Efficient electrocatalytic oxygen reduction by the ‘blue’ copper oxidase, laccase, directly attached to chemically modified carbons

Abstract

This discussion describes efforts to produce a stable, efficient electrocatalyst for four-electron O₂reduction through the direct attachment of fungal laccase, a ‘blue’ copper oxidase, to functionalised carbon electrode materials. Commercially available carbons, including fibrous and porous materials, offer important opportunities for achieving high conductivity over high surface areas that can be chemically functionalised. A promising approach for attaching laccase to a carbon surface is to use the diazonium coupling reaction to generate protrusive aromatic functionalities that can bind to hydrophobic residues close to the ‘blue’ Cu site: this site provides a fast, intramolecular electron relay into the buried trinuclear Cu active site that converts O₂ rapidly and cleanly to H₂O. This enhancement procedure makes possible the stable, direct electrocatalytic reduction of O₂ at high potential with high efficiency in terms of turnover frequency per enzyme active site engaged with the electrode. The absence of electron-transfer mediators and simplicity of electrode system reveals the more inherent characteristics of the electrocatalytic mechanism that are masked in the waveform when a mediator is used. The study includes experiments to assess the effects of methanol and chloride ions on laccase electrocatalysis, complementing studies carried out by other groups, particularly those in which laccase is embedded in an electron-mediating gel.