Quantum sized, thiolate-protected gold nanoclusters.

The scientific study of gold nanoparticles (typically 1-100 nm) has spanned more than 150 years since Faraday's time and will apparently last longer. This review will focus on a special type of ultrasmall (<2 nm) yet robust gold nanoparticles that are protected by thiolates, so-called gold thiolate nanoclusters, denoted as Au(n)(SR)(m) (where, n and m represent the number of gold atoms and thiolate ligands, respectively). Despite the past fifteen years' intense work on Au(n)(SR)(m) nanoclusters, there is still a tremendous amount of science that is not yet understood, which is mainly hampered by the unavailability of atomically precise Au(n)(SR)(m) clusters and by their unknown structures. Nonetheless, recent research advances have opened an avenue to achieving the precise control of Au(n)(SR)(m) nanoclusters at the ultimate atomic level. The successful structural determination of Au(102)(SPhCOOH)(44) and [Au(25)(SCH(2)CH(2)Ph)(18)](q) (q = -1, 0) by X-ray crystallography has shed some light on the unique atomic packing structure adopted in these gold thiolate nanoclusters, and has also permitted a precise correlation of their structure with properties, including electronic, optical and magnetic properties. Some exciting research is anticipated to take place in the next few years and may stimulate a long-lasting and wider scientific and technological interest in this special type of Au nanoparticles.