Monopeptide-Based Powder Gelators for Instant Phase-Selective Gelation of Aprotic Aromatics and for Toxic Dye Removal

Article abstract of DOI:10.1021/acs.langmuir.0c01101

Through a combinatorial screening of 35 possible phase-selective monopeptide-based organogelators readily made at low cost, we identified five of them with high gelling ability toward aprotic aromatic solvents in the powder form. The best of them (Fmoc-V-6) is able to instantly and phase-selectively gel benzene, toluene, and xylenes in the presence of water at room temperature at a gelator loading of 6% w/v. This enables the gelled aromatics to be separated by filtration and both aromatics and the gelling material to be recycled by distillation. We also identified Fmoc-I-16 as the best gelator for benzyl alcohol, and the corresponding organogel efficiently removes toxic dye molecules by 82-99% from their highly concentrated aqueous solutions. These efficient removals of toxic organic solvents and dyes from water suggest their promising applications in remediating contaminated water resources.

Full text of DOI:10.1021/acs.langmuir.0c01101

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Article
Monopeptide-based powder gelators for instant phase-
selective gelation of aprotic aromatics and for toxic dye removal
Zhongyan Li, Ziqing Luo, Jialing Zhou, Zecong Ye, Guang-
chuan Ou, Yanping Huo, Lin Yuan, and Huaqiang Zeng
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.0c01101 • Publication Date (Web): 23 Jul 2020
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Monopeptide-based powder gelators for instant phase-selective gelation
of aprotic aromatics and for toxic dye removal
Zhongyan Li,† Ziqing Luo,† Jialing Zhou,† Zecong Ye,‡ Guang-chuan Ou,† Yanping Huo,‡ Lin Yuan*† and Huaqiang Zeng*§
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† College of Chemistry and Bioengineering, Hunan University of Science and Engineering, Yongzhou, Hunan, China 425100
‡ Faculty of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, Guangdong, China 510006
§ Nanobio Lab, 31 Biopolis Way, The Nanos, Singapore 138669
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ABSTRACT: Through a combinatorial screening of 35 possible phase-selective monopeptide-
based organogelators readily made at lost cost, we identified five of them with high gelling ability
toward aprotic aromatic solvents in the powder form. The best of them (Fmoc-V-6) is able to
instantly and phase-selectively gel benzene, toluene and xylenes in the presence of water at room
temperature at gelator loading of 6% w/v. This enables the gelled aromatics to be separated by
filtration and both aromatics and the gelling material to be recycled by distillation. We also
identified Fmoc-I-16 as the best gelator for benzyl alcohol, and the corresponding organogel
efficiently removes toxic dye molecules by 82 - 99% from their highly concentrated aqueous
solutions. These efficient removals of toxic organic solvents and dyes from water suggest their
promising applications in remediating contaminated water resources.
INTRODUCTION
separated from water near-quantitatively by a simple filtration. This
certainly enables not only near-complete elimination of toxicity
caused by either oil or gelator molecules immiscible with water, but
also possible reclamation of treated oil. This noteworthy advantage
both intrinsic and unique to PSOGs has aroused intensive interests
from researchers worldwide, resulting in rapid development of
diverse types of PSOGs that have now emerged as promising oil-
scavenging and dye-removing materials.17-54
A critical prerequisite for any gelator to find practical
applications in spilled oil treatment especially on the large scale is
its ability to gel the oil with room temperature operations. This was
first achieved by our group in 2016 using a carefully devised
monopeptide-based gelator library,38 with their gelling ability and
particularly high solubility in environmentally friendly solvents
(e.g., ethanol and ethyl acetate) combinatorially optimizable. The
identified gelator is generally applicable to six types of
(un)weathered crude oils of varying viscosities. Still, these
solution-based gelators require flammable solvents for their
dissolution before application for oil gelation. In the effort to
minimize or even eliminate the use of flammable carrier solvents,
Sureshan,43 Zeng,38,44-45 Chaudhuri,46 and Song47 reported powder
gelators derived from sugar, monopeptide or naphthalene diimide
scaffolds for phase-selective room-temperature gelation of crude
oils.
Toxic organic solvents and dyes, having carcinogenic and
mutagenic effects, are widely found and used in various industry
sections (petrochemicals,1 paints,2 printing,3 pharmaceuticals,4
papermaking,5 textiles,6 chemical industry, garment industry,
cosmetics, etc). Both intentional and unintentional discharges of
organic or dye-containing waste solutions without appropriate
treatments have kept happening,1 not to mention accidental
discharges, plant fires or explosions.7 In particular, around 50000
tons of dyes are released into environment annually.8 These toxic
solvents or dye molecules, discharged into land or rivers but hard
to degrade in the natural environment, greatly threaten the
terrestrial aquatic systems by such as polluting local drinking
water.9
At the present, some common strategies to remove these toxic
pollutants from the environment include combustion,10 biochemical
treatment,11 chemical decomposition,12 membrane separation13 and
physical adsorption.14 However, these methods have some varying
limitations in practical applications. More specifically, a simple
burning not only causes environmental pollution but also is difficult
and costly on the large scale. While the biochemical treatment is
not matured at present, with pending biosafety issues and highly
variable treatment outcomes to be addressed,15 chemical
decomposition method is time-consuming and also may cause
secondary pollution as a result of introduced chemical reagents.
Thus, there is a need to develop alternative technologically simple
yet environmentally friendly approaches that offer practically
valuable features not seen in the existing methods.
Adding into this growing list of recently reported yet limitedly
available powder gelators,7,
43-47
in this work, we describe our
systematic efforts toward the discovery of some powerful powder
gelators for instant phase-selective gelation of aprotic aromatics
(e.g., benzene, toluene, xylene and nitrobenzene) at room
temperature. These aromatics, which are one important source of
environmental pollutions, nevertheless have remained much less
studied so far,7,47-52 especially when compared to more widely
studied petroleum oils.26-46 Additionally, we have also
Use of mostly hydrophobic phase-selective organogelators
(PSOGs) to achieve selective gelation of oil component from an oil-
water mixture was first proposed and demonstrated by
Bhattacharya and his co-workers in 2001.16 In principle, the
gelator-containing gelled oil, which stays afloat in water, can be
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demonstrated their usefulness in efficiently removing highly
concentrated dyes from dye-contaminated water.
Scanning electron microscopy (SEM). SEM images were
obtained using a field-emission scanning electron microscope
(JEOL JSM-6700F, Japan). A small amount of petrol gel was
placed on copper tape attached aluminum stub, and allowed to dry
overnight under ambient conditions. Sample was then sputter-
coated with a thin layer of Pt, and subjected to SEM observation at
an accelerating voltage of 20 kV.
Rheological study: Rheological studies of gels at biphasic
MGCs (BMGCs) were performed using an ARES-G2 rheometer
(TA Instruments, U.S.A.) equipped with a plate (8 mm diameter).
The gels were equilibrated at 25ºC between the plates that were
adjusted to a gap of 2.0 mm. The storage modulus (G’) and loss
modulus (G”) of gels were first measured in strain sweep (0.01–
100%) modes at a constant frequency of 1 Hz, followed by a
frequency scan of 1.0 to 100 rad/s under the controlled strain of
0.1%. Experiments were repeated twice to ensure the
reproducibility.
Dye adsorption measurement. The capacities by gels to absorb
dye from their aqueous solutions were determined using UV/vis
spectroscopy. And the final concentrations of the dyes in the
aqueous solutions were calculated following the Beer–Lambert law
(A = αlc, A is the absorbance of the dye at a certain absorption
wavelength, α is the extinction coefficient in the unit of L·mg-1·cm-
1, l is the path length of the incident light in the unit of cm, and c is
the concentration of the dye in solution). Crystal violet (CV),
rhodamine B and methylene blue were selected for investigation,
and the maximum absorption wavelengths for monitoring the
absorption process were 585, 555 and 664 nm, respectively. The
absorption efficiency (AE) was calculated using equation of RE =
(C1 – C0)/C0, in which C0 represents the initial concentration of the
dye in solution and C1 is the final concentration of the dye in the
presence of an adsorbing agent.
EXPERIMENTAL PROCEDURES
Reagents and compound characterizations. All the reagents
were obtained from commercial suppliers and used as received
unless otherwise noted. 1H NMR and 13C NMR spectra were
recorded using a Bruker Avance HD400 spectrometer with
tetramethylsilane as an internal standard. Mass spectra were
measured using an SHIMADZU LCMS-8030. The solvent signal
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of CDCl3 was referenced at δ = 7.26 ppm or 77 ppm for 1H and 13
C
NMR, respectively. Coupling constants (J values) were reported in
Hertz (Hz).
Typical synthetic procedure. Fmoc-Ala-OH (311 mg, 1.00
mmol), n-dodecylamine (185 mg, 1.0 mmol) and BOP (486 mg,
1.10 mmol) were dissolved in CH2Cl2/DMF (20 mL/10 mL) to
which DIEA (0.39 ml, 2.20 mmol) was added. The reaction mixture
was stirred for 20 h at room temperature. Solvent was removed in
vacuo and the crude product was dissolved in CH2Cl2 (50 mL),
washed with water (2 x 50 mL) and dried over Na2SO4 to give the
crude product, which was subject to flash column chromatography
(MeOH:CH2Cl2 = 1:25, v:v) to yield pure product F-A-12 as a white
solid. Yield: 445 mg, 93%. 1H NMR (400 MHz, CDCl3) δ 7.75 (d,
J = 7.5 Hz, 2H), 7.57 (d, J = 7.2 Hz, 2H), 7.39 (t, J = 7.3 Hz, 2H),
7.29 (t, J = 7.3 Hz, 2H), 6.35 (s, 1H), 5.67 (d, J = 6.1 Hz, 1H), 4.37
(d, J = 6.3 Hz, 2H), 4.29-4.13 (m, 2H), 3.28-3.14 (m, 2H), 1.50-
1.42 (m, 2H), 1.38 (d, J = 5.0 Hz, 3H), 1.32-1.18 (m, 18H), 0.92-
0.85 (m, 3H). 13C NMR (100 MHz, CDCl3) δ 172.28, 156.12,
143.76, 141.32, 127.79, 127.12, 125.08, 120.03, 67.08, 50.57,
47.12, 39.67, 31.94, 29.67, 29.62, 29.57, 29.48, 29.38, 29.30, 26.89,
22.72, 18.89, 14.16. MS-ESI: calculated for [M+Na]+
(C30H42O3N2Na): m/z 501.31, found: m/z 501.26.
Stable to inversion method for minimum gelation
concentration (MGC) determination. The MGCs of the peptide
gelators were determined using a dilution method. First, 10 mg of
each gelator was added to 0.2 mL of a known organic solvent in a
screw-capped glass vial. Then the formed gel system was
progressively diluted with a small amount of the tested solvent and
the heating-cooling process was repeated until no gel was formed.
The last concentration at which the gel state remained with no flow
observed within 30 s after inverting the sample vial was recorded
as the MGC value in the unit of % w/v (mg/100 µL). This process
was repeated more than three times for each gelator and the average
values were reported as the average MGC value.
Stable to inversion method for gelation in the powder state.
The gelation abilities of the powder gelators were determined by
adding a small amount of each gelator (10 mg) into an aromatic
solvent (0.2 mL) in a screw-capped glass vial, followed by gentle
shaking for 6 s and then resting at room temperature. The sample
was regarded as a gel if no flow was observed after inverting the
sample vial.
Method for phase-selective gelation by powder gelators.
Generally, a weighted amount of each peptide gelator was added to
a biphasic mixture of an appropriate organic solvent (0.5 mL) and
water (2.0 mL) in dry powder form via simple shaking at gelator
loading of 5% w/v at room temperature. Selective gelation of the
organic phase was achieved upon resting the sample at room
temperature. Gel was confirmed by complete absence of flow of the
aromatics by visual inspection.
RESULTS AND DISCUSSION
Molecular design of the gelator library. As illustrated in Fig 1,
the gelator library is derived from commercially available Fmoc-
protected (Fmoc = fluorenylmethyloxycarbonyl) amino acids (A, V,
L, I and F) with their N-terminus amidated via one step reaction
using straight alkyl chains of 4 to 16 carbon atoms (Fig. 1).
Combinations among five types of amino acids and seven types of
alkyl chains generate 35 simple monopeptide molecules. The
potential of these monopeptides to act as the gelator stems from the
ability of the two amide bonds to cross-link, via intermolecular H-
bonds, the molecules into one dimensionally aligned structures,
which further entangle with each other to produce 3D gelling
network for entrapment of liquid to be gelled. In fact, molecules
containing straight alkyl chains of 4 to 10 carbon atoms recently
Fig. 1 Structures of monopeptide-based PSOGs, with groups of R1 and R2
combinatorially variable to generate 35 library members.
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Inter-
columnar
packing
Fibers
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intertwining
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Phe
Porous gelling network for
solvent entrapment
1D H-
bonded stack
Fiber
C4H9
Fmoc
Fmoc-Phe-C4
Fig.2 Hypothetic hierarchical formation of supramolecular gelling network by one-dimensionally aligned H-bonded gelator molecules. Crystal structure of
Fmoc-Phe-C4, revealing the formation of well-defined 1D columnar structure, was recently reported by us.38
have been demonstrated to gel crude oils, via a hierarchically
formed supramolecular gelling network (Fig. 2), with moderate to
excellent gelling capacities, i.e., minimum gelation concentrations
(MGCs) to range from 0.29% w/v to 3.10% w/v (mg/100 μL), .38
Moreover, this simple modularly tunable molecular
monopeptide-based gelling scaffold turns out to be exceptionally
robust in that aromatic Fmoc can be replaced by a variety of both
aromatic39,41 and non-aromatic44 groups, invariably producing high
capacity PSOGs after a combinatorial screening of R1 and R2
groups.
With an aim to identify good PSOGs for room-temperature
gelation of aromatics,7,47 we have additionally included alkyl chains
having 12 to 16 carbon atoms for a systematic examination of side
chain effect on phase-selective gelling properties. The syntheses
and characterizations (i.e., 1H and 13C NMR spectra and mass
spectrometry) of these 15 new compounds can be found in the
Supporting Information.
Gelation properties determined by a heating-cooling process.
The gelation capacities of these 35 monopeptides were determined
by the conventional stable-to-inversion method via a heating-
cooling process. That is, after cooling to room temperature, gel
formation was confirmed by complete absence of gravitational flow
within 30 s when the gelling-containing glass vial was turned
upside-down. From the experimental results summarized in Table
1, at least two trends can be identified. For one, leucine-based
monopeptides tend to have a good solubility in aromatics, and only
the ones containing alkyl chains of 14 or 16 carbon atoms can
function as the gelators. For the other, protic aromatics (e.g., benzyl
alcohol and aniline) are more difficult to gel than aprotic ones (e.g.,
benzene, toluene, xylenes and nitrobenzene). This is because
aprotic solvents such as benzene won’t interfere with H-bonded 1D
columnar structure formed among gelator molecules, which
subsequently associated with each other to generate fibers of
various sizes. Further intertwining of fibers results in the formation
of gelling network. In protic solvents such as benzyl alcohol and
aniline that contain H-bond-competing hydroxyl groups, H-bonded
1D stacks are more difficult to form and their stabilization requires
additional assistance from aromatic - stacking as seen in
phenylalanine-derived gelators (e.g., Fmoc-F-n, n = 6, 8, 12, 14
and 16, Table 1) or enhanced VDW forces among elongated alkyl
chains as seen in gelators that carry long C14H29 or C16H32 alkyl
groups (e.g., Fmoc-aa-14 or 16, aa = A, V, I and L, Table 1).
The MGCs of the remaining 31 gelators (except for four leucine
derivatives) are 1.01% w/v - 4.23% w/v for benzene, 0.65% w/v -
3.69% w/v for toluene, 0.70% w/v - 4.02% w/v for xylenes and 1.11%
w/v - 4.53% w/v for nitrobenzene. Among five types of amino acids,
valine appears to perform generally better than other amino acids,
with isoleucine derivatives the second best. In particular, Fmoc-V-
n (n = 4, 6, 8 and 10) display high gelling capacities of 0.73% w/v
- 1.21% w/v toward benzene, toluene and xylenes.
Gelation properties determined in the powder form. The
potential of these gelators to gel the aromatics in the powder state
was then examined via shaking 0.2 mL aromatic solvent containing
gelator at 5% w/v loading for 6 s at room temperature, followed by
resting the solution for observing gel formation within a restrict
time frame of 1 h at room temperature. 7 out of 35 monopeptides
tested show room-temperature gelling abilities to varying extents in
the powder form (Table 2). Among them, Fmoc-A-6, Fmoc-V-6,
Fmoc-I-6 and Fmoc-I-8 all are able to gel benzene, toluene,
xylenes (a mixture of isomeric ortho, meta and para-forms) and
nitrobenzene within 10 min at room temperature. The gelling times
for the best powder gelator Fmoc-V-6 to gel benzene, toluene,
xylenes and nitrobenzene are 2, 2, 2 and 8 min, respectively.
With Fmoc-V-6, we have also investigated the impact of the
gelator loading amounts on the gelling time, and we compared four
loading amounts from 3% w/v to 6% w/v. The data presented in Fig.
3 clearly point to dramatic effects by minute increases in gelator
loading amount. More specifically, in all four aromatics studied, an
increase in gelator loading amount from by 3% w/v to by 6% w/v
shortens the gelling time by more than 88 times, and gelling times
for gelling benzene, toluene and xylenes are shortened to 6 s, 6 s, 6
s and 3 min, respectively. These indicate instant gelation of these
three aromatics at room temperature in the presence of Fmoc-V-6.
In further detail and take benzene as the example, the gelling
times are shortened from 125 min to 7 min by 16 times, from 7 min
to 2 min by 3.5 times, and from 2 min to 0.1 min by 20 times upon
respective increases of the loading amounts from 3% w/v to 6 %
w/v at an increment of 1% w/v. Similar drastic enhancements are
seen for both toluene and xylenes (Fig. 3). It might be worth
mentioning that powder-based PSOGs for gelling aromatics are
very limited.7,42-27 Recently, Yu7 and Song47 reported glucose- and
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Table 1. Gelation abilities (MGC values in % w/v, mg/100 μL)
Table 2. Gelling times (min) for achieving room-temperature
gelation of aromatics by peptide gelators in the powder form at
gelator loading of 5% w/v.
of gelators in five types of aromatics.a,b
Benzyl
alcohol
Nitro-
benzene
Benzene Toluene Xylenesc
Aniline
Benzyl
alcohol
Nitro-
benzene
Benzene Toluene Xylenesc
Aniline
S
2.75
4.15
3.12
3.59
3.68
1.82
2.51
2.41
1.65
4.53
4.50
Fmoc-F-4
Fmoc-F-6
N
N
5
2
8
2
12
2
N
8
N
N
Fmoc-A-6
Fmoc-V-6
Fmoc-V-12
Fmoc-V-16
Fmoc-I-4
Fmoc-I-6
Fmoc-I-8
3.50
S
PG
4.23
3.09
3.25
2.50
1.85
2.19
2.23
2.39
3.12
2.86
2.50
0.90
1.04
1.03
0.92
1.01
0.90
1.21
1.35
1.25
1.16
2.01
2.10
2.03
1.45
S
3.60
3.69
3.63
2.03
3.61
2.15
1.65
2.03
2.39
2.26
1.86
1.50
1.21
0.94
0.65
0.81
1.58
1.36
1.12
0.95
0.85
0.76
1.29
1.20
1.13
1.15
S
1.93
S
2.07
4.56
4.35
1.22
2.93
4.92
S
3.60
S
Fmoc-F-8
Fmoc-F-10
Fmoc-F-12
Fmoc-F-14
Fmoc-F-16
Fmoc-A-4
Fmoc-A-6
Fmoc-A-8
Fmoc-A-10
Fmoc-A-12
Fmoc-A-14
Fmoc-A-16
Fmoc-V-4
Fmoc-V-6
Fmoc-V-8
Fmoc-V-10
Fmoc-V-12
Fmoc-V-14
Fmoc-V-16
Fmoc-I-4
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N
5
5
6
60
T
PG
T
2.51
1.92
1.75
S
S
1.49
1.49
2.60
S
PG
T
35
6
23
PG
2
12
3
3.11
4.02
0.75
1.56
0.73
1.65
2.23
2.11
1.15
0.89
0.70
0.81
0.73
1.85
1.69
1.58
0.85
0.79
0.70
1.55
0.85
1.13
1.10
S
N
N
PG
10
10
N
2
T
N
4
2
2
N
S
S
a All the gelling times were determined by shaking the gelator-containing
solution for 6 s, followed by resting the sample at room temperature; gelling
times beyond 1 h were not recorded. b N = no gel formed, PG = partial gel
and T = turbid fluid state. c We have also determined the room-temperature
gelation of ortho, meta and para-xylenes using four gelators Fmoc-A-6,
Fmoc-V-6 Fmoc-I-6, and Fmoc-I-8 at the same gelator loading of 5% w/v.
And we found that the gelling capacities of the four gelators toward xylenes
are near-identical to those toward ortho, meta and para-xylenes.
S
S
S
S
S
S
S
S
S
2.50
1.61
S
S
3.43
2.78
S
2.88
2.07
2.33
1.77
2.49
2.49
1.06
1.48
1.75
1.72
1.68
1.99
1.29
1.95
1.60
S
S
S
S
S
S
4.34
2.51
1.71
1.15
S
S
2.40
1.52
S
S
4.80
S
Fmoc-I-6
S
Fmoc-I-8
S
S
Fmoc-I-10
Fmoc-I-12
Fmoc-I-14
Fmoc-I-16
Fmoc-L-4
Fmoc-L-6
Fmoc-L-8
Fmoc-L-10
Fmoc-L-12
Fmoc-L-14
Fmoc-L-16
S
3.34
1.43
1.11
S
1.63
1.08
S
S
S
S
S
S
S
Fig. 3 Times (min) taken for Fmoc-V-6 to gel benzene, toluene, xylenes
and nitrobenzene at room temperature in the powder form at gelator loading
amounts of 3% w/v to 6% w/v. Gelator-containing solution was first shaken
for 6 s before resting at room temperature until gel was formed.
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
3.33
2.61
3.55
S
2.63
1.88
2.13
2.10
2.10
2.02
1.75
2.13
3.01
2.44
a All the MGC values were determined by using a heating-cooling (stable-
to-inversion) method. b S = soluble and PG = partial gel at gelator loading
of 5% w/v. All gels formed at room temperature are stable at least for one
a PSOG in the treatment of an oil spillage. To demonstrate this
feasibility, the best powder gelator Fmoc-V-6 was taken for further
study at 5% w/v loading amount. Procedurewise, 25 mg of Fmoc-
V-6 was added into a biphasic systems comprising 0.5 mL benzene
and 2.0 mL water, and the resultant mixture was shaken for 6 s to
facilitate dispersion or dissolution in aromatics. Upon resting the
mixture for < 2 min, a gel was not only formed but also sufficiently
strong to support water that is 4.6 times of its own weight (Fig. 4a).
In the same way, toluene, xylenes and nitrobenzene can be gelled
within 2 min by 5% w/v of Fmoc-V-6 and, except for nitrobenzene
gells, are also able to support water 4.6 times of their own weights
(Fig. 4b-d). Since the density of nitrobenzene (1.2 g/cm3) is greater
than that of water, the nitrobenzene gel formed remained at the
bottom of the vial when the vial was inverted (Fig. 4d). We have
further studied three more volumetric ratios of benzene (0.25, 0.75
and 1.0 mL) over water (2 mL). In all three cases, instant phase-
selective gelation within 2 min were similarly observed. We have
c
month. A mixture of isomeric ortho, meta and para-forms of xylene
commercially available from Sigma
D-gluconic acetal-based powder PSOGs that can phase-selectively
solidify aniline/nitrobenzene (> 4.5% wt) and benzene/toluene/
xylene (1.8 – 2.3% wt), respectively, at room temperature within
minutes. In other words, our work reported here represents the 3rd
type of powder gelators for room-temperature phase-selective
gelation of aromatic solvents.
Phase-selective gelation of aromatics in the presence of water.
The ability to phase selectively gel aromatics in the presence of
water is one of key requirements for the practical application of
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(a)
(e)
(c)
(d)
(b)
(a)
(b)
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Filtration
Gelation
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(f)
G'
G''
G'
G''
Distillation
105
104
103
102
101
100
105
104
103
(d)
(e)
Collection
0.1
1
10
100
10 0 (Pa)
100
 (rad/s)
Fig. 4 (a)-(d) depict the gels phase-selectively formed from benzene,
toluene, xylenes and nitrobenzene at room temperature in the presence of
Fmoc-V-6 at 5% w/v. (e) SEM image of the as-formed benzene gels. (f)
Dynamic rheological studies of the as-formed benzene gels; G’ = storage
modulus, G” = loss modulus, frequency sweep = ω, and stress amplitude =
0.
Fig. 5 Schematic illustration of recovery of gelled xylenes and gelator
through filtration and distillation. (a) A mixture of water (10 mL) and
xylenes (5 mL). (b) Gel formation by Fmoc-V-6 (250 mg). (c) Separation
of the gels from water by filtration. (d) Distillation to recycle both xylenes
and gelator (e).
also further studied the possibility of using 5% wt of the powder
gelator to gel the emulsified aromatics (0.5 mL of benzene, toluene
or xylene) in water (2 mL). Again, in all three cases, we observed
instant gelation of emulsified aromatics within seconds (Supporting
movies 1-3). Additionally, such instant gelation was not affected by
pH (3, 7 or 11) or presence of NaCl at high concentration (35 g, 0.6
M), suggesting potential of its uses in real environment.
The SEM images of the as-formed benzene gels (Fig. 4e) as well
as toluene and xylene gels (Fig. S1) reveal extensive formation of
the fibrous structures, further assembling into a 3D gelling network
that trap the benzene molecules, via surface tension and capillary
forces, to turn liquid aromatics into a semi-solid gel state.
As shown in Fig. 4f, the facts that the storage modulus G’ of the
as-formed benzene gels is largely independent of frequency while
remaining much larger than loss modulus G’’ across the whole
frequency sweep range studied suggest excellent elasticity of the
gels. A relative high value of 7.0×103 Pa for the storage modulus
G’, which serves as an indicator of gel strength, demonstrates the
remarkable stiffness and strength of the benzene gels. Consistent
with the ability of the benzene gels to support the water layer (Fig.
4a), this high mechanic strength, together with facts that these as-
formed gels are stable for months and exhibit no thixotropy
property at room temperature, makes Fmoc-V-6 suitable for
application in oil spill treatment.
In addition, a likelihood to recycle both gelled aromatics and
gelator was demonstrated (Fig. 5). Briefly, 250 mg of Fmoc-V-6 in
the powder form was added into a biphasic system composed of 5
mL of xylenes and 10 mL of water. After shaking the mixture for 6
s and resting at room temperature for 1 min, the formed gels were
separated from water by filtration. Subsequent distillation recovers
both the gelled xylenes and gelator molecules nearly quantitatively.
Highly efficient dye absorption. With the rapid development of
our society, the heavy industrial discharge of water-soluble toxic
(a)
(b)
0.5 h
1.0 h
1.5 h
2.0 h
2.5 h
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0 h 2.5 h
400
450
500
550
600
650
700
l (nm)
(c)
(d)
0 h 10 h
0 h 8 h
Fig. 6 (a) Time-dependent adsorption spectra recorded at 585 nm for an
aqueous solution (2 mL), which contains the crystal violet dye at an initial
concentration of 250 mg/L, in the presence of 0.2 mL benzyl alcohol gel
formed using 2.0% w/v of Fmoc-I-16. (b)-(d) depict the concentration-time
correlations for (b) crystal violet at 585 nm, (c) for rhodamine B at 555 nm
(c) and (d) for methylene blue at 664 nm.
these toxic dyes from such as the sewage treatments has been a
constant focus in recent years.7,13,15,28,42,52-54 Since aniline,
nitrobenzene or other aromatic solvents are more toxic than benzyl
alcohol, benzyl alcohol was therefore chosen as a relatively safe
liquid for gel formation and as subsequent dye removal medium. In
this regard, we have tested all 35 monopeptides toward the gelation
of benzyl alcohol, and Fmoc-I-16 was identified to have the highest
gelling ability with a MGC value of 1.08% w/v toward benzyl
alcohol (Table 1). Therefore, Fmoc-I-16 was employed for
subsequent dye removal study. In a typical experiment, 0.2 mL
dyes to groundwater on the daily basis poses
a serious
environmental and economic problem,8 and effective removing of
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Table 3. Effects of pH and 0.6 M NaCl on the dye absorption
efficiency.
precipitating the gelator out. A Subsequent filtration near-
quantitatively recovers gelator by 96% for re-use.
In addition to these cationic dyes, we have also tested the ability
of both benzyl alcohol and aniline-based gels formed by Fmov-V-
6 toward absorbing anion dyes (i.e., Congo red and Trypan blue).
For reasons unclear to us, these gels turn out to be incapable of
extracting these anionic dyes from water.
Lastly, to check the effect gelators of different types might have
on the dye removal efficiency, we have measured and compared the
dye removal efficiencies by benzyl alcohol gels formed by the best
five gelators (Fmoc-I-16, Fmoc-F-16, Fmoc-A-16, Fmoc-V-16,
and Fmoc-L-16) at pH 7. Results summarized in Table 4 indicates
insignificant differences among the five gels across three dyes.
The above findings indicate that Fmoc-I-16 and its associated
benzyl alcohol gels can be used as an alternative sewage treatment
agent for removing the toxic dyes, particularly given that small
peptides are generally biodegradable.55
pH =3
98%
94%
80%
pH =7
pH =11
98%
Crystal violet
Rhodamine B
Methylene blue
99% (98%a)
97% (93%a)
82% (78%a)
96%
75%
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a Measured in the presence of NaCl at high concentration (35 g/L, 0.6 M)
Table 4. Dye absorption efficiencies by benzene gels formed by
the best five gelators at 2% wt at pH 7.
Crystal violet
99%
Rhodamine B
97%
Methylene blue
82%
Fmoc-I-16
Fmoc-F-16
Fmoc-A-16
Fmoc-V-16
Fmoc-L-16
96%
95%
81%
96%
95%
81%
97%
96%
82%
CONCLUSIONS
94%
93%
80%
In summary, a series of peptide gelators have been synthesized in
one step generally with high yields, which may satisfy the cost-
effective requirement of mass production in industrial scale for
practical applications. A combinatorial screening of a total of 35
candidates allowed us to identify Fmoc-V-6 as the best powder
gelator capable of instantly and phase-selectively gelling toluene,
toluene and xylenes in the presence of water in less than one minute
at room temperature at gelator loading of 6% w/v. In addition to
benzyl alcohol gel, formed using Fmoc-I-16 at loading of 2.0% w/v
was loaded into a 2 mL aqueous solution containing three different
dyes (i.e., crystal violet, rhodamine B and methylene blue) at a high
concentration of 250 mg/mL. The abilities of Fmoc-I-16 to remove
these dyes were monitored at 585 nm, 555 nm and 664 nm,
respectively, using UV/vis spectroscopy. The representative time-
dependent absorption curves for crystal violet are presented in Fig.
6a and the concentration-time correlations for the three dyes are
plotted in Fig. 6b-d. As evidenced from Fig. 6b-d, the dye removal
efficiencies from their aqueous solutions by benzyl alcohol gels
reach 99%, 97% and 82% for crystal violet, rhodamine B and
methylene blue after 2.5, 8.0 and 10.0 h, respectively. It might be
worth mentioning that the dye-absorbing capacity for methylene
blue plateaus at 82% around 10 h, after which time we observed the
same absorption efficiency of 82% at both 18 and 24 h. Moreover,
dye absorption efficiency of 97% was obtained at 12 h for a mixture
of dyes consisting of crystal violet and rhodamine B in equal weight.
Although we are not certain why the absorption times (i.e., 2.5 – 10
h) to reach the maximum absorptions vary substantially among the
three dyes, there are a number of preceding examples that suggest
dye-absorption process often is a time-consuming process, taking
common
advantages
associated
with
phase-selective
organogelators (e.g., simple filtration to separate formed gels from
water, and distillation to recover gelator for re-use), such a solvent-
free approach completely eliminates the toxicities and risks
associated with the flammable solvents required by solution-based
gelators. As such, Fmoc-V-6, which is possibly biodegradable,53
may offer practical benefits for its possible use in removing organic
pollutants from the environment. In addition, the gelator Fmoc-I-
16, having the highest gelling ability toward benzyl alcohol among
35 gelators studied, could combine with benzyl alcohol to produce
gels for efficiently removing dyes (crystal violet, rhodamine B and
methylene blue) form their highly concentrated aqueous solutions,
with removal efficiencies of 99%, 97% and 82%, respectively.
These findings let us to believe that these peptide gelators may find
promising use as effective sewage treatment agents in the future.
up to 24 h for achieving the maximum absorption7,28
.
Further investigations were then carried out to examine the
effects of pH or salt on the absorption capacity of the gelator (Table
3). We found variations in pH or the presence of NaCl at
concentration of as high as 0.6 M exert insignificant influence on
dye sorption efficiencies, which nevertheless reach the maximum
absorption capacities at neutral condition. At pH 7, inclusive of the
weight of benzyl alcohol, the maximum dye sorption capacities
were determined to be 10.14 mg/g for crystal violet, 10.02 mg/g for
rhodamine B, 1.93 mg/g for methylene blue.
Interestingly, we found that either MeOH or EtOH allows the
gelators to be recycled after dye absorption. Specifically, addition
of 1.0 mL of essentially non-toxic EtOH (or more toxic MeOH) into
0.2 mL of dye-absorbed benzyl alcohol gels turns the gels into
solution, dissolving away dye and benzyl alcohol while
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.langmuir.
Synthetic scheme, experimental procedures and a full
1
set of characterization data including H NMR, 13C
NMR and MS as well as SEM images of the as-formed
gels and supporting movies 1-3.
AUTHOR INFORMATION
Corresponding Authors
*Emails: hqzeng@ibn.a-star.edu.sg; tcyl431102@163.com
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Nanocomposite Deposited Membrane for Oil-in-Water Emulsion
Separation with in Situ Removal of Anionic Dyes and
Surfactants. Langmuir 2017, 33 (30), 7380-7388.
Notes
The authors declare no competing financial interest.
(14) Teng, W.; Bai, N.; Chen, Z.; Shi, J.; Fan, J.; Zhang, W.-x.,
Hierarchically Porous Carbon Derived from Metal-Organic
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designed? Chem. Soc. Rev. 2008, 37 (12), 2699-2715.
(19) Dawn, A.; Shiraki, T.; Haraguchi, S.; Tamaru, S.-i.; Shinkai, S.,
What Kind of “Soft Materials” Can We Design from Molecular
Gels? Chem. Asian J. 2011, 6 (2), 266-282.
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Organogelation and Hydrogelation of Low-Molecular-Weight
Amphiphilic Dipeptides: pH Responsiveness in Phase-Selective
Gelation and Dye Removal. Langmuir 2009, 25 (15), 8639-8648.
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ACKNOWLEDGMENT
This work was supported by the NanoBio Lab (Biomedical
Research Council, Agency for Science, Technology and Research,
Singapore), National Natural Science Foundation of China
(51772091), Natural Science Foundation of Hunan Province of
China (2017JJ3093 and 2018JJ3193), the construct program of
applied characteristic discipline in Hunan University of Science
and Engineering and The Technology Plan of Guangdong Province
(2019A050510042).
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