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Nanoparticles can catalyze many chemical transformations. Understanding their activity is important, but hampered by their structural heterogeneity. Our group has developed a single-molecule fluorescence approach to study nanoparticle catalysis at the single-particle and single-turnover resolution. We used single-molecule microscopy of fluorogenic reactions, a strategy pioneered in single-enzyme studies, and recently applied as well to micro- and nano-scale catalysts. In this approach, we detect the fluorescence of a catalytic product at the single-molecule level to monitor catalysis by individual Au-nanoparticles at single-turnover resolution under ambient solution conditions. We were able to quantify the inhomogeneous reactivity among individual Au-nanoparticles, reveal their differential selectivity between parallel reaction pathways, study their temporal catalytic dynamics that are coupled to their dynamic surface restructuring, and reveal their dynamic surface switching behaviors. Below I summarize some of our discoveries.

1) We found that for the catalytic product formation reaction, all Au-nanoparticles follow the Langmuir-Hinshelwood mechanism in heterogeneous catalysis -- the nanoparticle catalyzes the conversion of the substrate resazurin to the product resorufin while maintaining a fast substrate adsorption equilibrium. Although all following the same Langmuir-Hinshelwood mechanism, individual Au-nanoparticles have vastly different catalytic reactivity. This is manifested in the broad distribution of the catalytic rate constant of individual Au-nanoparticles. This quantification of heterogeneity in catalytic reactivity is unavailable from ensemble-averaged measurements.

2) For the dissociation of the product resorufin, the reaction kinetics includes two parallel reaction pathways: one a substrate-assisted product dissociation pathway, in which the nanoparticle binds a substrate first before the product leaves the particle surface, and the other a direct dissociation pathway. Strikingly, individual Au-nanoparticles have differential selectivity between the two parallel product dissociation pathways. Some particles prefer the substrate-assisted pathway, some prefer the direct pathway, and some take the two pathways equally. This differential selectivity between parallel pathways is hidden in ensemble-averaged measurements. We further found that tuning the average particle size also changes the selectivity of the nanoparticles between the two parallel reaction pathways.

3) Regarding temporal behaviors, we found that every Au-nanoparticle shows activity fluctuations. One manifestation of this activity fluctuation is the temporal variations of the rate of turnovers for a single particle. This temporal activity fluctuation is attributable to both catalysis-induced and spontaneous dynamic surface restructuring of each nanoparticle. Owing to their nanometer dimensions, nanoparticle surfaces are unstable and can reconstruct dynamically, especially under catalysis, where the constantly changing adsorbate-surface interactions can induce dynamic surface reconstruction.

The catalysis-induced nature of the activity fluctuations and the underlying surface restructuring are directly supported by the positive correlation between the activity fluctuation rates, i.e., the inverse of the correlation times, and the rate of turnovers ¾ for all Au-nanoparticles, the activity fluctuation rates increase with increasing rates of turnovers. We can further extrapolate the fluctuation rates linearly to zero rate of turnovers. The positive intercepts approximate the rates of spontaneous (as compared with catalysis-induced) surface restructuring dynamics for a Au-nanoparticle in an aqueous environment, corresponding to a timescale of 40-150 s. Consistently, the extrapolated spontaneous surface restructuring becomes slower when the particle size increases, as larger-sized particles have more stable surfaces. The determination of a timescale for nanoparticle surface restructuring dynamics in the absence of catalysis is exciting, because it is directly related to the energetics of nanoparticle surface atoms and because they are in general challenging to quantify, owing to the nanometer dimension and the heterogeneous surface structure of nanoparticles.

4) For the Au-nanoparticles, our single-particle single-turnover measurements also revealed their [S]-dependent surface catalytic behaviours, showing abrupt switching between a low and a high catalytic reactivity state at a certain substrate concentration. We do not yet know the molecular detail of the dynamic switching of Au-nanoparticle surface behaviors. But this behavior has strong implications in experimental studies of nanoparticle catalysts, or heterogeneous catalysts in general. It makes imperative to study heterogeneous catalysis at conditions relevant to real applications. Ultrahigh vacuum studies, for which many powerful spectroscopic techniques are available to provide rich information on catalytic mechanisms, should be complemented with high pressure, high concentration studies (e.g., in solution), to gain a full understanding of the catalytic properties, and for nanoparticles, single-particle resolution is necessary.