FUEL IDENTIFICATION WITH SURFACE ENHANCED RAMAN SPECTROSCOPY TAGS

TECHNICAL FIELD

[0001] The present invention relates to a method and system for using surface enhanced

Raman spectroscopy (SERS) tags or Raman Active dyes with an enhancing material to mark a quantity of fuel for identification and tracking purposes.

BACKGROUND OF THE INVENTION

[0002] There are several reasons why it is necessary or desirable to mark or tag a quantity of liquid fuel for identification purposes. In many instances, the simple labeling of a fuel container is not a sufficient identification technique. For example, it may be desirable to tag a quantity of fuel to distinguish the tagged supply from otherwise identical untagged supply. A need for the identification of a specific quantity of fuel may arise in the fields of supply management, inventory control, fuel taxation, the detection of fuel adulteration or theft and other purposes. In many cases a tag must be invisible to a casual observer, yet easily read by a properly equipped technician. In addition, a suitable tag or fuel marker must not compromise the effectiveness of the tagged supply as a fuel source. The present invention is directed toward meeting one or more of the needs identified above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003] Fig. 1 is a graph of the Raman spectrum of a fuel supply tagged with SERS nanoparticles;

[0004] Fig. 2 is a graph of the Raman spectrum of a diesel fuel supply tagged with

Raman active molecule dye in the presence of metal colloid; and

[0005] Fig. 3 is a graph of the Raman spectrum of a gasoline supply tagged with Raman active molecule in the presence of metal colloid.

[0006] Fig. 4 is a graph of the Raman spectrum of a fuel supply tagged with SERS beads.

[0007] Fig. 5 is a graph of the Raman spectrum of ethanol tagged with a Raman active dye soaked on a silver nanoparticle-based substrate.

[0008] Fig. 6 is a graph of the Raman spectrum of a diesel fuel supply tagged with a

SERS active dye at three separate concentrations on a KLARITE™ substrate.

[0009] Fig. 7 is a graph of the peak area versus concentration for the dye/substrate results of Fig. 6.

SUMMARY OF THE INVENTION

[0010] The present invention includes multiple embodiments of a method of identifying a quantity of fuel. The methods of identification disclosed herein rely upon the association of a substance having a known Raman spectrum with a quantity of fuel. In one embodiment, a nanoparticle including a SERS active core may be mixed into a fuel supply. In an alternative embodiment, a SERS active dye including a Raman active reporter molecule may be mixed with a quantity of fuel. The identification method further includes acquiring the Raman spectrum of the Raman active reporter molecule associated with the tag. If the quantity of fuel is tagged with a SERS active nanoparticle, the Raman spectrum may be acquired directly from a sample of the tagged fuel using a Raman spectrum reader. The SERS active nanoparticles may be separated from or concentrated within the fuel prior to acquiring the Raman spectrum. One method of concentrating a SERS active nanoparticle within fuel is with a centrifuge. Another concentration technique includes drying a quantity of tagged fuel on a suitable substrate. [0011] Alternatively, if the quantity of fuel is tagged with a dye having Raman active reporter molecules, the process of identifying the quantity of fuel may include mixing into a sample of the fuel a colloid of Raman enhancing metal particles. Suitable metals include, but are not limited to, silver or gold. Upon the contact of the Raman active reporter molecules with the surface of the colloidal metal, a Raman spectrum may be taken with a Raman spectrum reader. As an alternative to directly mixing a SERS active metal colloid into the dyed sample, a portion of the sample may be associated with a SERS active substrate.

[0012] Another embodiment of the present invention is a system for identifying a tagged quantity of fuel including a portable Raman spectrum reader and associated equipment.

DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention includes multiple alternative embodiments of a method of preparing a quantity of fuel for identification and subsequently identifying the fuel. Each embodiment of the method relies upon reading the Raman spectrum of a tag added to the fuel supply.

[0014] One disclosed embodiment includes mixing into a quantity of fuel surface enhanced Raman spectroscopy (SERS) nanotags. The characteristics and preparation of one type of SERS nanotag consistent with the present invention is described in U.S. Patent No. 6,514,767, entitled "Surface Enhanced Spectroscopy-Active Composite Nanoparticles," which patent is incorporated herein by reference. Identification of a fuel supply marked with SERS nanotags can be accomplished by acquiring the Raman spectrum of the SERS nanotag. The SERS nanotags may be separated from the fuel or concentrated prior to acquiring a Raman spectrum, to enhance the quality and intensity of the spectrum obtained. One method of separating the SERS nanotags from the fuel is the use of a centrifuge.

[0015] An alternative type of encapsulated nanoparticle suitable for fuel tagging is referred to as a SERS bead. A SERS bead includes a SERS active metal colloid core which has an appropriate Raman active dye adsorbed onto the colloid surface. The metal colloid core may be gold, silver, copper or any other metal which is Raman enhancing. The metal colloid core/SERS active dye combination may be coated with one or more polymer layers to create a SERS active bead as is described in [OX73 - what is EP or GB Patent No.?] which patent is incorporated herein by reference. As described in the [OX73] patent, encapsulation of the colloidal core into a polymer bead stabilizes the core and SERS label system. As described above, SERS beads may be directly dispersed in a quantity of fuel for tagging purposes. The SERS beads may be concentrated by centrifuge or other means prior to obtaining a spectrum. Alternatively the SERS bead, SERS nanotag or other SERS active nanoparticle may be localized on a substrate prior to Raman spectrum acquisition.

[0016] Another embodiment of the method of preparing a quantity of fuel for subsequent identification includes mixing Raman active reporter molecules directly into the fuel. The reporter molecules may thus be considered a Raman active dye. The natural Raman spectrum of virtually all suitable substances is, however, extremely weak. As is described in U.S. Patent No. 6,514,767, the Raman spectrum of a Raman active substance can be greatly enhanced by

bringing the substance into contact with certain metal surfaces. Accordingly, this embodiment may further include mixing into the previously dyed or tagged fuel a colloidal metal such as gold or silver, followed by the acquisition of the Raman spectrum. The Raman active molecules yield a strong Raman spectrum when brought into contact with the surface of the metal colloid. Colloid may be mixed into a relatively small sample of the tagged fuel. [0017] An alternative embodiment of the method of preparing a quantity of fuel for subsequent identification includes mixing Raman active reporter molecules directly into the fuel as described immediately above, but eliminates the need for a separate colloid addition. In this alternative, the marked fuel may be placed on or associated with a SERS active substrate prior to detection of the Raman spectrum. As described in certain examples below, the SERS active substrate could be mesoscopic or macroscopic. The SERS active substrate could be an immobilized colloid, a SERS active metal, a coated photonic lattice or any substance which will enhance a SERS signal when the marked fuel is placed upon it. A representative suitable SERS active substrate may be prepared by creating a photonic lattice of self-assembled silicon spheres. The spheres may be coated with gold or another Raman enhancing meta which may be excitation tuned to the desired SERS spectrum acquisition laser wavelength. Fuel marked with a SERS active dye may be drawn onto the substrate thus prepared. The marker which constitutes the SERS active dye will become active in the vicinity of the gold surface. The fuel/dye mixture thus associated with the substrate may be excited with an appropriate diode laser and a SERS spectrum acquired. The composition and preparation of a SERS active substrate which is suitable for the implementation for the present invention includes but is not limited to the specific materials listed above.

[0018] In summary, one general aspect of the disclosed method is based upon the acquisition of a Raman spectrum from discrete SERS active nanoparticles including but not limited to SERS nanotags and SERS beads. The nanoparticles are dispersed throughout the fuel when it is tagged and the nanoparticles may be separated or concentrated prior to the acquisition of a spectrum. Other broad aspects of the disclosed method include mixing Raman active reporter molecules directly with the fuel supply. Identification is accomplished by contacting the Raman reporter molecules with a metal colloid, and then obtaining a Raman spectrum, or by placing the marked fuel upon a SERS active substrate prior to acquiring the Raman signal.

[0019] Another aspect of the present invention includes a system for the identification of a quantity of fuel. The system includes a portable, possibly handheld, Raman spectrum reader and a container suitable for holding a sample taken from the fuel supply. If the fuel supply was tagged with SERS nanotags or SERS beads, the system may also include a small centrifuge, a substrate or other means for concentrating the nanotags at a specific location accessible to the Raman spectrum reader. Alternatively, if the fuel was tagged with Raman reporter molecules, the identification system may include either a supply of metal colloid such as gold or silver colloid which can be dispensed into and mixed with the sample or a suitable SERS active substrate for receiving the fuel prior to obtaining a spectrum with the Raman spectrum reader. [0020] As described above, the present invention may feature the use of encapsulated surface enhanced Raman scattering (SERS) tags. One type of encapsulated nanoparticle, referred to as a SERS nanotag, includes a metal nanoparticle, which metal is Raman enhancing; a Raman-active molecule (sometimes referred to as a SERS tag or reporter molecule) attached to, or associated with the surface of the nanoparticle; and an encapsulant, usually SiO ₂ (glass). The encapsulant surrounds both the metal nanoparticle and the Raman-active molecule. A particle prepared in this fashion has a measurable SERS spectrum. Although the invention is described in terms of SERS nanotags prepared from single nanoparticles, it is to be understood that nanoparticle core clusters or aggregates may be used in the preparation of SERS nanotags. Methods for the preparation of clusters of aggregates of metal colloids are known to those skilled in the art. The alternative use of sandwich-type particles is described in U.S. Patent No. 6,861,263, which patent is incorporated herein by reference.

[0021] SERS data may be obtained from the tags by illuminating the SERS nanotags with light having a suitable excitation wavelength. In the case of some reporter molecules suitable excitation wavelengths are in the range of about 600-1000 nm. In some embodiments, the excitation wavelengths are 632.8, 785, or 980 nm. Examples of reporter molecules include 4-mercaptopyridine (4-MP); trans-4, 4' bis(pyridyl)ethylene (BPE); quinolinethiol; 4,4'- dipyridyl, 1,4-phenyldiisocyanide; mercaptobenzamidazole; 4-cyanopyridine; 1', 3,3, 3', 3'- hexamethylindotricarbocyanine iodide; 3,3'-diethyltiatricarbocyanine; malachite green isothiocyanate; bis-(pyridyl)acetylenes; Bodipy, and isotopes thereof, including, for example, deuterated BPE, deuterated 4,4'-dipyridyl, and deuterated bis-(pyridyl)acetylenes; as well as pyridine, pyridine-d5 (deuterated pyridine), and pyridine- ¹⁵ N. A suitable excitation wavelength

is one at which the background noise component, generated by fluorescence from other fuel components is low enough to obtain a detectable SERS signal.

[0022] The SERS nanotags may comprise any nanoparticle core known in the art to be

Raman-enhancing. As used herein, the term "nanoparticle", "nanostructure", "nanocrystal", "nanotag," and "nanocomponent" are used interchangeably to refer to a particle, generally a metallic particle, having one dimension in the range of about 1 nm to about 1000 nm. In some embodiments, the metal nanoparticle core is a spherical or nearly spherical particle of 20-200 nm in diameter. In some embodiments the range is about 20 nm to about 50 nm, in some embodiments in the range of about 30 nm to about 100 nm. The tags may be polydisperse. That is, a group of tags may comprise tags with these ranges of diameters, but each tag need not have the same diameter.

[0023] Nanoparticles may be isotropic or anisotropic. Anisotropic nanoparticles may have a length and a width. In some embodiments, the length of an anisotropic nanoparticle is the dimension parallel to the aperture in which the nanoparticle was produced. In the case of anisotropic nanoparticles, in some embodiments, the nanoparticle has a diameter (width) of 350 nm or less. In other embodiments, the nanoparticle has a diameter of 250 nm or less and in some embodiments, a diameter of 100 nm or less. In some embodiments, the width is between 15 nm to 300 nm. In some embodiments, the nanoparticle has a length of about 10-350 nm. [0024] Nanoparticles include colloidal metal, hollow or filled nanobars, magnetic, paramagnetic, conductive or insulating nanoparticles, synthetic particles, hydrogels (colloids or bars), and the like. The nanoparticles used in the present invention can exist as single nanoparticles, or as clusters or aggregates of the nanoparticles. Clusters or aggregates may be formed by the addition of aggregating agents to the SERS nanotags.

[0025] It will also be appreciated by one of ordinary skill in the art that nanoparticles can exist in a variety of shapes, including but not limited to spheroids, rods, disks, pyramids, cubes, cylinders, nanohelixes, nanosprings, nanorings, rod-shaped nanoparticles, arrow-shaped nanoparticles, teardrop -shaped nanoparticles, tetrapod- shaped nanoparticles, prism-shaped nanoparticles, and a plurality of other geometric and non-geometric shapes. Another class of nanoparticles that has been described includes those with internal surface area. These include hollow particles and porous or semi-porous particles. Moreover, it is understood that methods to prepare particles of these shapes, and in certain cases to prepare SERS-active particles of these

shapes, have been described in the literature. While it is recognized that particle shape and aspect ratio can affect the physical, optical, and electronic characteristics of nanoparticles, the specific shape, aspect ratio, or presence/absence of internal surface area does not bear on the qualification of a particle as a nanoparticle.

[0026] Various systems can be used for detection of a Raman spectrum. A number of commercially available instruments may be used. For example, Raman Systems Inc., Enwave Optronics, Inc., Kaiser Optical Systems, Inc., InPhotonics, Inc., J-Y Horiba, Renishaw, Bruker Optics, Thermo Electron, Avalon, GE Ion Track, Delta Nu, Concurrent Analytical, Raman Systems, Inphotonics, Chemlmage, Jasco, Lambda Systems, SpectraCode, Savante, Real-Time Analyzers, Veeco, Witec, and other companies provide Raman spectrometers suitable for use in the present invention.

[0027] While the invention has been particularly shown and described with reference to a number of embodiments, it would be understood by those skilled in the art that changes in the form and details may be made to the various embodiments disclosed herein without departing from the spirit and scope of the invention and that the various embodiments disclosed herein are not intended to act as limitations on the scope of the claims.

EXAMPLES

[0028] The following examples are provided for illustrative purposes only and are not intended to limit the scope of the invention.

Example 1:

[0029] SERS nanotags prepared as described in U.S. Patent No. 6,514,767 were used to tag diesel fuel. Diesel Fuel of grade #2 was used for analysis. SERS nanotags were prepared to readily mix into diesel fuel or gasoline as follows: 1 mL of BPE-labeled tags (8 x 10 ¹¹ particles/mL) was centrifuged at 6000 rpm for 8 minutes. The supernatant was removed, 1 mL of dry tetrahydrofuran (THF) was added, and the process was repeated two additional times. 10 μL of the nanotags was mixed with 1 mL diesel fuel. Subsequently, the Raman spectra of graph 102 of Fig. 1 were acquired.

[0030] All spectra were acquired using a 785 nm excitation wavelength, coupled into an

InPhotonics head for excitation and collection of the scattered radiation. An Ocean Optics

USB2000 spectrometer was used for data acquisition. SENSERSee software was used to run the spectrometer.

[0031] Graph 102 shows the spectrum of the BPE-labeled SERS nanotags in diesel fuel as well as the diesel fuel background. The concentration of tags in this sample was 8 x 10 ⁹ particles/mL. The size of the signal obtained is consistent with the signal size seen in water for a similar concentration of tags.

[0032] Because the SERS nanotags can be separated from or concentrated within diesel fuel by centrifugation, the sensitivity of this measurement can be improved significantly if larger amounts of sample are processed. For example, if 10 mL of diesel fuel containing tags is centrifuged, the supernatant removed, and the sample resuspended to make a final volume of 1 mL, a 10x improvement in sensitivity may be obtained.

Example 2:

[0033] Raman active reporter molecules were used to tag diesel fuel and gasoline. Diesel

Fuel #2 and gasoline were used for analysis.

[0034] Trans- 1 ,2-bis(4-pyridyl)ethylene (BPE) and 4,4' -dipyridyl (dipy) were used as

Raman reporter molecules and prepared as follows: Dye solutions of BPE and dipy were prepared in THF. 10 μL of these solutions was mixed with 1 mL of both diesel fuel and gasoline to prepare the tagged solutions. These solutions were placed in glass vials and the Raman spectra of the reporter molecules in either diesel or gasoline were acquired. Subsequently, 200 μL of 50 nm diameter Au colloid (8 x 1010 particles/mL) was added to the vial. After shaking the vial several times another set of Raman spectra were acquired.

[0035] All spectra were acquired using a 785 nm excitation wavelength, coupled into an

InPhotonics head for excitation and collection of the scattered radiation. An Ocean Optics

USB2000 spectrometer was used for data acquisition. SENSERSee software was used to run the spectrometer.

[0036] Graph 104 of Fig. 2 shows the Raman spectra obtained as described above in diesel fuel. Graph 104 shows that both BPE and dipy can be used as SERS reporter molecules in diesel fuel. The spectra are of 5.5 μM BPE and 7.5 μM dipy (final concentrations in diesel fuel).

Neither spectrum can be detected in the solution before the addition of the Au colloid. Graph

106 of Fig. 3 illustrates that similar data may be acquired in gasoline. In this case the spectrum

is of 7.4 μM dipy in the gasoline. Gasoline has a more quiet background spectrum than diesel fuel, indicating that if all other factors were equivalent a lower detection limit should be possible in gasoline.

Example 3:

[0037] 0.125g of SERS beads prepared as described in [OX073] were used to tag 1 liter of fuel. An unconcentrated sample of the fuel was interrogated with a 3mW laser over a 5 second acquisition time. The selected excitation laser exhibits a laser spot volume of 0.06μl. The loading concentration described above results in approximately 2.5M beads per ml in suspension. This concentration translates to approximately 100 to 200 beads in the laser spot volume. Graph 108 of Fig. 4 illustrates a Raman spectra obtained in the above-described conditions. Although the SERS spectrum was within the detection limits, significantly enhanced sensitivity may be obtained by concentrating the beads, for example, by drying the beads onto a suitable substrate or otherwise localizing the beads for Raman spectrum interrogation.

Example 4:

[0038] Raman active reporter molecules (a Raman active dye) were used to tag ethanol.

The dye concentration used in this example was IxIO ^"10 Mol/1. Thus, one gram of Raman active reporter molecules could be used to label approximately 33x10 ⁶ liters of fuel. Prior to obtaining the Raman spectrum, a sample of the dyed ethanol was diluted and soaked onto a silver nanoparticle -based substrate. Graph 110 of Fig. 5 illustrates the Raman spectrum thus obtained.

Example 5:

[0039] SERS-440 Raman active dye was used to tag diesel fuel at three concentrations.

The three concentrations selected were IxIO ³ M, 5xlO ^"5 M and IxIO ⁶ M. The tagged diesel fuel was applied to a KLARITE™ substrate. A KLARITE™ substrate features the symmetrically designed nanometer scale patterning of a silicon surface that is coated in gold. The KLARITE™ substrate thus forms a photonic crystal that controls the surface plasmons that governs the Raman enhancement process.

[0040] The following protocol was followed in the KLARITE™ substrate experiments:

1) For each concentration 3 individual Klarite chips were measured.

2) For each chip, measurements were taken from different positions across the active area and combined and averaged to give on spectrum per chip (measurements were taken middle, top left, top right, bottom left and bottom right).

3) In order to maximise the signal, the integration time used for each concentration was optimized:

Ix 10 ^"3 M an integration time of 5 sec;

5xlO ^~5 M an integration time of lOsec;

IxICT ⁶ M an integration time of 15 or 20sec

Saturation occurred at the higher concentrations with longer acquisition times. All spectra have been divided by the appropriate exposure time so the amplitudes from each chip are directly comparable as shown in Figure 6.

4) Key peaks were observed at IxICT ⁶ M, but the ratios appeared to have changed (notably the peak around 1000 becomes the strongest).

[0041] As shown on Graph 112 of Fig. 6, usable Raman spectra was attained at each concentration. Graph 114 of Fig. 7 shows a plot of the peak area versus concentration for the triplicate measurements of the SERS-440 tag at each of the three concentrations.