Plasmonic Imaging of Electrochemical Reactions of Single Nanoparticles

Combining plasmonic and electrochemical biosensing methods

The idea of combining electrochemical (EC) and plasmonic biosensor methods was introduced almost thirty years ago and the potential of electrochemical-plasmonic (EC-P) biosensors has been highlighted ever since. Despite that, the use of EC-P biosensors in analytics has been rather limited so far and the search for unique applications of the EC-P method continues. In this paper, we review the advances in the field of EC-P biosensors and discuss the features and benefits they can provide. In addition, we identify the main challenges for the development of EC-P biosensors and the limitations that prevent EC-P biosensors from more widespread use. Finally, we review applications of EC-P biosensors for the investigation and quantification of biomolecules, and for the study of biomolecular and cellular processes.

Functional Plasmonic Microscope: Characterizing the Metabolic Activity of Single Cells via Sub-nm Membrane Fluctuations

Metabolic abnormalities are at the center of many diseases, and the capability to film and quantify the metabolic activities of a single cell is important for understanding the heterogeneities in these abnormalities. In this paper, a functional plasmonic microscope (FPM) is used to image and measure metabolic activities without fluorescent labels at a single-cell level. The FPM can accurately image and quantify the subnanometer membrane fluctuations with a spatial resolution of 0.5 μm in real time. These active cell membrane fluctuations are caused by metabolic activities across the cell membrane. A three-dimensional (3D) morphology of the bottom cell membrane was imaged and reconstructed with FPM to illustrate the capability of the microscope for cell membrane characterization. Then, the subnanometer cell membrane fluctuations of single cells were imaged and quantified with the FPM using HeLa cells. Cell metabolic heterogeneity is analyzed based on membrane fluctuations of each individual cell that is exposed to similar environmental conditions. In addition, we demonstrated that the FPM could be used to evaluate the therapeutic responses of metabolic inhibitors (glycolysis pathway inhibitor STF 31) on a single-cell level. The result showed that the metabolic activities significantly decrease over time, but the nature of this response varies, depicting cell heterogeneity. A low-concentration dose showed a reduced fluctuation frequency with consistent fluctuation amplitudes, while the high-concentration dose showcased a decreasing trend in both cases. These results have demonstrated the capabilities of the functional plasmonic microscope to measure and quantify metabolic activities for drug discovery.

Investigating the effect of volatility on the hygroscopicities of acetate nanoparticle aerosols by surface plasmon resonance microscopy

Under high relative humidity (RH) conditions, the release of volatile components (such as acetate) has a significant impact on the aerosol hygroscopicity. In this work, one surface plasmon resonance microscopy (SPRM) measurement system was introduced to determine the hygroscopic growth factors (GFs) of three acetate aerosols separately or mixed with glucose at different RHs. For Ca(CH3COO)2 or Mg(CH3COO)2 aerosols, the hygroscopic growth trend of each time was lower than that of the previous time in three cyclic humidification from 70% RH to 90% RH, which may be due to the volatility of acetic acid leading to the formation of insoluble hydroxide (Ca(OH)2 or Mg(OH)2) under high RH conditions. Then the third calculated GF (using the Zdanovskii-Stokes-Robinson method) for Ca(CH3COO)2 or Mg(CH3COO)2 in bicomponent aerosols with 1:1 mass ratio were 3.20% or 5.33% lower than that of the first calculated GF at 90% RH. The calculated results also showed that the hygroscopicity change of bicomponent aerosol was negatively correlated with glucose content, especially when the mass ratio of Mg(CH3COO)2 to glucose was 1:2, the GF at 90% RH only decreased by 4.67% after three cyclic humidification. Inductively coupled plasma atomic emission spectrum (ICP-AES) based measurements also indicated that the changes of Mg2+concentration in bicomponent was lower than that of the single-component. The results of this study reveal thatduring the efflorescence transitions of atmospheric nanoparticles, the organic acids diffusion rate may be inhibited by the coating effect of neutral organic components, and the particles aging cycle will be prolonged.

Optically Imaging In Situ Effects of Electrochemical Cycling on Single Nanoparticle Electrocatalysis

Single-nanoparticle studies often need one or a series of nanoparticle populations that are designed with differences in a nominally particular structural parameter to clarify the structure-activity relationship (SAR). However, the heterogeneity of various properties within any population would make it rather difficult to approach an ideal one-parameter control. In situ modification ensures the same nanoparticle to be investigated and also avoids complicating effects from the otherwise often needed ex situ operations. Herein, we apply electrochemical cycling to single platinum nanoparticles and optically examine their SAR. An electrocatalytic fluorescent microscopic method is established to evaluate the apparent catalytic activity of a number of single nanoparticles toward the oxygen reduction reaction. Meanwhile, dark-field microscopy with the substrate electrode under a cyclic potential control is found to be able to assess the electrochemically active surface area (ECSA) of single nanoparticles via induced chloride redox electrochemistry. Consequently, nanoparticles with drastically increased catalytic activity are discovered to have larger ECSAs upon potential regulation, and interestingly, there are also a few particles with decreased activity, as opposed to the overall trend, that all develop a smaller ECSA in the process. The deactivated nanoparticles against the overall enhancement effects of potential cycling are revealed for the first time. As such, the SAR of single nanoparticles when subjected to an in situ structural control is optically demonstrated.

An Ultrasensitive Plasmonic Sensor Based on 2D Ferroelectric Bi2 O2 Se

Plasmonic biosensing is a label-free detection method that is commonly used to measure various biomolecular interactions. However, one of the main challenges in this approach is the ability to detect biomolecules at low concentrations with sufficient sensitivity and detection limits. Here, 2D ferroelectric materials are employed to address the issues with sensitivity in biosensor design. A plasmonic sensor based on Bi2 O2 Se nanosheets, a ferroelectric 2D material, is presented for the ultrasensitive detection of the protein molecule. Through imaging the surface charge density of Bi2 O2 Se, a detection limit of 1 fM is achieved for bovine serum albumin (BSA). These findings underscore the potential of ferroelectric 2D materials as critical building blocks for future biosensor and biomaterial architectures.

Formation of Molecular Junctions by Single-Entity Collision Electrochemistry

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

Investigating the hygroscopicities of calcium and magnesium salt particles aged with SO2 using surface plasmon resonance microscopy

The hygroscopicities of calcium and magnesium salts strongly affect the environment and climate, but the aging products of these salts at high relative humidities (RHs) are still poorly understood. In this study, surface plasmon resonance microscopy (SPRM) was used to determine the hygroscopic growth factors (GFs) of Ca(NO₃)₂ and Mg(NO₃)₂ separately or mixed with galactose at different mass ratios at different RHs before and after aging. For all particles, the measured GFs showed no indication of deliquescence across the range of RHs tested, and overall hygroscopicity was clearly lower after than before aging. The Ca(NO₃)₂ and Mg(NO₃)₂ GFs at 90 % RH were 1.80 and 1.66, respectively, before aging and 1.33 and 1.42, respectively, after 4 h aging, meaning aging decreased the GFs by 26.11 % and 14.46 %, respectively. Aging decreased the hygroscopicity because insoluble or sparingly soluble substances (CaSO₃, CaSO₄, MgSO₃) formed and strongly changed the overall hygroscopicity. For bicomponent aerosols with different mass ratios, the GFs (calculated using the Zdanovskii-Stokes-Robinson method) of the other components except galactose at 90 % RH after 1 h aging were all lower, respectively, than the measured GFs of pure Ca(NO₃)₂ and Mg(NO₃)₂ after aging for 1 h, especially with the mass ratio of 1:2, their GFs have decreased by 14.63 % and 7.50 %, respectively. Subsequently, Ion chromatograms indicated that the peak area ratio of SO₄^2- to NO₃^- ratios were higher for the aged bicomponent particles than aged single-component particles, possibly because adding galactose improved the gas-liquid state stability during drying after the aging process and therefore promoted nitrate consumption and sulfate formation. The results indicated that organic components may play important roles in heterogeneous reactions between trace gases and multicomponent aerosols and should be considered in evaluating the impacts on submicron aerosol composition of high atmospheric SO₂ concentrations at high humidities.

Retrieval of refractive index of ultrafine single particle using hygroscopic growth factor obtained by high sensitive surface plasmon resonance microscopy

When exposed to different relative humidities (RHs), the optical properties of atmospheric aerosols will change because of changes in the aerosol particle size and complex refractive index (RI), which will affect haze formation and global climate change. The potential contributions of ultrafine particles to the atmospheric optical characteristics and to haze spreading cannot be ignored because of their high particle number concentrations and strong diffusibility; measurement of the optical properties of wet ultrafine particles is thus highly important for environmental assessment. Therefore, a surface plasmon resonance microscopy with azimuthal rotation illumination (SPRM-ARI) system is designed to determine the RIs of single particle aerosols with diameters of less than 100 nm in the hygroscopic growth process. Measurements are taken using mixed single particles with different mass ratios. The RIs of mixed single aerosols at different RHs are retrieved by measuring the scattering light intensity using the SPRM-ARI system and almost all the RIs of the bicomponent particles with different mass ratios decrease with increasing water content under high RH conditions. Finally, for each of the bicomponent particles, the maximum standard deviations for the retrieved RI values are only 2.06×10^-3, 3.08×10^-3 and 3.83×10^-3, corresponding to the NaCl and NaNO₃ bicomponent particles with a 3:1 mass ratio at 76.0% RH, the NaCl and glucose particles with a 1:3 mass ratio at 89.0% RH, and the NaCl and OA particles with a 1:1 mass ratio at 78.0% RH, respectively; these results indicate that the high-sensitivity SPRM-ARI system can measure the RI effectively and accurately.

Recent advances in surface plasmon resonance imaging and biological applications

Surface Plasmon Resonance Imaging (SPRI) is a robust technique for visualizing refractive index changes, which enables researchers to observe interactions between nanoscale objects in an imaging manner. In the past period, scholars have been attracted by the Prism-Coupled and Non-prism Coupled configurations of SPRI and have published numerous experimental results. This review describes the principle of SPRI and discusses recent developments in Prism-Coupled and Non-prism Coupled SPRI techniques in detail, respectively. And then, major advances in biological applications of SPRI are reviewed, including four sub-fields (cells, viruses, bacteria, exosomes, and biomolecules). The purpose is to briefly summarize the recent advances of SPRI and provide an outlook on the development of SPRI in various fields.

Recent advances in label-free imaging of cell-matrix adhesions

Cell-matrix adhesions play an essential role in mediating and regulating many biological processes. The adhesion receptors, typically transmembrane integrins, provide dynamic correlations between intracellular environments and extracellular matrixes (ECMs) by bi-directional signaling. In-depth investigations of cell-matrix adhesion and integrin-mediated cell adhesive force are of great significance in biology and medicine. The emergence of advanced imaging techniques and principles has facilitated the understanding of the molecular composition and structure dynamics of cell-matrix adhesions, especially the label-free imaging methods that can be used to study living cell dynamics without immunofluorescence staining. This highlight article aims to give an overview of recent developments in imaging cell-matrix adhesions in a label-free manner. Electrochemiluminescence microscopy (ECLM) and surface plasmon resonance microscopy (SPRM) are briefly introduced and their applications in imaging analysis of cell-matrix adhesions are summarized. Then we highlight the advances in mapping cell-matrix adhesion force based on molecular tension probes and fluorescence microscopy (collectively termed as MTFM). The biomaterials including polyethylene glycol (PEG), peptides and DNA for constructing tension probes in MTFM are summarized. Finally, the outlook and perspectives on the further developments of cell-matrix adhesion imaging are presented.

Intermediate-state imaging of electrical switching and quantum coupling of molybdenum disulfide monolayer

SignificanceThin transparent semiconductors of two-dimensional materials are attractive for the practical applications in next-generation nanoelectronic and optoelectronic devices. Probing the electron states and electrical switching mechanisms of a molybdenum disulphide monolayer with atomic-scale thickness (6.5 Å) allows us to unlock the full technological potential of this nanomaterial. We introduced a plasmonic phase imaging method to uncover the underlying mechanism and detailed switching dynamics of an electrical-state switching event. This dramatic phase change can be attributed to the reversible switching of classical electromagnetic coupling and quantum coupling effects interplaying between a single metal nanoparticle and molybdenum disulphide monolayer, and the transient intermediate states during the switching event can be directly imaged by a plasmonic technique.

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.

Plasmonic Imaging of Tuning Electron Tunneling Mediated by a Molecular Monolayer

Probing and tuning the electron tunneling in metal electrode-insulator-metal nanoparticle systems provide a unique vision for understanding the fundamental mechanism of electrochemistry and broadening the horizon in practical applications of molecular electronics in many electrochemical systems. Here we report a plasmonic imaging technique to monitor the local double-layer charging of individual Au nanoparticles deposited on gold electrode separated by monolayer of n-alkanethiol molecules. The thickness of molecular monolayer tunes the tunneling kinetics and conductivity, which predicts the heterogeneous behavior on the modified electrode surface for different electrochemical systems. We studied the distance dependence of the electron tunneling and double layer charging processes by a plasmonic-based electrical impedance microscopy. By performing fast Fourier transform analysis of the recorded plasmonic image sequences, we can quantify the interfacial impedance of single nanoparticles and the tunneling decay constant of molecular layer. We further observed the electron neutralization dynamics during single-nanoparticle collisions on different surfaces. This optical readout of electron tunneling demonstrates an imaging approach to determine the electrical properties of metal electrode-insulator-metal nanoparticle systems, which include the electron tunneling mechanism and local impedance.

Plasmonic Imaging of Electrochemical Reactions at Individual Prussian Blue Nanoparticles

Prussian blue is an iron-cyanide-based pigment steadily becoming a widely used electrochemical sensor in detecting hydrogen peroxide at low concentration levels. Prussian blue nanoparticles (PBNPs) have been extensively studied using traditional ensemble methods, which only provide averaged information. Investigating PBNPs at a single entity level is paramount for correlating the electrochemical activities to particle structures and will shed light on the major factors governing the catalyst activity of these nanoparticles. Here we report on using plasmonic electrochemical microscopy (PEM) to study the electrochemistry of PBNPs at the individual nanoparticle level. First, two types of PBNPs were synthesized; type I synthesized with double precursors method and type II synthesized with polyvinylpyrrolidone (PVP) assisted single precursor method. Second, both PBNPs types were compared on their electrochemical reduction to form Prussian white, and the effect from the different particle structures was investigated. Type I PBNPs provided better PEM sensitivity and were used to study the catalytic reduction of hydrogen peroxide. Progressively decreasing plasmonic signals with respect to increasing hydrogen peroxide concentration were observed, demonstrating the capability of sensing hydrogen peroxide at a single nanoparticle level utilizing this optical imaging technique.

Single-nanoparticle spectroelectrochemistry studies enabled by localized surface plasmon resonance

This review describes recent progress of spectroelectrochemistry (SEC) analysis of single metallic nanoparticles (NPs) which have strong surface plasmon resonance properties. Dark-field scattering (DFS), photoluminescence (PL), and electrogenerated chemiluminescence (ECL) are three commonly used optical methods to detect individual NPs and investigate their local redox activities in an electrochemical cell. These SEC methods are highly dependent on a strong light-scattering cross-section of plasmonic metals and their electrocatalytic characteristics. The surface chemistry and the catalyzed reaction mechanism of single NPs and their chemical transformations can be studied using these SEC methods. Recent progress in the experimental design and fundamental understanding of single-NP electrochemistry and catalyzed reactions using DFS, PL, and ECL is described along with selected examples from recent publications in this field. Perspectives on the challenges and possible solutions for these SEC methods and potential new directions are discussed.

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.

A rich catalog of C-C bonded species formed in CO2 reduction on a plasmonic photocatalyst

The understanding and rational design of heterogeneous catalysts for complex reactions, such as CO₂ reduction, requires knowledge of elementary steps and chemical species prevalent on the catalyst surface under operating conditions. Using in situ nanoscale surface-enhanced Raman scattering, we probe the surface of a Ag nanoparticle during plasmon-excitation-driven CO₂ reduction in water. Enabled by the high spatiotemporal resolution and surface sensitivity of our method, we detect a rich array of C₁-C₄ species formed on the photocatalytically active surface. The abundance of multi-carbon compounds, such as butanol, suggests the favorability of kinetically challenging C-C coupling on the photoexcited Ag surface. Another advance of this work is the use of isotope labeling in nanoscale probing, which allows confirmation that detected species are the intermediates and products of the catalytic reaction rather than spurious contaminants. The surface chemical knowledge made accessible by our approach will inform the modeling and engineering of catalysts.

X-ray-Based Techniques to Study the Nano-Bio Interface

X-ray-based analytics are routinely applied in many fields, including physics, chemistry, materials science, and engineering. The full potential of such techniques in the life sciences and medicine, however, has not yet been fully exploited. We highlight current and upcoming advances in this direction. We describe different X-ray-based methodologies (including those performed at synchrotron light sources and X-ray free-electron lasers) and their potentials for application to investigate the nano-bio interface. The discussion is predominantly guided by asking how such methods could better help to understand and to improve nanoparticle-based drug delivery, though the concepts also apply to nano-bio interactions in general. We discuss current limitations and how they might be overcome, particularly for future use in vivo.

Plasmonic Imaging of Oxidation and Reduction of Single Gold Nanoparticles and Their Surface Structural Dynamics

Gold nanoparticles (AuNPs) have been widely used in catalytic electrochemistry. Heterogeneity in size, shape, and surface sites leads to variable, particle-specific catalytic activities. Conventional electrochemical methods can only obtain the collective responses from all the catalytic nanoparticles on the electrode surface; the heterogeneity of particle performance will be averaged. Alternatively, plasmonic electrochemical imaging (PECi) is capable of imaging the electrochemical activity at individual nanoparticles. In this work, PECi was used to image the oxidation and reduction of the gold surface at individual AuNPs, and their associated structural alterations were successfully measured. We have studied the electrochemical responses from gold nanocubes, gold nanorods, and gold nanowires with PECi and observed different surface redox activities. We have also demonstrated the capability of monitoring the surface dynamics at individual AuNPs utilizing characteristic PECi derived cyclic voltammograms (CVs).

A Single-Shot Autofocus Approach for Surface Plasmon Resonance Microscopy

Surface plasmon resonance microscopy (SPRM) has been widely used as a sensitive imaging platform for chemical and biological analysis. The SPRM system inevitably suffers from focus inhomogeneity and drifts, especially in long-term recordings, leading to distorted images and inaccurate quantification. Traditional focus correction approaches require additional optical parts to detect and adjust focal conditions. Herein, we propose a deep-learning-based image processing method to gain autofocused SPRM images, without increasing the complexity of the optical systems. We trained a generative adversarial network (GAN) model with thousands of SPRM images of nanoparticles acquired at different focal distances. The trained model was able to directly generate focused SPRM images from single-shot defocused images, with no prior knowledge of the focus conditions during recording. Experiments using Au nanoparticles show that this method is effective in both static and time-lapse monitoring. The proposed autofocus technique thus provides an approach for improving the consistency among SPRM studies and for long-term monitoring.

Ligand-Free Fabrication of Ag Nanoassemblies for Highly Sensitive and Reproducible Surface-Enhanced Raman Scattering Sensing of Antibiotics

The assemblies of plasmonic nanoparticles (NPs) are the universal methods for enhancing their surface-enhanced Raman scattering (SERS) activities. However, the present methods suffer from the problems of poor reproducibility, complicated fabrication, or the adsorption of ligands on the surface, which limit their practical applications. In this work, by using a facile freeze-thaw method, we are able to fabricate the assemblies of Ag NPs with highly reproducible SERS activity without the use of ligands. Moreover, the Ag NPs can be well kept in a frozen state for a long time with few influences on the reproducibility (relative standard deviation, RSD ca. 7%), while those kept in colloid (4 °C) suffer from gradual surface oxidation and aggregation. Such a simple freeze-thaw method does not require the introduction of any ligands (or linkers) with long-term stability and reproducibility, implying its wide applications in practical SERS sensing.

Nanoimpacts at Active and Partially Active Electrodes: Insights and Limitations

While the electrochemical nanoimpact technique has recently emerged as a method of studying single entities, it is limited by requirement of a catalytically active particle impacting an inert electrode. We show that an active particle-active electrode can provide mechanistic insight into electrochemical reactions. When an individual Pt electrocatalyst adsorbs to the surface of a partially active electrode, further reduction of electrode-produced species can proceed on the nanocatalyst. Current transients obtained during hydrogen evolution allow simultaneous measurement of the Pt catalyst over different length scales, size dependency suggests H atom intercalation as a catalytic deactivation mechanism. Although results show that outer-sphere redox probes are unproductive for particle characterization, the breadth of inner-sphere electrochemical reactions makes this a promising method for understanding the properties of catalytic nanomaterials, one at a time.

Tracking Interfacial Dynamics of a Single Nanoparticle Using Plasmonic Scattering Interferometry

The ability to track interfacial dynamics of a single nanoparticle at the solution-solid interface is crucial for understanding physical, chemical, and biological processes, but it remains a challenge. Here, we demonstrated a plasmonic imaging technique that can track unlabeled nanoparticles at the solution-solid interface with high spatial and temporal resolutions. This technique is based on particle-induced interferometric scattering of a surface plasmonic wave, which results in a high vertical sensitivity. Using this ability, we tracked the trajectories of a single nanoparticle interacting with a surface, measured the hydrodynamically hindered diffusion of nanoparticles, and revealed the surface chemistry-dependent behavior of nanoparticles at the interface. The application for tracking formation of membranes from a lipid vesicle was demonstrated, indicating the potential for investigating a broad range of nano-objects at interfaces in a complex environment.

Plasmonic scattering imaging of single proteins and binding kinetics

Measuring the binding kinetics of single proteins represents one of the most important and challenging tasks in protein analysis. Here we show that this is possible using a surface plasmon resonance (SPR) scattering technique. SPR is a popular label-free detection technology because of its extraordinary sensitivity, but it has never been used for imaging single proteins. We overcome this limitation by imaging scattering of surface plasmonic waves by proteins. This allows us to image single proteins, measure their sizes and identify them based on their specific binding to antibodies. We further show that it is possible to quantify protein binding kinetics by counting the binding of individual molecules, providing a digital method to measure binding kinetics and analyze heterogeneity of protein behavior. We anticipate that this imaging method will become an important tool for single protein analysis, especially for low volume samples, such as single cells.

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.

Phase imaging of transition from classical to quantum plasmonic couplings between a metal nanoparticle and a metal surface

When a metal nanoparticle is brought near to a metal surface within electron tunneling distance (∼1 nm), classical electromagnetic coupling between the nanoparticle and the metal is expected to transition to quantum coupling. We show that this transition can be observed as a drastic phase change in the surface plasmon resonance (SPR) images of the gold nanoparticles. We study the transition by controlling the distance between the nanoparticles and electrode surface, modeling the impact of the transition on the SPR image in terms of a phase shift and demonstrating detection of microRNA based on the transition from classical to quantum coupling. The work shows that the quantum coupling can be directly visualized in SPR, and the extremely sensitive dependence of the transition on distance leads to a biosensing principle with SPR.

Observing atomic layer electrodeposition on single nanocrystals surface by dark field spectroscopy

Underpotential deposition offers a predominant way to tailor the electronic structure of the catalytic surface at the atomic level, which is key to engineering materials with a high activity for (electro)catalysis. However, it remains challenging to precisely control and directly probe the underpotential deposition of a (sub)monolayer of atoms on nanoparticle surfaces. In this work, we in situ observe silver electrodeposited on gold nanocrystals surface from sub-monolayer to one monolayer by designing a highly sensitive electrochemical dark field scattering setup. The spectral variation is used to reconstruct the optical “cyclic voltammogram” of every single nanocrystal for understanding the underpotential deposition process on nanocrystals, which cannot be achieved by any other methods but are essential for creating novel nanomaterials.

Underpotential deposition (UPD) is important to modify the surface properties of nanocrystals. Here, the authors show the application of in situ electrochemical dark field spectroscopy in identifying the UPD processes of silver on different facets of gold nanocrystals at the single nanoparticle level.

Single Entity Electrochemistry in Nanopore Electrode Arrays: Ion Transport Meets Electron Transfer in Confined Geometries

Electrochemical measurements conducted in confined volumes provide a powerful and direct means to address scientific questions at the nexus of nanoscience, biotechnology, and chemical analysis. How are electron transfer and ion transport coupled in confined volumes and how does understanding them require moving beyond macroscopic theories? Also, how do these coupled processes impact electrochemical detection and processing? We address these questions by studying a special type of confined-volume architecture, the nanopore electrode array, or NEA, which is designed to be commensurate in size with physical scaling lengths, such as the Debye length, a concordance that offers performance characteristics not available in larger scale structures.The experiments described here depend critically on carefully constructed nanoscale architectures that can usefully control molecular transport and electrochemical reactivity. We begin by considering the experimental constraints that guide the design and fabrication of zero-dimensional nanopore arrays with multiple embedded electrodes. These zero-dimensional structures are nearly ideal for exploring how permselectivity and unscreened ion migration can be combined to amplify signals and improve selectivity by enabling highly efficient redox cycling. Our studies also highlight the benefits of arrays, in that molecules escaping from a single nanopore are efficiently captured by neighboring pores and returned to the population of active redox species being measured, benefits that arise from coupling ion accumulation and migration. These tools for manipulating redox species are well-positioned to explore single molecule and single particle electron transfer events through spectroelectrochemistry, studies which are enabled by the electrochemical zero-mode waveguide (ZMW), a special hybrid nanophotonic/nanoelectronic architecture in which the lower ring electrode of an NEA nanopore functions both as a working electrode to initiate electron transfer reactions and as the optical cladding layer of a ZMW. While the work described here is largely exploratory and fundamental, we believe that the development of NEAs will enable important applications that emerge directly from the unique coupled transport and electron-transfer capabilities of NEAs, including in situ molecular separation and detection with external stimuli, redox-based electrochemical rectification in individually encapsulated nanopores, and coupled sorters and analyzers for nanoparticles.

Effect of the driving force on nanoparticles growth and shape: an opto-electrochemical study

Most protocols developed to synthesize nanoparticles (NPs) and to control their shape are inspired from nucleation and growth theories. However, to rationalize the mechanisms of the shape-selective synthesis of NPs, experimental strategies allowing to probe in situ the growth of NPs are needed. Herein, metal Au or Ag nanoparticles (NPs) are produced by reaction of a metallic ion precursor with a reversible redox reducer. The process is explored by an oxidative electrosynthesis strategy using a sacrificial Au or Ag ultramicroelectrode to both trigger the metallic ion generation and control the local concentrations of the different reactants. The effect of the driving force for the metallic ion reduction over metal NP growth dynamics is inspected in situ and in real time at the single NP level by high-resolution optical microscopy from the tracking of the Brownian trajectories of the growing NPs in solution. The NP reductive growth/oxidative etching thermodynamics, and consequently the NP shape, are shown to be controlled electrochemically by the reversible redox couple, while the intervention of an Au(i) intermediate ion is suggested to account for the formation of gold nanocubes.

Observing the structure-dependent electrocatalytic activity of bimetallic Pd-Au nanorods at the single-particle level

We developed an electrochemiluminescence (ECL) microscopy technique to image the structure-dependent electrocatalytic reactivity of bimetallic Pd-Au nanorods (NRs) at the single-particle level.

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.

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.

Plasmonic Measurement of Electron Transfer between a Single Metal Nanoparticle and an Electrode through a Molecular Layer

We study electron transfer associated with electrocatalytic reduction of hydrogen on single platinum nanoparticles separated from an electrode surface with an alkanethiol monolayer using a plasmonic imaging technique. By varying the monolayer thickness, we show that the reaction rate depends on electron tunneling from the electrode to the nanoparticle. The tunneling decay constant is ∼4.3 nm^-1, which is small compared to those in literature for alkanethiols. We attribute it to a reduced tunneling barrier resulting from biasing the electrode potential negatively to the hydrogen reduction regime. In addition to allowing study of electron transfer of single nanoparticles, the work demonstrates an optical method to measure charge transport in molecules electrically wired to two electrodes.

Single Nanoparticle Electrochemistry

Experimental techniques to monitor and visualize the behaviors of single nanoparticles have not only revealed the significant spatial and temporal heterogeneity of those individuals, which are hidden in ensemble methods, but more importantly, they have also enabled researchers to elucidate the origin of such heterogeneity. In pursuing the intrinsic structure-function relations of single nanoparticles, the recently developed stochastic collision approach demonstrated some early promise. However, it was later realized that the appropriate sizing of a single nanoparticle by an electrochemical method could be far more challenging than initially expected owing to the dynamic motion of nanoparticles in electrolytes and complex charge-transfer characteristics at electrode surfaces. This clearly indicates a strong necessity to integrate single nanoparticle electrochemistry with high-resolution optical microscopy. Hence, this review aims to give a timely update of the latest progress for both electrochemically sensing and seeing single nanoparticles. A major focus is on collision-based measurements, where nanoparticles or single entities in solution impact on a collector electrode and the electrochemical response is recorded. These measurements are further enhanced with optical measurements in parallel. For completeness, advances in other related methods for single nanoparticle electrochemistry are also included.

Detection of influenza virus by electrochemical surface plasmon resonance under potential modulation

In this study we report the development of a novel viral pathogen immunosensor technology based on the electrochemical modulation of the optical signal from a surface plasmon wave interacting with a redox dye reporter. The device is formed by incorporating a sandwich immunoassay onto the surface of a plasmonic device mounted in a micro-electrochemical flow cell, where it is functionalized with a monoclonal antibody aimed to a specific target pathogen antigen. Once the target antigen is bound to the surface, it promotes the capturing of a secondary polyclonal antibody that has been conjugated with a redox-active methylene blue dye. The methylene blue displays a reversible change in the complex refractive index throughout a reduction-oxidation transition, which generates an optical signal that can be electrochemically modulated and detected at high sensitivity. For proof-of-principle measurements, we have targeted the hemagglutinin protein from the H5N1 avian influenza A virus to demonstrate the capabilities of our device for detection and quantification of a critical influenza antigen. Our experimental results of the EC-SPR-based immunosensor under potential modulation showed a 300 pM limit of detection for the H5N1 antigen.

Label-free surface-sensitive photonic microscopy with high spatial resolution using azimuthal rotation illumination

Surface plasmon resonance microscopy (SPRM) with single-direction illumination is a powerful platform for biomedical imaging because of its wide-field, label-free, and high-surface-sensitivity imaging capabilities. However, two disadvantages prevent wider use of SPRM. The first is its poor spatial resolution that can be as large as several micrometers. The second is that SPRM requires use of metal films as sample substrates; this introduces working wavelength limitations. In addition, cell culture growth on metal films is not as universally available as growth on dielectric substrates. Here we show that use of azimuthal rotation illumination allows SPRM spatial resolution to be enhanced by up to an order of magnitude. The metal film can also be replaced by a dielectric multilayer and then a different label-free surface-sensitive photonic microscopy is developed, which has more choices in terms of the working wavelength, polarization, and imaging section, and will bring opportunities for applications in biology.

Real-time monitoring of electrochemical reactions on single nanoparticles by dark-field and Raman microscopy

Dark-field and Raman microscopy to probe the single NP electrochemistry in real time.

Science and technology of electrochemistry at nano-interfaces: concluding remarks

The Faraday Discussion on electrochemistry at nano-interfaces presented a platform for an incredibly diverse array of advances in electrochemical nanoscience and nanotechnology. In this summary, I have identified the factors which drive the development of the science and which ultimately support many impressive technological advances described. Prime among these are the emergence of new physical behaviors when device dimensions approach characteristic physical scaling lengths, the steadily increasing importance of surfaces as device dimensions shrink, and the capacity to fabricate and utilize structures which are commensurate in size with molecules, especially biomolecules and biomolecular complexes. In this Faraday Discussion we were treated to outstanding examples of each of these nanoscience drivers to produce new, and in many cases unexpected, electrochemical phenomena that would not be observed at larger scales. The main thrust of these collective activities has been to realize the promise implicit in several transformational experiments that were carried out in the last decades of the 20th century. Our task is not complete, and we can look forward to many additional developments springing from the same intellectual wellhead.