Improvement of integrated Ag/AgCl thin-film electrodes by KCl-gel coating for ISFET applications

To eliminate the need of a separate reference electrode and facilitate the use of ion-sensitive field-effect transistor (ISFET), an all-solid-state reference electrode integrated with ISFET in one chip has been developed. A novel agarose-stabilized KCl-gel membrane was introduced to serve both as a polymer-supported solid reference electrolyte and an ionic bridge for Ti/Pd/Ag/AgCl electrode. Calibrated against the commercial macro Ag/AgCl reference electrode the new miniaturized reference electrode shows its cell potential variation was ±0.9–1.4 mV (equivalent to about ±0.015–0.023 pH) in 1 h, less than 2 mV variation over pH 4–10, and almost insensitive to changes in Cl− ion concentration (about 0.02–0.25 mV/pKCl). The planar solid-state reference electrode also shows a very small offset voltage of 0.45 mV and reproducible to within ±0.5 mV among the batch fabricated electrodes. ISFET pH-sensor with integrated all-solid-state reference electrode characterized in standard pH solutions show a linear sensitivity of 56 mV/pH, these results agree very well with the theoretical value of 59 mV/pH. Dynamic response and hysteresis characteristics of the pH-sensor are also studied and discussed.