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 nanopartidies dramatically, but have little influence on their size.