Optical Imaging of Phase Transition and Li-Ion Diffusion Kinetics of Single LiCoO(2) Nanoparticles During Electrochemical Cycling

High-Throughput Identification of Single Nanoparticles via Electrochemically Assisted High-Resolution Plasmonic Scattering Interferometric Microscopy

The identification of nanoparticles within heterogeneous mixtures poses significant challenges due to the similarity in physical properties among different nanomaterials. Here, we present electrochemically assisted high-resolution plasmonic scattering interferometric microscopy (HR-PSIM). This technique allows for the high-throughput identification of nanoparticles by accurately measuring the refractive index of individual nanoparticles without interference from background signals. Through elimination of parabolic scattering interference and employing electrochemical modulation, HR-PSIM demonstrates high spatial resolution and stability against background noise, enabling the differentiation of nanoparticles with closely matched refractive indices, such as Au and Ag nanoparticles. The efficacy of this method is demonstrated through its application in real-time, label-free imaging of nanoparticle electrochemical activity, providing a platform for the precise and high-throughput characterization of nanomaterials. The robustness of our approach against electrochemical interference and its high spatial resolution mark a significant advancement in the field of nanomaterial analysis, promising wide-ranging applications in nanoparticle research and beyond.

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.

Direct imaging of dynamic heterogeneous lithium-gold interaction at the electrochemical interface during the charging/discharging processes

Lithium can smoothly plate on certain lithium alloys in theory, such as the Li-Au alloy, making the alloy/metal films promising current collectors for high energy density anode-free batteries. However, the actual performance of the batteries with alloy film electrodes often rapidly deteriorates. It remains challenging for current imaging approaches to provide sufficient details for fully understanding the process. Here, a "see-through" operando optical microscopic approach that allows direct imaging of Li-Au interaction with high spatiotemporal and chemical resolution has been developed. Through this approach, a two-step Li-Au alloying process that exhibits interesting complementary spatiotemporal evolution paths has been discovered. The alloying process regulates the nucleation of further Li deposition, while the Li nucleation sites generate pores on the electrode film. After several cycles, film rupture occurs due to the generation of an increased number of pores, thus explaining the previously unclear mechanism of poor cycling stability. We have also elucidated the deterioration mechanism of silver electrodes: the growth of defect pores in size, independent of the alloying process. Overall, this new imaging approach opens up an effective and simple way to monitor the dynamic heterogeneity of metal-metal interaction at the electrochemical interface, which could provide helpful insight for designing high-performance batteries.

Label-Free Optical Imaging of Nanoscale Single Entities

The advancement of optical microscopy technologies has achieved imaging of nanoscale objects, including nanomaterials, virions, organelles, and biological molecules, at the single entity level. Recently developed plasmonic and scattering based optical microscopy technologies have enabled label-free imaging of single entities with high spatial and temporal resolutions. These label-free methods eliminate the complexity of sample labeling and minimize the perturbation of the analyte native state. Additionally, these imaging-based methods can noninvasively probe the dynamics and functions of single entities with sufficient throughput for heterogeneity analysis. This perspective will review label-free single entity imaging technologies and discuss their principles, applications, and key challenges.

Nanoscale Luminescence Imaging/Detection of Single Particles: State-of-the-Art and Future Prospects

Single-particle-level measurements, during the reaction, avoid averaging effects that are inherent limitations of conventional ensemble strategies. It allows revealing structure-activity relationships beyond averaged properties by considering crucial particle-selective descriptors including structure/morphology dynamics, intrinsic heterogeneity, and dynamic fluctuations in reactivity (kinetics, mechanisms). In recent years, numerous luminescence (optical) techniques such as chemiluminescence (CL), electrochemiluminescence (ECL), and fluorescence (FL) microscopies have been emerging as dominant tools to achieve such measurements, owing to their diversified spectroscopy principles, noninvasive nature, higher sensitivity, and sufficient spatiotemporal resolution. Correspondingly, state-of-the-art methodologies and tools are being used for probing (real-time, operando, in situ) diverse applications of single particles in sensing, medicine, and catalysis. Herein, we provide a concise and comprehensive perspective on luminescence-based detection and imaging of single particles by putting special emphasis on their basic principles, mechanistic pathways, advances, challenges, and key applications. This Perspective focuses on the development of emission intensities and imaging based individual particle detection. Moreover, several key examples in the areas of sensing, motion, catalysis, energy, materials, and emerging trends in related areas are documented. We finally conclude with the opportunities and remaining challenges to stimulate further developments in this field.

Optical Approach for Mapping the Intercalation Capacity of Porous Electrodes

The intercalation capacity of a porous electrode in real batteries is not uniform spatially due to the inevitable structural and compositional inhomogeneity and site-dependent ion and electron transport features. Reliable methods to quantify the capacity distribution are highly desirable but absent so far in battery research. In this paper, a novel optical technique, oblique incident reflection difference (OIRD), was employed to monitor in situ the electrochemical ion (de)intercalation behavior of Prussian blue analogue (PBA) porous films. The OIRD signal responded synchronously to the ion (de)intercalation, and the change in the OIRD signal (ΔI) was positively correlated with the local electrochemical capacity, thereby enabling mapping of the spatially resolved ion storage capacity of the films. Optical analysis further showed that the OIRD response originated from the ion (de)intercalation-induced dielectric constant change of PBA films. This work therefore offers an intriguing in situ and spatially resolved tool for the study of rechargeable batteries.

Tracking Ion Transport in Nanochannels via Transient Single-Particle Imaging

The transport behavior of ions in the nanopores has an important impact on the performance of the electrochemical devices. Although the classical Transmission-Line (TL) model has long been used to describe ion transport in pores, the boundary conditions for the applicability of the TL model remain controversial. Here, we investigated the transport kinetics of different ions, within nanochannels of different lengths, by using transient single-particle imaging with temporal resolution up to microseconds. We found that the ion transport kinetics within short nanochannels may deviate significantly from the TL model. The reason is that the ion transport under nanoconfinement is composed of multi basic stages, and the kinetics differ much under different stage domination. With the shortening of nanochannels, the electrical double layer (EDL) formation would become the "rate-determining step" and dominate the apparent ion kinetics. Our results imply that using the TL model directly and treating the in-pore mobility as an unchanged parameter to estimate the ion transport kinetics in short nanopores/nanochannels may lead to orders of magnitude bias. These findings may advance the understanding of the nanoconfined ion transport and promote the related applications.

Iontronic microscopy of a tungsten microelectrode: "seeing" ionic currents under an optical microscope

Optical methods for monitoring electrochemical reactions at an interface are advantageous because of their table-top setup and ease of integration into reactors. Here we apply EDL-modulation microscopy to one of the main components of amperometric measurement devices: a microelectrode. We present experimental measurements of the EDL-modulation contrast from the tip of a tungsten microelectrode at various electrochemical potentials inside a ferrocene-dimethanol Fe(MeOH)2 solution. Using the combination of the dark-field scattering microscope and the lock-in detection technique, we measure the phase and amplitude of local ion-concentration oscillations in response to an AC potential as the electrode potential is scanned through the redox-activity window of the dissolved species. We present the amplitude and phase map of this response, as such this method can be used to study the spatial and temporal variations of the ion-flux due to an electrochemical reaction close to metallic and semiconducting objects of general geometry. We discuss the advantages and possible extensions of using this microscopy method for wide-field imaging of ionic currents.

Three-dimensional operando optical imaging of particle and electrolyte heterogeneities inside Li-ion batteries

Understanding (de)lithiation heterogeneities in battery materials is key to ensure optimal electrochemical performance. However, this remains challenging due to the three-dimensional morphology of electrode particles, the involvement of both solid- and liquid-phase reactants and a range of relevant timescales (seconds to hours). Here we overcome this problem and demonstrate the use of confocal microscopy for the simultaneous three-dimensional operando measurement of lithium-ion dynamics in individual agglomerate particles, and the electrolyte in batteries. We examine two technologically important cathode materials: LixCoO2 and LixNi0.8Mn0.1Co0.1O2. The surface-to-core transport velocity of Li-phase fronts and volume changes are captured as a function of cycling rate. Additionally, we visualize heterogeneities in the bulk and at agglomerate surfaces during cycling, and image microscopic liquid electrolyte concentration gradients. We discover that surface-limited reactions and intra-agglomerate competing rates control (de)lithiation and structural heterogeneities in agglomerate-based electrodes. Importantly, the conditions under which optical imaging can be performed inside the complex environments of battery electrodes are outlined.

Measuring the Pseudocapacitive Behavior of Individual V2O5 Particles by Scanning Electrochemical Cell Microscopy

V₂O₅ is a promising pseudocapacitive material for electrochemical energy storage with balanced power and energy density. Understanding the charge-storage mechanism is of significance to further improve the rate performance. Here, we report an electrochemical study of individual V₂O₅ particles using scanning electrochemical cell microscopy with colocalized electron microscopy. A carbon sputtering procedure is proposed for the pristine V₂O₅ particles to improve their structure stability and electronic conductivity. The achieved high-quality electrochemical cyclic voltammetry results, structural integrity, and high oxidation to reduction charge ratio (as high as 97.74%) assured further quantitative analysis of the pseudocapacitive behavior of single particles and correlation with local particle structures. A broad range of capacitive contribution is revealed, with an average ratio of 76% at 1.0 V/s. This study provides new opportunities for quantitative analysis of the electrochemical charge-storage process at single particles, especially for electrode materials with electrolyte-induced instability.

Plasmonic Scattering Microscopy for Label-Free Imaging of Molecular Binding Kinetics: From Single Molecules to Single Cells

Measuring molecular binding kinetics represents one of the most important tasks in molecular interaction analysis. Surface plasmon resonance (SPR) is a popular tool for analyzing molecular binding. Plasmonic scattering microscopy (PSM) is a newly developed SPR imaging technology, which detects the out-of-plane scattering of surface plasmons by analytes and has pushed the detection limit of label-free SPR imaging down to a single-protein level. In addition, PSM also allows SPR imaging with high spatiotemporal resolution, making it possible to analyze cellular response to the molecular bindings. In this Mini Review, we present PSM as a method of choice for chemical and biological imaging, introduce its theoretical mechanism, present its experimental schemes, summarize its exciting applications, and discuss its challenges as well as the promising future.

Plasmonic Probing Single-Cell Bio-Current Waves with a Shrinking Magnetite Nanoprobe

Probing of the single-cell level extracellular electron transfer highlights the maximum output current for microbial fuel cells (MFCs) at hundreds of femtoampere per cell, which is difficult to achieve by existing devices. Past studies focus on the external factors for boosting charge-extraction efficiency from bacteria. Here, we elucidate the intracellular factors that determine this output limit by monitoring the respiratory-driven shrinking kinetics of a single magnetite nanoprobe immobilized on a single Shewanella oneidensis MR-1 cell with plasmonic imaging. Quantified dissolving of nanoprobes unveils a previously undescribed bio-current fluctuation between 0 and 2.7 fA on a ∼40 min cycle. Simultaneously tracing of endogenous oscillations indicates that the bio-current waves are correlated with the periodic cellular electrokinesis. The unsynchronized electron transfer capability in the cell population results in the mean current of 0.24 fA per cell, significantly smaller than in single cells. It explains why the averaged output current of MFCs cannot reach the measured single-cell currents. This work offers a different perspective to improve the power output by extending the active episodes of the bio-current waves.

Optical Detection and Imaging of Nonfluorescent Matter at the Single-Molecule/Particle Level

Since the first optical detection of single molecules in 1989, single-molecule spectroscopy has developed rapidly and been widely applied in many areas. However, the vast majority of matter is extremely inefficient at emitting photons in our physical world, which seriously limits the applications of optical methods based on photoluminescence. In addition to indirect detection by fluorescence labeling, many efforts have been made to directly image nonfluorescent matter at the single-particle or single-molecule level in different ways based on the absorption or scattering interaction between light and matter. Herein, we review five popular methods for imaging nonfluorescent particles/molecules, including dark-field microscopy (DFM), surface plasmon resonance microscopy (SPRM), surface enhanced Raman microscopy (SERM), interferometric scattering microscopy (iSCAT), and photothermal microscopy (PTM). After summarizing the principles and applications of these methods, we compare the advantages and disadvantages of each method and describe further potential development and applications.

Spatiotemporal Mapping of Extracellular Electron Transfer Flux in a Microbial Fuel Cell Using an Oblique Incident Reflectivity Difference Technique

Extracellular electron transfer (EET) is a critical process involved in microbial fuel cells. Spatially resolved mapping of EET flux is of essential significance due to the inevitable spatial inhomogeneity over the bacteria/electrode interface. In this work, EET flux of a typical bioanode constructed by inhabiting Shewanella putrefaciens CN32 on a porous polyaniline (PANI) film was successfully mapped using a newly established oblique incident reflectivity difference (OIRD) technique. In the open-circuit state, the PANI film was reduced by the electrons released from the bacteria via the EET process, and the resultant redox state change of PANI was sensitively imaged by OIRD in a real-time and noninvasive manner. Due to the strong correlation between the EET flux and OIRD signal, the OIRD differential image represents spatially resolved EET flux, and the in situ OIRD signal reveals the dynamic behavior during the EET process, thus providing important spatiotemporal information complementary to the bulky electrochemical data.

Emerging Optical Microscopy Techniques for Electrochemistry

An optical microscope is probably the most intuitive, simple, and commonly used instrument to observe objects and discuss behaviors through images. Although the idea of imaging electrochemical processes operando by optical microscopy was initiated 40 years ago, it was not until significant progress was made in the last two decades in advanced optical microscopy or plasmonics that it could become a mainstream electroanalytical strategy. This review illustrates the potential of different optical microscopies to visualize and quantify local electrochemical processes with unprecedented temporal and spatial resolution (below the diffraction limit), up to the single object level with subnanoparticle or single-molecule sensitivity. Developed through optically and electrochemically active model systems, optical microscopy is now shifting to materials and configurations focused on real-world electrochemical applications.

Determining the depth of surface charging layer of single Prussian blue nanoparticles with pseudocapacitive behaviors

Understanding the hybrid charge-storage mechanisms of pseudocapacitive nanomaterials holds promising keys to further improve the performance of energy storage devices. Based on the dependence of the light scattering intensity of single Prussian blue nanoparticles (PBNPs) on their oxidation state during sinusoidal potential modulation at varying frequencies, we present an electro-optical microscopic imaging approach to optically acquire the Faradaic electrochemical impedance spectroscopy (oEIS) of single PBNPs. Here we reveal typical pseudocapacitive behavior with hybrid charge-storage mechanisms depending on the modulation frequency. In the low-frequency range, the optical amplitude is inversely proportional to the square root of the frequency (∆I ∝ f^−0.5; diffusion-limited process), while in the high-frequency range, it is inversely proportional to the frequency (∆I ∝ f⁻¹; surface charging process). Because the geometry of single cuboid-shaped PBNPs can be precisely determined by scanning electron microscopy and atomic force microscopy, oEIS of single PBNPs allows the determination of the depth of the surface charging layer, revealing it to be ~2 unit cells regardless of the nanoparticle size.

The surface charging layer in nanomaterials, which determines their pseudocapacitive behavior, is challenging to characterize. Here the authors perform Faradic electrochemical impedance spectroscopy measurements of single cuboid Prussian blue nanoparticles, displaying a hybrid charge storage mechanism, and determine the depth of the surface charging layer.

Formation of oxygen vacancies in Li2FeSiO4: first-principles calculations

The formation of oxygen vacancies could affect various properties of oxides. Herein we have investigated the formation energies of an oxygen vacancy (V_O) with the relevant charge states in bulk Pnma-Li₂FeSiO₄ using first-principles calculations. The formation energies of the V_O are essentially dependent on the atomic chemical potentials that represent the experimental conditions. The calculated formation energies of an oxygen vacancy in different charge states indicate that it would be energetically favorable to fully ionize the oxygen vacancy in Li₂FeSiO₄. The presence of V_O is accompanied by a distinct redistribution of the electronic charge densities only around the Fe and Si ions next to the O-vacancy site, which shows a very local influence on the host material arising from V_O. This local characteristic is also confirmed by the calculated partial densities of states (PDOS). We also studied the influence of substitutional (Mn_Fe and Co_Fe) and cation vacancy defects (i.e., V_Fe and V_Li) in the vicinity of an O-vacancy on the formation of an O-vacancy, respectively. We find that the calculated interaction energies between these defects and the oxygen vacancy are all negative, which implies that the formation of an oxygen vacancy becomes easier when the above defects are introduced. Compared to the substitutional defects, the interaction energies between the vacancy defects and the oxygen vacancy are significantly larger. Among them, the interaction energy between V_Fe and V_O is the largest.

A microwell array-based approach for studying single nanoparticle catalysis with high turnover frequency

Measuring the catalytical activities of single catalysts in the case of high turnover frequency (TOF, realistic conditions) is highly desirable to accurately evaluate the functional heterogeneities among individuals and to understand the catalytic mechanism. Herein, we report a microwell array-based method to in operando measure the photocatalytic kinetics of single CdS nanoparticles (NPs) with high TOF. This was realized by sealing individual CdS NPs into separated micrometer-sized polydimethylsiloxane wells, thus eliminating the diffusion of products among individuals in the case of high concentration of reactants. This method allowed us to monitor the activities of single catalysts with an average TOF up to 2.1 × 10⁵ s^-1. Interestingly, two types of catalytical behaviors were revealed during single CdS photocatalysis: a rapid decline in activity for most CdS NPs and an initial increase in activity followed by a decrease for a minor population of individuals. The developed method will facilitate the investigation of catalytic activities of single particles under realistic conditions and hold great potential in the fields of photo/electro-catalysts, enzymes, functional bacteria, and so on.

Electrochemical Surface Plasmon Resonance Spectroscopy for Investigation of the Initial Process of Lithium Metal Deposition

The initial process of Li-metal electrodeposition on the negative electrode surface determines the charging performance of Li-metal secondary batteries. However, minute depositions or the early processes of nucleation and growth of Li metal are generally difficult to detect under operando conditions. In this study, we propose an optical diagnostic approach to address these challenges. Surface plasmon resonance (SPR) spectroscopy coupled with electrochemical operation is a promising technique that enables the ultrasensitive detection of the initial stage of Li-metal electrodeposition. The SPR is excited in a thin copper film deposited on a glass substrate, which also serves as a current collector enabling electrochemical Li-metal deposition. For a propylene carbonate (PC)-based Li-ion battery electrolyte, under both cyclic voltammetry and constant-current operation, Li-metal deposition is readily detected by changes in the SPR absorption dip in the reflectance spectrum. Electrochemical SPR is highly sensitive to metal deposition, with a demonstrated capability of detecting an average thickness of approximately 0.1 nm, corresponding to a few atomic layers of Li. To identify the growth mechanism, the SPR reflectance spectra of various possible Li-metal deposition processes were simulated. Comparison of the simulated spectra with the experimental data found good agreement with the well-known nucleation and growth model for Li-metal deposition from PC-based electrolytes. The demonstrated operando electrochemical SPR measurement should be a valuable tool for basic research on the initial Li-metal deposition process.

Tracking the optical mass centroid of single electroactive nanoparticles reveals the electrochemically inactive zone

The inevitable microstructural defects, including cracks, grain boundaries and cavities, make a portion of the material inaccessible to electrons and ions, becoming the incentives for electrochemically inactive zones in single entity. Herein, we introduced dark field microscopy to study the variation of scattering spectrum and optical mass centroid (OMC) of single Prussian blue nanoparticles during electrochemical reaction. The "dark zone" embedded in a single electroactive nanoparticle resulted in the incomplete reaction, and consequently led to the misalignment of OMC for different electrochemical intermediate states. We further revealed the dark zones such as lattice defects in the same entity, which were externally manifested as the fixed pathway for OMC for the migration of potassium ions. This method opens up enormous potentiality to optically access the heterogeneous intraparticle dark zones, with implications for evaluating the crystallinity and electrochemical recyclability of single electroactive nano-objects.

Local Substrate Heterogeneity Influences Electrochemical Activity of TEM Grid-Supported Battery Particles

Understanding how particle size and morphology influence ion insertion dynamics is critical for a wide range of electrochemical applications including energy storage and electrochromic smart windows. One strategy to reveal such structure–property relationships is to perform ex situ transmission electron microscopy (TEM) of nanoparticles that have been cycled on TEM grid electrodes. One drawback of this approach is that images of some particles are correlated with the electrochemical response of the entire TEM grid electrode. The lack of one-to-one electrochemical-to-structural information complicates interpretation of genuine structure/property relationships. Developing high-throughput ex situ single particle-level analytical techniques that effectively link electrochemical behavior with structural properties could accelerate the discovery of critical structure-property relationships. Here, using Li-ion insertion in WO₃ nanorods as a model system, we demonstrate a correlated optically-detected electrochemistry and TEM technique that measures electrochemical behavior of via many particles simultaneously without having to make electrical contacts to single particles on the TEM grid. This correlated optical-TEM approach can link particle structure with electrochemical behavior at the single particle-level. Our measurements revealed significant electrochemical activity heterogeneity among particles. Single particle activity correlated with distinct local mechanical or electrical properties of the amorphous carbon film of the TEM grid, leading to active and inactive particles. The results are significant for correlated electrochemical/TEM imaging studies that aim to reveal structure-property relationships using single particle-level imaging and ensemble-level electrochemistry.

Single-Nanoparticle Coulometry Method with High Sensitivity and High Throughput to Study the Electrochemical Activity and Oscillation of Single Nanocatalysts

To explore nanocatalysts with high electro-catalytic performance and less loading of precious metals, efforts have been made to develop electrochemical methods with high spatial resolution at the single nanoparticle level. Herein, a highly sensitive single-nanoparticle coulometry method is successfully developed to study the electrochemical activity and oscillation of single PtTe nanocatalysts. Based on microbattery reactions involving the formic acid electro-oxidation and the deposition of Ag on the single PtTe nanocatalyst surface, this method enables the transition from the undetectable sub-fA electric signal of the formic acid electro-oxidation into strong localized surface plasmon resonance scattering signal of Ag detected by dark-field microscopy. The lowest limiting current for a single nanocatalyst is found to be as low as 25.8 aA. Different trends of activity versus the formic acid concentration and types of activity of the single nanocatalyst have been discovered. Unveiled frequency-amplitude graph shows that the two electrochemical oscillation modes of low frequency with high amplitude and vice versa coexist in a single PtTe nanocatalyst, indicating the abundantly smooth surfaces and defects of nanocatalysts. This conducted study will open up the new avenue for further behavioral and mechanistic investigation of more types of nanocatalysts in the electrochemistry community.

Electrogenerated chemiluminescence detection of single entities

Electrogenerated chemiluminescence, also known as electrochemiluminescence (ECL), is an electrochemically induced production of light by excited luminophores generated during redox reactions. It can be used to sense the charge transfer and related processes at electrodes via a simple visual readout; hence, ECL is an outstanding tool in analytical sensing. The traditional ECL approach measures averaged electrochemical quantities of a large ensemble of individual entities, including molecules, microstructures and ions. However, as a real system is usually heterogeneous, the study of single entities holds great potential in elucidating new truths of nature which are averaged out in ensemble assays or hidden in complex systems. We would like to review the development of ECL intensity and imaging based single entity detection and place emphasis on the assays of small entities including single molecules, micro/nanoparticles and cells. The current challenges for and perspectives on ECL detection of single entities are also discussed.

Vertical Diffusion of Ions within Single Particles during Electrochemical Charging

Determining the trajectory of ionic transport and diffusion within single electroactive nanomaterials is critical for understanding the charging kinetics and capacity fading associated with ion batteries, with implications for rational design of excellent-performance electrode materials. While the horizontal pathway of mass transport has been feasibly investigated by optical superlocalization methods and electron microscopes, determination on the vertical trajectory has proven a more challenging task. Herein, we developed dual-angle total internal reflection microscopy by simultaneously introducing different angle-dependent illumination depths to trace the optical centroid shifts of nano-objects in the vertical dimension. We first demonstrated the proof of concept by resolving the vertical moving trails of a nanosphere doing Brownian motion and subsequently explored the picture of mass transport in the interior of single Prussian blue (PB) particles during electrochemical cycling. The results indicated that the vertical centroids of single PB particles remained unchanged when ions were inserted or extracted, suggesting an outside-in ionic transport pathway instead of bottom-up trajectory that one would intuitively expect.

Progress in electrochemiluminescence of nanoclusters: how to improve the quantum yield of nanoclusters

Classification of nanoclusters and methods to improve their quantum yield and applications.

Moving Electrons Purposefully through Single Molecules and Nanostructures: A Tribute to the Science of Professor Nongjian Tao (1963-2020)

Electrochemistry intersected nanoscience 25 years ago when it became possible to control the flow of electrons through single molecules and nanostructures. Many surprises and a wealth of understanding were generated by these experiments. Professor Nongjian Tao was among the pioneering scientists who created the methods and technologies for advancing this new frontier. Achieving a deeper understanding of charge transport in molecules and low-dimensional materials was the first priority of his experiments, but he also succeeded in discovering applications in chemical sensing and biosensing for these novel nanoscopic systems. In parallel with this work, the investigation of a range of phenomena using novel optical microscopic methods was a passion of his and his students. This article is a review and an appreciation of some of his many contributions with a view to the future.

Sizing Single Particles at the Orifice of a Nanopipette

Developing new methods and techniques for the size analysis of particles in a solution is highly desirable not only for the industrial screening of particles but also for single biological entity analysis (e.g., single cells or single vesicles). Herein, we report a new technique for sizing single particles in a solution with a nanopipette. The rationale is essentially based on ion-current blockage when the particles approach the proximity of a nanopipette orifice. By rationally controlling the geometry of the nanopipette and the applied potential, the spike-type ion current transient generated from the motion of particles in the process of "collision and departure" is employed for sizing single particles. The results show that both the relative ion-current change (ΔI/I₀) and the dwell time (Δt) of spike-type transient are dependent on particle size. Differently, Δt is also related to an externally applied voltage. Statistical analysis shows that ΔI/I₀ is proportional to the particle diameter, and this linear relationship is further understood by finite-element simulations. This study not only provides a new principle for sizing single particles in a solution but also is helpful to understand the motion of a particle near the orifice of the nanopipette.

Accessing the Electrochemical Activity of Single Nanoparticles by Eliminating the Heterogeneous Electrical Contacts

While single nanoparticle electrochemistry holds great promise for establishing the structure-activity relationship (SAR) of electroactive nanomaterials, as it removes the heterogeneity among individuals, successful SAR studies remain rare. When one nanoparticle is seen to exhibit better performance than the others, it is often simply attributed to better activity of the particular individual. By taking the ion insertion reaction of Prussian blue nanoparticles as an example, here we show that the electrical contact between nanoparticles and electrode, a previously overlooked factor, was greatly distinct from one nanoparticle to another and significantly contributed to the apparent heterogeneity in the reactivity and cyclability. An individual nanoparticle with intrinsically perfect structure (size, facet, crystallinity, and so on) could be completely inactive, simply due to poor electrical contacts, which blurred the SAR and likely caused failures. We further proposed a sputter-coating method to enhance the electrical contacts by depositing an ultrathin platinum layer onto the sample. Such an approach was routinely adopted in scanning electron microscopy to improve the electron mobility between nanoparticles and substrate. Elimination of heterogeneous contacts ensured that the electrochemical activity of single nanoparticles can be accessed and further correlated with their structural features, thus paving the way for single nanoparticle electrochemistry to deliver on its promises in SAR.

Nanoscale optical imaging in chemistry

Single-molecule-level measurements are bringing about a revolution in our understanding of chemical and biochemical processes. Conventional measurements are performed on large ensembles of molecules. Such ensemble-averaged measurements mask molecular-level dynamics and static and dynamic fluctuations in reactivity, which are vital to a holistic understanding of chemical reactions. Watching reactions on the single-molecule level provides access to this otherwise hidden information. Sub-diffraction-limited spatial resolution fluorescence imaging methods, which have been successful in the field of biophysics, have been applied to study chemical processes on single-nanoparticle and single-molecule levels, bringing us new mechanistic insights into physiochemical processes. However, the scope of chemical processes that can be studied using fluorescence imaging is considerably limited; the chemical reaction has to be designed such that it involves fluorophores or fluorogenic probes. In this article, we review optical imaging modalities alternative to fluorescence imaging, which expand greatly the range of chemical processes that can be probed with nanoscale or even single-molecule resolution. First, we show that the luminosity, wavelength, and intermittency of solid-state photoluminescence (PL) can be used to probe chemical transformations on the single-nanoparticle-level. Next, we highlight case studies where localized surface plasmon resonance (LSPR) scattering is used for tracking solid-state, interfacial, and near-field-driven chemical reactions occurring in individual nanoscale locations. Third, we explore the utility of surface- and tip-enhanced Raman scattering to monitor individual bond-dissociation and bond-formation events occurring locally in chemical reactions on surfaces. Each example has yielded some new understanding about molecular mechanisms or location-to-location heterogeneity in chemical activity. The review finishes with new and complementary tools that are expected to further enhance the scope of knowledge attainable through nanometer-scale resolution chemical imaging.

Revealing the sub-50 ms electrochemical conversion of silver halide nanocolloids by stochastic electrochemistry and optical microscopy

Silver based ionic crystal nanoparticles (NPs) are interesting nanomaterials for energy storage and conversion, e.g. their colloidal solutions could be used as a reversible redox nanofluid in semi-solid redox flow cells. In this context, the reductive transformation of Brownian silver halide, AgX, NPs into silver NPs is probed by single NP electrochemistry, complemented by operando high resolution monitoring. However, their light sensitivity and poor conductivity make the operando monitoring of their chemical activity challenging. The electrochemical collisions of single AgX NPs onto a negatively biased electrode evidence a full conversion through multiple reduction steps within 3-10 ms. This is further corroborated by simulation of the conversion process and operando through a high resolution optical microscopy technique (Backside Absorbing Layer Microscopy, BALM). Both techniques are interesting strategies to infer at the single NP level the intrinsic charge capacity and charging rate of redox active Brownian nanomaterials, demonstrating the interest of the fast and reversible AgX/Ag system as a redox nanofluid.

Visualization of Electrochemical Cycling-Induced Dimension Change in LiMn2O4 Nanoparticles by High-Speed Atomic Force Microscopy

Exploring dynamic dimension change and lithium-ion diffusion kinetics of active nanoparticles is important to further improve the qualities of lithium-ion batteries (LIBs), such as the cycle life and charge rate. For advancing such research, an imaging technique that is capable of operating in an electrochemical environment with high spatial and temporal resolutions is really needed. In this work, we successfully developed electrochemical high-speed atomic force microscopy (EC-HS-AFM), which enabled nanoscale imaging at the rate of ∼1 frame/s during electrochemical cycling. The dimensional evolutions of LiMn₂O₄ single nanoparticles accompanying an insertion/extraction reaction of lithium ions were visualized. The surface area-potential hysteresis loops of the single nanoparticles at different sweep rates were quantitatively extracted from the successive HS-AFM images or video. The first-order derivative of the hysteresis loop was interestingly similar to the cyclic voltammetry (CV). Moreover, the EC-HS-AFM experiments confirmed that the utilization of the nanoparticles in the cathode can indeed improve the rate performance of the LIBs. These results demonstrated that EC-HS-AFM would be a promising tool to study dimensional evolutions and lithium-ion diffusion kinetics at a nanoscale.

In Situ Optical Monitoring of the Electrochemical Conversion of Dielectric Nanoparticles: From Multistep Charge Injection to Nanoparticle Motion

By shortening solid-state diffusion times, the nanoscale size reduction of dielectric materials-such as ionic crystals-has fueled synthetic efforts toward their use as nanoparticles, NPs, in electrochemical storage and conversion cells. Meanwhile, there is a lack of strategies able to image the dynamics of such conversion, operando and at the single NP level. It is achieved here by optical microscopy for a model dielectric ionic nanocrystal, a silver halide NP. Rather than the classical core-shrinking mechanism often used to rationalize the complete electrochemical conversion and charge storage in NPs, an alternative mechanism is proposed here. Owing to its poor conductivity, the NP conversion proceeds to completion through the formation of multiple inclusions. The superlocalization of NP during such heterogeneous multiple-step conversion suggests the local release of ions, which propels the NP toward reacting sites enabling its full conversion.

Spatially resolved electrochemistry enabled by thin-film optical interference

Electrochemical reactions occurring on the local surface can be spatially resolved by successive interferometric imaging of the nanochannel membrane coated electrode.

Optical imaging of the potential distribution at transparent electrode/solution interfaces

Optical monitoring of the electrode potential and imaging of its distribution on transparent electrodes are achieved by using OIRD technology.

High-Throughput Single-Nanoparticle-Level Imaging of Electrochemical Ion Insertion Reactions

Nanoparticle electrodes are attractive for electrochemical energy storage applications because their nanoscale dimensions decrease ion transport distances and generally increase ion insertion/extraction efficiency. However, nanoparticles vary in size, shape, defect density, and surface composition, which warrants their investigation at the single-nanoparticle level. Here we demonstrate a nondestructive high-throughput electro-optical imaging approach to quantitatively measure electrochemical ion insertion reactions at the single-nanoparticle level. Electro-optical measurements relate the optical density change of a nanoparticle to redox changes of elements in the particle under working electrochemical conditions. We benchmarked this technique by studying Li-ion insertion in hexagonal tungsten oxide (h-WO₃) nanorods during chronoamperometry and cyclic voltammetry. Interestingly, the optically detected current response revealed underlying processes that are hidden in the conventional electrochemical current measurements. This imaging technique may be applied to 13 nm particles and a wide range of electrochemical systems such as electrochromic smart windows, batteries, solid oxide fuel cells, and sensors.

Toward Real-Time Monitoring and Control of Single Nanoparticle Properties with a Microbubble Resonator Spectrometer

Optical microresonators have widespread application at the frontiers of nanophotonic technology, driven by their ability to confine light to the nanoscale and enhance light-matter interactions. Microresonators form the heart of a recently developed method for single-particle photothermal absorption spectroscopy, whereby the microresonators act as microscale thermometers to detect the heat dissipated by optically pumped, nonluminescent nanoscopic targets. However, translation of this technology to chemically dynamic systems requires a platform that is mechanically stable, solution compatible, and visibly transparent. We report microbubble absorption spectrometers as a versatile platform that meets these requirements. Microbubbles integrate a two-port microfluidic device within a whispering gallery mode microresonator, allowing for the facile exchange of chemical reagents within the resonator's interior while maintaining a solution-free environment on its exterior. We first leverage these qualities to investigate the photoactivated etching of single gold nanorods by ferric chloride, providing a method for rapid acquisition of spatial and morphological information about nanoparticles as they undergo chemical reactions. We then demonstrate the ability to control nanorod orientation within a microbubble through optically exerted torque, a promising route toward the construction of hybrid photonic-plasmonic systems. Critically, the reported platform advances microresonator spectrometer technology by permitting room-temperature, aqueous experimental conditions, which may be used for time-resolved single-particle experiments on non-emissive, nanoscale analytes engaged in catalytically and biologically relevant chemical dynamics.

In situ mapping of activity distribution and oxygen evolution reaction in vanadium flow batteries

Understanding spatial distribution difference and reaction kinetics of the electrode is vital for enhancing the electrochemical reaction efficiency. Here, we report a total internal reflection imaging sensor without background current interference to map local current distribution of the electrode in a vanadium redox flow battery during cyclic voltammetry (CV), enabling mapping of the activity and reversibility distribution with the spatial resolution of a single fiber. Three graphite felts with different activity are compared to verify its feasibility. In long-term cyclic voltammetry, the oxygen evolution reaction is proved to enhance activity distribution, and homogeneity of the electrode and its bubble kinetics with periodic fluctuation is consistent with the cyclic voltammetry curve, enabling the onset oxygen evolution/reduction potential determination. Higher activity and irreversibility distribution of the electrode is found in favor of the oxygen evolution reaction. This sensor has potential to detect in situ, among other processes, electrochemical reactions in flow batteries, water splitting, electrocatalysis and electrochemical corrosion.

Postrecording Pixel-Reconstruction Approach for Correcting the Lateral Drifts in Surface Plasmon Resonance Microscope

Surface plasmon resonance microscope (SPRM) sample stage inevitably suffers from lateral drifts as a result of many environmental factors including thermal fluctuation, mechanical vibration, and relaxation. It places great obstacles to time-lapsed imaging and measurements that need high spatial resolution or long recording time. Existing solutions often require experimental efforts such as the addition of optical markers together with piezoelectric stage-based active feedback configurations. Herein, we propose an all-digital, postrecording image-processing method to remove the lateral drift in a series of time-lapsed SPRM images. The method first calculates the value of lateral drift at subpixel accuracy by combining image cross-correlation analysis and superlocalization strategy. It subsequently reconstructed the drift-free image sequences in a pixel-by-pixel and frame-by-frame manner, according to the linear decomposition and reconstruction principle. This method purely relies on image processing, and it does not require any experimental efforts or hardware. In addition to SPRM, we further demonstrated the applicability of the present method in other types of optical imaging techniques including bright-field transmission microscope and dark-field scattering microscope.

Determining the Subnanometer Thickness of the Water-Depletion Layer at the Interface between Water and the Hydrophobic Substrate

Surface plasmon resonance (SPR) is one of the most popular and powerful techniques for label-free detecting and quantitatively analyzing the interfacial refractive index (RI). So far, most of the SPR measurements are mainly applied to detect the relative change of RI upon biological and chemical events occurring at the interface, while the determinations on the absolute value of RI remains challenging. However, the absolute value of RI has become increasingly urgent in some cases, such as the existence and physical properties of the water depletion layer (WDL). WDL refers to a subnanometer-thick layer with reduced density between water and the hydrophobic substrate. The detailed explanations of how water meets hydrophobic surface have been studied by several kinds of techniques for decades but it remains under debate. In this work, we successfully established a method to measure the absolute RI at a gold-liquid interface by surface plasmon resonance microscopy (SPRM) and 2D Fourier transformation image processing and further applied this method to study the existence and physical nature of WDL. It was found that a 0.6 nm thick WDL existed at the interface of water and the hydrophobic substrate, leading to a reduced refractive index of 1.3295 ± 0.0006 compared with the standard value of 1.3325. Our results further indicated that the WDL consisted of a uniform layer rather than numerous isolated surface nanobubbles that distributed at the interface with high density.

In Situ Observation of the Insulator-To-Metal Transition and Nonequilibrium Phase Transition for Li1-xCoO2 Films with Preferred (003) Orientation Nanorods

It is notoriously difficult to distinguish the stoichiometric LiCoO₂ (LCO) with a O3-I structure from its lithium defective O3-II phase because of their similar crystal symmetry. Interestingly, moreover, the O3-II phase shows metallic conductivity, whereas the O3-I phase is an electronic insulator. How to effectively reveal the intrinsic mechanism of the conductivity difference and nonequilibrium phase transition induced by the lithium deintercalation is of vital importance for its practical application and development. Based on the developed technology of in situ peak force tunneling atomic force microscopy (PF-TUNA) in liquids, the phase transition from O3-I to O3-II and consequent insulator-to-metal transition of LCO thin-film electrodes with preferred (003) orientation nanorods designed and prepared via magnetron sputtering were observed under an organic electrolyte for the first time in this work. Then, studying the post-mortem LCO thin-film electrode by using ex situ time-dependent XRD and conductive atomic force microscopy, we find the phase relaxation of LCO electrodes after the nonequilibrium deintercalation, further proving the differences of the electronic conductivity and work function between the O3-I and O3-II phases. Moreover, X-ray absorption spectroscopy results indicate that the oxidation of Co ions and the increasing of O 2p-Co 3d hybridization in the O3-II phase lead to electrical conductivity improvement in Li_1-xCoO₂. Simultaneously, it is found that the nonequilibrium deintercalation at a high charging rate can result in phase-transition hysteresis and the O3-I/O3-II coexistence at the charging end, which is explained well by an ionic blockade model with an antiphase boundary. At last, this work strongly suggests that PF-TUNA can be used to reveal the unconventional phenomena on the solid/liquid interfaces.