Cation-tuned stimuli-responsive and optical properties of supramolecular hydrogels

Article abstract of DOI:10.1002/asia.201500274

Hierarchical self-assembly of an amphiphilic tris-urea in aqueous media is shown. A mixture of the amphiphilic tris-urea and an alkaline solution gave a viscous solution composed of fibrous aggregates. This viscous solution transformed into supramolecular

Full text of DOI:10.1002/asia.201500274

DOI: 10.1002/asia.201500274
Communication
Self-Assembly
Cation-Tuned Stimuli-Responsive and Optical Properties of
Supramolecular Hydrogels
[
a]
[a]
[b]
[b]
Masamichi Yamanaka,* Kazushige Yanai, Yusuke Zama, Junko Tsuchiyagaito,
[c]
[b]
[b]
Masaru Yoshida, Ayumi Ishii, and Miki Hasegawa*
tion of three-dimensionally intertwined noncovalent net-
Abstract: Hierarchical self-assembly of an amphiphilic tris-
urea in aqueous media is shown. A mixture of the amphi-
philic tris-urea and an alkaline solution gave a viscous so-
lution composed of fibrous aggregates. This viscous solu-
tion transformed into supramolecular hydrogels, which
are capable of hierarchically organizing into higher-order
aggregates in response to several cationic triggers. The re-
sulting supramolecular hydrogels were relatively stiff and
[9–11]
works.
Supramolecular hydrogel formation is also a hier-
archical self-assembly, and the typical mechanism is as follows.
Regulated assembly of LMWHGs gives supramolecular polymer
strands at an early stage. Bundling and cross-linking of the
supramolecular polymer strands at a late stage provides a hy-
drogel. Both stages are integral for gelation, and hindered
progress of either stage does not afford a hydrogel. Various
supramolecular hydrogels that show a gel–sol phase transition
responsive to external stimuli, such as light, pH, and chemicals,
have been reported. Most of these phase transitions are ach-
ieved by regulating the formation of the supramolecular poly-
3
their storage moduli attained over 10 Pa. The stimuli-re-
sponsive and optical properties of the resulting hydrogels
were influenced by the cationic trigger. Proton and calci-
um ion triggers gave pH- and chemical stimuli-responsive
hydrogels, respectively. A terbium ion trigger also provid-
ed a highly luminescent hydrogel through energy transfer
from the tris-urea to terbium.
[12–20]
mer strands (the early stage of the gelation).
By contrast,
supramolecular hydrogel formation controlled at the late stage
[21,22]
of gelation is relatively rare.
In this paper, we report the hi-
erarchical self-assembly of a hexa-carboxylated amphiphilic
tris-urea. The supramolecular polymers constructed from tris-
urea in alkaline aqueous media transformed into supramolec-
ular hydrogels in response to several cationic triggers. The
characteristics of the resulting hydrogels were dependent on
the trigger. The proton-induced hydrogel showed reversible
Self-assembly is an effective and convenient method for pro-
[
1,2]
ducing complicated nano-architectures.
Supramolecular
polymers constructed from continuous self-assembly of well-
designed small molecules have attracted increased attention
not only for their attractive structure but also their potential
[21,22]
gel–sol phase transitions relative to pH.
Chemical stimulus
2
+
responsive characteristic was obtained for a Ca -induced hy-
[
3–5]
[25]
3+
applicability in a diverse range of fields.
One apparent chal-
drogel, while Tb -induced hydrogels showed a luminescent
[26–30]
lenge in supramolecular polymer chemistry is the control of
the hierarchy levels of self-assembly through external stimu-
property.
We have developed C -symmetrical tris-urea-based LMWHGs
3
[6–8]
[31–34]
li.
which yielded various types of supramolecular hydrogels.
Small molecules, that is, low-molecular-weight hydrogelators
On the basis of these structural features, we designed a C3-
symmetrical tris-urea with hexa-carboxylic acid 1. The inner hy-
drophobic and the outer hydrophilic structure of 1 induce
one-dimensional self-assembly into a supramolecular polymer
in aqueous media primarily through hydrophobic interaction.
The deprotonation ratio of the carboxyl groups of the tris-urea
alters the hydrophilicity of the supramolecular polymer. Syn-
(
LMWHGs) can immobilize an aqueous medium via the forma-
[
a] Prof. M. Yamanaka, K. Yanai
Department of Chemistry
Graduate School of Science
Shizuoka University
8
36 Ohya, Suruga-ku, Shizuoka 422-8529 (Japan)
Fax: (+81)54-237-3384
E-mail: yamanaka.masamichi@shizuoka.ac.jp
thesis of tris-urea 1 was achieved by condensation of triamine
[35]
2
and dimethyl 5-aminoisophthalate 3 in the presence of tri-
[
b] Y. Zama, J. Tsuchiyagaito, Dr. A. Ishii, Prof. M. Hasegawa
phosgene, followed by ester hydrolysis under alkaline condi-
College of Science and Engineering
Aoyama Gakuin University
1
tions (Scheme 1). The structure of 1 was identified by H and
13
5
-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258 (Japan)
C NMR spectroscopy and electrospray ionization mass spec-
E-mail: hasemiki@chem.aoyama.ac.jp
trometry (ESI-MS).
[
c] Dr. M. Yoshida
Mixtures of 1 in water or acidic solutions gave insoluble sus-
pensions even after heating. Conversely, a slightly viscous ho-
mogeneous fluid was obtained from heating a mixture of
Research Institute for Sustainable Chemistry
National Institute of Advanced Industrial Science and Technology (AIST)
3
-11-32 Kagamiyama, Higashihiroshima, Hiroshima 739-0046 (Japan)
1
and NaOH solution (10 mm each) (Figure 1a). Field-emission
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/asia.201500274.
scanning electron microscopy (FE-SEM) imaging of the mixture
Chem. Asian J. 2015, 10, 1299 – 1303
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Communication
1
showed uniform intertwining
nanofibers with diameters rang-
ing from 30 to 90 nm (Fig-
ure 1b). The fibrous aggregates
found in the viscous fluid (Fig-
ure 1a) and in the hydrogel (Fig-
ure 1b) resemble each other
substantially. This supports the
fact that hydrogelation is ach-
ieved by hierarchical self-assem-
bly to form cross-linked nanofib-
ers in response to a proton trig-
ger. Once formed, the hydrogel
of 1 was stable at ambient tem-
perature for at least one year.
The minimum gelation concen-
tration (MGC) of 1 was estimated
to be 3.0 mm. Hydrogelation of
Scheme 1. Synthesis of tris-urea 1.
equimolar fluids of 1 and NaOH
were induced by various acids,
such as sulfuric, nitric, and acetic acid. A reversible gel–sol
phase transition was accomplished by controlling the pH of
the mixture. Addition of an alkaline solution to the hydrogel
gave a viscous fluid. Regelation proceeded by adding an acidic
solution to the fluid. This hydrogelation is achieved by mixing
only two fluids, that is, an aqueous mixture of 1 and NaOH,
and an acidic solution, at ambient temperature. Therefore,
a shaped hydrogel could be easily prepared by using an ap-
propriate mold. The addition of aqueous HCl to an equimolar
mixture of 1 and NaOH (10 mm each) in a heart-shaped mold
gave a heart-shaped hydrogel (Figure S1, Supporting Informa-
tion). This hydrogel had sufficient stiffness to be picked up by
hand without degradation. Hydrogel droplets were formed by
adding droplets of aqueous HCl to an excess of an aqueous
mixture of 1 and NaOH. The hydrogel droplets in the mixture
retained their shape without swelling or diffusion (Figure S2,
Supporting Information).
Figure 1. Photographs (inset) and FESEM images of mixtures of a) 1 and
NaOH (10 mm each in H O); b) a)+HCl (0.75 equiv. to 1).
2
showed nanofibers with diameters ranging from 30 to 90 nm
Figure 1a). The fluid mixture was weakly acidic (pH 5.9). Based
on the pK values of isophthalic acid (pK =3.50, pK =4.50),
(
a
a1
a2
almost all of the carboxyl groups would be deprotonated at
that pH value. Electrostatic repulsion between the negatively
charged nanofibers would prevent further cross-linking in
higher hierarchy levels, thus disfavoring the formation of a hy-
drogel.
The viscoelastic features of the hydrogel were measured
using a rheometer. The storage moduli (G’) of the hydrogel of
1 were approximately seven to ten times higher than the loss
moduli (G’’). Both types of moduli were almost independent of
Acidification of the mixture causes protonation of the car-
boxylate groups in the nanofiber of 1. Consequently, attractive
carboxyl–carboxyl and carboxyl–carboxylate interactions be-
tween nanofibers increase the hierarchy of the self-assembly
to form a hydrogel state. An equimolar fluid mixture of 1 and
NaOH (10 mm each) was spontaneously transformed into a hy-
drogel upon addition of an appropriate amount of HCl
À1
the frequency from 0.01 to 10 rads . As shown in Figure 2a,
the G’ and G’’ values were dependent on the concentration of
À1
1. The G’ value at 1.0 rads was increased from 0.2 kPa to
4.2 kPa when the concentration of 1 was increased from 4 mm
À1
to 10 mm. A G’ value of 8.7 kPa (at 1.0 rads ) was observed in
a hydrogel with 15 mm of 1. Strain sweeps of the hydrogels
demonstrated an elastic response typical of supramolecular hy-
drogels (Figure 2b).
(
0.75 equiv. to 1) (Figure 1b). The addition of a large amount
of HCl (ꢀ0.75 equiv. to 1) also gave a hydrogel. However, the
addition of a small amount of HCl (<0.75 equiv. to 1) afforded
a partial gel. Acidic pH (ꢀ3.5) of the mixture was required to
form a hydrogel. Isophthalic acid groups of 1 existed as an
equivalent mixture of both protonated isophthalic acid and
mono-deprotonated isophthalate at pH 3.5. The resulting car-
boxyl–carboxyl and carboxyl–carboxylate interactions between
nanofibers would function as cross-links of the hydrogel. An
FESEM image of a xerogel of the semitransparent hydrogel of
Hydrogel formation in mixtures of 1 and alkaline solutions
were also triggered by a calcium salt. An equimolar fluid mix-
ture of 1 and NaOH (10 mm each) was spontaneously convert-
ed into a hydrogel by addition of CaCl (ꢀ0.5 equiv. to 1) (Fig-
2
ure 3a). The MGC of 1 was estimated to be 3 mm. A xerogel of
the CaCl -induced hydrogel of 1 showed uniform intertwining
2
of nanofibers that were approximately 90 nm in diameter (Fig-
ure 3a). The viscoelastic feature of the CaCl -induced hydrogel
2
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
tion). The G’ values of the CaCl -induced hydrogel of 1 were
2
slightly higher than those of the HCl-induced hydrogel of 1.
The G’ and G’’ values were independent of the concentrations
of CaCl , and similar rheological properties were observed in
2
hydrogels with various amounts of CaCl (Table S2, Supporting
2
Information).
2
+
Chelation between carboxylate groups and Ca raises the
hierarchy level of the self-assembly to a hydrogel state. The
gel–sol phase transition is initiated by adding [2,2,2]cryptand,
2
+
which forms a host–guest complex with the Ca and prevents
[36]
chelation within the assemblies of 1.
The addition of
5
equivalents of [2,2,2]cryptand to the hydrogel of 1, NaOH,
and CaCl (10 mm each) afforded a homogeneous fluid at am-
2
bient temperature within 5 min (Figure 3b). The hydrogel
spontaneously reconstructed with the addition of excess CaCl2
to the fluid.
Lanthanide ions also induced hydrogel formation in a mix-
ture of 1 and alkaline solutions, similar to the known fluores-
cent gels. The addition of more than 1 equivalent of Tb(OTf)3
(
OTf=CF SO ) to equimolar fluid mixtures of 1 and NaOH
3 3
(
10 mm each) spontaneously afforded hydrogels (Figure 4a). A
Figure 2. G’ and G’’values of a) HCl-induced hydrogel of 1 (4, 10, and
5 mm) with a frequency sweep; b) HCl-induced hydrogel of 1 (10 mm) with
a strain sweep; c) CaCl -induced hydrogel of 1 (10 mm) and Tb(OTf) -induced
Figure 4. a) Photograph (inset) and FESEM image of Tb(OTf)
gel of 1 (10 mm); b) Tb 3d XPS of a xerogel prepared from Tb(OTf)
hydrogel of 1 (solid line) and Tb(OTf) (dashed line).
3
-induced hydro-
1
3
-induced
2
3
3
hydrogel of 1 (10 mm) with a frequency sweep.
hydrogel of 1 with Gd(OTf) was also prepared. The MGC of
3
1
1
was estimated to be 3 mm. A xerogel of the hydrogel of
with Tb(OTf) also showed uniform intertwining of nanofibers
3
(
Figure 4a). Coordination bonds between carboxylate groups
3
+
and Tb raises the hierarchy level of the self-assembly to a hy-
drogel state. The viscoelastic feature of the Tb(OTf) -induced
3
hydrogel of 1 was measured. The G’ values of the Tb(OTf) -in-
3
duced hydrogel of 1 were approximately seven times higher
than the G’’ values, and both types of moduli were almost in-
À1
dependent of frequency from 0.01 to 10 rads (Figure 2c).
À1
The G’ value at 1.0 rads was 8.1 kPa in a hydrogel of 1 and
2
Figure 3. a) Photograph (inset) and FESEM image of the CaCl -induced hy-
Tb(OTf) (10 mm each). The G’ and G’’ values were independent
3
drogel of 1 (10 mm); b) chemical stimuli responsive gel–sol phase transition
of CaCl -induced hydrogel of 1.
of the concentrations of Tb(OTf) (Table S3, Supporting Infor-
3
2
mation). From the measurement of X-ray photoelectron spec-
troscopy (XPS), a xerogel prepared from Tb(OTf) -induced hy-
3
of 1 was measured. The G’ values of the CaCl -induced hydro-
drogel of 1 showed Tb 3d XPS bands at 1245 and 1280 eV (Fig-
2
gel of 1 were approximately seven times higher than the G’’
ure 4b). These bands appeared at a higher binding energy
values, and both types of moduli were almost independent of
side than those of Tb(OTf) alone (1243 and 1278 eV, respec-
3
À1
frequency from 0.01 to 10 rads (Figure 2c). The G’ and G’’
tively). Thus, the electron density localized on Tb in the hydro-
gel decreased in comparison with that in the case of Tb(OTf)3
due to coordination with 1. Coordination bond formation be-
tween Tb and 1 was also supported by the observation of N
values depended on the concentration of 1. The G’ values at
À1
1
.0 rads were 2.7, 5.6, and 9.5 kPa at concentrations of 4, 10,
and 15 mm of 1, respectively (Table S1, Supporting Informa-
1301
Chem. Asian J. 2015, 10, 1299 – 1303
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Tb ions intercalate into the
fibers; therefore, water does not
affect the luminescence even in
a hydrophilic atmosphere.
In conclusion, we have dem-
onstrated the hierarchical self-as-
sembly of amphiphilic tris-urea
1
a
1
. Cationic triggers transformed
homogeneous solution of
containing fibrous aggregates
into hydrogels. The characteris-
tics of the triggers were reflect-
ed in the properties of the hy-
2
+
drogels. Proton and Ca
trig-
gers gave pH- and chemical
stimuli-responsive hydrogels, re-
spectively. A luminescent hydro-
Figure 5. a) Photograph of UV irradiated Tb(OTf)
00 nm) and luminescence spectra (400–700 nm; lex =323 nm) of Tb(OTf)
the xerogel (dashed line); b) fluorescence (purple line) and phosphorescence (blue line) spectra of the Gd(OTf)
duced hydrogel of 1 (lex =323 nm); c) schematic representation of energy transfer pathway of Tb complex in the
hydrogel.
3
-induced hydrogel of 1 (insert) and electronic absorption (250–
-induced hydrogel of 1 (solid line) and
-in-
4
3
3+
gel was obtained by a Tb trig-
3
ger via the exited triplet state of
1
.
Acknowledgements
1s and C 1s XPS bands (Figure S3, Supporting Information).
Thus, the gelator 1 definitely coordinates with Tb ion even in
fibrous aggregates.
This work was supported by Grant-in-aid for the Scientific Re-
search on the Innovative Areas: ‘‘Fusion Materials’’ (Area No.
The Tb(OTf) -induced hydrogel of 1 and its xerogel showed
3
2206, No. 25107713 for MY, No. 25107730 for MH), Grant-in-aid
luminescence upon UV excitation (Figure 5a). Electronic ab-
for the Scientific Research (B) (No. 24310089 for MY), and the
Supported Program for the Strategic Research Foundation at
Private Universities (2013–2017 for MH).
sorption spectra localized on the p electronic system of the
Tb(OTf) -induced hydrogel of 1 appeared at about 290 and
3
3
23 nm as broad bands (Figure 5a). The ff emission bands at
5
4
89.2, 544.2, 582.7, and 620.8 nm were assigned to the D !
4
7
5
7
5
7
5
7
Keywords: energy transfer · gels · lanthanides · self-assembly ·
supramolecular polymers · ureas
F , D ! F , D ! F , and D ! F transitions, respectively
6
4
5
4
4
4
3
(l =323 nm). Absorption and luminescence bands of the xe-
ex
rogel well reproduced those of the hydrogel. Excitation spectra
monitored at each ff emission band corresponded to the elec-
tronic absorption bands (Figure S4, Supporting Information).
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Manuscript received: March 23, 2015
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