Light-induced transformation of citrate-stabilized silver nanoparticles: Photochemical method of increase of SERS activity of silver colloids

It is well known that silver sols are photochemically reconstructed. In this contribution we showed that light-induced transformation of citrate-stabilized silver sols may be used to cause their aggregation/agglomeration and hence their very efficient activation for surface-enhancement Raman scattering (SERS) measurements. In contrast to the chemical methods typically used for aggregation, there is no need to introduce additional compounds as aggregation agents. Light-induced aggregation is inhibited when the sol is deoxygenated. Moreover, photochemical transformation of Ag nanoparticles is practically stopped when the concentration of added citrate exceeds ca. 0.03 M (it means that addition of larger amount of citrates may be used for stabilization of even intensively illuminated silver colloids). The mechanism of light-induced aggregation of silver nanoparticles is proposed. Differences in the behavior of Ag nanoparticles with various diameters have been observed. In the case of silver sols containing “smaller” nanoparticles (with the average diameter of several nanometers) irradiation of the sol with the white light leads to formation of mixture of triangular, hexagonal and circular plates with large aspect ratio. Irradiation of such sols with color (narrowband) light leads to synthesis of more homogeneous nanoparticles. For example, illumination with the 562 nm radiation leads to formation of triangular prisms with the edge length of ca. 150 nm. Similar irradiation of “larger” spherical Ag nanoparticles with the average diameter of at least several tens of nanometers does not cause change of their size and shape.