Shape controlled biosynthesis of gold nanoprisms using Cinnamomum tamala leaf extract for mercuric ion sensing

Biosynthesis of metal nanoparticles using microbes, plants and plant extracts provides a viable alternative green approach for nanoparticle synthesis in comparison to conventional chemical approaches. The use of biosynthesis offers significant advantages over chemical synthesis in context of reduction of energy consumption, generation of non-hazardous by-products and the potential biocompatibility of nanomaterials. The idea behind the current work focuses on understanding the role of the physico-chemical environment in controlling the optical properties of gold nanoparticles synthesised using Cinnamomum tamala leaf extract. This control over the optical property was utilised to show the applicability of gold nanoparticles for mercuric ion sensing. This research therefore provides important information on the role of physical (extraction method, temperature and amount of reducing agent) and chemical (halide ions and surfactant) environment in controlling (i) the rate of reduction of gold ions, (ii) the tunability of optical properties, (iii) shape of nanoparticles, and (iv) size of nanoparticles. The work presented herein is divided into four sections each focussing on different aspects of research. Firstly, the important role of physical parameters in controlling the shape, size and optical property of nanoparticles is addressed. Primarily the method of extraction was assessed to provide experimental evidence for providing reproducible optical tuning of the gold nanoparticles. Further, as temperature is known to influence the rate of reduction of metal ions and reaction kinetics, synthesis of gold nanoparticles at different temperatures using different amount of plant extracts established its critical role in controlling the size and shape of gold nanoparticles.

The use of different temperatures allowed high degree of tunability in the surface plasmon resonance with lower temperatures promoting anisotropic growth of nanoparticles. This study, for the first time, also provided mechanistic understanding into the role of functional groups present in plant extract in reducing and stabilising the nanoparticles. Further, the role of externally added chemical agents in controlling the shape, size and optical properties of gold nanoparticles was studied. In particular, the effect of halide ions was addressed and its role in promoting specific facets of gold was studied. More importantly, the role of a surfactant (CTAB) during biosynthesis showed for the first time, branch-like super structures of gold providing clues into its role in assisting hierarchical assemblies. The final aspect of this thesis provided evidence of the practical applicability of biosynthesised gold nanoparticles in mercuric ion sensing. This was achieved by studying the change in the transverse and longitudinal surface plasmon resonance features in response to interaction of biosynthesised gold nanoparticles with mercury ions. This addresses a significant problem as obtaining monodisperse anisotropic nanoparticles is a challenge for mercury ion sensing and employment of both transverse and longitudinal SPR features for mercury ion sensing provides an opportunity to employ mixed shape population of gold nanoparticles for this application. The mechanistic understanding gained from these studies further suggested for the first time, the important role of phytochemicals present on the surface of nanoparticles in mercury ion binding leading to change in the SPR features.