Journal of Biomaterials and Nanobiotechnology, 2011, 2, 454-460
doi:10.4236/jbnb.2011.24055 Published Online October 2011 (http://www.SciRP.org/journal/jbnb)
β-Cyclodextrin Covalently Functionalized
Single-Walled Carbon Nanotubes: Synthesis,
Characterization and a Sensitive Biosensor
Platform
Yong Gao1, Yu Cao1, Guiling Song1, Yiming Tang1, Huaming Li1,2*
1College of Chemistry, Xiangtan University, Xiangtan, China; 2Key Laboratory of Polymeric Materials and Application Technology
of Hunan Province, Key Laboratory of Advanced Functional Polymeric Materials of College of Hunan Province, and Key Lab of
Environment-Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan, China.
Received July 30th, 2011; revised August 18th, 2011; accepted September 1st, 2011.
ABSTRACT
In this study, we presented the preparation of β-cyclodextrin (β-CD) covalently functionalized single-walled carbon nano-
tubes (SWCNTs) and its application in modifying the solid glass carbon electrode (GCE). Cyclic voltammetry (CV) method
was employed to evaluate the performance of the modified GCE. Solubility experiment indicated the conjugation of
SWCNTs and β-CD, SWCNTs-β-CD with 8 wt% β-CD content could be well dispersed in water. High-resolution transmis-
sion electron microscopy (HRTEM) demonstrated that the aggregated SWCNTs bundle were effectively exfoliated to small
bundle, even individual tube. The β-CD component was grafted on the side walls as well as tips of SWCNTs, and the grafted
β-CD component was not uniformly coated on the surface of SWCNTs. The CV measurements indicated the performance of
the GCE modified by SWCNTs-β-CD was better than that of the GCE modified by the hybrid of SWCNTs/β-CD, where
ascorbic acid (AA) and uric acid (UA) were selected as a prelimiltary substrate to evaluate it. The enhanced performance of
the modified GCE should be ascribed to the integration of the excellent electrocatalytic property of SWCNTs with the inclu-
sion ability of β-CD to analyte molecule.
Keywords: Single-Walled Carbon Nanotubes (SWCNTs), β-Cyclodextrin (β-CD), Electrochemical Sensor, Covalently
1. Introduction
Since the landmark paper on carbon nanotubes (CNTs)
in 1991 by Iijima [1], they have attracted great interdis-
ciplinary interest due to the extraordinarily electrical,
optical, and mechanical properties that CNTs possess
[2,3]. Nowadays, CNTs have the potential of being trans-
formed into new materials that will traverse a wide range
of applications, such as sensors, nanoelectronics, bio-
medical devices, and high-strength fibers [4-7]. However,
aggregated large nanotube bundles were insolubility in
routine solvent, which makes it difficult to manipulate
and utilize CNTs [8,9].
Two strategies, including noncovalent and covalent
modification, have been developed to modify the CNTs
to overcome the solubility limitations [10-16]. In the
cased of the noncovalent modification strategy, a rather
limited surfactants and compounds with benzene ring or
condensed aromatic ring are usually used to disperse the
CNTs bundle. The virtues of noncovalent modification
lie in the maintaining of the nanotube’s electronic struc-
ture and its relatively simple process [17]. With respect
to noncovalent modification, covalent functionalization
of CNTs provides a broad space in singling out the spe-
cies of compound, for a broad range of functional groups
could be incorporated onto the surface of CNTs by more
versatile modification approaches. Covalently function-
alized CNTs demonstrated wide applications in many
fields, such as in catalytic and biological applications etc
[18-22]. For example, Neelgund, et al. [18] reported the
synthesis of poly(lactic acid) functionalized CNTs-Pd
nanocatalyst by covalent grafting of poly(lactic acid)
onto CNTs and subsequent deposition of Pd nano- parti-
cles. The nanocatalyst demonstrated more effective
activity in the promotion of Heck cross-coupling reaction
between aryl halides and n-butyl acrylate. Karousis, et al.

β-Cyclodextrin Covalently Functionalized Single-Walled Carbon Nanotubes: Synthesis, Characterization and 455
a Sensitive Biosensor Platform
[22] reported that the preparation of water-soluble pep-
tidomimetics covalently grafted carbon nanotubes. The
prepared peptidomimetic functionalized CNT conjugates
showed extremely advantageous in the inhibition of in-
flammation or malignancy and potential future biological
applications in the area of drug delivery systems.
Cyclodextrins (CDs) are cyclic oligosaccharides with
D-(+)-glucose as the repeating unit coupled by 1,4-a-
linkages. α-, β-, and γ-CDs are commonly available
forms which consist of 6, 7, and 8 glucose units, respec-
tively. The most characteristic property of CDs is their
remarkable ability to form inclusion complexes with a
wide variety of guest molecules due to their different
cavity sizes. This property endowed CDs many wide-
spread applications not only in pharmaceutical chemistry,
food technology, analytical chemistry, chemical synthe-
sis, and catalysis [23-27], but also for constructing mo-
lecular devices and machines [28-32]. Therefore, the
combination of β-CD and CNTs is sure to generate sig-
nificant and interesting object for supramolecular chem-
istry, biomedicine and nanodevice construction. How-
ever, most studies were focused on the hybrids of
β-CD/CNTs due to its easy operation, and the studies
concerning the preparation of β-CD covalently function-
alized CNTs and its application were rare except for sev-
eral reports [17,33-37].
Herein, we presented the preparation of β-CD cova-
lently functionalized SWCNTs by a reaction sequence
involving oxidation and amidation reaction. The obtained
conjugation of SWCNTs and β-CD, SWCNTs-β-CD
showed an excellent stability in pure water. And then,
SWCNTs- β-CD was applied to modify the bare GCE.
The performance of modified GCE was investigated by
cyclic voltammetry (CV). Interestingly, the performance
of GCE modified by SWCNTs-β-CD conjugation was
better than that of GCE modified by the hybrids of
β-CD/SWCNTs under identical conditions. FTIR, TGA
as well as HRTEM were employed to characterize the
titled product. To the best of our knowledge, there is no
report on the modification of GCE with β-CD covalently
functionalized SWCNTs.
2. Experimental
2.1. Materials
SWCNTs with a purity of 60 wt% were obtained from
Shenzhen Nanotech Port Co., Ltd. (Shenzhen, China),
which were prepared by the chemical vapor deposition
(CVD) method. β-CD was purchased from Sigma-Al-
drich. p-Toluenesulfonyl (p-TsCl), 1,6-Hexanediamine
(HDA) were chemical grade. Freshly prepared AA and
UA solutions prior to measurements were used. All other
reagents and solvents were purchased from commercial
suppliers and used as received.
2.2. Oxidation of SWCNTs
In a typical experiment, a round bottom flask was char-
ged with 200 mg of SWCNTs and cooled in an ice bath.
Then, 20 mL of concentrated H2SO4/HNO3 (3:1 by vol-
ume) was slowly added. The mixture was subjected to
sonication at 0˚C for 30 min. The flask was stirred for 20
h at 20˚C. The dispersion was then carefully poured into
1000 mL of deionized water. The black slurry was va-
cuum filtered on a 0.22 µm Teflon membrane, resulting
in the formation of a black filter cake. The black filter
cake was then subjected to dispersion by sonication and
rinsed with deionized water until the pH value of the
filtrate was neutral. The product was dried under vac-
uum.
2.3. Preparation of
Mono-6-(hexanediamino)-β-cy-clodextrin
(HDA-β-CD)
HDA-β-CD was prepared by two successive chemical
processes. Firstly, Mono-6-OTs-β-CD was synthesized
according to the method reported in the literature [38]. In
a typical experiment, β-CD (30.0 g, 26.4 mmol) was
suspended in 250 mL of water, and 10 mL NaOH aque-
ous solution (8.2 mol·L–1) was added dropwise over 1 h.
When the suspension became homogeneous slightly yel-
low solution, 15 mL of p-TsCl acetonitrile solution (13.2
mol·L –1) was added dropwise over 2 h. This process re-
sulted in the formation of a white precipitate. After 2 h of
stirring at 25˚C, the precipitate was removed by filtration
under reduced press and the filtrate was refrigerated
overnight at 4˚C. The resulting white precipitate was
recrystallized from deionized water and dried under va-
cuum. 1H NMR (400 MHz, CDCl3): δ = 7.42 - 7.45 (2H,
=CHmeta), 7.74 - 7.77（2H，=CHortho）, 2.42（3H,CH3）,
3.83 - 3.68 (28H; C(3)–H, C(6)–H, C(5)–H), 3.51 - 3.25
(14H; C(2)–H, C(4)–H); IR (KBr, cm-1): 3382, 2927,
1029, 1320.
The preparation of HDA-β-CD was performed as de-
scribed by Liu, et al. [39]. In a typical experiment, 5.0 g
of Mono-6-OTs-β-CD (3.8 mmol) and 10.0 g (0.17 mol)
of HAD were charged into a bottom flask equipped with
a condenser. The mixture was stirred at 75˚C for 4 h, and
then the mixture was allowed to cool to room tempera-
ture. Subsequently, 300 mL of cold acetone was added,
resulting in the formation of a precipitate. The obtained
precipitate was dissolved in 10 mL of water-methanol
(v/v 3:1) mixture, and then poured into 300 mL of ace-
tone. This operation was repeated several times for the
complete removal of unreacted HAD. The obtained sam-

β-Cyclodextrin Covalently Functionalized Single-Walled Carbon Nanotubes: Synthesis, Characterization and
a Sensitive Biosensor Platform
456
3. Results and Discussion ple was dried at 50˚C for 72 h in a vacuum oven. 1H
NMR (400 MHz, CDCl3): δ = 4.90 (7H, C(1)–H), 3.83 -
3.68 (28H; C(3)–H, C(6)–H, C(5)–H), 3.51 - 3.25 (14H;
C(2)–H, C(4)–H), 2.89 (2H, –CH2NH–β-CD), 1.57 -
1.38 (8H, –(CH2)4–). IR (KBr, cm–1): 3382, 2927, 1050.
3.1. Synthesis of SWCNTs-β-CD
The synthesis of SWCNTs-β-CD was accomplished as
depicted in Scheme 1. SWCNTs were first oxidized by
concentrated H2SO4/HNO3 (3:1 by volume). This process
resulted in the incorporation of carboxyl groups on the
surface of SWCNTs.
2.4. Preparation of β-CD Covalently
Functionalized SWCNTs (SWCNTs-β-CD)
SWCNTs- β-CD was prepared by two successive chemi-
cal processes. In a typical experiment, 20 mg of oxidated
SWCNTs and 30 mL of fresh SOCl2 were added to a
round-bottom flask with a condenser and reacted at 65˚C
for 24 h under magnetic stirring. After cautious removal
of the residual SOCl2 under reduced press, 20 mL dried
DMF and 5.0 g of HDA-β-CD were quickly added, and
the flask was immersed in 60˚C oil bath for another 72 h
enduration with continuous stirring. The dispersion was
vacuum filtered on a 0.22 µm Teflon membrane and left
a black filter cake. The black filter cake was then sub-
jected to dispersion by sonication and rinsed with the
deionized water repeated. After centrifugation, the col-
lected crude product was dried in vacuum to give a black
solid of the SWCNTs-β-CD.
The introduction of carboxylic acid groups onto the
SWCNT surface was verified by the appearance of a
carboxyl stretch at 1719 cm–1 as well as a carboxylate ion
stretch at 1620 cm–1 in the FTIR spectrum of the oxi-
dized product [40], as shown in Figure 1B. Subsequently,
the carboxylic acid-functionalized tubes were activated
with SOCl2 followed by treatment with HDA-β-CD,
yielding SWCNTs-β-CD. The introduction of β-CD on
the surface of SWCNTs was proved by the appearance of
an –C–O–C– stretch at 1030 cm–1, –CONH– stretch at
1550 cm–1 as well as –CH2– stretch at 2987 cm-1 in the
FTIR spectrum (Figure 1C) [39].
The obtained SWCNTs-β-CD could be well dispersed
in the water. Figure 2 showed three vials containing
equal volume of solvent of water and equal mass of pure
SWCNTs (vial A), SWCNTs-β-CD (vial B) and 2 wt%
β-CD aqueous solution dispersed SWCNTs (vial C).
Clearly, SWCNTs is all completely insoluble in water,
whereas SWCNTs-β-CD in vial B formed a clear, dark-
brown solution that contained no discernable particulate
materials, and remains stable for a period of at least four
weeks. The dispersion formed by 2 wt% β-CD aqueous
solution dispersed SWCNTs in vial C was instability,
and the SWCNTs completely aggregated in 72 h.
2.5. Analysis and Characterization
FTIR spectra were recorded on a PE Spectrum One FTIR
spectrometer with KBr pellets. 1HNMR spectra were
recorded on a Bruker AV-400 NMR spectrometer in
deuterated solvents. Thermogravimetric analysis (TGA)
was carried out on a STA 449C instrument under a
flowing nitrogen atmosphere. Transmission electron mi-
croscopy (TEM) analysis was performed on a JEOL
2100 electron microscope. All the voltammetric experi-
ments were carried out with a CHI-660B electrochemical
workstation (Shanghai, China). A conventional three-
electrode system was employed, comprising a GCE (3
mm, diameter) as working electrode, a platinum wireas-
counter electrode, and a saturated calomel electrode as
reference electrode.
The content of β-CD on the surface of SWCNTs was
analyzed by TGA instrument, and the result was indi-
cated in Figure 3. For comparison, the TGA plot of pris-
tine SWCNTs and oxidiated SWCNTs were also shown
in Figure 3. If using the mass loss of the SWCNT- COOH
at 500˚C as the reference, the mass loss of SWCNTs-β-CD
was about 8 wt% at 500˚C.
Scheme 1. The synthesis protocol of SWCNTs-β-CD.

β-Cyclodextrin Covalently Functionalized Single-Walled Carbon Nanotubes: Synthesis, Characterization and 457
a Sensitive Biosensor Platform
Figure 1. FTIR spectra of the pristine SWCNTs (A),
SWCNT-COOH (B), and SWCNTs-β-CD (C).
Figure 2. Photograph of three separate SWCNTs samples in
water: (A) SWCNTs; (B) SWCNTs-β-CD; (C) 2 wt% β-CD
aqueous solution dispersed SWCNTs after 72 h.
Figure 3. TGA plots of the pristine SWCNTs (A), SWCNT-
COOH (B), SWCNTs-β-CD (C), acquired under nitrogen at
a ramp of 10˚C min–1.
The morphology of SWCNTs-β-CD was imaged by a
high resolution TEM, as depicted in Figure 4. Based on
TEM observation, SWCNTs bundle have been exfoliated
to small bundles, even individual tube. β-CDs were
grafted on the tips as well as sidewalls of the SWCNTs,
Figure 4. Representative TEM images of SWCNTs-β-CD.
which did not form a uniform coating layer on the sur-
face of exfoliated SWCNTs.
3.2. Electrochemical Sensor of AA and UA by
SWCNTs-β-CD Modified GCE
The GCE with a diameter of 3 mm was firstly treated by
standard procedure. And then, the modification of GCE
was accomplished by coating 5 μL of SWCNTs-β-CD
solution of water (0.5 mg·mL–1) onto the surface of fresh
treated GCE. As a control, 1 mg of pristine SWCNTs
was dispersed in 2 mL of 2 wt% β-CD aqueous solution
by ultrasonication, and then the dispersion was also ap-
plied to modify GCE. The modified GCEs were marked
with SWCNTs-β-CD/GCE and SWCNTs/β-CD/GCE,
respectively. The performances of modified GCEs were
evaluated by CV method, where AA and UA were se-
lected as a prelimiltary substrate. CVs for the AA and
UA mixture (1.0 mmol·L–1 AA + 1.0 mmol·L–1 UA) in
PBS (pH 7.0) at the bare GCE and at the modified GCEs
were shown in Figure 5. It could be observed that, in the
case of bare electrode, the voltammograms of AA and
UA exhibited just a small hump peak with the overlap-
ping potential. No cathodic peaks were observed for both
AA and UA, which indicated an electrochemically irre-
versible process (curve A). In the case of SWCNTs-β-
CD/GCE, however, two well-distinguished anodic peaks
at potentials of 8 and 379 mV were observed, corre-
sponding to the oxidation of AA and UA, respectively,
(curve B). Worth noticing is that the anodic peak poten-
tial difference between AA and UA is approximately 371
mV, which is much bigger than early reported methods
[41-43]. This suggested the electron transfer on the
modified GCE was facilitated. Additionally, substantial
increases in peak currents were also observed (curve B),
and the anodic current value of UA at the modified elec-
trode is much higher than that of AA. The reason for this
phenomenon may originate from the high concentration
of UA in the interface of modified electrode due to the
selective inclusion effect of β-CD. Since β-CD can read-
ily include the neutral UA than AA monoanionic species
at pH 7.0 [44]. Compared with curve B, the peak currents of
AA and UA at SWCNTs/β-CD/GCE (curve C) was
smaller than that of SWCNTs-β- CD/GCE, and its oxida

β-Cyclodextrin Covalently Functionalized Single-Walled Carbon Nanotubes: Synthesis, Characterization and 458
a Sensitive Biosensor Platform
Figure 5. Cyclic voltammograms for the mixture of 1.0 ×
10−3 mol·L-1 of AA and 1.0 × 10−3 mol·L-1 of UA in 0.1 mol·L-1
pH 7.0 PBS at (A) the bare GCE, (B) SWCNTs-β-CD/GCE,
(C) SWCNT /β-CD/GCE. Scan rate: 50 mV·s−1.
tive potentials shifted positively. This phenomenon
might be deriving from the reduced electron transfer rate
on electrode due to the continuous β-CDs layer absorbed
on the surface of SWCNTs. The experimental results
demonstrated that SWCNTs-β-CD colud be utilized to
construct a good sensor platform.
4. Conclusions
β-CD covalently functionalized SWCNTs has been pre-
pared. TEM demonstrated that SWCNTs bundle was
exfoliated into smaller bundle, even individual tube. The
introduced β-CD components were not uniformly coated
on the surface of SWCNTs. This morphology of SWC-
NTs-β-CD was benefited for the electron transfer when
SWCNTs- β-CD was utilized to modify the GCE. For the
selected prelimiltary substrates of AA and UA, CV mea-
surements indicated that the anodic peak potential dif-
ference between AA and UA was approximately 371 mV
on the SWCNTs-β-CD modified GCE, which was larger
than that of the GCE modified by the hybrids of
β-CD/SWCNTs under the identical condition. This sug-
gested the electron transfer was hampered on the hybrids
of β-CD/SWCNTs modified GCE due to the insulated
β-CD layer absorbed on the surface of SWCNTs.
5. Acknowledgements
Financial support from Program for NCET (NCET-07-
0731), NSFC (51072172), Key Project of Chinese Min-
istry of Education (209086), Scientific Research Fund of
Hunan Provincial Education Department (08B085), Open
Project Program of Key Laboratory of Advanced Func-
tional Polymeric Materials of Hunan Province (AFPM-
200904) and the Planned Science and Technology Pro-
ject of Hunan Province (2010WK2009) is greatly ac-
knowledged.
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