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 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 Ta?O, ISFETs is explained using this elaborated theory. It is shown that the electrolyte concentration has almost no influence on the sensitivity of Ta,O, ISFETs.
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,/3s, and the electrical surface capacitance, Cs. The ISFET sensitivity towards changes in the bulk pH is fully described by the ratio /3s/ C~. 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 ratio 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.
The spectroscopic technique for pH measurement is a well-established laboratory technique that can give high-accuracy pH values. Recent studies have shown the advantage of this technique over standard potentiometric methods for pH measurements in fresh water and seawater and also at high temperatures and pressures. However, a limitation of the spectroscopic technique is that a single pH dye is sensitive only over a narrow pH range. We have developed optimized dye mixtures that are both sensitive and accurate over a broad pH range. The measurement is robust and simple, requires a minimum of two wavelengths, and is independent of the volume of the dye mixture added. Optimization of the dye mixture formulation to maximize accuracy in a broad range of pH involves varying both the dye type and its mole fraction and also accounting for spectral noise. This technique has been successfully applied for in situ pH measurements of oilfield formation waters.
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