Scanning Electrochemical Microscopy Meets Optical Microscopy: Probing the Local Paths of Charge Transfer Operando in Booster-Microparticles for Flow Batteries.

Understanding the oxidation/reduction dynamics of secondary microparticles formed from agglomerated nanoscale primary particles is crucial for advancing electrochemical energy storage technologies. In this study, the behavior of individual copper hexacyanoferrate (CuHCF) microparticles is explored at both global and local scales combining scanning electrochemical microscopy (SECM), for electrochemical interrogation of a single, but global-scale microparticle, and optical microscopy monitoring to obtain a higher resolution dynamic image of the local electrochemistry within the same particle. Chronoamperometric experiments unveil a multistep oxidation/reduction process with varying dynamics. On the one hand, the global SECM analysis enables quantifying the charge transfer as well as its dynamics at the single microparticle level during the oxidation/reduction cycles by a redox mediator in solution. These conditions allow mimicking the charge storage processes in these particles when they are used as solid boosters in redox flow batteries. On the other hand, optical imaging with sub-particle resolution allows the mapping of local conversion rates and state-of-charge within individual CuHCF particles. These maps reveal that regions of different material loadings exhibit varying charge storage capacities and conversion rates. The findings highlight the significance of porous nanostructures and provide valuable insights for designing more efficient energy storage materials.

Electrochemical Tip-Enhanced Raman Spectroscopy for the Elucidation of Complex Electrochemical Reactions.

Tip-enhanced Raman spectroscopy (TERS) is an emerging nanospectroscopy technique whose implementation in situ/operando, namely, in the liquid phase and under electrochemical polarization (EC-TERS), remains challenging. The investigation of electrochemical processes at the nanoscale, in real time and over wide potential windows can be of particular interest but tedious when using EC-STM-TERS. This approach was successfully applied to the investigation of a well-established but yet complex system (a thiolated nitrobenzene derivative 4-NBM) whose reduction mechanism involves various multistep reaction paths, most likely pH-dependent. In light of the EC-TERS analysis carried out under specific conditions limiting the full (6 e-/6 H+) electrochemical reduction of 4-NBM and its photocoupling, a bimolecular electrochemical reaction path, difficult to evidence from the electrochemical response only, is proposed.

Redox-mediated electrochemiluminescence enhancement for bead-based immunoassay.

Electrochemiluminescence (ECL) is a highly sensitive mode of detection utilised in commercialised bead-based immunoassays. Recently, the introduction of a freely diffusing water-soluble Ir(iii) complex was demonstrated to enhance the ECL emission of [Ru(bpy)3]2+ labels anchored to microbeads, but a comprehensive investigation of the proposed 'redox-mediated' mechanism was not carried out. In this work, we select three different water-soluble Ir(iii) complexes by virtue of their photophysical and electrochemical properties in comparison with those of the [Ru(bpy)3]2+ luminophore and the TPrA co-reactant. A systematic investigation of the influence of each Ir(iii) complex on the emission of the Ru(ii) labels on single beads by ECL microscopy revealed that the heterogeneous ECL can be finely tuned and either enhanced up to 107% or lowered by 75%. The variation of the [Ru(bpy)3]2+ ECL emission was correlated to the properties of each Ir(iii)-based mediator, which enabled us to decipher the mechanism of interaction and define guidelines for the future design of novel Ir(iii) complexes to further enhance the ECL emission of bead-based immunoassays. Ultimately, we showcase the potential of this technology for practical sample analysis in commercial instruments by assessing the enhancement of the collective ECL intensity from a bead-based system.

Deciphering the Interplay between Local and Global Dynamics of Anodic Metal Oxidation.

The stark difference between global and local metal oxidation dynamics underscores the need for methodologies capable of performing precise sub-µm-scale and wide-field measurements. In this study, we present reflective microscopy as a tool developed to address this challenge, illustrated by the example of chronoamperometric Fe oxidation in a NaCl solution. Analysis at a local scale of 10 s of µm has revealed three distinct periods of Fe oxidation: the initial covering of the metal interface with a surface film, followed by the electrochemical conversion of the formed surface film, and finally, the in-depth oxidation of Fe. In addition, thermodynamic calculations and the quantitative analysis of changes in optical signal (light intensity), correlated with variations in refractive indexes, suggest the initial formation of maghemite, followed by its subsequent conversion to magnetite. The reactivity maps for all three periods are heterogeneous, which can be attributed to the preferential oxidation of certain crystallographic grains. Notably, at the global scale of 100 s of µm, reactivity initiates at the electrode border and progresses toward its center, demonstrating a unique pattern that is independent of the local metal structure. This finding underscores the significance of simultaneously employing sub-µm-precise, quantitative, and wide-field measurements for a comprehensive description of metal oxidation processes.

Optics Determines the Electrochemiluminescence Signal of Bead-Based Immunoassays.

Electrochemiluminescence (ECL) is an optical readout technique that is successfully applied for the detection of biomarkers in body fluids using microbead-based immunoassays. This technology is of utmost importance for in vitro diagnostics and thus a very active research area but is mainly focused on the quest for new dyes and coreactants, whereas the investigation of the ECL optics is extremely scarce. Herein, we report the 3D imaging of the ECL signals recorded at single microbeads decorated with the ECL labels in the sandwich immunoassay format. We show that the optical effects due to the light propagation through the bead determine mainly the spatial distribution of the recorded ECL signals. Indeed, the optical simulations based on the discrete dipole approximation compute rigorously the electromagnetic scattering of the ECL emission by the microbead and allow for reconstructing the spatial map of ECL emission. Thus, it provides a global description of the ECL chemical reactivity and the associated optics. The outcomes of this 3D imaging approach complemented by the optical modeling provide insight into the ECL optics and the unique ECL chemical mechanism operating on bead-based immunoassays. Therefore, it opens new directions for mechanistic investigations, ultrasensitive ECL bioassays, and imaging.

Dynamic Mapping of Electrochemiluminescence Reactivity in Space: Application to Bead-Based Assays.

As an electrochemical technique offering an optical readout, electrochemiluminescence (ECL) evolved recently into a powerful microscopy technique with the visualization of a wide range of microscopic entities. However, the dynamic imaging of transient ECL events did not receive intensive attention due to the limited number of electrogenerated photons. Here, the reaction kinetics of the model ECL bioassay system was revealed by dynamic imaging of single [Ru(bpy)3]2+-functionalized beads in the presence of the efficient tripropylamine coreactant. The time profile behavior of ECL emission, the variations of the ECL layer thickness, and the position of maximum ECL intensity over time were investigated, which were not achieved by static imaging in previous studies. Moreover, the dynamics of the ECL emission were confronted with the simulation. The reported dynamic ECL imaging allows the investigation of the ECL kinetics and mechanisms operating in bioassays and cell microscopy.

Three-dimensional operando optical imaging of particle and electrolyte heterogeneities inside Li-ion batteries.

Understanding (de)lithiation heterogeneities in battery materials is key to ensure optimal electrochemical performance. However, this remains challenging due to the three-dimensional morphology of electrode particles, the involvement of both solid- and liquid-phase reactants and a range of relevant timescales (seconds to hours). Here we overcome this problem and demonstrate the use of confocal microscopy for the simultaneous three-dimensional operando measurement of lithium-ion dynamics in individual agglomerate particles, and the electrolyte in batteries. We examine two technologically important cathode materials: LixCoO2 and LixNi0.8Mn0.1Co0.1O2. The surface-to-core transport velocity of Li-phase fronts and volume changes are captured as a function of cycling rate. Additionally, we visualize heterogeneities in the bulk and at agglomerate surfaces during cycling, and image microscopic liquid electrolyte concentration gradients. We discover that surface-limited reactions and intra-agglomerate competing rates control (de)lithiation and structural heterogeneities in agglomerate-based electrodes. Importantly, the conditions under which optical imaging can be performed inside the complex environments of battery electrodes are outlined.

Metal Core-Shell Nanoparticle Supercrystals: From Photoactivation of Hydrogen Evolution to Photocorrosion.

Gas nanobubbles are directly linked to many important chemical reactions. While they can be detrimental to operational devices, they also reflect the local activity at the nanoscale. Here, supercrystals made of highly monodisperse Ag@Pt core-shell nanoparticles are first grown onto a solid support and fully characterized by electron microscopies and X-ray scattering. Supercrystals are then used as a plasmonic photocatalytic platform for triggering the hydrogen evolution reaction. The catalytic activity is measured operando at the single supercrystal level by high-resolution optical microscopy, which allows gas nanobubble nucleation to be probed at the early stage with high temporal resolution and the amount of gas molecules trapped inside them to be quantified. Finally, a correlative microscopy approach and high-resolution electron energy loss spectroscopy help to decipher the mechanisms at the origin of the local degradation of the supercrystals during catalysis, namely nanoscale erosion and corrosion.

Fuelling electrocatalysis at a single nanoparticle by ion flow in a nanoconfined electrolyte layer.

We explore the possibility of coupling the transport of ions and water in a nanochannel with the chemical transformation of a reactant at an individual catalytic nanoparticle (NP). Such configuration could be interesting for constructing artificial photosynthesis devices coupling the asymmetric production of ions at the catalytic NP, with the ion selectivity of the nanochannels acting as ion pumps. Herein we propose to observe how such ion pumping can be coupled to an electrochemical reaction operated at the level of an individual electrocatalytic Pt NP. This is achieved by confining a (reservoir) droplet of electrolyte to within a few micrometres away from an electrocatalytic Pt NP on an electrode. While the region of the electrode confined by the reservoir and the NP are cathodically polarised, operando optical microscopy reveals the growth of an electrolyte nanodroplet on top of the NP. This suggests that the electrocatalysis of the oxygen reduction reaction operates at the NP and that an electrolyte nanochannel is formed - acting as an ion pump - between the reservoir and the NP. We have described here the optically imaged phenomena and their relevance to the characterization of the electrolyte nanochannel linking the NPs to the electrolyte microreservoir. Additionally, we have addressed the capacity of the nanochannel to transport ions and solvent flow to the NP.

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

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

Unsupervised Analysis of Optical Imaging Data for the Discovery of Reactivity Patterns in Metal Alloy.

Operando wide-field optical microscopy imaging yields a wealth of information about the reactivity of metal interfaces, yet the data are often unstructured and challenging to process. In this study, the power of unsupervised machine learning (ML) algorithms is harnessed to analyze chemical reactivity images obtained dynamically by reflectivity microscopy in combination with ex situ scanning electron microscopy to identify and cluster the chemical reactivity of particles in Al alloy. The ML analysis uncovers three distinct clusters of reactivity from unlabeled datasets. A detailed examination of representative reactivity patterns confirms the chemical communication of generated OH- fluxes within particles, as supported by statistical analysis of size distribution and finite element modelling (FEM). The ML procedures also reveal statistically significant patterns of reactivity under dynamic conditions, such as pH acidification. The results align well with a numerical model of chemical communication, underscoring the synergy between data-driven ML and physics-driven FEM approaches.

Electrochromic Corundum-like Compound Based on the Reversible (De)insertion of Lithium: Li2Ni2W2O9.

The search for efficient anodic electrochromic materials is essential to the development of electrochromic devices, such as smart windows. Magnetron-sputtered lithium-nickel-tungsten mixed oxides are good candidates to tackle this issue; however, they display a complicated microstructure, making it difficult to pinpoint the origin of their electro-optical properties. Herein, by exploring the Li2O-NiO-WO3 phase diagram, we obtained a new phase, Li2Ni2W2O9, that crystallizes in the orthorhombic Pbcn space group. This material can reversibly uptake/release 0.75 Li+ (31 mA h·g-1) when cycled between 2.5 and 5.0 V versus Li+/Li. Moreover, through operando optical microscopy, we show that this new phase is electrochromic, and crucial information can be accessed about the diffusion-limited insertion of lithium at the single-particle scale. This study sets the ground for future syntheses of electrochemically active materials crystallizing in the ramsayite structure and details how the electrochromic properties of battery materials can be used to shed some light on their electrochemical mechanisms.

Imaging the Footprint of Nanoscale Electrochemical Reactions for Assessing Synergistic Hydrogen Evolution.

This work proposes a novel method for measuring the intrinsic activity of single metal-based nanoparticles towards water reduction in neutral media at industrially relevant current densities. Instead of using gas nanobubbles as proxy, the method uses optical microscopy to track the local footprint of the reaction through the precipitation of metal hydroxide, which is associated to the local pH increase during electrocatalysis. The results show the electrocatalytic activities of different types of metal nanoparticles and bifunctionnal core-shell nanostructures made of Ni and Pt, and demonstrate the importance of metal hydroxide nano-shells in enhancing electrocatalysis. This method should be generalizable to any electrocatalytic reaction involving pH changes such as nitrate or CO2 reduction.

Bimodal Electrochemiluminescence Microscopy of Single Cells.

Electrochemiluminescence (ECL) microscopy is an emerging technique with new applications such as imaging of single entities and cells. Herein, we have developed a bimodal and bicolor approach to record both positive ECL (PECL: light-emitting object on dark background) and shadow label-free ECL (SECL: nonemissive object shadowing the background luminescence) images of single cells. This bimodal approach is the result of the simultaneous emissions of [Ru(bpy)3]2+ used to label the cellular membrane (PECL) and [Ir(sppy)3]3- dissolved in solution (SECL). By spectrally resolving the ECL emission wavelengths, we recorded the images of the same cells in both PECL and SECL modes using the [Ru(bpy)3]2+ (λmax = 620 nm) and [Ir(sppy)3]3- (λmax = 515 nm) luminescence, respectively. PECL shows the distribution of the [Ru(bpy)3]2+ labels attached to the cellular membrane, whereas SECL reflects the local diffusional hindrance of the ECL reagents by each cell. The high sensitivity and surface-confined features of the reported approach are demonstrated by imaging cell-cell contacts during the mitosis process. Furthermore, the comparison of PECL and SECL images demonstrates the differential diffusion of tri-n-propylamine and [Ir(sppy)3]3- through the permeabilized cell membranes. Consequently, this dual approach enables the imaging of the morphology of the cell adhering on the surface and can significantly contribute to multimodal ECL imaging and bioassays with different luminescent systems.

Electrochemiluminescence Amplification in Bead-Based Assays Induced by a Freely Diffusing Iridium(III) Complex.

Heterogeneous electrochemiluminescence (ECL) assays employing tri-n-propylamine as a co-reactant and a tris(2,2'-bipyridine)ruthenium(II) ([Ru(bpy)3]2+) derivative as an emissive label are integral to the majority of academic and commercial applications of ECL sensing. This model system is an active research area and constitutes the basis of successfully commercialized bead-based ECL immunoassays. Herein, we propose a novel approach to the enhancement of such conventional ECL assays via the incorporation of a second metal coordination complex, [Ir(sppy)3]3- (where sppy = 5'-sulfo-2-phenylpyridinato-C2,N), to the experimental system. By employing ECL microscopy, we are able to map the spatial distribution of ECL emission at the surface of the bead, from [Ru(bpy)3]2+ labels, and solution-phase emission, from [Ir(sppy)3]3-. The developed [Ir(sppy)3]3--mediated enhancement approach elicited a significant improvement (70.9-fold at 0.9 V and 2.9-fold at 1.2 V vs Ag/AgCl) of the ECL signal from [Ru(bpy)3]2+ labels immobilized on the surface of a polystyrene bead. This dramatic enhancement in ECL signal, particularly at low oxidation potentials, has important implications for the improvement of existing heterogeneous ECL assays and ECL-based microscopy, by amplifying the signal, opening new bioanalytical detection schemes, and reducing both electrode surface passivation and deleterious side reactions.