Water tuned nano/micro-structures in a redox-responsive supramolecular gel

Article abstract of DOI:10.1039/c4ra12247j

A redox-responsive chiral supramolecular gel based on coumarin-tailed cholesterol linked with disulfide was reported. It was found that the gelation was primarily driven by the combination of hydrogen bonding, π-π stacking, and van der Waals forces. Moreover, its assembled morphology could be regulated from nanofibers to micro flowers and ribbons by water on account of its amphiphilic nature.

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disulfide has already been adopted in the self-healing materials,
controlled release, and biosensor.9 Up to date, although many
redox-responsive LMOGs based on ferrocene group,10
Cu(I)/Cu(II),3i or tetrathiafulvalene (TTF) group3j have been
reported, the disulfide involved redox-responsive LMOGs are
still rare.11
Introduction
Supramolecular gels, which utilize the self-assembly of organic
molecules into entangled structures to immobilize the solvents,
have attracted immense interests over the past decades for their
potential applications in drug delivery, hybrid materials, oil
recovery, and electronic devices.1 In the family of
supramolecular gels, low-molecular-weight organogels
(LMOGs) show more superiorities because of their accurate
Results and Discussion
molecular
nanostructures, and stimuli-responsive properties.2 Specifically, Scheme 1. Reaction between cystamine dihydrochloride and
the stimuli-responsive gels are particularly appealing since their Boc anhydride afforded mono-protected amine 2 12
followed by
morphologies can be regulated easily in response to the external the treatment with coumarin-3-carboxylic acid chloride to give
stimuli, such as light, pH, ultrasound, ion, and . Subsequently was deprotected by trifluoroacetic acid to
oxidation/reduction.3 Although various assembly morphologies give
. Finally, the reaction between and cholesteryl
weight,
easy
modification,
multifarious The gelator 1 was synthesized in multiple steps as shown in
,
3
3
4
4
of gels have been reported, including fibers, ribbons, sheets, chloroformate was carried out in the presence of triethylamine
tubes, and flowers,4 the regulation of the morphologies is still to yield the coumarin-cholesterol conjugate
1, which was
challengeable. For example, Liu and co-workers reported that confirmed by H-, 13C NMR, ESI-MS, and HRMS (details see
metal ions including Na+, Li+, Ni2+, Eu3+ and Tb3+ could turn Supporting Information).
the assembled nanostructure into a uniform helical twist in
1
organogel.5a They also found that water could be used to tune
the assembled nanostructures from nanofibers to helical tapes,
helical tubes, and chiral nanotwists in organic solvents.5b Fang
and co-workers discovered nanofibers and microballs could be
obtained by changing the volume ratio of pyridine-methanol.6
As a continuance of our previous works,7 herein we
exploited
a novel redox-responsive gelator 1 based on
coumarin-tailed cholesterol linked with disulfide (Scheme 1)
and its self-assembly behavior was investigated (Figure 1). In
such a molecule, coumarin, cholesterol, and disulfide are used
as the sensor, gelation skeleton, and redox-responsive linker,
respectively. It is known that disulfide can be reduced by
dithiothreitol (DTT), glutathione (GSH) or PPh3 to sulfhydryl,
which could be oxidized back to disulfide with the treatment of
oxidants, such as oxygen, peroxide, and iodine.8 Therefore, Scheme 1 The synthetic route of compound
1
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Figure 1 Schematic illustration of the self-assembly process of the aggregates and the redox-responsive gel-sol transition triggered by DTT. The insets are photographs
of gel and collapsed sol.
As a general protocol,
and tested by “stable to inversion of the test tube” method.7b, 13 generated through the assembly of chiral molecules.15 The CD
The results showed that can form stable gel in DMF/H2O spectra of revealed a strong positive Cotton effect at 358 nm
1
was dissolved in various solvents
，
Generally, the chirality of supramolecular systems can be
1
1
from the ratio of volume 20:1 to 5:1 (Table S1 and Figure S1). in gel state, while no signal was detected in solution (Figure 2c).
Its viscoelastic behaviour was characterized by the rheological The signal intensity of the gel decreased gradually until
measurements, in which the storage modulus G' and the loss disappeared completely with the temperature increasing from
modulus G'' were measured as functions of angle frequency and
o
time sweep at 25 C, respectively. As shown in Figure 2a, G' is
around eight times greater than G'', revealing the indispensable
14
elastic behaviour of the gel.7c,
The fact that G' and G''
decreased with the extension of time indicates that gel
collapsed into the quasi-liquid state gradually (Figure 2b).14a
Moreover, the concentration-dependent sweep shows that G'
and G'' ascended with the increase of gelator concentration
(Figure S2).
Figure 2 Evolution of G' (black cube) and G'' (red dot) as functions of the (a) angle
frequency, and (b) time of the gel
1
(13 mg/mL, DMF/H2O =10:1, v/v); (c) CD
(8
spectra of in solution and gel; (d) temperature-dependent CD spectra of
1
1
Figure 3 SEM images of xerogels from
DMF/H2O (v/v): (a) 25:1, (b) 20:1, (c) 10:1, (d) 6:1, (e) 5:1, (f) 4:1; (g) TEM, and (h)
AFM images of xerogels from (10 mg/mL, DMF/H2O=10:1, v/v). Scale bars are
1 (10 mg/mL) at different volume ratio of
mg/mL, DMF/H2O=10:1, v/v).
1
500 nm for (a), 2 μm for (b, g), 5 μm for (c, d, e), 10 μm for (f), and 1 μm for (h).
2 | J. Name., 2012, 00, 1-3
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20 to 50 oC (Figure 2d). It indicates that the chirality is
transformed from gelator to the assembled aggregates during
its chiral packing process.5, 15
1
It is clearly seen that the gel turned opaque gradually with
the increase of water (Figure S1). Studies on the thermal
stability revealed that with the increase of water the minimum
gelation concentration (MGC) decreased, while the gel-to-sol
transition temperature (Tgel) increased (Figure S3). In order to
investigate how the water tunes the gel morphologies, scanning
electron microscopy (SEM), transmission electron microscopy
(TEM) and atomic force microscopy (AFM) were performed.
The results showed that a partial gel formed with the nanofibers
~30-50 nm in diameter when water is less than 5% in
DMF/H2O (Figure 3a). Whereas a few micro-scale flower-like
structures appeared co-existing with the nanofibers in the stable
gel when the ratio of water reached 5% (Figure 3b). The
number of uniformed micro-scale flower-like architectures
increased continuously with the increase of water in the mixed
solvents (Figure 3c). The flower-like architectures are ~5-40
μm in diameter, and the thickness of the ‘flower petals’ (arrow
in figure 3c) is ~30-60 nm, which is close to fiber diameter.
However, the further addition of water (6:1) led to collapse of
the flowers, consequently generating micro-scale thick ribbons
until all the flowers disappeared (Figure 3d-e). When water was
more than 20% in DMF/H2O, ribbons broke into fragments
resulting in the collapse of gel (Figure 3f). Moreover, the co-
existing of nanofibers and micro-flowers in Figure 3c can be
further confirmed by TEM and AFM (Figure 3g-h), in which
both of them have the similar sizes as the ones under SEM.
Apparently, the transformation from nanofibers to microflowers
is a result of the anisotropic arrangement of molecules induced
by the water content, consequently leading to the
transformation between partial gel, stable gel, opaque gel, and
collapsed gel.
Figure 4 (a) Temperature-dependent 1H NMR (600MHz) of
d7/D2O (10:1, v/v); (b) normalized UV-Vis and (c) fluorescence spectra of
M) in different volume ratio of DMF/H2O; (d) IR spectra of in solid and xerogel
(10 mg/mL) made from DMF/H2O (10:1, v/v); (e) XRD patterns of the xerogel (10
mg/mL) from in DMF/H2O (10:1, v/v). The inset is calculated 3D structure of 1.
1
(10 mg/mL) in DMF-
(10-4
1
1
To study the driving forces in the gelation process of 1, we
1
1
investigated the temperature-dependent H NMR, IR, UV-Vis,
and fluorescence spectroscopy. As the temperature increases,
the protons of coumarin (H1, H2, H3, H4, H5) shifted upfield
gradually as a result of the shielding effect, which strongly
revealed the π-π stacking between the coumarin moieties
(Figure 4a), while no change was observed of the aliphatic
protons (Figure S4). UV-Vis and fluorescence spectra provided
more information about the π-π stacking. As shown in Figure
4b, the absorption bands underwent a significant red shift from
295 to 300 nm with the increase of water content. Meanwhile,
Carbon atoms are presented in grey, oxygen atoms in red, nitrogen atoms in blue,
sulfur atoms in yellow, and hydrogens in white.
Additionally, IR spectra of a powder sample and xerogel of
1
were compared (Figure 4d). It showed that the stretching
vibrations of N-H moiety and C=O shifted from 3325, 1718 cm-
1 to 3333, 1721 cm-1, respectively, upon the gelation. This result
implied the formation of the intermolecular hydrogen bonding
during the gelation process.4b Furthermore, the C-H stretching
vibrations of alkyl groups appeared at a lower wavenumber
(2936 cm-1) in the xerogel compared with that in the powder
sample (2950 cm-1). It is known that the stretching vibrations of
alkyl groups can be used as a sensitive sensor for the order of
alkyl chains because of the association between the increase in
frequencies and band widths with the increasing numbers of
gauche defects and disorder.17 Therefore, the decrease in
wavenumbers from the powder sample to xerogel revealed an
increase of the van der Waals interactions among the
neighboring alkyl chains. It is apparent that the intermolecular
hydrogen bonding and van der Waals forces of alkyl chains
the addition of water to DMF solution of
1 led to a gradual
decrease in the intensity at 413 nm and an appearance of a new
band at 460 nm in fluorescence spectra (Figure 4c). Apparently,
the red shift ~47 nm of emission band should be due to the
excimer formation caused by π-π stacking between coumarin
groups.14a Both red shifts in UV-Vis and fluorescence spectra
strongly indicate the formation of “J”-type aggregates.16
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microflowers and microribbons on account of its amphiphilic
characteristic. Moreover, such supramolecular gel exhibited
excellent redox-responsive properties, which may be used in
controlled release and drug delivery. Our results point out the
potential self-assembled morphological control in area of
supramolecular gel.
Figure 5 (a) Molecular structure of compound 5; (b) SEM image of xerogel from 5
in DMF/H2O (10:1, v/v, 10 mg/mL), scale bar is 10 μm.
Acknowledgements
play crucial roles in the formation of supramolecular gel
besides the π-π stacking between the aromatic rings.
This work was supported by NNSF of China (nos. 21172130,
21472108) and NBRP of China (no. 2012CB8210601). JH is
thankful for the support of State Key Laboratory of Polymer
Physics and Chemistry, CIAC, China.
X-ray diffraction (XRD) afforded the direct insight into the
molecular packing pattern. As shown in Figure 4e, the
reflection peaks corresponding to d-spacing of 3.16, 1.58, 1.05,
0.79, 0.64 and 0.53 nm in the small angle region were observed,
following the ratio of 1: 1/2: 1/3: 1/4: 1/5: 1/6. It strongly
reveals the lamellar structure of the supramolecular gel. The
fact that the interlayer distance (3.16 nm) is close to the
Notes and references
a
Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical
Biology, Ministry of Education, Department of Chemistry, Tsinghua
University, Beijing 100084, China. E-mail: juyong@tsinghua.edu.cn.
extended molecular length of
1 (3.22 nm, Figure 4e, inset)
b
State Key Lab of Polymer Physics and Chemistry, Changchun Institute
indicates that the “J”-type aggregates are formed by using the
of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022,
China. E-mail: jhu@ciac.ac.cn
single molecule as a basic building block. Based on all the
above results, it is apparent that the free molecule
1 in DMF is
†
Electronic Supplementary Information (ESI) available: Synthesis and
structure characterization of compounds and ; Gelation behaviour of
in various solvents; Plots of Tgel against the concentration of
Temperature-dependent 1H NMR of
Concentration-dependent
rheological measurements of the gel; SEM images of ; ESI-MS
after the DTT addition. See DOI: 10.1039/
promoted to form ordered fibers upon the addition of water
primarily driven by van der Waals forces, hydrogen bondings,
and π-π stacking. These fibers can further entangle each other
to form 3D nest-like networks to trap solvent molecules (Figure
1, left). Whereas, on account of its amphiphilic nature, the
fibers tend to arrange more closely to form flower-like
aggregates bearing less hydrophobic areas with the increase of
water in the system.
1
5
1
1
;
1
;
5
spectrum of the gel
c000000x/
1
1
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We have exploited
a
novel redox-responsive chiral
supramolecular gel based on coumarin-tailed cholesterol linked
with disulfide, primarily driven by the combination of hydrogen
bonding, π-π stacking, and van der Waals forces. The gel
morphology could be regulated by water from nanofibers to
,
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