10744
J. Phys. Chem. B 1997, 101, 10744-10750
Surface-Functionalized Polyaniline Films E. T. Kang,* K. G. Neoh, and S. W. Huang Department of Chemical Engineering, National UniVersity of Singapore, Kent Ridge, Singapore 119260
S. L. Lim and K. L. Tan Department of Physics, National UniVersity of Singapore, Kent Ridge, Singapore 119260 ReceiVed: August 19, 1997; In Final Form: October 15, 1997X
The surfaces of emeraldine (EM) base films of polyaniline (PANi) have been functionalized by thermal graft copolymerization with acrylic acid (AAc) and N,N-dimethyl(methacryloylethyl)ammonium propanesulfonate (DMAPS). The AAc graft copolymerized EM films were further functionalized via covalent immobilization of glucose oxidase (GOD) and collagen. The so-immobilized GOD still retains a substantial amount of activity. The amphoteric DMAPS graft copolymerized EM base films are capable of exhibiting strong adhesive-free adhesion to one another and to another similarly modified dielectric film, such as a surface graft copolymerized poly(tetrafluoroethylene) (PTFE) film. Lap shear adhesion strengths on the order of 300 and 100 N/cm2 were obtained for the respective cases. Pristine PANi films of various intrinsic oxidation states are capable of undergoing spontaneous surface metallization through reduction of precious metal ions, such as gold and palladium, in acid solutions to their elemental forms. The rate of metal reduction is favored by a low intrinsic oxidation state of the polymer. The surface composition and microstructure of the polymer films after modification, functionalization, and metallization were characterized by angle-resolved X-ray photoelectron spectroscopy (XPS).
Introduction The synthesis and characterization of electroactive polymers have become one of the most important areas of research in polymer and materials science during the past two decades.1-4 Among these polymers, the century-old aniline polymers5,6 have been of particular interest because of their environmental stability,7-10 controllable electrical conductivity,11,12 and interesting redox properties associated with the chain nitrogens.13-15 The aniline polymers have the general formula [(sBsNHsBsNH)y(BsNdQdN)1-y]x, where B represents a benzenoid ring while Q represents a quinonoid ring. The intrinsic oxidation state of the polymer can be varied from the fully reduced leucoemeraldine state (LM, y ) 1) through the 50% intrinsically oxidized emeraldine state (EM, y ) 0.5) to the fully oxidized pernigraniline state (PNA, y ) 0). The 75% intrinsically oxidized polymer has been termed nigraniline (NA, y ) 0.75).5,6 The aniline polymers also exhibit crystallinity16,17 and solution- or counterion-induced processability.18-24 Furthermore, the electrical properties of aniline polymers can be substantially improved through secondary doping.25 The excellent processability and the presence of a number of intrinsic redox states have substantially enhanced the potential of the aniline polymers in practical applications, such as in corrosion protection of metals,26,27 light-emitting devices,28-30 and materials for electrodes and sensors.31-33 With respect to most practical applications, materials modification and functionalization, in particular those aiming at the surface and interface, will be necessary. It has been demonstrated that surface modification of polyaniline (PANi) films can be performed through graft copolymerization under relatively mild conditions.34,35 In the present work, the surfaces of PANi films were first modified by * To whom correspondence should be addressed. Fax: (65) 779-1936. E-mail:
[email protected]. X Abstract published in AdVance ACS Abstracts, November 15, 1997.
S1089-5647(97)02705-3 CCC: $14.00
thermally induced graft copolymerized with acrylic acid (AAc), and N,N-dimethyl(methacryloylethyl)ammonium propanesulfonate (DMAPS). The AAc polymer-modified PANi surfaces were further functionalized via covalent immobilization of a model enzyme and a model protein, such as glucose oxidase (GOD) and collagen. The possibility of achieving adhesive-free adhesion, not just between two surface graft copolymerized PANi films but also between a surface graft copolymerized PANi film and a similarly modified conventional polymer film, was also demonstrated. Finally, a process for the spontaneous metallization of a PANi surface in acid solutions of the metal ions was reported. The surface composition and microstructure after the modification and functionalization were determined by angle-resolved X-ray photoelectron spectroscopy (XPS). Experimental Section Materials. Polyaniline in its emeraldine (EM) salt form was first prepared by the oxidative polymerization of aniline by ammonium persulfate in 1 M HCl according to the method reported in the literature.36 It was then converted to the neutral EM base by treatment with excess 0.5 M NaOH. Free-standing and lightly cross-linked EM films of about 20 µm in thickness were prepared by heating the concentrated N-methylpyrrolidinone (NMP) gel solution (containing 8% EM base by weight) at 150 °C for about 6 h followed by exhaust pumping under reduced pressure. The EM base film so prepared has a tensile yield strength of about 120 N/cm2.37 Some of the EM base films were also converted to the 75% intrinsically oxidized base via a simple acid-base treatment15 or to the fully reduced LM by treatment with phenylhydrazine.5,6 The acrylic acid (AAc) monomer used for the graft copolymerization was obtained from Wako Pure Chemical Industries Ltd. of Tokyo, Japan. The N,Ndimethyl(methacryloylethyl)ammonium propanesulfonate (DMPAS) amphoteric monomer was prepared according to the method reported in the literature.38 The DMAPS molecule has the following chemical structure: CH2dC(CH3)COOC2H4N+© 1997 American Chemical Society
Polyaniline Films (CH3)2C3H6SO3-. The PTFE film having a thickness of 0.01 cm was purchased from Goodfellow Inc. of Cambridge, U.K. Water soluble 1-ethyl-(3,3-dimethylaminopropyl)carbodiimide hydrochloride (WSC) was purchased from Dojindo Laboratories of Kyoto, Japan. Glucose oxidase (GOD, EC1.1.3.4, type II, from Aspergillus niger, 23 000 units/g), peroxidase (POD, EC1.11.1.7, type I, from Horseradish, 290 000 units/g), β-D(+)-glucose, o-dianisidine, and collagen (type VII, acid soluble, from rat tail) were obtained from Sigma Chemical Co. BioRad dye reagent for protein assay (Catalog No. 500-0006) was obtained from Bio-Rad Laboratories Inc. The Dulbecco’s phosphate buffer solution or PBS (containing 8000 mg of NaCl, 200 mg of KCl, 1150 mg of anhydrous disodium phosphate, and 200 mg of anhydrous monopotassium phosphate in 1 L), used for the enzyme immobilization work, was freshly prepared. The solvent and other reagents were of analytical grade and were used without further purification. Graft Copolymerization. EM film strips of about 2.0 cm × 4.0 cm were used in all grafting experiments. Each EM film was immersed in an aqueous AAc solution or DMAPS solution of a predetermined concentration in a Pyrex glass ampule. The reaction mixture was thoroughly degassed with N2 before being kept in an 80 °C water bath for about 1 h. After each grafting experiment, the EM film was removed from the viscous homopolymer solution and washed with a jet of doubly distilled water. It was then immersed in a room-temperature water bath with continuous stirring for at least 48 h to remove the residual homopolymer. In the case of surface modification of PTFE films, the inert fluoropolymer substrate was first pretreated with Ar plasma followed by near-UV-light-induced graft copolymerization with AAc or DMAPS, according to the method reported earlier.39 Toluidine blue O (TB) uptake was also used to determine the total concentration, in weight per unit area, of the surface-grafted AAc polymer.40 Covalent Immobilization of Enzyme and Protein on the Surface-Modified EM Film. The method of GOD and collagen immobilization on the AAc graft copolymerized EM films is similar to that reported for the immobilization on conventional polymer substrates.40 For the covalent immobilization of GOD, the COOH groups of the grafted AAc polymer were preactivated for 1 h with WSC at 4 °C in 0.1 M PBS containing 5 mg/mL of WSC. The polymer films were then transferred to the 0.1 M PBS(+) (pH 7.4, with 0.02 M CaCl2 added) containing 4 mg/mL of the enzyme. The immobilization was allowed to proceed at 4 °C for 24 h. After that, the reversibly bound enzyme was desorbed in copious amounts of PBS(+) for 1 h at 25 °C. Physical adsorption of the enzyme was conducted in a similar manner except that the pretreatment of the film with WSC was omitted. A similar method was used for the covalent immobilization of collagen on the AAc graft copolymerized EM film. The amount of enzyme or protein immobilized on the EM base film was determined by the modified dye-interaction method,41,42 using the Bio-Rod protein dye reagent. The enzyme activity of the free and immobilized GOD was measured using the method reported in the Sigma Bulletin.43 The method is based on the spectrophotometric determination of the amount of hydrogen peroxide formed. The assay mixture (3 mL) consists of 0.1 mL of peroxidase (POD) solution (60 units/mL), 2.4 mL of dye buffer solution (0.2 mM of o-dianisidine solution in 0.05 M acetate buffer, pH ) 6), and 0.5 mL of β-D(+)-glucose solution (10 wt %). The reaction was initiated by adding 0.1 mL of free GOD solution to the assay mixture or by dipping GOD-immobilized EM film in the assay mixture. The initial rate of absorbance increase at 500 nm was used to evaluate the activities of the free and covalently
J. Phys. Chem. B, Vol. 101, No. 50, 1997 10745 bonded enzyme. In all cases, the activities of the enzyme were assayed after 5 min in the assay solution. Adhesion Strength Measurements. Adhesive-free adhesion between two surface graft coploymerized EM base films, or between surface graft copolymerized EM and PTFE films, was achieved by lapping the two film strips together in the presence of 3 µL of deionized water and under a constant load of 21 N at room temperature. The lapped area of the two films was kept at 0.5 cm × 0.1 cm unless otherwise stated. After a fixed adhesion (drying) time, the adhesion strength was determined by measuring the lap shear adhesion force using a microprocessor-controlled Rheometrics miniature materials tester (U.K.). All measurements were carried out at a crosshead speed of 10 mm/min. For each lap shear strength reported, at least three sample measurements were averaged. Spontaneous Metallization of PANi Surfaces. Spontaneous metallization of EM, LM, and NA films with Au and Pd was achieved by exposing the films to the aqueous acid solutions containing the respective precious metal ions. Palladium and gold solutions were prepared by diluting the respective palladium nitric acid and chloroauric acid standard solutions obtained from Aldrich Chenical Co. Each standard solution contains 1000 mg dm-3 of the precious metal. During the metallization process, the concentration of the precious metal ions in each solution was determined by UV-visible absorption spectroscopy, using a Shimadzu Model UV-160A spectrophotometer. Gold chloride has an absorption peak at about 312 nm, while palladium nitrate has an absorption peak at about 280 nm in the aqueous acid solution. Surface Characterizations. The polymer films after surface modification and functionalization were characterized by angleresolved X-ray photoelectron spectroscopy (XPS). XPS measurements were made on a VG ESCALAB MKII spectrometer with a Mg KR X-ray source (1253.6 eV photons) at a constant retard ratio of 40. The core-level signals were obtained at a number of photoelectron takeoff angles (R, with respect to sample surface) ranging from 20° to 75°. The X-ray source was run at a reduced power of 120 W (12 kV and 10 mA). All binding energies (BEs) were referenced to the C 1s neutral carbon peak at 284.6 eV. In peak synthesis, the line width (full width at half-maximum or fwhm) for the Gaussian peaks was maintained constant for all components in a particular spectrum. Surface elemental stoichiometries were determined from peakarea ratios, after correcting with the experimentally determined sensitivity factors, and were accurate to within (5%. The elemental sensitivity factors were determined using stable binary compounds of well-established stoichiometries. Results and Discussion Earlier XPS studies44 have shown that the quinonoid imine (dNs), benzenoid amine (sNHs), and the positively charged nitrogen in a PANi complex correspond respectively to N 1s peak components with binding energies (BE’s) at 398.2, 399.4, and >400 eV in the properly curve-fitted N 1s core-level spectrum. Thus, XPS provides a very useful and convenient tool for the study of the intrinsic structure and oxidation states of the aniline polymers and is particularly suited for the analysis of the present surface-functionalized PANi films. Figure 1 summarizes the schemes for interconversions among the various intrinsic oxidation states and protonation levels of PANi.44 1. Covalent Immobilization of Proteins. Previous studies34,35 have demonstrated that the EM base films are readily susceptible to near-UV or thermally induced graft copolymerization with cationic, anionic, and amphoteric monomers. Surface graft copolymerization with acrylic acid (AAc) readily
10746 J. Phys. Chem. B, Vol. 101, No. 50, 1997
Kang et al.
Figure 1. Interconversions among the various intrinsic oxidation states and protonation levels of PANi.
Figure 2. C 1s core-level spectra, obtained at photoelectron takeoff angle of 75°, of an AAc graft copolymerized EM base film (a) before and after mobilization of (b) GOD and (c) collagen.
gives rise to a self-protonated and semiconductive EM surface structure.34 Figure 3a shows the C 1s core-level spectrum of an EM base film after graft copolymerization in 10 wt % aqueous AAc solution. The presence of a distinct high-BE component at 288.7 eV, which is characteristic of the COO group,34 must have resulted from the surface-grafted AAc polymer. The C 1s core-level spectrum of the pristine (as-cast) EM film shows only a major neutral carbon component at 284.6 eV and a minor (
