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6490 Chem. Commun., 2012, 48, 6490–6492 This journal is c The Royal Society of Chemistry 2012
Cite this: Chem. Commun., 2012, 48, 6490–6492
A graphene–cobalt oxide based needle electrode for non-enzymatic
glucose detection in micro-dropletsw
Xuewan Wang,aXiaochen Dong,
aYanqin Wen,
bChangming Li,
aQihua Xiong
band
Peng Chen*a
Received 16th April 2012, Accepted 4th May 2012
DOI: 10.1039/c2cc32674d
A novel graphene–cobalt oxide hybrid needle-like electrode was
fabricated for non-enzymatic glucose detection. Taking advantage
of its small size, the needle electrode can probe glucose in a
micro-droplet with high sensitivity.
Various nanostructured materials have been used to develop
novel sensors in micro/nanoscale dimensions.1–4 Such minia-
turized sensors not only provide superior sensitivity but also
the ability of localized detection within a small volume. The
latter is particularly important when high spatial resolution in
detection is required or the availability of samples is limited
(e.g., for precious clinical samples).
The recently discovered graphene, a two-dimensional (2D)
monolayer of carbon atoms arranged in a hexagonal lattice,
adds new dimensions for the development of high-performance
sensors.5,6 In particular, graphene has been employed as electrode
material for electrochemical sensing owing to its unparalleled
charge carrier mobility, unique 2D structure, and high electro-
chemical potential.5,7 To endow graphene with various sensing
capabilities, it can be functionalized or hybridized with various
organic or inorganic nanomaterials.8–10
Herein, we demonstrated a novel graphene–Co3O4 hybrid
electrode constructed on a micropipette tip which can directly
detect glucose without the need for any enzymes with high
sensitivity and ability to probe a micro-droplet. Glass micro-
pipettes with tip diameter of B1 mm were pulled from boro-
silicate glass capillaries (inner diameter of 0.5 mm; outer
diameter of 1.0 mm) using a micropipette puller (Narishige,
Japan). Graphene films were grown on copper (Cu) foils at
1000 1C by chemical vapor deposition (CVD) using a mixture
of methane and hydrogen as carbon source.9,11 A polymethyl-
methacrylate (PMMA) layer was spin-coated on CVD grown
graphene. After etching away the underneath Cu foil, the
graphene–PMMA strip (0.5 mm wide and 1.0 cm long) was
transferred onto a glass micropipette with its tip being covered.12
Subsequently, the graphene-coated micropipette was dried at
100 1C. This was followed by removing the PMMA layer using
acetone and drying at 100 1C. The conductive graphene thin-
film strip was left on the micropipette serving as a needle-like
electrode. Silver conductive paint was then coated on the top
end of the graphene strip and extended along the pipette, in
order to assist the electrical wiring between the graphene
electrode and the recording apparatus.
Electrochemical deposition of Co(OH)2 was performed with
a CHI600D electrochemical workstation (Chenhua, China)
using the conventional three-electrode configuration. With
Co(NO3)2�6H2O (Sigma-Aldrich) solution (0.1 M) as the
electrolyte, a constant potential of �1.0 V vs. Ag/AgCl
reference electrode was applied to the graphene needle working
electrode for 100 s.13,14 Only the tip of the graphene coated
micropipette was immersed in the electrolyte and electro-
chemically deposited with Co(OH)2. After the electrochemical
deposition, the electrode was washed with deionized-water
(18 MO) and dried at 50 1C for 2 h. Finally, the pipette electrode
was heated to 400 1C at a heating rate of 3 1Cmin�1 and annealed
at 400 1C for 4 h in order to transform Co(OH)2 into Co3O4.15,16
Thus, the graphene–Co3O4 needle electrode was obtained.
The surface morphology of the graphene–Co3O4 needle
electrode was examined with scanning electron microscopy
(JSM-6700F, JEOL). As shown in Fig. 1a, the graphene coated
micropipette tip is uniformly and seamlessly covered by a layer
of regular Co3O4 nanostructure. Closer SEM inspection reveals
that the flower-like crystal structure of Co3O4 is formed from
electrochemical deposition (Fig. 1b and c). Such an open-
nanoporous flower-like structure offers a large specific surface
area (active area) and, at the same time, ensures unhindered
diffusion of ions and redox substances.
Fig. 1 SEM images of a graphene–Co3O4 needle electrode with
different magnifications.
aDivision of Bioengineering, School of Chemical and BiomedicalEngineering, Nanyang Technological University, 70 Nanyang Drive,637457, Singapore. E-mail: ChenPeng@ntu.edu.sg;Fax: +65 6791 1761; Tel: +65 6514 1086
b School of Physical and Mathematical Sciences, NanyangTechnological University, 21 Nanyang Link, 637371, Singapore
w Electronic supplementary information (ESI) available. See DOI:10.1039/c2cc32674d
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This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 6490–6492 6491
The X-ray diffraction analysis (Bruker D8 Advance
Diffractometer using Cu Ka radiation) was performed to
characterize the samples on a micropipette. The XRD spectrum
of graphene–Co(OH)2 exhibits two prominent peaks at
2 Theta (2y) value of 32.81 and 58.81, which are indexed as
(100) and (110) planes of a-Co(OH)2, respectively (Fig. 2a).13
No graphitic carbon diffraction peaks are observed due to the
relatively thicker Co(OH)2 layer on top. After annealing at
400 1C, Co(OH)2 is converted to Co3O4. The XRD spectrum
of graphene–Co3O4 presents the diffraction peaks at 2y value
of 31.21, 36.71, 38.41, 44.71, 55.31, 59.11 and 65.21 coinciding
with the (220), (311), (222), (400), (422), (511), and (440)
planes in the standard spectrum of Co3O4 crystal (JCPDS
42-1467). Evidently, the resulting Co3O4 is of high purity and
high crystallinity.
Fig. 2b presents the Raman spectrum of the graphene needle
electrode, obtained from the WITeck CRM200 Raman System
with laser excitation wavelength of 488 nm. Apart from the
graphene characteristic G and 2D bands, it also displays an
apparent D defect band at B1360 cm�1, which is likely due to
the scrambling and multi-folding of graphene at the micro-
pipette tip produced while removing PMMA with acetone.12
After deposition of Co3O4, the Raman spectrum of the hybrid
electrode shows four additional characteristic peaks (within
450 to 750 cm�1 range) corresponding to different vibrational
modes of crystalline Co3O4 (Eg, F2g1, F2g2 and Ag1). XRD
and Raman characterizations confirm the successful hybridi-
zation of graphene and Co3O4 and their attachment on a
micropipette tip.
Graphene is highly conductive and has a wide electro-
chemical window. Therefore, it is an ideal electrode material
as compared with metallic materials (e.g., gold). We thermal-
evaporated a 30 nm thick Au strip on a glass micropipette and
compared such a gold needle electrode with a graphene needle
electrode based on cyclic voltammograms (CV). NaOH
(0.1 M) solution was used for all electrochemical measure-
ments because low-strength alkaline solutions are optimal for
electrochemical activity of Co3O4 and its ability to mediate
enzymeless detection of glucose.17 Because of its low potential
window, Au electrode can dissolve at a high potential
(>800 mV).18 Even at low potentials, Au hydroxide and
oxides can form, which may degrade the electrode perfor-
mance or interfere with the electrochemical detection.19,20 The
oxidative and reductive peaks are evident in the CV curve of
the Au electrode (Fig. 3a).21 And these current peaks increase
sharply as the needle electrode immerses into the solution with
increasing depth. The intrinsic electrochemical reactions of Au
electrode (similarly for other metal electrodes) unavoidably
interfere with its ability in electrochemical detection.
In comparison, graphene is electrocatalytically inert and has
a large potential window (ca. 2.5 V in 0.1 M PBS at pH 7.0).22
Therefore, it is highly stable in solution and can detect
molecules with high oxidation or reduction potential. No
redox peaks are observed in the CV (0–0.65 V range) of the
graphene needle electrode (Fig. 3a). Its CV profile is capacitive
in nature and is less sensitive to the increasing immersion
depth of the electrode as compared with the Au needle
electrode. Furthermore, the high conductivity of graphene
ensures a fast charge transfer rate.5 Therefore, graphene is a
superior electrode material for electrochemical detection.
Fig. 3b demonstrates the CVs of a bare graphene needle
electrode and a graphene–Co3O4 hybrid needle electrode.
A pair of redox peaks is clearly observed from the hybrid
electrode, corresponding to the reversible conversion between
CoOOH and CoO2.23,24 The electrode is highly stable (o0.6%
variation after 100 CV sweeps). Fig. 3c shows the CVs of the
hybrid needle electrode at different scan rates. Both the
oxidative and reductive peak currents linearly scale with
the scan rate, indicating that the redox reaction of cobalt
oxide at the graphene surface is surface-controlled (Fig. 3d).
Detecting glucose is critical to the diagnosis and management
of type 2 diabetes. For most glucose sensors, the detection is
mediated by enzymes (e.g., glucose oxidase). The need for
enzyme proteins compromises the stability, reproducibility,
Fig. 2 (a) XRD patterns of graphene–Co(OH)2 and graphene–Co3O4
hybrids on a micropipette tip. (b) Raman spectra of graphene and
graphene–Co3O4 hybrid on a micropipette tip.
Fig. 3 (a) CVs of graphene and gold needle electrodes at a scan rate =
20 mV s�1, with the electrode immersion depth into solution varying
from 0.2 to 1 mm. The arrow indicates the increase of immersion
depth. (b) CVs of graphene and graphene–Co3O4 needle electrodes.
Scan rate = 20 mV s�1. (c) CVs of graphene–Co3O4 needle electrode
at different scan rates. (d) The plots of redox peak currents versus scan
rate.
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6492 Chem. Commun., 2012, 48, 6490–6492 This journal is c The Royal Society of Chemistry 2012
reusability and cost-efficiency of the sensor. Recently, it has
been demonstrated that Co3O4 nanostructures are able to
catalyse oxidation of glucose and therefore enable direct
electrochemical detection of glucose without the need for
any enzymes.17,25 Therefore, further electrochemical measure-
ment experiments were conducted. In the conventional three-
electrode configuration, the graphene–Co3O4 needle electrode
was used as the working electrode; a platinum plate as the
counter electrode; and an Ag/AgCl electrode as the reference.
As demonstrated in Fig. 4a, the increase of the anodic and
cathodic peak current of the graphene–Co3O4 needle electrode
is observed with the increase of glucose concentration. This is
due to the oxidation of glucose to gluconolactone catalyzed by
conversion of CoO2 to CoOOH: 2CoO2+C6H12O6 (glucose)-
2CoOOH + C6H10O6.17 Fig. 4b (inset) displays the ampero-
metric responses of the hybrid electrode (holding at 600 mV)
to successive addition of glucose at different concentrations.
The amperometric response is sensitive and fast. The dose–
response curve shows a linear response (R = 0.989) in the
glucose concentration range of 50 to 300 mM (Fig. 4b).
Furthermore, our electrode is able to detect glucose in serum
samples without being interfered by the presence of a large
variety of biomolecules, demonstrating its high selectivity and
potential for practical use (Fig. S1 in ESIw).In principle, the needle electrode (tip size B2 mm) is able to
probe the solution droplet of a picoliter volume (size of an
animal cell). In a proof-of-concept demonstration, a bare
graphene needle electrode and a graphene–Co3O4 needle
electrode was used as the counter electrode and working
electrode, respectively. In the amperometric recording mode
with the holding potential of 600 mV, the two electrodes
mounted on micromanipulators were inserted into a micro-
droplet (2 mL) of 0.1 M NaOH solution with observation
under an optical microscope (Fig. 4c). Upon addition of 2 mLof glucose solution (in 0.1 M NaOH) to reach the final glucose
concentration of 10 mM, an amperometric response was
triggered with a signal-to-noise ratio of B14 (Fig. 4d).
In summary, a needle-like graphene electrode was fabricated
by transferring a CVD grown graphene strip onto a glass
micropipette tip. Using electrochemical deposition followed by
thermal annealing, flower-like Co3O4 nanostructures with high
crystallinity were coated on the graphene needle electrode. The
obtained graphene–Co3O4 hybrid needle electrode was used
for enzymeless detection of glucose. Using a bare graphene
needle counter electrode and a graphene–Co3O4 hybrid working
electrode, glucose can be probed in a micro-droplet with a
lower detection limit o10 mM. In comparison with the
conventional planar electrodes or sensors, such a needle like
electrode enables sensitive detection in a small volume or
three-dimensionally addressable local-detection with high
spatial resolution.
This work is supported by an AcRF tier 2 grant from
Singapore Ministry of Education (MOE2011-T2-2-010).
Notes and references
1 R. Bogue, Sens. Rev., 2009, 29, 310.2 Y. X. Huang and P. Chen, Adv. Mater., 2010, 22, 2818.3 Y. X. Huang, D. Cai and P. Chen, Anal. Chem., 2011, 83, 4393.4 X. J. Huang and Y. K. Choi, Sens. Actuators, B, 2007, 122, 659.5 Y. X. Liu, X. C. Dong and P. Chen, Chem. Soc. Rev., 2012,41, 2283.
6 Y. X. Huang, X. C. Dong, Y. X. Liu, L. J. Li and P. Chen,J. Mater. Chem., 2011, 21, 12358.
7 C. Soldano, A. Mahmood and E. Dujardin, Carbon, 2010,48, 2127.
8 S. Campuzano and J. Wang, Electroanalysis, 2011, 23, 1289.9 Y. X. Huang, X. C. Dong, Y. M. Shi, C. M. Li, L. J. Li andP. Chen, Nanoscale, 2010, 2, 1485.
10 X. Huang, X. Y. Qi, F. Boey and H. Zhang, Chem. Soc. Rev., 2012,41, 666.
11 L. X. Liu, H. L. Zhou, R. Cheng, Y. Chen, Y. C. Lin, Y. Q. Qu,J. W. Bai, I. A. Ivanov, G. Liu, Y. Huang and X. F. Duan,J. Mater. Chem., 2012, 22, 1498.
12 C. H. Chen, C. T. Lin, Y. H. Lee, K. K. Liu, C. Y. Su, W. J. Zhangand L. J. Li, Small, 2012, 8, 43.
13 V. Gupta, T. Kusahara, H. Toyama, S. Gupta and N. Miura,Electrochem. Commun., 2007, 9, 2315.
14 T. Zhao, H. Jiang and J. Ma, J. Power Sources, 2011, 196, 860.15 Y. Z. Shao, J. Sun and L. Gao, J. Phys. Chem. C, 2009, 113, 6566.16 J. X. Zhu and Z. Gui, Mater. Chem. Phys., 2009, 118, 243.17 Y. Ding, Y. Wang, L. A. Su, M. Bellagamba, H. Zhang and Y. Lei,
Biosens. Bioelectron., 2010, 26, 542.18 P. Chen, B. Xu, N. Tokranova, X. J. Feng, J. Castracane and
K. D. Gillis, Anal. Chem., 2003, 75, 518.19 L. D. Burke, Electrochim. Acta, 1994, 39, 1841.20 K. E. Toghill and R. G. Compton, Int. J. Electrochem. Sci., 2010,
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S. K. Bhargava, Langmuir, 2009, 25, 3845.22 M. Zhou, Y. M. Zhai and S. J. Dong, Anal. Chem., 2009, 81, 5603.23 A. Salimi, R. Hallaj and S. Soltanian, Electroanalysis, 2009,
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Fig. 4 (a) CVs of the graphene–Co3O4 nanoneedle electrode in 0.1 M
NaOH at different concentrations of glucose. (b) Dose–response curve
and a representative amperometric response from the graphene–Co3O4
needle electrode. (c) Schematic illustration of amperometric detection
in a micro-droplet using two needle-electrodes. (d) The amperometric
response to the addition of 10 mM glucose into a 4 mL-droplet.
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