Discrete, monodispersed thiolated gold clusters. Synthesis, structure and application to electron microscopy

An important question in the field of Monolayer Protected Clusters (MPCs) is whether MPCs inherently fluctuate in size through collisions in solution or whether they can be synthesized and isolated as stable discrete entities. I will show that MPCs of the most common type (a gold core surrounded by organic thiols) can behave as discrete stable entities and that they can be synthesized as such. Several discrete products can be obtained by varying the synthetic parameters. Some of the clusters are stable for at least 6 months. We have studied the structural and optical properties of the clusters and present an atomic model from X-ray diffraction at 1.15 A resolution for one of them. The model shows 102 gold atoms and 44 para-mercaptobenzoic acid ligands. The structure supports an electronic closed shell model and suggests ways in which a geometrically closed shell model can be improved. Based on the structure, some simple rules can be derived for the prediction of related structures.;Furthermore, the availability of discrete, well-defined clusters opens the way to applications in nano-electronics, environmental sensing, memory devices, catalysis, drug delivery and imaging. I have explored the use of MPCs as markers for 3-D cryo-electron microscopy reconstruction, referred to as heavy atom electron microscopy (HEAVEM). In this technique, MPCs are covalently attached to single chain antibodies, which in turn bind to proteins of interest. Specific and rigid binding of MPCs to proteins permits better and faster alignment for reconstruction. It should allow sub-nanometer resolution for asymmetrical objects.