Method for quantitative surface-enhanced raman spectroscopy using a chemical reference

Abstract

A method for obtaining quantitative surface-enhanced Raman (SER) spectra that corrects for deficiencies of, and variations in, the materials and devices employed, especially the SER-active media utilized, employs a reference chemical, having an effective surface-enhanced Raman factor, of known concentration within the same SER experimental field of view as the analyte chemical being measured. Knowledge of the relative amounts of SER-scattering for the reference chemical and analyte chemical allows calculating the concentration of the latter to a high degree of accuracy and precision.

1 . A surface-enhanced Raman spectroscopic method for analysis of a solution containing an analyte chemical of unknown concentration, comprising the steps: providing a homogeneous test solution comprised of at least one analyte chemical, in an unknown concentration, and a reference chemical, having an effective surface-enhanced Raman factor, at a known concentration; providing a surface-enhanced Raman-active medium of selected composition; combining said test solution and said medium to provide a test sample; irradiating said test sample so as to produce, at a common field of view of said surface-enhanced Raman-active medium, a surface-enhanced Raman spectrum containing at least one band that is characteristic of said reference chemical and at least one band that is characteristic of said at least one analyte chemical; measuring the intensity of said at least one surface-enhanced Raman spectral band that is characteristic of said reference chemical in said test solution; measuring the intensity of said at least one surface-enhanced Raman spectral band that is characteristic of said at least one analyte chemical in said test solution; and utilizing said measured intensities, together with a known value of the intensity of said at least one surface-enhanced Raman spectral band, characteristic of said reference chemical at said known concentration, and a known value of the surface-enhanced Raman scattering efficiencies of said at least one analyte chemical and said reference chemical in selected concentrations and relative to one another to calculate said unknown concentration of said at least one analyte chemical.  
   
   
       2 . The method of  claim 1  wherein said reference chemical is of such composition that the intensity of its normal Raman response is increased by at least two orders of magnitude by the surface-enhanced Raman effect.  
   
   
       3 . The method of  claim 2  wherein said reference chemical produces surface-enhanced Raman spectral bands that do not substantially overlap said at least one characteristic surface-enhanced Raman spectral band of said at least one analyte chemical.  
   
   
       4 . The method of  claim 1  wherein the molecular size of said reference chemical is substantially smaller than the molecular size of said at least one analyte chemical.  
   
   
       5 . The method of  claim 4  wherein said molecular size of said reference chemical is such that it occupies no more than about one percent of the surface area of a metal constituting said surface-enhanced Raman active medium.  
   
   
       6 . The method of  claim 1  wherein said reference chemical comprises a thiocyanate or cyanide compound that is soluble in said test solution.  
   
   
       7 . The method of  claim 6  wherein said compound is a salt or an inorganic complex.  
   
   
       8 . The method of  claim 7  wherein said compound is selected from the group consisting of thiocyanate salts of sodium, potassium and calcium; cyanide salts of sodium, potassium and calcium; sodium ferrocyanide; potassium hexacyanoruthenate; and pentacyanoferrothiocyanate.  
   
   
       9 . The method of  claim 1  wherein said surface enhanced Raman-active medium comprises a metal selected from the group consisting of copper, gold, silver, nickel, and alloys and mixtures thereof.  
   
   
       10 . The method of  claim 9  wherein said metal is of particulate form, or is in the form of a surface having a morphology functionally equivalent to metal particles.  
   
   
       11 . The method of  claim 1  wherein said surface-enhanced Raman-active medium comprises a chemically synthesized porous structure.  
   
   
       12 . The method of  claim 11  wherein said surface-enhanced Raman-active medium comprises a sol-gel synthesized utilizing a silica-based, titania-based, or zirconia-based alkoxide, and at least one surface-enhanced Raman-active metal.  
   
   
       13 . The method of  claim 11  wherein the chemical reaction utilized to synthesize said porous structure comprises polymerization of at least one monomer that allows the inclusion of a surface-enhanced Raman-active metal.  
   
   
       14 . The method of  claim 1  wherein said surface-enhanced Raman-active medium comprises a mixture of a porous material and at least one surface-enhanced Raman-active metal.  
   
   
       15 . The method of  claim 14  wherein said porous material is effective to produce chemical separations or selective chemical extractions.  
   
   
       16 . The method of  claim 15  wherein said porous material is selected from the group consisting of sol-gels, silica gels, silica stabilized by zirconia, derivatized silica-based matrices, long-chain alkane particles, and derivatized long-chain alkane particles.  
   
   
       17 . The method of  claim 1  wherein collection of surface-enhanced Raman-scattered radiation, for making said measurements of Raman spectral band intensities, is effected from said common field of view of said surface-enhanced Raman-active medium, on an axis diametric to an axis of irradiation.  
   
   
       18 . The method of  claim 1  wherein said surface-enhanced Raman factor of said reference chemical, at said known concentration thereof, is of known value.  
   
   
       19 . A surface-enhanced Raman spectroscopic method comprising the steps: providing a homogeneous solution comprised of at least one analyte chemical and a reference chemical having an effective surface-enhanced Raman factor; providing a surface-enhanced Raman-active medium; combining said solution and said medium to provide a sample; irradiating said sample so as to produce, at a common field of view of said surface-enhanced Raman-active medium, a surface-enhanced Raman spectrum containing bands that are characteristic of each of said chemicals; measuring the intensity of at least one surface-enhanced Raman spectral band that is characteristic of said reference chemical; and utilizing said measured intensity as an indication of the Raman-enhancing effect of said surface-enhanced Raman-active medium.  
   
   
       20 . The method of  claim 19  wherein the concentration of said reference chemical in said solution is known and the concentration of said at least one analyte chemical in said solution is unknown; wherein the intensity of said at least one surface-enhanced Raman spectral band, characteristic of said reference chemical in said known concentration, is known; wherein the relative intensities of surface-enhanced Raman spectral bands characteristic of known concentrations of said analyte chemical and said reference chemical are known; and wherein said known reference chemical band intensity and said known relative intensities are utilized to calculate said unknown concentration of said at least one analyte chemical.  
   
   
       21 . A surface-enhanced Raman spectroscopic method for analysis of a test solution containing an analyte chemical in an unknown concentration, comprising the steps: providing a homogeneous test solution comprised of at least one analyte chemical having an effective surface-enhanced Raman factor, in an unknown concentration, and a Raman response-enhancing reference chemical in a known concentration; providing a surface-enhanced Raman-active medium of selected composition; combining said test solution and said medium to provide a test sample; irradiating said test sample so as to produce, at a common field of view of said surface-enhanced Raman-active medium, a surface-enhanced Raman spectrum containing at least one band that is characteristic of said reference chemical and at least one band that is characteristic of said at least one analyte chemical; measuring the intensity of said at least one surface-enhanced Raman spectral band that is characteristic of said reference chemical in said test solution; measuring the intensity of said at least one surface-enhanced Raman spectral band that is characteristic of said at least one analyte chemical in said test solution; dividing said measured intensity of said characteristic band of said at least one analyte chemical by said measured intensity of said characteristic band of said reference chemical to provide a ratio factor for eliminating the effects of parameters that cause variations in surface-enhanced Raman activity of said selected medium; and utilizing said factor to calculate said unknown concentration of said at least one analyte chemical.  
   
   
       22 . The method of  claim 21  wherein the surface-enhanced Raman scattering efficiencies of said reference chemical and of said at least one analyte chemical, relative to one another, are also utilized to calculate said unknown concentration of said at least one analyte chemical.  
   
   
       23 . The method of  claim 22  wherein, prior to carrying out said calculation of said unknown concentration, said relative scattering efficiencies are determined by measuring surface-enhanced Raman spectral band intensities of said reference chemical and said at least one analyte chemical using a standard sample containing a surface-enhanced Raman-active medium, having substantially said selected composition, and a standard solution containing known concentrations of said reference chemical and said at least one analyte chemical.  
   
   
       24 . The method of  claim 23  wherein said calculation of said unknown concentration is carried out by application of the equation:  
       [ AMeas ]=( I   SER   AMeas   /I   SER   RMeas )×( I   SER   RS   /I   SER   AS )×[ RMeas],   
     wherein AMeas stands for said at least one analyte chemical in said test solution, [AMeas] represents the concentration of said at least one analyte chemical in said test solution, I SER  stands for the measured intensity of the surface-enhanced Raman band used for said scattering efficiency determination, RMeas stands for said reference chemical in said test solution, [Rmeas] represents the concentration of said reference chemical in said test solution, RS stands for said reference chemical of known concentration in said standard solution, and AS stands for said at least one analyte chemical of known concentration in said standard solution; and wherein, the term (I SER   RS /I SER   AS ) provides said relative surface-enhanced Raman scattering efficiencies of said reference chemical and said at least one analyte chemical in said standard solution.  
   
   
       25 . A surface-enhanced Raman spectrographic method for analysis of a test solution containing an analyte chemical having an unknown property, comprising the steps: providing a reference solution comprised of a reference chemical having an effective surface-enhanced Raman factor, in a known concentration; providing a surface enhanced Raman-active medium of selected composition; combining said reference solution and said Raman-active medium to provide a reference sample; irradiating said reference sample so as to produce a surface-enhanced Raman spectrum containing at least one band that is characteristic of said reference chemical; measuring the intensity of said at least one characteristic surface-enhanced Raman spectral band; providing a homogeneous test solution comprised of said reference chemical and at least one analyte chemical having an unknown property; combining said test solution with a surface-enhanced Raman-active medium having substantially said selected composition, to provide a test sample; irradiating said test sample so as to produce, at a common field of view of said surface-enhanced Raman-active medium of substantially said selected composition, a surface-enhanced Raman spectrum containing at least one band that is characteristic of each of said reference chemical and said at least one analyte chemical; measuring the intensities of said characteristic bands of said chemicals in said test solution; and analyzing said test solution, to determine the unknown property of said analyte chemical, utilizing said intensity of said at least one spectral band, characteristic of said reference chemical and measured from said reference sample, as a measure of the Raman-enhancing scattering effect of said medium in said test sample.