Preparation of Colloidal Gold

Since their introduction by Faulk and Taylor (1 ), colloidal gold probes have become widely used for immunocytochemical staining at the electron microscopic level. Many different methods of producing colloidal gold sols have been published (2 –10 ). Gold sols are producing by boiling a solution of tetrachloroauric acid with a reducing agent. At the beginning of the reduction process, gold atoms are liberated from the chloroauric acid. The gold atoms aggregate forming microcrystals. As more chloroauric acid is reduced, the microcrystals grow in size until all of the chloroauric acid is reduced. The type of reducing agent and the concentration of components determine the ratio between nucleation and growth, and thus determine the final particle size. Gold sols can be made in the laboratory that have a particle size ranging from 2 to 40 nm, depending on the type and concentration of the reducing agent. The methods given below for various sizes of gold are relatively simple and very reproducible from batch to batch.