Monitoring chemical reactions at the gold/solution interface using atomic force microscopy

The adsorption of gold chloride onto a gold surface in aqueous solution, in the presence of excess citrate ions, and its subsequent reduction to Au has been monitored using an atomic force microscope. The force of interaction between a gold sphere and gold substrate in the presence of citrate (3.3 x 10(-4) M) at pH 6.3 is repulsive. However, this situation changes rapidly (<15 min) with the introduction of 1 x 10(-4) M AuCl4- and the force of interaction becomes attractive. Over a period of 7 h the measured long-range force of interaction again becomes repulsive as the gold chloride is reduced. Force-separation data indicate that multilayer adsorption of citrate occurs on the gold surface and that considerable counter-ion binding is associated with these adsorbed layers.