Nanocatalytic Mechanisms Investigated by Single Molecule Fluorescence Imaging at the Single-Particle Level

With the ever-increasing demands for energy and declining reserves of fossil fuels, efficient use of fossil fuels and energy extraction from alternative raw materials are critical to the sustainable future of humanity.Catalysis is one of the key technologies capable of helping address this energy challenge.Nanocatalysts are an integral part of catalysis technology, and they can catalyze chemical transformation for petroleum processing and energy conversions, which are more efficient than their bulk materials.With the rapid development of nanoscience, new nanocatalysts and novel catalytic properties continue to emerge.Tremendous work has been done in characterizing the catalytic properties of nanoparticles at the ensemble level.However, due to heterogeneous properties of nanoparticles in size, morphology or surface composition, traditional ensemble methods can only provide averaged behavior of numerous nanoparticles.Therefore, it is difficult to determine the reliable structure-activity relationship for individual nanoparticles.In addition, owing to the surface structural reconstruction, the active sites of nanocatalysts are variable under catalysis.Revealing the dynamic evolution of active sites is helpful to understand the process and mechanism of catalytic reaction.Consequently, developing a direct approach to study the nanocatalysis at the single-particle level and in real-time with sufficient spatiotemporal resolution is highly demanding.Recently, due to the single-molecule sensitivity and high spatiotemporal resolution, single-molecule fluorescence imaging (SMFI) has been proved to be an effective tool for studying the heterogeneous nanocatalysts at the single-particle level.By adopting fluorogenic reactions and monitoring the fluorescence signal of a product, the reaction process on a single nanoparticle can be followed in real-time at single-turnover resolution under steady-state reaction kinetics.In this review, we discuss recent processes of SMFI in investigating nanocatalysis at the single-particle level.The discussion focuses on the development and application of SMFI in probing catalytic mechanisms, including the size effect, facet effect, surface defects, plasmonic effect, activation energy, bimetallic effect, nanoconfined effect and catalytic communication.Finally, challenges and prospects of the SMFI for investigating nanocatalysis are put forward.