Principles and practice of hybrid pH sensors

Abstract

The fundamental questions underlying the performance and stability of any form of microelectronic ion-selective electrode are (i) the nature of the sensing mechanisms occurring at the electrode-test solution interface, i. e. the active surface and, (ii) the nature of the electronic and electrochemical processes involved in transmitting a change in potential at an active surface to the required electronics, i. e. the total transducing function. The crucial process requiring investigation is that which occurs at the interface between the ion-selective material, and in the case of hybrid type devices, the metal "back contact". This is particularly important when using materials such as pH-glasses which respond and conduct electricity by ionic mechanisms.

Fabrication details and procedures for thick-film pH sensors based on commercially available Corning 015 glass, were devised and are reported. Empirical studies evaluating their responses are detailed and the accompanying relevant theoretical treatments are included. Variations in sensitivities are categorised in terms of manufacturing parameters. Evaluation of the electrode processes for a variety of different electrical contacts was studied by a. c. impedance techniques. Activation energies for the principle processes were obtained and equivalent circuits proposed, based on the a. c. measurements. Incorporated within the different electrical contacts used were several systems which attempted to mimic in solid form the reversible system of a conventional pH-glass electrode.