Two-dimensional assembly and local redox-activity of molecular hybrid structures in an electrochemical environment

Electrochemical gating for single-molecule electronics with hybrid Au|graphene contacts

The single-molecular conductance of a redox active viologen molecular bridge between Au|graphene electrodes has been studied in an electrochemical gating configuration in an ionic liquid medium. A clear "off-on-off" conductance switching behaviour has been achieved through gating of the redox state when the electrochemical potential is swept. The Au|viologen|graphene junctions show single-molecule conductance maxima centred close to the equilibrium redox potentials for both reduction steps. The peak conductance of Au|viologen|graphene junctions during the first reduction is significantly higher than that of previously measured Au|viologen|Au junctions. This shows that even though the central viologen moiety is not directly linked to the enclosing electrodes, substituting one gold contact for a graphene one nevertheless has a significant impact on junction conductance values. The experimental data was compared against two theoretical models, namely a phase coherent tunnelling and an incoherent "hopping" model. The former is a simple gating monoelectronic model within density functional theory (DFT) which discloses the charge state evolution of the molecule with electrode potential. The latter model is the collective Kuznetsov Ulstrup model for 2-step sequential charge transport through the redox centre in the adiabatic limit. The comparison of both models to the experimental data is discussed for the first time. This work opens perspectives for graphene-based molecular transistors with more effective gating and fundamental understanding of electrochemical electron transfer at the single molecular level.

Binding of Sulfate-Terminated Surfactants with Different Alkyl Chain Lengths to Viologen Sites Covalently Embedded in the Interior of a Self-Assembled Monolayer on a Au Electrode

We investigated the binding of anionic surfactants of lower concentrations than their critical micelle concentrations (cmcs) to the cationic redox-active viologen site in the interior of a self-assembled monolayer (SAM) on a polycrystalline Au electrode. We embedded the viologen site in the midway of the alkyl chain to facilitate the ion-pairing binding, which depends on the oxidation state of the viologen. We found that the binding of anionic surfactants and inorganic anions causes a negative shift of the formal potential of the redox couple of the viologen radical cation/viologen dication in line with the binding equilibrium. In contrast, the anion binding was weak and trivial when viologens are located at the SAM surface, indicative of the enhancement of the binding by the electrostatic interaction in the microenvironment with the low dielectric constant. The negative shift of the formal potential of viologen in the interior was greater for the surfactants with longer alkyl chain lengths, indicative of the efficacy of the alkyl chain-chain interaction. The chain-length-dependent potential shift followed the linear Traube rule but with a smaller slope than that in the original rule. We also demonstrated that the conjugated layer of the viologen SAM with dodecyl sulfate at a lower concentration than the cmc completely blocks the direct electron transfer (ET) from the Au electrode to solution-phase Fe(CN)₆^3- but allows mediated ET around the formal potential of the viologen.

Tuning Single-Molecule Conductance by Controlled Electric Field-Induced trans-to-cis Isomerisation

External electric fields (EEFs) have proven to be very efficient in catalysing chemical reactions, even those inaccessible via wet-chemical synthesis. At the single-molecule level, oriented EEFs have been successfully used to promote in situ single-molecule reactions in the absence of chemical catalysts. Here, we elucidate the effect of an EEFs on the structure and conductance of a molecular junction. Employing scanning tunnelling microscopy break junction (STM-BJ) experiments, we form and electrically characterize single-molecule junctions of two tetramethyl carotene isomers. Two discrete conductance signatures show up more prominently at low and high applied voltages which are univocally ascribed to the trans and cis isomers of the carotenoid, respectively. The difference in conductance between both cis-/trans- isomers is in concordance with previous predictions considering π-quantum interference due to the presence of a single gauche defect in the trans isomer. Electronic structure calculations suggest that the electric field polarizes the molecule and mixes the excited states. The mixed states have a (spectroscopically) allowed transition and, therefore, can both promote the cis-isomerization of the molecule and participate in electron transport. Our work opens new routes for the in situ control of isomerisation reactions in single-molecule contacts.

Molecular Imaging of Viologen Adlayers and In Situ Monitoring Structural Transformations at Electrode-Electrolyte Interfaces

The effects of temperature and molecular concentration on the ordering of two-dimensional (2D) nanostructures have been investigated at the well-defined Au(111)-electrolyte interface. In comparison to the assembly of thiolated alkanes or hydrogen-bonded nonthiolated molecules, fabricating large aromatic thiolated molecules into a highly ordered adlayer on a surface remained a challenge. In this study, we demonstrated the importance of controlling the assembly conditions and procedures for the formation of ordered adlayers of redox-active viologen derivatives. The assembly conditions that were explored include the variation of molar concentration of assembly solutions, assembly time, and thermal annealing. We report that the optimal assembly conditions for creating highly ordered thiolated viologen derivatives on a Au(111)-(1 × 1) electrode surface are to limit the time in which the electrode is immersed in a deoxygenated 0.05 mM ethanolic viologen solution (preheated to 70 °C) to 45 s, followed by thermal annealing in absolute ethanol for 12 h. Highly ordered molecular adlayers were imaged by electrochemical scanning tunneling microscopy (STM), revealing the molecular packing of low-coverage adlayers. Furthermore, in situ STM combined with cyclic voltammetry (CV) allowed for the exploration of the structural transformation and potential limit of reductive and "oxidative" desorption of adlayers within the electrochemical potential range of the sample potential (E_S) from -0.95 V to -0.10 V vs SCE.

Observation of inhomogeneous plasmonic field distribution in a nanocavity

The progress of plasmon-based technologies relies on an understanding of the properties of the enhanced electromagnetic fields generated by the coupling nanostrucutres^1-6. Plasmon-enhanced applications include advanced spectroscopies^7-10, optomechanics¹¹, optomagnetics¹² and biosensing^13-17. However, precise determination of plasmon field intensity distribution within a nanogap remains challenging. Here, we demonstrate a molecular ruler made from a set of viologen-based, self-assembly monolayers with which we precisely measures field distribution within a plasmon nanocavity with ~2-Å spatial resolution. We observed an unusually large plasmon field intensity inhomogeneity that we attribute to the formation of a plasmonic comb in the nanocavity. As a consequence, we posit that the generally adopted continuous media approximation for molecular monolayers should be used carefully.

Nanoconfined self-assembly on a grafted graphitic surface under electrochemical control

Highly oriented pyrolytic graphite (HOPG) can be covalently grafted with aryl radicals generated via the electrochemical reduction of 3,5-bis-tert-butyl-diazonium cations (3,5-TBD). The structure of the grafted layer and its stability under electrochemical conditions were assessed with electrochemical scanning tunneling microscopy (EC-STM) and cyclic voltammetry (CV). Stable within a wide (>2.5 V) electrochemical window, the grafted species can be locally removed using EC-STM-tip nanolithography. Using dibenzyl viologen as an example, we show that the generated nanocorrals of bare graphitic surface can be used to study nucleation and growth of self-assembled structures under conditions of nanoconfinement and electrochemical potential control.

Investigating Nanoscale Electrochemistry with Surface- and Tip-Enhanced Raman Spectroscopy

The chemical sensitivity of surface-enhanced Raman spectroscopy (SERS) methodologies allows for the investigation of heterogeneous chemical reactions with high sensitivity. Specifically, SERS methodologies are well-suited to study electron transfer (ET) reactions, which lie at the heart of numerous fundamental processes: electrocatalysis, solar energy conversion, energy storage in batteries, and biological events such as photosynthesis. Heterogeneous ET reactions are commonly monitored by electrochemical methods such as cyclic voltammetry, observing billions of electrochemical events per second. Since the first proof of detecting single molecules by redox cycling, there has been growing interest in examining electrochemistry at the nanoscale and single-molecule levels. Doing so unravels details that would otherwise be obscured by an ensemble experiment. The use of optical spectroscopies, such as SERS, to elucidate nanoscale electrochemical behavior is an attractive alternative to traditional approaches such as scanning electrochemical microscopy (SECM). While techniques such as single-molecule fluorescence or electrogenerated chemiluminescence have been used to optically monitor electrochemical events, SERS methodologies, in particular, have shown great promise for exploring electrochemistry at the nanoscale. SERS is ideally suited to study nanoscale electrochemistry because the Raman-enhancing metallic, nanoscale substrate duly serves as the working electrode material. Moreover, SERS has the ability to directly probe single molecules without redox cycling and can achieve nanoscale spatial resolution in combination with super-resolution or scanning probe microscopies. This Account summarizes the latest progress from the Van Duyne and Willets groups toward understanding nanoelectrochemistry using Raman spectroscopic methodologies. The first half of this Account highlights three techniques that have been recently used to probe few- or single-molecule electrochemical events: single-molecule SERS (SMSERS), superlocalization SERS imaging, and tip-enhanced Raman spectroscopy (TERS). While all of the studies we discuss probe model redox dye systems, the experiments described herein push the study of nanoscale electrochemistry toward the fundamental limit, in terms of both chemical sensitivity and spatial resolution. The second half of this Account discusses current experimental strategies for studying nanoelectrochemistry with SERS techniques, which includes relevant electrochemically and optically active molecules, substrates, and substrate functionalization methods. In particular, we highlight the wide variety of SERS-active substrates and optically active molecules that can be implemented for EC-SERS, as well as the need to carefully characterize both the electrochemistry and resultant EC-SERS response of each new redox-active molecule studied. Finally, we conclude this Account with our perspective on the future directions of studying nanoscale electrochemistry with SERS/TERS, which includes the integration of SECM with TERS and the use of theoretical methods to further describe the fundamental intricacies of single-molecule, single-site electrochemistry at the nanoscale.

Single-Molecule Conductance of Viologen-Cucurbit[8]uril Host-Guest Complexes

The local molecular environment is a critical factor which should be taken into account when measuring single-molecule electrical properties in condensed media or in the design of future molecular electronic or single molecule sensing devices. Supramolecular interactions can be used to control the local environment in molecular assemblies and have been used to create microenvironments, for instance, for chemical reactions. Here, we use supramolecular interactions to create microenvironments which influence the electrical conductance of single molecule wires. Cucurbit[8]uril (CB[8]) with a large hydrophobic cavity was used to host the viologen (bipyridinium) molecular wires forming a 1:1 supramolecular complex. Significant increases in the viologen wire single molecule conductances are observed when it is threaded into CB[8] due to large changes of the molecular microenvironment. The results were interpreted within the framework of a Marcus-type model for electron transfer as arising from a reduction in outer-sphere reorganization energy when the viologen is confined within the hydrophobic CB[8] cavity.

Molecular Electronics: History and Fundamentals

The increasing difficulties of meeting ‘Moore’s Law’ rates of progress in conventional semiconductor electronics, coupled with the advent of methods capable of measuring the electronic properties of single molecules in a laboratory setting, have seen a surge of activity in the field of molecular electronics over the last decade. However, the concepts of molecular electronics are far from new, and the basic premise and ideas of molecular electronics have been shadowing those of solid-state semiconductor electronics since the middle of the 20th century. In this Primer Review, we introduce the topic of molecular electronics, drawing on some of the earliest expressions of the fundamental concepts, and summarizing key concepts to provide the interested reader with an entry to this fascinating field of science and emerging technology.

Conical Gradient Junctions of Dendritic Viologen Arrays on Electrodes

The three-dimensional construction of arrays of functional molecules on an electrode surface, such as organic semiconductors and redox-active molecules, is a considerable challenge in the fabrication of sophisticated junctions for molecular devices. In particular, well-defined organic layers with precise molecular gradients are anticipated to function as novel metal/organic interfaces with specific electrical properties, such as a space charge layer at the metal/semiconductor interface. Here, we report a strategy for the construction of a three-dimensional molecular array with an electrical connection to a metal electrode by exploiting dendritic molecular architecture. Newly designed dendritic molecules consisting of viologens (1,1′-disubstituted-4,4′-bipyridilium salts) as the framework and mercapto groups as anchor units form unique self-assembled monolayers (SAMs) on a gold surface reflecting the molecular design. The dendritic molecules exhibit a conical shape and closely pack to form cone arrays on the substrate, whereas, in solution, they expand into more flexible conformations. Differences in the introduction position of the anchor units in the dendritic structure result in apical- and basal-type cone arrays in which the spatial concentration of the viologen units can be precisely configured in the cones. The concentration in apical-type SAMs increases away from the substrate, whereas the opposite is true in basal-type SAMs.

Single-Molecule Electronics: Chemical and Analytical Perspectives

It is now possible to measure the electrical properties of single molecules using a variety of techniques including scanning probe microcopies and mechanically controlled break junctions. Such measurements can be made across a wide range of environments including ambient conditions, organic liquids, ionic liquids, aqueous solutions, electrolytes, and ultra high vacuum. This has given new insights into charge transport across molecule electrical junctions, and these experimental methods have been complemented with increasingly sophisticated theory. This article reviews progress in single-molecule electronics from a chemical perspective and discusses topics such as the molecule-surface coupling in electrical junctions, chemical control, and supramolecular interactions in junctions and gating charge transport. The article concludes with an outlook regarding chemical analysis based on single-molecule conductance.

Molecular ordering at electrified interfaces: Template and potential effects

A combination of cyclic voltammetry and in situ scanning tunneling microscopy was employed to examine the adsorption and phase transition of 1,1’-dibenzyl-4,4’-bipyridinium molecules (abbreviated as DBV²⁺) on a chloride-modified Cu(111) electrode surface. The cyclic voltammogram (CV) of the Cu(111) electrode exposed to a mixture of 10 mM HCl and 0.1 mM DBVCl₂ shows three distinguishable pairs of current waves P1/P’1, P2/P’2, and P3/P’3 which are assigned to two reversible electron transfer steps, representing the reduction of the dicationic DBV²⁺ to the corresponding radical monocationic DBV^+• (P1/P’1) and then to the uncharged DBV⁰ (P3/P’3) species, respectively, as well as the chloride desorption/readsorption processes (P2/P’2). At positive potentials (i.e., above P1) the DBV²⁺ molecules spontaneously adsorb and form a highly ordered phase on the c(p × √3)-precovered Cl/Cu(111) electrode surface. A key element of this DBV²⁺ adlayer is an assembly of two individual DBV²⁺ species which, lined up, forms a so-called “herring-bone” structure. Upon lowering the electrode potential the first electron transfer step (at P1) causes a phase transition from the DBV²⁺-related herring-bone phase to the so-called "alternating stripe" pattern built up by the DBV^+• species following a nucleation and growth mechanism. Comparison of both observed structures with those found earlier at different electrode potentials on a c(2 × 2)Cl-precovered Cu(100) electrode surface enables a clear assessment of the relative importance of adsorbate–substrate and adsorbate–adsorbate interactions, i.e., template vs self-assembly effects, in the structure formation process of DBV cations on these modified Cu electrode surfaces.

Regulating a benzodifuran single molecule redox switch via electrochemical gating and optimization of molecule/electrode coupling

We report a novel strategy for the regulation of charge transport through single molecule junctions via the combination of external stimuli of electrode potential, internal modulation of molecular structures, and optimization of anchoring groups. We have designed redox-active benzodifuran (BDF) compounds as functional electronic units to fabricate metal-molecule-metal (m-M-m) junction devices by scanning tunneling microscopy (STM) and mechanically controllable break junctions (MCBJ). The conductance of thiol-terminated BDF can be tuned by changing the electrode potentials showing clearly an off/on/off single molecule redox switching effect. To optimize the response, a BDF molecule tailored with carbodithioate (-CS2(-)) anchoring groups was synthesized. Our studies show that replacement of thiol by carbodithioate not only enhances the junction conductance but also substantially improves the switching effect by enhancing the on/off ratio from 2.5 to 8.

Nanoscale methods for single-molecule electrochemistry

The development of experiments capable of probing individual molecules has led to major breakthroughs in fields ranging from molecular electronics to biophysics, allowing direct tests of knowledge derived from macroscopic measurements and enabling new assays that probe population heterogeneities and internal molecular dynamics. Although still somewhat in their infancy, such methods are also being developed for probing molecular systems in solution using electrochemical transduction mechanisms. Here we outline the present status of this emerging field, concentrating in particular on optical methods, metal-molecule-metal junctions, and electrochemical nanofluidic devices.

Electrochemistry of single metalloprotein and DNA-based molecules at Au(111) electrode surfaces

We have briefly overviewed recent efforts in the electrochemistry of single transition metal complex, redox metalloprotein, and redox-marked oligonucleotide (ON) molecules. We have particularly studied self-assembled molecular monolayers (SAMs) of several 5'-C6-SH single- (ss) and double-strand (ds) ONs immobilized on Au(111) electrode surfaces via Au-S bond formation, using a combination of nucleic acid chemistry, electrochemistry and electrochemically controlled scanning tunnelling microscopy (in situ STM). Ds ONs stabilized by multiply charged cations and locked nucleic acid (LNA) monomers have been primary targets, with a view on stabilizing the ds-ONs and improving voltammetric signals of intercalating electrochemical redox probes. Voltammetric signals of the intercalator anthraquinone monosulfonate (AQMS) at ds-DNA/Au(111) surfaces diluted by mercaptohexanol are significantly sharpened and more robust in the presence than in the absence of [Co(NH3)6](3+). AQMS also displays robust Faradaic voltammetric signals specific to the ds form on binding to similar LNA/Au(111) surfaces, but this signal only evolves after successive voltammetric scanning into negative potential ranges. Triply charged spermidine (Spd) invokes itself a strong voltammetric signal, which is specific to the ds form and fully matched sequences. This signal is of non-Faradaic, capacitive origin but appears in the same potential range as the Faradaic AQMS signal. In situ STM shows that molecular scale structures of the size of Spd-stabilized ds-ONs are densely packed over the Au(111) surface in potential ranges around the capacitive peak potential.

Molecular self-assembly at metal-electrolyte interfaces

The self-assembly of molecular layers has become an important strategy in modern design of functional materials. However, in particular, large organic molecules may no longer be sufficiently volatile to be deposited by vapor deposition. In this case, deposition from solution may be a promising route; in ionic form, these molecules may even be soluble in water. In this contribution, we present and discuss results on the electrochemical deposition of viologen- and porphyrin molecules as well as their co-adsorption on chloride modified Cu(100) and Cu(111) single crystal electrode surfaces from aqueous acidic solutions. Using in situ techniques like cyclic voltametry and high resolution scanning tunneling microscopy, as well as ex-situ photoelectron spectroscopy data the highly ordered self-assembled organic layers are characterized with respect to their electrochemical behavior, lateral order and inner conformation as well as phase transitions thereof as a function of their redox-state and the symmetry of the substrate. As a result, detailed structure models are derived and are discussed in terms of the prevailing interactions.

Single-Molecule Conductance in a Series of Extended Viologen Molecules

Single-molecule conductance in a series of extended viologen molecules was measured at room temperature using a gold-molecule-gold scanning tunneling microscopy break junction arrangement. Conductance values for individual molecules change from 4.8 ± 1.2 nS for the shortest compound to 2.9 ± 1.0 nS for the compound with six repeating units and length of 11 nm. The latter value is almost 3 orders of magnitude higher than that reported for all-carbon-based aromatic molecular wires of comparable length. On the basis of the length of the molecules, an attenuation factor of only 0.06 ± 0.004 nm(-1) (0.006 ± 0.0004 Å(-1)) was obtained. To the best of our knowledge, this is the smallest value reported for the conductance attenuation in a series of molecular wires.

Electrochemical scanning tunneling microscopy and spectroscopy for single-molecule investigation

The technique of electrochemical scanning tunneling microscopy (ECSTM) and spectroscopy (ECSTS) for studying electron transport through single redox molecules is here described. Redox molecules of both biological and organic nature have been studied by this technique with the aim of understanding the transport mechanisms ruling the flow of electrons via a single molecule placed in a nanometer-sized gap between two electrodes while elucidating the role of the redox density of states brought about by the molecule. The obtained results provide unique clues to single-molecule transport behavior and support the concept of single-molecule electrochemical gating.

Molecular Rotors as Switches

The use of a functional molecular unit acting as a state variable provides an attractive alternative for the next generations of nanoscale electronics. It may help overcome the limits of conventional MOSFETd due to their potential scalability, low-cost, low variability, and highly integratable characteristics as well as the capability to exploit bottom-up self-assembly processes. This bottom-up construction and the operation of nanoscale machines/devices, in which the molecular motion can be controlled to perform functions, have been studied for their functionalities. Being triggered by external stimuli such as light, electricity or chemical reagents, these devices have shown various functions including those of diodes, rectifiers, memories, resonant tunnel junctions and single settable molecular switches that can be electronically configured for logic gates. Molecule-specific electronic switching has also been reported for several of these device structures, including nanopores containing oligo(phenylene ethynylene) monolayers, and planar junctions incorporating rotaxane and catenane monolayers for the construction and operation of complex molecular machines. A specific electrically driven surface mounted molecular rotor is described in detail in this review. The rotor is comprised of a monolayer of redox-active ligated copper compounds sandwiched between a gold electrode and a highly-doped P⁺ Si. This electrically driven sandwich-type monolayer molecular rotor device showed an on/off ratio of approximately 10⁴, a read window of about 2.5 V, and a retention time of greater than 10⁴ s. The rotation speed of this type of molecular rotor has been reported to be in the picosecond timescale, which provides a potential of high switching speed applications. Current-voltage spectroscopy (I-V) revealed a temperature-dependent negative differential resistance (NDR) associated with the device. The analysis of the device I–V characteristics suggests the source of the observed switching effects to be the result of the redox-induced ligand rotation around the copper metal center and this attribution of switching is consistent with the observed temperature dependence of the switching behavior as well as the proposed energy diagram of the device. The observed resistance switching shows the potential for future non-volatile memories and logic devices applications. This review will discuss the progress and provide a perspective of molecular motion for nanoelectronics and other applications.

Quasi-ohmic single molecule charge transport through highly conjugated meso-to-meso ethyne-bridged porphyrin wires

Understanding and controlling electron transport through functional molecules are of primary importance to the development of molecular scale devices. In this work, the single molecule resistances of meso-to-meso ethyne-bridged (porphinato)zinc(II) structures (PZn(n) compounds), connected to gold electrodes via (4'-thiophenyl)ethynyl termini, are determined using scanning tunneling microscopy-based break junction methods. These experiments show that each α,ω-di[(4'-thiophenyl)ethynyl]-terminated PZn(n) compound (dithiol-PZn(n)) manifests a dual molecular conductance. In both the high and low conductance regimes, the measured resistance across these metal-dithiol-PZn(n)-metal junctions increases in a near linear fashion with molecule length. These results signal that meso-to-meso ethyne-bridged porphyrin wires afford the lowest β value (β = 0.034 Å(-1)) yet determined for thiol-terminated single molecules that manifest a quasi-ohmic resistance dependence across metal-dithiol-PZn(n)-metal junctions.

An approach to measure electromechanical properties of atomic and molecular junctions

We describe a new setup for simultaneous measurements of force and current in conductive nanocontacts in a liquid environment with a high sampling rate and resolution. A lab-built current-to-voltage converter allows measurements of the current over seven orders of magnitude. As examples, we studied conductances and mechanical forces upon formation and breaking of gold atomic contacts and of two molecular junctions containing 1,2-di(4-pyridyl)ethyne (M1) and 1,4-di(4-pyridyl)buta-1,3-diyne (M2). We found that the forces required to deform or break gold atomic contacts depend critically on the surrounding medium. Further, they show non-linear behaviour in dependence of the number N of gold atoms detached. The electromechanical properties of the two types of molecular junctions upon stretching were analysed by correlating breaking forces with simultaneously measured junction conductances. A rather complex behaviour in a wide range of forces was discovered. Comparison of the current-probe atomic force microscopy experiments on the rupture of molecular junctions with STM-based break junction experiments enables the assignment of breaking forces of molecular junctions to the corresponding junction conductances.

Nonlinear charge transport in redox molecular junctions: a Marcus perspective

Redox molecular junctions are molecular conduction junctions that involve more than one oxidation state of the molecular bridge. This property is derived from the ability of the molecule to transiently localize transmitting electrons, implying relatively weak molecule-leads coupling and, in many cases, the validity of the Marcus theory of electron transfer. Here we study the implications of this property on the nonlinear transport properties of such junctions. We obtain an analytical solution of the integral equations that describe molecular conduction in the Marcus kinetic regime and use it in different physical limits to predict some important features of nonlinear transport in metal-molecule-metal junctions. In particular, conduction, rectification, and negative differential resistance can be obtained in different regimes of interplay between two different conduction channels associated with different localization properties of the excess molecular charge, without specific assumptions about the electronic structure of the molecular bridge. The predicted behaviors show temperature dependences typically observed in the experiment. The validity of the proposed model and ways to test its predictions and implement the implied control strategies are discussed.

Redox-switching in a viologen-type adlayer: an electrochemical shell-isolated nanoparticle enhanced Raman spectroscopy study on Au(111)-(1×1) single crystal electrodes

We reported the first application of in situ shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) to an interfacial redox reaction under electrochemical conditions. We construct gap-mode sandwich structures composed of a thiol-terminated HS-6V6H viologen adlayer immobilized on a single crystal Au(111)-(1×1) electrode and covered by Au(60 nm)@SiO(2) core-shell nanoparticles acting as plasmonic antennas. We observed high-quality, potential-dependent Raman spectra of the three viologen species V(2+), V(+●), and V(0) on a well-defined Au(111) substrate surface and could map their potential-dependent evolution. Comparison with experiments on powder samples revealed an enhancement factor of the nonresonant Raman modes of ∼3 × 10(5), and up to 9 × 10(7) for the resonance modes. The study illustrates the unique capability of SHINERS and its potential in the entire field of electrochemical surface science to explore structures and reaction pathways on well-defined substrate surfaces, such as single crystals, for molecular, (electro-)catalytic, bioelectrochemical systems up to fundamental double layer studies at electrified solid/liquid interfaces.

Single-molecule mapping of long-range electron transport for a cytochrome b(562) variant

Cytochrome b(562) was engineered to introduce a cysteine residue at a surface-exposed position to facilitate direct self-assembly on a Au(111) surface. The confined protein exhibited reversible and fast electron exchange with a gold substrate over a distance of 20 Å between the heme redox center and the gold surface, a clear indication that a long-range electron-transfer pathway is established. Electrochemical scanning tunneling microscopy was used to map electron transport features of the protein at the single-molecule level. Tunneling resonance was directly imaged and apparent molecular conductance was measured, which both show strong redox-gated effects. This study has addressed the first case of heme proteins and offered new perspectives in single-molecule bioelectronics.