Ion sensitivity from current hysteresis in InAs nanowire field-effect transistors functionalized with ionophore-doped fluorosilicone membranes

Abstract

Trapping of environmental charges in surface states typically dominates electrical transport in nanostructured field-effect transistors (FETs) applied as sensors. Such surface effects produce exceptional sensitivity, yet time dependencies on experimental timescales simultaneously results in hysteresis of FET conductance and signal instability. Whereas hysteresis is usually suppressed by means of chemical surface treatments, here we study it as a source of information for ion sensing. Ion-sensitive FETs were prepared by coupling InAs nanowires to fluorosilicone membranes doped with Na+ ionophores. From cyclic transfer characteristics in electrolytes of varying concentration, potentiometric and hysteretic calibration curves were obtained. The observed hysteresis was attributed to changes in membrane capacitance by redox reactions between ionized donor-like traps at the InAs surface and electroactive membrane constituents. Hysteresis was correlated to the ion potential through a model and demonstrated a filtering effect that stabilized the hysteretic response against potential drifts. Furthermore, the model elucidated the ability to modulate ion sensitivity by controlling the initial density of ionized traps via electrostatic polarization by the gate. In this mode of active operation, we demonstrate enhancement above the Nernstian limit with linear calibrations of (−77.5 ± 3.2 to −80.7 ± 3.0 mV/dec) despite the presence of non-equilibrium ion fluxes.