Sequential surface functionalization by polyoxometalates and lysine renders non-toxic gold nanoparticles strong antibacterial agents

We demonstrate sequential surface functionalization of tyrosine synthesised gold nanoparticles (AuNPs) with polyoxometalates (POMs) and lysine. Antibacterial activities of these AuNPs and their nanoconjugates were evaluated toward Gram negative bacteria Escherichia coli. AuNPs were found to be non-toxic to the E. coli but after their surface functionalization with POMs, these AuNPs showed antibacterial activity due to the oxidative stress to the bacterial cells induced by POMs present on the surface of AuNPs. Lysine modified AuNPs-POMs nanoconjugates exhibited significantly higher antibacterial activity as cationic amino acid lysine directs these nanoconjugates toward negatively charged bacterial cells and provoke rupture of cell wall/membrane. Short incubation time was enough to observe the bactericidal action. By varying the surface functionlization of non-toxic AuNPs with POMs and lysine, we have shown the different levels of antibacterial action and their mode of action.