Imaging Local Electric Field Distribution by Plasmonic Impedance Microscopy

Deep-Learning Driven, High-Precision Plasmonic Scattering Interferometry for Single-Particle Identification

Label-free probing of the material composition of (bio)nano-objects directly in solution at the single-particle level is crucial in various fields, including colloid analysis and medical diagnostics. However, it remains challenging to decipher the constituents of heterogeneous mixtures of nano-objects with high sensitivity and resolution. Here, we present deep-learning plasmonic scattering interferometric microscopy, which is capable of identifying the composition of nanoparticles automatically with high throughput at the single-particle level. By employing deep learning to decode the quantitative relationship between the interferometric scattering patterns of nanoparticles and their intrinsic material properties, this technique is capable of high-throughput, label-free identification of diverse nanoparticle types. We demonstrate its versatility in analyzing dynamic surface chemical reactions on single nanoparticles, revealing its potential as a universal platform for nanoparticle imaging and reaction analysis. This technique not only streamlines the process of nanoparticle characterization, but also proposes a methodology for a deeper understanding of nanoscale dynamics, holding great potential for addressing extensive fundamental questions in nanoscience and nanotechnology.

Imaging and Quantifying the Chemical Communication between Single Particles in Metal Alloys

The communication within particle agglomerates in industrial alloys can have a significant impact on the macroscopic reactivity, putting a high demand on the adaptation of wide-field methodologies to clarify this phenomenon. In this work, we report the application of correlated optical microscopies probing operando both local pH and local surface chemical transformation correlated with identical location scanning electron microscopy to quantify in situ the structure reactivity of particle agglomerates of foreign elements in the Al alloy. The optical operando analyses allow us (i) to reveal and quantify the local production of OH^- from proton and oxygen reduction at individual Si- or Fe-rich microparticles and (ii) to quantify (and model) the chemical communication between these active sites, within a few micrometer range, on the local chemical transformation of the material. Wide-field image analysis highlights the statistical importance of chemical communication that may introduce a new conceptual framework for the understanding of the mechanisms in related fields of charge transfer, electrocatalysis, and corrosion.

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.

Operando Scanning Electrochemical Probe Microscopy during Electrocatalysis

Scanning electrochemical probe microscopy (SEPM) techniques can disclose the local electrochemical reactivity of interfaces in single-entity and sub-entity studies. Operando SEPM measurements consist of using a SEPM tip to investigate the performance of electrocatalysts, while the reactivity of the interface is simultaneously modulated. This powerful combination can correlate electrochemical activity with changes in surface properties, e.g., topography and structure, as well as provide insight into reaction mechanisms. The focus of this review is to reveal the recent progress in local SEPM measurements of the catalytic activity of a surface toward the reduction and evolution of O₂ and H₂ and electrochemical conversion of CO₂. The capabilities of SEPMs are showcased, and the possibility of coupling other techniques to SEPMs is presented. Emphasis is given to scanning electrochemical microscopy (SECM), scanning ion conductance microscopy (SICM), electrochemical scanning tunneling microscopy (EC-STM), and scanning electrochemical cell microscopy (SECCM).

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.

Electrochemical Impedance Measurements in Scanning Ion Conductance Microscopy

Electrochemical impedance spectroscopy (EIS) is a versatile tool for electrochemistry, particularly when applied locally to reveal the properties and dynamics of heterogeneous interfaces. A new method to generate local electrochemical impedance spectra is outlined, by applying a harmonic bias between a quasi-reference counter electrode (QRCE) placed in a nanopipet tip of a scanning ion conductance microscope (SICM) and a conductive (working electrode) substrate (two-electrode setup). The AC frequency can be tuned so that the magnitude of the impedance is sensitive to the tip-to-substrate distance, whereas the phase angle is broadly defined by the local capacitive response of the electrical double layer (EDL) of the working electrode. This development enables the surface topography and the local capacitance to be sensed reliably, and separately, in a single measurement. Further, self-referencing the probe impedance near the surface to that in the bulk solution allows the local capacitive response of the working electrode substrate in the overall AC signal to be determined, establishing a quantitative footing for the methodology. The spatial resolution of AC-SICM is an order of magnitude larger than the tip size (100 nm radius), for the studies herein, due to frequency dispersion. Comprehensive finite element method (FEM) modeling is undertaken to optimize the experimental conditions and minimize the experimental artifacts originating from the frequency dispersion phenomenon, and provides an avenue to explore the means by which the spatial resolution could be further improved.

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

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

Effect of Substrate Permeability on Scanning Ion Conductance Microscopy: Uncertainty in Tip-Substrate Separation and Determination of Ionic Conductivity

Composite electrodes can significantly improve the performance of an electrochemical device by maximizing surface area and active material loading. Typically, additives such as carbon are used to improve conductivity and a polymer is used as a binder, leading to a heterogeneous surface film with thickness on the order of 10s of micrometers. For such composite electrodes, good ionic conduction within the film is critical to capitalize on the increased loading of active material and surface area. Ionic conductivity within a film can be tricky to measure directly, and homogenization models based on porosity are often used as a proxy. SICM has traditionally been a topography-mapping microscopy method for which we here outline a new function and demonstrate its capacity for measuring ion conductivity within a lithium-ion battery film.

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

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

Emerging tools for studying single entity electrochemistry

Electrochemistry studies charge transfer and related processes at various microscopic structures (atomic steps, islands, pits and kinks on electrodes), and mesoscopic materials (nanoparticles, nanowires, viruses, vesicles and cells) made by nature and humans, involving ions and molecules. The traditional approach measures averaged electrochemical quantities of a large ensemble of these individual entities, including the microstructures, mesoscopic materials, ions and molecules. There is a need to develop tools to study single entities because a real system is usually heterogeneous, e.g., containing nanoparticles with different sizes and shapes. Even in the case of "homogeneous" molecules, they bind to different microscopic structures of an electrode, assume different conformations and fluctuate over time, leading to heterogeneous reactions. Here we highlight some emerging tools for studying single entity electrochemistry, discuss their strengths and weaknesses, and provide personal views on the need for tools with new capabilities for further advancing single entity electrochemistry.

Temporal–spatial-resolved mapping of the electrical double layer changes by surface plasmon resonance imaging

An electrical double layer (EDL) is a specific distribution of ions at the electrolyte/electrode interface. As EDL plays a decisive role in the interfacial physical and chemical characteristics, a comprehensive and quantitative understanding of the EDL structure and its change dynamics is important for a wide range of fields, ranging from electrochemistry, energy storage and semiconductor materials to biotechnology. In this paper, we proposed a proof of concept method for temporal- and spatial-resolved mapping of the EDL structure and its change dynamics. A potential was applied on the interface and the potential induced ion re-arrangement process was monitored by surface plasmon resonance (SPR) imaging in real time. NaCl experiments were repeated six times and the coefficient of variation of the results was 5.17%, confirming the potential-induced SPR response. Experiments with different potential excitations, ion concentrations and species were performed and results indicated that the electron density change and ion re-arrangement contributed comparably to the potential induced SPR response. Additionally, the lateral distribution of the EDL formed at the interface between NaCl solutions and an Au film coated with arrays of 11-MUA spots was mapped. This method is temporally and spatially resolved, and thus has the potential to be a promising tool for EDL studies at heterogeneous interfaces.

This paper provides a novel method for temporal–spatial-resolved mapping of the electrical double layer changes at the heterogeneous interface.

Plasmonic Imaging of Electrochemical Impedance

Electrochemical impedance spectroscopy (EIS) measures the frequency spectrum of an electrochemical interface to resist an alternating current. This method allows label-free and noninvasive studies on interfacial adsorption and molecular interactions and has applications in biosensing and drug screening. Although powerful, traditional EIS lacks spatial resolution or imaging capability, hindering the study of heterogeneous electrochemical processes on electrodes. We have recently developed a plasmonics-based electrochemical impedance technique to image local electrochemical impedance with a submicron spatial resolution and a submillisecond temporal resolution. In this review, we provide a systematic description of the theory, instrumentation, and data analysis of this technique. To illustrate its present and future applications, we further describe several selected samples analyzed with this method, including protein microarrays, two-dimensional materials, and single cells. We conclude by summarizing the technique's unique features and discussing the remaining challenges and new directions of its application.

Imaging electric field dynamics with graphene optoelectronics

The use of electric fields for signalling and control in liquids is widespread, spanning bioelectric activity in cells to electrical manipulation of microstructures in lab-on-a-chip devices. However, an appropriate tool to resolve the spatio-temporal distribution of electric fields over a large dynamic range has yet to be developed. Here we present a label-free method to image local electric fields in real time and under ambient conditions. Our technique combines the unique gate-variable optical transitions of graphene with a critically coupled planar waveguide platform that enables highly sensitive detection of local electric fields with a voltage sensitivity of a few microvolts, a spatial resolution of tens of micrometres and a frequency response over tens of kilohertz. Our imaging platform enables parallel detection of electric fields over a large field of view and can be tailored to broad applications spanning lab-on-a-chip device engineering to analysis of bioelectric phenomena.

Detection of electric fields, central to chemical and biological processes, has been limited to measurements of current (e.g., electrodes) and secondary reporters (e.g., fluorescent dyes). Here, the authors demonstrate an optical platform capable of imaging electric field dynamics with high spatio-temporal resolution.

Plasmonic Imaging of Electrochemical Reactions of Single Nanoparticles

Electrochemical reactions are involved in many natural phenomena, and are responsible for various applications, including energy conversion and storage, material processing and protection, and chemical detection and analysis. An electrochemical reaction is accompanied by electron transfer between a chemical species and an electrode. For this reason, it has been studied by measuring current, charge, or related electrical quantities. This approach has led to the development of various electrochemical methods, which have played an essential role in the understanding and applications of electrochemistry. While powerful, most of the traditional methods lack spatial and temporal resolutions desired for studying heterogeneous electrochemical reactions on electrode surfaces and in nanoscale materials. To overcome the limitations, scanning probe microscopes have been invented to map local electrochemical reactions with nanometer resolution. Examples include the scanning electrochemical microscope and scanning electrochemical cell microscope, which directly image local electrochemical reaction current using a scanning electrode or pipet. The use of a scanning probe in these microscopes provides high spatial resolution, but at the expense of temporal resolution and throughput. This Account discusses an alternative approach to study electrochemical reactions. Instead of measuring electron transfer electrically, it detects the accompanying changes in the reactant and product concentrations on the electrode surface optically via surface plasmon resonance (SPR). SPR is highly surface sensitive, and it provides quantitative information on the surface concentrations of reactants and products vs time and electrode potential, from which local reaction kinetics can be analyzed and quantified. The plasmonic approach allows imaging of local electrochemical reactions with high temporal resolution and sensitivity, making it attractive for studying electrochemical reactions in biological systems and nanoscale materials with high throughput. The plasmonic approach has two imaging modes: electrochemical current imaging and interfacial impedance imaging. The former images local electrochemical current associated with electrochemical reactions (faradic current), and the latter maps local interfacial impedance, including nonfaradic contributions (e.g., double layer charging). The plasmonic imaging technique can perform voltammetry (cyclic or square wave) in an analogous manner to the traditional electrochemical methods. It can also be integrated with bright field, dark field, and fluorescence imaging capabilities in one optical setup to provide additional capabilities. To date the plasmonic imaging technique has found various applications, including mapping of heterogeneous surface reactions, analysis of trace substances, detection of catalytic reactions, and measurement of graphene quantum capacitance. The plasmonic and other emerging optical imaging techniques (e.g., dark field and fluorescence microscopy), together with the scanning probe-based electrochemical imaging and single nanoparticle analysis techniques, provide new capabilities for one to study single nanoparticle electrochemistry with unprecedented spatial and temporal resolutions. In this Account, we focus on imaging of electrochemical reactions at single nanoparticles.