Role of Surfactant in the Formation of Gold Nanoparticles in Aqueous Medium

The stability of gold nanoparticles is a major issue which decides their impending usage in nanobiotechnological applications. Often biomimetically synthesized nanoparticles are deemed useless owing to their instability in aqueous medium. So, surfactants are used to stabilize the nanoparticles. But does the surfactant only stabilize by being adsorbed to the surface of the nanoparticles and not play significantly in moulding the size and shape of the nanoparticles? Keeping this idea in mind, gold nanoparticles (GNPs) synthesized by l-tryptophan (Trp) mediated reduction of chloroauric acid (HAuCl4) were stabilized by anionic surfactant, sodium dodecyl sulphate (SDS), and its effect on the moulding of size and properties of the GNPs was studied. Interestingly, unlike most of the gold nanoparticles synthesis mechanism showing saturation growth mechanism, inclusion of SDS in the reaction mixture for GNPs synthesis resulted in a bimodal mechanism which was studied by UV-Vis spectroscopy. The mechanism was further substantiated with transmission electron microscopy. Zeta potential of GNPs solutions was measured to corroborate stability observations recorded visually. 1. Introduction Gold nanoparticles are important to bionanotechnology, owing to their easy synthesis, inert nature, good biocompatibility, and unique optical properties [1]. The tuning of optical properties of gold nanoparticles can be achieved by controlling the size of the nanoparticles [2]. Factors controlling the size of the gold nanoparticles are, namely, surface charge and double layer thickness [3]. Surface charge is again controlled by two criteria, nature and charge of ions adsorbed on the particles and surfactants used to stabilize the particles [4, 5]. Surfactants are used in the synthesis of gold nanoparticles when the reducing agent is not able to stabilize the particles [6]. The standard reducing agents which stabilize the gold nanoparticles in absence of any surfactant are sodium borohydride [7] and sodium citrate [8]. But for in-vivo applications these nanoparticles are of little use since the oxidised products of the reducing agents themselves show cytotoxicity [9, 10]. Thus, biomimetic routes of synthesis of gold nanoparticles are often implemented wherein a biomolecule is used as a reducing agent [11]. In recent years, many biomimetic synthetic [12–14] routes have been proposed to make the synthesized GNPs viable for bioapplication. For example, a variety of amino compounds have been explored as reducing agents which includes amino acids [15, 16], primary amines [17], peptides