Analysis of the factors determining the sensitivity of a miniaturized glucose biosensor made by codeposition of palladium and glucose oxidase onto an 8 mu m carbon fiber

The preparation of miniaturized glucose biosensors by the electrochemical codeposition of palladium and glucose oxidase (GOx) onto carbon fiber microelectrodes (8 mu m diameter) is described. The sensitivity of the resulting sensors was dependent on the amount of palladium salt in the deposition solution, with the optimal performance found for 2.0 mg K2PdCl6 per 5 ml. This is approximately one order of magnitude lower than found earlier (H. Sakslund et al. J. Electroanal. Chem., 374 (1994) 71) during the preparation of a similar electrode system using a glassy carbon disk (3 mm diameter). This difference in the optimal palladium salt concentration probably reflects that mass transport to the small cylindrical carbon fiber is more effective than to the large planar glassy carbon electrode. The dependence on the amount of GOx and the deposition time was similar for the two electrode systems, with the optimal response found when the amount of GOx was 26 mg or higher (per 5 mi) and the deposition time approximately 10 min. A long-term stability test demonstrated that the sensitivity decreased to 50% of its original value after 25 days and to 30% after 50 days. The morphology of the electrode surfaces was characterized by scanning electron microscopy.