Antimicrobial activity of fluorescent Ag nanoparticles.

UNLABELLED: The antimicrobial activity of fluorescent Ag nanoparticles of 1·5 nm (nAg-Fs) is demonstrated and compared with the other Ag nanoparticles of different shapes and size. The antimicrobial activity was evaluated using Gram-positive (Staphylococcus epidermidis NCIM2493 and Bacillus megaterium) and Gram-negative bacteria (Pseudomonas aeruginosa ATCC27853 and Escherichia coli) and fungal strains (Candida albicans and Aspergillus niger). Insights into the possible mechanism were investigated using fluorescence microscope and cytoplasmic materials release assay. The fluorescence microscopic measurements show that the nAg-Fs are localized at the centre of the cell, and 50% decrease in the fluorescence intensity was observed upon 2-h incubation. Maximum cytoplasmic release was observed with spherical Ag nanoparticles of 10 nm. Although the nAg-F shows minimum cytoplasmic release, it has the highest activity. The microbial killing effect of nAg-Fs is actually originates from its intracellular activity. The antimicrobial activity of nAg-Fs is significantly higher than the other synthesized nanoparticles of different shapes and size. The activity of the nanoparticles has been rationalized by considering the shape, size and surface structure of the particles. SIGNIFICANCE AND IMPACT OF THE STUDY: This study aims to demonstrate the size and shape-dependent antimicrobial activity of Ag nanoparticles. It is shown for the first time that the fluorescent Ag nanoparticles of 1·5 nm have superior antimicrobial activity with respect to the larger particles. The shape and size of the particles actually control their activity. The smaller particles can easily penetrate the cell wall and have pronounced activity. These findings may be useful in the development of potential antimicrobial agents.