ELECTRODEPOSITED GOD/BSA ELECTRODES - ELLIPSOMETRIC STUDY AND GLUCOSE-SENSING BEHAVIOR

Glucose oxidase (GOD)-immobilized Pt electrodes are prepared by the electrodeposition of GOD/bovine serum albumin (BSA) and subsequent crosslinking with glutaraldehyde. The thickness of the enzyme layer is controlled by the GOD/BSA concentration of the deposition solution, deposition potential and time. Among these deposition variables, the deposition potential has the most significant effect on the physical properties of the resulting enzyme layers and thus on the glucose-sensing behaviour. Ellipsometric study indicates that, when the deposition potential is in the oxygen-evolution region (1.0 and 1.2 V versus Ag/AgCl), the amounts of deposited proteins are larger and the packing density of the layer is higher than those deposited under the oxide-formation region (0.8 V) or double-layer region (-0.3 V). It is proposed that under the potential of the oxygen-evolution region, the pH decrease near the electrode surface induces precipitation of the proteins. At the other potential regions, however, the proteins seem to be physically adsorbed rather loosely. The GOD/BSA electrodes deposited at 1.2 V show stable current signals for more than 60 days on storage at 4 degrees C without significant reduction in current. However, the current signals gradually diminish for those electrodes deposited at 0.8 or -0.3 V. This behaviour has been attributed to differences in the deposition mechanism and the morphology of the resulting enzyme layers, which are mainly determined by the deposition potential. At 1.2 V the proteins are deposited in a denser way, thus the crosslinking seems to be effective. On the other hand, at 0.8 and -0.3 V, the proteins are loosely adsorbed and the crosslinking is not effective, which causes leaching of the enzyme molecules from the layer.