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Electrochemistry of Single Nanodomains Revealed by Three-Dimensional Holographic Microscopy.

Interest in nanoparticles has vigorously increased over the last 20 years as more and more studies show how their use can potentially revolutionize science and technology. Their applications span many different academically and industrially relevant fields such as catalysis, materials science, health, etc. Until the past decade, however, nanoparticle studies mostly relied on ensemble studies, thus leaving aside their chemical heterogeneity at the single particle level. Over the past few years, powerful new tools appeared to probe nanoparticles individually and in situ. This Account describes how we drew inspiration from the emerging fields of nanoelectrochemistry and plasmonics-based high resolution holographic microscopy to develop a coupled approach capable of analyzing in operando (electro)chemical reaction over one single nanoparticle. A brief overview of selected optical strategies to image NPs in situ with emphasis on scattering based methods is presented. In an electrochemical context, it is necessary to track particle behavior both in solution and near a polarized electrode, which is why 3D optical observation is particularly appealing. These approaches are discussed together with strategies to track NPs beyond the diffraction limit, allowing a much finer description of their trajectories. Then, the holographic setup is used to study electrochemically triggered Ag NP oxidation reaction in the presence of different electrolytes. Holography is shown to be a powerful technique to track and analyze the trajectory of individual NPs in situ, which further sheds light on in operando behaviors such as electrogenerated NP transport, aggregation, or adsorption. We then show that spectroscopy and scattering-based optical methods are reliable and sensitive to the point of being used to investigate and quantify NP (electro)chemical reactions in model cases. However, since real chemical reactions usually take place in an inherently complex environment, approaches based exclusively on optical imaging only reach their limitations. The strategy is then taken one step further by merging together electrochemical nanoimpact experiments with 3D optical monitoring. Previous strategies are validated by showing that in simple cases, these two independent ways of probing NP size and reactivity yield the same results. For more complicated reactions (e.g., multistep reactions), one must go beyond either technique by showing that the two approaches are perfectly complementary and that the two signals contain information of different natures, thus providing a much better characterization of the reaction. This point is illustrated by studying Ag NP oxidation (single or agglomerates) in the presence of a precipitating agent, where the actual oxidation is uncoupled from the dissolution of the particle, thus proving the point of our symbiotic approach.