Mediator-type biosensor for real-time wireless monitoring of blood glucose concentrations in fish

We developed a wireless mediator-type biosensor system to rapidly monitor blood glucose concentrations in free-swimming fish. Glucose oxidase was used to determine the glucose concentration, but fluctuations in the oxygen concentrations in the samples affected the measurements. In this work, we used a mediator, a synthetic electron acceptor, as a substitute in the oxidation–reduction reaction, thus ensuring that changes in the concentration of oxygen do not affect the measurements of glucose concentration performed by the sensor. We especially focused on the use of ferrocene as a mediator. The resulting enzyme sensor utilized a Pt–Ir wire (diameter 0.178 mm) as the working electrode and Ag/AgCl paste as the reference electrode. Chitosan was used to fix ferrocene and glucose oxidase to the working electrode. The characteristics of the sensor were tested, and it was found to be capable of measuring glucose independent of the oxygen concentration of the sample. Its reproducibility was also tested, and the results showed that the sensor was suitable for performing measurements in vivo. The sensor was then inserted into to eyeball interstitial sclera fluid in order to wirelessly monitor the glucose concentration in free-swimming fish. Measurements were taken for a total of 48 h. During these measurements, artificial stress was applied to the fish. The glucose concentrations of fish rise with their stress levels. The output current from the sensor gradually increased during the application of stress, which hinted that the stress was monitored by this system.