Size-controlled silver nanoparticles synthesized over the range 5-100 nm using the same protocol and their antibacterial efficacy

A systematic and detailed study for size-specific antibacterial efficacy of silver nanoparticles (AgNPs) synthesized using a co-reduction approach is presented here. Nucleation and growth kinetics during the synthesis process was precisely controlled and AgNPs of average size 5, 7, 10, 15, 20, 30, 50, 63, 85, and 100 nm were synthesized with good yield and monodispersity. We found the bacteriostatic/bactericidal effect of AgNPs to be size and dose-dependent as determined by the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of silver nanoparticles against four bacterial strains. Out of the tested strains, Escherichia coli MTCC 443 and Staphylococcus aureus NCIM 5201 were found to be the most and least sensitive strains regardless of AgNP size. For AgNPs with less than 10 nm size, the antibacterial efficacy was significantly enhanced as revealed through delayed bacterial growth kinetics, corresponding MIC/MBC values and disk diffusion tests. AgNPs of the smallest size, i.e., 5 nm demonstrated the best results and mediated the fastest bactericidal activity against all the tested strains compared to AgNPs having 7 nm and 10 nm sizes at similar bacterial concentrations. TEM analysis of AgNP treated bacterial cells showed the presence of AgNPs on the cell membrane, and AgNPs internalized within the cells.