A Theoretical Study of the Electrochemical Gate Effect in a STM-based biomolecular transistor

TL;DR

This paper provides a theoretical analysis of electrochemical gating in STM-based biomolecular transistors, deriving relations between potentials, analyzing potential distributions, and estimating gating efficiency for a specific protein system.

Contribution

It introduces a new theoretical relation linking local standard potential with electrode potentials and evaluates gating efficiency in a biomolecular transistor setup.

Findings

Linear potential dependence does not imply monotonic potential drop.

Calculated electrostatic potential distribution matches experimental data.

Estimated gating efficiency confirms effectiveness of electrochemical gating.

Abstract

ElectroChemical Scanning Tunneling Microscopy (ECSTM) is gaining popularity as a tool to implement proof-of-concept single (bio)molecular transistors. The understanding of such systems requires a discussion of the mechanism of the electrochemical current gating, which is intimately related to the electrostatic potential distribution in the tip-substrate gap where the redox active adsorbate is placed. In this article, we derive a relation that connects the local standard potential of the redox molecule in the tunneling junction with the applied electrode potentials, and we compare it with previously proposed relations. In particular, we show that a linear dependence of the local standard potential on the applied bias does not necessarily imply a monotonous potential drop between the electrodes. In addition, we calculate the electrostatic potential distribution and the parameters entering the derived relation for ECSTM on a redox metalloprotein (Azurin from P. Aeruginosa), for which experimental results exist. Finally, we give an estimate of the gating efficiency when the ECSTM setup including Azurin is interpreted as a single biomolecular wet transistor, confirming the effectiveness of the electrochemical gating for this system.