Bifunctional graphene/γ-Fe2O3 hybrid aerogels with double nanocrystalline networks for enzyme immobilization

Article abstract of DOI:10.1002/smll.201202923

Highly porous hosting materials with conducting (favorable to electron transfer) and magnetic (favorable to product separation) bicontinuous networks should possess great potentials for immobilization of various enzymes in the field of biocatalytic engineering, but the synthesis of such materials is still a great challenge. Herein, bifunctional graphene/γ-Fe₂O ₃ hybrid aerogels with quite low density (30-65 mg cm^-3), large specific surface area (270-414 m² g^-1), high electrical conductivity (0.5-5 × 10^-2 S m^-1), and superior saturation magnetization (23-54 emu g^-1) are fabricated. Single networks of either graphene aerogels or γ-Fe₂O ₃ aerogels are obtained by etching of the hybrid aerogels with acid solution or calcining of the hybrid aerogels in air, indicative of the double networks of the as-synthesized graphene/γ-Fe₂O₃ hybrid aerogels for the first time. The resulting bifunctional aerogels are used to immobilize β-glucuronidase for biocatalytic transformation of glycyrrhizin into glycyrrhetinic acid monoglucuronide or glycyrrhetinic acid, with high biocatalytic activity and definite repeatability. Graphene/γ- Fe₂O₃ hybrid aerogels with conducting (favorable to electron transfer) and magnetic (favorable to product separation) bicontinuous networks are fabricated for the first time and used to immobilize β-glucuronidase for the biocatalytic transformation of glycyrrhizin into glycyrrhetinic acid monoglucuronide or glycyrrhetinic acid, with high biocatalytic activity and definite repeatability. Copyright

Full text of DOI:10.1002/smll.201202923

Aerogels
Bifunctional Graphene/γ-Fe2O3 Hybrid Aerogels
with Double Nanocrystalline Networks for Enzyme
Immobilization
Liang Chen, Bin Wei, Xuetong Zhang,* and Chun Li*
H ighly porous hosting materials with conducting (favorable to electron transfer)
and magnetic (favorable to product separation) bicontinuous networks should
possess great potentials for immobilization of various enzymes in the field
of biocatalytic engineering, but the synthesis of such materials is still a great
challenge. Herein, bifunctional graphene/γ-Fe2O3 hybrid aerogels with quite
low density (30–65 mg cm−3 ), large specific surface area (270–414 m2 g−1 ), high
electrical conductivity (0.5–5 × 10−2 S m−1 ), and superior saturation magnetization
(23–54 emu g−1 ) are fabricated. Single networks of either graphene aerogels or γ-Fe2O3
aerogels are obtained by etching of the hybrid aerogels with acid solution or calcining
of the hybrid aerogels in air, indicative of the double networks of the as-synthesized
graphene/γ-Fe2O3 hybrid aerogels for the first time. The resulting bifunctional
aerogels are used to immobilize β-glucuronidase for biocatalytic transformation of
glycyrrhizin into glycyrrhetinic acid monoglucuronide or glycyrrhetinic acid, with
high biocatalytic activity and definite repeatability.
to industrial-scale thermal-insulation.[1–3] They are generally
1. Introduction
prepared from molecular precursors by sol-gel processing
and then by supercritical fluid drying or freeze drying to
replace the solvents in the wet gels with air.[4] In most cases
they are amorphous and metastable from the point of view
of their structural and thermodynamic properties.[5] So far
it is a great challenge for the sol-gel derived aerogels that
the parameters of their pores (e.g. size, geometry, order, etc.)
and solid walls (e.g. composition, size, geometry, crystal-
linity, etc.) of the aerogels can be respectively regulated at
will by tuning synthetic conditions during sol-gel processing,
and thus the structures and functionalities of the resulting
aerogels can be adjusted. Interpenetrating organic/inorganic
networks of resorcinol-formaldehyde (RF)/metal oxide
aerogels,[6] obtained in one pot through the catalytic effect
of gelling solutions of hydrated metal ions on the gelation
of RF have recently been demonstrated to provide such an
alternative.
Aerogels are a kind of highly porous three-dimensional
architectures with charming properties such as quite low
density, large open pore, high specific surface area, etc., and
thus have wide applications ranging from lab-scale catalysis
L. Chen, Prof. X. Zhang
School of Materials Science & Engineering
Beijing Institute of Technology
Beijing, 100081, PR China
E-mail: zhangxtchina@yahoo.com
B. Wei, Prof. C. Li
School of Life Science and Technology
Beijing Institute of Technology
Beijing, 100081, PR China
E-mail: lichun@bit.edu.cn
B. Wei
School of Chemistry and Chemical Engineering
Shihezi University
Shihezi 832003, Xinjiang, PR China
Graphene, the first prepared 2D nanocrystal,[7,8] has
stimulated ever-increasing attention in recent years owing to
its exceptional physical and chemical properties.[9–13] It can
be envisioned that graphene aerogels have great potentials
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L. Chen et al.
full papers
in many fields due to combination of their respective char- (30–65 mg cm−3 ), large specific surface area (270–414 m2 g−1 ),
acteristics of the graphene sheets and aerogels. Worsley high electrical conductivity (0.5−5 × 10−2 S m−1 ) and superior
et al.[14] have initially synthesized graphene aerogels with high saturation magnetization (23−54 emu g−1 ). To our surprise,
electrical conductivity by sol-gel polymerization of RF in an if anyone of two components of the hybrid aerogels have
aqueous suspension of graphene oxide (GO), and followed been completely removed by an appropriate treatment, the
by pyrolysis of the organic cross-links and thermal reduc- left one is still in original monolithic shape, indicating double
tion of the GO to graphene occurred simultaneously during networks existed within the resulting hybrid aerogels. The
pyrolysis after supercritical CO2 drying of GO-RF gels. Latter as-prepared graphene/Fe2O3 hybrid aerogels have been used
on, mechanically strong and electrically conductive graphene in immobilization of β-glucuronidase for biocatalytic trans-
aerogels[15] have been synthesized in our lab by supercritical formation of glycyrrhizin into glycyrrhetinic acid monoglu-
drying of hydrogel precursors synthesized from reduction of curonide or glycyrrhetinic acid. Results show that the target
GO with L-ascorbic acid. In situ incorporating of any other enzyme, immobilized to the hybrid aerogels through phys-
components with a controlled way is a simple but creative ical adsorption with the maximum loading amount up to
strategy to functionalize the as-synthesized graphene aero- 2.45 mg mg−1 , has exhibited high biocatalytic activity and
gels. For example, we have synthesized carbon nanotube/ definite repeatability. The work presented here, to the best
graphene hybrid aerogels[16] in our lab by supercritical CO2 of our knowledge, is the first report on graphene-based aero-
drying of their hydrogel precursors obtained from heating the gels with double networks. More importantly, this strategy
aqueous mixtures of graphene oxide and carbon nanotubes may give inspiration to synthesize a series of graphene-based
with vitamin C without stirring. Yu et al.[17] and Yan et al.[18] aerogels hybridized with other metal oxides and the resulting
have independently incorporated magnetic Fe3O4 nanoparti- bifunctional aerogels may immobilizing various enzymes for
cles into graphene hydrogel precursors and then freeze-dried biological catalysis, biofuel cell, etc.
the precursors for obtaining the corresponding graphene/
nanoparticle hybrid aerogels. In comparison with super-
critical drying, Freeze drying can generally keep the compo- 2. Results and Discussion
nents of the precursors from running off as no extraction with
2.1. Synthesis and Characterization of the Bifunctional
Aerogels
supercritical fluid has happened during drying of the aerogel
precursors, however in situ growth of the ice crystals can push
a lot of mesopores cracked to form macropores along the
freeze direction, which leads to low specific surface area (usu- Synthesis of the graphene/Fe2O3 hybrid aerogels was illus-
ally less than 100 m2 g−1 ) of the resulting aerogels.
trated in Figure 1. A fix amount of FeCl3•6H2O was added to
Fast development of biotechnology requires immobili- GO N,N-dimethylformamide (DMF) suspension to form the
zation of active biospecies, e.g., enzymes or cells on porous uniform mixture, then epichlorohydrin was added to above
matrix including mesoporous activated carbon, glass beads, mixture with stirring for a short while and then standing still
polymers, mesoporous silica, organic-inorganic hybrids, over a night to form the gel. Need to point our that, during the
etc.[19–22] The ability to form materials under aqueous, room- gel formation, if no GO existed in the mixture, the resulting
temperature conditions (at which biospecies are active) gel would be broken at the following solvent-exchange or
opens up the possibility to encapsulation of biologicals carbonization step, indicating that the existence of GO can
during sol-gel processing, and the main idea of the encap- greatly enhance the mechanical property of the resulting gel;
sulation technique has been easily adopted to aerogels. For and if no ferric chloride existed in the mixture, no gelation
example, cytochrome c has been stabilized in an aerogel would be observed, indicating that epichlorohydrin can not
matrix with over 80% of the protein remaining active in stimulate the gelation of the GO. The resulting graphene/iron
the final aerogel composite.[23] Lipase immobilized in silica oxide aerogel would be obtained as long as the above hybrid
aerogel has shown very high catalytic activity, in most cases gel was processed with solvent-exchange, supercritical CO2
much higher than that of the free lipase as it has a lower ten- drying and carbonization under Ar atmosphere in sequence.
dency to aggregate than that of free lipase when being dis-
X-ray powder diffraction (XRD) has been used to deter-
persed in solid aerogel matrix.[24] However in most cases the mine the crystalline structure of the resulting graphene/
gentle conditions associated with sol-gel chemistry are still Fe2O3 hybrid aerogels as shown in Figure 2a. There are six
too harsh for many biospecies and the entrapped biospecies peaks at 30.1, 35.5, 43.1, 53.4, 57.0 and 62.7° observed in the
usually retain only part of their native activity in fresh gels, hybrid aerogels, consistent with (220), (311), (400), (422),
which drive researchers to develop strategies to immobilize (511) and (440) planes of the standard XRD data for the
the biospecies within the aerogels after supercritical drying. cubic spinal crystal of the bulk maghemite (namely γ-Fe2O3,
In this manuscript we present a rational approach to syn- see JCPDS NO. 04-0755). Comparing the XRD patterns of
thesize crystalline graphene/γ-Fe2O3 hybrid aerogel mono- the hybrid aerogels with those of the graphene or Fe2O3
liths with conducting and magnetic bifunctionality by using powder with the same processing history, we can find that:
epichlorohydrin as a proton scavenger to initiate hydrolysis (1) no conventional peaks of graphene sheets have been
and polycondensation of ferric chloride in the presence detected, suggesting that the graphene sheets are homogene-
of GO, and followed by supercritical drying with CO2 and ously dispersed in the Fe2O3 matrix and overlapping of the
carbonizing in argon in sequence. The resulting graphene/ graphene sheets have not happened[25] and (2) the XRD pat-
Fe2O3 hybrid aerogel monoliths show quite low density terns of the hybrid aerogels are the same as that of the Fe2O3
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Hybrid Aerogels with Nanocrystalline Networks
of transition metal elements under laser
excitation.[31] X-ray photoelectron spec-
troscopy (XPS) has been further used to
determine the phase structure of the iron
oxide in the hybrid aerogels. In compar-
ison with GO, an additional Fe peak can
be observed from the graphene/iron oxide
hybrid aerogel as shown in Figure 2c, sug-
gesting that iron oxide has been embedded
within the graphene sheets. Figure 2d
presents the Fe2p XPS pattern of the
graphene/iron oxide hybrid aerogel, from
which it can be seen that Fe2p3/2 and
Fe2p1/2 are 710.7 eV and 724.8 eV, respec-
tively, with a satellite peak at 719.0 eV
(indicated with an arrow), characteristic
of Fe2O3.[32] Moreover, the changes in
chemical composition of the GO in hybrid
aerogel before and after carbonization
can also be acquired from XPS analysis.
For the GO powder, four types of carbon
bonds: C–C/C=C (284.5 eV), epoxy and
hydroxyl (286.5 eV), carbonyl (287.8 eV),
carboxyl (289.0 eV) can be observed[15] as
shown in Figure 2e. After carbonization,
the intensities of the peaks at 286.5, 287.8
and 288.7 eV are much lower, especially
Figure 1. Illustration of the synthetic route for single-network of the graphene and Fe2O3
aerogels and double-network of the hybrid graphene/Fe2O3 aerogels.
for the peak centered at 286.5 eV as shown in Figure 2f, sug-
gesting that most of the oxygen-containing functional groups
in GO have been removed successfully by thermal reduction
and majority of the conjugated bonds have been restored
accordingly.[33]
powder, indicating that the existence of graphene sheets can
not influence the crystallization of the iron oxide. Raman
spectroscopy has been used to determine the interaction
between components of the graphene/Fe2O3 hybrid aerogels
as shown in Figure 2b. Two prominent peaks at 1348 cm−1 and
1587 cm−1 can be observed from graphene powder, corre-
sponding to D and G band, respectively. It is well known that
D band is ascribed to the edges, other defects and disordered
carbon in the graphene sheets, and G band arises from the
zone centre E2g mode, corresponding to ordered sp2-bonded
carbon atoms in the graphene sheets.[26] Raman spectra of
graphene/Fe2O3 hybrid aerogels show similar D and G bands
to that of the graphene powder, indicating that the structure
of graphene is maintained after the carbonization proce-
dure.[27] The intensity ratios of D to G band (ID/IG) of the
hybrid aerogels increase from 0.84 to 0.98 as the mass ratios
of graphene to Fe2O3 decrease from 1:1 to 1:5, suggesting a
decrease in the size of in-plane sp2 domains of the graphene
sheets as the metal oxide fraction increases, similar to the
results reported elsewhere.[28,29] Furthermore, the G band of
the graphene/Fe2O3 hybrid aerogels show a visible blue-shift
in comparison with that of the graphene, indicating a strong
interaction between graphene sheets and iron oxide nanopar-
ticles, consistent with the observations related to metal oxide/
graphene composites in the literature.[30] For the iron oxide
powder with the same processing history, there are five bands
around 348, 500, 700, 1360 and 1580 cm−1 as shown in Figure 2b
and Figure SI1. This is the characteristic of Fe2O3 rather than
Fe3O4. However these characteristic bands of Fe2O3 can’t
be observed from the hybrid aerogels, which is probably
due to either special interaction between graphene sheets
and Fe2O3 nanoparticles or strong fluorescence background
Morphological structure of the resulting graphene/Fe2O3
hybrid aerogels has been verified by scanning electron micro-
scope (SEM) and transmission electron microscope (TEM) as
shown in Figure 3. Although the Fe2O3 aerogels synthesized
with the same processing parameters as those in our case has
been reported elsewhere,[34] we have not obtained monolithic
Fe2O3 aerogels in our lab probably due to their extremely
fragile attribute. However in the presence of graphene sheets
(actually its precursor GO before carbonization of the hybrid
aerogels) we have easily obtained monolithic graphene/
Fe2O3 hybrid aerogels as shown in Figure 3a, indicating that
2D carbonaceous sheets can greatly enhance the mechanical
property of the iron oxide aerogels. A lot of iron oxide nano-
particles with sphere-like morphology in the size of 10−30 nm
can be observed from both SEM (Figure 3b) and TEM
(Figure 3c) images. High resolution TEM study as shown in
Figure 3d has indicated that the resulting iron oxide nanopar-
ticles attached to the graphene sheets are polycrystalline with
characteristic diffraction pattern of the γ-Fe2O3, consistent
with the results revealed by XRD and XPS analyses. Particu-
larly, even after long time ultrasonication during TEM sample
preparation, the Fe2O3 nanoparticles in the hybrid aerogels
still can be tightly attached to the graphene sheets, suggesting
a strong interaction between graphene sheets and Fe2O3 nano-
particles, which agrees well with above-mentioned Raman
spectroscopy observation. We can envision that random
deposition of iron oxide nanoparticles onto both sides of the
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amount is at ca. 50 wt%, the BET specific
surface area of the as-made hybrid aerogel
reaches its maximum (see Table SI1). This
indicates that more iron oxide attached
onto the graphene sheets would decrease
the specific surface area of the resulting
hybrid aerogels, which is probably due
to that the Fe2O3 nanoparticles them-
selves tend to aggregate owing to their
strong magnetic behaviors.[41] The pore
size distributions obtained from the iso-
therms as shown in Figure 4b indicate a
number of mesopores with the diameters
approximately in the range 10∼12 nm
located in the hybrid aerogels. In addi-
tion, the total pore volume increases from
0.636 to 1.192 cm3 g−1 with the increase of
graphene content. The relative increase in
the pore volume in the resulting hybrid
aerogel comes from the relatively low
amount of Fe2O3 nanoparticles anchoring
on graphene sheets.
Magnetic performance of the graphene/
Fe2O3 hybrid aerogels were investigated
using a vibrating sample magnetometer
(VSM) at room temperature with an
applied magnetic field sweeping back and
forth between −20 KOe and +20 KOe.
It can be seen from Figure 4c that the
magnetic hysteresis loops of all samples
are S-shape. The saturation magnetization
(Ms) of the Fe2O3 powder is 67 emu g−1 ,
which is smaller than the reported value
(74 emu g−1 ) of the bulk Fe2O3.[34] The
decrease in Ms value of the Fe2O3 powder
can be attributed to size effect of Fe2O3
nanoparticles.[41] While the values of satu-
ration magnetization of graphene/Fe2O3
hybrid aerogels are smaller than that
of the Fe2O3 powder due to incorpora-
Figure 2. XRD patterns (a) and Raman spectra (b) of the graphene powder (1), graphene/
Fe2O3 hybrid aerogels with different mass ratios of graphene to Fe2O3: 1:1 (2), 1:2.5 (3),
1:5 (4), and Fe2O3 powder (5). All samples have been synthesized with the same processing
conditions; and XPS survey profiles of the graphene/Fe2O3 hybrid aerogel and GO (c), Fe 2p
XPS pattern of the graphene/Fe2O3 hybrid aerogel (d), C 1s XPS spectra of the pure GO (e) and
graphene/Fe2O3 hybrid aerogel (f). The hybrid aerogel used in (c–f) is with the mass ratio of
graphene to Fe2O3 1:2.5.
tion of magnetically inactive graphene
graphene sheets have produced a large amount of mesopores
(pore diameter in the range 2–50 nm), we will discuss below,
in the resulting hybrid aerogels. However we need to point
out that there are a great deal of macropores (pore diameter
>50 nm) in the order of ca. micrometer can be observed from
SEM images, which provides possibility that our hybrid aero-
gels can accommodate some large biomolecules, e.g., enzyme,
in these macropores.
The porous characteristics of the hybrid graphene/Fe2O3
aerogels have been further confirmed by nitrogen sorption
tests. As shown in Figure 4a, all the adsorption-desorption
curves exhibit type-IV isotherm with a H3 hysteresis loop,
suggesting a characteristic of open wedge-shaped mesopo-
rous structure.[35] The Brunauer-Emmett-Teller (BET) sur-
face areas of the investigated graphene/Fe2O3 hybrid aerogels
are calculated between 271 and 414 m2 g−1 , much higher than
that of other graphene/iron oxide composites reported else-
where (see Figure SI2).[18,36–40] When the graphene loading
sheets into iron oxide aerogels. The saturation magnetiza-
tion of the hybrid aerogels are further decreased from 54 to
23 emu•g−1 with the increase of the mass ratios of graphene to
Fe2O3 from 1: 5 to 1:1. However the saturation magnetization
values of our synthesized hybrid aerogels are much higher
than those of graphene/iron oxide composites reported else-
where[18,36–39] (see Figure SI3), which is probably ascribing
to double nanocrystalline networks of the hybrid aerogels
mentioned below. What’s more, all the magnetic rema-
nences (Mr) of the Fe2O3 powder and the graphene/Fe2O3
hybrid aerogels are nearly zero (the value of the former is
0.83 emu g−1 and values of the latter are in the range from
0.03 to 0.09 emu g−1 ). This indicates that there is almost no
remaining magnetization when the external magnetic field is
removed, suggesting that both Fe2O3 powder and graphene/
Fe2O3 hybrid aerogels exhibit superparamagnetic behaviors,
consistent with results reported elsewhere.[18,36–39] The super-
paramagnetic performance of the hybrid aerogels provides
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Hybrid Aerogels with Nanocrystalline Networks
graphene to Fe2O3 1:2.5 and 1:1 have the
close values in conductivity is one of the
strong evidence that the graphene sheets
have formed conducting network within
the resulting hybrid aerogel matrices.
However the conductivities of these
hybrid aerogels are two orders of magni-
tude lower than those of our synthesized
graphene aerogels reported elsewhere[15]
probably because of the higher junction
resistance of the conducting graphene net-
work, which results from the existence of
iron oxide nanoparticles embedded within
the graphene network observed from SEM
and TEM images.
2.2. Revealing of the Double Networks
for the Resulting Hybrid Aerogels
In order to further confirm the existence
of the conducting graphene network in
the hybrid aerogel matrices, the resulting
graphene/Fe2O3 hybrid aerogels with the
mass ratios of graphene to Fe2O3 from
1:1 to 1:2.5 were etched with 1.0 M HCl
Figure 3. Digital photo of the hybrid graphene/Fe2O3 aerogel under a magnet (a), SEM (b) and
TEM (c,d) images of the typical graphene/Fe2O3 hybrid aerogel. Inset in (d) is the selected
area electron diffraction (SAED) pattern of the hybrid aerogel.
to get rid of the Fe2O3 component and followed by secondly
supercritical CO2 drying, the monolithic graphene aerogels
were obtained as shown in the inset of Figure 5a; while the
resulting graphene/Fe2O3 aerogel with the mass ratio of
graphene to Fe2O3 1:5 was processed with the same proce-
dure, no monolithic graphene aerogel was obtained. These
results show that the graphene conducting network really
an easy way to handle them, as illustrated in Figure 3a, by
using an external magnetic field.
Conductivity of the resulting graphene/Fe2O3 aerogels has
been investigated by four-probe method at room temperature.
Figure 4d shows typical I−V curves of the hybrid aerogels with
different mass ratios of graphene to Fe2O3. It can be seen that
all the hybrid aerogels present linear I−V curves, indicating
that these aerogels are conductive. Resto-
ration of π-conjugated systems from GO
sheets upon thermal anneal,[42] confirmed
by XPS analysis as shown above, is the
reason for the electrical conductivity of
the hybrid aerogels. However the electrical
conductivity values of our synthesized
hybrid aerogels are greatly dependent on
the mass ratios of the graphene to Fe2O3.
When the mass ratio is 1:5, the elec-
trical conductivity of the resulting hybrid
aerogel is 4.97 × 10−3 S m−1 , and when
the mass ratio is 1:2.5, the electrical con-
ductivity of the resulting hybrid aerogel
is 4.63 × 10−2 S m−1 , one order of magni-
tude higher than that of the hybrid aerogel
with the mass ratio 1:5. When the mass
ratio is further increased to 1:1, the elec-
trical conductivity of the hybrid aerogel is
only 5.23 × 10−2 S m−1 , almost the same as
that of the hybrid aerogel with the ratio
1:2.5. Theoretical studies have shown that
graphene sheets have a quite low percola-
tion threshold (less than 0.5 wt%) and can
Figure 4. Nitrogen sorption isotherms (a), pore size distribution curves (b), magnetization
curves (c) and I−V curves (d) of the resulting graphene/Fe2O3 hybrid aerogels with different
form conductive network easily.[43] That
the hybrid aerogels with the mass ratios of mass ratios of graphene to iron oxide.
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further confirmed the existence of double
networks in the resulting graphene/Fe2O3
hybrid aerogels. For comparison, Yan et al.
have reported 3D graphene/Fe3O4 aero-
gels elsewhere.[18] We have repeated Yan’s
work and found that a graphene aerogel
monolith can be obtained after etching the
graphene/Fe3O4
aerogels,
suggesting
the presence of graphene network within
the graphene/Fe3O4 aerogels; while after
calcination process, we have obtained
Fe3O4 powders instead of Fe3O4 frame-
work, revealing that Fe3O4 network do not
exist within the resulting graphene/Fe3O4
aerogels; thereby we can conclude that this
graphene/Fe3O4 aerogels do not possess
double-network structure. Furthermore,
Yu et al. have synthesized graphene/α-
FeOOH aerogel elsewhere,[17] however
they have claimed that, after etching with
HCl to remove α-FeOOH component,
the graphene 3D network structure was
Figure 5. SEM images of graphene (a,b) and Fe2O3 (c,d) aerogels made by etching and
calcining the graphene/Fe2O3 hybrid aerogels, respectively. (The insets in (a) and (c) are broken down due to the severe reaggrega-
digital photos of the graphene and Fe2O3 aerogels, respectively).
tion of the graphene sheets, and thus we
conclude that this graphene/α-FeOOH
aerogel do not possess the double network, neither. These
comparisons have indicated that the double networks of the
graphene/Fe2O3 hybrid aerogels might be unique among the
graphene-based composite aerogels.
exists in the graphene/Fe2O3 hybrid aerogels with the mass
ratios of graphene to Fe2O3 from 1:1 to 1:2.5, consist with the
above results revealed by conductivity investigation. To our
surprise, if the resulting graphene/Fe2O3 hybrid aerogels with
the mass ratios of graphene to Fe2O3 from 1:1 to 1:5 were cal-
cined in air to get rid of the graphene component, the mono-
lithic Fe2O3 aerogels were obtained as shown in the inset of
Figure 5c, indicating the Fe2O3 magnetic network really exists
in the resulting graphene/Fe2O3 hybrid aerogels with the
mass ratios of graphene to Fe2O3 from 1:1 to 1:5, which might
be the reason that the saturation magnetization values of our
synthesized hybrid aerogels are much higher than those of
graphene/iron oxide composites reported elsewhere.[18,36–39]
The existence of conductive and magnetic networks in the
resulting graphene/Fe2O3 hybrid aerogel matrix has indicated
the resulting hybrid aerogels reported herein are bifunctional
ones with double networks.
Figure 5a,b present typical SEM images of the graphene
aerogel made by etching of the graphene/Fe2O3 hybrid
aerogel with 1.0 M HCl solution, from which a well-defined
and interconnected 3D porous graphene network can be
clearly verified. The partial overlapping or coalescence of
the graphene sheets has resulted in cross-linking to build the
resulting graphene aerogel. Furthermore, there is no obvious
Fe2O3 nanoparticles located within the graphene aerogel, sug-
gesting all the Fe2O3 nanoparticles attached to the graphene
sheets were completely removed and a graphene-only aerogel
was generated. Figure 5c and 5d present SEM images of the
Fe2O3 aerogel made by calcinating of the graphene/Fe2O3
hybrid aerogels in air, from which a well-defined and intercon-
nected 3D porous Fe2O3 network can be clearly verified. No
obvious graphene sheets existed in the remaining Fe2O3 aero-
gels, indicating all the graphene sheets embedded within the
Fe2O3 have been totally burned out. These observations have
The graphene and Fe2O3 aerogels obtained from the as-
made graphene/Fe2O3 hybrid aerogels were further inves-
tigated by XPS and XRD analyses as shown in Figure SI4.
After calcination of the as-prepared graphene/Fe2O3 hybrid
aerogel, no C composition can be detected by XPS survey
analysis in the remaining product as shown in Figure SI4a,
indicating the graphene sheets in the hybrid aerogels were
burned out completely, and no obvious difference in XRD
patterns has been observed between original graphene/Fe2O3
hybrid aerogel as shown in Figure 2a and the remaining Fe2O3
aerogel as shown in Figure SI4b, indicating that the remaining
aerogel is γ-Fe2O3 instead of α-Fe2O3. After etching of the as-
prepared graphene/Fe2O3 hybrid aerogel with HCl solution,
no Fe composition can be detected by XPS survey analysis
in the remaining product as shown in Figure SI4a, suggesting
Fe2O3 nanoparticles in the hybrid aerogels have been etched
out totally. Furthermore, double diffraction humps have
appeared at 24.4° and 44.1° in the XRD pattern as shown in
Figure SI4b, which could be the characteristic peaks of the
(002) and (100) planes of the graphite,[44] indicating that par-
tial overlapping of the graphene sheets has happened during
etching.
To further confirm the porous attribute of the resulting
graphene and Fe2O3 aerogels obtained from the as-made
graphene/Fe2O3 hybrid aerogels, nitrogen adsorption-
desorption isotherm were performed (see Figure SI5). Both
graphene and Fe2O3 aerogels exhibit a type-IV isotherm with
a H3 hysteresis loop, same as the graphene/Fe2O3 hybrid
aerogels, showing of a mesoporous structure. BET analysis
reveals that the resulting Fe2O3 aerogel has a rather large
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Hybrid Aerogels with Nanocrystalline Networks
etc.[46,47] GL is also widely used in food
industry as an important natural sweet-
ener. However, if the human body takes
in GL too much, it will produce certain
side effects, such as reducing the sodium
discharge of human body and increasing
the potassium discharge.[48] GL contains
a pentacyclic triterpenoid nucleus and
two distal glucuronides. It can be trans-
formed into glycyrrhetinic acid monoglu-
curonide (GAMG) or glycyrrhetinic acid
(GA) by β-glucuronidase (PGUS-E) as
shown in Figure 6a. GAMG is a kind of
high sweetness with low calorie functional
sweetener.[49] While GA serves as a very
important functional drug, it can inhibit
information delivery of the original cancer
cells and also possess a superior func-
tion of anti-viral infections, especially for
the carcinogenic viruses such as hepatitis
viruses, Epstein-Barr virus and HIV infec-
tion.[50] GA also can be used as an additive
for cosmetics due to its ability of scav-
enging oxygen free radical. In the previous
study, one of authors (C. Li) of this manu-
script and his co-authors have screened a
fungal strain, Penicillium purpurogenum
Li-3, which can use GL as a carbon source
and convert it into GAMG by expressed
β-glucuronidase.[51,52] C. Li et al. have also
cloned the gene (GenBank Accession No.
EU095019) and overexpressed PGUS-E in
Escherichia coli BL21.[53] After transfor-
mation of GL, separation of the reaction
components from the solution system has
been troubled by recovering of PGUS-E.
Herein we try to immobilize PGUS-E
to the as-made graphene/Fe2O3 hybrid
Figure 6. (a) Transformation of GL into GAMG and GA catalyzed by β-glucuronidase (PGUS-
E), (b) effect of time on the immobilization amount of PGUS-E to the graphene/Fe2O3 hybrid
aerogel, (c) SEM image of the hybrid aerogel after immobilization of the PGUS-E, (d) Nitrogen
element mapping image of the hybrid aerogel after immobilization of the enzyme and
(e) Raman spectra of the resulting hybrid aerogel (black line), aerogel-GA (red line), aerogel-
PGUS-E (blue line) and aerogel-GL (pink line) systems.
surface area of 110 m2 g−1 , which is higher than that of the
nanocrystalline iron oxide aerogels made at the same tem-
perature reported elsewhere.[34] In addition, the total pore
volume (0.43 cm3 g−1 ) of the resulting Fe2O3 aerogel is also
higher than that of the nanocrystalline iron oxide aero-
gels reported elsewhere.[34] Similarly, a BET surface area of
441 m2 g−1 can be obtained for the resulting graphene aerogel,
althoughthevalueisalittlelowerthanthatoftheself-assembled
graphene aerogel reported in our previous study,[15] the
graphene aerogel (13∼15 mg cm−3 ) made via etching of the as-
prepared graphene/Fe2O3 hybrid aerogel reported herein is
much lighter than that of the graphene aerogel (31 mg cm−3 )
reported in our previous study.[15]
aerogel and find that PGUS-E immobilized to the hybrid
aerogel has shown high catalytic activity and definite repeat-
ability in transformation of GL into GA with easy separation
of biocatalyst from the reaction mixture.
Loading capacity of the porous matrix can directly deter-
mine the catalytic effect of the immobilized enzyme. To
investigate the maximum adsorption capacity of the hybrid
aerogel for PGUS-E, we have explored relationship between
immobilization amount and adsorption time as shown in
Figure 6b. Results show that the PGUS-E can be immobi-
lized to the hybrid aerogel without any pretreatment, and the
maximum loading amount is about 2.45 mg mg−1 , which is
higher than those of the horseradish peroxidase immobilized
to the GO sheets.[54,55] SEM image as shown in Figure 6c
has indicated that the hybrid aerogel after immobilization of
enzyme biomacromolecules exhibits a smooth and compact
morphology, instead of a well-defined porous network as
observed in Figure 3b, which is probably due to the fact that
2.3. Immobilization of Enzyme Biomacromolecules
for Biocatalysis
Glycyrrhizin (GL), an important triterpenoid saponin,[45] the pores within aerogel have been filled with enzyme com-
is a drug with multi-pharmacological effects such as anti- pletely. Nitrogen element mapping as shown in Figure 6d has
inflammation, antivirus, anti-tumor, immuno-modulating indicated the uniform distribution of N element within the
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blue-shift to 1600 cm−1 , revealing that there is a strong inter-
action between enzyme and hybrid aerogel, which benefits
PGUS-E to move into the pores of the hybrid aerogel. By
the way, there are strong interactions as well between either
hybrid aerogel and GL or hybrid aerogel and GA as the G
bands of the hybrid aerogel after incorporation of either GL
or GA show apparent blue-shifts as well. However, blue-shift
of the G band for the hybrid aerogel incorporated with GL
is bigger than that for the hybrid aerogel incorporated with
GA, which means that, during enzyme catalytic reaction in
the co-existence of GL and GA, GL still can be adsorbed
into the pores of the hybrid aerogel to take place the reac-
tion as the interaction between the hybrid aerogel and GL is
higher than that between the hybrid aerogel and GA.
The catalytic performance of the immobilized PGUS-E
has been compared with those of the free PGUS-E and the
hybrid aerogel as shown in Figure 7a. For the free PGUS-E,
the conversion rate of the GL is up to 99.3% after 4-hour
reaction, indicating that the PGUS-E is a highly efficient bio-
catalyst for GL transformation.While for the graphene/Fe2O3
hybrid aerogel, only 3.74% conversion rate can be obtained
after 4 hours, which might be due to the pseudo-conversion
of the GL resulting from its physical adsorption to the hybrid
aerogel as no target products were observed from the reac-
tion mixture. For the immobilized PGUS-E, the conver-
sion rate of GL is up to 98.0% after the same reaction time,
almost the same as that of the free enzyme. These results
indicate that, after immobilization to the hybrid aerogel, the
target enzyme can keep its full bioactivity and can catalyze
the conversion of GL. However all the GL conversion rate
catalyzed by the immobilized PGUS-E is lower than that
catalyzed by the free enzyme after the same reaction period
as shown in Figure 7a, indicating that the reaction catalyzed
by the immobilized PGUS-E is slower than that catalyzed
by the free enzyme. Diffusion of the raw material (GL) into
the porous matrix and diffusion of the products (GAMG and
GA) out of the porous matrix might be the main reason for
observation of such slowing down transformation of GL into
GAMG and GA. Furthermore, the immobilized PGUS-E
and free enzyme have shared the same optimum pH and tem-
perature conditions for GL transformation (see Figure SI6),
suggesting that the enzyme structure has almost not changed
after immobilization of PGUS-E into hybrid aerogel and that
each component of the graphene/Fe2O3 hybrid aerogel has
no effects on the activity of the immobilized enzyme. To fur-
ther disclose the effect of the immobilization on the activity
of the enzyme, a comparative study on the apparent kinetic
parameters (Km, Vmax) of the reaction has been carried
out between the immobilized enzyme system and the free
enzyme system. The Lineweaver–Burk plot[56] as shown in
Figure 7b has suggested that the hydrolysis of GL catalyzed
by PGUS-E follows the Michaelis–Menten equation.[57] The
Figure 7. (A) Recorded curves of the GL conversion rate vs. reaction time
byusing the free PGUS-E, the hybrid aerogeland the immobilized PGUS-E
to the hybrid aerogel as the catalysts, respectively, (b) Lineweaver–Burk
plot for determination of apparent kinetic parameters of the hydrolysis
of GL catalyzed by free or immobilized PGUS-E, and (c) Arrhenius plot
for calculation of apparent activation energy of the hydrolysis of GL
catalyzed by free or immobilized PGUS-E.
hybrid aerogel matrix, indicative of uniform immobilization much lower value of Km have been observed from the immo-
of PGUS-E into the pores of the hybrid aerogel. To elucidate bilized PGUS-E system (see Table SI2) in comparison with
the driving force for PGUS-E being immobilized to the hybrid that observed from the free enzyme system, and the ratio of
aerogel, Raman spectroscopy has been used and the results Vmax/Km was about twice higher than that observed from the
are shown in Figure 6e. The G band of the graphene/γ-Fe2O3 free PGUS-E system, therefore the affinity capacity between
hybrid aerogel locates at 1594 cm−1 , after immobilization enzyme and GL substrate has been enhanced after enzyme
of enzyme into the aerogel, the G band shows an apparent immobilization, that is to say our hybrid aerogel can keep the
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Hybrid Aerogels with Nanocrystalline Networks
aerogel as a biocatalyst, we have designed
and carried out the following experiments:
the substrate (2 g L−1 GL) was catalyzed
by immobilized enzyme for 2 h and then
biocatalytic reaction was deliberately sus-
pended by separation of the immobilized
enzyme from the reaction mixture with a
magnet. At this time we could observe the
existence of a large quantity of GAMG
and a small quantity of GL in the reaction
mixture from the HPLC graph as shown
in Figure 8c. The supernatant was further
stirred for another 10 h in the absence
of the immobilized enzyme and then the
components of the mixture were analyzed
again by HPLC as shown in Figure 8d. It
was very interesting to see that almost all
substrate GL and intermediate product
GAMG have been transformed into the
final product GA, which ascribed to that
a small amount of the enzyme immobi-
lized to the hybrid aerogel has desorbed
into the solution, which has continued to
catalyze GL or GAMG transformation
into the final product GA. Partial desorp-
tion of the immobilized PGUS-E into the
solution might be another reason for that
GL conversion rate decreases significantly
Figure 8. A digital photo indicating that the graphene/Fe2O3 hybrid aerogel containing
PGUS-E has been separated from the reaction mixture by a magnet (a), illustration of GL
conversion vs. batch cycles when immobilized PGUS-E was used repeatedly (b), HPLC graph
of the reaction mixture catalyzed by the immobilized PGUS-E after 2 hours (c) and HPLC
graph of the supernatant in c reacted further in the absence of the immobilized PGUS-E after
10 hours (d).
with the increase of the batch cycles. These observations have
also indirectly shown that the loading of the PGUS-E into
the highly porous hybrid aerogel has been mainly governed
by the physical adsorption instead of the chemical bonding as
in the case of the latter the immobilized enzyme would not
desorb from the porous matrix to solution.
immobilized enzyme in an active conformation. Moreover,
the activation energy (Ea) of the PGUS-E catalyzed hydrol-
ysis of GL was determined by liner fit of the Arrhenius plot
as shown in Figure 7c, from which the Ea value can be calcu-
lated (see Table SI2). The Ea value for the hydrolysis of GL
by the immobilized PGUS-E was much lower than that of the
free PGUS-E, revealing that the hybrid aerogel might make
the formation of the ‘enzyme–substrate complex’ easier and
more stable due to its porous attribute.[58]
3. Conclusion
Furthermore, PGUS-E immobilized to the hybrid aerogel
can be easily separated from the reaction mixture by a
magnet as shown in Figure 8a and thus can be re-used for the
next batch, which shows great advantages over usual enzyme
immobilization strategies as the separation procedure used
here avoids long-time and multistep purification process.
The catalytic performance of the re-used biocatalyst against
re-use times has been plotted in Figure 8b, from which it can
be seen that, for the 2nd cycle, the conversion rate of the GL
remains 86.6%, while for the 7th cycle, the conversion rate of
GL decreases to 34.8%. The reason that GL conversion rate
decreases significantly with the increase of the batch cycles
probably attributed to the accumulation of the reaction prod-
ucts (GAMG and GA) covered on the surface of PGUS-E
immobilized to the porous matrix, which affects the catalytic
performance of the immobilized enzyme at the next batch.[59,60]
Obviously, immobilization of PGUS-E to the hybrid aerogel
can be re-used, and the re-use cycles are not very high for the
time being, but still superior than those of the other immo-
bilized enzymes reported elsewhere.[60–62] To further disclose
the essence that the conversion rate of GL decreases with
the cycle times when PGUS-E is immobilized to the hybrid
We have developed a general and practical approach to
synthesize graphene/Fe2O3 hybrid aerogels by supercritical
fluid drying and carbonizing of their hydrogel precursors,
which have been made by hydrolysis and polycondensa-
tion of FeCl3 in the presence of GO and epichlorohydrin in
DMF solution. The resulting graphene/Fe2O3 hybrid aero-
gels show quite low density (30–65 mg cm−3 ), large specific
surface area (270−414 m2 g−1 ), high electrical conductivity
(0.5–5 × 10−2 S m−1 ) and superior saturation magnetization
(23–54 emu g−1 ). In addition, we have also obtained intrinsic
graphene and Fe2O3 aerogels by etching of the resulting
graphene/Fe2O3 hybrid aerogels with 1.0 M HCl and calcining
of the resulting graphene/Fe2O3 hybrid aerogels in air atmos-
phere, respectively, which has directly confirmed the exist-
ence of the double skeleton networks in the as-synthesized
graphene/Fe2O3 hybrid aerogels. PGUS-E, one of the impor-
tant enzymes, can be immobilized to the graphene/Fe2O3
hybrid aerogels via physical adsorption with high biocata-
lytic activity and definite repeatability. This synthetic method
might be expanded to other graphene-based metal oxide
hybrid aerogels with double networks and the as-synthesized
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L. Chen et al.
full papers
[26] Y. Sun, C. Li, G. Q. Shi, J. Mater. Chem. 2012, 22, 12810.
[27] N. D. Phu, D. T. Ngo, L. H. Hoang, N. H. Luong, N. Chau, N. H. Hai,
J. Phys. D: Appl. Phys. 2011, 44, 345002.
graphene/metal oxide hybrid aerogels might be used as hosts
to immobilize other important enzymes for biocatalysis.
[28] C. Nethravathi, M. Rajamathi, Carbon 2008, 46, 1994.
[29] K. N. Kudin, B. Ozbas, H. C. Schniepp, R. K. Prudhomme,
I. A. Aksay, R. Car, Nano Lett. 2008, 8, 36.
[30] J. F. Shen, B. Yan, M. Shi, H. W. Ma, N. Li, M. X. Ye, J. Mater. Chem.
2011, 21, 3415.
[31] G. N. Yushin, S. Osswald, V. I. Padalko, G. P. Bogatyreva,
Y. Gogotsi, Diamond Relat. Mater. 2005, 14, 1721.
[32] X. Zhu, Y. Zhu, S. Murali, M. D. Stoller, R. S. Ruoff, ACS Nano
2011, 5, 3333.
Experimental Section
See the Supporting Information for a full explanation of the experi-
mental materials and methods.
[33] F. Meng, X. Zhang, B. Xu, S. Yue, H. Guo, Y. Luo, J. Mater. Chem.
2011, 21, 18537.
Supporting information
[34] J. W. Long, M. S. Logan, C. P. Rhodes, E. E. Carpenter, R. M. Stroud,
D. R. Rolison, J. Am. Chem. Soc. 2004, 126, 16879.
[35] S. B. Yang, X. L. Feng, K. Mullen, Adv. Mater. 2011, 23, 3575.
[36] B. Li, H. Cao, J. Shao, M. Qu, J. H. Warner, J. Mater. Chem. 2011,
21, 5069.
Supporting Information is available from the Wiley Online Library
or from the author.
[37] X. Yang, X. Zhang, Y. Ma, Y. Huang, Y. Wang, Y. Chen, J. Mater.
Chem. 2009, 19, 2710.
Acknowledgements
[38] J. Su, M. H. Cao, L. Ren, C. W. Hu, J. Phys. Chem. C 2011, 115, 14469.
[39] X. Y. Li, X. L. Huang, D. P. Liu, X. Wang, S. Y. Song, L. Zhou,
H. J. Zhang, J. Phys. Chem. C 2011, 115, 21567.
[40] G. M. Zhou, D. W. Wang, F. Li, L. L. Zhang, N. Li, Z. S. Wu, L. Wen,
G. Q. Lu, H. M. Cheng, Chem. Mater. 2010, 22, 5306.
[41] A. E. Berkowitz, W. J. Schuele, P. J. Flanders, J. Appl. Phys. 1968.
39, 1261.
The first two authors contributed equally to this work. This work
was financially supported by the National Natural Science Founda-
tion of China (20903009,21176028 and 20976014) and the Inno-
vation Program of the Beijing Institute of Technology.
[42] Z. Sui, X. Zhang, Y. Lei, Y. Luo, Carbon 2011, 49, 4314.
[43] J. Du, L. Zhao, Y. Zeng, L Zhang, F Li, P. Liu, C. Liu, Carbon 2011,
49, 1094.
[44] M. L. Chen, C. Y. Park, J. G. Choi, W. C. Oh, J. Korean Chem. Soc.
2011, 48, 147.
[45] L. Ruzicka, O. Jeger, W. Ingold, Helv. Chim. Acta. 1943, 26, 2278.
[46] K. Mizutani, T. Kambara, H. Masuda, Y. Tamura, Int Congress Ser.
1998, 1157, 225.
[47] J. Cinall, B. Morgenstem, G. Bauer, P. Chandra, H. Rabenau,
H. W. Doerr, Lancet 2003, 361, 2045.
[48] A. Chubachi, H. Wakui, K. Asakura, S. Nishimura, Y. Nakamoto,
A. B. Miura, Intern. Med. 1992, 31, 708.
[49] L. A. Baltina, Curr. Med. Chem. 2003, 10, 155.
[50] M. Ito, M. Tsurusawa, Z. Zha, S. Kawai, Takasaki, T. Fujimoto,
Cancer 1992, 69, 2176.
[ 5 1 ] S . F e n g , C . L i , X . X u , X . W a n g , J. Mol. Catal. B: Enzym. 2006, 43,
63.
[52] X. Wang, S. Feng, X. Wen, H. Li, C. Li, Chinese J. Bioprocess Eng.
2007, 5, 17.
[53] Z. K. Song, X. Y. Wang, G. Q. Chen, C. Li, J. Chem. Ind. Eng. 2008,
59, 3101.
[54] J. Zhang, F. Zhang, H. Yang, X. Huang, H. Liu, J. Zhang, S. Guo,
Langmuir 2010, 26, 6083.
[55] Y. Zhang, J. Zhang, X. Huang, X. Zhou, H. Wu, S. Guo, Small 2012,
8, 154.
[56] Y. L. Liu, J. Huangfu, F. Qi, I. Kaleem, W. E., C. Li, PloS One 2012,
DOI:10.1371/journal.pone.0030998.
[57] D. M. He, I. Kaleem, S. Y. Qin, D. Z. Dai, G. Y. Liu, C. Li, Process
Biochem. 2010, 45, 1916.
[58] W. Y. Lou, M. H. Zong, Y. Y. Zhang, H. Wu, Enzyme Microb Technol.
2004, 35, 190.
[59] S. Nakamoto, N. Machida, Water. Res. 1992, 26, 49.
[60] F. Zhang, B. Zheng, J. Zhang, X. Huang, H. Liu, S. Guo, J. Y. Zhang,
J. Phys. Chem. C 2010, 114, 8469.
[61] E. T. Hwang, B. Lee, M. Zhang, S. H. Jun, J. Shim, J. Lee, J. Kim,
M. B. Gu, Green Chem. 2012, 14, 1884.
[1] D. R. Rolison, Science 2003, 299, 1698.
[2] N. Gaponik, A. K. Herrmann, A. Eychmüller, J. Phys. Chem. Lett.
2012, 3, 8.
[3] J. Biener, M. Stadermann, M. Suss, M. A. Worsely, M. M. Biener,
K. A. Rose, T. F. Baumann, Energy Environ. Sci. 2011, 4, 656.
[4] U. Schubert, N. Hüsing, Angew. Chem. Int. Ed. 1998, 37, 22.
[5] A. C. Pierre, G. M. Pajonk, Chem. Rev. 2002, 102, 4243.
[6] N. Leventis, N. Chandrasekaran, A. G. Sadekar, C. S. Leventis,
H. Lu, J. Am. Chem. Soc. 2009, 131, 4576.
[7] A. K. Geim, K. S. Novoselov, Nat. Mater. 2007, 6, 183.
[8] C. Chen, Q. H. Yang, Y. Yang, W. lv, Y. Wen, P. X. Hou, M. Wang,
H. M. Cheng, Adv. Mater. 2009, 21, 3007.
[9] B. Luo, S. Liu, L. Zhi, Small 2012, 8, 630.
[10] Y. Chen, B. Zhang, G. Liu, X. Zhang, E. T. kang, Chem. Soc. Rev.
2012, 41, 4688.
[11] X. Wan, Y. Huang, Y. Chen. Acc. Chem. Res. 2012, 45, 598.
[12] D. Eder, Chem. Rev. 2010, 110, 1348.
[13] J. Feng, H. J. Zhang, Chem. Soc. Rev. 2012, DOI: 10.1039/
C2CS35069F.
[14] M. A. Worsley, P. J. Pauzauskie, T. Y. Olson, J. Biener, J. H. Satcher Jr.,
T. F. Baumann, J. Am. Chem. Soc. 2010, 132, 14067.
[15] X. Zhang, Z. Sui, B. Xu, S. Yue, Y. Luo, W. Zhan, B. Liu, J. Mater.
Chem. 2011, 21, 6494.
[16] Z. Y. Sui, Q. H. Meng, X. T. Zhang, R. Ma, B. Cao, J. Mater. Chem.
2012, 22, 8767.
[17] H. P. Cong, X. C. Ren, P. Wang, S. H. Yu, ACS Nano 2012, 6, 2693.
[18] W. Chen, S. Li, C. Chen, L. Yan, Adv. Mater. 2011, 23, 5679.
[19] G. G. Cristina, B. M. Angel, F. L. Roberto, C. R. Rafael, Adv. Synth.
Catal. 2011, 353, 2885.
[20] P. Jochems, Y. Satyawali, L. Diels, W. Dejonghe, Green Chem.
2011, 13, 1609.
[21] A. C. Moser, D. S. Hage, Bioanalysis 2010. 2, 769.
[22] K. Yasuhara, S. Miki, H. Nakazono, A. Ohta, J. Kikuchi, Chem.
Commun. 2011, 47, 4691.
[23] J. M. Wallace, J. K. Rice, J. J. Pietron, R. M. Stroud, J. W. Long,
D. R. Rolison, Nano Lett. 2003, 3, 1463.
[24] A. Karout, A. C. Pierre, J. Sol-Gel. Sci. Technol. 2009, 52, 276.
[25] S. B. Yang, X. L. Feng, S. Ivanovici, K. Müllen, Angew. Chem. Int.
Ed. 2010, 49, 8408.
[62] J. Cheng, S. M. Yu, P. Zuo, Water. Res. 2006, 40, 283.
Received: November 23, 2012
Published online: February 19, 2013
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