Anal. Chem. ISSO, 62, 2437-2441
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Fiber-optic Remote Sensor for in Situ Surface-Enhanced Raman Scattering Analysis Job M. Bello, V. Anantha Narayanan, David L. Stokes, and Tuan Vo-Dinh* Advanced Monitoring Development Group, Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6101
An in situ SERS fiber-optic system for remote sensing was developed for excitlng and collecting surface-enhanced Raman scattering (SERS) signals generated from a sensing probe havlng --coated mlcropartkks deposited on a glass plate support. The geometry of the excitation and cotiectlon fibers is a “head-on”arrangement, wtth the flbers on opposite sides of the SERS substrate and with the angle between the fibers equal to 180’. Remote measurements are performed with the SERS probe and the cdkctlon flbers Immemed in the anaiyte solution. The analytical data relevant to the performance of the SERS flbersptic system, such as spectral characteristics, limits of detection, signal response, and temporal behavior, are described.
INTRODUCTION The discovery of optical fibers as very efficient media for light transmission has had a significant impact in many areas of technology in recent years, especially in the telecommunications industry. Analytical chemists also have benefited from the discovery of optical fibers because they can be coupled to various spectroscopic instruments, thus opening new applications of spectroscopic methods, such as analysis of samples remote from the spectrometer, sampling in a hostile environment, and coupling several spectroscopic instruments together to a single detection unit (multiplexing). Early applications of fiber optics in the area of spectroscopy were limited to UV-visible absorption and fluorescence techniques. Recently, however, Raman spectroscopy is also gaining benefits from the use of optical fibers. Some of the uses of optical fibers in Raman spectroscopy have included collection of spontaneous Raman from a sample (I), transmission of coherent anti-Stokes Raman scattering generated in a flame to a remote spectrometer (2),and use as an illumination device for samples that were sensitive to a focused laser beam (3). In addition, a Raman probe ( 4 ) and a detailed study of the parameters governing the design of an optical Raman probe (5) have been reported recently. Raman techniques have certain advantages over UV-visible absorption or luminescence techniques because the Raman spectrum usually provides structural information, and Raman bands are narrower and highly resolved. A major limitation of Raman spectroscopy, however, is its low sensitivity, which is due to the small Raman scattering cross section. However, in 1974, Fleischmann and co-workers (6) reported that an enhancement in Raman signal by factors as high as lo7 is observed for molecules adsorbed on roughened metal surfaces. This new Raman technique named surface-enhanced Raman scattering (SERS), was later confirmed (9, and SERS has generated a significant amount of interest over the past years. Both electromagnetic and chemical theories have been developed to explain the SERS effect, and several excellent
* Author to whom correspondence should be addressed. 0003-2700/90/0382-2437$02.5010
papers and reviews (8-12) have discussed both theories; therefore, they will not be discussed in this paper. A wide variety of materials have been used to obtain SERS, including silver electrodes, sol solutions, silver island films, and silver-coated microparticles (13-18). In addition, a few publications dealing with the use of optical fibers in SERS have appeared in the literature recently (19-22). In these works, fiber optics were used for irradiating and collecting SERS signal generated from a silver electrode substrate. In the present work, we describe the development of a fiber-optic SERS monitor to perform remote analysis of SERS signals generated in solution with metal-covered microparticle-based probes. In this work, analytical data relevant to the performance of the solution SERS optical fiber system, such as spectral characteristics obtained with the setup, limits of detection, signal response, and behavior with time, are described.
EXPERIMENTAL SECTION Instrumentation. S E W measurementswere conducted with a SPEX Model 1403 double-grating spectrometer (SPEX Industries), equipped with a thermoelectrically cooled gallium arsenide photomultiplier tube (RCA, Model C31034), operated in the single-photon counting mode. Data storage and processing were performed with a SPEX Datamate computer. The monochromator bandpass was 3 cm-*. The 647.1-nm line of a krypton ion laser (Innova 70, Coherent) was used for excitation. The type of optical fiber (General Fiber Optics, Inc.) used was a soft plastic-clad silica fiber with a numerical aperture of 0.26. The fiber-optic holder was purchased from Newport Corp., and the translation stages were obtained from Oriel Corp. Chemicals. The 0.1-pm (type CR) agglomerate-free alumina used to prepare the substrates was provided by Baikowski International Corp. and used as received. p-Aminobenzoic acid (Aldrich),benzoic acid (Mallinckrodt),fluorescein isothiocyanate (MolecularProbes Co.), o-chlorophenol(Aldrich),o-bromophenol (Eastman Organic Chemicals),and 2,4-dinitrotoluene (Aldrich) were purchased at the purest available grade and were used as received. The alumina was suspended in high-performanceliquid chromatographygrade water (Burdick and Jackson), and sample solutions were prepared from spectroscopic grade ethanol (Warner-Graham Co.). Procedure. The preparation of the SERS substrates involved two steps. First, three drops of a 5% aqueous suspension of the alumina were deposited and then spread evenly on the surface of a precleaned rectangular glass strip (1 mm thick) cut from a microscope slide. The alumina-covered glass was then placed on a spinning device and spun to spread the alumina uniformly on the glass support. The second step involved the deposition of a 75-nm layer thickness of silver onto the alumina-coated glass strip. The deposition of the silver was done under vacuum at a pressure of -2 X lo4 Torr. Substrates were used immediately after preparation. Solution SERS measurements were performed as follows. Three milliliters of ethanol was placed in a small beaker. Next, the SERS probe was submerged into the ethanol solution and placed at the bottom of the beaker with the silver-coatedside of the plate facing upward and exposed to the bulk solution. Three microliters of an ethanol solution of the anal@ was then added to the ethanol solution. Afterward, the SERS signal was measured. 0 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 22. NOVEMBER 15, 1990
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Flgure 1.
Schematic diagram of the in situ SERS fiber optic system.
Blank spectra were obtained in the same manner a6 that of the analyte solution except that no analyte was added to the solvent.
RESULTS AND DISCUSSION Description of the 'Solution" SERS Optical Fiber System. Figure 1shows a schematic diagram of the in situ SERS fiber-optic system used in this work. In this system, a single optical fiber strand was used to deliver the laser beam into the sampling system, and another fiber was used to collect the scattered radiation. Also, in order to obtain a more efficient collection of the SERS signal, a 400-fim fiber was used for excitation, while the diameter of the collection fiber was 600 rm. By use of a smaller diameter fiber for excitation, the diameter of the beam that is focused onto the SERS substrate is smaller than the diameter of the collection fiber. Thus,most of the scattered radiation from the SERS substrate is contained within the acceptance cone of the collection fiber and collected by the fiber. As shown in Figure 1, after passing through the handpass filter, the laser beam was focused into one end of the excitation fiber by a microscope objective lens. Focusing of the beam into the excitation fiber was facilitated by having the fiber mounted on a fiber-optic holder with an X-Y-Z adjustment. The terminus end of the excitation fiber was then positioned on the bottom of a small beaker that contained the analyte solution and the SERS substrate. The fiber was positioned close to the beaker in order to contain the laser beam to a very small spot on the SERS substrate. The SERS substrate used in this work was prepared with a glass hacking so that the excitation and collection fiber could be positioned "head on-, with the fibers on opposite sides of the SERS substrate and with the angle between the fibers equal to 180' (Figure 1inset). As shown in Figure 1,the "head on" arrangement is very adaptable for in situ SERS measurement. In order to collect as much S E W signal as possible, the tip of the collection fiber was submerged into the analyte solution and was positioned very close to the SERS substrate (
