Antibacterial efficacy of silver nanoparticles of different sizes, surface conditions and synthesis methods.

Silver nanoparticles (Ag-nps) are used as a natural biocide to prevent undesired bacterial growth in clothing and cosmetics. The objective of this study was to assess the antibacterial efficacy of Ag-nps of different sizes, surface conditions, and synthesis methods against Escherichia coli, Ag-resistant E. coli, Staphylococcus aureus, methicillin-resistant S. aureus (MRSA), and Salmonella sp. Ag-nps samples were synthesized by: Base reduction with unmodified surfaces and used as synthesized ('unwashed'; 20, 50 and 80 nm) or after 20 phosphate buffer washes ('washed'; 20, 50 and 80 nm), or synthesized by laser ablation with carbon-stabilized surfaces ('carbon-coated'; 25 and 35 nm). Unwashed Ag-nps were toxic to all bacterial strains at concentrations between 3.0-8.0 μg/ml. The washed Ag-nps and carbon-coated Ag-nps were toxic to all bacterial strains except Ag-resistant E. coli at concentrations between 64.0-1024.0 μg/ml. Ag-resistant E. coli died only when treated with unwashed Ag-nps or its supernatant, both of which contained formaldehyde.