Single Particle Electrochemistry of Collision

Defect Chemistry in High-Voltage Cathode Materials for Lithium-Ion Batteries

High-voltage cathodes (HVCs) have emerged as a paramount role for the next-generation high-energy-density lithium-ion batteries (LIBs). However, the pursuit of HVCs comes with inherent challenges related to defective structures, which significantly impact the electrochemical performance of LIBs. The current obstacle lies in the lack of a comprehensive understanding of defects and their precise effects. This perspective aims to provide insights into defect chemistry for governing HVCs. The classifications, formation mechanisms, and evolution of defects are outlined to explore the intricate relationship between defects and electrochemical behavior. The pressing need for cutting-edge characterization techniques that comprehensively investigate defects across various temporal and spatial scales is emphasized. Building on these fundamental understandings, engineering strategies such as composition tailoring, morphology design, interface modification, and structural control to mitigate or utilize defects are thoroughly discussed for enhanced HVCs performance. These insights are expected to provide vital guidelines for developing high-performance HVCs for next-generation high-energy lithium-ion batteries.

Single Entity Electrocatalysis

Making a measurement over millions of nanoparticles or exposed crystal facets seldom reports on reactivity of a single nanoparticle or facet, which may depart drastically from ensemble measurements. Within the past 30 years, science has moved toward studying the reactivity of single atoms, molecules, and nanoparticles, one at a time. This shift has been fueled by the realization that everything changes at the nanoscale, especially important industrially relevant properties like those important to electrocatalysis. Studying single nanoscale entities, however, is not trivial and has required the development of new measurement tools. This review explores a tale of the clever use of old and new measurement tools to study electrocatalysis at the single entity level. We explore in detail the complex interrelationship between measurement method, electrocatalytic material, and reaction of interest (e.g., carbon dioxide reduction, oxygen reduction, hydrazine oxidation, etc.). We end with our perspective on the future of single entity electrocatalysis with a key focus on what types of measurements present the greatest opportunity for fundamental discovery.

Constraint Coupling of Redox Cascade and Electron Transfer Synchronization on Electrode-Nanosensor Interface for Repeatable Detection of Tumor Biomarkers

Quantitative analysis of up-regulated biomarkers in pathological tissues is helpful to tumor surgery yet the loss of biomarker extraction and time-consuming operation limited the accurate and quick judgement in preoperative or intraoperative diagnosis. Herein, an immobilization-free electrochemical sensing platform is developed by constraint coupling of electron transfer cascade on electrode-nanosensor interface. Specifically, electrochemical indicator (Ri)-labeled single-stranded DNA on electroactive nanodonor (polydopamine, PDA) can be responsively detached by formation of DNA complex through the recognition and binding with targets. By applying the oxidation potential of Ri, nanosensor collisions on electrode surface trigger a cascade redox cycling of PDA and Ri through synchronous electron transfer, which boost the amplification of current signal output. The developed nanosensor exhibit excellent linear response toward up-regulated biomarkers (miRNA-21, ATP, and VEGF) with low detection limits (32 fM, 386 pM, and 2.8 pM). Moreover, background influence from physiological interferent is greatly reduced by restricted electron transfer coupling on electrode. The practical applicability is illustrated in sensitive and highly repeatable profiling of miRNA-21 in lysate of tumor cells and tumor tissue, beneficial for more reliable diagnosis. This electrochemical platform by employing electron transfer cascades at heterogeneous interfaces offers a route to anti-interference detection of biomarkers in tumor tissues.

Investigation of the Single-Particle Scale Structure-Activity Relationship Providing New Insights for the Development of High-Performance Batteries

As electric vehicles, portable electronic devices, and tools have increasingly high requirements for battery energy density and power density, constantly improving battery performance is a research focus. Accurate measurement of the structure-activity relationship of active materials is key to advancing the research of high-performance batteries. However, conventional performance tests of active materials are based on the electrochemical measurement of porous composite electrodes containing active materials, polymer binders, and conductive carbon additives, which cannot establish an accurate structure-activity relationship with the physical characterization of microregions. In this review, in order to promote the accurate measurement and understanding of the structure-activity relationship of materials, the electrochemical measurement and physical characterization of energy storage materials at single-particle scale are reviewed. The potential problems and possible improvement schemes of the single particle electrochemical measurement and physical characterization are proposed. Their potential applications in single particle electrochemical simulation and machine learning are prospected. This review aims to promote the further application of single particle electrochemical measurement and physical characterization in energy storage materials, hoping to achieve 3D unified evaluation of physical characterization, electrochemical measurement, and theoretical simulation at the single particle scale to provide new inspiration for the development of high-performance batteries.

Recent Trends and Perspectives in Single-Entity Electrochemistry: A Review with Focus on a Water Splitting Reaction

Electrochemical measurements involving single nanoparticles have attracted considerable research attention. In recent years, various studies have been conducted on single-entity electrochemistry (SEE) for the in-depth analyses of catalytic reactions. Although, several electrocatalysts have been developed for H2 energy production, designing innovative electrocatalysts for this purpose remains a challenging task. Stochastic collision electrochemistry is gaining increased attention because it has led to new findings in the SEE field. Importantly, it facilitates establishing structure activity relationships for electrocatalysts by monitoring transient signals. This article reviews the recent achievements related to hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) using different electrocatalysts at the nanoscale level. In particular, it discusses the electrocatalytic activities of noble metal nanoparticles, including Ag, Au, Pt, and Pd nanoparticles, at the single-particle level. Because heterogeneity is a key factor affecting the catalytic activity of nanostructures, our work focuses on the influence of heterogeneities in catalytic materials on the OER and HER activities. These results may help to achieve a better understanding of the fundamental processes involved in the water splitting reaction.

Collision Oxidation Behavior of Silver Nanoparticles in Alkaline Solution

In recent years, silver nanoparticles (Ag NPs) have been used as positive electrode material for zinc/silver batteries, and the silver oxides formed during the charging process determine the discharge performance of batteries. Therefore, it is important to study the oxidation behavior of Ag NPs in alkaline solution. Single-nanoparticle collision is an important tool for analyzing oxidation behavior of individual nanoparticles. Based on thermodynamic information from collision events, it is known that oxidation products are potential-dependent and size-dependent. Based on dynamic information, including collisional peak shapes and duration time, it was observed that the Ag NP collision oxidation process changed from stepwise oxidation to direct oxidation as the potential increased or size decreased. This work provides guidance for application of Ag NPs in zinc/silver batteries and proposed a strategy for oxidation behavior of individual NP that could be tracked in situ through an all-encompassing view of thermodynamic and dynamic information.

Artificial Intelligence-Assisted Multiparameter Size Discrimination of Silver Nanoparticles through Electrochemical Collision

Single particle collision is an important tool for size analysis at the individual particle level; however, due to complex dynamic behaviors of nanoparticles on the surface of an electrode, the accuracy of size discrimination is limited. A silver (Ag) nanoparticle (NP) was chosen as the research target, and the dynamic behavior of Ag NPs was simplified by enhancing adsorption between Ag NP and Au ultramicroelectrode (UME) in alkaline media. Immediately after, accurate dynamic and thermodynamic information on single Ag NP was accurately extracted from collision events, including current intensity, transferred charge, and duration time. On the basis that there were differences between parameters of different-sized Ag NPs, multiparameter size discrimination was proposed, which improved the accuracy compared to single-parameter discrimination. More intriguingly, multiparameter analysis was combined with artificial intelligence, a tool adept at processing multidimensional data, for the first time. Finally, artificial intelligence-assisted multiparameter size discrimination was successfully used to intelligently distinguish mixed Ag NPs, with an optimal accuracy of more than 95%. To sum up, the artificial intelligence-assisted multiparameter method showed an excellent ability to quickly achieve the most accurate size discrimination of nanoparticles at the level of individual particle and provide an effective guidance for the application of nanoparticles.

A review of innovative electrochemical strategies for bioactive molecule detection and cell imaging: Current advances and challenges

Cellular heterogeneity poses a major challenge for tumor theranostics, requiring high-resolution intercellular bioanalysis strategies. Over the past decades, the advantages of electrochemical analysis, such as high sensitivity, good spatio-temporal resolution, and ease of use, have made it the preferred method to uncover cellular differences. To inspire more creative research, herein, we highlight seminal works in electrochemical techniques for biomolecule analysis and bioimaging. Specifically, micro/nano-electrode-based electrochemical techniques enable real-time quantitative analysis of electroactive substances relevant to life processes in the micro-nanostructure of cells and tissues. Nanopore-based technique plays a vital role in biosensing by utilizing nanoscale pores to achieve high-precision detection and analysis of biomolecules with exceptional sensitivity and single-molecule resolution. Electrochemiluminescence (ECL) technology is utilized for real-time monitoring of the behavior and features of individual cancer cells, enabling observation of their dynamic processes due to its capability of providing high-resolution and highly sensitive bioimaging of cells. Particularly, scanning electrochemical microscopy (SECM) and scanning ion conductance microscopy (SICM) which are widely used in real-time observation of cell surface biological processes and three-dimensional imaging of micro-nano structures, such as metabolic activity, ion channel activity, and cell morphology are introduced in this review. Furthermore, the expansion of the scope of cellular electrochemistry research by innovative functionalized electrodes and electrochemical imaging models and strategies to address future challenges and potential applications is also discussed in this review.

Formation of Molecular Junctions by Single-Entity Collision Electrochemistry

Controlling and understanding the chemistry of molecular junctions is one of the major themes in various fields ranging from chemistry and nanotechnology to biotechnology and biology. Stochastic single-entity collision electrochemistry (SECE) provides powerful tools to study a single entity, such as single cells, single particles, and even single molecules, in a nanoconfined space. Molecular junctions formed by SECE collision show various potential applications in monitoring molecular dynamics with high spatial resolution and high temporal resolution and in feasible combination with hybrid techniques. This Perspective highlights the new breakthroughs, seminal studies, and trends in the area that have been most recently reported. In addition, future challenges for the study of molecular junction dynamics with SECE are discussed.

Nanopore-Enabled Dark-Field Digital Sensing of Nanoparticles

In this study, we use nanopore arrays as a platform for detecting and characterizing individual nanoparticles (NPs) in real time. Dark-field imaging of nanopores with dimensions smaller than the wavelength of light occurs under conditions where trans-illumination is blocked, while the scattered light propagates to the far-field, making it possible to identify nanopores. The intensity of scattering increases dramatically during insertion of AgNPs into empty nanopores, owing to their plasmonic properties. Thus, momentary occupation of a nanopore by a AgNP produces intensity transients that can be analyzed to reveal the following characteristics: (1) NP scattering intensity, which scales with the sixth power of the AgNP radius, shows a normal distribution arising from the heterogeneity in NP size, (2) the nanopore residence time of NPs, which was observed to be stochastic with no permselective effects, and (3) the frequency of AgNP capture events on a 21 × 21 nanopore array, which varies linearly with the concentration of the NPs, agreeing with the frequency calculated from theory. The lower limit of detection (LOD) for NPs was 130 fM, indicating that the measurement can be used in applications in which ultrasensitive detection is required. The results presented here provide valuable insights into the dynamics of NP transport into and out of nanopores and highlight the potential of nanopore arrays as powerful, massively parallel tools for nanoparticle characterization and detection.

Mass Transport and Electron Transfer at the Electrochemical-Confined Interface

Single entity measurements based on the stochastic collision electrochemistry provide a promising and versatile means to study single molecules, single particles, single droplets, etc. Conceptually, mass transport and electron transfer are the two main processes at the electrochemically confined interface that underpin the most transient electrochemical responses resulting from the stochastic and discrete behaviors of single entities at the microscopic scale. This perspective demonstrates how to achieve controllable stochastic collision electrochemistry by effectively altering the two processes. Future challenges and opportunities for stochastic collision electrochemistry are also highlighted.

Three-Mediator Enhanced Collisions on an Ultramicroelectrode for Selective Identification of Single Saccharomyces cerevisiae

Selective detection of colliding entities, especially cells and microbes, is of great challenge in single-entity electrochemistry. Herein, based on the different cellular electron transport pathways between microbes and mediators, we report a three-mediator system [K₃Fe(CN)₆, K₄Fe(CN)₆, and menadione] to achieve redox activity analysis and selective identification of single Saccharomyces cerevisiae without the usage of antibodies. K₄Fe(CN)₆ in the three-mediator system will oxidize near the electrode surface and increase the local concentration of K₃Fe(CN)₆, which will promote the redox reaction of S. cerevisiae. The hydrophobic mediator─menadione─can selectively penetrate through the S. cerevisiae membrane and get access to its intracellular redox center and can further react with K₃Fe(CN)₆ in the bulk solution. In contrast, the mediator can only get access to the bacterial membranes of Escherichia coli and Staphylococcus aureus, which results in different electrochemical collision signals between the above microbes. In the three-mediator system, upward step-like collision signals were observed in S. cerevisiae suspension, which are related to their microbial redox activity. In comparison, E. coli or S. aureus only generated downward current steps because the blockage effect of mediator diffusion suppresses their redox activities. When S. cerevisiae co-existed with E. coli or S. aureus, transients generated by both blockage and redox activity were observed. The approach enables us to trace the collision behaviors of different microbes and distinguish their simultaneous collisions, which is the foundation for further application of electrochemical collision technique in the specific identification of single biological entities.

Learning from the Heterogeneity at Electrochemical Interfaces

Understanding the structure-activity relationship at electrochemical interfaces is crucial in improving the performance of practical electrochemical devices, ranging from fuel cells, electrolyzers, and batteries to electrochemical sensors. However, functional electrochemical interfaces are often complex and contain various surface structures, creating heterogeneity in electrochemical activity. In this Perspective, we highlight the role of heterogeneity in electrochemistry, especially in the context of electrocatalysis. Current methods for revealing the heterogeneity at electrochemical interfaces, including nanoelectrochemistry tools and single-entity approaches, are discussed. Lastly, we provide perspectives on what one can learn by studying heterogeneity and how one can use heterogeneity to design more efficient electrochemical devices.

On-Chip Electrokinetic Micropumping for Nanoparticle Impact Electrochemistry

Single-entity electrochemistry is a powerful technique to study the interactions of nanoparticles at the liquid-solid interface. In this work, we exploit Faradaic (background) processes in electrolytes of moderate ionic strength to evoke electrokinetic transport and study its influence on nanoparticle impacts. We implemented an electrode array comprising a macroscopic electrode that surrounds a set of 62 spatially distributed microelectrodes. This configuration allowed us to alter the global electrokinetic transport characteristics by adjusting the potential at the macroscopic electrode, while we concomitantly recorded silver nanoparticle impacts at the microscopic detection electrodes. By focusing on temporal changes of the impact rates, we were able to reveal alterations in the macroscopic particle transport. Our findings indicate a potential-dependent micropumping effect. The highest impact rates were obtained for strongly negative macroelectrode potentials and alkaline solutions, albeit also positive potentials lead to an increase in particle impacts. We explain this finding by reversal of the pumping direction. Variations in the electrolyte composition were shown to play a critical role as the macroelectrode processes can lead to depletion of ions, which influences both the particle oxidation and the reactions that drive the transport. Our study highlights that controlled on-chip micropumping is possible, yet its optimization is not straightforward. Nevertheless, the utilization of electro- and diffusiokinetic transport phenomena might be an appealing strategy to enhance the performance in future impact-based sensing applications.

Nanoconfined Electrochemical Collision and Catalysis of Single Enzyme inside Carbon Nanopipettes

Revealing the electrocatalytic features of single redox enzyme is significant to both fundamental biological processes and practical catalysis and sensing applications. Herein, we directly reveal the electrocatalytic current from a single enzyme inside the carbon nanopipettes via electrochemical collision strategies, based on the increased activity at nanoscale confinement. Besides the staircase current steps from surface blockage, discrete H₂O₂ oxidation and reduction current transients catalyzed by a single enzyme are also displayed and analyzed. The carbon nanopipette would increase the catalytic activities of enzymes and lead to a detectable current response, thus opening a new way to investigate the fundamental enzymatic mechanisms at the single enzyme level.

Stochastic Collision Electrochemistry from Single Pt Nanoparticles: Electrocatalytic Amplification and MicroRNA Sensing

Single-particle collisions have made many achievements in basic research, but challenges still exist due to their low collision frequency and selectivity in complex samples. In this work, we developed an "on-off-on" strategy based on Pt nanoparticles (PtNPs) that catalyze N₂H₄ collision signals on the surface of carbon ultramicroelectrodes and established a new method for the detection of miRNA21 with high selectivity and sensitivity. PtNPs catalyze the reduction of N₂H₄ on the surface of carbon ultramicroelectrodes to generate a stepped collision signal, which is in the "on" state. The single-stranded DNA paired with miRNA21 is coupled with PtNPs to form the complex DNA/PtNPs. Because PtNPs are covered by DNA, the electrocatalytic collision of N₂H₄ oxidation is inhibited. At this time, the signal is in the "off" state. When miRNA21 is added, the strong complementary pairing between miRNA21 and DNA destroys the electrostatic adsorption of DNA/PtNP conjugates and restores the electrocatalytic performance of PtNPs, and the signal is in the "on" state again. Based on this, a new method for detecting miRNA21 was established. It provides a new way for small-molecule sensing and has a wide range of applications in electroanalysis, electrocatalysis, and biosensing.

Current Lifetime of Single-Nanoparticle Collision for Sizing Nanoparticles

Accurate size analysis of nanoparticles (NPs) is vital for nanotechnology. However, this cannot be realized based on conventional single-nanoparticle collision (SNC) because the current intensity, a thermodynamic parameter of SNC for sizing NPs, is always smaller than the theoretical value due to the effect of NP movements on the electrode surface. Herein, a size-dependent dynamic parameter of SNC, current lifetime, which refers to the time that the current intensity decays to 1/e of the original value, was originally utilized to distinguish differently sized NPs. Results showed that the current lifetime increased with NP size. After taking the current lifetime into account rather than the current intensity, the overlap rates for the peak-type current transients of differently sized Pt NPs (10 and 15 nm) and Au NPs (18 and 35 nm) reduced from 73 and 7% to 45 and 0%, respectively, which were closer to the theoretical values (29 and 0%). Hence, the proposed SNC dynamics-based method holds great potential for developing reliable electrochemical approaches to evaluate NP sizes accurately.

Faradaic Counter for Liposomes Loaded with Potassium, Sodium Ions, or Protonated Dopamine

Collisional electrochemistry between single particles and a biomimetic polarized micro-liquid/liquid interface has emerged as a novel and powerful analytical method for measurements of single particles. Using this platform, rapid detection of liposomes at the single particle level is reported herein. Individual potassium, sodium, or protonated dopamine-encapsulated (pristine or protein-decorated) liposomes collide and fuse with the polarized micro-liquid/liquid interface accompanying the release of ions, which are recorded as spike-like current transients of stochastic nature. The sizing and concentration of the liposomes can be readily estimated by quantifying the amount of encapsulated ions in individual liposomes via integrating each current spike versus time and the spike frequency, respectively. We call this type of nanosensing technology "Faradaic counter". The estimated liposome size distribution by this method is in line with the dynamic light scattering (DLS) measurements, implying that the quantized current spikes are indeed caused by the collisions of individual liposomes. The reported electrochemical sensing technology may become a viable alternative to DLS and other commercial nanoparticle analysis systems, for example, nanoparticle tracking analysis.

Direct Observation of Amide Bond Formation in a Plasmonic Nanocavity Triggered by Single Nanoparticle Collisions

The real-time observation of chemical bond formation at the single-molecule level is one of the great challenges in the fields of organic and biomolecular chemistry. Valuable information can be gleaned that is not accessible using ensemble-average measurements. Although remarkably sophisticated techniques for monitoring chemical reactions have been developed, the ability to detect the specific formation of a chemical bond in situ at the single-molecule level has remained an elusive goal. Amide bonds are routinely formed from the aminolysis of N-hydroxysuccinimide (NHS) esters by primary amines, and the protocol is widely used for the synthesis, cross-linking, and labeling of peptides and proteins. Herein, a plasmonic nanocavity was applied to study aminolysis reaction for amide bond formation, which was initiated by single nanoparticle collision events between suitably functionalized free-moving gold nanoparticles and a gold nanoelectrode in an aqueous buffer. By means of simultaneous surface enhanced Raman spectroscopy (SERS) and single-entity electrochemistry (EC) measurements, we have probed the dynamic evolution of amide bond formation in the aminolysis reaction with 10 s of millisecond time resolution. Hence, we demonstrate that single-entity EC-SERS is a valuable and sensitive technique by which chemical reactions can be studied at the single-molecule level.

Fast electrodeposition of zinc onto single zinc nanoparticles

The zinc deposition reaction onto metallic zinc has been investigated at the single particle level through the electrode-particle collision method in neutral solutions, and in respect of its dependence on the applied potential and the ionic strength of a sulphate-containing solution. Depending on the concentration of sulphate ions in solution, different amounts of metallic zinc were deposited on the single Zn nanoparticles. Specifically, insights into the electron transfer kinetics at the single particles were obtained, indicating an electrically early reactant-like transition state, which is consistent with the rate-determining partial de-hydration/de-complexation process. Such information on the reaction kinetics at the nanoscale is of vital importance for the development of more efficient and long-lasting nanostructured Zn-based negative electrodes for Zn-ion battery applications.

Observing Host-Guest Interactions at Molecular Interfaces by Monitoring the Electrochemical Current

Macrocyclic cucurbit[n]uril (CB[n]) molecules have triggered renewed interest because of their outstanding capabilities as host molecules to selectively interact with a wide range of small guest molecules. Here, CB[7]-based host-guest interactions were investigated for a guest-modified nanoelectrode by monitoring the electrochemical current. A ferrocene (Fc)-terminated molecule immobilized on a gold nanoelectrode (GNE) showed suitable affinity with CB[7] when the effective exposing area of the GNE was between 5.3 and 12 μm² and the bias applied on the GNE was -500 mV. Monitoring the dynamics of nanoparticles (NPs) on a nanoelectrode provides new insights into the host-guest interactions at molecular interfaces.

Amphiphilic ligand modified gold nanocarriers to amplify lanthanide loading for ultrasensitive DELFIA detection ofCronobacter

Thiolated ethylenediaminetetraacetic acid and thiolated acylhydrazine-terminated ligands modified gold nanoflowers as amplified nanocarriers to increase the Ln³⁺labeling ratio for improving the sensitivity of traditional DELFIA.

High spatial resolution electrochemical biosensing using reflected light microscopy

If the analyte does not only change the electrochemical but also the optical properties of the electrode/solution interface, the spatial resolution of an electrochemical sensor can be substantially enhanced by combining the electrochemical sensor with optical microscopy. In order to demonstrate this, electrochemical biosensors for the detection of hydrogen peroxide and glucose were developed by drop casting enzyme and redox polymer mixtures onto planar, optically transparent electrodes. These biosensors generate current signals proportional to the analyte concentration via a reaction sequence which ultimately changes the oxidation state of the redox polymer. Images of the interface of these biosensors were acquired using bright field reflected light microscopy (BFRLM). Analysis showed that the intensity of these images is higher when the redox polymer is oxidized than when it is reduced. It also revealed that the time needed for the redox polymer to change oxidation state can be assayed optically and is dependent on the concentration of the analyte. By combining the biosensor for hydrogen peroxide detection with BFRLM, it was possible to determine hydrogen peroxide in concentrations as low as 12.5 µM with a spatial resolution of 12 µm × 12 µm, without the need for the fabrication of microelectrodes of these dimensions.