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

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

Spectroelectrochemical determination of thiolate self-assembled monolayer adsorptive stability in aqueous and non-aqueous electrolytes

Self-assembled monolayers (SAM) are ubiquitous in studies of modified electrodes for sensing, electrocatalysis, and environmental and energy applications. However, determining their adsorptive stability is crucial to ensure robust experiments. In this work, the stable potential window (SPW) in which a SAM-covered electrode can function without inducing SAM desorption was determined for aromatic SAMs on gold electrodes in aqueous and non-aqueous solvents. The SPWs were determined by employing cyclic voltammetry, attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), and surface plasmon resonance (SPR). The electrochemical and spectroscopic findings concluded that all the aromatic SAMs used displayed similar trends and SPWs. In aqueous systems, the SPW lies between the reductive desorption and oxidative desorption, with pH being the decisive factor affecting the range of the SPW, with the widest SPW observed at pH 1. In the non-aqueous electrolytes, the desorption of SAMs was observed to be slow and progressive. The polarity of the solvent was the main factor in determining the SPW. The lower the polarity of the solvent, the larger the SPW, with 1-butanol displaying the widest SPW. This work showcases the power of spectroelectrochemical analysis and provides ample future directions for the use of non-polar solvents to increase SAM stability in electrochemical applications.

Non-destructive and simultaneous development and enhancement of latent fingerprints on stainless steel based on the electrochromic effect of electrodeposited manganese oxides

Latent fingerprints, as one of the most frequently encountered traces in crime scene investigation and also one of the largest sources of forensic evidence, can play a critical role in determining the identity of a person who may be involved in a crime. Due to the invisible characteristic of latent fingerprints, exploring efficient techniques to visualize them (especially the ones resided on metallic surfaces) while retain the biological and chemical information (e.g., touch DNA) has become a multidisciplinary research focus. Herein we reported a new and highly sensitive electrochemical interfacial strategy of simultaneously developing and enhancing latent fingerprints on stainless steel based on synchronous electrodeposition and electrochromism of manganese oxides in a neutral aqueous electrolyte. By utilizing a specially designed device for electrochemical testing and image capture, a series of electrochemical measurements, physical characterization and image analysis have been applied to evaluate the feasibility, development accuracy and enhancement efficacy of the proposed electrochemical system. The qualitative and quantitative analysis on the in situ and ex situ fingerprint images indicates that the three levels of fingerprint features can be precisely developed and effectively enhanced. Forensic DNA typing has also been performed to reveal actual impact of the proposed electrochemical system on subsequent analysis of touch DNA in fingerprint residues. The ratio of detected loci after electrochemical treatment reaches up to 98.5 %, showing non-destructive nature of this fingerprint development and enhancement technique.

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.

Sixty years of electrochemical optical spectroscopy: a retrospective

Sixty years ago, Reddy, Devanatan, and Bockris performed the first in situ electrochemical ellipsometry experiment, which ushered in a new era in the study of electrochemistry, using optical spectroscopy. After six decades of development, electrochemical optical spectroscopy, particularly electrochemical vibrational spectroscopy, has advanced from a phase of immaturity with few methods and limited applications to a phase of maturity with excellent substrate generality and significantly improved resolutions. Here, we divide the development of electrochemical optical spectroscopy into four phases, focusing on the proof-of-concept of different electrochemical optical spectroscopy studies, the emergence of plasmonic enhancement-based electrochemical optical spectroscopic (in particular vibrational spectroscopic) methods, the realization of electrochemical vibrational spectroscopy on well-defined surfaces, and the efforts to achieve operando spectroelectrochemical applications. Finally, we discuss the future development trend of electrochemical optical spectroscopy, as well as examples of new methodology and research paradigms for operando spectroelectrochemistry.

Cascaded momentum-space polarization filters enabled label-free black-field microscopy for single nanoparticles analysis

Label-free optical imaging of single-nanometer-scale matter is extremely important for a variety of biomedical, physical, and chemical investigations. One central challenge is that the background intensity is much stronger than the intensity of the scattering light from single nano-objects. Here, we propose an optical module comprising cascaded momentum-space polarization filters that can perform vector field modulation to block most of the background field and result in an almost black background; in contrast, only a small proportion of the scattering field is blocked, leading to obvious imaging contrast enhancement. This module can be installed in various optical microscopies to realize a black-field microscopy. Various single nano-objects with dimensions smaller than 20 nm appear distinctly in the black-field images. The chemical reactions occurring on single nanocrystals with edge lengths of approximately 10 nm are in situ real-time monitored by using the black-field microscopy. This label-free black-field microscopy is highly promising for a wide range of future multidisciplinary science applications.

Ultrasensitive Plasmon-Enhanced Infrared Spectroelectrochemistry

IR spectroelectrochemistry (EC-IR) is a cutting-edge operando method for exploring electrochemical reaction mechanisms. However, detection of interfacial molecules is challenged by the limited sensitivity of existing EC-IR platforms due to the lack of high-enhancement substrates. Here, we propose an innovative plasmon-enhanced infrared spectroelectrochemistry (EC-PEIRS) platform to overcome this sensitivity limitation. Plasmonic antennae with ultrahigh IR signal enhancement are electrically connected via monolayer graphene while preserving optical path integrity, serving as both the electrode and IR substrate. The [Fe(CN)6 ]3- /[Fe(CN)6 ]4- redox reaction and electrochemical CO2 reduction reaction (CO2 RR) are investigated on the EC-PEIRS platform with a remarkable signal enhancement. Notably, the enhanced IR signals enable a reconstruction of the electrochemical curve of the redox reactions and unveil the CO2 RR mechanism. This study presents a promising technique for boosting the in-depth understanding of interfacial events across diverse applications.

Label-Free Optical Imaging of Nanoscale Single Entities

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

Surface Plasmon Resonance Biosensors: A Review of Molecular Imaging with High Spatial Resolution

Surface plasmon resonance (SPR) is a powerful tool for determining molecular interactions quantitatively. SPR imaging (SPRi) further improves the throughput of SPR technology and provides the spatially resolved capability for observing the molecular interaction dynamics in detail. SPRi is becoming more and more popular in biological and chemical sensing and imaging. However, SPRi suffers from low spatial resolution due to the imperfect optical components and delocalized features of propagating surface plasmonic waves along the surface. Diverse kinds of approaches have been developed to improve the spatial resolution of SPRi, which have enormously impelled the development of the methodology and further extended its possible applications. In this minireview, we introduce the mechanisms for building a high-spatial-resolution SPRi system and present its experimental schemes from prism-coupled SPRi and SPR microscopy (SPRM) to surface plasmonic scattering microscopy (SPSM); summarize its exciting applications, including molecular interaction analysis, molecular imaging and profiling, tracking of single entities, and analysis of single cells; and discuss its challenges in recent decade as well as the promising future.

In Situ Mapping of Activity Distribution of V(II)/V(III) and Onset Potential Distribution of Hydrogen Evolution Side Reaction in Vanadium Flow Batteries

Vanadium flow batteries (VFBs) face a challenge with the low reaction rates of the V(II)/V(III) redox couple, which limits the performance of VFBs. Additionally, the negative electrode in VFBs is often accompanied by the persistent hydrogen evolution reaction (HER), which is difficult to eliminate. Therefore, understanding the spatial distribution of activity on the negative electrode and the HER side reaction on the electrode surface is of critical importance. This study proposes a weak measurement imaging method to characterize the spatial distribution of surface activity and HER onset potential on the negative electrode in VFBs). This method enables the visualization and in situ detection of key parameters such as the absolute values of |ipa |, |ipc |, |∆E|, |ipc /ipa |, and the HER onset potential. By comparing three different types of graphite felts with varying activity levels, it validates the feasibility of this method. Furthermore, electrochemical stability tests are conducted to study the electrodes repeatability, uniformity, and durability. This method holds promise in guiding the design of electrodes with enhanced activity, good reversibility, minimized HER side reactions, and uniform distribution.

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

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

Intellectualized Visualization of Single-Particle Raman Spectra for Sensitive Detection and Simultaneous Multianalysis of Heavy Metal Ions

Easy-to-use, reliable, and real-time methods for detecting heavy metal ion contamination are urgently required, which is a primary concern for water pollution control and human health. However, present methods for this aim are still unable to achieve simultaneous multianalysis for complex real sample detection. Herein, an intellectualized vision-based single-nanoparticle Raman imaging strategy combined with ion-responsive functional nucleic acids (FNAs) was proposed to address these issues. We reported a correspondence between the concentration of the analytes and the density of particles (DOP) of specifically captured nanoparticles to achieve sensitive detection and simultaneous multianalysis of heavy metal ions. The specific detection of Pb2+ (Hg2+) was obtained with a detection linear range from 100 pM to 100 nM (from 500 fM to 100 nM) and limit of detections low to 1 pM (100 fM), with the advantages of good specificity, excellent homogeneity, and reproducibility. Furthermore, the differentiation of different heavy metal ions (Pb2+/Hg2+) was achieved, i.e., the simultaneous multianalysis, based on Raman imaging of the single particle and intelligent machine vision method. Finally, the Raman imaging assay was utilized for real sample analysis, and it provided a powerful and reliable tool for detecting trace Pb2+/Hg2+ in real water samples and facilitated the portable on-site monitoring of heavy metal ions.

Dynamic imaging of interfacial electrochemistry on single Ag nanowires by azimuth-modulated plasmonic scattering interferometry

Direct visualization of surface chemical dynamics in solution is essential for understanding the mechanisms involved in nanocatalysis and electrochemistry; however, it is challenging to achieve high spatial and temporal resolution. Here, we present an azimuth-modulated plasmonic imaging technique capable of imaging dynamic interfacial changes. The method avoids strong interference from reflected light and consequently eliminates the parabolic-like interferometric patterns in the images, allowing for a 67-fold increase in the spatial resolution of plasmonic imaging. We demonstrate that this optical imaging approach enables comprehensive analyses of surface chemical dynamics and identification of previously unknown surface reaction heterogeneity by investigating electrochemical redox reactions over single silver nanowires as an example. This work provides a general strategy for high-resolution plasmonic imaging of surface electrochemical dynamics and other interfacial chemical reactions, complementing existing surface characterization methods.

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

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

Progressive and instantaneous nature of lithium nucleation discovered by dynamic and operando imaging

The understanding of lithium (Li) nucleation and growth is important to design better electrodes for high-performance batteries. However, the study of Li nucleation process is still limited because of the lack of imaging tools that can provide information of the entire dynamic process. We developed and used an operando reflection interference microscope (RIM) that enables real-time imaging and tracking the Li nucleation dynamics at a single nanoparticle level. This dynamic and operando imaging platform provides us with critical capabilities to continuously monitor and study the Li nucleation process. We find that the formation of initial Li nuclei is not at the exact same time point, and Li nucleation process shows the properties of both progressive and instantaneous nucleation. In addition, the RIM allows us to track the individual Li nucleus's growth and extract spatially resolved overpotential map. The nonuniform overpotential map indicates that the localized electrochemical environments substantially influence the Li nucleation.

Reversible Regulation of Long-Distance Charge Transport in DNA Nanowires by Dynamically Controlling Steric Conformation

Understanding of DNA-mediated charge transport (CT) is significant for exploring circuits at the molecular scale. However, the fabrication of robust DNA wires remains challenging due to the persistence length and natural flexibility of DNA molecules. Moreover, CT regulation in DNA wires often relies on predesigned sequences, which limit their application and scalability. Here, we addressed these issues by preparing self-assembled DNA nanowires with lengths of 30-120 nm using structural DNA nanotechnology. We employed these nanowires to plug individual gold nanoparticles into a circuit and measured the transport current in nanowires with an optical imaging technique. Contrary to the reported cases with shallow or no length dependence, a fair current attenuation was observed with increasing nanowire length, which experimentally confirmed the prediction of the incoherent hopping model. We also reported a mechanism for the reversible CT regulation in DNA nanowires, which involves dynamic transitions in the steric conformation.

In Situ Detection of Electrochemical Reaction Surface Area by Optical Weak Measurement

The surface area is key to electrochemical systems, including those in electrocatalysis and energy storage. Studies have shown that the surface area of the electrocatalyst directly affects the electrochemical activity, adsorption performance, and stability of the electrocatalyst. This paper used an optical weak measurement (WM) method, which has little impact on the analyte, to measure the reaction surface area (RSA) that actually participated in the electrochemical reaction. Then compared the RSA obtained by the WM with the total surface area (TSA) obtained by the standard Brunauer-Emmett-Teller (BET) measurement and the active surface area (ASA) obtained by the electrochemical double-layer capacitance (EDLC) method. Their growth trend was consistent, indicating the reliability of the WM method. Compared with the above two methods, the WM method is an in situ detection and easy to operate experimentally, which can help researchers to consider the effect of surface area on electrocatalyst performance more rationally.

In Situ Confocal Fluorescence Lifetime Imaging of Nanopore Electrode Arrays with Redox Active Fluorogenic Amplex Red

Direct imaging of electrochemical processes on electrode surfaces is a central part of understanding spatially heterogeneous electrochemical processes on the surfaces. Herein, we report a strategy for the spatially resolved imaging of local faradaic processes on nanoscale electrochemical interfaces. This strategy is based on fluorescence lifetime imaging microscopy (FLIM) with the use of Amplex Red as a fluorogenic redox probe. After verifying the capability of Amplex Red for fluorescence lifetime imaging, we demonstrated the turn-on FLIM-based imaging of faradaic processes on the electrochemical interfaces of different dimensions. In particular, we achieved spatially resolved visualization of the local electrochemical processes occurring on even nanopore electrode arrays as well as conventional microelectrodes, including disk-shaped ultramicroelectrodes and interdigitated array microelectrodes.

Plasmon-Enhanced Electrochemiluminescence at the Single-Nanoparticle Level

Plasmon-enhanced electrochemiluminescence (ECL) at the single-nanoparticle (NP) level was investigated by ECL microscopy. The Au NPs were assembled into an ordered array, providing a high-throughput platform that can easily locate each NP in sequential characterizations. A strong dependence of ECL intensity on Au NP configurations was observed. We demonstrate for the first time that at the single-particle level, the ECL of Ru(bpy)₃ ²⁺ -TPrA was majorly quenched by small Au NPs (<40 nm), while enhanced by large Au ones (>80 nm) due to the localized surface plasmon resonance (LSPR). Notably, the ECL intensity was further increased by the coupling effect of neighboring Au NPs. Finite Difference Time Domain (FDTD) simulations conformed well with the experimental results. This plasmon enhanced ECL microscopy for arrayed single NPs provides a reliable tool for screening electrocatalytic activity at a single particle.

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

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

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.

Label-free imaging and biomarker analysis of exosomes with plasmonic scattering microscopy

Exosome analysis is a promising tool for clinical and biological research applications. However, detection and biomarker quantification of exosomes is technically challenging because they are small and highly heterogeneous. Here, we report an optical approach for imaging exosomes and quantifying their protein markers without labels using plasmonic scattering microscopy (PSM). PSM can provide improved spatial resolution and distortion-free image compared to conventional surface plasmon resonance (SPR) microscopy, with the signal-to-noise ratio similar to objective coupled surface plasmon resonance (SPR) microscopy, and millimeter-scale field of view as a prism-coupled SPR system, thus allowing exosome size distribution analysis with high throughput. In addition, PSM retains the high specificity and surface sensitivity of the SPR sensors and thus allows selection of exosomes from extracellular vesicles with antibody-modified sensor surfaces and in situ analyzing binding kinetics between antibody and the surface protein biomarkers on the captured exosomes. Finally, the PSM can be easily constructed on a popular prism-coupled SPR system with commercially available components. Thus, it may provide an economical and powerful tool for clinical exosome analysis and exploration of fundamental issues such as exosome biomarker binding properties.

Learning from the Heterogeneity at Electrochemical Interfaces

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

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

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

Label-Free Imaging of Single Proteins and Binding Kinetics Using Total Internal Reflection-Based Evanescent Scattering Microscopy

Single-molecule detection can push beyond ensemble averages and reveal the statistical distributions of molecular properties. Measuring the binding kinetics of single proteins also represents one of the critical and challenging tasks in protein analysis. Here, we report total internal reflection-based evanescent scattering microscopy with label-free single-protein detection capability. Total internal reflection is employed to excite the evanescent field to enhance light-analyte interaction and reduce environmental noise. As a result, the system provides wide-field imaging capability and allows excitation and observation using one objective. In addition, this system quantifies protein binding kinetics by simultaneously counting the binding of individual molecules and recording their binding sites with nanometer precision, providing a digital method to measure binding kinetics with high spatiotemporal resolution. This approach does not employ specially designed microspheres or nanomaterials and may pave a way for label-free single-protein analysis in conventional microscopy.

Emerging Optical Microscopy Techniques for Electrochemistry

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

Spatiotemporal-Resolved Hyperspectral Raman Imaging of Plasmon-Assisted Reactions at Single Hotspots

Raman spectroscopy facilitates the study of reacting molecules on single nanomaterials. In recent years, the temporal resolution of Raman spectral measurement has been remarkably reduced to the millisecond level. However, the classic scan-based imaging mode limits the application in the dynamical study of reactions at multiple nanostructures. In this paper, we propose a spatiotemporal-resolved Raman spectroscopy (STRS) technology to achieve fast (∼40 ms) and high spatial resolution (∼300 nm) hyperspectral Raman imaging of single nanostructures. With benefits of the outstanding electromagnetic field enhancement factor by surface plasmon resonance (∼10¹²) and the snapshot hyperspectral imaging strategy, we demonstrate the observation of stepwise Raman signals from single-particle plasmon-assisted reactions. Results reveal that the reaction kinetics is strongly affected by not only the surface plasmon-polariton generation but also the density of Raman molecules. In consideration of the spatiotemporal resolving capability of STRS, we anticipate that it provides a potential platform for further extending the application of Raman spectroscopy methods in the dynamic study of 1D or 2D nanostructures.

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.

Infrared surface plasmons on a Au waveguide electrode open new redox channels associated with the transfer of energetic carriers

Plasmonic catalysis holds promise for opening new reaction pathways inaccessible thermally or for improving the efficiency of chemical processes. We report a gold stripe waveguide along which infrared (λ₀ ~ 1350 nanometers) surface plasmon polaritons (SPPs) propagate, operating simultaneously as an electrochemical working electrode. Cyclic voltammograms obtained under SPP excitation enable oxidative processes involving energetic holes to be investigated separately from reductive processes involving energetic electrons. Under SPP excitation, redox currents increase by 10×, redox potentials decrease by ~2× and split in correlation with photon energy, and the charge transfer resistance drops by ~2× as measured using electrochemical impedance spectroscopy. The temperature of the working electrode was monitored in situ, ruling out thermal effects. Chronoamperometry measurements with SPPs modulated at 600 hertz yield a commensurately modulated current response, ruling out thermally enhanced mass transport. Our observations indicate opening of optically controlled nonequilibrium redox channels associated with energetic carrier transfer to the redox species.

Thermal Stability and Kinetics of Single I2@ZIF-8 Particles

Thermogravimetric analysis (TGA) is a key material characterization method for studying the thermal stability and thermochemical process. However, the common TGA for bulk samples lacks sufficient spatial information, which blurs the intrinsic thermal decomposition characteristic and limits the understanding of the structure-performance relationship. Here, we report a dark-field microscope (DFM) method for studying thermal desorption process of I₂ from I₂-loaded zeolitic imidazolate framework-8 (I₂@ZIF-8). Because of the high spatial resolution, DFM enables the imaging and tracking of the local mass loss of I₂ in single I₂@ZIF-8 particles at different reaction temperatures. We obtain from the DFM images the single-particle thermogravimetric and differential thermogravimetric curves to evaluate the inherent thermal stability of single I₂@ZIF-8 particles. We also find the heterogeneous thermal decomposition property among different I₂@ZIF-8 particles. Furthermore, we demonstrate the capacity of DFM to quantitatively determine thermal kinetics parameters such as the diffusion coefficient and activation energy of I₂ in individual and multiple ZIF-8 particles. These useful results are essential for developing high-efficient porous adsorbents for the capture of I₂.

Single-Particle Electrochemical Imaging Provides Insights into Silver Nanoparticle Dissolution at the Solution-Solid Interface

Dissolution of nanoparticles is an environmental interfacial process that affects the transformation of nanoparticles. Understanding the dissolution processes of nanoparticles is important to predict their fate in the aquatic environment. However, studying nanoparticle dissolution kinetics is still challenging since dissolution is usually coupled with nanoparticle aggregation. Here, we probed the dissolution process of Ag nanoparticles at the single-particle level by surface plasmon resonance microscopy. The single-particle imaging capability enabled us to classify Ag nanoparticles, measure the dissolution dynamics of single nanoparticles, and correlate the aggregation size with oxidation activity. Moreover, we studied the dual effect of natural organic matter on the dissolution of Ag nanoparticles and validated this result with real natural freshwater. Our study provides new insights into the dissolution of Ag nanoparticles, and this technique can be extended for other nanomaterials to evaluate their fate in aquatic environments.

Chemical-Based Surface Plasmon Resonance Imaging of Fingerprints

Fingerprints are extremely useful in personal identification; however, they are usually based on physical rather than chemical images because it remains a challenge to reveal a clear chemical fingerprint easily and sensitively. Herein, a surface plasmon resonance imaging (SPRi) method, combined with a chemically selective stepwise signal amplification (CS³A) strategy, is proposed to chemically image fingerprints with adjustable sensitivity and clarity. High-fidelity glucose-associated fingerprint images were obtained at five to seven cycles of CS³A based on the recognition reaction of concanavalin A (ConA) with dextran. The method is also extendable to image substances that possess and/or can be tagged with ConA- or dextran-recognizable groups. For demonstration, SPRi of carboxylic substances was conducted by amidating the carboxyl group with glucosamine to enable the ConA-based CS³A. Glucose- and carboxyl-based fingerprints were simultaneously and clearly imaged, allowing us to perform quantitative analysis of the representative of either glucose or amino acid (e.g., serine) or both. The curves measured from the standard spots were linear in the ranges of 1-4000 μM for glucose and 3.2-4000 μM for serine, with linear correlated coefficients of 0.9979 and 0.9962, respectively. It was then applied to the study of metabolic secretions in fingerprints during running exercise, yielding variation tendencies similar to those measured from sweat samples in the literature. As a noninvasive tool, the CS³A-coupled SPRi reveals both clear images of fingerprints and quantitative chemical information, and it is anticipated to become a competitive new method for chemically imaging fingerprints.

Dynamic single-molecule sensing by actively tuning binding kinetics for ultrasensitive biomarker detection

SignificanceThe detection of low-abundance molecular biomarkers is key to the liquid-biopsy-based disease diagnosis. Existing methods are limited by the affinity and specificity of recognition probes and the mass transportation of analyte molecules onto the sensor surfaces, resulting in insufficient sensitivity and long assay time. This work establishes a rapid and ultrasensitive approach by actively tuning binding kinetics and accelerating the mass transportation via nanoparticle micromanipulations. This is significant because it permits extremely sensitive measurements within clinically acceptable assay time. It is incubation-free, washing-free, and compatible with low- and high-affinity probes.

Preparation of Novel Magnetic Nanomaterials Based on "Facile Coprecipitation" for Developing Latent Fingerprints (LFP) in Crime Scenes

Recently, the application of novel nanomaterials, especially magnetic nanomaterials in the development of latent fingerprints (LFP), has become the hot focus for forensic scientists and criminal investigators. As a type of recyclable, environment-friendly material, Fe₃O₄ nanoparticles achieve a wonderful effect in visualization of LFP. We first report the synthesis and encapsulation of nano-Fe₃O₄ through "facile coprecipitation", (3-mercaptopropyl)triethoxysilane was covalently embedded into Fe₃O₄ nanoparticles, and the Fe₃O₄ core was encapsulated by the nanosilver to prepare novel magnetic nanomaterials (P-MNP@Ag) with the core-shell configuration. For comparison, the magnetic nanomaterials (S-MNP@Ag) were prepared by surface modification. Their composition, structure, and properties were characterized by SEM, TEM, XRD, IR, XPS, and VSM. Compared with commercially available gold powder, silver powder, bare magnetic powder, and prepared S-MNP@Ag, the development effect of LFP on different objects by using P-MNP@Ag had better performance, which presented the advantages of low background interference, high sensitivity, and clear secondary details in LFP. In the crime scenes of some influential cases, P-MNP@Ag had been applied to the visualization of LFP. The biometric identification of criminal suspects was confirmed through fingerprint comparison, which was highly affirmed by the public security department.

Combination of Capped Gold Nanoslit Array and Electrochemistry for Sensitive Aqueous Mercuric Ions Detection

Label-free surface plasmon resonance (SPR) detection of mercuric ions in various aqueous solutions, using capped gold nanoslit arrays combined with electrochemical (EC) sensing technique, is demonstrated. The nanoslit arrays are fabricated on flexible cyclo-olefin polymer substrates by a nanoimprinting lithography method. The EC and SPR signals for the investigation of current responses and transmission SPR spectra are simultaneously measured during metal ions electrodeposition. Glycerol-water solution is studied to evaluate the resonant peak wavelength sensitivity (480.3 nm RIU^-1) with a FOM of 40.0 RIU^-1 and the obtained intensity sensitivity is 1819.9%. The ferrocyanide/ferricyanide redox couple performs the diffusion controlled electrochemical processes (R² = 0.99). By investigating the SPR intensity changes and wavelength shifts of various mercuric ion concentrations, the optical properties are evaluated under chronoamperometric conditions. The sensors are evaluated in the detection range between 100 μM and 10 nM with a detection limit of 1 μM. The time dependence of SPR signals and the selectivity of 10 μM Hg²⁺ in the presence of 10 μM interfering metal ion species from Ca²⁺, Co²⁺, Ni²⁺, Na⁺, Cu²⁺, Pb^{2 +} and Mn²⁺ are determined. The capped gold nanoslit arrays show the selectivity of Hg²⁺ and the EC sensing method is effectively utilized to aqueous Hg²⁺ detection. This study provides a label-free detection technique of mercuric ions and this developed system is potentially applicable to detecting chemicals and biomolecules.

Spiers Memorial Lecture. Next generation nanoelectrochemistry: the fundamental advances needed for applications

Nanoelectrochemistry, where electrochemical processes are controlled and investigated with nanoscale resolution, is gaining more and more attention because of the many potential applications in energy and sensing and the fact that there is much to learn about fundamental electrochemical processes when we explore them at the nanoscale. The development of instrumental methods that can explore the heterogeneity of electrochemistry occurring across an electrode surface, monitoring single molecules or many single nanoparticles on a surface simultaneously, have been pivotal in giving us new insights into nanoscale electrochemistry. Equally important has been the ability to synthesise or fabricate nanoscale entities with a high degree of control that allows us to develop nanoscale devices. Central to the latter has been the incredible advances in nanomaterial synthesis where electrode materials with atomic control over electrochemically active sites can be achieved. After introducing nanoelectrochemistry, this paper focuses on recent developments in two major application areas of nanoelectrochemistry; electrocatalysis and using single entities in sensing. Discussion of the developments in these two application fields highlights some of the advances in the fundamental understanding of nanoelectrochemical systems really driving these applications forward. Looking into our nanocrystal ball, this paper then highlights: the need to understand the impact of nanoconfinement on electrochemical processes, the need to measure many single entities, the need to develop more sophisticated ways of treating the potentially large data sets from measuring such many single entities, the need for more new methods for characterising nanoelectrochemical systems as they operate and the need for material synthesis to become more reproducible as well as possess more nanoscale control.

Application of Tamm Plasmon Polaritons and Cavity Modes for Biosensing in the Combined Spectroscopic Ellipsometry and Quartz Crystal Microbalance Method

Low-cost 1D plasmonic photonic structures supporting Tamm plasmon polaritons and cavity modes were employed for optical signal enhancement, modifying the commercially available quartz crystal microbalance with dissipation (QCM-D) sensor chip in a combinatorial spectroscopic ellipsometry and quartz microbalance method. The Tamm plasmon optical state and cavity mode (CM) for the modified mQCM-D sample obtained sensitivity of ellipsometric parameters to RIU of Ψ_TPP = 126.78 RIU^-1 and Δ_TPP = 325 RIU^-1, and Ψ_CM = 264 RIU^-1 and Δ_CM = 645 RIU^-1, respectively. This study shows that Tamm plasmon and cavity modes exhibit about 23 and 49 times better performance of ellipsometric parameters, respectively, for refractive index sensing than standard spectroscopic ellipsometry on a QCM-D sensor chip. It should be noted that for the optical biosensing signal readout, the sensitivity of Tamm plasmon polaritons and cavity modes are comparable with and higher than the standard QCM-D sensor chip. The different origin of Tamm plasmon polaritons (TPP) and cavity mode (CM) provides further advances and can determine whether the surface (TPP) or bulk process (CM) is dominating. The dispersion relation feature of TPP, namely the direct excitation without an additional coupler, allows the possibility to enhance the optical signal on the sensing surface. To the best of our knowledge, this is the first study and application of the TPP and CM in the combinatorial SE-QCM-D method for the enhanced readout of ellipsometric parameters.

Planar photonic chips with tailored angular transmission for high-contrast-imaging devices

A limitation of standard brightfield microscopy is its low contrast images, especially for thin specimens of weak absorption, and biological species with refractive indices very close in value to that of their surroundings. We demonstrate, using a planar photonic chip with tailored angular transmission as the sample substrate, a standard brightfield microscopy can provide both darkfield and total internal reflection (TIR) microscopy images with one experimental configuration. The image contrast is enhanced without altering the specimens and the microscope configurations. This planar chip consists of several multilayer sections with designed photonic band gaps and a central region with dielectric nanoparticles, which does not require top-down nanofabrication and can be fabricated in a larger scale. The photonic chip eliminates the need for a bulky condenser or special objective to realize darkfield or TIR illumination. Thus, it can work as a miniaturized high-contrast-imaging device for the developments of versatile and compact microscopes.

The authors design a planar photonic chip with several multilayers of photonic band gaps and a region of dielectric nanoparticles for tailored angular transmission. They use it as sample substrate for high-contrast darkfield and total internal reflection microscopy on a conventional microscope.

Characterizing the Onset Potential Distribution of Pt/C Catalyst Deposition by a Total Internal Reflection Imaging Method

A catalytic electrode with extraordinary performances for hydrogen evolution reaction (HER) should achieve a low onset potential of the bulk electrode, as well as its uniform distribution. Herein, a total internal reflection imaging (TIRi) method to characterize the onset potential distribution of the catalytic electrode surface is presented. When the potential scans toward negative in a linear sweep voltammetry, the equivalent refractive index of the electrolyte on the electrode surface will decrease due to H₂ microbubbles generation, leading to the increase in optical intensity. Analysis of the relationship between the optical intensity and potential in each region results in the onset potential distribution. The TIRi method reveals poor uniformity and repeatability in the catalytic electrodes which are fabricated by depositing Pt/C catalysts on a porous carbon support with polymer binders (e.g., Nafion). Further electrochemical stability test also shows poor durability, whose HER onset potential deteriorates from the edge to the middle of these catalytic electrodes. The present TIRi method realizes direct visualization of the activity distribution on the bulk electrode surface, which provides a powerful tool for better fabrication and evaluation of large-area HER electrodes in industrial energy devices.

Localized surface plasmon resonance for enhanced electrocatalysis

Electrocatalysis plays a vital role in energy conversion and storage in modern society. Localized surface plasmon resonance (LSPR) is a highly attractive approach to enhance the electrocatalytic activity and selectivity with solar energy. LSPR excitation can induce the transfer of hot electrons and holes, electromagnetic field enhancement, lattice heating, resonant energy transfer and scattering, in turn boosting a variety of electrocatalytic reactions. Although the LSPR-mediated electrocatalysis has been investigated, the underlying mechanism has not been well explained. Moreover, the efficiency is strongly dependent on the structure and composition of plasmonic metals. In this review, the currently proposed mechanisms for plasmon-mediated electrocatalysis are introduced and the preparation methods to design supported plasmonic nanostructures and related electrodes are summarized. In addition, we focus on the characterization strategies used for verifying and differentiating LSPR mechanisms involved at the electrochemical interface. Following that are highlights of representative examples of direct plasmonic metal-driven and indirect plasmon-enhanced electrocatalytic reactions. Finally, this review concludes with a discussion on the remaining challenges and future opportunities for coupling LSPR with electrocatalysis.

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.

Imaging Sensor for the Detection of the Flow Battery Via Weak Value Amplification

Flow battery electrodes are vital for performing redox reactions, and an in-depth understanding of reaction kinetics and spatial distribution differences in electrodes is very important for improving the efficiency of electrochemical reactions. In this study, a reflection-type phase-sensitive weak measurement imaging system was developed for the detection of flow batteries. The phase difference between two polarization components in total internal reflection caused by electrode redox processes was measured by weak value amplification. The resulting refractive index resolution of the imaging system was estimated to be 2.8-4.2 × 10^-6 RIU. The real-time monitoring ability of the system was demonstrated by linear sweep voltammetry tests of vanadium redox batteries. Compared to traditional optical methods, the proposed weak measurement imaging sensor did not require coating, as it can be used in acid electrolytes of vanadium flow batteries. Meanwhile, the weak value amplification effect led to a higher resolution than the total internal reflection system shown in our previous work, thereby resulting in more accurate detection of electrochemical reactions. In sum, the proposed sensor looks very promising for the detection of electrochemical reactions in flow batteries, water splitting, electrochemical corrosion, and electrocatalysis.

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.