Metal particles in aqueous solution can be surface-modified by chemisorbed molecules and/or by a second deposited metal. The electron density in the metal particles is varied by controlled electron transfer from free radicals.’ These modifications lead to significant electronic changes in the metal particles, the result being changes in the optical properties and in chemical reactivity.
The physicochemical properties also change during the transition from the atomic to the metallic state with increasing particle size. In the case of silver, oligomeric non-metallic clusters can be stabilized in solution for days, time enough to study many of their properties. In other cases (Au, Cu, Pb), the clusters are short-lived; they are investigated by fast kinetic methods such as pulse radiolysis.