In-situ Voltage Tunneling Spectroscopy at Electrochemical Interfaces

Differential Electrochemical Conductance Imaging at the Nanoscale

Electron transfer in proteins is essential in crucial biological processes. Although the fundamental aspects of biological electron transfer are well characterized, currently there are no experimental tools to determine the atomic-scale electronic pathways in redox proteins, and thus to fully understand their outstanding efficiency and environmental adaptability. This knowledge is also required to design and optimize biomolecular electronic devices. In order to measure the local conductance of an electrode surface immersed in an electrolyte, this study builds upon the current-potential spectroscopic capacity of electrochemical scanning tunneling microscopy, by adding an alternating current modulation technique. With this setup, spatially resolved, differential electrochemical conductance images under bipotentiostatic control are recorded. Differential electrochemical conductance imaging allows visualizing the reversible oxidation of an iron electrode in borate buffer and individual azurin proteins immobilized on atomically flat gold surfaces. In particular, this method reveals submolecular regions with high conductance within the protein. The direct observation of nanoscale conduction pathways in redox proteins and complexes enables important advances in biochemistry and bionanotechnology.

Stabilizing nanometer scale tip-to-substrate gaps in scanning electrochemical microscopy using an isothermal chamber for thermal drift suppression

The control of a nanometer-wide gap between tip and substrate is critical for nanoscale applications of scanning electrochemical microscopy (SECM). Here, we demonstrate that the stability of the nanogap in ambient conditions is significantly compromised by the thermal expansion and contraction of components of an SECM stage upon a temperature change and can be dramatically improved by suppressing the thermal drift in a newly developed isothermal chamber. Air temperature in the chamber changes only at ~.2 mK/min to remarkably and reproducibly slow down the drift of tip-substrate distance to ~0.4 nm/min in contrast to 5-150 nm/min without the chamber. Eventually, the stability of the nanogap in the chamber is limited by its fluctuation with a standard deviation of ±0.9 nm, which is mainly ascribed to the instability of a piezoelectric positioner. The subnanometer scale drift and fluctuation are measured by forming a ~20 nm-wide gap under the 12 nm-radius nanopipet tip based on ion transfer at the liquid/liquid interface. The isothermal chamber is useful for SECM and, potentially, for other scanning probe microscopes, where thermal-drift errors in vertical and lateral probe positioning are unavoidable by the feedback-control of the probe-substrate distance.

Conductance Maps by Electrochemical Tunneling Spectroscopy To Fingerprint the Electrode Electronic Structure

We describe a methodology to perform reliable tunneling spectroscopy in electrochemical media. Sequential in situ tunneling spectra are recorded while the electrochemical potential of the electrode is scanned. Spectroscopic data are presented as conductance maps or conductograms that show the in situ electronic structure of an electrode surface while it undergoes an electrochemical reaction. The conductance map or conductogram represents the redox fingerprint of an electrode/liquid interface in a specific medium and can serve to predict its electrochemical behavior in a quantitative energy scale. The methodology is validated studying the reversible oxidation and passivity of an iron electrode in borate buffer, and we describe the main quantitative information that can be extracted concerning the semiconducting properties of the Fe passive film. This methodology is useful to study heterogeneous catalysis, electrochemical sensing and bioelectronic systems.