molecules and materials scatter light, a small fraction of the light
interacts in such a way that it allows scientists to determine their
detailed chemical makeup. This property, known as Raman scattering, is
used by medical researchers, drug designers, chemists and other
scientists to determine what materials are made of. An enormous
limitation in the use of Raman scattering has been its extremely weak
sensitivity. While it was discovered almost three decades ago that
roughened metallic surfaces could enhance Raman scattering signals by
factors of 1 million, this "surface-enhancement" effect has been
difficult to control, predict, and reproduce for practical sensing
applications. Now, Rice researchers have shown that nanoshells can
provide large, clean, reproducible enhancements of this effect, opening
the door for new, all-optical sensing applications.
"Not only did we find that nanoshells are extremely effective at
magnifying the Raman signature of molecules, we found each individual
nanoshell acts as an independent Raman enhancer," said lead researcher
Naomi Halas, the Stanley C. Moore Professor of Electrical and Computer
Engineering, Professor of Chemistry and Director of Rice's Laboratory
of Nanophotonics. "That creates an opportunity to design all-optical
nanoscale sensors -- essentially new molecular-level diagnostic
instruments -- that could detect as little as a few molecules of a
target substance, which could be anything from a drug molecule or a key
disease protein to a deadly chemical agent."
About 1/20th the size of a red blood cell, nanoshells are about the
size of a virus. They
are ball-shaped and consist of a core of
non-conducting glass that is covered by a metallic shell, typically
either gold or silver. The metal shell "captures" passing light and
focuses it, a property that directly leads to the enormous Raman
enhancements observed. Furthermore, nanoshells can be "tuned" to
interact with specific wavelengths of light by varying the thickness of
their shells. This tunability allows for the Raman enhancements to be
optimized for specific wavelengths of light.
Discovered by Halas at Rice in the 1990s, nanoshells are already being
developed for applications including cancer diagnosis, cancer therapy,
diagnosis and testing for proteins associated with Alzheimer's disease,
drug delivery and rapid whole-blood immunoassay.
In the current study, Halas and former graduate student Joseph B.
Jackson, now with Nanospectra Biosciences, Inc., created thin films of
nanoshells deposited atop plates of glass. Films with various densities
were studied, as were films containing both silver and gold nanoshells.
Through painstaking analysis, Halas and Jackson showed that the
nanoshells' 10 billion-fold increase in Raman effect was due entirely
to the interactions of light with individual nanoshells. This is
markedly different from the pattern exhibited by pure gold or silver
nanoparticle films. In that case, the Raman enhancement is an aggregate
effect, due to the presence of localized "junctions" or "hot spots"
between metallic regions of the metallic film substrate.
The finding that individual nanoshells can vastly enhance the Raman
effect opens the door for biosensor designs that use a single
nanoshell, something that could prove useful for engineers who are
trying to probe the chemical processes within small structures such as
individual cells, or for the detection of very small amounts of a
material, like a few molecules of a deadly biological or chemical
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