Catalytic reduction of graphene oxide nanosheets by glutathione peroxidase mimetics reveals a new structural motif in graphene oxide

Article abstract of DOI:10.1002/chem.201303339

A catalytic reduction of graphene oxide (GO) by glutathione peroxidase (GPx) mimics is reported. This study reveals that GO contains peroxide functionalities, in addition to the epoxy, hydroxyl and carboxylic acid groups that have been identified earlier. It also is shown that GO acts as a peroxide substrate in the GPx-like catalytic activity of organoselenium/tellurium compounds. The reaction of tellurol, generated from the corresponding ditelluride, reduces GO through the glutathione (GSH)-mediated cleavage of the peroxide linkage. The mechanism of GO reduction by the tellurol in the presence of GSH involves the formation of a tellurenic acid and tellurenyl sulfide intermediates. Interestingly, the GPx mimics also catalyze the decarboxylation of the carboxylic acid functionality in GO at ambient conditions. Whereas the selenium/tellurium-mediated catalytic reduction/decarboxylation of GO may find applications in bioremediation processes, this study suggests that the modification of GO by biologically relevant compounds such as redox proteins must be taken into account when using GO for biomedical applications because such modifications can alter the fundamental properties of GO.

Full text of DOI:10.1002/chem.201303339

FULL PAPER
DOI: 10.1002/chem.201303339
Catalytic Reduction of Graphene Oxide Nanosheets by Glutathione
Peroxidase Mimetics Reveals a New Structural Motif in Graphene Oxide
Amit A. Vernekar and Govindasamy Mugesh*[a]
Abstract:
A
catalytic reduction of
the glutathione (GSH)-mediated cleav-
age of the peroxide linkage. The mech-
anism of GO reduction by the tellurol
in the presence of GSH involves the
formation of a tellurenic acid and tel-
lurenyl sulfide intermediates. Interest-
ingly, the GPx mimics also catalyze the
decarboxylation of the carboxylic acid
functionality in GO at ambient condi-
tions. Whereas the selenium/tellurium-
mediated catalytic reduction/decarbox-
ylation of GO may find applications in
bioremediation processes, this study
suggests that the modification of GO
by biologically relevant compounds
such as redox proteins must be taken
into account when using GO for bio-
medical applications because such
modifications can alter the fundamen-
tal properties of GO.
graphene oxide (GO) by glutathione
ACHTUNGTRENNUNG
peroxidase (GPx) mimics is reported.
This study reveals that GO contains
peroxide functionalities, in addition to
the epoxy, hydroxyl and carboxylic acid
groups that have been identified earli-
er. It also is shown that GO acts as
a peroxide substrate in the GPx-like
catalytic activity of organoselenium/tel-
lurium compounds. The reaction of tel-
lurol, generated from the correspond-
ing ditelluride, reduces GO through
Keywords: decarboxylation · gluta-
thione peroxidase
· graphene ·
nanosheets · NMR spectroscopy ·
selenium
Introduction
Glutathione peroxidase (GPx) is a well-known selenocys-
teine-containing mammalian enzyme that acts as an antioxi-
dant and protects the cellular components from oxidative
damage by catalytically reducing harmful peroxides in the
presence of glutathione (GSH).[1] During the catalytic cycle,
the selenium moiety of the enzyme undergoes a series of ox-
idation and reduction reactions (Scheme 1). Initially, the se-
lenol group reacts with H2O2 to produce the corresponding
selenenic acid (E-SeOH), which further reacts with GSH to
produce the selenenyl sulfide intermediate (E-SeSG). A nu-
cleophilic attack of another GSH molecule at the sulfur
center of the E-SeSG intermediate regenerates the active
form of the enzyme (E-SeH) (Scheme 1).[2] In the overall re-
action, two equivalents of GSH are oxidized to glutathione
disulfide (GSSG) and H2O2 is reduced to water.
Scheme 1. Proposed catalytic mechanism of GPx, involving selenol, sele-
nenyl sulfide, and selenenic acid intermediates. GR=glutathione reduc-
tase.
During last three decades, several organoselenium com-
pounds have been reported to mimic the activity of
GPx.[3]Although the selenium moieties in some of these
compounds are quite different from that of GPx, these com-
pounds efficiently catalyze the reduction of H2O2 and other
related peroxides in the presence of GSH or aromatic
thiols.[3,4] In addition to organoselenium compounds, much
attention has been directed towards the design and synthesis
of organotellurium compounds that mimic the action of
GPx.[5] Singh and co-workers reported that diaryl ditellu-
AHCTUNGTREGrNNNU ides are much more efficient catalysts than the selenium
analogues in reducing H2O2 by using PhSH as a thiol co-sub-
strate.[6] Liu and co-workers showed that the GPx model, 2,
2’-ditellurobis(2-deoxy-b-cyclodextrin), exhibits high sub-
strate specificity and remarkable catalytic efficiency when 3-
carboxy-4-nitrobenzenethiol is used as a thiol substrate.[7]
Although several aliphatic and aromatic thiols have been
used as co-substrates for the peroxidase-like reactions of se-
lenium and tellurium compounds, the ability of these com-
pounds to catalyze the reduction of substrates other than
peroxides has not been studied. For the first time, we show
[a] A. A. Vernekar, Prof. Dr. G. Mugesh
Department of Inorganic and Physical Chemistry
Indian Institute of Science, Bangalore 560 012 (India)
Fax: (+91) 80 2360 1552
E-mail: mugesh@ipc.iisc.ernet.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201303339.
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that some of the GPx mimics exhibit new graphene oxide
(GO) reductase and decarboxylase activities in the presence
of GSH.
GO. According to this model, GO contains epoxy and hy-
droxyl as oxygen-containing functional groups. The revised
version of the Lerf–Klinowski model further indicated the
presence of carboxylic acid groups at the periphery of
GO.[23] Ajayan and co-workers revealed the presence of
five- and six-membered-ring lactols as oxygenated function-
alities on GO.[24] However, the new reactivity and properties
may originate from additional hidden functional groups on
GO. Recent studies on the local structure of GO by Saxena
et al. and others suggest that GO may contain peroxide link-
ages.[25] The use of GO as catalyst for the oxidation reactions
such as sulfoxidation,[26] alkene oxidation,[27a] alcohol oxida-
tion,[27a–b] amine oxidation,[28] hydrocarbon oxidation,[29] and
so on, under mild conditions, suggests the presence of oxi-
dizing functionalities such as peroxides. However, the spec-
troscopic techniques that are generally used to characterize
GO are not sufficient to distinguish the peroxide linkages
from epoxy groups. Therefore, chemical reactivity remains
a reliable tool for chemists to probe the presence of perox-
ide groups.
GO is a layered water-soluble material and a precursor
for the bulk-scale production of graphene, which has cap-
tured a tremendous interest in various fields.[8,9] Owing to its
interesting properties and ease of processability, GO finds
important use in polymers, ceramics, metal and thin-film
electronics, hydrogen-storage composites, and carbocataly-
sis.[10–16] GO has also been demonstrated as a chemothera-
peutic delivery vehicle and to possess intrinsic peroxidase-
like activity for highly selective and sensitive detection of
glucose.[17–19] Although these applications result from the re-
markable properties of GO, the studies on new properties of
GO appear to be challenging, which may widen its scope of
applications. Whereas the Lerf–Klinowski model[20] and the
Dꢁkꢂny model[21] suggest the probable structure of GO, the
fundamental structure of GO is complex and still under
debate. During our investigations on graphene-based nano-
materials,[22] we found that GO can act as a peroxide sub-
strate for GPx mimics. In the presence of GSH as a co-sub-
strate, catalytic activity could be achieved. Interestingly, the
diaryl ditelluride (1, Figure 2a), which has been shown to
reduce hydroperoxides in the presence of thiols,[6] effectively
catalyzes the reduction of GO in the presence of GSH. Al-
though this methodology yields good quality reduced graph-
Hummers method was employed to produce GO,[30] which
was ultrasonicated in water to yield a homogeneous disper-
sion of GO. Interestingly, in the preliminary test, GO re-
sponded positively for the peroxide starch/iodide reaction,
giving rise to a violet-colored dispersion (Figure S1, the Sup-
porting Information). We then envisaged that if peroxides
form the integral part of GO, it may serve as a new sub-
strate for GPx mimetics. Accordingly, when the GO disper-
sion was treated with small amount of 1 and GSH, a reduc-
tion of GO was observed at ambient temperature. In con-
trast, GSH alone was inefficient in reducing GO under iden-
tical conditions (Figure S2, the Supporting Information), in-
dicating that the tellurium compound is responsible for the
reduction in the presence of GSH. We further noticed that
the ditelluride catalyst could be recovered and reused at
least four times without a loss of its activity. The resulting
product was thoroughly characterized to check the catalytic
efficacy of the GPx mimetic (see below).
ACHTUNGTRENNUNGene oxide (RGO) nanosheets at ambient temperature, the
current investigation highlights a new property for both GO
and GPx mimics and reveals the presence of peroxide link-
ages on GO (Figure 1). This would have a significant impact
on its reactivity and behavior in broad range of applications
of GO, which need to be rationalized from the view point of
the proposed new structural motif.
To understand whether the reduction of GO by com-
pound 1 is similar to that of H2O2, we followed the reduc-
tion of GO by 1 by using a UV/Vis spectrometer. The de-
crease in absorbance of nicotinamide adenine dinucleotide
phosphate (NADPH) was followed at 340 nm in the pres-
ence of glutathione reductase (GR) in phosphate buffer
(pH 7.4) (Figure 2b and c). It is known that GR catalytically
reduces GSSG to GSH by using NADPH as cofactor. In the
control reactions, GSH or compound 1 alone in the absence
of GSH did not show any reduction of GO (Figure 2c).
These results suggest that GSH acts as cofactor in the cata-
lytic cycle. The steady-state kinetics of the reduction process
at different concentrations of GSH provided the Michaelis–
Menten curve, indicating enzyme-like behavior (Figure 2d).
From the plots of initial rate versus concentration of GSH
and the Lineweaver–Burk plots (Figure 2e), it can be con-
cluded that the reduction of GO by compound 1 follows an
enzyme-like mechanism. The Michaelis constant (KM) and
maximum velocity (Vmax) were found to be 7.3 mm and
Figure 1. Proposed structure of GO containing peroxide linkages and
epoxy-, hydroxyl-, and carboxylic acid groups.
Results and Discussion
Apart from the known, large sheet-like complex structures,
the chemical functionalities on GO has been the topic of in-
terest for quite some time. Among different models, the
Lerf–Klinowski[20] model, which is widely accepted, contrib-
utes to the understanding of the fundamental structure of
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Catalytic Reduction of Graphene Oxide Nanosheets
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Figure 2. a) Structures of compound 1 and glutathione; b) UV/Vis spectra showing the decrease in the absorbance of NADPH during the progress of the
reaction (different lines correspond to 4 min intervals); c) The exponential decrease of NADPH and the control reaction; d) The Michaelis–Menten
curve; e) Lineweaver–Burk plot corresponding to Figure 1d.
33.9 mmminÀ1, respectively. Although a high value of KM in-
dicates a weak binding of GO to the tellurium center during
the catalysis, it suggests that the release of reduced GO
from the catalytic centre is favored.
We have also investigated the GO reductase activity of
other GPx mimetics 2–7 (Figure 3a) and the natural GPx
enzyme for a comparison. The initial reduction rates (Fig-
ure 3b and c) indicate that the activities of compound 1 and
2 are much higher compared with their GPx-like activity
(H2O2 substrate). The higher catalytic activity of 1 and 2 can
be attributed to the presence of tertiary amine group that
acts as general base and facilitates the deprotonation of the
tellurol/selenol produced by GSH-mediated reduction of the
ditelluride/diselenide bond. In addition to the ditellurides (1
and 4) and diselenides (2 and 3), the cyclic compounds 5–7
also exhibited weak GO reductase activity. It should be
À
noted that the Se N covalent bond in ebselen (7) and relat-
ed compounds is readily cleaved by thiols such as GSH,
which is responsible for the GPx activity of these com-
pounds.[3] Similarly, the reaction of 1 and related ditellu-
AHCTUNGTREGrNNUN ides/diselenides with GSH produces the corresponding tel-
lurols/selenols through the formation of tellurenyl sulfide/se-
lenenyl sulfide intermediates.[3,6] Although GPx contains
a highly reactive selenocysteine residue, the enzyme was
unable to reduce GO effectively. This is probably due to the
fact that the large sheet-like structures of GO may not be
accessible to the active site of GPx.
The formation of tellurenyl sulfide (10) was confirmed by
125Te NMR spectroscopy. The peak observed at d=344 ppm
for compound 1 (Figure S3, the Supporting Information) dis-
appeared after the addition of an excess of GSH with a new
peak appearing at d=513 ppm (Figure S4, Supporting Infor-
mation) for the tellurenyl sulfide (10). The formation of 10
was also observed during the reduction of GO with 1 in the
presence of GSH (Figure S5, the Supporting Information).
Figure 3. a) Structures of other GPx mimics; b) Relative GPx-like activi-
ties in the reduction of GO using GSH alone and compounds 1–7 in the
presence of GSH; c) A comparison of the GO reductase activities of
GPx mimics.
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G. Mugesh and A. A. Vernekar
This probably indicates that the tellurol (8) rapidly reacts
with peroxide functionalities on GO. It has been shown that
thiols can open up the peroxide linkage in retinal pigment
epithelial bisretinoids by attacking at the electrophilic
carbon center.[31] Considering the presence of peroxide func-
tionalities on GO, the formation of tellurenic acid (9) was
expected upon treatment of 8 with GO (Path A, Figure 4a).
However, our attempts to detect 9 by using 125Te NMR spec-
troscopy were unsuccessful due to the facile reaction of
GSH with 9. The tellurol (8) is regenerated from 10 by reac-
tion with GSH. Further attack of GSH at the sulfur center
in the intermediate 11 results in the formation of a water
molecule and glutathione disulfide (GSSG). Interestingly,
after 12 h of reaction time, a peak corresponding to 1 reap-
peared, probably due to aerial oxidation of the tellurol (8)
(Figure S6, the Supporting Information).
To confirm that the tellurol (8) is actually responsible for
the reduction of GO, we reduced compound 1 with NaBH4
independently and then added to the mixture of GO and
GSH. In this reaction, a very rapid reduction of GO was ob-
served (see the video in the Supporting Information), which
also confirms the presence of peroxide linkages on GO. In
the absence of 1, a mixture of NaBH4 and GSH was unable
to reduce GO under identical conditions. When the tellurol
moiety in compound 8 was blocked by treatment with iodo-
acetic acid, no reduction of GO was observed. We further
Figure 4. a) A detailed mechanism showing the different pathways (A and B) for the reduction of GO in the presence of compound 1 and GSH; b) De-
convoluted C1s X-ray photoelectron spectra (XPS) of GO and RGO. GR=glutathione reductase.
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Catalytic Reduction of Graphene Oxide Nanosheets
FULL PAPER
noticed a rapid evolution of a gas from the reaction mixture,
which was confirmed as CO2 by gas chromatographic analy-
sis (Figure S7, the Supporting Information). The decrease in
the carboxyl functionalities is also observed in the C1s
X-ray photoelectron spectra (XPS) (Figure 4b) of the mate-
rial obtained after reduction. These observations suggest
that the GPx mimics not only reduce GO, but also act as an
efficient decarboxylase to remove the carboxylate function-
alities on GO at ambient temperature. The proposed mecha-
nism for the decarboxylation is given in Figure 4a (Path B).
When a similar experiment was carried out using dithio-
threitol (DTT) as a co-substrate instead of GSH, an immedi-
ate reduction of GO was observed. This is due to the stron-
ger reducing ability of DTT that can effectively reduce the
ditelluride bond in compound 1 to produce the tellurol (8).
Extremely slow reactions were observed with DTT or GSH
in the absence of 1. In both the control reactions, no materi-
al separated out in the reaction vials even after 10 days, indi-
cating that the reduction is ineffective. To understand
whether compound 1 is regenerated during the catalytic
cycle, we carried out the reduction of GO with a catalytic
amount of 1 under a nitrogen atmosphere. However, a rapid
reduction of GO was observed, indicating that the regenera-
tion of compound 1 is not required for the catalytic activity.
In addition to the demonstration of the new reactivity for
the GPx mimics, this method can also be used to prepare
RGO, which is a chemically generated form of graphene. It
is well-known that graphene is a 2D and atomically thin
sheet, which has interesting properties such as high mechan-
ical strength, high surface area, high conductivity, and so on.
Owing to the extraordinary properties, graphene finds re-
markable applications in nanoelectronics, photonics, cataly-
sis, biology, and so on.[32–34] To confirm the formation of
RGO under our experimental conditions, we further charac-
terized this material thoroughly by using spectroscopic and
microscopic methods and conductivity data.
band and broad G bands in the Raman spectra (Figure 5a).
Pristine graphite exhibits a G band as the only characteristic
feature of the first order scattering of the E2g mode at
1570 cmÀ1.[35] Treatment of GO by using 1-GSH mixture re-
sulted in an increase in the intensity of ID/IG ratio to a lower
extent compared with that of GO, indicating the restoration
of the sp2 conjugated network and formation of small aro-
matic domains. The ID/IG ratio (Table S1 in the Supporting
Information) obtained from other methods are higher than
that obtained from the reduction mediated by 1-GSH (ID/
IG =1.11). Recently, we reported that the GO reduced at
higher temperature by a thiol-based reducing agent, dithio-
threitol, shows an ID/IG ratio of about 1.06.[22]
From the powder X-ray diffraction (XRD) patterns (Fig-
ure 5b), it is evident that the GO is exfoliated due to the in-
troduction of oxygen-containing functional groups. Removal
of these functional groups through reduction by 1-GSH
shifted the peak to 2q=248, similar to the one observed for
the pristine graphite with an increase in the d-spacing of
0.38 nm. This indicates the formation of exfoliated RGO.
The reduction of GO was studied further with the help of
X-ray photoelectron spectroscopy (XPS). The deconvoluted
C1s XPS spectra of GO and RGO are given in Figure 4b.
À
The appearance of strong peaks corresponding to the C O
and C=O groups reveals that the GO is highly oxygenated.
On the other hand, the RGO produced after the reaction
À
exhibits weaker peaks for C O and C=O, indicating the ef-
fective reduction by 1-GSH at the ambient temperature.
The C/O ratio of the produced RGO was found to be 13.07.
A comparison of the C/O ratios of RGO material obtained
by various reduction methods is given in Table 1.
To illustrate the thermal stabilities of GO, RGO, and pris-
tine graphite, thermogravimetric analysis (TGA) was carried
out (Figure 5c). GO exhibits 11% decrease in weight due to
the removal of intercalated and adsorbed water molecules
between the layers of GO. At higher temperature (2008C),
a rapid decrease in the weight indicates the removal of
oxygen-containing functional groups in GO. Interestingly,
the TGA profile shows the higher thermal stability as the
observed weight loss for RGO at 2008C is only 4% under
Ar gas flow. However, at a higher temperature (6008C), the
bulk pyrolysis of carbon skeleton is responsible for a further
decrease in the weight, which is consistent with the TGA
profile of GO and graphite. It should be mentioned that the
reduction of GO with GSH alone at a higher temperature
shows a weight loss of ꢀ10% at 2008C,[44] which is signifi-
cantly higher than that observed for the reduction by 1-
GSH.
The dispersion of GO changed its color gradually from
brown to dark-brown and then finally to black within initial
30 min of reaction time (Figure S2, the Supporting Informa-
tion). In the UV/Vis absorption spectra (Figure S8, the Sup-
porting Information), GO exhibits the wavelength maxima
at 230 nm due to p–p* transition originating from C=C. The
broad band observed at 310 nm is attributed to the n–p*
transition resulting from the C=O groups. The reduction of
GO leads to a shift in the wavelength (to 265 nm), indicating
a bathochromic shift, which is also associated with a hyper-
chromic effect. The FTIR spectra (Figure S9, the Supporting
Information) also confirmed the formation of reduced
graph
ene oxide (RGO). The signature frequencies due to
Microscopic characterization of RGO by using SEM (Fig-
ure 5d) shows the randomly oriented exfoliated sheets of
few microns. The TEM image (Figure 5e) indicates the for-
mation of a silk-veil/wave-like structural morphology.[35]
Corrugation and scrolling, the intrinsic properties of graph-
the oxygen-containing functionalities such as carboxylic
acid, hydroxyl and epoxide/peroxide groups in GO are sig-
nificantly diminished upon reduction. This clearly indicates
that the distorted conjugation in GO is restored in RGO
after the reduction.
AHCTUNGTREGeNNUN ne, are responsible for the observed wave-like structure of
It is known that the oxidation of graphite by strong oxi-
dizing reagents introduces a large amount of oxygen-con-
taining functionalities as defects in GO, which exhibit D
the RGO nanosheets. It has been shown that these two in-
trinsic properties render the thermodynamic stability to the
nanosheets by bending.[45] The AFM image (Figure 5 f) with
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G. Mugesh and A. A. Vernekar
Figure 5. a) Raman spectra, b) X-ray diffraction (XRD) pattern, and c) thermogravimetry analysis (TGA) profile of pristine graphite (A), GO (B), and
RGO (C); d) Scanning electron microscopy (SEM) image of RGO showing the crumpled nanosheets. e) Transmission electron microscopy (TEM) image
showing RGO; f) Atomic force microscopy (AFM) image of RGO showing a single-sheet thickness of 0.664 nm; g) The stable dispersion of RGO in
DMF showing the Tyndall effect upon irradiating with red laser light, indicating the colloidal stability owing to hydrophobicity; h),i) Experiments to con-
firm the conductivity of RGO (functioning of LED and motor when RGO is connected in a circuit); the inset of h and i show RGO pellets used for the
experiments.
NMR spectra of GO (Figure S10, the Supporting
Information) shows the presence of a high amount
Table 1. Comparison of C/O ratios and electrical conductivities of graphene/RGO
prepared by various methods.
of oxygenated carbon nuclei. The spectra show
Reducing agent/
methods
C/O
ratio
Electrical
Reference
peaks at d=59.9 and 68.6 ppm, which can be as-
cribed to the carbon atoms bearing peroxide and
hydroxyl groups, respectively. The carbon that lacks
oxygen functional groups exhibits a peak at d=
130.3 ppm, whereas the carbonyl carbon exhibits
a peak at d=190.0 ppm. The carbonyl carbon of
the ester moiety also exhibits a small peak at d=
166.9 ppm, which is in agreement with an earlier
report.[24] On the other hand, due to the effective
reduction of GO at room temperature by com-
pound 1 in the presence of GSH, a broad peak was
observed at dꢀ106 ppm. These observations sug-
gest that the conjugation and formation of sp2 do-
mains are restored. The hydrophobic RGO was
highly dispersible in DMF through which a Tyndall
effect is observed upon irradiating with a red laser
beam (Figure 5g).
conductivity
AHCTUNGTERNNUNG ]
[SmÀ1
NaBH4
solvothermal
Na+EtOH
N2H4
l-cysteine
5.3
6.4
45
0.05
[36]
[37]
10.3
increased
–
18.6
7.9
–
200
0.124
53
2100
2300
3.6, 4.8, 267.8
82.3, 1660, 20200
544
[38]
[39]
[40]
[41]
[42]
[43]
tea
aluminium powder
Fe/HCl
NaOH, solvothermal
and two-step reduction
NaBH4, heating in H2SO4
and annealing at 11008C
ditelluride
4.78, >8.57, >246
[24]
13.07
present work
its height profile reveals the thickness of 0.664 nm. This
thickness is consistent with the reported value for the single
layer of graphene.[46] The 13C magic-angle spinning (MAS)-
The electrical conductivity of the RGO was
found to be 544 SmÀ1, indicating that the tellurol-mediated
reduction is efficient in generating RGO with good conduc-
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Catalytic Reduction of Graphene Oxide Nanosheets
FULL PAPER
(2 mgmLÀ1). An ethanolic solution of 1 (or 2–7) was then added immedi-
ately to this mixture, such that the concentration of catalyst was 1 mm in
the reaction mixture. This mixture was stirred, and at every 4 min inter-
val, an aliquot from the reaction mixture (30 mL) was added to a quartz
cuvette containing 0.4 mm NADPH and glutathione reductase (1.7
unitmLÀ1) in phosphate buffer (100 mm, pH 7.4). The decrease in absor-
AHCTUNGTREGbNNUN ance of NADPH at 340 nm was followed by using a UV/Vis spectropho-
tometer. The steady state kinetics experiments were carried out by using
a similar method at different concentrations of GSH.
tivity. A comparison of the electrical conductivities of graph-
ACHTUNGTRENNUNGene prepared from different methods is represented in
Table 1. The RGO produced by the present method was in-
troduced into a small circuit to glow a battery operated 1 W
light-emitting diode (LED) (Figure 5h) and a motor (Fig-
ure 5i). This suggests that the current methodology renders
high quality RGO sheets by the reduction of oxygen-con-
taining functionalities such as peroxide, hydroxyl, and car-
boxyl groups.
Quantitative production of RGO: GSH (40 mg, 0.130 mmol) was added
to a GO dispersion (20 mL, 2 mgmLÀ1). Diaryl ditelluride (1) (30 mg,
0.0573 mmol) dissolved in ethanol (2 mL) was then added to the above
dispersion at ambient temperature. The reaction mixture was stirred for
2 h and then GSH (40 mg, 0.130 mmol) was added. After stirring for 12 h
at room temperature, the mixture was centrifuged and washed with water
and ethanol several times to obtain good quality RGO in quantitative
yield.
Conclusion
We show, for the first time, that graphene oxide (GO) can
be reduced catalytically by glutathione peroxidase (GPx)
mimetics. GO acts as a peroxide substrate in the GPx-like
catalytic activity of organoselenium/tellurium compounds,
indicating that GO contains peroxide functionalities. A de-
tailed experimental investigation on a tellurium compound
indicates that the synthetic tellurol reduces GO through
cleavage of the peroxide ring. The mechanism of GO reduc-
tion by the tellurol in the presence of glutathione (GSH) in-
volves the formation of a tellurenic acid and tellurenyl sul-
fide intermediates. In addition to the rapid cleavage of per-
oxide linkages, the GPx mimics also catalyze the decarbox-
Acknowledgements
This study was supported by the Department of Science and Technology
(DST), New Delhi, India. G.M. acknowledges DST for the award of
a Swarnajayanti Fellowship. The authors also acknowledge the Institute
Nanoscience Initiative (INI) of Indian Institute of Science for providing
the TEM facilities. A.A.V. thanks the Council of Scientific and Industrial
Research (CSIR), New Delhi, India for a research fellowship.
ACHTUNGTRENNUNGylation of the carboxylic acid functionality in GO at ambient
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Received: August 26, 2013
Published online: November 4, 2013
16706
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