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Biosensors and Bioelectronics 26 (2011) 2756–2760

Contents lists available at ScienceDirect

Biosensors and Bioelectronics

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

hort communication

ickel oxide microfibers immobilized onto electrode by electrospinning andalcination for nonenzymatic glucose sensor and effect of calcinationemperature on the performance

ei Cao, Shu Guo, Huiyan Ma, Decai Shan, Shengxue Yang, Jian Gong ∗

ey Laboratory of Polyoxometalates Science of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, Jilin, PR China

r t i c l e i n f o

rticle history:eceived 25 June 2010eceived in revised form6 September 2010ccepted 6 October 2010vailable online 14 October 2010

a b s t r a c t

Nickel oxide microfibers (NiO-MFs) were directly immobilized onto the surface of fluorine tin oxide(FTO) electrode by electrospinning and calcination without using any immobilization matrix for nonen-zymatic glucose sensor. Morphology and structure of NiO-MFs were characterized by scanning electronmicroscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction pattern (XRD). Theelectrochemical and electrocatalytic performances of the NiO-MFs modified electrodes prepared at dif-

◦

eywords:ickel oxide microfiberslectrospinningalcination temperaturelucoseonenzymatic sensor

ferent calcination temperatures ranging from 300 to 500 C were evaluated by cyclic voltammetry (CV).The CV results have demonstrated that NiO-MFs modified electrode prepared at 300 ◦C displayed dis-tinct increase in electrocatalytic activity toward the oxidation of glucose, which is explored to developan amperometric nonenzymatic glucose sensor. The NiO-MFs prepared at 300 ◦C based amperomet-ric nonenzymatic glucose sensor has ultrasensitive current (1785.41 �A mM−1 cm−2) response and lowdetection limit of 3.3 × 10−8 M (signal/noise ratio (S/N) = 3), which are among the best values reported inliterature. Additionally, excellent selectivity and stability have also been obtained.

. Introduction

The development for glucose sensors with high sensitivity,ast response, good stability and low cost has become one of the

ost attractive subjects of investigation in electrochemistry dueo the practical medical needs and applications (Newman andurner, 2005; Wilson and Gifford, 2005). Although glucose biosen-ors with glucose oxidase have made a considerable progress, thenzyme electrodes have inevitable drawbacks, such as the chem-cal and thermal instabilities originated from the intrinsic naturef enzymes and the tedious fabrication procedures (Katakis andomínguez, 1995). In contrast, nonenzymatic electro-oxidationf glucose on metal electrodes such as platinum, gold and alloys advocated to avoid the disadvantages of enzyme electrodesVassilyev et al., 1985; Adzic et al., 1989; Sun et al., 2001). However,he high cost of the electrode materials may limit their commer-ial applications since the ability to produce glucose sensors in large

umbers at low cost is a major market requirement (Liu et al., 2009).herefore, numerous efforts have been focused on the use of low-ost metal oxide materials such as nickel oxide (NiO) (Berchmanst al., 1995; Shamsipur et al., 2010), copper oxide (Zhuang et al.,

∗ Corresponding author. Tel.: +86 431 85099765; fax: +86 431 85099668.E-mail address: gongj823@nenu.edu.cn (J. Gong).

956-5663/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.bios.2010.10.013

© 2010 Elsevier B.V. All rights reserved.

2008; Wang et al., 2010), and manganese oxide (Chen et al., 2008)to modify the electrodes for nonenzymatic electro-oxidation of glu-cose.

Among these relatively low-cost metal oxides, NiO materialsby virtue of natural abundance, low toxicity, good electrochemicalstability and catalytic properties are of particular interest, whichmake it suitable for the electrochemical sensors (Li et al., 2008;Hajjizadeh et al., 2008; Liu et al., 2009). It is well known that nano-or microstructured materials, especially wires and fibers exhibitpeculiar and fascinating electrochemical performance superior totheir bulk counterparts due to very large surface-to-volume ratio,dimensions comparable to the extension of surface charge region(Park et al., 2007). Recently, large quantities of NiO nanowireand nanofibers material have been reported (Yang et al., 2005;Guan et al., 2003). However, to our knowledge, most reports limitthe scope to their experimental production and characterization.There has less report on the investigation of NiO nanowires andnanofibers as electrochemical sensors.

Nowadays, with the rapid progress in the synthesis of elec-trospinning technique, the scope of its applications has been

broadened to the fields of chemically modified electrode (Yanget al., 2006). Great efforts have been made recently to fabricatemetal oxide nanofibers by combining electrospinning techniquewith subsequent thermal treatment for nonenzymatic sensor(Wang et al., 2009a,b; Wang et al., 2009a, b). However, in these

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F. Cao et al. / Biosensors and B

eports, immobilization matrix (Nafion solution) has been requiredo entrap metal oxide nanofibers for electrode attachment. Thisrocess is time-consuming, tedious, and usually leads to pooresults.

In this paper, a sol of poly(vinyl alcohol) (PVA)/nickel nitrateas directly electrospun onto the surface of FTO electrode. Then

alcination was used to remove the organic constituents of PVAnd convert the precursor fibers into pure NiO-MFs. Investigationhowed that the NiO-MFs could be well attached to the surfacef FTO after calcination when the content of inorganic precur-or was more than 50 wt%. Ultimately, NiO-MFs were immobilizednto the surface of FTO electrode without using any immobilizationatrix.It is worth mentioning that the calcination temperature has an

nfluence on the electrochemical and electrocatalytic performancesf the as-prepared electrode. At low temperature (300 ◦C), the NiO-Fs modified electrode presents the remarkable activity toward

lucose electro-oxidation, offering an excellent electrochemicalensing platform.

. Experimental

.1. Chemicals and reagent

Polyvinyl alcohol (PVA, Mw = 77,000), sodium hydroxideNaOH) and nickel nitrate were purchased from Beijing Chemi-al Co. (China). Glucose, l-ascorbic acid (AA), and dopamine (DA)ere obtained from Sigma–Aldrich. All chemicals were of analyt-

cal grade and used as received without any further purification.istilled water was used in all experiments.

.2. Preparation of NiO-MFs modified electrodes

First of all, FTO substrate (4 cm × 1 cm) was pretreated beforet was used for collecting the PVA/nickel nitrate composite fibers.t was sonicated in acetone, distilled water, concentrated NaOH in:1 (v/v) water/ethanol and distilled water for 10 min, respectively,nd dried with nitrogen gas. In order to control the electrode area1 cm × 1 cm), the FTO substrate was covered partly by a piece ofluminum foil. Secondly, in a typical procedure for electrospinning,g of nickel nitrate was added into 4 g of PVA solution (PVA 8.0 wt%)nder stirring for 12 h. The viscous sol of PVA with nickel nitrateomposite was contained in a plastic syringe and connected to aigh-voltage power supply for electrospinning. An electric poten-ial of 18 kV was applied between the orifice and the ground at aistance of 10 cm. After electrospinning of 10 min, the piece of alu-inum foil was removed from FTO surface. Finally, the precursor

TO substrates were calcined at the setting temperature (300, 400,nd 500 ◦C, respectively) at a rate of 10 ◦C/min and remained 3 h tobtain pure NiO-MFs modified electrodes.

.3. Apparatus

SEM images were obtained using a XL-30 ESEM FEG scanninglectron microscope operated at 20 kV with gold sputtered on sam-les with energy-dispersive X-ray analysis attached to SEM. XRDatterns were performed on a D/Max IIIC X-ray diffractometersing a Cu K� radiation source. All electrochemical measure-ents were performed on a CHI 800B electrochemical workstation

Shanghai, China) with a conventional three-electrode system com-osed of a platinum auxiliary, an Ag/AgCl (saturated KCl) reference,nd a NiO-MFs modified electrode. Amperometric measurementsere carried out in 15 mL of NaOH solution under continuous stir-

ing.

tronics 26 (2011) 2756–2760 2757

3. Results and discussion

3.1. Characterization of NiO-MFs

The morphology of the NiO-MFs modified electrode prepared at300 ◦C was observed directly by SEM as shown in Fig. S1. It can beseen that the average diameter of the microfibers is about 10 �mwith hundreds of micrometers in length. The reason for obtain-ing wider diameter fibers is due to the higher content of nickelnitrate in precursor sol solution, which is consistent with previousreport (Gong et al., 2003). The morphologies of the NiO-MFs modi-fied electrodes prepared at 400 and 500 ◦C have no obvious changecompared to that of the NiO-MFs modified electrode prepared at300 ◦C. The EDX characterization was applied to investigate eachelement existed in the microfibers, which reveals the presence ofoxygen and nickel (Fig. S2). This result indicates that the NiO-MFswere prepared successfully at 300 ◦C.

The XRD patterns of the microfibers prepared at 300, 400 and500 ◦C are illustrated in Fig. S3, respectively. All of them show fivecharacteristic diffraction peaks at 37.1◦ (1 1 1), 43.2◦ (2 0 0), 62.8◦

(2 2 0), 75.3◦ (3 1 1) and 79.3◦ (2 2 2) corresponding to pure cubicNiO, which coincide well with the literature value (JCPDS Card FileNo. 47-1049).

3.2. Electrochemical performance of NiO-MFs modified electrode

The cyclic voltammograms (CVs) of the bare FTO and NiO-MFsmodified electrodes in 0.1 M of NaOH solution in the potentialwindow ranging from 0.1 to 0.6 V with a scan rate 20 mV s−1 areshown in Fig. 1. As shown in Fig. 1A, comparing with that at FTOelectrode with only a small background current (curve a), a dra-matic increase of current signal is observed at NiO-MFs modifiedelectrode prepared at 300 ◦C (curve b), which can be ascribed tothe role of NiO-MFs in the increase of the electroactive surfacearea. The NiO-MFs modified electrode exhibits a pair of shoulderpeaks with the anodic and cathodic peaks (curve b), which areassigned to Ni(II)/Ni(III) redox couple forming in alkaline medium.Meanwhile, the peak currents in NiO-MFs modified electrodes pre-pared at 300, 400 and 500 ◦C (curves b, c and d) decrease graduallywith increasing the calcination temperature, indicating that thecalcination temperature has an influence on the electrochemicalperformance of NiO-MFs modified electrode. This phenomenonmay be explained clearly by previous reports about the effect ofcalcination temperature on the conductivity of NiO. Kohmoto et al.(2001) found that the electric resistivity of NiO semiconductorincreases substantially from ∼1 to 105 � cm with the increase ofannealing temperature. X. Wang et al. (2009) reported that theresistivity of NiO thin film increases from 45 to 1012 � cm and car-rier concentration of it decreases from 2.0 × 1017 to 7.1 × 1014 cm−3

with the increase of annealing temperature (623–923 K). As weknow that NiO is a semiconductor compound with p-type con-ductivity. The conductivity of p-type NiO is caused by the vacancyof Ni atoms and/or by the interstitial of oxygen atoms (Adler andFeinleib, 1970). However, oxygen atoms will lose from NiO at higherannealing temperature. The loss of oxygen atoms will cause thedecrease of carrier in p-type NiO semiconductor, ultimately caus-ing the decrease of the conductivity of NiO with the increase ofannealing temperature. Therefore, NiO-MFs prepared at lower tem-perature (300 ◦C) with higher conductivity exhibit enhanced peakcurrents.

Fig. 1B shows the CVs of NiO-MFs modified electrode prepared

at 300 ◦C in 0.1 M of NaOH solution at different scan rates. The redoxpeak currents increase linearly with increasing the scan rate from20 to 100 mV/s (Fig. 1C) with correlation coefficients (R2) of 0.9983(Ipa) and 0.9987 (Ipc), respectively, indicating that the redox pro-cess is a typical surface-controlled electrochemical process with a

2758 F. Cao et al. / Biosensors and Bioelectronics 26 (2011) 2756–2760

Fig. 1. (A) CVs of (a) the bare FTO electrode, (b) NiO-MFs modified electrode pre-pared at 300 ◦C, (c) at 400 ◦C and (d) at 500 ◦C in 0.1 M of NaOH solution at a scanrop

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Fig. 2. (A) CVs of (a and b) the bare FTO electrode and (c and d) NiO-MFs modifiedelectrode prepared at 300 ◦C in the absence and presence 2 mM of glucose in 0.1 Mof NaOH at a scan rate of 20 mV s−1; (B) CVs of the NiO-MFs modified electrode

positive end of the potential range is observed at NiO-MFs modi-

ate of 20 mV s−1; (B) CVs of NiO-MFs modified electrode prepared at 300 ◦C in 0.1 Mf NaOH solution at scan rates of 20, 40, 60, 80 and 100 mV/s, respectively; (C) thelot of peak current versus scan rate.

ast electron-transfer behavior (Shan et al., 2007; Shamsipur et al.,010).

.3. Electrocatalytic performance of glucose at NiO-MFs modifiedlectrode

The electrocatalytic performance of the bare FTO and NiO-MFs

odified electrodes toward the oxidation of glucose was investi-

ated by CVs in 0.1 M of NaOH solution in the presence of glucose at0 mVs−1. Fig. 2A shows the CVs behaviors in the absence (curves and c) and presence (curves b and d) of 2 mM of glucose, recorded

prepared at 400 ◦C in the absence and presence of 2 mM of glucose in 0.1 M of NaOHat a scan rate of 20 mV s−1; (C) CVs of the NiO-MFs modified electrode prepared at500 ◦C in the absence and presence of 2 mM of glucose in 0.1 M of NaOH at a scanrate of 20 mV s−1.

at the bare FTO (curves a and b) and NiO-MFs modified electrodesprepared at 300 ◦C (curves c and d). As seen, upon the addition of2 mM of glucose, no significant current response is obtained at bareFTO electrode, while the enhancement of the current toward the

fied electrode prepared at 300 ◦C, indicating that the NiO-MFs playa significant role in electrocatalytic detection of glucose. Fig. 2Band C displays the CVs behaviors of the NiO-MFs modified elec-trode prepared at 400 ◦C and at 500 ◦C, respectively, in the absence

ioelectronics 26 (2011) 2756–2760 2759

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Fig. 3. (A) Amperometric response of NiO-MFs modified electrode prepared at300 ◦C upon successive addition of glucose at low concentrations to 0.1 M of NaOHat an applied potential of 0.5 V; (B) amperometric response of (a) NiO-MFs modifiedelectrodes prepared at 300 ◦C, (b) at 400 ◦C and (c) at 500 ◦C upon successive addi-

F. Cao et al. / Biosensors and B

curve a) and presence (curve b) of 2 mM of glucose at the sameondition. Although NiO-MFs modified electrodes prepared at 300,00 and 500 ◦C all can catalyze the electro-oxidation of glucose,he redox peak currents in NiO-MFs modified electrodes preparedt 300 ◦C, 400 ◦C and 500 ◦C decrease gradually. As we expected, theiO-MFs modified electrode prepared at 300 ◦C presents remark-ble electrocatalytic performance for oxidation of glucose, offeringn excellent electrochemical sensing platform. Therefore, amper-metric determination of glucose at NiO-MFs modified electroderepared at 300 ◦C is further discussed in the following part.

.4. Amperometric performance of glucose at NiO-MFs modifiedlectrode

.4.1. Amperometric analysisThe superior electrocatalytic performance and easy fabrication

ake the NiO-MFs modified electrode prepared at 300 ◦C a goodlectrochemical sensing platform for amperometric determina-ion of glucose. Fig. 3A shows the amperometric responses of thes-prepared sensor to successive injection of glucose at low con-entrations. A well-defined, stable and fast amperometric responseithin 5 s can be observed at optimal potential 0.5 V. With the suc-

essive addition of glucose the sensor yields a step-like increasemperometric response. Fig. 3B displays a typical amperometricesponse (curves a, b and c) of glucose in 0.1 M of NaOH recordedt the NiO-MFs modified electrodes prepared at 300 ◦C, 400 ◦Cnd 500 ◦C, respectively. Amperometric response curves furtheremonstrate that NiO-MFs modified electrode prepared at 300 ◦Cresents good amperometric determination of glucose. Meanwhile,he calibration curve for the glucose sensor prepared at 300 ◦C isorked out, as shown in Fig. 3C. The sensor displays a linear range

rom 1 × 10−6 to 270 × 10−6 M glucose with a correlation coefficientf 0.998, a sensitivity of 1785.41 �A mM−1 cm−2 and a detectionimit of 3.3 × 10−8 M (S/N = 3). As listed in Table S1, the detec-ion limit and sensitivity for glucose determination at the NiO-MFs

odified electrode prepared at 300 ◦C are comparable and evenetter than those obtained at several nonenzymatic glucose sensoreported recently. The excellent sensing properties of our glucoseensor can be explained as follows. Firstly, NiO-MFs prepared at lowemperature (300 ◦C) with high conductivity exhibit enhanced elec-rochemical and electrocatalytic performances. The high sensitivitynd low detection limit can be attributed to the enhanced elec-rochemical and electrocatalytic performances. Secondly, NiO-MFsere immobilized onto electrode surface by directly electrospin-ing and calcination without using any immobilization matrix,hich offers a close contact of NiO-MFs to electrode. And there-

ore, direct electron transfer can be achieved between NiO-MFs andlectrode (Ding et al., 2010), which is also an important reason ofast and sensitive catalytic performances. Finally, this microfibertructure ensures that the glucose can infiltrate into the fibers eas-ly and react with the NiO-MFs sufficiently. The large surface andigh length/diameter ratio also provide more reaction sites on theurface of NiO-MFs and enhance the electron conduction along theength direction.

.4.2. Interferential analysisAs reported, many glucose sensors based on metal and alloy

lectrodes usually lost their activity because of being poisoned byhloride ions (Sun et al., 2001). The poisoning possibility of chlo-ide ions to the activity of NiO-MFs modified electrode preparedt 300 ◦C in glucose determination was also examined by adding

odium chloride in the supporting electrolyte in measurement. Theesult shows that the current response of the NiO-MFs modifiedlectrode prepared at 300 ◦C remains unchanged toward glucosexidation (data not shown), exhibiting good resistance to surfaceouling.

tion of glucose at 0.03 mM to 0.1 M of NaOH at an applied potential of 0.5 V; (C) thecalibration curve for the amperometric response of NiO-MFs modified electrode pre-pared at 300 ◦C. Error bars indicate standard deviations for triplicate measurementsat each glucose concentration.

AA and DA are the most important interferences normally co-exist with glucose in real samples (human blood). Consideringthat the concentration of glucose in the human blood is about 30times of AA or DA (Chen et al., 2008), the amperometric responsesof NiO-MFs modified electrode prepared at 300 ◦C toward the

addition of 3 mM of glucose, 0.1 mM of AA and 0.1 mM of DAwere examined in 0.1 M of NaOH solution at a potential of 0.5 V(vs. Ag/AgCl). The current responses produced by AA and DA are6.2 �A and 3.1 �A, only 3.12% and 1.56% compared to that of glu-cose (198.5 �A), respectively. Calculating sensitivity for each of

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Wilson, G.S., Gifford, R., 2005. Biosens. Bioelectron. 20, 2388–2403.Yang, G.C., Gong, J., Yang, R., Guo, H.W., Wang, Y.Z., Liu, B.F., Dong, S.J., 2006. Elec-

760 F. Cao et al. / Biosensors and B

he interferences is a more straightforward way to demonstrateelectivity of sensor. The amperometry results showed that the sen-itivity produced by AA and DA are 1371.77 �A mM−1 cm−2 and75.76 �A mM−1 cm−2, respectively, implying a good selectivity tolucose (1785.41 �A mM−1 cm−2).

.5. Stability and reproducibility of NiO-MFs modified electrode

The stability of the NiO-MFs modified electrode prepared at00 ◦C was evaluated by comparing the changes of current responsef the electrode stored in dry state in laboratory atmosphere beforend after 30 days with additions of 2 mM of glucose in 0.1 M of NaOHolution. It retains 96% of its initial response current after 30 days.oncerning the reproducibility, six NiO-MFs modified electrodesbtained at 300 ◦C were prepared with relative standard devia-ions (RSD) of 4.6% for the current response to 2 mM of glucose,ndicating a satisfied electrode-to-electrode reproducibility. Addi-ionally, the RSD of 2.4% (n = 8) for 2 mM of glucose demonstratedood intra-electrode reproducibility. Therefore, the NiO-MFs areood electrode material for preparation of sensitive, stable andeproducible amperometric sensors for glucose determination.

. Conclusions

In summary, NiO-MFs were immobilized onto FTO electrodey electrospinning and calcination without using any immobiliza-ion matrix for nonenzymatic glucose sensor. The results of CV andmperometry proved that calcination temperature has an influencen the electrochemical and sensing performances of as-preparedensors. The sensor prepared at low temperature (300 ◦C) exhib-ted ultrasensitive current (1785.41 �A mM−1 cm−2) response withower detection limit of 3.3 × 10−8 M, which can be attributed tohe enhanced electrochemical and electrocatalytic performances of

iO-MFs prepared at 300 ◦C. Further in-depth mechanism investi-ation of the influence of temperature on sensing performance ofiO-MFs is still needed and other metal-oxides microfibers directly

mmobilized on electrode by the simple preparation process forensor are expected to be extensively investigated.

tronics 26 (2011) 2756–2760

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.bios.2010.10.013.

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