A dual thermal and photo-switchable shrinking-swelling supramolecular peptide dendron gel

Article abstract of DOI:10.1039/c5cc08076b

A supramolecular dendron gel was fabricated through the co-assembly of an l-glutamic acid terminated amphiphilic dendron and a positively charged azobenzene derivative, which showed a dual thermal and photo-switched reversible volume phase transition or s

Full text of DOI:10.1039/c5cc08076b

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A dual thermal and photo-switchable shrinking–
swelling supramolecular peptide dendron gel†
Cite this:DOI: 10.1039/c5cc08076b
Fan Xie,a Long Qina and Minghua Liu*ab
Received 27th September 2015,
Accepted 10th November 2015
DOI: 10.1039/c5cc08076b
www.rsc.org/chemcomm
A supramolecular dendron gel was fabricated through the co-assembly
of an L-glutamic acid terminated amphiphilic dendron and a positively
charged azobenzene derivative, which showed a dual thermal and
photo-switched reversible volume phase transition or shrinking/
swelling.
Supramolecular gels, in which small molecules self-assembled
into entangled nanostructures and immobilized the solvents,
have been attracting continuous interest due to their stimuli-
responsiveness and many potential applications as smart
materials.1–5 Various stimuli such as the pH, metal ions, light
and sound have been applied to the supramolecular gels and
gel-to-sol or sol-to-gel transition was successfully realized.
However, stimuli-responsive hydrogel materials with a macro-
scopic volume phase transition or shrinking/swelling proper-
ties, which are very necessary in consideration of application in
drug release,6–9 biosensing,10,11 controlled microvalves,12–14
and self-healing,15–18 have been scarcely realized in supra-
molecular gels due to the inherent non-covalent bond nature.
Only some examples are reported. Hamachi et al.8,19 realized a
thermal and pH-responsive supramolecular hydrogel by using
glycosylated amino acetate as the gelator. We have developed a
metal ion triggered peptide dendron gelator.20 However, in all
these systems, the triggers of shrinking and swelling are greatly
Fig. 1 (A) Molecular structure of OGAc and AZOC2Py. (B) Schematic
illustration of the phase transition in the co-assembled processes under
multiple stimuli (photographs show the S-gel, swollen Gel 1 and Gel 2
respectively). Gelator OGAc is 0.13 wt%, the molar ratio of OGAc and
AZOC2Py is 5 : 1.
The hydrogel system is composed of an amphiphilic den-
involved in the gel system. A more important and remote stimulus dron terminated with three L-glutamic acid groups (OGAC) and
such as light to cause the gel shrinkage or swelling has not been a positively charged azobenzene derivative, as shown in Fig. 1.
successful in supramolecular gel systems. Here, combining the When OGAc and AZOC2Py were mixed in certain ratios, they
shrinkable merits of our dendron hydro-gelator and the photo- formed a transparent hydrogel initially. When such a hydrogel
active azobenzene, we present a strategy to construct a reversible was kept at a room temperature of 20 1C, the gel underwent
shrinking and swelling supramolecular gel system triggered by shrinkage to form a shrinking gel (S-gel), during which water
photoirradiation.
was expelled. Interestingly, when the S-gel was irradiated using
a UV light, the S-gel was swollen and formed a gel again (Gel 2).
Gel 2 is very stable and will not change when kept at room
temperature. However, if Gel 2 was subjected to visible light,
then Gel 2 would change into the S-gel again. Such shrinking
and swelling between the S-gel and Gel 2 can be reversibly
switched by alternate Vis/UV irradiation many times. In addi-
tion, as the basic property of the gel, the S-gel can be thermally
a CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute
of Chemistry, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of
China
b Collaborative Innovation Centre of Chemical Science and Engineering, Tianjin,
P. R. China. E-mail: liumh@iccas.ac.cn; Tel: +86-10-82615803
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cc08076b
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showed a crossover at 357 nm, and positive and negative valleys
at 331 nm and 427 nm, respectively. When the S-gel was
subjected to the UV light stimuli, the trans-azobenzene trans-
formed into the cis-form, the CD signals totally disappeared.
This phenomenon is the same as those of the self-assembled
chiral system where azobenzene was involved.23 This is because
the cis-form of azobenzene is difficult to be packed well and the
molecular chirality localized in the carbon atom could not be
transferred to the whole system. Upon the visible light irradia-
tion with Gel 2, the CD signals can reappear again, as shown
in Fig. 2D. Such a process can be repeated many times and the
CD signals changed subsequently. Fig. 2B shows a plot of
the intensity change of the CD signal for the co-assembly as
a function of the exposure cycles. In one direction, the shrunken
hydrogel was regulated by heating/cooling to obtain the swollen
Gel 1 with the transition from exciton-split to the positive CD
signal. In another direction, the shrunken hydrogel could trans-
form to swollen Gel 2 with the CD-silent signal. Thus, compared
to the chiroptical switch manifesting reversible supra-molecular
chirality between the gel and sol,24,25 a dual thermo- and photo-
Fig. 2 UV-Vis (A) and CD (C and D) spectra of OGAc co-assembled with
AZOC2Py. Gel 1 (red, regular triangle), S-gel (pink, cycle), Gel 2 (orange,
inverted triangle). (B) CD intensity as a function of the irradiation cycle
(331 nm, top; 427 nm, bottom). UV light (150 W at 365 nm) and visible light
(40 W compact fluorescent lamp, 4400 nm).
changed to Gel 1 upon heating in solution and cooling down to driven chiroptical switch was realized displaying reversible chirality
room temperature. Therefore with this system, a dual thermal change based on trans–cis isomerization of azobenzene and the
and photo-switchable shrinking/swelling gel can be realized. So assembling to dissembling process in the gel.
far many research studies have been devoted to the volume
In order to further clarify the assembling structure and the
phase transition in polymer systems. Here, we present the first packing during the shrinking/swollen process, AFM and TEM
example of a photo-switchable shrinking/swelling gel in a supra- of the corresponding xerogels were investigated, as shown in
molecular system.
Fig. 3 and Fig. S1 (ESI†).
The reversible change in the gel states and the corresponding
Long nanofibers were observed for the as-prepared gel.
shrinking/swelling process can be visualized or followed by spectro- After shrinking, the fibers become thicker and straight (Fig. 3B).
scopy. Fig. 2A shows UV-Vis spectral change of different gels in
comparison with that of the AZOC2Py aqueous solution. In solution,
the absorption peak of AZOC2Py appeared at 350 nm and 430 nm,
respectively. In the UV-Vis spectra of Gel 1, which is as-prepared by
cooling the mixed solution of OGAC and AZOC2Py to room tem-
perature, the maximum peak appears at 346 nm, which can be
ascribed to the p–p* transition of the trans-form azobenzene moiety
in an aggregated state. Upon resting to form the shrinking gel
(S-gel), the peak intensity increased and shifted further to 344 nm
due to the expelling of water. When the shrinkage approached
equilibrium after nearly 12 hours, the shrinkage ratio was estimated
to be 60% (ESI,† Table S1). Upon irradiation of the S-gel by the UV
light, the peak at 344 nm decreased, while a new peak at 420 nm
can be observed, which can be ascribed to the n–p* transition of
cis-form azobenzene.21 This indicated that upon UV irradiation,
azobenzene experiences a trans to cis-form transformation and
Gel 2 was composed mostly of the cis-form azobenzene. Upon
irradiation using visible light, Gel 2 changed into the S-gel and
the corresponding UV-Vis spectra back to the same one.
The OGAc gelator is chiral, we have also investigated the CD
spectral changes during the shrinkage and swelling of the gel in
Fig. 2C and D since CD spectra can provide much information
on the molecular packing in the chiral supramolecular system.22
Fig. 3 AFM (left) and TEM (right) images of OGAc co-assembled with
AZOC2Py. (A) Gel 1, (B) S-gel, (C) Gel 2. The AFM images were 5 mm  5 mm
In the CD spectra, a positive CD signal at 337 nm was observed
for Gel 1, indicating that the molecular chirality of OGAc was
transferred to the achiral AZOC2Py moiety. For the shrunken gel,
the strong exciton couplet Cotton effect is observed, which
and middle column was 2 mm  2 mm (arrows with different colours
represented different diameters). All the TEM images are 200 nm  200 nm
and the inset images show amplified structures in which the arrows
indicated the diameters of nanofibers.
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For the swelling Gel 2, many helical structures are observed bilayer units and AZOC2Py was attached at the headgroup of
(Fig. 3C), which seemed to be due to the helical entanglement OGAC. Due to the additional AZOC2Py in the headgroup the
of the nanofibers. Such changes in the chiral structures reflected a bilayer did not roll into a nanotube. Instead, they formed the
slightly different packing of the molecules in the gels.26 AFM nanofiber structure, which is from a multi-bilayer structure
analysis indicated that these nanofibers fell in the range of several composed of OGAC/AZOC2Py, as confirmed from AFM and
nanometers. TEM measurements further confirmed the thickness TEM. For the as-prepared Gel 1, AZOC2Py distributed separately
of the nanofibers or nanohelices, which fell in the range of on the surface of each bilayer through the electrostatic inter-
6.9–11 nm, in correspondence with the AFM measurements.
actions, as confirmed by the CD spectrum of Gel 1, in which
The X-ray diffraction patterns of three xerogels were further only a positive CD signal was observed. Since the azobenzene
investigated, as shown in Fig. S4 (ESI†). One diffraction peak moiety is hydrophobic and has a strong p–p stacking tendency,
was observed for all these xerogels, indicating the less ordered it aggregated when Gel 1 was kept at room temperature. As a
packing of the gelator molecules. The two theta (2y) values were result, water molecules were expelled from the fiber and the gel
observed to be 2.08, 2.28 and 2.08, respectively. According to shrunk. This can be verified from two points. Evidence is
the Braggs equation, the d-spacings were estimated to be 4.2, obtained from the CD spectrum of the S-gel, in which a strong
3.9, and 4.2 nm for the as-prepared, S-gel, and Gel 2, respec- exciton couplet was observed due to the p–p stacking of the
tively. Previously, we have reported that OGAc itself could azobenzene moiety. The other evidence is the change in the
self-assemble into a bilayer structure with a layer distance of layer distance, which was diminished upon shrinking. In addi-
3.9 nm. From the similarity of the two systems, it can be tion, the slight shift of the absorption maximum confirmed
suggested that a bilayer structure was essentially kept in the the aggregation of the azobenzene. All these suggested that the
present gel. However, upon adding AZOC2Py, both alkyl chain azobenzene moiety was aggregated during the gel shrinkage.
and the packing of the azobenzene seemed to be more tilted.
Upon photo irradiation of the S-gel, the trans–cis isomerization
We have further investigated the FT-IR spectra of the xerogels, for azobenzene was experienced. This process causes the volume
as shown in Fig. S5 (ESI†). It was found that upon shrinking, both change of the azobenzene moiety and the surrounding water was
the vibration bands assigned to COOÀ at 1600 cmÀ1 became more taken into the bilayer again and the S-gel swell into Gel 2. If visible
obvious, indicating the electrostatic interaction between AZOC2Py light was applied to Gel 2, the gel shrunk again due to the
and OGAC.
isomerization of azobenzene. Since AZOC2Py was attached to the
Based on the above results, a possible mechanism for the gel headgroup of OGAC, during such a change, the basic bilayer
shrinking and swelling can be proposed, as shown in Fig. 4. structures were not destroyed and the neighboring fibers were
The amphiphilic gelator OGAC formed basically interdigitated connected. Different from most of the gels containing the azobenzene
moiety,23 the present gel showed swelling than collapsing due to the
strong interaction between the OGAc molecules. Thus, we obtained
unambiguously a reversible shrinking/swelling supramolecular gel
using alternative UV/Vis photo-irradiation.
It should be noted that Gel 2 is different from Gel 1. Gel 2
did not show any CD spectrum and the azobenzene moiety is in
the cis form, while Gel 1 showed CD spectra and the azobenzene
moiety is in the trans form. Only in the trans form the chirality of
the molecules can be transferred to the whole nanostructure.
Moreover, there is a reversible change between Gel 1 and the
S-gel upon thermal treatment since Gel 1 was formed by cooling
the solution to room temperature.
We demonstrated a reversible shrinking/swelling behavior in
the supramolecular gel upon photoirradiation and thermal switch.
The gel was composed of an amphiphilic dendron and a positively
charged azobenzene derivative. It was confirmed that the amphi-
Fig. 4 Illustration of the hierarchical co-assembly of an amphiphilic philic gelator and the azobenzene moiety formed a multi-bilayer
dendron (OGAc) with the azobenzene derivative (AZOC2Py). Top: (a) thin
fibrils formed in a fresh hydrogel. (b) Thick fibrous bundles via strong p–p
stacking upon shrinking of Gel 1. (c) Helices obtained in the swollen
structure through the electrostatic interaction. Upon resting the
as-prepared gel at room temperature, the azobenzene moiety experi-
enced aggregation, which caused the shrinkage of the gel. Upon
hydrogel after UV-photoirradiation. Bottom: the packing of the azoben-
photoirradiation of the shrinking gel, the trans–cis isomerization
zene and OGAc. OGAc formed a bilayer structure, in which the azoben-
zene moiety was embedded. In the as-prepared gel, the azobenzene occurred, which led to the swelling of the shrunken gel. Such a
moiety separately existed due to a lower ratio (AZOC2Py/OGAc, 1 : 5).
During the shrinkage of the gel, the p–p stacking of the trans-azobenzene
increased and the neighboring fiber assembled together. Upon trans–cis
isomerization by UV light, the layer distance was slightly expanded. However,
process can be repeated. As a result, using the combination of
OGAC and the azobenzene derivative, we obtained a dual thermal
and photo-switchable shrinking–swelling supramolecular peptide
dendron gel. The work provided a new strategy to develop photo
switchable smart soft materials with volume phase transition.
the whole gel structure was retained due to strong interaction between the
OGAc gelators, thus causing the swelling of the gel rather than collapsing.
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This work was supported by the National Natural Science
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