Bacterial synthesis of silver sulfide nanoparticles

The metal-reducing bacterium Shewanella oneidensis MR-1 has been employed to obtain Ag2S nanoparticles from an aqueous solution of AgNO3 and Na2S2O3 at an ordinary temperature and pressure. The nanoparticles vary in size within 2–16 nm, and the fraction 6 to 12 nm in size constitutes about 70%. The maximum yield of nanoparticles in silver equivalent is 53%. Being visualized by transmission electron microscopy, the particles look like spheres with average diameters varying from 7 ± 2 nm to 9 ± 2 nm. The elemental composition of synthesized nanoparticles has been analyzed by energy-dispersive X-ray spectroscopy, and the estimated silver to sulfur atomic ratio is 2: 1. The presence of living bacterial cells is mandatory for the formation of Ag2S nanoparticles in the aqueous salt solution. Changes in the reaction conditions (reagent concentrations, temperature, and cell-incubation time in the reaction mixture) influence the yield of nanoparticles dramatically, but have little influence on their size.