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Electrochemical oxidationdetermination and voltammetricbehaviour of 4-nitrophenol based onCu2O nanoparticles modified glassycarbon electrodeHuanshun Yin a b , Yunlei Zhou c , Shiyun Ai a , Qiang Ma a ,Lusheng Zhu b & Linan Lu da College of Chemistry and Material Science, ShandongAgricultural University , Taian , 271018, Shandong , PR Chinab College of Resources and Environment, Shandong AgriculturalUniversity , Taian , 271018, Shandong , PR Chinac College of Life Science, Beijing Normal University , 100875,Beijing , Chinad School of Chemistry and Chemical Engineering, Liaoning NormalUniversity , Dalian , 116029, Liaoning , PR ChinaPublished online: 02 Nov 2011.

To cite this article: Huanshun Yin , Yunlei Zhou , Shiyun Ai , Qiang Ma , Lusheng Zhu & Linan Lu(2012) Electrochemical oxidation determination and voltammetric behaviour of 4-nitrophenol basedon Cu2O nanoparticles modified glassy carbon electrode, International Journal of EnvironmentalAnalytical Chemistry, 92:6, 742-754, DOI: 10.1080/03067319.2010.520123

To link to this article: http://dx.doi.org/10.1080/03067319.2010.520123

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Intern. J. Environ. Anal. Chem.Vol. 92, No. 6, 15 May 2012, 742–754

Electrochemical oxidation determination and voltammetric

behaviour of 4-nitrophenol based on Cu2O nanoparticles modified

glassy carbon electrode

Huanshun Yinab, Yunlei Zhouc, Shiyun Aia*,Qiang Maa, Lusheng Zhub* and Linan Lud

aCollege of Chemistry and Material Science, Shandong Agricultural University, Taian, 271018,Shandong, PR China; bCollege of Resources and Environment, Shandong Agricultural

University, Taian, 271018, Shandong, PR China; cCollege of Life Science, Beijing NormalUniversity, 100875, Beijing, China; dSchool of Chemistry and Chemical Engineering,

Liaoning Normal University, Dalian, 116029, Liaoning, PR China

(Received 24 December 2009; final version received 19 August 2010)

Cu2O nanoparticles (nano-Cu2O) modified glassy carbon electrode (GCE) wasfabricated and used to investigate the electrochemical behaviour of 4-nitrophenol(4-NP) by cyclic voltammetry (CV), chronoamperometry (CA), chronocoulome-try (CC) and differential pulse voltammetry (DPV). Compared with GCE, aremarkable increase in oxidation peak current was observed. It indicates thatnano-Cu2O exhibits remarkable enhancement effect on the electrochemicaloxidation of 4-NP. Under the optimised experimental conditions, the oxidationpeak currents were propotional to 4-NP concentration in the range from1.0� 10�6 to 4.0� 10�4mol L�1 with a detection limit of 5.0� 10�7mol L�1

(S/N¼ 3). The fabricated electrode presented good repeatability, stability andanti-interference. Finally, the proposed method was applied to determine 4-NP inwater samples. The recoveries for these samples were from 94.60% to 105.5%.

Keywords: Cu2O nanoparticles; 4-Nitrophenol; electrochemical oxidation; deter-mination; glassy carbon electrode

1. Introduction

In recent years, the ever-increasing number of organic toxic compounds being detected inhuman environment has raised attention about the contamination of environmentresources. Among various toxic compounds, nitrophenols are widely presented andpersistent in the environment, especially in waters. Many of these compounds havepotential to provoke carcinogenesis and mutagenesis in a grade that are considered to bepriority toxic pollutants by US Environmental Protection Agency (EPA) [1].4-Nitrophenol (4-NP) is amongst the highly hazardous and toxic phenols, which iswidely used as intermediate in the production of pesticides, herbicides, medicines and dyes.In the process of preparation and application of these compounds, 4-NP will be inevitablyreleased into the environment and cause severe environmental contamination. Thus, theinterest on its determination in environmental samples has led to the development ofseveral quantification methods, such as spectrophotometry [2], gas chromatography [3],

*Corresponding authors. Email: ashy@sdau.edu.cn; lushzhu@sdau.edu.cn

ISSN 0306–7319 print/ISSN 1029–0397 online

� 2012 Taylor & Francis

http://dx.doi.org/10.1080/03067319.2010.520123

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liquid chromatography [4], capillary electrophoresis [5] and electrochemical sensor [6–8].Among various determination techniques, electrochemical analysis should be a promisingdetermination technique with the advantage of simple operation, fast response, cheapinstrument, low consumption, time-saving qualities and in situ determination. Due to thehigh sensitivity and good selectivity, chemically modified electrodes (CMEs) show morepotential application than bare electrodes and attract more and more interest. Forpreparation of CMEs, it is vital to find or synthesise suitable modified material, which canenhance the determination sensitivity, or improve the anti-interference, or decrease thelimit of detection, preferably by simultaneous possessing these abilities. For this purpose,many materials have been investigated to modify electrode for 4-NP determination, suchas NaEon [6], boron [9], room temperature ionic liquid [7], multi-walled carbon nanotubes(MWCNTs) [10], single-walled carbon nanotubes (SWCNTs) [11,12], zeolite [13] andapatite [14]. Some satisfactory results have been obtained. However, there are still somedisadvantages in these reports. For example, the reduction of 4-NP was used in some ofprevious reports [6,7,10–14], which would be interfered by the dissolved oxygen molecule.Moreover, for the oxidation determination of 4-NP at boron-doped diamond (BDD)film electrode, the oxidation potential of 4-NP is more positive (1.52V) [9], whichwill be interferenced by some active substance in the real samples, and the practicalapplication will be limited. Therefore, it is still a challenge to seek the new electrodemodified material.

In recent years, cuprous oxide nanoparticles (nano-Cu2O), a p-type semiconductor, hasattracted more and more attention due to their high surface area, high adsorptivity andexcellent catalytic capability, which is considered to be a promising material in solarenergy [15], lithium ion battery [16], gas sensor [17] and catalysis [18]. Recently, it has beenfound that nano-Cu2O has potential application in CMEs as the electrode modifiedmaterial. For instance, Liu et al. investigated the electrochemical behaviour of dopamineat nano-Cu2O/GCE [19]. Compared with GCE, the oxidation response of dopamine waseffectively increased at nano-Cu2O/GCE, which was attributed to the catalytic activity ofnano-Cu2O. Hua et al. found that nano-Cu2O exhibited a sensitive electrocatalyticresponse for the oxidation of carbohydrates when it was used to modify sol-gel carboncomposite electrode [20]. All the above research proves that nano-Cu2O has potentialapplication in CMEs to enhance the electrochemical response and decrease the detectionlimit.

However, to the best of our knowledge, electrochemical determination of 4-NP bynano-Cu2O modified glassy carbon electrode using the oxidation process has not yet beenreported. Therefore, the main aim of this work is to investigate the electrochemicaloxidation behaviour of 4-NP on the nano-Cu2O modified glassy carbon electrode and itsdetermination using the oxidation signal, not the reduction signal. The proposed methodwas further applied to determine trace amounts of 4-NP in water samples.

2. Experimental

2.1 Reagents and apparatus

Nano-Cu2O (50 nm) was purchased from Beijing Nachen S&T Ltd. 4-NP was purchasedfrom Aladdin Reagent Co., Ltd. (China) and used as received. 0.1mol L�1 4-NP stocksolution was prepared with anhydrous ethanol and kept in darkness at 4�C. Workingsolutions were freshly prepared before use by diluting the stock solution. Phosphate buffer

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solution (PBS) was prepared by mixing the stock solution of 0.1mol L�1 NaH2PO4 and

Na2HPO4. Other chemicals were analytical reagent grade and all solutions were preparedwith redistilled deionised water from quartz.

Electrochemical experiments were performed with CHI660C electrochemical worksta-

tion (Shanghai Chenhua Co., China) with a conventional three-electrode cell. A bare ornano-Cu2O modified GCE (CHI104, d¼ 3mm) was used as working electrode.

A saturated calomel electrode (SCE) and a platinum wire were used as reference electrodeand auxiliary electrode, respectively. The image of scanning electron microscope (SEM)

was obtained at JSM-6360LV (JEOL, Japan). All the measurements were carried out atroom temperature.

2.2 Preparation of nano-Cu2O/GCE

Before modification, a bare GCE was polished with 0.05 mm alumina slurry on micro-clothpads, then washed successively with anhydrous alcohol and redistilled deionised water in

an ultrasonic bath and dried under nitrogen blowing before use. For preparation of the

modified electrode, 1.5mgmL�1 nano-Cu2O solution was first prepared with redistilleddeionised water, and then followed by ultrasonication for 30min to obtain a homoge-

neously dispersed solution. For preparing the modified electrode, 5 mL of nano-Cu2Osuspension was deposited on the fresh prepared GCE surface. After the solvent

evaporated, the electrode surface was thoroughly rinsed with redistilled deionised waterto wash away the unimmobilised modifier and dried under ambient condition. The

obtained electrode was noted as nano-Cu2O/GCE. The modified electrodes were stored at4�C in a refrigerator when not in use.

2.3 Analytical procedure

A certain volume of 4-NP stock solution (or water samples) and 10mL0.1mol L�1 PBS

(pH 6.0) was transferred into an electrochemical cell, and then the three-electrode systemwas installed on it. After an accumulation of 180 s at 0.40V, the cyclic voltammogram was

recorded from 0.40 to 1.20V. The differential pulse voltammogram was recorded from0.50 to 1.20V. The parameters are as follows, increment potential, 0.004V; pulse

amplitude, 0.05V; pulse width, 0.05 s; sample width, 0.0167 s; pulse period, 0.2 s; quiettime, 2 s.

3. Results and discussion

3.1 SEM characteristics of the modified electrode

The typical SEM image of nano-Cu2O/GCE was shown in Figure 1. It can be seen that theGCE was well covered by Cu2O nanoparticles. The porous and ragged three-dimensional

film of nano-Cu2O may allow the 4-NP molecules into the inner of the holes,increasing utilisation of the whole film. Moreover, the effective surface area of GCE

can be increased after electrode modification, which can increase the electrochemicalresponse of 4-NP.

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3.2 Electrochemical oxidation behaviour of 4-NP

The electrochemical oxidation behaviour of 4-NP was investigated at the bare GCE and

nano-Cu2O/GCE by cyclic voltammetry, and the results were shown in Figure 2. No redox

peak was observed in the absence of 4-NP at nano-Cu2O/GCE, indicating that nano-Cu2O

is non-electroactive in the selected potential region. When 4-NP was added into PBS, a

well-defined oxidation peak was obtained at both electrodes after accumulating 180 s at

0.4V, which can be attributed to the oxidation of 4-NP. However, no corresponding

Figure 2. Cyclic voltammograms of GCE (a) and nano-Cu2O/GCE (b,c) in the absence (b) andpresence (a,c) of 0.5mmol L�1 4-NP in 0.1 M PBS (pH 6.0). Scan rate: 100mV s�1. Accumulationtime: 180 s. Accumulation potential: 0.4V. Inset: Multiple scanning of 0.5mmol L�1 4-NP atnano-Cu2O/GCE.

Figure 1. SEM images of nano-Cu2O/GCE.

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reduction peak was observed in the following reverse scan from 1.4 to 0.4V, indicatingthat the oxidation of 4-NP is a totally irreversible electrode process under the aboveexperimental conditions. Compared with GCE, the oxidation peak current of 4-NPobtained at nano-Cu2O/GCE significantly increased. It is not difficult to believe that thisphenomenon should be attributed to the immobilised porous Cu2O nanoparticles, whichimprove the adsorbance amount of 4-NP onto the electrode surface and increase theelectrochemical oxidation response. This was further confirmed in section 3.4. However, itwas also found that the oxidation peak current of 4-NP decreased remarkably during thesuccessive cyclic voltammetric sweeps. After the second sweep, the peak current decreasedsignificantly, and the oxidation peak almost disappeared completely after the third sweep(inset of Figure 2). This phenomenon can be attributed to the oxidative products of 4-NP,or the polymers formed by the oxidative products, which deposited on the electrodesurface to block further access of 4-NP to the electrode, after original oxidation of 4-NP.Thus, in order to obtain the good sensitivity, the oxidation peak current in the first anodicsweep was recorded for 4-NP analysis in the following studies.

3.3 Optimisation of detection

Figure 3(a) showed the effect of the immobilised nano-Cu2O amount on the oxidationpeak current of 0.5mmol L�1 4-NP. It can be seen that the oxidation peak current of 4-NP

Figure 3. Effect of nano-Cu2O content (a) pH (b) accumulation time (c) and accumulation potential(d) on the electrochemical oxidation response of 4-NP in 0.1mol L�1 PBS.

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significantly increased with increasing the amount of nano-Cu2O from 0.0 to 1.5mgmL�1.When improving the immobilised amount of nano-Cu2O, more and more 4-NP could beadsorbed on the electrode surface. However, the oxidation peak current of 4-NP graduallydecreased when the amount of nano-Cu2O was higher than 1.5mgmL�1, which could becaused by the increase of thick film, decreasing the mass transfer rate, leading to thecurrent response decrease. Therefore, 1.5mgmL�1 nano-Cu2O was used in the followingexperiments.

The effect of solution pH on the peak current and peak potential of 0.5mmol L�1 4-NPwas investigated by cyclic voltammetry and the results were shown in Figure 3(b). It is veryclear that the highest peak current was obtained at pH 6.0. Therefore, this pH was used forfurther measurements. In addition, the maximum response to pH was lower than the pKa

of 4-NP (pKa¼ 7.16), which indicated that the non-dissociated 4-NP can be adsorbedbetter than the dissociated 4-NP on nano-Cu2O/GCE surface under the selectedexperimental conditions. The relationship between the oxidation peak potential (Epa)and pH was also shown in Figure 3(b). A linear shift of Epa towards negative potentialwith an increasing pH indicated that protons were directly involved in the oxidation of4-NP. It obeyed the equation of Epa(V)¼�0.048pHþ 1.24 (R¼ 0.9973). A slope of48mV/pH suggests that the number of electron transfer is equal with that of hydrogen ionstaking part in the electrode reaction [21].

Figure 3(c) and (d) illustrated the influence of accumulation time and accumulationpotential on the oxidation peak current of 0.25mmol L�1 4-NP. As can be seen inFigure 3(c), the oxidation peak current of 4-NP increased linearly with increasing theaccumulation time from 0 to 180 s. Then, the oxidation peak current of 4-NP increasedslightly when further extending accumulation time. With long accumulation time, moreand more 4-NP could be accumulated at the surface of nano-Cu2O/GCE. However, theaccumulation amount of 4-NP will increase very slightly when the amount of 4-NP isclosed to saturation on the electrode surface. Therefore, the oxidation response increasedslightly. Considering both sensitivity and work efficiency, the optimal accumulation timeof 180 s was employed in the further experiments. On the other hand, with accumulationpotential shifting from �0.10 to 0.40V at a fixed accumulation time of 180 s, the oxidationpeak current increased remarkably. Then, the decrease of peak current was observed withfurther positive shift of accumulation potential. Thus 0.40V was selected as optimalaccumulation potential.

The effect of scan rate on the oxidation of 4-NP was also investigated. Figure 4(a)showed the cyclic voltammograms of 0.5mmol L�1 4-NP at nano-Cu2O/GCE withdifferent scan rates. As can be seen in the inset of Figure 4(a), the oxidation peak currentincreased linearly with the square root of scan rate in the range of 20–300mV s�1 and canbe expressed as: Ipa(mA)¼�1.69v1/2 (mV s�1)þ 1.84 (R¼ 0.9979), which indicates that theoxidation of 4-NP at nano-Cu2O/GCE is a diffusion-controlled process. It seemed to beinconsistent with the accumulation characteristics of 4-NP mentioned above. We think thisresult can be attributed to the mixed mass transport regime. For proving it, therelationship of log Ipa versus logv was constructed in the range of 20 to 300mV s�1

(Figure 4(b)). The regression equation is log Ipa(mA)¼ 0.56 log v(mV s�1)þ 0.041(R¼ 0.9982). The slope of 0.56 is between the value of 0.5 associated with the semi-infinite diffusion of the electroactive species to the electrode and the value of 1 expected foran adsorbed electroactive species. This result indicated a mixed mass transport regime,with thin-layer diffusion being within the nano-Cu2O layer, and semi-infinite diffusionbeing outside the layer in solution [22,23]. From the SEM image of the modified electrode,

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the electrode surface can be thought of as a porous layer in which pockets of solution aretrapped in between multiple layers of nano-Cu2O. In the thin-layer cell, when 4-NP istrapped within the porous nano-Cu2O layer, the oxidation current is drawn as the electrontransfer becomes kinetically and thermodynamically favourable. However, since there is avery small amount of 4-NP available, it rapidly becomes depleted of 4-NP on the timescaleof the potential sweep, and the oxidation current decays simultaneously as 4-NP isexhausted within the thin-layer [22]. In addition, it also showed the similar electrochemical

Figure 4. (a) Cyclic voltammograms of 0.5mmol L�1 4-NP on nano-Cu2O/GCE with different scanrates. (a) to (i) were 20, 40, 60, 80, 100, 150, 200, 250 and 300mV s�1, respectively. Inset:Dependenceof the oxidation peak current on the square root of the scan rate. (b) The relationship between log Ipaand log v.

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behaviour with that obtained in section 3.2 above. In other words, only an oxidation peakwas observed even at low scan rate of 20mV s�1, suggesting that the electrochemicaloxidation process of 4-NP under selected experimental conditions is totally irreversible.

3.4 Chronoamperometry

Chronoamperometry was performed to evaluate the catalytic rate constant kcat of nano-Cu2O/GCE towards the oxidation of 4-NP based on the following equation [24]:

Icat=IL ¼ �1=2ðkcatc0tÞ

1=2

where Icat is the catalytic current of 4-NP at nano-Cu2O/GCE, IL is the limited current inthe absence of 4-NP and t is the time elapsed. The Chronoamperograms of 4-NP withdifferent concentrations in 0.1 M pH 6 PBS were shown in Figure 5(a). From the slopes ofthe plots of Icat/IL versus t1/2 (inset of Figure 5(a)), the average value of kcat was calculatedto be 347mol�1 L s�1.

The electrochemical effective surface areas of different electrodes were investigated bychronocoulometry using 0.1mM K3[Fe(CN)6] as model complex (the diffusion coefficientD of K3[Fe(CN)6] is 7.6� 10�6 cm2 s�1 [25]) based on the Anson equation of Q(t)¼2nFAcD1/2t1/2/�1/2þQads [26], where Qads is Faradic charge, A is electrochemical effectivesurface area, c is substrate concentration and D is diffusion coefficient. Other symbolshave their usual meanings. In this work, the slope of the linear relationship between Q andt1/2 were 3.915 (GCE) and 8.727 (nano-Cu2O/GCE) mC s�1/2. Therefore, A can becalculated to be 0.13 and 0.29 cm2 for GCE and nano-Cu2O/GCE, respectively. The resultsindicated that the electrode effective surface area was obviously increased after electrodemodification, which could enhance the adsorption amount of 4-NP, and further increasethe oxidation current.

In order to calculate the adsorption amount of 4-NP at GCE and nano-Cu2O/GCE,chronocoulometry was carried out in 0.1 M PBS (pH 6.0) containing 0.5mmol L�1 4-NPand the results were shown in Figure 5(b). After point-by-point background subtraction,the relationships between Q and t1/2 can be expressed as Q(mC)¼�12.74t1/2(s)þ 1.08(R¼ 0.9978) and Q(mC)¼�16.90t1/2(s)þ 1.299 (R¼ 0.9998) for GCE and nano-Cu2O/GCE, respectively. Therefore, based on the equation of Qads¼ nFAD, the adsorptionconcentration, D, can be obtained as 4.3� 10�11 and 2.32� 10�11mol cm�2 for GCE andnano-Cu2O/GCE, respectively. Though the adsorption concentration of 4-NP at nano-Cu2O/GCE is lower than that at GCE, the whole adsorption amount of 4-NP at nano-Cu2O/GCE (6.73� 10�12mol) is higher than that at GCE (5.59� 10�12mol). This resultindicated that the 4-NP adsorbance increased after electrode modification.

3.5 Calibration curve

The relationship between the oxidation peak current (Ipa) and 4-NP concentration atnano-Cu2O/GCE was investigated by differential pulse voltammetry and the results wereshown in Figure 6(a). Under the optimised experiment conditions, Ipa was proportional tothe concentration of 4-NP in the range from 1.0 to 400 mmolL�1. The regression equationcan be expressed as Ipa(mA)¼�0.0175c (mmolL�1)� 0.102 (R¼ 0.9985). The detectionlimit was estimated to be 5.0� 10�7mol L�1 (S/N¼ 3). The 4-NP detection performancesof the proposed method were compared with other electrochemical methods. The results

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were shown in Table 1. It can be seen that the nano-Cu2O/GCE offered a reasonable linearrange for 4-NP detection and the detection limit was lower than some of the previousreports. In addition, the fabrication method of nano-Cu2O/GCE is very simple, whichgreatly benefits the practical application. These also indicated that nano-Cu2O/GCEshould be an excellent platform for 4-NP determination and could be potentially used formonitoring the concentration of 4-NP.

The fabrication reproducibility was evaluated by comparing the oxidation peakcurrent of 10 mmolL�1 4-NP in 0.1M PBS (pH 6.0) at ten nano-Cu2O/GCEs

Figure 5. (a) Chronoamperograms of nano-Cu2O/GCE in 0.1M PBS containing (a) 0, (b) 0.05,(c) 0.2 and (d) 0.5mmol L�1 4-NP. Inset: Plot of Icat/IL versus t1/2. (b) Plot of Q-t curves of the bareGCE (a) and nano-Cu2O/GCE (b) after point-by-point background subtraction in 0.1M PBScontaining 0.5mmol L�1 4-NP. Inset: plot of Q-t1/2 curves obtained at the bare GCE (a) and nano-Cu2O/GCE (b).

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prepared independently. It is found that the relative standard deviation (RSD) is only3.72%, revealing that this method possesses good reproducibility. The stability of nano-Cu2O/GCE was also studied under the same conditions to reproducibility investigation.The electrodes were stored at 4�C in a refrigerator and their response to 10 mmolL�1 4-NPwas measured every 10 days. The prepared electrode retained about 94% of its initialcurrent response after 10 days and the response decreased to about 85% of the initial valueafter 30 days. Hence, the stability was acceptable.

The influences of some other substances on the oxidation signal of 4-NP were listed inTable 2. As expected, a great number of inorganic ions, such as 500-folds concentration of

Figure 6. Differential pulse voltammograms of nano-Cu2O/GCE in 4-NP solution with differentconcentrations. a-m: 0, 1, 5, 10, 20, 30, 40, 50, 80, 100, 200, 300 and 400mmolL�1. Inset: plot of thepeak current against the concentration of 4-NP.

Table 1. Performance comparison of the proposal electrode for 4-NP detection with otherelectrodes.

Electrode MethodLinear range(mmolL�1)

LODs(mmolL�1) Ref.

Nano-Cu2O/GCE DPV 0.25–150 0.5 This workNaEon/GCE DPV 20–230 17.1 [6]Pt/PAZ modiEed electrode SWV 30–90 8.23 [27]BDD film electrode DPV 50–1000 1.38 [9]RTIL-CPE CV 3–800 0.7 [7]MWCNTs/GCE LSV 2–4000 0.4 [10]Zeolite-CPE DPV 1.44–72 0.288 [13]Ti/TiO2/Au/HRP-MBelectrode

Amperometry 0.3–140 0.11 [8]

MWNT–Nafion/GCE DPV 0.1–10 0.04 [28]SWCNH/GCE LSV 0.05–10 0.011 [11]Apatite-CPE SWV 0.2–100 0.008 [14]SWNT/GCE DPV 0.01–5 0.0025 [12]

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Kþ, Mg2þ, Ca2þ, Ba2þ, Zn2þ, Cu2þ, Ni2þ, Al3þ, Fe3þ, F�, Cl�, NO�3 , SO2�3 , CO2�

3 , SO2�4 ,

have no influences on 4-NP signal, with deviations below 5%. Moreover, several phenoliccompounds were also tested for their effects on the 4-NP signal. It was found that100-folds concentration of 2,4-dinitrophenol, pyrocatechol, hydroquinone, 2-nitrobenzoicacid, 3-nitrobenzoic acid and 4-nitrobenzoic acid did not interfere with the oxidationsignal of 4-NP. However, 100-folds concentration of phenol, hydroxyphenol and2-nitrophenol were found to influence the determination of 4-NP. Nonetheless, theinterference investigation shows very good selectivity for the proposed electrode.

3.6 Determination of 4-NP in water samples

To evaluate the applicability of the proposed method, the recovery of 4-NP wasdetermined in river water and lake water, which were collected from Taian district (China).Before determination, the sample was filtered and the filtrate was collected. Then 1mL ofwater sample solution was added into 9mL PBS. Because no signals for 4-NP wereobserved when the samples were analysed, the standard addition method was used for the

Table 2. Influences of the coexisting substances on the determina-tion of 0.01mmol L�1 4-NP (n¼ 4).

Coexistingsubstance

Concentration(mmol L�1)

Relativeerror (%)

Kþ 5.0 2.23Mg2þ 5.0 1.43

Ca2þ 5.0 2.34

Ba2þ 5.0 2.25

Zn2þ 5.0 0.35

Cu2þ 5.0 1.69

Ni2þ 5.0 0.81

Al3þ 5.0 1.87

Fe3þ 5.0 3.58F� 5.0 3.79Cl� 5.0 1.37NO�3 5.0 2.23

CO2�3 5.0 3.36

SO2�3 5.0 4.57

SO2�4 5.0 1.92

Phenol 1.0 65.722-Nitrophenol 1.0 41002,4-Dinitrophenol 1.0 4.18Pyrocatechol 1.0 3.87Hydroxyphenol 1.0 29.13Hydroquinone 1.0 0.922-nitrobenzoic acid 1.0 2.863-nitrobenzoic acid 1.0 4.514-nitrobenzoic acid 1.0 3.74

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analysis of the prepared samples. Each sample solution was determined three times. Theresults were summarised in Table 3. The value of RSD is between 2.38% and 4.92%,indicating good reproducibility. In addition, the recovery of 4-NP was also determined,and the results are in the range from 94.60% to 105.5%, revealing that this method iseffective and reliable.

4. Conclusion

In this work, the electrochemical oxidation behaviour of 4-NP was investigated at nano-Cu2O/GCE. The results indicate that the electrochemical response of 4-NP could befacilitated by nano-Cu2O film due to its high surface area, high adsorptivity and excellentcatalytic capability. In addition, the fabrication process was easy and simple. The practicalapplication in determining water sample was satisfactory with the recovery ranging from94.60 to 105.5%.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 20775044) andthe Natural Science Foundation of Shandong province, China (Y2006B20).

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Table 3. Determination of 4-NP in water samples.

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Recovery(%)

River water (Nai river) 10 9.78 3.45 97.8020 19.67 2.83 98.3530 31.25 3.61 104.240 38.43 4.92 96.07

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