Effect of Surface Modification on the Fundamental Electrical Characteristics of Solution-Gated Indium Tin Oxide-Based Thin-Film Transistor Fabricated by One-Step Sputtering

Our solution-gated indium tin oxide (ITO)-based thin-film transistor (TFT) produced by single-step sputtering has great future potential in bioelectronics. In particular, chemical modifications of the ITO channel surface are expected to contribute to biomolecular recognition with ultrahigh sensitivity owing to a remarkably steep subthreshold slope (SS). In this study, we investigate the effect of a chemical modification of an aptamer as a receptor molecule at the ITO channel surface on the electrical characteristics of the solution-gated TFT. In this case, a SARS-CoV-2 aptamer is immobilized using a spacer molecule on an aryl diazonium monolayer that is electrochemically deposited with a radical scavenger. The monolayer is expected to not only passivate the ITO channel surface but also change the electron density in the ITO channel owing to the reduction reaction of aryl diazonium salts. Indeed, the electrochemical deposition of aryl diazonium salts decreases the leakage current through the ITO channel surface and provides a steep SS, which is near the thermal limit at 300 K, owing to the decrease in depletion layer capacitance. After the aptamer immobilization, the leakage current and SS unexpectedly return close to their original values before the surface modifications. This finding indicates that aptamer molecules should be carefully used because their negative charges would attract cations around the detection interface. Eventually, the solution-gated ITO-based TFT with the SARS-CoV-2 aptamer clearly responds to inactivated SARS-CoV-2 particles owing to the successful surface modification.