A versatile carbohydrate based gelator for oil water separation, nanoparticle synthesis and dye removal

Article abstract of DOI:10.1039/c6nj03520e

A versatile green gelator suitable for multiple applications is reported. Gelation of organic solvents in a significantly low gelation time (<5 s) is achieved. The effect of cooling and sonication on gelation time is investigated. Apart from organic solve

Full text of DOI:10.1039/c6nj03520e

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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A versatile carbohydrate based gelator for the oil water
separation, nanoparticles synthesis and dye removal
Chintam Narayana, Ravi Kant Upadhyay, Raman Chaturvedi and Ram Sagar∗
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A versatile green gelator suitable for multiple applications is reported. Gelation of organic solvents in a significantly low
gelation time (<5 sec) is achieved. The effect of the cooling and sonication on the gelation time is investigated. Apart from
organic solvents, gelator is capable of forming gel with the fuel oils such as diesel and petrol also, therefore, it has been
utilized for the separation of the oil from oil/water mixture through selective gelation of the oil. Gelator was also found to
be capable of forming hydrogel through rational control of the reaction conditions. Hydrogel prepared using gelator was
further explored as reaction medium for the growth of the gold nanoparticles. In the case of nanoparticles synthesis,
gelator served not only as capping agent but also as a reducing agent. By taking advantage of the dual functionality of the
gelator (capping and reducing agent), nanoparticles of both gold and silver were prepared in fluid medium also. The
organogel, prepared using toluene and gelator, was utilized for the removal of the waterborne synthetic dye rhodamine B.
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followed by removal of the oil swollen gel through physical
means such as filtration or using a scoop. Mukherjee and
coworkers have recently reported instant phase selective
Introduction
Owing to the wide application spectrum, ranging from
cosmetics to biomedical, a significant amount of research has
been devoted to the low molecular weight gelators (LMWGs).1-
gelation of oil from an oil/water mixture at room temperature
using two sugar-derived phase selective gelators (PSG).11
Carbohydrates are known to be excellent surface
passivating agents for the nanoparticles synthesis. A variety of
carbohydrates such as glucose, fructose, sucrose, starch and
3
A wide variety of LWMGs have been designed in order to
obtain gels suitable for different applications.2, 4 Recently low
molecular weight carbohydrates have garnered huge attention
as gelators.5-9 Under favorable optimized conditions these
carbohydrate based gelators (CBGs) form supramolecular
structure which can be useful for variety of applications such
chitosan have served as capping agents for the restricted
20
In few cases, apart from
growth of the nanoparticles.19,
capping molecules, carbohydrates have also acted as reducing
21
agents especially in metal nanoparticles synthesis.20,
as drug delivery, oil water separation, enzyme immobilization
10, 11
CBGs exhibit several
Engelbrekt and coworkers reported green synthesis of gold
nanoparticles using buffered glucose and starch solution.22 It
was found that phosphate or MES buffers play a crucial role in
the synthesis of nanoparticles. Gels have served very
efficiently as reaction medium for the confined growth of the
nanoparticles.23 In the case of gels, the chances of
agglomeration among synthesized nanoparticles are very less
since the nanoparticles, embedded in the gel matrix, are
deprived of free motion. Nanoparticles entrapped in semisolid
gel matrix have acquired considerable interest for various
biological and medical applications. These gel/nanoparticles
hybrids are highly useful systems for various applications in
biosensors, chiral catalysis, nonlinear optics and material
science.24-26
and nanomaterials preparation.6,
unique features such as ease of availability, environment
friendly and cost effectiveness etc, which makes these
preferred candidates of choice as gelator over other
conventional organic gelators.12,
13
A plethora of reports
mentioning use of various CBGs such as monosaccharide
derived carbamates, N-acetylglucosamine derivatives and
galactose derivatives have been documented.12 Wang et al.
reported N-linked carbamates gelators capable of forming gels
in both nonpolar (n-hexane) as well as polar (aqueous DMSO
and ethanol solution) solvents.5, 6
Recently several reports exploring CBGs for the removal of
oil from water through selective gelation of oil have been
surfaced.14-18 Selective gelation mediated oil removal process
involves solidification of the oil in form of gel by gelator
In present report, successful use of a new N-acetyl
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(d, J = 4.0 Hz, 1H, H-1), 4.08 (ddd, J = 3.6 Hz, 8.8 Hz, 10.0 Hz, 1H, H-
2), 3.68-3.59 (m, 2H, H-5, H-3), 3.54 (t, J = 8Hz, 1H, H-4), 3.43 (dd, J
= 2.8 Hz, 6.4 Hz, H-6a), 3.40 (s, 3H, OCH3), 3.34 (dd, J = 5.6 Hz, 10.0
Hz, 1H, H-6b), 2.75 (d, J = 1.8 Hz, 1H, OH), 2.04 (s, 3H, COCH3).
13CNMR (100 MHz, CDCl3): δ 172.0 (C=O), 143.9 (ArqC), 128.8,
128.0, 127.2 (ArC), 98.2 (C-1), 87.0 (CPh3), 74.3 (C-3), 72.8 (C-4),
70.2 (C-5), 63.8 (C-6), 55.1 (OCH3), 53.8 (C-2), 23.4 (COCH3). HRESI-
glucosamine based gelator for multiple applications such as
oil/water separation, metals (Au and Ag) nanoparticles
synthesis and removal of waterborne dye, has been
demonstrated. Organogel formation capability of the N-acetyl
glucosamine based gelator in different organic solvents is
monitored. Apart from organogels, hydrogels are also
prepared using the same gelator. Previously, carbohydrates
based gelators are mainly explored for the oil/water
separation whereas, in current report besides from oil/water
separation, application of the carbohydrate based gelator for
the synthesis of metals nanoparticles is also described.
Additionally, N-acetyl glucosamine (GlcNAc) based organogel is
also explored for the removal of rhodamine B dye from water.
MS (m/z): Calcd for C28H31NO6Na+, [M+Na]+
: 500.2044, found
500.2063.
Synthesis of Methyl 3,4-di-O-benzyl-6-O-triphenylmethyl-2-
acetamido-2-deoxy-α-D-gluco pyranoside (2a)
To a stirred solution of compound 2 (14 g, 29.32 mmol) in dry THF,
sodium hydride (3.5 g, 87.96 mmol) was added at 0 °C. After 20 min
benzyl bromide (10.4 mL, 87.96 mmol) was added drop wise and
the resulting mixture was allowed to heat at 80 °C with stirring for
1.5 h. After completion of reaction, the reaction mixture was
allowed to cool down at room temperature then it was quenched
by adding methanol (50 mL) and concentrated under reduced
pressure. The residue was dissolved in ethylacetate (500 mL)
washed with water (2 x 250 mL) and saturated brine solution (1 x
500 mL). The organic layer was dried over anhydrous sodium
sulfate, filtered and evaporated under reduced pressure to get the
yellow colour residue. The residue was washed with pentane to
furnish the compound 2a (Scheme S3) as brown sticky solid (15 g
78%). The compound 2a was used in next step without further
purification. A purified compound 2a was obtained through column
Experimental
The gelator (compound-3) was synthesized by exploring selective
protection and de-protection chemistry on the commercially
available N-acetyl-D-glucosamine (Scheme 1),
Synthesis of Methyl 2-acetamido-2-deoxy- D-glucopyranoside (1a)
Amberlite IR 120-H+ resin (20 g) was added to a pre-stirred solution
of N-acetyl glucosamine 1 (20 g, 90.41 mmol) in methanol (200 mL).
The resulting mixture was stirred at 80 °C for 24 h. After completion
of the reaction, reaction mixture was cooled down to room
temperature, and filtered to remove the resin. The filtrate was
evaporated under reduced pressure to obtain compound 1a
(Scheme S1) as white solid (19 g) in 89% isolated yield anomeric
20
1
chromatography (ethyl acetate) for analytical purpose. [α]D 75.8°
mixture (α:β, 9:1). HNMR (400 MHz, CD3OD): δ 4.65 (d, J = 3.5 Hz,
(c = 0.6, CHCl3).1HNMR (400 MHz, CDCl3): δ 7.51-7.49 (m, 6H, ArH),
7.37-7.18 (m, 17H, ArH), 6.89 (dd, J = 1.2 Hz, 6.4 Hz, 2H, ArH), 5.32
(d, J = 9.6 Hz, 1H, NH), 4.85 (d, J = 11.6 Hz, 1H, CH2Ph), 4.77 (d, J =
3.6 Hz, 1H, C-1), 4.67- 4.57 (m, 2H, CH2Ph), 4.32 (ddd, J = 3.6Hz, J =
6.4 Hz, 14.0 Hz, 2H, H-2, CH2Ph), 3.82 (dd, J = 10.0 Hz, 1H, H-4), 3.75
(dd, J = 2.8 Hz, 10.0 Hz, 1H, H-5), 3.63 (t, J = 8.8 Hz, 1H, H-3), 3.52
(dd, J = 1.6 Hz, 12.0 Hz, 1H, H-6a), 3.37 (s, 3H, OCH3), 3.24 (dd, J =
4.4 Hz, 10.0 Hz, 1H, H-6b), 1.86 (s, 3H, COCH3). 13CNMR
(100MHz,CDCl3): δ 169.9 (C=O), 144.0 (ArqC), 138.6, 137.9 (ArqC),
128.9, 128.6, 128.5, 128.3, 128.3, 127.9, 127.8, 127.0 (ArC), 98.6 (C-
1), 86.5 (CPh3), 80.4 (C-3), 78.9 (C-4), 75.1 (CH2Ph), 70.8 (C-5), 62.5
(C-6), 54.8 (OCH3), 52.8 (C-2), 23.6 (COCH3). HRESI-MS(m/z): Calcd.
for C42H43NO6Na+ [M+Na]+ :680.2983; found 680.2959.
1H, H-1α), 4.30 (d, J = 2.1 Hz, 0.1H, H-1β), 3.91 (dd, J = 3.6 Hz 10.8
Hz, 1H, H-2), 3.82 (dd, J = 2.0 Hz, 11.6 Hz, 1H, H-6), 3.69 (dd, J = 5.6
Hz, 11.6 Hz,1H, H-6), 3.63 (dd, J = 8.8 Hz ,10.0 Hz, 1H, H-3), 3.57
(ddd, J = 2 Hz, 5.6 Hz, 10Hz, 1H, H-5), 3.46 (s, 0.2H, OCH3-β), 3.37 (s,
3H, OCH3-α), 3.32 (dd, J = 3.8 Hz, 9.6 Hz, 1H, H-4), 3.26 (ddd, J = 2.2
Hz, 5.4 Hz, 9.6 Hz, 0.1H), 1.98 (s, NHAc-α, 3H), 1.97 (s, NHAc-β,
0.3H), 13C NMR (100 MHz, CD3OD): δ 173.6 (C=O), 103.5 (C-1β), 99.8
( C-1α), 78.0, 76.2, 73.6, 72.9, 72.3, (C-3, C-4, C-5), 62.7 (C-6), 55.4
(OCH3) 55.3 (C-2), 22.5 (COCH3); HRESI-MS (m/z): Calcd for
C9H17NO6H+, [M+H]+ : 236.1129, found 236.1128.
Synthesis of Methyl 6-O-triphenylmethyl-2-acetamido-2-deoxy-α-
D-glucopyranoside (2)
To a stirred solution of compound 1a (15.2 g, 64.68 mmol) in
anhydrous pyridine, trityl chloride (21.63 g, 77.61 mmol) and DMAP
(0.7 g, 6.46 mmol) were added and resulting mixture was stirred at
room temperature for 72 h. After completion of reaction, reaction
mixture was diluted with water (150 mL) and extracted with ethyl
acetate (2 x 250 mL), combined organic layer was washed with
copper sulfate solution (1 x 250 mL) followed by saturated brine
solution (1 x 200 mL). The organic layer was dried over anhydrous
sodium sulfate, filtered and evaporated under reduced pressure to
get the residue. The residue was purified by column
chromatography (ethyl acetate/methanol: 95:5) to furnish
compound 2 (Scheme S2) as amorphous solid (24.5 g) in 79%
Synthesis of Methyl 3,4 -di-O-benzyl-2-acetamido-2-deoxy-α-D-
glucopyranoside (3)
To a solution of compound 2a (10 g, 15.28 mmol) in dry CH2Cl2, a
33% solution of HBr in acetic acid (6.6 mL) was added at 0 °C drop
wise. The reaction mixture was stirred for 10 min at 0 °C.
Completion of the reaction confirmed by TLC, after which the
reaction mixture was quenched with ice cold water (50 mL) diluted
with CH2Cl2 (1 x 400 mL) and washed with water (1 x 100 mL),
saturated sodium bicarbonate solution (2 x 250 mL) and saturated
brine solution (1 x 100 mL). The combined organic layer was dried
over anhydrous sodium sulfate and filtered, evaporated under
reduced pressure which gave a residue. The residue was purified by
flash column chromatography (ethylacetate/hexane 80:20) which
20
isolated yield. [α]D 28.3° (c = 0.6, CHCl3); 1HNMR (400 MHz,
CDCl3): δ 7.47-7.21 (m, 15H, ArH), 5.91 (d, J = 8.4 Hz, 1H, NH), 4.70
2 | J. Name., 2012, 00, 1-3
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gave compound 3 (Scheme S4) as a white gelatinous solid, Further containing oil/water mixture which results in gelation of oil
re-crystallization with CH2Cl2 furnished the title compound 3 as a layer.
20
white solid (5 g) in 80% isolated yield. [α]D 108.3° (c = 0.6, CHCl3) ; Preparation of pristine hydrogel and Au nanoparticles
1HNMR (400 MHz, DMSO-d6): δ 8.08 (d, J = 8.0 Hz, 1H, NH) 7.36 – containing hydrogel
7.24 (m, 10H, ArH), 4.77 (t, J = 6.4 Hz, 1H, OH), 4.73- 4.60 (m, 4H, 2 x For the preparation of the hydrogel, 10 mg gelator was
CH2Ph), 4.54 (d, J = 3.6 Hz, 1H, H-1), 3.96 (ddd, J = 3.2 Hz, 10.4 Hz, dissolved in 1 mL of ethanol. Thereafter 3 mL of phosphate
12.8 Hz , 1H, H-2), 3.71 (dd, J = 4 Hz, 8.4 Hz, 1H, H-3), 3.66 (dd, J = 4 buffer solution (pH = 7.4) was added to the above prepared
Hz, 11.6 Hz, 1H, H-6a), 3.59-3.56 (m, 1H, H-6b), 3.51-3.47 (m, 2H, H- gelator solution. The resulting mixture was kept at room
4, H-5), 3.30 (s, 3H, OCH3), 1.85 (s, 3H, COCH3). 13CNMR (100MHz, temperature for 10 min which led to the formation of a gel.
CDCl3): δ 169.2 (C=O), 138.7, 138.4 (ArqC), 128.2, 128.1, 127.5,
In order to prepare Au nanoparticles containing hydrogel
127.4, 127.3 (ArC), 98.2 (C-1), 79.9, 78.0 (C-3, C-4), 73.9, 73.9 10 mg gelator was dissolved in 1 mL of ethanol. As obtained
(CH2Ph), 71.6 (C-5), 60.1 (C-6), 54.3 (OCH3), 52.6 (C-2), 22.5 (COCH3). gelator solution was mixed with 3 mL phosphate buffer (pH =
HRESI-MS (m/z) Calcd. for C23H29NO6Na+ [M+Na]+: 438.1887; found 7.4) and 0.2 mL 0.005M AuCl3 solution; the resulting reaction
438.1910.
mixture was left at room temperature for 10 min which
produced a violet color hydrogel. The violet color of the
hydrogel indicates the formation of Au nanoparticles. As
obtained Au nanoparticles containing hydrogel was dissolved
in water to prepare the Au nanoparticles suspension which
was further characterized.
Gelation of both polar and non polar organic solvents was
carried out by employing compound-3 as gelator.
Organogel preparation
5 mg gelator and measured amount of organic solvent
(according to critical gel concentration) were taken in a glass
vial and sonicated, gently heated to dissolve the gelator
completely. As obtained solution of the gelator was left at
room temperature till it transforms into the gel (Fig. 1). A
system was considered as gel if it doesn’t fell down on keeping
vial upside down. Table S1 lists all the organic solvents which
were gelled using the gelator along with the gel formation
time in different conditions and appearance of the gels.
In order to determine critical gel concentration (CGS) first 5
mg of the compound was dissolved in 0.9-1 mL of the test
liquid (solvent/oil) in a vial. If gel forms, then the gelator
solution was diluted by small fixed volume of same liquid and
gelation test was re-executed. This cycle of dilution and
gelation was repeated until the gel fell down on holding the
vial upside down. Thus, the maximum amount of the fluid that
can be contained by 5 mg of the gelator was measured which
was further used to calculate CGC as wt%.
Preparation of metals (Au and Ag) nanoparticles in solution
For the preparation of the Au nanoparticles, 5 mg gelator was
dissolved in 1 mL of ethanol. The resulting gelator solution was
mixed with 0.2 mL of 0.005M AuCl3 solution and 2mL of 20 mM
phosphate buffer solution (pH =7.4) which changed the color
of the solution from colorless to violet within few minutes. The
appearance of the violet color indicates the formation of the
Au nanoparticles. Since the amount of the gelator used is half
of the amount used in case of the hydrogel so formation of
hydrogel didn’t take place.
Similar to the Au nanoparticles for the preparation of the
Ag nanoparticles 5 mg gelator was dissolved in 1 mL of ethanol
and resulting solution was mixed with 2 mL of 0.005M AgNO3
solution. Thereafter few drops of 0.1M NaOH was added to
the above prepared reaction mixture which instantly changed
the appearance of the solution from colorless to yellow
confirming formation of the Ag nanoparticles.
Ultrasonication gel
Characterization details
A fixed amount of gelator was taken in a glass vial containing
organic solvent and heated slowly to dissolve the gelator
completely. Then vial was kept under probe sonicator till gel
formed.
TLC was visualized either by UV light or by treating with
phosphomolybdic acid in EtOH followed by heating. 1HNMR
and 13C NMR spectra were recorded by 400MHz spectrometer.
1H and 13C spectra were recorded by 400 MHz and 100MHz,
respectively. Proton chemical shifts are given in ppm relative
to the internal standard (tetramethylsilane) or referenced
relative to the solvent residual peaks (CHCl3: δ 7.26; DMSO: δ
2.50; CD3OD: δ 3.31). Multiplicity was stated as follows: s
Gelation test at cooling
5 mg gelator was taken in 5 mL vial containing 0.9 mL of
measured solvent, as prepared reaction mixture was sonicated
and heated slowly to dissolve the gelator completely, then the
vial was kept on an Ice bath (0°C to 5°C) which leads to gel
formation.
Oil water separation using gelator
In the case of organic solvents such as toluene and benzene,
the gelator was easily getting dissolved however for the
dissolution of gelator in diesel, petrol and crude oil an
additional solvent tetrahydrofuran (THF) was required. First 5
mg of compound-
3
was dissolved in THF and as obtained
in THF was added to a glass vial
Figure 1 Organogels of (a) benzene (b) toluene (c) o-Xylene (d) m-xylene (e) p-xylene (f)
solution of the compound-
3
mesitylene (g) 1,
benzene (k) diesel.
2
dichlorobenzene (h) chlorobenzene (i) bromobenzene (j) Iodo
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(Singlet); d (doublet); t (triplet); q (quartet), m (multiplet): dd
(doublet of doublet); dt (doublet of triplet); td (triplet of
doublet); ddd (doublet of doublet of doublet), etc. Coupling
constants (J) were reported in Hz. Flash chromatography was
performed by silica gel 100-200 and 230-400 mesh. Optical
rotation was recorded using Polarimeter kruss P300. High
resolution mass spectra determined from quadrapole/TOF
mass spectrometer with an ESI source. FTIR measurements
were performed using ATR FTIR spectroscopy (Thermo
Scientific NicoletTMIsTM5 FTIR) with diamond ATR accessory.
Gel samples were taken out from the closed vial and after
subtle drying to remove physically absorbed solvents gels were
mounted on the ATR crystal to record spectra. Rheological
measurements were executed on Anton Paar MCR 302
rheometer equipped with steel coated parallel-plate geometry
(50 mm diameter). The gap distance was fixed at 1 mm and a
solvent-trapping device was placed above the plate to prevent
solvent evaporation. All measurements were done at 25°C.
Frequency sweep experiments were carried out from 0.1 to
600 rad s-1 at a constant stress of 0.1%, fit within the linear
visco-elastic region (LVER). UV-vis spectrum of the gelator was
recorded by (Thermo Scientific Evolution-201). The
Morphology of the gels was examined using scanning electron
microscopic (SEM) Zeiss EVO40. A small amount of gel was
placed on a carbon tape pasted to a copper grid and the gel
was subjected to ambient drying, as prepared sample was
coated with gold. The shape and size of the fibers present in
hydrogel and metals nanoparticles was viewed using HR-TEM
Figure 2 SEM images of (a1, a2) toluene gel (b1, b2) Hydrogel.
important criterion which decides the usefulness of the gelator
for practical large scale application;27 gelators capable of
forming gels in less time are highly desirable. In several
previously documented reports carbohydrates based gelator
are mentioned to be taking a very long time (several
minutes)16, 17 for the gel formation which discourages their
wide scale application. In order to shorten the gelation time
different new carbohydrate based gelators have been
designed and modifications in the gelation conditions have
also been introduced.11 In the current study, we have
proposed two strategies, rapid cooling and ultrasonication, in
order to expedite the gelation process. For the rapid cooling of
solution, obtained after dissolution of the gelator in the
(Tecnai G2 20 twin, Tecnai 200 KV twin microscope). For the solvent, it was transferred on an ice bath which drastically
reduced the gel formation time and the gelation completed
within few seconds. A significant lower gel formation time was
recorded for gel prepared through cooling on ice bath
compared to the gel obtained through cooling at ambient
temperature (Table S1). For instance, in the case of toluene, at
room temperature, the gel formation time was recorded to be
3 minutes 54 seconds while ice bath cooling accelerated the
gel formation process and it completed within 52 seconds
which clearly indicates that the cooling temperature dictates
the rate of gel formation. A similar decrease in the gel
formation time was observed for other solvents too. SEM
images of the toluene containing gel confirmed the presence
of interconnected network of fibers (Fig. 2).
TEM viewing sample was prepared through drop casting
method in which drop of the metals nanoparticles
a
suspension was drop dried on the copper grid.
Results and discussion
Gelator (compound-3) exhibited excellent gelation ability and
was able to form gel with different organic solvents even at a
very low concentration 0.38-0.7% (W/V). However, among α
and β anomers of the compound-
gelation property while the β anomer forms precipitate in
most of the organic solvents. Compound- formed highly
3 only α anomer exhibits
3
strong gels with various organic solvents including benzene,
toluene, o-xylene, m-xylene, p-xylene, mesitylene etc (fig. 1). It
is well known fact that in the case of LMWGs, non-covalent
forces, such as van der Waals attraction, π-π stacking and
hydrogen bonding, among gelator molecules or gelator and
solvent, underpin the formation of gels.12 In the present case,
the gelator molecule consists of several aromatic moieties and
also readily forms gel with aromatic ring containing solvents,
which suggests that π-π stacking is mainly responsible for the
gel formation. Apart from aromatic rings, the hydroxyl group is
also present in the gelator molecule which endows it with the
capability of undergoing hydrogen bonding.
In general practice sonication is usually considered for the
re-dispersion of the aggregates, however, it has been found
that in given right conditions sonication can also aggregate
molecules in assemblies of minimum energy which can lead to
the formation of gels; these gels are also referred as sonogels.
28, 29 Sonication assisted gelation is not a general phenomenon;
it is limited to only a few gelators. In the present case, a
substantial decrease in the rate of gelation of the organic
solvents by the compound-
3 is observed when probe
sonication was applied during gelation. For almost all the
organic solvents the gel formation time for sonication assisted
gelation was recorded to be lower than 5 seconds, which,
according to the best of our knowledge, is the lowest gelation
Owing to the low CGC, compound-3 can be considered as
super-gelator. Apart from CGC, gel formation time is another
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Figure 5 TEM images of the Au nanoparticles containing hydrogel.
Figure 3 Frequency sweep measurements of the pristine and nanoparticles containing
distillation. Akin to organic solvents in case of diesel and petrol
also cooling reduced the gelation time substantially. Room
gels.
time among N-acetyl glucosamine based gelators (Table S1).
temperature gelation of the diesel took ~3 minutes while in
Previously, it has been reported that sonication disrupts the
the case of cooling assisted gelation, gel formation completed
within a minute, similar observation was made in case of
petrol also which makes gelator highly suitable for the
recovery of the fuel oil spills from the sea water in geographic
regions with low atmospheric temperature. Gelator formed a
very strong gel with both diesel and petrol so the chances of
re-dissolution of the gel during mechanical agitation are very
less. In addition to the strength and stability these gels are also
thermo-reversible in nature and can be converted back to fluid
form by simple heating and further gel stage could be restored
through cooling. Fluid-gel cycles were repeated multiple times
and no significant change in the gelation ability of the gelator
was observed insinuating high stability and thermo-reversible
nature of the gels. All gels were found to be stable at room
temperature and didn’t experience any significant physical
change during storage for long duration.
intramolecular
hydrogen
bonding
and
promotes
intermolecular hydrogen bonding,28 in present case also we
believe that the sonication driven quick alignment of the
molecules into energy favorable orientations which promote
intermolecular hydrogen bonding among gelator molecules, is
the viable reason behind the quick formation of the gel.
Besides aromatic nonpolar solvents, gelator was also able to
form gel with polar organic solvents such as ethanol and
DMSO (Fig. S5).
Mechanical strength is an extremely important property of
the gels which decides their potential application. In order to
probe the mechanical strength of the gels rheology analysis
was carried out (Fig. 3). Frequency sweep measurement of the
toluene gel (0.5% W/V) is shown in figure 3. It is well
documented that for strong gels oscillatory frequency
measurements exhibit a weak frequency dependent storage
(G’) and loss (G”) modulus throughout the frequency range
while the value of G’ always remains higher than the G”.30-32 In
present case also we have made the same observation, both
G’ and G” merely changed throughout the frequency range
and G’>G” through the entire frequency range (Fig.3).
The gelation ability of the gelator is not restricted to the
organic solvents and oils but it is also capable of forming
hydrogels under specified conditions which further expands its
application domain. Hydrogels were prepared through rational
control of the buffer solution/gelator/ethanol ratio. For the
preparation of hydrogel, 10 mg gelator was dissolved in 1 mL
ethanol, as prepared gelator solution was mixed with
phosphate buffer (3 mL) and left at room temperature until gel
formation completed (Fig. 4d). Formation of the hydrogels
requires buffer and ethanol in a very specific quantity so the
formation of hydrogel in oil/water separation experiments is
not feasible. SEM images of the hydrogel revealed that similar
to the toluene gel it is also composed of interconnected fibers
(Fig. 2b1, b2). Further efforts were also made to exploit
hydrogel as reaction medium for the growth of Au
nanoparticles. For the preparation of Au nanoparticles
containing hydrogel, 0.2 mL of Auric chloride (0.005M) was
added to the reaction mixture during hydrogel formation. It
Furthermore, apart from organic solvents, compound-3 is
capable of forming gels with fuels such as diesel and petrol
also; therefore, this gelator can be an interesting aspirant
material for the separation of oil from water or vice versa
through selective gelation of the oils (Fig. 4a-c). For the phase
selective gelation of the diesel and petrol from water, the
solution of gelator in the tetrahydrofuran (THF) (16mg/0.2 mL)
was added to the both fuel oils (0.8 mL) which led to the
formation of very thick gels. These gels can be separated out
using a spatula and the fuel oils can be recovered through
was found that compound-3 not only served as gelator but
also acted as a reducing agent which eliminated the need of
external reducing agent for the reduction of gold salt into the
Au nanoparticles. The dual functionality of the compound-
makes it an enticing material for the preparation of the
nanoparticles. Figure shows the TEM images of Au
3
5
Figure 4 (a-c) Removal of petrol from petrol/water mixture using gelator (d) pristine
nanoparticles containing hydrogel, it can be clearly seen from
the figure that the hydrogel exhibits fibrous structure with an
hydrogel (e) Au nanoparticles containing hydrogel.
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Figure 8 Removal of the waterborne dye using toluene gel (inset a-dye solution, b-
after removal of dye from water).
compound-3 serves better as capping agent in case of Au
nanoparticles compare to the Ag nanoparticles. UV-vis
spectrum of the Au nanoparticles consists of a broad peak
centered at 540 nm which is characteristic surface Plasmon
peak for Au nanoparticles (Fig. 7). The absorption spectrum of
the Ag nanoparticles also exhibits a broad surface Plasmon
peak around 450 nm (Fig. 7). Compare to the Au nanoparticles,
surface Plasmon resonance peak in case of Ag nanoparticles is
wide suggesting higher agglomeration which is corroborated
by the TEM images also. With time a monotonic increase in the
absorbance is observed for both metal nanoparticles.
Figure 6 TEM images of the (a1, a2) Au nanoparticles (b1, b2) Ag nanoparticles.
individual fiber having length in micrometers and diameter in
the range of 40-60 nm (Fig. 5). Au nanoparticles, imbedded in
the fibers of hydrogel, can be easily noticed, the size of the Au
nanoparticles is in the range of 5-10 nm. TEM images confirm
that the Au nanoparticles are distributed uniformly throughout
the fibers in form of agglomerates.
Compound-3 was utilized as surface passivating agent as
The removal of waterborne pollutants such as organic
solvents, oils and toxic synthetic dyes from water is very
crucial from the point of view of remediation of polluted
water.33 Organic synthetic dyes are found to be responsible for
various health deteriorating effects on the mankind. It has
been reported previously that carbohydrate based PSG can be
utilized for the removal of the dyes from dyed water/oil
mixture.17, 34 In PSG based dye removal process, first, transfer
of dye from water to nonpolar solvent takes place thereafter
oil can be solidified in form of gel using gelator and finally
using simple filtration solid gel containing oil and dye can be
recovered from the oil/dyed water mixture. We have also
well as green reducing agent for the preparation of Au and Ag
nanoparticles in the fluid medium also. Figure 6 (a1, a2 and b1,
b2) show the TEM images of the Au and Ag nanoparticles
prepared by taking advantage of the dual functionality of the
compound-3. In case of Au nanoparticles spherical particles
having particle size in the range of 10-20 nm were obtained
(Fig. 6a2). Similar to Au, in case of Ag nanoparticles also
spherical particles were obtained, however, in addition to
spherical particles, few particles having elongated structure
were also visible (Fig. 6b2). The particle size of Ag
nanoparticles was estimated in the range of 20-50 nm which is
higher compare to the Au nanoparticles. Apart from higher
particle size, Ag nanoparticles are more agglomerated
compare to the Au nanoparticles. These results suggest that
successfully utilized compound-3 for the dye removal from
oil/dyed water mixture. However, in present case, the
extraction of dye from water into organic solvent was allowed
to precede post gelation unlike previously published report
where first dye was extracted into the organic solvents and
then gelation was carried out. For dye absorption experiments
5 mg of compound-
heated until it dissolved completely, as obtained clear solution
of compound- in toluene was left for 5 minutes at room
3 was added to a 1 mL of toluene and
3
~
temperature which led to the formation of gel. Thereafter 10
mL of rhodamine B dye solution was added to a test tube
containing above prepared toluene gel. UV-vis spectrum of the
dye solution, taken out from the reaction mixture at a regular
interval, was recorded in order to monitor the removal of the
dye. Absorption spectra revealed that up to 96% of the dye can
be removed using toluene gel (Fig. 8). Results suggest that the
gelator can be utilized efficiently for the removal of not only oil
Figure 7 UV-Vis spectra of the Au and Ag nanoparticles.
6 | J. Name., 2012, 00, 1-3
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In conclusion, a multifunctional gelator, which is capable of
forming both organogel and hydrogel, is reported. The gelator
could produce highly strong gel in a very short gelation time.
Through the rational implementation of the strategies such as
ice bath assisted cooling and sonication, the gelation of
different solvents was achieved in few seconds. Particularly,
through sonication, gelation of different solvents was achieved
within 5 seconds which is probably the shortest gelation time
for N-acetyl glucosamine based gelators. In addition to organic
solvents, the gelator was proven to be equally efficient for the
gelation of the oils such as diesel and petrol which qualifies it
as a suitable material for the oil spill recovery application.
Organogel was also successfully utilized for the removal of
waterborne synthetic dye which makes gelator useful for the
environmental applications such as water treatment also. The
hydrogel of the same gelator was explored as growth medium
for the fabrication of Au nanoparticles, where besides acting as
a structural unit, gelator also served as a reducing agent.
Synthesis of Au and Ag nanoparticles was also carried out in
fluid medium using gelator as stabilizing cum reducing agent.
Overall, instead of developing separate molecules for
different applications herein we have identified one single
molecule with wide possible application spectrum.
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Acknowledgements
Authors are thankful to the Shiv Nadar University (SNU) for
providing required lab facilities. Chintam Narayana is thankful
to Council of Scientific and Industrial Research (CSIR), India for
junior research fellowship. Authors are also thankful to
Department of Science and technology (DST) India for financial
support (DST-EMR/2014/000320).
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