ISFETbased

ISFET-based refers to ion-sensitive field-effect transistor sensors, a class of devices that use a MOS structure with an ion-selective membrane or layer replacing the conventional gate electrode. In an ISFET, ionic activity in the sample modulates the surface potential at the membrane/oxide interface, which in turn modulates the transistor’s channel conductance. The transduced signal is an electrical current that can be read with standard electronic instrumentation, enabling direct conversion of chemical information into an electrical readout.

Operating principle and readout: The ion-selective membrane defines the gate environment, and changes in the concentration of a target ion alter the interfacial potential in a way described by the Nernst equation. This potential shift changes the threshold or transconductance of the device, producing a measurable change in drain current at a fixed drain-source voltage. For pH-sensitive ISFETs, the response is typically near the Nernstian slope of about 59 mV per pH unit at 25 °C under ideal conditions. In practice, a stable reference electrode is often used to fix the solution potential, or an integrated on-chip reference can be employed.

History and scope: The ISFET concept was introduced in the 1970s by Piet Bergveld, paving the way for silicon-based ion sensing. Since then, ISFETs have evolved with CMOS integration, allowing compact, low-cost, multiplexed sensing. While many ISFETs are used for pH measurements, the sensing surface can be coated with various ion-selective membranes to detect ions such as Na+, K+, Ca2+, Cl−, and others.

Applications and advantages: ISFET-based sensors are used in biotechnology, clinical diagnostics, environmental monitoring, and food analysis. They offer advantages such as miniaturization, compatibility with silicon electronics, fast response, and potential for on-chip multiplexing. Challenges include drift and temperature sensitivity, long-term membrane stability, fouling, and the need for periodic calibration.