Sensing characteristics of tyrosinase immobilized and tyrosinase, laccase co-immobilized platinum electrodes

Abstract

Tyrosinase was covalently immobilized on platinum electrode according to the method we developed for laccase (Bull. Korean Chem. Soc. 2002, 23(7), 385) and p-chlorophenol, p-cresol, and phenol could be detected with sensitivities of 334, 139 and 122 nA/μM and the detection limits of 1.0, 2.0, and 2.5 μM, respectively. The response time (t90%) is 3 seconds for p-chlorophenol, and 5 seconds for p-cresol and phenol. The optimal pHs of the sensor are in the range of 5.0-6.0. This sensor can tolerate at least 500 times repeated injections of p-chlorophenol with retaining 80% of initial activity. In case of tyrosinase and laccase co-immobilized platinum electrode, the sensitivities are 560 nA/μM for p-phenylenediamine (PPD) and 195 nA/μM for p-chlorophenol, respectively. The sensitivity of the bi-enzyme sensor for PPD increases 70% compared to that of only laccase immobilized one, but the sensitivity for p-chlorophenol decreases 40% compared to that of only tyrosinase immobilized one. The sensitivity increase for the bi-enzyme sensor for PPD can be ascribed to the additional catalytic function of the co-immobilized tyrosinase. The sensitivity decrease for p-chlorophenol can be explained by the "blocking effect" of the co-immobilized laccase, which hinders the mass transport through the immobilized layer. IfPPD was detected with the electrode that had been used for p-chlorophenol, the sensitivity decreased 20% compared to that of the electrode that had been used only for PPD. Similarly, if p-chlorophenol was detected with PPD detected electrode, the sensitivity also decreased 20%. The substrate-induced conformation changes of the enzymes in a confined layer may be responsible for the phenomena.