In situ wiring of single molecules into an electrical circuit via electrochemical distance tunneling spectroscopy

The electrical conductance of single n-alkanethiol and alpha,omega-alkanedithiol molecules was measured via in situ distance tunneling spectroscopy in aqueous 0.1 M KOH solution. The statistical analysis of the conductance values show that the alpha,omega-alkanedithiol molecule trapped in the STM break junction can adopt two distinct geometries that result in "lower" and "higher" conductivity values. In contrast, n-alkanethiol molecules trapped in the junction show only a single conductivity value characteristic for a particular molecule. Furthermore, the "lower" conductivity value determined for alpha,omega-alkanedithiol is virtually identical to the electrical conductivity of the n-alkanethiol containing the same number of atoms in the backbone. Moreover when the STM tip is polarized to electrochemical potential preventing a chemical reaction between the terminal -SH group and Au, only "lower" conductivity values are observed for alpha,omega-alkaneditiols.     

Three‐State Single‐Molecule Naphthalenediimide Switch: Integration of a Pendant Redox Unit for Conductance Tuning

We studied charge transport through core‐substituted naphthalenediimide (NDI) single‐molecule junctions using the electrochemical STM‐based break‐junction technique in combination with DFT calculations. Conductance switching among three well‐defined states was demonstrated by electrochemically controlling the redox state of the pendent diimide unit of the molecule in an ionic liquid. The electrical conductances of the dianion and neutral states differ by more than one order of magnitude. The potential‐dependence of the charge‐transport characteristics of the NDI molecules was confirmed by DFT calculations, which account for electrochemical double‐layer effects on the conductance of the NDI junctions. This study suggests that integration of a pendant redox unit with strong coupling to a molecular backbone enables the tuning of charge transport through single‐molecule devices by controlling their redox states.     

The principles of bipolar electrochemistry and its electroanalysis applications

The present study reports the wireless technique that generates asymmetric reactivity on the surface of the conducting substrate without any direct electrical connection in the electrolyte solution by inducing external power. In recent years, bipolar electrochemical systems have received special attention that they are used for new kinds of electrochemical applications ranging from electrodeposition to electroanalytical chemistry. Bipolar electrochemistry is a unique technique because of the lack of direct electrical connection to the bipolar electrode. In this perspective article, we first illustrate the concept and history of the bipolar electrochemistry as well as their application based on the open and closed bipolar configuration in different fields.     

Optical-facilitated single-entity electrochemistry

Single-entity electrochemistry focusing on the study of transient electrochemical process at the confined interface, has become a promising field that addresses questions from multi-disciplines such as cellular biology, material chemistry, organic chemistry, etc. It offers the fruitful information hidden in bulk electrochemical measurements. As the optical techniques improve in spatial and temporal resolution, the combination of electrochemistry with optical microspectroscopy provides more comprehensive information of single-entity electrochemistry. Herein, we review recent progress made in optical–electrochemical measurements covering three aspects from the precise localization and temperature measurements of single compartments, to the in-situ tracking of dynamic behaviors of single nanoparticles in electrochemical process, and to the monitoring confinement-controlled electrochemistry at the single molecule/ion level. The review demonstrates how these optical methods are innovatively integrated with single-entity sensing. It also reveals how these optical–electrochemical combinations push single-entity electrochemistry forward.     

Charge transport in single Au / alkanedithiol / Au junctions: coordination geometries and conformational degrees of freedom

Recent STM molecular break-junction experiments have revealed multiple series of peaks in the conductance histograms of alkanedithiols. To resolve a current controversy, we present here an in-depth study of charge transport properties of Au|alkanedithiol|Au junctions. Conductance histograms extracted from our STM measurements unambiguously confirm features showing more than one set of junction configurations. On the basis of quantum chemistry calculations, we propose that certain combinations of different sulfur-gold couplings and trans/gauche conformations act as the driving agents. The present study may have implications for experimental methodology: whenever conductances of different junction conformations are not statistically independent, the conductance histogram technique can exhibit a single series only, even though a much larger abundance of microscopic realizations exists.     

Reduction-induced switching of single-molecule conductance of fullerene derivatives

The effect of electrochemical reduction on the single-molecule conductance of fullerene C60 derivatives was studied by scanning tunneling microscopy. Three types of C60 derivatives were synthesized, a monoadduct having an amino-terminated linker and two bisadducts having two linkers at different positions (trans2 and trans3). Each C60 derivative was immobilized on a gold surface by an amino-gold linkage, confirmed by infrared reflection-absorption spectroscopy. The immobilized C60 derivatives showed reversible and multiple reduction peaks in the cyclic voltammogram in dimethylformamide (DMF) at almost the same potentials as those in solution, showing the redox properties of the molecules are intact on gold. Single-molecule conductances of the bisadducts, which can span between a scanning tunneling microscopy (STM) tip made of gold and substrate with the two linkers, were determined by the STM break-junction measurements in water and DMF. The conductances were 6.1+/-4.5 nS in water and 4.9+/-1.7 nS in DMF for the trans2 bisadduct and 8.4+/-3.4 nS in water and 7.9 nS+/-2.8 in DMF for the trans3 bisadduct. By using a potential-controlled STM setup, the tunneling current through a single molecule was recorded with sweeping the potentials of the tip and substrate. The trans2 bisadduct showed significant changes in the current when the reductions of the C60 moiety occur. Some current curves showed multiple peaks, and the other curves showed stepwise increase and decrease at the C60 reduction and subsequent reoxidation. Statistical analysis afforded stepwise switching of the conductance as the average behavior and suggested that the electron tunneling through the C60 derivative is enhanced as it accepts electrons.     

电化学门控调节具有平行路径的单分子电路中电子传输

电化学门控已成为一种可行且高效调节单分子电导的方法。在本研究中,我们证实了具有两个平行苯环的单分子电路中电子传输可以通过电化学门控控制。首先,我们利用STM-BJ技术以金为电极构筑了具有两条平行路径的单分子结。与单条路径的单分子结相比,两条路径的分子结由于具有增强性量子干涉效应,具有2.82倍的电导值。进一步地,我们利用电化学门控对具有两个平行苯环的单分结的电导进行调控,获得了333%·V^-1调节比。结合DFT计算,发现在E=E_F附近的V形透射系数谱图导致了实验观测的电导门控行为。本研究揭示了具有平行路径的单分子电路的电化学门控行为,并为设计高性能分子器件的分子材料提供了新的途径。     

Adsorption and electrochemical activity: an in situ electrochemical scanning tunneling microscopy study of electrode reactions and potential-induced adsorption of porphyrins

The effect of adsorption on molecular properties and reactivity is a central topic in interfacial physical chemistry. At electrochemical interfaces, adsorbed molecules may lose their electrochemical activity. The absence of in situ probes has hindered our understanding of this phenomenon and electrode reactions in general. In this work, classical electrochemistry and electrochemical scanning tunneling microscopy (EC-STM) were combined to provide molecular level insight into electrochemical reactions and the molecular adsorption state at the electrolyte-electrode interface. The metal-free porphyrin 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (TPyP) adsorbed on Au(111) in 0.1 M H(2)SO(4) solution was chosen as a model system. TPyP is found to irreversibly adsorb on Au(111) over a wide range of potentials, from -0.25 to 0.6 V(SCE). The adsorption state of TPyP has a dramatic effect on its electrochemistry. Preadsorbed, oxidized TPyP displays no well-defined cathodic peaks in cyclic voltammograms in sharp contrast to solution-phase TPyP. Our present work provides direct, molecular level evidence of the electrochemically "invisible" species. Electrochemical activity of absorbed species is recovered by allowing the oxidized molecule sufficient time (tens of minutes) to reduce. The redox state of adsorbed TPyP also affects the nature of the adsorption. Oxidized species can apparently only form monolayers. However, multilayers, stable enough to be imaged by STM, can form when the adsorbed TPyP is in the reduced state. This suggests that by controlling the electrochemistry one can either promote or suppress the formation of multilayers.     

电化学STM技术在金属腐蚀科学中的应用及研究进展

本文介绍了电化学STM在金属腐蚀电化学研究领域的技术优势.其最大特点是可以在金属腐蚀环境中和原子/分子水平上,实时、原位、三维空间观察,并可以通过电位控制表面电极过程.综述了电化学STM在钝化膜结构、缓蚀剂的自组装膜、金属表面原子的活性溶解等揭示腐蚀缓蚀机理的研究领域中所起的重要作用以及最新研究进展.     

Structure-property relationships in redox-gated single molecule junctions--a comparison of pyrrolo-tetrathiafulvalene and viologen redox groups

We demonstrate that the electrical "switching" behavior of single molecules connected between two electrode contacts can be controlled by altering their structure and electrochemical characteristics. The electrical properties of gold|molecule|gold single molecule junctions incorporating HS(CH2)6-X-(CH2)6SH, where X = viologen (4,4'-bipyridinium) or pyrrolotetrathiafulvalene, are determined using a scanning tunneling microscopy based technique. The switching behavior, controlled through a tuneable electrochemical gate, changes from an on-off response (viologen) to an off-on-off response (pyrrolotetrathiafulvalene) on changing the central redox group. In contrast, the electrical properties of junctions incorporating redox-inactive HS(CH2)6-1,4-C6H4-(CH2)6SH do not alter significantly as a function of applied potential.     

Scanning tunneling spectroscopy in an ionic liquid

Molecular redox levels can be used to modulate tunneling currents through single or small numbers of molecules and induce molecular electronic device function. While most of these devices require cryogenic conditions, room temperature operation has been demonstrated by using electrochemical gating in aqueous environments. The latter have, however, serious shortcomings with a view on their relatively high volatility and narrow stability ranges in terms of potential. Here we report the first-time use of an ionic liquid, 1-butyl-3-methylimidazoliumhexafluorophosphate (BMI), as an electrochemical gate in a Scanning Tunneling Microscope (STM) configuration. Ionic liquids are known to have a very low vapor pressure, and accessible potential ranges are in principle large, up to 6 V. In a proof-of-principle experiment, we show how a heteroleptic redox-active Os bisterpyridine complex (Ossac) can be brought to exhibit both transistor and diode function in this novel environment at room temperature. This renders ionic liquids an attractive gating medium for configurations where back-gating is difficult to implement (e.g., break-junction techniques) or experimental conditions prohibit the use of aqueous or organic electrolyte media (vacuum or high temperatures). From an applied perspective, they represent a step toward solid-state molecular electronics with electrochemical gating.     

In situ electrochemical scanning tunnelling microscopy investigation of structure for horseradish peroxidase and its electricatalytic property

Immobilization of protein molecules is a fundamental problem for scanning tunnelling microscopy (STM) measurements with high resolution. In this paper, an electrochemical method has been proved to be an effective way to fix native horseradish peroxidase (HRP) as well as inactivated HRP from electrolyte onto a highly oriented pyrolytic graphite (HOPG) surface. This preparation is suitable for both ex situ and in situ electrochemical STM (ECSTM) measurements. In situ STM has been successfully employed to observe totally different structures of HRP in three typical cases: (1) in situ ECSTM reveals an oval-shaped pattern for a single molecule in neutral buffer solution, which is in good agreement with the dimension determined as 6.2×4.3×1.2. nm³ by ex situ STM for native HRP; (2) in situ ECSTM shows that the adsorbed HRP molecules on HOPG in a denatured environment exhibit swelling globes at the beginning and then change into a V-shaped pattern after 30 min; (3) in situ ECSTM reveals a black hole in every ellipsoidal sphere for inactivated HRP in strong alkali solution. The cyclic voltammetry results indicate that the absorbed native HRP can directly catalyse the reduction of hydrogen peroxide, demonstrating that a direct electron transfer reduction occurred between the enzyme and HOPG electrode, whereas the corresponding cyclic voltammograms for denatured HRP and inactivated HRP adsorbed on HOPG electrodes indicate a lack of ability to catalyse H₂O₂ reduction, which confirms that the HRP molecules lost their biological activity. Obviously, electrochemical results powerfully support in situ STM observations.     

Organized Monolayers of Biological Macromolecules on Au(111) Surfaces

Single-crystal electrochemistry and scanning tunneling microscopy directly in aqueous electrolyte solution (in situ STM) are established in physical electrochemistry but new in studies of adsorption and interfacial electrochemistry of biological macromolecules. These high-resolution techniques have now been applied comprehensively to proteins and other biomolecules in recent studies, discussed in this report. Focus is on three systems. The first one is a pair of amino acids, cysteine and cystine. These are strongly adsorbed via thiolate and disulfide, respectively, with identical reductive desorption and in situ STM patterns. Long-range lateral order can be imaged to molecular resolution. The amino acids are also reference molecules for the blue single-copper protein Pseudomonas aeruginosa azurin. This protein assembles in two well-defined orientations. One applies on bare Au(111) to which the protein is linked via its surface disulfide group. This orients the copper center away from the electrode surface. The other mode is by hydrophobic interactions with variable-length alkanethiols self-assembled on Au(111). In this mode the copper center is directed towards the surface. Adsorption and long-range electron tunneling in both modes have been characterized in detail using different electrochemical and spectroscopic techniques, as well as STM. Other data show that penta-(A–T) oligonucleotide adsorbed via a covalently bound thiol linker also displays reductive desorption and in situ STM to molecular resolution. The three systems thus appear to open new perspectives for broader use of high-resolution electrochemical techniques in comprehensive investigations of large biological molecules.     

Single-molecule conductance through multiple π-π-stacked benzene rings determined with direct electrode-to-benzene ring connections

Understanding electron transport across π-π-stacked systems will help to answer fundamental questions about biochemical redox processes and benefit the design of new materials and molecular devices. Herein we employed the STM break-junction technique to measure the single-molecule conductance of multiple π-π-stacked aromatic rings. We studied electron transport through up to four stacked benzene rings held together in an eclipsed fashion via a paracyclophane scaffold. We found that the strained hydrocarbons studied herein couple directly to gold electrodes during the measurements; hence, we did not require any heteroatom binding groups as electrical contacts. Density functional theory-based calculations suggest that the gold atoms of the electrodes bind to two neighboring carbon atoms of the outermost cyclophane benzene rings in η(2) fashion. Our measurements show an exponential decay of the conductance with an increasing number of stacked benzene rings, indicating a nonresonant tunneling mechanism. Furthermore, STM tip-substrate displacement data provide additional evidence that the electrodes bind to the outermost benzene rings of the π-π-stacked molecular wires.     

Measuring Level Alignment at the Metal–Molecule Interface by In Situ Electrochemical 13C NMR

A new technique to measure energy‐level alignment at a metal–molecule interface between the Fermi level of the metal and the frontier orbitals of the molecule is proposed and experimentally demonstrated. The method, which combines the electrochemistry of organo‐ligand‐stabilized Au nanoparticles with ¹³C NMR spectroscopy (i.e. in situ electrochemical NMR), enables measuring both occupied and unoccupied states.     

A core-shell strategy for constructing a single-molecule junction

Understanding the effects of intermolecular interactions on the charge-transport properties of metal/molecule/metal junctions is an important step towards using individual molecules as building blocks for electronic devices. This work reports a systematic electron-transport investigation on a series of "core-shell"-structured oligo(phenylene ethynylene) (Gn-OPE) molecular wires. By using dendrimers of different generations as insulating "shells", the intermolecular π-π interactions between the OPE "cores" can be precisely controlled in single-component monolayers. Three techniques are used to evaluate the electron-transport properties of the Au/Gn-OPE/Au molecular junctions, including crossed-wire junction, scanning tunneling spectroscopy (STS), and scanning tunneling microscope (STM) break-junction techniques. The STM break-junction measurement reveals that the electron-transport pathways are strongly affected by the size of the side groups. When the side groups are small, electron transport could occur through three pathways, including through single-molecule junctions, double-molecule junctions, and molecular bridges between adjacent molecules formed by aromatic π-π coupling. The dendrimer shells effectively prohibit the π-π coupling effect, but at the same time, very large dendrimer side groups may hinder the formation of Au-S bonds. A first-generation dendrimer acts as an optimal shell that only allows electron transport through the single-molecule junction pathway, and forbids the other undesired pathways. It is demonstrated that the dendrimer-based core-shell strategy allows the single-molecule conductance to be probed in a homogenous monolayer without the influence of intermolecular π-π interactions.     

Electrochemical plasmonic sensors

The enormous progress of nanotechnology during the last decade has made it possible to fabricate a great variety of nanostructures. On the nanoscale, metals exhibit special electrical and optical properties, which can be utilized for novel applications. In particular, plasmonic sensors including both the established technique of surface plasmon resonance and more recent nanoplasmonic sensors, have recently attracted much attention. However, some of the simplest and most successful sensors, such as the glucose biosensor, are based on electrical readout. In this review we describe the implementation of electrochemistry with plasmonic nanostructures for combined electrical and optical signal transduction. We highlight results from different types of metallic nanostructures such as nanoparticles, nanowires, nanoholes or simply films of nanoscale thickness. We briefly give an overview of their optical properties and discuss implementation of electrochemical methods. In particular, we review studies on how electrochemical potentials influence the plasmon resonances in different nanostructures, as this type of fundamental understanding is necessary for successful combination of the methods. Although several combined platforms exist, many are not yet in use as sensors partly because of the complicated effects from electrochemical potentials on plasmon resonances. Yet, there are clearly promising aspects of these sensor combinations and we conclude this review by discussing the advantages of synchronized electrical and optical readout, illustrating the versatility of these technologies.     

STM studies of single molecules: molecular orbital aspects

As a fundamental and frequently referred concept in modern chemistry, the molecular orbital plays a vital role in the science of single molecules, which has become an active field in recent years. For the study of single molecules, scanning tunneling microscopy (STM) has been proven to be a powerful scientific technique. Utilizing specific distribution of the molecular orbitals at spatial, energy, and spin scales, STM can explore many properties of single molecule systems, such as geometrical configuration, electronic structure, magnetic polarization, and so on. Various interactions between the substrate and adsorbed molecules are also understood in terms of the molecular orbitals. Molecular engineering methods, such as mode-selective chemistry based on the molecular orbitals, and resonance tunneling between the molecular orbitals of the molecular sample and STM tip, have stimulated new advances of single molecule science.     

《碳纳米材料电化学》专辑序言 ——方兴未艾的电化学能量转换和存储先进材料和技术

可以预见,在相当一段时期内,能源和环境将是全球发展的两大主题. 其实,人类对能源的获取方式将对地球的生态环境和人类未来的生存状态和生活方式产生重要影响. 正因为如此,世界各国正在大力发展可再生能源和清洁能源. 电化学能源是将化学能高效转变为电能的一种能量转换方式,它历史悠久,但不断被改进和创新,尤其是近年来得到了较快的发展. 目前,电化学能源转换和存储器件主要包括一次电池（如锌锰电池等）、二次电池（如铅酸电池、镍氢电池、锂离子电池等）、燃料电池、金属-空气电池以及超级电容器等. 电化学能源和其它可再生能源相互补充、交叉利用将是未来清洁能源的主要发展方向.