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. INTRODUCTION Nanotechnology is the most promising field to generate new applications in medicine. However, only a small number of nano-products are currently in use for medicinal purposes (Chen and Schluesener, 2008). Nanomaterials often show unique and significantly changed physico-chemical and biological properties compared to their bulk counterparts, which makes them useful for various applications (Li et. al., 2001). In this context, gold, silver and copper have been used predominantly for the synthesis of stable dispersion of nanoparticles, which are useful in areas such as photography, catalysis, biological labelling, photonics, optoelectronics and surface-enhanced Raman scattering (SERS) detection etc. (Smith et. al., 2006; Kearns et. al., 2006). Therefore, based on the nanomaterials application, the surface chemistry, internal composition and other physical properties have to be designed in a specific manner to achieve desired action (Walczyk et al., 2010). In this context, many nanomaterials have H. Daima, PR. Selvakannan, S.K. Bhargava, V. Bansal 2 been developed for antibacterial activities, in which the majority has focussed on the toxicity of the inorganic component (Neal, 2008), while a minority discuss the surface charge induced toxicity (Xia et al., 2008). Most of the nanomaterials synthesis routes typically employ toxic chemicals and organic solvents for their synthesis, which limit their applicability in medicine and biological applications while green synthesis methods include mixed-valence polyoxometallates, polysaccharide, irradiation and biological synthesis (Sarma et. al., 2009). Recently, due to the emergence of antibiotics-resistance bacteria and limitations of the use of antibiotics the clinicians have returned to silver wound dressing containing varying level of silver (Chopra et. al., 2007). Ag ion is known to deactivate cellular enzymes and DNA by coordinating to electron-donating groups such as thiols, carboxylates, amides, imidazoles, indoles, hydroxyls etc (Feng et. al., 2000). Silver is also known to create pits in bacterial cell walls, leading to enhanced permeability and cell death (Sondi and Salopek-sondi 2004). On the other hand, gold nanoparticles generally are considered to be biologically inert but can be engineered to posses chemical or photothermal functionality (Pissuwan et. al., 2010). Surface coating of nanoparticles can also affect antibacterial properties as these coating can impart charge to the particles (positive/negative) and can stabilize them against aggregation and deposition. There are some reports to explore toxic effects from positively charged nanoparticles, but these effects were not observed when the same particles were coated with negatively charged functional groups (Wiesner et al., 2006). If antibacterial composition had the same charge as the bacterial cells, this induced repulsion and prevented contact but changing the charge of formulation using surface coatings considerably improved the activity of the formulation (Hamouda and Baker, 2000). Thus, antibacterial property of the nanomaterials depends on various parameters such as surface charge, metal composition, dose, incubation time and partial oxidation capacity of nanomaterials. Therefore, nanoparticles based antibacterial formulations should have the suitable combination of nanoparticles composition, counter ions for initial interaction with bacterial cell wall, surface charge and functionalization to render them as generic materials in antibacterial applications. Considering all these facts and increasing focus on greener routes towards nanomaterial synthesis, we have synthesised Au, Ag and bimetallic Au-Ag nanoparticles of different compositions by reduction of aqueous gold and silver ions using amino acid under photochemical conditions. Amino acids have certain reactive side groups that can reduce gold and silver ions into their atoms, they are zwitterionic (charge can be varied by varying the pH), their biological presence and structured shell of amino acids bound to the surface of nanoparticles could resemble enzymes/ proteins, which are key reasons to use amino acids. The amine and carboxyl groups of the amino acids on the surface of these nanoparticles could help in anchoring these nanoparticles with any biological surface through polyvalent interactions, similar to enzymatic interaction with any substrate. These properties, when combined with the toxicity of nanoparticles in a cooperative manner, could be used to produce generic materials for antibacterial applications. Using amino acid as reducing and functionalizing agent for the synthesis of nanoparticles rather than noxious chemical or polymeric reducing and functionalizing agents, is a major step towards greener synthesis of nanomaterials (Dickerson et al., 2008). H. Daima, PR. Selvakannan, S.K. Bhargava, V. Bansal 3 In this present study we have used threonine amino acid as a reducing and capping agent in the presence of PVA to synthesis gold, silver and their bimetallic nanoparticles under photochemical conditions. Threonine reduces gold and silver ions into their respective nanoparticles through alcoholic group as follow: