Probing the catalytic activity and heterogeneity of Au-nanoparticles at the single-molecule level.

Nanoparticles can catalyze many important chemical transformations in organic synthesis, pollutant removal, and energy production. Characterizing their catalytic properties is essential for understanding the fundamental principles governing their activities, but is challenging in ensemble measurements due to their intrinsic heterogeneity from their structural dispersions, heterogeneous surface sites, and surface restructuring dynamics. To remove ensemble averaging, we recently developed a single-particle approach to study the redox catalysis of individual Au-nanoparticles in solution. By detecting the fluorescence of the catalytic product at the single-molecule level, we followed the catalytic turnovers of single Au-nanoparticles in real time at single-turnover resolution. Here we extend our single-nanoparticle studies to examine in detail the activity and heterogeneity of 6 nm spherical Au-nanoparticles. By analyzing the statistical properties of single-particle reaction waiting times across a range of substrate concentrations, we directly determine the distributions of kinetic parameters of individual Au-nanoparticles, including the rate constants and the equilibrium constants of substrate adsorption, and quantify their heterogeneity. Large activity heterogeneity is observed among the Au-nanoparticles in both the catalytic conversion reaction and the product dissociation reaction, which are typically hidden in ensemble-averaged measurements. Analyzing the temporal fluctuation of catalytic activity of individual Au-nanoparticles further reveals that these nanoparticles have two types of surface sites with different catalytic properties-one type-a with lower activity but higher substrate binding affinity, and the other type-b with higher activity but lower substrate binding affinity. Each Au-nanoparticle exhibits type-a behavior at low substrate concentrations and switches to type-b behavior at a higher substrate concentration, and the switching concentration varies largely from one nanoparticle to another. The heterogeneous and dynamic behavior of Au-nanoparticle catalysts highlight the intricate interplay between catalysis, structural dispersion, variable surface sites, and surface restructuring dynamics in nanocatalysis.