Achieving oxygen reduction at high positive potentials with fast heterogeneous electron transfer is desirable for the biocathode of fuel cells based on enzymes. Here, we present an effective interface for obtaining direct electron transfer from a laccase (Lac)-based cathode for O2 reduction, starting from a potential very close to the redox equilibrium potential of the oxygen/water couple. The interface between Lac and single-walled carbon nanotubes was improved by modification with the steroid biosurfactant sodium cholate. The heterogeneous electron-transfer rate between the type-1 Cu site of Lac and the modified electrode was determined to be 3000 s(-1). The electron-transfer rate was sensitive to the side chain of the steroid biosurfactant, and the rate decreased more than 10-fold when sodium deoxycholate was used as the side chain.

The rate of heterogeneous direct electron transfer of laccase immobilized on single-walled carbon nanotube (SWCNT) and carbon paper electrodes was evaluated by cyclic voltammetry and background-current-corrected steady-state linear voltammetry. These rates indicated that the molecular orientation of laccase immobilized on the SWCNT electrode was more favorable for direct electron transfer, than that of laccase immobilized on the carbon paper electrode. The inhibition of the bioelectrocatalytic O 2 reduction current of the two electrodes by chloride and fluoride were tested. The results indicated differing inhibition mechanisms by these two halides. Laccase immobilized on the SWCNT electrode exhibited high stability and high resistance to chloride inhibition.

The type and coverage of biosurfactants adsorbed on single-walled carbon nanotubes strongly influence the direct electron transfer reaction of laccase.