Development of an Ion-Sensitive Field Effect Transistor Based on PVC Membrane Technology with Improved Long-Term Stability

An NH+4-ISFET sensor based on PVC membrane technology with improved long-term stability has been developed. As a new approach, the plasticizer (tetra-n-undecyl) 3,3′,4,4′-benzhydroltetracarboxylate (ETH2112) was used in membrane preparation. Its lipophilic nature provides a restricted diffusion of the membrane components to the external solution and 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 durability and stability of the sensor were enhanced by the application of a graphite-epoxy layer as an internal reference between the gate area and 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 NH+4-ISFET with a long-term stability of three months and a sensitivity of -58.7 ± 2.3 mV decade-1in a linear range of 10-5-0.1 mol dm-3has been developed. The application of this sensor to a continuous-flow system has confirmed the feasibility of the technological approach proposed.