Tyrosine mediated Gold, Silver and their alloy nanoparticles synthesis: Antibacterial activity toward Gram positive and Gram negative bacterial strains

Shape and size-controlled synthesis of metal and metal alloy nanoparticles have gained significant attention due to their unique physico-chemical properties. Most of these synthesis routes have thus far explored use of toxic chemicals for metal nanoparticles synthesis, which limit their biological applications. With the increasing focus on eco-friendly routes towards nanomaterials synthesis and their biological applications, we show that metal (gold and silver) and their alloy nanoparticles with controlled composition can be synthesized using tyrosine amino acid. Antimicrobial activities of these nanoparticles were tested against model Gram positive bacteria Staphylococcus albus and Gram negative bacteria Escherichia coli. Irrespective of metal composition, tyrosine synthesized nanoparticles including AuNPs were found to be highly toxic towards S. albus, while antibacterial activity against E. coli was exhibited only by AgNPs and Ag containing alloys. The toxicity of tyrosine synthesized nanoparticles towards S. albus arises due to its ability to hydrolyze the peptidoglycan cell wall. In the case of E. coli, these nanoparticles showed toxicity due to the partial oxidation of Ag0 into Ag+ ions. The alloy nanoparticles toxicity depends on the silver fraction towards E. coli while all nanoparticles irrespective of composition have a similar effect on S. albus. UV-vis, TEM, AAS and zeta potential measurements were used to characterize these nanoparticles and their influence on the observed antimicrobial properties. SEM was used to study morphological changes and bactericidal action of these nanoparticles. The SEM results confirmed that treated E. coli and S. albus cells were damaged, showing formation of big holes and cell wall lysis.