Fluorescent single-walled carbon nanotubes following the 1,3-dipolar cycloaddition of pyridinium ylides

Article abstract of DOI:10.1021/ja903712f

Pyridinium ylides generated from simple Kroehnke salts undergo a 1,3-dipolar cycloaddition to single-walled carbon nanotubes (SWNTs) offering a simple and convenient method for the covalent modification of carbon nanotubes. The indolizine functionalized SWNTs generated, emit blue light when excited at 335 nm. The location and distribution of the functional groups was determined by AFM using electrostatic interactions with gold nanoparticles. While resonance Raman spectroscopy showed that the 1,3-dipolar cylcloaddition of the pyridinium ylides to the nanotube surface was selective for metallic and large diameter semiconducting SWNTs. The indolizine functionalized SWNTs were further characterized using FTIR, UV-vis-NIR, TGA-MS, and XPS.

Full text of DOI:10.1021/ja903712f

Published on Web 07/01/2009
Fluorescent Single-Walled Carbon Nanotubes Following the
1,3-Dipolar Cycloaddition of Pyridinium Ylides
Mustafa K. Bayazit and Karl S. Coleman*
Department of Chemistry, Durham UniVersity, South Road, Durham DH1 3LE, United Kingdom
Received May 7, 2009; E-mail: k.s.coleman@durham.ac.uk
Abstract: Pyridinium ylides generated from simple Kro¨hnke salts undergo a 1,3-dipolar cycloaddition to
single-walled carbon nanotubes (SWNTs) offering a simple and convenient method for the covalent
modification of carbon nanotubes. The indolizine functionalized SWNTs generated, emit blue light when
excited at 335 nm. The location and distribution of the functional groups was determined by AFM using
electrostatic interactions with gold nanoparticles. While resonance Raman spectroscopy showed that the
1,3-dipolar cylcloaddition of the pyridinium ylides to the nanotube surface was selective for metallic and
large diameter semiconducting SWNTs. The indolizine functionalized SWNTs were further characterized
using FTIR, UV-vis-NIR, TGA-MS, and XPS.
nanotube surface have included radical,12,13 cycloaddition,14,15
and oxidation reactions.16,17 The chemical modification of CNTs
Introduction
Carbon nanotubes (CNTs) have impressive electrical, thermal,
and mechanical properties and many potential applications
ranging from composite materials, energy storage, sensors, and
field emission devices to nanoscale electronic components have
been envisaged.1,2 Unfortunately, due to strong intertube van
der Waals forces CNTs, in particular single-walled carbon
nanotubes (SWNTs), have a tendency to aggregate together into
bundles, resulting in low solubility in both aqueous and
nonaqueous solvents, making their handling and processing
difficult. For this reason, much effort has been spent on
introducing organic and inorganic functional groups to the
surface of nanotubes to improve solubility and or provide a site
for attachment of polymers, biomolecules, or other material.
We have recently shown that the simple addition of tertiary
phosphines to SWNTS can improve the solubility of the
nanotubes in common organic solvents.3 Noncovalent modifica-
tion of CNTs, often resulting in increased solubility, can be
achieved by using surfactants,4 polymer wrapping5,6 or by
π-stacking of pyrenes, or other polycyclic aromatic compounds,
onto the nanotube surface.7-11 Whereas, covalently, strategies
to improve solubility and to introduce functional groups to the
has been extensively reviewed.18
One of the most versatile routes to the covalent chemical
modification of carbon nanotubes is perhaps 1,3-dipolar cy-
cloadditions. In particular, the cycloaddition of azomethine
ylides pioneered by Prato.19 The reactive azomethine ylides used
are typically generated in situ by decarboxylation of immonium
salts derived from the thermal condensation of glycines with
aldehydes,20 or to a lesser extent by thermal ring-opening of
aziridines.21 The introduction of pyrrolidines to the surface of
CNTs has been used extensively to aid in the transfection of
CNTs into mammalian cells and for the delivery of pharmaco-
logically active molecules such as peptides, MTX, and ampho-
(9) Chen, R. J.; Zhang, Y.; Wang, D.; Dai, H. J. Am. Chem. Soc. 2001,
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H. P.; Qin, A. J.; Lam, J. W. Y.; Zheng, Q.; Tang, B. Z. Macromol-
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R. C. J. Am. Chem. Soc. 2003, 125, 14893.
(14) Tagmatarchis, N.; Prato, M. J. Mater. Chem. 2004, 14, 437.
(15) Azamian, B. R.; Coleman, K. S.; Davis, J. J.; Hanson, N.; Green,
M. L. H. Chem. Commun. 2002, 366.
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Haroz, E.; Kuper, C.; Tour, J. M.; Ausman, K. D.; Smalley, R. E.
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M. Chem. Mater. 2007, 19, 1076.
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Fluorescent SWNTs Using Cycloaddition of Ylides
A R T I C L E S
tericin B.22,23 Other 1,3-dipolar cycloaddition reactions to carbon
nanotubes have involved ozone,24,25 nitrile imines,26 and nitrile
oxides.27
SWNT-Indolizine (Microwave Heating) (2b). Using a method
similar to that described above, SWNTs (10 mg) were dispersed
in N,N-dimethylformamide (15 mL) using mild sonication in an
ultrasonic bath (Ultrawave U50, 30 - 40 kHz) for 5 min. The
pyridinium salt (1) (1.026 g, 4.17 mmol) and triethylamine (0.58
mL, 4.177 mmol) was then added to the dispersion. The reaction
mixture was then heated to 150 °C at 2 bar pressure, in a heat and
pressure resistant vessel, with microwaves for 1 h (150 W for 5
min followed by 20 W for 50 min at 2.54 GHz) using a Biotage
Initiator Sixty. The SWNTs were washed and isolated as described
above to afford SWNT-indolizine (2b).
Characterization. AFM. Samples for AFM analysis were
produced by drop deposition onto freshly cleaved mica of the
corresponding solution of SWNTs (∼0.005 mg mL-1) in N,N-
dimethylformamide produced by sonication in an ultrasonic bath
(Ultrawave U50, 30-40 kHz) for 15 min. Samples were dried in
air before imaging in tapping mode using a Digital Instruments
Multimode AFM with a Nanoscope IV controller.
XPS. XPS studies were performed at NCESS, Daresbury
laboratory using a Scienta ESCA 300 hemispherical analyzer with
a base pressure under 3 × 10-9 mbar. The analysis chamber was
equipped with a monochromated Al KR X-ray source (hν ) 1486.6
eV). Charge compensation was achieved (if required) by supplying
low energy (<3 eV) electrons to the samples. XPS data were
referenced with respect to the corresponding C 1s binding energy
of 284.5 eV which is typical for carbon nanotubes.17 Photoelectrons
were collected at a 45° takeoff angle, and the analyzer pass energy
was set to 150 eV giving an overall energy resolution of 0.4 eV.
TGA-MS. Thermogravimetric analysis-mass spectrometry
(TGA-MS) data were recorded on 1-3 mg of sample using a
Perkin-Elmer Pyris I coupled to a Hiden HPR20 mass spectro-
meter. Data were recorded in flowing He (20 mL min-1) at a ramp
rate of 10 °C min-1 to 900 °C after being held at 120 °C for 30
min to remove any residual solvent.
UV-vis-NIR Spectroscopy. The UV-vis-NIR absorption spec-
tra were recorded on a Perkin-Elmer Lambda 900 spectrometer.
The samples were prepared by dispersing the nanotube material in
N,N-dimethylformamide (∼0.03 mg mL-1) by sonication in an
ultrasonic bath (Ultrawave U50, 30-40 kHz) for 5 min followed
by filtration through a plug of cotton wool to remove particulates
after allowing the solution to stand for 2 h.
Although the derivativization of CNTs is highly desirable,
the challenge remains to achieve sufficient functionalization of
the CNT surface to ensure ease of processing or facilitate the
attachment of active molecules or particles, while avoiding
significant degradation of the structure which could compromise
the exciting properties of the material.28,29 Herein, we report a
simple and convenient nondestructive modification of SWNTs
using 1,3-dipoar cycloaddition of pyridinium ylides, readily
prepared from simple Kro¨hnke salts. X-ray photoelectron
spectroscopy (XPS), thermogravimetric analysis-mass spec-
trometry (TGA-MS), atomic force microscopy (AFM), UV-vis-
NIR, FTIR and Raman spectroscopy have been employed to
characterize the functionalized material.
Experimental Section
Material Preparation. SWNTs. Purified SWNTs produced by
the HiPco method and supplied by Unidym, USA, were further
purified by heating in air at 400 °C, then soaking in 6 M HCl
overnight, followed by filtration over a polycarbonate membrane
(0.2 µm), and washing with copious amounts of high-purity water
until pH-neutral. The purified SWNTs were annealed under vacuum
(10-2 mbar) at 900 °C to remove residual carboxylic acid functional
groups and any adsorbed gases or solvents.
N-(Ethoxycarbonylmethyl)-pyridinium Bromide (1). The py-
ridinium salt (1) was prepared following a modified literature
procedure.30 Pyridine (100 mmol) was added to ethyl 2-bromoac-
etate (110 mmol) and the mixture stirred for 12 h at room
temperature. The resulting off-white solid was washed with diethyl
ether (3 × 20 mL) to remove excess pyridine and ethyl 2-bro-
moacetate to afford the pyridinium bromide salt (1) (22.64 g, 92%)
1
as confirmed by H and 13C NMR and mass spectrometry.
SWNT-Indolizine (Conventional Heating) (2a). SWNTs (10
mg) were dispersed in N,N-dimethylformamide (15 mL) using mild
sonication in an ultrasonic bath (Ultrawave U50, 30-40 kHz) for
5 min and the dispersion heated to 140 °C. The pyridinium salt (1)
(1.026 g, 4.17 mmol) was then added to the dispersion followed
by triethylamine (0.58 mL, 4.17 mmol) after 30 min. The reaction
mixture was refluxed for 5 days, and the functionalized SWNTs
were collected via filtration through a PTFE membrane (0.2 µm).
The solid SWNTs were then transferred to a cellulose thimble, and
impurities and unreacted reagents were removed by Soxhlet
extraction using acetonitrile for 18 h. The SWNTs were then
dispersed in deionized water (50 mL) and filtered through a PTFE
membrane (0.2 µm), dispersed in acetone (50 mL) and filtered
through a PTFE membrane (0.2 µm), and finally dispersed in
ethanol (50 mL) and filtered through a PTFE membrane (0.2 µm)
and dried overnight at 120 °C to afford SWNT-indolizine (2a).
Raman Spectroscopy. Raman spectra were recorded using a
Jobin Yvon Horiba LabRAM spectrometer in a back scattered
confocal configuration using He/Ne (632.8 nm, 1.96 eV), Nd:YAG
(532 nm, 2.33 eV) or diode (785 nm, 1.58 ev) laser excitation. All
spectra were recorded on solid samples over several regions and
were referenced to the silicon line at 520 cm-1
.
FTIR Spectroscopy. Infrared spectra were recorded on thick
films using a Perkin-Elmer Spectrum 100 equipped with a Pike
ATR fitted with a Ge crystal.
Fluorescence Spectroscopy. Fluorescence spectra were recorded
on a Perkin-Elmer LS55 luminescence spectrometer using an
excitiation wavelength of 335 nm. Samples were prepared by
dispersing SWNTs in N,N-dimethylformamide (0.1 mg mL-1), and
allowing them to settle for 8 h followed by filtration.
(22) Pantarotto, D.; Briand, J.-P.; Prato, M.; Bianco, A. Chem. Commun.
2004, 16.
(23) Kostarelos, K.; Lacerda, L.; Pastorin, G.; Wu, W.; Wieckowski, S.;
Luangsivilay, J.; Godefroy, S.; Pantarotto, D.; Briand, J.-P.; Muller,
S.; Prato, M.; Bianco, A. Nat. Nanotechnol. 2007, 2, 108, and
references therein.
Results and Discussion
(24) Mawhinney, D. B.; Naumenko, V.; Kuznetsova, A.; Yates, J. T., Jr.;
Liu, J.; Smalley, R. E. J. Am. Chem. Soc. 2000, 122, 2383.
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Langa, F. J. Phys. Chem. B 2004, 108, 12691.
Purified SWNTs were reacted with pyridinium ylides gener-
ated in situ by the addition of base to the Kro¨hnke salt
N-(ethoxycarbonylmethyl)-pyridinium bromide (1) to afford
SWNTs with indolizine groups covalently bound to the nanotube
surface (2), Scheme 1. The reaction can be carried out using
both conventional (2a) and microwave heating (2b) with the
later resulting in significantly shorter reaction times (5 days
versus 1 h). The use of microwaves to accelerate cyloaddition
reactions on a carbon nanotube surface was recently reported
for the cycloaddition of aziridines.21 The 1,3-dipolar cycload-
dition of the pyridinium ylide, to the SWNT surface, with the
(27) Alvaro, M.; Atienzar, P.; de la Cruz, P.; Delgado, J. L.; Troiani, V.;
Garcia, H.; Langa, F.; Palkar, A.; Echegoyen, L. J. Am. Chem. Soc.
2006, 128, 6626.
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Haddon, R. C. Science 1998, 282, 95.
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Bayazit and Coleman
Scheme 1. Schematic Representation of the 1,3-Dipolar
Cycloaddition of a Pyridinium Ylide to SWNTs to form an
Indolizine
Figure 1. FTIR spectra of purified SWNTs (black), indolizine function-
alized SWNTs (2a) prepared by conventional heating (red) and indolizine
functionalized SWNTs (2b) prepared by microwave heating (blue).
nanotube acting as the dipolarphile, is believed to occur with
the pyridinium ylide first adding to the nanotube surface to form
a pyrrolidine ring, closely followed by the addition of a second
ylide to the addendum on the nanotube surface to afford an
indolizine, Scheme 1. The driving force for the double addition
of the ylide is likely to be aromatization of the resulting
indolizine formed. A similar result occurs in the absence of
SWNTs with the pyridinium ylide generated from (1) undergo-
ing a 1,3-dipolar cycloaddition with a second pyridinium ylide
as evidenced by electrospray mass spectrometry (see Figure S1,
in Supporting Information). Indolizine derivatives are particu-
larly interesting as they are well-known for exhibiting a variety
of pharmacologically desirable properties, including cardiovas-
cular, anti-inflammatory, and antioxidant properties,31 as well
as having known fluorescent properties and being used in sensor
applications.32,33
Figure 2. Normalized (at 400 nm) UV-vis-NIR spectra, recorded in N,N-
dimethylformamide, of purified SWNTs (black), indolizine functionalized
SWNTs (2a) prepared by conventional heating (red) and indolizine
functionalized SWNTs (2b) prepared by microwave heating (blue).
600 °C compared to ∼7% for purified SWNTs, Figure 3. This
corresponds to the presence of ∼1 functional group for every
133 and 96 carbon atoms, respectively. The microwave condi-
tions giving slightly higher levels of functionalization. The peak
of mass loss is ∼300 °C, where, although the parent ion of the
indolizine is not observed, fragments relating to the ethyl ester
group (OEt, 45 amu) and N-methylpyridinium (94 amu) were
detected by mass spectrometry. Further support and quantifica-
tion of the level of functionalization is provided by XPS.
Measurements, Figure 4, show the presence of a N 1s peak at
a binding energy of 398.7 eV characteristic of sp2 nitrogen.38
Elemental composition analysis shows the presence of ∼2
atomic percent (at.%) of nitrogen in the sample which is
FTIR of the indolizine functionalized SWNTs (2a) and (2b),
Figure 1, shows the presence of the ester group (ν(CO) 1730
cm-1) and two bands characteristic of an indolizine at 1632 and
34
1541 cm-1
.
The UV-vis-NIR spectra of (2a) and (2b) are
shown in Figure 2, the characteristic features due to the van
Hove singularities are clearly present but are suppressed, as
expected, when compared to purified SWNTs indicative of
chemically modified nanotubes.13,35-37 The degree of function-
alization was probed by gravimetric analysis. TGA-MS of (2a)
and (2b) show a weight loss of 17 and 22%, respectively, at
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Fluorescent SWNTs Using Cycloaddition of Ylides
A R T I C L E S
Figure 5. Normalized and offset fluorescence spectra of the indolizine
formed from the 1,3-dipolar cycloaddition of the pyridinium ylide, generated
from the pyridinium salt (1), with a second pyridinium ylide in the absence
of SWNTs (black), indolizine functionalized SWNTs (2a) prepared by
conventional heating (red), indolizine functionalized SWNTs (2b) prepared
by microwave heating (blue) and the control experiment of mechanically
mixing purified SWNTs with the indolizine generated from the cycloaddition
of one pyridinium ylide to another (magenta).
Figure 3. TGA-MS data (10 °C min-1) of purified SWNTs (black),
indolizine functionalized SWNTs (2a) prepared by conventional heating
(red) and indolizine functionalized SWNTs (2b) prepared by microwave
heating (blue). MS trace (green) of N-methylpyridinium (94 amu) and
(magenta) ethyl ester fragment (OEt, 45 amu) given off during heating.
Resonance Raman spectroscopy has been used extensively
to study SWNTs.39-44 A wealth of information regarding the
nanotube chirality, electronic type and degree of chemical
functional groups (or defects) present can be obtained. A
typical Raman spectrum of SWNTs can be divided into four
regions; the RBM (radial breathing mode) typically 100-400
cm-1, the disorder or D band from 1280-1320 cm-1, the
graphene or G band (tangential mode) 1500-1600 cm-1 and
the G′ (or D*) band ∼2600 cm-1. The RBM is inversely
proportional to the diameter of the nanotube [ωRBM ) A +
(B/dt), where A ) 223.5 nm cm-1, B ) 12.5 cm-1, and dt is
the nanotube diameter in nm] which in turn can be related
to the chirality of the SWNT as dt ) (aꢀ(n2 + m2 + nm))/
π, where n and m are the chiral integers and a is the lattice
vector of graphene () d(c-c)ꢀ3, where d(c-c) is the C-C bond
length in nm). The D band is dispersive (shifts to higher
frequency as the excitation energy increases) and is linked
to the reduction in symmetry of the SWNTs as result of
functionalization or the presence of defects, and the ratio of
this band with the tangential band (G) is widely used to assess
the degree chemical modification in SWNTs.13,35-37 The
Figure 4. N1s photoelectron spectrum of indolizine functionalized SWNTs
(2b) prepared by microwave heating.
equivalent to approximately one indolizine group per 83 CNT
carbon atoms and is in close agreement with the values
determined by TGA-MS.
tangential mode can be split into two components ω+ and
G
ω- associated with vibrations along the nanotube axis and
Given that the fluorescent properties of indolizines are
well-known,32,33 it is perhaps no surprise that the indolizine
functionalized SWNTs (2a) and (2b) emit light upon excitation,
Figure 5. Excitation of (2a) and (2b) at 335 nm results in a
blue emission at 416 nm. Unfortunately, quantum yields were
not determined due to the strong scattering and absorption of
the SWNTs, which made it difficult to compare absolute
emission intensities and true quantum efficiencies. However, it
was possible to optically image the functionalized SWNTs as a
solid material using epi-fluorescence microscopy (See Figure
S2 of the Supporting Information), suggesting that the quantum
efficiency is sufficiently high for practical applications such as
monitoring transfection into cells. Control experiments mechani-
cally mixing the free indolizine, prepared via the 1,3-dipolar
addition of the pyridinium ylide to a second pyridinium ylide,
with purified SWNTs followed by washing with water and
acetone, showed no sign of fluorescence.
G
vibrations perpendicular to the nanotube axis, respectively.45
The G′ (or D*) band is a two-phonon second order Raman
scattering process that is pressure dependent and has been
used to monitor stress transfer in carbon nanotube compos-
ites.46 The Raman spectra (excited at 632.8 nm and normal-
(39) Strano, M. S. J. Am. Chem. Soc. 2003, 125, 16148.
(40) Nair, N.; Kim, W.-J.; Usrey, M. L.; Strano, M. S. J. Am. Chem. Soc.
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Figure 6. Raman spectra (632.8 nm, 1.96 eV) of purified SWNTs (black),
indolizine functionalized SWNTs (2a) prepared by conventional heating
(red) and indolizine functionalized SWNTs (2b) prepared by microwave
heating (blue) normalized at the G-band.
Figure 7. RBM region of the Raman spectra (632.8 nm, 1.96 eV) of
purified SWNTs (black), indolizine functionalized SWNTs (2a) prepared
by conventional heating (red) and indolizine functionalized SWNTs (2b)
prepared by microwave heating (blue) normalized at 251 cm-1
.
ized to the G-band intensity) of pristine SWNTs and the
cycloaddition products (2a) and (2b) are shown in Figure 6.
As expected the modified SWNTs (2a) and (2b) have an
enhanced D-band at ∼1350 cm-1 when compared with
unmodified SWNTs, with an ID/IG ratio of 0.18 and 0.27,
respectively, indicative of groups attached to the surface of
the nanotubes.13,35-37 Close examination of the RBMs in the
Raman spectra shows some small differences between the
chemically functionalized and pristine SWNTs. From modi-
fied Kataura plots,39 we can establish that excitation at 632.8
nm (1.96 eV) brings into resonance both metallic and
semiconducting SWNTs with a diameter range of 0.52 to
1.24 nm, whereas 532 nm (2.33 eV) and 785 nm (1.58 eV)
excitation brings into resonance predominately metallic
SWNTs (0.84 to 0.99 nm) and semiconducting SWNTs (0.64
to 1.17 nm), respectively, assuming a 50 meV resonance
window.47 Raman spectroscopy at 632.8 nm (1.96 eV), Figure
7, of the unmodified SWNT material in the RBM region
shows five peaks at ∼187, 197, 209, 236, and 251 cm-1, the
latter two being significantly less intense. Using the modified
Kataura plots of Strano,39 these bands are assigned to (12,6),
(13,4), (11,5), (12,1), and (10,3) SWNTs, the first three being
metallic and the remaining two semiconducting nanotubes.
Cycloaddition to form (2a) and (2b) results in changes in
intensity of some of the bands. By normalizing the spectra
to the band at 251 cm-1 (which is essentially unchanged after
reaction), it is clear that the semiconducting SWNTs are
relatively unchanged upon functionalization, whereas the
bands assigned to metallic SWNTs are significantly reduced
in intensity, Figure 7. Excitation at 532 nm (2.33 eV) brings
into resonance predominately metallic SWNTs and the
spectrum of unmodified SWNTs, Figure 8, shows two bands
at 187 and 273 cm-1 which can be assigned to (12,6) and
(9,3) respectively; the two possible weak bands at 234 and
253 cm-1 are ignored. The assignment of the (12,6) SWNTs
is tentative, as this nanotube chirality would be expected to
be just off resonance using 2.33 eV excitation. Raman spectra,
Figure 8. RBM region of the Raman spectra (532 nm, 2.33 eV) of purified
SWNTs (black), indolizine functionalized SWNTs (2a) prepared by
conventional heating (red) and indolizine functionalized SWNTs (2b)
prepared by microwave heating (blue) normalized at 273 cm-1
.
normalized at 273 cm-1, of the functionalized SWNTs (2a)
and (2b), Figure 8, show a significant decrease in the band
at 187 cm-1 assigned to larger diameter metallic SWNTs. In
contrast, excitation at 785 nm (1.58 eV) brings into resonance
predominately semiconducting SWNTs and the spectrum of
unmodified SWNTs, Figure 9, shows five bands at 202, 212,
222, 230, and 256 cm-1 which can be assigned (9,8), (9,7),
(10,5), (11,3), and (10,2) respectively; the possible weak band
at 245 cm-1 is ignored. Note the (10,2) assignment, although
the best fit, is tentative, as this nanotube chirality would be
just off resonance when excited at 1.58 eV. The Raman
spectra of the functionalized SWNTs (2a) and (2b), normal-
ized at 265 cm-1, show a significant decrease in the band at
202 cm-1 assigned to larger diameter semiconducting SWNTs
and a somewhat smaller decrease in the bands at 212, 222,
and 230 cm-1 attributed to smaller diameter semiconducting
SWNTs, Figure 9. The use of resonance Raman for the
assessment of selectivity in reactions with SWNTs is
particularly useful. As a SWNT becomes functionalized, the
electronic properties of the material change and the nanotube
moves off resonance, and an apparent decrease in intensity
of the normally resonance enhanced RBM is observed.40-44
(47) The value of 50 meV for the resonance window was estimated from
spectral line widths shown in the following: Bachilo, S. M.; Strano,
M. S.; Kittrell, C.; Hauge, R. H.; Smalley, R. E.; Weisman, R. B.
Science 2002, 298, 2361. However, it is noted that a value of 40 meV
is stated by Strano in ref 39.
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10674 J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009

Fluorescent SWNTs Using Cycloaddition of Ylides
A R T I C L E S
Scheme 2. Schematic Representation of the Electrostatic
Interaction of Gold Colloids with Indolizine Functionalized SWNTs
(2a,b) for AFM Functional Group Tagging Experimentsa
Figure 9. RBM region of the Raman spectra (785 nm, 1.58 eV) of purified
SWNTs (black), indolizine functionalized SWNTs (2a) prepared by
conventional heating (red) and indolizine functionalized SWNTs (2b)
prepared by microwave heating (blue) normalized at 265 cm-1
.
From the results displayed here, it is clear that excitation at
632.8 nm shows that the cycloaddition reaction is selective
for metallic SWNTs where 532 excitation shows that
selectivity is toward larger diameter metallic SWNTs.
Interestingly, 785 nm excitation shows that the reaction is
not limited to metallic SWNTs as larger diameter semicon-
ducting SWNTs react. Thus from the resonance Raman data,
it is clear that the 1,3-dipolar cycloaddition of pyridinium
ylides is selective for metallic and larger diameter semicon-
ducting SWNTs. This is interesting as typically smaller
diameter SWNTs are often predicted to be more reactive than
their larger counterparts due to the greater degree of curvature
and π-orbital misalignment present.48 However, larger di-
ameter semiconducting SWNTs have been shown to be more
reactive than smaller diameter SWNTs to diazonium salts
due to the latter having a larger band gap and thus a large
region where the density of states (DOS) of the SWNT is
zero.40 The selectivity displayed by the 1,3-dipolar cycload-
dition reaction discussed here is similar, metallic more
reactive than semiconducting and large diameter semicon-
ducting more reactive than small diameter SWNTs, and thus
the reactivity is likely to be correlated with band gap. This
fits with a mechanism that first involves electron transfer from
the SWNT to the 1,3-dipole, which is in agreement with
metallic SWNTs, which have a finite DOS at the Fermi level,
being more reactive than semiconducting SWNTs.
a (i) N,N-Dimethylethylenediamine, 100 °C, 15 h. (ii) MeI, room
temperature, 15 h. (iii) Aqueous solution of citrate stabilized gold colloids.
Figure 10. A typical tapping mode AFM height image of an indolizine
functionalized SWNT (2a,b) with positively charged tertiary amines after
exposure to citrate stabilized Au colloids (4-6 nm). The image shown is
1.9 × 1.9 µm with a z scale of 0-5 nm. The Au colloids can be seen (light
colored features) decorating the complete length of the nanotube.
Tagging of Functional Groups for AFM Visualization. De-
finitive characterization of SWNTs functionalized with organic
groups using conventional spectroscopy can be difficult and
requires extensive use of control experiments to rule out physical
adsorption. However, using chemical tagging techniques, we
have shown in the past that it is possible to exploit the chemistry
of the attached organic group to anchor nanoparticles to the
surface of the SWNTs.15,16 This allows the functional groups
to be visualized by atomic force microscopy (AFM) and
provides information on distribution and localization of func-
tional groups on the nanotube surface. Here, we have converted
the ester group on the indolizine functionalized SWNTs (2a,b)
to an amide with a terminal tertiary amine, by reaction with
N,N-dimethylethylenediamine, which was subsequently quater-
narized by reaction with iodomethane, Scheme 2. The positively
charged indolizine functionalized SWNTs were then deposited
on a mica surface, where they were exposed to citrate stabilized
gold colloids 4-6 nm in diameter, Figure 10. The electrostatic
interaction of the negatively charged gold nanoparticles with
the positively charge functionalized SWNTs is clear with the
gold particles, visible as light features on the AFM height image,
showing the presence and distribution of the indolizine func-
tional groups on the nanotube surface.
Conclusions
We have shown that it is possible for SWNTs to undergo
1,3-dipolar cycloaddition, under conventional or microwave
(48) Niyogi, S.; Hamon, M. A.; Hu, H.; Zhao, B.; Bhowmik, P.; Sen, R.;
Itkis, M. E.; Haddon, R. C. Acc. Chem. Res. 2002, 35, 1105.
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A R T I C L E S
Bayazit and Coleman
Acknowledgment. We thank Dr. Graham Beamson of the
National Centre for Electron Spectroscopy and Surface Analysis
(NCESS) at Daresbury Laboratory for technical assistance and
useful discussions on the XPS measurements, Doug Carswell for
recording the TGA data, and the EPSRC (EP/E025722/1) and
TUBITAK (MKB) for funding.
heating conditions, with pyridinium ylides readily generated
from simple and easy to prepare Kro¨hnke salts. The addition
of the pyridinium ylides to the SWNT surface results in the
formation of an indolizine group, an important class of
compounds that have interesting pharmacological and lumi-
nescent properties. The indolizine functionalized SWNTs
were shown to emit in the blue (416 nm) when excited at
335 nm. Interestingly, using resonance Raman spectroscopy,
we have been able to show that the 1,3-dipolar cylcloaddition
to form the indolizines is selective for metallic and large
diameter semiconducting SWNTs. This is in agreement with
a mechanism first involving donation of electrons from the
nanotube to the 1,3-dipole. AFM tagging experiments
exploiting the electrostatic interaction of citrate stabilized
gold nanoparticles with indolizines that were deliberately
positively charged showed that the functional groups were
present on the entire length of the carbon nanotubes.
Supporting Information Available: Electrospray mass spec-
trum of free indolizine prepared from the pyridinium ylide
generated from (1) undergoing a 1,3-dipolar cycloaddition with
a second pyridinium ylide (Figure S1). Fluorescence image of
indolizine functionalized SWNTs Figure S2. This material is
available free of charge via the Internet at http://pubs.acs.
org.
JA903712F
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10676 J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009