Dual-responsive recurrent self-assembly of a supramolecular polymer based on the host-guest complexation interaction between β-cyclodextrin and azobenzene

Article abstract of DOI:10.1039/c7nj05042a

In this work, we report a supramolecular polymer based on the host-guest interaction between β-cyclodextrin groups in β-CD-PEG and azobenzene groups in PAE-g-Azo. The supramolecular polymer PAE-g-Azo@β-CD-PEG could self-assemble into a regular nanostructu

Full text of DOI:10.1039/c7nj05042a

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Dual-responsive recurrent self-assembly of supramolecular
polymer based on the host-guest complexation interaction
between β-cyclodextrin and azobenzene
Received 00th January 20xx,
Accepted 00th January 20xx
Ping Shen,a Liyan Qiu*a
DOI: 10.1039/x0xx00000x
In this work, we report a supramolecular polymer based on the host-guest interaction between β-cyclodextrin groups in
β-CD-PEG and azobenzene groups in PAE-g-Azo. The supramolecular polymer PAE-g-Azo@β-CD-PEG could self-assemble
into regular nanostructure in aqueous solution due to its proper amphiphilicity. Benefiting from the pH-responsiveness of
PAE and the photo-responsive trans-to-cis isomerization of Azo, the supramolecular assemblies apparently show pH and
photo responsive properties. Furthermore, the combination of the pH regulation and the irradiation of UV light or visible
light induced the recurrent disassembly and reassembly of the supramolecular polymer. This dual-responsive
supramolecular polymer has great prospects in various practical applications such as intelligent nanomaterials and
nanotechnology.
www.rsc.org/
assembled morphologies of guest or host molecules by taking
advantage of this unique character. For instance, Kim et al.22
synthesized a dendron amphiphile having a pyrene apex,
1 Introduction
In recent years, supramolecular polymers have attracted great
interest in various fields due to their potential applications in
smart materials1-3, nanodevices4,5 and drug delivery6-8. In
contrast to conventional covalent-connected polymers, which
usually involve tedious and time-consuming synthetic
procedures, supramolecular polymers can be easily built by
straightforwardly connecting polymeric segments together via
noncovalent interactions, such as metal-coordination9, π-π
interactions10, hydrogen bonding11, electrostatic interactions12
and host-guest interactions13-15 as long as these polymers were
well tailored. Among them, host-guest interactions have been
the most widely utilized for fabricating supramolecular
polymers due to their selective recognition, abundant stimuli-
responsiveness, and remarkable capability in forming stable
complexes16-20. Compared with other macrocyclic hosts, such
as calixarenes, pillararenes, crown ethers and cucurbiturils,
cyclodextrin is the most popular host in supramolecular
chemistry and can interact with a variety of guest molecules21.
As we know, once the hydrophobic guest moieties of a
macromolecule are included into the cavities of cyclodextrins,
the solubility of whole supramolecular polymers would be
changed. Therefore, it is possible to obtain self-assembled
nanoparticles with certain morphologies or regulate the self-
which could self-assemble into vesicles in aqueous solution,
and found that the vesicular morphology could be converted
to nanotubes when the dendron was capped with β-
cyclodextrin (β-CD) through the host-guest interaction
between β-CD and pyrene. Harada and co-workers23 reported
that the micelles formed by alternating copolymers composed
of dodecyl vinyl ether and sodium maleate units would be
dissociated when α-cyclodextrin was introduced to the system,
which resulted from the increase of the hydrophilicity of the
polymers after the complexation between α-cyclodextrin and
dodecyl group. Zhu et al.24 designed and prepared a star
polymer comprising an α-cyclodextrin core and multiple
adamantane termined PHB arms, which could self-assemble
into uniform nanovesicles when a small quantity of DM-β-CD
was added.
Moreover, it’s very interesting that lots of supramolecular
polymers are responsive to external stimuli such as
temperature15,18,25, pH variance16,26-28, electrochemical redox29,
30 and photoirradiation31-33. For example, Xia et al.16 reported a
supramolecular polymer constructed by the host-guest
interactions between pillar[5]arene and imidazolium cation,
which significantly showed pH-responsive properities. Liu and
co-workers34 utilized the coupling reaction between CD-
functionalized hyperbranched polyglycerol and Azo-
functionalized
hyperbranched
poly(3-ethyl-3-
oxetanemethanol) to fabricate
a
supramolecular Janus
hyperbranched polymer and it could be dissociated by UV
irradiation. However it is worth noting that though stimuli-
responsive supramolecular polymers have been widely
investigated in the past few decades, there are only a few
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research have been focused on the supramolecular polymers
that are both responsive to pH and photoirradiation.
Herein, we designed and prepared a pH and photo dual
responsive supramolecular polymer PAE-g-Azo@β-CD-PEG
based on the host-guest interaction between the hydrophobic
azobenzene-contained poly(β-amino ester) (PAE-g-Azo) and
hydrophilic PEGlated β-cyclodextrin (β-CD-PEG). As we know,
azobenzene can undergo trans-to-cis photoisomerization
under UV light irradiation and only trans-Azo can form stable
complex with β-CD. Thus, azobenzene and its derivatives are
wildly used in the construction of photo-responsive
supramolecular assemblies15, 25, 34-36. To our best knowledge,
most work was focused on the azo-contained amphiphilic
polymers, while azo-contained hydrophobic polymer like this
PAE-g-Azo was seldom reported. Meanwhile, poly(β-amino
ester) (PAE) is a typical pH-responsive polymer containing
tertiary amino groups in its backbone and can undergo
hydrophilic-to-hydrophobic transition as the pH increases
beyond its pKb due to the deionization of tertiary amino
groups37. By combining the properties of PAE and Azo, the
supramolecular assemblies formed by PAE-g-Azo@β-CD-PEG
were expected to be obviously responsive to pH and
photoirradiation.
To
prove
our
assumption,
the
supramolecular assemblies were constructed first by mixing
PAE-g-Azo and β-CD-PEG, then their size and morphology
altering with the pH values and UV light or visible light
irradiation was tracked.
Scheme 1. Synthesis route of (A) the host polymer β-CD-PEG and (B) guest polymer
2 Experimental
PAE-g-Azo.
2.1 Materials
2.2 Polymer synthesis
Poly(ethylene glycol) methyl ether acrylate (PEGMEA, Mn =
480 g⋅
mol−1) was purchased from Sigma-Aldrich Co. (St Louis,
2.2.1 Synthesis of mono-6-thio-β-cyclodextrin (β-CD-SH). β-CD-
SH was synthesized from β-CD according to the previous
literature with a little modifications38
MO, USA) and used directly as received. β-cyclodextrin (β-CD)
was obtained from Aladdin Chemistry Co., Ltd. (Shanghai,
China) and purified by recrystallization thrice from deionized
(
Scheme 1A). β-CD (50 g,
44 mmol) was suspended in deionized water (417.6 ml), and
water followed by dried under vacuum at 120 °C for 48 h 16.7 ml NaOH aqueous solution (5.47 g, 136.7 mmol) was
before use. 1,4-butanediol diacrylate (4-BD) was obtained
from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). 3-
amino-1-propanol (3-AP) and dimethylphenylphosphine
(DMPP) were purchased from Sigma-Aldrich Co. (St Louis, MO,
USA) and stored under nitrogen atmosphere. Succinic
anhydride (SA), p-aminoAzobenzene (Azo), 4-dimethylamino-
added dropwise to the mixture within 10 min under magnetic
stirring so that the suspension gradually became
homogeneous. p-Toluenesulfonyl chloride (12.5 g, 66 mmol)
dissolving in 25 ml acetonitrile was added at a very low rate to
the above solution within ice-water bath to ensure that the
substitution occurred at the C6 position of β-CD. After stirring
2 h at ambient temperature, the pH value of the solution was
adjusted to 6-7 by hydrochloric acid. 30 min later, the crude
product precipitated out of the solution and was collected by
suction filtration. After recrystallization three times from water
to remove unreacted p-toluenesulfonyl chloride and β-CD, the
intermediate product β-CD-OTs was dried to constant weight
in a vacuum oven at 100 °C for 48 hours. Next, the mixture of
β-CD-OTs (2 g) and thiourea (2 g) was dissolved in 100 ml 80%
(v/v) methanol-water and heated at reflux for 48 h under
nitrogen atmosphere. The solvent of the reaction mixture was
removed in vacuum. Then, 100 ml acetone was added to the
white solid and stirred for an hour. After filtration, the
pyridine
(DMAP),
p-toluenesulfonyl
chloride,
N,N-
dicyclohexylcarbodiimide (DCC) and trichloroethylene (TCE)
were purchased from Aladdin Chemistry Co., Ltd. (Shanghai,
China). Anhydrous N,N-dimethylformamide (DMF) was
purchased from InnoChem Science & Technology Co., Ltd.
(Beijing, China). All other regents of analytical grade were from
local suppliers and used as received without further
purification.
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resultant precipitate was dissolved in 100 ml 10 wt% NaOH anhydrous DMF in a flask. In order to activate the carboxyl
solution and stirred for 5 h at 50 °C. When the reaction groups of Azo-COOH, the solution was stirred at 0 °C for 1 h.
mixture was adjusted to pH 2 with 1 N HCl solution, a pale Then Azo-COOH (6 g, 20 mmol) dissolving in 20 ml DMF was
yellow solution was formed. Then 5 ml TCE was added to the added dropwise into the solution. The reaction mixture was
solution and stirred overnight. The resulting white precipitate proceed for 48 h at room temperature under vigorous stirring.
was recovered by suction filtration and recrystallized three After filtered to remove the reaction byproduct of dicyclohexyl
times with water to give a white solid (β-CD-SH).
urea (DCU), the DMF solvent was removed by vacuum
distillation and the product was dialyzed against DMF for 48 h
to remove free Azo-COOH. The resulting solution was
precipitated in diethyl ether for 3 cycles and subsequent drying
under vacuum to get PAE-g-Azo.
2.2.2 Synthesis of poly(ethylene glycol)-β-cyclodextrin (β-CD-
PEG). β-CD-PEG was synthesized via thio-ene click reaction
between PEGMEA and β-CD-SH, with dimethylphenyl-
phosphine (DMPP) as a catalyst (Scheme 1A). As a typical
synthesis, β-CD-SH (4.02 g, 3.5 mmol) and PEGMEA (2.4 g,5
mmol) were dissolved in 20 ml anhydrous DMSO in a 100 ml
flask. After purging with nitrogen for 30 minutes to eliminate
the oxygen, DMPP (50 μl, 0.35 mmol) in 2 ml DMSO was then
added dropwise into the system under nitrogen atmosphere.
The flask was sealed under a nitrogen atmosphere and placed
in an oil bath for 24 h at 40 °C. After removing most of the
solvent by a rotary evaporator, the remaining portion was
2.3 Structural identification
1H-NMR spectra of the samples were recorded on a Bruker
DMX500 spectrometer operating in the Fourier transform
mode at ambient temperature with DMSO-d6 or CDCl3 as the
solvent. Tetramethylsilane was used as the internal reference.
FTIR spectrometer, recorded on a NICOLET 200SXV Infrared
Spectrophotometer at room temperature, was utilized to
inspect the changes of the main chemical functional group
precipitated into acetone to give a white solid. Then the solid during the synthesis process. The solid samples were
thoroughly pressed into potassium bromide pellets. The
number average molecular weight (Mn) and polydispersity
index (PDI) of PAE and PAE-g-Azo were evaluated by a GPC
machine equipped with a Waters 1515 pump and a Waters
2414 differential refractive index detector coupled to a Styrage
TM HT3 GPC column of 300 nm in length and 7.3 mm in
diameter. DMF was used as an eluent at a flow rate of 1.0 ml
min-1 at 40 °C. Calibration of the molecular weight of the
polymer was based on standard polystyrenes. The molecular
weight of β-CD-PEG was determined by MALDI-TOF analysis
was dissolved in a small amount of deionized water and
dialyzed against water with dialysis membrane for 48 h to
remove the unreacted substrate. After dialysis, the resulting
solution was dried by a freeze-dryer system to obtain the final
product β-CD-PEG.
2.2.3 Synthesis of carboxylated p-aminoAzobenzene (Azo-
COOH). Carboxylated p-aminoazobenzene (Azo-COOH) was
synthesized by the reaction of p-aminoazobenzene and
succinic anhydride in accordance with the previous literature39
conducted on
a Bruker Ultra FLEX MALDI-ToF mass
(Scheme 1B). Briefly, under nitrogen atmosphere, p-amino-
spectrometer equipped with a 337 nm nitrogen laser. The
sample was dissolved in deionized water and mixed with 2,5-
dihydroxybenzoic acid before drying.
azobenzene ( 2 g, 10.2 mmol), DMAP (244 mg, 2.0 mmol) and
pyridine (1.6 mL, 20.4 mmol) were dissolved in 30 ml
anhydrous acetone in a 100 ml round flask. SA (2 g, 20.4 mmol)
dissolving in 10 ml acetone was slowly added to the aforesaid
flask. Then the reaction was carried out at 60 °C for 6 hours. At
the end of the reaction, the solution was cooled to room
temperature and the resulting product precipitated out of the
reaction mixture. After suction filtration, the residue was
washed three times with acetone and dried under vacuum for
24 h to get an orange-yellow solid Azo-COOH.
2.4 Measurement of pH-responsive property of PAE-g-Azo
2.4.1 pH titration measurement. PAE-g-Azo was dissolved in
deionized water containing HCl at a concentration of 1 mg/ml.
The pH values of the solution was gradually increased by
dropping 0.1 M NaOH aqueous solution at an increment of 50
μl. A pH meter was used to monitor the exact pH values of the
solution.
2.2.4 Synthesis of poly(β-amino ester)-graft-Azo. Poly(β-amino
2.4.2 Optical measurement. The optical transmittances of
ester)-graft-Azo was prepared by esterification reaction the PAE-g-Azo solutions (1 mg/ml) at various pH were
measured on a UV–Vis spectrophotometer (PuXi General
Instrument Co., Beijing, China) at 500 nm.
between PAE and Azo-COOH (Scheme 1B). Firstly, PAE was
synthesized via Michael addition polymerization according to
the literature.40 Briefly, 3-AP (4.6 g, 60 mmol) and 4-BD (12 g,
60 mmol) were added quickly into a fire-dried flask under
nitrogen atomosphere. Then the flask was put into an oil bath
at 100 °C with stirring for 5 h. After cooling to room
temperature, the viscous liquid was dissolved in DCM and
precipitated twice into an excess amount of diethyl ether. The
purified product was dried under vacuum for 48 h at room
temperature. Then, PAE (5.48 g, 20 mmol), DCC (8.24 g, 40
mmol), DMAP (0.2 g, 2 mmol) were dissolved in 30 ml
2.5 Measurement of photo-responsive property of PAE-g-Azo
PAE-g-Azo was dissolved in DMSO to obtain a homogeneous
solution and then irradiated with 365nm UV light using a UV
LED irradiator (UVP, B-100AP, λ0= 365 nm) followed by 450nm
visible light irradiation using a Vis LED irradiator(CCS, λ0 =450
nm). The UV-vis spectra of the solution during the irradiation
process was measured by UV-Vis spectrophotometer at room
temperature.
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solution. Then the solution was adjusted to pH7 to induced the
reassembly of the supramolecular assemblies. TEM was used
to monitored the morphology transition during the above
process.
2.6 Preparation and characterization of supramolecular
Assemblies
The supramolecular assemblies with different β-CD/Azo molar
ratios were prepared by Jiang’s method41. In
a typical
experiment, PAE-g-Azo was dissolved in THF to obtained a
homogeneous solution with a concentration of 2 mg/ml. Then,
0.1 ml PAE-g-Azo THF solution was slowly injected into a 1.9 ml
solution of β-CD-PEG in water (1:1 molar ratio of β-CD/Azo)
over 10 min and stirred for 30 min at room temperature. The
bluish opalescence of the micelle solution indicated the
formation of the assembled particles. The final micelle solution
has a concentration of 0.1 mg/ml.
The particle size and size distribution of the assemblies were
measured by dynamic light scattering spectrophotometry
using a Zetasizer Nano ZS90 instrument (Malvern Instruments,
UK). All samples were measured using a 633 nm laser light
with a fixed scattering angel of 90° and the detection
temperature was kept at 25 °C during the whole experiment.
The morphology of the assemblies were verified on a JEM-
1200EX TEM with an 80 kV accelerating voltage. The samples
for TEM observation were prepared by dipping the freshly-
prepared aqueous solution onto copper grids coated with
carbon and thin films and then dried at ambient temperature.
There was no staining agents used in TEM images.
The host-guest interaction between PAE-g-Azo and β-CD-
PEG was characterized by 2D-NOESY 1H NMR spectra,
fluorescence spectra and UV-Vis spectra measurements. The
2D-NOESY 1H NMR spectra containing PAE-g-Azo and β-CD-PEG
was recorded on a Bruker DMX500 spectrometer with D2O as
the deuterated solvent. UV-vis spectra of supramolecular
assemblies solution (0.1 mg/ml) with different β-CD/Azo molar
ratio were carried out using a UV-vis spectrophotometer (PuXi
General Instrument Co., Beijing, China) at room temperature.
The fluorescence spectra of supramolecular assemblies
solutions (0.1 mg/ml) with different β-CD/Azo molar ratios
were measured on a FP-6000 (Jasco, Japan) fluorescence
spectrometer. The excitation wavelength was 350 nm and the
scanning range was from 380 to 600 nm.
Figure 1.
1H NMR spectra of (A) β-CD-OTs, (B) β-CD-SH, (C) β-CD-PEG.
2.7 Dual-responsive recurrent behaviors of supramolecular
assemblies
2.7.1 pH-responsive behavior. To measure the pH-
responsive behavior of the supramolecular assemblies, the
fresh prepared micelle solution was first adjusted to pH3 by
0.1 N HCl solution and then monitored by DLS and TEM.
3 Result and discussion
3.1 Synthesis of the host polymer β-CD-PEG
β-CD-PEG was synthesized via thio-ene click reaction between
thiolated β-CD and acrylate-terminated PEG as shown in
Scheme 1A. Tosylated β-CD (β-CD-OTs) was adopted as an
effective intermediate to synthesize mono-substituted
cyclodextrin derivative mono-6-thio-β-CD (β-CD-SH). The click
reaction between the vinyl group of PEGMEA and the thio
group of β-CD-SH was initiated by DMPP at ambient
temperature to produce β-CD-PEG. The structures of β-CD-OTs
2.7.2 UV-induced disassembly. For the disassembly
experiment, the fresh prepared micelle solution was directly
irradiated with 365 nm UV light for 30 min using a UV LED
irradiator under stirring. 30 min later, the supernatant was
1
directly monitored by TEM and then freeze-dried for H NMR
analysis. The precipitates on the magnetic stick were also
collected for 1H NMR analysis.
2.7.3 Vis-pH-induced reassembly. The reassembly process of
1
and β-CD-SH were characterized by H-NMR spectra and FT-IR
spectra. As shown in Figure 1A, the peaks at δ=7.75 ppm, 7.45
ppm and 2.45 ppm were ascribed to the protons of the
attached tosyl group, suggesting the success synthesis of β-CD-
OTs. After thiolation reaction, it is clear to see that a peak at
the supramolecular assemblies was studied by two steps. First,
the mixture obtained after UV irradiation was adjusted to pH3
under 450nm visible light irradiation to get a homogeneous
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2.08 ppm in Figure 1B appeared, which belongs to the protons
of thio groups, while the corresponding peaks of tosyl group
disappeared completely. Moreover, comparing the FT-IR
spectra of β-CD-SH and β-CD-OTs in Figure 2, an absorption
peak at 2602 cm-1 appeared after thiolation reaction, which
was ascribed to the stretching vibration bands of thio groups,
further suggesting that β-CD has been thiolated. Finally β-CD-
PEG was prepared by the thio-ene click reaction between β-
CD-SH and acrylate-terminated PEG. As can be seen from the
1NMR spectrum in Figure 1C, the peaks at 2.08 ppm assigned
to the protons of thio groups disappeared completely,
confirming the occurrence of the thio-ene reaction between
acrylate-terminated PEG and β-CD-SH. Furthermore, as shown
in Figure S1, the MALDI-TOF spectrum of β-CD-PEG showed
the molecular weight increase significantly in comparison with
β-CD (Mn=1135), which means that β-CD-PEG was successfully
synthesized.
Figure 3. 1H NMR spectrum of (A) Azo-COOH, (B) PAE, (C) PAE-g-Azo.
Figure 2. FT-IR spectra of (A) β-CD, (B) β-CD-OTs, (C) β-CD-SH, (D) β-CD-PEG.
3.2 Synthesis of the guest polymer PAE-g-Azo
The synthetic route of PAE-g-Azo is illustrated in Scheme 1B
.
PAE-g-Azo was synthesized by the esterification reaction
between carboxyl-terminated aminoazobenzene (Azo-COOH)
and poly(β-amino ester) (PAE) at ambient temperature. Azo-
COOH was prepared by the carboxylation reaction between p-
aminoAzobenzene and succinic anhydride. As shown in Figure
3A, the signals at δ=7.67 ppm, 7.85 ppm were assigned to the
protons of azobenzyl group and the signals at 2.55 ppm and
2.63 ppm were assigned to the methylene protons, indicating
Figure 4. (A) The potentiometric titration, and (B) transmittance at 500 nm of the PAE-
the successful synthesis of Azo-COOH. PAE was prepared by g-Azo solutions dependent on the different pH values.
means of Michael addition polymerization. It can be seen from
Figure 3B that all the protons of PAE could be observed. PAE-g-
Azo was prepared by DCC coupling between PAE and Azo-
COOH. It’s apparent to see the characteristic peaks of Azo and
PAE segments in Figure 3C, suggesting that Azobenzene groups
were successfully attached to PAE. Moreover, the substitution
degree of azobenzene moieties was about 33% according to
the ratio of integral area of the peaks at 7.0-8.0 ppm on
azobenzene moieties to the peaks at 4.0 ppm on PAE. In
addition, the molecular weight of PAE and PAE-g-Azo was 5214
g/mol and 7455 g/mol from GPC result, also confirming the
successful modification of azobenzene groups(Figure S2).
3.3 Characterization of the stimuli-responsiveness of PAE-g-Azo
3.3.1 pH-responsiveness of PAE-g-Azo. The pH-responsive
behavior of PAE-g-Azo was evaluated by acid-base titration
method and by monitoring the UV-Vis transmittance at
different pH in water. As seen in Figure 4A, the pKb of PAE-g-
Azo was about 5 when the pKb value was defined as the
inflection point of the titration curve. From the transmittance
result in Figure 4B, it can be seen that PAE-g-Azo could be
dissolved in water to form a clear solution below pH 5 while
the transmittance of the solution significantly decreased with
the increase of pH values and the solution suddenly became
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turbid. This phenomenon could be attributed to the of azobenzene and cyclodextrin moieties was recorded at
deprotonation of tertiary amine groups. All these results ambient temperature in D2O. As shown in Figure 6, the
demonstrated that PAE-g-Azo segment exhibited pH- appearance of crosspeaks (in green squares) from dipolar
responsive properity.
interaction between the protons of azobenzene moieties (7.4-
7.8 ppm) and the internal H3 and H5 protons of β-CD (3.5-3.8
ppm) were obviously detected, indicating that azobenzene
moieties deeply penetrated into the cavities of β-CD.
Consequently the corresponding amphiphilic supramolecular
polymer PAE-g-AZO@β-CD-PEG has been successfully obtained.
To further confirm the formation of the supramolecular
polymer PAE-g-Azo@β-CD-PEG, we adopted the approach of
UV-Vis spectra and fluorescence spectra to demonstrate the
complex between PAE-g-Azo and β-CD-PEG. As shown in
Figure 7A, the PAE-g-Azo solution displays a typical absorption
of trans-Azo at 355 nm. When the concentration of PAE-g-Azo
was maintained at 0.1 mg/ml, upon addition of β-CD-PEG, the
absorption peak of trans-Azo at 355 nm increased remarkably.
According to other researchers’ study, this phenomenon was
attributed to the increasement of the molar extinction
coefficient of trans-Azo43-46, suggesting that azobenzene group
entered into the cavity of β-CD. Figure 7B shows the
fluorescence spectra of PAE-g-Azo solution with different
amounts of β-CD-PEG. The fluorescence intensity at 420 nm
increased obviously with the increase of β-CD/Azo ratio. This
phenomenon can be interpreted as the complexation of β-CD
and Azo destroyed the aggregation of Azo groups and
consequently weakened the fluorescence quenching25,47. All
these results confirmed that the supramolecular polymer has
been successful prepared.
3.3.2 Photo-responsiveness of PAE-g-Azo. Polymers
containing azobenzene groups usually exist photo-
responsiveness because azobenzene can undergo reversible
photoisomerization under UV or visible light irradiation, which
will cause a change in UV-vis spectra of the polymer25, 35, 36
.
Thus, the photo-responsiveness of PAE-g-Azo was
characterized by UV-Vis spectrophotometer in DMSO. As
shown in Figure 5A, upon UV irradiation, the absorption band
ascribed to π-π* transition of trans-Azo at around 350 nm was
significantly decreased while the absorption band ascribed to
n-π* transition of cis-Azo near 440 nm was slightly increased
with increasing UV irradiation time, indicating that trans-Azo
was photoisomerized to cis-Azo. Upon subsequent 450nm
visible light irradiation, as shown in Figure 5B, the absorption
band at around 350 nm obviously increased and the
absorption band at 440 nm concomitantly decreased, meaning
that trans-Azo was transformed back to cis-Azo. All these
results demonstrated that PAE-g-Azo segment exhibited
photo-responsive property.
Figure 5. UV-vis spectra of PAE-g-Azo in DMSO as a function of time (A) upon 365 nm
UV light irradiation and (B) subsequently upon 450 nm visible light irradiation.
3.4 Formation of the amphiphilic supramolecular Polymer PAE-g-
Azo@β-CD-PEG based on host-guest interaction
Benefiting from their dynamic-tunable properties and
extensive applications, supramolecular polymers constructed
by host-guest complexation have attracted increasing
attention in recent years16-18. β-CD is one of the most popular
host substances because of its brilliant abilities in
encapsulating guest molecule in its hydrophobic cavity21. Due
to the well-documented host-guest complexation interaction
between azobenzene and β-CD, we constructed a novel
amphiphilic supramolecular polymer with β-CD/Azo junction
by mixing hydrophobic PAE-g-Azo and hydrophilic β-CD-PEG in
aqueous solution.
Figure 6. (A) 2D 1H NOESY spectrum of supramolecular polymer PAE-g-Azo@β-CD-PEG
in D2O.
1
To confirm the formation of supramolecular polymer, 2D H
nuclear Overhouser effect spectroscopy (NOESY) was used to
evidence the host-guest complexation between β-CD-PEG and
PAE-g-Azo. NOESY is an universal method to characterize host-
guest interactions and its cross-peaks can identify protons
undergoing ‘‘through space’’ dipolar interactions which are
significant at interaction distances of <4A°42. Thus, a reference
spectrum of PAE-g-Azo and β-CD-PEG with an equimolar ratio
Figure 7. UV-Vis spectra (A) and Fluorescence spectra (B) of PAE-g-Azo (0.1 mg/ml) in
the absence and presence of β-CD-PEG (n(CD):n(Azo)= 0, 1/8, 1/4, 1/2, 1/1 from a to e )
6 | J. Name., 2012, 00, 1-3
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with little β-CD-PEG is still insoluble in water and tended to be
wrapped into vesicles formed by the amphiphilic PAE-g-Azo
complexed with more β-CD-PEG. It must be noted that the
entrapped vesicles can be formed even at a low β-CD/Azo
molar ratio as 1/4 (Figure 8Ca). Thus, in our case, we chose the
β-CD/Azo molar ratio at 1:1 to study the self-assembly
behavior.
Figure 9. (A) DLS data of the supramolecular assemblies at pH 3 and pH 7; (B)TEM
image of the supramolecular assemblies at pH 3; (C) Photograph of supramolecular
assemblies solution before and after irradiation with 365 nm UV-light irradiation; (D)
TEM image of supernatant; (E) TEM image after adjusting to pH3 under visible light; (F)
TEM image after subsequently adjusting to pH 7.
Figure 8. (A) DLS data of the supramolecular assemblies at 1/1 β-CD/Azo molar ratio; (B)
Diameter of supramolecular assemblies (0.1 mg/ml) at different β-CD/Azo molar ratios;
(C) TEM images of supramolecular assemblies at different β-CD/Azo molar ratios: (a)
1/4, (b) 1/2, (c) 1/1, (d) 2/1, (e) 5/1, and ( f ) the enlarged TEM image of (c).
3.6 Dual-responsive behaviors of the supramolecular assemblies
3.5 Self-assembly of the supramolecular PAE-g-Azo@β-CD-PEG
3.6.1 pH-responsive behaviors of the Supramolecular
Assemblies. Because of the pH-responsiveness of PAE-g-Azo,
the supramolecular assemblies formed by PAE-g-Azo@β-CD-
PEG were expected to show pH-responsive behavior. As shown
in Figure 9A and 9B, when the pH value was adjusted to 3 by
HCl, the size of the supramolecular assemblies increased
obviously from 110 nm to 160 nm and the regular entrapped
vesicles changed into irregular aggregates completely. The
reason for this change was that the protonation of PAE-g-Azo
will cause the transformation of the polymer from
hydrophobic to hydrophilic, which led to the transition of the
self-assembled morphology.
It is well known that amphiphilic polymer can undergo self-
assembly in selective solution to form nanostructure with
various morphologies such as spheres, vesicles, cylinders and
hierarchical assemblies48. The morphologies and sizes of the
assemblies are mainly determined by the polymer composition
and the nature of the common solvent. Considering the high
hydrophobicity of PAE-g-Azo segment, β-CD-PEG was
introduced as the hydrophilic segment via the host-guest
interaction between β-CD and Azo to improve the solubility of
PAE-g-Azo in water. The amphiphilic supramolecule of PAE-g-
Azo@β-CD-PEG with different β-CD/Azo molar ratio was
constructed and then self-assembled into nanoparticles.
Dynamic light scattering (DLS) was used to measure the size
and size distribution of the supramolecular aggregates. As
shown in Figure 8A, the hydrodynamic diameter of the
supramolecular assemblies at 1:1 β-CD/Azo molar ratio was
about 110 nm and the relative average polydispersity was 0.15.
More interestingly, it is apparent to see that the β-CD/Azo
molar ratio had little impact on the size of the supramolecular
aggregates in Figure 8B. A similar phenomenon has been
reported by the previous literature49. The morphologies of the
supramolecular aggregates were further observed by
transmission electron microscopy (TEM). As shown in Figure
8C, the supramolecular assemblies with different β-CD/Azo
molar ratio all displayed a toroidal shape with a size of about
100 nm, like vesicles with a spherical particle entrapped inside,
3.6.2 UV-induced Disassembly of the Supramolecular
Assemblies. It is well known that trans-Azo can be isomerized
to cis-Azo when exposed to UV light and only trans-Azo can
interact with β-CD, whereas cis-Azo cannot34. Thus, we
assumed that the supramolecular assemblies formed by PAE-g-
Azo@β-CD-PEG could be disassociated by UV irradiation. The
disassembly process of the supramolecular assemblies was
investigated by irradiating the micelle solution with 365 nm UV
light. As shown in Figure 9C, without irradiation, as discussed
above, the supramolecular amphiphilic polymer could self-
assemble into entrapped vesicles in water to form a clear
solution. Upon irradiation with 365 nm UV light for 30 minutes
under stirring, the micelle solution was transformed from
yellow to colorless. Meanwhile, some yellow precipitates were
observed on the magnetic stick, which were ascribed to the
water-insoluble PAE-g-Azo segment based on the 1H NMR
result in Figure S3. Besides, the 1H NMR spectra of the residual
substance in supernatant is the same as that of β-CD-PEG
which were called entrapped vesicles as previously reported50-
52
.
The possible reason for this phenomenon is the
inhomogeneous complexation between β-CD and azobenzene
group according to Tong’s theory52. The PAE-g-Azo complexed
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(
Figure S4) and there was no regular entrapped vesicles exit in
4 Conclusions
the transparent solution after UV irradiation as observed by
In summary, we have designed and synthesized a novel pH-
and photo- responsive supramolecular polymer based on the
noncovlent host-guest coupling reaction between the
hydrophobic guest polymer PAE-g-Azo and the hydrophilic
host polymer β-CD-PEG. The obtained supramolecular polymer
could self-assemble into regular spherical entrapped vesicles in
water, which was revealed by DLS and TEM. The conformation
and morphology of the suparmolecular entrapped vesicles
could be adjusted by pH and photoirradiation. When in an
acidic environment(pH<pKb), the supramolecular entrapped
vesicles disappeared and some irregular and larger particles
appeared. Furthermore, by irradiating with UV light or visible
light, the supramolecular assemblies can be dissociated or
reassembled. It can be envisaged that this dual-responsive
supramolecular polymer may have potential appications in
many fields, especially in intelligent nanomaterials and
nanotechnology. Moreover, we believe that this work will cast
a new light on the design and construction of supramolecular
polymer with multi-responsiveness.
TEM
(Figure 9D). Thus, it can be concluded that the
supramolecular entrapped vesicles were dissociated into
water-soluble β-CD-PEG segment and PAE-g-Azo precipitates
by irradiating with 365 nm UV light because of the
isomerization of Azo groups.
3.6.3 Vis-pH-induced reassembly of the supramolecular
assemblies. As we know, the photoisomerization of
azobenzene is reversible and cis-Azo can be transformed to
trans-Azo by visible light irradiation. Thus we envision that the
supramolecular polymer PAE-g-Azo@β-CD-PEG could be
reconstructed by visible light irradiiation and further form
regular nanoparticles. Since PAE-g-Azo precipitated out of the
solution after UV irradiation, the obtained mixture could not
reassemble simply by visible light irradiation. In order to make
the mixture reassemble into regular nanoparticles, we utilized
its pH-responsive solubility discussed above to redissolve the
precipitated PAE-g-Azo. As shown in Figure 9E, by adjusting
the mixture solution to pH3 and subsequently irradiated with
450nm visible light, PAE-g-Azo could be redissolved into water
and then interact with β-CD-PEG to form some irregular
aggregates the same as that in Figure 9B. Subsequently
adjusting the mixture to pH 7, supramolecular entrapped
vesicles can be reconstructed (Figure 9F). The influence of β-
CD/Azo molar ratio on the dual-responsive recurrent behaviors
of the supramolecular assemblies has also been studied. We
found that the β-CD/Azo molar ratio didn’t affect the dual-
responsive recurrent behaviors of the supramolecular
assemblies, and the supramolecular assemblies could be
reconstructed either at 1/2 or 2/1 β-CD/Azo molar ratio
Conflicts of interest
There are no conflicts to declare.
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Table of content
A novel supramolecular polymer PAEꢀgꢀAzo@βꢀCDꢀPEG was constructed, which significantly
displayed pHꢀ and photoꢀ dualꢀresponsive recurrent selfꢀassembly behaviors.