Marie Freebody, email@example.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
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.