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DOI: 10.1002/elan.201300356

Environmentally Friendly Reduction of Graphene Oxide

Using Tyrosine for Nonenzymatic Amperometric H2O2
Detection
Qi Wang,[a, b] Musen Li,[b] Sabine Szunerits,[a] and Rabah Boukherroub*[a]

Abstract: Herein, we report on the simultaneous reduc- urements. The rGO/Tyr nanomaterial displayed good
tion and functionalization of graphene oxide (GO) with electrocatalytic activity for the detection of hydrogen per-
environmentally friendly tyrosine through the direct reac- oxide (H2O2) with a wide linear range from 100 mM to
tion of GO with tyrosine (Tyr) at 100 8C for 24 h. The re- 2.1 mM, a detection limit of 80 mM and a sensitivity of
sultant material (rGO/Tyr) has been characterized using 69.07 mA mM 1 cm 2. In addition, the rGO/Tyr electrode
UV-vis spectrometry, FTIR spectroscopy, X-ray photo- showed good selectivity for H2O2 detection in the pres-
electron spectroscopy (XPS), and electrochemical meas- ence of uric acid, ascorbic acid, dopamine and glucose.
Keywords: Reduced graphene oxide · Tyrosine · Nonenzymatic · Hydrogen peroxide · Amperometric detection

1 Introduction there has been recently a huge focus on the development

of nonenzymatic sensors for the detection of H2O2 and
Application of electrochemical methods for the detection other biorelevant species [5].
and determination of dissolved species has generated Graphene, a one atomic layer, 2D material is a unique
huge attention over decades. To broaden electrochemical combination of high surface area, good electrical conduc-
sensing to a wider range of analytes, to increase sensitivi- tivity, mechanical flexibility and chemical stability suita-
ty and to achieve better selectivity, there is a continuous ble for a wide range of applications. Furthermore, chemi-
development of new electrode materials [1]. cally reduced graphene oxide (rGO) has a high density of
Reactive oxygen species (ROS), including hydroxyl edge-plane-like defect sites, which might provide many
radicals (HO·), superoxide anion (O2 ) and hydrogen per- active sites for electron transfer to chemical and biologi-
oxide (H2O2) are produced in various physiological pro- cal species [6]. These properties have made graphene an
cesses and can be used as an early indicator for cytotoxic effective biosensing interface of different biomolecules
events and cellular disorders [2, 3]. Among all ROS, H2O2 and biologically relevant molecules such as H2O2, glucose,
is the most stable species and can be particularly harmful dopamine, ascorbic acid, uric acid, protein, DNA, choles-
since it can diffuse across membranes through water terol, histidine, organosulfate pesticides, nicotinamide ad-
channels and cause biological modifications such as per- enine dinucleotide (NADH), etc.[5, 6].
oxidation of cell membrane lipids, DNA bases and back- Graphene has also shown promise in nonenzymatic
bone hydroxylation at distal areas [2]. H2O2 is also a by- H2O2 detection. Li et al. have prepared a nonenzymatic
product of many oxidative biological reactions, including H2O2 sensor based on manganese dioxide (MnO2)/gra-
those of glucose oxidase, cholesterol oxidase, alcohol ox- phene oxide (GO) nanocomposite with a good activity in
idase, galactose oxidase, etc., and an essential mediator in alkaline medium [7]. The sensor displayed a detection
food, pharmaceutical, clinical, industrial and environmen- limit of 0.8 mM and a long-term stability. The good perfor-
tal analyses [4]. Thus it is of prime importance to design mance of the sensor was attributed to the high surface
biosensors for the detection of H2O2 with high sensitivity,
good stability, large detection range and good selectivity. [a] Q. Wang, S. Szunerits, R. Boukherroub
By taking advantage of the sensitivity and selectivity of Institut de Recherche Interdisciplinaire (IRI, USR CNRS
enzymatic reactions, highly sensitive electrochemical de- 3078), Universit Lille 1, Parc de la Haute Borne, 50 Avenue
tection schemes of H2O2 were usually achieved by modi- de Halley, BP 70478, 59658 Villeneuve dAscq, France
fying the surface of electrodes with enzymes. While enzy- tel: + 33 3 62 53 17 24; fax: + 33 3 62 53 17 01
*e-mail: rabah.boukherroub@iri.univ-lille1.fr
matic biosensors for H2O2 detection have shown good
performances with low detection limits, there are several [b] Q. Wang, M. Li
Key Laboratory for Liquid-Solid Structural Evolution and
drawbacks associated with enzyme-modified electrodes Processing of Materials
such as the high cost of enzymes, long-term stability and (Ministry of Education), Shandong University, Jinan 250061,
complexity of immobilization [4]. Furthermore, the P. R. China
enzyme activity is highly affected by the temperature and Supporting Information for this article is available on the
the pH of the sensing medium. To alleviate these hurdles, WWW under http://dx.doi.org/10.1002/elan.201300326

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area of GO combined with the good catalytic activity of The detailed experimental conditions are reported in re-
MnO2 nanoparticles. Zhang et al. developed a novel elec- cently published work [33, 34]. A homogeneous yellow
trocatalytic sensor based on functionalized reduced gra- brown suspension (0.5 mg/mL) of GO sheets in water was
phene oxide (rGO) with conducting polypyrrole graft co- achieved by ultrasonication for 3 h.
polymer, poly(styrenesulfonic acid-g-pyrrole) (PSSA-g-
PPY/rGO) via p p noncovalent interaction [8]. The
nanocomposite showed high electrocatalytic activity 2.3 Reduction of GO Using Tyrosine
toward H2O2 oxidation in neutral media with a detection
limit of 10 nM. Graphene/metal nanoparticles have also 10 mM l-tyrosine was added to 0.5 mg/mL homogeneous
been investigated for enzymeless detection of H2O2 [9– water suspension of GO nanosheets. The mixture was
27]. The sensors showed detection limits ranging from kept under stirring for 24 h at 100 8C. The resulting black
50 nM to 35 mM, depending on the metal nature, the dep- precipitate was separated from the aqueous supernatant
osition mode and the supporting electrode material. Gra- by centrifugation at 14 000 rpm for 20 min. After washing
phene decorated with CdS nanocrystals has been success- with ethanol (3 times) and Milli-Q water (3 times), the
fully applied for electrochemiluminescence (ECL) detec- precipitate was dried in an oven (80 8C) and then dis-
tion of H2O2. The sensor exhibited a linear range from persed in DMF with the aid of ultrasonication for 30 min.
5 mM to 1 mM with a detection limit of 1.7 mM with excel-
lent reproducibility and long-term stability [28]. Similarly,
sensors based on Prussian blue/GO [29], Nafion/GO/ 2.4 Sample Characterization
Co3O4 [30] and 3D carbon micropillars/rGO [31] have
2.4.1 UV-Vis Measurements
achieved good performances for enzyme-free detection of
H2O2. Zhang et al. reported on the use of graphene quan- Absorption spectra were recorded using a Perkin Elmer
tum dots (GQDs), 30 nm in diameter, assembled on Au Lambda UV/Vis 950 spectrophotometer in plastic cuv-
electrode for the detection of H2O2 in human breast ade- ettes with an optical path length of 10 mm. The wave-
nocarcinoma cell line MCF-7. The H2O2 release was trig- length range is 200–800 nm.
gered by injecting phorbol myristate acetate (PMA),
a compound that can induce H2O2 generation in cells
[32].
2.4.2 FTIR Spectroscopy
In this paper, we report on simultaneous reduction/
functionalization of graphene oxide using environmental- Fourier transform infrared (FTIR) spectra were recorded
ly friendly reagent, tyrosine (Tyr). This one-step reaction using a ThermoScientific FTIR instrument (Nicolet 8700)
consists of a simple reaction of GO with Tyr at 100 8C for at 4 cm 1. Dried GO or rGO samples (1 mg) were mixed
24 h. The electrochemical activity of the resulting materi- with KBr powder (100 mg) in an agate mortar. The mix-
al was exploited for the detection of H2O2. The nonenzy- ture was pressed into a pellet under 10 tons load for 2–
matic sensor was fabricated based on rGO/Tyr deposited 4 min, and the spectrum was recorded immediately. Six-
on glassy carbon electrode (GCE). It exhibited a good teen accumulative scans were collected. The signal from
sensitivity of 69.07 mA mM 1 cm 2 with a linear range a pure KBr pellet was subtracted as the background.
from 100 mM to 2.1 mM and a limit of detection of
80 mM.
2.4.3 X-Ray Photoelectron Spectroscopy
2 Experimental X-ray photoelectron spectroscopy (XPS) measurements
2.1 Materials were performed with an ESCALAB 220 XL spectrometer
from Vacuum Generators featuring a monochromatic Al
All chemicals were reagent grade or higher and were Ka X-ray source (1486.6 eV) and a spherical energy ana-
used as received unless otherwise specified. Graphite lyzer operated in the CAE (constant analyzer energy)
powder (< 20 mm), potassium permanganate (KMnO4), mode (CAE = 100 eV for survey spectra and CAE =
sulfuric acid (H2SO4), hydrogen peroxide (H2O2), dime- 40 eV for high-resolution spectra), using the electromag-
thylformamide (DMF), l-tyrosine, potassium chloride netic lens mode. The surface was prepared by depositing
(KCl), potassium ferrohexacyanide [K4Fe(CN)6], potassi- 100 mL of the GO suspension in water or rGO in DMF
um ferricyanide [K3Fe(CN)6], ruthenium hexamine (1 mg/mL) onto silicon wafers and heated at 80 8C to
(Ru(NH3)6)3 + , indium tin oxide (ITO, sheet resistivity: remove the solvent. The detection angle of the photoelec-
15–25 W/sq) were purchased from Aldrich. trons is 308, as referenced to the sample surface. After
subtraction of the Shirley-type background, the core-level
2.2 Preparation of Graphene Oxide (GO) spectra were decomposed into their components with
mixed Gaussian-Lorentzian (30 : 70) shape lines using the
GO nanosheets were produced from natural graphite CasaXPS software. Quantification calculations were per-
powder by an improved Hummers and Offeman method. formed using sensitivity factors supplied by PHI.

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2.4.4 Electrochemical Measurements toxic and potentially explosive chemical. To avoid using
hydrazine, many environmentally friendly and high-effi-
Electrochemical experiments were performed using an cient reductants have been developed and used for the re-
Autolab potentiostat 20 (Eco Chemie, Utrecht, The Neth- duction of GO, including vitamin C, amino acids, reduc-
erlands). An Ag/AgCl (Bioanalytical Systems, Inc.) elec- ing sugars, alcohols, hydroiodic acid, reducing metal
trode was used as reference electrode, and a platinum powder, sodium citrate, tea, lysozyme, dopamine, etc.
wire as counter electrode. Cyclic voltammetry measure- [37]. We have recently reported on simultaneous reduc-
ments were performed in aqueous solutions of tion and functionalization of GO using tetrathiafulvalene
Fe(CN)63 /4 (5 mM) containing 0.1 M KCl, 5 mM (TTF) [38, 39] and dopamine [40, 41] derivatives.
Ru(NH3)63 + /0.1 M KCl, as well as in PBS (pH 7.4) in the Here we describe an easy and environmentally friendly
absence and presence of H2O2. chemical method for the reduction and noncovalent func-
The rGO/Tyr modified GC electrodes were prepared tionalization of GO using tyrosine (Figure 1). The reac-
by casting 30 mL of rGO/Tyr-DMF suspension (0.5 mg/ tion of an aqueous solution of GO and tyrosine at 100 8C
mL) onto the GCE (A = 0.28 cm2) followed by heating at for 24 h produced a black precipitate that can be easily
80 8C until full DMF evaporation. separated from the supernatant via centrifugation. The
Chronoamperometric detection of H2O2 on the rGO/ success of the reaction was confirmed by UV-vis absorp-
Tyr modified GC electrode was performed under N2-satu- tion spectra (Figure 2). The UV-vis spectrum of GO
rated steady-state condition in stirring phosphate buffered shows two peaks at 228 nm and ~ 300 nm corresponding
saline (pH 7.4) by applying a constant potential of 0.5 V to p p* transitions of aromatic C=C bond and n p* tran-
to the working electrode. When the background current sitions of C=O bond in GO, respectively. It is can be
became stable (after 60 s), a subsequent addition of H2O2 clearly seen that the absorption peak red-shifted from 228
was realized and the current was measured. to 270 nm upon reaction with tyrosine at 100 8C for 24 h.
The presence of a peak at around 220 nm is most likely
3 Results and Discussion due to the incorporation of the tyrosine moieties in the

The reduction of GO has been a subject of intense efforts

in the last decade. Many efforts have been focused on the
preparation of chemically modified GO and graphene
composites aiming at a better processability of graphene
and modulating at wish its electronic, optical and electro-
chemical properties [35]. Functionalization of graphene
enables indeed this material to be processed by solvent-
assisted techniques and prevents its agglomeration and
thus maintaining the inherent properties of graphene cru-
cially important for their end application.
Various approaches have been investigated for the re-
duction or reduction/functionalization of GO using chem-
ical, electrochemical, thermal or photochemical means
[36]. Hydrazine monohydrate is still the most widely used
reductant, mainly due to its strong reduction activity to
eliminate most oxygen-containing functional groups of
GO and its ability to yield stable rGO aqueous disper-
sions. However, with hydrazine as the reducing agent, its
residual trace may strongly decrease the performance of Fig. 2. UV-vis spectra of GO before and after reduction with
rGO-based devices. In addition, hydrazine is a highly tyrosine at 100 8C for 24 h.

Fig. 1. Schematic illustration of the reduction of GO using tyrosine.

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reduced GO. Indeed, free tyrosine displays a strong ab-
sorption peak at 223 nm (Supporting Information Fig-
ure S1). In addition, the intensity of the absorption tail in
the region > 400 nm has significantly increased. The result
suggests that the GO nanosheets have been reduced and
the electronic conjugation within the GO nanosheet was
restored upon reaction with tyrosine.
The reduction and functionalization of GO with tyro-
sine was further evidenced by FTIR spectroscopy
(Figure 3). The chemically exfoliated GO contains a varie-
ty of functional groups such as hydroxyl (C OH), epox-
ide (C O C), carbonyl (C=O), and carboxyl (COOH)
groups usually present at the defects and edges of the
sheets. The transmission FTIR spectrum of the exfoliated
GO displays a broad and strong band at ~ 3400 cm 1 as-
signed to the vibration of hydroxyl groups and/or ad-
sorbed water molecules. Furthermore, bands due to C=O
( COOH) vibration, OH deformation, and C O (alkoxy)
and C O (epoxy) stretching modes are visible at 1744,
1407, 1230 and 1087 cm 1, respectively. A band at
~ 1626 cm 1 assigned to C=C stretching modes is also
present in the FTIR spectrum of the initial GO. After re-
action with tyrosine at 100 8C for 24 h, most of the vibra-
tions due to oxygen-related functional groups disap-
peared. The FTIR spectrum is dominated by a broad
band at ~ 1570 cm 1 due to C=C stretching modes, sug-
gesting that the aromatic network has been restored upon
reaction with tyrosine. A small and broad band is also
present at 1700 cm 1 most likely due to residual carbonyl
groups and/or to carbonyl functions introduced through
tyrosine incorporation in the rGO network through p p
interactions (Figure 3).
X-ray photoelectron spectroscopy (XPS) analysis was
performed on GO before and after its reaction with tyro-
sine at 100 8C for 24 h to gain further information on the
chemical transformations occurred on its surface. The C1s
core level XPS spectrum of GO nanosheets is displayed
in Figure 4 A and can be deconvoluted into four compo-
nents with binding energies at about 283.9, 285.1, 286.8

Fig. 4. High resolution X-ray photoelectron spectroscopy (XPS)

of GO before (A) and after (B) reaction with tyrosine at 100 8C
for 24 h. (C) corresponds to N1s core level spectrum of rGO/Tyr
composite.

and 287.8 eV assigned to sp2-hybridized carbon, C H/C

C, C O and C=O species, respectively. The C/O ratio of
GO is 2.03 comparable to reported data in the literature
[34]. After reaction of GO with tyrosine, analysis of the
resulting composite indicates significant changes in the
C1s core level spectrum (Figure 4 B). The C1s exhibits
bands at 283.9 (sp2-hybridized carbon), 285.1 (C H/C C),
287.8 (C O/C N) and 290.7 eV (COOH). The intensity
of the band at 283.9 eV increased significantly compared
Fig. 3. Transmission FTIR spectra of GO before and after re- to GO, suggesting that the sp2 network has been restored
duction with tyrosine at 100 8C for 24 h. during the process (Figure 4 B). The ratio of carbon to

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oxygen increased to 6.07. Indeed, GO is a good electron tives on the rGO matrix was further confirmed by the
acceptor that can be easily reduced in the presence of presence of nitrogen in the XPS spectrum at ~ 400 eV
electron donors. Tyrosine seems to act as a good reducing (Figure 4 C).
agent for GO, as previously seen for other amino acids The electrochemical properties of the rGO/Tyr compo-
such dopamine [40] and l-ascorbic acid, getting at the site were evaluated by cyclic voltammetry using
same time oxidized to a quinone-type product (Figure 1) Fe(CN)63 /4 (Figure 5 A) and Ru(NH3)63 + (Figure 5 B) as
[42]. In a control experiment, an aqueous solution of GO redox couples. Figure 5 A displays the CVs of ITO before
(without tyrosine) was heated at 100 8C for 24 h and its and after drop casting a film of rGO/Tyr using
oxidative degree was determined by XPS. The result was Fe(CN)63 /4 as redox couple. The ITO electrode shows
comparable to that of GO with an increased C/O ratio to a peak separation, DE of 189 mV, which increased upon
2.8 due to a slight decrease in the C-O content. However, coating with rGO/Tyr to 302 mV. Similarly, rGO/Tyr dis-
the component due to sp2-hybridized carbon remained played a comparable electrochemical behavior using
rather unchanged. The result suggests that water alone is Ru(NH3)63 + as redox couple with a DE of 110 mV for
not sufficient to reduce GO to rGO under these experi- bare ITO and 165 mV after coating ITO with rGO/Tyr
mental conditions. Incorporation of tyrosine or its deriva- (Figure 5 B). For both redox couples, the detected cur-
rents on rGO/Tyr exceeded that of a bare ITO with a rela-
tively strong capacitive component.
To assess the electrocatalytic activity of rGO/Tyr for
hydrogen peroxide (H2O2) reduction, linear sweep vol-
tammetry was performed on the rGO/Tyr/GC electrode
in PBS solution (pH 7.4) by addition of H2O2 (Fig-

Fig. 5. Cyclic voltammograms of (A) ITO (grey) and ITO/rGO/ Fig. 6. (A) Linear sweep voltammograms of rGO/Tyr/GCE in
Tyr (red) in 5 mM Fe(CN)63 /4 /0.1 M KCl, scan rate: 100 mV/S; PBS solution (pH 7.4) with different concentrations of H2O2 : 0
(B) ITO (grey) and ITO/rGO/Tyr (red) in 5 mM Ru(NH3)63 + (a), 200 mM (b), 800 mM (c), 1 mM (d), 5 mM (e). Scan rate:
/0.1 M KCl, scan rate: 50 mV/s. 50 mV/s. (B): Calibration curve (linear up to 2 mM).

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rapidly to the substrate and the current rose steeply to
reach a stable value. At the applied potential of 0.5 V,
the cathode current of the sensor increased dramatically
and achieved 95 % of the steady-state current within 25 s,
indicating a fast amperometric response behavior. The
calibration curve of the sensor is shown in the inset. A
linear detection range from 0.1 to 2.1 mM with I (mA) =
1.5 20.72  [H2O2] (r = 0.9995) is obtained resulting in
an estimated sensitivity of 69.07 mA mM 1 cm 2. A detec-
tion limit of 80 mM at a signal-to-noise ratio of 3 was ach-
ieved using the rGO/Tyr/GCE sensor. This value is lower
than those reported for metal nanoparticles/rGO sensors
(Table 1). However given the ease of preparation of the
electrode material and the good sensitivity of detection,
there is a room for improvement of the detection limit by
incorporating metal or metal oxide nanoparticles in the
rGO/Tyr matrix.
Fig. 7. Amperometric response curve at rGO/Tyr/GCE polar- It is well known that some co-existing electroactive
ized at 0.55 V vs. Ag/AgCl with subsequent addition of H2O2 species such as uric acid (UA), ascorbic acid (AA), dopa-
(200 mM). Inset: Calibration curve ([H2O2] = 1.34 20.72x, R =
mine (DA) and glucose in real samples will affect the
0.999).
sensor response. These species generally show serious in-
terference for electrochemical H2O2 detection, which
ure 6 A). Addition of increasing amounts of H2O2 resulted limits the practical application of the sensor. Herein, the
in an increase of the reduction peak at 0.5 V with influence of common interference species such as UA,
a linear range up to ~ 2.1 mM (Figure 6 B). AA, DA and glucose was thus investigated using their
Chronoamperometric detection of H2O2 was performed relevant physiological levels by chronoamperometry [12].
in addition. Figure 7 A exhibits the typical current time Figure 8 exhibits the amperometric response to the con-
plot of the rGO/Tyr/GCE in PBS solution (pH 7.4) on secutive injection of 2 mM H2O2 and 0.1 mM of interfer-
successive step change of H2O2 concentrations under opti- ing species: UA, AA and DA, and 5 mM glucose. The
mized conditions. When an aliquot of H2O2 was added working potential was hold at 0.5 V. An obvious in-
into the stirring PBS solution, the electrode responded crease of the current was observed when 2 mM H2O2 was

Table 1. Comparison of the performance of various nonenzymatic H2O2 sensors.

Electrode Detection limit Sensitivity Linear range Reference
1 2
GO/MnO2 0.8 mM 38.2 mM mM cm 5–600 mM [7]
PSSA g PPY/rGO 10 nM 673 mM mM 1 cm 2 3  10 8–2.8  10 5 M [8]
AgPd/rGO 1.4 mM – 0.01–1.4 mM [10]
Pt/rGO 0.8 mM – 2.5–6650 mM [9]
PtNPs/PANI/rGO 50 nM 857 mA mM 1 cm 2 – [24]
AuEPG 0.1 mM 75.9 mA mM 1 cm 2 0.5 mM–4.9 mM [21]
AuNPs/rGO paper 2 mM 236.8 mA mM 1 cm 2 0.005–8.6 mM [23]
AuNPs/rGO 6 mM 3 mA mM 1 cm 2 20–280 mM [15]
AgNPs/PQ11/rGO 28 mM – 100 mM–40 mM [22]
AuNPs/POM/rGO 1.54 mM 58.87 mA mM 1 cm 2 5  10 6–1.8  10 2 M [27]
AgNCs/rGO 3 mM 183.5 mA mM 1 cm 2 2  10 5–1  10 2 M [11]
AgNPs/rGO 1.80 mM – 0.1–60 mM [12]
AgNPs/rGO 0.5 mM – 0.1–100 mM [13]
AgNPs/rGO 31.3 mM – 100 mM–100 mM [14]
AgNPs/Aniline/rGO 7.1 mM – 100 mM–80 mM [16]
AgNPs/rGO/AuE 1.9 mM – 0.1 mM–20 mM [17]
AgNPs/rGO 3.6 mM – 0.1–100 mM [18]
AgNPs/PDDA/rGO 35 mM – 100 mM–41 mM [20]
PtNPs/PMAA/rGO 80 nM – 1 mM–500 mM [25]
PtAuNPs/rGO/CNTs 0.6 mM – 2.0–8561 mM [26]
PdNPs/rGO 0.05 mM – 0.1 mM to 1.0 mM [19]
CdS/rGO 1.7 mM – 5 mM–1 mM [28]
PB/GO 0.122 mM 408.7 mA mM 1 cm 2 5.0  10 6–1.2  10 3 M [29]
Nafion/EGO/Co3O4 0.3 mM 560 mA mM 1 cm 2 – [30]
3D carbon micropillars/rGO 0.07 mA mM 1 cm 2 250 mM–5.5 mM [31]
GQDs/Au 0.7 mM – 0.002–8 mM [32]

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over 0.1 to 2.1 mM linear range. In addition, the sensor
showed good selectivity for H2O2 detection in the pres-
ence of interfering species such as uric acid, ascorbic acid,
dopamine and glucose. The technique developed in this
work can be easily applied for simultaneous reduction of
GO and metal salts for the preparation of rGO/metal
nanoparticles composites and thus holds promise for the
fabrication of highly sensitive nonenzymatic sensors.

Acknowledgements
R. B. and S. S. gratefully acknowledge financial support
from the Centre National de Recherche Scientifique
(CNRS), the University Lille 1 and Nord Pas de Calais
region. S. S. thanks the Institut Universitaire de France
(IUF) for financial support. Q. W. thanks the Natural Sci-
ence Foundation of China, under Grant No. 50972078 and
the Chinese Government for the China Scholarship
Council Award.

Fig. 8. Chronoamperometry curves of the rGO/Tyr/GCE ex-

posed to H2O2 (2 mM), UA (0.1 mM), AA (0.1 mM), DA
(0.1 mM), glucose (5 mM), and H2O2 (2 mM). Applied potential:
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