Research Express@NCKU - Articles Digest (Volume 7 Issue 6)

1.Introduction
Gold complex nanostructures are a major research area in the field of unusual physical properties, including work on biological imaging labels, nanophotonics, tunable magnetic properties, entangled quantum states, quantum information processing, and potential applications in solar cells and biosensors. There are two convenient syntheses for the preparation of Au nanoparticles (NPs), the Brust-Schiffrin method and chemical reduction. The Brust-Schiffrin method is, however, a toxic approach. The development of Au NPs synthesized by the chemical reduction method in a water-based solution is limited because of size and shape problems.

In this study, we investigated the synthesis and optical properties of gold nano-sea-urchin. We synthesized gold nano-sea-urchin in a dark field by using a friendly-environmentally method. Then optical properties of gold nano-sea-urchin were recorded not only in the solution via UV-vis spectrum, but also on the indium-tin-oxide (ITO) glass substrate via ellipsometry. We found that the surface plasmon resonance (SPR) of gold nano-sea-urchin interacts with the substrate strongly. Compared with the SPR of spherical Au NPs, the SPR of gold nano-sea-urchin splits one mode to two modes and blue-shifts evidently.

2.Experiment
Au NPs were prepared by chemical-reduction method with HAuCl_4(aq) and Na-cit solution. 1.5 ml of freshly prepared 0.34 M trisodium-citrate _(aq) solution was added to 15 ml of 0.5 mM HAuCl_4(aq) solution at room temperature under vigorous stirring. Furthermore, 45 ml of 1M NaCl_(aq) solution were added to gold colloid to synthesize gold nano-sea-urchin in dark field. The ITO glass substrate was immersed in Au colloid for 6hr to adsorb Au NPs.

3.Results & discussion
The size of Au NPs has recently been synthesized to about 10 nm by the chemical reduction method. Morphologies of Au NPs were observed by TEM. The Au particles synthesized with 87.5 % 1 M NaCl_(aq) addition without being photoinduced are nano-sea-urchin in high-quantity. (Fig. 1a)) The size of the Au nano-sea-urchin is about 210 nm in Fig. 1(b). The morphology of the Au nano-sea-urchin is shown in Fig. 1(b). The radial symmetrical body of Au particles with nano-scaled “spines” gives it the appearance of a nano-scaled “sea urchin”. The diffraction pattern of nanoscale spines on the gold nano-sea-urchin is shown in Fig. 1(d). The structure of the spines on the nano-sea-urchin belongs to the face center cubic (FCC) phase and it has a poly-crystalline structure. The size of nano-scaled spines is 23 nm in diameter and 81 nm in length in Fig. 1(c).

Figure 1 (a) Morphology and (b) zoom-in image of Au nano-sea-urchin and (c) nano-scaled spines on Au nano-sea-urchin and (d) diffraction pattern.

According to Mie theory, the SPR is related to the onset of quantum size and shape effects of Au NPs. Fig. 2 shows the intensity of increases SPR for Au nano-sea-urchin (grown from Au NPs) without being photoinduced. The SPR peak red-shifts from 540 to 560 nm as the size of Au nano-sea-urchin increases, and is thus enhanced by the Au nano-sea-urchin. The direction of Au NPs growth is anisotropic without irradiating. The radial symmetrical growth occurs and is observed as a nano-scaled sea urchin.

Figure 2 Time-dependent spectra showing the conversion of Au NPs to nano-sea-urchin without photoinduced method.

The absorption coefficient of Au nano-structure on ITO glass substrate is shown in Fig. 3. The dielectric constants of air, thanol, and ITO are 1, 1.88, and 2.89, respectively. The appearance of these two peaks is due to the asymmetric dielectric environment experienced by Au NPs. For the upper interface, the gold nano-structure is in contact with air or ethanol. For the bottom interface, the gold nano-structure is in contact with the ITO glass substrate. When the Au NPs are on the surface of the ITO glass substrate, the splitting peak of SPR is not predominantly due to the weak interaction of SPR mode with the substrate. Au nano-sea-urchin can thus enhance the interaction of SPR mode with the substrate.

Figure 3 Absorption spectra of (a) Au NPs and (b) Au nano-sea-urchin in ethanol and air environment.

4.Conclusion
In our research, we used only trisodium-citrate with the addition of sodium-chloride to synthesize a high-quality of gold nano-sea-urchin, which is thus an environment-friendly technology. Moreover, we demonstrated that isotropic SPR make the direction of Au NPs growth isotropically in the photoinduced mechanism in a water-based solution. The interaction of SPR with the substrate is reinforced by the gold nano-sea-urchin. These results have potential applications for the enhanced-Raman scattering, optical communications, and solar cells.

Acknowledgments
This work was financially supported by the National Science Council of Taiwan, the Republic of China, grant No. NSC 94-2120-M-006-006, which is gratefully acknowledged.