Abstract
AbstractImpedance 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/gmof 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/gmcorresponding 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 thegm(VGS) 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.
Highlights
The detection of biomolecules with label-free biosensors is of great interest because it has the capability of providing fast and low-cost analytical results.[1]
We first consider the pH response of unmodified ion-sensitive field-effect transistors (ISFETs), and later we study the detection of biomarkers by using surfacefunctionalized ISFETs
We have investigated the electrical response of the ISFET to pH with electrochemical impedance spectroscopy (IS)
Summary
The detection of biomolecules with label-free biosensors is of great interest because it has the capability of providing fast and low-cost analytical results.[1]. ISFETs are metal-oxide-semiconductor field-effect transistors (MOSFETs) where the gate solid-state conducting electrode is substituted by a liquid solution In their unmodified form, they are sensitive to the pH of the solution. A Faradaic impedance appears when there is a (redox) charge exchange, which typically behaves to capacitance in parallel to a resistance, showing a semicircle in the Nyquist plot.[17] If no charge exchange exists, a non-Faradaic impedance is obtained, which is essentially a capacitive element. In their normal operation, ISFETs are intrinsically non-Faradaic devices, because there is no charge transfer through the insulating gate.
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