Gold nanoparticles have gained increased worldwide research interest during the recent years owing to large surface-to-volume ratio and the increased surface activity ^[1]. Gold nanoparticles have attracted wide attention due to their potential applications in catalysis,electrical conductivity,optical properties,etc.,^{[2, 3, 4, 5, 6, 7]}. As for the preparation of gold nanoparticles,many synthetic methods have been developed to form and to stabilize gold nanoparticles among which the reduction of chloroauric acid (HAuCl₄) ^[8] is the most widely used,but abundant chlorine ions exist by these methods. In some areas,it can be time-consuming and expensive to remove residual chloride ions. There have been few studies on adopting other gold compounds to synthesize gold nanoparticles.

In this research,we proposed sodium gold sulfite (Na₃Au(SO₃)₂),without chloride,to synthesize the gold nanoparticles without the introduction of other reducing reagents by using polyvinyl pyrrolidone (PVP) as a protective reagent. During the reaction process,sodium sulfite in the sodium gold sulfite molecule was converted to sodium sulfate,which has high water solubility and low adverse impact. The products formed in a few seconds to afford different shapes and sizes. The gold nanoparticles dispersed in this system have a high colloidal stability and a narrow size distribution. 2. Experimental

Chloroauric acid (HAuCl₄) was prepared by my research team. Ammonia water (NH₃·H₂O),sodium sulfite (Na₂SO₃) are all guaranteed reagents purchased from Tianjin Chemical Corporation (Tianjin,China). All solutions were prepared with redistilled water. Na₃Au(SO₃)₂ was prepared as follows: NH₃·H₂O was added to HAuCl₄ solution,generating an orange precipitate. After repeatedly washing with redistilled water,sodium sulfite was added,and after a short time,a light yellow transparent sodium gold sulfite solution was obtained ^[9].

Preparation of gold nanoparticles: 3 mL of 1 g/L PVP and 40 mL of water were added to 1 mL of 0.5 mmol/L ^{[Na₃Au(SO₃)₂]} aqueous solution,and then the mixture stirred for 1 min. Varying the temperature from 10℃ to 90℃,dilute sulfuric acid or sodium hydrate was added to regulate the solution at pH 1 ~ 9,this mixture solution was stirred for 1 min. The resulting gold nanoparticle solution was used for further characterization.

The physical characteristic of the resulting gold nanoparticles was characterized by transmission electron microscope. Samples for TEM were prepared by placing a drop of gold solution of ultrasonically dispersed product on a copper grid and then dried at room temperature. TEM images were performed using F-20 (HITACHI,Japan) operated at an accelerating voltage of 120 kV. UV-vis absorption spectra were performed by using a commercial UV-2600 spectrophotometer at room temperature with 1 cm optical path length. 3. Results and discussion

Table 1 illustrates Na₃Au (SO₃)₂ is not reduced to atomic Au below temperature 90℃ and pH 1. In other conditions, Na₃Au(SO₃)₂ has no obvious change in a few seconds,including the mixing solution of Na₃Au(SO₃)₂ and PVP at 90℃,which illustrates the reducing agent is Na₃Au (SO₃)₂ at pH 1. In this reaction process,the reaction of ionic Au(I) to atomic Au was complete,and SO₃^2- was converted to Na₂SO₄. The excess of sulfate in an aqueous solution of gold nanoparticles may cause their aggregation ^[10],however,the concentration of the sulfate byproduct is so low that the gold nanoparticles have a good stability in solution. During the reaction,gold nanoparticles were prevented from aggregating by the dispersion affects of PVP. The reaction equation of this system is as follow:

Table 1

Table 1 The reaction of Na3Au (SO3)2 at different pH values and temperatures,‘‘+’’ denotes Na3Au (SO3)2 was reduced,‘‘-’’denotes no obvious change.

Table 1 Oligonucleotides designed in the present study.a

The different sizes of gold nanoparticles were obtained by using different concentrations of Na₃Au(SO₃)₂ aqueous solution. The color of the solution changes slowly from transparent to shallow purple depending on the concentrations of the Na₃Au(SO₃)₂ aqueous solution. This change indicates the reduction of total ionic Au(I) to atomic Au is accomplished,and gold nanoparticles were formed.

Fig. 1 illustrates the examples of the TEM micrographs and the particle size distribution for samples of gold nanoparticles prepared as described in Section 2 ^[9]. The surface morphology of the particles is nearly spherical and the average size of resulting particles is about 6 nm. Some previous research reported UV-vis spectrophotometry as an effective method to monitor the evolution of metal species in the synthesis of metal particles ^{[11, 12, 13]}. Fig. 1 shows the UV-vis absorption spectrum of gold nanoparticles obtained by our method. All UV-vis spectra were recorded at room temperature against the same reference sample. A strong absorption peak at approximately 520 nm indicates the formation of gold nanoparticles with an average size of particles of about 6 nm identical to the results of the TEM images. It is worthwhile mentioning that Na₃Au(SO₃)₂ can be reduced to form gold nanoparticles,because we have not introduced other reducing reagents.

Fig. 1

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Fig. 1.TEM images (left) and particle size distribution (right) showing characteristic of gold nanoparticles.

The absorption peak maximum is related to the size of the particles. The absorption spectra obtained for gold particles using different concentrations of Na₃Au(SO₃)₂ are shown in Fig. 2. With the increase of (Na₃Au(SO₃)₂),the synthesis of gold nanoparticles showed a different absorption peak at 524,525, 547,and 550 nm,respectively. It has been well established that the maximum wavelength of nanoparticles depends on size and shape,so this directly influences a shift of the absorption peak strongly,where larger gold particle sizes will cause an absorption peak maximum shift to longer wavelengths,lower frequency,and lower energies ^{[14, 15, 16]}. This result indicates that with the increase concentration of (Na₃Au(SO₃)₂),the diameter of gold nanoparticles increases. Gold nanoparticles size could be controlled by adjusting the concentration of Na₃Au(SO₃)₂. When ^{[Na₃Au(SO₃)₂]} = 0.1 mmol/L,the gold nanoparticles have a smaller average size and a narrow size distribution.

Fig. 2

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Fig. 2.Comparison of UV-vis spectra with concentration of 0.01 mmol/L [Na3Au (SO3)2] (left) and different concentrations of Na3Au (SO3)2 (right).

1. The gold nanoparticles were produced directly from a new gold salt Na₃Au(SO₃)₂ simply by the reduction of Au(I) with the method of intra-molecular reduction.
2. Colloidal gold of about 6 nm particle size was prepared in optimum conditions: T = 90℃,pH 1,[Na₃Au(SO₃)₂] = 0.01 mmol/L,[PVP]/[Na₃Au(SO₃)₂] = 0.3,stirring speed of 200 rpm, the reaction time is 1 min.
3. The size of resultant gold nanoparticles can be controlled by the concentration of gold precursor introduced. Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (No. 20976201) and the Graduate’s Scientific Research Foundation of Central South University of Forestry and Technology (No. CX2012B04).