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Sensors and Actuators B 215 (2015) 337–344

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

jo ur nal home page: www.elsev ier .com/ locate /snb

o–Cu alloy nanoparticles decorated TiO2 nanotube arrays for highlyensitive and selective nonenzymatic sensing of glucose

.V. Suneesha, Vidhu Sara Vargisa, T. Ramachandrana, Bipin G. Nairb,

.G. Satheesh Babua,∗

Department of Sciences, Amrita Vishwa Vidyapeetham, Amritanagar P.O., Coimbatore 641112, IndiaAmrita School of Biotechnology, Amrita Vishwa Vidyapeetham, Amritapuri, Clappana P.O., Kollam 690525, India

r t i c l e i n f o

rticle history:eceived 2 December 2014eceived in revised form 10 March 2015ccepted 12 March 2015vailable online 7 April 2015

eywords:

a b s t r a c t

A nonenzymatic glucose sensor was fabricated by electrodepositing cobalt rich cobalt–copper alloynanoparticles (Co–CuNPs) on vertically aligned TiO2 nanotube (TDNT) arrays. For this, TDNT arrays withtube diameter of 60 nm were synthesized by electrochemical anodization. The composition of the elec-trodeposited alloy was optimized based on the electrocatalytic activity towards glucose oxidation. This isachieved by controlling the concentration of electrolyte and time of deposition. Chemical composition ofthe optimized Co–Cu alloy nanoparticles is determined to be Cu0.15Co2.84O4 with fcc crystalline structure.

−1 −2

obalt–copper alloyanofloweronenzymatic glucose sensoriO2 nanotube arrayslectrodepositionnodization

The sensor showed two linear range of detection with high sensitivity of 4651.0 �A mM cm up to5 mM and 2581.70 �A mM−1 cm−2 from 5 mM to 12 mM with a lower detection limit of 0.6 �M (S/N = 3).The sensor is highly selective to glucose in the presence of various exogeneous and endogeneous inter-fering species and other sugars. The response of the sensor towards blood serum was in good agreementwith that of commercially available glucose sensors.

© 2015 Elsevier B.V. All rights reserved.

. Introduction

In the present scenario of increase in diabetics globally, quicknd effective estimation and monitoring of glucose level in bloodecomes highly essential. Several earlier reports are available relat-

ng to the enzymatic estimation of glucose [1–6] which is veryelective and sensitive but suffer from poor storage stability andlso from thermal and chemical instability. In this context use ofon-enzymatic sensors assumes great importance and research inevelopment of such sensors is being carried out all over the world.he focus of their research is on making sensors that possess highelectivity, great accuracy, good mechanical, thermal and chemicaltability and above all of relatively low cost [7–11]. In line with thishe primary aim of our research is to develop an electrochemicalon-enzymatic sensor that has all the above said properties.

Non-enzymatic electrochemical sensors employing preciousetals like platinum, gold and their alloys [12,13] are reported

o show excellent performance but sometimes their cost is pro-ibitive, unaffordable by many. Further platinum and platinumased alloys are prone to get poisoned by some of the products

∗ Corresponding author. Tel.: +91 9442368632; fax: +91 422 2686274.E-mail address: tgsatheesh@gmail.com (T.G. Satheesh Babu).

ttp://dx.doi.org/10.1016/j.snb.2015.03.073925-4005/© 2015 Elsevier B.V. All rights reserved.

in the analytes and also interference from other biomolecules likeascorbic acid, dopamine etc. has been observed [14]. Common andcheaper metals like copper, cobalt and nickel and their oxides areshown to possess good electrocatalytic activity towards the oxida-tion of glucose, in alkaline medium [15–18]. The sensors based onthese metals have been very well reviewed [19].

Further it has been established that incorporating these metalsand their oxides in nano dimension onto suitably surface modi-fied substrate such as titanium results in highly enhanced responsein terms of current, reduction in overpotential, etc., while sensingglucose. For instance, sensors based on titanium dioxide nano tubearrays (substrate) decorated with nano particles of Cu [20], Pt [21],Ni [22,23] and Cu–Ni [24] sense glucose with excellent selectivityand high degree of sensitivity. Bimetallic and alloy nano particlesare reported to have good electro catalytic activity and enhancedstability, due to the synergistic effect of one to the other. Suchsystems include Ni–Co [25], Pt–Ni [26], Pt–Cu [27], Cu–Ni [24,28],Pt–Pb [29], Pt–Pd [30], Pt–CuO [31] and Pt–Au [32,33].

This work demonstrates the development and testing of a highlysensitive nonenzymatic glucose sensor. The sensor was fabricated

by depositing Co–Cu alloy electrocatalyst on TDNT arrays. It wassubjected to rigorous performance tests, in terms of sensitivity,selectivity, reproducibility, storage stability, real-sample analysisand found to function excellent. The sensitivity of the sensor

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38 P.V. Suneesh et al. / Sensors an

eveloped is much higher than that of the many recent reportsrom our group and other groups.

. Experimental

.1. Materials and methods

Titanium foil (Ti, 0.25 mm thick), glucose (Glu), ascorbic acidAA), dopamine (DA), uric acid (UA), acetamidophenol (AP),ructose, galactose, maltose, lactose, urea and creatinine were pur-hased from Sigma Aldrich. CoCl2·6H2O, CuSO4·5H2O, boric acid,Cl and NaOH were of analytical grade from FINAR chemicals, Indiand used without further purification. All stock solutions exceptric acid were prepared in Millipore water (15.2 M� cm, Millipore,ermany) and stored at 4 ◦C when not in use. Uric acid stock solu-

ion was prepared in 0.01 M NaOH.

.2. Morphological and electrochemical measurements

Surface morphology of the modified electrodes was studiedsing Variable Pressure Field Emission Scanning Electron Micro-cope (FESEM, Hitachi SU6600). The surface compositions of eachlectrode were analyzed using Energy Dispersive SpectroscopyEDS) attached with FESEM. The crystallinity of the electrode sur-ace was established using X-ray powder diffractometer (BrukerXS D8).

All electrochemical measurements were carried out on CHI60C electrochemical workstation (CH Instruments, TX, USA) with

three electrode cell. A platinum wire as counter and TDNT orodified TDNT electrodes, titanium plate and glassy carbon (GC,

mm dia.) were used as working electrode. All the potential mea-urements were made with respect to Ag/AgCl (saturated KCl)eference electrode. Cyclic voltammograms (CV) were recorded onDNT arrays at a scan rate of 0.05 V/s in an electrolyte of 380 mMoric acid and 380 mM KCl containing with 250 mM cobalt chlo-ide, 15 mM copper sulphate and 250 mM cobalt chloride + 15 mMopper sulphate at a pH of 4 to study the deposition potential. CVnd linear sweep voltammetry (LSV) experiments were performedn the modified electrodes in 0.15 M NaOH with and without glu-ose at 0.05 V/s scan rate in order to study the electrooxidationf glucose. Chronoamperometric experiments were carried out in

constantly stirred solution of 0.15 M NaOH by injecting glucosend other species.

.3. Fabrication of sensor electrode

Titanium foil was anodized from a solution containing 0.15 Mmmonium fluoride in glycerol and water mixture with a ratio0:10 (v/v, %) to obtain TDNT arrays and annealed at 500 ◦C for

h [23,34,35]. On to this TDNT copper was electrodeposited from solution containing 15 mM copper sulphate, 380 mM boric acidnd 380 mM KCl to obtain CuNP/TDNT (nano dimension confirmedy morphology later). Similarly cobalt was electrodeposited from50 mM cobalt chloride, 380 mM boric acid, 380 mM KCl contain-

ng bath to make (CoNP/TDNT). Cobalt–copper alloy was depositedrom a solution of 250 mM of cobalt chloride, 380 mM boric acidnd 380 mM KCl electrolyte containing varying concentrations ofopper sulphate (5–25 mM) to get Co–CuNP/TDNT. Duration oflectrodeposition was 30 s under an applied potential of −1 V, theH of the electrolytes were adjusted to 4 using dilute H2SO4. After

lectrodeposition the electrodes were oxidized in 0.15 M NaOHolution by cycling the potential between −1.0 V and +1.0 V for0 times at 0.05 V/s scan rate. These electrodes were washed andtored in distilled water.

ators B 215 (2015) 337–344

2.4. Real sample analysis

For estimation of glucose in human blood, serum wasextracted from human blood samples by centrifugation at 15,294 g(12,000 rpm) for 30 min. 50 �L of the serum samples were injectedinto 4 mL constantly stirred solution of 0.15 M NaOH. The amper-ometry was done with blank and with addition of 50 �L of standardglucose solution at +0.6 V. This was compared with commercialJohnson and Johnson One Touch Select test strips.

3. Results and discussion

3.1. Electrochemical deposition of cobalt, copper andcobalt–copper alloy

The CV in electrolyte containing cobalt ions (Fig. 1A) shows ahuge rise in cathodic current at −1.0 V (a) corresponds to the reduc-tion of cobalt ions and its corresponding oxidation peak ‘b’ appearsat −0.16 V during the anodic sweep. Nucleation loop formed dur-ing the reverse scan indicates that the deposition of cobalt on TDNTrequires an over potential for nucleation [36]. CV obtained in cop-per electrolyte (Fig. 1B) shows two cathodic peaks at −0.16 V (a)and −0.24 V (b) correspond to the reduction of Cu2+ to Cu+ and Cu+

to Cu respectively. The peak ‘c’ at −0.8 V corresponds to the bulkreduction of copper. The peak obtained during the anodic sweepis due to the oxidation of deposited copper. CV in Co–Cu binaryelectrolyte shows three reduction peaks and they can be assignedto the reduction of copper (a,b) and cobalt ions (c) by compar-ing with the peaks obtained for individual metal ion electrolytes(Fig. 1C). Similar reduction peaks were reported for Co–Cu alloyelectrodeposition by various authors [37–39]. There is the forma-tion of nucleation loop but the cobalt reduction potential is slightlyshifted towards positive, which could be attributed to the alloy for-mation. The peaks obtained during anodic scan can be assignedfor the oxidation of cobalt (d) and for cobalt-copper alloy (e). It isobvious from the above discussion that in order to achieve a code-position of cobalt and copper on TDNT array, a minimum of −0.9 Vis required. Hence for all deposition studies −1.0 V is used.

3.2. Morphological and elemental characterization

Fig. 2 presents the FESEM images of the TDNT, CoNP/TDNT,CuNP/TDNT and Co–CuNP/TDNT electrodes. Vertically aligned TiO2nanotube arrays of average tube diameter 60 nm are clearly seen inFig. 2A. The nanotubes are highly uniform and distributed all overthe surface. Synthesis of TiO2 nanotubular structures and the mech-anism of tube formation are well reported and reviewed [34,40].FESEM images of the CoNP/TDNT electrode (Fig. 2B) show spher-ical nanoparticles which are decorated with flake like structuresand are uniformly distributed over the electrode surface. Similarmorphology is reported by Lee et al. [41]. The cobalt nanoflakeshave an average thickness of 15 nm. Fig. 2C shows that Cu depositobtained on TDNT is highly non-uniform and it can be attributed tothe non uniformity of TDNT surface at the nano regime. The FESEMof Co–CuNP/TDNT on the surface shows flower like structures. Thepresence of TiO2, cobalt, copper and oxygen on the electrode surfacewas established by EDS analysis.

The XRD patterns of the modified electrodes are shown in Fig. 3.Annealed TDNT (A) is present in anatase form with tetragonalphase and various crystal planes are identified by comparing withJCPDS data card 89-4921. Cobalt in CoNP/TDNT (B) occurs as Co3O4

with fcc lattice structure (JCPDS data card 43-1003). The intensediffraction patterns confirm that the oxide of cobalt is highly crys-talline. Monoclinic CuO crystals were identified on CuNP/TDNT(JCPDS data card 89-5895) surface (C). Various peaks of Cu and

P.V. Suneesh et al. / Sensors and Actuators B 215 (2015) 337–344 339

Fig. 1. Cyclic voltammograms recorded on TDNT arrays at a scan rate of 0.05 V/s in an electrolyte of 380 mM boric acid and 380 mM KCl containing (A) 250 mM cobalt chloride,( e at a

Ca(tm

B) 15 mM copper sulphate and (C) 250 mM cobalt chloride + 15 mM copper sulphat

o were identified on Co–CuNP/TDNT (JCPDS data card 78-2172)

nd the alloy is present in Cu0.15Co2.84O4 (fcc) crystalline phaseD). The decrease in intensity of the peaks can be attributed tohe lower crystallinity of the alloy compared to the individual

etals.

Fig. 2. FESEM images of TDNT (A), CoNP/TDNT (B), CuNP/TDNT (C) and Co

pH of 4.

3.3. Electrochemical oxidation of glucose

Fig. 4A shows the CVs recorded in 0.15 M NaOH containing6.25 mM glucose on CuNP/TDNT, CoNP/TDNT and Cu–CoNP/TDNTelectrodes. Even though copper is a good electrocatalyst for

–CuNP/TDNT (D) electrodes prepared by electrochemical methods.

340 P.V. Suneesh et al. / Sensors and Actuators B 215 (2015) 337–344

), CoN

gocC+rwscaSntic

cfoirtt

F

Fig. 3. X-ray diffraction patterns of annealed TDNT (A

lucose oxidation; a well defined oxidation peak was not obtainedn CuNP/TDNT electrode. This may be due to the very low con-entration of copper on the electrode surface. It is found that onoNP/TDNT electrode, glucose undergoes oxidation at around0.6 V but the peak was not well defined. Similar results wereeported by various authors [42,43,41]. A well defined anodic peakas obtained on the Co–CuNP/TDNT electrode at +0.45 V corre-

ponding to the oxidation of glucose. This tremendous increase inurrent response corresponding to the oxidation of glucose can bettributed to the synergistic effect of both Co and Cu nanoparticles.imilar observations were reported recently on copper–cobaltanostructures modified graphene oxide [38]. It is interestinghat the presences of very small amount of Cu along with Concreases the electrocatalytic activity and in turn the responseurrent.

To study the role of TDNT arrays, sensor electrodes were fabri-ated using other substrates like GC and Ti plate following similarabrication procedure. The response obtained is compared with thatf Co–CuNP/TDNT electrode (Fig. 4B). Well defined oxidation peaks obtained on all these electrodes but the magnitude of current

esponse on Co–CuNP/TDNT electrode is several times higher thanhe response on sensor based on Ti and GC. Further the overpoten-ial for glucose oxidation is less on Co–CuNP/TDNT when compared

ig. 4. CVs recorded on modified TDNT electrodes (A) and different electrodes modified w

P/TDNT (B), CuNP/TDNT (C) and Co–CuNP/TDNT (D).

with that on other two electrodes. The enhancement in sensorproperties may be attributed to the increased surface area of thesensor electrode which in turn due to the nanotubular structuresof TDNT.

In order to study the influence of amount and composition ofCo–Cu alloy towards glucose oxidation, various electrodes werefabricated by varying the deposition parameters such as time ofdeposition and composition of electrolyte solution. The depositiontime was varied from 10 s to 120 s and the sensitivity of the elec-trodes was compared. It is found that the sensor obtained after30 s of electrodeposition showed maximum sensitivity and fur-ther increase in deposition time did not show an improvementin response, rather it exhibited a shift in oxidation potentials tomore positive values. Thus deposition for 30 s was considered asthe optimum.

The composition of the alloy was varied by varying the concen-tration of copper sulphate in the electrolyte from 5 mM to 25 mMkeeping cobalt chloride concentration constant. The alloy deposi-tion was performed from each electrolyte at a constant potential of−1.0 V vs Ag/AgCl (sat. KCl) for 30 s. All the electrodes were tested

in 0.15 M NaOH containing 6.25 mM glucose and the sensitivity wascompared. It was found that the alloy obtained from the bath con-taining 15 mM copper sulphate showed highest sensitivity and a

ith Co–CuNP (B) in 0.15 M NaOH with 6.25 mM glucose at a scan rate 0.05 V/s.

P.V. Suneesh et al. / Sensors and Actu

Table 1Materials and conditions used for electrodeposition of Co–Cu alloy.

Cobalt chloride 250 mMCopper sulphate 15 mMBoric acid 380 mMPotassium chloride 380 mM

pH 4Bath temperature RT

lg

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response obtained for the biomolecules are listed in Table 2 consid-

Fa

Deposition potential −1 V vs Ag/AgCl (sat. KCl)Deposition time 30 s

inear response. Thus the optimized alloy deposition conditions areiven in Table 1.

.4. Quantitative determination of glucose

LSVs recorded on the Co–CuNP/TDNT electrode in 0.15 M NaOHolution with increasing concentrations of glucose are depicted inig. 5A. There are two linear ranges, the first up to 5 mM (Fig. 5B)ith a sensitivity of 4651 �A mM−1 cm−2 with linear regression

quation jp (mA cm−2) = 0.7350 + 4.651C (mM) where regressionoefficient r = 0.9970, standard deviation � = 0.6631 for N = 9 mea-urements. The second is between 5 mM and 12 mM (Fig. 5C)ith sensitivity of 2581 �A mM−1 cm−2, regression equation jp

mA cm−2) = 11.47 + 2.5817C (mM), r = 0.9966, � = 0.4633 for N = 11.he oxidation potential shifts towards more positive on increas-ng the concentration of glucose. This shows that there is someinetic limitation between redox sites of the electrode and glucoses reported on nickel oxide by Shamsipur et al. [44]. The sensitivitynd upper range of glucose detection is higher than the correspond-ng values reported for other bimetallic systems [24,25,28,31].

The effect of scan rate on the glucose oxidation current wastudied on the Cu–CoNP/TDNT electrode and the plot of peak cur-ent vs scan rate is shown in Fig. 5D. It was found that the responseurrent is increasing linearly with scan rate. This suggests that

ig. 5. (A) LSVs recorded on Co–CuNP/TDNT electrode in 0.15 M NaOH with increasing cond (C) calibration plot up to 12 mM glucose concentrations. (D) Study on the effect of sc

ators B 215 (2015) 337–344 341

glucose oxidation on the alloy surface obeys Nernstian conditionfor adsorption controlled reactions [45].

Further, the statistical lower detection limit was calculatedby performing CV in 0.15 M NaOH for 30 times. The populationstandard deviation was calculated and the lower detection limitwas obtained as 0.6 �M with a signal to noise ratio of 3.

Steady state current response was obtained on theCo–CuNP/TDNT electrode at different applied potentials andthe sensitivity and the upper detection limits are compared(Fig. 6A) and is found that at +0.6 V the sensor showed highercurrent density and is able to detect concentrations up to 12 mM.At +0.5 V the current density is lower and can detect glucose up to6 mM. The response at +0.4 V is still lower and could detect onlyup to 2.5 mM concentrations of glucose. At +0.7 V the electroderesponse is lower than that of +0.6 V and is greatly affected withgas evolution and could detect glucose concentration up to 5 mMwithout noise. Thus +0.6 V was chosen as the optimum potentialfor the amperometric determination of glucose. All further testingwas carried out at this potential. The response was measured forvarious concentrations of glucose and is shown in Fig. 6B. It isclear from the figure that the sensor could detect glucose frommicromolar to millimolar levels.

3.5. Effect of interfering species

The serious drawback faced by the nonenzymatic electrochem-ical sensors is their lack of specificity. Easily oxidizable moleculespresent along with glucose in biological fluids such as ascorbic acid,uric acid and dopamine interfere with the sensor response. We havealso tested possible interference from other sugars present in bio-logical fluids at their physiological concentrations. The percentage

ering the response of glucose as 100%. The results show that thecommon endogenous and exogenous interfering species and othersugars will not influence the response of the sensor.

ncentrations of glucose at a scan rate of 0.05 V/s, (B) the calibration plot up to 5 mMan rate on glucose oxidation current in 0.15 M NaOH containing 6.25 mM glucose.

342 P.V. Suneesh et al. / Sensors and Actuators B 215 (2015) 337–344

Fig. 6. (A) Comparison of sensitivities and upper detection limit of the Co–CuNP/TDNT at different applied potentials. (B) Steady state amperometric response of the sensortowards various ranges glucose concentrations and (C) determination of glucose in blood serum at an optimized detection potential of +0.6 V. Analyses were performed in0.15 M NaOH under constant stirring.

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.6. Reproducibility and repeatability of the sensor

Glucose sensor response measured on five electrodes fabricatedsing the same procedure shows less than 2% variation among theesults and which confirms the high reproducibility of the sensorabrication. The sensor was preserved in distilled water at roomemperature (25 ± 2 ◦C) when not in use. The sensor stability wasested for a period of three months. A small shift in oxidation poten-ial was observed and the current response decreased by 8% afterhree months. This enhanced stability may be attributed to the highhermodynamic stability of oxides of cobalt and copper and also due

o the extra stability provided by the alloy.

able 2teady state current response (in percentage) obtained for different biomoleculesn Co–CuNP/TDNT electrode at 0.6 V in 0.15 M NaOH solution.

Biomolecule Physiologicallevel (mM)

Testedconcentration(mM)

Percentagecurrentresponse

Glucose 4.4–6.6 6.25 100Fructose 0.4 0.4 5Maltose 0.4 0.4 5Galactose 0.2 0.2 −3Lactose 0.029 0.029 −4Ascorbic acid 0.125 0.125Uric acid 0.000125 0.000125Acetamidophenol 0.125 0.125 NegligibleCreatinine 0.258 0.258 (<1)Urea 0.6 0.625Chloride 0.13 0.205

3.7. Comparison of the sensor characteristics with existingsensors

The response of Co–Cu TDNT sensor towards glucose was com-pared with other recently reported sensors in Table 3 [46–53]. It isevident that the sensor shows very high sensitivity when comparedwith the other sensors. The lower applied potential, wide range ofdetection (up to 12 mM), lower detection limit of 0.6 �M and highselectivity towards glucose makes the sensor superior among theother recently reported nonenzymatic glucose sensors.

3.8. Real sample analysis

Serum glucose levels were tested using the fabricated sensor.Blood serum was injected to 0.15 M NaOH solution at an appliedpotential of +0.6 V followed by standard glucose solutions. Thereis a slight time dependant decrease in current after the additionof serum samples when compared with the pure glucose solu-tions. This may be attributed to the effect of protein impuritiespresent in the serum. The current response obtained for bloodserum is compared with that of the glucose solution (Fig. 6C) andthe serum glucose concentration is calculated. Five replicate mea-surements with the Co–CuNP/TDNT and commercial sensor stripsfor the serum samples showed a standard deviation of 3.033 and2.0 respectively. This shows the high precision among the glucosesensor values in mg/dL by the developed sensor. The student t-test

values implies that the mean value (�) lie with 95% probabilitywithin � ± 3.57 mg/dL. Also the mean value of the measurementsby the Co–CuNP/TDNT and the commercial sensor shows only adifference of 2.8 mg/dL (Johnson and Johnson One Touch Select).

P.V. Suneesh et al. / Sensors and Actuators B 215 (2015) 337–344 343

Table 3Comparison of Co–CuNP/TDNT sensor response with recently reported nonenzymatic glucose sensors.

Sensorelectrode

Sensitivity(�A mM−1 cm−2)

Appliedpotential (V)

Linear range(mM)

Limit ofdetection (�M)

Interferingmoleculestested

Ref.

Ni nanoparticle/carbon nanofiber paste 3.3 +0.6 0.002–2.5 1 Cl− [46]Ni nanowire arrays 1043 +0.55 0.0005–7.0 0.1 AA and UA [47]Over-oxidized polypyrrole/Pd/Si 370 +0.08 24 2 AA and UA [48]Ni/Al layered double hydroxide on Ti 24.45 +0.7 10.0 5 [49]CuO flowers and nanorods 371.43 and 709.52 +0.6 0.004–8 4 AA and DA [50]FeOOH nanowire 12.13 0.015–3 AA and DA [51]Cu–CNTs–GCE 17.76 0.0007–3.5 0.21 AA, UA and AP [52]NiO/Pt/ERGO 668.2 +0.6 0.00005–5.66 0.2 AA, UA, AP and lactic

acid[53]

Co–CuNP/TDNT 4651.0 and 2581.7 +0.6 V 12 0.6 AA, DA, UA, AP, Cl− This work

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. Conclusion

Cobalt–copper alloy nanoparticles modified TiO2 nanotuberrays were successfully employed for the nonenzymatic deter-ination of glucose. The sensor showed analytical features such

s high sensitivity, good stability, reproducibility and wide rangef detection. The sensor is highly selective to glucose in presencef other sugars and common interfering molecules. The enhancedensor performance is attributed to the synergistic effect of Co andu nanoparticles to catalyze the electrooxidation of glucose andhe large surface area provided by the TiO2 nanotube arrays. Serumlucose levels obtained with the sensor are in agreement with com-ercial glucose sensors.

cknowledgments

T.G. Satheesh Babu and Bipin Nair gratefully acknowledge theepartment of Biotechnology (DBT), Government of India for thenancial support (Sanction Nos. BT/PR14849/MED/32/157/2010nd BT/PR4076/MED/32/221/2011). Suneesh P.V. thanks Council ofcientific and Industrial Research (CSIR) for the financial supportnder CSIR-SRF Direct scheme (Sanction No. 09/1034(0001)/2011-MR-I dated 10/02/11). Authors sincerely thank the servicesendered by Centre for Microscopy, NIT Calicut and SAIF-STIC,ochin University of Science and Technology, Kerala.

eferences

[1] J.S.Y. Chia, M.T. Tan, P.S. Khiew, J.K. Chin, C.W. Siong, A bio-electrochemicalsensing platform for glucose based on irreversible, non-covalent pi–pi func-tionalization of graphene produced via a novel, green synthesis method, Sens.Actuators B: Chem. 210 (2015) 558–565.

[2] X. Kang, J. Wang, H. Wu, I.A. Aksay, J. Liu, Y. Lin, Glucoseoxidase–graphene–chitosan modified electrode for direct electrochemistryand glucose sensing, Biosens. Bioelectron. 25 (2009) 901–905.

[3] C . Atan, E. Karakus , Novel zinc oxide nanorod and chitosan-based electrochem-ical glucose biosensors for glucose assay in human serum samples, Sensor Lett.12 (2014) 1613–1619.

[4] Y. Lin, L. Hu, Y. Lu, L. Guo, Electrochemical glucose biosensor with improvedperformance based on the use of glucose oxidase and Prussian Blue incorpo-rated into a thin film of self-polymerized dopamine, Sens. Actuators B: Chem.210 (2015) 513–518.

[5] K. Hyun, S.W. Han, W.-G. Koh, Y. Kwon, Direct electrochemistry of glucose oxi-dase immobilized on carbon nanotube for improving glucose sensing, Int. J.Hydrogen Energy 40 (2015) 2199–2206.

[6] N.C. Foulds, C.R. Lowe, Enzyme entrapment in electrically conducting polymers.Immobilisation of glucose oxidase in polypyrrole and its application in amper-ometric glucose sensors, J. Chem. Soc. Faraday Trans. 1: Phys. Chem. Condens.Phases 82 (1986) 1259–1264.

[7] A. Heller, B. Feldman, Electrochemical glucose sensors and their applications

in diabetes management, Chem. Rev. 108 (2008) 2482–2505.

[8] M.-S. Steiner, A. Duerkop, O.S. Wolfbeis, Optical methods for sensing glucose,Chem. Soc. Rev. 40 (2011) 4805–4839.

[9] S. Liu, B. Yu, T. Zhang, A novel non-enzymatic glucose sensor based on NiOhollow spheres, Electrochim. Acta 102 (2013) 104–107.

[

and common sugars

10] Y. Lin, F. Lu, Y. Tu, Z. Ren, Glucose biosensors based on carbon nanotube nano-electrode ensembles, Nano Lett. 4 (2004) 191–195.

11] J. Wang, Electrochemical glucose biosensors, Chem. Rev. 108 (2008) 814–825.12] Y.Y. Song, D. Zhang, W. Gao, X.H. Xia, Nonenzymatic glucose detection by using

a three-dimensionally ordered, macroporous platinum template, Chem. Eur. J.11 (2005) 2177–2182.

13] Y. Bai, W. Yang, Y. Sun, C. Sun, Enzyme-free glucose sensor based on a three-dimensional gold film electrode, Sens. Actuators B: Chem. 134 (2008) 471–476.

14] K.E. Toghill, R.G. Compton, Electrochemical non-enzymatic glucose sensors: aperspective and an evaluation, Int. J. Electrochem. Sci. 5 (2010) 1246–1301.

15] E. Reitz, W. Jia, M. Gentile, Y. Wang, Y. Lei, CuO nanospheres based nonenzy-matic glucose sensor, Electroanalysis 20 (2008) 2482–2486.

16] Y. Zhang, L. Su, D. Manuzzi, H.V.E. de los Monteros, W. Jia, D. Huo, et al., Ultrasen-sitive and selective non-enzymatic glucose detection using copper nanowires,Biosens. Bioelectron. 31 (2012) 426–432.

17] P.V. Suneesh, K. Chandhini, T. Ramachandran, Bipin.G. Nair, T.G. SatheeshBabu, Tantalum oxide honeycomb architectures for the development of a non-enzymatic glucose sensor with wide detection range, Biosens. Bioelectron. 50(2013) 472–477.

18] T.G.S. Babu, T. Ramachandran, B. Nair, Single step modification of copper elec-trode for the highly sensitive and selective non-enzymatic determination ofglucose, Microchim. Acta 169 (2010) 49–55.

19] M.M. Rahman, A. Ahammad, J.-H. Jin, S.J. Ahn, J.-J. Lee, A comprehensive reviewof glucose biosensors based on nanostructured metal-oxides, Sensors 10 (2010)4855–4886.

20] S. Luo, F. Su, C. Liu, J. Li, R. Liu, Y. Xiao, et al., A new method for fabricat-ing a CuO/TiO2 nanotube arrays electrode and its application as a sensitivenonenzymatic glucose sensor, Talanta 86 (2011) 157–163.

21] Y. Wang, J. Chen, C. Zhou, L. Zhou, Y. Kong, H. Long, et al., A novel self-cleaning,non-enzymatic glucose sensor working under a very low applied potentialbased on a Pt nanoparticle-decorated TiO2 nanotube array electrode, Elec-trochim. Acta 115 (2014) 269–276.

22] S. Yu, X. Peng, G. Cao, M. Zhou, L. Qiao, J. Yao, et al., Ni nanoparticles decoratedtitania nanotube arrays as efficient nonenzymatic glucose sensor, Electrochim.Acta 76 (2012) 512–517.

23] C. Wang, L. Yin, L. Zhang, R. Gao, Ti/TiO2 nanotube array/Ni composite electrodesfor nonenzymatic amperometric glucose sensing, J. Phys. Chem. C 114 (2010)4408–4413.

24] X. Li, J. Yao, F. Liu, H. He, M. Zhou, N. Mao, et al., Nickel/Copper nanopar-ticles modified TiO2 nanotubes for non-enzymatic glucose biosensors, Sens.Actuators B: Chem. 181 (2013) 501–508.

25] S.S. Mahshid, S. Mahshid, A. Dolati, M. Ghorbani, L. Yang, S. Luo, et al.,Electrodeposition and electrocatalytic properties of Pt/Ni–Co nanowires fornon-enzymatic glucose detection, J. Alloys Compd. 554 (2013) 169–176.

26] H. Gao, F. Xiao, C.B. Ching, H. Duan, One-step electrochemical synthesis ofPtNi nanoparticle-graphene nanocomposites for nonenzymatic amperometricglucose detection, ACS Appl. Mater. Interfaces 3 (2011) 3049–3057.

27] X. Cao, N. Wang, S. Jia, Y. Shao, Detection of glucose based on bimetallic PtCunanochains modified electrodes, Anal. Chem. 85 (2013) 5040–5046.

28] R. Qiu, X.L. Zhang, R. Qiao, Y. Li, Y.I. Kim, Y.S. Kang, CuNi dendritic material: syn-thesis, mechanism discussion, and application as glucose sensor, Chem. Mater.19 (2007) 4174–4180.

29] J. Wang, D.F. Thomas, A. Chen, Nonenzymatic electrochemical glucose sensorbased on nanoporous PtPb networks, Anal. Chem. 80 (2008) 997–1004.

30] X. Bo, J. Bai, L. Yang, L. Guo, The nanocomposite of PtPd nanoparticles/onion-likemesoporous carbon vesicle for nonenzymatic amperometric sensing of glucose,Sens. Actuators B: Chem. 157 (2011) 662–668.

31] K. Dhara, J. Stanley, B.G. Nair, T. Ramachandran, T.G. Satheesh Babu, Pt-CuOnanoparticles decorated reduced graphene oxide for the fabrication of highly

sensitive non-enzymatic disposable glucose sensor, Sens. Actuators B: Chem.195 (2014) 197–205.

32] B. Singh, F. Laffir, T. McCormac, E. Dempsey, PtAu/C based bimetallic nanocom-posites for non-enzymatic electrochemical glucose detection, Sens. ActuatorsB: Chem. 150 (2010) 80–92.

3 d Actu

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[

[

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[

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[

[

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44 P.V. Suneesh et al. / Sensors an

33] J. Wang, H. Gao, F. Sun, C. Xu, Nanoporous PtAu alloy as an electrochemicalsensor for glucose and hydrogen peroxide, Sens. Actuators B: Chem. 191 (2014)612–618.

34] C.A. Grimes, G.K. Mor, TiO2 Nanotube Arrays: Synthesis, Properties, and Appli-cations, Springer, 2009.

35] J. Wang, Z. Lin, Anodic formation of ordered TiO2 nanotube arrays: effects ofelectrolyte temperature and anodization potential, J. Phys. Chem. C 113 (2009)4026–4030.

36] L. Gao, L. Wang, T. Qi, Y. Li, J. Chu, J. Qu, Electrodeposition of aluminum fromAlCl3/Et3NHCl ionic liquids, Acta Phys.-Chim. Sinica 24 (2008) 939–944.

37] E. Gómez, A. Llorente, X. Alcobe, E. Vallés, Electrodeposition for obtaininghomogeneous or heterogeneous cobalt-copper films, J. Solid State Electrochem.8 (2004) 82–88.

38] L. Wang, Y. Zheng, L. Xingping, L. Zhuang, L. Sun, Y. Song, Dendritic copper-cobalt nanostructures/reduced graphene oxide-chitosan modified glassycarbon electrode for glucose sensing, Sens. Actuators B: Chem. 195 (2014) 1–7.

39] M. Gu, Initial stages of the electrocrystallization of Co–Cu alloys on GCE fromthe Co rich electrolytes, Electrochim. Acta 52 (2007) 4443–4448.

40] M. Paulose, K. Shankar, S. Yoriya, H.E. Prakasam, O.K. Varghese, G.K. Mor, et al.,Anodic growth of highly ordered TiO2 nanotube arrays to 134 �m in length, J.Phys. Chem. B 110 (2006) 16179–16184.

41] K.K. Lee, P.Y. Loh, C.H. Sow, W.S. Chin, CoOOH nanosheets on cobalt substrate asa non-enzymatic glucose sensor, Electrochem. Commun. 20 (2012) 128–132.

42] C.-W. Kung, C.-Y. Lin, Y.-H. Lai, R. Vittal, K.-C. Ho, Cobalt oxide acicular nanorodswith high sensitivity for the non-enzymatic detection of glucose, Biosens. Bio-electron. 27 (2011) 125–131.

43] X. Wang, X. Dong, Y. Wen, C. Li, Q. Xiong, P. Chen, A graphene–cobalt oxidebased needle electrode for non-enzymatic glucose detection in micro-droplets,Chem. Commun. 48 (2012) 6490–6492.

44] M. Shamsipur, M. Najafi, M.-R.M. Hosseini, Highly improved electrooxidationof glucose at a nickel (II) oxide/multi-walled carbon nanotube modified glassycarbon electrode, Bioelectrochemistry 77 (2010) 120–124.

45] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Fundamentals and Applica-tions, 2001.

46] Y. Liu, H. Teng, H. Hou, T. You, Nonenzymatic glucose sensor based on renewableelectrospun Ni nanoparticle-loaded carbon nanofiber paste electrode, Biosens.Bioelectron. 24 (2009) 3329–3334.

47] L.-M. Lu, L. Zhang, F.-L. Qu, H.-X. Lu, X.-B. Zhang, Z.-S. Wu, et al., A nano-Ni basedultrasensitive nonenzymatic electrochemical sensor for glucose: Enhancing

sensitivity through a nanowire array strategy, Biosens. Bioelectron. 25 (2009)218–223.

48] J. Shi, P. Ci, F. Wang, H. Peng, P. Yang, L. Wang, et al., Nonenzymatic glucosesensor based on over-oxidized polypyrrole modified Pd/Si microchannel plateelectrode, Biosens. Bioelectron. 26 (2011) 2579–2584.

ators B 215 (2015) 337–344

49] X. Li, J. Liu, X. Ji, J. Jiang, R. Ding, Y. Hu, et al., Ni/Al layered doublehydroxide nanosheet film grown directly on Ti substrate and its applica-tion for a nonenzymatic glucose sensor, Sens. Actuators B: Chem. 147 (2010)241–247.

50] X. Wang, C. Hu, H. Liu, G. Du, X. He, Y. Xi, Synthesis of CuO nanostructures andtheir application for nonenzymatic glucose sensing, Sens. Actuators B: Chem.144 (2010) 220–225.

51] C. Xia, W. Ning, A novel non-enzymatic electrochemical glucose sensor modi-fied with FeOOH nanowire, Electrochem. Commun. 12 (2010) 1581–1584.

52] X. Kang, Z. Mai, X. Zou, P. Cai, J. Mo, A sensitive nonenzymatic glucose sensorin alkaline media with a copper nanocluster/multiwall carbon nanotube-modified glassy carbon electrode, Anal. Biochem. 363 (2007) 143–150.

53] M. Li, X. Bo, Z. Mu, Y. Zhang, L. Guo, Electrodeposition of nickel oxide andplatinum nanoparticles on electrochemically reduced graphene oxide filmas a nonenzymatic glucose sensor, Sens. Actuators B: Chem. 192 (2014)261–268.

Biographies

Mr. P.V. Suneesh received his BSc and MSc degrees in Chemistry from CalicutUniversity, Calicut, Kerala, India. Currently he is a PhD student in Amrita VishwaVidyapeetham, Coimbatore, Tamil Nadu, India. His research interests focuses ondevelopment of electrochemical biosensors, nanomaterials and batteries.

Ms. Vidhu Sara Vargis received B.Sc degree in botany from Kerala Universityand M.Sc degree in Biotechnology from School of Biotechnology, Amrita VishwaVidyapeetham. Now she is doing her PhD in Amrita University. Her research focuseson electrochemical immunosensors and nanomaterials.

Dr. T. Ramachandran received B.Sc, M.Sc and PhD degree from Madras Univer-sity. He had post doctoral research at Georgetown University (Washington, USA).Presently he is a Professor in Amrita Vishwa Vidyapeetham. His research interestsare Industrial Electrochemical Processes, Fuel cells and Biosensors.

Dr. Bipin G. Nair received a B.Sc degree from Gujarat University, M.Sc and PhDdegree from Maharaja Sayajirao University of Baroda. Four years Post doctoralresearch in University of Tennessee, Memphis. Now he is Professor and Dean and inSchool of Biotechnology, Amrita Vishwa Vidyapeetham.

Dr. T.G. Satheesh Babu received B.Sc degree from Calicut University, M.Sc degreefrom Gandhigram University and he received PhD degree from Amrita University.Now he is Associate Professor in Amrita Vishwa Vidyapeetham. His research focuseson synthesis of nanomaterials, Biosensors and Lab on a chip.