Preparation and stabilization of monodisperse colloidal gold by reduction with aminodextran

Stable concentrated aqueous dispersions of monodisperse gold nanoparticles were prepared using diethylaminoethyl-dextran as reductant and stabilizer. The effectiveness of dextran as reducing agent was strongly affected by the pH. In alkaline solutions, the Au(III) species were rapidly and completely reduced, yielding stable dispersions of spherical uniform gold nanoparticles. Their modal diameter could be adjusted from 18 to 40 nm by varying the pH, temperature, and the Au3+/dextran ratio. In contrast, under acidic conditions (pH approximately 4.0) the reduction was very slow, favoring the formation of large gold crystals of other shapes. A general mechanism explaining the reducing and stabilizing actions of polysaccharides in general, and of diethylaminoethyl-dextran in particular, is proposed.