Optical methods for studying local electrochemical reactions with spatial resolution: A critical review

Monitoring Electrochemical Dynamics through Single-Molecule Imaging of hBN Surface Emitters in Organic Solvents

Electrochemical techniques conventionally lack spatial resolution and average local information over an entire electrode. While advancements in spatial resolution have been made through scanning probe methods, monitoring dynamics over large areas is still challenging, and it would be beneficial to be able to decouple the probe from the electrode itself. In this work, we leverage single molecule microscopy to spatiotemporally monitor analyte surface concentrations over a wide area using unmodified hexagonal boron nitride (hBN) in organic solvents. Through a sensing scheme based on redox-active species interactions with fluorescent emitters at the surface of hBN, we observe a region of a linear decrease in the number of emitters against increasingly positive potentials applied to a nearby electrode. We find consistent trends in electrode reaction kinetics vs overpotentials between potentiostat-reported currents and optically read emitter dynamics, showing Tafel slopes greater than 290 mV·decade-1. Finally, we draw on the capabilities of spectral single-molecule localization microscopy (SMLM) to monitor the fluorescent species' identity, enabling multiplexed readout. Overall, we show dynamic measurements of analyte concentration gradients on a micrometer-length scale with nanometer-scale depth and precision. Considering the many scalable options for engineering fluorescent emitters with two-dimensional (2D) materials, our method holds promise for optically detecting a range of interacting species with exceptional localization precision.

Dual-Mode Imaging of Dynamic Interaction between Bubbles and Single Nanoplates during the Electrocatalytic Hydrogen Evolution Process

Gas bubble formation at electrochemical interfaces can significantly affect the efficiency and durability of electrocatalysts. However, obtaining comprehensive details on bubble evolution dynamics, particularly their dynamic interaction with high-performance structured electrocatalysts, poses a considerable challenge. Herein, dual-mode interference/total internal reflection fluorescence microscopy is introduced, which allows for the simultaneous capture of the evolution pathway of bubbles and the 3D motion of nanoplate electrocatalysts, providing high-resolution and accurate spatiotemporal information. During the hydrogen evolution reaction, the dynamics of hydrogen bubble generation and their interactions with single nanoplate electrocatalysts at the electrochemical interface are observed. The results unveiled that, under constant potential, bubbles initially manifest as fast-moving nanobubbles, transforming into stationary microbubbles subsequently. The morphology of stationary nanoplates regulates the trajectories of these moving nanobubbles while the pinned microbubbles induce the motion of the electrocatalysts. The dual-mode microscopy can be employed to scrutinize numerous multiphase electrochemical interactions with high spatiotemporal resolution, which can facilitate the rational design of high-performance electrocatalysts.

Surface-Enhanced Raman Spectroscopy and Electrochemistry: The Ultimate Chemical Sensing and Manipulation Combination

One of the lessons we learned from the COVID-19 pandemic is that the need for ultrasensitive detection systems is now more critical than ever. While sensors' sensitivity, portability, selectivity, and low cost are crucial, new ways to couple synergistic methods enable the highest performance levels. This review article critically discusses the synergetic combinations of optical and electrochemical methods. We also discuss three key application fields-energy, biomedicine, and environment. Finally, we selected the most promising approaches and examples, the open challenges in sensing, and ways to overcome them. We expect this work to set a clear reference for developing and understanding strategies, pros and cons of different combinations of electrochemical and optical sensors integrated into a single device.

Complex electrochemiluminescence patterns shaped by hydrodynamics at a rotating bipolar electrode

Electrochemiluminescence (ECL) is a powerful analytical approach that enables the optical readout of electrochemical processes. Over the last few years, ECL has gained considerable attention due to its large number of applications, including chemical sensing, bioanalysis and microscopy. In these fields, the promotion of ECL at bipolar electrodes has offered unprecedented opportunities thanks to wireless electrochemical addressing. Herein, we take advantage of the synergy between ECL and bipolar electrochemistry (BE) for imaging light-emitting layers shaped by hydrodynamics, polarization effects and the nature of the electrochemical reactions taking place wirelessly on a rotating bipolar electrode. The proof-of-principle is established with the model ECL system [Ru(bpy)3]2+/tri-n-propylamine. Interestingly, the ECL-emitting region moves and expands progressively from the anodic bipolar pole to the cathodic one where ECL reactants should neither be generated nor ECL be observed. Therefore, it shows a completely unusual behavior in the ECL field since the region where ECL reagents are oxidized does not coincide with the zone where ECL light is emitted. In addition, the ECL patterns change progressively to an "ECL croissant" and then to a complete ring shape due to the hydrodynamic convection. Such an approach allows the visualization of complex light-emitting patterns, whose shape is directly controlled by the rotation speed, chemical reactivity and BE-induced polarization. Indeed, the bipolar electrochemical addressing of the electrode breaks the circular symmetry of the reported rotating system. This unexplored and a priori simple configuration yields unique ECL behavior and raises new curious questions from the theoretical and experimental points of view in analytical chemistry. Finally, this novel wireless approach will be useful for the development of original ECL systems for analytical chemistry, studies of electrochemical reactivity, coupling microfluidics with ECL and imaging.

Nanoscale Luminescence Imaging/Detection of Single Particles: State-of-the-Art and Future Prospects

Single-particle-level measurements, during the reaction, avoid averaging effects that are inherent limitations of conventional ensemble strategies. It allows revealing structure-activity relationships beyond averaged properties by considering crucial particle-selective descriptors including structure/morphology dynamics, intrinsic heterogeneity, and dynamic fluctuations in reactivity (kinetics, mechanisms). In recent years, numerous luminescence (optical) techniques such as chemiluminescence (CL), electrochemiluminescence (ECL), and fluorescence (FL) microscopies have been emerging as dominant tools to achieve such measurements, owing to their diversified spectroscopy principles, noninvasive nature, higher sensitivity, and sufficient spatiotemporal resolution. Correspondingly, state-of-the-art methodologies and tools are being used for probing (real-time, operando, in situ) diverse applications of single particles in sensing, medicine, and catalysis. Herein, we provide a concise and comprehensive perspective on luminescence-based detection and imaging of single particles by putting special emphasis on their basic principles, mechanistic pathways, advances, challenges, and key applications. This Perspective focuses on the development of emission intensities and imaging based individual particle detection. Moreover, several key examples in the areas of sensing, motion, catalysis, energy, materials, and emerging trends in related areas are documented. We finally conclude with the opportunities and remaining challenges to stimulate further developments in this field.

Toward the Detection Limit of Electrochemistry: Studying Anodic Processes with a Fluorogenic Reporting Reaction

Recently, shot noise has been shown to be an inherent part of all charge-transfer processes, leading to a practical limit of quantification of 2100 electrons (≈0.34 fC) [ Curr. Opin. Electrochem. 2020, 22, 170-177]. Attainable limits of quantification are made much larger by greater background currents and insufficient instrumentation, which restricts progress in sensing and single-entity applications. This limitation can be overcome by converting electrochemical charges into photons, which can be detected with much greater sensitivity, even down to a single-photon level. In this work, we demonstrate the use of fluorescence, induced through a closed bipolar setup, to monitor charge-transfer processes below the detection limit of electrochemical workstations. During this process, the oxidation of ferrocenemethanol (FcMeOH) in one cell is used to concurrently drive the oxidation of Amplex Red (AR), a fluorogenic redox molecule, in another cell. The spectroelectrochemistry of AR is investigated and new insights on the commonplace practice of using deprotonated glucose to limit AR photooxidation are presented. The closed bipolar setup is used to produce fluorescence signals corresponding to the steady-state voltammetry of FcMeOH on a microelectrode. Chronopotentiometry is then used to show a linear relationship between the charge passed through FcMeOH oxidation and the integrated AR fluorescence signal. The sensitivity of the measurements obtained at different timescales varies between 2200 and 500 electrons per detected photon. The electrochemical detection limit is approached using a diluted FcMeOH solution in which no faradaic current signal is observed. Nevertheless, a fluorescence signal corresponding to FcMeOH oxidation is still seen, and the detection of charges down to 300 fC is demonstrated.

Emerging Optical Microscopy Techniques for Electrochemistry

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

Determining the depth of surface charging layer of single Prussian blue nanoparticles with pseudocapacitive behaviors

Understanding the hybrid charge-storage mechanisms of pseudocapacitive nanomaterials holds promising keys to further improve the performance of energy storage devices. Based on the dependence of the light scattering intensity of single Prussian blue nanoparticles (PBNPs) on their oxidation state during sinusoidal potential modulation at varying frequencies, we present an electro-optical microscopic imaging approach to optically acquire the Faradaic electrochemical impedance spectroscopy (oEIS) of single PBNPs. Here we reveal typical pseudocapacitive behavior with hybrid charge-storage mechanisms depending on the modulation frequency. In the low-frequency range, the optical amplitude is inversely proportional to the square root of the frequency (∆I ∝ f^−0.5; diffusion-limited process), while in the high-frequency range, it is inversely proportional to the frequency (∆I ∝ f⁻¹; surface charging process). Because the geometry of single cuboid-shaped PBNPs can be precisely determined by scanning electron microscopy and atomic force microscopy, oEIS of single PBNPs allows the determination of the depth of the surface charging layer, revealing it to be ~2 unit cells regardless of the nanoparticle size.

The surface charging layer in nanomaterials, which determines their pseudocapacitive behavior, is challenging to characterize. Here the authors perform Faradic electrochemical impedance spectroscopy measurements of single cuboid Prussian blue nanoparticles, displaying a hybrid charge storage mechanism, and determine the depth of the surface charging layer.

Single-nanoparticle spectroelectrochemistry studies enabled by localized surface plasmon resonance

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

Single-Nanoparticle Coulometry Method with High Sensitivity and High Throughput to Study the Electrochemical Activity and Oscillation of Single Nanocatalysts

To explore nanocatalysts with high electro-catalytic performance and less loading of precious metals, efforts have been made to develop electrochemical methods with high spatial resolution at the single nanoparticle level. Herein, a highly sensitive single-nanoparticle coulometry method is successfully developed to study the electrochemical activity and oscillation of single PtTe nanocatalysts. Based on microbattery reactions involving the formic acid electro-oxidation and the deposition of Ag on the single PtTe nanocatalyst surface, this method enables the transition from the undetectable sub-fA electric signal of the formic acid electro-oxidation into strong localized surface plasmon resonance scattering signal of Ag detected by dark-field microscopy. The lowest limiting current for a single nanocatalyst is found to be as low as 25.8 aA. Different trends of activity versus the formic acid concentration and types of activity of the single nanocatalyst have been discovered. Unveiled frequency-amplitude graph shows that the two electrochemical oscillation modes of low frequency with high amplitude and vice versa coexist in a single PtTe nanocatalyst, indicating the abundantly smooth surfaces and defects of nanocatalysts. This conducted study will open up the new avenue for further behavioral and mechanistic investigation of more types of nanocatalysts in the electrochemistry community.

Utilizing waste corn-cob in biosynthesis of noble metallic nanoparticles for antibacterial effect and catalytic degradation of contaminants

In the present study, cost-effective, and environmentally friendly fabrication of silver and gold nanoparticles was performed by using aqueous extract of waste corn-cob. The formation of the metallic nanoparticles (MNPs) was optimized by UV-Vis method. The phytoconstituents were responsible for reduction of silver and gold ions to silver nanoparticles (CC-AgNPs) and gold nanoparticles (CC-AuNPs) which were demonstrated by Fourier-transform infrared (FTIR) spectroscopy while formation of AgCl was attributed to the presence of chloride ions in the aqueous extract. The crystalline nature of the AgNPs, AgCl, and AuNPs was confirmed using the X-ray diffraction (XRD) and selected area electron diffraction (SAED) patterns. Morphological studies showed that the synthesized CC-AgNPs existed in spherical shape with the size ranging from 2 to 28 nm possessing an average value of 11 nm while CC-AuNPs were present in the multiple shapes with size ranging from 5 to 50 nm possessing an average value of 35 nm. For studies on bioactive application, the CC-AgNPs exhibited a high antibacterial activity against three bacterial strains including Salmonella typhimurium, Bacillus cereus, and Staphylococcus aureus. In addition, the catalytic efficiency of MNPs was investigated for reduction of o-, m-, p-nitrophenols, and degradation of organic dyes including Eosin Y and Rhodamine 6G. The rate constants calculated from the kinetical data revealed that the biosynthesized nanoparticles are excellent catalysts in potential applications for treatment of wastewater. Graphical abstract .

Spatially resolved electrochemistry enabled by thin-film optical interference

Electrochemical reactions occurring on the local surface can be spatially resolved by successive interferometric imaging of the nanochannel membrane coated electrode.