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Sensors and Actuators B 183 (2013) 381– 387

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators B: Chemical

journa l h om epage: www.elsev ier .com/ locat e/snb

iO-decorated single-walled carbon nanotubes for high-performanceonenzymatic glucose sensing

guyen Quoc Dung, Dewyani Patil, Hyuck Jung, Jimin Kim, Dojin Kim ∗

epartment of Materials Science and Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea

a r t i c l e i n f o

rticle history:eceived 8 February 2013eceived in revised form 5 April 2013ccepted 7 April 2013vailable online 15 April 2013

a b s t r a c t

Porous nickel-decorated single-walled carbon nanotubes (Ni-SWCNTs) were grown on indium tin oxide(ITO)-coated glass substrate through the arc-discharge method. Oxidation followed at 400 ◦C in the airfor 2 h to synthesize the NiO-SWCNT/ITO electrode. The Ni nanoparticles distributing among the SWCNTswere oxidized to become NiO as the effective sensing probes. The surface morphology and crystallinestructure of the NiO-SWCNT/ITO electrode were characterized by SEM, EDS and Raman spectroscopy. The

eywords:ickel oxideanoparticlesingle-walled carbon nanotubeslucose sensing

response of the NiO-SWCNT/ITO electrode to glucose was examined using the electrochemical method.The obtained cyclic voltammograms revealed the electrocatalytic behavior of the NiO-SWCNT/ITO elec-trode with glucose in 0.1 M NaOH solution. The amperometric response of the NiO-SWCNT/ITO electrodeto glucose was investigated at a potential of 0.5 V in 0.1 M NaOH that showed a linear range of thedetection from 1 �M to 1000 �M, as well as a low detection limit of 0.3 �M and a high sensitivity at907 �A mM−1 cm−2.

. Introduction

Glucose detection is important for the diagnosis and treat-ent of diabetes mellitus, as well as for various applications in

he food industry [1–3]. Although third-generation enzymatic glu-ose biosensors with high sensitivity and selectivity have beeneveloped, limitations such as the lack of stability, the varietyf requirements for the enzyme immobilization process, low-emperature storage and operation, inadequate control of pH, andigh toxicity in the environment have motivated further explo-ation of non-enzymatic glucose sensors [4–6]. Metal or metalxide nanoparticles of electrochemical catalysts, such as Pd, Pt, Au,i, NiO, and CuO, have been studied as non-enzymatic glucose-

ensing materials. Carbon nanotubes (CNTs) have been used toupport probe materials, as well as transducers, because of the one-imensional nanostructural nature of their high surface areas andlectrical conductivity [7,8]. In particular, the electrochemical oxi-ation of glucose in alkaline media using Cu and Ni electrodes hasevealed high signal currents due to the catalytic effects of suchlectrodes [4,5,9].

The common requirements for fabricating high-sensitivity com-

osites include highly porous hierarchical nanostructures for amoother contact with target elements in the solution as shownn the laboratory [9]. Various techniques have been attempted to

∗ Corresponding author. Tel.: +82 42 821 6639; fax: +82 42 823 4224.E-mail address: dojin@cnu.ac.kr (D. Kim).

925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.snb.2013.04.018

© 2013 Elsevier B.V. All rights reserved.

combine probe materials and CNTs to create non-enzymatic glu-cose sensors [8,10–12]. In the present paper, we report a one-stepsynthesis process for a non-enzymatic glucose sensor combiningNiO probes and single-walled CNTs (SWCNTs). The in situ arc-discharge method for SWCNTs [13,14], in which the catalytic metalNi is intercalated in the SWCNT bundles during synthesis, was usedto synthesize the glucose-probing sensor. This method has threeadvantages. First, the Ni nanoparticles as the sensing probe mate-rial are well dispersed among the SWCNT bundles of the carriertransport channel. Second, the Ni content can be controlled bythe Ni content in the graphite source and/or by the gas speciesin the environment. Third, the interwoven SWCNTs indicate thehigh porosity of the hierarchical structure. We have also shownthat the fabricated nanostructure sensor displays a sensing per-formance that is as commendable as other nano sensors installed[9,15,16].

2. Experiment

2.1. Reagents and materials

Analytical reagent-grade chemicals were used with de-ionizedwater in this study. Ascorbic acid (AA), d-glucose, sodium hydrox-ide (NaOH) and human serum were purchased from Sigma–Aldrich,

Yongin, Korea. Uric acid (UA), methanol and hydrochloric acid wereobtained from Alfa Aesar, Seoul, Korea. Indium tin oxide (ITO)-coated glass substrates (Samsung Corning Co., Korea) with a sizeof 0.5 cm × 2.0 cm and a sheet resistance of ∼6.5 �/sq were used.

382 N.Q. Dung et al. / Sensors and Actuators B 183 (2013) 381– 387

g of sy

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Scheme 1. The processin

.2. Preparation of the NiO-SWCNT/ITO electrodes

The NiO-decorated SWCNT synthesis conducted in this studyas based on catalytic SWCNT synthesis using the arc-dischargeethod developed in the laboratory [14]. The procedure of elec-

rodes fabrication is schematically shown in Scheme 1. A carbonraphite rod containing Ni, Mo, and Fe wires was used as therc-discharge source. More Ni nanoparticles were scattered in theWCNT bundles with the arc discharge in hydrogen environmenthan in helium environment. An initial metal particle content of

pproximately 15 wt.% in Helium (He) environments was increasedo approximately 39 wt.% in hydrogen environments [14]. Hence,WCNTs, with the inclusion of Ni particles, were grown in situ

ig. 1. (a) SEM images of NiO/SWCNTs before methanol treatment and (b) after methanatalyst particles.

nthesizing the electrode.

in an arc-discharge chamber onto the ITO electrode substratemounted on the chamber wall [13]. The samples underwent oxida-tion at 400 ◦C to remove amorphous carbon and oxidize Ni to NiO.The synthesized NiO-SWCNTs were characterized by high poros-ity attributed to steric hindrance among SWCNTs deposited onthe substrate, as shown in Fig. 1a. However, porosity also pre-vents the efficient adhesion of SWCNTs onto the substrate. Thus,adhesion promotion was conducted through methanol treatment,in which methanol was dropped onto the surface of the SWCNTelectrode and then heated at 100 ◦C for 10 min on a hot plate.

The pores subsequently shrunk to a few tens of nanometers insize because of agglomeration among the SWCNTs, as shown inFig. 1b. The NiO-SWCNT/ITO electrode was compared with two

ol treatment. (c) TEM image of the synthesized SWCNTs showing distribution of

N.Q. Dung et al. / Sensors and Actuators B 183 (2013) 381– 387 383

DS an

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Fig. 2. (a) Raman spectrum and (b) E

ther kinds of electrodes. One is the SWCNT/ITO electrode, whichas obtained by immersing the NiO-SWCNT/ITO electrode in con-

entrated hydrochloric acid (HCl) for 5 min to remove all catalyticarticles (NiO, MoO, and FeO). The other is the NiO/ITO thin-filmlectrode, which was fabricated via the sputter deposition of Ninto the ITO substrate followed by oxidation at 400 ◦C for 2 h in air.ll the electrodes used had a working area of 0.5 cm × 0.5 cm, whichas established by covering the unnecessary area by an insulating

poxy.

.3. Structural characterization and electrochemicaleasurements

The structure of the synthesized electrodes was examinedy field-emission scanning electron microscopy (FE-SEM; JSM-000F, JEOL), transmission electron microscopy (TEM, JEM-2100F,EOL), Raman spectroscopy (JY Lab Ram System, 514.5 nm linef an argon-ion laser), and energy-dispersive spectroscopy (EDS).lectrochemical measurements were performed using Potentio-tat/Galvanostat (EG&G Princeton Research Model 263A, USA) andontrolled using the electrochemistry software program, Pow-rsuite 2.55, and 0.1 M NaOH electrolyte solution composed of

three-electrode setup. The fabricated electrodes were used ashe working electrode, platinum as the counter-electrode, and

saturated calomel electrode (SCE) as the reference electrode.he cyclic voltammetry was used to study the redox behaviorf NiO-SWCNT/ITO electrode. The optimum potential applied formperometric detection of glucose was determined via hydro-ynamic voltammograms in which the currents flowing thelectrode was measured with varying potential in the absence andresence of glucose under stirring of magnetic bar.

. Results and discussion

.1. Characterization of the NiO-SWCNT electrode

The plan-view morphologies of NiO-SWCNTs/ITO electrodes, asbserved by FE-SEM before and after the methanol treatment, areompared in Fig. 1a and b. The pore sizes of the interwoven SWCNTundles exhibited a broad spectrum of sizes ranging from tens ofanometers to micrometers in the as-synthesized state, whereashe diameters of the SWCNT bundles were typically ∼20 nm inize. After the methanol treatment, the pores shrunk to tens ofanometers in size or to the size of the bundles (Fig. 1b). Theorous structure achieved a higher signal by enhancing the dif-

usion and flow of electrolytes through the SWCNT bundles. A TEMmage shows the morphology of the catalytic metal particles dis-ributing among the SWCNT bundles (Fig. 1c). The Raman spectrumerived for the NiO-SWCNTs is shown in Fig. 2a. The spectrum

alysis of NiO-SWCNTs/ITO electrode.

revealed peaks only for SWCNTs. The broad peak was at 189 cm−1,which is the radial breathing mode related to the SWCNT diame-ter [17]. A sharp G-band peak was observed for crystalline graphite(1595 cm−1) with a shoulder peak at 1572 cm−1, and a weak D-bandpeak (1340 cm−1) was observed for defective carbon, indicating thegood crystalline quality of the synthesized SWCNTs. The EDS spec-trum of NiO-SWCNTs is shown in Fig. 2b, which clearly indicatesthe presence of Ni and C. Fe was also detected, as well as the ele-ments from the ITO glass (Ca, Si, O, and In, among others). Sinceno glucose signals from Fe or Mo have been previously reported inNaOH solution [18], we assumed the detected signals came fromNiO.

3.2. Electrochemical properties

The electrocatalytic properties of the NiO-SWCNT/ITO electrodewere investigated in the alkaline medium of 0.1 M NaOH electrolytevia cyclic voltammetry. Ni-based electrodes are known to undergofouling under repeated cyclic sweeping of the potential in CV mea-surements, and the corresponding changes in the peak currentdensity were explained by the changes in the crystal structures ofthe nickel oxide and nickel hydroxide [11,19,20]. CV scanning wasthus repeated for the NiO-SWCNT/ITO electrode from 0.0 V to 0.8 Vat a scan rate of 20 mV/s, until a stationary state was reached. Thisprocess is summarized in Fig. 3a, which illustrates a well-definedpair of anodic and cathodic current peaks due to Ni(II)/Ni(III) redoxreactions [8]. However, the illustration signifies the shift of theanodic and the cathodic current density peaks to a higher potentialwith repeated cyclic biasing. The trend proves that electrocatalyticreactions mainly occur on the surface of NiO nanoparticles, ratherthan on SWCNTs and/or on other catalytic metal oxide particlesof iron oxide (FexOy) and molybdenum oxide (MoO3). The peakcurrent density changed might be due to change in the crystal struc-tures of the nickel hydroxide and nickel oxyhydroxide as reportedin ref. [11]. The stationary state could be obtained after 50 scanswhen the crystal structures became stable. We further show thatNiO nanoparticles decorating the SWCNT bundles enhance sensorperformance.

The effect of the potential scan rate on the oxidation and reduc-tion peak currents was investigated through scan rate variationsfrom 10 mV/s to 500 mV/s (Fig. 3b). The peak currents against thesquare root of the potential scan rate are plotted in Fig. 3c. The lin-earity proves the diffusion-controlled kinetics of the CV reaction atthe electrode surface, which was also demonstrated by other Ni-based electrodes [11,21]. The electrochemical oxidation of glucose,

measured in 0.1 M NaOH solution with varying glucose concentra-tions from zero to 5 mM at a scan rate of 20 mV/s, is represented bythe increase of the oxidation peak current, whereas the cathodicpeak current decreases with increasing glucose concentration

384 N.Q. Dung et al. / Sensors and Actuators B 183 (2013) 381– 387

F b) CV

5 an rate

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the inset of Fig. 5d for the response of the sensor with 0.1 mMglucose (the data dots are 1 s interval). The sensitivity of theelectrode, as calculated from the slope of the calibration curve,

ig. 3. (a) CV of NiO-SWCNTs/ITO electrode after 20, 30, 50, 90 consecutive cycles. (00 mV/s). (c) Plot of anode current and cathode current vs the square root of the sc

Fig. 3d). The results indicate that glucose was oxidized via theedox coupling [11]. Furthermore, the shift to a higher potential at

higher glucose concentration reflects the diffusion-limited reac-ion of glucose on the electrode surface [11,12,22]. All the resultsre in agreement with those of previous reports and suggested thenvolvement of the catalytic reaction [NiO + OH− → NiO(OH) + e− ori(OH)2 + OH− → NiO(OH) + H2O + e−] and glucose oxidation reac-

ion [NiO(OH) + glucose → Ni(OH)2 + glucolactone].

.3. Sensing properties

Hydrodynamic voltammetry was used to reduce the opti-um operational potential for glucose sensing. The hydrodynamic

oltammograms of NiO-SWCNT/ITO, compared with those ofWCNT/ITO and NiO/ITO thin-film electrodes, were measuredn 0.1 M NaOH with varying potential for 0.1 mM glucose. Theesponse currents plotted in Fig. 4 are the differences in the cur-ent densities before and after the addition of glucose. The highestesponse current of the SWCNT/ITO electrode was observed at theotential of 0.55 V, whereas for the NiO/ITO thin film and the NiO-WCNT/ITO electrodes were at 0.5 V. The bias potentials for theighest responses are near the half-cell potential (E0

1/2) that can

e determined by E01/2 = (Epc + Epa)/2 in Fig. 3a, where Epc is the

eduction potential peak and Epa is the oxidation potential peak.he decrease of the current density at the potential above theeak bias voltage can be explained by two reasons: (1) the glu-ose oxidation interfered by the oxidation of water and (2) theoisoning caused by the products of glucose oxidation at highotentials. The overlapping of the oxidation peaks for glucose andater was reported in our previous report for SWCNTs/ITO elec-

rode [9]. The most remarkable observation is the much higheresponse of the NiO-SWCNT/ITO electrode than that of the other

lectrodes. The small signal and high sensitivity to glucose of theiO-SWCNT/ITO structure may be explained by the roles of SWC-Ts and NiO nanoparticles as a suitable electron transport channelnd large surface-area catalyst, respectively. CNTs or graphite is

of NiO-SWCNTs/ITO electrode with different scan rate (10, 20, 30, 50, 100, 200, 300,. (d) CV of the NiO-SWCNTs/ITO electrode at different glucose concentration (mM).

assumed to lack a glucose-detecting capacity [7,8]. In actuality,pure-SWCNT electrodes obtained by etching in acid solution maystill contain unsolved intercalated NiO particles among the bun-dles, and these particles can produce the signals. This issue mustbe explored separately. Based on the hydrodynamic voltammetryresults, a potential of 0.5 V was identified in succeeding glucosedetection by amperometric measurements.

Fig. 5 shows amperometric glucose detection by the NiO-SWCNT/ITO electrode in 0.1 M NaOH solution at 0.5 V withsuccessive additions of glucose. The sensor electrode exhibitedlinearity for glucose sensing from 1 �M to 900 �M, with an excel-lent correlation coefficient of 0.999. The steady state currentswere obtained at fast response times of about 2 s as shown by

Fig. 4. The response current of NiO/SWCNTs/ITO, NiO/ITO thin film, and SWC-NTs/ITO measured in 0.1 M NaOH for 100 �M glucose at different potentials.

N.Q. Dung et al. / Sensors and Actuators B 183 (2013) 381– 387 385

Fig. 5. Amperometric response of NiO-SWCNTs/ITO electrode measured at 0.5 V in 0.1 M NaOH solution with addition of glucose. Results are measurement in the range of( rent de remena

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It showed negligible change in the response current density fortwo weeks reflecting a good stability of the nonenzymatic glucosesensor.

a) 1–10 �M, (b) 10–100 �M, and (c) 100–1500 �M. (d) Shows the summarized currror bar at each glucose concentration is the relative standard deviation for measut 1 s intervals.

as 907 �A mM−1 cm−2, with a linear regression equation of = 14.46 + 907 C (�A mM−1 cm−2), where j is the current density and

is the glucose concentration, and the detection limit was about.3 �M, which was determined by the concentration at which theesponse current density is 3-fold of the noise level (or signal-to-oise ratio = 3).

The synthesized sensors were compared with those of otherlucose sensors in terms of the type of electrolyte, applied poten-ial, linear range of detection, limit of detection, and sensitivity,s shown in Table 1. The NiO-SWCNT/ITO electrode was partic-larly superior in terms of sensitivity and low limit detectiongain due to the scattered nano-particulate catalyst of Ni and aigh-conductivity current path assisted by the SWCNTs. The NiOanoparticles of tens of nm distributing among the porous SWCNTsill signify the nano-size effect.

.4. Sensor stability and specificity

The specificity of the NiO-SWCNT/ITO electrode was tested byeasuring its response together with interfering species of ascorbic

cid (AA), uric acid (UA), and sucrose at the operational potential of.5 V. The specificity of the electrode is presented in Fig. 6 by adding0 �M of AA, UA, and sucrose in 0.1 M NaOH solution containing00 �M glucose. The additional signals from the addition of AA,A, and sucrose were 1.1%, 2.7%, and 1.9%, respectively. Consider-

ng the AA and UA concentrations in the blood were less than 1/30 ofhe glucose, the NiO-SWCNT/ITO electrode should be able to detectlucose in the blood. The NiO–SWCNT/ITO sensor electrode waslso tested for the determination of glucose in human serum sam-

les. A serum solution of 100 �L was diluted in a 25 ml 0.1 M NaOHolution. The amperometric response was recorded at a potential of.5 V. The glucose concentration of 5.07 mM in serum sample wassed as stock solution that was determined in our previous report

ensity vs. glucose concentration for estimation of the linearity and sensitivity. Thets by three samples. Inset in (d) shows a response of the sensor to 0.1 mM glucose

[9]. Seven serum samples were prepared from the stock solutionby adding water or glucose. Measurements were done three timesfor each serum sample and relative standard deviations (SDs) werecalculated as summarized in Table 2. The reproducibility of sensingfor six different electrodes made from the same batch prepara-tion was also examined with 0.1 mM glucose. Standard deviationof 5.95% was obtained, and this proved good reproducibility ofthe sensors. The long-term stability was also examined by mea-suring response every day for two weeks with 0.1 mM glucose.

Fig. 6. The specificity of NiO/SWCNTs/ITO electrode examined by measuringresponses to interfering species of AA, UA, and sucrose at a potential 0.5 V in 0.1 MNaOH.

386 N.Q. Dung et al. / Sensors and Actuators B 183 (2013) 381– 387

Table 1Analytical parameters obtained for different glucose sensors.

Electrode Electrolyte Applied potential (V) Linear range Limit of detection (�M) Sensitivity (�A mM−1 cm−2) Ref.

Ni/MWCNTs hybrid 0.1 M NaOH +0.6 Up to 17.5 mM 0.89 67.2 [11]Ni-BDD 1 M NaOH +0.48 1–10 mM 2.7 1040 [10]NiCFP electrode 0.1 M NaOH +0.6 2 �M–2.5 mM 1 420.4 [8]CuO-NFs/GCE 0.1 M NaOH +0.65 6 �M–3.5 mM 0.8 431.3 [23]CuO nano sphere/GCE 0.1 M NaOH +0.6 1 �M–2.55 mM 1 404.53 [24]Gold nanowire array electrode 0.1 M NaOH −0.4 1–10 mM 50 309.0 [6]Ni nanowires array 0.1 M NaOH +0.55 0.5 �M–7Pt nanoporous PBS (7.4) +0.4 50 �M–7

NiO-SWCNTs/ITO 0.1 M NaOH +0.5 1–900 �M

Table 2Glucose sensing in human blood serum samples having different glucose concen-trations. Intended and measured concentrations are listed up with relative standarddeviations.

Sample NiO-SWCNT/ITO glucose sensor

Intendedconcentration (mM)

Measured byconcentration (mM)

SD (%)

1 5.07 5.10 1.612 2.54 2.44 6.463 2.98 2.70 4.364 3.62 3.52 3.415 4.23 4.3 3.986 4.61 4.64 5.37

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. Conclusions

The arc-discharge synthesis of SWCNTs was conducted to cre-te a high-performance glucose sensor. The technique is basedn the use of the NiO catalyst nanoparticles that were dispersedell among porous SWCNT bundles. The NiO nano-particles aresed as nano-probes for non-enzymatic glucose detection, withhe entangled SWCNT bundles functioning as highly conductiveransport channels. The porosity produced by the SWCNT entan-lement provides space for glucose facile diffusion to NiO particles.he NiO-decorated SWCNT electrode exhibited high sensitivity at07 �A mM−1 cm−2, a low detection limit at 0.3 �M, and a linearange from 1 �M to 900 �M. The results are considerably betterhan thin-film type NiO electrodes. Furthermore, comparison withure-SWCNTs confirmed the effect of NiO nanoparticles for high-ensing performance. Appropriate selectivity from AA, UA, anducrose was examined. The test with serum samples shows repro-ucible measurements suggesting the possible application of thelectrode to actual diagnoses.

cknowledgement

This research was supported by the Basic Research program (No.012R1A1A4A01011671) of the National Research Foundation oforea.

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Nguyen Quoc Dung received his master of materials science in InternationalTraining Institute for Materials Science (ITIMS) at Hanoi University of Technology,Vietnam, in 2006. He is now in Ph.D. course in materials science and engineering,

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N.Q. Dung et al. / Sensors and

hungnam National University, Korea. His current interests are nano-structureaterials and their applications to bio- and chemicalsensors.

ewyani Patil is a post doctoral fellow in Department of Materials Sci-nce and Engineering, Chungnam National University, Daejeon, Southorea. She did her Ph.D. in physics from North Maharashtra Univeristy,

algaon, India in 2010. Her research interests are synthesis of nanostruc-ured materials and their application to gas sensors, biosensors and humidityensors.

yuck Jung received his Ph.D in Materials Science and Engineering from Chung-am National University, Korea, in 2013. He is currently a researcher at the Nanoaterials and Application Lab, Chungnam National University, Korea. His cur-

ent interests include fabrication and characterization of carbon nanotube and

tors B 183 (2013) 381– 387 387

its application to nano-electronic devices of transparent conductive film andsensors.

Jimin Kim received his degree in master of materials science and engineering fromChungnam National University, Korea, in 2013. His current interests include syn-thesis and application of semiconducting nanowires.

Dojin Kim received his Ph.D in Materials Science and Engineering from the Uni-versity of Southern California, USA, in 1989. He is a professor at the School of

Nanotechnology and the Department of Materials Science and Engineering inChungnam National University, Korea. His current research interests are synthe-sis of carbon nanotube and metal oxides and applications to electronic devices offield emitters, solar cells, gas sensors, so on. He is also interested in semiconductornanostructures growth and applications.