(138a) Bio-Nanocomposite Catalysts for Direct Electron Transfer in Bio-Electrochemical Systems

Bioelectrochemical devices such as biosensors and biological-fuel cells require effective electrical communication between the enzyme redox active center and the engineered material surface. The traditional strategy to use soluble redox mediators to shuttle electrons between the enzyme and electrode suffers huge overpotential loss associated with mediator redox reactions and diffusion processes. Coupled with the poor stability of the soluble mediators, this strategy limits long term practical applications. An alternative strategy is to link the enzymes and electrodes using a ?direct electron transfer' mechanism. Direct electron transfer (DET) between the enzyme and electrode provides great potential for miniaturization, stability and high power output in biological fuel cells. However, effectively attaining the desired electrical connection inside the enzyme nano-constructs for DET is a significant challenge. Carbon nanotubes (CNTs) are efficient conductive carriers for DET between the enzyme redox centers and the electrode surface. The enzyme-nanotube matrix entrapped within bio-mimetically synthesized silica particles has been proven to enhance the bio-electrocatalytic activity of glucose oxidase. In this work a phenol oxidase (laccase) from fungi (Trametes versicolor) has been used to catalyze the oxygen reduction reaction for the fuel cell cathode. Two types of electron carriers were studied in this work: carbon nanotubes and conducting polymers. For CNT based nanostructures, the biocatalyst layers were prepared in a matrix of carbon nanotube and Nafion on different base porous electrode materials. For conducting polymer based structures, the laccase was co-immobilized with polypyrrole on the electrode surface resulting in a 3d porous matrix of polypyrrole entrapping laccase molecules. Cyclic voltammetry results reveal that both types of nanostructures are electro-catalytically active for oxygen reduction reaction (OCP > 0.45 V vs. AgCl) indicating the presence of DET pathways in these structures. Continued optimization of the physical and electrochemical properties of the structures could result in several-fold increase in catalytic current densities.