Electrocatalysis of L-Cysteine/Laccase Modified Nanogold Cathode for Oxygen Reduction

Abstract

L-Cysteine functionalized nanogold particles were synthesized with hydrogen tetrachloroaurate tetrahydrate and L-cysteine as raw materials.The functionalized nanogold particles were attached to the surface of gold disk electrode and then were covalently coupled with laccase.Electrochemical behavior of this laccase-based electrode in deaerated phosphate buffer solution and its electrocatalytic effect for oxygen reduction were investigated by cyclic voltammetry.Furthermore,the performance of this laccase based cathode as oxygen sensor was evaluated: oxygen detection limit of this sensor and its affinity towards oxygen(characterized with Michaelis-Menten constant KM) were measured with chronoamperometry.The long-term usability,thermal stability of O2 sensor together with the relationship between pH value and catalytic current were also studied.The results indicate that a direct electron transfer between the redox active centre T2 of laccase and the conductive functionalized nanogold particle occurs without any additional electron relay(formal potential of redox peaks:192.5 mV vs AgCl/Ag),and oxygen reduction occurs at a potential similar to that of T3 redox centre in laccase(780 mV vs NHE).The affinity of the laccase based sensor towards O2 was relatively high(KM=216.4 μmol/L) and the detection limit of oxygen was 0.22 μmol/L.Catalytic current for oxygen reduction can still retain approximately 78% of the initial value after storage at 4 ℃ refrigerator for 60 days.However,this laccase based O2 sensor displayed inferior thermal stability,and its catalytic current corresponding to O2 reduction was strongly influenced by pH value in buffer solution:when pH value of the buffer solution comes close to physiological condition,near all catalytic activity towards O2 was lost.