A REFERENCE ELEMENT BASED ON A SOLID-STATE STRUCTURE

A new reference element for ISFET measurements was introduced. The element is based on a multilayer structure consisting of a fluoride sensitive layer, LaF3, a porous fluoride salt layer, CaF2 and a polymer membrane, cellulose-2,4-acetate. Constant potential is achieved by the established dissolution equilibrium in the CaF2 layer in combination with the membrane acting as a diffusion barrier. The structure can be prepared on ISFETs of different design resulting in solid-state reference field-effect transistors (REFETs). The individual layers of the structure were characterized concerning their special requirements. Particular attention is given to the investigation of the polymer membrane (permeation, impedance, pH sensitivity). The stable operation of the reference structure is shown in the pH range 4-9 up to 65 days in solution. Differential measurement with the final REFET and pF-ISFETs were carried out to determine fluoride concentrations up to 3 x 10(-3) M.