We report one pot synthesis of uniform and stable polyvinyl pyrolidone (PVP) protected gold nanoparticles (Au NPs) using environmental friendly, glycerol as reducing agent. The effect of the presence of a capping agent (PVP) and the concentration of reactants (glycerol, tetra chloroauric acid, and NaOH) on the size and homogeneity of the Au NPs formed were investigated. Highly stable and well - distributed Au NPs were obtained at higher concentration of NaOH in the presence of PVP with a clear dependence of the size and the concentration of glycerol, NaOH and the presence of capping agent, whereas, large heterogeneous Au NPs were obtained in absence of PVP. The particle morphology, size and crystallinity were characterized using UV-Vis spectroscopy, transmission electron microscopy and X-ray diffraction techniques. The catalytic performance of as synthesized Au NPs for the reduction of o-nitro aniline was investigated in aqueous solution. The pseudo-first-order rate constants were also calculated for the catalytic reaction.
In recent decades there in growing interest in the synthesis of metal nanoparticles due to their unique properties, which are significantly different from the behavior of the respective bulk material [1,2]. Gold nanoparticles (Au NPs) have wide attraction because of their electronic, biosensing, plasmonic, photonics, catalysis, biomedical and surface-enhanced Raman scattering (SERS) properties [2-7]. Most of the chemical methods reported in the literature for the synthesis of Au NPs often involve use of toxic reducing agents (such as sodium borohydride, hydrazine, etc.) and harsh reaction parameters like high temperatures in the polyol method [8-10]. Glycerol, also known as glycerin, is commonly used in the pharmacological application and its derivative used in the synthesis of drugs [
Few studies have been reported in the literature demonstrating the formation of silver and Au NPs using glycerol as reducing agent at low temperature. Genc et al. have shown low temperature method to obtain monodispersed Au NPs using glycerol-incorporated nanosized liposome, where glycerol, is incorporated on both the external and internal polar surfaces of liposome encapsulating chloroauric acid, facilitates the reduction of Au(III) to form Au(0) atoms and subsequently nanoparticles [
As water is the most beneficial solvent it will be of interest to study the effect of water in glycerol on the formation of NPs. To the best of our knowledge, there is no report demonstrating the effect of concentration of glycerol, NaOH and capping agent on the formation of Au NPs at room temperature without requiring any additional reactants. In this paper we report environmentally friendly, low temperature method to prepare Au NPs without using any external reducing agent. The formed particles were further studied for their catalytic application in reduction of o-nitro aniline.
Tetra chloroauric acid, glycerol, PVP (Mol. wt. 40000), sodium hydroxide, o-nitro aniline and sodium borohydride were purchased from Sigma-Aldrich and used as received. Millipore purified water having 18.2 MΩ electrical resistivity was used for making solutions.
Absorption measurements were carried out on a Jasco V- 650 spectrophotometer. The spectra were recorded at room temperature using 1 cm quartz cuvette. Samples for transmission electron microscopy (TEM) were prepared by putting a drop of the colloidal solution on a copper grid coated with a thin amorphous carbon film placed on a filter paper. The excess solvent was removed using a filter paper, and letting the solvent evaporate at room temperature. TEM characterization was carried out using a Phillips CM 200 electron microscope with working voltage 200 kV having magnification 34,000 to 78,000. Particle size was measured by TEM photograph and calculated the size by considering at least 100 particles. XRD measurement was carried out on precipitated nanoparticles using Philips X’pert Pro machine with monochromatised CuKα X-ray source operated at 20 kV and 30 mA.
All the solutions were prepared freshly in order to avoid any photochemical reactions and experiments were carried out in an aerated condition. Gold sol was prepared by mixing the required concentration of NaOH (5 × 10−4 M - 1 × 10−3 M) and chloroauric acid solution (1 × 10−4 M - 1 × 10−3 M) in either neat glycerol or glycerol-water mixtures [80 to 20% (v/v) glycerol in water] at room temperature in the absence or presence of PVP [0.05% - 0.1% (w/v)]. Gradual formation of different color with time indicates the formation of Au NPs (Scheme 1). The experiments were repeated at least three times and found to be within the experimental error of ± 5%.
The catalytic reduction reaction was carried out in 5 ml volumetric flask. In a typical reaction excess of ice cold NaBH4 (3 × 10−2 M) was mixed with o-nitro aniline (2.3 × 10−4 M) solution in water. This was followed by the addition of Au NPs. The absorption spectra were recorded immediately after mixing with a time interval of 2 min in a scanning range of 200 - 600 nm at 25˚C till the yellow colored o-nitro aniline solution became colorless (from 4 - 25 min depending upon the concentration of catalyst used). The experiments were repeated at least three times and found to be within the experimental error of ± 10%.
Metal nanoparticles (especially Ag, Au and Cu) exhibit a unique UV-Vis absorption band derived from collective oscillation of conduction electrons upon interaction with electromagnetic radiation, known as surface plasmon resonance (SPR) [
was observed, after 2 min of shaking, by change in the color of the solution from light yellow to colorless to pinkish violet.
ticles are nonspherical including nanorod with average particle size 30 nm. This non spherical nature of the nanoparticles caused the absorption at longer wavelength in absence of stabilizing agent. Yang et al. have shown the formation of anisotropic Au NPs of average diameter of 50 - 100 nm without using any stabilizing agent [