Recognition-Induced Supramolecular Porous Nanosphere Formation
from Cyclodextrin Conjugated by Cholic Acid
Yu Liu,* Yan-Li Zhao, and Heng-Yi Zhang
Department of Chemistry, State Key Laboratory of Elemento-Organic Chemistry, Nankai UniVersity,
Tianjin 300071, People’s Republic of China
ReceiVed September 6, 2005. In Final Form: January 22, 2006
A supramolecular porous nanosphere is constructed from amphiphilic cholic acid-modified cyclodextrin triggered
by guest sodium 1-naphthylamino-4-sulfonate and is comprehensively characterized by nuclear magnetic resonance
that the porous nanosphere with the radius of 25-35 nm has moderate nitrogen adsorption ability. Further NMR,
circular dichroism, and the fluorimetric titrations on the self-assembling behavior in aqueous solution reveal that the
substituting group of the guest molecule and pH values are the key to induce the formation of the porous nanosphere.
The combined processes of molecular-scale recognition and
self-assembly offer a powerful route to the development of both
refined supramolecular systems and, more generally, new
technological devices based on controlling and analyzing
of nanometer-sized porous spheres has found numerous ap-
plications in drug delivery/targeting and extraction,5nanoreac-
tors,6and functional materials.7Therefore, the characterization
formed from synthetic receptors has been shown to be crucial
for the potential application of these systems. While the general
on assembling inorganic compounds or inorganic-organic
nanospheres of organic aggregates have rarely been reported
thus far, to the best of our knowledge. Cyclodextrins (CDs) and
their derivatives are excellent building blocks to create su-
be also constructed by using amphiphilic molecules with
we describe a new method for the preparation of porous
(1,4-SNS). We also demonstrate that the strong host-guest
inclusion interaction, the shape of the guest molecule, and pH
values are the key factors to induce the formation of fullerene-
2. Experimental Section
Materials. ?-CD of reagent grade was recrystallized twice from
water and dried in vacuo at 95 °C for 24 h prior to use.
N,N-Dimethylformamide (DMF) was dried over calcium hydride
for 2 days and then distilled under a reduced pressure prior to use.
Dicyclohexylcarbodiimide (DCC) and cholalic acid were com-
mercially available and used without further purification. 1,4-SNS
from Tokyo Kasei and used as received. Mono[6-O-(p-toluene-
sulfonyl)]-?-CD (6-OTs-?-CD) was prepared by the reaction of
p-toluenesulfonyl chloride with ?-CD in alkaline aqueous solution.
Then, 6-OTs-?-CD was converted to mono(6-aminoethylamino-6-
* To whom correspondence should be addressed. Telephone: +86-22-
23503625. Fax: +86-22-23503625. E-mail: email@example.com.
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10.1021/la052434j CCC: $33.50© 2006 American Chemical Society
Published on Web 03/03/2006
deoxy)-?-CD in 70% yield upon heating in excess ethylenediamine
at 70 °C for 7 h.12
Instruments. Fluorescence spectra were recorded in a conven-
tional quartz cell (10 × 10 × 40 mm) at 25.0 ( 0.1 °C on a JASCO
FP-750 fluorescence spectrometer with the excitation and emission
slits of 5 nm width. Circular dichroism (CD) and UV-vis spectra
were recorded in a conventional quartz cell (light path of 10 mm)
spectrophotometer equipped with a PTC-348WI temperature con-
troller to keep the temperature at 25.0 °C. Elemental analysis was
performed on a Perkin-Elmer 2400C instrument.
Mercury VX300 spectrometer. Transmission electron microscopy
(TEM) experiments were performed using a Philips Tecnai G2 20
by depositing a drop of the suspension onto a holey carbon grid.
Light Scattering Experiments. A Brookhaven Instruments BI-
200SM goniometer and a BI9000AT correlator were used in static
and dynamic light scattering experiments for vortically polarized
light at λ ) 514.5 nm with an (Innova 304) Ar+ion laser. The
experiments were performed at 25 °C in aqueous solution by using
toluene as the standard reference. The laser power was maintained
at 200 mW.
When interparticle interactions and hydrodynamic corrections
were neglected, the hydrodynamic radii (Rh) of nanospheres were
measured in pure water at 25 °C by dynamic light scattering with
various concentrations (c ) 16.7, 12.5, 10.3, and 8.4 × 10-4mol
dm-3). The average hydrodynamic radius was obtained with five
independent experiments for each concentration. Furthermore, the
molecular weight (M) and gyration radius (Rg) of the nanospheres
M and Rgusing the classical equation13
where K ) 4π2n2(dn/dc)2/(NAλ04), RWis the scattering intensity, NA
cm3g-1), c is the concentration, n is the refractive index of the
versus sin2(θ/2), the molecular weight MWand gyration radius Rg
could be calculated from the intercept and slope, respectively.
Gas Adsorption Measurements. The sorption isotherm mea-
surement of nitrogen was performed at 298 K, by using Thermo
Finnigan (Sorptomatic 1990) automatic volumetric adsorption
was placed in the quartz tube, and then, prior to measurements, the
sample was dried under high vacuum at 323 K for 10 h to remove
the solvated water molecules. The adsorbate was dosed into the
sample tube; then the change of the pressure was monitored; and
the amount of adsorption was determined by a decrease in pressure
at the equilibrium state.
Preparation of Mono[6-cholaminoethyleneamino-6-deoxy]-
?-CD (1). To a solution of DMF (30 mL) containing 1.2 g of mono-
(6-aminoethylamino-6-deoxy)-?-CD and 0.26 g of DCC was added
0.45 g of cholic acid in the presence of a small amount of 4 Å
molecular sieves. The reaction mixture was stirred for 2 days in an
ice bath and another 2 days at room temperature and then allowed
to stand for 1 h. The precipitate was removed by filtration, and the
filtrate was poured into 300 mL of acetone. The precipitate was
collected and subsequently purified on a Sephadex G-25 column
ppm (d, 7H). Elemental analysis calcd for C68H114O38N2‚8H2O: C,
47.71; H, 7.66; N, 1.64. Found: C, 47.68; H, 7.72; N, 1.49.
Preparation of Nanospheres. An aqueous solution of 1,4-SNS
(1 mmol, 10 mL) was added to an aqueous solution of amphiphilic
CD 1 (1 mmol, 10 mL) and stirred at room temperature for 2 h. The
solution was dried by rotary evaporation to yield a thin film in a
Then, water of 8 mL was added, and the sample solution was kept
at room temperature for 0.5 h and filtrated by using a 0.45 µm
syringe filter after standing for 1 day. The filtrate was evaporated
the filtration to give the nanospheres.
3. Results and Discussion
1 was prepared by the condensation of cholic acid with mono-
(6-aminoethylamino-6-deoxy)-?-CD in 25% yield. When 1 was
dispersed into a thin film of water by using a sonication bath for
1 h at room temperature, no nanosphere was obtained from the
solution. Interestingly, when an aqueous solution of 1,4-SNS
SNS complex, the length of the complex is about 3 nm. The
× 10-4mol dm-3. Moreover, replacement of 1,4-SNS with 2,6-
DNS in the experiment did not produce nanosphere.
Light Scattering, TEM, and Gas Adsorption. To confirm
the size and average molecular weight of nanospheres, the light
scattering experiments are performed in aqueous solution at 25
°C. As can be seen from Table 1, dynamic light scattering of the
(Rh) of the CD aggregates is in the range of 15-30 nm and their
in each aggregate. On the other hand, the ratio of the gyration
radius (Rg) to the hydrodynamic radius (Rh) is in the range of
0.97-1.03, which suggests a hollow or porous formation of the
of 1/1,4-SNS complexes.14
Furthermore, TEM experiments give direct evidence for the
TEM images of the 1/1,4-SNS aggregates, revealing numerous
nanospheres with a radius of 25-35 nm, which is consistent
with the results of the light scattering experiments. From the
the microstructure of nanospheres. There are a lot of hole
structures with a diameter of 3-10 nm in the nanosphere.
(12) Petter, R. C.; Salek, J. S.; Sikorski, C. T.; Kumaravel, G.; Lin, F.-T. J.
Am. Chem. Soc. 1990, 112, 3860-3868.
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ed.; Academic Press: San Diego, CA, 1991; Chapter 1.
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New York, 1971; 56-70.
2+ ...]+ 2A2c
Table 1. Hydrodynamic Radius (Rh), Gyration Radius (Rg), and
Molecular Weight (M) of the Nanosphere of 1/1,4-SNS with
Different Concentrations in Pure Water at 25 °C
5.0 × 105
5.0 × 105
5.1 × 105
5.3 × 105
Supramolecular Porous Nanosphere FormationLangmuir, Vol. 22, No. 7, 2006 3435
experiments, and some relative reports,15-18we could deduce a
possible self-assembly mode of 1/1,4-SNS complexes in the
nanospheres, which are illustrated as in Figure 3. It is well-
known that the primary driving force for the aggregation with
the cholesterol groups is due to the one-dimensional stacking of
the cholesterol moieties.15In present system, the CD complexes
while the hydrophobic cholic acid moieties cluster inside.
nitrogen adsorption isotherm was carried out at 298 K. A rapid
increase in the amount of adsorbed gas was observed with an
of guest gas molecules into the holes of nanospheres.19In a
gas adsorption ability. The result obtained shows that the
Teller (BET) surface area. Moreover, the adsorption and
desorption experiments with nitrogen traced almost the same
isotherms, which indicates that the porous structure is retained
appreciable gas adsorption ability, one possible explanation for
the phenomenon of the gas adsorption is that the holes of
gas adsorption ability. Usually, some inorganic materials or
inorganic-organic hybrid materials could show higher gas
adsorption ability.20In the present work, we determined the gas
we would like to exploit a new approach for studying the gas
adsorption abilities of porously organic materials.
NMR, Induced Circular Dichroism (ICD) Spectra, and
pH Effects. To investigate the role of guest 1,4-SNS for the
formation of the nanospheres, we first performed a 2D NMR
experiment of 1 in D2O. As shown in Figure 4, the rotating-
(15) (a) Kawano, S.-i.; Fujita, N.; Shinkai, S. Chem. Commun. 2003, 1352-
1353. (b) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori,
T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664-6676.
(c) Ishi-i, T.; Iguchi, R.; Snip, E.; Ikeda, M.; Shinkai, S. Langmuir 2001, 17,
C.; Chen, L.; Swanson, B.; Whitten, D. G. J. Am. Chem. Soc. 2000, 112, 2399-
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Int. Ed. 2006, 45, 456-460.
Chem. Int. Ed. 2004, 43, 192-195.
(20) Fe ´rey,G.;Mellot-Draznieks,C.;Serre,C.;Millange,F.;Dutour,J.;Surble ´,
S.; Margiolaki, I. Science 2005, 309, 2040-2042.
Figure 1. Suggested pathways for the formation of 1/1,4-SNS nanospheres.
Figure 2. Typical TEM images of the nanospheres with 1/1,4-SNS.
3436 Langmuir, Vol. 22, No. 7, 2006 Liu et al.
of 1 (1.8 × 10-3mol dm-3) displayed clear nuclear Overhauser
effect (NOE) cross-peaks between the H3 protons of ?-CD and
H18 (peaks A) and H14 (peaks C) protons of the cholic acid
moiety, as well as between the H3/H5 of ?-CD and H21 proton
(peaks B). The results suggest that the cholic acid moiety in 1
in Figure 1. Consequently, the self-included conformation
prevents the hydrophobic segments in 1 from undergoing
intermolecular association to form the nanospheres. It is
documented that the complex stability constant (KS) for the
complexation of ?-CD with cholate is 0.41 × 104M-1in
phosphate buffer aqueous solution (pH 7.2) at 25.0 °C.21On the
a strict size-fit relationship and relatively stronger hydrophobic
interaction;22therefore, we selected 1,4-SNS and 2,6-DNS as
molecular receptors to explore the function of the guest to the
of the 1:1 complex of 1 with 1,4-SNS/2,6-DNS in phosphate
constants for the stoichiometric 1:1 complexation of 1 with 1,4-
SNS and 2,6-DNS are obtained by the fluorimetric titrations,
and the corresponding KSvalues are 4.19 × 104and 1.04 × 105
M-1in phosphate buffer aqueous solution (pH 7.2) at 25.0 °C,
respectively. These values are larger than the KSvalue for the
complexation of ?-CD with cholate; therefore, both of the guest
molecules should be included into the CD cavity to change the
conformation of the cholic-acid-modified ?-CD 1.
CD chiral cavity. As can be seen from Figure 5, the stepwise
addition of CD 1 of up to 0.72 mM to an aqueous solution of
1,4-SNS (34 µM) causes a gradual increase of the ICD intensity
and corresponding CD signals appear in ca. 238 nm (positive),
284 nm (negative), 336 nm (positive), and 401 nm (negative).
Because neither 1,4-SNS nor 1 displays any appreciable CD
therefore deduce that 1,4-SNS is included in the cavity of CD,
according to the principles of CD spectra of CD complexes.23
(21) Liu, Y.; Yang, Y.-W.; Cao, R.; Song, S.-H.; Zhang, H.-Y.; Wang, L.-H.
J. Phys. Chem. B 2003, 107, 14130-14139.
(22) Inoue, Y.; Hakushi, T.; Liu, Y.; Tong, L.-H.; Shen, B.-J.; Jin, D.-S. J.
Am. Chem. Soc. 1993, 115, 475-481.
(23) (a) Kajta ´r, M.; Horvath-Toro, C.; Kuthi, E.; Szejtli, J. Acta Chim. Acad.
G. J. Am. Chem. Soc. 2001, 123, 5240-5248.
Figure 3. Probable self-assembly mode of 1/1,4-SNS complexes in the nanospheres.
Figure 4. ROESY spectrum of 1 (1.8 × 10-3mol dm-3) in D2O
at 25.0 °C with a mixing time of 300 ms.
Figure 5. (a) Absorption and (b) CD spectra of 1,4-SNS (3.4 ×
10-5mol dm-3) upon the addition of 1 (from a to i ) 0, 0.40, 0.80,
1.60, 2.40, 3.20, 4.00, 5.60, and 7.20 × 10-4mol dm-3) in aqueous
solution at 25.0 °C.
Supramolecular Porous Nanosphere FormationLangmuir, Vol. 22, No. 7, 2006 3437
Further evidence for the conformations of complexes 1/1,4-
SNS and 1/2,6-DNS was obtained by the 2D NMR study. The
ROESY spectrum (Figure 6a) of 1 with 1,4-SNS in D2O shows
the NOE cross-peaks between the H3 of ?-CD and the Hd/Heof
1,4-SNS and the ones between the H5 of ?-CD and the Hd/He
and Hcof 1,4-SNS. However, there are no NOE cross-peaks
found between the H3/H5 of ?-CD and the protons of the cholic
acid moiety or between protons of 1,4-SNS and the protons of
must be excluded from the hydrophobic cavity of ?-CD by 1,4-
SNS, as illustrated in Figure 6b. Moreover, the 2D NMR
experiment of 1 with 2,6-DNS in D2O shows that not only is
2,6-DNS included in the cavity of ?-CD but the cholic acid
that there is an inclusion equilibrium between the cholic acid
moiety of 1 and the 2,6-DNS molecule at the cavity of ?-CD.
Therefore, it is reasonable that the 1/2,6-DNS complex cannot
acid group in 1,4-SNS are located at the primary sides of ?-CD;
therefore, they can prevent hydrophobic cholic acid segments in
1 from self-including into the cavity of ?-CD, making the
formation of nanospheres become a natural process.
we performed the self-assembly experiments of the 1/1,4-SNS
complex under the different pH values (pH ) 2.2, 6.2, 7.2, and
10.0). The obtained results indicated that the nanospheres could
be formed in pH 6.2, 7.2, and 10.0, while it could not be formed
in pH 2.2. Furthermore, the binding abilities of 1 with 1,4-SNS
are also determined with different pH values by the fluorimetric
titrations. The corresponding KSvalues for the complexation of
1 with 1,4-SNS are as follows: the spectral changes are too
weak to calculate the KSvalue for pH 2.2; 1.98 × 104M-1for
pH 6.2; 4.19 × 104M-1for pH 7.2; and 5.89 × 104M-1for pH
10.0. When the pH value was 2.2, the amino group of 1,4-SNS
is fully protonated; therefore, 1,4-SNS could not be effectively
included in the CD cavity to exclude the cholic acid moiety.
group of the tether24in 1 are partially protonated. Therefore,
group of 1,4-SNS and the imido group of CD 1’s tether, which
for the formation of the nanoclusters. When the pH value was
acid moiety from the CD cavity to form the nanoclusters.
In summary, we have prepared a class of porous nanospheres
from amphiphilic cholic-acid-modified ?-CD 1 using 1,4-SNS
and ICD studies as well as the KS values indicate that a
combination of the strong host-guest inclusion interaction and
for the study of CDs in supramolecular self-assembled nano-
spheres but also may suggest a potential application of the
nanosphere in drug/gas entrapment and release.
(90306009, 20421202, and 20572052) and the Tianjin Natural
Science Fund (05YFJMJC06500), which are gratefully acknowl-
edged. We thank Dr. Xiao-Peng Bai (Columbia University) for
his help in the preparation of final manuscript. We also thank
the referees for their highly valuable suggestions regarding the
This work is supported by NNSFC
isotherm and TEM images of the porous nanospheres. This material is
available free of charge via the Internet at http://pubs.acs.org.
(24) Liu, Y.; Yang, Y.-W.; Yang, E.-C.; Guan, X.-D. J. Org. Chem. 2004, 69,
Figure 6. (a) ROESY spectrum of 1 (2.1 × 10-3mol dm-3) with
1,4-SNS (2.4 × 10-3mol dm-3) in D2O at 25.0 °C with a mixing
3438 Langmuir, Vol. 22, No. 7, 2006 Liu et al.