Revisiting Silver Nanoparticle Chemical Synthesis and Stability by Optical Spectroscopy

Silver nanocolloids (SNC) are materials useful for wide application in chemistry and biology which are usually prepared by chemical reduction of a silver salt. Monodisperse SNC sols are relatively difficult to prepare compared to gold nanocolloids and few data are available on the optimization of the synthetic conditions as well as on the possible evolution of the nanocolloids after synthesis. To document this, we have carried out a series of chemical syntheses of SNC by the citrate and borohydride methods by varying the reductant/AgNO3 ratio. We characterized the colloids by UV-vis. spectroscopy immediately after completion of the synthetic process and during several months of storage. The spectroscopic data collected were verified for conformity with Mie theory and we retained only the data fitting to spherical nanoparticles for further analysis. Our results indicate that the SNC prepared by the citrate method contain large particles (diameters around 40 nm) which remain stable during storage. In contrast, the borohydride method generates smaller SNC. During storage, these nanoparticles were found to experience alternating nucleation and Ostwald ripening phases, which were not necessarily dependent on further silver reduction but rather on particle-particle interactions, stabilizing only after several months, depending on the synthetic conditions. We conclude that the citrate method generates stable SNC that can be safely used after synthesis, although a safe use during storage of SNC produced by borohydride reduction depends heavily on the synthetic conditions. Keywords: Silver nanocolloids, Chemical synthesis, Stability, Spectroscopic characterization