Size-dependent antibacterial activity for laser-generated silver nanoparticles

Silver nanoparticles (Ag NPs) have been used widely for antibacterial applications; however, the effects of their sizes on antibacterial activities and toxicities to human cells, particularly for the laser-generated Ag NPs, are not fully understood. In this study, sucrose gradient centrifugation was used to separate laser-generated Ag NPs into different fractions by size. Transmission electron microscopy was used to analyze the size distribution of the Ag NPs, and well diffusion method was used to evaluate the antibacterial activity of the Ag NP fractions against the Escherichia coli. Results showed that the antibacterial effects of laser-generated Ag NPs inversely correlated to the particle size. Among Ag NP fractions with average sizes ranging 19–47 nm, the 19-nm Ag NPs presented the highest bactericidal effect. The smaller sized laser Ag NPs also significantly induced the generation of reactive oxygen species when applied to E. coli, compared with that of the larger sized laser Ag NPs. Cytotoxicity analysis revealed that the different sized laser-generated Ag NPs were not significantly toxic to the human fibroblasts and lung epithelial cells in a 72-h in vitro cell culture period. Understanding the size-dependent functional properties of the laser-generated Ag NPs helps informing the designs for future applications of the laser-generated Ag NPs.