Colloidal metal films as a substrate for surface-enhanced spectroscopy

Colloidal films of gold and silver were prepared on glass or quartz slides. The slides were derivatized with (3-mercaptopropyl)trimethoxysilane and subsequently reacted with aqueous metal colloids for variable time periods. The formation of the sulfur-metal bond provides a stable colloidal film on the surface. Because of the electrostatic interaction between individual particles, a semiregular structure is produced, as can be seen from electron micrographs. The unique property of the colloidal film is that they possess the optical properties of colloidal metals and the convenience of solid substrates. The effect of the dielectric constant of solvents on the optical frequencies, as well as the specific interaction of the solvent molecules with the metal on the plasmon resonances, was examined in detail. The colloidal films exhibit strong enhancement of Raman scattering and fluorescence emission from molecules adsorbed on the surface. Enhancement of fluorescence was observed for fluorescein-labeled molecules spaced 0-200 A away from the surface. These substrates can be used in a number of analytical applications, such as surface-enhanced spectroscopies as well as for fundamental studies of plasmon resonances in small metal particles.