The synthesis and characterization of gold nanoparticles in water-soluble polymers A

A thesis submitted to the University of Zululand for the Degree of Doctor of Philosophy in the Department of Chemistry, Faculty of Science and Agriculture, 2010.

Nanomaterials with critical dimensions at the nanoscale currently attract considerable attention as they show distinct chemical and physical properties that are dependent on their size and shape. A number of techniques have been used to synthesize metal nanoparticles. This thesis presents modified methods for the synthesis of gold nanoparticles, CdSe and Au-CdSe hybrid nanoparticles, capped with water soluble polymers. Chapter one reviews some basic concepts of nanostructured materials. The electronic structure and properties of gold nanoparticles are discussed. The general synthetic routes of gold nanoparticles are also reviewed with special emphasis on their preparation in water soluble polymers. A few applications of nano-gold including electronics, catalysis and biomedical applications are presented. Finally the characterization techniques used in this study are discussed. Chapter two describes the synthesis of gold nanoparticles from various chemical reduction techniques. These techniques have yielded a combination of both spherical and anisotropic shaped nanoparticles. Different reducing agents, such as sodium borohydride, presence of sodium hydroxide, ascorbic acid, sodium citrate, tri-sodium citrate and hydrogen peroxide have been used in the reduction of Au3+ ions to Au0. Water soluble polymers such as poly vinylpyrrolidone (PVP), poly vinyl alcohol (PVA) and organic ligands tri-n-octylphosphine oxide (TOPO) and octadecylamine (ODA) were used as capping agents for the gold nanoparticles. Seed-mediated and two-phase techniques were also employed to yield anisotropic nanorods of gold. Different structure-directing agents and phase-transfer agents were used in the formation of anisotropic nanoparticles. The optical properties of the gold nanoparticles were studied by UV/Vis spectroscopy. The structure and morphology of the as prepared particles was determined using powder X-ray diffraction (XRD) and electron microscopy, respectively. In Chapter three the anisotropic gold nanoparticles synthesized by a UV-irradiation technique through the interaction of HAuCl4 and a stabilizing agent, poly (vinyl pyrrolidone) (PVP) are presented. The effect of irradiation time on the size and shape of gold nanoparticles was investigated by UV-Visible spectroscopy and Transmission Electron Microscopy (TEM). The other effects that were varied include; capping agent, polymer concentration, precursor concentration, lamp wavelength, solvent, and lastly the presence of citric acid in other reactions. The absorption spectra of all samples show a broad band with the characteristic surface plasmon resonance (SPR) peak visible at around 530 nm. The presence of an additional low intensity absorption peak at a longer wavelength suggests the presence of non-spherical nanoparticles. The TEM measurements show evidence of particle shapes such as spheres, hexagons, decahedrons and truncated triangles as the reaction time was varied. Chapter four describes the synthesis of CdSe semiconductor nanoparticles at room temperature in the presence of PVP as the capping agent. Different cadmium sources were used and the mole ratios of the starting materials were varied in order to obtain monodispersed CdSe nanoparticles. L-cysteine and PVP-capped Au-CdSe hybrid nanostructures were synthesized at room temperature. The UV-Vis of these hybrid materials showed gold absorption features for the PVP-capped Au-CdSe while for cysteine capped hybrid particles very little features were observed on the spectrum. The denser spherical gold particles are distinctly visible in the TEM images. The high resolution TEM clearly showed the lattice fringes that confirm the crystallinity of the PVP capped Au-CdSe hybrid nanoparticles.

National Research Foundation, AuTEK – Mintek,Royal Society (UK), ICMR-JNCSR (USA-India) and the University of Zululand.