Surface-enhanced Raman scattering substrate for fuel oil analysis

Abstract

A surface-enhanced Raman scattering (SERS) substrate is provided. The SERS substrate includes a transparent substrate and a nanocomposite composition. The nanocomposite composition includes a silver-loaded silica (AgSiO.sub.2) nanocomposite having a silica core and a silver/silica shell disposed around the silica core and a zeolitic material having a nano porous structure. The silver/silica shell contains silver nanoparticles uniformly distributed therein. The AgSiO.sub.2 nanocomposite is uniformly disposed on a surface of the zeolitic material. The nanoparticles of the AgSiO.sub.2 nanocomposite are spherical and have a mean particle size of 100 to 500 nanometers (nm). A method of obtaining a Raman spectrum of a sulfur-containing compound in a mixing composition is also provided.

Claims

1. A surface-enhanced Raman scattering (SERS) substrate for fuel oil analysis, comprising: a transparent substrate; and a nanocomposite composition comprising: a silver-loaded silica (AgSiO.sub.2) nanocomposite having a silica core and a silver/silica shell disposed around the silica core; and a zeolitic material having a nano porous structure; wherein the silver/silica shell contains silver nanoparticles uniformly distributed therein; wherein the AgSiO.sub.2 nanocomposite is uniformly disposed on a surface of the zeolitic material; wherein the nanoparticles of the AgSiO.sub.2 nanocomposite are spherical and have a mean particle size of 250 to 350 nanometers (nm); wherein the silver/silica shell of the AgSiO.sub.2 nanocomposite has a mean thickness of 50 to 350 nm and the silver nanoparticles in the silver/silica shell of the AgSiO.sub.2 nanocomposite have a mean particle size of 10 to 50 nm; and wherein the SERS substrate has a specific surface area in a range of 150 to 300 m.sup.2/g, a cumulative specific pore volume in a range of 0.2 to 0.3 cm.sup.3/g, and an average pore diameter of 4 to 7 nm.

2. The SERS substrate of claim 1, wherein the silver nanoparticles are at least one selected from the group consisting of nanospheres, nanorods, nanostars, nanotriangles, nanoprisms, nanocubes, nanofibers, nanoplates, nanowires, nanotetrahedrons, nanocrystals, nanohexagons, nanodisks, nanoribbons, nanocylinders, nanogranules, nanowhiskers, nanoflakes, nanofoils, nanopowders, nanoholes, nanobelts, nanourchins, nanoflowers, nanoislands, and nanomeshes.

3. The SERS substrate of claim 1, wherein the AgSiO.sub.2 nanocomposite has: a specific surface area in a range of 10 to 100 square meter per gram (m.sup.2/g); and a cumulative specific pore volume in a range of 0.1 to 0.15 cubic centimeter per gram (cm.sup.3/g).

4. The SERS substrate of claim 1, wherein the AgSiO.sub.2 nanocomposite comprises from about 5 to 25 weight percentage (wt. %) of silver.

5. The SERS substrate of claim 1, wherein a weight ratio of the silver-loaded silica nanocomposite to the zeolitic material in the nanocomposite ranges from about 1:5 to 5:1.

6. The SERS substrate of claim 1, wherein the zeolitic material has a silicon-to-aluminum molar ratio of greater than 10:1.

7. The SERS substrate of claim 1, wherein the zeolitic material has: a specific surface area in a range of 300 to 400 m.sup.2/g; a cumulative specific pore volume in a range of 0.15 to 0.2 cm.sup.3/g; and an average pore diameter of 3 to 6 nm.

8. The SERS substrate of claim 1, has a detection limit of 110.sup.9 molar (M) for a sulfur-containing compound.

9. The SERS substrate of claim 1, wherein the transparent substrate comprises a glass substrate, and wherein the glass substrate is at least one selected from the group consisting of a fluorine doped tin oxide (FTO) coated glass substrate, a tin doped indium oxide (ITO) coated glass substrate, an aluminum doped zinc oxide (AZO) coated glass substrate, a niobium doped titanium dioxide (NTO) coated glass substrate, an indium doped cadmium oxide (ICO) coated glass substrate, an indium doped zinc oxide (IZO) coated glass substrate, a fluorine doped zinc oxide (FZO) coated glass substrate, a gallium doped zinc oxide (GZO) coated glass substrate, an antimony doped tin oxide (ATO) coated glass substrate, a phosphorus doped tin oxide (PTO) coated glass substrate, a zinc antimonate coated glass substrate, a zinc oxide coated glass substrate, a ruthenium oxide coated glass substrate, a rhenium oxide coated glass substrate, a silver oxide coated glass substrate, and a nickel oxide coated glass substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A more complete appreciation of this disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

(2) FIG. 1A is a schematic flow chart of a method of forming a surface-enhanced Raman scattering (SERS) substrate containing a silver-loaded silica/H-ZSM-5 nanocomposite, according to certain embodiments of the present disclosure;

(3) FIG. 1B is a schematic flow chart of a method of obtaining a Raman spectrum of an analyte in a mixing composition, according to certain embodiments of the present disclosure;

(4) FIG. 2A is a scanning electron microscopy (SEM) image of silver-loaded silica (AgSiO.sub.2) nanoparticles at magnification of 1 micrometer (m), according to certain embodiments of the present disclosure;

(5) FIG. 2B is a SEM image of the AgSiO.sub.2 nanoparticles at magnification of 500 nanometers (nm), according to certain embodiments of the present disclosure;

(6) FIG. 2C is a SEM image of a zeolitic material, Z-150, at magnification of 1 m, according to certain embodiments of the present disclosure;

(7) FIG. 2D is a SEM image of the Z-150 at magnification of 500 nm, according to certain embodiments of the present disclosure;

(8) FIG. 2E is a SEM image of AgSiO.sub.2-Z-150 nanocomposite at magnification of 1 m, according to certain embodiments of the present disclosure;

(9) FIG. 2F is a SEM image of the AgSiO.sub.2-Z-150 nanocomposite at magnification of 500 nm, according to certain embodiments of the present disclosure;

(10) FIG. 3A is a graph depicting energy dispersive spectroscopy (EDS) analysis of the AgSiO.sub.2, according to certain embodiments of the present disclosure;

(11) FIG. 3B is a graph depicting EDS analysis of the Z-150, according to certain embodiments of the present disclosure;

(12) FIG. 3C is a graph depicting EDS analysis for the AgSiO.sub.2-Z-150 nanocomposite, according to certain embodiments of the present disclosure;

(13) FIG. 4A is a graph depicting Fourier transform infrared (FT-IR) spectra of the AgSiO.sub.2 nanoparticles, the Z-150 and the AgSiO.sub.2-Z-150 nanocomposite, according to certain embodiments of the present disclosure;

(14) FIG. 4B is a graph depicting powder X-ray diffraction (P-XRD) patterns of the AgSiO.sub.2 nanoparticles, the Z-150, and the AgSiO.sub.2-Z-150 nanocomposite, according to certain embodiments of the present disclosure;

(15) FIG. 5 is a graph depicting Brunauer-Emmett-Teller (BET) isotherms resulting from N.sub.2 physisorption on the AgSiO.sub.2 nanoparticles, the Z-150, and the AgSiO.sub.2-Z-150 nanocomposite, according to certain embodiments of the present disclosure;

(16) FIG. 6A is a graph depicting Raman spectra of pure dimethyl sulfoxide (DMSO) compared with dibenzothiophene (DBT) analyte (0.1 molar (M)) without (normal Raman) and with (SERS) the AgSiO.sub.2 nanoparticles, both in the DMSO medium, according to certain embodiments of the present disclosure;

(17) FIG. 6B is a graph depicting SERS spectra the DBT analyte, at various molar concentrations, using the AgSiO.sub.2 nanoparticles in the DMSO solvent, according to certain embodiments of the present disclosure;

(18) FIG. 6C is a graph depicting a linear correlation of different concentrations of the DBT with SERS intensities corresponding to SERS peaks appearing at 1602 cm.sup.1, 1556 cm.sup.1 and 1476 cm.sup.1, using the AgSiO.sub.2 nanoparticles in the DMSO solvent, according to certain embodiments of the present disclosure;

(19) FIG. 7A is a graph depicting suppression of fluorescence background in diesel samples using various Si/Al.sub.2 ratios of H-ZSM-5 (Z-30, Z-80, Z-150 and Z-280), according to certain embodiments of the present disclosure;

(20) FIG. 7B shows Raman spectra of pure diesel compared with the DBT analyte (0.1 M) without (normal Raman), with the (SERS) AgSiO.sub.2 nanoparticles, with the Z-150, and with the (SERS) AgSiO.sub.2-Z-150 nanocomposite, all in diesel medium, according to certain embodiments of the present disclosure;

(21) FIG. 8A is a graph depicting SERS spectra of diesel with Z-150, Z-150 spiked with DBT analyte, and the AgSiO.sub.2-Z-150 nanocomposite spiked with the DBT analyte, according to certain embodiments of the present disclosure;

(22) FIG. 8B is a SERS spectra in the region 900-1800 cm.sup.1 for various molar concentrations of the DBT analyte with the AgSiO.sub.2-Z-150 nanocomposite, in diesel, according to certain embodiments of the present disclosure;

(23) FIG. 8C is a graph depicting a linear correlation between the concentration of the DBT analyte in diesel sample with SERS intensities, according to certain embodiments of the present disclosure;

(24) FIG. 9 is a bar graph depicting reproducibility of the SERS response in diesel with 0.1 M DBT using the AgSiO.sub.2-Z-150 nanocomposite, according to certain embodiments of the present disclosure;

(25) FIG. 10 is a graph depicting reproducibility of SERS responses with 0.1 M DBT spiked in the AgSiO.sub.2-Z-150 composite, in diesel, at 1633 cm.sup.1 band, according to certain embodiments of the present disclosure;

(26) FIG. 11A is an optimized geometrical structure of the DBT, according to certain embodiments of the present disclosure;

(27) FIG. 11B is a graph depicting calculated Raman spectrum of the DBT performed at the B3LY P/6-311g (d, p) level of theory, according to certain embodiments of the present disclosure; and

(28) FIG. 12 is a graph depicting the method of forming the SERS substrate containing the silver-loaded silica/H-ZSM-5 nanocomposite, according to certain embodiments of the present disclosure.

DETAILED DESCRIPTION

(29) In the drawings, like reference numerals designate identical or corresponding parts throughout the several views. Further, as used herein, the words a, an and the like generally carry a meaning of one or more, unless stated otherwise.

(30) Furthermore, the terms approximately, approximate, about, and similar terms generally refer to ranges that include the identified value within a margin of 20%, 10%, or preferably 5%, and any values there between.

(31) Aspects of the present disclosure are directed to a nanocomposite composed of silver-loaded silica nanoparticles, and a zeolitic material for use as a surface-enhanced Raman scattering (SERS) substrate. The substrate of the present disclosure is sensitive and is effective in the detection of sulfur-containing compounds in fuel samples, such as petrol and diesel. Although the description provided herein refers to the use of the substrate for detecting a class of sulfur containing compounds, such as dibenzothiophene (DBT), it may be understood by a person skilled in the art, that aspects of the present disclosure may be used for identification of other sulfur-containing compounds having similar chemical structures as well, albeit with a few variations, as may be obvious to a person skilled in the art. Compounds containing other chalcogenides such as selenium and tellurium may also be detected using the SERS substrate described herein. The silver nanoparticles in the nanocomposite enhances Raman signals associated with the DBT molecules, while the zeolitic material helps in reducing a fluorescence background, resulting in a low detection level (up to 10.sup.7 molar (M)).

(32) According to an aspect of the present disclosure, a SERS substrate is described. The SERS substrate includes a transparent substrate and a nanocomposite composition. The transparent substrate or the substrate includes a glass substrate. In some embodiments, the glass substrate is at least one selected from the group consisting of a fluorine doped tin oxide (FTO) coated glass substrate, a tin doped indium oxide (ITO) coated glass substrate, an aluminum doped zinc oxide (AZO) coated glass substrate, a niobium doped titanium dioxide (NTO) coated glass substrate, an indium doped cadmium oxide (ICO) coated glass substrate, an indium doped zinc oxide (IZO) coated glass substrate, a fluorine doped zinc oxide (FZO) coated glass substrate, a gallium doped zinc oxide (GZO) coated glass substrate, an antimony doped tin oxide (ATO) coated glass substrate, a phosphorus doped tin oxide (PTO) coated glass substrate, a zinc antimonate coated glass substrate, a zinc oxide coated glass substrate, a ruthenium oxide coated glass substrate, a rhenium oxide coated glass substrate, a silver oxide coated glass substrate, and a nickel oxide coated glass substrate.

(33) In certain embodiments, the substrate may have a thickness of less than or equal to about 3 mm, for example, ranging from about 0.1 mm to about 2.5 mm, from about 0.3 mm to about 2 mm, from about 0.7 mm to about 1.5 mm, or from about 1 mm to about 1.2 mm, including all ranges and subranges therebetween. Other ranges are also possible.

(34) The nanocomposite composition includes a silver-loaded silica (AgSiO.sub.2) nanocomposite having a silica core, and one or more layers of silver/silica shell disposed around the silica core. In some embodiments, the AgSiO.sub.2 nanocomposite includes silver in the form of silver ions and silver nanoparticles. In some further embodiments, the silver nanoparticles dispersed in the A g-SiO.sub.2 nanocomposite are uniformly dispersed/distributed in the silver/silica shell with little or no aggregation. In some preferred embodiments, this may provide an advantage since agglomeration or aggregation of the AgSiO.sub.2 nanocomposite can affect the properties, including transparency, of the SERS substrate. In some further preferred embodiments, the silver nanoparticles of the AgSiO.sub.2 nanocomposite are uniformly distributed in the silver/silica shell. In some most preferred embodiments, the weight percentage of silver in AgSiO.sub.2 nanocomposite is about 5-25 wt. %, preferably about 7.5 to 22.5 wt. %, preferably about 10 to 20 wt. %, preferably about 12.5 to 17.5 wt. %, or even more preferably about 15-20 wt. %. Other ranges are also possible.

(35) In some embodiments, the silver nanoparticles can be in the form of nanospheres, nanorods, nanostars, nanotriangles, nanoprisms, nanocubes, nanofibers, nanoplates, nanowires, nanotetrahedrons, nanocrystals, nanohexagons, nanodisks, nanoribbons, nanocylinders, nanogranules, nanowhiskers, nanoflakes, nanofoils, nanopowders, nanoholes, nanobelts, nanourchins, nanoflowers, nanoislands, and nanomeshes. In a preferred embodiment, the nanoparticles are spherical and have a mean particle size of 100 to 500 nm, preferably 150 to 450 nm, preferably 200 to 400 nm, preferably 250 to 350 nm, and yet more preferably about 300 nm. In some further preferred embodiments, the nanoparticles have a mean particle size of 10 to 50 nm, more preferably 20 to 40 nm, and yet more preferably 25 to 35 nm. Other ranges are also possible.

(36) The AgSiO.sub.2 nanocomposite may be amorphous or crystalline. In some embodiments, it is preferred that the AgSiO.sub.2 nanocomposite are amorphous in nature. In some further embodiments, the silica core of the AgSiO.sub.2 nanocomposite has a mean diameter of 50 to 150 nanometers (nm), more preferably 80 to 130 nm, and yet more preferably 100 to 120 nm. In some more preferred embodiments, the silver/silica shell of the AgSiO.sub.2 nanocomposite has a mean thickness of 50 to 350 nm, more preferably 100 to 300 nm, yet more preferably 150 to 250 nm, and even more preferably about 200 nm. Other ranges are also possible.

(37) The AgSiO.sub.2 nanocomposite can be prepared by any of the methods conventionally known in the art; however, it is desirable for the AgSiO.sub.2 nanocomposite to possess the following properties: a specific surface area in a range of 10 to 100 square meter per gram (m.sup.2/g), more preferably 20 to 80 m.sup.2/g, and yet more preferably 30 to 75 m.sup.2/g; a cumulative specific pore volume in a range of 0.1 to 0.15 cubic centimeter per gram (cm.sup.3/g), more preferably 0.12 to 0.14 cm.sup.3/g; and an average pore diameter of 6 to 10 nm, more preferably 7 to 9 nm, and yet more preferably 8 to 8.5 nm.

(38) The nanocomposite composition further includes a zeolitic material having a nano-porous structure. In some embodiments, the AgSiO.sub.2 nanocomposite is uniformly disposed on a surface of the zeolitic material. In some further embodiments, a weight ratio of the AgSiO.sub.2 nanocomposite to the zeolitic material in the nanocomposite ranges from about 1:5 to 5:1, preferably about 1:3 to 3:1, or more preferably about 1:1. Zeolites are alumino-silicate nanoporous, crystalline solids having regular arrays of molecule-sized nanopores that allow for shape- and size-selective adsorption, diffusion, and reaction of adsorbed guest molecules. The zeolites are generally shown by xM.sub.2/n O.Math.Al.sub.2O.sub.3.Math.ySiO.sub.2.Math.zH.sub.2O, where M represents an ion-exchangeable metal ion, which is usually the ion of a monovalent or divalent metal; n corresponds to the valence of the metal; x is a coefficient of the metal oxide; y is a coefficient of silica; and z is the number of water of crystallization. Various kinds of zeolites having different component ratio, fine pore diameter, and specific surface area are known. In some embodiments, the zeolitic material of the present disclosure has a specific surface area in a range of 300 to 400 m.sup.2/g, more preferably 320 to 380 m.sup.2/g; a cumulative specific pore volume in a range of 0.15 to 0.2 cm.sup.3/g, more preferably 0.16 to 0.18 cm.sup.3/g; and an average pore diameter of 3 to 6 nm, more preferably 4 to 5.5 nm. Other ranges are also possible.

(39) One of the key factors that may affect the properties of the SERS substrate is the ratio of silica to alumina in the zeolitic material. In some embodiments, the zeolitic material is Zeolite Socony Mobil-5 (ZSM-5). H-ZSM-5 (Z) is a class of zeolites composed of several pentasil units linked together by oxygen bridges to form the pentasil chains. Such a class is characterized with a high silicon-to-aluminum ratio. In an embodiment, the weight ratios of silica to alumina (Si/Al.sub.2) in ZSM-5 are greater than 10:1. More specifically, weight ratios of silica to alumina in the ZSM-5 are in a range of 30:1 to 280:1, more specifically, 30:1, 80:1, 150:1, and 280:1. In a preferred embodiment, the weight ratio of silica to alumina in ZSM-5 is 150:1. Other ranges are also possible.

(40) The crystalline structures of the zeolitic material, the AgSiO2 nanocomposite, and a silver-loaded silica/zeolite nanocomposite may be characterized by powder X-ray diffraction (P-XRD), respectively. In some embodiments, the XRD patterns are collected in a Bruker D8 Advance diffractometer equipped with a Cu-K radiation source (=0.15406 nm) for a 2 range extending between 5 and 80, preferably 15 and 70, further preferably 30 and 60 at an angular rate of 0.005 to 0.04 s1, preferably 0.01 to 0.03 s1, or even preferably 0.02 s1. In some embodiments, the zeolitic material has at least a first intense peak with a 2 theta (0) value in a range of 5 to 15 in a powder X-ray diffraction (P-XRD) spectrum, as depicted in FIG. 4B. In some embodiments, the zeolitic material has at least a second intense peak with a 20 value in a range of 20 to 30 in the P-XRD spectrum, as depicted in FIG. 4B. Other ranges are also possible.

(41) In some embodiments, the AgSiO.sub.2 nanocomposite has at least a first intense peak with a 2 theta (0) value in a range of 25 to 35 in a powder X-ray diffraction (P-XRD) spectrum, as depicted in FIG. 4B. In some preferred embodiments, the AgSiO.sub.2 nanocomposite has at least a second intense peak with a 20 value in a range of 40 to 50 in the P-XRD spectrum, as depicted in FIG. 4B. Other ranges are also possible.

(42) In some further embodiments, the silver-loaded silica/zeolite nanocomposite has at least a first intense peak with a 2 theta () value in a range of 5 to 15 in a powder X-ray diffraction (P-XRD) spectrum, as depicted in FIG. 4B. In some preferred embodiments, the silver-loaded silica/zeolite nanocomposite has at least a second intense peak with a 20 value in a range of 20 to 30 in the P-XRD spectrum, as depicted in FIG. 4B. In some further preferred embodiments, the silver-loaded silica/zeolite nanocomposite has at least a third intense peak with a 20 value in a range of 30 to 35 in the P-XRD spectrum, as depicted in FIG. 4B. Other ranges are also possible.

(43) Additionally, the structures of the zeolitic material, the AgSiO.sub.2 nanocomposite, and the silver-loaded silica/zeolite nanocomposite are also characterized by the Raman spectroscopy as depicted in FIG. 4A. Raman spectra over the range of 200 to 3900 cm.sup.1 were obtained by using a HORIBA Scientific LabRAM HR Evolution Raman spectrometer. A HeNe laser source equipped with two gratings (600 and 1800) with automated switching, and a CCD detector working at 633 nm excitation wavelength with 100% laser power was used for the measurements. An acquisition time of 25 s with 2 accumulations was set for the Raman spectra collection for bare and modified hematite films. In some embodiments, the zeolitic material has at least a first intense peak in a range of 800 to 1200 cm.sup.1, as depicted in FIG. 4A. In some embodiments, the zeolitic material has at least a second intense peak in a range of 2600 to 2900 cm.sup.1, as depicted in FIG. 4A. Other ranges are also possible.

(44) In some further embodiments, the AgSiO.sub.2 nanocomposite has at least a first intense peak in a range of 450 to 750 cm.sup.1, as depicted in FIG. 4A. In some preferred embodiments, the A g-SiO.sub.2 nanocomposite has at least a second intense peak in a range of 800 to 1500 cm.sup.1, as depicted in FIG. 4A. Other ranges are also possible.

(45) In some further preferred embodiments, the silver-loaded silica/zeolite nanocomposite has at least a first intense peak in a range of 450 to 750 cm.sup.1, as depicted in FIG. 4A. In some more preferred embodiments, the silver-loaded silica/zeolite nanocomposite has at least a second intense peak in a range of 800 to 1500 cm.sup.1, as depicted in FIG. 4A. In some most preferred embodiments, the silver-loaded silica/zeolite nanocomposite has at least a third intense peak in a range of 2600 to 3000 cm.sup.1, as depicted in FIG. 4A. Other ranges are also possible.

(46) Moreover, the structures of the zeolitic material, the AgSiO.sub.2 nanocomposite, and the silver-loaded silica/zeolite nanocomposite are also characterized by nitrogen adsorption/desorption measurements. In some embodiments, the nitrogen adsorption/desorption measurements are collected in a Micromeritics Tristar 3000, and results (e.g., surface area and pore size) are calculated using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJ H) methods. In some embodiments, the zeolitic material has a specific surface area in a range of 300 to 420 m.sup.2/g, more preferably 320 to 400 m.sup.2/g, and yet more preferably 340 to 380 m.sup.2/g, as depicted in FIG. 5. In some further embodiments, the zeolitic material has a cumulative specific pore volume in a range of 0.1 to 0.25 cm.sup.3/g, preferably 0.15 to 0.2 cm.sup.3/g, or more preferably 0.16 to 0.18 cm.sup.3/g, as depicted in FIG. 5. In some preferred embodiments, the zeolitic material has an average pore diameter of 3 to 6 nm, preferably 3.5 to 5.5 nm, preferably 4 to 5 nm, or even more preferably about 4.5 nm, as depicted in FIG. 5. Other ranges are also possible.

(47) In some embodiments, the silver-loaded silica/zeolite nanocomposite has a specific surface area in a range of 180 to 260 m.sup.2/g, more preferably 200 to 240 m.sup.2/g, and yet more preferably 210 to 230 m.sup.2/g, as depicted in FIG. 5. In some further embodiments, the silver-loaded silica/zeolite nanocomposite has a cumulative specific pore volume in a range of 0.15 to 0.35 cm.sup.3/g, preferably 0.2 to 0.3 cm.sup.3/g, or more preferably 0.23 to 0.27 cm.sup.3/g, as depicted in FIG. 5. In some preferred embodiments, the silver-loaded silica/zeolite nanocomposite has an average pore diameter of 3 to 9 nm, preferably 4 to 8 nm, preferably 5 to 7 nm, or even more preferably about 6 nm, as depicted in FIG. 5. Other ranges are also possible.

(48) In some embodiments, the AgSiO.sub.2 nanocomposite has a specific surface area in a range of 10 to 80 m.sup.2/g, more preferably 25 to 65 m.sup.2/g, and yet more preferably 40 to 50 m.sup.2/g, as depicted in FIG. 5. In some further embodiments, the AgSiO.sub.2 nanocomposite has a cumulative specific pore volume in a range of 0.1 to 0.2 cm.sup.3/g, preferably 0.11 to 0.15 cm.sup.3/g, or more preferably 0.12 to 0.13 cm.sup.3/g, as depicted in FIG. 5. In some preferred embodiments, the AgSiO.sub.2 nanocomposite has an average pore diameter of 6 to 12 nm, preferably 7 to 11 nm, preferably 8 to 10 nm, or even more preferably about 9 nm, as depicted in FIG. 5. Other ranges are also possible.

(49) The nanocomposite composition of the present disclosure serves as an effective SERS substrate for sensitive detection of sulfur containing compounds in fuels and/or other hydrocarbon-based matrices. Suitable examples of sulfur-containing compounds include, but are not limited to, thiophene, DBT, benzothiophene, 2-methylbenzothiophene, 2,4-dimethylthiophene, 3-methylthiophene, 2-methylthiophene, 4,6-dimethyldibenzothiophene, 2,4,6-trimethyldibenzothiophene, and 2,3,4,7-tetramethylbenzothiophene, or a combination thereof. In a preferred embodiment, the SERS substrate of the present disclosure is effective in detection of DBT, with a detection limit of 110.sup.9 molar (M).

(50) In some embodiments, the SERS substrate has a specific surface area in a range of 150 to 300 m.sup.2/g, more preferably 180 to 250 m.sup.2/g, and yet more preferably 200 to 230 m.sup.2/g. In some further embodiments, the SERS substrate has a cumulative specific pore volume in a range of 0.2 to 0.3 cm.sup.3/g, preferably 0.22 to 0.28 cm.sup.3/g, or more preferably 0.24 to 0.26 cm.sup.3/g. In some embodiments, the SERS substrate has an average pore diameter of 4 to 7 nm, preferably 4.5 to 6.5 nm, preferably 5 to 6 nm, or even more preferably about 5.5 nm. Other ranges are also possible.

(51) FIG. 1A illustrates a schematic flow chart of a method 100A of forming the SERS substrate. The method includes preparing the nanocomposite composition by following steps 102A-112A. The order in which the method 100A is described is not intended to be construed as a limitation, and any number of the described method steps can be combined in any order to implement the method 100A. Additionally, individual steps may be removed or skipped from the method 100A without departing from the spirit and scope of the present disclosure.

(52) At step 102A, the method 100A includes mixing a first silica precursor compound, an alcohol, and an ammonia solution (such as ammonium hydroxide) to form a silica mixture. In some embodiments, a volume ratio of the first silica precursor compound to the alcohol is in a range of 1:1 to 8:1, preferably 2:1 to 7:1, preferably 3:1 to 6:1, or even more preferably 4:1 to 5:1. A volume ratio of the first silica precursor compound to the ammonia solution is in a range of 2:1 to 8:1, preferably 3:1 to 7:1, preferably 4:1 to 6:1, or even more preferably about 5:1. Other ranges are also possible.

(53) In some embodiments, the first silica precursor compound includes tetraethyl orthosilicate (TEOS), and tetramethyl orthosilicate (TMOS). In some further embodiments, the alcohol may include methanol, ethanol, n-propyl alcohol, isopropyl alcohol, and t-butyl alcohol. In some preferred embodiments, the alcohol has a formula (I)

(54) ##STR00002## where R.sub.1 and R.sub.2 are each independently selected from the group consisting of hydrogen, alkanes, alkenes, alkynes, cyclic alkanes, cyclic alkenes, cyclic alkynes, and aromatics having in the range of 1 to 20 carbon atoms. The ammonia solution has a concentration of 0.5 to 2 M, more preferably 0.8 to 1.8 M, and yet more preferably 1 to 1.5 M. Other ranges are also possible.

(55) At step 104A, the method 100A includes sonicating a second silica precursor compound with the silica mixture and mixing to form a modified silica mixture. In some embodiments, the second silica precursor compound includes alkoxysilane compound. As used herein, the term alkoxysilane compound refers to the silane compound which includes alkoxy radicals. The alkoxysilane compound is at least one selected from the group consisting of trimethoxy (octadecyl) silane, octadecyltrichlorosilane and octyldimethylchlorosilane. In some embodiments, the alkoxysilane compound may include Si(OMe).sub.4, MeSi(OMe).sub.3, MeSi(OCH.sub.2CH.sub.2OMe).sub.3, ViSi(OMe).sub.3, PhSi(OMe).sub.3 and PhSi(OCH.sub.2CH.sub.2OMe).sub.3. A volume ratio of the first silica precursor compound to the second silica precursor compound is in a range of 1:3 to 3:1, preferably 1:2 to 2:1, or even more preferably about 1:1. Other ranges are also possible.

(56) At step 106A, the method 100A includes dropwise adding a silver salt solution to the modified silica mixture and cooling to form a crude mixture containing a silver-loaded silica nanocomposite precipitate. The silver-loaded silica nanocomposite precipitate can be obtained by performing centrifugation in a range of 9,000-11,000 rotations per minute (rpm), preferably 9,500-10,500 rpm, more preferably about 10,000 rpm for about 5-90 minutes, preferably about 15-80 minutes, preferably about 25-70 minutes, preferably about 35-60 minutes, or even more preferably about 45 minutes at 5-30 C., preferably 5-20 C., or even more preferably about 10 C. In some embodiments, the silver salt is at least one salt selected from the group consisting of silver nitrate, silver sulfate, silver carbonate and silver chloride. In some embodiments, the silver salt may include silver 2,4-pentanedionate, silver acetate, silver benzoate hydrate, silver bis(trifluoromethylsulfonyl)imide, silver bromide, silver chromate, silver cyanate, silver cyclohexanebutyrate, silver diethyldithiocarbamate, silver heptafluorobutyrate, silver hexafluoroantimonate, and silver hexafluoroarsenate. The zeolitic precursor compound includes ammonium zeolite having a silicon-to-aluminum molar ratio of greater than 10:1. Other ranges are also possible.

(57) At step 108A, the method 100A includes separating the silver-loaded silica nanocomposite precipitate from the crude mixture and drying to form the silver-loaded silica (AgSiO.sub.2) nanocomposite. The separation of the silver-loaded silica nanocomposite precipitate from the crude mixture can be done by using any method used or known in the prior art.

(58) At step 110A, the method 100A further includes calcinating a zeolitic precursor compound at a temperature of at least 500 C. to form the zeolitic material. Calcination of the zeolitic precursor compound can be performed for 1-12 hours, preferably 3-9 hours, or even more preferably 5-7 hours at a temperature in a range of 300-1000 C., preferably 400-800 C., or even more preferably 500-600 C. Other ranges are also possible.

(59) At step 112A, the method 100A further includes mixing the silver-loaded silica nanocomposite with the zeolitic material to form the nanocomposite composition. In some embodiments, a weight ratio of the silver-loaded silica nanocomposite to the zeolitic material in the nanocomposite composition ranges from about 1:5 to 5:1, preferably about 1:4 to 4:1, preferably about 1:3 to 3:1, preferably about 1:2 to 2:1, or even more preferably about 1:1. Other ranges are also possible.

(60) FIG. 1B illustrates a schematic flow chart of a method 100B of obtaining a Raman spectrum of an analyte in a mixing composition. The order in which the method 100B is described is not intended to be construed as a limitation, and any number of the described method steps can be combined in any order to implement the method 100B. Additionally, individual steps may be removed or skipped from the method 100B without departing from the spirit and scope of the present disclosure.

(61) At step 102B, the method 100B includes contacting the mixing composition containing the analyte with the SERS substrate to form a sample. The mixing composition includes hydrocarbons having atmospheric boiling points that match the atmospheric boiling points of at least one of gasoline, kerosene, and diesel fuel. In some embodiments, the hydrocarbons may include benzene and naphthalene derivatives, dodecane, nonane, decane, and undecane. In some further embodiments, the hydrocarbons may also contain one or more sulfur-containing compounds, that form the analyte. Suitable examples of the analyte include, but are not limited to, thiophene, DBT, benzothiophene, 2-methylbenzothiophene, 2,4-dimethylthiophene, 3-methylthiophene, 2-methylthiophene, 4,6-dimethyldibenzothiophene, 2,4,6-trimethyldibenzothiophene, and 2,3,4,7-tetramethylbenzothiophene, or a combination thereof. In a preferred embodiment, the analyte is DBT. In some most preferred embodiments, the analyte has a Raman scattering signal that is enhanced relative to that of the analyte without contacting with the SERS substrate.

(62) At step 104B, the method 100B includes exposing the sample to laser light such that a portion of the laser light is scattered by the sample to form scattered light. In some embodiments, the laser light has a wavelength of 600 to 650 nm, more preferably 620 to 640 nm for detecting the sulfur-containing compound with a detection limit of 110.sup.9 M. Other ranges are also possible.

(63) At step 106B, the method 100B includes detecting the scattered light. In some embodiments, the scattered light may be detected by using various software or methods known in the art such as nephelometry, or turbidimetry. The intensity of the scattered light is indicative of the concentration of the analyte in the sample. In other words, a higher intensity of scattered light is indicative of a greater concentration of the analyte in the sample. The SERS substrate of the present disclosure may be used for detection of sulfur-containing compounds including DBT.

EXAMPLES

(64) The following examples demonstrate exemplary embodiments of the SERS substrate described herein. The examples are provided solely for illustration and are not to be construed as limitations of the present disclosure, as many variations thereof are possible without departing from the spirit and scope of the present disclosure.

Example 1: Chemicals and Reagents

(65) Silver nitrate (CAS Number: 7761-88-8, Purity>99%), tetraethyl orthosilicate (CAS Number: 78-10.sup.4, Purity>98%), trimethoxy (octadecyl) silane (CAS Number: 3069-42-9, Purity>96%), chloroform (CAS Number: 67-66-3, Purity>99.5%), ethanol (CAS Number: 64-17-5, Purity>99.9%), dimethyl sulfoxide (CAS Number: 67-68-5, Purity>99.9%), dibenzothiophene (CAS Number: 132-65-0, Purity>99%) were purchased from Sigma-Aldrich and used without further purification. Diesel oil (D2 Grade) was used for the analysis. ZSM-5 (ammonia form) material was purchased from Zeolyst International.

Example 2: Synthesis of AgSiO.SUB.2 .Nanocomposites

(66) 50 milliliters (mL) tetraethyl orthosilicate, as a silane agent for the silica nanoparticles, was dissolved in 15 mL ethanol analytical grade for the manufacturing of silver/silica (AgSiO.sub.2) nanocomposites. After being homogenized under stirring for 1 hour, the tetraethyl orthosilicate was hydrolyzed by adding 10 mL NH 40H (1.0 M) under the N.sub.2 atmosphere while stirring at room temperature (RT) for 5 hours. The mixtures were progressively reacted to obtain particle size increment and a transparent colloidal system. In addition, 10 mL of trimethoxy (octadecyl) silane was added to the system under vigorous stirring for surface modification. The mixture was subjected to ultrasonic treatment for 2 hours at 60 degrees Celsius ( C.), followed by magnetic stirring for 6 hours at 65 C. A dropwise addition of 10 mL of 0.1 M silver nitrate solution was added to this mixture. After cooling for 3 hours, the Ag-loaded core-shell AgSiO.sub.2 nanocomposite material (AgSiO.sub.2) (13.25 wt. %) was precipitated by centrifugation at 10000 rpm for 40 minutes at 10 C.

Example 3: Conversion of NH 4-ZSM-5 to H-ZSM-5 Materials (Z)

(67) Various silica-alumina ratios of the H-ZSM-5 (Si/Al.sub.2=30, 80, 150, and 280) denoted as Z-30, Z-80, Z-150, and Z-280, respectively, were obtained after calcining NH 4-ZSM-5 at 550 C. for 6 hours. The zeolites thus prepared were used to study the effect on the SERS enhancement. The P-XRD pattern for the MFI structure confirmed the framework of such materials.

Example 4: Preparing the Combined AgSiO.SUB.2.-Z-150 Nanocomposite

(68) A 1:1 ratio of the AgSiO.sub.2 and Z-150 was collected in a vial for characterization by FT-IR, XRD, and N.sub.2-sorption. Notably, the mixture (1:1 ratio of the AgSiO.sub.2 and Z-150) was kept in hexane for 10 minutes and then dried at 100 C. for an hour before subjecting to field emission scanning electron microscopy (FE-SEM) analysis. The prepared AgSiO.sub.2-Z-150 material was confirmed to contain 6.33 wt. % of silver based on ICP-OES analysis.

Example 5: Characterization of the Nanomaterials

(69) P-XRD was carried out using a Bruker D8 Advance diffractometer equipped with Ni-filtered Cu K radiation. The diffractograms were recorded from 2 theta () of 5 to 80 at a step size of 0.02 with a dwell time of 0.5 seconds. Nitrogen adsorption/desorption measurements were performed using a Micromeritics Tristar 3000. Before each measurement, the samples were degassed under N.sub.2 at 150 C. for 3 hours. The surface area and pore size were calculated using BET and Barrett-Joyner-Halenda (BJ H) methods. To explore the morphological characteristics of the prepared AgSiO.sub.2, Z-150, and combined AgSiO.sub.2-Z-150 materials, FE-SEM images were recorded in the solid phase using a Thermo Scientific Quattro microscope at 10 kilovolts (kV) accelerating voltage and varied magnification and spatial resolution. The FT-IR spectra were acquired in the region of 3900-500 cm.sup.1 using a Perkin Elmer spectrometer, where the sample was loaded in powder form. The ICP-OES Horiba ULTIMA 2 instrument quantified the percentages of metals.

Example 6: Raman and SERS Measurements

(70) The Raman and SERS spectra were acquired using a HORIBA LabRAM HR Evolution spectrometer with a HeNe laser source, two gratings (600 and 1800) with automated switching, a charge-coupled device (CCD) detector, and the red laser line (633 nm) as an excitation source. The substrate dispersed with the nanocomposite composition was prepared by dispersing 10 milligrams (mg) of the nanocomposite in 5 mL of dimethyl sulfoxide (DMSO) or diesel. A 1 cm diameter and 1 cm depth glass cuvette were used to hold the sample solution, which included a 1:1 volume ratio of the target molecule and the nanocomposite. The sample was focused using the microscope's 50 objective lens and laser power of 25%. SERS spectra were taken with four accumulations and a 20-second acquisition duration during all trials in the present study. Before being exposed to SERS analysis, all sample solutions were dissolved either in DMSO or diesel.

Example 7: Characterization of the AgSiO.SUB.2., Z-150 and AgSiO.SUB.2.-Z-150 Nanostructures

(71) The FT-IR, powder-XRD, and FE-SEM analyses confirmed the characteristics and morphology of the synthesized core-shell AgSiO.sub.2 nanocomposite. Moreover, the Z-150 and A g-SiO.sub.2Z-150 nanocomposites employed in the SERS investigation were characterized using the BET N.sub.2-sorption technique to identify the underlying structure-absorbance relationship. The SEM images of the Z-150 and AgSiO.sub.2-Z-150 nanocomposite, along with the energy dispersive spectroscopy (EDS), are presented in FIGS. 2A-3C The SEM images of the AgSiO.sub.2 nanocomposite are displayed in FIGS. 2A-2F, which shows the silver-silica core (AgSiO.sub.2 nanocomposite) is spherical with almost no sign of aggregation (FIG. 2A and FIG. 2B). The size of the obtained spheres of the AgSiO.sub.2 nanocomposite is in the order of 300 nm (FIG. 2B). Comparing the morphologies of the fresh Z-150 (FIG. 2C and FIG. 2D) with the hybrid AgSiO.sub.2Z-250 nanocomposite (in FIG. 2E and FIG. 2F) evidenced for the successful combination of the hybrid nanocomposite. Additionally, the EDS analysis was utilized to determine the chemical composition of the materials. EDS spectra showed that the sample is composed of silicon, oxygen, and silver for the AgSiO.sub.2 composite (FIG. 3A); the sample is composed of silicon, oxygen, and aluminum for the Z-150 (FIG. 3B); and the sample is composed of silicon, oxygen, silver, and aluminum for the AgSiO.sub.2-Z-150 (FIG. 3C), which confirmed the expected constituent components of the nanostructured materials.

(72) The FT-IR spectrum of the powdered AgSiO.sub.2 nanocomposite (FIG. 4A) confirmed that the spherical core is composed of a (SiOSi) framework, as well as for the Z-150. The average infrared intensities associated with (SiOSi/Al) bands can be traced for the AgSiO.sub.2-Z-150. The band range at 700-1100 cm.sup.1 showed the vibrations of SiO and AlO, while the band range at 950-1250 cm.sup.1 showed the vibrations of the asymmetry of SiO and AlO within the framework. Additionally, the characterization of the two-dimensional (2-D) structure of the AgSiO.sub.2 and three-dimensional (3-D) structure of Z-150 nanomaterials by XRD is required to verify the crystallinity of the hybrid nanocomposite. The addition of the AgSiO.sub.2 to the zeolitic platform did not affect the crystallinity of the original structure, as clearly depicted in FIG. 4B, indicating that the original nanostructured precursors retain corresponding chemical and physical properties within the newly formed nanocomposite. Notably, in the hybrid nanocomposite, AgSiO.sub.2-Z-150, the presence of AgSiO.sub.2 diffraction prominent peaks is labeled with asterisks (in FIG. 4B), and thereby remaining peaks originated from the Z-150 that is within MFI-framework.

(73) The surface areas of the materials have been determined based on the BET analysis (FIG. 5) and are listed in Table 1.

(74) TABLE-US-00001 TABLE 1 Vibrational assignment of the three critical SERS peaks of the DBT appearing in diesel media, along with the respective detection limits, linear dynamic ranges, and regression coefficients. SERS peak (cm.sup.1) Assignment LOD LDR R2 1611 CC stretching 1.0 10.sup.7 10.sup.1 10.sup.6 0.973 1333 CH in-plane bending, CC 1.0 10.sup.6 10.sup.1 10.sup.6 0.955 stretching 1039 CS stretching 1.0 10.sup.5 10.sup.1 10.sup.5 0.914

(75) The 2-D silica material has a considerably lower surface area. It is well-established that the generation of metal nanoparticles at silica surfaces induces an even lower surface area than pristine silica material, as metal nanoparticles tend to occupy the silica pores. Hence, the AgSiO.sub.2 surface area was found to be 47 m.sup.2/g. The surface area of the pristine material, Z-150, was found to be 368 m.sup.2/g, which is higher than the case of the AgSiO.sub.2-Z-150 nanocomposite. Notably, the higher and lower surface area of the zeolitic and silver silica materials were combined, resulting in an average surface area material. The BJH analysis was further employed to compare the pore sizes of the three materials (Table 2).

(76) TABLE-US-00002 TABLE 2 Textural properties of materials based on the BET and BJH analyses. BET Surface Area BJH pore volume BJH pore width Material (m.sup.2 g.sup.1) (cm.sup.3 g.sup.1) (nm) AgSiO.sub.2 45 0.127 8.78 Z-150 368 0.175 4.54 AgSiO.sub.2Z-150 215 0.248 5.75

(77) It can be noticed that the pore volume improved when the Z-150 was combined with the core-shell silica. The surface textural properties suggested that adsorption and absorption have taken place between the surfaces of the two materials while combining the Z-150 and the AgSiO.sub.2. The overall surface assessment confirmed a combination of type-IV and type-l isotherms characteristic of meso-nano porous materials.

Example 8: SERS Detection of DBT in DMSO

(78) In the preliminary study, a 0.1 M concentration of the DBT analyte in dimethyl sulfoxide (DMSO) solvent was subjected to SERS analysis using the AgSiO.sub.2 as substrate (FIG. 6A). Spectrum 602 corresponds to plain DMSO, while spectrum 604 corresponds to 0.1 M DBT dissolved in DMSO where the new Raman peaks appeared are assigned to the DBT. In particular, the non-SERS prominent DBT peak shown at 1599 cm.sup.1 is assigned to the aromatic CC stretching vibration. Such a peak can be interesting to explore further using an appropriate SERS substrate. Spectrum 606 depicts the SERS effect, in which the DBT in DMSO is enhanced by employing the AgSiO.sub.2. The peaks are enhanced due to the possible analyte interaction with the nanomaterial. Such a significant enhancement observed in the Raman intensities results from the high plasmonic light field resonance upon laser excitation of the roughened silver-modified surface.

(79) Further analysis has been carried out with a different range of molar concentrations of the DBT in DMSO, namely from 10.sup.1 to 10.sup.8 M (FIG. 6B). The best representative SERS lines in the present approach were those whose intensities got significantly enhanced. The most conspicuous SERS peak of the DBT is the one appearing at 1602 cm.sup.1 (aromatic CC stretching corresponding to 1599 cm.sup.1 in the typical Raman spectrum), as illustrated in FIG. 6B. Nonetheless, the other two SERS peaks at 1556 cm.sup.1 and 1476 cm.sup.1 have exercised noticeable intensity enhancements and are assigned to ring deformation and CH asymmetric stretching vibration, respectively. The linear relationship of SERS response with the concentration of the DBT for these three peaks is presented in FIG. 6C. The SERS band at 1602 cm.sup.1 shows linearity in a wide range of the DBT concentrations (10.sup.1 to 10.sup.8 M) with a correlation coefficient of R.sup.2=0.9819, and limit of detection (LOD) as 10.sup.9 M was achieved.

(80) The Raman enhancement factor (EF) is a critical metric for assessing nanomaterial signal enhancement performance using the well-established method reported in the literature with the formula given in Equation (1):

(81) $\begin{array}{cc}EF=\frac{I_{\mathrm{SERS}}}{I_{\mathrm{NRS}}}\frac{C_{\mathrm{NRS}}}{C_{\mathrm{SERS}}}& \left(1\right)\end{array}$
where I.sub.SERS is the SERS mode intensity in the presence of the DBT and the substrate, and C SERS is the sample concentration, while I.sub.NRS is the normal Raman mode intensity in the presence of the DBT without substrate, and C.sub.NRS is the sample concentration. The area of the 1611 cm.sup.1 Raman peak was utilized as the intensity of I.sub.NRS or I.sub.SERS because the present band is the strongest symmetric peak compared with others in the spectra. Consequently, the EF turned out to be 1.5910.sup.6.

Example 9: SERS Detection of DBT in Diesel

(82) The study was further extended for the DBT sensing in real diesel media based on the results above. Silver-loaded silica core-shell was proven to serve as an active SERS substrate for sensing the DBT; therefore, the silver-loaded silica core-shell could be a potential candidate for real sample applications. The SERS analysis of the DBT in diesel was performed without DMSO solvent. However, the spectral response (FIG. 7A, spectrum 706 corresponding to DSL_Z-80) encountered a significant fluorescence background in most of the key DBT peaks and resulted in a broadening in the spectral range around 700 cm.sup.1. The fluorescence effect is believed to be caused by diesel constituents. Hence, a zeolitic formula could be used as a fluorescence suppressor in a diesel medium to proceed with the SERS analysis.

(83) Several SERS analyses of spiked DBT in diesel samples have been performed by introducing the Z-150, as shown in FIG. 7B. Spectra 720 and 722 correspond to plain diesel and the DBT in diesel, respectively, in which a high fluorescence effect is encountered. Spectrum 712 corresponds to diesel with the Z-150, where the fluorescence background is reduced. Further, spectrum 714 corresponds to the DBT with the Z-150, where no enhancement of analyte peaks has appeared at 1615 cm.sup.1, while spectrum 718 corresponds to the DBT in diesel with the AgSiO.sub.2, where the DBT peak is enhanced. Still, fluorescence at the same time increased tremendously. The fluorescence dropped after adding the Z-150 with A gS (1:1) into the DBT in diesel (FIG. 7B, spectrum 716), and the DBT peak's enhancement was retained. Spectrum 702 corresponds to bare diesel. Before this, different Si/Al.sub.2 ratios of the H-ZSM-5 (Z-30, Z-80, Z-150, and Z-280 correspond to spectra 704, 706, 708, 710) have been examined in diesel (FIG. 7A) to select the optimal composition that reduces the fluorescence interference in the Raman spectrum of diesel, and the Z-150 form was found to be the best fluorescence suppressant.

(84) Furthermore, FIG. 8A focused on the main outcome of the results from FIGS. 7A-7B. It can be noted that diesel-like oils are associated with high fluorescence that could be reduced using the Z-150 nanoporous material. The SERS spectra of the DBT analyzed in diesel using the combined AgSiO.sub.2-Z-150 material (FIG. 8B) revealed a set of prominent Raman peaks different than those identified in the DMSO solvent. It indicated that the presence of Z nanostructure influenced the interaction of the DBT with the silver nanoparticles.

(85) The three key SERS lines that showed the most noticeable enhancement was 1039 cm.sup.1 (CS stretching), 1333 cm.sup.1 (CH bending and CC stretching), and 1611 cm.sup.1 (CC ring stretching). However, the calibration using the band at 1611 cm.sup.1 provides better results than the other two calibration lines (FIG. 8C). It should be mentioned that there is a shift of the peak obtained in diesel media than those obtained in DMSO. The band obtained at 1602 cm.sup.1 in DMSO was shifted to 1611 cm.sup.1 in diesel media. The linear DBT concentration dependency with SERS spectral response was analyzed for three peaks in the concentration range of 0.1 M to 10.sup.6 M. The LOD was 10.sup.7 M while the limit of quantification (LOQ) was 10.sup.6 M in the diesel sample at 1611 cm.sup.1 was successfully achieved and corresponds to the ring stretching mode. The EF value in diesel using the AgSiO.sub.2-Z-150 substrate was computed as EF=7.7210.sup.4 The concentration dependency responses of the key SERS peaks of the DBT in diesel were evaluated, and the corresponding LOD, linear dynamic range, and correlation coefficient (R.sup.2) are listed in Table 1.

(86) The reproducibility is a key element of the SERS substrate and corresponding analytical performance. The reproducibility of the AgSiO.sub.2-Z-150 substrate has been investigated for the 0.1 M concentration of the DBT spiked in commercial diesel (FIG. 9). The five consecutive experiments were based on the selected region from 1500 to 1800 cm.sup.1 where the DBT most prominent peak at 1611 cm.sup.1 appeared (FIG. 10). The relative standard deviation (RSD) for the parallel SERS measurements was <2% that indicates the developed SERS substrate and proposed analytical method exhibit a good reproducibility.

Example 10: Vibrational Analysis of Raman and SERS Spectra of DBT

(87) Vibrational assignments of the Raman and SERS spectra of the DBT were carried out based on density functional theory (DFT). The DBT peaks in DMSO solvent appeared at 3060, 1599, 1557, 1477, 1317, 1233, 1130, 501, and 403 cm.sup.1, while three prominent bands appeared at 1615, 1330, and 1034 cm.sup.1 in diesel oil media in the presence of the Z-150. From DFT, the molecular structure of the DBT was optimized (FIGS. 11A-11B) using the B3LY P functional introduced by Becke and Lee et al. [A. D. Becke, Density-functional exchange-energy approximation with correct asymptotic behavior, Physical Review A. 38 (1988) 3098.; C. Lee, W. Yang, R. G. Parr, Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density, Physical Review B. 37 (1988) 785, incorporated herein by reference] and the cost-effective basis set 6-311g (d,p) for moderately small organic molecules [M. Haroon, I. Abdulazeez, T. A. Saleh, A. A. Al-Saadi, SERS-based trace-level quantification of sulindac: Spectroscopic and molecular modeling evaluation, Journal of Molecular Liquids. 312 (2020) 113402, incorporated herein by reference].

(88) Its frequency calculations were computed at the same level of theory and were scaled using a scale factor of 0.98, and the scaling technique used in the present work was developed by Baker and co-workers [J. Baker, A. A. Jarzecki, P. Pulay, Direct scaling of primitive valence force constants: An alternative approach to scaled quantum mechanical force fields, Journal of Physical Chemistry A. 102 (1998) 1412-1424, incorporated herein by reference]. The Raman bands of the DBT were predicted at 3105, 1617, 1568, 1586, 1338, 1245, 1142, 1037, 501, and 410 cm.sup.1 which are attributed to CH symmetric stretching, CC stretching, ring deformation, CH asymmetric stretching, CC stretching, out-of-plane CH bending, in-plane CH bending, CS stretching, ring wagging, and ring torsion, respectively, as shown in Table 3.

(89) Notably, DFT calculations predicted several Raman peaks of the DBT in the absence of DMSO solvent. Still, experimentally some of the Raman lines associated with the DBT are suppressed in the presence of DMSO. The positions of spectral peaks of the DBT are slightly shifted in DMSO and diesel media due to bond length changes in the DBT as a result of analyte interaction with the AgSiO.sub.2. The Raman band shift was observed in two scenarios (a) to higher wave numbers and (b) to lower wave numbers in the presence of A gSiO2. The shift of the Raman band to higher wavenumbers indicates short bond length among atoms and vice versa [C. R. Andrew, T. M. Loehr, J. Sanders-Loehr, H. Y eom, J. Selverstone Valentine, B. Gran Karlsson, N. Bonander, G. van Pouderoyen, G. W. Canters, Raman Spectroscopy as an Indicator of CuS Bond Length in Type 1 and Type 2 Copper Cysteinate Proteins, J Am Chem Soc. 116 (1994) 11489-11498, incorporated herein by reference; L. Popovi, D. De Waal, J. C. A. Boeyens, Correlation between Raman wavenumbers and PO bond lengths in crystalline inorganic phosphates, Journal of Raman Spectroscopy. 36 (2005) 2-11, incorporated herein by reference]. In DMSO solvent, DBT Raman bands appeared at 3060, 1599, 1557, 1477, 1317, 1233, 1130, 1024, 501, and 403 cm.sup.1, which were shifted in SERS to 3058, 1602, 1556, 1476, 1319, 1230, 1133, 1022, 492 and 407 cm.sup.1, respectively. In diesel media, Raman bands appeared at 1615, 1330, and 1034 which were shifted in the SERS spectrum to 1611, 1333, and 1039 cm.sup.1, respectively (Table 3).

(90) TABLE-US-00003 TABLE 3 Vibrational assignment based on the literature and DFT calculation for DBT in DM SO. Predicted Raman SERS DFT (cm.sup.1) (cm.sup.1) (cm.sup.1) Vibrational Assignment 3105 3060 3058 CH symmetric stretching 1617 1599 1602 CC stretching 1568 1557 1556 Ring deformation 1586 1477 1476 CH asymmetric stretching 1338 1317 1319 CH in-plane bending, CC stretching 1245 1233 1230 CH out-of-plane bending 1142 1130 1133 CH in-plane bending 1037 1024 1022 CS stretching 501 501 492 Ring deformation 410 403 407 Ring torsion

(91) The present disclosure provides the nanostructured silver-loaded silica (AgSiO.sub.2) mixed with H-ZSM-5 (Z) materials used for the SERS-based detection of the trace DBT concentrations in diesel oil samples, as shown in FIG. 12. Among the different compositions of zeolitic materials screened, the Si/Al.sub.2=150 ratio best suppresses the fluorescence background, leading to reliable monitoring of the DBT in diesel. The relationship of Raman peak intensities and amount of the DBT in diesel is established in the range from 10.sup.1 to 10.sup.6 M with LOD of 10.sup.7 and LOQ of 10.sup.6 M at 1611 cm.sup.1. Nonetheless, the AgSiO.sub.2 is a useful SERS substrate for detecting the DBT in the non-fluorescent DMSO environment with a LOD of as low as 10.sup.9 M. The vibrational assignments of SERS peaks of the DBT have been proposed based on the literature values and DFT computation. The present SERS-based spectroanalytical method is fast, reliable, and non-destructive. A 1:1 ratio AgSiO.sub.2-Z-150 nanocomposite for the SERS analysis can be extended to examine other sulfur-containing compounds in oil samples.

(92) Numerous modifications and variations of the present disclosure are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.