Tethering Osmium Complexes within Enzyme Films on Electrodes to Provide a Fully Enzymatic Membrane-Less Glucose/Oxygen Fuel Cell

Enzyme electrodes based on cross-linking bilirubin oxidase or a Streptomyces coelicolor laccase (SLac) and osmium redox complexes possessing an amine-terminated molecular tether at graphite electrodes can produce current for oxygen reduction under pseudo-physiological conditions. Here we report on enzyme electrodes for oxygen reduction by co-immobilization of SLac, polyallylamine (PAA) and an [osmium(2,2′-bipyridine)2(4-aminomethylpyridine)Cl]+ complex. Enzyme electrodes prepared by incorporation of added multi-walled carbon nanotubes as support produce oxygen reduction current densities of 0.8 mA cm−2 in oxygen saturated, 150 mM NaCl, 50 mM phosphate buffer solution at 37°C, a 3-fold increase in oxygen reduction current densities over those prepared without multi-walled carbon nanotubes. Membrane-less glucose-O2 fully enzymatic fuel cells are assembled by combination of the SLac-based enzyme electrode as a cathode, with glucose-oxidizing anodes, based on either a pyrroloquinoline quinone (PQQ) or FAD-dependent glucose dehydrogenase (GDH) and tetherable osmium redox complexes. The fuel cell based on selection of PQQGDH enzyme electrode as anode provides a maximum power density of 66 μW cm−2 in 5 mM glucose, 150 mM NaCl, phosphate buffer solution at 37°C, dropping to 37 μW cm−2 in human serum, the highest reported power density to date for an enzymatic fuel cell operating in serum.