Orientation of Laccase on Charged Surfaces. Mediatorless Oxygen Reduction on Amino- and Carboxyl-Ended Ethylphenyl Groups

Abstract

The gold electrode surface has been functionalized by phenyl groups ending with aminoethyl or carboxyethyl entities. The functionalization was achieved by the electrochemical reduction of 4-aminoethylbenzene- or 4-carboxyethylbenzene-diazonium salts. The laccase layers were studied by polarization modulation infrared spectroscopy (PMIRRAS), linear sweep voltammetry, and electrochemical quartz microbalance. The kind of surface groups controlling the orientation of the enzyme on the surface was revealed by PMIRRAS. The orientation influenced the enzyme activity on its turn. The direct, mediatorless electrocatalytic current due to the oxygen reduction was observed starting from +0.60 V versus Ag/AgCl at pH 4.5 for laccase immobilized on the amine-ended layer. The observed potential value is close to the redox potential of the laccase T1 center. The electrocatalytic current values were equal to 4 mA/cm. Immobilization of laccase on the carboxyethyl groups leads to smaller electrocatalytic current values. The onset of the electrocatalytic current was down-shifted to +0.56 V.