Membrane design and photocuring encapsulation of flatpack based ion-sensitive field effect transistors

Abstract

A flatpack mounted ion-sensitive field effect transistor (ISFET) device fixed in a Perspex flow cell provides a compact flow injection analysis (FIA) manifold. This allows for the injection of small sample volumes and rapid substitution of one ISFET sensor with another. Encapsulation of the chip was facilitated by the use of a “dry film” DuPont Riston photoresist material, which reproducibly patterned the encapsulant around the gate regions.

Analytical conditions have been optimised using conventional solvent-cast PVC matrix membranes. As alternatives to these, an examination of a series of photopolymerisable methacrylate membranes showed butyl methacrylate to be a viable system. This has the advantage of ease of application to the FET gate and possible elimination of uneven membranes arising from evaporation of the solvent normally used to apply PVC. The butyl methacrylate-dioctylphenyl phosphonate-calcium bis{di[4-(1,1,3,3-tetramethylbutyl)phenyl] phosphate} sensor system yields good flatpack mounted ISFET devices which perform well in the above FIA manifold. It has the expected response characteristics for the calcium ion-sensing system with lifetimes >21 d.