Direct detection of enzyme-catalyzed products by FET sensor with ferrocene-modified electrode

An FET-based biosensor with a ferrocene-modified gold electrode detects the enzyme-produced electrons by using mediators that transfer the electrons from the enzyme to the sensor. Since an extended-gate FET sensor with a light-shielding mask can be operated without a light-shielding box, a small portable instrument will soon be realised. However, when the FET sensor detected enzyme-catalyzed products with the mediators under light conditions, measurements fluctuated due to photo-reduction of the mediators, resulting in decreased sensitivity. To improve sensitivity by reducing the fluctuation, we developed a procedure for directly detecting enzyme-catalyzed products without using the mediators. The key technique used in this procedure was a measurement technique using our developed potential-keeping method, in which the modified electrode of the FET sensor was oxidised by ferricyanide solution to make its surface the same high potential every time, and this high potential was kept until measurement because of the high input impedance of the FET structure. After this method was applied, the interfacial potential of the gold electrode decreased depending on the amount of enzyme-catalyzed products due to the ferrocene molecules immobilised on the gold electrode directly reacting with the products. The results obtained in light conditions indicated that model compounds of the products were detected from 10 μM to 10 mM with the Nernstian response of 59.2 mV/decade. Also, this method was applied to pesticide detection by using the enzyme inhibition by pesticide, and 5 ppb of diazinon was successfully detected by using only a sensor chip.