Durable poly(vinyl chloride) matrix ion-selective field effect transistors for nitrate ions

A nitrate/ion-selective field effect transistor (ISFET) with a conventional ion-exchanger using trioctylmethylammonium nitrate (TOMA-NO3) showed unstable potential response because of the poor adhesion of the ion-sensing membrane onto the Si3N4 insulator thin film of the ISFET device. In order to improve the adhesion onto the device, highly lipophilic nitrate ion-exchangers such as tridodecylmethylammonium nitrate (TDMA-NO3) and tetradecylmethylammonium nitrate (TTMA-NO3) were used. The ISFETs with nitrate-sensing membranes composed of 70 wt % the ionexchanger and 30 wt% PVC were prepared by dip-coating onto the gate region of the ISFET device. The nitrate/ISFETs with TDMA-NO3 and TTMA-NO3 showed the slope of 55 mV per decade in the range from 10-4.5 M to 100 M and that of 56 mV in the range 10-5 M100 M, respectivery. Both nitrate/ISFETs showed the response time of some seconds, stable potential responses and almost the same selectivity corresponding to “Hofmeister's series”. The life-times of the ISFETs with TDMA-NO3 and TTMA-NO3 were about 3 and 7 weeks, respectively.