A water-soluble pillar[6]arene: Synthesis, host-guest chemistry, and its application in dispersion of multiwalled carbon nanotubes in water

Article abstract of DOI:10.1021/ja306399f

The first water-soluble pillar[6]arene was synthesized. Its water solubility can be reversibly controlled by changing the pH. This solubility control was used in reversible transformations between nanotubes and vesicles and dispersion of multiwalled carbon nanotubes in water.

Full text of DOI:10.1021/ja306399f

Communication
pubs.acs.org/JACS
A Water-Soluble Pillar[6]arene: Synthesis, Host−Guest Chemistry,
and Its Application in Dispersion of Multiwalled Carbon Nanotubes
in Water
Guocan Yu, Min Xue, Zibin Zhang, Jinying Li, Chengyou Han, and Feihe Huang*
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Chemistry, Zhejiang University, Hangzhou
310027, P. R. China
S
* Supporting Information
achieved by noncovalent modification of this amphiphilic guest
ABSTRACT: The first water-soluble pillar[6]arene was
synthesized. Its water solubility can be reversibly
controlled by changing the pH. This solubility control
was used in reversible transformations between nanotubes
and vesicles and dispersion of multiwalled carbon
nanotubes in water.
with the water-soluble pillar[6]arene host, which caused a
transformation from nanotubes to vesicles. Adjusting the pH to
make the solution acidic protonates the carboxylate groups on
both rims of the pillar[6]arene, affording insoluble carboxylic
acid groups. On the contrary, by increasing the pH value, the
−COOH units are deprotonated again. Therefore, the solubility
of the pillar[6]arene can be reversibly controlled just by changing
the pH of the solution, resulting in the reversible transformation
of the host−guest system between nanotubes and vesicles.
Furthermore, its confined hydrophobic cavity was utilized to
interact with neutral guest molecule G2 through hydrophobic
interactions. This host−guest system can be used in pH-
controlled reversible dispersion of multiwalled carbon nanotubes
(MWNTs) in water, improving their applications in many fields,
such as controllable nanocatalysis and controllable nanocarriers.
The pH-responsiveness of WP6 was verified by fluorescence
experiments. The fluorescence intensity at 330 nm correspond-
ing to the characteristic peak for WP6 was almost unchanged
when the solution pH was changed from 10.5 to 7.3 but
decreased rapidly when the solution pH was changed from 7.3 to
6.8. On the other hand, no significant intensity changes were
observed when the pH was decreased from 6.8 to 2.6 (Figure S16
in the Supporting Information). The opposite phenomena were
observed when the pH was increased from 2.5 to 10.6 (Figure
S17). The pH-controlled solubility of WP6 was also confirmed
acrocycles such as crown ethers,1 cyclodextrins,2
M
calixarenes,3 and cucurbiturils4 have attracted much
interest because of their applications in a broad range of fields,
including memory storage, smart supramolecular polymers, drug
delivery systems, sensors, protein probes, and functional
nanodevices.5 Pillar[n]arenes, mainly including pillar[5]arenes6
and pillar[6]arenes,7 are new macrocyclic hosts made up of
hydroquinone units linked by methylene (−CH2−) bridges at
their 2- and 5-positions. Their syntheses, conformational
mobility, derivatization, host−guest complexation with different
guests, and self-assembly have recently been actively explored.6,7
With the current emphasis on “environment-friendly chemistry”,
the search for water-soluble hosts that could accommodate
water-insoluble guest molecules within their confined hydro-
phobic cavities has gained momentum.8 Water-soluble pillar[5]-
arenes have been demonstrated to have interesting host−guest
chemistry.6c,i,k In this work, the first water-soluble pillar[6]arene,
WP6, was synthesized by introducing carboxylate anionic groups
on both rims (Scheme 1). By etherification of 1,7e methox-
1
using H NMR spectroscopy. Proton signals from WP6 (1.00
mM) could be easily collected, but when the water-soluble host
precipitated from D2O after acidification of the solution, the
signals disappeared. When the solution was made basic, the
Scheme 1. Synthetic Route to WP6
1
precipitated host redissolved in the solution, and the H NMR
signals reappeared. These results showed that the solubility of the
pillar[6]arene could be easily controlled simply by changing the
pH.
To investigate higher-order self-assembled aggregates in water,
an aqueous solution of G1 was prepared with a concentration of
1.00 × 10−4 M, which is higher than its critical aggregation
concentration (CAC). As indicated by transmission electron
microscopy (TEM) (Figure 1a), regular nanotubes were
observed with an average diameter of ∼300 nm. Interestingly,
upon addition of 1 equiv of WP6, vesicles with a diameter of
∼300 nm were observed (Figure 1b), drastically different from
ycarbonylmethoxy-substituted pillar[6]arene 2 was synthesized.
The hydrolysis of 2 under basic conditions afforded carboxylic
acid-substituted pillar[6]arene 3. By treatment with 1 equiv of
sodium hydroxide in aqueous solution, water-soluble pillar[6]-
arene WP6 was obtained.
WP6 strongly binds organic pyridinium salt G1 in water,
mainly driven by hydrophobic and electrostatic interactions.
Successful manipulation of self-assembling nanostructures was
Received: July 1, 2012
© XXXX American Chemical Society
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Journal of the American Chemical Society
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Figure 1. (a−c) TEM images of (a) G1 aggregates, (b) WP6⊃G1
aggregates, and (c) WP6⊃G1 aggregates after the solution pH was
adjusted to 6.0. (d−h) Fluorescence microscopy images (10 μm × 10
μm) of (d) G1 aggregates, (e) 5 s and (f) 10 s after addition of 1 equiv of
WP6, and (g) 10 s and (h) 30 s after the solution pH was adjusted to 6.0.
(i) DLS data of WP6⊃G1 aggregates at pH >7.0.
Figure 2. Schematic representations of (top) the reversible trans-
formations between G1 nanotubes and WP6⊃G1 vesicles and (bottom)
the molecular structures of WP6 and G1.
the hollow nanotubes formed by G1 alone. These regular
nanotubes with the same diameter reappeared when the solution
pH was adjusted to 6.0 (Figure 1c). We also used fluorescence
microscopy to monitor the changes in the self-assembled
structures (Figure 1d−h). Tubular assemblies formed by G1
could be clearly seen (Figure 1d). Upon addition of WP6, the
self-assembled nanotubes of G1 were rapidly transformed into
vesicles. It took ∼10 s to transform the nanotubes into vesicles
completely. When the solution pH was adjusted to acidity, the
WP6⊃G1 vesicles were gradually transformed back into
nanotubes. Moreover, when the pH was higher than 7.0, vesicles
rather than nanotubes formed in solution. Therefore, the self-
assembly of this host−guest system could be reversibly
controlled between nanotubes and vesicles by changing the
pH. As shown by dynamic light scattering (DLS), the main
diameter distribution of the WP6⊃G1 vesicles was ∼312 nm
(Figure 1i), in good agreement with the TEM image in Figure 1b.
The packing of G1 in the nanotubes (Figure 2) was studied by
UV−vis spectroscopy and X-ray diffraction (XRD). The blue
shift upon dilution (Figure 3a) indicated an H-aggregation form,9
suggesting that adjacent pyrene aromatic rings undergo
considerable overlap through π−π stacking interactions (Figure
2). The bilayer structure of the membrane was confirmed by
XRD and small-angle X-ray scattering (SAXS). The bilayer
thickness bilayer was calculated to be 4.8 nm (Figure S19a),
which is equal to the length of two G1 molecules with antiparallel
packing and overlapped pyrene rings (Figure 2).
We then investigated whether the bilayer structure was
damaged after complexation. It was verified by XRD and SAXS
that the structure was retained with a bilayer thickness of ∼4.8
nm after complexation (Figure S19b). A mechanism is proposed
to explain why the shape of the G1 aggregates is transformed
from nanotubes to vesicles after complexation with WP6 (Figure
2). The microassembled structure of the aggregates formed by
the two distinct bilayers is determined by the curvature of the
membrane. Typically, high membrane curvature favors nano-
tubular structures, while a vesicular structure is dependent on a
low-curvature membrane.5i Prior to complexation with WP6, G1
Figure 3. (a) UV−vis absorption spectra of aqueous solutions of G1 at
different concentrations. (b) Fluorescence emission spectra of G1 and
WP6⊃G1 at WP6 and G1 concentrations of 4.00 × 10−5 M.
self-assembles in aqueous solution to form a highly ordered
bilayer of pyrenyl groups through π−π stacking interactions,
generating high curvature and forming a tubular structure
(Figure 2). Upon complexation, WP6 is inserted into the
membrane of the G1 nanotubes to form 1:1 [2]pseudorotaxanes
through hydrophobic and electrostatic interactions without
changing the thickness of the membrane. Because of the steric
hindrance and the electrostatic repulsion generated upon
insertion of the WP6 molecules, the straight G1 arrays along
the axis become curved, resulting in the formation of a vesicular
structure with low curvature (Figure 2). As WP6 precipitates
from water after acidification of the solution, G1 dethreads from
the cavity of the host and forms nanotubes again. This structural
transformation from nanotubes to vesicles was revealed through
fluorescence microscopy (Figure 1d−h). Figure 1e reveals the
coexistence of nanotubes and vesicles upon addition of WP6,
indicating a nanotube-to-vesicle transformation. The nanotubes
disappeared completely within ∼10 s after addition of WP6,
being replaced by vesicular structures. It should also be noted
that a typical intermediate state between vesicles and nanotubes
could also be observed by adjusting the solution pH to 6.0
(Figure 1g), supporting the mechanism mentioned above.
The complexation of G1 with WP6 was further studied by 1H
NMR spectroscopy. Because of the relatively poor solubility of
B
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Journal of the American Chemical Society
Communication
G1 in water, 1-octylpyridinium bromide (OPB) was used as a
model compound. Upon addition of WP6, significant chemical
shift changes of the OPB protons appeared (Figure S20),
providing strong evidence of WP6−OPB interactions. Also, a
mole ratio plot based on fluorescence titrations (Figure S22)
showed that the complex formed by WP6 and OPB has 1:1
stoichiometry, and the association constant (Ka) in water was
calculated to be (3.26 0.28) × 105 M−1 using a nonlinear curve-
fitting method (Figure S23).
Further evidence for the formation of WP6⊃G1 was obtained
from UV−vis absorption spectroscopy. Upon addition of 1 equiv
of WP6, the spectrum exhibited a broad absorption above 400
nm that corresponds to the characteristic absorption of the
charge-transfer complex.10 On the other hand, a notable red shift
was observed (Figure S24), which indicated electronic
communication between WP6 and G1. Fluorescence spectra
(Figure 3b) showed that making the solution acidic changed the
water-soluble −COO− groups of WP6 to insoluble −COOH
groups, resulting in the decomplexation of WP6⊃G1. However,
this 1:1 [2]pseudorotaxane formed again when the solution was
made basic, and this process could be reversibly cycled many
times (Figure S25).
Figure 4. (top) TEM images of (a) MWNTs, (b) G2 and MWNTs, and
(c) WP6⊃G2/MWNT complexes and photographs of the correspond-
ing mixtures with water. (bottom) Illustration of the pH-responsive
solubility of the MWNTs in the presence of WP6⊃G2.
Carbon nanotubes (CNTs) have been the subject of great
interest because of their extraordinary electronic, mechanical,
and adsorption properties and applications in diverse areas of
nanoscience.11 However, their lack of solubility in solvents
presents a considerable impediment to harnessing their
applications. Noncovalent supramolecular approaches to
functionalize the sidewalls of CNTs can preserve their unique
properties.12 Among these approaches based on noncovalent
interactions, easily prepared pyrene derivatives with a large
variety of functional groups have been used to solubilize the
CNTs through π−π interactions.11b The interaction with pyrene
molecules partially exfoliates the CNT bundles and brings
individual tubes into solution. These supramolecular approaches
do not form any covalent bonds but instead form only π−π
interactions and thus only weakly perturb the nanotube
conjugated system. In these systems, the solubility of the
CNTs is hard to control, resulting in the limitation of their
application to some degree. Here we used the confined
hydrophobic cavity of WP6 to interact with a neutral guest G2
containing a π-rich pyrenyl ring through hydrophobic
interactions. As shown in Figure S26, the association constant
between WP6 and G2 was estimated to be (8.04 0.68) × 104
M−1 using the Stern−Volmer equation.13 The solubility of this
host−guest system can be controlled by adjusting the pH of the
solution. Therefore, the solubility of the MWNTs can be
controlled easily, and this process can also be achieved reversibly.
We found that MWNTs were dispersed very well in an
aqueous solution of WP6⊃G2 through simple sonication but
could not be dispersed by G2 alone. The black solution of the
WP6⊃G2/MWNT complex was homogeneous and stable
(Figure 4) and could stand for >1 month without significant
change. This observation indicates that WP6⊃G2 plays an
important role in the solubilization of MWNTs. Moreover,
MWNTs precipatated when the solution pH was made acidic
and could be dispersed in water again when the solution pH was
raised above 7.0. Thus, this dispersion could be reversibly
controlled simply by changing the solution pH.
G2, no changes occurred, and the MWNTs were still aggregated
(Figure 4b). In the case of WP6⊃G2/MWNT complexes, the
hydrophobic section containing the pyrenyl ring attached to the
surface of the MWNT, and the hydrophilic part containing the
alkyl chain was included in the cavity of WP6 dispersed in the
water. Therefore, well dispersed MWNTs were distinguished
(Figure 4c). SEM images of individual nanotubes (Figure S28)
were in good agreement with the above TEM results.
To confirm the complexation between MWNTs and the
WP6⊃G2 host−guest complex, fluorescence spectroscopy was
employed to compare the difference in fluorescence emission
between the solution of WP6⊃G2 and the aqueous dispersion of
MWNTs. As shown in Figure S29, a solution of WP6⊃G2
displayed characteristic fluorescence emission of pyrenyl groups
with high intensity. However, after MWNTs were dispersed in
that solution, strong fluorescence quenching was observed. As
previously reported, when pyrene is bound to a CNT through
π−π stacking, it suffers fluorescence quenching as a result of
energy transfer from pyrene to the CNT.11b Since the only
difference between the solution of WP6⊃G2 and the dispersion
of MWNTs was the introduction of MWNTs, the quenching is
attributed to WP6⊃G2−MWNT interactions.
Additional proof for WP6⊃G2−MWNT interactions came
from UV−vis−NIR spectra of WP6⊃G2/MWNT complexes
(Figure S30). Upon addition of a WP6⊃G2 solution (molar ratio
1:1), the absorbance range from 600 to 900 nm corresponding to
the characteristic absorptions of CNTs increased gradually.11d
Usually these absorptions are not observable because of the poor
solubility of CNTs. Upon addition of WP6⊃G2, the appearance
of the characteristic absorptions of CNTs clearly indicated the
homogeneous dispersion of the MWNTs in aqueous solution.
Moreover, we employed thermogravimetric analysis to see
whether the WP6⊃G2 host−guest complex modified the
MWNTs and to calculate the content of WP6⊃G2 in the
system (Figure S31). WP6⊃G2/MWNT hybrids were prepared
by sonication of WP6⊃G2 (20 mg, 0.009 mmol) in H2O (5 mL)
with 1 mg of MWNTs. During the sonication, the aqueous
solution changed from colorless to black, indicating solubiliza-
tion of MWNTs. After the sonication, insoluble MWNTs were
Direct evidence for WP6⊃G2−MWNT interactions was
obtained using TEM and scanning electron microscopy
(SEM). TEM images of pristine MWNTs showed the presence
of large aggregates of nanotubes (Figure 4a). Upon addition of
C
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Journal of the American Chemical Society
Communication
(4) (a) Gom
́
ez-Kaifer, W.; Ong, M.; Kaifer, A. E. Org. Lett. 2002, 4,
removed by centrifugation. The supernatant was dialyzed against
H2O for 2 weeks to remove excess free WP6⊃G2 from the
solution. Another sample of the original untreated MWNTs was
also studied as a control experiment. Compared with the original
MWNTs, the prepared sample of WP6⊃G2/MWNTs under-
went 44.6% weight loss up to 650 °C, corresponding to the
decomposition temperature of MWNTs, which provided
additional evidence of the interaction between WP6⊃G2 and
MWNTs. These results indicate that WP6⊃G2 has a strong
ability to keep MWNTs well-dispersed for a long time, and the
solubility of MWNTs can be reversibly controlled.
In summary, we have successfully synthesized the first water-
soluble pillar[6]arene. In contrast to the tubular aggregates
formed by amphiphilic molecule G1, the host−guest complex
WP6⊃G1 self-assembled into vesicles. The reversible trans-
formation between nanotubes and vesicles could be easily
controlled by changing the pH. Furthermore, the confined
hydrophobic cavity of WP6 could be ultilized to interact with a
neutral guest containing a pyrenyl group. This host−guest
complex can be used in the pH-controlled reversible dispersion
of MWNTs in water. The new recognition motif based on the
water-soluble pillar[6]arene has potential applications in many
fields, including supramolecular polymers, nanoelectronics,
sensors, and drug and gene delivery systems.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details and supporting data. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
fhuang@zju.edu.cn
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (20834004, 91027006, 21125417), the
Fundamental Research Funds for the Central Universities
(2012QNA3013), the Program for New Century Excellent
Talents in University, and the Zhejiang Provincial Natural
Science Foundation of China (R4100009).
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