Stable Thin-Film Reference Electrode on Plastic Substrate for All-Solid-State Ion-Sensitive Field-Effect Transistor Sensing System

Solid-state thin-film reference electrodes (REs) were fabricated on plastic substrate based on a titanium/gold/silver/silver chloride (Ti/Au/Ag/AgCl) multi-layer metal structure for ion-sensitive field-effect-transistor (ISFET)-based sensing systems. A porous structure polyvinyl butyral membrane was formed on top to maintain a constant concentration of chloride as well as acting as a bridge between the electrolyte inside the membrane and the test solution. Excellent measurement stability was achieved with the fabricated RE, with a small drift rate of the open circuit potential less than 1.7 mV per hour. An all-solid-state ISFET sensing system was thus able to be built using the RE for pH measurement, showing excellent reproducibility. The system was finally applied for measuring different beverages, and the measurement results agreed well with those obtained with a commercial pH meter.