Tubular Structures Self-Assembled from a Bola-Amphiphilic Pillar[5]arene in Water and Applied as a Microreactor

Article abstract of DOI:10.1021/acs.orglett.5b01910

Various nanomorphologies were obtained by simply changing the fabrication conditions, such as the pH of the system, different solvent, or different concentration, of bola-amphiphilic pillar[5]arene Bola-AP5. Importantly, hybrid microtubules as a microreac

Full text of DOI:10.1021/acs.orglett.5b01910

Letter
pubs.acs.org/OrgLett
Tubular Structures Self-Assembled from a Bola-Amphiphilic
Pillar[5]arene in Water and Applied as a Microreactor
Rener Chen,† Huajiang Jiang,† Haining Gu,§ Qizhong Zhou,*,† Zhen Zhang,‡ Jiashou Wu,†
and Zhengneng Jin*,†
‡
†Department of Chemistry and School of Life Science, Taizhou University, Taizhou 318000, China
§Department of Chemistry, Zhejiang University, Hangzhou 310027, China
S
* Supporting Information
ABSTRACT: Various nanomorphologies were obtained by simply changing the
fabrication conditions, such as the pH of the system, different solvent, or different
concentration, of bola-amphiphilic pillar[5]arene Bola-AP5. Importantly, hybrid
−
microtubules as a microreactor were successfully prepared by directly reducing AuCl4
on the surface of Bola-AP5-based tubular structures in water.
elf-assembly of nanomaterials to form well-ordered hier-
archical structures provides great opportunities in optical,
self-assemble into tubes but formed micelles or gel-like
structures. Otherwise, tubular structures could be obtained
immediately when Bola-AP5 was dissolved in THF and dropped
into water at a high concentration. Importantly, hybrid
microtubules as a microreactor were successfully prepared by
S
electronic, and magnetic materials and devices.1 Among a variety
of molecular building blocks for self-assembly, such as block
copolymers, peptide derivatives, and lipid molecules, macrocyclic
amphiphiles,2 consisting of rigid cycles and flexible functional
groups, are excellent candidates for creating well-defined
supramolecular structures in selective solvents.
−
directly reducing AuCl4 on the surface of Bola-AP5-based
tubular structures in water (Scheme 1).
Bola-AP5 was successfully obtained by refluxing ester-
functionalized pillar[5]arene and 1,4-butanediamine in toluene
with a 45% yield. When Bola-AP5 was dissolved in pure water, its
CAC value was about 4.00 × 10−4 mol/L (see the Supporting
Information, Figure S4), and its amphiphilic properties change
upon the addition of H+. In a typical experiment, 1 mg of Bola-
AP5 was first dissolved in 10 mL of water upon ultrasonic
treatment at room temperature for 30 min. Then HCl was
utilized to control the pH of the system. Figure 1 shows the
dynamic light scattering (DLS) studies of Bola-AP5 in water
under different pH conditions. The average diameter of the
assemblies was about 100 or 120 nm when the system pH was 5
or 7, respectively (Figure 1c,d). However, the diameter of the
assemblies decreased sharply to 5 nm when the system pH
decreased to 4 (Figure 1b). When the system pH decreased to 2,
the diameter of the assemblies was smaller than 2 nm (Figure 1a),
implying that Bola-AP5 was monodispersed in this condition.
The DLS studies above indicated that changing the pH of the
system could induce the morphology transformations of Bola-
AP5. Similar behavior was observed when the experiments were
conducted under controlled pH using buffer solutions (see the
Supporting Information Figure S13). The hydrochloride salt of
As a new class of macrocycles after crown ethers,3 cyclo-
dextrins,4 calixarenes,5 cucurbiturils,6 and other macrocyclic
hosts,7 pillar[n]arenes8 consist of repeating hydroquinone units
linked by methylene (−CH2−) bridges at their 2,5-positions with
pillar-like architectures and electron-donating cavities, and their
syntheses, conformational mobility, derivatization, host−guest
complexation, self-assembly in water or organic solvents, and
applications have been widely explored recently in various fields.
Huang and co-workers synthesized the first amphiphilic
pillararene and found that it could self-assemble into vesicles
and then further assemble into stable microtubes in water.9
Although several amphiphilic pillararenes have been synthe-
sized,10 there are two main problems in the syntheses and
application of amphiphilic pillar[5]arene. First, the yield of
asymmetric pillar[5]arene is too low.9 Second, when they self-
assemble in water, the intermolecular interactions are not
strong.10 How to synthesize amphiphilic pillararenes with strong
intermolecular interactions and high yields is very important in
the development of macrocyclic chemistry.
Herein, we report the synthesis of a new bola-amphiphilic
pillar[5]arene (Bola-AP5, Scheme 1) with 10 amino groups and
10 amide groups as hydrophilic groups. When Bola-AP5 was
dissolved in water with a concentration approximately at its
critical aggregation concentration (CAC), it self-assembled into
vesicles then into stable microtubes after 2 weeks. However, if the
system pH is too low or the concentration is too high, it could not
Received: July 3, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01910
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Scheme 1. Synthetic Routes and Chemical Structure of Bola-
Amphiphilic Pillar[5]arene Bola-AP5 and Cartoon
Representation of the Formation of Different Nanostructures
as Microreactor
Figure 2. TEM images of Bola-AP5 self-assembled in water with
different pH values: (a) pH = 7; (b) pH = 5; (c) pH = 4. SEM image (d)
and cross section (e) of Bola-AP5 self-assembled in water for 4 weeks
when the system pH was ∼7.
Bola-AP5 molecules, suggesting that the vesicles have multilayer
walls (see the Supporting Information, Figure S5). However, the
assemblies turned into small solid micelles with a diameter of
about 5 nm when the system pH decreased to 4 (Figure 2c).
When the system pH further decreased to 2, we could not find
regular assemblies. More interestingly, the vesicles could
transform into well-defined fibers with 1 mm length and 500
nm width after 4 weeks (Figure 2d). Furthermore, from the cross
section of one fiber, we can see that the fibers are, in fact,
microtubes (Figure 2e). The UV−vis spectrum also confirmed
the further self-assembly of the vesicles in water (see Figure S6).
In view of the pH-induced transformation of vesicles to
micelles, one can envision that hydrophilic small molecules can
be encapsulated within the interior of the Bola-AP5-based
vesicles when the system pH value was about 7 and can release
the encapsulated molecules when the system pH decreases. As
shown in Figure S7, little entrapped calcein was observed over a
period of 12 h, indicating that these Bola-AP5-based vesicles
were very stable. However, when the system pH of the calcein-
loaded vesicles was decreased to 4, the encapsulated calcein
environment was released rapidly and completely. This is
because the amine groups on Bola-AP5 were quaternized when
the pH of the system decreased, so the hydrophilic parts of the
molecule increased, which triggered the vesicles to transform
into a micellar structure (see Figure S8).2a
After investigating the morphologies controlled by the pH of
the system, we then studied solvent influence on the self-
assembly process. Bola-AP5 can be well-dissolved in THF. In a
typical experiment, 1 mg of Bola-AP5 was first dissolved in 30 μL
of THF, and then 120 μL of water was poured into the above
solution followed by ultrasonic treatment at room temperature
for 5 min. Figure S8 shows that the size distribution of the
assemblies is very broad from 100 to >1000 nm. Then SEM
(Figure 3a) and TEM images (Figure 3b) of the obtained
assemblies confirmed that Bola-AP5 self-assembled into fibers in
the THF/water solution immediately. This is quite different from
the self-assembly of Bola-AP5 in pure water. Further
investigation of the vertical section of a fiber showed that the
fibers are actually microtubes (see Figure S9).
Figure 1. DLS results of Bola-AP5 dissolved in water with different pH
values: (a) pH = 2, ∼1.5 nm; (b) pH = 4, ∼5 nm; (c) pH = 5, ∼100 nm;
(d) pH = 7, ∼120 nm.
Bola-AP5 did not show self-assembly in water (see the
Supporting Information Figure S14).
To gain better insight into the self-assembly of Bola-AP5 in
water with different pH values, the resultant assemblies were
analyzed on the microcosmic level by transmission electron
microscopy (TEM) images and scanning electron microscopy
(SEM) images. As shown in Figure 2a,b, when the system pH was
7 and 5, the peripheries and center colors of the assemblies were
obviously different, indicating that the assemblies were hollow
spheres. Moreover, the wall thicknesses of these vesicles were
about 20 nm, which corresponds to a multiextended length of
The solution concentration has been reported to exert a huge
influence on the assembly morphology by changing the speed of
aggregation.11 In this work, different amounts of Bola-AP5 were
chosen to assemble in water (pH = 7.0) with ultrasonic treatment
at room temperature for 20 min and then standing for 2 weeks.
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DOI: 10.1021/acs.orglett.5b01910
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Furthermore, from the vertical section of one hybrid microtube,
we can see that there were no nanoparticles in the interior of the
tubes (Figure 5e).
After we prepared the hybrid microtubes successfully, we
further used them as recycled catalysts for reduction of
nitroaniline by NaBH4. The catalytic activity of AuNP micro-
tubes is shown in Table 1. All of the substrates were reduced with
Table 1. Reducing Nitro Aromatic Compounds Catalyzed by
AuNP Microtubes
Figure 3. (a) SEM and (b) TEM images of Bola-AP5 self-assembly in a
THF/water solution.
We found that Bola-AP5 could self-assemble into floccules at all
concentrations. However, further SEM studies showed when the
concentration of Bola-AP5 was too small (0.1 × CAC) or too
high (10 × CAC), it could not self-assemble into microtubes but
did form gel-like structures (Figures 4a,c). When the
entry
cycle 1
cycle 2
cycle 3
cycle 4
1
2
3
o
p
q
96%
97%
99%
95%
97%
99%
95%
96%
99%
96%
97%
99%
yields of about 95−99% in only 30 min (see Figures S10-S12).
Moreover, the AuNP microtube catalyst can be easily recovered
and reused for many catalytic cycles without any loss of reactivity
and conversion ratio. We were unable to conduct control
experiments with gold nanoparticles that were not supported on
a microtube.
In conclusion, various characterization methods, including 1H
NMR, DLS, UV−vis spectroscopy, TEM, and SEM, were
performed to investigate the self-assembly behaviors of a new
amphiphilic pillar[5]arene in water. When dissolved in water, the
morphology of its assemblies transformed from vesicles to
micelles with the decrease of pH. When the pH value was 5−7,
the vesicles further self-assembled into well-defined microtubes
after 2 weeks. However, when Bola-AP5 was dissolved in THF
first and then dropped into water, it self-assembled into
microtubes immediately. More interestingly, the concentration
of Bola-AP5 has a huge influence on the assembly morphology.
Only when the concentration was close to its CAC could it
further self-assemble into microtubes. While under other
concentrations, it only self-assembled into gel-like structures.
Importantly, hybrid microtubules as a microreactor were
successfully prepared by directly reducing AuCl4− on the surface
of Bola-AP5-based tubular structures in water. This work
provides a new method to directly create microreactors and
will have potential applications in industry.
Figure 4. SEM images of Bola-AP5 self-assembly in water with different
concentrations: [C] = (a) 0.1 CAC, (b) 1 CAC, and (c) 10 CAC.
concentration of Bola-AP5 was approximately at its CAC, it
self-assembled into well-defined microtubes (Figure 4b). This
result is consistent with the pH-induced self-assembly (Figure
2d).
As is well-known, amino groups, carboxylic groups, or sulfydryl
groups can act as stabilizers for preparing gold nanoparticles. The
obtained nanotubes in this work have numerous amino groups
on their exterior surfaces. Therefore, we can prepare hybrid
microtubes by reducing HAuCl4 on the surface of the tubular
structures directly. As shown in Figure 5, gold nanoparticles were
distributed on the surfaces of the microtubes, and the hybrid
microtubes remained in their original tubular structures.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b01910.
Synthetic details, self-assembly of Bola-AP5 in water, and
controlled release of small molecules (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: qizhongchou@tzc.edu.cn.
*E-mail: zn_jin@tzc.edu.cn.
Figure 5. (a) SEM and (b) enlarged SEM images of AuNP microtubes
(for this sample, we were unable to spray gold nanoparticles before SEM
studies). TEM images of AuNP microtubes (c−e).
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.5b01910
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Organic Letters
Letter
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ACKNOWLEDGMENTS
■
This work was supported by the Natural Science Foundation of
China (21172166 and 21402137), Natural Science Foundation
of Zhejiang Province (LY14B020012 and LY12B07003), and
China Postdoctoral Science Foundation (No. 2013M541456).
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DOI: 10.1021/acs.orglett.5b01910
Org. Lett. XXXX, XXX, XXX−XXX