Fabrication of amperometric xanthine biosensors based on direct chemistry of xanthine oxidase

The construction of amperometric xanthine biosensor by immobilization of xanthine oxidase (XOD) on the multi-wall carbon nanotubes (CNTs) modified glassy carbon (GC) electrode surface was investigated. The direct chemistry of XOD was accomplished and the formal potential was about − 0.465 V (vs SCE). The heterogeneous electron transfer rate constant was evaluated to be 2.0 ± 0.3 s^− 1. The xanthine biosensor based on XOD entrapped in silica sol–gel (SG) thin film on CNTs-modified GC electrode surface was also investigated. The XOD still maintains its activity to xanthine. The amperometric response to xanthine showed a linear relation in the range from 0.2 µM to 10 µM and a detection limit of 0.1 µM (S/N = 3). The enzyme electrode retained 95% of its initial activity after 90 days of storage. The sensor exhibited high sensitivity, rapid response and good long-term stability.