Radiolytic preparation of ultrafine colloidal gold particles in aqueous solution: Optical spectrum, controlled growth, and some chemical reactions

Orange- or yellow-orange-colored aqueous dispersions of ultrafine gold part icles (similar to 2 nm) result from the gamma-irradiation of deaerated solu tions containing hydrolyzed AuCl4- and poly(vinyl alcohol) or poly(vinylpyr rolidone), respectively. The particles have a weak and very broad plasmon a bsorption band with maximum in the 490 to 500 nm range, i.e., at substantia lly shorter wavelengths than expected from classical Mie theory using bulk dielectric data; moreover, the specific UV absorption increases as the part icles become smaller. The particles are used as seeds in the radiolytic red uction of added Au(CN)(2)(-) to yield larger particles of any desired size and improved monodispersity. The particles partially react with oxygen. Che misorption of S-mercapto propionic acid strongly affects the electronics of the particles, as indicated by the changes in optical absorption. The earl y stages of AuCl4- reduction are also investigated. The reduction occurs es sentially in two steps: (1) formation, and (2) reduction of Au+. A 203-nm a bsorption band is tentatively attributed to Au+. Without further irradiatio n, Au+ disappears thermally within hours to yield larger particles (20-70 n m).