Rongchao Jin of Carnegie Mellon University in Pittsburgh, Pennsylvania, USA, and colleagues have obtained an X-ray structure of the gold nanoparticle, Au133, which comprises 133 gold atoms and 52 surface-protecting molecules. It is the biggest nanoparticle structure resolved with X-ray crystallography. "With X-ray crystallography, we were able to see very beautiful patterns, which was a very exciting discovery. These patterns only show up when the nanoparticle size becomes big enough," Jin explains. The experiments revealed that during formation the nanoparticles self-assemble into three layers within each particle: the gold core, the surface molecules that protect it and the interface between the two. The X-ray data reveal the core to be icosahedral and the interface between the core and the surface-protecting molecules to be composed of a layer of sulfur atoms bonded to the gold atoms stacked into helical ladder-like structures. The outer layer of molecules have carbon tails that self-assemble into fourfold swirls.