Historical Introduction to Gold Colloids, Clusters and Nanoparticles

Colloidal gold is a suspension of sub-micrometre-sized particles of gold in a fluid either water or an organic solvent. Although the gold colloids cannot be viewed using optical microscopy, the sol has an intense colour (red for particles less than 100 nm or blue/purple for larger particles). The unique optical, electronic and molecular recognition properties of gold colloids have attracted substantial interest in recent years. The properties and applications of colloidal gold particles strongly depend upon their size and shape. For example, rod-like particles have both transverse and longitudinal absorption peaks, and the anisotropy of their shapes influences their self-assembly. Gold colloids and nanoparticles have found applications in electron microscopy, electronics, nanotechnology, materials science and medicine. The development of straightforward syntheses of gold colloids in organic solvents has had a major impact on the field and the development of etching and focusing techniques has led to the isolation of some monodispersed crystalline samples which have been characterised at the atomic level. Simultaneously the isolation of molecular cluster compounds of gold, initially stabilised by phosphine and more recently organothiolato ligands, has resulted in the characterisation at the atomic level of metal particles with 3–100s of atoms. These developments have provided interesting insights into the relationships between colloids and clusters. As the diameters of these species approach the nanoscale, interesting chemical, physical and catalytic properties have emerged.