Glucose oxidation by osmium redox polymer mediated enzyme electrodes operating at low potential and in oxygen, for application to enzymatic fuel cells

Graphite electrodes modified with a redox polymer, [Os(4,4′- dimethoxy-2,2′-bipyridine)2(polyvinyl imidazole)10Cl]Cl (E°′ = −0.02 V vs Ag/AgCl), crosslinked with glucose oxidising enzymes and various amounts of multi-walled carbon nanotubes are investigated for current generation in the presence of glucose in physiological buffer solutions. Enzyme electrodes based on glucose oxidase and FAD-dependent glucose dehydrogenase are compared in the presence and absence of oxygen. The highest glucose oxidation currents are produced from enzyme electrodes containing 68% w/w multi-walled carbon nanotubes in the deposition matrix. The FAD-dependent glucose dehydrogenase and glucose oxidase enzyme electrodes provide similar current density of ∼0.8 mA cm−2 in de-oxygenated 50 mM phosphate-buffered saline at 37 °C containing 5 mM glucose concentration. Current densities under the same conditions, but in the presence of oxygen are 0.50 mA cm−2 and 0.27 mA cm−2, for glucose dehydrogenase and glucose oxidase enzyme electrodes, respectively, with decreased currents a result of oxygen reduction by the redox polymer in both cases, and oxygen acting as a co-substrate for the glucose oxidase-based electrodes. Application of the anodes in membrane-less enzymatic fuel cells is demonstrated by connection to cathodes prepared by co-immobilisation of [Os(2,2′-bipyridine)2(polyvinyl imidazole)10Cl]Cl redox polymer, Myrothecium verrucaria bilirubin oxidase and multi-walled carbon nanotubes on graphite electrodes. Power densities of up to 270 μW cm−2 are achieved, showing promise for in vivo or ex vivo power generation under these conditions.