ISFET (Ion-Sensitive Field-Effect Transistor)-based Enzymatic Biosensors for Clinical Diagnostics and their Signal Conditioning Instrumentation

Abstract

Metal-oxide-semiconductor field-effect transistors (MOSFETs) are the key components of silicon ICs (integrated circuits). Due to the potential of these devices to provide large, label-free, low-cost, disposable arrays of sensors that are easily integrated in portable ‘point-of-care’ instrumentation, efforts have been devoted worldwide, over the past three decades, towards exploitation of field-effect mechanism in biological sensors. Most of this work concerned the development of the ion-sensitive field-effect transistor (ISFET). A major advantage of ISFET is that it operates under equilibrium conditions. Current does not flow across the biological layer due to the presence of the insulating layer on top of the semiconductor. Although ISFET is primarily a pH sensor, creation of a gate potential by enzyme-promoted reactions of analyte biomolecules leading to pH changes near its gate surface, have resulted in the development of a diversity of biosensors for medical applications. In this paper, sensing mechanisms of different biosensors based on ISFET structure for medical diagnostics are described. In order to obtain a measuring signal, the ISFET has to be associated with an interface and readout circuit. During measurements with ISFET, the drain-source voltage and the drain-source current are maintained constant while the changes in gate-source voltage are determined. Introducing the source-drain follower circuit, the Wheatstone bridge method for eliminating the influence of temperature on measurements is explained. The paper presents an overview of the current scenario and ongoing developments covering from the sensing device to the instrument. ISFET fabrication at CEERI, Pilani is briefly presented, and significant achievements are highlighted.