Threonine amino acid mediated photochemical synthesis of Au, Ag and Bimetallic Au-Ag nanoparticles with antibacterial activity

Controlled synthesis of metal and bimetallic alloy nanoparticles has gained significant attention due to their unique physico-chemical properties and their potential important applications in the area of medicinal microbiology, drug and DNA delivery, optics and electronics etc. Most of these synthesis routes typically employ toxic chemicals and organic solvents for their synthesis, which limit their applicability in biomedicine. With the increasing focus on greener routes towards nanomaterial synthesis, we have synthesised gold (Au), silver (Ag) and their bimetallic (Au-Ag) nanoparticles of different compositions by reduction of aqueous gold and silver ions using L-threonine amino acid under photochemical conditions. To characterize these mono and bimetallic nanoparticles, UV-vis spectroscopy, transmission electron microscopy and atomic absorption spectroscopy analysis were carried out. Further, we investigated antimicrobial activities of these mono (Au and Ag) and bimetallic (Au-Ag) nanoparticles against medicinally important Gram positive bacteria Staphylococcus albus and Gram negative bacteria Escherichia coli. Threonine-reduced Ag nanoparticles were found to have higher bactericidal effect than Au nanoparticles against both bacterial strains. In the case of bimetallic nanoparticles, as we decrease the molar fraction of silver relative to gold, the bactericidal activity also shows linear decrease. We also found that different counter anions present in the precursor salts had little influence on the antimicrobial profile of nanomaterials. Mechanism of antimicrobial action was correlated to the surface charge of particles, their size and composition.