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“Smartdust” shines new light on SERS

Photonics Spectra
Sep 2010
Marie Freebody, marie.freebody@photonics.com

XIAMEN, China – Layering dust onto a surface you wish to study may seem counterintuitive, but scientists in China and the US claim that their “smartdust” will enable Raman spectroscopy to be used on any surface.

Surface-enhanced Raman scattering (SERS) is used widely for detecting molecules and trace materials. Although the powerful spectroscopy technique is popular among those working in inspection and analysis, the enhanced Raman signals needed for SERS are generated efficiently only by textured metal surfaces such as gold, silver or copper. This severely limits the breadth of possible applications of the technique.

Now, Zhongqun Tian at Xiamen University and Zhong Lin Wang at Georgia Institute of Technology in Atlanta have developed a specially synthesized dust that can be applied to any surface to obtain high-quality Raman spectra.


Pictured is a transmission electron microscopy image of a gold nanoparticle uniformly coated with a 3-nm-thick silica layer. Courtesy of Zhong Lin Wang, Georgia Institute of Technology.


Results of the study were published online March 18, 2010, in the scientific journal Nature. The dust comprises gold nanoparticles with an ultrathin shell of silica or alumina and forms the basis of a new technique dubbed shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS).

The researchers say SHINERS will be useful in materials and life sciences as well as for the inspection of food, drugs, explosives and pollutants.


This image shows the SHINERS method used for the detection of surface contaminants on fruit. Courtesy of professor Zhongqun Tian, Xiamen University.


“We are optimistic that, along with the rapid development of portable Raman instruments, SHINERS will be a simple, fast, nondestructive and portable characterization tool,” said Wang, a member of the School of Materials Science and Engineering at the institute.

SERS relies on strong plasmon resonance at the surface of the analytic material to generate a Raman signal. Because only a few metals can provide a sufficient signal, a number of approaches have been explored to extend the technique to other surfaces.

Tip-enhanced Raman spectroscopy (TERS) is one of those. It employs a nanoscale gold tip placed very close to the surface to act as a Raman signal amplifier. The drawback is that the Raman spectra recorded are dominated by molecules adsorbed at the tip with the highest SERS activity, which may provide misleading information.

For SHINERS, each nanoparticle of dust could act as a gold tip in the TERS system, and with about 1000 “tips” within the laser spot, the Raman intensity is substantially increased. What’s more, each gold nanoparticle is protected with a chemically and electrically inert shell so that the Raman signal comes solely from the probed substrate.

“The SHINERS approach is much simpler and more cost-effective in comparison with TERS,” Wang said. “It can be widely applied in probing surface composition, adsorption and processes of diverse materials, from single crystals to nanoparticles, from semiconductors to living biological systems.”

Furthermore, given the availability of preparing monodispersed gold or silver nanoparticles, together with the diversity of shell materials, Wang believes that there is tremendous scope and potential for studying surfaces of different morphologies as well as of natural and artificial objects in a highly flexible way compared with today’s surface Raman spectroscopy.


GLOSSARY
raman spectroscopy
That branch of spectroscopy concerned with Raman spectra and used to provide a means of studying pure rotational, pure vibrational and rotation-vibration energy changes in the ground level of molecules. Raman spectroscopy is dependent on the collision of incident light quanta with the molecule, inducing the molecule to undergo the change.  
tip-enhanced raman spectroscopy
A spectroscopic technique that combines the high chemical specificity of Raman scattering and signal sensitivity provided by surface plasmon resonance enhancement, with the nanoscale spatial resolution of scanning probe microscopy (SPM). Supported by a sharp metallized-SPM tip, tip-enhanced Raman spectroscopy (TERS) enables label-free detection of surface components in multicomponent samples with ultrahigh spatial resolution, making it suitable for studying nanomaterials.
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