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

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6 Y. X. Huang, X. C. Dong, Y. X. Liu, L. J. Li and P. Chen,J. Mater. Chem., 2011, 21, 12358.

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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.

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Biosens. Bioelectron., 2010, 26, 542.18 P. Chen, B. Xu, N. Tokranova, X. J. Feng, J. Castracane and

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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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