"breathing" vesicles with jellyfish-like on-off switchable fluorescence behavior

Article abstract of DOI:10.1002/anie.201206362

Controlled, deep breathing: Polymeric vesicles that exhibit reversible pH-induced "breathing" behavior accompanied by switchable fluorescence (see picture) were prepared through the aqueous self-assembly of an amphiphilic block copolymer. Mechanistic stud

Full text of DOI:10.1002/anie.201206362

Angewandte
Chemie
DOI: 10.1002/anie.201206362
Vesicles
“Breathing” Vesicles with Jellyfish-like On–Off Switchable
Fluorescence Behavior**
Ruijiao Dong, Bangshang Zhu, Yongfeng Zhou,* Deyue Yan, and Xinyuan Zhu*
Nowadays, synthetic vesicles serve as excellent model mem-
branes to mimic the dynamic and structural features of cells in
a blossoming field coined cytomimetic chemistry, as reviewed
by Menger et al.[1] Most studies have been based on lipid
vesicles (liposomes). For example, Sackmann and co-workers
demonstrated liposome budding and fission induced by
heating and osmotic deflation.[2] Menger and co-workers
reported the fusion, birthing, foraging, healing, separation,
endocytosis, adhesion, and aggregation of liposomes.[3] Ewing
and co-workers showed the exocytosis of liposomes.[4] Keating
and co-workers observed liposome budding.[5] Polymeric
vesicles (polymersomes) have also been used in cytomimetic
chemistry. For example, Eisenberg and co-workers described
the fusion, fission, and breathing of polymersomes.[6] Our
group[7] reported the real-time fusion, fission, and aggregation
of giant hyperbranched polymer vesicles. Thus, generally
speaking, great progress has been made in cytomimetic
chemistry, and synthetic vesicles have proved to be valuable
model systems for investigating the mechanisms of these
cellular processes, especially in the discrimination of essential
membrane activities from protein interference.
Until now, all reported cytomimetic processes have been
limited to shape transformations of the vesicle membranes,
and no function has been generated during these processes.
However, in many cellular processes or living organisms there
is a concomitant function expression during membrane
deformation. For example, some jellyfish show on–off switch-
able fluorescence behavior in response to the swelling and
shrinkage of the membrane during the breathing process:[8]
fluorescence is quenched when the jellyfish breathes in, and
strong green fluorescence is emitted when the jellyfish
breathes out (Figure 1A). The molecular basis for this
concomitant breathing and light-emitting process is the
oxidation- or deoxidation-induced conformation transforma-
[*] Dr. R. Dong, Prof. Y. Zhou, D. Yan, X. Zhu
School of Chemistry and Chemical Engineering, State Key Labo-
ratory of Metal Matrix Composites, Shanghai Jiao Tong University
800 Dongchuan Road, Shanghai 200240 (China)
Figure 1. The breathing processes of A) jellyfish and B) vesicles accom-
panied by highly reversible green-fluorescence quenching and recovery.
C) Schematic representation of the amphiphilic diblock copolymer
E-mail: yfzhou@sjtu.edu.cn
Prof. B. Zhu, X. Zhu
Instrumental Analysis Center, Shanghai Jiao Tong University
800 Dongchuan Road, Shanghai 200240 (China)
E-mail: xyzhu@sjtu.edu.cn
PEG-b-PDMA-Azo and the vesicle structure.
[**] This research was sponsored by the National Natural Science
Foundation of China (21074069, 91127047, 20974062, 30700175),
the National Basic Research Program (2009CB930400,
2012CB821500, 2013CB834506), and China National Funds for
Distinguished Young Scientists (21025417).
tion of the membrane-incorporated fluorescent proteins (see
section 5 of the Supporting Information). Thus, to push
forward the development of cytomimetic chemistry, it is
necessary to combine vesicular morphological transforma-
tions with dynamic functional changes. That is, cytomimetic
vesicle deformation should trigger or be accompanied by the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206362.
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Communications
expression of a function, such as the generation of light-
emitting signals as seen in jellyfish. To our knowledge,
a concomitant morphological and functional transformation
in cytomimetic chemistry has not been described to date.
As inspired by jellyfish, we report herein a smart “breath-
ing” polymeric vesicle with reversible on–off switchable
fluorescence behavior. The polymeric vesicles were prepared
through the aqueous self-assembly of an amphiphilic diblock
copolymer consisting of many fluorescent chromophores of
dimethylaminoazobenzene (DMA-Azo groups). They exhib-
ited pH-responsive breathing behavior involving the swelling
and shrinkage of the vesicles. Furthermore, in analogy with
jellyfish, the fluorescence is quenched when the vesicles
breathe in, whereas a strong green fluorescence is emitted
when the vesicles breathe out (Figure 1B). The jellyfish-like
breathing and light-emitting behavior could be induced
reversibly many times by the alternate addition of HCl and
NaOH. Mechanistic investigations showed that this behavior
originates from reversible conformation transformations of
the DMA-Azo chromophores as induced by protonation or
deprotonation.
We first attempted to synthesize the azobenzene-contain-
ing amphiphilic diblock copolymer by using brominated
poly(ethylene glycol) (PEG-Br) as the macroinitiator to
initiate the atom-transfer radical polymerization (ATRP) of
readily obtained 4-(4-dimethylaminophenylazo)phenyl acry-
late (monomer 1; see Scheme S1 in the Supporting Informa-
tion). Unfortunately, the block copolymer was not formed
owing to the low polymerization activity and great steric bulk
of monomer 1. To overcome this problem, we introduced
a long alkyl-chain spacer (hexyl) into monomer 1 and
replaced the acrylate group with methacrylate. The resulting
highly reactive monomer 6-(4-(4-dimethylamino phenylazo)-
phenoxy)hexyl methacrylate (monomer 2; see Schemes S2
and S3 and Figures S1–S3 in the Supporting Information)
underwent ATRP with PEG-Br to give the well-defined
target amphiphilic diblock copolymer PEG-b-PDMA-Azo
(Mn = 21.4 kDa, Mw/Mn = 1.07; see Figures S4 and S5) with
thermotropic liquid-crystalline behavior (see Figure S6).
Furthermore, the diblock copolymers tend to self-assemble
in water as a result of their amphiphilic nature. The critical
aggregation concentration (CAC) was determined to be
approximately 0.04 mgmLÀ1 (see Figure S7) by measurement
of the UV absorbance at 317 nm of azobenzene chromo-
phores in the diblock copolymers at increasing concentra-
tions.
The morphology and size of the aggregates were inves-
tigated by atomic force microscopy (AFM), transmission
electron microscopy (TEM), and dynamic laser scattering
(DLS) measurements. The AFM image in Figure 2A shows
spherical particles with a diameter-to-height ratio larger than
20, which indicates a thin-layered and collapsed vesicle
structure.[9] The presence of the concave feature (white
arrows in the inset of Figure 2A) in the deformed particles
indicates a possible hollow structure of the spherical particles.
Moreover, the contrast difference between the particle skin
and the inner pool in the TEM image of the self-assembled
structures shows that these particles are unilamellar vesicles
(Figure 2B).[10] The hollow structure of the vesicles was also
Figure 2. A) AFM image, B) TEM image, and C) DLS profile of the self-
assembled vesicles in the initial state (pH 7). D) AFM image, E) TEM
image, and F) DLS profile of the vesicles at pH 4. The inset in (A) is
a 3D image of the vesicles deformed by the external force of the AFM
tip. The insets in (B) and (E) are magnified images.
confirmed by the holes observed in the particles (gray arrow
in Figure 2B; see also Figure S8). The average size of the
vesicles according to the AFM and TEM images is approx-
imately 120 nm, which is consistent with the hydrodynamic
diameter (Dh) of approximately 123 nm determined by DLS
(Figure 2C). The vesicles are flexible and can deform along
the force direction under the pressure of the AFM tip even in
a dried state (see Figure S9).[11] They are also very stable and
can be kept in water without any change in either morphology
or size for at least half a year. The vesicles are also stable in
phosphate-buffered saline (PBS) buffer (pH 7.4) at 378C
according to the real-time DLS analysis (see Figure S10).
The molecular packing structure in the vesicles was
further studied by TEM and UV spectroscopy. The average
vesicle-wall thickness, as determined from TEM images on
the basis of 60 vesicles, is approximately 15 nm. The PDMA-
Azo chain in PEG45-b-P(DMA-Azo)47 was calculated to be
about 12 nm if it adopts a rigid conformation. Therefore, the
diblock copolymers are thought to pack in an interdigitated
arrangement to form a bilayer structure. As further evidence
for this packing mode, the UV absorption spectrum of the
vesicles in water showed a pronounced blue shift when
compared to that obtained in the good solvent THF (Fig-
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The groups of Eisenberg,[13] Lecommandoux,[14] Klok,[14]
Armes,[15] and others[16] have done excellent work on the
development of stimuli-responsive vesicle systems. The
polymeric vesicles obtained in this study demonstrated
a pH-induced “breathing” behavior. They are pH-responsive
since the DMA-Azo groups have a pKa value of about 3.6.[17]
At an initial pH value of 7, the average vesicle diameter is
about 120 nm, and the vesicle-wall thickness is about 15 nm.
When the pH value decreased to 4, AFM showed a diameter
of approximately 236 nm (Figure 2D), TEM a diameter of
approximately 240 nm (Figure 2E), and DLS a diameter of
approximately 233 nm (Figure 2F). Thus,
a remarkable
expansion of the vesicle volume by a factor of about 7
occurs. Meanwhile, the vesicle-wall thickness was halved to
approximately 8 nm according to the TEM image (Fig-
ure 2E). Furthermore, when the pH value was increased to
12, the vesicles shrunk to nearly their initial size (ca. 112 nm),
and their walls reverted back to their original thickness (ca.
15 nm). This vesicle expansion and shrinkage accompanied by
the thinning and thickening of the vesicle walls is reversible
for at least four cycles upon the alternate addition of HCl and
NaOH to switch the pH value between 4 and 12 (Figure 3A).
The vesicle morphology also remained constant during the
four cycles according to the TEM (see Figure S11) and AFM
images obtained afterwards (see Figure S13). Such highly
reversible pH-induced swelling and shrinkage of the vesicles
is reminiscent of the reported “breathing” behaviors of
vesicle systems.[13b,16b] In this case, the acid-induced swelling
can be regarded as a “breathing in” process, whereas the base-
induced shrinkage is a “breathing out” process.
The “breathing” mechanism of the vesicles can be
explained as shown in Figure 3B. The vesicles possess
a compact or thick bilayer wall structure at pH 7 or 12.
When the pH value is decreased to 4, the PDMA-Azo
segments are protonated (about a 33% degree of protona-
tion) and change from hydrophobic to hydrophilic. As
a result, the interchain electrostatic repulsive force compels
the vesicles to expand to some extent to lower their
interaction free energy. This expansion corresponds to vesicle
swelling, the so-called “breathing in” process. Meanwhile,
spreading of the protonated PDMA-Azo chains from the
vesicle core to the corona leads to thinning of the vesicle wall.
However, the vesicles are not disassembled at pH 4 owing to
the protonation degree of the PDMA-Azo chains of only
33%. Inversely, when the pH is increased to 12, a deprotona-
tion process occurs. Subsequently, the electrostatic repulsive
force in the vesicles decreases and finally disappears, and the
PDMA-Azo chains spreading in the vesicle coronae become
hydrophobic again and retract back into the vesicle cores.
Thus, vesicle shrinkage and vesicle-wall thickening occur; the
vesicles revert back to their original state in the “breathing
out” process.
Figure 3. A) Reversible size change of the vesicles as detected by DLS
upon the alternate addition of HCl (pH 4) and NaOH (pH 12). Black
bars represent the mean values (n=4). B) Schematic representation of
the pH-induced reversible “breathing” process of the vesicles upon the
protonation and deprotonation of PDMA-Azo blocks in response to
the pH value of the solution. C) UV/Vis spectra of PEG-b-PDMA-Azo in
THF and in H2O (c=0.05 mgmLÀ1). D) Fluorescence decay curve of
the vesicles in H2O (c=0.20 mgmLÀ1) with excitation at 380 nm and
detection at 536 nm after proper deconvolution of the instrument
response function (IRF). E) Steady-state fluorescence spectra of the
copolymer in H2O/THF mixed solvents with different volume ratios
(c=0.20 mgmLÀ1) at an excitation wavelength of 380 nm. The insets
in (E) are fluorescent images of the solutions.
ure 3C). This result indicates that the p-conjugated DMA-
Azo chromophores form an H-type aggregate in the vesi-
cles.[12] However, since the UV absorption band is relatively
broad and weak, the DMA-Azo groups in the vesicles may be
arranged in a less optimal mode of H-aggregation. On the
basis of these data, we propose the vesicular self-assembly
mechanism shown in Figure 1C.
The so-prepared PEG-b-PDMA-Azo vesicles also dem-
onstrate interesting fluorescence properties. PEG-b-PDMA-
Azo is nonfluorescent in the good solvent THF owing to the
non-emissive nature of azobenzene.[18] However, the fluores-
cence of the copolymer solution was enhanced remarkably by
the addition of water (Figure 3E). The higher the water
content, the stronger was the fluorescence intensity. The
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fluorescence was enhanced by a factor of about 130 at a water
volume ratio of 95%. The introduction of water leads to
vesicular self-assembly, and the formation of vesicles triggers
the enhanced fluorescence. Evidently, this diblock copolymer
has the characteristics of aggregation-induced emission (AIE)
systems.[19] In good agreement with the fluorescence analyses,
the photographs in the inset of Figure 3E clearly show that
the solution of the diblock copolymer in THF hardly exhibits
any apparent fluorescence, whereas the aqueous vesicular
solution presents strong green fluorescence. As further
evidence, Figure 3D shows that the vesicles in water have
a lifetime of about 2.2 ns. In contrast, owing to its extremely
weak fluorescence, the fluorescence lifetime of PEG-b-
PDMA-Azo in THF could not be determined.
The AIE effect of the obtained vesicles can be explained
in terms of three factors. First, the intramolecular vibrational
or torsional motions of DMA-Azo groups are greatly
restricted after vesicle formation as a result of the stacking
of hydrophobic PDMA-Azo chains, which is beneficial to the
enhancement of fluorescence efficiency.[19] Second, the trans-
to-cis photoisomerization is hindered to some extent in the
vesicles, since the azobenzene chromophores are confined in
the bilayer membranes. Thus, the efficiency of nonradiative
relaxation is greatly reduced, which leads to the strong
fluorescence.[20] Third, as mentioned above, although the
DMA-Azo chromophores form H-aggregates in the vesicles,
they are relatively disordered or twisted, which is also
beneficial to the enhancement of fluorescence.
Next, we explored the fluorescence properties of the
vesicles in response to the pH-induced “breathing” process. In
the UV spectrum of the vesicular solution (Figure 4A), an
enhanced absorption band around 560 nm (black arrow) was
observed at pH 4, and this band disappeared again as the
pH value was adjusted to 12. Accordingly, the solution also
changed from brown at pH 4 to orange at pH 12 (inset in
Figure 4A). The orange solution exhibited strong green
fluorescence, whereas only very weak fluorescence signals
were observed from the brown solution (Figure 4B). Fig-
ure 4C shows the corresponding fluorescence spectra for
these processes. Upon the addition of HCl, the fluorescence
intensity of the vesicular solution gradually decreased; it was
lower than the initial value by a factor of approximately 4.5 at
pH 4. The fluorescence intensity then increased gradually as
the pH value was increased to 12 by the addition of NaOH.
The fluorescence quenching and recovery of the vesicular
solution can be recycled reversibly many times by the
alternate addition of HCl and NaOH (Figure 4D). Thus, the
vesicle system exhibits a pH-induced fluorescence quenching
and recovery in response to the “breathing” process. The
fluorescence is turned off when the vesicle breathes in at
pH 4, and the fluorescence is turned on when the vesicle
breathes out at pH 12. This behavior is very similar to the
breathing process of a jellyfish (Figure 1A).
Figure 4. A) UV/Vis spectra of the vesicles at pH 4, 7, and 12
(c=0.05 mgmLÀ1). The inset shows photographs of the vesicular
solution at pH 4 (right) and 12 (left). B) Schematic representation of
the protonation and deprotonation of PEG-b-PDMA-Azo in the vesicles
by the alternate addition of HCl and NaOH. Fluorescent images of the
vesicular solution at pH 4 (right) and 12 (left) are also shown.
C) Steady-state fluorescence spectra of the vesicular solution at
pH values between 4 and 12 at an excitation wavelength of 380 nm.
D) Reversible pH-induced change in the fluorescence of the polymer at
536 nm upon the alternate addition of HCl (pH 4) and NaOH (pH 12;
c=0.20 mgmLÀ1).
The central question is the mechanism for the pH-
dependent fluorescence properties. First, we believe the
fluorescence quenching under acidic conditions is related to
protonation and the formation of charge-transfer complexes
between the vesicle-forming polymers. The protonated
DMA-Azo chromophores (DMAH+-Azo) have a high elec-
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Chemie
Angew. Chem. 2002, 114, 1406 – 1408; Angew. Chem. Int. Ed.
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tron-accepting ability, whereas the unprotonated DMA-Azo
chromophores have an electron-donating ability. Therefore,
the polymers tend to form charge-transfer complexes based
on donor–acceptor interactions (Figure 4B); the formation of
these complexes leads to fluorescence quenching.[21] The
occurrence of such a mechanism involving complex formation
was proved by UV/Vis and steady-state-fluorescence meas-
urements of a solution of the PEG-b-PDMA-Azo monomer
in THF upon the addition of TFA (see Figure S16). The
fluorescence quenching of the vesicles under acidic conditions
might also be related to the increase in conformational
freedom of the polymers in the vesicles as a result of
protonation. Both vesicle swelling and vesicle-wall thinning
occur under acidic conditions, and thus the restriction of
azobenzene chromophores in the vesicle core decreases. As is
characteristic of AIE system, such a change in the molecular
packing of the vesicles will result in fluorescence quenching.
In summary, we have prepared a novel class of polymeric
vesicles that exhibit a pH-induced “breathing” behavior
accompanied by jellyfish-like on–off switchable fluorescence
through the aqueous self-assembly of a diblock copolymer of
PEG-b-PDMA-Azo. During the “breathing in” process under
acidic conditions, swelling of the vesicles and thinning of the
vesicle walls are accompanied by fluorescence quenching as
a result of the protonation process. Inversely, during the
“breathing out” process under alkaline conditions, shrinking
of the vesicles and thickening of the vesicle walls are
accompanied by recovery of the strong green fluorescence
as a result of the deprotonation process. The occurrence of
changes in the light-emitting properties of the polymer upon
membrane deformation during the breathing process is
reminiscent of the breathing behavior of jellyfish. Therefore,
we believe we have developed a smart vesicle system with
controllable and dynamic functions. More importantly, this
study may extend cytomimetic chemistry from the mere
morphological transformation of membranes to a combina-
tion of cytomimetic morphology with the concomitant
expression of a function.
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Received: August 8, 2012
Revised: September 12, 2012
Published online: October 12, 2012
Keywords: block copolymers · breathing · fluorescence ·
.
self-assembly · vesicles
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