Potentially-implantable, amperometric glucose sensors with mediated electron transfer: improving the operating stability.

The major problems with existing amperometric enzyme electrodes for glucose sensing are oxygen sensitivity and output drift. The recently described miniature glucose sensors using immobilised ferrocene (dicyclopentadienyl iron) to mediate electron transfer from glucose oxidase to a base electrode are oxygen-independent but are often unstable. In this study, we test the hypothesis that the stability of ferrocene-based sensors can be markedly improved by better retention of the enzyme at the electrode. Sensors with graphite foil as the base electrode, dimethylferrocene as the mediator and carbodiimide/glutaraldehyde for enzyme immobilisation lost most of their activity when calibration curves were compared before and after 18 h operation in vitro at 37 degrees C (mean current decrease at 20 mmol/litre glucose was 91%). This sensor type with a covering cellulose membrane also lost activity (mean current decrease of 40% after 18 h). Electrodes in which the base sensor was platinum and the enzyme was covalently attached to agarose lost no activity when operated either at 700 mV without mediator (as a hydrogen peroxide detector) or at 160 mV with mediator. We conclude that both ferrocene-mediated and hydrogen peroxide-detecting amperometric glucose sensors with a high density of covalently-linked enzyme have an operating stability in vitro which may render them suitable for clinical use as, for example, an overnight hypoglycaemia alarm.