On the impedance spectroscopy of field-effect biosensors

Impedance spectroscopy is an electrochemical technique widely used for the electrical characterization of the behavior of biomaterials in all kinds of biosensors. Field-effect devices, and in particular ion-sensitive field-effect transistors (ISFETs), have been extensively studied as transducers for biosensing. They however have not been much analyzed with impedance spectroscopy, because they typically generate non-Faradaic capacitance measurements, since there is no charge transfer through the insulating gate in contact with the liquid solution. We have recently experimentally shown that Faradaic-like impedance spectroscopy spectra can be obtained with ISFET devices, allowing high-sensitivity measurements for different pH and biomarker concentrations. In this paper, we report that both Faradaic-like and non-Faradaic impedance behavior can be well understood by a DC and small-signal AC model of the ISFET working in different conditions. Faradaic-like impedance measurements are described by the operation of the ISFET in subthreshold conditions. A Nyquist plot semicircle is obtained, corresponding to the transimpedance 1/g<inf>m</inf> of the ISFET in parallel to the gate capacitances. We show that this behavior is independent of the presence of membrane material on the gate surface. In these conditions, the change in the semicircle diameter for different pH or biomarker concentrations can be understood by the change of 1/g<inf>m</inf> corresponding to a threshold voltage shift of the transistor. This description is illustrated with our recent results for pH measurements and the detection of tumor necrosis factor-α with functionalized devices in standard solutions in the concentration range of 1–20 pg/ml. The use of the impedance spectroscopy technique takes advantage of the exponential behavior of the g<inf>m</inf>(V<inf>GS</inf>) curves in the subthreshold (weak inversion) operation of the ISFET. This results in a large signal amplification, where a small change in the threshold voltage results in a large change in the impedance spectrum, thus achieving an increased precision in the measurement of the device response to changes in the analyte concentration.