Synthesis of gold nanoparticles prepared for the first time by Michael Faraday and later further explained by Turkevich. It involves the reaction of chloroauric acid with small amounts of sodium citrate solution. Gold nanoparticles with smaller size will form due to citrate ions act as a reducing agent and as well as capping agent. By varying the concentration of goldchloride and citrate, different size of gold nanoparticles will form.
Basic information on the synthesis of gold nanoparticles by following the links:
We have very recently improved the Turkevich-protocol in terms of reproducibility, uniformity and monodispersity of the AuNP (diameter ~ 12 nm) as well as scaling up.