Credit: © 2007 AAAS

Difficulties in preparing a homogeneous sample of gold nanoparticles mean that, until now, there has been little information on their structural properties. However, researchers from Stanford and Colorado Universities in the US have managed to isolate and crystallize a gold–thiolate cluster, which has allowed them to determine the X-ray structure, yielding unexpected results.

By systematically varying the reaction conditions, Roger Kornberg and colleagues1 synthesized gold nanoparticles — each coated with a layer of sulphur-containing organic molecules (p-mercaptobenzoic acid, p-MBA) — of sufficiently uniform size to grow large single crystals. X-ray diffraction studies revealed that the clusters contained 102 gold atoms surrounded by a rigid surface layer comprising 44 p-MBA molecules. The majority of the gold atoms in each particle assemble into a truncated decahedron, as expected, but 13 of them form an equatorial band around the rest. This band has no apparent symmetry and therefore imparts chirality to the core, possibly owing to the interactions with the p-MBA molecules.

This arrangement of a core of atoms with polar caps and an equatorial band doesn't appear to be in keeping with the idea of discrete clusters being formed by geometrical shell closing. The chemical and physical properties of these particular nanoparticles will need to be explored to explain the basis of the gold packing and gold–sulphur interactions observed.