Gold nanoparticles are the most stable and widely used metal nanoparticles. Extraction of gold metal was started during 5th Millennium B.C. Soluble gold appeared during Ca. 400 B.C. Colloidal gold is still used for colouring and decoration purposes. The Lycurgus Cup (4th Century B.C) is a best example which is ruby red in transmitted light and green in reflected light due to the presence of gold colloids. Gold colloids were used for aesthetic (decoration) and curative purposes. During 1857 Faraday reported on the formation of deep-red solutions of colloidal Au by reduction of an aqueous solution of AuCl4 by phosphorus in CS2 (two phase system). Thin films prepared from dried colloidal solutions have shown reversible colour changes upon mechanical compression.
Preparation
All chemical methods for the preparation of gold nanoparticles are based on reduction of Au(III) derivatives, but in general it is difficult to control the size of the nanoparticles, and in the absence of stabilizing agents the particles tend to aggregate. Alkylammonium salts can be used to stabilize nanoparticles, as they form a sort of electrical double layer on the surface. Such an interaction is weak and the gold surface remains reactive. For this reason, these nanoparticles are not very stable and cannot be separated from the solution from which they have been prepared. The synthesis is usually performed in a two phase (water/organic solvent) system. Alkylammonium salts like TOAB (tetraoctylammonium bromide), besides passivating the gold surface, perform the task of phase transfer agents towards the Au(III) species to the organic phase. Au nanoparticles, as well as other metal and metal oxide nanoparticles can be prepared inside a suitable block copolymer micelle. Here reduction of the metal salt is performed with usual reagents such as H2, NaBH4 and hydrazine.
The size of the NP can be controlled from the stoichiometric ratio of the reactants employed in the synthesis. For example, the following ratios HAuCl4 : TOAB : thiol : NaBH4 = 1 : 3 : 2 : 10 afford NP of diameter around 5 nm.Gold clusters are made up of a defined number of Au atoms. Such a number is based on the dense packing of atoms taken as spheres, each atom being surrounded by 12 nearest neighbors. Thus, the smallest cluster contains 13 atoms, and the following layers contain 10n2+2 atoms (n = layer number). Clusters containing 55, 147, 309, 561, 923, 1415 and up to 2057 atoms (n=2–8) have been isolated. Larger aggregates constitute the fuzzy frontier between clusters and colloids (Au NP), the latter being objects with some dispersity described by a histogram of size distribution determined using TEM data.