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Electrochimica Acta 117 (2014) 379– 384

Contents lists available at ScienceDirect

Electrochimica Acta

jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta

se of copper electrode for melamine quantification in milk

illiam R. de Araujo, Thiago R.L.C. Paixão ∗

nstituto de Química - Universidade de São Paulo São Paulo, SP, 05508-900, Brazil

r t i c l e i n f o

rticle history:eceived 18 October 2013eceived in revised form5 November 2013ccepted 27 November 2013vailable online 11 December 2013

eywords:ood adulteration

a b s t r a c t

We report the development of a simple analytical method that offers reagentless sensing capabilitiesby using a copper sensor for monitoring melamine. In this method, the anodic dissolution of the copperelectrode is initiated electrochemically to release copper ions, which form the melamine chloride copperionic pair at the electrode surface. After this process, the melamine chloride copper ionic pair can bereduced electrochemically at the surface of the electrode and quantified under the optimized conditions.This protocol eliminates the necessity to contaminate the sample with a copper salt, making the methodproposed herein more environmentally benign than other techniques. The elucidation of the electro-chemical reduction process of the melamine copper ionic pair was investigated with an electrochemical

lectrochemical sensorainted milkopper electrodeelamine quantification

quartz crystal microbalance (EQCM). The quantification of melamine in milk was performed using differ-ential pulse voltammetry, where the peak current response was found to increase linearly with melamineconcentration over the range 5–90 �mol L−1. The repeatability of the electrode response was evaluated as1% (n = 20), and the detection limit of the proposed method was estimated to be 0.85 �mol L−1 (3�/slope).The accuracy of the proposed method was evaluated using milk samples and an addition and recoveryprotocol.

. Introduction

Melamine (2,4,6-triamino-1,3,5-triazine; C3H6N6; MEL) isdded to raw milk products to enhance the apparent protein con-ent, since the nitrogen content in melamine is 66%. Melamine alsomproves the nitrogen concentration in milk resulting in a falseppearance of a higher level of protein by the Kjeldahl method [1].ince 2007, two cases of milk adulteration involving the addition ofelamine and its analogs ammeline, ammelide, and cyanuric acid

ave been recorded. The first case resulted in a major outbreak ofenal disease, and was associated with the deaths of dogs and cats inhe United States owing to the contamination of pet food products2,3]. In 2008, melamine contamination in milk and infant formu-as was reported, which caused kidney stones and renal failure in

any people; by the end of 2008, approximately 300 thousand peo-le were affected, and some child deaths were also reported. Hence,

simple and reliable method for detecting MEL level in milk is nec-ssary to control this situation, which has become a global healthoncern [2,4].

Several different methods have been reported for

elamine quantification, such as spectrophotometry [5,6],

hemiluminescence [2,7,8], electrochemistry [9–14], and chro-atography with UV or mass spectroscopy (MS) detectors [15–18].

∗ Corresponding author. Tel.: +55 11 30919150.E-mail address: trlcp@iq.usp.br (T.R.L.C. Paixão).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.11.160

© 2013 Elsevier Ltd. All rights reserved.

Most electrochemical methods for melamine quantification usea biosensor (enzyme) and molecular imprinting techniques[9,10,19]. However, the biosensor performance depends on thestability of the enzyme layer, and therefore, recalibration of thesensor is necessary. In addition, the thickness of the enzymelayer can limit the analytical signal and response time becauseof the impeded transport of the analyte through the enzymelayer. Methods that use molecularly imprinted electrochemicalsensors require different steps (polymerization of monomer andtemplate elution steps) for the accurate printing of analyte inthe modified layer to obtain the desired selectivity, making suchmethods complex and time-consuming. Zhu et al. [12] developedan electroanalytical method based on the oxidation of a melaminecopper complex using an electrode modified with multiwalledcarbon nanotubes. In Zhu et al. [12], the authors intentionallyspiked the sample with an excess of copper ions to form an elec-troactive complex through coordination of the copper salt to MELin non-acid experimental conditions. Therefore, the developmentof a cheap, fast, in situ method using a non-modified electrodeand a selective electrochemical method is necessary for analyzingmelamine levels in milk.

In this work, we report our efforts to develop a rapid, inex-pensive, and accurate analysis method based on a non-modified

electrode for the quantification of melamine levels. Copper-baseddisposable devices, which are cheap, practical, and miniaturizable[20], can also be used to develop a portable method for fast in situdetection of melamine; use of copper-based device also makes it

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80 W.R. de Araujo, T.R.L.C. Paixão / El

nnecessary to intentionally add a copper salt to the sample. Thisethod is based on the electrochemical monitoring of the reduc-

ion of the melamine copper chloride ionic pair (MELH+[CuCl2]−),ormed in situ through the dissolution of the copper electrode inhe melamine-containing medium. The proposed technique wassed to quantify the melamine levels in milk samples without the

nterference of organic substances that are normally present in thisatrix.

. Experimental

.1. Chemicals, materials, and samples

All solid reagents were of analytical grade and were used with-ut further purification. Ascorbic acid, sodium phosphate, sodiumetraborate, acetic acid, phosphoric acid, boric acid, potassiumhloride, and sodium hydroxide were obtained from Merck (Darm-tadt, Germany). Melamine was obtained from Sigma–AldrichSteinheim, Germany). Solutions were prepared by dissolving theeagents in deionized water, which was processed with a water-urification system (18.2 M� cm) (Direct-Q® 5 Ultrapure Waterystems, Millipore, MA, USA). The milk samples were purchasedrom local supermarkets.

.2. Electrodes and instrumentation

A �AUTOLABIII (Eco Chemie, The Netherlands), with data acqui-ition software available from the manufacturer (GPES 4.9.007ersion), was used for electrochemical measurements. Homemadeg/AgCl (saturated KCl) [21] electrodes and Pt wires were useds the reference and counter electrodes, respectively. The workinglectrode was made from a copper rod encapsulated in polytetraflu-roethylene. The working electrode surface was polished with anlumina suspension (0.3 mm, Alfa Aesar, MA, USA) on a microloth polishing pad between electrochemical measurements, andas washed thoroughly with deionized water. For the electro-

hemical quartz crystal microbalance experiments, the workinglectrode was a 6-MHz AT-cut piezoelectric quartz crystal with aiameter of 7 mm and a piezo-active electrode area of 0.385 cm2

AUTOLAB). The frequency resonance shift was used to calculateass change by using the Sauerbrey equation [22] �f = –K�m,here the integral sensitivity constant, K (0.0815 ng cm−2 Hz−1),as initially obtained by electrodeposition from a copper sulfate

olution (0.5 mol L−1) in an acidic medium (0.25 mol L−1 H2SO4).his cupric ions solution was also used to obtain the copperlectrodes employed in all experiments; the copper was elec-rodeposited at E = –0.25 V for 10 s. The copper deposition waserified by measuring the mass/charge ratio (3.31 × 10−4 g C−1),hich corresponds to a molecular mass of 63.9 g mol−1 (using the

araday law). This value is in agreement with the theoretical one63.54 g mol−1), indicating the efficacy of copper electrodeposition.his procedure was carried out before each electrochemical exper-ment, with previous copper layers being removed by cleaning thelectrode surface with a cotton swab with a 10% (v/v) HNO3 solu-ion.

.3. Electrochemical analysis

The analytical standard solution of melamine was diluted withulfuric acid solution 0.25 mol L−1. For electrochemical melaminenalysis, differential pulse voltammetry was performed, with the

oltage potential ranging from 0.1 to −0.5 V. The optimum valuesor the step and amplitude were 0.01 V and 0.2 V, respectively, andhe optimal time for preconditioning by applying the 0.1 V potentialor generating cuprous ions in solution, i.e., the melamine copper

imica Acta 117 (2014) 379– 384

ionic pair, was evaluated as 120 s. All the measurements were per-formed without the prior removal of oxygen. The milk sampleswere analyzed directly, without any prior dilution or pretreatment,but with simple addition of melamine to the matrix.

3. Results and discussion

As reported previously in the literature [23–25], the melaminecopper complex could be formed ex situ with copper(I) and (II)salts in alkaline conditions. Zhu et al. [12], using glassy carbon elec-trodes modified with multiwalled carbon nanotubes, showed thatthe copper(I) complex formed ex situ with melamine could be oxi-dized at 0.054 V in alkaline conditions. To eliminate the need tointentionally contaminate the sample with a copper salt and tonot use a chemically modified electrode, we evaluated a proce-dure involving the anodic dissolution of metallic copper to formthe copper melamine ionic pair at the electrode surface in acidconditions. Moreover, we tried to monitor the reduction of the cop-per melamine ionic pair instead of the oxidation of the melaminecopper complex as proposed by the Zhu et al. [12] because of thepossible of the oxidation process of the metallic copper overlappedwith the oxidation of the copper melamine complex in alkalineconditions and non-formation of the melamine copper complexat acid conditions due to a competition with protonation of theamine group. Therefore, cyclic voltammograms were first recordedfrom–0.6 to 0.1 V, and subsequently, the potential was swept inthe reverse potential scan to reduce the copper melamine ionicpair. The forward scan was used to form cuprous ions in the vicin-ity of the electrode surface when the potential required for copperdissolution (E > 0.0 V) was reached, resulting in the formation ofthe melamine copper ionic pair. Using this approach, we recordedcyclic voltammograms in the presence and absence of melamine inelectrolyte solutions with different pH values (Fig. 1).

As shown in Fig. 1 (1A to 5A), no changes were observed in thevoltammetric profile when melamine was added to the solution,indicating that no melamine copper ionic pair was formed. On theother hand, a sharp peak could be observed in the reverse scannear–0.15 V (Fig. 1 (1B)) when chloride was added to the solution.This sharp peak appearing in the presence of chloride is an indi-cation of the formation of the melamine copper chloride ionic pairand its reduction at the copper surface. Even in the presence of chlo-ride, the other pH conditions studied (above pH = 4.6) did not showany indication of the melamine copper chloride ionic pair (Fig. 1(2B to 5B)), demonstrating the necessity of acid conditions (insteadof the analytical conditions reported for the oxidation of melaminecopper complex by Zhu et al. [12]) and the addition of chloride forthe formation of the melamine copper ionic pair.

According Pietrzyk et al. [14], the pKa of melamine in aque-ous solutions is around 5. At the optimal pH conditions (below topKa value), melamine is protonated at the amino group or iminoprotonated form of the acid-base equilibrium in aqueous medium.Birinci et al. [26] article, they proposed a separation of palladiumions of other metals, through the formation of ionic pair betweenchloro-palladate complex and melamine resin in acid or slightlyacid conditions. Additionally, Aydin et al. [27] reported a similarwork with separation and recovery of gold(III), through the samemechanism and reports that the ionic interaction occurs by proton-ated amines group. Copper(I) ion can be classified as a borderlinebetween hard and soft and this cation has properties to react ashard cations via electrostatic interactions due to a gain in entropycaused by changes in orientation of hydration water molecules

[28]. Others works shown the use of melamine resins for chela-tion and extraction of various metals, including copper [26,29].These authors point out that amino resins including N donor atomsas chelating resins have ionic interaction/ion exchange properties

W.R. de Araujo, T.R.L.C. Paixão / Electrochimica Acta 117 (2014) 379– 384 381

F shed

0 1 phosN mog

bc

mstmi(ittwetc

F(0p

ig. 1. Cyclic voltammograms obtained using copper electrode in the absence (da.25 mol L−1 sulfuric acid, (2) 0.10 mol L−1 acetate buffer (pH = 4.6), (3) 0.10 mol L−

aOH (pH = 14). Scan rate: 100 mV s−1. Figures in the right column represent voltam

y protonated amines [26] at ambient temperature and pressureonditions.

The amount of chloride added to the solution was further opti-ized in an attempt to enhance the detectability of melamine. Fig. 2

hows cyclic voltammograms recorded with different concentra-ions of chloride in the solution before and after the addition of

elamine. As seen in Fig. 2, when the concentration of chloriden the solution increases from 0.01 mol L−1 (Fig. 2A) to 0.1 mol L−1

Fig. 2B), the faradaic signal for the melamine copper ionic pairncreases, again indicating the necessity of chloride in the mediumo form the melamine copper chloride ionic pair. However, whenhe concentration increases by a factor of 10 (Fig. 2C) compared

−1

ith that in Fig. 2B (chloride at 0.1 mol L ), the signal for thelectrochemical reduction of melamine decreases. This can be jus-ified by a competition between the formation of a copper chlorideomplex and a melamine copper chloride ionic pair, which may

ig. 2. Cyclic voltammograms obtained using copper electrode in the absencedashed lines) and presence (solid lines) of 1 mmol L−1 melamine. Electrolyte used:.25 mol L−1 sulfuric acid with (A) 0.01 mol L−1 potassium chloride, (B) 0.1 mol L−1

otassium chloride, and (C) 1 mol L−1 potassium chloride. Scan rate: 100 mV s−1.

lines) and presence (solid lines) of 0.5 mmol L−1 melamine. Electrolytes used: (1)phate buffer (pH = 7.2), (4) 0.10 mol L−1 borate buffer (pH = 9.1), and (5) 1 mol L−1

rams (1B–5B) recorded in presence of chloride; final concentration: 0.1 mol L−1.

decrease the electrochemical signal for the melamine copper ionicpair.

To understand the electrochemical process occurring at theelectrode surface, we performed some experiments using an elec-trochemical quartz crystal microbalance (EQCM). Fig. 3A showsa typical voltammetric curve obtained by sweeping the potentialfrom–0.6 V to the positive potential region 0.1 V in the presence ofmelamine and by simultaneously measuring the frequency shiftsas the potential is scanned (�f values were calculated and areshown in Fig. 3B); the plots of �f versus potential in the pres-ence and absence of melamine have been reported. As mentionedabove, when scanning the potential to more positive values, we

observed the electrochemical process for the oxidation of metal-lic copper to cuprous ions (Equation 1) in the electrolyte solution(region 1 in Fig. 3A). In the absence of melamine (Fig. 3B, dashedline), this region shows an increase in frequency (i.e., a decrease in

Fig. 3. Cyclic voltammetry and �f versus potential plots for copper electrodepositedquartz crystal in 0.25 mol L−1 sulfuric acid + 0.1 mol L−1 potassium chloride solutionbefore (dashed line) and after (solid line) addition of melamine (final concentra-tion = 1 mmol L−1). Scan rate = 50 mV s−1.

3 ectrochimica Acta 117 (2014) 379– 384

mrTeo

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Fig. 4. Differential pulse voltammograms obtained with copper electrode in elec-trolyte 0.25 mol L−1 sulfuric acid + 0.1 mol L−1 potassium chloride (dashed line) and

82 W.R. de Araujo, T.R.L.C. Paixão / El

ass) in this potential region. By calculating the theoretical �m/qatio (using Equation 1), a value of 6.59 × 10−4 g C−1 is obtained.his result is close to that obtained from Fig. 3B, region 1, with anxperimental value of 6.68 × 10−4 g C−1, confirming the dissolutionf copper.

u � Cu+ + e− (1)

Even with respect to the electrochemical process in the absencef melamine, for potentials above 0.05 V (region 2 in Fig. 3B (dashedines)), we observed a large decrease in frequency (i.e., increasen mass), which could be attributed to the deposition of copperhloride salt adsorbed on the electrode surface (Equation 2). Similarehaviour was reported by Doblhofer et al. [30].

u+ + Cl−� CuCl(solid) Ksp = 1.72 × 10−7(25 ◦C) (2)

here Ksp is a solubility product constant of reactionEquation 2 gives a theoretical �m/q value of 3.68 × 10−4 g C−1.

he analysis of experimental data showed a value of.73 × 10−4 g C−1, confirming the formation of CuCl at thelectrode surface; this decrease in frequency continues up tohe potential value of 0.0 V in the backward scan. In region 3, aatio of �m/q of 4.95 × 10−4 g C−1 was observed experimentally,hich can be associated with the formation of adsorbed copper

hloride (Equations 2 and 3) owing to the large amount of chloridevailable in the solution. From the experimental value of �m/q,e can propose a percentage ratio of 65:35 for CuCl and CuCl2−

theoretical �m/q = 4.97 × 10−4 g C−1).

u+ + 2Cl−� CuCl2−(adsorbed at the electrode surface) (3)

In region 4 (Fig. 3B, dashed line) from 0.0 to –0.15 V, webserved an increase in frequency (mass decrease) related tohloride desorption during the reduction of the copper chlo-ide adsorbed species in Equations 4 and 5. The experimentalalue of �m/q was 4.76 × 10−4 g C−1, which is in agreement withm/qtheoretical = 4.78 × 10−4 g C−1 for a combination of the reactions

iven by Equations 4 and 5 (percentage ratio of 70:30 for CuCl anduCl2−).

uCl(solid) + e−� Cu0 + Cl− (4)

uCl2−(adsorbed at the electrode surface) + e−� Cu0 + 2Cl− (5)

The influence of melamine in the voltammetric behavior andelationship of �f versus potential can be observed in Fig. 3A andB, respectively. In the voltammetric profile, it is possible to observe

n increase in current during the copper dissolution process (for-ard scan) and copper re-deposition process (backward scan). The

ncrease in current indicates the influence of melamine on the ratef this process, i.e., the formation of the melamine copper chloride

Scheme 1. Schematic view of formation and reduc

5, 15, 25, 45, 60, and 90 �mol L−1 melamine (solid lines). Parameters: step 0.01 V andamplitude 0.2 V. Inset: Calibration curve obtained from differential pulse voltam-mograms.

ionic pair. In evaluating the changes in �f, a small increase in �f(decrease of mass) could be noted, which is related to the release ofcopper into the solution, suggesting the formation of a stable ionicpair between melamine and copper chloride.

For potentials higher than 0.05 V (region 2 in Fig. 3B (solid line)),we observed a large decrease in frequency (i.e., increase in mass)compared with that in the absence of melamine, which could beattributed to the formation of the ionic pair MELH+[CuCl2]− (Equa-tion 7).

MEL + H+� MELH+pKa = 5 (6)

[CuCl2]− + MELH+� MELH+[CuCl2]−(at electrode surface) (7)

In region 2, an experimental ratio of �m/q equal to6.65 × 10−4 g C−1 was observed, which is higher than the valueobtained without melamine; this can be associated with the forma-tion of adsorbed copper chloride plus the adsorbed MELH+[CuCl2]−

ionic pair. In region 3, an experimental ratio of �m/q of6.43 × 10−4 g C−1 was observed, which is very similar to that foundin region 2, showing that there is competition between copperchloride and MELH+[CuCl2]− ionic pair formation on the electrodesurface.

In the potential range varying from 0.0 to–0.15 V (region 4),we observed an increase in frequency (mass decrease), which washigher than that for the experiment performed in the absence ofmelamine; this is related to the chloride and melamine desorption

tion of melamine copper chloride ionic pair.

W.R. de Araujo, T.R.L.C. Paixão / Electrochimica Acta 117 (2014) 379– 384 383

Table 1Evaluation of some possible interfering.

Interfering Current signal/mA

Blank (27.7 ± 0.8)Ascorbic Acid (26.8 ± 0.7)

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Table 2Comparison between detection limit, potential and electrodes used in the literatureand in the proposed method for electrochemical melamine analysis.

References LD/nmol L−1 E/V Sensor

[11] 800 0.30 preanodized screen-printed carbonelectrode

[13] 360 0.20 molecularly imprinted polymer filmmodified glassy carbon electrode

[19] 9.6 0.23 oligonucleotides film modified goldelectrode

[14] 5 - piezoelectric microgravimetry (acoustic)chemosensor using a molecularlyimprinted polymer film

[12] 2 0.07 glassy carbon electrode modified withmultiwalled carbon nanotubes

Sucrose (26.9 ± 0.6)Lactose (26.1 ± 1.1)

oncentration of the interference specie = 100 �mol L−1

uring the reduction of the copper-adsorbed species (experimentalalue of �m/q = 6.64 × 10−4 g C−1, Equations 4 and 8). Additionally,t can be observed in both graphs of �f versus potential (Fig. 3B) thatt the end of the scan (–0.6 V) there is a small increase in frequencyompared to that at the beginning of the experiment, which maye related to the loss of copper ions by diffusion into the solution,hich could not then be re-deposited.

ELH+[CuCl2]−(atelectrodesurface)+e− � Cu0(redeposited) + MELH+Cl−

+ Cl− (8)

On the basis of the above mentioned conclusions, a scheme forhe reduction of the melamine copper ionic pair at copper surfacesn the acid medium is proposed. The proposed mechanism involveshe anodic oxidation of Cu in the first step (step 1) followed byhe formation of the melamine copper chloride ionic pair (step 2),nd the electrochemical reduction of the melamine copper chlorideonic pair (step 3). The proposed mechanism is shown in Scheme 1or the electrode process, is in agreement with the results obtainedy the EQCM and reports in the literature about melamine ionic

nteraction with metal ions in acidic medium [26–29].Understood the mechanism of the formation and reduction of

he melamine copper chloride ionic pair, we changed the voltam-etric technique to differential pulse voltammetry in order to

chieved better analytical figures of merit; the optimized valuesor this technique were: step 0.01 V and amplitude 0.2 V. Before thisptimization process, an analytical curve was constructed (Fig. 4)or melamine concentrations ranging from 5 to 90 �mol L−1 underhe optimized conditions.

As can be seen in Fig. 4, the peak potential shifts toward negativeirection with the increase of melamine concentration, indicatinghat a previously adsorption step of the melamine or melamineopper chloride ionic pair at the electrode surface is necessaryor the electroreduction process and depending of the kineticsf this process a peak potential shifts can occurs. Zhu et al. [12]eported some similar behaviour for the oxidation of melamineopper complex. Linearity was observed for this concentrationange, and the straight line was in accordance with the equationI/A) = 3.32 × 10−4 + 98.6 (C/�mol L−1) (R2 = 0.995). The detectionimit was calculated by multiplying the standard deviation oflank measurements (the blank solution contained 50 �mol L−1

elamine) by three, and then dividing this by the slope of thenalytical curve (3�/slope). The detection limit was estimated toe 0.85 �mol L−1. The repeatability (or short-term stability) of theroposed method was investigated by performing 20 repetitionsf differential pulse voltammogram measurements at 50 �mol L−1

elamine, which yielded a relative standard deviation of 1%.Interference experiments were conducted to determine

hether other species that could be present in milk might affecthe melamine copper chloride ionic pair reduction signal (Table 1).hree substances were tested: ascorbic acid, sucrose, and lactose.he results given in Table 1 show that none of these substancesnder the experimental conditions evaluated had any faradaic

urrent signal in the region of the reduction of the melamineopper chloride ionic pair.

One of the advantages of the proposed method, as previ-usly mentioned is that a non-modified electrode is used for the

ProposedMethod

850 -0.16 copper electrode without anypretreatment

melamine analysis, besides the possibility to fabricate and usedisposable copper, as reported in literature [20,31] for melaminequantification in real samples. Table 2 shown a comparison of theproposed method with some other analytical methods reported inthe literature. As reported in Table 2, the proposed method can beused for the quantification of melamine in low potential and it doesnot require any pretreatment of the electrode surface.

Recovery tests were also used to evaluate the accuracy of theproposed method. Recovery tests on the three untreated differentmilk samples were performed. In these samples, melamine wasspiked at 200 �mol L−1, and the obtained recovery values werebetween 94% and 103%, confirming the good accuracy and absenceof the matrix effect for this proposed method. As a conclusion, theproposed method was shown to be an efficient technique for thequantification of melamine in milk (limited of detection = 850 nmolL−1, safety limits for ingestion = 4 �mol L−1 in the EU) without theneed to pretreat the milk samples with an extraction procedure orto modify the electrode surface as reported by Zhu et al. [12].

4. Conclusions

In this study, we have demonstrated that a non-modifiedcopper electrode in acidic conditions is suitable for monitoringmelamine through the electrochemical reduction of the ionic pairMELH+[CuCl2]− formed by the dissolution of the copper electrodein a medium containing melamine and chloride. The proposedmethod is simple, rapid, and inexpensive, gives accurate resultsand there is the possibility to use of copper disposable devices,enabling the determination of melamine levels in milk sampleswithout the interference of organic substances that are normallypresent in milk.

Acknowledgments

The authors are grateful to FAPESP (Fundac ão de Amparo àPesquisa do Estado de São Paulo), Grant Numbers: 2012/10612-5 and 2011/19903-5, CAPES, and CNPq (Conselho Nacionalde Desenvolvimento Científico e Tecnológico), Grant Numbers:470919/2011-6 and 302700/2011-0, for financial support.

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