Parallel Potentiometric and Capacitive Response in a Water-Gate Thin Film Transistor Biosensor at High Ionic Strength

Abstract

We show that an SnO2-based water-gate thin film transistor (WGTFT) biosensor responds to a waterborne analyte, the spike protein of the SARS-CoV-2 virus, by a parallel potentiometric and capacitive mechanism. We draw our conclusion from an analysis of transistor output characteristics, which avoids the known ambiguities of the common analysis based on transfer characteristics. Our findings contrast with reports on organic WGTFT biosensors claiming a purely capacitive response due to screening effects in high ionic strength electrolytes, but are consistent with prior work that clearly shows a potentiometric response even in strong electrolytes. We provide a detailed critique of prior WGTFT analysis and screening reasoning. Empirically, both potentiometric and capacitive responses can be modelled quantitatively by a Langmuir‒Freundlich (LF) law, which is mathematically equivalent to the Hill equation that is frequently used for biosensor response characteristics. However, potentiometric and capacitive model parameters disagree. Instead, the potentiometric response follows the Nikolsky-Eisenman law, treating the analyte ‘RBD spike protein’ as an ion carrying two elementary charges. These insights are uniquely possible thanks to the parallel presence of two response mechanisms, as well as their reliable delineation, as presented here.

A ‘Frugal’ EGFET Sensor for Waterborne H2S

“…Section 3.1 and Section S2 . A common non-Nernstian response model for EGFET and WGTFT (water-gate thin film transistor) sensors is based on the Langmuir–Freundlich (LF) adsorption isotherm [ 33 , 35 , 43 , 44 , 45 , 46 ], Equation (3): …”

“…However, its ‘normally on’ property, affordable price (<1 £/unit) and well documented and consistent performance characteristics [ 30 ] have made it somewhat of a default choice for EGFET applications, e.g. , [ 33 , 34 , 35 ]. A voltage from source V CG is not directly applied to the LND150 gate, but to CG.…”

“…The LND150 is not purpose-built for EGFET sensors, but is manufactured for generic electronic engineering purposes like high voltage switches, precise constant current sources, and voltage ramp generators [30] by several manufacturers, including Supertex Inc. (Sunnyvale, CA, USA) and MICROCHIP (Chandler, AZ, USA). However, its 'normally on' property, affordable price (<1 £/unit) and well documented and consistent performance characteristics [30] have made it somewhat of a default choice for EGFET applications, e.g., [33][34][35]. A voltage from source V CG is not directly applied to the LND150 gate, but to CG.…”

“…This is widely used for the immobilisation of biosensitisers (enzymes, antibodies, and similar proteins) to Au electrodes, reviewed, e.g., by Zuilhof [32]. A typical example is cysteamine, H 2 N-C 2 H 4 -SH, which first binds to an Au electrode via its thiol (-SH) group, and then may condense at its amine (-NH 2 ) group with an activated carboxyl group of a biosensitiser, e.g., an immunoglobulin [33]. Here we show that affinity between thiols and Au extends to H 2 S, which can be seen as a thiol, R-SH, with R = H. We use this affinity to demonstrate a novel sensor for H 2 S aq : Immersing an Au wire as an electrode into an EDL capacitor pool filled with water containing H 2 S aq leads to a surface potential without any further sensitisation.…”

Advanced Field-Effect Sensors

“…Within the ISFET sensor family, water-gate thin-film transistors (WGTFT) [16], which incorporate an electric double layer capacitor into the transistor itself and use the aqueous sample as an active part of the field-effect device, have been widely used for the detection of aqueous analytes such as organic pollutants, ions, viruses, and hormones. AlQahtani and coworkers present a WGTFT biosensor with a parallel potentiometric and capacitive response that utilizes SnO 2 as the semiconductor, chosen for its good stability and immunity to electrochemical doping [17]. The response is based on the analysis of the output characteristics rather than the transfer characteristics.…”