DEVELOPMENT OF AN ION-SENSITIVE FIELD-EFFECT TRANSISTOR BASED ON PVC MEMBRANE TECHNOLOGY WITH IMPROVED LONG-TERM STABILITY

An NH4+-ISFET sensor based on PVC membrane technology with improved lo ng-term stability has been developed. As a new approach, the plasticiz er (tetra-n-undecyl) 3,3',4,4'-benzhydroltetracarboxylate (ETH2112) wa s used in membrane preparation. Its lipophilic nature provides a restr icted diffusion of the membrane components to the :external solution a nd improves membrane adhesion to the gate area of the ISFET. The good performance of this plasticizer was confirmed by comparison with usual plasticizers applied in standard ISE technology. Moreover, the durabi lity and stability of the sensor were enhanced by the application of a graphite-epoxy layer as an internal reference between the gate area a nd the PVC membrane. This composite layer permits the reduction of the optical sensitivity and improves the adherence of the PVC membrane to the ISFET surface. Furthermore, this composite layer acts as a plug, preventing the entrance of water upon the encapsulant-chip interface, thus protecting electrical connections from moisture. As a result an N H4+-ISFET with a long-term stability of three months and a sensitivity of -58.7 +/- 2.3 mV decade(-1) in a linear range of 10(-5)-0.1 mol dm (-3) has been developed. The application of this sensor to a continuou s-flow system has confirmed the feasibility of the technological appro ach proposed.