Kyu‐Sung Han

This tutorial review covers recent developments in using single-molecule fluorescence microscopy to study nanoscale catalysis. The single-molecule approach enables following catalytic and electrocatalytic reactions on nanocatalysts, including metal nanoparticles and carbon nanotubes, at single-reaction temporal resolution and nanometer spatial precision. Real-time, in situ, multiplexed measurements are readily achievable under ambient solution conditions. These studies provide unprecedented insights into catalytic mechanism, reactivity, selectivity, and dynamics in spite of the inhomogeneity and temporal variations of catalyst structures. Prospects, generality, and limitations of the single-molecule fluorescence approach for studying nanocatalysis are also discussed.

This review discusses the latest advances in using single-molecule microscopy of fluorogenic reactions to examine and understand the spatiotemporal catalytic behaviors of single metal nanoparticles of various shapes including pseudospheres, nanorods, and nanoplates. Real-time single-turnover kinetics reveal size-, catalysis-, and metal-dependent temporal activity fluctuations of single pseudospherical nanoparticles (<20 nm in diameter). These temporal catalytic dynamics can be related to nanoparticles' dynamic surface restructuring whose timescales and energetics can be quantified. Single-molecule super-resolution catalysis imaging further enables the direct quantification of catalytic activities at different surface sites (i.e., ends vs. sides, or corner, edge vs. facet regions) on single pseudo 1-D and 2-D nanocrystals, and uncovers linear and radial activity gradients within the same surface facets. These spatial activity patterns within single nanocrystals can be attributed to the inhomogeneous distributions of low-coordination surface sites, including corner, edge, and defect sites, among which the distribution of defect sites is correlated with the nanocrystals' morphology and growth mechanisms. A brief discussion is given on the extension of the single-molecule imaging approach to catalysis that does not involve fluorescent molecules.

In chain-growth polymerization, a chain grows continually to reach thousands of subunits. However, the real-time dynamics of chain growth remains unknown. Using magnetic tweezers, we visualized real-time polymer growth at the single-polymer level. Focusing on ring-opening metathesis polymerization, we found that the extension of a growing polymer under a pulling force does not increase continuously but exhibits wait-and-jump steps. These steps are attributable to the formation and unraveling of conformational entanglements from newly incorporated monomers, whose key features can be recapitulated with molecular dynamics simulations. The configurations of these entanglements appear to play a key role in determining the polymerization rates and the dispersion among individual polymers.