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.