Photocontrolled self-assembly and disassembly of block ionomer complex vesicles: A facile approach toward Supramolecular Polymer nanocontainers

Article abstract of DOI:10.1021/la9023844

A new concept of designing a photocontrollable supramolecular polymer nanocontainer through the electrostatic association between an azobenzene-containing surfactant (AzoC10) and a double-hydrophilic block ionomer, polyethylene glycol)-b-poly(acrylic acid

Full text of DOI:10.1021/la9023844

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© 2009 American Chemical Society
Photocontrolled Self-Assembly and Disassembly of Block Ionomer Complex
Vesicles: A Facile Approach toward Supramolecular Polymer
Nanocontainers
Yapei Wang,† Peng Han,† Huaping Xu,† Zhiqiang Wang,† Xi Zhang,*,† and
Alexander V.Kabanov‡
†Key Lab of Organic Optoelectronics & Molecular Engineering Department of Chemistry, Tsinghua University,
Beijing 100084, P. R. China, and ‡Department of Pharmaceutical Sciences and Center for Drug Delivery and
Nanomedicine, University of Nebraska Medical Center, 985830 Nebraska Medical Center, Omaha, Nebraska
68198-5830
Received July 3, 2009
A new concept of designing a photocontrollable supramolecular polymer nanocontainer through the electrostatic
association between an azobenzene-containing surfactant (AzoC10) and a double-hydrophilic block ionomer, poly-
(ethylene glycol)-b-poly(acrylic acid) (PEG43-PAA153), is described. Such a block ionomer complex can self-assemble in
aqueous solution and form vesicle-like aggregates, which are composed of a poly(ethylene glycol) corona and a
poly(acrylic acid) shell associated with azobenzene-containing surfactant. The photoisomerization of azobenzene
moieties in the block ionomer complex can reversibly tune the amphiphilicity of the surfactants, inducing the
disassembly of the vesicles. Such block ionomer complex vesicles are further evaluated as nanocontainers capable to
encapsulate and release guest solutes on demand controlled by light irradiation. For example, the vesicles encapsulating
the fluorescein sodium display clear spherical images observed by fluorescence microscopy. However, such fluorescence-
marked images disappear after releasing the solute from the vesicles triggered by the UV light. Such novel materials are
of both basic and practical significance, especially as prospective nanocontainers for cargo delivery.
Introduction
controlled drug delivery through tuning the amphiphilicity of the
block copolymers.2 Generally, the drug species encapsulated in or
attached to the polymeric assemblies can be released via rever-
sible3 or irreversible4 disassembly of the hydrophobic core-form-
ing segments. However, the self-assembly of amphiphilic block
copolymers usually involves the use of organic solvents and
suffers from complicated preparation processes. More than a
decade ago Kataoka’s group5 and Kabanov’s group6 developed a
concept for preparing block copolymer assemblies on the basis of
electrostatic interactions. This new family of polymeric assemblies
is formed by double-hydrophilic block copolymers, containing
ionic and nonionic water-soluble segments (block ionomers), and
can incorporate many charged polymers, including synthetic
polyions,7 enzymes,8 DNA,9 RNA,10 and others. One great
advantage of this approach is that such assemblies are formed
in water, and no organic solvent is required for their preparation.
Moreover, the block ionomers with appropriate molecular weight
and composition can also form micelle-like or vesicle-like aggre-
gates. The basic mechanism of the formation of such polymeric
assemblies involves the core precipitation of the charged blocks of
block ionomers with the oppositely charged polyions. Besides the
Self-assembly of amphiphilic block copolymers induces forma-
tion of nanosized polymeric micelles or vesicles, which have been
widely explored as carriers for enzymes or nonbiological catalysts
as well as containers for drug or gene delivery.1 Since amphiphi-
licity is the principal basis of such self-assembly, some approaches
have been developed to modulate the polymeric assemblies for
*Corresponding author. E-mail: xi@mail.tsinghua.edu.cn.
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Langmuir 2010, 26(2), 709–715
Published on Web 07/24/2009
DOI: 10.1021/la9023844 709

Article
Wang et al.
block ionomer-polyion systems, Kabanov et al. proposed a
simple method to prepare block ionomer complexes by electro-
static complexation of block ionomers with oppositely charged
surfactants.11 Such block ionomer complex can be depicted as an
amphiphilic supramolecular block copolymer, in which the non-
ionic block functions as the hydrophilic part while the electro-
static complex of the ionic block and aggregated surfactant
counterions serves as the hydrophobic part. It is known that by
introducing stimuli-responsive moieties onto the surfactants, the
surfactant aggregates can be tuned toward controllable disassem-
bly.12 Herein, for the first time we demonstrate a possibility of
controlled self-assembly and disassembly of the block ionomer
and surfactant complex vesicles through tuning the amphiphili-
city of the surfactants.
It is well-known that the trans- and cis-forms of azobenzene-
bearing surfactant have different critical micelle concentration
(cmc), and this can be used to realize the destruction and
formation of the micellar structure in solution upon alternatively
treatment of UV light and visible light.13 Inspired by this work, we
attempted to introduce the azobenzene-bearing surfactant into
the block copolymer complex and to develop a new concept of
stimuli-responsive block ionomer complex. In addition, we won-
dered if the complex is able to form photocontrolled vesicle-like
structure and this stable vesicle-like structure can be used as
polymeric nanocontainer for cargo delivery. For this purpose, in
the present work, we prepared a positive-charged azobenzene-
containing surfactant, 1-[10-(4-phenylazophenoxy)decyl]triethyl-
amine bromide (AzoC10), and a double-hydrophilic block
copolymer of poly(ethylene glycol)-block-poly(acrylic acid)
(PEG43-PAA153). We have demonstrated that the electrostatic
complexation between the single-tail surfactant and the PAA
block of the block copolymer leads to the spontaneous formation
of block ionomer complex vesicles. The block ionomer complex
vesicles are found to disassemble when the azobenzene moiety
changes from trans- to cis-form as a result of UV light irradiation.
Furthermore, such block ionomer complex vesicles can be used as
nanocontainers for encapsulation and release of guest molecules
controlled by UV light irradiation.
Figure 1. Synthetic route of AzoC10.
mixture was kept stirring for 8 h at 60 °C under an Ar atmosphere.
After evaporation of acetonitrile, the yellow solid was dissolved in
tetrahydrofuran. The solution was added stepwise with magnetic
stirring to a plenty of petroleum ether, and the yellow precipitate
was filtered and dried in a vacuum oven. Final characteristics of
AzoC10 were as follows: 1H NMR (solvent: CDCl3): δ 7.99-7.90
(q, 4H), δ 7.52-7.46 (m, 3H), δ 7.02-7.00 (d, 2H), δ 4.07-4.03
(t, 2H), δ 3.54-3.47 (q, 6H), δ 3.29-3.24 (t, 2H), δ 1.83 -1.32 (m,
25H). 13C NMR (solvent: CDCl3): δ 161.89, 152.70, 146.82,
130.47, 129, 14, 124, 94, 122,60, 114,82, 68.40, 57.68, 53.67,
29.38. 29.30, 29.21, 26.56, 26.01, 22.21, 8.23. MS: [M-Br]þ
calculated, 438.35; found, 438.33. Anal. Calcd for C28H44BrN3O
(AzoC10): C, 64.85; H, 8.55. Found: C, 64.88; H, 8.50.
1H NMR spectra were recorded using a JEOL JNM-ECA300
spectrometer. The electrospray ionization mass spectrometry
(ESI-MS) was performed using a PE SciexAPI 3000 spectrometer.
UV/vis and fluorescence spectra were obtained using Hitachi
U-3010 and Hitachi F-7000 spectrophotometers, respectively.
A high-pressure mercury lamp with an optical fiber and an
intensity of 900 mW/cm2 was used as the irradiation light source
for photoisomerization of azobenzene. The two band-pass filters
with the wavelengths 365 ( 10 and 450 ( 10 nm were used to
produce the UV light (365 nm) and visible light (450 nm),
respectively. To reduce the error in the kinetic measurement of
photoisomerization, the solutions were irradiated in an in situ
mode using the UV/vis spectrophotometer with the optical fiber.
Additionally, to avoid the heat generated by light irradiation,
the optical fiber was kept 3 cm away from the sample. The
transmission electron microscopy (TEM) was performed on a
JEMO 2010 electron microscope, operating at an acceleration
voltage of110 kV. The samples werepreparedbydrop-coatingthe
aqueous solution on the carbon-coated copper grid and stained
with 0.2% phosphotungstic acid hydrate before TEM observa-
tion. The dynamic light scattering (DLS) measurements were
performed using ALV/DLS/SLS-5022F laser light scattering
measurement system. The light wavelength was set at 633 nm,
which is much longer than the absorption band of trans- or cis-
azobenzenegroup. The fluorescent imageswerecaptured using an
OLYMPUSBX 51 fluorescence microscope by straightly casting
the block ionomer complex incorporating FNa, before or after
UV irradiation, on a clean glass.
Experimental Section
Materials and Instruments. All reagents and solvents were
purchased from Beijing Chemical Reagent Co., China. Triethy-
lamine was dried by NaOH before use. The block copolymer
PEG43-PAA153 was synthesized by atom transfer radical polym-
erization (ATRP).14 AzoC10 was synthesized according to the
synthetic route shown in Figure 1, in which the synthesis of
compounds 1 and 2 has been described before.12c
Synthesis of 1-[10-(4-Phenylazophenoxy)decyl]triethylamine
Bromide (AzoC10). Excessive triethylamine (0.5 mL) was added
to compound 2 (0.1425 g, 0.3 mmol)-acetonitrile solution. The
(11) (a) Bronich, T. K.; Kabanov, A. V.; Kabanov, V. A.; Yu, K.; Eisenberg, A.
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Yu, K.; Eisenberg, A. J. Am. Chem. Soc. 1998, 120, 9941. (c) Bronich, T. K.; Popov, A.
M.; Eisenber, A.; Kabanov, V. A.; Kabanov, A. V. Langmuir 2000, 16, 481.
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Ohkubo, T.; Sakai, H.; Abe, M. J. Am. Chem. Soc. 2004, 126, 12282. (b) Boomer, J.
A.; Thompson, D. H. Chem. Phys. Lipids 1999, 99, 145. (c) Wang, Y.; Ma, N.; Wang,
Z.; Zhang, X. Angew. Chem., Int. Ed. 2007, 46, 2823. (d) Liu, Y.; Jessop, P. G.;
Cunningham, M.; Eckert, C. A.; Liotta, C. L. Science 2006, 313, 958. (e) Jiang, Y.;
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(13) (a) Kunitake, T.; Nakashima, N.; Shimomura, M.; Okahata, Y.; Kano, K.;
Ogawa, T. J. Am. Chem. Soc. 1980, 102, 6642. (b) Sakai, H.; Matsumura, A.;
Yokoyama, S.; Saji, T.; Abe, M. J. Phys. Chem. B 1999, 103, 10737. (c) Orihara, Y.;
Matsumura, A.; Sito, Y.; Ogawa, N.; Saji, T.; Yamaguchi, A.; Sakai, H.; Abe, M.
Langmuir 2001, 17, 6072.
Preparation of the Block Ionomer Complex. To prepare
the block ionomer complex, two precursors of AzoC10 and
PEG43-PAA153 were separately dissolved in water, and their
concentrations were 2.0 ꢀ 10-4 M and 0.06 mg/mL, respectively.
0.5 mL of PEG43-PAA153, in which the molar concentration of
carboxylate group is 7.3 ꢀ 10-4 M, was added in a tube, following
withaddition of AzoC10 solution and wateruntil the total volume
is 5 mL. The composition of the mixture (Z value) was expressed
(14) (a) Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921. (b) Niu, H.;
Zhang, L.; Gao, M.; Chen, Y. Langmuir 2005, 21, 4205.
710 DOI: 10.1021/la9023844
Langmuir 2010, 26(2), 709–715

Wang et al.
Article
Scheme 1. Schematic Illustration of the Self-Assembly of the Block Ionomer Complex Vesicles
as the molar ratio of the positive-charged AzoC10 and negative-
charged carboxylate groups of PEG43-PAA153, Z = [AzoC10]/
[COO-]. For example, when 2 mL of AzoC10 was mixed with
0.5 mL of PEG43-PAA153, and the mixture was further diluted
by water to 5 mL, the final concentrations of carboxylate and
AzoC10were7.3 ꢀ 10-4 and 8.0ꢀ 10-4 M, respectively, and the Z
value in the mixture solution was around 1.1.
Turbidity Measurements. The turbidity was measured at
different Z values during titration of the corresponding polymers
with AzoC10 in aqueous solution. The absorbance of the mix-
ture solution at 550 nm was measured by a UV/vis spectrometer.
The turbidity was calculated as turbidity = 1 - 10-A, where A is
the UV absorbance of the mixture.
Kinetic Photoisomerization of AzoC10. The reversible
photoisomerization of azobenzene moiety is induced by alternat-
ing UV (365 nm) and visible light (450 nm) irradiation. Usually
the photoisomerization of azobenzene group proceeds as the
first-order reaction. The first-order plot for trans-cis photoi-
somerization is determined by the following equation:
Figure 2. Turbidity of the mixtures of AzoC10 with the block
copolymer PEG43-PAA153 and homopolymer PAA as a function
of the molar ratio between AzoC10 and carboxylate groups of the
polymer(Zvalue). [COO-] = 7.3ꢀ 10-5 M.TheUVmeasurement
was carried out by gradually increasing the concentration of
AzoC10.
ðA0 -AeqÞ
ln
¼ ktt
FNa was complete as confirmed by lack of fluorescence in the
filtrate. No loss of the block copolymer was postulated because
the pore size of semipermeable bag was small enough to prevent
block copolymers from passing through. Based on this, the final
Z value of the block ionomer complex was calculated using
AzoC10 concentration (3.5 ꢀ 10-4 M) in the residue deter-
mined by the UV spectra, the initial amount of PEG43-PAA153
(0.06 mg), and the volume of the residue solution which was
measured as 9.8 mL after the dialysis.
ðAt -AeqÞ
where A0, At, and Aeq are the initial absorbance, the absorbance at
time t, and the absorbance at the photostationary state of azo-
benzene group of AzoC10 at 340 nm, respectively; kt is the rate
constant of the trans-cis isomerization. For the cis-trans conver-
sion of azobenzene, the kinetic equation needs to be changed to
ðAeq -A0Þ
ln
¼ kct
ðAeq -AtÞ
Results and Discussion
in which kc is the rate constant of the cis-trans photoisomeriza-
tion.
Formation of Block Ionomer Complex Vesicles. The
strategy employed to prepare block ionomer complex vesicles is
illustrated in Scheme 1. The azobenzene-containing surfactant,
AzoC10, and the block copolymer, PEG43-PAA153, used herein
as its sodium salt, are both water-soluble. Therefore, all experi-
ments were carried out in aqueous solutions at pH 9.0. The
concentrations of the reagents were set to ensure that at every
Z value the concentration of AzoC10 was much lower than
its cmc, which was measured as high as 1.3 ꢀ 10-4 M. This
allowed avoiding self-assembly of AzoC10 alone. However, in the
Loading of Fluorescent Molecules. Taking fluorescein so-
dium (FNa) as an example, the loading procedure is described
as follows. First, three components, 1 mL of FNa (1ꢀ10-4 M),
4 mL of AzoC10(2 ꢀ 10-4 M), 1 mLof PEG43-PAA153 (0.06mg/
mL), and 4 mL of water were simply mixed together. The Z value
in this mixture was equal to 1.1. Second, after stirring for half an
hour, the mixture solution was dialyzed against water to remove
the excess of FNa, which did not incorporate into vesicles. After
4 day dialysis (water changed twice every day), the separation of
Langmuir 2010, 26(2), 709–715
DOI: 10.1021/la9023844 711

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Wang et al.
Figure 3. TEM observation of block ionomer complex vesicles (a) at Z=0.25, (b) Z = 0.5, and (c) Z=1 and the size distribution of block
ionomer complex vesicles (d) at Z=0.25, (e) Z=0.5, and (f) Z=1. The concentration of PEO43-PAA153 is kept at 6 ꢀ 10-3 mg/mL, which
corresponds to [COO-] = 7.3 ꢀ 10-5 M.
presence of the PEG43-PAA153 due to electrostatic complexation
with PAA segment AzoC10 aggregated becoming a part of the
block ionomer complex.
The critical Z value corresponding to the onset of the formation
of the block ionomer complex was estimated from the turbi-
dity measurements. As shown in Figure 2, the turbidity of the
PEG43-PAA153 and AzoC10 mixtures starts to increase slightly
at Z = 0.25, suggesting formation of the colloidal aggregates.
Even more pronounced are the turbidity changes in the case of
the homopolymer PAA (Mw = 2000), which was used as the
positive control to observe its association with AzoC10. In this
case the onset of the turbidity increase was also observed at Z=
0.25. Furthermore, at Z = 0.5 the dispersion became highly turbid
due to precipitation of an insoluble complex of PAA and AzoC10.
In contrast, the complexes of PEG43-PAA153 and AzoC10 did
not precipitate, and the dispersion remained relatively clear at Z as
high as 1.25. The difference in the aggregation behavior of
the complexes formed by the block copolymer and homopoly-
mer is noteworthy and suggests that contrary to PAA the
Figure 4. Photoisomerization of AzoC10 in the block ionomer
complex vesicles (Z = 1). [PEG43-PAA153] = 0.06 mg/mL;
[AzoC10] = 8 ꢀ 10-5 M which is below its cmc (1.3 ꢀ 10-4 M).
PEG43-PAA153 forms with AzoC10 small colloidal aggregates,
which remain stable in dispersion. The TEM microphotographs
The TEM data suggest that the block ionomer complexes
undergo drastic structural changes upon irradiation-induced
suggest that these aggregates at various Z values are vesicles
of nanoscale size (Figure 3). The formation of the vesicles was
photoisomerization of the surfactant. Specifically, the vesicles
also independently confirmed using fluorescence probes as
formed by the block copolymer and trans-form of AzoC10 in
the case of Z=1 appeared to be partially destroyed after UV
irradiation (Figure 5a,b). This was accompanied by a substantial
described further.
Photocontrolled Self-Assembly and Disassembly of the
Vesicles. The data in Figure 4 demonstrate that the molecules of
increase in the particle size. As shown in Figure 5c, before UV
AzoC10 incorporated the block ionomer complex can undergo
irradiation, the main diameter distribution of the block ionomer
reversible photoisomerization. Specifically, upon irradiation with
complex was around 80 nm. After UV irradiation, the number
UV light of 365 nm for 300 s, the absorption band at around
intensity decreased dramatically and the size of the particles
340 nm decreased remarkably. Concomitantly the absorption
increased to several hundred nanometers. Such remarkable
band around 430 nm increased. These absorption bands at
changes in the morphology and size before and after UV irradia-
tion must be related to the photoisomerization of the azobenzene
340 and 430 nm are ascribed to π-π* and n-π* transitions,
respectively. Therefore, the change of the absorption bands is
indicativeof the photoisomerization of AzoC10 fromtrans- to cis-
form induced by UV irradiation. The opposite transition was
moiety of AzoC10. A similar phenomenon was demonstrated by
Zhao et al. in their azo-copolymers.15 It is remarkable that the
visible light irradiation of the block ionomer complex subjected to
observed upon irradiation of the cis-form by the visible light of
450 nm for 900 s. In this case the π-π* absorption increased while
the n-π* absorption decreased. This suggests that AzoC10
inverts from cis- back to trans-form.
(15) Tong, X.; Wang, G.; Soldera, A.; Zhao, Y. J. Phys. Chem. B 2005, 109,
20281.
712 DOI: 10.1021/la9023844
Langmuir 2010, 26(2), 709–715

Wang et al.
Article
Figure 5. TEM images of the block ionomer complex (Z = 1)
(a) before UV irradiation, (b) after UV irradiation for 300 s, and
(c) DLS results:(I) beforeUVirradiation;(II) after UV irradiation.
(d) The recovery of block ionomer complex after further visible
light irradiation for 900 s.
prior UV irradiation appears to induce the recovery of the vesicle-
like structures (Figure 5d). Moreover, we have tried two cycles to
confirm the reversibility of assembly and disassembly.
Figure 6. Formation of block ionomer complex vesicles (Z = 0.5)
is indicated by (a) TEM observation and (b) DLS. (c) The vesicles
are disrupted upon UV irradiation for 300 s. (d) TEM observation
and (e) DLS result of the reassembled block ionomer complex
vesicles by further visible light irradiation for 900 s.
To understand if the ratio between the block copolymer and
AzoC10 can influence self-assembly and disassembly of vesicle-
like aggregates, we changed the composition of the mixture to
Z = 0.5; i.e., the concentration of AzoC10 is the half of that of
carboxylate groups. As indicated by TEM observation in
Figure 6a, the vesicles can also form under this condition. As
shown by DLS, the main diameter distribution of the vesicles
is around 70 nm (Figure 6b). Different from the previous case
of Z = 1, the block ionomer complex vesicles at Z = 0.5 can be
completely disassembled upon UV irradiation (Figure 6c). The
complete disassembly is also supported by DLS, which shows
no DLS signal in this case. Therefore, the block ionomer
complex at Z = 0.5 is suitable for guaranteeing the fully reversible
change of the vesicles before and after UV irradiation. When
irradiated by visible light, the block ionomer complex vesicles can
be reassembled again, as expected. (Figure 6d). However, as well
as confirmed by DLS (Figure 6e), the size distribution increases
slightly comparing with the original vesicles without light irradia-
tion, although the mechanism behind is not fully understood.
The self-assembly and disassembly of block ionomer complex
vesicles are also reflected by different photoisomerization kinetics
of AzoC10 under various conditions. As shown in Table 1, the
initial trans-cis photoisomerization rate constants (kt) of
AzoC10 in the presence of PEG43-PAA153 are always lower than
those of the absence of the copolymer. However, the cis-trans
photoisomerization rate (kc) of AzoC10 in the presence of the
block copolymer after UV irradiation remains similar to that of
free AzoC10. These data indicate that AzoC10 become densely
packed upon incorporation in the block ionomer complex,
responsible for the significant decrease of kt. In the case of cis-
form of AzoC10, it cannot well form aggregates with the block
copolymer, which agrees well with the situation of disassembly as
discussed above.
Table 1. Photoisomerization of AzoC10 at Different Concentration in
the Absence or Presence of the Block Copolymer
C(AzoC10) [M]
kt [s-1
]
kc [s-1
]
2 ꢀ 10-5 M
0.0840
0.0256
0.0501
0.0220
0.0321
0.0152
0.0083
0.0084
0.0050
0.0044
0.0027
0.0031
2 ꢀ 10-5 M (PEG43-PAA153, Z = 0.25)
4 ꢀ 10-5 M
4 ꢀ 10-5 M (PEG43-PAA153, Z = 0.5)
8 ꢀ 10-5 M
8 ꢀ 10-5 M (PEG43-PAA153, Z = 1)
the Methods section. As expected, the UV and fluorescence
spectra clearly indicate that FNa remained in the vesicles after
the dialysis (Figure 7a,b). Notably, part of AzoC10 was lost
during the dialysis, resulting in the effective decrease of Z value
from 1 to around 0.5. Nevertheless, as discussed above, the
vesicles can be formed at such Z value, as shown in Figure 7c.
Successful loading of FNa in the block ionomer complex vesicles
is reinforced by fluorescence microscopy, as shown in Figure 7e.
With the excitation wavelength at 500 nm, clear spherical aggre-
gates were captured, which should be ascribed to the block
ionomer complex vesicles encapsulating FNa. The apparently
larger size of vesicles observed compared to TEM image may be
due to the luminescence effect. Interestingly, after the UV
irradiation of the block ionomer complexes the punctuate fluor-
escence was greatly diminished, suggesting that most of the
vesicles were disintegrated which is confirmed by TEM observa-
tion (Figure 7d), and simultaneously, FNa was released
(Figure 7f). Additionally, there was no effect of UV irradiation
on the fluorescence intensity of FNa, indicating that there is no
photobleaching that could contribute to a decrease of fluores-
cence in Figure 7d.
Loading and Release of Solutes in the Vesicles. FNa and
sodium 1-pyrenesulfonate (PySO3Na) were used as model fluor-
escent probes to evaluate whether the block ionomer complex
vesicles can encapsulate and release guest molecules. The block
ionomer complexes were prepared in the presence of the solutions
of these probes. The mixtures were then dialyzed as described in
We have also confirmed that the block ionomer com-
plex vesicles can be applied for loading of PySO3Na. Contrary
Langmuir 2010, 26(2), 709–715
DOI: 10.1021/la9023844 713

Article
Wang et al.
Figure 7. Encapsulation of and release of FNa in block ionomer complex vesicles demonstrated by (a) UV spectrum, (b) fluorescence
spectrum (λex = 485 nm), and (c, d) TEM images; (e, f) fluorescence microscopy images of the block ionomer complexes prepared in presence
of FNa (a-c, e) before and (d) after UV irradiation.
trans-azobenzene. Indeed, as seen in Figure 8a, PySO3Na showed
weak fluorescence in the presence of the block ionomer complex
vesicles. However, after UV irradiation, the fluorescence in-
creased, as shown in Figure 8b, because cis-azobenzene does
not quench PySO3Na. Moreover, upon visible light irradia-
tion, the probe fluorescence decreased (Figure 8c), which is
consistent with the restoration of the trans-form of AzoC10.
However, since after the recovery the fluorescence intensity did
not quench completely to the same level than that observed before
UV-light irradiation, we speculate that part of PySO3Na was
irreversibly released from the interior of the block ionomer
complex vesicles.
Conclusion
Figure 8. Fluorescence spectra (λex = 339 nm) of the block iono-
mer complex vesicles encapsulating PySO3Na: (a) before UV
irradiation, (b) after UV irradiation, and (c) after visible light
irradiation.
In conclusion, we have demonstrated for the first time that a
block ionomer complex can be obtained by electrostatic com-
plexation between block copolymer and photoresponsive azo-
benzene-containing surfactant. More importantly, this block
ionomer complex can form UV-responsive vesicles, which can
be explored as supramolecular polymer nanocontainers for con-
trolled loading and release of solutes. This represents a simple
method for fabricating stimuli-responsive block ionomer complex
vesicles. The method can be adopted for introducing different
azobenzene-containing surfactants and even other stimuli-re-
sponsive moieties. By introducing stimuli-responsive surfac-
tants into the block ionomer complex, one may broaden the
application of this new class of supramolecular materials. Such
to FNa, PySO3Na has poor solubility in water and in
the dispersions of the block ionomer complexes most likely
would localize in the hydrophobic areas formed by the surfac-
tant molecules. Since PySO3Na can be quenched by trans-azo-
benzene moiety (mainly because of energy transfer),16 the
fluorescence quenching of PySO3Na can be indicative of interac-
tion of this probe with the surfactant structures containing
€
(16) Puntoriero, F.; Ceroni, P.; Balzani, V.; Bergamini, G.; Vogtle, F. J. Am.
Chem. Soc. 2007, 129, 10714.
714 DOI: 10.1021/la9023844
Langmuir 2010, 26(2), 709–715

Wang et al.
Article
novel materials are of both basic and practical significance,
especially as prospective nanocontainers for drug delivery.
acknowledges the financial support of his work on block ionomer
complexes by the United States National Science Foundation
(DMR 0513699).
Acknowledgment. This work was financially supported by the
National Basic Research Program of China (2007CB808000),
National Natural Science Foundation of China (NSFC), and
the joint project between NSFC and DFG (TRR61). We thank
Mr. Zhenhua Jiang for the design of cartoon figures. A.V.K.
Supporting Information Available: Cmc measurement of
AzoC10 and details for the kinetic photoisomerization of
AzoC10 in different conditions. This material is available
free of charge via the Internet at http://pubs.acs.org.
Langmuir 2010, 26(2), 709–715
DOI: 10.1021/la9023844 715