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Biosensors and Bioelectronics 25 (2009) 708–714

Contents lists available at ScienceDirect

Biosensors and Bioelectronics

journa l homepage: www.e lsev ier .com/ locate /b ios

hree-dimensional network films of electrospun copper oxide nanofibersor glucose determination

en Wang, Lili Zhang, Shengfu Tong, Xin Li, Wenbo Song ∗

ollege of Chemistry, Jilin University, Changchun 130012, PR China

r t i c l e i n f o

rticle history:eceived 8 June 2009eceived in revised form 4 August 2009ccepted 7 August 2009vailable online 15 August 2009

eywords:

a b s t r a c t

Copper oxide nanofibers (CuO-NFs) prepared by electrospinning and subsequent thermal treatment pro-cesses were demonstrated for the first time for glucose non-enzymatic determination. The structuresand morphologies of CuO-NFs were characterized by transmission electron microscopy (TEM), scanningelectron microscopy (SEM) and X-ray diffraction spectrum (XRD). Different dispersants were utilizedfor the suspension preparation and effects of ultrasonic time on the films electrode fabrication wereinvestigated in detail. The assay performances to glucose were evaluated by cyclic voltammetry (CV) and

−1 −2

opper oxide nanofiberslectrospunlectrocatalysislucose sensor

chronoamperometry (I–t). Results revealed a high sensitivity (431.3 �Am M cm ), fast response (about1 s), long-term stability and excellent resistance towards electrode fouling in the glucose determinationat +0.40 V. The improved performances of CuO-NFs films electrode for electro-oxidation glucose wereascribed to the high surface-to-volume ratio, complex pore structure, extremely long length of the as-prepared CuO-NFs, and the excellent three-dimensional network structure after immobilization. Results

elecctroc

in this study suggest thatmicrofabrication of bioele

. Introduction

Great efforts have been made recently to fabricate one-imensional (1-D) metal oxide nanomaterials due to their specialorphologies, compositions, chemical and physical properties

Chen et al., 2008; Lu et al., 2006; Shen et al., 2009). A variety ofethods including electrochemical deposition (Takahashi et al.,

004), hydrothermal process (Pan et al., 2001), electrospinningFan and Whittingham, 2007), vapor phase growth (Shen et al.,006), and solution phase growth (Xue et al., 2005) have beeneveloped for preparation of 1-D metal oxide nanomaterials. Inhe meantime, a large quantity of 1-D metal oxide nanomate-ials with different components and varied structures, such asanowires, nanobelts, nanotubes, nanorings and nanofibers, haveeen reported. However, most reports limited the scope to theirxperimental production and characterization, only few reportsecently emerged concerning the applications of 1-D metal oxideanomaterials in catalysis and electroanalysis.

One of the most important applications of 1-D metal oxide

anomaterials is to develop their potential in chemical sensingr biosensing, profiting from their large surface-to-volume ratiosnd a Debye length comparable to their dimensions. Gas sensorsFan et al., 2004), humidity sensors (Wan et al., 2004) and biosen-

∗ Corresponding author. Tel.: +86 431 85168352; fax: +86 431 85167420.E-mail address: wbsong@jlu.edu.cn (W. Song).

956-5663/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.bios.2009.08.013

trospun CuO-NFs is a promising 1-D nanomaterial for further design andhemical nanodevices for glucose determination.

© 2009 Elsevier B.V. All rights reserved.

sors (Curreli et al., 2005) fabricated with 1-D metal oxide-basednanomaterials have been reported recently. To our best knowledge,application of these kind materials for electrochemical sensoring isrelatively difficult due to the challenges to prepare the dispersiblephases of nanomaterials from their corresponding powders (Gabig-Ciminska et al., 2004). In present study, glycol was utilized asdispersant to prepare the three-dimensional (3-D) network filmsof copper oxide nanofibers (CuO-NFs), which were produced bycombination of electrospinning technique with subsequent ther-mal treatment, for designing electrochemical enzymeless glucosesensors.

Reliable and fast determination of glucose is of considerableimportance in biotechnology, clinical diagnostics and food indus-try. Various techniques such as chemiluminescence (Bostick andHercules, 1975), chromatography (Lehnhardt and Winzler, 1968)and electrochemistry (Cass et al., 1984) were applied in glu-cose determination, in which, electrochemical methods, especiallyamperometry, have been proved to be a powerful approach andattracted much attention.

The majority of the well-known amperometric sensors for glu-cose monitoring are enzyme-based ones (Wang, 2001, 2008), andnew strategies were particularly explored in the designing of glu-

cose enzyme sensors over the last several decades (Crespilho etal., 2009a; Okahata et al., 1988). However, insufficient stabilityoriginated from the intrinsic nature of the enzyme to both pHand temperature is hardly overcome. In recent years, more andmore attempts have been made to determine glucose concentra-

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W. Wang et al. / Biosensors an

ion without using enzymes (Bai et al., 2008; Salimi and Roushani,005; Wang et al., 2008a). Most enzymeless electrochemical glu-ose sensors rely on the properties of electrode materials, on whichlucose is oxidized directly. Carbon, platinum, and gold (Li et al.,007; Salimi and Roushani, 2005) have been widely explored as theandidates in the hope of developing effective enzyme-free sensors.owever, most of these electrodes have drawbacks of low sensitiv-

ty and poor selectivity, attributed to the surface poisoning resultedrom the adsorbed intermediates and chloride. Pulsed electrochem-cal detection was applied to remove the adsorbed products, butrought dissolution of the electrode and over prolonged period ofse in flow-through cells (Robert and Johnson, 1994). Therefore, it

s important to develop novel electrode materials with high sen-itivity, stability and interference-free in glucose non-enzymaticetermination.

The direct electro-oxidation of glucose on transition metals andheir alloys is greatly enhanced compared with others, attributedo the catalytic effect resulting from the multi-electron oxida-ion mediated by surface metal oxide layers (Luo et al., 1991;

ho and Johnson, 2001; Salimi and Roushani, 2005). Moreover,ransition metals such as Cu and Ni are economical and can oxi-ize carbohydrates directly without surface poisoning. In contrasto Cu and Ni, which are unstable and easily oxidized in air andolution, their oxide (or hydroxide) materials are relatively stableJiang et al., 2002; Wang et al., 2005). Copper oxide with virtuef natural abundance, low production cost, good electrochemicalnd catalytic properties is of particular interest, which makes ituitable for the application in electrical, optical and photovoltaicevices, heterogeneous catalysis, magnetic storage media, gas sens-

ng, field-emission emitters, lithium ion electrode materials and soorth (Gao et al., 2004; Rakhshani et al., 1996; Zhang et al., 2006).

Recent progress in nanoscience and nanotechnology revealedhat the catalytic properties of electrode materials at nanoscaleegime are strongly influenced by their morphologies and dimen-ional constraints (Cao et al., 2001). The performances oflectrochemical detectors such as mass transport, catalytic ratesnd signal to noise current ratios can be improved by adjustingurface morphology of the electrode materials. Various strategiesor enzyme immobilization and novel electrode materials with highurface area were developed to improve the performances of glu-ose in both enzyme sensors and enzymeless sensors at nanometerevel. For example, M.A. Brett et al. (Crespilho et al., 2006; Crespilhot al., 2008) reported a new method for glucose oxidase immobiliza-ion based on a combination of LbL self-assembly, redox mediatorlectrodeposition and cross-linking. The resulted nanodevice wasuccessfully applied as a biosensor for the amperometric measure-ent of glucose with improved performances. Zhuang et al. (2008)

eported a new method for direct growth of CuO nanowires on au substrate for glucose detection with an improved sensitivity.

One of our research interests focuses on exploiting novel elec-rocatalysts for sensor fabrication especially metal-based catalystsor glucose electro-oxidation (Li et al., 2009; Liu et al., 2006; Tongt al., 2009). In this work, we demonstrate for the first time thereparation of 3-D network films of electrospun CuO-NFs and itspplication for direct glucose determination. The morphologies andtructures of the as-prepared CuO-NFs were characterized by trans-ission electron microscopy (TEM), scanning electron microscopy

SEM) and X-ray powder diffraction (XRD). Different dispersantsere utilized for the suspension preparation and the effects ofltrasonic time on the films electrode preparation were investi-ated in detail. The activity of CuO-NFs films electrode towards

lucose electro-oxidation and its sensing performances were eval-ated by cyclic voltammetry (CV) and chronoamperometry (I–t).resent study provides a simply controlled test-bed for furtheresign and microfabrication of electrospun CuO-NFs bioelectro-hemical nanodevices for glucose determination.

lectronics 25 (2009) 708–714 709

2. Experimental

2.1. Chemicals and reagent

Polyvinyl alcohol (PVA, Mw = 70,000–75,000), Triton-X100 andNafion (5%) were purchased from Aldrich. The carbohydratesincluding d-glucose and others, oxalic acid (OA), ascorbic acid (AA),uric acid (UA), ethanol, cupric acetate, sodium hydroxide and therest inorganic chemicals were purchased from Beijing ChemicalPlant (Beijing, China). Standard glucose samples containing vari-ous concentrations of glucose and other possible interfering species(AA, UA and chloride ions) at normal physiological levels were pre-pared. All chemicals were of analytical grade and used as received.All solutions were prepared with redistilled water before experi-ments.

2.2. Preparation of CuO-NFs

CuO-NFs were fabricated through electrospinning and followedby calcination process. In a typical procedure (Li and Xia, 2004;Wang et al., 2008b), 0.4 g cupric acetate was added slowly into 7.6 gPVA aqueous solution (PVA 7.9 wt%) with 0.01 g Triton-X100. Thesolution was kept under vigorous magnetic stirring for 12 h, anda viscous gel was obtained. The as-prepared gel was loaded into asyringe and connected to a high-voltage power supply for electro-spinning. An electric potential of 15 kV was applied between theorifice and the ground at a distance of 20 cm. Calcination (500 ◦C inair for 5 h) was used to treat the as-spun composite fibers to removethe organic constituents of PVA and convert the precursor into pureCuO-NFs.

2.3. Preparation of the CuO-NFs film electrodes

In order to immobilize the CuO-NFs onto the surface of a glasscarbon electrode (GCE, 3 mm diameter), “casting” suspensions ofa certain concentration of CuO-NFs in water, ethanol and glycolwere prepared by sonicating the suspension for different time soas to thoroughly disperse the CuO-NFs. A 5 �L aliquot of this cast-ing suspension was then cast onto the pretreated GCE or indium tinoxide (ITO) surface, and the solvent were allowed to evaporate atinfrared lamp leaving the CuO-NFs immobilized onto the electrodesurface. 5 �L 1 wt% Nafion solution was finally applied to serve bothas permselective membrane and entrapment matrix for the immo-bilization of CuO-NFs. The as-prepared CuO-NFs films electrode wasabbreviated as CuO-NFs-GCE.

2.4. Apparatus

The crystal structures of the products were determined by XRDusing an X-ray diffractometer (Siemens D5005, Munich, Germany).The morphologies of the electrospun nanofibers were viewed bySEM (SHIMADZU SSX-550, Japan). TEM observations (JEM-2000EX,JEOL) were performed with an accelerating voltage of 200 kV. Allelectrochemical measurements were performed on a CHI 660Aelectrochemical workstation (CH instrument, USA) with a con-ventional three-electrode system composed of a platinum wire asauxiliary electrode, a saturated calomel electrode (SCE) as referenceelectrode, and CuO-NFs modified GCE as working electrode.

3. Results and discussion

3.1. Characterization of the CuO-NFs

The morphologies of the as-prepared CuO-NFs were observeddirectly by SEM. It could be seen clearly from Fig. 1A that the as-prepared CuO-NFs have a diameter ranging from 90 to 240 nm

710 W. Wang et al. / Biosensors and Bioe

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Fig. 1. SEM image (A), TEM image (B) and XRD pattern (C) of CuO-NFs.

ith tens of micrometers in length in the area examined. Theetailed morphologies and size of the as-prepared CuO-NFs wereurther characterized and analyzed by TEM. Result revealed thathe CuO-NFs were accumulated by CuO nanoparticles (Fig. 1B) withomplex pore structure, which may have the potential to providearge surface area and high surface energy for catalytic reaction.he diameter of CuO-NFs was about 170 nm, which was in goodgreement with that of the SEM characterization. To confirm thetructure and the phase composition of the sample, XRD analysisas carried out. Fig. 1C shows a typical XRD spectrum collected

or the as-prepared sample. All of the diffraction peaks can bessigned to the monoclinic structured CuO (JCPDS 05-0661), andhe sharp peaks indicate that the products are perfectly crystal-ized. The major peaks located at 2� = 35.5◦ and 38.7◦ indexed as1 1 1)–(0 0 2) and (1 1 1)–(2 0 0) planes, respectively, are the char-cteristics for the pure phase monoclinic CuO crystallites.

.2. Characterization of the immobilized CuO-NFs

Electrospinning has been extensively investigated as an efficientnd versatile approach to produce long polymer fibers with the

lectronics 25 (2009) 708–714

diameter ranging from tens of nanometers to several micrometers(Li and Xia, 2004). Subsequent carbonization of the electrospunpolymer fibers at high temperature could be utilized to prepareinorganic oxide nanofibers with the proper precursor. Althoughmetal oxide-based nanofibers are widely fabricated via electrospin-ning, their applications in electrochemical sensors are extremelylimited, mainly due to the difficulty in preparing the dispersiblenanofiber suspensions in suitable dispersants. In present paper,different dispersants, involving water, ethanol and glycol wereapplied for the CuO-NFs suspension preparation and the subse-quent electrode fabrication. Note that same weight percentage ofthe as-prepared CuO-NFs was used in the study. The resulting elec-trodes were referred as a, b and c, respectively. It is observed thatelectrode c exhibited higher electrochemical activity for glucosethan the others under the same condition (Fig. 1, Supplemen-tary Material). Glycol, with the higher viscidity and lower volatility,seemed favorable to form a well-proportioned suspension of CuO-NFs than the others, and resulted in a uniform CuO-NFs film.Influence of the casting dosage (5–15 �L) of the CuO-NFs sus-pension (5 mg mL−1) on the electrochemical activity of the filmelectrodes was also studied. It can be seen that the oxidation cur-rent of glucose at CuO-NFs films electrode did not always increasewith increasing CuO-NFs dosage, but has a maximum at the CuO-NFs films electrode prepared by casting 10 �L CuO-NFs suspension(Fig. 2, Supplementary Material). Thus, 10 �L suspension of theas-prepared CuO-NFs was employed in the following work.

In order to investigate the effect of ultrasonic time on themorphologies and electrochemical property of the CuO-NFs filmselectrode, 10 �L suspension of 5 mg mL−1 CuO-NFs in glycol expe-rienced different ultrasonic time were dropped onto the surfaceof a pre-treated ITO slide. Fig. 2A shows the typical SEM surfacemorphology of the CuO-NFs films prepared by ultrasonic 5 min,and a uniform 3-D network-like structure was obtained. Fig. 2Band C shows surface morphologies of CuO-NFs films obtained byprolonged ultrasonication for 30 and 60 min. Continuous break-down of CuO-NFs was observed obviously in the sonication process.From Fig. 2B, the excellent 1-D CuO-NFs were exfoliated to shortnanofibers. From Fig. 2C, short CuO-NFs were continuously exfoli-ated with large aggregates appeared through durative sonication,and 3-D network-like structure was no longer obtained. Resultsreveal that 1-D nanofibers can form 3-D network structure, whichprovides large effect electrode surface and facile electron trans-port for catalytic application. Fig. 2D shows the CVs of glucoseelectro-oxidation at the CuO-NFs films prepared at different ultra-sonic time. Obviously, the CuO-NFs obtained at 5 min sonication(curve a) exhibited higher catalytic activity than those of prolongedsonication times (curves b and c), potentially due to the significantincrease of the effective electrode surface by the 3-D porous struc-ture in the CuO-NFs films and the facilitation of the analyte diffusioninto the films.

3.3. Electro-oxidation of glucose at CuO-NFs-GCE

Electro-oxidation of glucose was examined in 0.1 M NaOH solu-tion in the presence of glucose in the potential window rangingfrom 0 to +0.65 V. As shown in Fig. 3, only a small backgroundcurrent was observed at the Nafion covered GCE in 0.1 M NaOH(curve a), while a dramatic increase of current signal toward thepositive end of the potential range was observed at CuO-NFs-GCE(curve c), which was ascribed to the role of CuO-NFs in the increaseof the electroactive surface area. No obvious redox currents were

observed at Nafion-GCE in the presence of glucose (curve b), sug-gesting the direct oxidation of glucose at Nafion-GCE was unable.Comparing with that at Nafion-GCE, a significant oxidation of glu-cose starting at ca. +0.15 V (vs. SCE) with a shoulder peak at ca.+0.40 V was observed at CuO-NFs-GCE (curve d). The glucose oxida-

W. Wang et al. / Biosensors and Bioelectronics 25 (2009) 708–714 711

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ig. 2. The morphologies of CuO-NFs films fabricated on ITO surfaces with ultrasonlectrodes fabricated at ultrasonic time of 5 min (a), 30 min. (b) and 60 min. (c) in g

ive response at the CuO-NFs films electrode is comparable withhat at the copper-based electrodes previously reported (Liu etl., 2006; Tong et al., 2009; Xu et al., 2006). A series of CVs wereecorded at various concentrations of glucose. With the increase of

lucose concentration, the oxidative current increased (Fig. 3, Sup-lementary Material).

The glucose electro-oxidation mechanism at copper-based elec-rode remains somewhat controversial till now. However, the

ig. 3. Cyclic voltammograms of GCE (a and b) and CuO-NFs-GCE (c and d) in 0.1 MaOH in the absence (a and c) and presence (b and d) of 0.6 mM glucose, respectivelyt 50 mV s−1.

e of 5 min (A), 30 min (B) and 60 min (C); (D) electrocatalytic effect of CuO-NFs filmowards 0.6 mM glucose in 0.1 M NaOH at 50 mV s−1.

Cu(II)–Cu(III) redox couple, which has been reported to be stronglydependent upon the hydroxide concentration, was consideredplaying an important role for carbohydrate oxidation. Hydroxylradicals are probably formed in the OH− oxidation at Cu(III) cat-alytic centre during the potential window, which react with theorganic molecules through the abstraction of a hydrogen atom fromthe carbon in �-position with respect to –OH group (Casella et al.,1996). The reproducible electrocatalytic activity could be obtainedby experiencing the reverse potential scan.

In order to study the fouling behavior of this modified electrode,repetitive cyclic scans were performed in 0.1 M NaOH with 0.6 mMglucose. It was observed that after repetitive cycles, the glucoseoxidative peak current did not change, indicating that the inter-mediates or reaction products were not deposited on the electrodesurface. Thus, electrode fouling did not take place here.

3.4. Amperometric performance of the CuO-NFs-GCE to glucose

3.4.1. Selection of the optimal measurement conditionAs mentioned previously, alkaline electrolyte is required for

electro-oxidation of carbohydrates on Ni and Cu as a resultof their electrocatalytic effect mediated by Ni(OH)2/NiO(OH) orCu(OH)2/CuO(OH) redox couples (Luo et al., 1991). The effect of

NaOH concentration on glucose electro-oxidation (0.6 mM) at CuO-NFs-GCE was investigated in this study. The concentrations of NaOHused in this paper were 0.001, 0.01, 0.1 and 0.5 M (Fig. 4, Supple-mentary Material). When the concentration of NaOH was 0.001 M,almost no distinguishable oxidative current was generated, while

7 d Bioelectronics 25 (2009) 708–714

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12 W. Wang et al. / Biosensors an

ith NaOH concentration increasing, an obvious oxidative peakas obtained with the oxidative potential negatively shifted, and

he oxidative current largely enhanced in the meanwhile. To avoidhe high alkaline in glucose detection, 0.1 M NaOH was applied ashe supporting electrolyte in the following section. To understand ifhe dissolved oxygen was involved in the electrocatalytic oxidationrocess, amperometric responses of the CuO-NFs films electrodeo the successive addition of 0.2 mM glucose to 0.1 M NaOH withr without saturated O2 were examined. The current responses ofuO-NFs-GCE to glucose (Fig. 5, Supplementary Material) remainedhe same magnitude when the solution was bubbled with nitrogen,ndicating that oxygen was not involved in this reaction.

The choice of the detection potential is necessary to achievehe lowest detection limit and avoid the electrochemical interfer-ng species. The amperometric current response of CuO-NFs-GCEn the presence of 0.2 mM glucose in a stirring electrolyte solu-ion was measured at a constant interval of 0.05 V from +0.20 Vo +0.65 V (Fig. 6, Supplementary Material). The oxidation currentf glucose at CuO-NFs-GCE increased with increasing the potentialbviously. However, the signal-to-background ratio (S/B) increasedntil the potential reached +0.40 V, and then decreased with fur-her increase in potential due to the dramatic increase in theaseline current at a higher potential. Thus, a constant poten-ial of +0.40 V was selected and employed for all the subsequentmperometric detection for a high sensitive determination. Thisotential is 200 mV more negative than those of CuO nanorodundles (Batchelor-McAuley et al., 2008) and CuO nanospheresReitz et al., 2008) modified electrodes, 50 and 150 mV moreegative than those in our prevenient reports (Li et al., 2009;iu et al., 2006; Tong et al., 2009). The low detection potentialill significantly diminish the influence of those easily oxidizable

pecies.

.4.2. Amperometric analysisAmperometric analysis were carried out at +0.40 V at CuO-

Fs-GCE by consecutive injection of glucose to 0.1 M NaOH.ell-defined, stable and fast amperometric responses were

bserved in Fig. 4A. The time required to reach the steady-stateurrent was about 1 s, indicating a rapid oxidative process, which isuch faster than those in similar determination (Safavi et al., 2009;

ong et al., 2009; Umar et al., 2009). Fig. 4B demonstrated a goodinear responses of the electrocatalytic current of glucose at theuO-NFs-GCE in a wide range 6 × 10−6–2.5 × 10−3 M (correlationoefficient 0.998). The detection limit was 8 × 10−7 M (signal-to-oise ratio of 3). The sensitivity of the present non-enzymaticlucose sensor is 431.3 �Am M−1 cm−2, which is higher than otherimilar glucose detecting constructions (Ozcan et al., 2008; Reitz etl., 2008; Safavi et al., 2009). The high sensitivity and fast responsere attributed to the fact that 1-D CuO-NFs formed an ordered 3-

network to shuttle electron between glucose and the workinglectrode, thus greatly enhance the electron transfer reaction oflucose.

The reproducibility and stability of CuO-NFs-GCE were also eval-ated. Under the same fabrication condition used in this study,ve electrodes were prepared with a RSD of 7.5% for the steady-tate current to 0.2 mM glucose at +0.40 V, indicating a satisfiedeproducibility of this procedure. The RSD of the steady-state cur-ent response for six individually repetitive tests containing 0.1 mMlucose at CuO-NFs-GCE was 4.2%, confirming a good reproducibil-ty of the electrode. The current response decayed by 7.4% after 2

onths storage, demonstrating a long-term storage stability of the

lectrode. The high stability and reproducibility of CuO-NFs-GCEn glucose electro-oxidation resulted from the chemical stability ofuO-NFs and network-like structure formed on GCE surface, whichrevented the copper-based materials from conglomeration andhe interference of oxygen in the air.

Fig. 4. (A) Amperometric response of CuO-NFs-GCE with successive additions of0.1 mM glucose to 0.1 M NaOH at +0.40 V and (B) calibration curve for the ampero-metric responses of CuO-NFs-GCE to glucose.

3.4.3. Interferential analysisThe poisoning possibility of chloride ions to the activity of CuO-

NFs-GCE in glucose determination was examined by adding sodiumchloride in the supporting electrolyte in measurement. The lin-ear response for glucose at CuO-NFs-GCE remains almost constant(data not shown), demonstrating that the electrode can be usedin the presence of high concentration chloride ions. AA and UA arethe most important interferences normally co-exist with glucose inblood plasma. The normal physiological level of glucose is 3–8 mMand the interfering species such as AA and UA are about 0.1 mM.Interference tests were carried out by adding 3.0 mM glucose, fol-lowed with additions of 0.1 mM AA, 0.1 mM UA and other possibleinterference such as ethanol and OA in 0.1 M NaOH solution con-taining 0.2 M NaCl (Fig. 5). Results revealed that AA and UA causedneglectable interference in physiological level, ethanol and OA didnot cause any observable interference. Other carbohydrates thatcan be electrocatalytically oxidized at CuO-NFs electrode were alsoinvestigated, and the results were presented in Table 1, Supple-mentary Material. The response generated by glucose was morepronounced than d-fructose, sucrose, malt dust at the same con-centration level. In view of the much lower normal physiologicalconcentration of other carbohydrates than glucose, the responsegenerated by other carbohydrates can be neglected, implying agood selectivity to glucose at CuO-NFs-GCE.

3.4.4. Sample analysisTo verify the reliability of the CuO-NFs-GCE, standard glucose

samples were employed in the determination of glucose. Differentstandard glucose samples were measured at +0.40 V in 0.1 M NaOH

W. Wang et al. / Biosensors and Bioe

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ig. 5. Amperometric responses of CuO-NFs-GCE with successive additions of 3 mMlucose, 0.1 mM AA, 0.1 mM UA, 10 mM ethanol and 10 mM OA to 0.1 M NaOHontaining 200 mM NaCl at +0.40 V.

nd the results were listed in Table 2, Supplementary Material). Thisable showed that the results obtained by the proposed sensor wereatisfactory and agreed closely with their real values, revealing theeliability for glucose determination.

Semiconducting nanowires and carbon nanotubes (Bestemant al., 2003; Crespilho et al., 2009b; Cui et al., 2001; Dekker,999) are well-known promissing 1-D nanomaterials in leadingo conceptual new miniaturization strategies in the electron-cs and computer industry, due to their characteristic featuresor application in molecular electronics (Tans et al., 1998). Forxample, Chiquito et al. (2007) reported recently the tunableroperties of magneto-transport from metallic to a nonmetallicharacter by decreasing the dimensions of individual Sn dopedn2O3 nanowires. Frank N. Crespilho demonstrated the possibil-ty of converting biological activity to an electrochemical signalt low overpotential by immobilizing glucose oxidase enzymen individual indium tin oxide nanowires attached on gold con-acts deposited on top of a microchip (Crespilho et al., 2009b).ased on above results, a promising future for 1-D CuO-NFs nano-aterials prepared in design and microfabrication of CuO-NFs

ioelectrochemical nanodevices with geometry at the nanometerevel for glucose determination can be anticipated. Developmentnd investigations on these nanoscale electrochemical devicesay not only open new tools for electrochemical studies at the

ingle-molecule level, but also provide database for further explo-ation in bioelectrochemical devices, such as biofuel cells andiosensors.

. Conclusion

Electrospun CuO-NFs with high surface-to-volume ratio, com-lex pore structure and extremely long length were detailed

nvestigated for direct electrocatalytic oxidation of glucose inhis paper. Different dispersants were utilized for the suspensionreparation and effects of ultrasonic time on the film electrodeabrication were investigated in detail for fabrication of enzyme-ess glucose sensors. The CuO-NFs modified electrode showshe superiorities of fast response, good stability, high sensitiv-ty, good selectivity and resistance towards electrode fouling to

lucose detection, attributed to the large surface area, chemi-al stability and 3-D network structure of the CuO-NFs films.ll these results suggest that CuO-NFs is a promising electrodeaterial for fabrication of amperometric glucose enzymeless sen-

ors.

lectronics 25 (2009) 708–714 713

Acknowledgement

This work was supported by National Natural Science Foun-dation of China under Grant 20543003 and Scientific ResearchFoundation for Returned Overseas Chinese Scholars, State Educa-tion Ministry of China.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.bios.2009.08.013.

References

Bai, Y., Sun, Y.Y., Sun, C.Q., 2008. Biosensors and Bioelectronics 24 (4), 579–585.Batchelor-McAuley, C., Du, Y., Wildgoose, G.G., Compton, R.G., 2008. Sensors and

Actuators B - Chemical 135 (1), 230–235.Besteman, K., Lee, J.O., Wiertz, F.G.M., Heering, H.A., Dekker, C., 2003. Nano Letters

3 (6), 727–730.Bostick, D.T., Hercules, D.M., 1975. Analytical Chemistry 47 (3), 447–452.Cao, Y.W., Jin, R., Mirkin, C.A., 2001. Journal of the American Chemical Society 123

(32), 7961–7962.Casella, I.G., Cataldi, T.R.I., Guerrieri, A., Desimoni, E., 1996. Analytica Chimica Acta

335 (3), 217–225.Cass, A.E., Davis, G., Francis, G.D., Hill, H.A., Aston, W.J., Higgins, I.J., Plotkin, E.V.,

Scott, L.D., Turner, A.P., 1984. Analytical Chemistry 56 (4), 667–671.Chen, P.C., Shen, G.Z., Zhou, C.W., 2008. IEEE Transactions on Nanotechnology 7 (6),

668–682.Chiquito, A.J., Lanfredi, A.J.C., de Oliveira, R.F.M., Pozzi, L.P., Leite, E.R., 2007. Nano

Letters 7 (5), 1439–1443.Crespilho, F.N., Esteves, M.C., Sumodjo, P.T.A., Podlaha, E.J., Zucolotto, V., 2009a.

Journal of Physical Chemistry C 113 (15), 6037–6041.Crespilho, F.N., Ghica, M.E., Florescu, M., Nart, F.C., Oliveira, O.N., Brett, C.M.A., 2006.

Electrochemistry Communications 8 (10), 1665–1670.Crespilho, F.N., Ghica, M.E., Gouveia-Caridade, C., Oliveira, O.N., Brett, C.M.A., 2008.

Talanta 76 (4), 922–928.Crespilho, F.N., Lanfredi, A.J.C., Leite, E.R., Chiquito, A.J., 2009b. Electrochemistry

Communications, doi:10.1016/j.elecom. 2009.07.006.Cui, Y., Wei, Q., Park, H., Lieber, C.M., 2001. Science 293 (17), 1289–1292.Curreli, M., Li, C., Sun, Y.H., Lei, B., Gundersen, M.A., Thompson, M.E., Zhou, C.W.,

2005. Journal of the American Chemical Society 127 (19), 6922–6923.Dekker, C., 1999. Physics Today 52 (5), 22–28.Fan, Q., Whittingham, M.S., 2007. Electrochemical and Solid State Letters 10 (3),

A48–A51.Fan, Z.Y., Wang, D.W., Chang, P.C., Tseng, W.Y., Lu, J.G., 2004. Applied Physics Letters

85 (24), 5923–5925.Gabig-Ciminska, M., Los, M., Holmgren, A., Albers, J., Czyz, A., Hintsche, R., Wegrzyn,

G., Enfors, S.O., 2004. Analytical Biochemistry 324 (1), 84–91.Gao, X.P., Bao, J.L., Pan, G.L., Zhu, H.Y., Huang, P.X., Wu, F., Song, D.Y., 2004. Journal

of Physical Chemistry B 108 (18), 5547–5551.Jiang, X.C., Herricks, T., Xia, Y.N., 2002. Nano Letters 2 (12), 1333–1338.Lehnhardt, W.F., Winzler, R.J., 1968. Journal of Chromatography A 34 (4), 471–

479.Li, D., Xia, Y.N., 2004. Advanced Materials 16 (14), 1151–1170.Li, X., Zhu, Q.Y., Tong, S.F., Wang, W., Song, W.B., 2009. Sensors and Actuators B -

Chemical 136 (2), 444–450.Li, Y., Song, Y.Y., Yang, C., Xia, X.H., 2007. Electrochemistry Communications 9 (5),

981–988.Liu, H.Y., Su, X.D., Tian, X.F., Huang, Z., Song, W.B., Zhao, J.Z., 2006. Electroanalysis 18

(21), 2055–2060.Lu, J.G., Chang, P.C., Fan, Z.Y., 2006. Materials Science and Engineering R-Reports 52

(1–3), 49–91.Luo, P., Zhang, F., Baldwin, R.P., 1991. Analytica Chimica Acta 244, 169–178.Mho, S.I., Johnson, D.C., 2001. Journal of Electroanalytical Chemistry 500 (1–2),

524–532.Okahata, Y., Tsuruta, T., Ijiro, K., Ariga, K., 1988. Langmuir 4 (6), 1373–1375.Ozcan, L., Sahin, Y., Turk, H., 2008. Biosensors and Bioelectronics 24 (4), 512–

517.Pan, Z.W., Dai, Z.R., Wang, Z.L., 2001. Science 291 (5510), 1947–1949.Rakhshani, A.E., Makdisi, Y., Mathew, X., 1996. Thin Solid Films 288 (1–2), 69–

75.Reitz, E., Jia, W.Z., Gentile, M., Wang, Y., Lei, Y., 2008. Electroanalysis 20 (22),

2482–2486.Robert, R.E., Johnson, D.C., 1994. Electroanalysis 6 (3), 193–199.Safavi, A., Maleki, N., Farjami, E., 2009. Biosensors and Bioelectronics 24 (6),

1655–1660.Salimi, A., Roushani, M., 2005. Electrochemistry Communications 7 (9), 879–887.Shen, G., Chen, P.C., Ryu, K., Zhou, C., 2009. Journal of Materials Chemistry 19 (7),

828–839.Shen, G.Z., Bando, Y., Chen, D., Liu, B.D., Zhi, C.Y., Golberg, D., 2006. Journal of Physical

Chemistry B 110 (9), 3973–3978.

7 d Bioe

T

TT

U

W

W

WW

Xue, D.S., Zhang, L.Y., Gui, A.B., Xu, X.F., 2005. Applied Physics A - Materials Science

14 W. Wang et al. / Biosensors an

akahashi, K., Limmer, S.J., Wang, Y., Cao, G.Z., 2004. Journal of Physical ChemistryB 108 (28), 9795–9800.

ans, S.J., Verschueren, A.R.M., Dekker, C., 1998. Nature 393, 49–52.ong, S., Jin, H., Zheng, D., Wang, W., Li, X., Xu, Y., Song, W., 2009. Biosensors and

Bioelectronics 24 (8), 2404–2409.mar, A., Rahman, M.M., Al-Hajry, A., Hahn, Y.B., 2009. Electrochemistry Communi-

cations 11 (2), 278–281.

an, Q., Li, Q.H., Chen, Y.J., Wang, T.H., He, X.L., Gao, X.G., Li, J.P., 2004. Applied

Physics Letters 84 (16), 3085–3087.ang, D.B., Song, C.X., Hu, Z.S., Fu, X., 2005. Journal of Physical Chemistry B 109 (3),

1125–1129.ang, J., 2001. Electroanalysis 13 (12), 983–988.ang, J., 2008. Chemical Reviews 108 (2), 814–825.

lectronics 25 (2009) 708–714

Wang, J., Thomas, D.F., Chen, A., 2008a. Analytical Chemistry 80 (4), 997–1004.

Wang, W., Huang, H.M., Li, Z.Y., Zhang, H.N., Wang, Y., Zheng, W., Wang, C., 2008b.Journal of the American Ceramic Society 91 (11), 3817–3819.

Xu, Q., Zhao, Y., Xu, J.Z., Zhu, J.J., 2006. Sensors and Actuators B - Chemical 114 (1),379–386.

and Processing 80 (2), 439–442.Zhang, J.T., Liu, J.F., Peng, Q., Wang, X., Li, Y.D., 2006. Chemistry of Materials 18 (4),

867–871.Zhuang, Z.J., Su, X.D., Yuan, H.Y., Sun, Q., Xiao, D., Choi, M.M.F., 2008. Analyst 133 (1),

126–132.