A novel description of ISFET sensitivity with the buffer capacity and double-layer capacitance as key parameters

The pH sensitivity of ISFETs arises from interactions of protons with ISFET gate surface sites. This sensitivity is described by a new simpler model with the intrinsic buffer capacity and the differential capacitance as key parameters. The obtained expression is independent of the models used for the chemical surface equilibria and the charge profile in the solution. The general expression for the sensitivity is elaborated using the site-binding theory and the Gouy-Chapman-Stern theory. The relatively high sensitivity of Ta2O5 ISFETs is explained using this elaborated theory. It is shown that the electrolyte concentration has almost no influence on the sensitivity of Ta2O5 ISFETs

A general model to describe the electrostatic potential at electrolyte oxide interfaces

Colloid chemists have proposed several theories to describe the charging mechanism of metal oxides in electrolyte solutions and the resulting electrical double layer at the oxide surface. In this paper a new general theory to describe the electrostatic potential at the metal oxide electrolyte solution interface is presented. This theory describes the variations of the electrostatic potential as a function of the differential double layer capacitance and the intrinsic buffer capacity. ISFET measurements are interpreted using this theory, and it is shown that these measurements can differentiate between the theories for the double layer and the theories for the charging mechanism for the oxide

The remarkable similarity between the acid-base properties of ISFETs and proteins and the consequences for the design of ISFET biosensors

Studying the acid-base properties of protein molecules led us to reconsider the operational mechanism of ISFETs. Based on the site-dissociation model, applied to the amphoteric metal oxide gate materials used in ISFETs, the sensitivity of ISFETs is described in terms of the intrinsic buffer capacity of the oxide surface, ßs, and the electrical surface capacitance, Cs. The ISFET sensitivity towards changes in the bulk pH is fully described by the ratio ßs/Cs. Practical measurements support this theoretical approach. The new approach to the description of the acid-base properties of ISFETs is analogous to the classical description of the acid-base properties of protein molecules. The acid-base titration of proteins is also determined by the ration between the intrinsic buffer capacity and the electrical double layer capacitance. In addition to the amazing conclusion that ISFET surfaces and protein molecules behave in a similar way with respect to their acid-base properties, further conclusions are drawn with respect to the possibility of protein characterization by means of dynamic measurements with protein covered ISFETs. Design rules are given for this type of biosensors, based on the theoretical understanding of the acid-base behaviour of both sensor parts