Nanogold Particles Produced by NaBH4Reduction of Gold Salt in the Presence of Laponite Sol

Abstract

In the last few decades, there has been considerable effort in preparing and characterizing nanostructured materials. Layered inorganic solids such as polysilicates, double hydroxide, perovskites and clay minerals are important constituents of these assemblies because of their ability to provide interlamellar space, and their large active surface area. Laponite is a synthetic polycrystalline clay similar in structure and composition to natural hectorite of the smectite group. The sodium ions in the central layer of laponite are exchangeable, and in aqueous dispersions, these ions diffuse into the water, and plate-like particles with negatively charged faces are formed. Among nanoparticles, colloidal metal or metal oxide particles have been intensively studied because of their unique optical and catalytic properties and their biomedical applications. There have been many reports on the synthesis of nanoparticle/clay composites. Metal hydroxides of iron, chromium, cobalt, manganese and cerium in acetic acid solutions were used to exchange the sodium ions in laponite, and their corresponding nanometal oxidelaponite composite products were obtained by calcination of adsorptions of these precursor solutions at 500 C in air. The preparation and characterization of nanogold/laponite composite were examined in this study. For this purpose, gold(III) chloride solution in dilute HCl was reduced with sodium borohydride in the presence of laponite. UV-vis spectra of different concentrations of nanogold sols are shown in Figure 1. In Figures 1(a) and (b), very broad absorption peaks are observed, and apparent surface plasmon resonance absorptions of nanogold particles are not observed due to the very low concentration of nanogold particles. Alternatively, characteristic surface plasmon resonance absorption of nanogold particles can be observed at about 520 nm in high concentrations of nanogold (Figures 1(c), (d) and (e)). With the increase of gold content, the absorption peak position is shifted toward higher wavelength, from 518 nm to 521 nm. This red shift can be related to the growth of nanogold particles with the increase of gold content. It is known that the surface plasmon resonance absorption of gold nanoparticles is very sensitive to particle aggregation and variations in surroundings, and strongly influenced by chemical modifications of the particle surfaces. The growth of nanogold particles has also been observed in TEM micrographs, as shown in Figure 2. The particle size is changed from about 3 nm to about 15-20 nm. With a low concentration of nanogold particles, not many particles were observed, and these results agree well with those of UV-vis spectra, Figure 1. UV-vis absorption spectra of gold colloids in the presence of laponite as a function of HAuCl4 concentrations: (a) 0.05, (b) 0.25, (c) 0.50, (d) 2.5, (e) 5.0 mmol.