Sensitive layer, selectivity potentiometric sensors

The concept of light addressable potentiometric sensors (LAPS) was introduced in 1988 [67], LAPS is a semiconductor-based sensor with either electrolyte-insulator-semiconductor (EIS) or metal-insulator-semiconductor (MIS) structure, respectively. Figure 4.13 illustrates a schematic representation of a typical LAPS with EIS structure. A semiconductor substrate (silicone) is covered with an insulator (Si02). A sensing ion-selective layer, for instance, pH-sensitive S3N4, is deposited on top of the insulator. The whole assembly is placed in contact with the sample solution. 

For the establishment of the receptor selectivity, different chemical interactions can be used. For example, the output of potentiometric sensors is strongly influenced by the equilibrium of the analyte with the sensitive layer of the chemosensor. Polymeric matrix membrane-based ion-selective electrodes are utilizing the concentration-dependent extraction of the analyte in the organic layer, while the analyte-dependent shift of potential of ion-selective sensors based on electrodes of the second kind can be described by the solubility product of the hardly soluble salt and the resulting Nemst equation of the electrochemical base reaction. Further equilibria can be predicated on ion exchanges, complexation, or adsorption effects. The interplay of analyte and receptor is determined by... 

In comparison to voltammetry, with heated potentiometric sensors, advantages are not obvious at first glance. Anyhow, there are chances to tune such sensors in order to achieve higher sensitivity, faster response and better selectivity. Of course, experiments are restricted to all-solid ISEs which can be heated indirectly. An interesting example was presented [19], where a heated copper disc had been modified by an ionophore layer. The slope of the sensor was strongly increased by heating, and the detection Unfit was decreased by half an order of magnitude. 

Among potentiometric enzyme sensors, the urea enzyme electrode is the oldest (and the most important). The original version consisted of an enzyme layer immobilized in a polyacrylamide hydrophilic gel and fixed in a nylon netting attached to a Beckman 39137 glass electrode, sensitive to the alkali metal and NHj ions [19, 2A Because of the poor selectivity of this glass electrode, later versions contained a nonactin electrode [20,22] (cf. p. 187) and especially an ammonia gas probe [25] (cf. p. 72). This type of urea electrode is suitable for the determination of urea in blood and serum, at concentrations from 5 to 0.05 mM. Figure 8.2 shows the dependence of the electrode response... 

---