Differential Electrochemical Conductance Imaging at the Nanoscale

Exploring scanning electrochemical probe microscopy in single-entity analysis in biology: Past, present, and future

Scanning Electrochemical Probe Microscopy (SEPM) shows significant potential promise for analyzing localized electrochemical activity at biological interfaces of single entities. Utilizing various SEPM probe manipulations allows real-time monitoring of the morphology and physiological activities of single biological entities, offering vital electrochemical insights into biological processes. This review focuses on the application of five SEPM techniques in imaging single biological entities, highlighting their unique advantages in the observation and quantitative evaluation of biological morphology. Specifically, these techniques not only enable high-resolution imaging of single biological structures but also allow for quantitative analysis of their response behavior. Additionally, the integration of Artificial Intelligence (AI) is discussed to improve data processing and image analysis, potentially advancing SEPM technology towards automation. Although still in an early stage, AI integration opens new avenues for deeper single-entity analysis. This review aims to offer an interdisciplinary perspective and encourage advancements in SEPM-based imaging and analytical techniques, contributing to the bioanalytical field.

Shallow conductance decay along the heme array of a single tetraheme protein wire

Multiheme cytochromes (MHCs) are the building blocks of highly conductive micrometre-long supramolecular wires found in so-called electrical bacteria. Recent studies have revealed that these proteins possess a long supramolecular array of closely packed heme cofactors along the main molecular axis alternating between perpendicular and stacking configurations (TST = T-shaped, stacked, T-shaped). While TST arrays have been identified as the likely electron conduit, the mechanisms of outstanding long-range charge transport observed in these structures remain unknown. Here we study charge transport on individual small tetraheme cytochromes (STCs) containing a single TST heme array. Individual STCs are trapped in a controllable nanoscale tunnelling gap. By modulating the tunnelling gap separation, we are able to selectively probe four different electron pathways involving 1, 2, 3 and 4 heme cofactors, respectively, leading to the determination of the electron tunnelling decay constant along the TST heme motif. Conductance calculations of selected single-STC junctions are in excellent agreement with experiments and suggest a mechanism of electron tunnelling with shallow length decay constant through an individual STC. These results demonstrate that an individual TST motif supporting electron tunnelling might contribute to a tunnelling-assisted charge transport diffusion mechanism in larger TST associations.

Understanding molecular and electrochemical charge transfer: theory and computations

Electron, proton, and proton-coupled electron transfer (PCET) are crucial elementary processes in chemistry, electrochemistry, and biology. We provide here a gentle overview of retrospective and currently developing theoretical formalisms of chemical, electrochemical and biological molecular charge transfer processes, with examples of how to bridge electron, proton, and PCET theory with experimental data. We offer first a theoretical minimum of molecular electron, proton, and PCET processes in homogeneous solution and at electrochemical interfaces. We illustrate next the use of the theory both for simple electron transfer processes, and for processes that involve molecular reorganization beyond the simplest harmonic approximation, with dissociative electron transfer and inclusion of all charge transfer parameters. A core example is the electrochemical reduction of the S2O82- anion. This is followed by discussion of core elements of proton and PCET processes and the electrochemical dihydrogen evolution reaction on different metal, semiconductor, and semimetal (say graphene) electrode surfaces. Other further focus is on stochastic chemical rate theory, and how this concept can rationalize highly non-traditional behaviour of charge transfer processes in mixed solvents. As a second major area we address ("long-range") chemical and electrochemical electron transfer through molecular frameworks using notions of superexchange and hopping. Single-molecule and single-entity electrochemistry are based on electrochemical scanning probe microscopies. (In operando) scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) are particularly emphasized, with theoretical notions and new molecular electrochemical phenomena in the confined tunnelling gap. Single-molecule surface structure and electron transfer dynamics are illustrated by self-assembled thiol molecular monolayers and by more complex redox target molecules. This discussion also extends single-molecule electrochemistry to bioelectrochemistry of complex redox metalloproteins and metalloenzymes. Our third major area involves computational overviews of molecular and electronic structure of the electrochemical interface, with new computational challenges. These relate to solvent dynamics in bulk and confined space (say carbon nanostructures), electrocatalysis, metallic and semiconductor nanoparticles, d-band metals, carbon nanostructures, spin catalysis and "spintronics", and "hot" electrons. Further perspectives relate to metal-organic frameworks, chiral surfaces, and spintronics.

Single-molecular protein-based bioelectronics via electronic transport: fundamentals, devices and applications

Biomolecular electronics is a rapidly growing multidisciplinary field that combines biology, nanoscience, and engineering to bridge the two important fields of life sciences and molecular electronics. Proteins are remarkable for their ability to recognize molecules and transport electrons, making the integration of proteins into electronic devices a long sought-after goal and leading to the emergence of the field of protein-based bioelectronics, also known as proteotronics. This field seeks to design and create new biomolecular electronic platforms that allow for the understanding and manipulation of protein-mediated electronic charge transport and related functional applications. In recent decades, there have been numerous reports on protein-based bioelectronics using a variety of nano-gapped electrical devices and techniques at the single molecular level, which are not achievable with conventional ensemble approaches. This review focuses on recent advances in physical electron transport mechanisms, device fabrication methodologies, and various applications in protein-based bioelectronics. We discuss the most recent progress of the single or few protein-bridged electrical junction fabrication strategies, summarise the work on fundamental and functional applications of protein bioelectronics that enable high and dynamic electron transport, and highlight future perspectives and challenges that still need to be addressed. We believe that this specific review will stimulate the interdisciplinary research of topics related to protein-related bioelectronics, and open up new possibilities for single-molecule biophysics and biomedicine.

Nanoscale electrochemical charge transfer kinetics investigated by electrochemical scanning microwave microscopy

We show how microwave microscopy can be used to probe local charge transfer reactions with unprecedented sensitivity, visualizing surface reactions with only a few hundred molecules involved. While microwaves are too fast under classical conditions to interact and sense electrochemical processes, this is different at the nanoscale, where our heterodyne microwave sensing method allows for highly sensitive local cyclic voltammetry (LCV) and local electrochemical impedance spectroscopy (LEIS). LCV and LEIS allow for precise measurement of the localized charge transfer kinetics, as illustrated in this study for a ferrocene self-assembled monolayer immersed in an electrolyte. The theoretical analysis presented here enables a consistent mapping of the faradaic kinetics and the parasitic contributions (nonfaradaic) to be spectrally resolved and subtracted. In particular, this methodology reveals an undistorted assessment of accessible redox site density of states associated with faradaic capacitance, fractional surface coverage and electron transfer kinetics at the nanoscale. The developed methodology opens a new perspective on comprehending electrochemical reactivity at the nanoscale.

Distance and Potential Dependence of Charge Transport Through the Reaction Center of Individual Photosynthetic Complexes

Charge separation and transport through the reaction center of photosystem I (PSI) is an essential part of the photosynthetic electron transport chain. A strategy is developed to immobilize and orient PSI complexes on gold electrodes allowing to probe the complex's electron acceptor side, the chlorophyll special pair P700. Electrochemical scanning tunneling microscopy (ECSTM) imaging and current-distance spectroscopy of single protein complex shows lateral size in agreement with its known dimensions, and a PSI apparent height that depends on the probe potential revealing a gating effect in protein conductance. In current-distance spectroscopy, it is observed that the distance-decay constant of the current between PSI and the ECSTM probe depends on the sample and probe electrode potentials. The longest charge exchange distance (lowest distance-decay constant β) is observed at sample potential 0 mV/SSC (SSC: reference electrode silver/silver chloride) and probe potential 400 mV/SSC. These potentials correspond to hole injection into an electronic state that is available in the absence of illumination. It is proposed that a pair of tryptophan residues located at the interface between P700 and the solution and known to support the hydrophobic recognition of the PSI redox partner plastocyanin, may have an additional role as hole exchange mediator in charge transport through PSI.

RNA BioMolecular Electronics: towards new tools for biophysics and biomedicine

The last half-century has witnessed the birth and development of a new multidisciplinary field at the edge between materials science, nanoscience, engineering, and chemistry known as Molecular Electronics. This field deals with the electronic properties of individual molecules and their integration as active components in electronic circuits and has also been applied to biomolecules, leading to BioMolecular Electronics and opening new perspectives for single-molecule biophysics and biomedicine. Herein, we provide a brief introduction and overview of the BioMolecular electronics field, focusing on nucleic acids and potential applications for these measurements. In particular, we review the recent demonstration of the first single-molecule electrical detection of a biologically-relevant nucleic acid. We also show how this could be used to study biomolecular interactions and applications in liquid biopsy for early cancer detection, among others. Finally, we discuss future perspectives and challenges in the applications of this fascinating research field.

Attoampere Nanoelectrochemistry

Electrochemical microscopy techniques have extended the understanding of surface chemistry to the micrometer and even sub-micrometer level. However, fundamental questions related to charge transport at the solid-electrolyte interface, such as catalytic reactions or operation of individual ion channels, require improved spatial resolutions down to the nanoscale. A prerequisite for single-molecule electrochemical sensitivity is the reliable detection of a few electrons per second, that is, currents in the atto-Ampere (10-18 A) range, 1000 times below today's electrochemical microscopes. This work reports local cyclic voltammetry (CV) measurements at the solid-liquid interface on ferrocene self-assembled monolayer (SAM) with sub-atto-Ampere sensitivity and simultaneous spatial resolution < 80 nm. Such sensitivity is obtained through measurements of the charging of the local faradaic interface capacitance at GHz frequencies. Nanometer-scale details of different molecular organizations with a 19% packing density difference are resolved, with an extremely small dispersion of the molecular electrical properties. This is predicted previously based on weak electrostatic interactions between neighboring redox molecules in a SAM configuration. These results open new perspectives for nano-electrochemistry like the study of quantum mechanical resonance in complex molecules and a wide range of applications from electrochemical catalysis to biophysics.

Direct Electrochemical Enzyme Electron Transfer on Electrodes Modified by Self-Assembled Molecular Monolayers

Self-assembled molecular monolayers (SAMs) have long been recognized as crucial “bridges” between redox enzymes and solid electrode surfaces, on which the enzymes undergo direct electron transfer (DET)—for example, in enzymatic biofuel cells (EBFCs) and biosensors. SAMs possess a wide range of terminal groups that enable productive enzyme adsorption and fine-tuning in favorable orientations on the electrode. The tunneling distance and SAM chain length, and the contacting terminal SAM groups, are the most significant controlling factors in DET-type bioelectrocatalysis. In particular, SAM-modified nanostructured electrode materials have recently been extensively explored to improve the catalytic activity and stability of redox proteins immobilized on electrochemical surfaces. In this report, we present an overview of recent investigations of electrochemical enzyme DET processes on SAMs with a focus on single-crystal and nanoporous gold electrodes. Specifically, we consider the preparation and characterization methods of SAMs, as well as SAM applications in promoting interfacial electrochemical electron transfer of redox proteins and enzymes. The strategic selection of SAMs to accord with the properties of the core redox protein/enzymes is also highlighted.

Many-Body State Description of Single-Molecule Electroluminescence Driven by a Scanning Tunneling Microscope

Electron transport and optical properties of a single molecule in contact with conductive materials have attracted considerable attention because of their scientific importance and potential applications. With the recent progress in experimental techniques, especially by virtue of scanning tunneling microscope (STM)-induced light emission, where the tunneling current of the STM is used as an atomic-scale source for induction of light emission from a single molecule, it has become possible to investigate single-molecule properties at subnanometer spatial resolution. Despite extensive experimental studies, the microscopic mechanism of electronic excitation of a single molecule in STM-induced light emission has yet to be clarified. Here we present a formulation of single-molecule electroluminescence driven by electron transfer between a molecule and metal electrodes based on a many-body state representation of the molecule. The effects of intramolecular Coulomb interaction on conductance and luminescence spectra are investigated using the nonequilibrium Hubbard Green's function technique combined with first-principles calculations. We compare simulation results with experimental data and find that the intramolecular Coulomb interaction is crucial for reproducing recent experiments for a single phthalocyanine molecule. The developed theory provides a unified description of the electron transport and optical properties of a single molecule in contact with metal electrodes driven out of equilibrium, and thereby, it contributes to a microscopic understanding of optoelectronic conversion in single molecules on solid surfaces and in nanometer-scale junctions.

Bottom-Up Self-Assembled Supramolecular Structures Built by STM at the Solid/Liquid Interface

One of the lines of research on organic devices is focused on their miniaturization to obtain denser and faster electronic circuits. The challenge is to build devices adding atom by atom or molecule by molecule until the desired structures are achieved. To do this job, techniques able to see and manipulate matter at this scale are needed. Scanning tunneling microscopy (STM) has been the selected technique by scientists to develop smart and functional unimolecular devices. This review article compiles the latest developments in this field giving examples of supramolecular systems monitored and fabricated at the molecular scale by bottom-up approaches using STM at the solid/liquid interface.

Approaches to single-molecule studies of metalloprotein electron transfer using scanning probe-based techniques

The single-molecule properties of metalloproteins have provided an intensely active research area in recent years. This brief review covers some of the techniques used to prepare, measure and analyse the electron transfer properties of metalloproteins, concentrating on scanning tunnelling microscopy-based techniques and advances in attachment of proteins to electrodes.