Evaluation of the toxicities of silver and silver sulfide nanoparticles against Gram-positive and Gram-negative bacteria.

In this study, the endogenous lipid signalling molecules, N-myristoylethanolamine, were explored as a capping agent to synthesise stable silver nanoparticles (AgNPs) and Ag sulphide NPs (Ag2S NPs). Sulphidation of the AgNPs abolishes the surface plasmon resonance (SPR) maximum of AgNPs at 415 nm with concomitant changes in the SPR, indicating the formation of Ag2S NPs. Transmission electron microscopy revealed that the AgNPs and Ag2S NPs are spherical in shape with a size of 5-30 and 8-30 nm, respectively. AgNPs and Ag2S NPs exhibit antimicrobial activity against Gram-positive and Gram-negative bacteria. The minimum inhibitory concentrations (MIC) of 25 and 50 μM for AgNPs and Ag2S NPs, respectively, were determined from resazurin microtitre plate assay. At or above MIC, both AgNPs and Ag2S NPs decrease the cell viability through the mechanism of membrane damage and generation of excess reactive oxygen species.