Images acquired with a JEOL Ltd. scanning transmission
electron microscope show silver nanoparticles with an
average size of 15 nm (left) and 45 nm (top). The
smaller particles provide greater enhancement of the
Raman signal in SERS experiments. Courtesy of the
Journal of Physical Chemistry C.
spectrographic equipment. Furthermore,
laser ablation produces more uniform
particle sizes and apparently works with
materials that are not greatly affected by
typical acids used in chemical erosion
ICP-OES is not the only form of spectroscopy performed on materials in solution. Once relegated to testing particles
splayed out on wide-open metal substrates, Raman spectroscopy now also is
being used to explore substances in aqueous environments. In traditional surface-enhanced Raman spectroscopy (SERS),
investigators use a silver or gold film to
enhance the otherwise very weak Raman
signal that is emitted by a test material.
Performing SERS on a flat substrate is one
thing, but using it to study particles in
cells or other aqueous solutions requires
getting the metal particles into place and
understanding how they will act.
A number of researchers are starting to
extend the utility of SERS-like signal
enhancement by inverting the relationship between the test subject and the substrate.
Creating metal nanoparticles and placing them near the particles of interest in
their liquid home environment provides as
much Raman signal enhancement as using
a metallic substrate. This knowledge, however, was not enough for Caryn S. Seney
and Brittany M. Gutzman of Mercer University in Macon, Ga., and their colleague,
Russell H. Goddard of Valdosta State University, also in Georgia. They wanted to
know whether the size of metallic nanoparticles had an influence on the Raman
The group created silver nanoparticles
ranging from 15 to 160 nm across, placed
them in solution with BPE, a standard
compound used in Raman studies of solutions, and applied an argon-ion laser made
by Coherent Inc. to excite them. As the
scientists reported in the Jan. 8 issue of
the Journal of Physical Chemistry C, the
Raman effect is indeed tied to the size of
the silver particles.
According to the researchers, SERS activity generally is inversely proportional to
particle size – the smaller the particle, the
higher the Raman signal. In their study,
they found that the 15-nm silver particles
had the highest signal enhancement, and
that the highest effective laser wavelength
to obtain the SERS signal was 390 nm.