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 recon sider the operational mechanism of ISFETs. Based on the site-dissociat ion model, applied to the amphoteric metal oxide gate materials used i n ISFETs, the sensitivity of ISFETs is described in terms of the intri nsic buffer capacity of the oxide surface, beta(s), and the electrical surface capacitance, C-s. The ISFET sensitivity towards changes in th e bulk pH is fully described by the ratio beta(s)/C-s. Practical measu rements support this theoretical approach. The new approach to the des cription of the acid-base properties of ISFETs is analogous to the cla ssical description of the acid-base properties of protein molecules. T he acid-base titration of proteins is also determined by the ratio bet ween the intrinsic buffer capacity and the electrical double layer cap acitance. In addition to the amazing conclusion that ISFET surfaces an d protein molecules behave in a similar way with respect to their acid -base properties, further conclusions are drawn with respect to the po ssibility 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 be haviour of both sensor parts.