Nanoparticle Enhanced Optical Spectroscopy

1
Nanoparticle Enhanced Optical Spectroscopy
Left artists view of a nanoparticle-array with
many hot-junctions in between neighboring
particles, which are responsible for its
extremely high SERS enhancing power. Center SEM
image and TEM image (inset) of a Ag-nanoparticle
array on an AAO substrate. Right Integrated
Raman intensity of adenine at 739 cm-1 as a
function of interparticle gap width (W) for
different Ag/AAO substrates. Inset shows a
typical SERS spectrum of adenine.
2
When a molecule is irradiated by a green light,
it could emit very weak yellow light. This is
because a very small fraction of the green light
induces the vibration of the molecule and loses
part of its energy, or equivalently, the
frequency of the green light is decreased and its
wavelength is increased to become a yellow light.
Such shift in the frequency of the scattered
light is characteristic of the vibration
frequency of a molecule, similar to the
uniqueness of ones fingerprint. The identity of
a molecule can be identified by this so-called
Raman Spectroscopy. Prof. C. V. Raman, an
Indian scientist, was awarded the Nobel Prize in
Physics in 1930 for the discovery. Sponsored by
the National Science and Technology Program for
Nanoscience and Nanotechnology, a team of
researchers from the Institute of Atomic and
Molecular Sciences, Academia Sinica, National
Taiwan University, and National Yang-Ming
University, fabricated very ordered and
close-packed Ag nanoparticle arrays using
nanochannels on anodic aluminum oxide as the
template. When such a unique nanoparticle-array
is exposed to a laser, the field intensity on the
surface of and in the gaps between nanoparticles
is enhanced tremendously. For molecules adsorbed
on the array, their Raman signal intensity could
be enhanced by millions or even hundreds of
million times, therefore their chances of being
detected by Raman spectroscopy are greatly
improved. This technique based on
nanoparticle-array-enhanced Raman spectroscopy is
being exploited to detect various
environmentally, biologically, and bio-medically
important molecules. (Advanced Materials 18,
491-495 (2006))