Electrocatalytic Oxidation of Hydrogen Peroxide on Poly(m-toluidine)-Nickel Modified Carbon Paste Electrode in Alkaline Medium

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Electrocatalytic Oxidation of Hydrogen Peroxide onPoly(m-toluidine)-Nickel Modified Carbon Paste Electrode inAlkaline MediumReza Ojani,* Jahan-Bakhsh Raoof, Roghaieh Babazadeh

Electroanalytical Chemistry Research Laboratory, Department of Analytical Chemistry, Faculty of Chemistry, MazandaranUniversity, Babolsar, Iran*e-mail: [email protected]

Received: January 28, 2009Accepted: December 23, 2009

AbstractThe poly(m-toluidine) film was prepared by using the repeated potential cycling technique in an acidic solution at thesurface of carbon paste electrode. Then transition metal ions of Ni(II) were incorporated to the polymer by immersionof the modified electrode in a 0.2 M NiSO4, also the electrochemical characterization of this modified electrodeexhibits stable redox behavior of the Ni(III)/Ni(II) couple. The electrocatalytic ability of Ni(II)/poly(m-toluidine)/modified carbon paste electrode (Ni/PMT/MCPE) was demonstrated by electrocatalytic oxidation of hydrogenperoxide with cyclic voltammetry and chronoamperometry methods in the alkaline solution. The effects of scan rateand hydrogen peroxide concentration on the anodic peak height of hydrogen peroxide oxidation were alsoinvestigated. The catalytic oxidation peak current showed two linear ranges with different slopes dependent on thehydrogen peroxide concentration and the lower detection limit was 6.5 mM (S/N¼ 3). The catalytic reaction rateconstant, (kh), was calculated 5.5� 102 M�1 s�1 by the data of chronoamperometry. This modified electrode has manyadvantages such as simple preparation procedure, good reproducibility and high catalytic activity toward thehydrogen peroxide oxidation. This method was also applied as a simple method for routine control and can beemployed directly without any pretreatment or separation for analysis cosmetics products.

Keywords: Electrocatalytic oxidation, Poly(m-toluidine), Hydrogen peroxide, Ni/PMT/MCPE

DOI: 10.1002/elan.200900068

1. Introduction

Development of a highly selective and sensitive determi-nation system for hydrogen peroxide is one of the topicalproblems in analytical chemistry [1], since H2O2 is animportant analytical target in the field of biochemistry [2],clinical chemistry [3, 4], food chemistry [5] and environ-mental chemistry [6 – 8]. Detection of hydrogen peroxidewhich is a by product in an enzymatic reaction is importantin the field of biosensor fabrication [9].

Many methods such as spectrophotometry [10 – 13],fluorimetry [13 – 15], fluorescence [16], chemiluminescence[17 – 20] and electrochemical methods [21 – 24] have beendeveloped for this purpose, and some of them were directedto the analysis of hydrogen peroxide during photodegrada-tion reactions [10 – 11]. Electrochemical methods have beenproved to be an inexpensive and effective way for hydrogenperoxide determination.

The direct reduction or oxidation of hydrogen peroxide atbare electrode is not suited for analytical application due toslow electrode kinetics and high overpotentials required forredox reactions of H2O2 on many electrodes materials. Highoverpotential is necessary for the oxidation of H2O2 (aroundþ0.65 V) on a bare carbon electrode [25]. For this reason,

redox mediators have been widely used in order to decreasethe overpotential and increasing the electron transferkinetics. Different electron transfer mediators such as,cobalt phthalocyanine [26], water soluble days [27], plati-num and iridium [28], vanadium doped zirconias [29],Prussian blue [30], iron hexacyanoosmate(II) [31], nickelSchiff base complex [32] and copper complex [33] have beenused for determination of hydrogen peroxide. Furthermore,immobilization of different peroxidase enzymes has beenused for fabrication hydrogen peroxide biosensors [34 – 37].Although modified electrodes have been shown interestingability toward hydrogen peroxide detection, they alsodisplay many problems related to the immobilization ofthe mediator and its toxicity, low sensitivity and stability, andmediator leakage. In addition, high cost, low reproducibility,and poor repeatability are also the disadvantages of thesesensors. Hence, developing simple and reliable methods forfabrication of novel sensor for hydrogen peroxide detectionhave been always a goal for research groups. Different metaloxide particles and nanoparticles such as; manganeousoxide [38], zirconium oxide [39], tungsten oxide [40], ironoxide [41] and nickel oxide [42, 43] have been successfullyused for immobilization of enzymes and proteins and theirapplications in fabrication of hydrogen peroxide biosensor.

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Furthermore, electrodes modified with other transitionmetals or metal oxides such as; silver [44], platinum [45],gold [46], iridium oxide films [47, 48], rhodium [49], indium-tin oxide [50], ruthenium [51], metal hexacyanoferrate [52],cobalt(II) phthalocyanine – cobalt(II) porphyrin pentamer[53], methylene blue/silicon oxide nanocomposition [54]and cobalt oxide/tetraruthenated cobalt – porphyrin com-posite [55] have been also used for micromolar detection ofH2O2.

Recent research has demonstrated that coating of elec-trode surface with polymeric films is an attractive approachbecause not only enhancing the power and scope of electro-chemically modified electrode [56, 57] but also the electrodesensitivity and selectivity is increased.

Electrochemical polymerization offers the advantages ofreproducible deposition in terms of film thickness andloading, allowing the immobilization procedure of a nickel-based electrocatalyst very simple and reliable [58, 59].

The operation mechanism of such chemically modifiedcarbon paste electrodes depends on the properties of themodifier materials used to promote selectivity and sensitiv-ity towards the target species [60]. The most popularelectrode to study hardly soluble compounds is based on acarbon paste. This kind of electrode is inexpensive andpossesses many advantages such as low background current,wide range of potential application and easy fabrication.Surface renewal and modification are also simple [61].

Previously, we combined the advantageous features ofpolymer modification, dispersion of metallic particles intoan organic polymer and carbon paste technology byconstruction of a poly(1-naphthylamine)/nickel modifiedcarbon paste electrode for successful electrocatalytic oxi-dation of some carbohydrates [66], and also we constructedcarbon paste electrode modified by nickel ion dispersed intopoly(1,5-diaminonaphthalene) film with capable of chelat-ing nickel ions with an extra free amine group and thisnickel-modified polymeric carbon paste electrode wasuseful toward the electrocatalytic oxidation of methanol inalkaline medium [67].

In the present work, we combine the above mentionedadvantages feature again. So that, the poly(m-toluidine) wasprepared by electropolymerization of aqueous solution ofmonomer at the surface of carbon paste electrode usingalkaline media. Then nickel ions were incorporated into thepolymeric matrix by immersion of polymeric modifiedelectrode in a nickel sulfate solution. Efficiency of thisnickel-modified polymeric carbon paste electrode towardthe electrocatalytic oxidation of hydrogen peroxide wasinvestigated. The high stability and electrocatalytic activityof this new substrate for the hydrogen peroxide oxidation, aswell as its application for the preparation of a new sensor fordetermination of hydrogen peroxide in cosmetics productsare described.

2. Experimental

2.1. Reagent and Materials

M-Toluidine and hydrogen peroxide from Fluka was used asreceived. Sodium hydroxide obtained in analytical gradefrom Merck and used without further purification. Highviscosity paraffin (density: 0.88 g cm�3) from Fluka wasutilized as the pasting liquid for the carbon paste electrode.Graphite powder (particle diameter: 0.1 mm) from Merckwas employed as the working electrode (WE) substrate. Thesolvent used in this work was once distilled water.

2.2. Apparatus

The electrochemical experiments were carried out using apotentiostat/galvanostat (Sama 500-C ElectrochemicalAnalysis System, Sama, Iran) coupled with a Pentium IVpersonal computer to gain the data. A platinum plate wasused as the auxiliary electrode. A carbon paste and a doublejunction Ag jAgCl jKCl (3 M) electrodes were used asworking and reference electrodes, respectively.

2.3. Preparation of the Working Electrode

A mixture of graphite powder and paraffin was blended byhand mixing with a mortar and pestle for preparation ofcarbon paste. The resulting paste was then inserted in thebottom of a glass tube (internal radius: 0.5 mm). Theelectrical connection was implemented by a copper wirelead fitted into the glass tube. A fresh electrode surface wasgenerated rapidly by extruding a small plug of the paste witha stainless steel rod and smoothing the resulting surface onwhite paper until a smooth shiny surface is observed.

3. Results and Discussion

3.1. Preparation of Poly(m-toluidine) Modified CarbonPaste Electrode (PMT/MCPE)

It is well documented in the literature that poly(m-toluidine) (PMT) films could be prepared at the surface ofPt-coated titanium electrode [68]. In this work, electro-polymerization at the surface of CPE using consecutivecyclic voltammetry (for 8 cycles) between 0 and þ1.1 V at50 mV s�1 was performed in 0.1 M perchloric acid contain-ing solution 1 mM m-toluidine (not shown). At the firstcycle in the forward scan, oxidation of monomer occursabout 0.93 V vs. Ag jAgCl jKClsat and in the reverse scan,one reduction peak appear at the potential 0.33 V vs. Ag jAgCl jKClsat related to the polymer formed, respectively. Atthe second cycle, also one new oxidation peak at thepotential 0.41 V related to the polymer can be observed. Theelectrode was then removed, rinsed with water and the sideswiped with soft tissue paper. The redox behavior of the

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produced film was strongly dependent on the pH of theelectrolyte solution (Scheme 1). The obtained polymershowed a well defined redox behavior in acidic supportingelectrolyte solution, whereas it behave as a nonelectroactivein alkaline media (0.1 M NaOH) in the potential range from0.1 V to 0.7 V (not shown).

However, the film was not degraded under these exper-imental conditions and the electrode response was recov-ered when the electrode is immersed in acidic supportingelectrolyte solution.

3.2. Incorporation of Ni(II) Ions into Poly(m-toluidine)Film

For incorporate the Ni(II) ions into PMT film, the freshlymodified electrode was placed in the supporting electrolytesolution for 24 hours, to attain more stability. After this timethe potential was cycled from 0.1 to 0.7 V in 0.1 M NaOHsolution, and then it was placed at open circuit in a wellstirred aqueous solution of 0.2 M NiSO4. Accumulation ofnickel ions was carried out by complex formation betweenNi(II) and amine site in the polymer backbone [69, 72], for agiven of time (ta, accumulation time). The polarizationbehavior was examined in 0.1 M NaOH solution using cyclicvoltammetry technique. This technique demonstrated theelectrochemical reactivity of the surface.

3.3. Electrochemical Behavior of Ni(II)/PMT/MCPEElectrodes in Alkaline Solution

Figure 1A shows electrochemical response of PMT/MCPEand Ni/PMT/MCPE in 0.1 M NaOH solution. As it can beseen in Figure 1A, PMT/MCPE system does not show anyelectrochemical response but a well defined redox peaks isobserved for Ni/PMT/MCPE system in the potential rangeof 0.1 – 0.7 V. This peak is attributed to the oxidation ofNi(II) to Ni(III) having a peak potential at 0.48 V andreduction of Ni(III) to Ni(II) with a peak potential at 0.42 V.The charge transferred for the cathodic and anodic process-es (Qa/Qc¼ 1.07) is very similar indicating that the systemcould be chemically reversible. However, the anodic peakcurrent height is some more higher than that the cathodicone. Some authors [73] have proposed tentative explan-ations for the electropolymerization and further electro-chemical behavior of nickel planar complexes in alkalinemedium. According to these reports, the attachment ofnickel complexes to the electrode surface and the electro-chemistry of the resulting polymeric films in alkalinesolutions are related to the oxidation of OH� anions. Ashas been reported [74 – 77], nickel hydroxide may generally

exist in two different crystallographic forms designed a-Ni(OH)2 and b-Ni(OH)2 which are hydrous and anhydrous,respectively. In addition [77, 78], the oxidation of nickelhydroxide gives two other varieties of oxyhydroxides, b andg, which explains the existence of the two reduction peaksduring the backward sweep. The a-form is known to beunstable, and when a-Ni(OH)2 is formed at the initial stageof electrooxidation of the Ni electrode, it is further slowlyconverted to the b-Ni(OH)2. It was reported by Desilvestro

Fig. 1. (A) Electrochemical response of electrodes: (a) PMT/MCPE, (b) Ni/PMT/MCPE in 0.1 M NaOH solution. (B) Electro-chemical response of PMT/MCPE in 0.1 M NaOH solution withv¼ 15 mV s�1 to (a) 0.0 M and (b) 1.0 mM hydrogen peroxide andNi/PNMA/MCPE to (c) 0.0 mM and (d) 1.0 mM, hydrogenperoxide.

Scheme 1. The redox behavior of the PMT film in acidic supporting electrolyte solution.

Electrocatalytic Oxidation of Hydrogen Peroxide

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et al. [79, 80] that both the electrodeposited nickel hydrox-ide and nickel oxyhydroxide phases are believed to be un-stoichiometric. Therefore, the peak potentials change withthe increase of cycling numbers. A similar behavior can beexpected for electrodes modified with films derived frompoly-[Ni(II)-PMT].

The surface coverage of immobilized active substance i.e.Ni(II), in the film was evaluated from charge measurementunder the current-potential curve as exhibited in Fig-ure 1Ab. The value of G (G¼Q/nFA) for Ni/PMT/MCPEsystem was found to be 7.38� 10�2 mol cm�2.

3.4. Electrocatalytic Oxidation of Hydrogen Peroxide atthe Surface of Ni/PMT/MCPE

In the route of this study, at first oxidation of hydrogenperoxide was studied at PMT/MCPE by cyclic voltammetryin 0.1 M NaOH solution in the potential range from 0.1 to0.7 V vs. Ag jAgCl jKCl (3 M). Figure 1B shows the elec-trochemical response of the electrode (a) in the absence ofhydrogen peroxide, (b) in the presence of 1.0 mM hydrogenperoxide. Figure 1Bc shows the electrochemical response ofNi/PMT/MCPE system in alkaline solution which exhibitedwell defined anodic and cathodic peaks. The observed peaksare devoted to Ni(II)/Ni(III) redox couple. As can be seen,upon hydrogen peroxide addition there is an increase in theanodic peak current and an decrease in the cathodic peakcurrent (Fig. 1Bd). This behavior is typical of that expectedfor mediated oxidation as follows:

2 Ni(OH)2þ 2 OH�! 2 NiOOHþ 2 H2Oþ 2 e� E

2 NiOOHþH2O2! 2 Ni(OH)2þ 1/2 O2þH2O C’

According to the above reactions oxidation of hydrogenperoxide undergoes two processes, the first one correspondsto the formation of Ni(III) species with a reversible trans-formation of Ni(OH)2/NiOOH on Ni/PMT/MCPE and inthe second processes in the presence of hydrogen peroxide anew peak which having large peak current is appeared, itmight be concluded from the appearance of the new anodicpeak that hydrogen peroxide takes place after the oxidationreaction (E). In other words, the new anodic peak a2 resultsfrom the anodic oxidation of Ni(OH)2 formed from reaction(C�). Crystallographic form of this Ni(OH)2, represented byb, is different from that of the a-Ni(OH)2 involved in theanodic peak a1. The corresponding electrode reactioninvolved in the anodic peak a2 might be:

b-Ni(OH)2þOH�!NiOOHþH2Oþ e�

Therefore, the Ni(OH)2 produced in reaction (C’) might beb-Ni(OH)2 [81]. This is consistent with different redoxpotentials of a-Ni(OH)2/NiOOH and b-Ni(OH)2/NiOOH[82], i.e. a-Ni(OH)2 is converted to NiOOH at a lowerpotential than b-Ni(OH)2 to NiOOH. It shows that hydro-gen peroxide is oxidized and accompanied by the trans-

formation of NiOOH to Ni(OH)2. Therefore NiOOHprobably acts as an electrocatalyst. It can be seen fromreaction (E) and (C’) that NiOOH can be considered as amediator if hydrogen peroxide is present in the solution.

3.5. Effect of Scan Rate on the Anodic Peak Height

Cyclic voltammograms of Ni/PMT/MCPE following theaddition of 10 mM hydrogen peroxide at different scan rateswere shown in Figure 2A. It can be seen from this Figurethat, with increasing scan rate, the peak potential for thecatalytic oxidation of hydrogen peroxide shifts to increas-ingly positive potentials, suggesting a kinetic limitation inthe reaction between the redox sites of the Ni/PMT/MCPEand hydrogen peroxide. However, the oxidation current forhydrogen peroxide increased linearly with the square root ofthe scan rate by linear regression equation y¼ 20.84 xþ1.908, R2¼ 0.998 (not shown), suggesting that the reactionis mass transfer controlled. Also, a plot of the scan ratenormalized current (I V�1/2) vs. scan rate (Fig. 2B), exhibits ashape typical of an EC’ process.

Values of ana (where a is the transfer coefficient and na isthe number of electrons involved in the rate determiningstep) were calculated for the oxidation of hydrogenperoxide at both the modified and unmodified carbon pasteelectrodes, according to the equation [83]:

ana¼ 0.048/Ep�Ep/2

where Ep/2 is the potential corresponding to Ip/2. The valuesfor ana were found to be 0.49 and 0.27 for the oxidation ofhydrogen peroxide at the surface of the modified andunmodified electrodes, respectively. These values clearlyshow that not only is the overpotential for the hydrogenperoxide oxidation reduced at the surface of the Ni/PMT/MCPE but also the rate of the electron transfer process isgreatly enhanced; this phenomenon is confirmed by thelarger Ip values recorded during cyclic voltammetry at theNi/PMT/MCPE.

3.6. Chronoamperometric Studies

Double potential step chronoamperometry as well as otherelectrochemical methods were employed to gain moreinformation about the examined electrochemical processes.Figure 3A shows the current – time curves of Ni/PMT/MCPE at 0.7 (first potential step) and 0.3 V (secondpotential step) vs. Ag jAgCl jKCl (3 M) and at variousconcentrations of hydrogen peroxide.

As can be seen in Figure 3B the forward and backwardpotential step chronoamperometry of the modified elec-trode in blank solution shows an almost symmetricalchronoamperogram having almost equal charge, which isconsumed for the oxidation and reduction of surfaceconfined Ni(II)/Ni(III) sites. Anyway, in the presence ofhydrogen peroxide, the charge value associated with the




forward chronoamperometry, Qf, is greater than of thatobserved for the backward, Qb.

The rate constant for the chemical reaction betweenhydrogen peroxide and redox sites of Ni/PMT/MCPEsystem can be evaluated by chronoamperometry accordingto the methods described in literature [84]:

IC/IL¼ g1/2 [p1/2 erf (g1/2)þ exp(�g1/2)/g1/2]

where, IC is the catalytic current of Ni/PMT/MCPE system inthe hydrogen peroxide presence, IL is the limiting current inthe absence of hydrogen peroxide and g¼ khcot (co is thebulk concentration of hydrogen peroxide) is the argumentof the error function. In the case where g exceeds 2, the errorfunction is almost equal to 1 and the above equation can bereduced to:

IC/IL¼p1/2 g1/2¼p1/2 (khcot)1/2

where, kh and t are the catalytic rate constant (cm3 mol�1 s�1)and time elapsed (s) respectively. From the slope of IC/IL

versus t1/2 plot, the value of kh can be simply calculated for agiven concentration substrate. In fact, plot of IC/IL versus t1/2

gives a straight line (y¼ 4.182 xþ 3.727, R2¼ 0.994) con-structed from the chronoamperograms of Ni/PMT/MCPEin the absence and presence of 4 mM hydrogen peroxide(not shown), that from the slope, the mean value of kh isdetermined to be 5.5� 102 M�1 s�1.

3.7. Electrocatalytic Determination of Hydrogen Peroxide

Figure 4 shows the cyclic voltammograms of the Ni/PMT/MCPE in the presence of hydrogen peroxide. As shown, the

Fig. 2. (A) Cyclic voltammograms of the Ni/PNMA/MCPE inthe presence of 10 mM hydrogen peroxide in 0.1 M NaOHsolution at scan rates: (a) 10, (b) 15, (c) 20, (d) 25, (e) 30, (f) 40, (g)60, (h) 80 and (i) 100 mV s�1. (B) Plot of I v�1/2 vs. v. Fig. 3. (A) Chronoamperogram obtained at the Ni/PNMA/

MCPE in the presence of (a) 0, (b) 4 and (c) 20 mM hydrogenperoxide. (B) Dependence of Q vs. t derived from the data ofchronoamperogram of (a) and (c).




anodic peak current dependence to hydrogen peroxideconcentration in the range of 8� 10�3 – 2� 101 mM. Fig-ure 4A clearly shows that the plot of peak current vs.hydrogen peroxide concentration is constituted from twolinear segments with different slopes, corresponding to two

different ranges of substrate concentration. We ascribe thisto a change in catalytic reaction conditions arising from theformation of oxygen gas bubbles at the surface of themodifier, as has already been reported by Scharf andGrabner [85]. Indeed, at low substrate concentrations, the

Fig. 4. Current – potential curves of Ni/PMT/MCPE for electrocatalytic oxidation of hydrogen peroxide at the scan rate of 15 mV s�1 in0.1 M NaOH solution with different concentrations of hydrogen peroxide (a) 0.0, (b) 0.008, (c) 0.03, (d) 0.05, (e) 0.08, (f) 0.1, (g) 0.4, (h)0.6, (i) 0.8, (j) 1, (k) 4, (l) 6 and (m) 8 mM. Plots of the electrocatalytic current as a function of hydrogen peroxide concentration in theranges of (A) 8 to 400 mM (GEU: gas evolution unaffected zone; GEA: gas evolution affected zone), (B) 0.008 to 0.1 mM, (C) 0.1 to20 mM.




gas formed, being negligible, has no effect on the diffusion ofhydrogen peroxide towards electrode surface [gas evolutionunaffected (GEU) zone]. While, at high concentrations ofhydrogen peroxide, gas evolution at the electrode surfaceslackened to some extent the normal diffusion of substrate[gas evolution affected (GEA) zone]. At concentrationshigher than 0.10 mM, there appears to be a leveling of theresponse, due most probably to the kinetic limitations.Figures 4B and C illustrates the first and second part of thepeak current vs. hydrogen peroxide to a range of 8� 10�3 to1� 10�1 and 1� 10�1 to 2� 101 mM respectively. The linearregression equation for the concentration range from 8�10�3 to 1� 10�1 and 1� 10�1 to 2� 101 mM is obtained as I(mA)¼ 29.40 C (mM)þ 5.27, R2¼ 0.999 and I (mA)¼12.58 C (mM)þ 6.375, R2¼ 0.999 respectively. The detec-tion limit for the range of 8� 10�3 to 1� 10�1 mM and 1�10�1 to 2� 101 mM is estimated to be 6.5 and 2.2� 101 mMrespectively, when the signal to noise ratio is 3.

3.8. Amperometric Detection of Hydrogen Peroxide

Oxidation of hydrogen peroxide at the surface of Ni/PMT/MCPE was also studied by hydrodynamic amperometry,which is one of the most widely employed techniques forbiosensors. In amperometric detection, the potential ap-plied to the working electrode directly affects the sensitivity,detection limit and stability of the detection method.Amperometric experiments were implemented in stirredsolution of 0.1 M NaOH. The potential of the Ni/PMT/MCPE was kept at 0.75 V during successive addition of20 mL of 1 mM hydrogen peroxide. The resulting currentversus time curve for a series of replicate additions ofhydrogen peroxide at a lower concentration range of 7�10�6 M to 2.9� 10�5 M is shown in Figure 5. Inset in Figure 5shows the corresponding calibration plot with a correlationcoefficient of 0.9976. The amperogram recorded clearlyshows that the current response increases steeply onincreasing the concentration of hydrogen peroxide in thesolution. The limit of detection was estimated to be 0.98 mM,

Fig. 5. Amperometric response of the Ni/PMT/MCPE electrode to successive injections of 20 mL of 1 mM stock solution of hydrogenperoxide in 0.1 M NaOH at the applied potential of 0.75 V. Sampling time: 20 s. Inset shows the calibration plot for hydrogen peroxideusing amperometry.

Table 1. Comparison of the efficiency of some mediators used in electrocatalysis of hydrogen peroxide.

Electrode Electrocatalytic mediator pH Electrocatalyticeffect(mV)

LOD (mM) LDR (mM) References

GC Cobalt (II)-tartrate complexes 11.6 – – 700 – 4� 103 [82]CPE Manganese oxide 7.4 200 2 100 – 690 [83]Pt Pt/silver nanoparticles 7 120 1 1.25 – 1� 103 [84]SWCNH paste electrode [a] Activated SWCNH 7 – 50 500 – 1� 105 [85]GC rotating disk electrode Fe3O4 3 – 7.6 Up to 4000 [86]

7 – 7.4 5000CPE PNMA (SDS)/Co 13 400 18 (CV) 30 – 12� 103 [87]CPE Ni (II)/PMT 13 400 6.54 (CV) 8 – 100 Present work

22.4 (CV) 1� 102 – 2� 104

0.98 (CHA) 7 – 29

[a] Singlewalled carbon nanohorn.




based on an S/N¼ 3. However, the electrocatalysis ofhydrogen peroxide in this work has been compared withother research works (Table 1). The results show that Ni/PMT/MCPE film is comparable with other pervious medi-ators.

3.9. Stability and Reproducibility of Ni/PMT/MCPE

We investigated a chronoamperogram with a large timewindow of our modified electrode in presence of hydrogenperoxide (not shown), the decrease in current is relativelyslow and, when the time is above 100 s, the current reaches arelatively stable value, which is still about 52.3% of theinitial current. It is obvious that the Ni/PMT/MCPE exhibitsa high stability toward hydrogen peroxide oxidation. Wealso verified the long-term stability of Ni/PMT/MCPE bymeasuring its response for hydrogen peroxide oxidationafter 1 and 2 weeks of storage in dry conditions. Theelectrode retains 91 and 83% of its initial response,respectively. To examine the reproducibility of the Ni/PMT/MCPE electrode, repetitive measurements were car-ried out in solution containing 2� 10�4 M hydrogen perox-ide. A relative standard deviation of 4.25% was obtainedfor 3 successive measurements. Such stability and reprodu-cibility seems to be acceptable for most practical applica-tions.

3.10. Real Sample Analysis

In order to demonstrate the catalytic oxidation of hydrogenperoxide in a real sample, we examined this ability indetermining hydrogen peroxide in Acid Peroxide Creamthat is used as hair dye liquidator. The determination ofhydrogen peroxide in a sample was carried out by thestandard addition method for prevention of any matrixeffect. Figure 6A shows related voltammetry for thispurpose. As can be seen in this Figure, adding hydrogenperoxide to the solution of 2000 order diluted Acid PeroxideCream by 0.1 M NaOH solution at the surface of Ni/PMT/MCPE (curve b) caused an increase in the oxidation peakheight (curve c to l). Thus, the peak was attributed tohydrogen peroxide oxidation. Also Figure 6B is a diagram ofIpa versus hydrogen peroxide concentration with error barsthat shows the linear region usable for determination ofhydrogen peroxide. By this method hydrogen peroxideconcentration in Acid Peroxide Cream was about 1.45 M.Accuracy was examined by comparison of the data obtainedfrom this method with a recognized common method fordetermination of hydrogen peroxide (mangenometry titra-tion in acidic media by KMnO4). The results from thestatistical calculation indicate good agreement between themean values (t-test) and precision (f-test) for two methods(for p¼ 0.05).

4. Conclusions

A carbon paste electrode modified with a NiOOH film hasbeen prepared in an alkaline solution using cyclic voltam-metry. This Ni/PMT/M CPE can catalyze the oxidation ofhydrogen peroxide via a surface layer mediated chargetransfer. The kinetic process of the catalytic reaction can beexplained using cyclic voltammetry and chronoamperom-etry. The catalytic oxidation peak current showed two linearranges with different slopes dependent on the hydrogenperoxide concentration because of the establishment ofoxygen gas bubbles at the surface of the modifier. The

Fig. 6. (A) Cyclic voltammograms: (a) of the Ni/PMT/MCPE in0.1 M NaOH solution, (b) aþ 2000 order diluted solution ofsample, (c-l) same (b) after adding hydrogen peroxide (c) 0.1, (d)0.2, (e) 0.3, (f) 0.4, (g) 0.5, (h) 0.6, (i) 0.9, (j) 1.2, (k) 1.5 and (l)2.5 mM, respectively, v¼ 15 mV s�1. (B) Plot of Ipa as a function ofadded hydrogen peroxide concentration to sample.




kinetic parameters such as charge transfer coefficient, a, andthe catalytic reaction rate constant, kh, were also determinedusing cyclic voltammetry and chronoamperometry datarespectively. Suitability of this proposed voltammetricmethod for routine analysis of real sample was demonstrat-ed. The fabrication of the Ni/PMT/MCPE electrode thoughsimple, exhibited good stability and reproducibility.

It can be inferred from these results that the presence ofthe dispersion of nickel particles into an organic polymerand carbon paste technology facilitates the detection ofhydrogen peroxide at low concentration level.

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