AnalyticalMethods

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Electroanalysis and Bio electrochemistry

Engineering and Biotechnology, National

Section 3, Chung-Hsiao East Road, Taipei

ms15.hinet.net; Fax: +886-2-27025238; Tel:

Cite this: Anal. Methods, 2014, 6, 4686

Received 15th March 2014Accepted 1st May 2014

DOI: 10.1039/c4ay00628c

www.rsc.org/methods

4686 | Anal. Methods, 2014, 6, 4686–4

A novel voltammetric p-nitrophenol sensor basedon ZrO2 nanoparticles incorporated into amultiwalled carbon nanotube modified glassycarbon electrode

Balamurugan Devadas, Muniyandi Rajkumar, Shen-Ming Chen* and Pin-Chun Yeh

Herein, we report the incorporation of zirconium oxide (ZrO2) nanoparticles into functionalized multiwalled

carbon nanotubes (fMWCNTs/ZrO2) to form a nanocomposite film via a simple and clean in situ method

based on the electrochemical redox reaction of zirconyl chloride (ZrOCl2). The electrocatalytic

properties and surface morphology of the as-prepared nanocomposite were studied using cyclic

voltammetry, electrochemical impedance spectroscopy and field emission scanning electron

microscopy. The as-prepared fMWCNTs/ZrO2 nanocomposite-modified glassy carbon electrode shows

a prominent electrocatalytic activity towards the voltammetric determination of p-nitrophenol. The

presence of fMWCNTs in the film enhances the surface coverage concentration and also increases the

electron transfer rate constant of the ZrO2 nanoparticles. The modified electrode has a linear range of

2–26 mM for p-nitrophenol. The proposed film was also used successfully for the voltammetric

determination of p-nitrophenol in river and tap water samples with a linear range of 0–24 mM. A well-

defined peak for the detection of p-nitrophenol in water samples has proved this fMWCNTs/ZrO2

nanocomposite-modified electrode to be a successful sensor material. The proposed film has long-term

stability.

1. Introduction

In recent years, nanoparticles and well-organized low-dimen-sional nanostructures have received overwhelming attentiondue to their unique capabilities of enhancing mass transport,facilitating catalysis, increasing surface area, and controllingthe microenvironment of electrodes.1 In addition, their largesurface-to-volume ratio gives them excellent electrocatalyticproperties, making them of interest in electroanalysis.2 Nano-particles of ZrO2, an inorganic oxide, have been shown to be anideal material for the immobilization of biomolecules withoxygen-containing groups because of the thermal stability,chemical inertness, lack of toxicity, and affinity for oxygen-containing groups of ZrO2.3 These ZrO2 nanoparticles alsoprovide a three-dimensional stage and some restricted orien-tations, which can be tailored to direct electron transferbetween protein molecules and the conductor surface.4 Owingto these attractive properties, ZrO2 has been used in electro-chemical sensor applications. In recent years, composites ofZrO2 with carbon materials have been used in chemical and

Laboratory, Department of Chemical

Taipei University of Technology, No. 1,

106, Taiwan (ROC). E-mail: smchen78@

+886-2-27017147

691

biosensor applications.5,6 Multiwalled carbon nanotubes(MWCNTs) have also been applied extensively in various eldsas a result of their chemical and mechanical properties. Inaddition, metal nanoparticles decorated with CNTs have beenused in various applications such as gas sensors, nano-electronics and heterogeneous catalysis.7 Various methods havebeen used to decorate CNTs withmetal nanoparticles, includingelectro-less deposition,8 thermal deposition,9 vapor deposi-tion,10 and electrodeposition.11 Modied electrodes based onCNTs decorated with metal nanoparticles have been reported inelectrochemical sensor applications.12,13

Nitrophenols (NPs) are a class of man-made, toxic, inhibitoryand biorefractory organic compounds. NPs are used extensivelyin the production of pesticides, dyes and pharmaceuticals. Inparticular, p-nitrophenol (p-NP) is a toxic derivative of parathioninsecticide, which is considered to be a hazardous waste andpriority toxic pollutant by the US Environmental ProtectionAgency.14 Hence there is a need to detect p-NP, not only inindustrial wastewater, but also in freshwater and marine envi-ronments. Owing to its high stability and solubility in water, thedegradation treatment of wastewater contaminated with p-NP isdifficult using traditional techniques and generally requires along period of incubation. Therefore it is important to developsimple and effective methods for the determination of traceamounts of p-NP in aqueous solutions to monitor the

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degradation of p-NP or to protect water resources and foodsupplies from pollution.15,16

Electrochemical methods17�21 have received considerableattention in the determination of NPs because of theiradvantages over chromatographic22 and spectroscopicmethods, including their simple operation, fast response andin situ detection. The selectivity and sensitivity in electro-chemical detection are strongly dependent on the micro-structure and properties of the electrode materials. Fewworkers are currently focusing on nanostructured materials23

or chemically modied electrodes24,25 for use in electro-chemical sensors. We report here a simple electrodepositionmethod to prepare a functionalised MWCNT/ZrO2 (fMWCNT/ZrO2) nanocomposite-modied electrode. The redox complexwith the nanostructured materials was used for the selectiveelectrocatalytic determination of p-NP. This composite wasprepared on glassy carbon electrodes (GCEs) and indium-tinoxide electrodes using a simple two-step process by drop-casting fMWCNTs followed by electrochemical reduction in azirconyl chloride solution in phosphate-buffered saline (PBS)at pH 7. The as-prepared nanocomposite-modied electrodewas characterized using the surface analysis technique of eldemission scanning electron microscopy (FESEM) and theelectrochemical techniques of cyclic voltammetry (CV) andelectrochemical impedance spectroscopy (EIS). ThefMWCNTs/ZrO2 nanocomposite-modied electrode wassuccessfully used to determine p-NP without any interferenceand was quantied in various real-system water samples.

2. Experimental2.1. Apparatus

The electrochemical measurements, such as CV and linear-sweep voltammetry (LSV), were performed using a CHI 1205Aelectrochemical analyzer. A conventional three-electrode elec-trochemical cell was used at room temperature with a GCE(surface area 0.07 cm2) as the working electrode, an Ag/AgCl(saturated KCl) electrode as the reference electrode and a plat-inum wire as the counter electrode. The reported potentials areall referred to the standard Ag/AgCl (saturated KCl) referenceelectrode. The surface morphology of the lm was studied usinga eld emission scanning electron microscope (Hitachi, Japan).Electrochemical impedance measurements were made using aZAHNER impedance analyzer (ZAHNER Elektrik, Germany).

Fig. 1 Cyclic voltammograms for the electrodeposition of ZrO2

nanoparticles on fMWCNT nanocomposite-modified GCE in pH 7 PBScontaining 5 � 10�3 M ZrOCl2. The potential scan was performedbetween 0.7 and �1.1 V for ten cycles at a scan rate of 20 mV s�1.

2.2. Materials

The zirconyl chloride octahydrate, MWCNTs and p-NP werepurchased from Sigma-Aldrich. The p-NP solution was freshlyprepared each day. The other chemicals used in this investiga-tion were of analytical-reagent grade (99%) (Merck). Watersamples were collected from a nearby river, and tap water wascollected from the laboratory. All the solutions were preparedusing double-distilled water. Electrocatalytic studies werecarried out in 0.05 M PBS (pH 7). Pure nitrogen gas was purgedthrough all the experimental solutions to remove dissolvedoxygen.

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2.3. Preparation of fMWCNTs and electrochemicalfabrication of the nanocomposite-modied electrode

The pristine (commercial) MWCNTs are difficult to disperse inaqueous media to produce a stable homogenous solution, andthis is due to their hydrophobic nature. Acid treatment istherefore used to produce nanoparticles with hydrophilicproperties. The MWCNTs were pre-treated and functionalizedas reported previously by suspending 150 mg of the MWCNTs ina mixture of concentrated sulfuric acid and nitric acid (3 : 1, v/v)and sonicating for 2 h. The nanotube mat obtained was lteredusing a 0.45 mm hydrophilized PTFE membrane, then lteredand further washed with deionized water until it reached a pHof 7. It was then stored and dried under vacuum.23 A 10 mgsample of the fMWCNTs obtained by this method was dissolvedin 10 ml of water and ultrasonicated for 6 h to obtain uniformdispersion. This process not only produces fMWCNTs withhydrophilic properties, but also helps to break down the largerbundles of the fMWCNTs into smaller bundles.24

The ZrO2-modied GCE was prepared by a simple electro-chemical deposition method. In a typical procedure, 5 ml of theas-prepared fMWCNTs were drop-cast onto the GCE and air-dried in an oven at 30 �C. The fMWCNTs were then placed intoan electrochemical cell containing 4 ml of 5.0 mM ZrOCl2 in0.05 M PBS solution (pH 7). Ten consecutive CV cycles wererecorded in the potential range 0.7 to �1.1 V at a scan rate of20 mV s�1 to obtain stable voltammograms.25 The resultingfMWCNTs/ZrO2 lm-modied GCE was then rinsed withdouble-distilled water and used for further electrochemicalstudies.

3. Results and discussion3.1. Electrochemical characterization

Fig. 1 shows the CV process for the deposition of ZrO2 nano-particles on the fMWCNT-modied GCE. Fig. 1 shows that anobvious reduction peak appeared during the rst cycle, indi-cating the reduction of ZrOCl2. During normal electrodeposi-tion, a linear increasing current is observed with repeatedscanning. The steep increase in the cathodic and anodiccurrents in the potential range 0.7 to �1.1 V corresponds to the

Anal. Methods, 2014, 6, 4686–4691 | 4687

Fig. 2 Scan rate studies for the fMWCNTs/ZrO2 nanocomposite-modified GCE at pH 7. The scan rate varies from 0.01 to 0.1 V s�1. Insetshows a current versus scan rate plot at pH 7.

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complex redox behavior of ZrOCl2 on the fMWCNT-modiedsurface. The formation of ZrO2 on the electrode surface can beexpressed by the following reaction mechanism:

ZrOCl2 + e� / ZrOCl + Cl� (1)

2ZrOCl + 2H2O / 2ZrO2 + 2HCl + H2 (2)

In the next step, the prepared fMWCNTs/ZrO2 nano-composite-modied electrode was transferred into PBS (pH 7)for scan rate studies. Fig. 2 shows the cyclic voltammogramsobtained at the fMWCNTs/ZrO2 composite electrode in N2-saturated PBS solution (pH 7) at different scan rates. The peakcurrents (Ipa) increased linearly with an increase in scan ratebetween 0.01 and 0.1 V s�1. This indicated that the electrontransfer process occurring at the fMWCNTs/ZrO2 compositelm is a surface-conned process. The peak current versus scanrate plot is shown as the inset to Fig. 2. The Ipa shows a linearrelationship with scan rate (R2 ¼ 0.998).

3.2. EIS investigation of electrochemical behavior of variouslm-modied electrodes

The Nyquist plots of the modied lms were studied using EIS.Randles' equivalent circuit model was used to t experimentalparameters such as the electron transfer resistance (Ret), the

Fig. 3 Electrochemical impedance spectra of fMWCNTs/ZrO2 nano-composite-modified GCE (line a) and bare GCE (line b) in 5 mM[Fe(CN)6]

3�/4� in PBS (pH 7).

4688 | Anal. Methods, 2014, 6, 4686–4691

solution resistance (Rs) and the double layer capacity (Cdl). Fig. 3shows the real and imaginary parts of the electrochemicalimpedance spectra for the bare GCE and the fMWCNTs/ZrO2

nanocomposite-modied GCE recorded in 5 mM [Fe(CN)6]3�/4�

in PBS (pH 7). The semicircles obtained at lower frequenciesindicate the diffusion-limited electron transfer process and athigher frequencies the charge transfer limited process. Fig. 3(line a) represents the Nyquist plot for the fMWCNTs/ZrO2

nanocomposite lm. The fMWCNTs/ZrO2 nanocomposite-modied GCE shows a small semicircular region with a very lowelectron transfer resistance value (Ret ¼ 50 Z0 U), indicating therapid electron transfer of the lm due to the high conductivityof the fMWCNTs. Fig. 3 (line b) (the bare GCE) shows a largesemicircle with a high electron transfer resistance value (Ret ¼175 Z0 U). These results show that the small semicircular regionof the fMWCNTs/ZrO2 nanocomposite lm has a very goodelectrochemical activity compared with the bare GCE. Thecomposite lm could therefore be efficiently used for variouselectrocatalytic reactions.

3.3. Effect of pH

The effect of pH on the fMWCNTs/ZrO2 nanocomposite-modi-ed electrode was investigated in PBS at different pH valuesranging between 1 and 13 (Fig. 4). As shown in Fig. 4A, theanodic and cathodic peak potentials were shied towards the

Fig. 4 (A) Cyclic voltammogram of ZrO2 nanoparticles incorporatedinto fMWCNT nanocomposite-modified GCE under deoxygenatedconditions at different pH values ranging from 1 to 13. The inset showsthe influence of solutions at different pH values versus potential. (B)Cyclic voltammogram of fMWCNT/ZrO2 nanocomposite-modifiedGCE in solutions of 25 mM p-NP at different pH values.

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negative side with increases in the pH of the solution (from 1 to13). The plot of pH versus E0 (inset) shows the linear dependenceover the whole pH range. The linear regression equation is E0

(V) ¼ 0.3957 (V) � 0.056 pH (V pH�1). The value of the slope ofthe plot,�0.056 V pH�1, is close to the theoretical value (�0.058V pH�1) for reversible processes with equal numbers of elec-trons and protons. The fMWCNTs/ZrO2 nanocomposite-modi-ed electrode shows well-dened, stable and enhanced redoxpeaks at pH 7 (inset, Fig. 4); we therefore selected pH 7 for allour electrocatalytic experiments.

Fig. 4B shows the cyclic voltammogram of the catalyticreduction of p-NP (25 mM) in PBS with different pH values at ascan rate of 50 mV s�1. An enhanced reduction peak wasobserved at pH 7 in the PBS solution. This reduction peakpotential shied towards a more negative potential in solutionswith a basic pH. The peak potential moves towards a morepositive value in acidic solutions. A very small reduction peakwas observed at lower pH values (pH 1). These results indicatethat the p-NP reduction reaction was pH dependent. The linearregression equation based on the calibration plot was E0 (V) ¼0.291 (V)� 0.052 pH (V pH�1). Hence, we decided to carry out allthe electrocatalytic studies at pH 7.

3.4. Morphological studies

Fig. 5 shows the FESEM images of fMWCNTs, ZrO2 andfMWCNTs/ZrO2 coated on an indium-tin oxide (ITO) electrode.Fig. 5A shows the tubular network of bundles of fMWCNTs witha uniform thickness of 3–4 nm. Fig. 5B shows the sponge-likeZrO2 nanoparticles coagulated with each other over the ITOelectrode surface. Fig. 5C shows an image of fMWCNTs/ZrO2

nanoparticles on the ITO substrate; the ZrO2 nanoparticles arehomogenously incorporated throughout the tubular network ofthe nanotubes and the nanoparticles are almost uniformlydistributed throughout the nanotube layer. It is clearly evident

Fig. 5 FESEM images of (A) fMWCNTs, (B) ZrO2 and (C) fMWCNTs/ZrO2 films at different magnifications.

This journal is © The Royal Society of Chemistry 2014

from these results that the electrodeposited ZrO2 nanoparticleswere uniformly incorporated onto the fMWCNTs.

3.5. Voltammetric determination of p-NP

The fMWCNTs/ZrO2 nanocomposite-modied GCE was able tobe used directly for the voltammetric detection of p-NP. It can beseen that, relative to the bare GCE, the fMWCNTs/ZrO2 nano-composite-modied GCE (Fig. 6A) shows well-dened electro-catalytic peaks for the detection of p-NP (peak a). Modicationwith fMWCNTs alone gave separate peaks which were lessdistinct than those seen using the fMWCNTs/ZrO2 lm (peak b).It is worth noting that the fMWCNTs/ZrO2-modied GCE showsreduction peak potentials for p-NP roughly 150 and 180 mVlower than those for the ZrO2 GCE (peak c) and the bare GCE(peak d). These CV results clearly show the capability of theproposed fMWCNTs/ZrO2 lm for the detection of p-NP. Thismay be due to the presence of ZrO2 nanoparticles which act aselectroactive centres for the detection and determination ofthese compounds. In addition, the use of fMWCNTs as anotherlayer on the surface clearly supports surface enhancement andthe selective detection of p-NP. Scheme 1 illustrates the fabri-cation of the fMWCNTs/ZrO2 nanocomposite lm on the elec-trode surface.

In the next step, LSV was used for the selective detection ofp-NP. Fig. 6B shows the LSV curves of p-NP for various

Fig. 6 (A) Cyclic voltammograms obtained for (a) fMWCNT/ZrO2, (b)fMWCNT, (c) ZrO2 and (d) bare GCEs at pH 7 in a solution containing25 mM p-NP at a scan rate of 50 mV s�1. (B) LSV of fMWCNT/ZrO2

nanocomposite-modified electrode for different concentrations ofp-NP (laboratory sample) at pH 7. Inset shows current versusconcentration plot for p-NP.

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Scheme 1 Schematic representation of single-step fabrication of thefMWCNTs/ZrO2 nanocomposite-modified electrode.

Fig. 7 LSV obtained using fMWCNTs/ZrO2 nanocomposite-modifiedGCE for the detection of p-NP in (A) tap water and (B) river water. Theinsets in (A) and (B) show current versus concentration plot for p-NP(real samples).

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concentrations of the fMWCNTs/ZrO2 nanocomposite-modiedelectrode at 50 mV s�1. The reduction peak current occurs at apotential of about �0.65 V, corresponding to the reduction ofp-NP. The reduction peak currents of p-NP increase linearly withincreasing concentrations of p-NP in the range 2–26 mM. Theseresults show that the fMWCNT/ZrO2 nanocomposite-modiedGCE has specic electrocatalytic activity towards p-NP, whichmay be the main reason for the successful detection. Basedon the calibration graph, the linear regression equation forp-NP can be expressed as I (mA) ¼ 0.8527C (mM) + 12.796(R2 ¼ 0.9902). The linear response shows the stability andpromising electrocatalytic application of the proposed lm. TheCV and LSV results indicate that the fabricated fMWCNTs/ZrO2

nanocomposite-modied GCE can be used for the voltammetricdetermination of p-NP without any fouling effects. A compar-ison of the analytical parameters of the fMWCNTs/ZrO2 nano-composite-modied GCE with other p-NP sensors is given inTable 1.

3.6. Determination of p-NP in different water samples

The fMWCNTs/ZrO2 nanocomposite-modied GCE has beenused for the determination of p-NP in water samples. Riverwater collected from a nearby area and tap water collected fromthe laboratory tap were used for the analysis of real samples.The freshly collected water samples (pH 6.7) were ltered

Table 1 Comparison of the electrochemical detection of p-NP using va

Modication method Detection limit (mM) Linear ra

Nanogold/GCEa 8 10–1000Poly(propyleneimine)–gold/GCE 0.45 0.61–625Nanoporous gold — 0.25–10 mMWCNT/GCEb 0.4 2–4000Nano-Cu2O/Pt electrode 0.1 10–1000Silver particles/GCE 0.5 1.5–140fMWCNTs/ZrO2/GCE

c 0.03 2–26

a GCE: glassy carbon electrode. b MWCNT/GCE: multiwalled carbon nancarbon nanotube.

4690 | Anal. Methods, 2014, 6, 4686–4691

several times using Whatman lter paper (grade 1) and thenused in further experiments. Fig. 7A shows the LSV for thedetermination of p-NP in tap water using the fMWCNTs/ZrO2

nanocomposite-modied GCE at a scan rate of 50 mV s�1. Foreach addition of tap water containing various concentrations(0–20 mM) of p-NP, the reduction peak increased gradually.Similarly, the addition of river water samples (0–16 mM) for thedetection of p-NP is shown in Fig. 7B. The current versusconcentration plot for p-NP in water samples is shown in theinsets of Fig. 7A and B. The dependence of the LSV response inthe linear range on the concentration of p-NP in tap water isexpressed as I (mA) ¼ 0.3307C (mM) + 7.4549 (R2 ¼ 0.982); forriver water this is I (mA)¼ 0.152 (mM) + 8.510 (R2 ¼ 0.977). Basedon these results, it is clear that the fabricated GCE can detect p-NP in real samples.

rious types of modified electrodes

nge (mM) Correlation coefficient Stability Reference

— 95% (7 days) 260.9994 — 27

g dm�3 0.9941 — 280.9965 — 290.9985 — 300.9800 — 310.9902 98% (7 days) This work

otube/glassy carbon electrode. c fMWCNT: functionalized multiwalled

This journal is © The Royal Society of Chemistry 2014

Fig. 8 (a–g) CV response of fMWCNTs/ZrO2 nanocomposite-modi-fied GCE in PBS (pH 7) over a 10 day period when the sample wasstored in the open air at room temperature.

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3.7. Repeatability, reproducibility and stability studies

The repeatability of the fMWCNTs/ZrO2 nanocomposite-modi-ed lm for the detection of p-NP was evaluated using LSV. TheLSV was recorded in PBS (pH 7) at a scan rate of 100 mV s�1 inthe presence of p-NP at 20 mM. The fabricated GCE shows goodrepeatability with a relative standard deviation (RSD) of 4.5%(n¼ 10). It also shows good reproducibility, with an RSD of 3.5%for ve successive individual measurements. The repeatabilityand reproducibility values conrmed that the fabricated lmwas suitable for the detection of p-NP. The stability of the lmwas examined by storing it in the open air at room temperature(Fig. 8). Aer 10 days it continued to show stable behavior, withonly a gradual decrease (5%) from the initial current values.These results show that the modied electrode has goodrepeatability and reproducibility for the detection of p-NP.

4. Conclusions

In conclusion, we have successfully fabricated a fMWCNTs/ZrO2

nanocomposite lm by a simple electrodeposition method. Thefabricated nanocomposite lm was characterized by FESEM,which showed that the as-prepared ZrO2 nanoparticles werestrongly incorporated into the fMWCNT GCE. The electro-chemical activities of the fMWCNTs/ZrO2 nanocomposite lmwere examined using CV and EIS. The nanocomposite-modiedGCE has a highly electroactive surface area, which is well suitedto the voltammetric detection of p-NP. The proposedfMWCNTs//ZrO2 nanocomposite-modied GCE remarkablysuppressed interference effects and showed well-denedreduction peaks for the determination p-NP. The fMWCNTs/ZrO2 nanocomposite-modied GCE was found to be able todetect p-NP in tap and river water samples.

Acknowledgements

This work was supported by the Ministry of Science and Tech-nology, Taiwan (ROC).

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