; Yang, Y.; Wang, C.; Yu, L.; Shao, B.; Qiao, M. Colloid Surf. B 2007, 60, 243–249.

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Figure 2. Fluorescence microscopy images after reaction of FITCT59 peptide with surfaces modified with (a) APTES and (b) GA.

time with corresponding photo images. When APTES- or GAmodified substrate was used, the PPyCl film was thicker than that prepared using T59 peptide-immobilized substrate. Approximated deposition rates in the late stage (during 3-7 h of reaction) are 16.4 nm/hour on the ATPES-modified surface, 14.5 nm/hour on the GA-modified surface, and 8.2 nm/hour on the T59 peptideimmobilized surface. The deposition rate was approximately half times lower on the peptide-coated surface. The faster deposition by a factor ∼2 on the APTES- and GA- surfaces is in agreement with previous report on the hydrophobic surface.17 However, considering the PPy deposition behavior on carboxyl surface similar to those on hydrophobic surfaces, the thinner PPy film must be influenced by the peptides on the surface. Only a little difference of the film deposition is observed at the early stage of PPyCl deposition at around 1 h. This slight difference indicates that the polymerization rates on both hydrophobic and peptidedecorated surfaces are not much different at the early stage of reaction because of the preferential adsorption of pyrrole monomer to the peptide layer. This result is consistent with the previous results of PPyCl film morphology. The difference of PPyCl film thickness with deposition time is due to the “memory effect” of the interface that was suggested in the literature.21 The apparent colors of the deposited films varied with substrate. The color of the film changed from yellow to light blue with Langmuir 2009, 25(19), 11495–11502

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Figure 3. SEM images of the PPyCl films deposited (a) on bare silicon wafer, (b) after modification with GA, and (c) after immobilization of T59 peptide.

the increase in thickness when the substrate was modified by APTES and GA. However, when peptide-immobilized substrate was used, the color of the wafer changed little with PPy film deposition. Color changes of PPyCl films may be related to the different degree of oxidation of PPy. Polypyrrole thin film is known to be an electrochromic material whose optical absorption is influenced by the degree of oxidation.27 When PPy is doped with chlorine anions, the optical absorption band shifts to a lower energy part by the oxidation such that the PPyCl film is looked blue.28 In contrast, a transparent and yellowish color of PPyCl film on the peptide-immobilized surface suggests the reduced state of PPyCl. We speculate that peptides changed the redox state of PPy because migration of dopant molecule is influenced by the surface functionality.29 Adhesion Strength of PPyCl Films. The adhesion strength of the deposited PPyCl film was strongly influenced by surface modification of the substrate. The adhesion strength of PPyCl films were measured by the pull-off test. The forces required to pull the PPyCl film from the substrate are presented in Figure 5. The adhesion force of PPyCl film to bare silicon substrate (27) Thompson, B. C.; Schottland, P.; Zong, K.; Reynolds, J. R. Chem. Mater. 2000, 12, 1563–1571. (28) Mortimer, R. J.; Dyer, A. L.; Reynolds, J. R. Displays 2006, 27, 2–18. (29) Errachid, A.; Caballero, D.; Crespo, E.; Bessueille, F.; Pla-Roca, M.; Mills, C. A.; Teixidor, F.; Samitier, J. Nanotechnology 2007, 18, 485301.

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Figure 4. Growth of PPyCl film thickness with deposition time and corresponding photo images of PPyCl films.

could not be tested because an even and continuous film could not be prepared. When the substrate was hydrophobic after modification by APTES or GA, the adhesion force of PPyCl film with approximate thickness of 40 nm is around 36.2 N. The DOI: 10.1021/la901326q

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Figure 5. (a) Adhesion forces of PPyCl films prepared on differently modified surfaces and (b) corresponding photo images after tape peeling.

adhesive force mainly originates from the hydrophobic attraction between the PPyCl and APTES or GA.20,21 The PPyCl film after detachment from the GA-modified substrate is presented in the Figure 5b. As seen in the picture, most of the PPyCl films were pulled off of the substrate and stuck to the adhesive tape. The adhesion force was significantly higher when the PPyCl film was deposited on either carboxylate-terminated or T59-immobilized substrates. The PPyCl film on this carboxylate-modified surface showed an adhesion force of around 45 N/cm2. Since the adhesion of PPyCl to hydrophilic surfaces is stronger than to hydrophobic surfaces, the larger adhesive force of the carboxylate surface is reasonable.30 The adhesion force of PPyCl to the T59 peptideimmobilized surface is remarkable. The PPyCl film adhered so strongly to the T59 surface that it would not peel off even at the maximum applicable force of instrument, 50 N/cm2. The difference in adhesion strength between carboxylate-terminated and peptide-immobilized surfaces is easily observed in the pictures in Figure 5b. The PPyCl film on the T59 peptide-immobilized surface was not peeled off at all after the pull-off test, in contrast to the partially peeled-off PPyCl film on the carboxylate surface. The strong adhesion of the PPyCl film is due to the specific (30) Rozsnyai, L. F.; Wrighton, M. S. Chem. Mater. 1996, 8, 309–311. (31) Willett, R. L.; Baldwin, K. W.; West, K. W.; Pfeiffer, L. N. Proc. Nat. Acad. Soc. 2005, 102, 7817–7822.

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interaction between T59 peptide and PPyCl. A similar adhesive property of Asp to heterogeneous surface is reported elsewhere.31 In detail, the carboxylate of Asp side chain in T59 peptide interacts with the positively charged PPyCl backbone.22 To explain the stronger binding force of peptide-immobilized surface, the charge-charge attractive force between the Asp side chain and the PPyCl backbone should be larger than the attractive force between the prepared carboxyl groups and the PPyCl backbone. However, if there is little difference of attraction between Asp side chain and carboxyl groups, potential factors to contribute to the higher adhesion would be the folding and the hydrophobic amino acids of peptide. Peptide folding entangling the PPyCl backbone could provide additional mechanical strength. Though the peptide used here is only 12-mer, but it is long enough to influence on the morphologies of associated materials.32,33 The hydrophobic amino acid close to C-terminus (Phe-Val-Ile) could increase adhesion force since they can associate with PPyCl in the molecular level. As observed above, the preferential adsorption of pyrrole monomer was expected so that the molecular association could remain even after the polymerization. This molecular (32) Banerjee, I. A.; Yu, L.; Matsui, H. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 14678–14682. (33) Naik, R. R.; Stringer, S. J.; Agarwal, G.; Jones, S. E.; Stone, M. O. Nat. Mater. 2002, 1, 169–172.

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Article Table 2. Electrical Conductivity of Each PPyCl Film Prepared on Different Surfaces modified surface of substratea APTES

conductivity (S/cm) 0.47 ( 0.06 a PPyCl film thickness on each substrate: 82 ( 6.0 nm.

level association was not considered in previous report22 in which the peptides were bound on the electrochemically deposited PPyCl film. In every surface, the adhesion force decreases as the PPyCl film becomes thicker. It is probably because the influence of the interface on the PPyCl diminishes with increase of the thickness. Since the interface property directly influences on the polymer conformation on the first several layers,17 the adhesion force would decrease with thickness. Electrical Conductivity of PPyCl Films. The most notable property of PPyCl film prepared on the peptide-immobilized surface is its improved conductivity. The electrical conductivities of PPyCl films prepared on different surfaces were examined. The electrical conductivities of the films are shown in Table 2. Conductivities of PPyCl film on the APTES-, GA-, and carboxylmodified surfaces were in the same order under 1 S/cm. However, the PPyCl film deposited on the T59 peptide-immobilized surface showed remarkably increased electrical conductivity. This significant increase in conductivity does not come from the surface current of the peptide layer, which was examined in another control experiment (data not shown). The possibility of differential doping between samples was not considered here because the polymerization conditions were the same. In the detailed inspection, the PPyCl film conductivity on the hydrophilic carboxyl surface is lower than that on the hydrophobic surfaces. The low conductivity of hydrophilic surface is consistent with the previous report.17 The hydrophobic surface would influence on several layers of PPy increasing π-conjugation in the PPy backbone. The highly increased conductivity of peptide-immobilized surface may come from more extended π-conjugation chain length and ordered structure of the PPy backbone. Ordering of polymer backbone on a molecular scale is achieved through the microenvironmental interaction of monomer with the molecules at the interface.34,35 It is expected that the carboxylate residue of Asp in T59 peptide would attract the positively charged N-H group of the PPyCl, ordering the backbone. In addition, we speculate that the hydrophobic amino acids near the C-terminus of T59 peptide contributes to the ordering of hydrophobic pyrrole monomer like the molecular alignment of monomers in a micromicelle polymerization system.35 This molecular association of pyrrole monomer with peptide was suggested above, which is in agreement with the conductivity results, too. To confirm the molecular ordering of PPyCl, FT-IR spectra of PPyCl films prepared on various surfaces were investigated. Specifically, the absorbance intensity ratios between CdC and C-C bonds were calculated to examine the degree of π-conjugation in the PPyCl film. In Figure 6, changes of C-C peak at 1480 cm-1 and CdC peak at 1560 cm-1 are clearly present. The PPyCl films prepared on the hydrophobic GA, ATPES, and carboxyl surfaces show considerable peak height corresponding to a C-C bond. In contrast, the C-C bond peak height decreased notably in the PPyCl film prepared on the T59-immobilized surface. The intensity ratio between CdC and C-C peaks indicates increased (34) Yan, F.; Xue, G.; Zhou, M. J. Appl. Polym. Sci. 2000, 77, 135–140. (35) Yan, F.; Zheng, C.; Zhai, X.; Zhao, D. J. Appl. Polym. Sci. 1998, 67, 747– 754.

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GA

COOH

T59 peptide

0.58 ( 0.08

0.20 ( 0.01

6.50 ( 1.5

Figure 6. FT-IR spectra of PPyCl films deposited on various surfaces; PPyCl films on surfaces modified with (a) ATPES, (b) GA, (c) carboxyl group, and (d) T59 peptide. Table 3. Intensity Ratio between CdC (at 1560 cm-1) and C-C (at 1480 cm-1) Peaks modified surface of substrate

I(CdC)/I(C-C)

ATPES

GA

COOH

T59 peptide

9.33

11.7

4.389

781.4

π-conjugation in the PPyCl film on the T59-imobilized surface, as shown in Table 3. Thus, it is clear that the T59 influence on the molecular structure of PPyCl film caused a reduction in C-C bond formation during the polymerization. The above FT-IR results suggest that both electrical and hydrophobic interactions between the peptide and PPyCl film are key factors in influencing molecular structure and improving electrical conductivity. Both hydrophobicity and electrostatic properties of peptide influenced on the conformation structure of PPyCl, and resulted in continuous and highly conductive film with remarkable adhesion force to the substrate. The more detailed roles of the peptide’s hydrophobic and electrical moieties in its influence on PPyCl films will be examined in the future using variants of the T59 peptide.

Conclusion Deposition of polypyrrole on nonconductive substrate was achieved by using a designer peptide as a binder molecule linking the substrate and polypyrrole macromolecules. The designer peptide influenced the growth and the conjugation length of PPyCl film, improving electrical conductivity. In addition, the adhesion force of PPyCl film to the peptide-immobilized surface was much larger than that of PPyCl film to carboxylate-terminated surfaces. The enhanced adhesion and conductivity are probably due to the molecular interaction between the designer peptide and polypyrrole backbone, which is likely both electrical and hydrophobic in nature. Several control experiments proved both hydrophobic and electrical molecular attractions to be critical in changing the physical and electrical properties of the DOI: 10.1021/la901326q

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surface. The designer peptide could be useful in fabricating biocompatible polymer-based conducting materials. Acknowledgment. This work was supported by Korea Science and Engineering Foundation (KOSEF M1075502000108N5502-00110).

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Supporting Information Available: Figures showing topological images and height profiles of the PPyCl films used in film thickness measurement and text describing the procedures. This material is available free of charge via the Internet at http://pubs.acs. org.

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