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Electrochimica Acta 55 (2010) 3035–3040

Contents lists available at ScienceDirect

Electrochimica Acta

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reparation of Cerium (III) 12-tungstophosphoric acid/ordered mesoporousarbon composite modified electrode and its electrocatalytic properties

in Liua, Jean Chrysostome Ndamanishab, Jing Baia, Li-ping Guoa,∗

Faculty of Chemistry, Northeast Normal University, Changchun 130024, PR ChinaUniversité du Burundi, Institut de pédagogie appliquée, B.P 5223 Bujumbura, Burundi

r t i c l e i n f o

rticle history:eceived 11 November 2009eceived in revised form 13 January 2010ccepted 15 January 2010vailable online 22 January 2010

eywords:eteropoly-acidrdered mesoporous carbon

a b s t r a c t

In this work, a novel structured Cerium (III) 12-tungstophosphoric acid (CePW)/ordered mesoporouscarbon (OMC) composite is synthesized. The characterization of the material by the Fourier transforminfrared spectroscopy (FTIR), X-ray powder diffraction (XRD), Raman spectroscopy, scanning electronmicroscopy (SEM), X-ray photoelectron spectroscopy (XPS) and electrochemical characterization showsthat the novel CePW/OMC composite has improved properties based on the combination of CePWand OMC properties. CePW/OMC can be used to modify the glassy carbon (GC) electrode and theCePW/OMC/GC modified electrode shows an enhanced electrocatalytic activity. This property can beapplied in the determination of some biomolecules. Especially, the detection and determination of the

lectrocatalysisuanine

guanine (G) in the presence of adenine (A) is achieved. The catalytic current of G versus its concentrationshows a good linearity with two good linear ranges from 4.0 × 10−6 to 8.0 × 10−5 M and from 8.0 × 10−5

to 1.9 × 10−3 M (correlation coefficient = 0.999 and 0.996) with a detection limit of 5.7 × 10−9 M (S/N = 3).The linear range for adenine is 4.0 × 10−6–7.0 × 10−4 M with a detection limit of 7.45 × 10−8 M. With goodstability and reproducibility, the present CePW/OMC/GC modified electrode should be a good modelfor constructing a novel and promising electrochemical sensing platform for further electrochemicaldetection of other biomolecules.

. Introduction

Heteropoly-compounds have attracted much attention becausef their application in chemical processes as heterogeneous andomogeneous catalysts [1,2]. These compounds have been founduitable promoters in various electrochemical processes. Theydrogen evolution reaction and the electrochemical deposition ofhotoconductive semiconductor compounds [1–4] are some exam-les. In recent years, there has been significant interest in theevelopment of heteropoly-acids (HPAs) as multifunctional mate-ials and they have been widely used in analytical and clinicalhemistry, biochemistry (electron transport inhibition), medicineantitumoral, antiviral and even anti-HIV activity) and solid-stateevices [5–8]. The most commonly used heteropoly-acid is theeggin type dodecatungstophosphoric acid (TPA). Among the

eteropoly-acids, tungstophosphoric acid, H3-[P(W3O10)4] (PWA)

s a Keggin type which has very interesting properties, such as theigh stability of their redox states, the high proton conductivitynd the possibility of multiple electron transfer [9,10]. Cerium (III)

∗ Corresponding author. Tel.: +86 431 85099762; fax: +86 413 85099762.E-mail address: guolp078@nenu.edu.cn (L.-p. Guo).

013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2010.01.056

© 2010 Elsevier Ltd. All rights reserved.

12-tungstophosphoric acid (CePW) has a keggin structure and canhave such properties.

On the other hand, ordered mesoporous carbons (OMCs) arenovel carbon materials and their considerable properties, such asuniform and tailored pore structure, high specific surface area, largepore volume and chemical inertness have been reported in theliterature [11,12]. Those carbon materials that, sometimes, showa similar electrochemical behavior as carbon nanotubes (CNTs)[13] can be used in the electrochemical determination of somemolecules [13,14]. In our previous work, we have found that elec-trocatalytic properties of OMC can be improved by incorporation ofsome substances [14]. Therefore, the incorporation of Cerium (III)12-tungstophosphoric acid (CePW) in OMC may result in a newcomposite with improved properties. A combination of the elec-trochemical properties of Cerium (III) 12-tungstophosphoric acidand OMC can then help to develop a novel electrochemical sensingplatform.

In the present work, a novel CePW/OMC composite based on

the incorporation of Cerium (III) 12-tungstophosphoric acid inordered mesoporous carbon is synthesized and used in the studyof some biomolecules. Among them guanine is especially studiedand determined at such CePW/OMC modified electrode becauseof its important significance in the bioanaytical chemistry and life

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ciences [15,16]. This study is also due to the fact that the elec-roactive groups of the DNA oxidation were adenine and guanineesidues [17], which exhibits slow direct electron transfer on theare working electrode with low sensitivity for DNA detection.

. Experimental

.1. Reagents and apparatus

Cerium (III) nitrate (Ce(NO3)3·6H2O) and phosphotungstic acidPWA, H3PW12O40·nH2O (water content 10.2 wt%), purity higherhan 99%) were obtained from Standard Chemical Reagent Com-any of Tianjin (China). Potassium ferricyanide (K3Fe(CN)6) wasurchased from Chemical Reagent Company of Shanghai (China).uanine (G), Acetaminophen (AP), Dopamine (DA), and Adenine

A) were obtained from Sigma. All other chemicals not mentionedere were of analytical reagent grade and used as received. Double-istilled water was used throughout. The 0.1 M phosphate bufferolution (PBS), which was made up from Na2HPO4, NaH2PO4, and3PO4, was employed as a supporting electrolyte.

Electrochemical experiments were carried out using a CHI 830blectrochemical Analyzer (CH Instruments, Shanghai Chenhuanstrument Corporation, China) in a conventional three-electrodeell. The working electrode used was glassy carbon (GC) electrodeModel CHI104, 3 mm diameter). A platinum electrode was takens the counter electrode and an Ag/AgCl (in saturated KCl solu-ion) electrode served as the reference electrode. All potentials inhis paper were measured and reported versus Ag/AgCl. All mea-urements were carried out at room temperature (20 ± 2 ◦C). Smallngle X-ray diffraction (XRD) patterns were obtained on an X-rayiffractor D4 instrument (Brucker, Germany) with Cu-Ka radia-ion (� = 0.15406 nm) operating at 40 kV and 20 mA. Raman spectraere recorded at ambient temperature on a Renishaw Raman

ystem model 1000 spectrometer with an argon-ion laser at anxcitation wavelength of 514.5 nm. Infrared spectrum of the sam-le was recorded with Nicolet Magna 560 FTIR Spectrometer withBr plate. The SEM images were determined with a Philips XL-30SEM operating at 3.0 kV. Energy-dispersive X-ray spectra (EDX)ere collected from an attached Oxford Link ISIS energy-dispersive

pectrometer fixed on a JEM-2010 electron microscope operated at00 kV.

.2. Preparation of CePW/OMC/GC electrode

SBA-15, as the template, was prepared as discussed in the liter-ture [18] using Pluronic P123 (non-ionic triblock copolymer, EO20O70 EO20) as a surfactant and TEOS (tetraethoxysilane) as a silicaource. OMC was synthesized according to the previous reportedork [2] with sucrose as the carbon source.

Fig. 1. (A) FTIR spectra of OMC (a), CePW (b) and CePW/OMC (c). (

ta 55 (2010) 3035–3040

GC electrodes were polished before each experiment with 1,0.3 and 0.05 �m alumina powder, respectively, rinsed thoroughlywith doubly distilled water between each polishing step. Then itwashed successively washed with a solution of nitric acid + acetone(V: V = 1:1) and doubly distilled water in ultrasonic bath and driedin air.

OMC (40.5 mg) were dispersed in 5 mL anhydrous alcohol byultrasonic agitation for about 1 h. The PWA (43.2 mg) was addedto the suspension. 5 mL of anhydrous alcohol containing Ce(NO3)3(8.6 mg) were slowly added to the mixture under stirring [19]. Thesolution was stirred at 20 ◦C for 72 h and then dried in the kiln at70 ◦C for 12 h. The homogeneous CePW/OMC nanocomposites werethen obtained.

CePW/OMC (5 mg) was dispersed in 10 mL of N, N-dimethylformamide (DMF), 2 �L of the CePW/OMC suspensionwas cast onto the surface of the well-polished glassy carbonelectrode and the solvent was allowed to dry under an infraredlamp for 10 min. The CePW/OMC/GC modified electrode was thenobtained.

3. Results and discussion

3.1. Structural characterization

Fig. 1A shows the IR spectra of OMC and CePW/OMC. The bandsat 1077, 981, 894 and 821 cm−1 are attributed to the asymmetricstretching vibration of P–O1 bond in the central PO4 tetrahedron,the stretching vibration of W O2, W–O3–W and W–O4–W groups,respectively [20]. The four bands are observed in the spectrum ofCePW (Fig. 1A curve b) and that of CePW/OMC (Fig. 1A curve c), indi-cating the immobilization of CePW on OMC. In Fig. 1A curve b, theprimary Keggin structure of CePW can be identified by four charac-teristic IR bands appearing within the range 500–1200 cm−1. Onecan note that the OMC alone (Fig. 1A curve a) shows no bands corre-sponding to the spectrum of CePW. It is important to note that thecharacteristic IR bands of CePW/OMC (Fig. 1A curve c) appeared atalmost the same positions without significant band shifts comparedto those of CePW (Fig. 1A curve b), indicating that CePW speciesare incorporated in OMC. This demonstrates that CePW species inthe CePW/OMC still keep the Keggin structure. Accordingly, theproperties of CePW are preserved in CePW/OMC.

Fig. 1B curve a shows powder X-ray diffraction (XRD) patterns ofOMC. The ordered arrangement of carbon nanorods can be observedby the well-resolved XRD peaks, which can be assigned to (1 0 0),

(1 1 0) diffractions of hexagonal (p6mm) structure. The long-rangeperiodic carbon structure is due to the interconnecting carbon spac-ers. The XRD peaks (Fig. 1B curve a) demonstrate that the carbonnanorods are rigidly interconnected into a highly ordered hexago-nal array by the carbon spacers, which are the inverse replica of the

B) The powder XRD patterns of OMC (a) and CePW/OMC (b).

L. Liu et al. / Electrochimica Acta 55 (2010) 3035–3040 3037

CePW

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Fig. 2. Raman spectra of

BA-15. As shown in Fig. 1B curve b, the XRD pattern of CePW/OMCxhibits the characteristic peaks of OMC (2� = 0.94◦) and loadingf CePW gives rise to a decrease in intensity of the peak at 0.94◦

2�). This indicates that a part of CePW/OMC has lost the long-ange order of OMC [21]. However, it can be noted that the orderedtructure is still preserved in CePW/OMC. Moreover, no patterns ofny bulk CePW crystal phase are observed for the fresh compositeaterials, indicating that the CePW is finely dispersed on the OMC

upporter [4].

Fig. 2 shows the Raman spectra of CePW/OMC (a) and OMC (b).

he two spectra exhibit the presence of D and G bands, locatedt 1384 cm−1 (disorder mode) and 1600 cm−1 (tangential) [22].he D band at around 1384 cm−1 is associated with the presencef defects in the graphite layer. The peak at 1600 cm−1 corre-

Fig. 3. SEM images of OMC (a) and CePW/OMC (b), (c) E

/OMC (a) and OMC (b).

sponds to the Raman-active E2g, which results from the vibrationmode corresponding to the movement in opposite directions oftwo neighboring carbon atoms in a single crystal graphite sheet.Furthermore, the relative intensity ratio of the D and G bands(ID/IG ratio) is proportional to the number of defective sites in thegraphite carbon [23]. From Fig. 2, it is clear, that the ID/IG ratio ofCePW/OMC is much larger than that of OMC, indicating that thereare more significant edge-plane-like defective sites on the surfaceof CePW/OMC than on the surface of OMC. In our previous work

[24] we have shown that the edge plane-like defective sites (EDSs)are very important in the electrocatalytic activity of OMC. Thereforethe incorporation of CePW in CePW/OMC is very important.

The morphologies of OMC and CePW/OMC were further charac-terized by SEM. In Fig. 3a, a delicate structure of OMC was observed

nergy-dispersive X-ray spectrum of CePW/OMC.

3038 L. Liu et al. / Electrochimica Ac

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ig. 4. CVs obtained at GC electrode, OMC/GC electrode and CePW/OMC/GC elec-rode in 5 mM K3Fe(CN)6/0.1 M KCl solution at a scan rate of 50 mV s−1.

n the SEM image with high magnification, which showed OMCas made up of carbon nanorods. Compared with OMC, CePW/OMC

xhibited similar morphologies (Fig. 3b) which indicates that, afterhe loading of CePW, the global particle shape and the delicate

tructure of the OMC host were maintained. The energy-dispersive-ray (EDX) spectrum (Fig. 3c) gives the proof for the existence ofe, P, and W in CePW/OMC.

Fig. 4 shows the cyclic voltammograms (CVs) for different elec-rodes in 5 mM K3Fe(CN)6/0.1 M KCl. According to Randles–Sevcik

ig. 5. CVs for 1.0 × 10−4 M guanine (a), 5.0 × 10−4 M acetaminophen (b), 1.0 × 10−4 M dopalectrode and CePW/OMC/GC electrode in 0.05 M PBS (pH 7.0). Scan rate: 50 mV s−1.

ta 55 (2010) 3035–3040

equation [25], the apparent electroactive surface area calculatedfor CePW/OMC/GC, OMC/GC and GC is 0.094, 0.080 and 0.069 cm2,respectively. This indicates that the electrochemical reactivity ofCePW/OMC/GC electrode is better than that of OMC/GC and GCelectrodes because CePW/OMC/GC electrode has lager apparentelectroactive surface area. Also, the smallest value of the poten-tial difference (�Ep) between the anodic and cathodic peaks isobserved at CePW/OMC/GC electrode, suggesting the best elec-trochemical reaction ability and fastest electron transfer kineticsfor this modified electrode. This implies that the edge plane sitesand CePW nanoparticles on CePW/OMC play an important role inaccelerating the heterogeneous electron transfer kinetics.

3.2. Electrochemical response of CePW/OMC/GC electrode toguanine, acetaminophen, dopamine and adenine

The electrochemical responses of guanine (G), acetaminophen(AP), dopamine (DA), and adenine (A) were studied at differ-ent electrodes in pH 7.0 PBS as shown in Fig. 5. Compared tothe electrochemical response of OMC/GC and GC electrodes, theelectrocatalytic activity of CePW/OMC/GC electrode towards G,AP, DA, and A enhances significantly, indicating more favorableelectrochemical activity than OMC/GC and GC electrodes. Lowerovervoltages for oxidation (or reduction) potentials as well aslarger faradic currents were achieved at CePW/OMC/GC electrode,

predicting that the CePW/OMC/GC electrode has more favorableelectron transfer kinetics than GC, CePW/GC and OMC/GC elec-trode. In this work, the electrocatalytic activity of CePW/GC aloneis not obvious perhaps because CePW does not adhere well to theGC electrode as in the other conditions.

mine (c) and 1.0 × 10−4 M adenine (d) at GC electrode, CePW/GC electrode, OMC/GC

L. Liu et al. / Electrochimica Acta 55 (2010) 3035–3040 3039

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Fig. 7. Linear sweep voltammograms of various concentrations of guanine at theCePW/OMC/GC electrode in 0.05 M PBS (pH 7.0) (from a to k). Scan rates: 50 mV s−1.

mination of G and A can be then achieved. It is therefore possibleto determine quantitatively without any considerable influence oneach other at a larger range of concentration. This speculation isconfirmed in Fig. 9 where the concentration of G is kept constant

ig. 6. CVs of 1.0 × 10−4 M guanine at CePW/OMC/GC electrode in 0.05 M PBS (pH.0) at scan rates of 5, 10, 30, 50, 70, 100, 150, 200, 250, 300, 350, 400, 450, 500 mV s−1

from a to n). Inset: the relationship between the peak currents and the square rootf scan rates.

Such ability of CePW/OMC not only suggests it could be a kindf more advanced electrode material for routine electroanalyticalnvestigations but also indicates a potential of CePW/OMC to be of

ide range of sensing applications. Because the electrocatalysis ofhe electroactive compounds at the CePW/OMC/GC electrode in thisork should be a good model for constructing a novel and promis-

ng electrochemical sensing platform for further electrochemicaletection of other biomolecules.

.3. Electrochemical determination of guanine

As an example, the electrochemical behavior of guanine wastudied. The influence of the scan rate on guanine oxidation athe CePW/OMC/GC modified electrode was investigated by cyclicoltammetry (Fig. 6). At scan rates in the range from 0.005 to.5 V s−1, the oxidation peak currents of the CePW/OMC/GC elec-rode in guanine solution increased linearly with the square root ofhe scan rates, indicating a diffusion-controlled process [25].

We also studied the effect of different pH on the guanine oxida-ion at the CePW/OMC/GC electrode (data not shown). It was foundhat the increase of the solution pH leads to the negative shift ofeak potential, which shows a linear variation of potential with pH.

n the pH range of 4–9, the maximum peak currents are observedt pH 7. So, the electrochemical activity of guanine is highest atH 7. Simultaneously, pH 7 approaches the physiological pH valuend is also more suited to the operational requirements of manyiosensors.

As described above, CePW/OMC/GC electrode can effectivelyatalyze the oxidation of G, and it appears likely that an ampero-etric detection of G by CePW/OMC/GC electrode is possible. Linear

weep voltammetry was adopted to detect the guanine. It is clearrom Fig. 7 that there is a good linear correlation between the peakurrent (Ip) and concentration (C) of G. In the inset of Fig. 7, theodified electrode shows two good linear ranges from 4.0 × 10−6

o 8.0 × 10−5 M and from 8.0 × 10−5 to 1.9 × 10−3 M (correlationoefficient = 0.999 and 0.996) with a detection limit of 5.7 × 10−9 MS/N = 3), the linear regression equation is Ip = 1.006C + 7.598 × 10−6

nd Ip = 0.027C + 8.784 × 10−5. This detection limit is lower than theeported ones in the literature [26–28] and such low detection limitan be attributed to the efficient combination of the properties ofMC and CePW nanoparticles.

The influence of the adenine, another important purine baseound in the deoxyribonucleic acid was also studied. Fig. 8 showshe cyclic voltammograms of guanine and adenine in 0.05 M PBSpH 7.0). One can note that the response at the bare GC electrodes very poor and the biomolecules cannot be detected efficiently. In

Fig. 8. CVs for 1.0 × 10−4 M guanine and 1.0 × 10−4 M adenine at GC andCePW/OMC/GC electrode in 0.05 M PBS (pH 7.0). Scan rate: 50 mV s−1.

contrast, at the CePW/OMC/GC electrode the peak current signalsof guanine and adenine enhance significantly with their oxidativepotential moving negatively. Moreover, the voltammetric signalsdue to G and A are well separated and the simultaneous deter-

Fig. 9. Linear sweep voltammograms of 5 × 10−5 M guanine co-existed with variousconcentrations of adenine at the OMC electrode (from a to i). Inset figure: a linearcorrelation between peak current versus concentration of adenine.

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ith different concentration of A. The anodic peak current dueo A oxidation increases linearly with increasing A concentrationnd the correlation coefficient is 0.997 whereas the peak currentue to G remains almost constant. The linear range for adenine

s 4.0 × 10−6–7.0 × 10−4 M with a detection limit of 7.45 × 10−8 M.

his indicates that the presence of even high concentration of ade-ine has no influence on the guanine determination.

We also investigated the stability and reproducibil-ty of CePW/OMC/GC electrode. The eight freshly preparedePW/OMC/GC electrodes were used to measure 5 × 10−5 M gua-ine in PBS. All the electrodes exhibited similar current responsesnd a relative standard deviation (R.S.D.) of 2.8% was obtained,ndicating that the CePW/OMC/GC electrode is considerably stablend repeatable. After being stored at 277 K for two weeks, theesponse of CePW/OMC/GC electrode to guanine was almostnchanged, and this suggests a good stability.

. Conclusion

The ordered mesoporous carbon was functionalized witherium (III) 12-tungstophosphoric acid by its immobilization on theaterial. The results above showed that the electrochemical and

lectrocatalytic properties of CePW/OMC composite are improved.he material is able to be used to modify an electrode that cane used to investigate the electrochemistry of some biomolecules.s example, electrocatalytic properties of the CePW/OMC/GC mod-

fied electrode towards guanine, adenine, acetaminophen andopamine have been studied and the modified electrode showsast response and good electrocatalytic activity. To show that the

odified electrode can be applied in the simultaneous detectionf biomolecules, the detection and determination of guanine in

he presence of adenine was achieved. The modified electrodehowed also good stability and reproducibility, indicating that itan be a promising electrochemical sensing platform for furtherlectrochemical detection of other biomolecules based on the elec-rocatalysis of the electroactive compounds.

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Acknowledgment

The authors gratefully acknowledge the financial support by theNational Natural Science Foundation of China (No. 20875012).

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