Microfabricated Liquid Junction Ag/AgCl Reference Electrode and Its Application to a One-Chip Potentiometric Sensor

A liquid-junction Ag/AgCl reference electrode was microfabricated. A silver thin-film pattern was covered with a polyimide protecting layer with a 50-μm-wide slit at the center of the pattern, and the AgCl layer was grown from there into the silver layer. A liquid junction was formed with a photocurable hydrophilic polymer. The electrolyte layer was formed by screen-printing a paste containing fine KCl powder. A silicone rubber passivation covered the entire area except for the pad and the end of the junction. The novel thin-film Ag/AgCl element could maintain its expected potential for longer than 30 h in a saturated KCl solution. The completed miniature liquid-junction reference electrode could maintain a stable potential level within ±1 mV for longer than 100 h with the aid of a poly(vinylpyrrolidone) (PVP) matrix in the electrolyte layer. Fluctuation of the potential was less than 0.1 mV at each moment. Dependency on external KCl concentration and pH was shown to be insignificant. The miniature reference electrode was integrated with an iridium oxide pH indicator electrode. No significant difference was observed between the potential obtained with the miniature reference electrode and that obtained with a macroscopic commercial Ag/AgCl electrode, which confirmed that the miniature reference electrode was applicable to one-chip potentiometric sensors.