Noble metal nanoparticles are an important class of catalyst, however they are inherently heterogeneous. This makes studying them at the single-particle level desirable, as it then becomes possible to quantify these differences. A high-throughput fluorescence microscopy technique was developed based on catalytic conversion of non-fluorescent reactants to a fluorescent product on a gold catalyst surface. A single-molecule, sub-diffraction resolution imaging method was used to spatially resolve the product locations to ~25 nm accuracy. By overlaying the optical imaging results onto an SEM image of the same nanoparticles, correlation of structure and activity was achieved for individual nanoparticles. Since this technique can monitor the activity of many particles simultaneously, it was used to resolve sub-populations of both size and activity within a heterogeneous sample. This can be used to aid in future catalyst design.
Also, by studying particles larger than the spatial resolution of this technique, it becomes possible to resolve intraparticle activity distributions. By studying ~500 nm long gold nanorods, it was found that there is higher activity in the center which decays linearly along the length of the rods to the two ends. This activity gradient was attributed to an underlying surface defect density gradient originating from the initial growth kinetics of the nanorod. The surface defects were proposed to be the catalytically most active sites on the nanorods.