Chemistry: analytical and immunological testing – Optical result
Reexamination Certificate
2000-11-02
2003-09-23
Snay, Jeffrey (Department: 1743)
Chemistry: analytical and immunological testing
Optical result
C436S171000, C422S082050, C356S301000
Reexamination Certificate
active
06623977
ABSTRACT:
BACKGROUND OF THE INVENTION
The ability to detect and identify trace quantities of chemicals has become increasingly important in virtually every scientific discipline, ranging from part per billion analyses of pollutants in sub-surface water to analysis of cancer treatment drugs in blood serum. Surface-enhanced Raman spectroscopy (SERS) has proven to be one of the most sensitive methods for performing such chemical analyses by the detection of a single molecule (see Nie, S. and S. R. Emory, “Probing Single Molecules and Single Nanoparticles by Surface Enhanced Raman Scattering”,
Science,
275,1102 (1997)). A Raman spectrum, similar to an infrared spectrum, consists of a wavelength distribution of bands corresponding to molecular vibrations specific to the sample being analyzed (the analyte). For example, appended
FIG. 3
(to be discussed more fully below) shows the infrared and Raman spectra of adenine. In the practice of Raman spectroscopy, the beam from a light source, generally a laser, is focused upon the sample to thereby generate inelastically scattered radiation, which is optically collected and directed into a wavelength-dispersive spectrometer in which a detector converts the energy of impinging photons to electrical signal intensity.
Historically, the very low conversion of incident radiation to inelastic scattered radiation limited Raman spectroscopy to applications that were difficult to perform by infrared spectroscopy, such as the analysis of aqueous solutions. It was discovered in 1974 however that when a molecule in close proximity to a roughened silver electrode is subjected to a Raman excitation source the intensity of the signal generated is increased by as much as six orders of magnitude. (see Fleischmann, M., Hendra, P. J., and McQuillan, A. J., “Raman Spectra of Pyridine Adsorbed at a Silver Electrode,”
Chem. Phys. Lett,
26, 123, (1974), and Weaver, M. J., Farquharson, S., Tadayyoni, M. A., “Surface-enhancement factors for Raman scattering at silver electrodes. Role of adsorbate-surface interactions and electrode structure,”
J. Chem. Phys.,
82, 4867-4874 (1985)). The mechanism responsible for this large increase in scattering efficiency has been the subject of considerable research (see Section Four: Theory in “Surface-Enhanced Raman Scattering,” [M. Kerker and B. Thompson Eds.]
SPIE,
MS 10, also p. 225 (1990)). A description of the theory is given by B. Pettinger in “Light Scattering by Adsorbates at Ag Particles; Quantum-Mechanical Approach for Energy Transfer Induced Interfacial Optical Processes Involving Surface Plasmons, Multipoles, and Electron-hole Pairs,”
J. Chem. Phys.,
85, 7442-7451 (1986). Briefly, incident laser photons couple to free conducting electrons within the metal which, confined by the particle surface, collectively cause the electron cloud to resonate. The resulting surface plasmon field provides an efficient pathway for the transfer of energy to the molecular vibrational modes of a molecule within the field, and thus generates Raman photons (see “Surface-Enhanced Raman Scattering; Section Four: Theory”, supra).
The described phenomenon occurs however only if the following three conditions are satisfied: (1) that the free-electron absorption of the metal can be excited by light of wavelength between 250 and 2500 nanometers (nm), preferably in the form of laser beams; (2) that the metal employed is of the appropriate size (normally 5 to 1000 nm diameter particles, or a surface of equivalent morphology), and has optical properties necessary for generating a surface plasmon field; and (3) that the analyte molecule has effectively matching optical properties (absorption) for coupling to the plasmon field (see Weaver,
J. Chem. Phys.,
82, 4867-4874 (1985), and Pettinger,
J. Chem. Phys.,
85, 7442-7451 (1986), supra). Although limited signal enhancement has been observed for the other coinage metals, such as nickel and platinum, as well as for alloys containing one or more of the coinage metals, as a practical matter the foregoing conditions restrict SERS to the Periodic Table Group IB metals, copper, gold, and silver, with diameters between 5 and 200 nm (see Pettinger,
J. Chem. Phys,
85, 7442-7451 (1986) supra, and Wang, D. -S., and Kerker, M., “Enhanced Raman Scattering by Molecules Adsorbed at the Surface of Colloidal Spheroids,”
Physical Review B.,
24, 1777-1790 (1981)). The SERS method has been used to measure the spectra of adenine on a silver-doped sol-gel coated glass substrate, and has achieved signal increases of six orders of magnitude, as shown by appended FIG.
3
(
c
).
Analyses for numerous chemicals and biochemical by SERS has been demonstrated using: (1) activated electrodes in electrolytic cells (see Lombardi, D. R., C. Wang, B. Sun, A. W. Fountain III, T. J. Vickers, C. K. Mann, F. R. Reich, J. G. Douglas, B. A. Crawford, and F. L. Kohlasch,
Appl. Spectrosc.
48, 875-833 (1994); Storey, J. M. E., Shelton, R. D., Barber, T. E., and Wachter, E. A., “Electrochemical SERS Detection of Chlorinated Hydrocarbons in Aqueous Solutions,”
Appl. Spectrosc.,
48, 1265-1271 (1994); Freeman, R. D., Hammaker, R. M., Meloan, C. E., and Fately, W. G., “A detector for liquid chromatography and flow injection analysis using SERS,”
Appl. Spectrosc.,
42, 456-460 (1988); Angel, S. M., L. F. Katz, D. D. Archibold, L. T. Lin, D. E. Honigs,
App. Spectrosc.
42, 1327 (1988); and Vo-Dinh, T., Stokes, D. L., Li, Y. S., and Miller, G. H., “Fiber-Optic Sensor Probe For In-Situ Surface-Enhanced Raman Monitoring,” SPIE, 1368, 203-209 (1990)); (2) activated silver and gold colloid reagents (see Berthod, A., J. J. Laserna, and J. D. Winefordner, “SERS on silver hydrosols studied by flow injection analysis”,
Appl. Spectrosc.
41, 1137-1141 (1987) 42, 456-460 (1988) and Angel, S. M., L. F. Katz, D. D. Archibold, L. T. Lin, D. E. Honigs,
Appl. Spectrosc.
42, 1327 (1988); and (3) activated silver and gold substrates (see Vo-Dinh, SPIE, 1368, 203-209 (1990), and Storey, J. M. E. Barber, T. E., Shelton, R. D., Wacher, E. A., Carron, K. T., and Jiang, Y. “Applications of Surface-Enhanced Raman Scattering (SERS) to Chemical Detection”,
Spectroscopy,
10(3), 20-25 (1995). None of the foregoing techniques is capable of providing quantitative measurements, however, and consequently SERS has not gained widespread use.
More specifically, the first technique referred to uses electrodes that are “roughened” by changing the applied potential between oxidation and reduction states; it is found that the desired metal surface features (roughness) cannot be reproduced faithfully from one procedure to the next, and the method is also limited to electrolyte solutions. In the second technique, colloids are prepared by reducing a metal salt solution to produce metal particles, which in turn form aggregates. Particle size and aggregate size are strongly influenced by initial chemical concentrations, temperature, pH, and rate of mixing, and again therefore the desired features are not reproducible; also, the method is limited to aqueous solutions. Finally, the third technique mentioned uses substrates that are prepared by depositing the desired metal onto a surface having the appropriate roughness characteristics. To permit the analysis, the sample is preferably dried on the surface to concentrate the analyte on the active metal, and once again replication is difficult to achieve; the colloids and substrates are further limited moreover in that the chemical interaction of the analyte and the SER-active metal is not reversible, thus precluding use of the materials for repeat measurements. The relative merits of the three methods described above, for preparing SER-active surfaces, have been further reviewed by K. L. Norrod, L. M. Sudnik, D. Rousell, and K. L. Rowlen in “Quantitative comparison of five SERS substrates: Sensitivity and detection limit,”
Appl. Spectrosc.,
51, 994-1001 (1997).
SUMMARY OF THE INVENTION
It is therefore the broad object of the present invention to provide a novel method for preparing a SERS-active material that, in ge
Farquharson Stuart
Lee Yuan-Hsiang
Nelson Chad
Dorman Ira S.
Real-Time Analyzers, Inc.
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