There are three types of ion-selective electrodes which are of use in biosensors:
Glass electrodes for cations (e.g. normal pH electrodes) in which the sensing element is a very thin hydrated glass membrane which generates a transverse electrical potential due to the concentration-dependent competition between the cations for specific binding sites. The selectivity of this membrane is determined by the composition of the glass. The sensitivity to H+ is greater than that achievable for NH4+,
Glass pH electrodes coated with a gas-permeable membrane selective for CO2, NH3 or H2S. The diffusion of the gas through this membrane causes a change in pH of a sensing solution between the membrane and the electrode which is then determined.
Solid-state electrodes where the glass membrane is replaced by a thin membrane of a specific ion conductor made from a mixture of silver sulphide and a silver halide. The iodide electrode is useful for the determination of I- in the peroxidase reaction (Table 6.2c) and also responds to cyanide ions.