Despite the frequent use of silver nanoparticles in consumer products and medical treatments, their reactivity and degradation in aqueous suspensions are still under debate. Here we elucidate this reactivity by an in situ opto- and spectro-electrochemical approach. Using dark-field microscopy coupled to a spectrophotometer and to an electrochemical cell, redox reactions of individual silver nanoparticles are studied in the presence of chloride. The intensity and spectral position of the plasmon resonance of an individual particle are tracked simultaneously in real time during cyclic voltammetry. They both change almost instantaneously with the detected current in a chemically reversible way. Thus, it is evidenced that the intensity decrease of the optical signal at the silver peak position is caused by the reversible formation of silver chloride and not by dissolution of silver. Moreover, at large positive potentials, further transformation to silver oxide or chlorite is revealed spectroscopically, although the electrochemical current is hidden by water and chloride oxidation. Thus, the combination of electrochemistry with dark-field microscopy and hyperspectral imaging is introduced as a new tool for real-time analysis of (electro-)chemical reactions of nanoparticles on a single-entity level.

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Since nanoparticles are frequently used in commercial applications, there is a huge demand to obtain deeper insights into processes at the nanoscale. Especially, catalysis, chemical and electrochemical reaction dynamics are still poorly understood. Thus, simultaneous and coupled opto-and spectro-electrochemical dark-field microscopy is used to study in situ and operando the electrochemically driven dissolution mechanism of single silver nanoparticles in the presence of nitrate ions as non-complexing counter-ions, herein. Hyperspectral imaging is used to probe the intrinsic localized surface plasmon resonance of individual silver nanospheres before, during and after their electrochemical oxidation on a transparent indium tin oxide (ITO) electrode. Furthermore, optical video imaging was performed for additional information. Based on the complete loss of spectral information and intensity, a dissolution of the particles during the reaction was concluded. This way it is revealed that the dissolution of individual particles proceeds over several seconds, indicating a hindrance by the nitrate ions. Only electrochemical analysis does not provide this insight as the measured current does not allow distinguishing between successive fast dissolution of one particle after another or slow dissolution of several particles in a concerted manner. For comparison, experiments were performed in the presence of chloride ions. It was observed that the silver chloride formation is an instantaneous process. Thus, it is possible to study and define the reaction dynamics on the single nanoparticle level in various electrochemical systems and electrolyte solutions. Accordingly, operando opto-and spectro-electrochemical studies allow us to conclude, that the oxidation of silver to solvated silver cations is a kinetically slow process, while the oxidation to silver chloride is fast. We propose this approach as a new method to study electrocatalyst materials, their transformation and degradation under operando conditions.

The effect of surface orientations on the formation of iridium oxide species during the oxygen evolution reaction (OER) remains yet unknown. Herein, we use a needle‐shaped iridium atom probe specimen as a nanosized working electrode to ascertain the role of the surface orientations in the formation of oxide species during OER. At the beginning of electrolysis, the top 2–3 nm of (024), (026), (113), and (115) planes are covered by IrO−OH, which activates all surfaces towards OER. A thick subsurface oxide layer consisting of sub‐stoichiometric Ir−O species is formed on the open (024) planes as OER proceeds. Such metastable Ir−O species are thought to provide an additional contribution to the OER activity. Overall, this study sheds light on the importance of the morphological effects of iridium electrocatalysts for OER. It also provides an innovative approach that can directly reveal surface species on electrocatalysts at atomic scale.

Redox magnetohydrodynamics (RMHD) microfluidics is coupled with dark-field microscopy (DFM) to offer high-throughput single-nanoparticle (NP) differentiation in situ and operando in a flowing mixture by localized surface plasmon resonance (LSPR) and tracking of NPs. The color of the scattered light allows visualization of the NPs below the diffraction limit. Their Brownian motion in 1-D superimposed on and perpendicular to the RMHD trajectory yields their diffusion coefficients. LSPR and diffusion coefficients provide two orthogonal modalities for characterization where each depends on a particle's material composition, shape, size, and interactions with the surrounding medium. RMHD coupled with DFM was demonstrated on a mixture of 82 ± 9 nm silver and 140 ± 10 nm gold-coated silica nanospheres. The two populations of NPs in the mixture were identified by blue/green and orange/red LSPR and their scattering intensity, respectively, and their sizes were further evaluated based on their diffusion coefficients. RMHD microfluidics facilitates high-throughput analysis by moving the sample solution across the wide field of view absent of physical vibrations within the experimental cell. The well-controlled pumping allows for a continuous, reversible, and uniform flow for precise and simultaneous NP tracking of the Brownian motion. Additionally, the amounts of nanomaterials required for the analysis are minimized due to the elimination of an inlet and outlet. Several hundred individual NPs were differentiated from each other in the mixture flowing in forward and reverse directions. The ability to immediately reverse the flow direction also facilitates re-analysis of the NPs, enabling more precise sizing.