Tailored synthesis of various nanomaterials by using a graphene-oxide-based gel as a nanoreactor and nanohybrid-catalyzed C-C bond formation

Article abstract of DOI:10.1002/asia.201402695

New graphene oxide (GO)- based hydrogels that contain vitamin B₂/B₁₂ and vitamin C (ascorbic acid) have been synthesized in water (at neutral pH value). These gel-based soft materials have been used to synthesize various metal nanoparticles, including Au, Ag, and Pd nanoparticles, as well as nanoparticle-containing reduced graphene oxide (RGO)-based nanohybrid systems. This result indicates that GO-based gels can be used as versatile reactors for the synthesis of different nanomaterials and hybrid systems on the nanoscale. Moreover, the RGO-based nanohybrid hydrogel with Pd nanoparticles was used as an efficient catalyst for C-C bond-formation reactions with good yields and showed high recyclability in Suzuki-Miyaura coupling reactions.

Full text of DOI:10.1002/asia.201402695

FULL PAPER
DOI: 10.1002/asia.201402695
Tailored Synthesis of Various Nanomaterials by Using a Graphene-Oxide-
À
Based Gel as a Nanoreactor and Nanohybrid-Catalyzed C C Bond
Formation
Abhijit Biswas and Arindam Banerjee*[a]
Abstract: New graphene oxide (GO)-
based hydrogels that contain vita-
min B2/B12 and vitamin C (ascorbic
acid) have been synthesized in water
(at neutral pH value). These gel-based
soft materials have been used to syn-
thesize various metal nanoparticles, in-
cluding Au, Ag, and Pd nanoparticles,
as well as nanoparticle-containing re-
duced graphene oxide (RGO)-based
nanohybrid systems. This result indi-
cates that GO-based gels can be used
as versatile reactors for the synthesis of
different nanomaterials and hybrid sys-
tems on the nanoscale. Moreover, the
RGO-based nanohybrid hydrogel with
Pd nanoparticles was used as an effi-
À
cient catalyst for C C bond-formation
reactions with good yields and showed
high recyclability in Suzuki–Miyaura
coupling reactions.
Keywords: cross-coupling
·
graphene · nanoparticles · nano-
reactors · vitamins
Introduction
ious nanomaterials, including metal nanoparticles (Au, Ag,
and Pd) and nanohybrids with RGO and metal nanoparti-
cles. Thus, there is a real need for the design and construc-
tion of gel-based soft materials that can act as reactors for
making different nanomaterials and hybrid systems in aque-
ous medium.
Supramolecular gels that are formed by trapping a large
number of solvent molecules belong to an important class of
soft materials that have fascinating applications in several
fields, including cell cultures, tissue engineering, light har-
vesting, regenerating medicine, controlled drug release, oil-
spill recovery, and the synthesis of metal nanoclusters.[1]
Some of these gel matrices have been used to create hybrid
gels that contain carbon-based nanomaterials.[2] Graphene
oxide (GO), or reduced graphene oxide (RGO), is an amaz-
ing carbon-based nanomaterial that contains various reac-
tive functional groups (including hydroxy, carboxylic acid,
and epoxy groups) and can act as a wonderful building
block to create different supramolecular entities, including
gels.[3] Shi and co-workers synthesized a GO-based hydrogel
by using a polymer and DNA.[4] Zhang and co-workers de-
veloped graphene-oxide-based hydrogels by using glucono-
d-lactone,[5a] whilst the same group also made a significant
contribution to graphene-based gels.[5b,c] There have been
several other reports on GO- and RGO-based nanohybrid
systems,[6] although there are only a few examples of the use
of micro- or nanomaterials as a nanoreactors for photocatal-
ysis and other applications.[7] However, none of these afore-
mentioned examples employed GO-containing-hydrogel-
based nanoreactors for the tailored “green synthesis” of var-
Herein, vitamin-containing (vitamin B2/B12) GO-based
gel-phase materials have been found to be versatile reactors
for the formation of different metal nanoparticles (Au, Ag,
and Pd). Furthermore, RGO-based hydrogels were also fab-
ricated through the in situ reduction of the corresponding
metal ions and GO in the presence of another vitamin, as-
corbic acid (vitamin C). The merit of this system is that judi-
cial choice of the constituents of the gel-based material, that
is, 1) water (as a medium), 2) GO and vitamin B2/B12 (as ge-
lator molecules), and 3) ascorbic acid (as the reducing
agent), influence the reactivity of the gel, such that one of
these vitamins (B2 or B12) promotes gelation, whereas the
other vitamin (vitamin C) acts as a reducing agent to reduce
GO into RGO and other metal salts into their correspond-
ing metal nanoparticles within the hydrogel matrix. Thus,
this gel system holds great promise as a nanoreactor for the
formation of various nanoparticles and/or RGO-containing
nanohybrid systems by using a simple “green chemical” ap-
proach (Scheme 1).
The Suzuki–Miyaura coupling reaction is a well-estab-
À
lished method for the creation of C C bonds by using a pal-
ladium catalyst.[8] A number of examples of Suzuki–Miyaura
[a] A. Biswas, Prof. Dr. A. Banerjee
Department of Biological Chemistry
Indian Association for the Cultivation of Science
Jadavpur, Kolkata-700032 (India)
Fax : (+91)33-2473-2805
À
C C coupling reactions have been reported with palladium
nanoparticle catalysts. Amatore and co-workers reported
that Au@Pd nanoparticles acted as an efficient catalyst in
Suzuki–Miyaura cross-coupling reactions.[9] Recently, it was
found that GO- or RGO-supported Pd nanoparticles
showed superior catalytic activity to Pd nanoparticles with-
E-mail: bcab@iacs.res.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201402695.
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Abhijit Biswas and Arindam Banerjee
Results and Discussion
GO can form hydrogel matrixes in the presence of other
molecules through p–p stacking and/or hydrogen-bonding
interactions by utilizing large polyaromatic surfaces and
other oxygenated functional groups (OH, COOH, and
epoxy groups) under appropriate conditions. Two biological-
ly important molecules, vitamin B12 and vitamin B2, have
been separately introduced as cross-linking agents into dis-
persions of GO in aqueous medium, which ultimately leads
to the formation of stable hydrogels at room temperature
(see the Supporting Information, Figure S1). Both of these
vitamins (B12 and B2), which contain NH2 and OH function-
alities, can form strong hydrogen bonds with the OH and
COOH groups of GO sheets. Moreover, each of these vita-
mins contains aromatic moieties that can interact with the
polyaromatic surface of GO through p–p stacking interac-
tions. Fluorescence emission spectroscopy of vitamin-B2-
containing GO-based hydrogels has been performed to ex-
amine the p–p stacking interactions (Figure 1). Free vita-
Scheme 1. Schematic representation of the GO gel and its function as
a versatile reactor for making different nanomaterials, including metal-
nanoparticle-containing RGO-based nanohybrid gels.
out a GO/RGO support, owing to their high thermal and
chemical stability, as well as to the large specific surface
area of GO and RGO.[10] In addition, RGO was insoluble in
most solvent, which enabled the effective removal of the
catalyst owing to the highly hydrophobic nature of RGO.
Mꢁlhaupt and co-workers synthesized GO–Pd and CDG–Pd
nanocomposite materials as air-stable catalysts and success-
fully applied them to the Suzuki–Miyaura coupling reac-
tion.[11] El-Shall and co-workers prepared graphene-support-
ed Pd nanoparticles that showed high catalytic activity in
Suzuki–Miyaura coupling reactions with high turnover fre-
quencies.[12] Gao and co-workers developed Ag–Pd@rGO bi-
metallic nanoparticles catalysts that exhibited high catalytic
activity in Suzuki–Miyaura coupling reactions.[13] Recently,
Camp et al. reported a Suzuki–Miyaura coupling reaction by
using in situ generated glucose-derived palladium nanoparti-
cles.[14] However, none of these examples included Suzuki–
Miyaura reactions by using a palladium-nanoparticle-con-
taining RGO-based hydrogel as a catalyst. A major problem
associated with Pd-nanoparticle-containing catalysts is the
gradual loss of their catalytic activity, owing to the need for
cumbersome separation of the catalyst from the reaction
mixture. To circumvent this drawback, it is necessary to use
a new catalyst that is easily separable from the reaction mix-
ture. To the best of our knowledge, herein, we report the
first palladium-nanoparticle-containing RGO–vitamin B2-
based hydrogel as a catalyst for Suzuki–Miyaura coupling
reactions. The use of this catalyst has several advantages:
1) it uses relatively inexpensive chemicals for the catalyst
preparation in an environmentally friendly approach; 2) it
employs a facile, straightforward procedure for the separa-
tion of the catalyst from the reaction mixture; and 3) the hy-
drogel catalyst showed excellent reusability (up to 15 times)
without the significant loss of catalytic activity.
Figure 1. Fluorescence emission spectra of vitamin B2 and GO–vitamin B2
in water; excitation: 488 nm.
min B2 exhibited a fluorescence emission maximum at
532 nm upon excitation at 488 nm. However, considerable
quenching of the emission maximum was observed in a vita-
min-B2-containing GO-based hydrogel at the same excita-
tion wavelength, which indicated the presence of p–p stack-
ing interactions between the GO sheets and the molecules
of vitamin B2 in the hydrogel. The minimum gelation con-
centrations (MGCs) of the vitamin-B12- and vitamin-B2-con-
taining hydrogels were 0.20% (w/v) and 0.28% (w/v), re-
spectively, thus suggesting that each of these vitamin mole-
cules acted as a good cross-linker between the GO sheets to
form the gels. To gain an insight into the morphologies of
the GO-based hydrogels, TEM analysis was performed (see
the Supporting Information, Figure S2). TEM images of
these hydrogels revealed the presence of large self-assem-
bled GO nanosheets in the supramolecular hydrogel. Inter-
estingly, the GO–vitamin B12/B2 hydrogels could be easily
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reduced in the presence of another vitamin, ascorbic acid
(vitamin C), to form a reduced graphene oxide (RGO)-
based hydrogel. The merit of using vitamin C is that the
entire reduction process is environmentally friendly, because
vitamin C is nontoxic and, during the reduction, no gaseous
side-products are produced. In this transformation, vita-
min C does not disturb the gel network. However, the gel
material shrinks slightly at the time of reduction, but still
contains 70% water in the RGO–vitamin hydrogel.
The formation of RGO in the presence of ascorbic acid
was examined by Raman spectroscopy, which is a powerful
tool for characterizing carbon-based materials. Two funda-
mental vibration modes were observed in the spectrum of
the RGO-based gel at 1345 and 1586 cmÀ1, which corre-
sponded to the D and G bands of RGO, respectively
(Figure 2). The higher intensity ratio of the D/G bands for
the vitamin-B2-containing RGO-based gel suggested the for-
mation of more sp2 domains in RGO after the reduction of
GO in the gel phase.
Figure 3. Frequency dependence of the dynamic storage modulus (G’)
and the loss modulus (G’’) of GO- and RGO-based hydrogels in the pres-
ence of vitamin B2 or vitamin B12.
ing GO gels. The higher G’ value (129 KPa) of the RGO–vi-
tamin B12 gel than the RGO–vitamin B2 gel (115 KPa) at
a fixed angular strength of 103 radsÀ1 was due to a narrowing
of the interlayer distance in RGO. Therefore, the stiffness of
these gels followed the order: RGO–vitamin B12 >RGO–vi-
tamin B2 >GO–vitamin B12 >GO–vitamin B2.
Ascorbic acid (vitamin C) can readily reduce some metal
salts into their corresponding metal nanoparticles
(MNPs).[15] Thus, we anticipated that, by utilizing this reduc-
ing property of ascorbic acid, we could obtain metal-nano-
particle-fabricated RGO-based nanohybrid systems. More-
over, the free space within the 3D cross-linked network
system in the hydrogels and also the in situ reduction of the
metal ions provided good nucleation and growth of the
nanoparticles. Herein, GO–vitamin/MNPs and RGO–vita-
min/MNPs nanohybrid hydrogels were obtained by in situ
reduction of the corresponding metal ions (Au, Ag, and Pd)
and GO (Scheme 1). Both of these hydrogel formations
(GO–vitamin/MNPs and RGO–vitamin/MNPs) were assist-
ed by vitamin C. The formation of metal nanoparticles
within the hydrogel matrix was characterized by TEM,
SAED, and EDX analyses. TEM images (Figure 4) revealed
that these nanoparticles were spherically shaped and uni-
formly distributed within the GO and RGO nanosheets.
EDX analysis (see the Supporting Information, Figure S3)
also showed the presence of metal nanoparticles within the
hydrogels. SAED analysis (see the Supporting Information,
Figure S4) showed a ring pattern that corresponded to each
of these nanoparticles (Au, Ag, and Pd).
The metal-nanoparticle-anchored RGO–vitamin hydrogel
was self-supported and free-standing and, hence, it could be
used as a nanocatalyst. Thus, the Pd-nanoparticle-containing
RGO–vitamin B2 nanocomposite hydrogel (Pd/RGO–vita-
min B2) was tested as a nanocatalyst in Suzuki–Miyaura cou-
pling reactions. The in situ synthesized and well-dispersed
Pd nanoparticles within the graphene sheets showed good
catalytic activity in Suzuki–Miyaura reactions at relatively
low temperatures in an aerobic environment with good
yields (Table 1). One of the most striking advantages of this
catalyst is its recovery process and recyclability. The efficien-
Figure 2. Raman spectrum of a dried sample of a vitamin B2-containing
RGO-based hydrogel.
The viscoelastic properties of these hydrogels were exam-
ined by measuring their rheological properties at a fixed
concentration of 1.8% (w/v; Figure 3). The storage modulus
(G’) and loss modulus (G’’) were monitored as a function of
angular frequency under a fixed strain of 0.1%. The G’
values of the vitamin-B12- and vitamin-B2-containing gels
were 56.4 and 41.4 KPa, respectively, at a fixed angular
strength of 103 radsÀ1. So, the vitamin-B12-containing gel
was stronger than the vitamin-B2-containing gel, because vi-
tamin B12 bound more strongly with GO than vitamin B2.
Although both vitamins (B12 and B2) contained NH2 and
OH functional groups and aromatic moieties for participat-
ing in hydrogen-bonding interactions and p–p interactions
with GO, respectively, the number of hydrogen bonds for as-
sociation and gelation was higher in the GO–vitamin B12
pair than in the GO–vitamin B2 pair. In these RGO gels, the
gel strength was dramatically higher than in the correspond-
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Abhijit Biswas and Arindam Banerjee
Table 1. Suzuki–Miyaura coupling reactions between aryl halides and
aryl boronic acids catalyzed by a Pd-nanoparticle-containing RGO–vita-
min B2 hydrogel.[a]
Entry
Aryl
halide
Aryl boronic
acid
Product
Yield
[%][b]
1
2
93
94
3
4
5
97
96
95
Figure 4. TEM images of A) Ag, B) Au, and C) Pd-nanoparticle-contain-
ing GO-based hydrogels and D) Ag, E) Au, and F) Pd-nanoparticle-con-
taining RGO-based hydrogels.
cy of the catalyst is dependent on the isolation and recovery
of the catalyst from the reaction mixture and, therefore, its
reusability also depends on how successfully it is recovered.
In most of the reported applications of such systems in het-
erogeneous catalysis, the catalyst has been removed through
filtration or centrifugation[6g,8a,c,e] and these techniques can
be attributed to some loss of catalytic activity.
6
97
Sun and co-workers recently reported the formation of
semi-heterogeneous palladium-nanoparticle-supported
7
8
9
97
91
93
a
Fe3O4/sulfonated-graphene catalyst, which was efficiently re-
movable owing to the presence of magnetic nanoparticles in
the system.[16] Herein, we used the Pd-nanoparticle-contain-
ing RGO–vitamin B2 hybrid hydrogel as a potential catalyst
to test the recyclability of the catalyst. This catalyst re-
mained in its gel form until the end of the reaction and,
thus, it was easily separable from the reaction mixture by
using a spatula (Figure 5). Therefore, there was no loss of
catalyst for the subsequent reaction. Moreover, the catalyst
contained graphene sheets. Owing to its large polyaromatic
surface, the graphene sheets could promote the adsorption
of reacting molecules through p–p interactions. Therefore,
[a] Reaction conditions: aryl halide (1 mmol), aryl boronic acid
(1.2 mmol), K2CO3 (3 mmol), water/EtOH (6 mL), Pd/RGO–vitamin B2
1
catalyst (0.73 g), 608C. [b] Yield of isolated product (as determined by H
and 13C NMR spectroscopy).
such adsorption provided a high effective concentration of
the aryl halides and aryl boronic acids near the Pd nanopar-
ticles. This hydrogel catalyst could be easily recycled nine
times without a significant loss of yield (see the Supporting
Information, Figure S6A). After the ninth cycle, the yield of
the reaction decreased slightly. This result may be due to ag-
gregation of the nanoparticles (see the Supporting Informa-
tion, Figure S7A). However, increasing the reaction temper-
ature to 808C caused a remarkable increase in the yield and
the catalyst could be used 15 times efficiently without any
significant loss of catalytic activity (see the Supporting Infor-
mation, Figure S6B).
Figure 5. Photographs of the recovered catalyst. The hydrogel can be ef-
fortlessly separated from the reaction mixture by using a spatula.
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RGO-based hydrogel was obtained by heating the GO gel in the pres-
ence of ascorbic acid at 808C for a few minutes.
Conclusion
A simple and elegant method for the synthesis of various
nanoparticles and nanohybrid systems has been developed
by using a new vitamin-containing GO-based hydrogel as
a nanoreactor. An ascorbic-acid-containing GO-based gel
acted as a highly efficient nanoreactor for the synthesis of
RGO gels, metal-nanoparticle-containing GO gels, and
metal-nanoparticle-containing RGO nanohybrid gels. One
of these hybrid hydrogels (the Pd-nanoparticle-containing
RGO–vitamin B2 gel) acted as a highly efficient catalyst for
Suzuki–Miyaura coupling reactions. This hydrogel catalyst
offers several advantages, including: 1) the use of inexpen-
sive and non-hazardous starting materials; 2) good stability
over the course of the reaction; 3) it catalyzed Suzuki–
Miyaura coupling reactions under aerobic conditions at rela-
tively low temperatures in high yields; and 4) it could be
easily separated from the reaction mixture, thereby showing
remarkable recyclability. This versatile hydrogel-based reac-
tor for the synthesis of various nanomaterials holds great
promise for the creation of advanced gel-based materials for
the tailored production of new functional materials for cat-
alysis and other applications.
Synthesis of metal-nanoparticle-containing RGO-based hydrogels
First, an aqueous solution (0.1 mL) of the metal salt (HAuCl4/AgNO3/
Na2PdCl4; 1 mgmLÀ1
) was added to an aqueous dispersion of GO
(0.6 mL, 11.8 mgmLÀ1) and thoroughly sonicated. Then, ascorbic acid (vi-
tamin C, 0.2 mL, 100 mgmLÀ1) was added to the mixture, followed by vi-
tamin B2 (0.2 mL, 4.5 mgmLÀ1) and the mixture was again sonicated thor-
oughly. Then, the mixture was heated at 808C for a few minutes to
obtain the metal-nanoparticle-containing RGO-based hydrogels. Herein,
we expect that all of the metal ions were reduced in situ by the ascorbic
acid within the hydrogel matrix.
Catalyst preparation
First, an aqueous solution (0.1 mL) of a Pd2+ salt (8.17 mm) was added to
an aqueous dispersion of GO (0.6 mL, 11.8 mgmLÀ1) and the mixture
was thoroughly sonicated. Then, ascorbic acid (vitamin C, 0.2 mL,
100 mgmLÀ1) was added, followed by vitamin B2 (0.2 mL, 4.5 mgmLÀ1
)
and the mixture was sonicated for a few minutes. Then, the mixture was
heated at 808C to obtain the Pd-nanoparticle-containing RGO–vita-
min B2 hydrogel catalyst.
General procedure for the Suzuki–Miyaura coupling reactions
In a Suzuki coupling reaction, an aryl halide (1 mmol), an aryl boronic
acid (1.2 mmol), and K2CO3 (3 mmol) were dissolved in a water/EtOH
mixture (1:1, 6 mL). Then, the catalyst (0.73 gm, 0.012 wt% Pd) was
added and the reaction was stirred at 608C. The progress of the reaction
was monitored by thin-layer chromatography. After completion of the re-
action (by TLC), the catalyst was removed by a spatula and washed thor-
oughly with MeOH and water. The mixture was extracted with EtOAc
(3ꢂ10 mL). Evaporation of the solvent and purification by column chro-
matography on silica gel provided the pure product, which was character-
Experimental Section
Reagents and materials
1
ized by H and 13 C NMR spectroscopy.
l-Ascorbic acid, H2SO4, KMnO4, sodium nitrate, H2O2, and the solvents
were purchased from SRL, India. Others materials were purchased from
Sigma–Aldrich.
Synthesis of graphene oxide
Acknowledgements
Graphene oxide (GO) was synthesized from graphite powder (<30 mm)
by using a modified Hummers method.[17] In a typical synthesis, graphite
powder (0.5 g) was added to concentrated H2SO4 (20 mL) and the mix-
ture was stirred. Then, sodium nitrate (NaNO3, 0.25 g) was added and
the mixture was cooled to 08C. Under vigorous agitation, KMnO4 (1.5 g)
was gradually added to this mixture and the temperature was kept at less
than 208C to prevent overheating. Then, the resulting mixture was trans-
ferred into a 358C water bath and stirred for 30 min, during which time
a brownish gray paste was formed. Then, the mixture was diluted by the
addition of water (30 mL). During the addition of the water, the temper-
ature was raised to 908C and this temperature was maintained for
15 min. Then, the solution was mixed with warm water (80 mL) and 30%
H2O2 (0.5 mL) was added to ensure the completion of the reaction. The
warm solution was filtered and washed thoroughly with HCl and warm
water. The solid was dispersed in distilled water (100 mL) by sonication.
Then, the solution was centrifuged at 3000 rpm for 15 min. The filtrate
was re-suspended by using sonication and centrifuged again at
20000 rpm. The re-suspension/centrifugation process was repeated sever-
al times. Finally, the centrifuged viscous graphene oxide was collected
and freeze-dried under vacuum to afford a solid powder.
A.B. thanks the CSIR (New Delhi, India) for financial assistance.
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Received: June 20, 2014
Published online: September 15, 2014
Chem. Asian J. 2014, 9, 3451 – 3456
3456
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim