Watching A Single Nanoparticle Catalyst in Action Metal nanoparticles are important catalysts for processing petroleum for the chemical industry, oxidizing carbon monoxide for pollutant removal, and converting chemical energy to electricity in fuel cells. However, current preparation methods generate populations of nanoparticle catalysts with a dispersion of structures and surface active sites, which makes understanding the catalytic properties of nanoparticles challenging when looking at the collected behaviors of many nanoparticles. To address this challenge, Cornell researchers have devised a way to watch the actions of a single nanoparticle in catalysis in real time at single-reaction resolution. They designed a catalytic probe reaction that generates fluorescent product molecules, and by detecting the fluorescence of the product one molecule at a time in real time, they monitored how a single 6-nm gold nanoparticle converts a reactant to a product and how a product leaves the nanoparticle surface. They discovered diverse catalytic “personalities” among individual gold nanoparticles, which differ from one particle to another and for a single particle, vary from time to time. These nanoparticle- and time-dependent behaviors inform the intricate interplay between catalysis, heterogeneous reactivity, particle surface reconstruction in nanocatalysis, a technological area crucial for tackling the current global energy challenge. Schematic of single-molecule fluorescence detection of individual catalytic reactions of resazurin conversion to resorufin by single gold nanoparticle catalysis. Background trajectory: real-time single-turnover trajectory of a single gold nanoparticle. Graphics by A. Kalininskiy.