WWRC 91-05
Surface-Enhanced Raman Spectroscopy with Abrasively Modified Fiber Optic Probes
Introduction
Raman spectroscopy is a very attractive technique for in
situ monitoring of remote environments with optical fibers.
The major advantage of Raman over its vibrational counterpart,
IR spectroscopy, is the use of visible radiation. Raman
can be excited with visible light, which has a high transmission
in optical fibers. Furthermore, an important application of
remote, optical fiber sensors is water quality analysis and water
does not interfere with Raman spectroscopy. The advantage
of Raman over other visible spectroscopies (fluorescence and
absorbance spectroscopy) is the large amount of structural
information about the analyte that can be gleaned from Raman
spectra. Normal Raman, resonance Raman, and SERS
fiber optic probes have been constructed and shown to provide
good sensitivity in special cases (1, 2). However, in many
situations samples will contain fluorescent impurities or
analyte concentrations that are too low to be detected with a
probe based on normal or resonance Raman scattering.
We have shown that it is possible to use an indicator
molecule to flag the presence of an analyte through characteristic
changes in the indicator's Raman spectrum (3). In
order to improve sensitivity, the indicator molecule was chosen
such that resonance Raman was possible. A further improvement
in enhancement was achieved by placing the indicator
directly on roughened silver substrates and using
surface-enhanced Raman spectroscopy (SERS). In our test
case of metal ions as the analyte and Eriochrome Black T as
the indicator we found a 3-s detection limit on the order of
1 X 10-6 M. The selectivity was found to be superior to
absorption spectroscopy. These studies were performed on
carefully fabricated 1 in. square SERS substrates.
In this Technical Note we would like to describe a method
by which we can obtain strong SERS spectra by using an
abrasively modified optical fiber. This represents an
important step toward the fabrication of remote, fiber optic
probes. Previously described SERS fiber optic probes have
focused on the spatially separated triad of excitation fiber,
SERS substrate, and collection fiber. The SERS substrates
have been either specially prepared roughened surfaces (2a, b)
or electrode surfaces (2c). A particularly interesting study
by Bello and Vo Dihn showed that one could excite SERS
through relatively thick silver films (2a). We have used this
to advantage in removing spurious Raman lines that form in
the fiber due to excitation of vibrational modes in the fiber
material and possible fluorescence excited in the media
surrounding the fiber.
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