Abstract:
A sensor for performing surface enhanced Raman spectroscopy (SERS) includes a sensor body having a throughbore; a window mounted to the sensor body that is coterminous with the throughbore; surface enhanced Raman scattering structure mounted to the window; an optical energy source for generating an optical excitation signal; a first optical fiber mounted in the throughbore for directing the optical excitation signal through the surface enhanced Raman scattering (SERS) structure; a second optical fiber mounted in the throughbore for receiving primary Raman emissions generated when an analyte in contact with the surface enhanced Raman scattering structure is irradiated by the optical excitation signal; and an optical detector for generating an optical signal representing the primary Raman emissions.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of commonly assigned U.S. patent application Ser. No. 09/593,675, filed Jun. 14, 2000, now U.S. Pat. No. 6,406,777, and entitled A METAL AND GLASS STRUCTURE FOR USE IN SURFACE ENHANCED RAMAN SPECTROSCOPY AND METHOD FOR FABRICATING SAME. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to the field of Raman spectroscopy, and more particularly, to a sensor for detecting chemicals both in gas and liquid environments using surface enhanced Raman spectroscopy. 
     Raman spectroscopy is an emission technique that involves inelastic scattering of incident laser energy and results in spectral peaks that are frequency shifted from the incident energy. The Raman bands arise from changes in polarizability in a molecule during vibration. As a result, virtually all organic molecules display a characteristic Raman emission. Therefore, a Raman sensor would not be limited to a specific class of molecules as is the case for the laser induced fluorescence (LIF) sensor. Raman spectrometry allows the fingerprinting of species present and is structurally specific. The inherently high resolution of Raman spectra often permits the analysis of several components in a mixture simultaneously. 
     The advent of inexpensive, portable Raman spectrometers has seen renewed interest in the area of Raman spectrometry. This new generation of spectrometers employs fiber-optic probes, holographic notch filters for rejection of the Rayleigh line, a single grating monochromator, and a charge-coupled device (CCD) detector for multichannel detection. These spectrometers contain a minimum of optical components as compared to conventional Raman instrumentation resulting in high throughputs; and, once coupled to a laser and spectrometer, optical-fiber probes require no further alignment. 
     Despite the advantages of Raman spectroscopy over other spectroscopic techniques and the technological advances in the area of Raman spectrometry, Raman spectroscopy is, inherently, an insensitive technique. To achieve detection limits in the low ppm range would require either the use of a multiple pass cell or long acquisition times. In the 1970s, it was discovered that Raman scattering from molecules adsorbed on such noble metals as silver, copper, and gold can be enhanced by as much as 10 6  to 10 7 . This phenomenon, called surface enhanced Raman spectroscopy (SERS), is still not understood despite intensive theoretical and experimental research. It is believed that more than one mechanism is involved in the SERS phenomenon. Initially, the SERS technique was used as a means to probe adsorption at metal interfaces both in electrochemical and gas-phase environments. This technique has proven useful in deducing the effects of interfacial structure and reactivity on the adsorption process. However, the sensitivity of the technique as well as its exceptional spectral selectivity has made SERS attractive for a broad range of analytical applications. SERS can be used for trace organic analysis and as a detection method in gas chromatography, liquid chromatography, and thin layer chromatography. Electrochemical SERS and SERS of chemically modified surfaces have been used to detect aromatic compounds and chlorinated hydrocarbons, organic contaminants of environmental concern, in the ppm concentration range. 
     There are many applications in which detection of particular chemical species or analytes is desirable, as for example, hydrocarbons that may be present in ground water, toxic vapors in industrial environments, explosives, metal ions, narcotics, toxic anions, and chemical warfare agents. 
     However, a problem with optical fiber based SERS systems is that the optical excitation signal, and Raman emissions received by the collection optics can generate secondary Raman emissions in the optical fibers. Therefore, a need exists for an optical fiber based SERS sensor for detecting analytes of interest which is not affected by secondary Raman emissions within the excitation and collection fibers. A further need exists for an optical fiber based SERS sensor that may be deployed in physically challenging environments, such as at sea and in terrestrial bore holes. 
     SUMMARY OF THE INVENTION 
     The present invention provides a sensor for performing surface enhanced Raman spectroscopy (SERS) that includes a sensor body having a throughbore; a window mounted to the sensor body that is coterminous with the throughbore; a surface enhanced Raman scattering structure mounted to the window; an optical energy source for generating an optical excitation signal; a first optical fiber mounted in the throughbore for directing the optical excitation signal through the surface enhanced Raman scattering (SERS) structure; a second optical fiber mounted in the throughbore for receiving primary Raman emissions generated when an analyte in contact with the surface enhanced Raman scattering structure is irradiated by the optical excitation signal; and an optical detector for generating an optical signal representing the primary Raman emissions. A long pass filter is optically spliced in series with each second optical fiber for filtering out optical signals having wavelengths that are less than a predetermined wavelength. The sensor also includes a bandpass filter optically spliced to the first optical fiber for attenuating any secondary Raman emissions that may be stimulated in the first optical fiber by the optical excitation signal. A first lens collimates the excitation signal from the first optical fiber and another lens focuses the excitation signal onto the external surface of the SERS structure, i.e., at a SERS surface/liquid interface or SERS surface/gas interface when the sensor is being utilized. The sensor may further include electrodes for polarizing the SERS surface to attract analytes to the surface. The sensor may further include a liquid detector for determining when the sensor body is in contact with a liquid. Another embodiment of the invention includes electrodes in the vicinity of the SERS structure for performing electrochemical SERS. 
     The surface enhanced Raman scattering structure includes: a glass substrate having a roughened surface; an adhesion layer formed on the roughened surface; metal islands formed on the adhesion layer that define a metal patterned structure; and a self-assembled monolayer formed over the metal islands. 
     The sensor eliminates background interferences arising from fiber emissions, operates at long lengths of fiber (30+ meters), is able to do multiple samplings, is easily deployable, and withstands the shock and vibration associated with deployment in subsurface environments. The sensor may be used to detect subsurface pollutants of environmental concern, in particular BTEX, chlorinated hydrocarbons, anionic nutrients, metal ions, narcotics, explosive materials, and agents of chemical warfare. 
    
    
     These and other advantages of the invention will become more apparent upon review of the accompanying drawings and specification, including the claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a first embodiment of a fiber optic sensor for performing surface enhanced Raman spectroscopy that includes a SERS structure mounted to the window of the sensor. 
     FIG. 2 shows transmission curves for the bandpass filter of FIG. 1 operating when the excitation light signal has a wavelength of 852 nm. 
     FIG. 3 shows transmission curves for the long pass filter of FIG. 1 operating when the excitation light signal has a wavelength of 852 nm. 
     FIG. 4 illustrates a second embodiment fiber optic sensor for performing surface enhanced Raman spectroscopy that further includes a liquid level detector. 
     FIG. 5 illustrates a third embodiment of a fiber optic sensor for performing surface enhanced Raman spectroscopy that includes electrodes for performing electrochemical SERS. 
     FIG. 6 illustrates a third embodiment of a fiber optic sensor for performing surface enhanced Raman spectroscopy that includes electrodes for performing SERS, where one of the electrodes is in ohmic contact with the SERS structure. 
     Throughout the several view, like elements are referenced using like references. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the present invention is directed to a sensor  10  for performing surface enhanced Raman spectroscopy. The sensor  10  includes a sensor body  12  having a throughbore  14  in which is positioned a fiber optic bundle  16  comprised of collection optical fibers  18 , excitation optical fiber  20 , collimating lens  22 , focusing lens  24 , window  26 , and a Surface Enhanced Raman Spectroscopy (SERS) structure  28  that is described in commonly assigned U.S. patent application Ser. No. 09/593,675, filed Jun. 14, 2000, now U.S. Pat. No. 6,406,777, and entitled A METAL AND GLASS STRUCTURE FOR USE IN SURFACE ENHANCED RAMAN SPECTROSCOPY AND METHOD FOR FABRICATING SAME, incorporated herein by reference. Filter  30  is optically aligned and spliced to excitation optical fiber  20 ; and a filter  32  is optically aligned and spliced to each of collection optical fibers  18 . Bushing  36  is threaded into sensor body  12  to secure the fiber optic bundle  16  within the bore  14  of the sensor body  12 . An O-ring  38  may be interposed between bushing  36  and sensor body  12  to provide a watertight seal therebetween. Window  26  on which SERS structure  28  is bonded may be secured to sensor body  12  using adhesives, not shown, or by mechanical means, such as flanges or clamps. 
     In the operation of sensor  10 , optical energy source  40  emits a light signal  42  that is directed to propagate through excitation optical fiber  20 . Optical energy source  40  may be implemented as a krypton ion laser, near infrared (IR) diode laser, or Nd:YAG laser that generates light signals having wavelengths in the range, by way of example, from 647 to 1064 nm. Optical filter  30  is a bandpass filter that removes Raman emissions that may be excited within excitation optical fiber  20  by light signal  42 . Light signal  42  is emitted from the polished end of excitation optical fiber  20  and then is collimated by lens  22  and focused by lens  24  onto the external surface  77  of the SERS structure  28 , which is a SERS surface/liquid or SERS surface/vapor interface when sensor  10  is being utilized. Next, focused and collimated light signal  42  passes through window  26  and SERS structure  28  and is then emitted from sensor body  12  through the SERS structure  28  into the environment  44 , which for example, may be a liquid or gas in which an analyte of interest may be present. 
     If an analyte of interest is present in environment  44 , then the interaction of light signal  42  and the analyte in the presence of SERS structure  28  stimulates the generation of primary Raman emissions  48  that are transmitted through window  26 , focusing lens  24 , and collimating lens  22 , and then enter collection optical fibers  18 . Primary Raman emissions  48  are directed by collection optical fibers  18  through long pass filters  32  which block the Rayleigh line, thereby preventing excitation of secondary Raman emissions in collection optical fibers  18 . The primary Raman emissions  48  are directed to optical detector  50  which detects the spectral components of signals  48 . Secondary Raman emissions are generally defined as Raman emissions that are not stimulated by irradiation of the analyte by optical excitation signal  42 . Optical detector  50  generates signal  52  that represents the primary Raman emissions  48 , particularly, the spectral components of primary Raman signals  48 . In response to receiving signal  52 , processor  54  determines the identity of the analyte that resulted in the generation of primary Raman emissions  48 , as for example, by comparing the value of signal  52  to values stored in a look-up table implemented in processor  54 . If the value of signal  52  falls within a predetermined difference between the value of signal  52  and a reference value stored in the look-up table, then the processor  54  generates an output signal  56  that causes display  58  to present the identity the detected analyte, i.e., particular chemical associated with the reference value. The look-up table may include reference values for many chemical species of interest, thereby providing sensor  10  with the capability for identifying a host of chemical species that may cause SERS structure  28  to stimulate primary Raman emissions  48 . 
     By way of example, collection optical fibers  18  and  20 , and filters  30  and  32  are available as a commercial package from Visionex, Inc., and may be selected for specific excitation wavelengths. FIGS. 2 and 3 shows transmission curves for the bandpass filter  30  and long pass filters  32 , respectively, operating when excitation light signal  42  has a wavelength of 852 nm. FIG. 2 shows that the bandpass filter  30  has a very narrow bandpass centered about 850 nm and a full width, half maximum of value of 7 nm. FIG. 3 shows that the long pass filter  32  sharply passes light having wavelengths of about 868 nm or higher, but sharply attenuates light having shorter wavelengths than that. 
     Referring to FIG. 4, the sensor may further include a liquid detector comprised of wire leads  60  and  62 , each having for example, a 0.5 mm diameter, and a volt meter  68 . The wire leads  60  and  62  each may be mounted through with bores, not shown, in sensor body  12  and secured to the sensor body with epoxy. The ends  64  and  66  of wire leads  60  and  62 , respectively, extend a distance d beneath the optical output end of the sensor body. The distance d may typically be in the range of 1 to 3 mm. Wire leads  60  and  62  are connected, for example, to a +5 V power supply, not shown, and are preferably made of platinum or platinum alloys to provide the leads with excellent chemical resistance. In general, voltmeter  68  will read approximately +5 V if the optical output and detection end  69  of sensor body  12  is not immersed in a liquid. However, if optical output and detection end  69  of sensor body  12  is immersed in a liquid  71 , the voltmeter  68  will display a reading of about 0 V, because any conductivity of liquid  71  will cause a short circuit between wire leads  60  and  62 . The liquid sensor is important because it provides a means by which one may determine whether the sensor body  12  comes into contact with a liquid environment, as for example, in applications where sensor body  12  is lowered into bore holes, tubes, and the like. 
     Referring to FIG. 5, sensor  10  may be employed to perform electrochemical SERS and includes counter electrode  70 , working electrode  72 , reference electrode  74 , and potentiostat  76 . Counter electrode  70  is preferably made of platinum or platinum alloys because platinum has excellent chemical resistance. However, the embodiment of sensor  10  shown in FIG. 5 does not include a SERS structure  28 . Working electrode  72  is preferably made of silver, gold, or copper since those materials exhibit excellent SERS enhancement, i.e., they show a SERS effect when roughened. Reference electrode  74  preferably is made of silver coated with silver chloride. Electrodes  70 ,  72 , and  74  may be implemented as wires, where electrodes  70  and  74  have a 0.5 mm diameter and electrode  72  has a 2 mm diameter. The electrodes  70 ,  72 , and  74  are fitted through bores, not shown, in the sensor body  12  and may be secured with epoxy. In electrochemical SERS, adsorption of analytes of interest onto the working electrode  70  is induced by varying the potential of working electrode  72 , as for example, between +2.2 and −2.2 volts. The surface of working electrode  72  may be roughened in-situ in the presence of the analyte by performing repeated oxidation/reduction cycles (ORCs). The ORCs are performed by allowing the potentiostat to cycle the working electrode  72  between +0.2 and −0.3 volts, vs. the reference electrode  74  for approximately 10 minutes at a scan rate of 0.2 V/s. Since the adsorption isotherm of an analyte on an electrode surface is potential dependent, controlling the applied potential to working electrode  72  offers some degree of selectivity as to the types of analytes that may be detected. Example of analytes that may be detected using electrodes  70 ,  72 , and  74  of sensor  10  to perform electrochemical SERS include, but are not limited to naphthalene, toluene, and benzene. 
     FIG. 6 illustrates another embodiment of sensor  10  wherein working electrode  72  is  18  placed in ohmic contact with the external surface  77  of SERS structure  28 , preferably using  19  mechanical fastening means, such as clamps, not shown, so that a potential may be applied to the  20  surface  77  in order to attract ions from the analyte of interest and thus, increase the sensitivity of sensor  10 . Mechanical fastening means are preferred over an electrically conductive epoxy because epoxy may generate its own Raman emissions if excited by light signal  42  that could contaminate Raman emissions  48 .