Environmental Friendly New Method for the Production of New Metal Nanostructures for Optical Enhancement

The most efficient and cost effective methods for fabrication and characterization of metal nanoparticles involves colloidal chemistry, from which it has been inferred that the shape, size, size distribution, and stability of nanoparticles depends on specific preparation methods, especially on the reducing agents used. Chemical methods use the salt of the metal, a reducing and a protective agent, dissolved in water or any other appropriate solvent. In this context, the use of macromolecules seems to be advantageous, since they offer control over the rate of the oxidation process, and may act both as the reducing and protecting agents [1]. In this communication we use of fulvic acid (FA) in the synthesis of gold nanoparticles, demonstrating unprecedented control of particle size and shape, by varying the experimental conditions. FA is the most water-soluble fraction of humic substances (HS), which are the main components of organic matter of soils and sediments in waters [2, 3]. The formation of colloidal gold particles by HS was observed by Machesky in his studies concerning the mechanisms of interaction of Au with HS in the natural environment [4, 5]. In the present work, gold nanoparticles of different shapes and sizes are produced through the reaction of FA and gold tetrachloric acid. FA-Au films prepared by casting are used as substrates for analytical applications of optical enhancement such as surface-enhanced Raman scattering (SERS)[6]. Gold nanoparticles were produced by mixing 20 mL of 0.01% HAuCl 4 (Aldrich) water solution with an equal volume of fulvic acid solution (250 mg L-1) at three different pHs (5, 8 and 11) and concentrations. In order to study the effect of pH and FA concentration on the shape and size of the gold nanoparticles, we employed high-resolution transmission electron microscopy (HRTEM). Figure 1 shows that the size of the colloidal particles increases as the pH decreases. For pH 11 the size of the nanoparticles is in the range from 4 to 10 nm (Figure 2a). The colloids at pH 8 produce a slightly larger size range, from 10 to 15 nm (Figure 2b). At pH 5 the micrographs show two different sizes, hexagonal and triangular particles of ~50 nm and truncated triangles of ~200 nm (Figure 2c-d). In summary, controlling the pH of FA, nanoparticles of distinctly different sizes and different shapes can be produced. The control of size and shape of gold nanoparticles that can be used embedded in fulvic acid …