In situ measurement of activity and mass transfer effects in enzyme immobilized electrodes

Enzyme catalyzed biofuel cells have been proposed as an alternative to transition metal catalysts for power generation as they oxidize alcohols at relatively low overpotential without the production of detrimental carbon monoxide, and are capable of operation at lower temperatures [Palmore GT, Bertschy H, Bergens SH, Whitesides GM. A methanol/dioxygen biofuel cell that uses NAD⁺-dependent dehydrogenase as catalysts: application of an electro-enzymatic method to regenerate nicotinamide adenine dinucleotide at low overpotentials. J Electroanal Chem 1998;443:155-61]. Additionally, the immobilization procedure prevents internal leakage or cross-contamination of electron mediators. However, full realization of the membrane-less biofuel cell as a power source requires a three-dimensional boundary structure which balances the overall effective surface area against porosity, thus ensuring the maximum number of catalyst sites are available without suffering the blockage of fuel transport, which occurs if the pore size is too small. In this work, a simple and in situ method using a simplified diffusion model is presented to estimate the total activity immobilized enzyme in the absence of mass transfer effects. The method, which also calculates a combined mass transfer parameter including an effective diffusion coefficient, models the reactant concentration at the enzyme surface using bulk concentrations which then can be measured in situ by spectrophotometric detection or ex situ by HPLC analysis. The method was then applied to evaluate two methods of alcohol dehydrogenase electrode fabrication: direct adsorption to carbon felt and entrapment within the conductive polymer polypyrrole. Results showed that direct adsorption provided 26 times the activity versus the method of direct entrapment and better mass transfer characteristics. Correlation of these results to scanning electron micrographs suggested that the polypyrrole entrapment method lacked the expected diffusive pathways and most likely expelled enzyme during growth of the film.