A single-molecule electrical approach for amino acid detection and chirality recognition

Magneto-electrochemical method for chiral recognition of amino acid enantiomers

Chiral recognition of enantiomers with identical mirror-symmetric molecular structures is important for the analysis of biomolecules, and it conventionally relies on stereoselective interactions in chiral chemical environments. Here, we develop a magneto-electrochemical method for the enhanced detection of chiral amino acids (AAs), that combines the advantages of the high sensitivity of electrochemiluminescent (ECL) biosensors and chirality-induced effects under a magnetic field. The ECL difference between L- and D-enantiomers can be amplified over 35-fold under a field of 3.5 kG, and the chiral discrimination can be achieved in dilute AA solutions down to the nM level. The field-dependent ECL and chronocoulometry measurements suggest that chiral AAs can lock the spins on their radicals and thus enlarge the ECL change under applied magnetic fields (magneto-ECL, MECL), which explains the field-enhanced chiral discrimination of AA enantiomers. Finally, a detailed protocol is demonstrated for the identification of unknown AA solutions, in which the species, chirality and concentration of AAs can be determined simultaneously from the 2D plots of the ECL and MECL results. This work benefits the development of field-assisted detection methods and represents a promising and universal strategy for the comprehensive analysis of chiral biomolecules.

Chiral carbon dots derived from tryptophan and threonine for enantioselective sensing of L/D-Lysine

Presently, most fluorescent probes for amino acid enantiomers detection require metal ions participation, which greatly increases the detection steps and costs, and affects the accuracy of detection results. To solve this problem, a dual pattern recognition sensor of chiral carbon dots (L-Try-Thr-CDs) with a quantum yield of 36.23 % was prepared by a one-step solvothermal method for the highly selective detection of lysine (Lys) enantiomers. Under optimal experimental conditions, the fluorescence and circular dichroism (CD) signals of the obtained L-Try-Thr-CDs could rapidly and effectively responded to L-Lys with limits of detection (LOD) of 16.51 nM and 24.38 nM, respectively, much lower than previously reported sensors. Importantly, the L-Try-Thr-CDs as a dual-mode sensor could not only detect amino acid enantiomers and simplify the steps, but also avoid inaccurate detection results due to unstable metal ions. Furthermore, the L-Try-Thr-CDs could detect L-Lys in living cells via a fluorescence microscope because of their excellent fluorescence characteristics and low toxicity. These results indicated that the dual-mode sensor not only provided a practical strategy for the design of new fluorescent probes, but also possessed outstanding application prospects in the accurate detection of lysine enantiomers.

Understanding Emergent Complexity from a Single-Molecule Perspective

Molecules, with structural, scaling, and interaction diversities, are crucial for the emergence of complex behaviors. Interactions are essential prerequisites for complex systems to exhibit emergent properties that surpass the sum of individual component characteristics. Tracing the origin of complex molecular behaviors from interactions is critical to understanding ensemble emergence, and requires insights at the single-molecule level. Electrical signals from single-molecule junctions enable the observation of individual molecular behaviors, as well as intramolecular and intermolecular interactions. This technique provides a foundation for bottom-up explorations of emergent complexity. This Perspective highlights investigations of various interactions via single-molecule junctions, including intramolecular orbital and weak intermolecular interactions and interactions in chemical reactions. It also provides potential directions for future single-molecule junctions in complex system research.

Single-Molecule Electrical Profiling of Peptides and Proteins

In recent decades, there has been a significant increase in the application of single-molecule electrical analysis platforms in studying proteins and peptides. These advanced analysis methods have the potential for deep investigation of enzymatic working mechanisms and accurate monitoring of dynamic changes in protein configurations, which are often challenging to achieve in ensemble measurements. In this work, the prominent research progress in peptide and protein-related studies are surveyed using electronic devices with single-molecule/single-event sensitivity, including single-molecule junctions, single-molecule field-effect transistors, and nanopores. In particular, the successful commercial application of nanopores in DNA sequencing has made it one of the most promising techniques in protein sequencing at the single-molecule level. From single peptides to protein complexes, the correlation between their electrical characteristics, structures, and biological functions is gradually being established. This enables to distinguish different molecular configurations of these biomacromolecules through real-time electrical monitoring of their life activities, significantly improving the understanding of the mechanisms underlying various life processes.

A chiral metal cluster triggers enantiospecific electronic transport

Chirality is a geometric property of matter that can be present at different scales, especially at the nanoscale. Here, we investigate the manifestation of chirality in electronic transport through a molecular junction. Spinless electronic transport through a chiral molecular junction is not enantiospecific. However, when a chiral metal cluster, C3-Au34, is attached to the source electrode, a different response is obtained in spinless electronic transport between R and L systems: this indicates the crucial role of chiral clusters in triggering enantiospecific spinless electronic transport. In contrast, when an achiral metal cluster, C3v-Au34, is attached, no change in conductance occurs between enantiomeric systems. Using the non-equilibrium green's function method, we characterized this phenomenon by calculating the transmission and conductance of spin-unpolarized electrons. Our theoretical results highlight the importance of metal clusters with specific sizes and chiral structures in electronic transport and support previously published experimental results that exhibited enantiospecific scanning tunneling measurements with intrinsically chiral tips.

Real-time detection of 20 amino acids and discrimination of pathologically relevant peptides with functionalized nanopore

Precise identification and quantification of amino acids is crucial for many biological applications. Here we report a copper(II)-functionalized Mycobacterium smegmatis porin A (MspA) nanopore with the N91H substitution, which enables direct identification of all 20 proteinogenic amino acids when combined with a machine-learning algorithm. The validation accuracy reaches 99.1%, with 30.9% signal recovery. The feasibility of ultrasensitive quantification of amino acids was also demonstrated at the nanomolar range. Furthermore, the capability of this system for real-time analyses of two representative post-translational modifications (PTMs), one unnatural amino acid and ten synthetic peptides using exopeptidases, including clinically relevant peptides associated with Alzheimer's disease and cancer neoantigens, was demonstrated. Notably, our strategy successfully distinguishes peptides with only one amino acid difference from the hydrolysate and provides the possibility to infer the peptide sequence.

Recent Progress in Detecting Enantiomers in Food

The analysis of enantiomers in food has significant implications for food safety and human health. Conventional analytical methods employed for enantiomer analysis, such as gas chromatography and high-performance liquid chromatography, are characterized by their labor-intensive nature and lengthy analysis times. This review focuses on the development of rapid and reliable biosensors for the analysis of enantiomers in food. Electrochemical and optical biosensors are highlighted, along with their fabrication methods and materials. The determination of enantiomers in food can authenticate products and ensure their safety. Amino acids and chiral pesticides are specifically discussed as important chiral substances found in food. The use of sensors replaces expensive reagents, offers real-time analysis capabilities, and provides a low-cost screening method for enantiomers. This review contributes to the advancement of sensor-based methods in the field of food analysis and promotes food authenticity and safety.

Evolution of Single-Molecule Electronic Interfaces

Single-molecule electronics can fabricate single-molecule devices via the construction of molecule-electrode interfaces and also provide a unique tool to investigate single-molecule scale physicochemical processes at these interfaces. To investigate single-molecule electronic devices with desired functionalities, an understanding of the interface evolution processes in single-molecule devices is essential. In this review, we focus on the evolution of molecule-electrode interface properties, including the background of interface evolution in single-molecule electronics, the construction of different types of single-molecule interfaces, and the regulation methods. Finally, we discuss the perspective of future characterization techniques and applications for single-molecule electronic interfaces.

Photostable yellow emissive carbon dots for iron-mediated reversible sensing of biothiols and cellular imaging

The different levels of biothiols in cells can not only screen cancer cells but also play a selective role in killing cancer cells. Therefore, accurate monitoring of biothiol in cancer cells is of great research significance. Herein, novel yellow emission CDs (Y-CDs) were prepared by a simple hydrothermal method using 2, 5-dihydroxyterephthalic acid (DHTA) as precursors. The Y-CDs as a highly efficient dual-mode sensor could detect Fe3+ and biothiols by colorimetric and fluorescence signals. Especially, with the addition of L-Cysteine, the quenched fluorescence could be quickly restored within 2 min and the detection limit was as low as 31.65 nM. Additionally, this sensor was utilized to sense biothiols in actual samples and living cells due to its eminent biocompatibility. Finally, the Y-CDs were successfully applied not only as fluorescent ink for message encryption but also as a portable solid hydrogels sensor for the detection of Fe3+ and biothiols. Therefore, these results suggested that Y-CDs could serve as a promising sensor for Fe3+ and biothiol detection in early cancer screening.

Controlling piezoresistance in single molecules through the isomerisation of bullvalenes

Nanoscale electro-mechanical systems (NEMS) displaying piezoresistance offer unique measurement opportunities at the sub-cellular level, in detectors and sensors, and in emerging generations of integrated electronic devices. Here, we show a single-molecule NEMS piezoresistor that operates utilising constitutional and conformational isomerisation of individual diaryl-bullvalene molecules and can be switched at 850 Hz. Observations are made using scanning tunnelling microscopy break junction (STMBJ) techniques to characterise piezoresistance, combined with blinking (current-time) experiments that follow single-molecule reactions in real time. A kinetic Monte Carlo methodology (KMC) is developed to simulate isomerisation on the experimental timescale, parameterised using density-functional theory (DFT) combined with non-equilibrium Green's function (NEGF) calculations. Results indicate that piezoresistance is controlled by both constitutional and conformational isomerisation, occurring at rates that are either fast (equilibrium) or slow (non-equilibrium) compared to the experimental timescale. Two different types of STMBJ traces are observed, one typical of traditional experiments that are interpreted in terms of intramolecular isomerisation occurring on stable tipped-shaped metal-contact junctions, and another attributed to arise from junction‒interface restructuring induced by bullvalene isomerisation.

Precise Detection, Control and Synthesis of Chiral Compounds at Single-Molecule Resolution

Chirality, as the symmetric breaking of molecules, plays an essential role in physical, chemical and especially biological processes, which highlights the accurate distinction among heterochiralities as well as the precise preparation for homochirality. To this end, the well-designed structure-specific recognizer and catalysis reactor are necessitated, respectively. However, each kind of target molecules requires a custom-made chiral partner and the dynamic disorder of spatial-orientation distribution of molecules at the ensemble level leads to an inefficient protocol. In this perspective article, we developed a universal strategy capable of realizing the chirality detection and control by the external symmetry breaking based on the alignment of the molecular frame to external stimuli. Specifically, in combination with the discussion about the relationship among the chirality (molecule), spin (electron) and polarization (photon), i.e., the three natural symmetry breaking, single-molecule junctions were proposed to achieve a single-molecule/event-resolved detection and synthesis. The fixation of the molecular orientation and the CMOS-compatibility provide an efficient interface to achieve the external input of symmetry breaking. This perspective is believed to offer more efficient applications in accurate chirality detection and precise asymmetric synthesis via the close collaboration of chemists, physicists, materials scientists, and engineers.

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.

Construction of chiral capillary electrochromatography microsystems based on Aspergillus sp. CM96

Novel chiral capillary electrochromatography (CEC) microsystems were constructed based on Aspergillus sp. CM96. As a newly discovered intrinsic characteristic of the cell, cell chirality occupies an essential position in life evolution. Aspergillus sp. CM96 spore (CM96s) was chosen as a proof of concept to develop chiral capillary columns. Interestingly, various types of amino acid (AA) enantiomers were baseline separated under the optimized conditions. Furthermore, the time-dependent chiral interactions between AAs and CM96s were explored in a wider space. Pectinases generated from Aspergillus sp. CM96 fermentation were immobilized onto graphene oxide-functionalized capillary silica monoliths for separating AA enantiomers. Molecular docking simulations were performed to explore chiral separation mechanisms of pectinase for AA enantiomers. These results indicated that Aspergillus sp. CM96-based CEC microsystems have a significant advantage for chiral separation.

Beyond Stereoisomeric Effects: Exploring the Importance of Intermolecular Electron Spin Interactions in Biorecognition

This work shows that electron spin polarization and stereoisomeric effects make comparable contributions to the enantioselective binding of amino acids. Magneto-electrochemical quartz crystal microbalance methods are used to study the adsorption of chiral amino acids onto a monolayer film of chiral molecules that are spin polarized by an underlying ferromagnetic substrate. The direction of the electron spin polarization affects both the kinetics and thermodynamics of the enantiospecific adsorption of the amino acids. Comparison of these data with the circular dichroism (CD) spectra of the amino acid adsorbates shows that the CD spectrum of the interacting group provides a good figure-of-merit for predicting the contributions of electron spin to the intermolecular interaction. These findings demonstrate the importance of electron spin in enantioselective intermolecular interactions between chiral amino acids and represent a paradigm shift for how selectivity should be viewed in biorecognition.

Chemical Sensors using Single-Molecule Electrical Measurements

Driven by the digitization and informatization of contemporary society, electrical sensors are developing toward minimal structure, intelligent function, and high detection resolution. Single-molecule electrical measurement techniques have been proven to be capable of label-free molecular recognition and detection, which opens a new strategy for the design of efficient single-molecule detection sensors. In this review, we outline the main advances and potentials of single-molecule electronics for qualitative identification and recognition assays at the single-molecule level. Strategies for single-molecule electro-sensing and its main applications are reviewed, mainly in the detection of ions, small molecules, oligomers, genetic materials, and proteins. This review summarizes the remaining challenges in the current development of single-molecule electrical sensing and presents some potential perspectives for this field.

Advances in single-molecule junctions as tools for chemical and biochemical analysis

The development of miniaturized electronics has led to the design and construction of powerful experimental platforms capable of measuring electronic properties to the level of single molecules, along with new theoretical concepts to aid in the interpretation of the data. A new area of activity is now emerging concerned with repurposing the tools of molecular electronics for applications in chemical and biological analysis. Single-molecule junction techniques, such as the scanning tunnelling microscope break junction and related single-molecule circuit approaches have a remarkable capacity to transduce chemical information from individual molecules, sampled in real time, to electrical signals. In this Review, we discuss single-molecule junction approaches as emerging analytical tools for the chemical and biological sciences. We demonstrate how these analytical techniques are being extended to systems capable of probing chemical reaction mechanisms. We also examine how molecular junctions enable the detection of RNA, DNA, and traces of proteins in solution with limits of detection at the zeptomole level.

Graphene-molecule-graphene single-molecule junctions to detect electronic reactions at the molecular scale

The ability to measure the behavior of a single molecule during a reaction implies the detection of inherent dynamic and static disordered states, which may not be represented when measuring ensemble averages. Here, we describe the building of devices with graphene-molecule-graphene single-molecule junctions integrated into an electrical circuit. These devices are simple to build and are stable, showing tolerance to mechanical changes, solution environment and voltage stimulation. The design of a conductive channel based on a single molecule enables single-molecule detection and is sensitive to variations in physical properties and chemical structures of the detected molecules. The on-chip setup of single-molecule junctions further offers complementary metal-oxide-semiconductor (CMOS) compatibility, enabling logic functions in circuit elements, as well as deciphering of reaction intermediates. We detail the experimental procedure to prepare graphene transistor arrays as a basis for single-molecule junctions and the preparation of nanogapped carboxyl-terminal graphene electrodes by using electron-beam lithography and oxygen plasma etching. We describe the basic design of a molecular bridge with desired functions and terminals to form covalent bonds with electrode arrays, via a chemical reaction, to construct stably integrated single-molecule devices with a yield of 30-50% per chip. The immobilization of the single molecules is then characterized by using inelastic electron tunneling spectra, single-molecule imaging and fluorescent spectra. The whole protocol can be implemented within 2 weeks and requires users trained in using ultra-clean laboratory facilities and the aforementioned instrumentation.

Characterization and Application of Supramolecular Junctions

The convergence of supramolecular chemistry and single-molecule electronics offers a new perspective on supramolecular electronics, and provides a new avenue toward understanding and application of intermolecular charge transport at the molecular level. In this review, we will provide an overview of the advances in the characterization technique for the investigation of intermolecular charge transport, and summarize the experimental investigation of several non-covalent interactions, including π-π stacking interactions, hydrogen bonding, host-guest interactions and σ-σ interactions at the single-molecule level. We will also provide a perspective on supramolecular electronics and discuss the potential applications and future challenges.

Facile synthesis of nitrogen-doped carbon dots as sensitive fluorescence probes for selective recognition of cinnamaldehyde and l-Arginine/l-Lysine in living cells

The disorder of amino acid metabolism and the abuse of small molecule drugs pose serious threats to public health. However, due to the limitations of existing detection technologies in sensing cinnamaldehyde (CAL) and l-Arginine/l-Lysine (l-Arg/l-Lys), there is an urgent need to develop new sensing strategies to meet the severe challenges currently facing. Herein, nitrogen-doped carbon dots (N-CDs) were developed using a simple one-pot hydrothermal carbonization method. These N-CDs exhibited numerous distinctive characteristics such as excellent photoluminescence, high water dispersibility, favorable biocompatibility, and superior chemical inertness. Strikingly, the as-prepared CDs as a highly efficient fluorescent probe possessed significant sensitivity and selectivity toward CAL and l-Arg/l-Lys over other analytes with a low detection limit of 58 nM and 16 nM/18 nM, respectively. The fluorescence of N-CDs could be quenched by CAL through an electron transfer process. Then, the strong electrostatic interaction between l-Arg/l-Lys and N-CDs induced the efficient fluorescence recovery. More importantly, the outstanding biosafety and excellent analyte-responsive fluorescence characteristics of N-CDs have also been verified in living cells as well as in serum and urine. Overall, the N-CDs had a wide application prospect in the diagnosis of amino acid metabolic diseases and small molecule drug sensing.

Supramolecular Enhancement of Charge Transport through Pillar[5]arene-Based Self-Assembled Monolayers

Intermolecular charge transport is one of the essential modes for modulating charge transport in molecular electronic devices. Supermolecules are highly promising candidates for molecular devices because of their abundant structures and easy functionalization. Herein, we report an efficient strategy to enhance charge transport through pillar[5]arene self-assembled monolayers (SAMs) by introducing cationic guests. The current density of pillar[5]arene SAMs can be raised up to about 2.1 orders of magnitude by inserting cationic molecules into the cavity of pillar[5]arenes in SAMs. Importantly, we have also observed a positive correlation between the charge transport of pillar[5]arene-based complex SAMs and the binding affinities of the pillar[5]arene-based complexation. Such an enhancement of charge transport is attributed to the efficient host-guest interactions that stabilize the supramolecular complexes and lower the energy gaps for charge transport. This work provides a predictive pattern for the regulation of intermolecular charge transport in guiding the design of next generation switches and functional sensors in supramolecular electronics.

Single-Molecule Peptide Identification Using Fluorescence Blinking Fingerprints

The ability to identify peptides with single-molecule sensitivity would lead to next-generation proteomics methods for basic research and clinical applications. Existing single-molecule peptide sequencing methods can read some amino acid sequences, but they are limited in their ability to distinguish between similar amino acids or post-translational modifications. Here, we demonstrate that the fluorescence intermittency of a peptide labeled with a spontaneously blinking fluorophore contains information about the structure of the peptide. Using a deep learning algorithm, this single-molecule blinking pattern can be used to identify the peptide. This method can distinguish between peptides with different sequences, peptides with the same sequence but different phosphorylation patterns, and even peptides that differ only by the presence of epimerized residues. This study builds the foundation for a targeted proteomics method with single-molecule sensitivity.

Single-molecule nano-optoelectronics: insights from physics

Single-molecule optoelectronic devices promise a potential solution for miniaturization and functionalization of silicon-based microelectronic circuits in the future. For decades of its fast development, this field has made significant progress in the synthesis of optoelectronic materials, the fabrication of single-molecule devices and the realization of optoelectronic functions. On the other hand, single-molecule optoelectronic devices offer a reliable platform to investigate the intrinsic physical phenomena and regulation rules of matters at the single-molecule level. To further realize and regulate the optoelectronic functions toward practical applications, it is necessary to clarify the intrinsic physical mechanisms of single-molecule optoelectronic nanodevices. Here, we provide a timely review to survey the physical phenomena and laws involved in single-molecule optoelectronic materials and devices, including charge effects, spin effects, exciton effects, vibronic effects, structural and orbital effects. In particular, we will systematically summarize the basics of molecular optoelectronic materials, and the physical effects and manipulations of single-molecule optoelectronic nanodevices. In addition, fundamentals of single-molecule electronics, which are basic of single-molecule optoelectronics, can also be found in this review. At last, we tend to focus the discussion on the opportunities and challenges arising in the field of single-molecule optoelectronics, and propose further potential breakthroughs.

TiO2 Nanotubes Decorated with CdSe Quantum Dots: A Bifunctional Electrochemiluminescent Platform for Chiral Discrimination and Chiral Sensing

Chiral analysis is of significant importance for living organisms since chirality is the fundamental phenomenon in nature. In this work, a bifunctional electrochemiluminescent (ECL) platform is constructed for chiral discrimination and chiral sensing. 3-Mercaptopropionic acid-functionalized CdSe quantum dots (CdSe QDs) are combined with aminated TiO₂ nanotubes (NH₂-TiNTs) via amidation. The resultant CdSe QDs/TiNTs display significantly enhanced ECL signals due to the synergistic effect between CdSe QDs and TiNTs, which are then used for the chiral discrimination of the isomers of nine chiral amino acids (AAs) in the presence of d-AA oxidase (DAAO). DAAO can selectively catalyze the oxidation of d-AAs to generate H₂O₂, which acts as the coreaction reagent and triggers the ECL signals of CdSe QDs/TiNTs, and thus, the isomers of the nine chiral AAs can be effectively discriminated. In addition, the as-constructed ECL platform can also be used for the sensitive detection of d-AAs in the presence of DAAO with a wide linear range and a low limit of detection. These findings suggest that the CdSe QDs/TiNTs can work as a bifunctional ECL platform (chiral discrimination and chiral sensing), which might be an advanced ECL platform for biomedical applications.

Highly sensitive and low-power consumption metalloporphyrin-based junctions for COx detection with excellent recovery

The development of cost-effective and eco-friendly sensor materials is needed to realize the application of detectors in daily life-such as in the internet of things. In this regard, monitoring air pollutants such as carbon monoxide (CO) and carbon dioxide (CO₂), mainly emitted by anthropogenic sources from daily human activities, is of great importance. In particular, developing a susceptible and portable CO₂ sensor raises a dilemma because of the chemical inertness and non-polarity of CO₂ molecules. We find that porphyrin-based materials, exploited by nature in biological systems, are a playground to search for such sensor materials. Using density functional non-equilibrium Green's function formalism, we fully screen all 3d metalloporphyrin (MPor) based devices to find efficient CO and CO₂ gas sensors. Our detailed analysis of the adsorption energy, molecular orbitals, transmission spectra, sensitivity, and recovery time reveals that the nature of central M alters the efficiency of MPor gas detectors. We find that CO and CO₂ can be monitored using, respectively, CoPor- and TiPor-based devices. The estimated sensitivity is around 100%, along with a fast recovery time at very low bias voltages (V ≥ 0.5 V), which turn metalloporphyrins into promising candidates for the widespread development of enhanced CO and CO₂ sensors awaiting further experimental validations.

Measuring conductance switching in single proteins using quantum tunneling

Interpreting the electrical signatures of single proteins in electronic junctions has facilitated a better understanding of the intrinsic properties of proteins that are fundamental to chemical and biological processes. Often, this information is not accessible using ensemble and even single-molecule approaches. In addition, the fabrication of nanoscale single-protein junctions remains challenging as they often require sophisticated methods. We report on the fabrication of tunneling probes, direct measurement, and active control (switching) of single-protein conductance with an external field in solution. The probes allowed us to bridge a single streptavidin molecule to two independently addressable, biotin-terminated electrodes and measure single-protein tunneling response over long periods. We show that charge transport through the protein has multiple conductive pathways that depend on the magnitude of the applied bias. These findings open the door for the reliable fabrication of protein-based junctions and can enable their use in future protein-embedded bioelectronics applications.

Single-Molecule Junction: A Reliable Platform for Monitoring Molecular Physical and Chemical Processes

Monitoring and manipulating the physical and chemical behavior of single molecules is an important development direction of molecular electronics that aids in understanding the molecular world at the single-molecule level. The electrical detection platform based on single-molecule junctions can monitor physical and chemical processes at the single-molecule level with a high temporal resolution, stability, and signal-to-noise ratio. Recently, the combination of single-molecule junctions with different multimodal control systems has been widely used to explore significant physical and chemical phenomena because of its powerful monitoring and control capabilities. In this review, we focus on the applications of single-molecule junctions in monitoring molecular physical and chemical processes. The methods developed for characterizing single-molecule charge transfer and spin characteristics as well as revealing the corresponding intrinsic mechanisms are introduced. Dynamic detection and regulation of single-molecule conformational isomerization, intermolecular interactions, and chemical reactions are also discussed in detail. In addition to these dynamic investigations, this review discusses the open challenges of single-molecule detection in the fields of physics and chemistry and proposes some potential applications in this field.

A Review of Sensors and Biosensors Modified with Conducting Polymers and Molecularly Imprinted Polymers Used in Electrochemical Detection of Amino Acids: Phenylalanine, Tyrosine, and Tryptophan

Recently, the studies on developing sensors and biosensors-with an obvious interdisciplinary character-have drawn the attention of many researchers specializing in various fundamental, but also complex domains such as chemistry, biochemistry, physics, biophysics, biology, bio-pharma-medicine, and bioengineering. Along these lines, the present paper is structured into three parts, and is aimed at synthesizing the most relevant studies on the construction and functioning of versatile devices, of electrochemical sensors and biosensors, respectively. The first part presents examples of the most representative scientific research focusing on the role and the importance of the phenylalanine, tyrosine, and tryptophan amino acids, selected depending on their chemical structure and their impact on the central nervous system. The second part is dedicated to presenting and exemplifying conductor polymers and molecularly imprinted polymers used as sensitive materials in achieving electrochemical sensors and biosensors. The last part of the review analyzes the sensors and biosensors developed so far to detect amino acids with the aid of conductor polymers and molecularly imprinted polymers from the point of view of the performances obtained, with emphasis on the detection methods, on the electrochemical reactions that take place upon detection, and on the electroanalytical performances. The present study was carried out with a view to highlighting, for the benefit of specialists in medicine and pharmacy, the possibility of achieving and purchasing efficient devices that might be used in the quality control of medicines, as well as in studying and monitoring diseases associated with these amino acids.

Accurate Single-Molecule Kinetic Isotope Effects

An accurate single-molecule kinetic isotope effect (sm-KIE) was applied to circumvent the inherent limitation of conventional ensemble KIE by using graphene-molecule-graphene single-molecule junctions. In situ monitoring of the single-molecule reaction trajectories in real time with high temporal resolution has the capability to characterize the deeper information brought by KIE. The C-O bond cleavage and the C-C bond formation of the transition state (TS) were observed in the Claisen rearrangement through the secondary kinetic isotope effect, demonstrating the high detection sensitivity and accuracy of this method. More interestingly, this sm-KIE can be used to determine TS structures under different electric fields, revealing the multidimensional regulation of the TS. The detection and manipulation of the TS provide a broad perspective to understand and optimize chemical reactions and biomimetic progress.

Highly Sensitive, Selective and Low-Power Consumption Metalloporphyrin−Based Junctions for Nitrogen Monoxide Detection with Excellent Recovery

Research interest in chemical gas detection has been directed towards developing highly selective bio-inspired and eco-friendly materials that allow the integration of sensors in daily human life, such as the...

Silicon - single molecule - silicon circuits

In 2020, silicon - molecule - silicon junctions were fabricated and shown to be on average one third as conductive as traditional junctions made using gold electrodes, but in some instances to be even more conductive, and significantly 3 times more extendable and 5 times more mechanically stable. Herein, calculations are performed of single-molecule junction structure and conductivity pertaining to blinking and scanning-tunnelling-microscopy (STM) break junction (STMBJ) experiments performed using chemisorbed 1,6-hexanedithiol linkers. Some strikingly different characteristics are found compared to analogous junctions formed using the metals which, to date, have dominated the field of molecular electronics. In the STMBJ experiment, following retraction of the STM tip after collision with the substrate, unterminated silicon surface dangling bonds are predicted to remain after reaction of the fresh tips with the dithiol solute. These dangling bonds occupy the silicon band gap and are predicted to facilitate extraordinary single-molecule conductivity. Enhanced junction extendibility is attributed to junction flexibility and the translation of adsorbed molecules between silicon dangling bonds. The calculations investigate a range of junction atomic-structural models using density-functional-theory (DFT) calculations of structure, often explored at 300 K using molecular dynamics (MD) simulations. These are aided by DFT calculations of barriers for passivation reactions of the dangling bonds. Thermally averaged conductivities are then evaluated using non-equilibrium Green's function (NEGF) methods. Countless applications through electronics, nanotechnology, photonics, and sensing are envisaged for this technology.

Capturing the Rotation of One Molecular Crank by Single-Molecule Conductance

Unveiling the internal dynamics of rotation in molecular machine at single-molecule scale is still a challenge. In this work, three crank-shaped molecules are elaborately designed with the conformational flipping between syn and anti fulfilled by two naphthyl groups rotating freely along 1,3-butadiynyl axis. By investigating the single-molecule conductance using scanning tunnelling microscope break junction (STM-BJ) technique and theoretical simulation, the internal rotation of these crank-shaped molecules is well identified through low and high conductance corresponding to syn- and anti-conformations. As demonstrated by theoretically computational study, the orbital energy changes with the conformational flipping and influences the intraorbital quantum interference, thus eventually modulating the single-molecule conductance. This work demonstrates single-molecule conductance measurement to be a rational approach for characterizing the internal rotation of molecular machines.