Surface-enhanced Raman spectroscopy (SERS).
Raman methods are also useful in evaluating metallic nanoparticles, which can be used in nanomedicines, diagnostics, or biomedical
imaging. Scientists at the National Institute of Standards and Technology (NIST) recently used surface-enhanced Raman spectroscopy
(SERS) to test the properties of nanostars or star-shaped gold nanoparticles. They found that the nanostars had better optical
qualities than other nanospheres commonly used for Raman enhancement, thereby making them for a range of applications, including
disease diagnostics. (3).
SERS relies on metallic nanoparticles, most commonly gold and silver, to amplify signals from molecules present in only trace
quantities. The NIST researchers tested the optical properties of the nanostars using two target molecules: 2-mercaptopyridine
and crystal violet, which were selected because of their structural similarity to biological molecules and their large number
of delocalized electrons, a characteristic that lends itself to SERS, according to NIST. The researchers found that the Raman
signal of 2-mercaptopyridine was 100,000 times stronger when nanostars were present in the solution. The nanostars also enhanced
the signature of crystal violet and delivered a signal about 10 times stronger than nanorods, another type of nanosphere.
Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS).
Researchers at the Georgia Institute of Technology in Atlanta and Xiamen University in Xiamen, China, recently reported on
what they have classified as shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), a method which they believe
could be developed into a nondestructive and flexible portable characterization tool for drug and food inspections as well
as for characterizing living cells through in-situ detection of cell-wall proteins (4).
Their research addressed the limitations of SERS by obtaining high quality Raman spectra of various molecules adsorbed at
platinum and silver single-crystal surfaces with different facets in an electrochemical environment. In SHINERS, the Raman
signal is amplified by gold nanoparticles with an ultrathin silica or alumina shell. A monolayer of these nanoparticles, or
so-called "smart dust," is spread over the surface to be probed. The ultrathin coating prevents the nanoparticules from agglomerating,
protects the SERS-active nanostructure from contact with the probed material, and allows the "smart dust" to conform to different
contours and morphologies of a given substrate. The researchers used a three-dimensional finite-difference time-domain method
to simulate the related enhancement. The results showed that SHINERS can be applied in probing surface composition, adsorption,
and processes of diverse objects and morphologies. It may also be applied to general spectroscopy such as IR spectroscopy,
sum frequency generation, and fluorescence.
The NIST and the National Cancer Institute's Nanotechnology Characterization Laboratory (NCL) are engaged in ongoing research
in the characterization of nanomaterial formulations for cancer treatments. Some examples of their work include isotope dilution
mass spectrometric analysis of a nanoemulsion submitted by NCL for gadolinium content; the development of a method based on
size-exclusion chromatography with inductively coupled plasma–mass spectrometry detection to quantitatively distinguish between
free and complexed forms of gadolinium; and analysis of tissue samples for toxicological study of gold nanoparticles.
1. S. Webster and K.J Baldwin, Pharm. Technol. Europe, http://pharmtech.findpharma.com/pharmtech/Analytical/Raman-as-a-PAT-Tool/ArticleStandard/Article/detail/173525/, accessed June 21, 2010.
2. R.L. Green and C.D Brown, Pharm. Technol. 33 (10), 72–82 (2008).
3. E. Nalbant Esenturk and A.R. Hight Walker, J. Raman Spec. 40 (1), 86–91 (2008).
4. J. Feng et al., Nature
426 (7287), 392–395 (2010).