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.
Highlights
Following the invention of the ‘ion-sensitive field effect transistor’ (ISFET) by Bergveld in 1970 [1], the sensor community has developed an entire ‘family’ of field effect-based potentiometric sensors for waterborne analytes
We present here a water-gate thin film transistor (WGTFT) biosensor with a parallel potentiometric and capacitive response despite using a strong electrolyte as a gate medium, together with an improved quantitative understanding
We introduce a new analysis of WGTFT response characteristics, based on output rather than transfer characteristics
Summary
Following the invention of the ‘ion-sensitive field effect transistor’ (ISFET) by Bergveld in 1970 [1], the sensor community has developed an entire ‘family’ of field effect-based potentiometric sensors for waterborne (aqueous) analytes. In principle, this opens a second, capacitive sensing window in addition to the potentiometric mode to which ISFET and EGFET are limited. The immunoglobuline selectively binds to the SARS-CoV-2 virus via the viruses’ superficial RBD spike protein, and we here characterise this WGTFT under exposure to the isolated RBD spike protein of the SARS-CoV-2 virus, rather than the SARS-CoV-2 virus itself This avoids working with a high-level biohazard, while still serving the purpose of our study, which is the detailed investigation of response modes in WGTFT biosensors. We provide a detailed critique of the previously reported apparent absence of potentiometric response in WGTFT biosensors, which we find unsound both theoretically and experimentally, and issue recommendations for future work with transistor-based biosensors
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