7382 Chem. Commun., 2010, 46, 7382–7384 This journal is c The Royal Society of Chemistry 2010

Noncovalent functionalization of multiwall carbon nanotubes by

methylated-b-cyclodextrins modified by a triazole groupw

Bastien Leger,abd Stephane Menuel,abd David Landy,ac Jean-Francois Blach,abd

Eric Monflierabd

and Anne Ponchel*abd

Received 23rd July 2010, Accepted 17th August 2010

DOI: 10.1039/c0cc02761h

Multiwall carbon nanotubes have been efficiently suspended into

water thanks to methylated b-cyclodextrins (CDs) containing a

triazole group, itself substituted in the 4-position by hydrophilic

moieties.

In recent years, carbon nanotubes (CNTs) have received

increasing interest due to their unique structural, mechanical

and electronic properties1 and have found applications in a

number of advanced technological areas including sensors,2

nanocomposites,3 molecular devices,4 electronic materials5

and biomedical carriers.6 Their structures are usually classified

into two categories, i.e. the singlewall and multiwall carbon

nanotubes (SWNTs7 and MWNTs,8 respectively). Due to the

presence of van der Waals inter-tube forces, CNTs have an

inherent tendency to exist as bundles (agglomerates), resulting

in poor dispersion and surface area contact, which consequently

limits their practical applications in aqueous solvents. To

circumvent these problems, two strategies can be employed.

The first one involves the sidewall functionalization of CNTs

by oxidative acid treatment or covalent grafting of organic

molecules having hydrophilic substituents9 while the second

one involves the noncovalent functionalization of the sidewalls

using appropriate dispersing agents.10 This last approach has

the advantage of not altering the intrinsic mechanic and

electronic properties of CNTs. In this context, native and

functionalized cyclodextrins (water-soluble cyclic oligo-

saccharides formed by glucopyranose units; CDs) have recently

emerged as multifunctional supramolecular agents capable of

dispersing CNTs by noncovalent interaction and introducing

molecular recognition sites into the materials.11 Thus, Harada

et al.11d and Stoddart et al.11e have demonstrated that

pyrene-modified b-cyclodextrins could efficiently stabilize

aqueous SWNT suspensions by means of favorable p–pstacking interactions between the polyaromatic part of the

CD and the sidewall of SWNT, and more interestingly, that

the CD cavities remain available to form host–guest complexes

with organic molecules in the aqueous solution.

The present work describes the effects of partially

methylated b-CD derivatives, containing a 1,4-disubstituted

1,2,3-triazole unit, on the dispersion of CNTs in water. Our

idea was to take advantage of the presence of the triazole

group to facilitate the stabilization of the carbon nanotube

particles in water through p–p interactions. Unexpectedly, we

have found that the ability of these CD derivatives to disperse

CNTs strongly depended on the nature of the substituent

attached to the triazole group.

The 1,2,3-triazole-linked b-CD derivatives investigated in

this study have been synthesized in high yields by reaction of

a partially methylated mono-6-azido-b-CD with the corres-

ponding water-soluble alkynyl derivatives (procedure in

ESIw).12 All methylated b-CD derivatives are mono-substituted

on the primary face by a triazole group, itself substituted in the

4-position by a hydrophilic functional group. A sulfonato group

(CDTSO3Na) or hydroxy groups (CDTOH or CDTpolyOH)

have been chosen to ensure a high water solubility to the CD

derivatives (Fig. 1).

Experiments have been conducted with Graphistrength

Arkema MWNT washed with aqueous HNO3 prior to use.

The MWNT dispersions were prepared as follows: 1.2 mg of

the MWNT (SBET = 213 m2 g�1, procedure in ESIw) were

suspended in 10 mL of 0.87 mM aqueous methylated b-CDderivative solution and then sonicated for 30 min.

The stability of different aqueous dispersions of MWNT

after one month has been evaluated from the photographs

shown in Fig. 2. For comparison, additional dispersion tests

have been performed with methylated-b-CD (RAME-b-CD)zand a methylated-b-CD containing only one sulfobutyl ether

group (CDSBE).13 As expected, the MWNT particles are falling

down to the bottom of the vessel with these non-triazole CD

derivatives (Fig. 2b and c). In the case of b-CDs bearing a

triazole ring, a clear sedimentation of the MWNT is observed

Fig. 1 Structure of the methylated b-CD derivatives. DS: average

number of methyl groups per glucopyranose unit.

aUniv Lille Nord de France, F-59000 Lille, France.E-mail: anne.ponchel@univ-artois.fr; Fax: +33 3-2179-1755;Tel: +33 3-2179-1754

bU Artois, UCCS, Faculte des Sciences Jean Perrin,Rue Jean Souvraz, SP 18, F-62300 Lens, France

cULCO, UCEIV, F-59140 Dunkerque, FrancedCNRS, UMR 8181, Francew Electronic supplementary information (ESI) available: Chemicals,synthesis and analytical data of CDTpolyOH and CDTSO3Na,characterization methods, T-ROESY spectrum of CDTOH, TGAanalyses and FTIR spectra. See DOI: 10.1039/c0cc02761h

COMMUNICATION www.rsc.org/chemcomm | ChemComm

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This journal is c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 7382–7384 7383

with CDTOH (Fig. 2d) whereas stable suspensions are

obtained with CDTpolyOH and CDTSO3Na (Fig. 2e and f).

The long-time stability of these dispersions has been further

confirmed by Transmission Electron Microscopy (TEM)

measurements. Indeed, the TEM images of the CDTSO3Na–

MWNT system clearly reveal the presence of individual

MWNT or thin carbon bundles (Fig. 3). The diameter of the

individual carbon nanotube was estimated to be 10–20 nm

with mean lengths on the order of several micrometres, in

good agreement with the data provided by the manufacturer.

The above results indicated that the attachment of triazole

group to the CD ring did not necessarily ensure the MWNT

dispersion in water. Indeed, a rapid sedimentation of MWNT

has been observed with CDTOH whereas stable suspensions

are obtained with CDTSO3Na and CDTpolyOH. These

results were rationalized by performing 2D T-ROESY NMR

experiments on the 1,2,3-triazole-linked methylated b-CDderivatives in D2O solution. Whereas the T-ROESY spectra

of the CDTSO3Na and CDTpolyOH exhibit no marked

signal, the spectrum of CDTOH clearly shows cross-peaks

between the CD and triazole protons, proving the existence of

inclusion phenomena (Fig. 4 and Fig. S1 in ESIw). In fact, two

different signals for the triazole proton (Ht) are observed at

d = 7.80 and 7.95 ppm, indicating the presence of different

species in solution. The broadest signal at 7.95 ppm that

correlates with the CD protons is attributed to a self-included

structure. This structure is probably the consequence of the

partial capping of the CD cavity by the substituted triazole

group (Fig. 5, species A). The signal at 7.80 ppm is attributed

to the expected non-included form of CDTOH (Fig. 5, species B).

Furthermore, the presence of supramolecular oligomeric

species cannot be excluded. Indeed, the triazole ring of

species B could be embedded in the cavity of another type B

cyclodextrin and could also form another species (Fig. 5,

species C). Consequently, the rapid sedimentation observed

with CDTOH could be due to the partial inclusion of triazole

ring of CDTOH into the CD cavity. In fact, it seems that

highly hydrophilic substituents such as sulfonato or poly-

hydroxyl groups are required to counterbalance the inherent

tendency of the triazole group to be included into the CD

cavity and remain available to interact with the carbon

surface.

The difference of affinity of CDTOH and CDTSO3Na for

the MWNT surface has also been evidenced by thermo-

gravimetric analysis of CDTOH–MWNT and CDTSO3Na–

MWNT materials (Fig. 6). These hybrid materials were

directly recovered from the aqueous suspensions by filtration

and rinsed with a known volume of water to remove the excess

of weakly bound cyclodextrins. In both cases, we observe a

weight loss in the 200–400 1C temperature range, which

corresponds to the thermal decomposition of the residual

oligosaccharides (Fig. 6b and c).14 It is worth mentioning that,

even after several washings of the MWNT particles by water,

an important amount of CDTSO3Na is still adsorbed onto the

Fig. 2 Photographs of the MWNT aqueous dispersions after one

month without CD (a) and with RAME-b-CD (b), CDSBE (c),

CDTOH (d), CDTpolyOH (e) and CDTSO3Na (f).

Fig. 3 TEM images of the aqueous MWNT suspension after one

month (CDTSO3Na–MWNT) at different magnifications (a) 200 nm,

(b) 50 nm.

Fig. 4 T-ROESY 1H NMR spectrum (300 MHz) of a solution

containing CDTOH at 36 mmol L�1 in D2O at 298 K.

Fig. 5 Schematic representations of CDTOH supramolecular

complexes.

Fig. 6 Thermogravimetric profiles under N2 atmosphere (heating

rate of 2 1C min�1) of the following materials: (a) MWNT,

(b) CDTOH–MWNT and (c) CDTSO3Na–MWNT.

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7384 Chem. Commun., 2010, 46, 7382–7384 This journal is c The Royal Society of Chemistry 2010

surface (290 mmol g�1). Note that this value is about 3.5 times

greater than that obtained from the solid resulting from the

aqueous suspension of CDTOH (79 mmol g�1). This last result

confirms that stronger interactions between the modified CD

and the carbon surface are obtained when the triazole unit

attached to the b-CD is not included inside the CD cavity.

Finally, spectroscopic evidences of the adsorption and

interaction of the triazole ring of CDTSO3Na on MWNT

have also been obtained by ATR-FTIR characterization of the

hybrid material recovered by filtration. Indeed, a strong

decrease in the intensity of the vibrational modes of the

triazole cycle is observed in the 1700–1250 cm�1 region

[CQC (1580 and 1450 cm�1), NQN (1425 cm�1) and C–N

(1300 cm�1)]. Note that the presence of adsorbed CDTSO3Na

has been clearly evidenced by the presence of the oligosaccharide

bands in the 1250–750 cm�1 region (Fig. S3 in ESIw).Fluorescence experiments have also confirmed the interaction

between the triazole group and the carbon nanotube (Fig. 7).

Indeed, the fluorescence signal associated with the triazole ring

of CDTSO3Na (i.e. 350–500 nm) notably decreased when

increasing amounts of MWNT were added to the aqueous

solution of CDTSO3Na. This phenomenon can be explained

by a change in the p electron transfer between the triazole and

the sp2 nanotube structure. Similar quenching effects have

already been described in the literature by using fused aromatic

compounds functionalized by water soluble dendrimers15 or

adamantane/cyclodextrin inclusion complexes16 to solubilize

SWNT.

In conclusion, we have demonstrated that methylated

b-CDs modified by a triazole group, itself substituted by

highly hydrophilic moieties, can constitute a new class of

MWNT dispersing agents. However, the hydrophilic groups

linked to triazole should be carefully chosen to ensure a good

dispersion of MWNT in aqueous solution. We believe that

these CD derivatives will provide new possibilities to introduce

recognition sites on the CNTs surface and, consequently,

extend the applications of the CNTs in the fields of chemistry,

biology, medicine and electronic materials.

The authors are grateful to Olivier Gardoll (UCCS,

University of Lille, France) for the TG analyses and to Dr

Christine Lancelot (UCCS, University of Lille, France) for

the TEM analyses. Roquette Freres (Lestrem, France) is

gratefully acknowledged for generous gifts of cyclodextrins.

Notes and references

z The RAME-b-CD was a native b-CD partially O-methylated withstatistically 1.8 OH groups modified per glucopyranose unit.

1 (a) P. M. Ajayan, Chem. Rev., 1999, 99, 1787; (b) D. Tasis,N. Tagmatarchis, A. Bianco and M. Prato, Chem. Rev., 2006,106, 1105.

2 P. G. Collins, K. Bradley, M. Ishigami and A. Zettl, Science, 2000,287, 1801.

3 D. M. Guldi, G. M. A. Rahman, F. Zerbetto and M. Prato, Acc.Chem. Res., 2005, 38, 871.

4 J. Yan, H. Zhou, P. Yu, L. Su and L. Mao, Adv. Mater., 2008, 20,2899.

5 Q. Cao and J. A. Rogers, Adv. Mater., 2009, 21, 29.6 (a) L. Jiang, L. Gao and J. Sun, J. Colloid Interface Sci., 2003, 260,89; (b) R. Bandyopadhyaya, E. Nativ-Roth, O. Regev andR. Yerushalmi-Rozen, Nano Lett., 2002, 2, 25; (c) J. C. Grunlan,L. Liu and O. Regev, J. Colloid Interface Sci., 2008, 317, 346.

7 S. Niyogi, M. A. Hamon, H. Hu, B. Zhao, P. Bhowmik, R. Sen,M. E. Itkis and R. C. Haddon, Acc. Chem. Res., 2002, 35, 1105.

8 R. Andrews, D. Jacques, D. Qian and T. Rantell, Acc. Chem. Res.,2002, 35, 1008.

9 (a) L. Meng, C. Fu and Q. Lu, Prog. Nat. Sci., 2009, 19, 801;(b) V. Datsyuk, M. Kalyva, K. Papagelis, J. Parthenios, D. Tasis,A. Siokou, I. Kallitsis and C. Galiotis, Carbon, 2008, 46, 833;(c) Y. P. Sun, K. Fu, Y. Lin and W. Huang, Acc. Chem. Res., 2002,35, 1096; (d) C. A. Dyke and J. M. Tour, Chem.–Eur. J., 2004, 10,812.

10 (a) Y. L. Zhao and J. F. Stoddart, Acc. Chem. Res., 2009, 42, 1161;(b) A. Di Crescenzo, D. Demurtas, A. Renzetti, G. Siani, P. DeMaria, M. Meneghetti, M. Pratod and A. Fontana, Soft Matter,2009, 5, 62; (c) T. H. Kim, C. Doe, S. R. Kline and S. M. Choi,Adv. Mater., 2007, 19, 929.

11 (a) J. Chen, M. J. Dyer and M. F. Yu, J. Am. Chem. Soc., 2001,123, 6201; (b) K. Liu, H. Fu, Y. Xie, L. Zhang, K. Pan andW. Zhou, J. Phys. Chem. C, 2008, 112, 951; (c) T. Ogoshi,T. A. Yamaguchi, Y. Nakamoto and A. Harada, Chem. Lett.,2007, 36, 1026; (d) T. Ogoshi, Y. Takashima, H. Yamaguchi andA. Harada, J. Am. Chem. Soc., 2007, 129, 4878; (e) Y. L. Zhao,L. Hu, J. F. Stoddart and G. Gruner, Adv. Mater., 2008, 20, 1910.

12 (a) M. Mourer, F. Hapiot, E. Monflier and S. Menuel,Tetrahedron, 2008, 64, 7159; (b) M. Mourer, F. Hapiot, S. Tilloy,E. Monflier and S. Menuel, Eur. J. Org. Chem., 2008, 5723.

13 P. Blach, D. Landy, S. Fourmentin, G. Surpateanu, H. Bricout,A. Ponchel, F. Hapiot and E. Monflier, Adv. Synth. Catal., 2005,347, 1301.

14 F. Trotta, M. Zanetti and G. Camino, Polym. Degrad. Stab., 2000,69, 373.

15 C. Backes, C. D. Schmidt, F. Hauke, C. Bottcher and A. Hirsch,J. Am. Chem. Soc., 2009, 131, 2172.

16 S.-Z. Zu, X.-X. Sun, Y. Liu and B.-H. Han, Chem.–Asian J., 2009,4, 1562.

Fig. 7 Fluorescence emission spectra of aqueous solutions of

CDTSO3Na–MWNT with lexc = 327 nm at different MWNT

concentrations: 0 (a), 2 (b), 4 (c), 6 (d), 8 (e) and 10 (f) mg L�1.

[CDTSO3Na] = 15 mM.

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