The Role of Relative Capacitances in Impedance Sensing with Organic Electrochemical Transistors

Abstract The organic electrochemical transistor (OECT) has attracted interest for use in biosensor technology due to its ability to transduce ionic to electronic signals and operate in aqueous environments. While OECTs have been broadly applied for biosensing and impedance characterization of biological systems, there is still no consensus on the ideal geometries, relative capacitances, and operational conditions for specific sensing scenarios. Here it is shown that for impedance sensing with a capacitive layer on the gate, gate‐limited OECTs produce the largest sensor response. An equivalent circuit model is used to study frequency response with non‐permeable and ion‐permeable membranes added to the gate and found that the transistor configuration, with respect to gate and channel capacitances, able to produce the largest sensor signal is determined by the capacitance to be sensed as well as the membrane permeability. The findings are applied to design a gold gate OECT capable of detecting formation of a lipid bilayer on the gate. The results indicate that high transconductance OECTs typically considered attractive do not deliver the largest sensor signals when used for impedance sensing. Results are presented in settings similar to those used in practical experiments, thereby providing guidance on how to best design OECTs for impedance biosensing.