Furthermore, an increasingnumber of sensing and diagnostic technologies make use of electrochemistry,and interfacial charge transfer is at the heart of living systems,with renewed recognition of the importance of local bioelectricalphenomena in cell biology.4 Finally, sinceelectrochemistry deals with mass transport and reactions at interfaces,the subject provides a framework for addressing and describing interfacialprocesses generally.To acquire a deeper understandingof electrochemical systems requires techniques that address nanoscaleaspects of electrochemistry, 6 We are witnessing radical changes to the wayin which electrochemists perform and analyze experiments; this reviewarticle thus focuses on recent advances in frontier nanoscale electrochemistryand imaging techniques that address many of these key targets andare well-placed to embrace other aspects in the near future. Our goalis to provide an overview of the present state-of-the-art in high-throughputnanoelectrochemistry and microscopy and signpost promising new avenuesfor nanoscale electrochemical methods.Scanning electrochemical probe microscopies(SEPMs) have playeda pivotal role in advancing small-scale electrochemistry. SEPMs usean electrochemical probe (micro/nanoelectrode or pipet) to quantifyand map local interfacial fluxes of electroactive species and havefound increasingly wide applications. Our contribution to the Fundamentaland Applied Reviews in Analytical Chemistry 2019 (ref ) discuss9 where the goal is to detect and analyze, inter alia, single molecules, single cells, and individual particles (and othernanoobjects), as well as to break down the response of complex electrodesurfaces and interfaces into a set of simpler elementary features,e.g., terraces, step edges, grain boundaries, etc. Each of the areaswe have selected is benefiting from similar developments in experimentalcapability and analysis tools; there are also efforts to integratetechniques and ideas from each of these areas. Furthermore, thereis considerable overlap in the types of processes and phenomena thatare studied with these different techniques, and so bringing togethera discussion and comparison of the methodology is beneficial. We consider:(i)13 we highlight here the present state-of-the art and the importanceof nanopores as a platform for high-throughput electroanalysis. Electrochemicalnanoimpacts is another platform providing high-throughput electroanalysisat the level of individual nanoparticles (NPs). This methodology allowsNP counting and characterization by recording at high throughput theelectrochemical signature associated with a series of NP collisions,on a miniaturized electrode. The technique has allowed explorationof a wealth of electrochemical or physical  phenomena at the nanoscale. However, the potentialof the methodology to resolve structure–activity is much lessexplored and difficult to achieve, unless the methodology is enhancedwith integrated operando microscopy. Thus, electrochemicalnanoimpacts will be discussed in parts of this article, but not addressedin a specific section, especially since recent research efforts inthis area have been exhaustively summarized in authoritative reviews.17Nanopores that areused in a widerange of fields spanning biophysics, bioanalytical sensors, and waterdesalination applications. They exploit the high-throughput and selectivetransport and screening of chemical species offered by the electricfield and molecular confinement in nanopores and nanopipettes. Whilescholarly reviews of nanopore electrochemistry have been publishedrecently,(ii)18 and scanning electrochemical cell microscopy(SECCM).19 These two scanned probe techniquesshare the same type of easy-to-make and characterize probes that arereadily deployed at the nanoscale (10s nm sized probes). Scanningelectrochemical microscopy (SECM) is not addressed in this review,as high-throughput nanoscale resolution imaging by SECM is still rareand challenging to implement. Moreover, the third edition of the classictext on SECM, providing a comprehensive review of the field, has recentlybeen assembled by authorities in the field.20The use of nanopipettesas localelectrochemical probes in scanning ion conductance microscopy (SICM)(iii)operando with nanoscale resolution, using superlocalizationprinciples. They are now expanding to a wide range of electrochemicalsystems, as illustrated by several recent reviews.26 Their easy hyphenation with complementary electrochemical or structuralanalyses make them particularly compelling tools for high-throughputcharacterization as we discuss herein.Advanced opticalmicroscopy techniquesthat have increasingly been used for imaging electrochemical interfaces In selecting materialfor inclusion in this article, we considerdistinct, but related areas, spanning nanopores/pipettes, nanopipetteprobe microscopy, and widefield imaging. These methods are becominga natural choice for high-throughput single-entity nanoelectrochemistry,28 The general nanoporereadout relies on resistive pulse sensing, where the nanopore linkstwo compartments filled with an electrolyte solution with electrodesimmersed in each compartment. Upon the application of a constant voltage,analytes are translocated through the nanopore causing a temporarymodulation in the measured ionic current. The characteristic of thecurrent perturbations, such as current magnitude, duration, and frequency,provide information about the physicochemical properties of the analyte and its interactions withthe nanopore  have been employed in the analysis of inorganic colloids and nanoparticles.50 The most successful application of nanopore technology, however,is in the field of nucleic acid sequencing, as demonstrated by thecommercialization of nanopore sequencing devices from Oxford NanoporeTechnologies Ltd.51 This technology relieson the integration of biological nanopores with advanced electronics,and some recent applications of biological nanopores were reviewedrecently.52The development of nanoporetechnology in the last decades has enabled numerous applications insingle-entity research.nanopore 1A.29−31 In this section, we highlight recent work enablinghigh-throughputsingle-entity detection with nanopores and implications for single-entityresearch. We present key aspects to allow for high-throughput analysisand discuss the integration of nanopores with microfluidic/opticalsetups, and the application of machine-learning algorithms to processnanopore data and to enable high-resolution single-entity analysis.30 These methods have broughtgreat flexibility in tuning the size and geometry of nanopores usinga range of substrate materials, e.g., SiNx, SiO2, graphene, polymers, and glass capillaries.30 Capitalizing on this fabrication technology,nanopore devices can be scaled-up to produce multiple nanopores integratedin one device or the fabrication of nanopore arrays. Wen et al. employed30–100 nanopores in a suspended SiNx membrane for the detection of nanoparticles  drilling, and controlled dielectricbreakdown.articles 1B.53 Simntrolled 1C,58,59 60 and modern CMOSfabrication processes can favor new emerging strategies for the fabricationof arrayed nanopores integrated with the electronics readout.61With the expansion of arrayed nanoporeconfigurations, the requirement for parallelization of the measurementset up becomes evident64 While the complexity of samples investigatedby nanopore-based sensors is increasing at a rapid pace, so too isthe nanopore measurement throughput. New strategies are thereforeneeded for real-time analysis and to explore new discriminants forthe detection and characterization of complex analyte mixtures. Inorder to classify each signal, various features are extracted fromthe current–time waveforms recorded. Conventionally, the peakheight and the width of the signal are used to discriminate moleculesduring nanopore translocation.64 However,additional features can be used to enhance signal classification,such as the peak area, rise and decay time, frequency of events, andsubpeak levels, to name just a few.62 Suchfeatures can serve as the input for machine learning algorithms toenhance the readout efficiency and lay the ground for enhanced analyteidentification and classification, as exemplified in An importantaspect in expanding the high-throughput capabilities of nanopore platformsis the implementation of novel data analysis strategies and machine-learningalgorithms. Ionic current characteristics are routinely used in nanoporemeasurements to extract physical information about the analytes investigated.65 Hu et al. demonstratedthe use of a machine learning algorithm to obtain automated classificationof metabolites using a nanopore platform.66 Taniguchi et al. reported the identification of several polystyrenenanoparticles by combining solid-state nanopore sensing with a machinelearning algorithm.67 The same group hasdemonstrated that such approach can also be employed for accuratedetection of different types of viruses.68 A further example of a machine learning approach for discriminatinganalytes in mixtures is the use of the learning time-series shapelets,as exemplified by Wei et al.69 Here, themaximum discriminative features corresponding to each analyte thatform a short segment of time-series data are extracted as shapeletsignals and use the shapelet-transformed representation in order toclassify the analytes investigated.Several studies onnanopore-based sensing have demonstrated the implementation of postprocessingmachine learning algorithms for quantitative and qualitative applications.The general approach for the development of machine learning algorithmsis to build a training process, including several stages such as dataimporting, extraction of defined features, model training, and evaluationof the model.A key aspect in improvingthe readout throughput of nanopores isenabling real-time analysis and classification readout efficiency.Moreover, synergetic efforts to define servers with commonly useddatabases can contribute greatly to the advancement of nanopore dataanalysis. Complementing ionic current signals with information obtainedfrom other complementary techniques  would generate a platform with high-throughput readoutfor single-molecule investigations.72 Varongchayakul et al. presented a standalone microfluidic-nanoporesetup that supports on-chip sample preparation, purification, andsingle-molecule nanopore measurements  techniques.29The small footprint of nanopores and thepossibility to integrate them with complementary nanostructures allowstheir interfacing with nonelectrical readouts for increased throughput.Verschueren et al. proposed a setup comprising an inverted-bowtieplasmonic nanopore to achieve both ionic current and transmitted lightdetection of DNA translocation.etection 1F.10 Lasin situ redox reactions,isolating the nanoparticle behavior from the bulk colloidal motion.81 Furthermore, by monitoring SERS in conjunction with amperometry,enhanced information about the transport and capture of single nanoparticlescan be obtained, as exemplified in 82 The asymmetric geometry of the conical nanopipette and its surfacecharge generates distinctive electrochemical properties enhanced bythe nanoconfinement. The asymmetric ion transport under diluted electrolytesolution gives rise to the ion current rectification (ICR) effect.84 This effect has be further developed into a nanopore sensing approachwhere the characteristic current–voltage curves can be modulatedby the changes on the surface of the nanopore induced by the presenceof analytes . This signal dependence on the nanopipette–sampleseparation can be used as active feedback to map the topography ofthe sample. Korchev, Klenerman, and co-workers further refined thetechnique to allow topographical mapping of living cells immersedin culture media, broadening the applications of SICM to biology.97 SICM has been applied mainly in biological research,102 as depicted in 103 provides a collectionof historical developments and recent advances in SICM, as does anauthoritative review.18Scanningion conductancemicroscope (SICM) is a scanning-probe technique where a glass nanopipette,filled with an electrolyte solution, is scanned over a sample immersedin an electrolyte while monitoring the ion current between a quasi-referencecounter electrode (QRCE) inside the nanopipette and a QRCE in thereservoir.110 Recentadvances in optical methods and SPM demonstrated the tracking of individualmolecules in living systems and the mapping of single cells with highspatial resolution. These experiments generate a very large amountof data whose processing requires advanced and efficient data analysispipelines.114 Compared to the subsecond image acquisition times of confocal laserscanning microscopy, SICM often requires several seconds or minutesto acquire a single topographical scan.115 More efficient hardware and software designs are required to develophigh-speed imaging able to resolve fast dynamic processes. Furthermore,beyond imaging, the collection of sufficient samples/data to reachstatistical significance still represents a significant challengedue to the lack of automation and the need for skilled operators.In the following sections, we will discuss the most recent developmentsto overcome these limitations and creative applications of SICM insingle-entity research.In biological science, high-throughputmethods are needed to dissect biological processes regulated by thousandsof molecules whose concentrations quickly evolve as a response tospecific stimuli or perturbations. Understanding the function andinteraction of these biological entities at the nanoscale becomescritical to better address the bulk responses that we typically observe,although acquiring sufficient data to obtain the required statisticalsignificance has been challenging.117 In SPM, high-throughput atomic force microscopy (AFM) was achievedby developing smart image processing algorithms that allow multiparametricanalysis of single entities and by developing high-speed imaging.Ridolfi et al. developed an AFM-based nanomechanical screening techniquethat allows nanomechanical and morphological characterization of thousandsof individual extracellular vesicles.113 Konrad et al. developed a high-throughput pipeline based on AFMimaging with large fields of view and automated, multiparameter imageprocessing for the investigation of nucleosome conformation.112 More generally, the development of high-speedAFM has allowed imaging of dynamic structures with unprecedented temporal,and spatial resolution and new processing algorithms are laying thegroundwork to multiplexed and high-throughput analysis of biologicaldynamic processes.119High-throughputimaging enablesmeasurement of the dynamics of cellular pathways, such as human genomepositioning factors or DNA damage and repair in immune cells.122 As a result, the probe travels many micrometers along the z direction; thus, increasing the speed along z is the clear solution to improve imaging speed. Generally, piezoelectricactuators with resonance frequencies of tens of kHz and travel rangeof tens of micrometers are used for SICM. One limitation in usingpiezo actuators with higher frequency is imposed by the correspondingshort travel range. The probe travel range needs to be at least 10–20μm to scan convoluted biological surfaces , andthis range limits the range of resonance frequencies available forthe piezo actuator. Furthermore, the slow response of the active feedbackresults in an overshooting of the z piezo actuator,which becomes more prominent when the speed of actuation increases.Zhu et al. discussed recent hardware advances highlighting the benefitsof using a second stacked piezoelectric actuator with a higher resonancefrequency which compensates for the overshoot due to the slow feedbackresponse.18 Another limitation to high-throughputSICM is the x–y scan areawhich needs to be small enough to reach high temporal and spatialresolutions.Conceptually similarlyto high-speed AFM, high-throughput imaging in SICM is synonymous withhigh-speed scanning that can be achieved by optimizing the scanningroutines and/or improving the hardware design. From the hardware perspective,the scanning speed is limited by the resonance frequency of the piezoelectricactuators and the time response of the electronics. Modern SICM ismainly based on the hopping mode, where the probe is retracted toa preset hopping height after each approach.102 This results in an increased temporal resolutiondue to flat subareas being imaged with a lower spatial resolution(higher speed) than rougher areas that contain the features of interest.Algorithms are being developed that allow the number of pixels acquiredper experiment to be minimized (to decrease acquisition time) andreconstruct the final image in high resolution in a postprocessingstep.124From the software perspective, imaging speed hasbeen improvedby adopting optimized scanning regimes which drastically reduce thenumber of pixels acquired. A common example is the compression algorithmwhere the scan area is divided into small subarea regions of interest,and the resolution of each subarea is set according to the local roughnessestimated by measuring the four corners of the subarea.125 They synchronized the z-piezoactuator for nanopipette vertical positioning with another piezo usedto vertically move the objective of a confocal microscope to achieveconfocal autofocusing. The system allowed topography measurementsof areas as large as 4 μm × 4 μm with a scanningspeed as fast as 18 s/frame, while simultaneously acquiring confocalmicroscopy images of insulin granules inside and on the surface ofsingle cells. The same group then imaged the kinetics of virus-likeparticle formation on the membrane of living cells, demonstratingthat the particles can reach full size in 3–5 min.105Implementingan optical microscopy read-out is a further instrumentationsolution to leverage the imaging throughput issue. Moreover, the opticalreadout (see dedicated section below) provides a complementary imagingof the objects (or function). Bednarksa et al. used correlative high-speedSICM and confocal microscopy to measure the kinetics of exocytosisof single granules of insulin from the top surface of single β.126 usedcorrelative ultrafast topographical mapping and multielectrode arrayrecordings to simultaneously measure the morphology of cardiomyocytesand distribution of action potentials during a full contraction cycle.Navikas et al.127 integrated super-resolutionoptical fluctuations imaging to SICM to achieve correlative 3D microscopyof single cells. In their study, they used high-speed SICM with apixel acquisition rate of 200 Hz to image cytoskeletal actinin dynamicson living cells. Hagemann et al.128 combinedstimulated emission depletion and SICM to measure correlative surfacetopography and distribution of cytoskeletal actin. Gesper129 used SICM and fluorescence correlation spectroscopyto assess diffusion in cell membranes. Their findings revealed thatcell surface roughness is unevenly distributed, with the plasma membraneabove the nucleus being the smoothest.130Simeonov and Schäffer131 They demonstrated continuous surface topography measurementof large areas (80 μm × 80 μm) with good spatialresolution (512 × 512 pixels) and temporal resolution (from 0.5s/frame to 20 min/frame depending on scan area). In earlier work,similar large scan areas were achieved by Zhuang et al. by developinga stitching algorithm able to “stitch” different scansin a postprocessing step.123Leitao et al. developed a high-speed SICM based on high-bandwidthlarge-scale piezo actuator able to perform time-resolved, long-termtopographical mapping of living eukaryotic cells.132 Wang etal.133 assessed the morphological and nanomechanicalproperties of intestinal tumor cells using high-speed SICM showingthat highly metastatic cells exhibit unique topographical and nanomechanicalfeatures.Recently,Zhuang’s group developed an SICM imaging modeusing double-barrel nanopipettes and an adaptive sensitivity methodto enhance the imaging rate.134 an AFM with a microfluidicchannel built in the cantilever, to longitudinally sample RNA fromthe cytoplasm of a living cell followed by transcriptomic analysis.135 With this approach, highly skilled operatorsperform hundreds of single-cell extractions in a semiautomated fashion,followed by molecular and data analyses. Similarly, SICM systems arestill semiautomated and heavily rely on the operator skills, limitingupscaling possibilities.97The advent of next-generation sequencingconsiderably changed themeaning of high-throughput biology, massively scaling up the informationgenerated with a typical assay. Scanning probe techniques are excellenttools for live investigations of cells, but they lack throughput dueto a strong operator dependence and the intrinsic serial acquisition system. Chen et al. recently published a ground-breakingstudy using FluidFM,99 Tóth etal. advanced the nanobiopsy technique, first introduced by Actis etal. to perform cytoplasmic extractions from single cells.136 An unsupervised algorithm based on self-organized maps (SOMs) allowedcompartmental transcriptome profiling between different nanobiopsylocations within the same cell.107 Buryet al. used an adaptation of the technique to perform subcellularbiopsies from human tissue with micrometer precision.137 Yu et al.138 useda similar approach to extract femtoliter volumes of cytoplasm fromsingle cells for downstream time-of-flight secondary ion mass spectrometry(ToF-SIMS) analysis, expanding the applications of nanopipettes to in situ mass spectrometry.139 Similar techniques have been developed to deliver molecules insidesingle cells with high spatial resolution.144The pastfew years brought a series of advances that are increasingly movingSICM toward automation.145 developed a fully automated nanofountain probe electroporation systembased on angular approach SICM.146 A fullyconvolutional network was used to spatially localize cells in theoptical field of view and to tag the position of nuclei. The systemfacilitated the successful delivery of a controlled amount of Cas9nuclease to knockout specific genes in a variety of cell types ina fully automated manner. Automation partially overcomes some of thekey problems with manual probe-based approaches, allowing the manipulationof tens of cells with the same nanopipette.145 Hu et al. used an Al2O3-coated nanopipetteas an SICM probe to detect intracellular reactive oxygen species (ROS).Their system is based on a computer-vision algorithm for automaticdetection of cell nuclei and lateral nanopipette positioning. TheSICM-current feedback allows automated nanopipette vertical positioningon cell membrane while cell membrane penetration and intracellularROS sensing is enabled by continuous monitoring of the ROS current.147Despite these studies being linked to high-throughput downstreambiological analysis, the number of collectable samples is still limitedby the lack of automation. Mukherjee et al.148.A multiplexed localized electroporationdevice in conjunctionwith automated image segmentation and analysis based on artificialintelligence (AI) swiftly improved molecular delivery and transfectionconditions for a variety of adherent and suspension cell types. TheAI pipeline provides automated measurements of delivery/transfectionefficiency and assists in the analysis of crucial cell morphologicaltraits to pinpoint factors that promote high efficiency while alsomaintaining cell viability and health.106 They combined a delivery system based on angular approachSICM with light-sheet microscopy to controllably deliver and tracksingle molecules within single cells with unprecedented resolutionand control. The technique was used to deliver amyloid-β aggregateson the surface of single macrophages to investigate the cellular responsetriggered by the aggregates. Similarly, Simonis et al.140 developed a portable and easy-to-build systemto allow nanoinjections in single cells without the need for bulkyand custom-made equipment, which would limit its use to equipped laboratoriesand highly skilled operators.These examples show how SICM could be coupled with AI algorithmsto increase automation. Nonetheless, high-throughput and multiplexingcan also be achieved, combining SICM with super resolution microscopy.Li et al. recently developed a novel multifunctional imaging systemcapable of high-speed 3D detection at single-molecule resolution withinliving cells and accurate delivery of single proteins to specificsites inside the cytoplasm or on the cell membrane.109 The device was used for high spatiotemporalresolution monitoring of extracellular pH by precisely positioningthe nanoprobe near the cell surface under SICM feedback control. Thesetypes of measurements used a double-barrel nanopipette, with one barrelfor SICM and the other for potentiometric measurements.149 The same system was applied recently to quantifythe chemical environment around single phytoplankton cells and tocompare it to the conditions of bulk seawater, expanding the use ofSICM to environmental sciences.150 Allthese studies underpin the potential of SICM as a noninvasive toolfor single-cell manipulation and show the future directions that thetechnology will take to advance high-throughput biological investigationsin life sciences.High-resolution multiparametricimaging enabled by SICM was achievedalso for local pH mapping. Zhang et al. developed a label-free pH-sensitivenanoprobe made from a self-assembled nanomembrane that assembles ina zwitterion-like fashion at the tip of a nanopipette.152 or interfacial ionic concentration.153 When this is recognized and SICM is used asan electrochemical cell, for current–potential measurements,or potential pulse chronoamperometry, for example, it becomes a powerfulmultifunctional tool for surface and interfacial science, and electrochemistryin particular. With careful tuning and varying of the applied potentialbetween the SICM tip and bath for each pixel in a scan, with the potential–timewaveform aided by finite element method (FEM) modeling, it is possibleto map the topography and other interfacial properties synchronously.152The working principleof SICM for topographical mapping relies on the reduced ion flow  generated by the proximity of a nanopipetteto the investigated substrate immersed in an electrolyte solution.However, the nanopipette is also sensitive to other physicochemicalproperties of the substrate, such as surface charge154 As briefly highlighted above, the permselectivity of the conicalnanopipette to cations (in this case) in solution generated by theEDL at the glass wall,154 leads to ICR,with strong rectification effects when the aperture diameter is inthe same range of the Debye-length of the EDL.157 Similarly, charged surfaces immersed in an electrolyte solutionpossess an EDL, which affects ion transport through a nanopipettewhen positioned close to the surface, resulting in nonlinear current–potentialresponses,160 due to surface-induced rectification.161 A key aspect of these measurementsis to be able to deconvolute surface charge and topography.151 A number of SICM modes are now available which recognize that thecurrent is relatively insensitive to surface charge at very smallbias and much more sensitive at extreme bias.161 Thus, quantitative SICM charge mapping makes use of multipotentialmeasurements at each pixel.162When a glass nanopipette is immersedin an aqueous electrolyte solution, an EDL builds at the glass walldue to the negative charge generated by the deprotonation of silanolgroups in solution, for a wide range of pH.165 A double-barrel nanopipette was also used as the SICM probe to measure independently surface potential and topography, where onebarrel was used to record the open-circuit potential while the otherbarrel was used to measure topography.167A recent technique, termed “quantitative surface chargeconductivity microscopy” takes advantage of the potential dependenceof surface-induced rectification. In this method, the same area isscanned under positive and negative bias (with the same amplitude)and the difference between the two maps is used to calculate surfacecharge density on the substrate solving the Poisson and Nernst–Planckequations which describe ion fluxes in the system.172 The technique allows conductance measurements at the nanometer scaleacross membranes with high spatial resolution. To improve the spatialresolution of SICM and to expand the range of applications to molecular/ionicflux sensing,176 ion channel probe-SICM has been developed, where a biological ionchannel is inserted within a lipid bilayer formed at the nanopipettetip.Double-barrelnanopipettes have been used to probe ion transportat the nanoscale. Potentiometric SICM uses a multielectrode setupto probe conductance and transport across membranes.177 Surface charge measurementswith SICM have been performed for various types of samples, from polymersto minerals, metal substrates, and electrodes.180 Zhu et al.181 mapped the surface chargeof the clay mineral dickite, revealing a distinct pH-independent negativecharge on the basal surface and a positive charge on step edges thatincreased as the bulk pH decreased from 6 to 3. Tao et al.178 measured localized surface charge of highlyoriented pyrolytic graphite (HOPG) after the introduction of surfacedefects by plasma irradiation, finding that the topography of HOPGafter irradiation showed more irregularities which were attributedto higher local surface charge compared to the pristine structure.Jin et al.182 found that locations withhigher surface charge and stronger photocatalytic activity were responsiblefor irregular height profiles on TiO2 nanotubes. The photocatalyticdegradation of organic pollutants (rhodamine B) by TiO2 has been investigated using SICM, providing an in situ technique for local investigation of photocatalytic processes .151 The first application of charge mapping in biology usedSICM to map surface charge at Zea mays root hairs,demonstrating their high negative surface charge density, and at humanadipocytes, finding different surface charge distributions acrosstheir surface, including regions of positive charge that potentiallypinpointed the location of key proteins (amino terminus) in the cellmembrane, which are considered to mediate fatty acid uptake and performother functions.187 SICM surface chargemapping has been extended to PC-12 cells,162 HeLa cells exposed to the thinning-agent dimethyl sulfoxide (DMSO),188 and human hair exposed to different conditioningtreatments.189 Recently, Cremin et al.190 used SICM to map local ion properties of livebacterial strains  developed previously and illustratedin 201 Similar electrochemical sensors based on carbon fibers have beendeveloped to accurately measure the signal distortion due to the gapat the interface between the tissue and the sensor, providing benefitto research based on electrode–tissue interfaces.202Single-cell electrochemical investigations usingnanopipette technologies are not limited to the surface of the cellmembrane but can be performed inside the cell with minimal disruptionof cell viability.le cells4C. Simil203 Making use of the same pipet probes as SICM, the sample comes incontact only with the hanging electrolyte meniscus, formed at thepipet tip pipettes were used for SECCM experiments.204 In this configuration, a QRCE is inserted in each barrel, filledwith the chosen electrolyte  and an additional voltage, applied betweenthe QRCEs and the WE, facilitates the implementation of a varietyof electrochemical techniques, within the confines of the miniaturecell. Voltammetry experiments217 and chronoamperometry experiments218 are usedto investigate the local electrochemical activity and to induce andmonitor phase changes  techniqueshave shed light into localized corrosion processes.225 Electrochemical impedance spectroscopy (EIS) measurements were alsodemonstrated, by combining SECCM with a frequency response analyzer.221Recently, single-barrel pipet SECCM probes have been mostpopular.ed pipet 5B. It is230 or the interference-based optical monitoring of the electrode–electrolyteinterface.231 Such substrates were usedfor the carrier generation-tip collection (CG-TC) SECCM method, mappingcharge carrier diffusion over laser-excited areas.228 Other technical advancements have led to the capacity forregulating the electrolyte pressure inside the pipet probe232 or substituting with solid electrolyte.233 Environmental control during the experimenthas been another major point of interest.238 Oil has been used to cover the substrate and restrict gas exchangefrom the meniscus,;225 while small containersconnected to a gas supply were utilized to establish specific atmosphericconditions. Recently, SECCM measurements have been conducted insidea glovebox,239 and aided by a thorough understanding of the behavior of nonaqueouselectrolyte media,240 they have set thegroundwork for future work into battery materials.The easily customizable SECCM platformhas allowed coupling tovarious instruments and methods 5A. OpticiR drop) was studied in single- and double-barrel pipet configurations.241 Analytical treatments and simulations wereconsidered. Another crucial element in the experiment is the stabilityof the Ag/AgCl QRCE, particularly during long measurements. Followingprevious scrutiny of typical SECCM configurations which provided recommendationsfor successive long-time imaging free from interference from the QRCE,96 the effect of Ag+ contamination onthe estimation of open-circuit potential of aluminum alloy was investigated,and mitigation strategies for the use of a NaCl system were proposed.242 The stability of the pipet tip meniscus andits impact on surface imaging quality has been discussed elsewhere.232 The application of a controlled pressure tothe back end of the pipet probe allowed the adjustment of the meniscusshape with respect to unique characteristics of the sample.Part of thepopularity of SECCM as an analytical technique comesfrom a straightforward interpretation of the recorded data, basedon the confinement of the reaction volume within a simple geometry.Theoretical and technical aspects of the experimental system havebeen covered since the introduction of the technique, with some furtherdevelopments recently. The ohmic potential drop .upstudy 6A.208 An218 In this mode, a double-barrel probe is used so that themeniscus cell can be landed on the membrane irrespective of its iontransfer characteristics. The driving force for ion transport is apotential applied between the SECCM probe and a counter electrodeon the trans-membrane side. The technique was used to probe and visualizelocal proton transport across CVD graphene-on-Nafion membranes, forwhich proton transport activity was detected from atomic-scale defects,which were analyzed in detail to reveal the size from the currentresponse, to larger cracks and tears. We anticipate that this techniquecould find myriad applications for imaging transport sites in membranesand coatings. Indeed, a recent application showcased the detectionof nanometric pinhole defects across a multilayer aryl film on anelectrode, formed by aryldiazonium reduction.245 As a model case for probing defects in protective coatings,detection sensitivity was examined with respect to the pipet and pinholesize.While a major focus of SECCM is on electron transferprocesses,a new local electrochemical ion (proton) pump mode of SECCM has beendeveloped to image proton transfer across membranes.246 SECCM was further employed to interrogate theelectrochemical activity of Fe(CN)63–/4– at boron-doped diamond (BDD). The surfaces of single-crystal particles was explored in detail by separatelyresolving the contribution of DA adsorption from that of intrinsicelectrode kinetics. Further work on the DA system was carried outby a multiscale SECCM approach:249 glassycarbon electrode surfaces were functionalized utilizing a larger pipetprobe and subsequently examined with a smaller one in imaging mode imaging to count the numberof particles probed.251Structure–activity relationships, governingthe local electrochemical behavior of screen-printed carbon electrodes,were inspected with SECCM, followed by colocated correlative analysisvia SEM and Raman spectroscopy.ing mode6C. This 252 and noted correlation with local grain orientation.Further work by Wang et al. reintroduced the rarely used continuousscanning voltammetric mode of SECCM, originally developed by Güellet al.,253 and obtained similar resultsto Güell on (ferrocenylmethyl) trimethylammonium oxidationon the basal surface of HOPG.207 They furtherused the technique to reveal the grain orientation dependence of hydrogenevolution reaction (HER) on platinum, employing the powerful combinationof SECCM and electron backscatter diffraction (EBSD), which is becomingan increasingly popular methodology.254Wang et al.developed a method to map the local potential of zerocharge (PZC), through repeated SECCM approaches at a series of potentials,217 Phaseseparation yielded conductive P3HT dots in a contiguous PMMA layer,presenting an ultramicroelectrode array for investigation by SECCM.The electron transfer kinetics for the oxidation of 1,1′-ferrocenedimethanol(FcDM) were determined by voltammetric imaging. Comparison to macroscalemeasurements revealed that the latter are dominated by bulk resistanceeffects.The local electrochemical current flowing through a thin organiccoating is strongly dependent on the coating’s electrical conductivity.This was demonstrated from the nanoscale activity imaging of indiumtin oxide (ITO) electrodes coated with blends of conductive (P3HT)and insulating (PMMA) polymer.255 High-index orientation grains demonstrated higher ratesof electrochemical hydrogen absorption, and this methodology shouldbe valuable in examining this process at a range of materials, forexample in studies of the mechanical degradation of structural metalsdue to hydrogen embrittlement.Optimal sites for electrochemical hydrogen absorptionon polycrystallinepalladium electrodes were discovered using spatially resolved SECCMvoltammetry in tandem with colocated EBSD analysis.SECCMis becoming an increasingly populartechnique in the field of metal corrosion. By confining the reactionwithin the probe meniscus, one can follow the dissolution at a nanoscopicscale and relate it to corresponding local variations in the materialcomposition and structure. Importantly, corrosion-related processescan only be initiated upon, or after, meniscus contact, enabling thestudy of short time, transient, phenomena, and the initiation of processes.Surface electrochemical mapping, especially in conjunction with complementarycolocated characterization, can provide a wealth of information aboutsuch structure–activity relations.213 The anodic current response was probed andcorrelated with grain orientation via colocated EBSD. In addition,typically encountered MnS inclusions were identified in the sample,and their characteristic behavior was determined  calculations, and the corrosion susceptibility was thusrationalized by the energy required to remove one Fe atom at the surfaceof a lattice. A study of Zn corrosion234 was carried out, involving controlled atmospheric conditions (O2 or Ar atmosphere) with the SECCM meniscus surrounded by adodecane oil layer deposited on the sample. The latter provided bettercontrol against excessive electrolyte spreading during cathodic processes,which can occur when there is a significant increase in local pH inthe meniscus. Furthermore, the oil/aqueous electrolyte/metal systemhas been of special interest to the automotive industry and furtherdiversifies the application of SECCM to interesting liquid/liquid/solidsystems in general.A model exemplar systemis the corrosion susceptibility of low-carbon steel in neutral medium.termined 7A. In su257 For this system, the approach potential was shown to have an effecton subsequent electrochemical measurements. Experiments on the samesystem demonstrated dependence of corrosion behavior on the grainorientation.258 An extended pipet bodywas proposed to accommodate a three-electrode setup and reduce contaminationfrom the Ag/AgCl reference electrode, which appears more problematicthan typical in this particular system.Corrosion of polycrystalline Al alloy AA7075-T73, under a NaClmedium, was examined in two related studies. First, the choice ofthe approach potential was scrutinized.2 or Ar atmosphere to highlight key electrochemical processes underpinninglocalized Cu corrosion across a wide range of surface structural facetsand grain boundaries identified by colocated EBSD.225 In a follow-up study employing voltammetric mode SECCM,the efficiency of an oil-soluble derivative of the classic Cu corrosioninhibitor benzotriazole was quantified for both anodic (copper dissolution)and cathodic processes, presented against structural variations ofthe surface.259 Another model system exploredby a similar approach was the dissolution of Ag260 and its local reaction kinetics. Voltammetric mapping andadditional colocated AFM characterization allowed extensive analysisof the dissolution mechanism and the prominent role of surface grainboundaries.The crystallographicorientation-dependent corrosion of Cu hasbeen studied, again utilizing dodecane oil, to achieve a triple-phaseenvironment. Chronopotentiometry measurements were performed 7B under 261 The underlyingNi grain orientation matched to differences in the tendency of theNiO oxide film to break down at a particular Ni dissolution potential.As verified by colocated ToF-SIMS imaging, it was shown that locationswith thinner oxide film were, counterintuitively, more resistant tobreakdown. The results were explained and supported by a model thatdrew on classical nucleation theory.262Extending the types of corrosion phenomena amenableto study bySECCM, the performance of the native oxide film over a bare metalbase has been probed.263 Electrochemical impedance spectra were acquired at confined areasacross the Mg sample surface, and a distribution of relaxation timeanalysis revealed time-dependent interface processes. In a work thatcovers corrosion in archeological artifacts,264 purposefully prepared Ag–Cu alloy samples were studied ina variety of neutral solutions of different ion species. Localizedelectrochemical examination, across the alloy grain boundaries andgeneral inhomogeneities, shed light onto the intergranular corrosionmechanism that would affect ancient artifacts of similar composition.Combining both direct and alternating current polarization,single-crystalMg corrosion was probed at the microscale.SECCM has been widely used to studyphase formation, owing to the ability to directly induce change withinconfined volumes, in a controlled manner. An attribute of SECCM isthat measurements can be made at thousands of sites on a substrate,either under the same conditions or with conditions different at eachspot or a portion of spots. This provides large data sets for detailedstatistical analysis, and this aspect emphasizes the high-throughputnature of such SECCM measurements.265 Miniaturizationof the working electrode surface enables the characterization of thenucleation of single nanobubbles.266 Aclassic experiment for the creation of a H2 nanobubbleis to reduce H+, on a Pt disk nanoelectrode immersed inH2SO4 solution, by sweeping the electrode potentialnegative of E0 (H+/H2). With theincrease of local H2 concentration, a bubble eventuallynucleates and covers the electrode surface. This is mirrored in therecorded current, where an increase in current magnitude with drivingpotential eventually results in an abrupt decrease upon bubble nucleation.As a prototype of phase transition,gas bubble nucleation is commonly explored, both to progress theoreticalunderstanding and improve real-life, critical applications. The objectivecould be to promote or restrict the evolution of gas, or to removeand guide it away from the active surface. The study of how bubblenucleation and growth is mediated by nanoscale dynamics is consideredof major significance.267 where confinement is naturally achieved withinthe meniscus cell. Similarities and differences—mainly relatedto the pipet aperture size and geometry—to the nanoelectrodesetup were reported, and FEM simulations assisted in determining therelationship between the measured faradaic current and the local gasconcentration. As mentioned above, an advantage of SECCM is the possibilityof making measurements at several thousand different “electrodes”,defined by the meniscus contact, in one experimental run. Changingthe electrode material from Au to MoS2, or structuringthe surface,268 provided further insightinto bubble nucleation and elimination dynamics and the overall bubblestability. The latter factor was assessed by chronoamperometric measurementsat a fixed cathodic potential, resulting in periodic current oscillations,arising from the nucleation–growth–detachment lifecycleof a bubble. The effect of surfactants on electrochemically generatednanobubbles has been examined,269 and theywere shown to promote HER and bubble formation on a Pt electrode,while also being critical in stabilizing H2 bubbles whenusing submicrometer radius SECCM pipettes. Electrochemically inertSiO2 nanospheres on a smooth glassy carbon surface wereutilized as H2 bubble nucleants,270 when they were individually encapsulated and probed within separateSECCM landing sites. The peak voltammetric current was related tolocal gas supersaturation conditions over a range of nucleant sizes;and the role of the nanoconfinement geometry—between the nanospheresand the surface—was highlighted via theoretical analysis andFEM simulations. This study demonstrates the attraction of SECCM fornucleation studies, in being able to quantitatively assess a widerange of nucleation sites quickly in a combinatorial fashion.A similar experiment has been applied in an SECCM configuration7C,267 wh+ reduction was performed.211 In this case, the pipet configuration allowed monitoringof bubble evolution via the ionic current flowing between the twobarrels. Aided by FEM simulations, the relationship between the measuredcurrent peak and the local gas concentration was clarified. Probingdistinctive particles in a spatially isolated manner is a recurringand powerful theme in SECCM studies.NP nucleants dispersed on a catalytically inert substrate are representativeof the typical nanocatalyst systems used in gas-evolving reactions.Individual Pt NPs on HOPG were encapsulated by a double-barrel pipetmeniscus, and H2 bubbles were monitored over the surface of polycrystallinePt by a double-barrel probe in a native voltammetric mapping mode.214 Across the scanning pattern, the recorded response—relatedto the gas concentration and the activation energy required for nucleation—exhibitedvariations; however, it was notably not correlated with crystal grainsor grain boundaries.Gas evolution was alsoexplored with respect to the heterogeneousactivity of the electrode surface. Nucleation and growth of H271 Thenondestructive nature of the experimental procedure allowed the depositionof samples onto delicate substrates, by means of the residue leftbehind after probe retraction. The method played an important partin the successful in situ TEM observation of singlecatalyst particles during ethanol oxidation. In a similar fashion,tailored deposition of cobaloxime complex salts was achieved on carbonnanomembranes by means of SECCM arrays.272 The formation and the quality of alkanethiol self-assembled monolayerswas observed electrochemically by SECCM.273 The reaction was confined within the area wetted by the nanopipettemeniscus, while the process was monitored via the diminishing activityof the Fe(CN)63–/4– redox species.Moving to the solid phase, SECCM has beenused to deposit preformedstructures from solution or to form a solid phase from solution ona substrate. Electrochemistry can act as the driving force of phasechange or simply assist in the process. SECCM serves as a powerfuldeposition tool in its own right as evidenced by its use to immobilizetarget catalysts onto special chip substrates for subsequent liquid-celltransmission electron microscopy.275 Pt NPs were electrodeposited on carbon-coated TEM grid supports.276 A straightforward and high-throughput patterneddeposition process provided separated ensembles of deposits, readyto be imaged under TEM and the structures related to experimentalparameters .SECCM has been particularly powerful in the study of the earlystages of metal nucleation and growth. Building on initial work,277 Cu nanostructureshave also been produced by electrodeposition from SECCM nanopipettes.278 The limits of this electrochemical additivemanufacturing  process were explored,while it was fine-tuned to achieve feature sizes of just 25 nm.Cu electrodepositionis also a popular system for SECCM studies.Together with SEM, AFM, and XPS, SECCM was used to map the distributionof Cu nucleation activity on glassy carbon electrodes, to reveal awide diversity of electrochemical activity which was subject to detailedstatistical analysis.3 crystalswere precipitated from solution,231 withinan SECCM meniscus, by electrochemically changing the local pH. Situatedon a semitransparent ITO electrode, the landing spots were concurrentlymonitored with optical (interference reflection) microscopy, revealingspatially diverse growth regimes across the electrode surface. Thiswork opens up new possibilities for studying the nucleation and growthof insulating materials  on surfaces using SECCMwith direct optical visualization of the process. The optical microscopyset up could be further expanded to other optical techniques suchas Raman microscopy.In a case of indirectly driving phase change, CaCO2, MoS2, and WSe2),283 graphene and graphene derivatives,285 hexagonal boron nitride (h-BN), etc.,286 have generated great interest in catalysisapplications, especially in the area of photocatalysis and electrocatalysis.SECCM is ideally suited for studying the electrochemistry of suchmaterials, as small flakes or regions of a 2D material can be targeteddirectly and the properties of those regions can be deduced by a rangeof colocated complementary techniques, e.g., Raman microscopy, AFM,etc.,287 to reveal the relationship betweenactivity and electronic and structural properties of the material.Recent contributions showcasing electronic property manipulation—achievedby structural modifications, 2D support effects, and rational controlof atomic defects—have provided insight toward understandingthe interfacial charge transfer chemistry, ion transport, and carriertransport. These studies have tended to focus on simple outer sphereelectron transfer processes, for which a range of one-electron transferredox probes can be used. Ru(NH3)63+/2+ is proving to be increasingly popular as a redox probe due to itsstability and well-characterized electrochemistry and because theformal potential is close to the electroneutrality point of grapheneand related materials,288 and so it isvery sensitive to any changes in electronic structure of the materialsstudied. We cover such studies in this section, along with electrocatalyticprocesses at 2D materials. Photoelectrochemical studies are also ofincreasing importance in 2D materials,289 and some innovative measurements based on SECCM are reported inthe next section.Benefitingfrom unique structuresand electronic properties, two-dimensional (2D) materials, includingtransition metal dichalcogenides .290 Faster kinetics on thinner stacked layers were observed in all casesattributed to a narrower electron tunneling barrier in low-layer stackedTMDCs, resulting from changes of band gap as a function of layer numbersof TMDCs. A similar experimental trend was obtained for chemical vapordeposition (CVD)-grown graphene on a copper substrate, where experimentson Ru(NH3)63+/2+ were accompaniedby a detailed, complementary computational and theoretical analysisof outer-sphere electron transter.210 Decreasingkinetics in the order of monolayer > bilayer > multilayer oncopperwas opposite to the trend for graphene on silicon oxide substrates,287 due to the key role of the copper substrate dominating the electronicproperties. The relative rates on the monolayer versus bilayer graphenewere shown to agree quantitatively with predictions for adiabaticelectron transfer; the graphene layer acts as a barrier to ET at theelectrode/electrolyte interface, with bilayer graphene having a biggereffect than monolayer graphene. These studies further highlight howthe metal support has a profound effect on electron transfer at 2Dmaterials,292 which was also demonstratedin SECCM studies of electrocatalysis of HER at h-BN comparing Cu andAu supports.293SECCM and AFM were used to quantify the effectof layer number (∼5) on electron transfer kinetics in stacksof four different 2D TMDCs variants (MoS3)63+/2+ at twisted bilayergraphene supported on h-BN, where the electron transfer kinetics wereshown to be strongly dependent on the interlayer twist angle (θm).222 θm was variedbetween 0.2° and 5.0°, and the electrochemistry was studiedwith SECCM. Around the “magic angle region”, ca. 1.1°,there is a significant increase in the density of electronic states(DOS) which leads to a significant enhancement in electron transferkinetics. This study opens up a new dimension of topological defectsin 2D heterostructures that can be explored with the combination ofSECCM and correlative multimicroscopy.294One of the most excitingrecent studies of outer-sphere electrontransfer at graphene materials considered the electrochemistry ofRu shows catalytic activity. Thestandard current density was measured to be about an order of magnitudelower on the basal surface than at the edge planes. SECCM was alsoutilized as an efficient patterning tool, to create localized controllabledefects by local anodization of the surface .249 These studies revealed the importanceof defects in the structure as transport pathways, ranging from tearsand cracks to atomic-scale vacancies.245SECCM is a powerful method for detecting surface inhomogeneitiesand defects in 2D materials . This was already highlighted for graphene on Nafionand films on electrodes in 299 SECCM was applied to isolate the faradaic and nonfaradaic current,and the edge of N-rGO (rich in pyridine and pyrrole N) presented higherHER activity.297 Aided by DFT calculations,the pyridinic N in nitrogen and phosphorus codoped graphene was suggestedto be the active site for HER.285 The combinationof SECCM measurements and DFT calculations suggested that three-dimensionalcurved graphene possesses a high density of topographical defectsand is able to accommodate the chemical dopants on the curved lattice,which exhibited higher electrocatalytic activity than surroundingflat regions.284Heteroatom doping of 2D materials is an effective strategyforcatalysis enhancement; charge accumulations—in pyridine, pyrrole,and quaternary N, across N-reduced graphene oxide (rGO)—arereported to increase catalytic activity.300 Measurements were made before and after the addition ofthe electron hole scavenger, H2O2, at high spatialresolution along the electrode surface.SECCM is gaining in popularityfor local photoelectrochemistry studies, as a tool to locally probethe activity of relevant photoelectrode materials. The charge injectionyield at hematite films, during photoelectrochemical water splitting,was interrogated at the nanoscale with SECCM.2230 utilized an ITO substrate as a semitransparentcontact and intermittent illumination from underneath in order tomap the photoelectrochemical reaction rate with SECCM probes. Resultsshowed a local activity dependence on the sheet thickness and thesignificance of defect features. A multiscale SECCM method alloweddeliberate introduction of high-activity defects on WSe2 and subsequent photoelectrochemical characterization of the samesample.229 Along with topographical informationand FEM simulations, the approach shed light on the morphology ofthe engineered defects and quantified local HER kinetics. Elsewhere,recording the spatially heterogeneous photoelectrochemical behaviorof single or multiple layers of MoS2281 with SECCM complemented photoluminescence spectroelectrochemistrymeasurements  electron transfer sites, and a broaddistribution of lower activity on the nanoscale. Interestingly, thesenanoscale measurements suggested that macroscopic outer sphere voltammetry,on typical time scales, should be faster than observed in practice.This difference between nanoscale and macroscopic kinetics suggestedthat macroscale measurements are dominated by uncompensated resistances,most likely lateral conductivity in the ITO film under electrochemicaloperation, also observed in SECM measurements on a larger length scale.3032D materials canserve as coatings for semiconductor surfaces,and other work305 shape,306 porosity,282 facets,308 and morphology309 . Surface featuresor defects  can act as active sites and have significant contributionto the overall response. Identifying the local characteristics helpsto understand the reaction mechanism and reveal the active sites forthe rational design of highly efficient catalysts.SECCM has been appliedand proven to be a powerful technique to elucidate the structure–activityrelationship of a wide range of electrochemically active materialsin various electrochemical reactions. In 3(HITP)2,)227 are attracting considerableinterest as electrocatalysts.310 The highspatial resolution of SECCM and intrinsically high mass transportfor gaseous reactants254 was hugely beneficialin lifting mass transport limitations and revealing conductive MOFsorders of magnitude more active than previously recognized, pavingthe way to optimizing their use in larger-scale devices. SECCM, combinedwith other operando microscopies, was applied toinvestigate the oxygen evolution reaction (OER) activity of single-crystallineβ-Co(OH)2 platelet particles.206 Simultaneous topography and OER current density maps werecollected, demonstrating an inactive basal plane and dominant edgeactivity at pristine plates ,where spatially heterogeneous electrocatalytic activity was observed.This was attributed to variations in the nanoporosity of the thinfilm, after excluding contributions from composition, chemical structure,and surface roughness.282 This was revealedby analyzing the capacitive current in these measurements, to estimatethe local electroactive (wetted) surface area in situ from the SECCM response.SECCM has alsobeen applied to electrodeposited amorphous filmsof molybdenum sulfides  and nano-octahedra (ODs), obtained via electrospraying,243 and expressing {100} and {111} crystal planeswere screened for HER activity. The NCs exhibited higher electrocatalyticactivity than ODs, in agreement with macroscale measurements. However,the NCs showed considerable particle-to-particle variations in catalyticactivity, which is only revealed by targeted nanoscale measurements.212SECCM offers a unique platform toisolate and analyze the electrochemicalresponse of single metal NPs.2 reduction reaction (eCO2RR) by the samegroup.208 The RDs showed superior activity and selectivity compared withODs and high-index TDPs. Results were in good agreement with the macroscalemeasurements, but the SECCM-derived TOF was an order of magnitudehigher than the macroscale-derived TOF, due to the high mass transportrate of SECCM (see above).In subsequent work, gold nanorhombicdodecahedra (RDs), ODs, andnanotruncated ditetragonal prisms (TDPs), expressing {110}, {111},and {310} crystal planes, were investigated for the electrochemicalCO306 Other combinatorial analyses were performed in order toinvestigate the heterogeneity between, and within, single particles.Cu2O octahedrons with {111} facets showed higher activitythan cubes with {100} facets. The poor inherent electric conductivitywithin Cu2O nanocrystals, resulting in low overall activity,was also underlined.307SECCM was used to target borohydrideoxidation at Au nanoparticlesof controlled shape, which offered a variety of facets, and resultedin facet-dependent electrocatalytic activity.311 Increased OER activity was obtained at body sites, and longer nanorodswith higher body share were beneficial in catalyst design. In a seminalwork toward automation of SECCM operation, an optically targeted electrochemicalcell microscopy method was established to perform guided measurementson individual Cu nanoparticles (dedicated regions of interest).305 In that case, no correlation was observed betweenelectrocatalytic activity and particle size. In a study intendingto probe the OER activity of single NiFe2O4 superparticles,a size-sensitive electrochemical response was reported.304 A particular composition effect (related tothe Au content of NiFe2O4–Au particles)was only demonstrated at particle sizes smaller than 800 nm.Single hematitenanorods of different lengths, with tip and bodypart corresponding to {110} and {001} facets, respectively, were preparedto examine face-dependent OER activity.215 The Ag/AgCl QRCE was reported to be less stableat high current densities in alkaline solutions. A noninterferingOs2+/3+ complex was used as an internal reference redoxcouple (in solution) to correct for QRCE potential drift, and thevoltammograms of encapsulated ZIF-67 particles were normalized withrespect to the active surface area to exclude the current contributionfrom the substrate surface within the wetted area. A follow-up studyexplored the significant structural  and compositional changes of singleCo3O4 particles at high current density in alkalinesolution.309 Supported by TEM characterization,this study provided a view of structural transformations during electrochemicalprocesses, at the single-particle level.Experimental considerations were highlighted in studies of OERat single zeolitic imidazolate framework (ZIF-67)-derived particles.312 This study opens up the use of SECCM to targeting the solidelectrolytes, e.g., the electrode/electrolyte interface in a solid-statebattery, or the electrode/electrocatalyst/membrane interface in afuel cell.An interesting applicationof SECCM concerned the electrochemicalanalysis of Pt nanoparticles (70 nm in diameter) positioned at a buriedinterface between an HOPG support and a Nafion membrane. ElectrocatalyticHER activity was investigated as a function of membrane thickness,with the highest activity at 200 nm thick films.2RR, with one example already highlighted above. Theattributes ofSECCM for such studies and the complementarity of the SECCM techniqueto related advanced voltammetric methods were recently reviewed.313 SECCM provided evidence to support the directlink between local configurations of Sn/rGO and eCO2RRperformance.314 Activity at three siteswas considered: the Sn particles, the rGO surface, and the boundarybetween Sn and rGO. A controlled atmosphere of Ar or CO2 during the experiment was essential in order to confirm that theinterface between Sn particles and rGO promotes CO2 reductionand suppresses proton reduction to hydrogen. Additionally, a synergeticeffect in the catalytic mechanism of Sn/rGO was emphasized, with CO2 activity measured from two sources: (i) directly, from theCO2 adsorbed on the Sn surface, and (ii) indirectly, fromthe CO2 adsorbed on oxidized function groups of rGO, continuouslymigrating to the Sn surface to supply the reaction and enhance thecatalytic activity.SECCM is finding application in several studies ofeCO2RR, OER, etc.). The combined, high-throughputapproach has proven to be a powerful tool toward interpreting structure–activityrelations. Mariano et al. studied eCO2RR on polycrystallinegold315 and reported enhanced eCO2RR activity around grain boundaries of specific geometries, whichcan be attributed to an altered lattice strain and increase of dislocationdensity. In a follow-up work, SECCM and high angular resolution EBSDwere employed to investigate the origin of enhanced electroreductionactivity at the slip band around the Σ3 grain boundary regionand defect-rich areas  < (100) < (110). The facets (941) and (741), in particular,displaying higher step and kink feature density, resulted in higherelectroreduction activity. A naturally formed copper-passivating layer(Cu(OH)2, CuO, and Cu2O) shows strong grain-dependentstripping behavior. Sequential cyclic voltammetry cycles verifiedthat the surface oxides could be removed down to an electrochemicallyundetectable level after a single cycle pretreatment, which eliminatescontribution from the passive layer in studies of eCO2RRat copper.SECCM with colocated EBSD maps has beenused extensively to screenthe local response over multiple crystallographic orientations andgrain boundaries of pseudo-single-crystal materials  for a range of processes of local surface processes within the SECCMmeniscus.in situ imaging to break down the averagedresponse at a macroscopic battery electrode, from a combination ofactive materials, binder, and additives, enabling investigation ofbattery materials down to the single-particle (entity) level.318 The performance of lithium-ion battery (LIB)cathode materials has been the subject of a number of studies. SECCMwas first employed (in aqueous solution) to investigate localizedredox activity on composite LiFePO4 (LFP) in 2014,219 revealing heterogeneous reaction rates dependingon the local composition. More recently, SECCM was used to evaluatemetal oxide coating effects on Li+ (de)intercalation atLiCoO2.319 It was found thatZrO2 coating can improve cycle durability at the expenseof reaction rate. Ion transport at isolated LiFePO4 particleswas also investigated, demonstrating fairly homogeneous Li+ deintercalation on a single particle  to investigate the facet-dependent Li+ electrochemicalreactivity and diffusion coefficient at Li4Ti5O12 thin-film electrodes.238 They also achieved fast charging/discharging of single LiFePO4 particles at a scan rate of 1 V s–1 andcharacterized a metastable state of LixFePO4 , as a modelclass of aprotic solvents, for SECCM measurements under conditionscomparable to those of battery operation.LixFePO4 9C.235 In recent work, SECCM measurements were coupled with enhancedRaman spectroscopy monitoring via shell-isolated nanoparticles  were used to perform local cyclic voltammetryon (111)-facet silicon. SECCM/SHINERS coupling sheds light onto thestages of a continuously evolving SEI through tweaking the numberof cycles and potential cycling range. Finally, the charge transferresistance on individual LFP particles was quantified, as a modelsystem for exploring battery charging/discharging at high rate.322 The results affirmed the significance of dischargelevel and electrolyte solution type (aqueous vs organic solvent here).Three different types of HOPG, having differentstep densitiesand step heights, were utilized as a model system to investigate solidelectrolyte interphase (SEI) formation with SECCM in a glovebox. SEIis formed at the surface of LIB negative electrodes due to reductionand decomposition of electrolytes, and this dynamic process is crucialfor battery performance. Slow scan rate SECCM voltammetry revealedthat step edges promote electrolyte reduction resulting in a morepassivating SEI layer than on the basal plane.Optical microscopymethods have long been used in physics and biology,for example, to quantify the action of forces by monitoring the motionof objects and to identify the biological functions or structure ofsubentities of living microorganisms. Optical microscopies have beenconsidered less in chemistry or electrochemistry. One of the reasonsis that the visible region of light used to obtain an optical imageis not usually the most informative spectral range for identifyingchemical structures or chemical transformations. This view has changedconsiderably over the last decades due to progress in (a) the syntheticdesign of molecules or nanoobjects with switchable optical propertiesin the visible range; (b) the development of highly sensitive andrapid photodetectors, e.g., able to detect the ultimate limit of asingle photon; and (c) the development of strategies enabling imaging at higher resolution (such as super-resolutionmicroscopy) and higher sensitivity (even label-free).324 Here we focus on new methodologies able to unravelgreater levels of complexity in terms of electrochemical processes/information,including visualizing simultaneous phenomena, as well as new experimentaland image-processing methodologies.For detailsof the progressand the operational principles and applications of optical microscopyfor imaging electrochemical systems, we refer to a number of recentreviews.2) containing thousands of pixels within a millisecond time scale. Second, the imaging process is usuallynot perturbing to the electrochemical process and does not requireaccounting for invasive contributions  of a nearby local electrochemical probe.Thus, optical microscopy provides a unique means to observe operando, and at high throughput, electrochemical processesin conditions near to those of real electrochemical systems.However, imaging electrochemical processes with an optical wide-fieldmicroscope involves catching the optically visible footprint of theresult/consequence of an electrochemical reaction. Thus, the localoptical information collected at each pixel of the optical image carries(and may be converted) into local electrochemically relevant information.26 electrochemicalluminescence (ECLM),325 or Raman-based(RM) microscopies, usually require labeled tracers (fluorescent orRaman active molecules or quantum dots). Label-free microscopies aresensitive to the local change in refractive index associated withan electrochemical reaction. This was mostly addressed from the changein absorbance for an electrochromic object, or change in light extinction,for example for plasmonic NPs. For uncolored dielectric objects, lightvariations in the refractive index can also be probed by interferometricimaging mode, as exploited, for example, by the interferometric scattering(iSCAT) or other equivalent microscopes.328Quantitative electrochemical imaging from optical microscopy istherefore demanding, as the data incorporate the chemical complexityof the systems and also requires the appropriate analysis or conversionof the optical image. As a consequence of the former point, opticalmicroscopy has first tackled model (or simpler) systems and made useof basic light detection principles. Some of these are recalled in The imageformation requires a significant change of the local optical propertiesrelative to the background level. Sensitive detection and imagingthen mostly rely on the ability to image a system with the lowestbackground level.22 SMLM was used to monitor the dynamics of molecularadsorption and electrochemical or catalytic processes at various nanoscaleinterfaces, such as NPs or nanobubbles.335The concept of superlocalization (enabling super-resolution imaging)consists of resolving the center of mass or centroid, by a Gaussianfit procedure, of the optical feature representing the imaged object.Single fluorescent molecular probe sensitivity is reached by FM, inwhat is known as single-molecule localization microscopy (SMLM).337 At nonplasmonic (transparent)electrodes, TIR illumination allows tracking the growth of nanodendriteswith nanometer spatial resolution.339 It has particularlybeen used to image the formation of EDL at individual NPs.327Removal of the background incident light is usually achievedthrough oblique incidence illumination .Total internal reflection (TIR) incidence allows observation, withhigher sensitivity, of the few 200–300 nm adjacent to the illuminatedinterface. Different single-object studies rely on superlocalizingthe position of single objects in this illuminated region. TIR illuminationis also used with label-free microscopies. It enables the observationof any type of nanoobject deposited on or near a thin plasmonic Auelectrode in the surface plasmon resonance microscopy (SPRM) configuration.328 The visualization principlerelies, as sketched in Therehas been increasedinterest in interferometric-based strategies. They are exploited indifferent reflection-based microscopy configurations, the most popularbeing the iSCAT.325 As it relies on the reaction-transport of electrogeneratedspecies, ECLM is pertinent to imaging the adhesion sites of singlebiological cells.345 Interferometric measurements can also be implemented.346 Ultrasensitive cameras are now capable of reachingthe ultimate detection of single photons. ECLM is then ideal for imagingobjects or reactions nearby the electrode region with single-photonsensitivity.347Recent efforts have sought to push the detection limitof ECLM, sincethe light emission event is triggered by an electrochemical reactionand therefore does not require an illuminating light source. ECLMis therefore operated under almost ideal dark conditions, though underthe constraining conditions of producing reactive species at an electrodeunder strongly oxidative (or reductive) potentials.352 The changes in optical signature during an electrochemical experimentcarry valuable chemical information, complementary to the electrochemicalsignal. In particular, characteristic times of the optical signature variations can be associated and correlatedto different (electro)chemical transformations . Several strategies have also been proposed,to convert the optical signature variation quantitatively into anoptically inferred electrochemical curve, such as an optical-voltammogram.356 Light absorbance, or the extinction wavelength (for plasmonic NPs),is usually proportional to a local concentration or amount of electrochemicallytransformed material, i.e., an electrochemical charge.357 The electrochemical current then often tracksthe time derivative of local optical transients. For more complexsituations, optical models have been developed to simulate the changein optical intensity associated with the electrogenerated growth ofNPs,353 the conversion of NPs,359 or nanobubbles360 from electrodes orelectrocatalytic processes at NPs.The most straightforward information in anoptical image is the object position along its 2D or 3D trajectory toward a polarized electrode, whichcorrelates to the electrochemical current in electrochemical nanoimpactstudies.ex situ, it provides a complementary imaging  for a large number of imaged objects from which structure–opticalresponse relationships can be obtained. A large number of objectsoften need to be analyzed, and automated image analysis routines canbe used.356 These routines, often employingmachine learning strategies, are now able to compare/recognize automaticallyimages obtained from different microscopies, in order to find thesame features, and classify them according to their activity, shape,or structure.Another meansto extract quantitative information from optical images is to comparethem to colocated correlative complementary higher-resolution microscopies,such as electron microscopy  images. Even if such correlative imaging is obtained 361 This area grew from the meticulousexamination of electrochemical transients associated with the reactivecollision of individual nanoobjects onto an electrode; optical observationwas rapidly implemented not only to see these individual collisionevents but also to provide insightful complementary information. Followingthe seminal works of Crooks362 and Tao,363 two types of information can be gathered. First,high-throughput, wide-field-of-view imaging of the electrode regionallows observation of where and how nanoobjects encounter the electrode.Spatiotemporal localization of a wide variety of individual objects canbe achieved with all microscopies, including those supplemented withspectroscopic interrogation,365 in 2D or more recently in 3D.366 Second,the optical signal associated with the individual objects is scrutinized,as it provides further complementary information about the electrochemicaltransformation of the isolated object or in its surrounding environment.Therenewed interest in optical microscopy in electrochemistry is intimatelyrelated to the expansion of the field of single-entity electrochemistry.367 By observingoptically the opposite pole electrode surface by FM, one can obtainan image of the reaction of interest of the nonfluorogenic species.The imaging strategy can be coupled with both electroactive or a pH-sensitivefluorogenic species that fluoresces upon (electro)chemical transformation.369Emission based microscopystrategies coupled to electrochemical actuation have been successfullyused over the past few years for mapping electron transfer processeswith very high spatial resolution. This approach reveals the heterogeneityof electrode reactivity. To enable the imaging of electron transferat nonfluorogenic species, Zhang et al. proposed a method using abipolar electrode system to directly couple a conventional electrochemical reaction on one pole of the bipolar electrode tothe reverse electrochemical  conversion of a fluorogenicspecies on the opposite pole.370 The strategy can be employed to image a wide range of electrochemicalprocesses or serve as an indirect quantitative fluorescence sensorfor nonfluorogenic redox labels for cellular biomarkers371 or respiration.372 The pixel in the image is given by the size and density of the bipolarelectrode arranged in arrays. Resolution at the micrometer scale issufficient to image biomarkers bound at cell membranes,373 while the use of nanoelectrode arrays improvesresolution.374As a single-electron transfer can result in multiple photonemission, transducing electrochemical events into light emission candramatically enhance the detection sensitivity up to single electrontransfer events. Using an ECL reaction at the detection pole allows,in principle, higher sensitivity (lower noise).375 Single-moleculeECL imaging also considerably increases the spatial resolution ofECLM down to a few hundred nanometers by applying the concepts ofsuper-resolution microscopy. This approach enabled the facet and defect-dependentelectrochemical activity of Au nanorod and nanoplates to be determined.376ECL microscopy was used at the level of single-moleculeor single-event imaging. The advantages of this approach are the near-zerooptical noise level imaging and the absence of photobleaching usuallyencountered in single-molecule fluorescence microscopy studies. Single-moleculesensitivity allowed detection and imaging of the position of singleproteins at the cell membrane.32+, oxidation reaction at electrodes.By using large excess of coreactant to ensure the excited state generation(and light emission) is confined near the electrode surface, and collectingand counting single photons over time resulting from single reactionevents, Dong et al. implemented such dynamic single-photon countingto draw super-resolved images of electrodes and cells.344 The strategy was used to map the Ru(bpy)32+ oxidation kinetics, analyzed to produce thestandard rate constant, k0, and the standardpotential, E0, over a catalytic gold microplatecatalyst surface. Super-resolution images of these descriptors areprovided in 347 This study revealed that the moreactive sites, characterized by higher reaction rate constants, arelocated at the edges of the catalyst. By comparing the TOF maps, constructedat different potentials, they also highlighted the formation of Auoxide on the catalysts at higher overpotential, resulting in a decreasein activity.Single-photon ECLM was employed to resolvespatially and temporally single trisruthenium(II)complex, Ru(bpy)operando molecular transportfrom/to electrode regions. The most commonly imaged targets are theelectroactive species engaged in the reaction: fluorogenic electroactiveor pH sensitive species, light absorbing or refracting reaction productsor substrates, and scattering NPs. Recent strategies are devoted tothe transport of the electrolyte itself (ion fluxes) during the electrodereaction: from fluorogenic or Raman-active electrolyte ions to fluorogenicelectroinactive molecules or particles, which are used to monitorthe accessibility or flow of the solvent to porous electrodes.Theheterogeneity ofcharge transfer is not just restricted to the polarized electrodeor its electroactive domains, but it propagates, through mass transfer,within diffusion distances. Optical microscopy methodologies havealso been developed to map the transport of optically active speciesinvolved in electrochemical reactions. It is indeed a unique, simpleinstrument enabling imaging 379 illustrated recently from the3D diffusion layer build-up during enzymatic reactions.380 The instruments are sufficiently mature totackle the transport of active species in real energy storage or conversionsystems, such as porous electrodes381 forelectrolyzers or batteries384 under operation.3D mapping is also possible through confocal imaging ,+ ions during (dis)charging currents withinindividual micrometric electroactive LiCo oxide or NbW oxide microparticles.383 Merryweather et al. suggested from the dynamic analysis of theseion propagations that the charge and discharge processes do not followthe same trajectory within the particle. These differences are discussedbased on the difference in conductivities in each state of the particle,which is then manifested locally.382 Themethod was extended to imaging in the presence of the usual electrodecomponents (polymer binder and carbon paste) and developed to enable3D probing inside the particles using confocal visualization.384 Complemented with 2-photon fluorescence excitation,the local distribution of the LiPF6 electrolyte could beimaged around the particles, revealing inhomogeneous electrolyte diffusionas a result of the geometry and porosity of the carbon/binder matrixsurrounding the particles.Refractivebased microscopy was recently used to image the localtransport of Li3 NPs, i.e., indirectly the lithium ion intercalation process,385 via changes in the optical density. From theexperiments, it was concluded that pseudocapacitive charge storagedepended on the NP morphology. The same methodology was also appliedin a comprehensive series of studies from Wang’s group on the(indirect) visualization of mass transport of potassium ions insidesingle Prussian blue particles during their reversible electro-chromicconversion to Prussian white.386 Optical data suggested a less intuitive shell-to-core transformationmechanism355 and evidenced partial conversiondue to an inactive zone inside some of the studied particles.386 The same material was further investigatedby coupling optical visualization to EIS to probe the depth of thesurface charging layer during the same redox conversion.387As the electrochemicaltransformation ofa single entity usually provokes a significant modification of itsintrinsic optical properties, optical imaging can be employed to followthe conversion process, to identify reaction intermediate, and todecipher mechanistic routes. The conversion of electrochromic NPscan be monitored from local absorbance imaging. For instance, Evanset al. monitored and quantified the conversion of single WO359 The approach was also used to discriminatebetween surface charging and compositional evolution during the electrochemicalconversion of Ag into AgCl NPs that was previously imaged by interferometricreflection microscopy.358 Again, the conversiondoes not follow the same intermediates and dynamics during oxidationor reduction. The reduction of the less conductive AgCl NP startswith heterogeneous surface inclusion before a core–shell transformationoperated. As the visualization also provided spatial information,the NPs were shown to move on the electrode during multistep chargeinjection of the electrochemical conversion. It was hypothesized thatthe local halide ions released during the conversion propelled theNPs.358Optical phase retrieval from SPRM allowsa quantitative direct evaluation of refractive index at the single-NPlevel. This strategy also brings selectivity to label-free imagingfor discriminating between different types of chemistry of NPs 10C.359 T388 A case study is the electrochemical deposition of noblemetals such as Ag, Au, or Cu, for which the nucleation and growthmechanisms are still under intensive investigation. Dark-field spectroscopicmicroscopy (DFM) detected the facet-dependent deposition of atomiclayers of silver on single gold NPs.357 Visualizing operando deposition processes alsogives access to the evolution of the surface nucleation density.391 As the local scattering intensity is linked to the NP volume, italso allows sizing single NPs over time and the measurement of theirgrowth dynamics.393 By differentiating the NP volume fluctuation, one can further calculatean “optical current” related to the growth of singleNPs.391Electrochemical reaction canlead to the creation of nanostructures that can be imaged at the earlystage of their formation by optical microscopies.391 Co,392 Li,395 Mg,396 and Zn339 as metal and/or as metal oxides and hydroxides . These studies have implications in electrocatalysisand battery processes, as well as for studying the electrocrystallizationof metal oxides for energy conversion and storage. For instance, opticalmicroscopy, employed as an “optical microbalance”, evidencedthe direct influence of zinc hydroxide sulfate precipitation/dissolutionduring the dissolution/deposition of the MnO2 electrodeassociated with its discharge/charge in an aqueous Zn–MnO2 battery environment.397Optical microscopy has been employedfor imaging the electrodeposition of more complex materials such asNi,400 the electrodeposition of mono- or bimetallic shells  or semiconductor shells  around AuNPs.402Optical visualization can also be a “helping hand”in the elaboration of heteronanostructures, as exemplified by themonitoring of the electrochemically assisted deposition of shellsof various composition around Au NPs. These studies take advantageof the sensitivity of the localized surface plasmon resonance spectrumof the NP to alteration of its surface composition. The electrodepositionis then tracked from the change in color of the NP observed by DFM.It was used to track amalgamation of Au with Hg,403 One way of analyzing single NP dissolution kinetics from opticalimages consists of evaluating the characteristic duration of the decreaseof scattered light with time by DFM. The same strategy was used  on the dissolution rate. The strategy is sound asa means of evaluating the CO2 storage capacity of biomineralizingalgae. A key general feature, when studying dissolution/growth atmicroscopic single entities, is that mass transport (diffusion) iswell-defined and high, thus enabling the measurement of fast kineticswith simple platforms.406Optical monitoring of thedissolution kinetics of single entitiesby the same methodology is also possible. The electrodissolution ofsingle Ag NPs continues to be actively investigated.was used 10D to im363The accumulation of products atthe vicinity of a nanoscale electro-catalyst sometimes provokes achange in the local refractive index of the surrounding medium thatcan further be detected by optical microscopy.408 Optical microscopies are ideal for monitoringdynamically and under operating conditions the dynamics of formationand growth of nanobubbles on surfaces.410 Nanobubbleswere imaged by single-molecule fluorescence microscopy using fluorescentprobes that are trapped at the liquid–gas interface,333 even forming molecular aggregates.335 Using this methodology, the Zhang group imagedthe hydrogen spillover occurring at gold nanoplates supported on ITOelectrodes during OER331 and the delayedproduction of H2 nanobubbles at AuPd electrode during HERthat could be explained by the ability of palladium to store hydrogen.411Electrocatalytic reactions such as hydrogen or oxygenevolution reaction often produce nanobubbles at the electrode surfacethat nucleate and grow once the local environment becomes saturatedby gas molecules  during OER caused by the formation of O2 nanobubbles between the electrode and the catalysts and furtherused this hopping behavior as an indicator of catalytic activity .383 Dynamic imaging of the localstate of charge (Li+ ion front) in an NbW oxide microparticleduring lithiation and delithiation reveals phase separations withinthe particle. + diffusion coefficient on the state of charge. Phase separationfrom charging the particle at extreme rates of delithiation  can even lead to particle dislocationor rupture, also detected optically from the appearance of the darkdot at 283 s and propagating later from the left to the right of theparticle in 383Theelectrochemical conversion or processes of ion insertion during energystorage material charge or discharge can be accompanied by considerablephysical transformation. Relative volume expansion/shrinking as lowas a few percent can be probed from the superlocalization of individualmicroparticle edges during its conversion 2+ ion solution in neutral pH.391 Owing to the negative potential needed for the reduction of theNi2+ ions, the electrodeposition of metallic Ni NP competeswith the electrogeneration of OH– from water reductionat the electrode. The latter results in the precipitation of Ni(OH)2 nanocrystals. The competition between metallic Ni and Ni(OH)2 NP formation is distinguished by interference reflectionmicroscopy: the former appear as bright optical features, while thelatter appear as dark-contrasted features.391 The use of electrolyte microdroplets, produced from micropipettes,allows probing the electrochemical reaction over different regionsof the electrode (a transparent ITO surface). Interestingly, the resultsstrongly depend on the local electroactivity of the ITO electrodeitself, as either a mixture of Ni and Ni(OH)2, or a singlepopulation of Ni(OH)2 NPs can be formed. The contributionof each competitive route also depends on the electrode potential(and Ni2+ ion concentration) and can be identified by comparingthe dynamic growth of each nanomaterial population. In essence, identifyingcompetitive paths requires analysis of optical images of objects ordomains presenting different chemical structures. Operando optical monitoring needs to be compared with multiple complementaryand correlative microscopies of the electrode at identical locations.As illustrated in operando optical images. A supervised machine learning analysis and featurerecognition in the different sets of images enabled identificationand classification by size and chemistry of the different electrogeneratednanoobjects.356A typical exampleof the use of optical microscopy to investigate parallel or competingchemical reactions is the monitoring of electrochemical processesoccurring within the potential regions associated with electrolytedischarge, as illustrated by the electrodeposition of Ni NPs fromNi2+ solutionby 5 mM H2SO4 allowed inspection of the hydrogenevolution reaction at the ITO surface and revealed its competitionwith In metal formation.303 Optically,In NPs and H2 nanobubbles appear with similar contrastwithin the same negative potential region and disappear upon backward(positive) polarization. Their identification within optical imagesis obtained by superlocalizing the nucleation and mobility of thedifferent objects on the electrode. Over successive potential cyclesthe In NPs are nucleated and  reoxidized at the same location. In contrast, the H2 nanobubbles nucleate at stochastic sites over repeated cycles. Moreover,the growth/dissolution of bubbles on the electrode is also detectedas a mobile optical feature, which highlights the pinning of the bubbleat electrode surface defects. It is noteworthy that In2O3 reduction and H2 generation compete forH+ ions, resulting in segregated regions of micrometricsize for each population over the same electrode.Confining the electrochemicalreaction with micropipettes, in an SECCM type experiment, within microsizedregions of the electrode reveals the great heterogeneity of the electroactivityof ITO for metal electrogeneration versus water reduction. Indeed,a third population of NPs was also found corresponding to the reductionof ITO into metallic In NPs. Replacing the Ni422 Label-free optical microscopies are now ableto image NP densities up to 109 NPs per cm2.They have allowed imaging of the dynamics of NPs, from onset, duringthe electrodeposition of Ag,422 Cu,390 or Ni (vs Ni(OH)2)NPs,391 or the electrocrystallization ofCaCO3 particles.231The nucleation/growth ofnew phases, as in electrodeposition or electrocrystallization processes,is often associated with collective behaviors. These can be understoodfrom high-throughput optical microscopy that allows observation ofindividual entities and the ensemble. Optical observation revealedintrinsic growth kinetics of each individual NP within an ensemble,along with apparent progressive growth associated with the broad distributionof NP nucleation or onset times.389 Whenthere is significant reactant depletion, the nucleation of new NPsis delayed in regions near existing ones and the growth is sloweddown.390Collectivebehaviors are related to the overlapping of the diffusion (concentrationboundary) layers to sustain NP growth. Chemical communication betweenNPs, from overlapping of diffusion fields, could be evaluated fromthe distribution of sizes of the Voronoi  cell occupiedby each NP during their growth.3 nano/microparticles.231 The formation of such crystals is due to thelocal alkalization of the Ca2+-containing electrolyte owingto OH– electrogeneration during ORR. The reactionwas monitored by interference reflection microscopy, watching themeniscus of an SECCM microscale-sized electrochemical cell  dissolutionrate than expected based on Faraday’s law. This mismatch suggeststhe MnO2 reduction results in a partial dissolution ofthe MnO2 layer accompanied by the precipitation of a foreignhydroxide layer from the electrolyte. This solid electrolyte interphaseformation, which is detected optically, may impact the overall cyclabilityof the electrode material.397As alsohighlighted in SECCM multiscale measurements, sometimes, the opticallyinferred information does not match the macroscopic electrochemicalbehaviors. For example, the cyclic voltammogram for the electrodepositionof a single Ni NP can be inferred from the optical monitoring presentedin Sincethe role of individual entities is intimately related to that of theensemble, multimicroscopy identical location imaging approaches areessential as we have described herein. As also described in the SECCMsection , such de426 The identificationof Ni versus Ni(OH)2 NPs formation used a machine learningclustering based on optical, SEM, and EDX images colocalized by automatizedpattern recognition algorithms.356 Machinelearning strategies are also intensively applied in optical microscopyto make these tools quantitative.428Obviously, this requires large and multidimensionaldata sets from which trend lines, correlation, and classificationbecomes less straightforward for a single operator. The handling oflarge data sets and their analysis is now met by data science andmachine learning approaches, which are also impacting the automatizedanalyses of images. Recent reviews discuss the introduction of machinelearning approaches for identifying the different components of anelectrode material in batteries.5 or spectroscopic (TERS)429 probes. In the context of high-throughput imaging,while maintaining atomic or nanometer-scale image resolution, it ispossible to image surfaces under electrochemical operation at a rateof 30 images per second , with so-called video-rate imaging-STM orAFM.430 This approach has enabled studiesof different single-crystal electrode surfaces and associated surfacedynamic changes for model systems. Examples include the formationof surface adsorbates and their surface diffusion (or migration) underpolarization, e.g., sulfur adsorbates on Br-coated Ag(100),431 or on Br- or Cl-coated Cu(100),432 or CO onto Pt(111).434 The strategy is currently extended to more complex situations illustratedby the observation with video-AFM of (i) the dynamics of nanobubbleson electrodes during water electrolysis435 or (ii) a wide variety of dynamic restructurings of model Cu(100)catalyst electrode surface during electrochemical CO2 reduction.436 In the latter case, under open-circuit potential,an epitaxial oxide layer is formed atop the Cu electrode. Under moderatereduction potentials the Cu catalyst is transformed from smoothlycurved surface to rectangular terraced surface, while at higher cathodicpotentials the density of step edges (uncoordinated Cu sites) is drasticallyincreased.436 Such measurements can belinked effectively to SECCM-EBSD236 -EBSD236  to provi442 reporting the use of in situ electron microscopyfor electrochemical energy storage/conversion. Applications in thisarea have been facilitated by the mass production of liquid cell (LC)chips, thin enough to obtain high spatial resolution imaging. Commercialcells are now available, consisting of two sufficiently thin and robustSiNx windows, 10–30 nm thick, providinggood transparency to the electron beam chemical reactions, or the appropriate geometric configurationof the electrode to obtain reliable electrochemical measurements.447Recent years havewitnessed an increasing number of studies andreviewsron beam 12A. Elecoperando imaging technique.448 Its advantage is the possibility to perform multiple characterizations analysis at the same location to enable, inter alia: better resolution of morphology, detection ofcrystal structures by electron diffraction, to allow the analysisof deformation in nanocrystals, and chemical identification by X-rayspectroscopy. A significant focus has been toward the operando evaluation of the durability of materials under electrochemicalconditions. LC-TEM is well-suited to evaluate structural changes ofnanomaterials for battery electrodes during dis/charging (Li450 or Na ion batteries451), or of nanocatalysts under electrocatalytic conditions. In additionto dissolution kinetics , in situ TEM provides a dynamicanalysis of the object structures at the nanoscale, especially duringtheir corrosion induced by electrochemical operation. Electrocatalyticoxidations provide, in fact, aggressive conditions that can lead tothe dissolution/corrosion of various nanomaterials, such as cobaltoxide NPs during the oxygen evolution reaction,452 or boron nitride NPs during ethanol oxidation  surfacesolution 12D. It pIn this article we have sought to showcase considerable developmentsin key aspects of nanoelectrochemistry arising from advances in robustnanoscale electrochemical devices and imaging techniques that arereadily implemented. Nanoscale imaging of surface dynamics now encompassesthe wide variety of subjects covered by electrochemistry, from moleculesto materials, from sensors to energy storage or conversion, and theintersection of the subject with the life sciences. Imaging electrochemicalprocesses at the nanometer scale or detecting individual nano-objects(even molecules) in an electrochemical situation is becoming muchmore routine. The challenge, now and in the future, is to increasethe throughput of knowledge through the development of strategiesto both increase the volume and proportion of relevant data from anexperiment (avoiding redundancy) and for the analysis of larger datasets. To this end, our article has explored efforts made in high-throughputelectrochemical analysis and imaging, focused on the past few years.in situ) microscopy and spectroscopy techniques,SECCM is expected to open up fascinating understanding of energy storage, conversion, electrosynthesis, and corrosion at the nanoscale.The application of SECCM to these problems is attractive because theseprocesses are intricately related in real-world nanostructured materials.While we have touched upon several SPM techniques used in electrochemistry,nanoscale electrochemical imaging  is mostreadily achieved through nanopipette-based probes, i.e., imaging bySICM and SECCM. SECCM is now used by increasing numbers of researchgroups in a plethora of systems impacting all fields of electrochemistryand materials. In its present form, and with further developments,including intelligent scan routines and integration with complementary.We envision the increasing translation of nanopore developments tothe SICM to enable high-throughput imaging of interfaces at high spatialand temporal resolution with high chemical sensitivity. We have describedherein enhanced information from nanopores through the use of arraydevices and more detailed analysis of nanopore signals, as well asmultifunctionality from the incorporation of additional electrochemicaland optical detection strategies.operando a wealth of electrochemicalprocesses with nanoscale resolution, drawing on the concepts of superlocalization.While liquid-cell TEM offers higher resolution for in situ and operando visualization, there are some compromiseson cell design (and mass transport), limitations on integrating thetechnique with complementary methods, and issues around the effectof the electron dose on the solvent and processes studied. We thusexpect optical methods to become an important option for the mainbody of electrochemists, especially if seeking quick initial insights.Optical microscopy strategiesoffer a fast, simple, and cost-effectiveway to monitor The ultimate single-molecule imaging limit is commonly achievedfor fluorescent probes, but it is now at hand with label-free imagingbased on promising developments in interferometric scattering microscopy.Recent advances have shifted the technique from model plasmonic systemsto more complex situations, including real battery electrode materials,or to inspect further chemical complexity, such as competing chemicalreactions. Furthermore, the wide field of view enables monitoringof thousands of nanoentities simultaneously, not just to resolve thebehavior of single entities, but also to address the question of inter-entitycommunications.The development of holistic views of nanoscaleelectrochemistrywill increasingly make use of correlative multimicroscopy approaches.We have illustrated this aspect of nanoelectrochemistry throughoutthis article with recent examples that show how the level of structural,chemical, and functional  information on a givensystem is massively increased. Storing, handling, analyzing, and interpretingthese data will require a shift in the field toward data science andartificial intelligence, through machine learning. The next generationof nanoscale electrochemistry and electrochemical imaging approacheswill draw on automatized multi-image cross-correlation and automatizedimaging tools, which will increase throughput and accelerate the discoveryand rational development of electrochemical technologies so urgentlyneeded.