Micro Urea Sensor Using Semiconductor and Enzyme-immobilizing Technologies

Abstract

A micro enzyme FET for sensing urea has been developed using semiconductor and enzymeimmobilizing technologies. The enzyme FET was composed of an ISFET (Ion Sensitive Field Effect Transistor) which had been developed as a pH sensor and of the urease-immobilized membrane in which urease was immobilized on the cellulose membrane by the cross-linking method with glutaraldehyde. As pH in the solution was changed by the urea-urease reaction, the pH change depending on the concentration of urea was detected by the ISFET. The measurement of urea in the solution was carried out using the differential mode between the enzyme FET and a reference FET on which urease was not immobilized. It was found that the differential mode of measurement automatically compensated the temperature and ambient pH changes in the solution and stable responses to the various concentrations of urea were obtained. By the use of the calibration curve obtained by the present sensor, it was possible to determine the urea concentration in the range 5 X 10-5-1x 10-2 g/ml. The response time of the enzyme FET was about 1 minute, and was considered to be limited by both urea-urease reaction rate and OH- diffusion time through the cellulose membrane. The life time of the present sensor was 28 days with the data fluctuation within ±3 mV at the output level of 20mV, depending on the activity of the immobilized urease. The performance characteristics of the present enzyme FET were similar to those reported previously using conventional urea sensors without ISFETs. Some advantages can be emphasized for enzyme FETs; i. e., miniaturization and multifunctioning are possible using the integrated circuit technology. Utilizing those advantages, various types of FET biosensors will be developed and applied for clinical uses.