Aggregation of Silver Hydrosols Prepared in Air

Abstract

The chemistry involved in the preparation and activation of silver hydrosols was monitored by pH, potential, and surface charge measurements, by absorption spectra in the visible region, and by surface-enhanced Raman spectroscopy (SERS). The activation, displayed by a red-shift of the colloid absorption at 392 nm, corresponds to a partial aggregation of elementary silver particles as displayed by transmission electron micrographs. The participation of carbonate species in the chemistry of hydrosols handled in air was made obvious by titration with strong acids. The colloid destabilization was performed either with protons, by adding strong non-complexing acids (HNO3, HClO4), or with low concentrations of Cu2+. In contrast, HCl determined a stabilization of hydrosols related to the complexing affinity of Cl−toward silver. The successive addition of Cu(NO3)2and HCl allowed a balance between all the chemical reactions and a very efficient activation process. Whereas the chemical reactants used are nominally inorganic, at total concentrations lower than 10−3M,the activated hydrosols display anomalous SER spectra which were previously assigned to organic molecules.