A tripeptide-based self-shrinking hydrogel for waste-water treatment: Removal of toxic organic dyes and lead (Pb2+) ions

Article abstract of DOI:10.1039/c7cc01774j

A triphenylalanine-based superhydrogel shows automatic syneresis (self-compressing properties) with time and this self-shrinking behavior has been successfully utilized to remove toxic lead ions and organic dyes from waste-water efficiently with the abili

Full text of DOI:10.1039/c7cc01774j

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
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A Tripeptide-based Self-Shrinking Hydrogel for Waste-Water
Treatment: Removal of Toxic Organic Dyes and Lead (Pb ) Ions
Received00th January 20xx,
2+
a
a
a
b
a,
Shibaji Basak,‡ Nibedita Nandi,‡ Subir Paul, Ian W Hamley and Arindam Banerjee *
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
1
1f
A
triphenylalanine-based superhydrogel shows automatic external chemicals.
So, there is a need to explore
syneresis (self-compressing property) with time and this self- supramolecular hydrogels that show shrinking property
shrinking behavior has been successfully utilized to remove toxic without any external stimulus. In the course of our
lead ions and organic dyes from waste-water efficiently with the investigation to study the self-assembly and gelation behavior
ability to re-use for a few times.
of self-assembling peptide-based molecules, a tripeptide-
based amphiphile has been discovered to form a hydrogel at
pH 7.4 and it has been found to exhibit a remarkable self-
shrinking property with time (Fig. 1b).
1
Supramolecular hydrogels composed of low molecular weight
gelators (LMWGs) are a rapidly expanding area of recent
research due to their wide range of applications including
2
controlled release of biologically active compounds and drugs,
3
cell culture and tissue engineering, nano-particles and nano-
4
5
clusters synthesis, catalysis and other purposes. Peptides are
particularly useful candidates to study self-assembly and
gelation as these molecules are bio-compatible and
6
biodegradable in nature as well as incorporating a diversity of
amino acids which leads to versatility in the control of peptide
sequence. Among the peptide gels so far reported it was found
that hydrophobic amino acids especially phenylalanine have an
7
important role in the gelation process. However, use of
Fig. 1 (a) The chemical structure of the amphiphile MF. (b) Image showing transparent
fresh hydrogel obtained from MF (at 1.5 mM concentration) and 7-day aged opaque
gel upon syneresis. (c) Photographs of dye (Brilliant blue) containing hydrogel before
phenylalanine has a limitation as it reduces the solubility of the
8
gelator molecule in aqueous medium . The hydrophobicity of
9
2+
gelators often leads to ‘syneresis’,
a
phenomenon and after syneresis. (d) Photographs of Pb ion containing hydrogel before and after
involvingthe expulsion of the gelling solvent from the gel syneresis.
phase and ultimately macroscopic contraction of the gel,
which occurs within several hours. This phenomenon has been
Water pollution caused by toxic organic dyes and leaching of
heavy metal ions including lead (Pb), cadmium (Cd), mercury
1
0
mostly observed for many sol-gel based polymeric systems.
In contrast, there are only a few reports on stimuli-responsive
syneresis in supramolecular hydro- or organogels driven by
(
Hg) and others from industrial waste is a great threat to
12
modern society. The dye effluents have a negative impact on
the immune system, reproductive systems as well as they
12
1
1a,b
11c
external stimuli including pH,
1d
metal ion,
photo-
1
11e
irradiation,
mechanical pressure
or in the presence of
exhibit potential geno-toxicity and cardio-toxicity. On the
other hand, lead has a deleterious effect on human health
including renal, reproductive, central nervous and
a.
Department of Biological Chemistry, Indian Association for the Cultivation of
Science, Jadavpur, Kolkata, 700032 (India) Fax: (+ 91) 33-2473-2805, E-mail: hematopoietic systems through the enhancement of oxidative
bcab@iacs.res.in.
Department of Chemistry, University of Reading, Whitenights, Reading, RG6, 6AD,
UK.
Electronic Supplementary Information (ESI) available: [Experimental section,
Instrumentation, Synthetic procedures, NMR, HRMS, FE-SEM, TEM, FTIR, XRD, XPS,
SAXS, Spectroscopic and Rheological studies.] See DOI: 10.1039/x0xx00000x
13
stress. There is thus a prime interest for the development
b.
and analysis of environmentally friendly materials to protect
the environment by removing toxic pollutants from
contaminated water. There are some methods to remove
pollutants from waste-water using functionalized CNTs,
‡
These two authors contributed equally
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4
photocatalysis, reverse osmosis and others.
In this shift and steady increase in fluorescence of ANS upon
11e
connection, supramolecular hydrogels are wonderful materials binding
with the gel fibres clearly suggest that the
due to their highly porous fibrous network structure and large hydrophobic nature of the gel system is increasing with time
5
1
surface areas in the gel matrix to trap the toxic pollutants.
and this ultimately causes the more and more proximity of the
There are a few examples of polymer based and graphene gel fibres within the gel matrix by expelling water molecules
containing hydrogels for the removal of lead ions from waste- from the system.
1
6
water. However, none of these above mentioned hydrogels Field Emission Scanning Electron Microscopy (FE-SEM) was
exhibit a self-shrinkage property. Herein, we report a novel carried out for the xerogels obtained from freshly prepared gel
triphenylalanine-based hydrogel that can offer an easy and and the 7 days aged gel. Fig.2b, c show that both of these
effective way to remove water-soluble pollutants to have clean xerogels appeared to contain well-developed numerous
and safe water upon instant syneresis. In this study, the intertwined nano-fibres (average width 25 nm to 45 nm) in
2
+
hydrogel has a good capacity to entrap toxic Pb ions as well their self-assembled states. Comparison between these two
as toxic organic dyes (Methylene blue, Brilliant blue) from images (freshly prepared and 7 days aged gel) indicates that
waste-water (Fig. 1c, d). The scavenging capacity of removing the morphology of the fibrous network does not notably
1
1e
toxins from contaminated water is excellent as an appreciable change upon syneresis.
2
+
amount (1995mg) of Pb ions can be removed by per gram of
the gelator from waste-water. To the best of our knowledge, it
2
is the highest Pb ion absorption capacity by a hydrogel
+
1
6a
reported so far. Moreover, only a small amount of the
gelator is required (< 1 mg/mL) to prepare this superhydrogel
and it is recyclable several times to efficiently remove toxic
substances from waste-water.
The tripeptide-based amphiphilic gelator MF (Fig. 1a) was
observed to self-assemble in phosphate buffer of pH 7.4. The
gel is stable in the pH range 7.0 to 8.5 and it is also thermo-
reversible in nature. The gelator molecule MF has three
phenylalanine moieties bearing hydrophobic π-core, thus
making the molecule highly hydrophobic in nature. Therefore,
it is hard to dissolve the gelator in aqueous medium and it
requires 15-20 minutes of heating on a hot plate followed by Fig. 2(a) Amount of expelled water with time shows the dynamic shrinkage behaviour
of the hydrogel obtained from MF. Image showing FE-SEM images obtained (b)
cooling at room temperature (25 °C) to obtain a transparent
immediate after gel formation and (c) after 7 days of syneresis. (d) ANS binding
hydrogel. The minimum gelation concentration (MGC) was
fluorescence assay of the gel shows an increase in fluorescence during syneresis. (e)
found to be 1.2mM (0.08 % wt/v) to form a self-supporting gel.
SAXS data obtained from the xerogels (obtained from freshly prepared, 4-day and 7-
The gel melting temperature (T ) at MGC was found to be 56 day aged gels).
gel
°
C. The temperature vs. concentration plot (Fig.S4) of the
freshly prepared hydrogel shows very high thermal stability.
Surprisingly, a more interesting feature of volume phase
transition - ‘syneresis’ - was observed for this hydrogel. The
hydrogel is started to shrink immediately after its formation
followed by the expulsion of water from the gel matrix. The
freshly prepared gel was transparent in nature, while the aged
gel gradually became opaque upon shrinking (Fig. S5). The
shrinking capacity of the hydrogel was quantitatively evaluated
by measuring the volume of the expelled water from the
shrunken gel as well as volume of the shrunken gel itself. It
was observed that the volume of the expelled water gradually
increases with time and after 7 days the volume of the
hydrogel contracts to about 75 % of its original volume by
releasing about 80% clear water from 2 mL of hydrogel (1.5
mM) (Fig.2a). The shrinkage behaviour of this hydrogel is
thermal, pH and pressure dependent in nature (Fig. S6).
To get insight into the mechanical strength and flow behaviour
of the hydrogels, rheological experiments were performed for
the fresh hydrogel, and 4-day and 7-day aged gels. The
frequency sweep experiments (Fig. S7a) suggest that the
storage modulus of these hydrogels are very similar and in the
3
4
order of 10 -10 Pa. The oscillatory stress sweep experiment
Fig. S7b) also shows that the stiffness of the freshly prepared
(
gel is more than that of the 4-day and 7-day aged gels and it is
evident that syneresis does have a negative effect on the
mechanical strength of the gel and the exact reason for that is
yet to be explored.
The slow shrinkage of the hydrogel inspired us to look into the
detailed packing pattern of the gelator molecules within the
hydrogel. In small angle X-ray scattering (SAXS) experiment, a
broad peak is observed at 39.3 Å, which becomes more
pronounced with time as indicated in Fig. 2e. This length (39.3
Å) is greater than the molecular length of a single gelator
molecule (calculated to be 28 Å), but shorter than double the
calculated molecular length indicating an interdigitated
arrangements of the molecules in the gel phase. In the wide
angle powder X-ray diffraction (WXRD) pattern of the xerogels
The syneresis of the hydrogel may be due to the extreme
hydrophobic nature of the nanofibres obtained from the self-
assembling gelator molecules containing three hydrophobic
Phe residues. An attempt has been made to explore the
mechanism of shrinkage behaviour by using a dye (ANS)
binding assay fluorimetrically (Fig. 2d). A time dependent blue
2
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(fresh and aged), the appearance of peaks at 2θ = 19.2° (d = the hydrogel matrix (Fig.1c). Within 1 hr. these hydrogels
4.5 Å) and 2θ = 22.5° (d =3.7 Å) respectively suggest a β-sheet started to shrink by expelling clear water (Fig.1c). UV-visible
like structure and π-π stacking of the phenyl rings in the spectra (Fig.3a,b) shows that a negligible number of the dye
hydrogel states (Fig. S8). In FTIR study (Fig. S9), peaks at 3416 molecules are present in the expelled water part compared to
-1
-1
cm and 3292cm are associated with non-hydrogen bonded the initial concentration of the dye molecules present in the
and hydrogen bonded amide N-H stretching respectively and hydrogels. The highest loading capacity for both these dyes is
-1
the peak at 1640 cm is due to amide carbonyl group given in the Table S1. The dye removal capacity was found to
stretching. This clearly indicates the presence of an be remarkably high for both of these tested dyes (99.8%).
2
+
intermolecular hydrogen bonded sheet-like extended Moreover, removal of all these toxic substances (Pb ion and
backbone structure in the gel states (Fig. S9). organic dyes) was also tested in a mixture of these three
The shrinkage property can be used as an easy method to substances and all these substances are removed leaving
remove toxic metal ions such as lead. An aliquot 10 μL of 66.2 behind clear water. This indicates that this tripeptide-based
mM solution of lead nitrate Pb(NO ) was added to the hot gelator molecule has great potential in future in waste-water
3
2
2+
solution of amphphile MF at 1.5 mM concentration and the treatment. Moreover, the removal of toxic substances (Pb
whole solution was sonicated for few seconds. A white ions and organic dyes) together in a mixture was also tested
precipitate was formed that dispersed homogeneously into and all these substances are separated sequestered (Fig. S12).
the whole solution. After cooling the solution to room
temperature a translucent hydrogel was formed. Interestingly,
2
+
this white-coloured opaque hydrogel containing Pb ions
started to shrink within an hour of its formation by expelling
2
clear water (without Pb ions). The separation of white
+
2
+
hydrogel leaving behind colourless water indicates that Pb
ions got entrapped into the self-compressed hydrogel network
Fig.1d). The effect of anion was also tested by taking lead
(
2
+
perchlorate (Pb(ClO ) ) as a Pb ion source. However, no
4
2
significant anion effect was found as Pb(ClO4)2 also got
separated from the solution (Fig. S10). The whole study can be
represented as a schematic diagram as in Fig. S11.
Atomic absorption spectroscopic (AAS) was performed to
2
+
determine the efficiency of the hydrogel for Pb ion
separation. An amount 20μL of the shrinkage gel and 20 μL of
the expelled water weretaken respectively to prepare 20 ml
2
+
solutions to measure the Pb ion concentration (ppm). These
solutions were stirred for 4 h at room temperature, followed
2
+
Fig. 3 Fig. 3 The UV-visible spectra of (a) Brilliant blue and (b) Methylene blue
containing gel and the solution after syneresis showing the absence of the
corresponding dyes. TEM images show elemental mapping of Pb (element) within gel
network (c) before and (d) after shrinkage of lead containing hydrogel [The presence of
by filtration. The concentration of the Pb ions in the initial
solution was determined through AAS using a standard
calibration curve. However, the results are striking as the
2
+
freshly made hydrogel contains a Pb ion concentration of red dots indicate the distribution of Pb (element) in the gel]. (e) A schematic
.9285 ppm whereas Pb ion concentration in the expelled representation showing the entrapment of toxic dye molecules and Pb2+ ions within
2
+
2
2
+
the hydrogel matrix upon syneresis.
water part is 0.0469 ppm. This indicates that the Pb ion
concentration is negligible in the water phase compared to the
2
+
2
+
aged and shrunken hydrogel as the concentration of Pb ions
in the aqueous phase is insignificant. This performance is
1
better than previous reports as the efficiency and extent of
We were curious to know whether Pb ions interact with the
gelator peptide or not and to examine this, shrunken and aged
6a
2
gels containing Pb ions were taken out from the vial and
+
2
+
Pb ions removal from water by using our gelator is improved
98.4%). To the best of our knowledge, this is the first report of
were placed in a saturated solution of EDTA-Na . Interestingly,
2
(
it was found that as time elapsed, the white coloured gel was
gradually transformed to a transparent gel through its
periphery (Fig. S13). This observation can be due to the
a peptide-based supramolecular superhydrogel that shows a
fascinating self-compressing property through the expulsion of
water. It also demonstrates the potential usability of this
hydrogelator in waste-water treatment by removing toxic
2
+
leakage of Pb ions from the gel matrix upon complexation
2
+
with EDTA. So, it can be envisaged that Pb ions may be
physically absorbed or entrapped in the three-dimensional
porous network structure of the hydrogel without any
2
+
heavy metal ions (Pb ) and environmentally harmful organic
dyes and Fig. 3e represents a schematic diagram for the entire
phenomenon.
2
+
complexation with the gelator molecule and that is why Pb
ions can be easily removed with a strong complexation with
For the dye removal study 10 μL of a 1mM solution of Brilliant
blue and Methylene blue was added to a 1.5 mM 2 mL
hydrogel in phosphate buffer respectively. Upon cooling,
hydrogels were formed and the dye molecules were trapped in
2
+
EDTA. To explore the distribution of Pb ions in the gel
network, elemental mapping of Pb (element) was performed
by FE-SEM and TEM studies before and after the shrinkage of
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2
+
lead-containing gel. Fig. S14 and 3c, d reveal that Pb ions are
randomly distributed within the gel matrix in a nonspecific
manner as these elements are found both on nanofibres as
well as within the pores of the entangled nanofibres of the gel
matrix. The FTIR data also supports this result as there is a not
significant change in FT-IR spectral pattern before and after
Hamley, R. H. Barnes, K.-A. Karatzas, D. Hermida-
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4
2
+
addition of Pb ions into the gel (Fig. S15). The XPS data (Fig.
2
S16) reveal the possibility of the entrapment of Pb ions
+
2
+
18
within the gel network as hydroxide of Pb ions during
syneresis.
(
c)E. Pazos, E. Sleep, C. M. R. Ṕerez, S. S. Lee, F. Tantakitti and
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shrunken gels were treated with saturated potassium
hydrogen sulphate solution which was extracted with ethyl
5
(
,
1
1967.
5
6
7
M. Tena-Solsona, J. Nanda, S. Díaz-Oltra, A. Chotera, G.
Ashkenasy and B. Escuder, Chem. Eur. J., 2016, 22, 6687.
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Chem. Int. Ed., 2015, 54, 5705.
1
5e
acetate following the standard methods by pH variation . It
was found that the highly water soluble pollutants (dye and
metal ions) remain in the aqueous phase and the gelator
molecules were extracted through the organic phase. As
shown in Fig. S17 by using the above stated method, we can
use the gelator molecule for a maximum four cycles with only
nominal loss of the gelator compound.
(a) W. Liyanage and B. L. Nilsson, Langmuir, 2016, 32, 787;
(
b) M. Pellach, S. Mondal, L. J. W. Shimon, L. Adler-
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In summary, a tripeptide-based potent hydrogelator has been
discovered, which forms a super-hydrogel at a very low
concentration. This hydrogel exhibits a remarkable time-
dependent self-shrinking property by expelling water
molecules. Moreover, this hydrogel has been successfully
utilized for efficient removal of pollutants including toxic heavy
9
1
L. L. Hench and J. K. West, Chem. Rev., 1990, 90, 33.
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Table of content
A tripeptide-based supramolecular automatically self-shrinking superhydrogel has been
2+
discovered for efficient removal of toxic organic dyes and Pb ions from waste-water.