U.S. patent application number 12/700509 was filed with the patent office on 2010-08-19 for fluorescent carbon nanotube compositions deposited on surfaces.
This patent application is currently assigned to WILLIAM RICE MARSH UNIVERSITY. Invention is credited to Sergei M. Bachilo, Eric Christopher Booth, R. Bruce Weisman.
Application Number | 20100209632 12/700509 |
Document ID | / |
Family ID | 34375242 |
Filed Date | 2010-08-19 |
United States Patent
Application |
20100209632 |
Kind Code |
A1 |
Weisman; R. Bruce ; et
al. |
August 19, 2010 |
Fluorescent Carbon Nanotube Compositions Deposited on Surfaces
Abstract
The present invention is directed toward fluorescent inks and
markers comprising carbon nanotubes. The present invention is also
directed toward methods of making such inks and markers and to
methods of using such inks and markers, especially for security
applications (e.g., anti-counterfeiting). Such inks and markers
rely on the unique fluorescent properties of semiconducting carbon
nanotubes.
Inventors: |
Weisman; R. Bruce; (Houston,
TX) ; Bachilo; Sergei M.; (Houston, TX) ;
Booth; Eric Christopher; (Hammond, LA) |
Correspondence
Address: |
WINSTEAD PC
P.O. BOX 50784
DALLAS
TX
75201
US
|
Assignee: |
WILLIAM RICE MARSH
UNIVERSITY
Houston
TX
|
Family ID: |
34375242 |
Appl. No.: |
12/700509 |
Filed: |
February 4, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10572720 |
Aug 1, 2006 |
7682523 |
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PCT/US04/28603 |
Sep 2, 2004 |
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12700509 |
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60500394 |
Sep 5, 2003 |
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Current U.S.
Class: |
428/29 ; 428/199;
428/323; 977/742; 977/750; 977/902 |
Current CPC
Class: |
H01F 1/0045 20130101;
C09K 11/65 20130101; Y10T 428/25 20150115; B82Y 25/00 20130101;
B82Y 30/00 20130101; C09D 11/50 20130101; C09K 11/02 20130101; Y10T
428/24835 20150115 |
Class at
Publication: |
428/29 ; 428/199;
428/323; 977/902; 977/742; 977/750 |
International
Class: |
B32B 9/00 20060101
B32B009/00; B44F 1/12 20060101 B44F001/12; C09K 11/65 20060101
C09K011/65 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] This invention was made with Government support under grant
number CHE-9900417 awarded by the National Science Foundation. The
Government has certain rights in the invention.
Claims
1. A fluorescent composition comprising: a substrate; and carbon
nanotubes deposited on the substrate; wherein the carbon nanotubes
are fluorescent and have diameters of less than about 3 nm; wherein
the carbon nanotubes have a visible excitation and an emission
following the visible excitation; and wherein the carbon nanotubes
comprise a fluorescent ink that is formulated to adhere to the
substrate.
2. The fluorescent composition of claim 1, wherein the fluorescent
ink further comprises a surfactant.
3. The fluorescent composition of claim 1, wherein the fluorescent
ink further comprises a polymer.
4. The fluorescent composition of claim 1, wherein the carbon
nanotubes are dispersed.
5. The fluorescent composition of claim 1, wherein the carbon
nanotubes are selected from the group consisting of single-wall
carbon nanotubes, multi-wall carbon nanotubes, double-wall carbon
nanotubes, and combinations thereof.
6. The fluorescent composition of claim 1, wherein the carbon
nanotubes comprise single-wall carbon nanotubes.
7. The fluorescent composition of claim 1, wherein the carbon
nanotubes comprise an essentially homogenous population of carbon
nanotubes; wherein the essentially homogenous population comprises
a property selected from the group consisting of type, dimension,
and species.
8. The fluorescent composition of claim 1, wherein the carbon
nanotubes comprise separated carbon nanotubes; wherein the
separated carbon nanotubes have fluorescence properties tuned
within a range of excitation and emission wavelengths.
9. The fluorescent composition of claim 1, wherein the carbon
nanotubes are homogenized by electronic type.
10. The fluorescent composition of claim 1, wherein the carbon
nanotubes are chemically derivatized.
11. The fluorescent composition of claim 1, wherein the carbon
nanotubes are deposited on the substrate in patterned form.
12. The fluorescent composition of claim 1, wherein the substrate
comprises a material selected from the group consisting of paper,
natural fibers, synthetic fibers, metals, polymeric materials,
ceramics, glasses, and combinations thereof.
13. The fluorescent composition of claim 1, wherein the substrate
is pretreated to facilitate adhesion of the fluorescent ink.
14. The fluorescent composition of claim 1, wherein the substrate
is a security document.
15. The fluorescent composition of claim 1, wherein the substrate
is a currency.
16. The fluorescent composition of claim 1, wherein the carbon
nanotubes are invisible when deposited on the substrate.
17. The fluorescent composition of claim 1, wherein the emission
comprises a near-infrared emission.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S. patent
application Ser. No. 10/572,720, filed Aug. 1, 2006, which is a 35
U.S.C. .sctn.371 National Stage entry of PCT Application serial no.
PCT/US2004/028603, filed Sep. 2, 2004, which claims priority to
U.S. Provisional Patent Application Ser. No. 60/500,394, filed Sep.
5, 2003. These priority applications are each incorporated by
reference herein in their entirety.
FIELD OF THE INVENTION
[0003] The present invention relates generally to fluorescent inks
and markers, especially for security applications. More
specifically, the invention relates to such inks and markers
comprising carbon nanotubes.
BACKGROUND OF THE INVENTION
[0004] The issues of authentication and counterfeit deterrence can
be important in many contexts. Bills of currency, stock and bond
certificates, credit cards, passports, driver's licenses, as well
as many other legal documents (e.g., deeds, wills, etc.) all must
be reliably authentic to be useful. Museums and art galleries face
such challenges when authenticating works of art. Additionally,
consumer products and other articles of manufacturing, such as
pharmaceuticals, books, movies, software, etc., are frequently the
subject of counterfeiting in the form of "pirated" versions or
"knock-offs."
[0005] A wide variety of attempts have been made to limit the
likelihood of counterfeiting. Most such attempts tend to
incorporate a unique identifier into the potentially counterfeited
item. The addition of fluorescent compounds to inks and dyes has
long been a technique used by governments and banks for
anti-counterfeiting purposes. Likewise, fluorescent compounds can
be incorporated or otherwise associated with other articles for
identification and/or anti-piracy purposes. See, e.g., U.S. Pat.
Nos. 4,558,224 and 6,246,061.
[0006] Fluorescence, being a subset of photoluminescence (PL),
occurs when a material is irradiated with electromagnetic radiation
(EM), at least some of which is absorbed. Fluorescence refers to
the subsequently re-emitted radiation of wavelength other than that
which was absorbed. Typically, such emission, or fluorescence, is
red-shifted to longer wavelengths relative to the incident or
absorbed radiation, such emission can also be described as being
Stokes shifted. The terms "fluorescence," "luminescence," and
"photoluminescence," will be used synonymously herein.
[0007] Fluorescent compounds typically used in such above-described
applications are generally organic molecules that fluoresce in the
visible region of the EM spectrum when irradiated with ultra-violet
(UV) light. There is, however, a constant need for both new and
better fluorescent compounds to a) stay ahead of the would-be
counterfeiters, and b) to expand the breadth of such marking and
authentication techniques, wherein such fluorescent compounds offer
a more unique optical signal and/or yield themselves to processing
and operating conditions unsuitable for existing fluorescent
compounds.
BRIEF DESCRIPTION OF THE INVENTION
[0008] The present invention is directed toward fluorescent inks
and markers comprising carbon nanotubes, and to methods of using
carbon nanotubes (CNTs) as fluorescent identifiers for
anti-counterfeiting and authentication purposes.
[0009] Generally, the fluorescent inks of the present invention
comprise a dispersion or suspension of CNTs in a liquid (i.e.,
solvent) medium. Such a dispersion may further comprise surfactant
species and/or other traditional ink components. Such inks may be
referred to herein as "nanotube inks." Note that such inks are but
a subset of the fluorescent markers of the present invention that
can be attached to, incorporated into, or otherwise associated with
an article for which identification and/or authentication is deemed
important, generally at some point in the future.
[0010] Methods of using CNTs as fluorescent identifiers generally
rely on a knowledge of their photoluminescence properties and on
techniques of incorporating and/or attaching such species into
and/or to articles being marked or tagged. Note that the terms
"marker" and "taggant" (and their verb conjugates) will be used
synonymously herein.
[0011] Generally, the fluorescence is effected by irradiating the
item or article comprising CNTs with visible light (i.e., radiation
in the visible region of the EM spectrum). The fluorescence is then
detected in the near infrared (NIR) region of the EM spectrum.
Depending on the embodiment, such detection can be of a qualitative
or quantitative nature. In some embodiments, the detection involves
imaging. Such imaging can be spectral or even multi-spectral.
[0012] In some embodiments of the present invention, the CNTs are
chemically derivatized. Such chemical derivatization expands the
range of solvents and solvent systems that can be employed to
generate the suspension of single-wall carbon nanotubes as utilized
in the present invention. Such chemical derivatization can be
removed via thermal and/or chemical treatments subsequent to
printing such inks and/or incorporating such markers.
[0013] In some embodiments, the CNTs are homogenized by electronic
type according to a separation procedure. Generally, this
translates to a concentration of one electronic type within a
mixture of types (e.g., increasing the amount of semiconducting
CNTs with respect to metallic and semi-metallic CNTs). Thus, in
some embodiments, the population of CNTs for a particular
application may be largely semiconducting CNTs with a small range
of bandgaps.
[0014] In some embodiments, through such above-described chemical
derivatizations and/or separation procedures, "designer"
compositions of CNTs can be used in which the photoluminescence
properties of the CNT-based inks and markers are tuned within a
range of excitation and emission wavelengths. This provides for an
almost limitless variety of unique inks and markers with which to
incorporate into, and/or associate with, articles for
identification, anti-counterfeiting, and authentication purposes.
In some embodiments, the fluorescence characteristics of a
population of CNTs is varied by modulating the parameters of the
CNT synthesis.
[0015] In some embodiments, the invention is drawn to a suspension
of CNTs, such as a suspension of single-walled carbon nanotubes
(SWNTs), wherein the suspension serves as an invisible ink. In some
embodiments, this ink is an aqueous suspension. When dried, this
nanotube ink is virtually invisible. However, the nanotube ink will
fluoresce when illuminated with light of an appropriate wavelength;
for instance it will glow in the near-infrared when illuminated
with visible light of the appropriate wavelength. If partly or
fully structure-separated nanotube samples are used, then one can
prepare inks that have distinct wavelengths of excitation and
emission.
[0016] In some embodiments of the present invention, a dilute
aqueous surfactant suspension of CNTs, such as SWNTs, is applied to
paper or cloth using flowing ink pens, inkjet printers, etc.,
wherein such a suspension (dispersion) is the ink. After drying,
the "ink" can be illuminated with visible light matching a second,
third, or higher van Hove optical transition of the semiconducting
carbon nanotubes. This yields a luminescence emission at a
corresponding first van Hove wavelength in the near-infrared.
Resulting images (in the case of written words, shapes, and/or
patterns) can be visualized in the near-infrared using appropriate
near-infrared detection equipment (e.g., an InGaAs camera).
Spectral filtering can also distinguish different nanotube species
in the ink, because each will show distinct absorption and emission
wavelengths. This latter aspect is highly relevant in embodiments
wherein pluralities of nanotubes have been manipulated to be
concentrated in a particular type species within the greater
collection of CNTs within the ink or marker.
[0017] In some embodiments, the compositions (inks and markers) of
the present invention are used as anti-counterfeiting markings for
high-value items, such as currency. Nanotubes of different
diameters can be used to prepare various inks for selectively
inscribing different denominations of bills. For example, a $100
bill would exhibit fluorescence only with a specific combination of
excitation and observation wavelengths; a $50 bill with a different
combination, etc. This spectral selectivity feature can be used
with or without imaging detection.
[0018] In some embodiments, the compositions and methods of the
present invention are used to provide spectral "bar coding" for
non-contact identification of items, such as clothing. Combinations
of different nanotube inks can be applied to merchandise at the
factory and then detected remotely by an infrared scanner for
inventory-taking, identification at a sales counter, or theft
control. The selective use of several different nanotube species
provides many possible combinations of emission wavelengths that
can be used to generate spectral bar code identifiers.
[0019] In some embodiments, the compositions of the present
invention are used in currency as replacements for the magnetic
identifiers currently used to identify different denominations
(such as by integration of the nanotube inks into the currency), in
machines, such as, for example, vending machines. Such an
application would rely on optical detection rather than magnetic
detection to differentiate the bills. Alternatively, such optical
identifiers of the present invention can be used in combination
with existing identifiers (e.g., magnetic materials and/or
fluorescent dyes). Numerous other applications for such
inks/markers exist.
[0020] The present invention provides improvement over the existing
fluorescent identifiers in that the unique excitation and emission
wavelengths of these nanotube inks and markers cannot be simulated
by conventional fluorescent materials. Furthermore, the region of
the EM spectrum in which these inks and markers fluoresce is
generally inaccessible with other fluorophores. Also, there is
virtually no background emission in the near-infrared, so only tiny
quantities of nanotubes are required for marking. Furthermore,
other fluorescent ink materials do not offer the variety of
wavelength-specific forms that can provide the added information
and security of nanotube ink. Finally, numerous methods of inducing
luminescence and known detection systems capable of detecting the
emission can be employed.
[0021] The foregoing has outlined rather broadly the features of
the present invention in order that the detailed description of the
invention that follows may be better understood. Additional
features and advantages of the invention will be described
hereinafter which form the subject of the claims of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] For a more complete understanding of the present invention,
and the advantages thereof, reference is now made to the following
descriptions taken in conjunction with the accompanying drawings,
in which:
[0023] FIG. 1 is a fluorescence spectral analysis of an aqueous
(D.sub.2O) suspension of SWNTs obtained using a single-wavelength
excitation (651 nm), wherein the SWNTs are surfactant-suspended
with sodium dodecylsulfate (SDS), and wherein a deconvolution of
the peaks illustrates the manner in which the fluorescence is
highly unique to a particular collection of CNTs such that each one
of the deconvoluted peaks in the figure is the result of a
different semiconducting SWNT species being present, the particular
species being indicated by the n,m indices above each peak;
[0024] FIG. 2 depicts fluorescence spectra of two batches of SWNTs
produced by the same reactor, but under slightly different
synthesis conditions, that yield different fluorescence signatures
when irradiated with 660 nm radiation from a diode laser source,
wherein the relative quantities of particular SWNT semiconducting
species within Batch 1 (dashed line) differ from those within Batch
2 (solid line);
[0025] FIG. 3 illustrates a near-infrared photograph of an
embodiment wherein single-wall carbon nanotubes are applied to a
surface (as ink) and illuminated with light in the visible region
to effect photoluminescence in the near-infrared;
[0026] FIG. 4 is a fluorescence spectrum of SWNT fluorescence
markers that have been integrated into a PMMA host, wherein
excitation is at 669 nm from a diode laser; and
[0027] FIG. 5 depicts the fluorescence spectra of a single sample
of SWNT (HiPco, Rice University), taken with three different
excitation wavelengths, wherein excitation wavelengths are as
follows: trace a, 669 nm; trace b, 573 nm; and trace c, 723 nm.
DETAILED DESCRIPTION OF THE INVENTION
[0028] The present invention is directed toward fluorescent inks
and markers comprising carbon nanotubes, and to methods of using
carbon nanotubes (CNTs) as fluorescent identifiers for
anti-counterfeiting and authentication purposes.
[0029] Carbon nanotubes (CNTs) comprising multiple concentric
shells and termed multi-wall carbon nanotubes (MWNTs) were
discovered by Iijima in 1991 [Iijima, Nature 1991, 354, 56].
Subsequent to this discovery, single-wall carbon nanotubes (SWNTs),
comprising a single graphene rolled up on itself, were synthesized
in an arc-discharge process using carbon electrodes doped with
transition metals [Iijima, S.; Ichihashi, T. Nature 1993, 363, 603;
and Bethune et al. Nature 1993, 363, 605]. These carbon nanotubes
(especially CNTs with diameters less than about 3 nm, e.g., SWNTs)
possess unique mechanical, electrical, thermal and optical
properties, and such properties make them attractive for a wide
variety of applications. See Baughman et al., Science, 2002, 297,
787-792.
[0030] The diameter and chirality of individual CNTs are described
by integers "n" and "m," where (n,m) is a vector along a graphene
sheet which is conceptually rolled up to form a tube. When
|n-m|=3q, where q is an integer, the CNT is a semi-metal (bandgaps
on the order of milli eV). When n-m=0, the CNT is a true metal and
referred to as an "armchair" nanotube. All other combinations of
n-m are semiconducting CNTs with bandgaps typically in the range of
0.5 to 1.5 eV. See O'Connell et al., Science, 2002, 297, 593. CNT
"type," as used herein, refers to such electronic types described
by the (n,m) vector (i.e., metallic, semi-metallic, and
semiconducting). CNT "species," as used herein, refers to CNTs with
discrete (n,m) values. It is the semiconducting CNTs that possess
fluorescence properties that make them useful as the optical
identifiers of the present invention.
[0031] All known preparative methods lead to polydisperse materials
of semiconducting, semimetallic, and metallic electronic types. See
M. S. Dresselhaus, G. Dresselhaus, P. C. Eklund, Science of
Fullerenes and Carbon Nanotubes, Academic Press, San Diego, 1996;
Bronikowski et al., Journal of Vacuum Science & Technology
2001, 19, 1800-1805; R. Saito, G. Dresselhaus, M. S. Dresselhaus,
Physical Properties of Carbon Nanotubes, Imperial College Press,
London, 1998. As such, a primary hurdle to the widespread
application of CNTs, and SWNTs in particular, is their manipulation
according to electronic structure [Avouris, Acc. Chem. Res. 2002,
35, 1026-1034].
[0032] Recent advances in the solution phase dispersion [Strano et
al., J. Nanosci. and Nanotech., 2003, 3, 81; O'Connell et al.,
Science, 2002, 297, 593-596] along with spectroscopic
identification using bandgap fluorescence [Bachilo et al., Science,
2002, 298, 2361; and commonly-assigned U.S. patent application Ser.
No. 10/379,273] and Raman spectroscopy [Strano, Nanoletters 2003,
3, 1091] have greatly improved the ability to monitor electrically
distinct nanotubes as suspended mixtures and have led to definitive
assignments of the optical features of semiconducting [Bachilo et
al., Science, 2002, 298, 2361], as well as metallic and
semi-metallic species [Strano, Nanoletters, 2003, 3, 1091]. Indeed,
such spectroscopic assignments can provide a background for the
optical bar coding of the present invention.
[0033] Shown in FIG. 1 is a fluorescence spectral analysis of an
aqueous (D.sub.2O) suspension of SWNTs using a single-wavelength
excitation (651 nm), wherein the SWNTs are surfactant-suspended
with sodium dodecylsulfate (SDS). Deconvolution of the peaks
illustrates the manner in which the fluorescence is highly unique
to a particular collection of CNTs. Each one of the deconvoluted
peaks in the figure is the result of a different semiconducting
SWNT species being present, the particular species being indicated
by the n,m indices above each peak. Emission intensity for each
peak is a function of the relative concentration of the particular
species providing for a particular peak.
[0034] Techniques of chemically functionalizing CNTs have greatly
facilitated the ability to manipulate these materials, particularly
for SWNTs which tend to assemble into rope-like aggregates [Thess
et al., Science, 1996, 273, 483-487]. Such chemical
functionalization of CNTs is generally divided into two types: tube
end functionalization [Chen et al., Science, 1998, 282, 95-98], and
sidewall functionalization [PCT publication WO 02/060812 by Tour et
al.; Holzinger et al., Angew. Chem. Int. Ed, 2001, 40, 4002-4005;
Khabashesku et al., Acc. Chem. Res., 2002, 35, 1087-1095]. Most
recently, SWNTs were shown to be selectively functionizable,
providing a chemical route to their separation. See Strano et al.,
Science, 2003, 301, 1519-1522; and commonly-assigned International
Patent Application No. PCT/US04/24507.
[0035] Carbon nanotube chemistry has been described using a
pyramidization angle formalism [S, Niyogi et al., Acc. of Chem.
Res., 2002, 35, 1105-1113]. Here, chemical reactivity and kinetic
selectivity are related to the extent of s character due to the
curvature-induced strain of the sp.sup.2-hybridized graphene sheet.
Because strain energy per carbon is inversely related to nanotube
diameter, this model predicts smaller diameter nanotubes to be the
most reactive, with the enthalpy of reaction decreasing as the
curvature becomes infinite. While this behavior is most commonly
the case, the role of the electronic structure of the nanotubes in
determining their reactivity is increasingly important--especially
when desiring selectivity among a population of similar-diameter
CNTs (such as is often the case with SWNT product). Furthermore,
because such structure is highly sensitive to chiral wrapping,
chemical doping, charged adsorbates, as well as nanotube diameter,
there exists a considerable diversity among these various pathways
in addition to a simple diameter dependence, and with implications
for separating CNTs by type.
[0036] Other methods with which CNTs can be separated by type have
been reported. Such techniques include dielectrophoresis [Krupke et
al., Science, 2003, 301, 244-347], selective precipitation
[Chattophadhyay et al., J. Am. Chem. Soc., 2003, 125, 3370-3375],
ion-exchange chromatography [Zheng et al., Nature Mater., 2003, 2,
338-342], and complexation/centrifugation [Chen et al., Nano Lett.,
2003, 3, 1245-1249].
[0037] Carbon nanotubes (CNTs), according to the present invention,
include, but are not limited to, single-wall carbon nanotubes
(SWNTs), multi-wall carbon nanotubes (MWNTs), double-wall carbon
nanotubes, buckytubes, fullerene tubes, tubular fullerenes,
graphite fibrils, and combinations thereof. Such carbon nanotubes
can be of a variety and range of lengths, diameters, number of tube
walls, chiralities (helicities), etc., and can be made by any known
technique including, but not limited to, arc discharge [Ebbesen,
Annu. Rev. Mater. Sci. 1994, 24, 235-264], laser oven [Thess et
al., Science 1996, 273, 483-487], flame synthesis [Vander Wal et
al., Chem. Phys. Lett. 2001, 349, 178-184], chemical vapor
deposition [U.S. Pat. No. 5,374,415], wherein a supported [Hafner
et al., Chem. Phys. Lett. 1998, 296, 195-202] or an unsupported
[Cheng et al., Chem. Phys. Lett. 1998, 289, 602-610; Nikolaev et
al., Chem. Phys. Lett. 1999, 313, 91-97] metal catalyst may also be
used, and combinations thereof. While not intending to be bound by
theory, it is believed that the CNTs exhibiting photoluminescence
in accordance with the present invention typically have diameters
less than about 3 nm.
[0038] Depending on the embodiment, the CNTs can be subjected to
one or more processing steps. In some embodiments, the CNTs have
been purified. Exemplary purification techniques include, but are
not limited to, those by Chiang et al. [Chiang et al., J. Phys.
Chem. B 2001, 105, 1157-1161; Chiang et al., J. Phys. Chem. B 2001,
105, 8297-8301]. In some embodiments, the CNTs have been cut by a
cutting process. See Liu et al., Science 1998, 280, 1253-1256; Gu
et al., Nano Lett. 2002, 2(9), 1009-1013; Haddon et al., Materials
Research Society Bulletin, 2004, 29, 252-259. The terms "carbon
nanotube" and "nanotube" will be used interchangeably herein.
[0039] Depending on the embodiment, the CNTs used in the inks,
markers, and methods of the present invention can be separated by
length, diameter, type or species and/or chemically derivatized
according to any of the above-described separation and/or chemical
derivatization methods. "Separation," as defined herein, generally
involves the concentration of CNTs of a particular type, dimension,
or species. The extent of such separation, and the level at which
it is carried out, can lead to essentially homogeneous populations
of CNTs comprising a particular type, dimension, or species of
CNT.
[0040] Generally, the fluorescent inks of the present invention
("nanotube inks") comprise a dispersion or suspension of CNTs in a
liquid (i.e., solvent) medium. Such a dispersion may further
comprise surfactant species and/or other traditional ink
components. Fluorescent markers of the present invention are simply
compositions CNTs with established photoluminescent properties that
can be attached, incorporated into, and/or otherwise associated
with an article for the purpose of identification and/or
authentication. In some embodiments, the marker compositions may
further comprise material other than CNTs, e.g., polymer.
[0041] In some embodiments, the CNTs within a nanotube ink or
marker have been homogenized or separated by one or more of the
above-described techniques. In some embodiments, separation and/or
chemical derivatization techniques, such as those described above,
are used to generate unique, designer mixtures of nanotubes of
varying type, dimension and/or species. In some embodiments, it is
merely enough to predetermine the fluorescence properties of the
CNT mixture in a qualitative or quantitative manner. In some
embodiments, the process parameters for the synthesis of the CNTs
are altered so as to produce CNTs with a slightly different
fluorescence signature. Indeed, there is greater flexibility for
unique fluorescence signatures from mixtures than from homogenous
populations (of CNTs). Referring to FIG. 2, it can be seen that
SWNTs produced by the same reactor, but under slightly different
synthesis conditions, yield different fluorescence signatures when
irradiated with 660 nm radiation from a diode laser source, wherein
the relative quantities of particular SWNT semiconducting species
within Batch 1 (dashed line) differ from those within Batch 2
(solid line).
[0042] Suitable solvent media for nanotube inks and markers
include, but are not limited to, water, alcohols, alkanes,
N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO),
o-dichlorobenzene, benzene, xylenes, toluene, mesitylene,
tetrahydrofuran, chloroform, dichloromethane, FREONs (general class
of halocarbons, primarily fluorinated hydrocarbons), supercritical
fluids (SCFs, such as supercritical CO.sub.2), and combinations
thereof.
[0043] Surfactants, according to the present invention, can be any
chemical agent which facilitates the dispersion of carbon nanotubes
in water or other solvent media. Surfactants include ionic
(cationic and anionic) surfactants and non-ionic surfactants.
Suitable surfactants include, but are not limited to, sodium
dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS),
sodium octylbenzene sulfonate, TRITON X-100, TRITON X-405,
dodecyltrimethylammonium bromide (DTAB), and combinations thereof.
Such surfactants can aid in maintaining the stability of the
dispersion and/or facilitating the wetting of a surface by the
nanotube ink.
[0044] In some embodiments, the CNTs are dispersed in a superacid
(e.g., oleum) or other intercalating media. See Ramesh et al., J.
Phys. Chem. B, 2004, 108, 8794-8798.
[0045] In some embodiments, the CNTs, as markers, are incorporated
into a polymer host as a composite or blend material, wherein the
CNTs have a predetermined fluorescence signature. This blend can
then be used to fabricate articles of manufacture, objects, or
parts. In some embodiments, polymer fibers comprising such CNT
fluorescence markers are fabricated. Such fibers can be used to
make paper, currency, textiles, etc. Generally, but not always, the
material or article into which the CNT fluorescence markers are
being blended should be transparent to both the excitation and
emission wavelengths used to detect and analyze the fluorescence
signature.
[0046] Methods of using the fluorescent nanotube inks of the
present invention can generally comprise the steps of: 1)
depositing a suspension of CNTs onto a surface, generally in the
form of words, shapes, and/or patterns, and 2) removing the
solvent. Provided that relatively small quantities of the CNTs are
actually transferred to the surface, such words, shapes, and/or
patterns can be said to be invisible and the nanotube ink referred
to as an invisible ink. Such methods typically also can comprise
the steps of 3) irradiating the ink with a visible light source,
and 4) viewing the resulting fluorescence with a NIR optical
viewer, such as an InGaAs camera or other such device.
[0047] As above, CNTs within such a suspension (dispersion) may
previously have been subjected to separation and/or chemical
derivatization techniques to generate homogenous and/or designer
mixtures of CNTs. In some embodiments, where the CNTs have been
subjected to chemical derivatization, a further step of thermal
and/or chemical defunctionalization is used to cause the CNTs to
revert back to their original underivatized state [Bahr et al., J.
Mater. Chem., 2002, 12, 1952-1958]. Depending on the embodiment,
and for use as an ink, it is generally sufficient to merely have a
qualitative knowledge of the fluorescent properties of the CNTs
within the mixture, such that no manipulation of the CNTs by type,
dimension (length, diameter), or species is necessary for their use
as inks.
[0048] The suspension of CNTs generally has the CNTs dispersed in a
suitable solvent medium. As described above, there is considerable
variety in the selection of such media. Often, a surfactant is
added to provide for or enhance the suspension of CNTs.
[0049] In some embodiments of the present invention, the process of
forming a mixture of surfactant-suspended carbon nanotubes
comprises a homogenizing step. A homogenizing step, according to
the present invention, can be any method which suitably homogenizes
the mixture and renders at least some of the carbon nanotubes
encapsulated in micellar-like assemblies.
[0050] In some embodiments of the present invention, the process of
forming an mixture of surfactant-suspended carbon nanotubes further
comprises ultrasonic assistance. Ultrasonic assistance can be
provided by either an ultrasonic bath or an ultrasonic horn
sonicator, typically operating at a power from between about 200 W
to about 600 W. The duration of such ultrasonic assistance
typically ranges from about 1 min to about 20 min.
[0051] In some embodiments of the present invention, the mixture of
surfactant-suspended carbon nanotubes is centrifuged to separate
the surfactant-suspended nanotube material from other material. In
such embodiments, the other material gravitates to the bottom and
the surfactant-suspended carbon nanotubes are decanted. In some
embodiments of the present invention, the centrifugation is
provided by an ultracentrifuge, and centrifugation is performed
with an intensity which ranges generally from about 10,000 rpm to
about 90,000 rpm, and for a duration which ranges generally from
about 1 hour to about 6 hour.
[0052] In some embodiments, one or more additional materials are
added to the suspension of CNTs (the ink). Such additional
materials may include, dyes, binders, traditional fluorescent inks,
magnetic materials, nanoparticles, or other materials used in the
formulation of inks.
[0053] Surfaces or substrates, according to the present invention,
include but are not limited to, paper, natural or synthetic fibers,
metals, polymeric materials, ceramics, glasses, etc. In some
embodiments, the surface is pretreated to facilitate adhesion of
the ink. Such pretreatments can be of a chemical (e.g., etching) or
physical (e.g., plasma) nature.
[0054] Depositing the suspension of CNTs, as nanotube ink, onto a
surface can be by way of any number of standard printing
techniques. Such techniques include, but are not limited to, inkjet
printing, screen printing, lithographic techniques, brushing,
spraying, flowing ink pens, stamping, and combinations thereof.
[0055] As an ink, such deposition can generally be in some
patterned form such as words and/or shapes and symbols. In some
embodiments, however, this nanotube ink is invisible to the naked
eye.
[0056] Solvent removal generally involves an evaporative process.
Such evaporative processes can be facilitated by heat, vacuum,
and/or other processes.
[0057] In some embodiments, after solvent removal, an additional
treatment is applied to the deposited ink. Such additional
treatments generally serve to protect the integrity of the words,
shapes or symbols printed on a surface. Such treatments may
comprise a lamination, e.g., the deposition of a polymer or glass
film over the deposited ink, wherein the deposited polymer of glass
is transparent to both the excitation and emission wavelengths
required to induce and detect fluorescence.
[0058] Irradiation of the deposited ink can be done with a variety
of visible light sources. Such sources provide the excitation
required for fluorescence and can be monochromatic or polychromatic
in nature. In some embodiments, the excitation source is a laser.
In some embodiments, the excitation source has a wavelength near or
above 750 nm so as to be essentially invisible itself.
[0059] Viewing or detecting the fluorescent emission, which has a
frequency in the near infrared region of the EM spectrum, is
generally done with an near infrared viewer or camera, such as an
InGaAs camera or imager. It is generally not necessary, in the case
of such inks, to resolve the spectral information, but this can be
done with spectral imaging techniques when desired.
[0060] Methods of using CNTs as fluorescent identifiers (e.g.,
spectral bar codes) rely on a knowledge of their photoluminescence
properties and on techniques of incorporating and/or attaching such
species to articles being marked or tagged. Generally, such methods
comprise the steps of: 1) providing a plurality of carbon nanotubes
with unique, predetermined photoluminescence characteristics; and
2) incorporating the carbon nanotubes into articles as optical
identifiers to form optically tagged articles. Methods of using
CNTs as fluorescent identifiers may further comprise the steps of:
3) irradiating the optically tagged articles with EM radiation; and
4) detecting photoluminescence from the carbon nanotubes for the
purpose of identifying the optically tagged article.
[0061] The use of CNTs compositions as markers generally requires a
thorough understanding of their fluorescence properties. In some
embodiments, the CNTs have been subjected to separation and/or
chemical derivatization techniques to generate homogenous and/or
designer mixtures of CNTs. In some embodiments, where the CNTs have
been subjected to chemical derivatization, a further step of
thermal and/or chemical defunctionalization is used to cause the
CNTs to revert back to their original, underivatized state. In some
embodiments, CNTs are synthesized as unique mixtures with unique
fluorescent properties. Regardless of how the CNTs have been
synthesized and/or processed, the fluorescence signature (i.e.,
spectra) for such CNTs is carefully evaluated (i.e., predetermined)
prior to marking or tagging articles or objects with the CNT
fluorescent markers.
[0062] In some embodiments, the CNT markers can be suspended in a
solvent medium and applied as an ink (as above). Such inks could
contain multiple levels of information, wherein the shapes and/or
words contain one level of information and additional levels of
spectral information can be contained within the CNT marker
compositions within the inks.
[0063] In some embodiments, the markers are incorporated into a
host material, wherein the host material is generally transparent
to the excitation and emission wavelengths with which the CNTs
fluoresce. Typically, such host materials are polymeric in nature,
but they can also be ceramic or glass. The CNT markers can be
attached to an article either directly or in a host material. In
some embodiments, the CNTs markers are incorporated into a host
material that makes up an article or manufacture. For example, CNT
markers could be blended into synthetic fibers which are then used
to make articles of clothing. In some embodiments, the host
material is liquid or fluid.
[0064] Irradiating the CNT fluorescent markers can be done with a
variety of visible light sources. Such sources provide the
excitation required for fluorescence and can be monochromatic or
polychromatic in nature. In some embodiments, the excitation source
is a laser. Suitable lasers sources include, but are not limited
to, solid state diode lasers, HeNe lasers, Ar lasers, Kr lasers,
and combinations thereof. In some embodiments, greater
differentiation between sets of CNTs is afforded by the use of two
or more discrete excitation wavelengths.
[0065] Detection of the emission can be qualitative in nature using
spectral filters and such. More typically, however, detection is
such that the spectral signature of the CNT markers is resolved,
thus providing a high level of identification. Such spectral
resolution is typically provided via spectroscopic gratings and NIR
detectors. Suitable NIR detectors include, but are not limited to,
photodiodes, photomultipliers, one- or two-dimensional photodiode
arrays, or CCD or CMOS cameras based on semiconductors such as Si,
Ge, or InGaAs.
[0066] Applications for the nanotube inks and markers of the
present invention include, but are not limited to, authentication
of currency, security documents, passports, drivers licenses,
pharmaceuticals, clothing and other consumer goods, books, art, and
combinations thereof. Such inks and markers can be used in quality
or process control to identify batches. Such inks or markers could
also be used in leak detection or other similar applications.
Additionally such inks and/or markers could be used in combination
with other methods of authentication and identification such as
magnetic devices, strips or labels.
[0067] The following examples are provided to more fully illustrate
some of the embodiments of the present invention. It should be
appreciated by those of skill in the art that the techniques
disclosed in the examples which follow represent techniques
discovered by the inventors to function well in the practice of the
invention, and thus can be considered to constitute exemplary modes
for its practice. However, those of skill in the art should, in
light of the present disclosure, appreciate that many changes can
be made in the specific embodiments that are disclosed and still
obtain a like or similar result without departing from the spirit
and scope of the invention.
Example 1
[0068] This Example serves to illustrate a manner in which nanotube
inks can be used according to some embodiments of the present
invention.
[0069] SWNTs (HiPco, Rice University) were dispersed in an aqueous
solution of SDS surfactant by an accepted process of high-shear
mixing, ultrasonic agitation, and ultracentrifugation to create a
nanotube ink.
[0070] The nanotube ink suspension was used to fill the reservoir
of a flowing ink drafting pen, which was then used with a drafting
template to write the characters "SWNT" onto a piece of office
paper manufactured with a mild gloss coating. The ink was then
allowed to dry such that the resulting characters were
approximately 5 mm in height and with a mass per character of
approximately 10 nanograms. Such a small amount of SWNTs renders
the ink invisible to the naked eye.
[0071] Upon irradiation with monochromatic light of 671 nm
wavelength, the letters were seen to fluoresce, emitting radiation
in the NIR. This emission was detected with a camera with a
detection range of 1125-1700 nm. While invisible to the naked eye,
FIG. 3 is an image generated by this NIR camera with an exposure of
6 video frames.
Example 2
[0072] This Example serves to illustrate how CNT fluorescent
markers can be integrated into host materials like polymers.
[0073] SWNTs (HiPco, Rice University) were blended into a
poly(methylmethacrylate) (PMMA) matrix by ultrasonic dispersion of
SWNTs into a xylene solution of PMMA. Evaporation of the xylene
gave an optically clear solid containing fluorescent SWNTs.
[0074] FIG. 4 is a fluorescence spectrum of these SWNT markers
which have been integrated into the PMMA host, wherein excitation
is at 669 nm from a diode laser. Each peak indicated by an arrow
corresponds to fluorescence from a different SWNT species within
the sample.
Example 3
[0075] This Example serves to illustrate excitation selectively
within a CNT sample comprising a variety of CNT species.
[0076] In the case of SWNTs, due to excitation selectivity, only a
subset of SWNT types will be detectible with some "standard"
excitation wavelength, such as 660 to 670 nm (the region where some
common semiconductor diode lasers emit). Many nanotube types that
might be present in the sample may be hidden or exhibit an emission
intensity which is too low to be detected, especially where such
peaks lie close to intense peaks activated with such "standard"
excitation wavelengths. The application of additional excitation
wavelengths can possibly reveal these typically "hidden" peaks (and
the semiconducting SWNT species that produce them).
[0077] FIG. 5 depicts the fluorescence spectra of the same SWNT
sample (HiPco, Rice University), taken with three different
excitation wavelengths, wherein excitation wavelengths are as
follows: trace a, 669 nm; trace b, 573 nm; and trace c, 723 nm.
[0078] It can be seen from FIG. 5 that the relative intensities of
the peaks change when the excitation frequency is changed,
revealing a selectivity to the excitation. Such excitation
selectivity can be exploited in anti-counterfeiting
applications.
[0079] All patents and publications referenced herein are hereby
incorporated by reference. It will be understood that certain of
the above-described structures, functions, and operations of the
above-described embodiments are not necessary to practice the
present invention and are included in the description simply for
completeness of an exemplary embodiment or embodiments. In
addition, it will be understood that specific structures,
functions, and operations set forth in the above-described
referenced patents and publications can be practiced in conjunction
with the present invention, but they are not essential to its
practice. It is therefore to be understood that the invention may
be practiced otherwise than as specifically described without
actually departing from the spirit and scope of the present
invention as defined by the appended claims.
* * * * *