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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