Aqueous, unprotected gold nanoparticles were prepared from HAuCl4 using a water-soluble benzoin (Irgacure-2959) as a photochemical source of strongly reducing ketyl radicals. This rapid method provides spatiotemporal control of nanoparticle generation, while light intensity can be used to control particle size. The particles are stable for months and do not require any of the conventional (S, N, or P) stabilizing ligands, although these can be readily incorporated if required.

Colloidal or heterogeneous nanocatalysts can improve the range and diversity of Cu(I)-catalysed click reactions and facilitate catalyst separation and reuse. Catalysis by metal nanoparticles raises the question as to whether heterogeneous catalysts may cause homogeneous catalysis through metal ion leaching, since the catalytic process could be mediated by the particle, or by metal ions released from it. The question is critical as unwanted homogeneous processes could offset the benefits of heterogeneous catalysis. Here, we combine standard bench scale techniques with single-molecule spectroscopy to monitor single catalytic events in real time and demonstrate that click catalysis occurs directly at the surface of copper nanoparticles; this general approach could be implemented in other systems. We use 'from the mole to the molecule' to describe this emerging idea in which mole scale reactions can be optimized through an intimate understanding of the catalytic process at the single-molecule-single catalytic nanoparticle level.

The ketone-photoinduced formation of Au, Ag, and Cu nanoparticles from their corresponding ions in solution has been carried out using benzoin photoinitiators. Ketones are good photosensitizers for nanoparticle synthesis not because of the energy they can absorb or deliver, but rather because of the reducing free radicals they can generate. Efficient photochemical nanoparticle generation thus requires a careful selection of substrates and experimental conditions such that free radical generation occurs with high quantum efficiency, where metal ion precursors do not inhibit radical formation. A key consideration to achieve nanoparticle synthesis with short exposure times is to minimize excited-state quenching by metal ions. Applications of nanostructures in catalysis require control of the nanoparticle characteristics, such as dimension, morphology, and surface properties. Part of this article describes the strategies to modify photochemically prepared particles. Finally, we illustrate some of the nanoparticle applications that interest us, with some emphasis on plasmon-mediated processes.

Single molecule fluorescence microscopy reveals that copper-in-charcoal--a high performance click catalyst- has remarkably few catalytic sites, with 90% of the charcoal particles being inactive, and for the catalytic ones the active sites represent a minute fraction (∼0.003%) of the surface. The intermittent nature of the catalytic events enables subdiffraction resolution and mapping of the catalytic sites.

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