Nanostructured carbon electrodes for laccase-catalyzed oxygen reduction without added mediators

Reduction of dioxygen catalyzed by laccase was studied at carbon electrodes without any added mediators. On bare glassy carbon electrode (GCE) the catalytic reduction did not take place. However, when the same substrate was decorated with carbon nanotubes or carbon microcrystals the dioxygen reduction started at 0.6 V versus Ag/AgCl, which is close to the formal potential of the laccase used. Four different matrices: lecithin, hydrophobin, Nafion and lipid liquid-crystalline cubic phase were employed for hosting fungal laccase from Cerrena unicolor. The carbon nanotubes and nanoparticles present on the electrode provided electrical connectivity between the electrode and the enzyme active sites. Direct electrochemistry of the enzyme itself was observed in deoxygenated solutions and its catalytic activity towards dioxygen reduction was demonstrated. The stabilities of the hosted enzymes, the reduction potentials and ratios of catalytic to background currents were compared. The boron-doped diamond (BDD) electrodes prepolarized to high anodic potentials exhibited behavior similar to that of nanotube covered GCE pointing to the formation of nanostructures during the anodic pretreatment. BDD is a promising substrate in terms of potential of dioxygen reduction, however the catalytic current densities are not large enough for practical applications, therefore as shown in this paper, it should be additionally decorated with carbon particles being in direct contact with the electrode surface.