The relevance of shape and size of Au55 clusters.

This critical review deals with the history of Au55(PPh3)12Cl6 and its derivatives from the very beginning in 1981 to date. Au55 clusters obtain their special interest from their ultimate size and their ideal cuboctahedral structure. They are part of the family of so-called full-shell clusters, particles with perfectly completed geometries, also represented by icosahedral Au13 clusters. Bare as well as ligand protected Au55 clusters not only exhibit special chemical and physical stability, but draw their attention particularly from their unique electronic properties. Single electron switching at room temperature becomes possible, giving rise for development of applications in future nanoelectronic devices. A predominantly size-determined property of the 1.4 nm particles becomes obvious with respect of biological response. Au55 clusters indicate an unusual cytotoxicity which seems to be caused by the unusually strong interaction between the 1.4 nm particles and the major grooves of DNA. Only marginally smaller or larger particles show drastically reduced toxicity, whereas significantly larger gold nanoparticles are completely non-toxic. Both, the electronic perspectives as well as the relevance in toxicology are at very early stages of development (75 references).