Influence of the Counterions of Cetyltrimetylammonium Salts on the Surfactant Adsorption onto Gold Surfaces and the Formation of Gold Nanoparticles

We report the effects of counterion species on the adsorption of cetyltrimetylammonium salts (C16TAX) onto gold surfaces and the subsequent formation of gold nanoparticles in the C16TAX surfactant solution. The surfactant adsorption onto gold surfaces was examined by using quartz crystal microbalance (QCM) and atomic force microscopy (AFM). The counterion species (X) examined in this study were as follows: X = Br^-, Cl^-, NO₃^-, F^-, OH^-, and SO₄^2-. The adsorption onto the gold surface depended on the counterion species of C16TAXsthe frequency shift of QCM decreased in the following order: X = Br^- > NO₃^- ~ SO₄^2- > Cl^- > F^- > OH^-. The chemical reduction of gold(III) in the surfactant solutions of C16TAX with use of 2,2-iminodiethanol as a mild reducing agent produced anisotropic gold nanoparticles, depending on the surfactant counterion. The high-affinity adsorption of Br^-, NO₃^-, and Cl^- on gold surfaces produced anisotropic gold nanoparticles, while only spherical gold nanoparticles were obtained for weakly bound counterions such as SO₄^2-, F^-, and OH^-. In contrast, the chemical reduction with hydrazine proceeded rapidly such that the shape of the gold nanoparticles was uncontrollable; this resulted in the formation of spherical gold nanoparticles, irrespective of the surfactant counterion.