Stimuli-responsive supramolecular polymer network based on bi-pillar[5]arene for efficient adsorption of multiple organic dye contaminants

Article abstract of DOI:10.1039/d0nj02524k

A novel supramolecular polymer network gel (NT), based on bi-pillar[5]arene, which can adsorb and separate organic dyes, has been efficiently constructed. First, a novel supramolecular gelator, NBP5 (host), and a tripodal guest, TG (guest), were successfu

Full text of DOI:10.1039/d0nj02524k

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PAPER
Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
prepared via a microwave-assisted hydrothermal route using
H2O2 and KMnO4 as oxidizing agents
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Stimuli-responsive supramolecular polymer network based on bi-
pillar[5]arene for efficient multiple organic dye contaminants
adsorption
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Tai-Bao Wei*, Li-Hua Qi, Qin-Peng Zhang, Wen-Huan Zhang, Hong Yao, You-Ming Zhang and Qi
Lin*
A novel supramolecular polymer networks gel (NT) based on bi-pillar[5]arene, which could adsorb and separate organic
dyes were efficiently constructed. First, a novel supramolecular gelator, NBP5 (host), and a tripodal guest, TG (guest), were
successfully designed and synthesized. Then, host NBP5 and guest TG could construct a stable supramolecular polymer
2
networks gel, NT, via π-π interactions, hydrogen bonding interactions and host-guest interactions in DMSO/H O solution.
Interestingly, NT shows multi-responsiveness toward outer stimuli, such as temperature, mechanical, concentration, pH
and competitive guest. More importantly, NT could carry out adsorption and separation of organic dyes such as methylene
blue, crystal violet , methyl orange, orange I, sudan I and sudan Ⅱ by the complex intermolecular interactions. The
adsorption rates of this gels for dyes are in the range of 83.15%-97.56%, indicating nice adsorption property. Moreover,
the xerogel of NT could desorp in ethanol solution, showing good recyclability. The multi-responsiveness and adsorption
separation properties are based on the multi-interaction sites that we rationally introduced into the NT. Therefore, it is a
convenient way for the preparation of supramolecular polymer networks-based multiple functional smart materials.
9
suitability for diverse dyes. Up to now, many materials such as
1
0
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Introduction
graphene aerogels (GAs), clay and modified clay, activated
carbon, zeolites, synthetic fibers materials,14 metal organic
1
2
13
Organic dyes are widely used in industries such as food,
paper, cosmetics, pharmaceuticals, leather industries and so
on. However, many organic dyes are toxic and even
carcinogenic. Toxic dyes can enter the food chain through
water bodies, eventually affecting health of human and
animal. Moreover, even low concentrations (<1 ppm) of dyes
1
5
frameworks and so on, were explored as adsorbents to
remove organic dyes from wastewater. However, most of
those materials show high adsorption capacity in high
concentrated dyes wastewater, but poor efficiency in low
concentrated dye wastewater. In addition, the high cost is also
a main barrier which limits the practical application of present
1
2
can seriously affect the clarity and oxygen solubility of water
bodies. And can also pose significant threats to the ecosystem.
Owing to dyes being thermal stability, light stability, stable to
oxidizing agents, difficult to degrade and high solubility in
water, treatment of organic dyeing wastewater is a major
1
6
adsorbents. So it is still a great practical significance to
develop cheap and efficient adsorbents.
In the past few decades, supramolecular polymer networks
(
SPNs) represent a new multi-functional materials that have
1
7
many potential applications in fields such as sensing, drug
3
4
challenge. Currently, numerous methods such as adsorption,
1
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19
20
21
delivery, recognition, molecular adsorption, separation
chemical oxidation,5 photolytic degradation, biological
6
2
2
and catalysis. Commonly, SPNs is constructed from a large
amount of low-molecular weight monomer components
through intermolecular non-covalent interactions, including
host-guest inclusion interactions, H-bonding interactions,
C-H · · · π interactions, π - π stacking interactions,26 metal
7
8
treatment and membrane separation have been developed
to remove organic dyes from wastewater. Among these
methods, adsorption is considered as the most attractive
technology because it has the advantages of high feasibility,
high efficiency, economic feasibility, fewer by-products,
simplicity of operation, high recyclability as well as the wide
2
3
24
25
coordination hydrophobic/hydrophilic
interactions,27
2
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29
30
interactions, electrostatic interactions, Van der Waals and
so on. Interestingly, the diversity and dynamic controllability of
self-assembling interactions and diverse self-assembly systems
affords the SPNs multi-stimuli response properties and flexible
a. Key Laboratory of Eco-functional Polymer Materials of the Ministry of Education;
Key Laboratory of Eco-environmental Polymer Materials of Gansu Province;
College of Chemistry and Chemical Engineering, Northwest Normal University,
Lanzhou, Gansu, 730070, P. R. China. E-mail: weitaibao@126.com;
linqi2004@126.com. Address here.
Electronic Supplementary Information (ESI) available: Synthesis and
characterization of compounds; SEM image of supramolecular polymer network
gel and so on. See DOI: 10.1039/x0xx00000x
3
1
32
nature. Compared with metal-organic frameworks (MOFs),
33
covalent organic frameworks (COFs) other frameworks, SPNs
have the merits of easy preparation and functionalization. This
makes it an excellent multifunctional materials with unlimited
†
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development potential in various fields. Although there are
many reports on SPNs, it is still a big challenge to construct
SPNs with various method of sself-assembly and special
functions.
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DOI: 10.1039/D0NJ02524K
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Pillar[n]arenes as a new family of macrocyclic host
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molecules, reported in 2008 by Ogoshi, present unique
structures and easily functionalized properties. Importantly,
the pillar[n]arenes provide rich and diverse supramolecular
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assembly driving interactions, which could also serve as a
new platform to prepare a intriguing supramolecular systems.
However, due to the rigid structure of pillar[n]arenes and the
relatively weak binding capacity, there are only a few reports
on the construction of SNPs based on pillar[n]arene. This will
greatly hinder the development of SPNs based on
pillar[n]arenes. Therefore, it is a great significance and value to
develop a new SNPs based on pillar[n]arene.
Figure 1 (a) Temperature-dependent fluorescent spectra of
NT (Sol) and NT (Gel) in the DMSO/H O solution; (b)
2
Fluorescent “ Gel-Sol ” cycles of NT, controlled by cooling
and heating (λex =360nm).
Then, supramolecular polymer networks NT was prepared
by dissolving host NBP5 (7mg) and guest TG (0.5mg) in
DMSO/H
than 43 ℃ followed by cooling to room temperature. After
guest TG was added to DMSO/H O (v : v = 8 : 2) solutions of
2
O solution (0.1mL), and the temperature is not lower
Inspired by this,37 we designed and synthesized a bi-
pillar[5]arene with multi-interactions sites. The bi-
pillar[5]arene was rational introduced into SNPs through the
host-guest interactions and intermolecular interactions, as a
new supramolecular multi-functional material to develop its
potential applications. The design strategy are the following:
First, we introduced the acylhydrazone group into
pillar[5]arene molecule as fluorophore, hydrogen bonding sites
and connecting bridge between the two pillar[5]arene. Then,
the object of the tripod structure was introduced to provide a
strong assembly complexing ability and a hydrogen bonding
sites. Finally, the stable SNPs NT were successfully constructed
from bi-pillar[5]arene NBP5 and the guest TG in mixed solvent
2
host NBP5, the fluorescence intensity of mixture will be
enhanced (Figure S16), which indicated that there are host-
guest collaborations between TG and NBP5.
Additionally, as shown in Figure 1a, NT has weak
2
fluorescence emission in hot DMSO/H O (T>Tgel); however,
with the temperature dropping below (T<Tgel), the blue
fluorescence intensity will gradually increase and reached a
steady state within short time, which indicated that the
38
fluorescence emission of NT was AIE. Meanwhile, the
gelation properties of NT were explored in various solvents are
summarized in Table S1. The critical gel concentration (the
lowest amounts of NBP5 needed to sustain gel formation) of
2
of DMSO/H O. Studies have shown that there are host-guest
interactions, π - π stacking interactions and intermolecular
hydrogen bonding interactions present in NT. Due to various
intermolecular interactions, NT shows strong blue
fluorescence emission and multi-stimuli response. Most
importantly, the NT shows adsorption separation properties
for organic dyes pollutants from aqueous solutions. Therefore,
this work contributes to the construction of a supramolecular
polymer networks based on bi-pillar[5]arene, which has
potential application in adsorption of organic dyes.
-1
NT is determined to 7% (w/v, 10mg·mL = 1%). Contemporary,
the phase transition temperature (Tgel) of NT is approximately
40-43 ℃. Moreover, it is interesting to find that NT showed
reversible transition (gel-sol) in response to temperature and
performed many times with small fluorescence efficiency loss
(Figure 1b). In addition, the Tyndall effect experiment was
carried out to measure the critical aggregation concentration
(CAC) of supramolecular aggregates. The critical aggregation
-5
concentrations were measured to be 1.0 × 10 M in
DMSO/H
2
O (v : v = 8 : 2) binary solution. Meanwhile, we have
Results and discussion
At the first, a novel gelator (NBP5) decorated with two
identical pillar[5]arene tails was first synthesized by rationally
connecting two aldehyde functionalization pillar[5]arene
moiety group via 2,6-pyridine dihydrazide. The compound
1
13
NBP5 and TG were characterized by H NMR, C NMR, ESI
mass. (Figure S1-S14).
Then, supramolecular polymer networks NT was prepared
by dissolving host NBP5 (7mg) and guest TG (0.5mg) in
DMSO/H
than 43 ℃ followed by cooling to room temperature. After
guest TG was added to DMSO/H O (v : v = 8 : 2) solutions of
2
O solution (0.1mL), and the temperature is not lower
2
host NBP5, the fluorescence intensity of mixture will be
enhanced (Figure S16), which indicated that there are host-
guest collaborations between TG and NBP5.
1
Figure 2 H NMR titration spectrum (600 MHz, 298K) of
at various equivalents of TG:
a) 0 equiv.; (b) 1.0 equiv.; (c) 2.0 equiv.; (d) 4.0 equiv.
NBP5 (10.0 mM) in DMSO-d
(
6
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Figure 3 2D NOESY NMR spectrum (600 MHz, 298 K) of
0.0 mM NBP5 and TG in DMSO-d solution.
Figure 4
d6 at various concentrations: (a) 1.0 mM; (b) 5.0 mM; (c)
0.0 mM; (d)20.0 mM; (e) 40.0 mM.
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H NMR spectra (600MHz, 298 K) of NT in DMSO-
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estimated the association constant (K ) of the complexation
between NBP5 and TG at room temperature in DMSO solution
is about 2.14 ×10 M according to the absorption titration
experiments (Figure S17). Therefore, NT gel has potential
applications in temperature fluorescent switches.
To study the assembly mechanism of NBP5 and TG, H NMR
titration experiment was carried out (Figure 2). With the
increasing amount of TG, the signals of protons H
NBP5 shifted upfield gradually. What’s more, the signals of
a
intermolecular hydrogen bonding interactions between the bi-
pillar[5]arene group of adjacent molecules. Meanwhile, the
signals of protons H on TG shifted downfield, which also could
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be attributed to the intermolecular hydrogen bonding
interactions between the TG of adjacent molecules.
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-1
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In SEM, host NBP5 showed a flake structure (Figure S19a),
guest TG showed a rod-like structure (Figure S19b) and NT
showed a regular porous network structure (Figure S19c),
which also supported the idea that NBP5 and TG could
assemble into supramolecular polymer networks by π - π
stacking interactions, hydrogen bonding interactions and host-
guest interactions. According to th above research, we
speculated about the possible self-assembly mechanism of
supramolecular polymer networks gel NT. First, host NBP5 and
guest TG could assemble into a novel supramolecular polymer
networks through host-guest interactions. Then, the
supramolecular polymer networks could form stable
supramolecular polymer networks gel NT through π-π stacking
interactions and intermolecular hydrogen bonding interactions
a
and Hb,c on
protons H
protons H
1
on TG shifted upfield. Meanwhile, the signals of
on TG shifted downfield. These results confirmed
2
that both the the pyridinium groups of TG were partially
included in the cavity of NBP5 through π - π stacking
interactions, C-H···π interactions and C-H···O interactions. At
the same time, in the corresponding 2D NOESY NMR
experiments (Figure 3), there are four correlations (A, B, C and
D) were observed between the protons H
NBP5 and the protons H and H on TG, which implied the
pyridinium groups of TG were included into the cavity of bi-
pillar[5]arene NBP5.
Moreover, from the concentration-dependent 1H NMR
39
a
and Hb,c on cavity of
1
2
(
Scheme 1).
spectra (Figure 4), all signal peaks shifted rapidly and became
broad at high concentration, which clear confirmed the
formation of high-molecular-weight aggregates driven by host-
Supramolecular assembly systems become sensitive to the
external environment due to the existence of host-guest
40
guest interactions between NBP5 and TG.
Moreover, the assembly process of NBP5 and TG was also
investigated by the small-angle XRD patterns. As shown in
Figure S18, the small-angle XRD patterns of NBP5 suggested
that there was no long-range order in NBP5. However, the
xerogel of NT exhibited well-resolved XRD patterns that were
characteristic of the long-range ordering of the molecules.
These results verified that NBP5 and TG could assemble into a
stable gel. For the xerogel of NT, the d-spacings of 3.80Å (2θ =
2
3.08°) confirmed the π-π stacking interactions existed in the
assembly process of NBP5 and TG. Moreover, hydrogen
bonding interactions also were involved in the stacking process
1
of NT. In H NMR titration (Figure 2), the signals of protons H
and H on NBP5 shifted downfield, indicating that there are
g
Scheme 1 Proposed assemble mechanism of NT
f
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interaction, intermolecular hydrogen bonding interactions and
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π-π stacking interactions in the system. On this foundation,
we carefully studied the multi- stimulus response behavior of
NT (Figure 5). As we all know, the supramolecular host-guest
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assembly system has a reversible response to temperature.
At high temperature, supramolecular assembly system NT
transforms into a clear and transparent solution. Subsequently,
the solution was cooled below the melting temperature, and a
light yellow gel formed again and reached a stable state. It is
because the host-guest interactions and intermolecular
hydrogen bonding interactions will be destroyed at high
temperature, resulting in the decomposition and
transformation of supramolecular assembly system NT into
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Simultaneously, the gel NT exhibited reversible thixotropic
behavior through mechanical stimulation (Figure 5). The vial
containing the gel was shaken for 10 seconds, and it was
observed that the gel turned into flowing white turbid solutio.
And then stand for 5 min, the gel state recovered and
remained stable. It shows that gel NT has good mechanical
response performance. Similarly, this gel NT could also
achieved reversible gel-sol phase transition several times in
Figure 6 (a) Chemical structures of the organic dyes used
to test the adsorption behavior of NT; (b) Cartoon
representation of adsorption process; (c) The adsorption
separation and desorption of various organic dyes by NT.
guest adiponitrile had stronger competitive binding with bi-
pillar[5]arene than TG, resulting in TG free from the cavity of
bi-pillar[5]arene. The SEM also showed the cross-linked
networks morphology of NT was destroyed and turned to an
unordered block structure after addition of adiponitrile (Fig.
S20a). Based on the above studies, supramolecular polymer
networks gel have great research value as stimuli-responsive
4
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the case of concentrate-dilute.
It is known that the pH conditions had a notable effect on
the formation of intermolecular hydrogen bonding
interactions. The NT also showed the corresponding acid-base
responsiveness.45 As shown in Fig. 5, after 5µL triethylamine
was added into NT, the NT gradually collapses and becomes
white turbid solution. SEM images also showed that the cross-
linked networks of gel NT was destroyed, and an irregular
massive morphology was observed (Figure S20b). These results
suggested that intermolecular hydrogen bonding interactions
and π-π stacking interactions have been destroyed. Next, the
4
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materials.
Organic dye contamins that are considered to be poisonous
exist everywhere in our daily life. The most urgent task is to
remove such dye molecules from effluents. Considering NT
contains porous network in its structure, the adsorption
properties were explored using various dye adsorption and
separation experiments in water. As shown in Fig. 6, six
organic dyes with different charges, namely, positively charged
methylene blue, crystal violet, negatively charged orange,
orange I and neutral sudan I, sudan Ⅱ were selected to
evaluate the abilities of NT in the adsorption and separation of
dyes. All the measurements were performed in aqueous
solutions at room temperature under constant stirring. The
adsorption rate experiments were performed by immersing
3
same amount of CH COOH was added into the above turbid
solution, the gel returned to its original gel state and remained
stable. The SEM image also records the change of the gel
system morphology from disordered block morphology to
regular microsphere morphology (Figure S20c).
Furthermore, if the competitive guest adiponitrile was
added to NT at high temperature, the solution could not form
1
.0mg as-synthesized NT powder into 10.0mL of different
organic dyes aqueous solutions. During the UV-vis absorbance
measurements, in units of 30 minutes, 2.0mL of the upper
clear part of the solution was picked up periodically and
poured back to the original solution system instantly after the
measurement. Changes in the UV-vis spectrum were observed
(
Figure 7a, Figure S21): the absorbance intensity decreased
with immersion time. And the dramatic decolorization changes
could be also easily monitored by the naked eye (Figure 6c).
After adsorption, assuming a Beer-Lambert relationship
between concentration and absorbance intensity (Figure
S22), the concentration was calculated by comparing the UV-
vis absorbance to the appropriate calibration curve. The
removal efficiency of dye was calculated by the following
4
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Figure 5 Photographs of gel-sol transitions of gel NT
triggered by a variety of stimuli.
gel NT after cooling. The fundamental reason was that the equation:
4
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Figure 7 (a) The absorbance of each dye changes with
time; (b)The time-dependent adsorption efficiency of
each organic dye; (c) The equilibrium uptake percentage
and amount of organic dye adsorbed per gram of NT; (d)
the percentage removal efficiency of each organic dye
determined after 30 min contact.
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Figure 8 Partial H NMR spectra of NT in DMSO-d
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with
MB (a) NT (b) 3 equiv.; (c) MB.
The NT powder attached with the color of the dye returns to
its original state at an objective speed (Figure 6c).
Subsequently, after filtration and vacuum-drying, the NT
xerogel was recovered. Regardless of reasonable quality loss,
NT powder can achieve recycling. In addition, the SEM of the
regenerated NT xerogel still kept up the porous structure,
confirming its good stability (Figure S25). This implies that
supramolecular polymer networks gel NT can be considered
potential materials for dye adsorption and selective separation
-
1
-1
0 v
(1) where C (mg·L ) and C (mg·L ) are the initial and
residual concentrations of the organic dyes in the stock
solutions and filtrates, respectively. And E is the percentage of
adsorption.
(C
0
 C
0
v)
(1)
E 
100%
C
-1
(
2) where C
aqueous solution at the beginning, t-time, respectively. The friendliness.
adsorption capacity Q (dye removal per gram of NT of at time,
The adsorption mechanism of NT was also investigated by
(dye adsorbed per gram of NT at equilibrium, NMR, 2D diffusion-ordered NMR spectroscopy (2D-NOESY
0 t
and C (mg·L ) are concentrations of dye in with the merits of convenience, rapidity and environmental
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1
t
H
-
1
mg·g ) and Q
e
-1
mg·g ) were calculated according to the following equations:
NMR) and scanning electron microscopy (SEM) analysis. Taking
MB as an example, as shown in Figure 8, after a serious of MB
(
C
0
 C
t
)
(2)
Q
t

V
m
was added into the NT solution (DMSO-d
6
), the signals of
(
C
0
 C )
m
e
(3)
Qe

V
protons H and H of MB and the signals of protons H and H
5
6
a
b,c
-1
of bi-pillar[5]arene shifted upfield. Meanwhile, the signals of
protons H and H of TG shifted downfield, which indicating
e
where C (mg·L ) represents concentration of dye in
1
2
aqueous solution at equilibrium; m (g) is the mass of NT and V
49
that π - π stacking interactions and hydrogen bonding
interactions involved in NT and MB. At same time, in the
NOESY spectrum (Figure S24), cross-peaks A and B revealed
the correlations between the protons signals of MB and those
of protons of NBP5, indicating that the C-H···π interactions, π-
π stacking interactions and electrostatic interactions which
existed in MB and the bi-pillar[5]arene groups. Furthermore,
the above proposed adsorption mechanism was also
supported by SEM. As shown in Figure S25, after adsorption
MB into the NT, the porous network structures of the NT
xerogel changed into a irregular block, which could be ascribed
to dye molecules that was adsorbed by the NT xerogel.
According to these mechanism researches, it is clear that the
rationally introduced multi-interactions sites such as
electrostatic, π · · · π stacking interactions, H-bonding
interactions, C-H · · · π interactions, and host-guest inclusion
interactions sites. It plays play a vital role in the separation of
organic dyes.
(L) is the volume of dye solution.
In addition, corresponding plots of the organic dyes removal
efficiency as a function of time (Figure 7b) and net percentage
of organic dyes removal efficiency (Figure 7d) were
constructed. In SEM (Figure S23), after the adsorption, the
network structure used disappeared, indicating that there is
adsorption behavior between NT and dyes. According to these
results, the NT xerogel shows fast adsorption (5-30min) and
high organic dyes removal efficiency (up to 97.56%). In order
to compare the adsorption capacity of the gel NT and the host
NBP5, the two powders were added to the same dye solution.
After 30 minutes, the dye solution containing NT faded, while
the dye solution containing host NBP5 remained unchanged.
Desorption experiment was processed with fresh,
completely dye filled NT by immersing in organic solvent
ethanol and slightly heated for about 20min. With the passage
of time, the dye is dispersed in the organic solution ethanol,
and the color of the solution changes to the color of the dye.
This journal is © The Royal Society of Chemistry 20xx
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Y. Zhou, L.C. Liu, G. Xu, C. Yan, J. Nanosci. NanotVeiecwhnAroticl.le, 2O0nl1in8e,
Conclusions
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In conclusion, a novel supramolecular polymer networks gel 11 A. Kausar, M. Iqbal, A. Javed, K. Aftab, Z.I.H. Nazli, H. N.
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NT was been successfully constructed by directly assembling
bi-pillar[5]arenebased gelator NBP5 and tripodal guest TG.
Interestingly, the NT exhibited strong aggregation induced
emission (AIE) and multi-responsiveness toward outer stimuli,
1
which can facilitate its application as smart material. 14 M. Sultan, Environ. Chem. Lett., 2017, 15, 347.
0
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Importantly, the NT xerogels could effectively adsorb and
separat various organic dyes from aqueous solution. The
removal efficiency towards MB dyes exceeds 97%, and the
adsorption exhibits high efficiency at low equilibrium
concentration which has big significance to treat practical
wastewater. In addition, NT xerogels shows excellent
application potential in recycling. This original research not
only develops
a
simple, efficient method for the
supramolecular polymer networks, but also provides a cheap
and sustainable approach for organic dyes separation. We
expec that this original research would provide a broad
platform for the design and construction of advanced and
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Conflicts of interest
There are no conflicts to declare.
2
Acknowledgements
2
4919.
This work was supported by the National Natural Science
Foundation of China (Nos. 21662031; 21661028; 21574104),
the Program for Changjiang Scholars and Innovative Research
Team in University of Ministry of Education of China (IRT
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DOI: 10.1039/D0NJ02524K
Graphical Abstract
Stimuli-responsive supramolecular polymer network based on
bi-pillar[5]arene for efficient multiple organic dye contaminants
adsorption
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Tai-Bao Wei*, Li-Hua Qi, Qin-Peng Zhang, Wen-Huan Zhang, Hong Yao, You-Ming
Zhang and Qi Lin*
Text:
A novel supramolecular polymer networks gel have been successfully prepared via bi-pillar[5]arene and
tripodal guest, which could self-assemble to form a supramolecular polymer networks gel at high concentration,
exhibiting multiple stimuli-responsiveness and efficient adsorption of organic dyes in aqueous solution.