Chapter 29 Enzymatically Synthesized Electronic and Photoactive Materials
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Jayant Kumar , Wei Liu , Soo-Hyuong Lee , Suizhou Yang , Sukant Tripathy , and Lynne Samuelson 1
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Center for Advanced Materials, Department of Chemistry and Physics, University of Massachusetts at Lowell, Lowell, M A 01854 Materials Science Team, U.S. Army Soldier and Biological Chemical Command, Soldier Systems Center, Natick, M A 01760 2
Electronic and photo-active polymers as a class of advanced materials have attracted a lot of interest during last two decades. Typically these materials are synthesized chemically or electrochemically under harsh conditions such as extremely low pH, toxic catalysts and byproducts. New enzymatic approaches have been developed for the synthesis of electro and photo active polymers such as polyanilines, polyazophenols, and polypyrene derivatives. These enzymatically synthesized materials show interesting optical and electronic properties.
Introduction Peroxidase-catalyzed oxidation of phenols and anilines has been extensively investigated due to its importance in the synthesis of electronic and photoactive polymer in an environmentally benign way (1-3). In the presence of H O , peroxidase, such as horseradish peroxidase (HRP) can oxidize phenols and anilines to generate corresponding radicals. These radicals may couple together through radical coupling and radical transfer to form oligomers and polymers. In 2
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378 a suitable solvent system, polymers such as polyanilines and polyphenols with modest molecular weight may be realized. Typically, the enzymatically synthesized polyanilines (in both organic and aqueous solutions) are not electrically active due to the low molecular weight and presence of branched structure (4). Recently a new enzymatic approach was developed to synthesize water-soluble conducting polyaniline under mild conditions using sulfonated polystyrene (SPS) as a template (5). The properties of this polyaniline/SPS complex are comparable to previously reported chemically synthesized and self-doped sulfonated polyaniline. The use of the anionic polyelectrolytes as templates in peroxidase-catalyzed polymerization of aniline demonstrates the first enzymatic synthesis of conducting polyaniline, and opens the door for the synthesis of electrical active conducting polymers through biological approach. Peroxidase-catalyzed synthesis involves a reaction mechanism that results in a direct ring-to-ring coupling of phenol and aniline monomers. The resulting polymers may have an aromatic backbone structure, with interesting electrical and optical properties. By using chromophore functionalized phenols or anilines as substrates, phenol-based (or aniline-based) macromolecular dyes with interesting optical or electrical properties (depending on the chromophore) may be synthesized. This approach offers the possibility to build in substantial chromophore density in the polymers. For example, by using azo-functionalized phenol and aniline as monomers, photo-active polyaniline and polyphenol have been enzymatically synthesized by our group (5). These biologically derived azopolymers have almost 100% dye content. Since both ortho and meta couplings occur through the phenol ring, a high articulated nanostructured macromolecular dye is realized, leading to large free volume and poor packing of the azo chromophores. In this paper, the enzymatic synthesis of conducting polyaniline, and macromolecular dyes such as polyazophenols and poiypyrenes is discussed.
Experimental Materials. Horseradish peroxidase (HRP) (EC 1.11.1.7) (200 unit/mg) was purchased from Sigma with RZ > 2.2. A stock solution of 10 mg/ml in 0.1 M phosphate buffer (PH = 6.0) was prepared. The para substituted azophenol monomers were synthesized following typical procedure. All other chemicals and solvents used were commercially available, of analytical grade ör better and used as received. Enzymatic polymerization of aniline. The.enzymatic polymerization of aniline was typically carried out at room temperature in a 30 ml, 0.1 M sodium
In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
379 phosphate buffer solution of pH 4.3 which contained a 1:1 molar ratio of SPS to aniline, (6 mM) SPS (based on the monomer repeat unit) and 6mM aniline. SPS was added first to the buffered solution, followed by addition of the aniline with constant stirring. To the solution, 0.2 ml of HRP stock solution (10 mg/ml) was then added. The reaction was initiated by the addition of a stoichiometric amount of H 0 under vigorous stirring. To avoid the inhibition of HRP due to excess H 0 , diluted H 0 (0.02 M) was added dropwise, incrementally, over 1.5 hours. After the addition of H 0 , the reaction was left stirring for at least one hour and then the final solution was dialyzed (cutoff molecular weight of 2000D) against pH 4.3 deionized water overnight to remove any unreacted monomer, oligomers and phosphate salts. 2
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Enzymatic polymerization of azophenols. Enzymatic polymerization of 4phenylazophenol was carried out at room temperature in a 100 ml, 50% acetone and 50% 0.01 M sodium phosphate buffer mixture, which contained 2.0 g of 4phenylazophenol. To this solution, 2.0 ml of HRP stock solution was added. The reaction was initiated by the addition of H 0 . To avoid the inhibition of HRP due to excess H 0 , a diluted stoichiometric amount of H 0 (0.2 M) was added incrementally under vigorous stirring over a three hour time period. After the addition of H 0 , the reaction was left stirring for one more hour. The yellow precipitates formed during the reaction were then collected with a Buchner funnel, washed thoroughly with the mixed solvent of 20% acetone and 80% water (v/v) to remove any residual enzyme, phosphate salts and unreacted monomers and then vacuum dried for 24 hours. Similar experimental conditions were used for CH 0, N 0 and CN substituted azophenols. Polymerization of the sulfonated and carboxylic substituted monomers was carried out in a mixture of 80% phosphate buffer and 20% acetone. The other conditions are similar to that described previously. In these reactions, no precipitates were formed and the resulting polymers were solubilized in the reaction media. The polymer solutions were dialyzed against deionized water for 24 hours using a dialysis bag (SPECTRUM®) with a molecular weight cut off of 3000 to remove unreacted monomer and phosphate salts. The resulting dialyzed solution was then condensed and dried in a vacuum oven at 50 °C. 2
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Enzymatic polymerization of 4-hydroxypyrene. The enzymatic polymerization of 4-hydroxypyrene was performed as a similar procedure as described above for the azophenols in the solution of 50% ethanol and water mixture. The synthesized polymers were formed as precipitates, and purified by centrifuging and washing. Optical quality polymer film preparation. The H, CN, N 0 , and CH 0 substituted azophenol polymers were soluble in most polar organic solvents. 2
In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
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380 These polymers were dissolved in spectroscopic grade dioxane, and then filtered through a 0.45 μιη membrane. The solutions were then spin-coated onto glass slides. The film thickness was controlled to 0.2-2.0 μιη by adjusting the solution concentration and spin speed. The spin-coated films were then dried under vacuum for 24 h at 40-50 °C and stored in a desiccator until further studies. Since the sulfonated and carboxylic substituted polyazophenols were watersoluble, deionized water was used as the solvent to dissolve these polymers (pH 11 water was used for the carboxylic substituted polyazophenol). The solutions were alsofilteredthrough a 0.45 μπι membrane and the films were fabricated on glass substrates at temperature of 70 °C. The thickness of all spin-coated films was measured by using a Dektak ΠΑ surface profiiometer. Surface relief grating formation. Surface relief gratings (SRG) were holographically recorded by a simple two-beam interference apparatus at 488 nm from an argon ion laser under ambient conditions with a typical laser intensity of 300 mW/cm . The formation process of the grating was probed by monitoring the first order diffraction of a lower power He/Ne laser beam at 633 nm, at which the absorption is negligible. After the holographic gratings were recorded, the surface relief structures of the gratings on the polymer films were imaged by atomic force microscopy (AFM, Autoprobe Cp, Park Scientific Instruments) under ambient conditions. A 100 μπι scanner in the contact mode under a scan rate of 1 Hz was used in these measurements. 2
Results and Discussion Enzymatically synthesized conducting polyaniline. The enzymatic polymerization of aniline in micelle solution was performed as shown schematically in Scheme 1. This approach is based on preferential electrostatic alignment of aniline monomer onto an anionic template to minimize branching and promote a linear polyaniline chain growth. Since aniline has a pK of 4.63 (
