Single-Molecule Catalysis Mapping Quantifies Site-Specific Activity and Uncovers Radial Activity Gradient on Single 2D Nanocrystals

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.

Unexpectedly high thermal stability of Au nanotriangle@mSiO2 yolk-shell nanoparticles

The shape of Au nanoparticles (NPs) plays a crucial role for applications in, amongst others, catalysis, electronic devices, biomedicine, and sensing. Typically, the deformation of the morphology of Au NPs is the most significant cause of loss of functionality. Here, we systematically investigate the thermal stability of Au nanotriangles (NTs) coated with (mesoporous) silica shells with different morphologies (core-shell (CS): Au NT@mSiO2/yolk-shell (YS): Au NT@mSiO2) and compare these to 'bare' nanoparticles (Au NTs), by a combination of in situ and/or ex situ TEM techniques and spectroscopy methods. Au NTs with a mesoporous silica (mSiO2) coating were found to show much higher thermal stability than those without a mSiO2 coating, as the mSiO2 shell restricts the (self-)diffusion of surface atoms. For the Au NT@mSiO2 CS and YS NPs, a thicker mSiO2 shell provides better protection than uncoated Au NTs. Surprisingly, the Au NT@mSiO2 YS NPs were found to be as stable as Au NT@mSiO2 CS NPs with a core-shell morphology. We hypothesize that the only explanation for this unexpected finding was the thicker and higher density SiO2 shell of YS NPs that prevents diffusion of Au surface atoms to more thermodynamically favorable positions.

One-Shot Dual-Detection-Based Single-Molecule Super-Resolution Imaging Method for Real-Time Observation of Spatiotemporal Catalytic Activity Variations on the Plasmonic Gold Nanoparticle Surface

Understanding the relationship between the surface properties of a single plasmonic nanoparticle and its catalytic performance is critical for developing highly efficient nanocatalysts. In this study, a one-shot dual-detection-based single-molecule super-resolution imaging method in the evanescent field was developed to observe real-time spatiotemporal catalytic activity on a single plasmonic gold nanoparticle (AuNP) surface. The scattering intensity of AuNPs and the fluorescence of resorufin molecules produced on the AuNP surface were obtained simultaneously to investigate the relationship between nanoparticles and catalytic reactions at a single-molecule level. Chemisorbed adsorbates (i.e., catalytic product and resorufin) changed the electron density of individual AuNPs throughout the catalytic cycle, resulting in the fluctuation of the scattering intensity of individual AuNPs, which was attributed to the electron transfer between reactant resazurin molecules and AuNPs. The increase in the electron density of individual AuNPs affected the catalytic reaction rate. Furthermore, sequential mapping of individual catalytic events at the subdiffraction limit resolution was completed for real-time surface dynamics and spatiotemporal activity variations on the single AuNP surface. The developed method can aid in understanding surface-property-dependent catalytic kinetics and facilitate the development of nanoparticle-based heterogeneous catalysts at subdiffraction limit resolution.

Spatial Distributions of Single-Molecule Reactivity in Plasmonic Catalysis

Plasmonic catalysts have the potential to accelerate and control chemical reactions with light by exploiting localized surface plasmon resonances. However, the mechanisms governing plasmonic catalysis are not simple to decouple. Several plasmon-derived phenomena, such as electromagnetic field enhancements, temperature, or the generation of charge carriers, can affect the reactivity of the system. These effects are convoluted with the inherent (nonplasmonic) catalytic properties of the metal surface. Disentangling these coexisting effects is challenging but is the key to rationally controlling reaction pathways and enhancing reaction rates. This study utilizes super-resolution fluorescence microscopy to examine the mechanisms of plasmonic catalysis at the single-particle level. The reduction reaction of resazurin to resorufin in the presence of Au nanorods coated with a porous silica shell is investigated in situ. This allows the determination of reaction rates with a single-molecule sensitivity and subparticle resolution. By variation of the irradiation wavelength, it is possible to examine two different regimes: photoexcitation of the reactant molecules and photoexcitation of the nanoparticle's plasmon resonance. In addition, the measured spatial distribution of reactivity allows differentiation between superficial and far-field effects. Our results indicate that the reduction of resazurin can occur through more than one reaction pathway, being most efficient when the reactant is photoexcited and is in contact with the Au surface. In addition, it was found that the spatial distribution of enhancements varies, depending on the underlying mechanism. These findings contribute to the fundamental understanding of plasmonic catalysis and the rational design of future plasmonic nanocatalysts.

Getting the Most Out of Fluorogenic Probes: Challenges and Opportunities in Using Single-Molecule Fluorescence to Image Electro- and Photocatalysis

Single-molecule fluorescence microscopy enables the direct observation of individual reaction events at the surface of a catalyst. It has become a powerful tool to image in real time both intra- and interparticle heterogeneity among different nanoscale catalyst particles. Single-molecule fluorescence microscopy of heterogeneous catalysts relies on the detection of chemically activated fluorogenic probes that are converted from a nonfluorescent state into a highly fluorescent state through a reaction mediated at the catalyst surface. This review article describes challenges and opportunities in using such fluorogenic probes as proxies to develop structure-activity relationships in nanoscale electrocatalysts and photocatalysts. We compare single-molecule fluorescence microscopy to other microscopies for imaging catalysis in situ to highlight the distinct advantages and limitations of this technique. We describe correlative imaging between super-resolution activity maps obtained from multiple fluorogenic probes to understand the chemical origins behind spatial variations in activity that are frequently observed for nanoscale catalysts. Fluorogenic probes, originally developed for biological imaging, are introduced that can detect products such as carbon monoxide, nitrite, and ammonia, which are generated by electro- and photocatalysts for fuel production and environmental remediation. We conclude by describing how single-molecule imaging can provide mechanistic insights for a broader scope of catalytic systems, such as single-atom catalysts.

How Heterogeneity Affects Cooperative Communications within Single Nanocatalysts

Catalysis remains one of the most essential methods in chemical research and industry. Recent experiments have discovered an unusual phenomenon of catalytic cooperativity, when a reaction at one active site can stimulate reactions at neighboring sites within single nanoparticles. While theoretical analysis established that the transport of charged holes is responsible for this phenomenon, it does not account for inhomogeneity in the structural and dynamic properties of single nanocatalysts. Here, we investigate the effect of heterogeneity on catalytic communications by extending a discrete-state stochastic framework to random distributions of the transition rates. Our explicit calculations of spatial and temporal properties of heterogeneous systems in comparison with homogeneous systems predict that the strength of cooperativity increases, while the communication lifetimes and distances decrease. Monte Carlo computer simulations support theoretical calculations, and microscopic arguments to explain these observations are also presented. Our theoretical analysis clarifies some important aspects of molecular mechanisms of catalytic processes.

Plasmonic Cavity-Catalysis by Standing Hot Carrier Waves

Manipulating active sites of catalysts is crucial but challenging in catalysis science and engineering. Beyond the design of the composition and structure of catalysts, the confined electromagnetic field in optical cavities has recently become a promising method for catalyzing chemical reactions via strong light-matter interactions. Another form of confined electromagnetic field, the charge density wave in plasmonic cavities, however, still needs to be explored for catalysis. Here, we present an unprecedented catalytic mode based on plasmonic cavities, called plasmonic cavity-catalysis. We achieve direct control of catalytic sites in plasmonic cavities through standing hot carrier waves. Periodic catalytic hotspots are formed because of localized energy and carrier distribution and can be well tuned by cavity geometry, charge density, and excitation angle. We also found that the catalytic activity of the cavity mode increases several orders of magnitude compared with conventional plasmonic catalysis. We ultimately demonstrate that the locally concentrated long-lived hot carriers in the standing wave mode underlie the formation of the catalytic hotspots. Plasmonic cavity-catalysis provides a new approach to manipulate the catalytic sites and rates and may expand the frontier of heterogeneous catalysis.

Mechanistic Insights Gained by High Spatial Resolution Reactivity Mapping of Homogeneous and Heterogeneous (Electro)Catalysts

The recent development of high spatial resolution microscopy and spectroscopy tools enabled reactivity analysis of homogeneous and heterogeneous (electro)catalysts at previously unattainable resolution and sensitivity. These techniques revealed that catalytic entities are more heterogeneous than expected and local variations in reaction mechanism due to divergences in the nature of active sites, such as their atomic properties, distribution, and accessibility, occur both in homogeneous and heterogeneous (electro)catalysts. In this review, we highlight recent insights in catalysis research that were attained by conducting high spatial resolution studies. The discussed case studies range from reactivity detection of single particles or single molecular catalysts, inter- and intraparticle communication analysis, and probing the influence of catalysts distribution and accessibility on the resulting reactivity. It is demonstrated that multiparticle and multisite reactivity analyses provide unique knowledge about reaction mechanism that could not have been attained by conducting ensemble-based, averaging, spectroscopy measurements. It is highlighted that the integration of spectroscopy and microscopy measurements under realistic reaction conditions will be essential to bridge the gap between model-system studies and real-world high spatial resolution reactivity analysis.

Plate-Like Colloidal Metal Nanoparticles

The pseudo-two-dimensional (2D) morphology of plate-like metal nanoparticles makes them one of the most anisotropic, mechanistically understood, and tunable structures available. Although well-known for their superior plasmonic properties, recent progress in the 2D growth of various other materials has led to an increasingly diverse family of plate-like metal nanoparticles, giving rise to numerous appealing properties and applications. In this review, we summarize recent progress on the solution-phase growth of colloidal plate-like metal nanoparticles, including plasmonic and other metals, with an emphasis on mechanistic insights for different synthetic strategies, the crystallographic habits of different metals, and the use of nanoplates as scaffolds for the synthesis of other derivative structures. We additionally highlight representative self-assembly techniques and provide a brief overview on the attractive properties and unique versatility benefiting from the 2D morphology. Finally, we share our opinions on the existing challenges and future perspectives for plate-like metal nanomaterials.

Amalgamation of DNAzymes and Nanozymes in a Coronazyme

Artificial enzymes such as nanozymes and DNAzymes are economical and stable alternatives to natural enzymes. By coating Au nanoparticles (AuNPs) with a DNA corona (AuNP@DNA), we amalgamated nanozymes and DNAzymes into a new artificial enzyme with catalytic efficiency 5 times higher than AuNP nanozymes, 10 times higher than other nanozymes, and significantly greater than most of the DNAzymes on the same oxidation reaction. The AuNP@DNA demonstrates excellent specificity as its reactivity on a reduction reaction does not change with respect to pristine AuNP. Single-molecule fluorescence and force spectroscopies and density functional theory (DFT) simulations indicate a long-range oxidation reaction initiated by radical production on the AuNP surface, followed by radical transport to the DNA corona, where the binding and turnover of substrates take place. The AuNP@DNA is named coronazyme because of its natural enzyme mimicking capability through the well-orchestrated structures and synergetic functions. By incorporating different nanocores and corona materials beyond DNAs, we anticipate that the coronazymes represent generic enzyme mimics to carry out versatile reactions in harsh environments.

In situ quantitative single-molecule study of site-specific photocatalytic activity and dynamics on ultrathin g-C3N4 nanosheets

Graphitic carbon nitride (g-C₃N₄) has attracted extensive research attention in recent years due to its unique layered structure, facile synthetic route, visible-light-responsive nature, and excellent photocatalytic performance. However, an insightful investigation of site-specific catalytic activities and kinetics on g-C₃N₄ is still warranted. Here, we fabricated ultrathin g-C₃N₄ nanosheets through thermal exfoliation. The optimized sample exhibits a high specific surface area of 307.35 m² g^-1 and a remarkable H₂ generation activity of 2008 μmol h^-1 g^-1 with an apparent quantum efficiency of 4.62% at λ = 420 nm. Single-molecule fluorescence microscopy was applied for the first time to spatially resolve the reaction heterogeneities with nanometer precision (∼10 nm). The catalytic kinetics (i.e., reactant adsorption, conversion, and product dissociation) and temporal activity fluctuations were in situ quantified at individual structural features (i.e., wrinkles, edges, and basal planes) of g-C₃N₄. It was found that the wrinkle and edge exhibited superior photocatalytic activity due to the intrinsic band modulation, which are 20 times and 14.8 times that of the basal plane, respectively. Moreover, due to the steric effect, the basal plane showed the highest adsorption constant and the lowest direct dissociation constant. Density functional theory (DFT) simulations unveiled the adsorption energies of reactant and product molecules on each structure of g-C₃N₄, which support our experimental results. Such investigation would shed more light on the fundamental understanding of site-specific catalytic dynamics on g-C₃N₄, which benefits the rational design of 2D layered materials for efficient solar-to-chemical energy conversion.

Electronic Tuning of Gold Nanoparticle Active Sites for Reduction Catalysis

Electronic tuning of active sites in heterogeneous catalysis with organic ligands remains challenging since the ligands are often bound to the most active sites on the catalysts' surfaces. In this work, gold nanoparticles, which are on average less than 2 nm in diameter, are synthesized with strongly binding thiol and phosphine ligands and have measurable quantities of accessible sites on their surfaces in both cases. Triphenylphosphine (TPP) is used as the phosphine ligand, while triphenylmethyl mercaptan (TPMT) serves as the thiol ligand. Phosphines are chosen because they are electron-donating ligands when bound to Au, and thiols are selected because they are electron-withdrawing on the Au surface. X-ray photoelectron spectroscopy (XPS) results show differences in the Au 4f binding energies between the TPP- and TPMT-bound Au nanoparticles. Fourier transform infrared spectroscopy (FTIR) measurements of bound CO indicate that the TPP-bound Au nanoparticles are more electron-rich than the TPMT-bound Au nanoparticles. The number of binding sites on the surface is quantified using 2-naphthalenethiol titration experiments. It is observed that the number of binding sites on the thiol and phosphine-bound Au nanoparticles is similar in both cases. The Au nanoparticles are used for three different reactions: resazurin reduction, CO oxidation, and benzyl alcohol oxidation. For both CO oxidation and benzyl alcohol oxidation, which are performed with the ligands attached, TPP- and TPMT-bound nanoparticles are both catalytically active. However, for resazurin reduction, the TPMT-bound Au nanoparticles are not active, while the TPP-bound Au nanoparticles are catalytically active. These results illustrate that the catalytic activity can be tuned using bound organic ligands with different electronic properties for reduction reactions using Au nanoparticle catalysts.

Synthesis of Highly Accessible and Reactive Sites in Gold Nanoparticles Using Bound Bis(Diphenylphosphine) Ligands

While bound organic ligands provide steric protection against aggregation for metallic nanoparticles in solution, they can block a large fraction of the surface atoms which are needed for binding in catalysis and sensing applications. In this work, highly accessible Au nanoparticles ligated with bis(diphenylphosphine) molecules are synthesized and characterized in solution. Characterization is performed using high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), ultraviolet-visible (UV-Vis) spectroscopy, and fluorescence chemisorption experiments. These synthesized nanoparticles are accessible to a 2-napthalenethiol (2-NT) probe molecule in solution. The highest 2-NT accessibility is observed when using 1,1-bis(diphenylphosphino)methane (dppm) ligand where 61 % of the total gold atoms are accessible. It is hypothesized that increasing the rigidity of the bis(diphenylphosphine) ligand increases the number of binding sites on the Au nanoparticles. These nanoparticles are catalytically active for resazurin reduction, and the resazurin reduction rate scales with the number of binding sites.

Plasmonic imaging of the layer-dependent electrocatalytic activity of two-dimensional catalysts

Studying the localized electrocatalytic activity of heterogeneous electrocatalysts is crucial for understanding electrocatalytic reactions and further improving their performance. However, correlating the electrocatalytic activity with the microscopic structure of two-dimensional (2D) electrocatalysts remains a great challenge due to the lack of in situ imaging techniques and methods of tuning structures with atomic precision. Here, we present a general method of probing the layer-dependent electrocatalytic activity of 2D materials in situ using a plasmonic imaging technique. Unlike the existing methods, this approach was used to visualize the surface charge density and electrocatalytic activity of single 2D MoS₂ nanosheets, enabling the correlation of layer-dependent electrocatalytic activity with the surface charge density of single MoS₂ nanosheets. This work provides insights into the electrocatalytic mechanisms of 2D transition metal dichalcogenides, and our approach can serve as a promising platform for investigating electrocatalytic reactions at the heterogeneous interface, thus guiding the rational design of high-performance electrocatalysts.

In Situ Imaging of Catalytic Reactions on Tungsten Oxide Nanowires Connects Surface-Ligand Redox Chemistry with Photocatalytic Activity

Semiconductor nanocrystals are promising candidates for generating chemical feedstocks through photocatalysis. Understanding the role of ligands used to prepare colloidal nanocrystals in catalysis is challenging due to the complexity and heterogeneity of nanocrystal surfaces. We use in situ single-molecule fluorescence imaging to map the spatial distribution of active regions along individual tungsten oxide nanowires before and after functionalizing them with ascorbic acid. Rather than blocking active sites, we observed a significant enhancement in activity for photocatalytic water oxidation after treatment with ascorbic acid. While the initial nanowires contain inactive regions dispersed along their length, the functionalized nanowires show high uniformity in their photocatalytic activity. Spatial colocalization of the active regions with their surface chemical properties shows that oxidation of ascorbic acid during photocatalysis generates new oxygen vacancies along the nanowire surface. We demonstrate that controlling surface-ligand redox chemistry during photocatalysis can enhance the active site concentration on nanocrystal catalysts.

Computational Aspects of Single-Molecule Kinetics for Coupled Catalytic Cycles: A Spectral Analysis

Catalysis from single active sites is analyzed using methods developed from single-molecule kinetics. Using a stochastic Markov-state description, the observable properties of general catalytic networks of reactions are expressed using an eigenvalue decomposition of the transition matrix for the Markov process. By the use of a sensitivity analysis, the necessary eigenvalues and eigenvectors are related to the energies of controlling barriers and wells located along the reaction routes. A generalization of the energetic span theory allows the eigenvalues to be computed from several activation energies corresponding to distinct barrier-well pairings. The formalism is demonstrated for model problems and for a physically realistic mechanism for an alkene hydrogenation reaction on a single-atom catalyst. The spectral analysis permits a hierarchy of timescales to be identified from the single-molecule signal, which correspond to specific relaxation modes in the network.

Surface Engineering of Palladium Nanocrystals: Decoupling the Activity of Different Surface Sites on Nanocrystal Catalysts

The existence of various surface active sites within a nanocrystal (NC) catalyst complicates understanding their respective catalytic properties and designing an optimal catalyst structure for a desired catalytic reaction. Here, we developed a novel approach that allows unequivocal investigation on the intrinsic catalytic reactivity of the edge and terrace atoms of NCs. Through the comparison of the catalytic behaviors of edge-covered Pd NCs, which were prepared by the selective deposition of catalytically inactive Au atoms onto the edge sites of rhombic dodecahedral (RD) Pd NCs, with those of the pristine RD Pd NCs toward alkyne hydrogenation and Suzuki-Miyaura coupling reactions, we could decouple the activity of the edge and {110}-plane atoms of the Pd NCs without uncertainties. We expect that this study will provide an opportunity to scrutinize the surface properties of various NC catalysts to a more precise level and devise ideal catalysts for intended catalytic reactions.

Encapsulation within a coordination cage modulates the reactivity of redox-active dyes

Confining molecules within well-defined nanosized spaces can profoundly alter their physicochemical characteristics. For example, the controlled aggregation of chromophores into discrete oligomers has been shown to tune their optical properties whereas encapsulation of reactive species within molecular hosts can increase their stability. The resazurin/resorufin pair has been widely used for detecting redox processes in biological settings; yet, how tight confinement affects the properties of these two dyes remains to be explored. Here, we show that a flexible Pd^II₆L₄ coordination cage can efficiently encapsulate both resorufin and resazurin in the form of dimers, dramatically modulating their optical properties. Furthermore, binding within the cage significantly decreases the reduction rate of resazurin to resorufin, and the rate of the subsequent reduction of resorufin to dihydroresorufin. During our studies, we also found that upon dilution, the Pd^II₆L₄ cage disassembles to afford Pd^II₂L₂ species, which lacks the ability to form inclusion complexes - a process that can be reversed upon the addition of the strongly binding resorufin/resazurin guests. We expect that the herein disclosed ability of a water-soluble cage to reversibly modulate the optical and chemical properties of a molecular redox probe will expand the versatility of synthetic fluorescent probes in biologically relevant environments.

Operando Imaging of Chemical Activity on Gold Plates with Single-Molecule Electrochemiluminescence Microscopy

Classical electrochemical characterization tools cannot avoid averaging between the active reaction sites and their support, thus obscuring their intrinsic roles. Single-molecule electrochemical techniques are thus in high demand. Here, we demonstrate super-resolution imaging of Ru(bpy)₃ ²⁺ based reactions on Au plates using single-molecule electrochemiluminescence microscopy. By converting electrochemical signals into optical signals, we manage to achieve the ultimate sensitivity of single-entity chemistry, that is directly resolving the single photons from individual electrochemical reactions. High spatial resolution, up to 37 nm, further enables mapping Au chemical activity and the reaction kinetics. The spatiotemporally resolved dynamic structure-activity relationship on Au plates shows that the restructuring of catalysts plays an important role in determining the reactivity. Our approach may lead to gaining new insights towards evaluating and designing electrocatalytic systems.

Application of One-Dimensional Nanomaterials in Catalysis at the Single-Molecule and Single-Particle Scale

The morphology of nanomaterials has a great influence on the catalytic performance. One-dimensional (1D) nanomaterials have been widely used in the field of catalysis due to their unique linear morphology with large specific surface area, high electron-hole separation efficiency, strong light absorption capacity, plentiful exposed active sites, and so on. In this review, we summarized the recent progress of 1D nanomaterials by focusing on the applications in photocatalysis and electrocatalysis. We highlighted the advanced characterization techniques, such as scanning tunneling microscopy (STM), atomic force microscopy (AFM), surface photovoltage microscopy (SPVM), single-molecule fluorescence microscopy (SMFM), and a variety of combined characterization methods, which have been used to identify the catalytic action of active sites and reveal the mechanism of 1D nanomaterials. Finally, the challenges and future directions of the research on the catalytic mechanism of single-particle 1D nanomaterials are prospected. To our best knowledge, there is no review on the application of single-molecule or single-particle characterization technology to 1D nanomaterial catalysis at present. This review provides a systematic introduction to the frontier field and opens the way for the 1D nanomaterial catalysis.

Super-Resolution Electrogenerated Chemiluminescence Microscopy for Single-Nanocatalyst Imaging

Electrogenerated chemiluminescence microscopy (ECLM) provides a real-time imaging approach to visualize the surface-dependent catalytic activity of nanocatalysts, which helps to rationalize the design of catalysts. In this study, we first propose super-resolution ECLM that could measure the facet- and site-specific activities of a single nanoparticle with nanometer resolution. The stochastic nature of the ECL emission makes the generation of photons obey Poisson statistics, which fits the requirement of super-resolution radial fluctuation (SRRF). By processing an SRRF algorithm, the spatial resolution of ECL images achieved ca. 100 nm, providing more abundant details on electrocatalytic reactivities at the subparticle level. Beyond conventional wide-field ECL imaging, super-resolution ECLM provided the spatial distribution of catalytic activities at a Au nanorod and nanoplate with scales of a few hundred nanometers. It helped uncover the facet- and defect-dependent surface activity, as well as the dynamic fluctuation of reactivity patterns on single nanoparticles. The super-resolution ECLM provides high spatiotemporal resolution, which shows great potential in the field of catalysis, biological imaging, and single-entity analysis.

Visualizing the Spatial Heterogeneity of Electron Transfer on a Metallic Nanoplate Prism

The involvement between electron transfer (ET) and catalytic reaction at the electrocatalyst surface makes the electrochemical process challenging to understand and control. Even ET process, a primary step, is still ambiguous because it is unclear how the ET process is related to the nanostructured electrocatalyst. Herein, locally enhanced ET current dominated by mass transport effect at corner and edge sites bounded by {111} facets on single Au triangular nanoplates was clearly imaged. After decoupling mass transport effect, the ET rate constant of corner sites was measured to be about 2-fold that of basal {111} plane. Further, we demonstrated that spatial heterogeneity of local inner potential differences of Au nanoplates/solution interfaces plays a key role in the ET process, supported by the linear correlation between the logarithm of rate constants and the potential differences of different sites. These results provide direct images for heterogeneous ET, which helps to understand and control the nanoscopic electrochemical process and electrode design.

Nanoscale cooperative adsorption for materials control

Adsorption plays vital roles in many processes including catalysis, sensing, and nanomaterials design. However, quantifying molecular adsorption, especially at the nanoscale, is challenging, hindering the exploration of its utilization on nanomaterials that possess heterogeneity across different length scales. Here we map the adsorption of nonfluorescent small molecule/ion and polymer ligands on gold nanoparticles of various morphologies in situ under ambient solution conditions, in which these ligands are critical for the particles’ physiochemical properties. We differentiate at nanometer resolution their adsorption affinities among different sites on the same nanoparticle and uncover positive/negative adsorption cooperativity, both essential for understanding adsorbate-surface interactions. Considering the surface density of adsorbed ligands, we further discover crossover behaviors of ligand adsorption between different particle facets, leading to a strategy and its implementation in facet-controlled synthesis of colloidal metal nanoparticles by merely tuning the concentration of a single ligand.

Adsorption is a fundamentally important process but challenging to quantify, especially at the nanoscale. Here, the authors map the adsorption affinity and cooperativity of various ligands on single gold nanoparticles and discover adsorption crossover behaviors between different facets, leading to a strategy to control particle shape.

Single-Particle Catalytic Analysis by a Photon Burst Counting Technique Combined with a Microfluidic Chip

Single-particle catalytic analysis plays an important role to understand the catalytic mechanism of nanocatalysts. Currently, some methods are used to study the relationship between single-particle catalytic activity and morphology. However, there is still lack of a simple and rapid analysis method for evaluating the catalytic activity of an individual nanocatalyst that freely moves in solution. Here, we proposed a novel single-particle catalytic analysis method for investigating the catalytic activity of a free nanocatalyst. Its working principle is based on the photon burst counting analysis on fluorescent catalytic products of an individual nanocatalyst combined with a microfluidic chip. In this study, we used the reduction reaction of resazurin (RZ) to resorufin (RF) catalyzed by gold nanoparticles (GNPs) as a model. When nonfluorescent RZ molecules in one microchannel of the microfluidic chip mixed with the GNPs flowing in another channel under the control of flow rates, each individual photon burst from the catalytic product RF by GNPs was measured in real time with a constructed flow single-particle catalytic analysis (SPCA) system. With the method, the obtained intensity of each photon burst reflects the capacity of a particle to catalyze RZ molecules into RF(s). The number of photon burst within sampling time reflects the particle number of GNPs with catalytic activity. The experimental conditions including the mixing mode of the nanocatalyst and the substrate, the flow rate, RZ concentration, and detection time were optimized. Finally, the method was successfully used to study the catalytic activity of GNPs with different sizes and morphologies.

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.

Tuning Electrogenerated Chemiluminescence Intensity Enhancement Using Hexagonal Lattice Arrays of Gold Nanodisks

Electrogenerated chemiluminescence (ECL) microscopy shows promise as a technique for mapping chemical reactions on single nanoparticles. The technique's spatial resolution is limited by the quantum yield of the emission and the diffusive nature of the ECL process. To improve signal intensity, ECL dyes have been coupled with plasmonic nanoparticles, which act as nanoantennas. Here, we characterize the optical properties of hexagonal arrays of gold nanodisks and how they impact the enhancement of ECL from the coreaction of tris(2,2'-bipyridyl)dichlororuthenium(II) hexahydrate and tripropylamine. We find that varying the lattice spacing results in a 23-fold enhancement of ECL intensity because of increased dye-array near-field coupling as modeled using finite element method simulations.

Plasmonic metal nanostructures: concepts, challenges and opportunities in photo-mediated chemical transformations

Plasmonic metal nanostructures (PMNs) are characterized by the plasmon oscillation of conduction band electron in response to external radiation, enabling strong light absorption and scattering capacities and near-field amplification. Owing to these enhanced light-matter interactions, PMNs have garnered extensive research interest in the past decades. Notably, a growingly large number of reports show that the energetics and kinetics of chemical transformations on PMNs can be modified upon photoexcitation of their plasmons, giving rise to a new paradigm of manipulating the reaction rate and selectivity of chemical reactions. On the other hand, there is urgent need to achieve clear understanding of the mechanism underlying the photo-mediated chemical transformations on PMNs for unleashing their full potential in converting solar energy to chemicals. In this perspective, we review current fundamental concepts of photo-mediated chemical transformations executed at PMNs. Three pivotal mechanistic questions, i.e., thermal and nonthermal effects, direct and indirect charge transfer processes, and the specific impacts of plasmon-induced potentials, are explored based on recent studies. We highlight the critical aspects in which major advancements should be made to facilitate the rational design and optimization of photo-mediated chemical transformations on PMNs in the future.

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.

Single Particle Approaches to Plasmon-Driven Catalysis

Plasmonic nanoparticles have recently emerged as a promising platform for photocatalysis thanks to their ability to efficiently harvest and convert light into highly energetic charge carriers and heat. The catalytic properties of metallic nanoparticles, however, are typically measured in ensemble experiments. These measurements, while providing statistically significant information, often mask the intrinsic heterogeneity of the catalyst particles and their individual dynamic behavior. For this reason, single particle approaches are now emerging as a powerful tool to unveil the structure-function relationship of plasmonic nanocatalysts. In this Perspective, we highlight two such techniques based on far-field optical microscopy: surface-enhanced Raman spectroscopy and super-resolution fluorescence microscopy. We first discuss their working principles and then show how they are applied to the in-situ study of catalysis and photocatalysis on single plasmonic nanoparticles. To conclude, we provide our vision on how these techniques can be further applied to tackle current open questions in the field of plasmonic chemistry.

Single-Molecular Catalysis Identifying Activation Energy of the Intermediate Product and Rate-Limiting Step in Plasmonic Photocatalysis

Plasmon-mediated photocatalysis provides a novel strategy for harvesting solar energy. Identification of the rate-determining step and its activation energy in plasmon-mediated photocatalysis plays critical roles for understanding the contribution of hot carriers, which facilitates rational designation of catalysts with integrated high photochemical conversion efficiency and catalytic performance. However, it remains a challenge due to a lack of research tools with spatiotemporal resolution that are capable of capturing intermediates. In this work, we used a single-molecule fluorescence approach to investigate a localized surface plasmon resonance (LSPR)-enhanced photocatalytic reaction with subturnover resolution. By introducing variable temperature as an independent parameter in plasmonic photocatalysis, the activation energies of tandem reaction steps, including intermediate generation, product generation, and product desorption, were clearly differentiated, and intermediate generation was found to be the rate-limiting step. Remarkably, the cause of the plasmon-enhanced catalysis performance was found to be its ability of lowering the activation energy of intermediate generation. This study gives new insight into the photochemical energy conversion pathways in plasmon-enhanced photocatalysis and sheds light on designing high-performance plasmonic catalysts.

Shapes of epitaxial gold nanocrystals on SrTiO3 substrates

Morphological control of gold nanocrystals is important as their catalytic and optical properties are highly shape dependent.

Dark-field spectroscopy: development, applications and perspectives in single nanoparticle catalysis

Dark-field microscopy (DFM) is an effective method to detect the scattering signal from single nanoparticles. This technique could break through the 200 nm limit resolution of ordinary optical microscopes. It even can observe the submicron particles of 20-200 nm. Moreover, from 2000, DFM was coupled with a spectrometer to measure the scattering spectra of single silver nanoparticles. Then, dark-field spectroscopy becomes a very important plasmon spectroscopy technique for single nanoparticles. Usually, plasmonic nanoparticles are the major research target, because they have unique optical properties due to their localized surface plasmon resonance (LSPR), which can be influenced by many factors, such as composition, size, morphology, the refractive index of the surrounding medium etc. When surface chemical reactions occur on a single nanoparticle, it could induce the variation of these factors. Then, the structure-activity relationship for these nanoparticle catalysts can be studied at a single nanoparticle level and in real time. This review mainly summarized the development of dark-field spectroscopy, spectrometers, light sources, and other accessories, which greatly improved the imaging capabilities of dark-field spectroscopy. Meanwhile, the applications of dark-field spectroscopy in single-particle catalysis such as chemocatalysis, photocatalysis, electrocatalysis and biocatalysis are also reviewed.

Super-Resolution Microscopy Reveals Shape and Distribution of Dislocations in Single-Crystal Nanocomposites

With their potential to offer new properties, single crystals containing nanoparticles provide an attractive class of nanocomposite materials. However, to fully profit from these, it is essential that we can characterise their 3D structures, identifying the locations of individual nanoparticles, and the defects present within the host crystals. Using calcite crystals containing quantum dots as a model system, we here use 3D stochastic optical reconstruction microscopy (STORM) to locate the positions of the nanoparticles within the host crystal. The nanoparticles are shown to preferentially associate with dislocations in a manner previously recognised for atomic impurities, rendering these defects visible by STORM. Our images also demonstrate that the types of dislocations formed at the crystal/substrate interface vary according to the nucleation face, and dislocation loops are observed that have entirely different geometries to classic misfit dislocations. This approach offers a rapid, easily accessed, and non-destructive method for visualising the dislocations present within crystals, and gives insight into the mechanisms by which additives become occluded within crystals.

A mechanistic examination of salting out in protein-polymer membrane interactions

Developing a mechanistic understanding of protein dynamics and conformational changes at polymer interfaces is critical for a range of processes including industrial protein separations. Salting out is one example of a procedure that is ubiquitous in protein separations yet is optimized empirically because there is no mechanistic description of the underlying interactions that would allow predictive modeling. Here, we investigate peak narrowing in a model transferrin-nylon system under salting out conditions using a combination of single-molecule tracking and ensemble separations. Distinct surface transport modes and protein conformational changes at the negatively charged nylon interface are quantified as a function of salt concentration. Single-molecule kinetics relate macroscale improvements in chromatographic peak broadening with microscale distributions of surface interaction mechanisms such as continuous-time random walks and simple adsorption-desorption. Monte Carlo simulations underpinned by the stochastic theory of chromatography are performed using kinetic data extracted from single-molecule observations. Simulations agree with experiment, revealing a decrease in peak broadening as the salt concentration increases. The results suggest that chemical modifications to membranes that decrease the probability of surface random walks could reduce peak broadening in full-scale protein separations. More broadly, this work represents a proof of concept for combining single-molecule experiments and a mechanistic theory to improve costly and time-consuming empirical methods of optimization.

Deciphering nanoconfinement effects on molecular orientation and reaction intermediate by single molecule imaging

Nanoconfinement could dramatically change molecular transport and reaction kinetics in heterogeneous catalysis. Here we specifically design a core-shell nanocatalyst with aligned linear nanopores for single-molecule studies of the nanoconfinement effects. The quantitative single-molecule measurements reveal unusual lower adsorption strength and higher catalytic activity on the confined metal reaction centres within the nanoporous structure. More surprisingly, the nanoconfinement effects on enhanced catalytic activity are larger for catalysts with longer and narrower nanopores. Experimental evidences, including molecular orientation, activation energy, and intermediate reactive species, have been gathered to provide a molecular level explanation on how the nanoconfinement effects enhance the catalyst activity, which is essential for the rational design of highly-efficient catalysts.

From Optical to Chemical Hot Spots in Plasmonics

In recent years, the possibility to induce chemical transformations by using tunable plasmonic modes has opened the question of whether we can control or create chemical hot spots in these systems. This can be rationalized as the reactive analogue of the well-established concept of optical hot spots, which have drawn a great deal of attention to plasmonic nanostructures for their ability to circumvent the far-field diffraction limit of conventional optical elements. Although optical hot spots can be mainly defined by the geometry and permittivity of the nanostructures, the degrees of freedom influencing their photocatalytic properties appear to be much more numerous. In fact, the reactivity of plasmonic systems are deeply influenced by the dynamics and interplay of photons, plasmon-polaritons, carriers, phonons, and molecular states. These degrees of freedom can affect the reaction rates, the product selectivity, or the spatial localization of a chemical reaction. In this Account, we discuss the oportunities to control chemical hot spots by tuning the cascade of events that follows the excitation and decay of plasmonic modes in nanostructures. We discuss a series of techniques to spatially map and image plasmonic nanoscale reactivity at the single photocatalyst level. We show how to optimize the reactivity of carriers by manipulating their excitation and decay mechanisms in plasmonic nanoparticles. In addition, the tailored generation of non-thermal phonons in metallic nanostructures and their dissipation is shown as a promise to understand and exploit thermal photocatalysis at the nanoscale. Understanding and controlling these processes is essential for the rational design of solar nanometric photocatalysts. Nevertheless, the ultimate capability of a plasmonic photocatalyst to trigger a chemical reaction is correlated to its ability to navigate through, or even modify, the potential energy surface of a given chemical reaction. Here we reunite both worlds, the plasmonic photocatalysts and the molecular ones, identifying different energy transfer pathways and their influence on selectivity and efficiency of chemical reactions. We foresee that the migration from optical to chemical hot spots will greatly assist the understanding of ongoing plasmonic chemistry.

Ultrasensitive Three-Dimensional Orientation Imaging of Single Molecules on Plasmonic Nanohole Arrays Using Second Harmonic Generation

Recently, fluorescence-based super-resolution techniques such as stimulated emission depletion (STED) and stochastic optical reconstruction microscopy (STORM) have been developed to achieve near molecular-scale resolution. However, such a super-resolution technique for nonlinear label-free microscopy based on second harmonic generation (SHG) is lacking. Since SHG is label-free and does not involve real-energy level transitions, fluorescence-based super-resolution techniques such as STED cannot be applied to improve the resolution. In addition, due to the coherent and non-isotropic emission nature of SHG, single-molecule localization techniques based on isotropic emission of fluorescent molecule such as STORM will not be appropriate. Single molecule SHG microscopy is largely hindered due to the very weak nonlinear optical scattering cross sections of SHG scattering processes. Thus, enhancing SHG using plasmonic nanostructures and nanoantennas has recently gained much attention owing to the potential of various nanoscale geometries to tightly confine electromagnetic fields into small volumes. This confinement provides substantial enhancement of electromagnetic field in nanoscale regions of interest, which can significantly boost the nonlinear signal produced by molecules located in the plasmonic hotspots. However, to date, plasmon-enhanced SHG has been primarily applied for the measurement of bulk properties of the materials/molecules, and single molecule SHG imaging along with its orientation information has not been realized yet. Herein, we achieved simultaneous visualization and three-dimensional (3D) orientation imaging of individual rhodamine 6G (R6G) molecules in the presence of plasmonic silver nanohole arrays. SHG and two-photon fluorescence microscopy experiments together with finite-difference time-domain (FDTD) simulations revealed a ∼10⁶-fold nonlinear enhancement factor at the hot spots on the plasmonic silver nanohole substrate, enabling detection of single molecules using SHG. The position and 3D orientation of R6G molecules were determined using the template matching algorithm by comparing the experimental data with the calculated dipole emission images. These findings could enable SHG-based single molecule detection and orientation imaging of molecules which could lead to a wide range of applications from nanophotonics to super-resolution SHG imaging of biological cells and tissues.