Green Synthesis of Gold Nanoparticles Using Glycerol as a Reducing Agent

Pradnya Nalawade, Tulsi Mukherjee, Sudhir Kapoor

Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai, India.

DOI: 10.4236/anp.2013.22014                

Abstract

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.

Keywords

Gold Nanoparticles; Glycerol; Synthesis; Catalytic Reaction; Characterization

1. Introduction

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 [11]. It is also used as a sweetener in food industry and due to its hygroscopic nature it is widely used as dehydrating and moistening agent [12]. Besides that it is easily biodegradable in aerobic conditions thus can be replaced by traditional reducing agent.

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 [13]. Singh et al. have reported the formation of nickel nanoparticles in glycerol at 100˚C by using hydrazine hydrate in alkaline medium [14]. Nisaratanaporn and Wongsuwan prepared silver powders of particle size more than 63 nm using silver alkoxide as silver ion precursor and glycerol as a reducing agent at high temperature [15]. Grace and Pandian reported the synthesis of spherical and prism shaped Au NPs in glycerol at both reflux and microwave conditions [16].

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.

2. Materials and Methods

2.1. Materials

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.

2.2. Characterization

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.

2.3. Method

2.3.1. Synthesis of Au NPs in Glycerol or Water:

Glycerol System

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%.

2.3.2. Catalytic Reduction of o-Nitro Aniline

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%.

3. Results and Discussion

3.1. Formation Au NPs in Neat Glycerol

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) [2]. The absorption maximum of SPR depends on the shape and size of the particles [2]. This optical property of noble metal NPs is exploited for many applications [2]. In the present work Au NPs were prepared by simply adding NaOH to the tetra chloroauric acid solution in neat glycerol. Formation of nanoparticles

Scheme 1. Synthesis of gold nanoparticles.

was observed, after 2 min of shaking, by change in the color of the solution from light yellow to colorless to pinkish violet. Figure 1 shows the typical changes in absorption UV-Vis spectra of the Au NPs in neat glycerol. The initial peaks at 505 nm due to the surface plasmon resonance of Au NPs which is red shifted up to 536 nm along with formation of new band at longer wavelength (748 nm) with time, this might be due to formation of the anisotropic Au NPs which always exhibit two to three SPR bands depending on their shape compared with single SPR for small spherical nanocrystals [17]. As can be seen from the TEM image of the Au NPs, prepared in neat glycerol (Figure 1(b)), that the formed par-

Figure 1. (a) Variation in the surface plasmon absorption band of Au NPs formed on mixing 5 × 10⁻⁴ M HAuCl₄ and 1 × 10⁻³ M NaOH in neat glycerol (i) after 3 min (ii) after 5 min (iii) after 48 min (iv) after 24 hr (v) after 6 days; (b) Corresponding TEM image and electron diffraction pattern.

Figure 2. XRD pattern of Au NPs.

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 [18]. The colloidal stability of particles prepared in neat glycerol was not good as they starts to aggregate after about 24 hrs. Figure 2 shows the XRD pattern in the 2q range 30˚ - 70˚ for Au NPs prepared in neat glycerol. The patterns exhibit peaks due to diffraction from (111), (200) and (220) planes of metallic gold (JCPDS No. 04-0784). The XRD pattern suggests the formation of crystalline Au NPs with face-centred cubic (fcc) structure. Broadening of XRD peaks clearly indicate that the samples are nanocrystalline in nature. The particle size (d) was also calculated from XRD line broadening data, after instrumental line broadening correction, using Scherrer formula, kλ = d.β.cosθ, where λ is wavelength of the X-ray used, and it is 1.5406 Å, β is the angular line width at half maximum intensity, θ is Bragg diffraction angle and k is a constant, and is equal to 0.9 [19]. The particle size calculated from Figure 2, was found to be ~25 nm.

3.2. Effect of PVP Concentration

Figure 3 also shows the effect of concentrations of PVP on UV-Vis optical absorption spectra of the Au NPs sols prepared by addition of 1 × 10⁻³ M NaOH and 5 × 10⁻⁴ M tetra chloroauric acid in neat glycerol. PVP was found to be a very efficient stabilizer for the stabilization of Au NPs. Even a small concentration of PVP (0.05%) was sufficient to stabilize the Au NPs. When concentration of PVP decreased to 0.01% the red shift of absorption maxima was observed which might be due to the increase in particle size of the Au NPs. However, concentration of PVP (³0.05%) has no significant effect on stability as well as size, as Au NPs prepared with different concentrations are equally stable for more than a year. Since the particles were stable in the sol form XRD could not be carried out, nevertheless, TEM and selective area electron diffraction (SAED) confirmed the presence of fcc Au NPs (results not shown).

Conflicts of Interest

The authors declare no conflicts of interest.

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