Sidewall Carboxylic Acid Functionalization of Single-Walled Carbon Nanotubes

Article abstract of DOI:10.1021/ja037746s

The reactions of single-walled carbon nanotubes (SWNTs) with succinic or glutaric acid acyl peroxides in o-dichlorobenzene at 80-90 °C resulted in the addition of 2-carboxyethyl or 3-carboxypropyl groups, respectively, to the sidewalls of the SWNT. These acid-functionalized SWNTs were converted to acid chlorides by derivatization with SOCl₂ and then to amides with terminal diamines such as ethylenediamine, 4,4′ -methylenebis(cyclohexylamine), and diethyltoluenediamine. The acid-functionalized SWNTs and the amide derivatives were characterized by a set of materials characterization methods including attenuated total reflectance (ATR) FTIR, Raman and solid state ¹³C NMR spectroscopy, transmission electron microscopy (TEM), and thermal gravimetry-mass spectrometry (TG-MS). The degree of SWNT sidewall functionalization with the acid-terminated groups was estimated as 1 in 24 carbons on the basis of TG-MS data. In comparison with the pristine SWNTs, the acid-functionalized SWNTs show an improved solubility in polar solvents, for example, alcohols and water, which enables their processing for incorporation into polymer composite structures as well as for a variety of biomedical applications.

Full text of DOI:10.1021/ja037746s

Published on Web 11/15/2003
Sidewall Carboxylic Acid Functionalization of Single-Walled
Carbon Nanotubes
Haiqing Peng, Lawrence B. Alemany, John L. Margrave,* and
Valery N. Khabashesku*
Contribution from the Department of Chemistry and the Center for Nanoscale Science and
Technology, Rice UniVersity, 6100 Main Street, Houston, Texas 77005-1892
Received August 4, 2003; E-mail: khval@rice.edu
Abstract: The reactions of single-walled carbon nanotubes (SWNTs) with succinic or glutaric acid acyl
peroxides in o-dichlorobenzene at 80-90 °C resulted in the addition of 2-carboxyethyl or 3-carboxypropyl
groups, respectively, to the sidewalls of the SWNT. These acid-functionalized SWNTs were converted to
acid chlorides by derivatization with SOCl
2
and then to amides with terminal diamines such as
ethylenediamine, 4,4′-methylenebis(cyclohexylamine), and diethyltoluenediamine. The acid-functionalized
SWNTs and the amide derivatives were characterized by a set of materials characterization methods
including attenuated total reflectance (ATR) FTIR, Raman and solid state 13C NMR spectroscopy,
transmission electron microscopy (TEM), and thermal gravimetry-mass spectrometry (TG-MS). The degree
of SWNT sidewall functionalization with the acid-terminated groups was estimated as 1 in 24 carbons on
the basis of TG-MS data. In comparison with the pristine SWNTs, the acid-functionalized SWNTs show an
improved solubility in polar solvents, for example, alcohols and water, which enables their processing for
incorporation into polymer composite structures as well as for a variety of biomedical applications.
5
3c,f
6
Introduction
alkyllithium and Grignard reagents or diamines, and reac-
7
8
tions of SWNTs with aryl diazonium salts, azomethine ylides,
nitrenes, and organic radicals.
tionalizations have been most extensively explored through
oxidation routes
1
Since their discovery in 1993, single-walled carbon nano-
tubes (SWNTs) have become an area of wide ranging research
activity due to their exceptional chemical and physical proper-
ties. To take advantage of the remarkable tubular framework
structure of SWNTs in various applications, particularly in the
engineering of multifunctional materials, the SWNTs need to
be derivatized with organic functional groups that can provide
a high binding affinity and selectivity through formation of either
hydrogen or covalent bonds. For medical and biological
applications, the SWNTs must be chemically derivatized with
hydrophilic substituents, such as those containing terminal
hydroxyl or carboxylic acid groups. These functional groups
are also necessary for providing sites for covalent integration
of the SWNTs into organic/inorganic polymer structures to
produce nanotube-reinforced composite materials.2
9
9-11
The SWNT open-end func-
3
d,e,12,13
to form shortened nanotubes bearing
carboxylic acid end groups that have been further derivatized
(
3) (a) Hirsch, A. Angew. Chem., Int. Ed. 2002, 41, 1853. (b) Bahr, J. L.; Tour,
J. M. J. Mater. Chem. 2002, 12, 1952. (c) Khabashesku, V. N.; Billups,
W. E.; Margrave, J. L. Acc. Chem. Res. 2002, 35, 1087. (d) Sun, Y.-P.;
Fu, K.; Lin, Y.; Huang, W. Acc. Chem. Res. 2002, 35, 1096. (e) Niyogi,
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Haddon, R. C. Acc. Chem. Res. 2002, 35, 1105. (f) Khabashesku, V. N.;
Margrave, J. L. Chemistry of Carbon Nanotubes. In The Encyclopedia of
Nanoscience and Nanotechnology; Nalwa, S., Ed.; American Scientific
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B 2002, 106, 12144.
(
4) Mickelson, E. T.; Huffman, C. B.; Rinzler, A. G.; Smalley, R. E.; Hauge,
R. H.; Margrave, J. L. Chem. Phys. Lett. 1998, 296, 188.
(
5) (a) Boul, P. J.; Liu, J.; Mickelson, E. T.; Huffman, C. B.; Ericson, L. M.;
Chiang, I. W.; Smith, K. A.; Colbert, D. T.; Hauge, R. H.; Margrave, J.
L.; Smalley, R. E. Chem. Phys. Lett. 1999, 310, 367. (b) Saini, R. K.;
Chiang, I. W.; Peng, H.; Smalley, R. E.; Billups, W. E.; Hauge, R. H.;
Margrave, J. L. J. Am. Chem. Soc. 2003, 125, 3617.
In addition to trying to meet these technological demands,
the development of functionalization methods for nanotubes is
also of fundamental importance for gaining a greater knowledge
of chemical reactivity for materials with nanoscale size and
(
6) Stevens, J. L.; Huang, A. Y.; Peng, H.; Chiang, I. W.; Khabashesku, V.
N.; Margrave, J. L. Nano Lett. 2003, 3, 331.
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E.; Tour, J. M. J. Am. Chem. Soc. 2001, 123, 6536.
(
8) (a) Georgakilas, V.; Kordatos, K.; Prato, M.; Guldi, D. M.; Holzinger, M.;
Hirsch, A. J. Am. Chem. Soc. 2002, 124, 760. (b) Georgakilas, V.;
Tagmatarchis, N.; Pantarotto, D.; Bianco, A.; Briand, J.-P.; Prato, M. Chem.
Commun. 2002, 3050. (c) Pantarotto, D.; Partidos, C. D.; Graff, R.;
Hoebeke, J.; Briand, J.-P.; Prato, M.; Bianco, A. J. Am. Chem. Soc. 2003,
3
shape. However, in light of the most recent reviews, the number
of documented studies in this new research field of function-
alization chemistry of carbon nanotubes is not yet substantial.
Reports have appeared of SWNT sidewall functionalizations
1
25, 6160.
(
9) (a) Holzinger, M.; Vostrowsky, O.; Hirsch, A.; Hennrich, F.; Kappes, M.;
Weiss, R.; Jellen, F. Angew. Chem., Int. Ed. 2001, 40, 4002. (b) Holzinger,
M.; Abraham, J.; Whelan, P.; Graupner, R.; Ley, L.; Hennrich, F.; Kappes,
M.; Hirsch, A. J. Am. Chem. Soc. 2003, 125, 8566.
4
using fluorine, followed by subsequent interactions with
(
1) (a) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (b) Bethune, D. S.;
Kiang, C. H.; de Vries, M. S.; Gorman, G.; Savoy, R.; Vazquez, J.; Beyers,
R. Nature 1993, 363, 605.
(10) Peng, H.; Reverdy, P.; Khabashesku, V. N.; Margrave, J. L. Chem. Commun.
2003, 362.
(11) Ying, Y.; Saini, R. K.; Liang, F.; Sadana, A. K.; Billups, W. E. Org. Lett.
2003, 5, 1471.
(
2) Zhu, J.; Kim, J.; Peng, H.; Margrave, J. L.; Khabashesku, V. N.; Barrera,
E. V. Nano Lett. 2003, 3, 1107 and references therein.
(12) Banerjee, S.; Wong, S. S. Nano Lett. 2002, 2, 49.
15174
9
J. AM. CHEM. SOC. 2003, 125, 15174-15182
10.1021/ja037746s CCC: $25.00 © 2003 American Chemical Society

Functionalization of Single-Walled Carbon Nanotubes
A R T I C L E S
Scheme 2. Preparation of SWNT Amide Derivative 3
Scheme 1. Chemical Routes for Preparation of SWNT-Derivatives
2a and 2b
Scheme 3. Preparation of SWNT Amide Derivative 4
1
3,14
by reactions with thionyl chloride and long-chain amines
or by esterification.15
Until now, the most successful functionalization of carbon
nanomaterials with carboxylic acid-terminated moieties has been
with fullerene C60 through a two-step process (Bingel 2+1
cycloaddition reaction followed by deesterification) yielding
carboxylated methanofullerene structures.16 A similar process
was shown to be much less efficient when applied to pristine
SWNTs because of their reduced reactivity to carbene addition
via a Bingel-type reaction.17 With carbon nanotubes, the
carboxylic acid functions have been generated so far only on
Scheme 4. Preparation of SWNT Amide Derivative 5
their open ends and to some extent on the partially etched
3
(“unzipped”) sidewalls through treatment with various oxidants.
In the present work, we demonstrate a one-step method for
derivatization of SWNTs that is based on free radical addition
of alkyl groups terminated with a carboxylic acid. This method,
which is nondestructive to the sidewall, utilizes the organic acyl
peroxides of dicarboxylic acids, HOOC(CH2)nC(O)OO(O)C-
(CH2)nCOOH (1a, n ) 2; 1b, n ) 3), as precursors for the
corresponding “functional” radicals. These precursors were
chosen on the basis both of the well-known, long time use of
peroxides as radical initiators in polymerization reactions and
also of our successful sidewall functionalization of SWNTs by
addition of alkyl or phenyl radicals thermally generated from
organic acyl peroxides10 (later confirmed by Ying et al. ). As
compared to other radical sources, organic peroxides can offer
more kinds of functional groups and are more conveniently
available at relatively lower costs. Acyl peroxides, RC(O)OO-
these carboxyalkyl radicals, generated in situ from peroxides
1
a and 1b, have reacted with the SWNTs to produce sidewall
acid-functionalized SWNT-derivatives 2a and 2b, respectively
(Scheme 1).
11
The attached carboxylic acid groups in 2a and 2b have been
chemically characterized by formation of amide derivatives in
the course of subsequent reactions with thionyl chloride and
then with several terminal diamines, such as ethylenediamine,
4,4′-methylenebis(cyclohexylamine), and diethyltoluenediamine
(Schemes 2-4). Instrumental analyses of the sidewall-func-
tionalized SWNTs 2a,b and their amide derivatives have been
carried out with the help of both now commonly used optical
(O)CR, where R could be aliphatic, aromatic, or another group,
readily decompose to release carbon dioxide and form free
radicals R upon mild heating.18 Succinic acid peroxide, 1a,
decomposes to form a HOOCCH2CH2COO‚ radical,1
9-21
which
3
can subsequently lose CO2 to yield a 2-carboxyethyl radical.
Glutaric acid peroxide, 1b, would be expected to yield a
spectroscopy and microscopy methods as well as solid state
13
C NMR spectroscopy and thermal gravimetry-mass spectros-
3
-carboxypropyl radical via a similar route. In the present study,
copy. The present paper provides a detailed discussion of these
data.
(
(
(
13) (a) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.;
Haddon, R. C. Science 1998, 282, 95. (b) Hamon, M. A.; Chen, J.; Hu, H.;
Chen, Y.; Itkis, M. E.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. AdV.
Mater. 1999, 11, 834.
14) Liu, J.; Rinzler, A. G.; Dai, H.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.;
Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias, F.;
Shon, Y.-S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280,
253.
15) (a) Riggs, J. E.; Guo, Z.; Carroll, D. L.; Sun, Y.-P. J. Am. Chem. Soc.
000, 122, 5879. (b) Sun, Y.-P.; Huang, W.; Lin, Y.; Fu, K.; Kitay-
gorodskiy, A.; Riddle, L. A.; Yu, Y. J.; Carroll, D. L. Chem. Mater. 2001,
3, 2864.
16) (a) Lamparth, I.; Hirsch, A. J. Chem. Soc., Chem. Commun. 1994, 1727.
b) Hirsch, A.; Lamparth, I.; Karfunkel, H. R. Angew. Chem., Int. Ed. Engl.
994, 33, 437.
Experimental Section
Materials. The raw SWNTs used in this study were produced by
the HiPco process in Professor Richard Smalley’s Carbon Nano-
technology laboratory at Rice University. Such prepared HiPco SWNTs
have an average diameter of 1 nm. The iron impurity in the HiPco
SWNTs was removed by wet air oxidation and subsequent hydrochloric
acid rinse as described previously.22 The final purification step was
the annealing of the SWNTs at 800 °C in an argon atmosphere for 2
h. The iron residue in the purified SWNTs was found by TGA to be
about 1 wt %. The SEM/EDX analysis yielded 0.2 at. % iron content
in the same material.
1
2
1
(
(
(
1
17) (a) Kini, V. U.; Khabashesku, V. N.; Margrave, J. L. Rice Quantum Institute
Sixteenth Annual Summer Research Colloquium; August 9, 2002; Abstr. p
2
5. (b) Coleman, K. S.; Bailey, S. R.; Fogden, S.; Green, M. L. H. J. Am.
Chem. Soc. 2003, 125, 8722.
Succinic and glutaric anhydrides, used as precursors for the
preparation of dicarboxylic acid acyl peroxides, and ethylenediamine
were purchased from Aldrich. 4,4′-Methylenebis(cyclohexylamine) was
(18) Organic Peroxides; Swern, D., Ed.; Wiley-Interscience: New York, London,
Sydney, Toronto, 1971; Vol. II, p 799.
(19) Fieser, L. F.; Turner, R. B. J. Am. Chem. Soc. 1947, 69, 2338.
(20) Pettit, G. R.; Houghton, L. E. J. Chem. Soc. C 1971, 509.
(21) Anufriev, V. P.; Novikov, V. L.; Maximov, O. B.; Elyakov, G. B.; Levitsky,
D. O.; Lebedev, A. V.; Sadretdinov, S. M.; Shvilkin, A. V.; Afonskaya,
N. I.; Ruda, M. Y.; Cherpachenko, N. M. Bioorg. Med. Chem. Lett. 1998,
(22) Chiang, I. W.; Brinson, B. E.; Huang, A. Y.; Willis, P. A.; Bronikowski,
M. J.; Margrave, J. L.; Smalley, R. E.; Hauge, R. H. J. Phys. Chem. B
2001, 105, 8297.
8
, 587.
J. AM. CHEM. SOC.
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A R T I C L E S
Peng et al.
supplied by Air Products, and diethyltoluenediamine was purchased
from Resolution Performance Products.
The solid state, magic angle spinning (MAS) NMR studies were
1
3
done on a Bruker AVANCE-200 NMR spectrometer (50.3 MHz C,
1
Synthetic Procedures. To prepare succinic and glutaric acid acyl
peroxides (1a and 1b), 10 g of succinic or glutaric anhydride fine
powder was added to 20 mL of ice cold 8% hydrogen peroxide and
stirred for 30 min until all of the powder dissolved and a white gellike
solution formed. The solution was filtered onto a 1-µm pore size PTFE
membrane (Cole Palmer) to leave a deposit which was washed with a
small amount of water and air-dried for 10 min. The white peroxide
products were transferred from the membrane to a glass vial and
vacuum-dried at room temperature for 24 h. Approximately 6.5 g of
each peroxide, 1a and 1b, was obtained using this one-step procedure
200.1 MHz H). MAS spectra were obtained with each sample packed
in a 4-mm outer diameter rotor, which was spun at 7.5 kHz for the
pristine SWNTs and at 11.0 kHz for the functionalized SWNTs. This
2
3
results in spinning sidebands well outside the sp and sp centerband
regions of interest (149 ppm upfield or downfield from the centerband
for pristine SWNTs and 219 ppm upfield or downfield from a
centerband for the functionalized SWNTs). Each spectrum was obtained
with a 4.5-µs 90° 13C pulse and a 32.9-ms, proton-decoupled free
induction decay (FID), followed by a 180-s relaxation delay for the
SWNTs or a 45-s relaxation delay for each of the functionalized
SWNTs. In the experiments, we used a total of 1320 scans (66.0 h) for
the SWNTs, 7152 scans (89.5 h) for the SWNT-derivative 2a, 4984
scans (62.3 h) for the SWNT-derivative 2b, and 3560 scans (44.5 h)
for the SWNT amide derivative 3. Each FID was processed with 50
Hz (1 ppm) of line broadening. The resulting spectrum was phase
corrected. A fourth-order polynomial was then applied to the baseline
over the region from δ315 to δ-70 to create a nearly flat baseline
after the polynomial was subtracted from the spectrum. With much
23
reported long ago by Clover et al. ATR-FTIR spectra of both peroxides
a and 1b show similar features: a broad band in the 3000-3500 cm-1
region due to the carboxylic O-H stretches, peaks of the C-H
1
-
1
-1
stretchings in the 2850-3000 cm range, absorptions near 1700 cm
characteristic of the acid carbonyl groups, and pairs of peaks at 1812
and 1779 cm assigned to the peroxide carbonyls. The solid state 13
NMR spectrum of 1a shows three methylene carbon peaks at 29.9,
8.8, and 24.7 ppm and three carbonyl carbon peaks at 181.7, 179.7,
and 168.7 ppm. The spectrum of 1b shows three methylene peaks at
3.0, 28.4, and 19.0 ppm and two carbonyl peaks at 182.2 and 170.3
ppm.
To prepare acid-functionalized SWNTs (2a and 2b), the following
procedure was developed. Purified SWNTs (50 mg) were placed in a
50-mL flask filled with 50 mL of dry o-dichlorobenzene and sonicated
17 W/55 kHz bath, Cole Palmer) for 30 min to obtain a SWNT
-1
C
2
2 2 n
more of the functionalized C60 sample, C60-{CH CH COOH} ,
24a
3
available, a 7-mm OD rotor could be used, which was spun at 7.0
kHz. The spectrum was obtained with a 3.8-µs 90° 13C pulse and a
32.9-ms, proton-decoupled FID, followed by either a 15-s relaxation
delay (10 440 scans, 43.6 h) or a 45-s relaxation delay (2132 scans,
26.7 h) to make a rough determination of the relative relaxation rates
2
(
1
13
of the various types of carbons. H- C cross polarization/magic angle
spinning (CPMAS) spectra of acid acyl peroxides 1a and 1b were
quickly obtained with 5 kHz MAS, a 1-ms contact time, 32.9-ms FID,
and 5-s relaxation delay. 13C chemical shifts are relative to the carbonyl
carbon of glycine at 176.5 ppm.2
suspension solution. The latter was heated at 80-90 °C for 10 days
while each day 0.5 g of peroxide 1a or 1b was added. After the reaction
was complete, the suspension was cooled and poured into a 500-mL
Erlenmeyer flask containing a large amount of tetrahydrofuran and
sonicated for 15 min. The obtained solution was filtered using a 0.2-
µm pore size PTFE membrane (Cole Palmer). Functionalized SWNTs
4b
Results and Discussion
2
a or 2b, respectively, were collected on the membrane, then placed
Reactions of SWNTs with Peroxides 1a and 1b. As
compared to the fullerenes, particularly C60, the SWNTs are
significantly less reactive due to the much lower curvature of
their sidewalls built of graphene cylinders. For this reason, the
SWNTs require a much longer reaction time (days vs hours)
for addition of acyl peroxide-generated free radicals to achieve
in 100 mL of ethanol, sonicated for 20 min, and then filtered again.
During the filtration, a large amount of ethanol was repeatedly used to
completely wash off the unreacted peroxides and the reaction byprod-
ucts. Finally, the functionalized SWNTs 2a and 2b were vacuum-dried
at 70 °C overnight.
The SWNT amide derivative 3 (Scheme 2) was prepared from 20
mg of acid-functionalized SWNT 2a placed in a dry 100-mL flask to
which 20 mL of thionyl chloride was added, and the mixture was stirred
for 12 h. The reaction mixture was then vacuum filtered through a
significant sidewall derivatization, as we observed in our recent
10,24a
studies of several peroxides.
In the present work, the
carboxyalkyl radicals were thermally produced from dicarbox-
ylic acid acyl peroxides 1a and 1b (Scheme 1), which have
half-lives of 1 h at 90 °C according to information from Atofina
Chemicals. On the basis of that, this particular temperature was
chosen for conducting the SWNT functionalization reactions.
Because of the inductive effect of the carboxyl group, the
reactivity of these carboxyalkyl radicals toward SWNTs was
0
.2-µm pore size membrane, and the obtained precipitate was flushed
with a large amount of dry acetone. The solid precipitate was dried in
air and then placed in a 100-mL flask containing 20 mL of ethylene-
diamine. The mixture was stirred at room temperature for 12 h and
then poured into a large amount of ethanol and sonicated for another
1
0 min. The batch was filtered through a 0.2-µm pore size membrane
and flushed with a large amount of ethanol. The SWNT-derivative 3
was collected on a membrane and dried overnight in a vacuum oven at
somewhat reduced as compared to undecyl and phenyl radicals,
7
0 °C. A similar procedure was applied for preparation of amide 4
10,24a
studied earlier.
Therefore, in this work, we used a larger
(
Scheme 3) by treating 2b with SOCl and 4,4′-methylenebis(cyclo-
2
excess of the peroxide precursor relative to SWNT (∼10:1
hexylamine). Unlike the syntheses of 3 and 4 at room temperature, the
synthesis of amide derivative 5 from 2b required heating at 120 °C for
weight ratio) to facilitate the addition reaction (Scheme 1).
Characterization of SWNT Derivatives by Optical Spec-
troscopy. Raman spectroscopy has proven to be an important
tool for characterization of both pristine SWNTs and their
sidewall-functionalized derivatives. In the Raman spectrum of
1
2 h during the reaction with diethyltoluenediamine (Scheme 4).
Characterization. The Raman spectra of purified and functionalized
SWNTs were collected with a Renishaw 1000 microraman system with
a 780-nm laser source. For the ATR-FTIR spectral measurements, a
Thermo Nicolet Nexus 870 FTIR system with an ATR accessory was
employed. The spectra in the UV-vis-NIR range were taken using a
Shimadzu 3101 PC UV/Vis/NIR spectrometer. The thermal degradation
and volatile products evolution analyses were performed with a TGA/
MS instrument which includes a Q500 Thermal Gravity Analyzer
coupled with a Pffeifer Thermal Star mass spectrometer.
-1
purified SWNTs (Figure 1A), the tangential mode at 1594 cm
is the dominant feature. The bands of the radial breathing mode
-1
at 213, 230, and 265 cm indicate the diameter distribution of
(
24) (a) Reverdy, P.; Peng, H.; Khabashesku, V. N.; Margrave, J. L. Rice
Quantum Institute Sixteenth Annual Summer Research Colloquium; August
9, 2002; Abstr. p 19. (b) Hayashi, S.; Hayamizu, K. Bull. Chem. Soc. Jpn.
(23) Clover, A. M.; Houghton, A. C. Am. Chem. J. 1904, 32, 55.
1991, 64, 685.
1
5176 J. AM. CHEM. SOC. VOL. 125, NO. 49, 2003
9

Functionalization of Single-Walled Carbon Nanotubes
A R T I C L E S
Figure 1. Raman spectra of SWNT materials: (A) pristine SWNTs, (B) 2a, (C) 2b, (D) SWNT residue after TGA of 2a, (E) 3, (F) 4, (G) 5.
HiPco SWNTs. When the sidewalls of the SWNTs are co-
to that of 2a, shown in Figure 2B, indicating a significant
alteration of the electronic properties of the nanotubes through
sidewall functionalization.
valently modified, as in the case of SWNT-{CH2CH2COOH}x
-
1
(
(
2a), the appearance of a prominent Raman peak at ∼1296 cm
3
Figure 1B) due to the sp states of carbon demonstrates
Heating the functionalized SWNTs 2a and 2b in argon to
800 °C at the rate of 10 °C/min in a TGA system dramatically
the disruption of the aromatic system of π-electrons, as
established previously.2
,3,15
The breathing mode bands become
3
reduced the intensity of the sp carbon mode in the Raman
weakened in this spectrum, probably because the functional
groups attached to the sidewalls hinder the radial breathing
oscillation motion.
spectrum of the nanotube material collected after pyrolysis
(Figure 1D). These data indicate that defunctionalization of 2a
and 2b takes place upon heating, which is in accord with
It should be noted that Raman bands at 265 and 230
cm- were observed to weaken most with respect to other
peaks, which can be explained by a higher reactivity of
smaller diameter SWNTs. The Raman features of SWNT-
previous data on thermal degradation properties of other
1
5b,6,7,10
sidewall-functionalized SWNTs.
The smaller diameter
(less than 1 nm) nanotube derivatives are probably getting
“unzipped” under 700-800 °C, which could explain why the
-
1
{
CH2CH2CH2COOH}x (2b) (Figure 1C) are similar to those of
Raman peak of the radial breathing mode at 265 cm did not
3
2
a (Figure 1B) except for a slightly lower intensity of the sp
recover in the spectrum of 2a after TGA.
mode. This can be related to a somewhat lower number of
covalent attachments created on the sidewall surface of 2b as
compared to 2a.
The modification of the sidewalls in 2a was also evident in
the UV-vis-NIR absorption spectra (Figure 2) from the
complete loss of the van Hove band structure typical for pristine
SWNTs. The spectrum of the SWNT derivative 2b was similar
Although the infrared spectra of pristine SWNTs are feature-
less, FTIR spectroscopy using the ATR accessory is very
informative for studying the functional groups attached to the
sidewalls of the SWNTs. Figure 3A shows the infrared spectrum
-1
of the SWNT-derivative 2a. The peaks in the 2800-3050 cm
region are characteristic of C-H stretches, and the broad
-
1
shoulder band in the 3100-3600 cm region is characteristic
J. AM. CHEM. SOC.
9
VOL. 125, NO. 49, 2003 15177

A R T I C L E S
Peng et al.
Figure 2. UV-vis-NIR spectra of (A) pristine SWNTs and (B) SWNT derivative 2a.
Figure 3. ATR-FTIR spectra of functionalized SWNTs: (A) 2a, (B) 3, (C) 4, (D) 5.
of acid O-H stretches. The dominant peak at 1708 cm-1 can
stretching mode of the amide moiety. The carbonyl peak in 3
-
1
-1
be clearly assigned to the acid carbonyl stretching mode. The
shifts to 1691 cm (as compared to 1708 cm for 2a) as a
result of the peptide C(dO)NH linkage formation. The latter is
also characterized by the in-plane amide NH deformation mode
-1
broad band at 1384 cm can be identified as the C-H bending
-
1
mode, and a broad peak at 1149 cm is identified as the C-O
stretching mode. A similar infrared spectrum was obtained for
the carboxypropyl derivative 2b.
-1
-1
observed at 1542 cm . The other prominent peak at 1639 cm
can be assigned to the NH2 scissoring mode in 3. The C-H
stretchings in 3 are only slightly enhanced as compared to 2a
(Figure 3A).
The functionalized SWNTs 2a and 2b can further react with
thionyl chloride and terminal diamines to form the corresponding
amides (Schemes 2-4), thus providing chemical evidence for
the presence of carboxylic acid groups in the side chain. These
reactions were carried out in analogy with the known chemistry
The SWNT-derivative 2b was also found to be readily
converted into the amide derivative 4 by SOCl2 and 4,4′-
methylenebis(cyclohexylamine) treatment at room temperature
in accordance with Scheme 3. In the infrared spectrum of 4
(Figure 3C), the features of the methylene C-H stretches at
of the carboxylic acid groups created on the open ends of
SWNTs by oxidizing treatment.1
3-15
In the FTIR spectrum
-1
(
Figure 3B) of the SWNT-{CH2CH2CONHCH2CH2NH2}x (3),
2915 and 2848 cm appear greatly enhanced. The peak at 1711
-
1
cm and a broad band at ∼3400 cm-1 are assigned to the CdO
-1
a new intense peak at 3284 cm can be assigned to the N-H
15178 J. AM. CHEM. SOC.
9
VOL. 125, NO. 49, 2003

Functionalization of Single-Walled Carbon Nanotubes
A R T I C L E S
Figure 4. Ion current versus time (temperature) plots obtained for ions
(
a-e, m/z) detected in the course of TG-MS analysis of evolution products
from SWNT-derivative 2a.
Figure 5. TEM image of the specimen of SWNT-{CH2CH2CONHC6H10-
CH2C6H10NH2}x (4). Inset shows a zoomed-on picture of a single func-
tionalized nanotube.
and N-H stretching modes, respectively. The NH2 scissor
motion can be most likely associated with the peak observed at
all fragments originate from the same parent (molecular) ion.
The detected fragment evolutions started at about 170 °C, which
is comparable to that for methylated, butylated, and hexylated
-
1
1
626 cm .
In contrast, 2b gave only negligible signs of reaction with
diethyltoluenediamine at room temperature. However, the
amidation reaction (Scheme 4) did occur upon heating at 120
SWNTs (∼160 °C) and is relatively low as compared to
15,16
phenylated SWNTs (∼250 °C).
We assume that all of the
°
C, yielding the amide product 5, whose infrared spectrum is
weight loss occurring during fragmentation is due to pyrolysis
of the side-chain groups. On the basis of this assumption, we
estimate the degree of SWNT sidewall functionalization as one
functional group for every 24 nanotube carbons.
-
1
shown in Figure 3D. The bands at 2950 and 2867 cm as well
as those at 2916 and 2837 cm can be assigned to the CH
stretches of the CH3 and CH2 groups, respectively. The peaks
-
1
-
1
of the carbonyl stretching mode at 1711 cm and the NH
Microscopy Analysis. Figure 5 shows a typical TEM image
of the SWNT derivative 4 specimen placed on lacey carbon-
coated copper grid. This image shows that the covalent
attachment of functional groups produces a “bumpy” sidewall
morphology for the nanotubes of 4 in the bundles observed. A
zoomed image of a single nanotube found in the same specimen
of 4 (inset in Figure 5) indicates the attached long-chain moieties
which tend to bend and stretch along the SWNT sidewalls.
The atomic force microscopy studies of the carboxyalkylated
SWNTs 2a also revealed their predominantly bundled morphol-
ogy, with the bundle sizes ranging from 10 to 20 nm. Within
these bundles, the functionalized nanotubes are most likely held
together through intramolecular hydrogen bonds formed by the
side-chain terminal carboxylic acid groups.
Solubility. A quantitative comparison of the solubility of
pristine SWNTs and the SWNT-derivative 2a in several polar
solvents was performed as follows: 25 mg of SWNT material
was dispersed in 200 mL of the selected solvent by sonication
for 2 h; the dispersion was left to settle for 48 h; then the
decanted top 150 mL of the solution was filtered; and the
precipitate collected from the filter was weighed. The solubility
data obtained (Table 1) show that, as compared to the pristine
nanotubes, the solubility of functionalized SWNTs 2a is
significantly improved. Isopropyl alcohol was found to be the
best solvent for SWNT-derivative 2a.
-
1
stretching mode at ∼3400 cm again appear weak. The bands
-
1
at 1621, 1451, and 1375 cm can be assigned to the NH2
25
scissor, CH deformation, and phenyl ring modes, respectively.
The SWNT derivatives 3-5 show characteristic peaks in the
Raman spectra (Figure 1E-G) similar to carboxyalkyl SWNTs
2
a and 2b. This confirms that the reactions of 2a and 2b with
terminal diamines result in the chemical modification of their
side-chain moieties and not the sidewalls.
Thermal Gravimetry/Mass Spectrometry Analysis. Ad-
ditional evidence for covalent attachment of carboxyalkyl groups
to the SWNTs was provided by thermal degradation analyses
(TGA) of 2a and 2b in the 50-800 °C range coupled with on-
line monitoring of the volatile products by a mass spectrometer.
These experiments were carried out with 15 mg of 2a or 2b
placed in a TGA pan and heated at 10 °C/min up to 800 °C in
a flow of argon. Figure 4 presents the ion current versus time
curves for parent and fragment ions originating from the
evolution products detaching from the side chains of SWNT-
derivative 2a.
The evolution curves were obtained for the parent ion (m/z
7
3) of the detaching CH2CH2COOH radical and for the fragment
ions with the m/z 72 {CH2CH2COO}, m/z 45 {COOH}, m/z 44
CO2}, and m/z 29 {CH3CH2}. All curves for the fragment ions
{
were observed to peak at the same time (evolution temperature)
as the evolution curve for the parent ion, which indicates that
The photographs in Figure 6 clearly show the great solubility
difference between pristine nanotubes and SWNT-derivative 2a
in isopropyl alcohol. However, the functionalized SWNTs were
not sufficiently soluble to allow useful solution state 13C NMR
(
25) Lin-Vien, D.; Colthup, N. B.; Fatelley, W. G.; Grasselli, J. G. The Handbook
of Infrared and Raman Characteristic Frequencies of Organic Molecules;
Academic Press Inc.: San Diego, CA, 1991; p 299.
J. AM. CHEM. SOC.
9
VOL. 125, NO. 49, 2003 15179

A R T I C L E S
Peng et al.
Table 1. Solubility Data (mg) for Pristine SWNT and
SWNT-Derivative 2a
solvent, 150 mL
pristine SWNT
SWNT-{CH
2
CH
2
COOH}
x
(2a)
o-dichlorobenzene
isopropyl alcohol
water
3.5
7.0
not soluble
not soluble
not soluble
16.0
15.2
15.1
1
% NH4OH in water
Figure 6. Photographs ofthe SWNT materials dispersed in isopropyl
alcohol: (A) acid-functionalized SWNT 2a, (B) pristine SWNTs.
Figure 7. MAS 13C NMR spectra of SWNTs: (A) pristine, (B) 2b, (C)
2a, (D) 3.
spectra to be obtained. Therefore, we have carried out these
studies in the solid state.
Solid State 13C NMR Studies. Solid state 13C NMR studies
three different samples of MWNTs give a considerably broader
1
3
27,31,32
isotropic C NMR signal at δ121-130.
observation of an isotropic C NMR signal at δ123 for the
purified HiPco SWNTs is reasonable (Figure 7A).
Thus, the
have been done by several research groups to further charac-
terize both pristine nanotubes and their derivatives. These reports
1
3
13
have dealt with the C spin-lattice relaxation behavior of
MWNTs,2
6b,27
28
29,30d-f
However, this signal is considerably narrower (half-height
line width is only about 9 ppm) than those observed previously,
which suggests that the purified HiPco SWNTs have at least
one of the following attributes: more uniform composition (with
respect to diameter, length, and chirality) and packing of the
tubes into bundles, relatively low metal content, or relatively
few defects.
Functionalization with the (CH2)nCOOH moieties (n ) 2 or
3) creates different local environments on the nanotube and
clearly broadens this signal (Figure 7B and C). The peak
maximum also appears to shift slightly upfield. The Raman
functionalized MWNTs, SWNTs,
and
lithium-intercalated SWNTs;2 the H spin-spin relaxation
9c
1
28
behavior of functionalized MWNTs; and the chemical envi-
ronment of nanotubes26,27,29b,30a,d-f
and their oxidized (carboxyl-
28,30a,e,f
substituted) derivatives before and after heating.
addition, the calculated 1 C chemical shift anisotropy tensors
In
3
30b,c,e,f
of metallic and semiconductor SWNTs have been reported.
These papers indicate that five different samples of SWNTs
1
3
29b,30a,d-f
give an isotropic C NMR signal at δ121-126,
while
(
26) (a) Maniwa, Y.; Sato, M.; Kume, K.; Kozlov, M. E.; Tokumoto, M. Carbon
996, 34, 1287. (b) Maniwa, Y.; Hayashi, M.; Kumazawa, Y.; Tou, H.;
Kataura, H.; Ago, H.; Ono, Y.; Yamabe, T.; Tanaka, K. AIP Conf. Proc.
1
(Figure 1B and C), UV-vis-NIR (Figure 2), and IR (Figure
1998, 442 (Electronic Properties of NoVel MaterialssProgress in Molecular
3
A) spectra of 2a and 2b provide clear evidence for the
Nanostructures), 87.
attachment of aliphatic functionality to SWNT sidewalls, but
the methylene groups give very weak signals at best in the MAS
(
(
(
27) Kishinevsky, S.; Nikitenko, S. I.; Pickup, D. M.; van-Eck, E. R. H.;
Gedanken, A. Chem. Mater. 2002, 14, 4498.
28) Xu, M.; Huang, Q.; Chen, Q.; Guo, P.; Sun, Z. Chem. Phys. Lett. 2003,
1
3
1
3
75, 598.
C NMR spectra. If the corresponding H NMR signal is very
29) (a) Tang, X.-P.; Kleinhammes, A.; Shimoda, H.; Fleming, L.; Bennoune,
broad and shifted, decoupling may not be effective, because it
would cause the methylene carbon signals to be severely
broadened. Additional studies are planned to probe this further.
Functionalization also results in the appearance of a carbonyl
signal at about δ172, which is exceptionally shielded for a
K. Y.; Bower, C.; Zhou, O.; Wu, Y. Mater. Res. Soc. Symp. Proc. 2000,
5
93 (Amorphous and Nanostructured Carbon), 143. (b) Tang, X.-P.;
Kleinhammes, A.; Shimoda, H.; Fleming, L.; Bennoune, K. Y.; Sinha, S.;
Bower, C.; Zhou, O.; Wu, Y. Science 2000, 288, 492. (c) Zhou, O.;
Shimoda, H.; Gao, B.; Oh, S.; Fleming, L.; Yue, G. Acc. Chem. Res. 2002,
3
5, 1045.
(
30) (a) Goze-Bac, C.; Holzinger, M.; Jourdain, V.; Latil, S.; Aznar, R.; Bernier,
P.; Hirsch, A. Mater. Res. Soc. Symp. Proc. 2001, 633 (Nanotubes and
Related Materials), A11.2.1-A.11.2.6. (b) Latil, S.; Goze Bac, C.; Bernier,
P.; Henrard, L.; Rubio, A. Mater. Res. Soc. Symp. Proc. 2001, 633
3
3
13
carboxylic acid. Such a C chemical shift is more typical of
-1
an ester or anhydride, but the IR absorption at 1708 cm (Figure
A) is clearly more consistent with a carboxylic acid. Further-
(
Nanotubes and Related Materials), A14.13.1-A14.13.6. (c) Latil, S.;
Henrard, L.; Goze Bac, C.; Bernier, P.; Rubio, A. Phys. ReV. Lett. 2001,
6, 3160. (d) Goze Bac, C.; Latil, S.; Vaccarini, L.; Bernier, P.; Gaveau,
3
more, acid treatment, which would be expected to hydrolyze
an ester or anhydride, did not affect this IR peak. Not only is
8
P.; Tahir, S.; Micholet, V.; Aznar, R.; Rubio, A.; Metenier, K.; Beguin, F.
Phys. ReV. B 2001, 63, 100302-1. (e) Goze Bac, C.; Bernier, P.; Latil, S.;
Jourdain, V.; Rubio, A.; Jhang, S. H.; Lee, S. W.; Park, Y. W.; Holzinger,
M.; Hirsch, A. Curr. Appl. Phys. 2001, 1, 149. (f) Goze-Bac, C.; Latil, S.;
Lauginie, P.; Jourdain, V.; Conard, J.; Duclaux, L.; Rubio, A.; Bernier, P.
Carbon 2002, 40, 1825.
(31) Rao, C. N. R.; Govindaraj, A.; Satishkumar, B. C. Chem. Commun. 1996,
1525.
(32) Tang, B. Z.; Xu, H. Macromolecules 1999, 32, 2569.
15180 J. AM. CHEM. SOC.
9
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Functionalization of Single-Walled Carbon Nanotubes
A R T I C L E S
the carbonyl signal at δ172 exceptionally shielded, it also is
much broader than the carboxyl signals in the CPMAS spectra
of the precursor peroxides. Thus, the π-system of the nanotube
itself appears to exert a significant shielding and broadening
effect on the carbonyl carbon of the substituent.
to the aromatic groups; the intensity of these methylene protons
is also greatly reduced relative to the intensity of the more
remote protons. In another polymer-wrapped SWNT, the poly-
1
(vinylpyrrolidone) polymer gives no detectable H NMR
3
5d
signals, while in an example where a -CH2NH2-substituted
13
There is a precedent for such an effect in solid state C NMR
crown ether interacts with carboxyl groups on the SWNT, the
and especially in solution state H and 19F NMR spectra of
1
methylene side-chain resonances are broadened to the point of
1
13
35e
functionalized nanotubes. In the H- C CPMAS spectrum of
a MWNT bearing CONH(CH2)17CH3 substituents, the signals
for the two CH2 groups closest to the nanotube were broadened
almost disappearing.
As compared to these illustrative
examples of relatively large molecules interacting with (e.g.,
wrapping around) SWNTs, it is hard to imagine how small,
carboxyl-substituted free radicals could do so.
28 1
beyond detection. H NMR has been used to see the interaction
34a
between the substituent and the SWNT. The signals of the
protons next to the nanotube sidewalls are shifted upfield in
correlation with the distance from the sidewall of the SWNT
functionalized with the aziridino substituent >N-CO-O-
CH2CH2-(OCH2CH2)n-CH2CH2CH2CH3, and those signals are
The broadening and shifting of resonances is consistent with
3
6a,b
36c,d
experimental
and theoretical
work indicating a large
diamagnetic susceptibility resulting from delocalized electrons
(a π-electron ring current) in nanotubes. While C60 has five-
37
membered rings with paramagnetic (also called paratropic )
3
4b
38
also broadened. The same phenomenon has been observed
ring currents, perfect SWNTs do not have five-membered rings
1
in the H NMR spectrum of a SWNT functionalized with a
in their sidewall framework, although such rings can be present
as defects. The absence of such rings could reasonably be
34c
carbene. A recent report on other aziridino-substituted SWNTs
1
36a
also indicates that the H NMR signals are shifted upfield and
expected to lead to a larger diamagnetic susceptibility. That
broadened in the functionalized SWNT.9 In the F NMR
spectrum of a SWNT bearing n-C8F17 substituents, the signals
for the two CF2 groups closest to the nanotube are broadened
beyond detection.9 Finally, in 2D NMR spectra of a SWNT
coupled to a bioactive peptide, a decrease and a broadening of
the signal intensities were observed for the amino acid residues
b
19
the resonances of substituents become more shielded would also
be consistent with the presence of diamagnetic (also called
3
7
diatropic ) ring currents in the network of six-membered rings
a
1
in nanotubes. The H NMR spectrum of the fulleroid C61H2, a
methanoannulene, provides a classic example of the sensitivity
of substituent chemical shifts to the local magnetic field.3 (In
this case, the proton above a five-membered ring is deshielded
by its paratropic ring current, while the proton above a six-
membered ring is shielded by its diatropic ring current.37,38a,39b)
Clearly, much work remains to be done to gain a better
understanding of the effects of the substantial diamagnetic
susceptibility of functionalized nanotubes on the acquisition,
appearance, and interpretation of NMR spectra, particularly in
the solid state.
9a
8c
approaching the aromatic tube walls.
Broadening and shifting of resonances has also been observed
for noncovalently functionalized nanotubes. The major inter-
action between a n-C10H21O-substituted poly(p-phenylene-
ethynylene) and a SWNT is believed to be π-stacking; the
signals for the methylene protons closest to the SWNT are
broadened and shifted upfield by 0.5 ppm, while the phenylene
protons are too broad to be detected.3 Similarly, π-stacking
between a pyrene derivative and a SWNT has also been
proposed to explain the small upfield shift of the pyrenyl
protons.3 In an example where a polymer, a n-C8H17O-
substituted poly(m-phenylenevinylene), wraps around the SWNT,
π-π interactions are believed to be responsible for the
broadening and shifting of resonances.3 Broadening is par-
ticularly noticeable for the aromatic and vinyl protons of the
polymer backbone and the protons of the octyloxy chains closest
5a
Thus, it is not unreasonable to observe in the spectra of 2a
and 2b that the signals of the methylene carbons, which are
even closer than the carbonyl carbon to the nanotube, are shifted
even further upfield and broadened almost beyond detectability.
For the sample of 2a, a very weak broad signal consistent with
aliphatic carbon is centered at about δ20, while for 2b, signals
from aliphatic carbons are essentially undetectable, presumably
because less derivatization occurred, as indicated in the Raman
spectra of 2a and 2b (Figure 1B and C).
5b
5c
(
33) Relevant carboxyl 13C chemical shift data on model compounds with Cquat
CH CH COOH, Cquat-CH CH CH COOH, or similar groups include: (a)
,4-dimethylhexanoic acid, δ180.6 in CDCl relative to TMS: Aurell, M.
J.; Domingo, L. R.; Mestres, R.; Mu n˜ oz, E.; Zaragoz a´ , R. J. Tetrahedron
relative
-
SWNTs can be sidewall functionalized with long-chain
2
2
2
2
2
1
0
4
3
n-alkyl (e.g., n-C11H23 ) or (CH2)nCOOH groups, and a MAS
1
3
C NMR spectrum could be obtained under quantitative
1
999, 55, 815. (b) 4,4,6-trimethylheptanoic acid, δ180.5 in CDCl
3
1
3
to TMS: Katritzky, A. R.; Zhang, S.; Hussein, A. H. M.; Fang, Y.; Steel,
P. J. J. Org. Chem. 2001, 66, 5606. (c) 5,5-dimethylhexanoic acid,
δ180.3: Taber, D. F.; Ruckle, R. E., Jr. J. Am. Chem. Soc. 1986, 108,
conditions. In a direct C pulse experiment with a sufficiently
28
long relaxation delay, the integrated intensity of the distinctive
methyl signal or carboxyl signal relative to the integrated area
of the methylene signals would then indicate the number of
methylene carbon signals that had not been detected. Such an
7
3
686. (d) 5,5-diphenylpentanoic acid, δ179.9 in CDCl relative to TMS:
Foubelo, F.; Lloret, F.; Yus, M. Tetrahedron 1992, 48, 9531.
34) (a) Holzinger, M.; Hirsch, A.; Hennrich, F.; Kappes, M. M.; Dziakova,
A.; Ley, L.; Graupner, R. AIP Conf. Proc. 2002, 633 (Structural and
Electronic Properties of Molecular Nanostructures), 96. (b) Abraham, J.;
Hirsch, A.; Hennrich, F.; Kappes, M.; Dziakova, A.; Graupner, R.; Ley,
L. AIP Conf. Proc. 2002, 633 (Structural and Electronic Properties of
Molecular Nanostructures), 92. (c) Holzinger, M.; Hirsch, A.; Bernier, P.
AIP Conf. Proc. 2001, 591 (Electronic Properties of Molecular Nano-
structures), 337.
(
(36) (a) Wang, X. K.; Chang, R. P. H.; Patashinski, A.; Ketterson, J. B. J. Mater.
Res. 1994, 9, 1578. (b) Ramirez, A. P.; Haddon, R. C.; Zhou, O.; Fleming,
R. M.; Zhang, J.; McClure, S. M.; Smalley, R. E. Science 1994, 265, 84.
(c) Lu, J. P. Phys. ReV. Lett. 1995, 74, 1123. (d) Haddon, R. C. Nature
1997, 388, 31.
(37) B u¨ hl, M.; Hirsch, A. Chem. ReV. 2001, 101, 1153.
(38) (a) Pasquarello, A.; Schl u¨ ter, M.; Haddon, R. C. Science 1992, 257, 1660.
(b) Haddon, R. C. Science 1993, 261, 1545. (c) Pasquarello, A.; Schl u¨ ter,
M.; Haddon, R. C. Phys. ReV. A 1993, 47, 1783. (d) Haddon, R. C. Nature
1995, 378, 249.
(
35) (a) Chen, J.; Liu, H.; Weimer, W. A.; Halls, M. D.; Waldeck, D. H.; Walker,
G. C. J. Am. Chem. Soc. 2002, 124, 9034. (b) Nakashima, N.; Tomonari,
Y.; Murakami, H. Chem. Lett. 2002, 638. (c) Star, A.; Stoddart, J. F.;
Steuerman, D.; Diehl, M.; Boukai, A.; Wong, E. W.; Yang, X.; Chung,
S.-W.; Choi, H.; Heath, J. R. Angew. Chem., Int. Ed. 2001, 40, 1721. (d)
O’Connell, M. J.; Boul, P.; Ericson, L. M.; Huffman, C.; Wang, Y.; Haroz,
E.; Kuper, C.; Tour, J.; Ausman, K. D.; Smalley, R. E. Chem. Phys. Lett.
(39) (a) Suzuki, T.; Li, Q.; Khemani, K. C.; Wudl, F. J. Am. Chem. Soc. 1992,
114, 7301. (b) Prato, M.; Suzuki, T.; Wudl, F.; Lucchini, V.; Maggini, M.
J. Am. Chem. Soc. 1993, 115, 7876.
2
001, 342, 265. (e) Kahn, M. G. C.; Banerjee, S.; Wong, S. S. Nano Lett.
002, 2, 1215.
2
J. AM. CHEM. SOC.
9
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A R T I C L E S
Peng et al.
analysis would be predicated upon the methyl or carboxyl carbon
at the end of the long chain being detected at full intensity
because that carbon is far from the nanotube. As a practical
matter, this analysis would be easier to do with a long-chain
n-alkyl substituent than with a long-chain (CH2)nCOOH sub-
stituent because the much larger chemical shift anisotropy of
the carboxyl group would require carefully measuring the
intensity of its relatively weak spinning sidebands in addition
to the intensity of the centerband.
Finally, even after one takes into account the small number
of scans with a long relaxation delay for the purified SWNTs
1, their spectrum exhibits (Figure 7A) a relatively low S/N as
compared to the derivatized SWNTs 2b, 2a, and 3 (Figure 7B-
D). The relatively low S/N for 1 is consistent with the relatively
1
3
29,30e,f
long C T1 values attributed to semiconducting SWNTs.
Conclusion
We have developed an efficient one-step method for covalent
attachment of alkyl groups terminated with a carboxylic acid
to the sidewalls of carbon nanotubes. The procedure utilizes
easily accessible diacyl peroxides containing terminal carboxylic
acid groups. Further functionalization of the carboxyl groups
on the nanotube side chains with terminal amines and diamines
has been demonstrated. These types of sidewall functionaliza-
tions impart improved solubility and processing to the nanotubes,
which may enable their incorporation into polymer composite
materials through chemical bonding to the surrounding matrix
via terminal functional groups on the nanotubes. These func-
tional groups also provide sites for covalent attachment of
biologically active moieties, for example, amino acids, peptides,
proteins, and oligonucleotides, to the nanotubes for drug delivery
and biosensor applications.
Comparison of the 13C MAS spectra of SWNT-{CH2CH2-
COOH}x (2a) and C60-{CH2CH2COOH}x (6)40 shows that the
π-system of the nanotube causes more shielding and broadening
of the carbonyl carbon signal than does the π-system of C60. In
the spectrum of C60-derivative 6, the carbonyl signal is observed
33
at δ177, only about 3 ppm upfield of that in model compounds.
As compared to the π-system of SWNT-derivative 2a, the
π-system in 6 is clearly smaller and curves away from the
substituent. In addition, C60 exhibits a magnetic susceptibility
some 30 times smaller (on a per gram basis) than that of
3
6a
nanotubes. Thus, less shielding and less broadening of the
carbonyl carbon in C60-derivative 6 than in SWNT-derivative
2
a is reasonable. This observation is also consistent with
functionalization along the sidewall, rather than at the curved
end, of the SWNT. A qualitatively similar effect on chemical
shift has been noted on the relative shielding of the pyrrolidine
protons generated when C60 and SWNT are subjected to the
The solid state 13C NMR studies of functionalized nanotubes
have shown that the side-chain carbons exhibit a notable
broadening and shifting of resonances consistent with experi-
mental and theoretical work indicating a large diamagnetic
susceptibility resulting from delocalized electrons (a π-electron
ring current) in nanotubes. These effects were found to be
stronger than in similarly functionalized C60, where the π-system
is clearly smaller and curves away from the substituent.
8
a
same 1,3-dipolar cycloaddition of azomethine ylides. It is not
obvious how one would differentiate -(CH2)nCOOH function-
alization of an isolated defect site in a SWNT from such
functionalization of a long intact π-system, as the carbonyl
carbon could be far enough from the defect site to feel the effect
of a π-electron ring current near the carbonyl carbon.
Functionalization of the SWNTs with CH2CH2CONHCH2-
CH2NH2 groups has surprisingly little effect on the line width
of the signal from the nanotube carbons in the spectrum of
SWNT-derivative 3 (Figure 7D). The amide group gives a signal
at δ174. Signals from the methylene carbons are clearly stronger
than in the spectra of 2a and 2b. The increased intensity
presumably results from the relatively remote NHCH2CH2NH2
methylene carbons, whose signals appear upfield of what might
be expected.
13
Additional solid state C NMR investigations of functionalized
nanotubes will clearly be of interest both in fundamental and
in applied studies.
Acknowledgment. This work was supported by the Texas
Advanced Technology Program and The Robert A. Welch
Foundation of Texas. Funding for the 200 MHz spectrometer
was provided by the Office of Naval Research (grant N00014-
9
6-1-1146). We thank Mr. Yunxuan Xiao for discussions and
assistance in obtaining the solid state NMR data.
(
40) Peng, H.; Margrave, J. L.; Khabashesku, V. N.; Alemany, L. B., unpublished
results.
JA037746S
15182 J. AM. CHEM. SOC.
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