Materials Science and Engineering C 47 (2015) 273–280

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Materials Science and Engineering C

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Development of a novel MWCNTs–triazene-modified carbon pasteelectrode for potentiometric assessment of Hg(II) in theaquatic environments

Mohammad Hossein Mashhadizadeh ⁎, Soleyman Ramezani, Mohammad Kazem RofoueiFaculty of Chemistry, Kharazmi University, Tehran, Iran

⁎ Corresponding author.E-mail address: mashhadizadeh@khu.ac.ir (M.H. Mash

http://dx.doi.org/10.1016/j.msec.2014.11.0110928-4931/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 17 January 2013Received in revised form 11 October 2014Accepted 6 November 2014Available online 7 November 2014

Keywords:Modified carbon paste electrodeMulti-walled carbon nanotubePotentiometric sensorMercury(II)Triazene

In this approach, a new chemically modified carbon paste electrode was assembled for potentiometric assay ofmercury(II) ion in the aqueous environments. Hereby, MWCNTs were used in the carbon paste composition tomeliorate the electrical conductivity and sensitivity of the carbon paste owing to its exceptional physicochemicalcharacteristics. Likewise, participation of theBEPT as a super-selective ionophore in the carbon paste compositionboosted significantly the selectivity of the modified electrode towards Hg(II) ions over a wide concentrationrange of 4.0 × 10−9–2.2 × 10−3 mol L−1 with a lower detection limit of 3.1 × 10−9 mol L−1. Besides, Nernstianslope of the proposed sensor was 28.9 (±0.4) mV /decade over a pH range of 3.0–5.2 with potentiometric shortresponse time of 10 s. In the interim, by storing in the dark and cool dry place during non-usage period, theelectrode can be used for at least 30 days without any momentous divergence of the potentiometric response.Eventually, to judge about its practical efficiency, the arranged sensor was utilized successfully as an indicatorelectrode for potentiometric titration of mercury(II) with standard solution of EDTA. As well, the quantitativeanalysis of mercury(II) ions in some aqueous samples with sensible accuracy and precision was satisfactorilyperformed.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Arguably, it has been assumed that about 5500 tons of mercuryenters the global atmosphere annually from all sources includingnatural, anthropogenic (human activity), and oceanic emission. Notonly mercury is a menace to human health, but also it is detrimentalto microorganisms, plants and animals [1]. It may cause kidney toxicity,neurological damage, paralysis, chromosome breakage, and birthdisorders. Elemental mercury and short-chain alkyl mercurial areknown to be responsible for causing damage to the sensory parts ofthe central nervous system [2]. Because of the abovementioned reasons,accurate quantitative surveillance of the mercury ion in the environ-ments covering human, animals, and even plants seems absolutelycrucial.

Hereby, various analytical techniques have been used fordetermination of mercury at low concentrations, but the mostcommonly used ones comprise cold vapor technique coupled withatomic absorption spectroscopy (CV-AAS) and atomic fluorescencespectroscopy (CV-AFS), inductively coupled plasma atomic emissionspectrophotometry (ICP-AES), and inductively coupled plasma massspectrometry (ICP-MS) [3–9]. These instrumental techniques, owing

hadizadeh).

to its good selectivity and low detection limit, have a special acceptance.Nevertheless, their relatively high instrumental and/or running costs aswell as intricate instrument setup make it difficult to employ thesehyphenated techniques for routine speciation analysis.

In contrast, ion selective electrodes (ISEs) due to its unparalleledproperties including high operation speed, affordability, simplepreparation process, simple instrumentation, relatively short responsetime, acceptable sensitivity and selectivity, and wide dynamic ranges,can be a worthy alternative to the instrumental techniques [10,11].Among the ISEs' basic materials, carbon (graphite) past is definitelypopular because it is non-toxic, chemically inert, easily attainable atminimal expenses, and is especially apt for preparing an electrodematerial modified with admixtures of other compounds thus givingthe electrode certain pre-determined properties. Electrodes made inthisway are highly selective sensors for both inorganic electrochemistryand organic electrochemistry [12–14]. Moreover, these electrodes havesome advantages such as, ease of preparation and use, renewal of sur-face, robustness, stability of response, low Ohmic resistance, no needof internal solution (compared with membrane electrodes) and suit-ability for a variety of sensing, and detection applications [15–17].

Unquestionably, the operation mechanism of such chemicallymodified carbon paste electrodes (CMCPEs) depends chiefly on thechemical nature and properties of the modifying additives used toboost selectivity and sensitivity of the desired electrode towards the

274 M.H. Mashhadizadeh et al. / Materials Science and Engineering C 47 (2015) 273–280

target species. Among all modifying additives of the CMCPEs, theionophores (so-called ion-carrier) play a pivotal role in promoting theselectivity and sensitivity of the modified sensors. Hitherto, manyorganic synthetic compounds were reported as Hg(II)-carrier withsome specific features, which qualifies them as an appropriateionophore for mercury(II) ion [18–21]. It should be pointed out that;all of the ion selective compoundswere expected to interact with targetions (Lewis acid) via coordinative centers owning some basic donoratoms (Lewis base) in its structure based on HSAB rule of Pearson.

Recent reports have shown that, triazene compounds based on thediazoamino groups (\N_N\NH\) as an electron donor active site(Lewis base) in its structure, have a strong tendency to form relativelystable complexes with the mercury(II) ions as a Lewis acid. In some ofthese reports, the interaction of triazene ligand with Hg(II) ion in thesolution, and crystal structure of the formed complexes betweenthem, have been investigated byX-ray andUV–Vis techniques to upheldthe significant interaction between the ligand and mercury(II) ion[22–26].

In recent years, MWCNTs is one of the most widely usednanomaterials to promote the performance of chemically modifiedsensors and biosensors. CNTs display fascinating electronic and opticalproperties that are distinguished from the other carbonaceousnanomaterials [27]. Particularly, the properties of ordered structurewith high aspect ratio, ultra-lightweight, high mechanical strength,high electrical and thermal conductivity, metallic or semi-metallicbehavior, high surface area, modifiable sidewall, and high reactivitymake CNTs ideal candidates for constructing sensors and biosensorswith high performances [28]. Therefore, CNT-based sensors generallyhave higher sensitivities, lower limits of detection, and faster electrontransfer kinetics accordingly faster response time than traditionalcarbon electrodes [29].

In the present research, the synthetic ligand of BEPT (Fig. 1) besidesMWCNTs was utilized as modifying additives to meliorate the CPEcharacteristics for potentiometric determination of mercury(II) ions.Importantly, the influence of the carbon paste ingredient on thepotential response of the Hg(II) sensor was investigated. Using the

Fig. 1. Chemical structure of the BEPT used as ionophore and its Hg ML2 complex [22].

CNTs based on its exclusive properties and the special triazenecompound (BEPT) as a sensitive ionophore for mercury(II) in thepaste composition causes to improve some basic functional propertiessuch as Nernstian slope, limit of detection, linear range, and responsetime of the modified CPE. The proposed method was finally applied inthe potentiometric determination of mercury ions spiked in someaqueous samples.

2. Experimental

2.1. Chemical reagent and materials

All analytical reagent grade chemicals and double distilled waterwere used for preparing all aqueous solutions. The graphite powder(GP) and paraffin oil (PO) (Merck, Germany) were of high purity andused for the preparation of the carbon pastes. Nitrate salts of variousmetal ions with high purity were purchased from Merck. Themulti-walled carbon nanotubes (MWCNTs) with 8–15 nm ODand ~50 μm lengths were supplied (Neutrino Co., Iran) and usedwithout any initial manipulation. The Hg(II)-carrier of BEPT wassynthesized and purified according to the reported procedure [26]. Astock solution of 2.2 × 10−2 mol L−1 Hg(II) was prepared by dissolvingappropriate amount of Hg(NO3)2·H2O in deionized water. Then, theobtained solution was diluted to 25.0 mL in a volumetric flask and thepH of the resulted solution was adjusted at ~2.0 by adding HNO3

(1.0 mol L−1). More dilute sample solutions were preparedfreshly by appropriate dilution of the stock solution with water.All the other metal nitrate solutions were freshly prepared byaccurate dilution from their stock solution of 0.01 M, with deionizedwater.

2.2. Instrumentation

All potentiometric measurements were made with a pH/mV meter(Metrohm-827, Switzerland) using proposed sensor as an indicatorelectrode in conjunction with an Ag/AgCl, (KCl 3.5 M) (Azar electrode,Iran) as reference electrode. UV–Vis absorption spectrumwas recordedwith a UV–Vis spectrometer (Perkin-Elmer model Lambda 25, USA). Ifneeded the solutions were homogenized by an ultrasonic instrument(Bandelin UW 3200, Germany).

2.3. Construction of the modified carbon paste electrode

The general process of makingmodified carbon paste electrode is asfollows: different amounts of the ionophore BEPT alongwith appropriateamount of graphite powder, paraffin oil and MWCNTs were thoroughlymixed. Then, in order to achieve the maximum uniformity of the carbonpaste, the resulted mixture was suspended in an appropriate amount ofthe acetone and was homogenized by an ultrasonic homogenizer for5 min. After complete evaporation of the solvent, a portion of theremained paste mixture was packed carefully into the polyethylenetube tip with 2.5 mm i.d. A copper wire was inserted into the oppositeend of the tube, to establish electrical contact. The electrode surface wassmoothed with soft paper. Scraping out the old surface and replacingthe carbon paste produced a new surface. It is not recommended toemploy freshly made CPEs immediately, but they should be left unusedfor a certain time (12–24 h) to allow their final homogenization toproceed. This process of “self-homogenization” has been confirmedexperimentally; freshly homogenized carbon pastes often exhibit arather unstable behavior [30]. The fresh surface of the modified CPEwas preconditioned by drenching in the 1.0 × 10−4 mol L−1 of Hg(II)solution for 1 h at pH 5.5, and then the electrode was rinsed in distilledwater for 15 min.

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2.4. Electrode system and emf measurement

All electromotive force (emf) measurements were carried out withthe following cell assembly:

Ag=AgCl sð Þ; KCl 3Mð Þ sample solution carbon paste electrodejk :

The calibration graph was drawn by plotting the potential, E, versusthe logarithm of the concentrations of various metal ions. Theperformance of the electrodes was investigated by measuring theemfs of mercury(II) nitrate solutions, which is prepared with aconcentration, range of 1.0 × 10−3 to 1.0 × 10−10 mol L−1 by serialdilution. All measurements were carried out at ~25 °C and pH 4.0. ThepHs of the test solutions containing mercury ion were adjusted about4.0 at the required value by the addition of 1.0 mol L−1 nitric acid.

2.5. Spectrophotometric study

Necessarily, the UV–Vis spectrophotometric method was used formore appraisal of the BEPT ligand interaction with different cations inmethanol solution. In this way, very useful data such as stoichiometryand stability of the formed complexes for evaluating the interaction ofBEPT with different ions were obtained. For this purpose, the UV–Visspectra of 2.0 mL of 5.0 × 10−5 mol L−1 methanolic solution of the

0.1

0.3

0.5

0.7

0.9

250 300 350 400 450 500 550

Abs

.

Wavelength (nm)

(a)

0.35

0.45

0.55

0.65

0.75

0.85

0 0.5 1 1.5 2 2.5 3

Abs

.

[Hg2+]/[L]

(b)

Fig. 2. a) Electronic absorption spectra of BEPT (5.0 × 10−5M) inMeOH in the presence ofincreasing amounts of Hg(II), b) absorbance versus [Hg(II)]/[BEPT] molar ratio plots.

BEPT in the wavelength range of 250–550 nm were obtained. Then,varying amounts of the different metal ions were added to the initialligand solution and the changes in the spectra were recorded for moreinvestigations. As can be seen in Fig. 2a, shrinkage in the absorptionintensity of the ligand in thewavelength of 373 nm and simultaneously,shifting to the401 nmat the equivalent point that belongs to theHg(II)–BEPT complex, signify a good interaction of the mentioned ligand withthe mercury ion.

Likewise, the depicted mole ratio plot (absorbance versus[Hg2+]/[BEPT]) at the wavelength of 373.0 nm for BEPT has beenshown in the inset in Fig. 2. As shown, the absorbance–mole ratio plotrevealed a level off at ([Hg2+]/[BEPT]) molar ratio of about 0.5,accentuating the formation of 1:2 (ML2) complex in the methanolsolution (Fig. 1).

Besides, the complexation of the BEPT with a number of metal ionswas also investigated and the formation constants of the resulting 1:2complexes were evaluated by the non-linear least-squares curve-fitting program KINFIT [31] and the resulted data were gathered inTable 1. As the contents of Table 1 suggests, the BEPT ligand has greatdesire to form the most stable complex selectively with Hg(II) ion (logβ2 = 8.82 ± 0.03) in the presence of interfering foreign ions. Thisphenomenon can be quite regular based on the existence of threenitrogen and two oxygen as active donor atoms in the structure of theBEPT ligand that interact selectively with the mercury(II) ion in thepresence of the foreign ions (Fig. 1).

Therefore, all the results agree consistently with the fact that, theBEPT compound, is expected to act as a selective ionophore for thepreparation of CMCPE for stripping voltammetric determination ofHg(II) ion (See Fig. 3).

3. Results and discussion

Because of the presence of O and N donor atoms in its structure, theBEPT ligand can play key role of a suitable ion carrier in the ISEs'structure to sensitive and selective assay heavy metal ions. Because ofinsolubility in water, the BEPT compound has adequately lipophilicnature to prevent it from leaching into the solution overlaying theelectrode surface. Additionally, as previously mentioned, dependingon the solubility of the ligand in the chemically inert and volatile organicsolvents, the ligand and carbon powder mixture could easily behomogeneous by dissolving in a suitable solvent. Hence, the BEPT andcarbon powder mixture can dissolve in the inert volatile solvents toinduce high uniformity to the modified paste. The electromotive forceresponses perceived for all other cations are much lower than thatanticipated by the Nernst equation. The exceptional selectivity towardsHg(II) must be associated to the coordinate interaction between BEPTand Hg(II) ions via donor atoms of O and N based on HSAB rule aswell as exchange kinetics of the resulting Hg(II)–BEPT complex.Therefore, based on the results acquired in primary investigations,the BEPT was effectively used as a neutral carrier to organize anHg(II)-selective CPE for sensitive potentiometric quantification ofmercury(II) concentration in the aqueous solutions contaminatedby some interfering ions.

Table 1Formation constants of different metal–BEPT com-plexes in methanol.

Cation Log β2

Hg(II) 8.82(±0.03)Cu(II) 2.49(±0.05)Ag(I) 2.15(±0.03)Zn(II) 1.45(±0.02)Cd(II) 2.98(±0.04)Co(II) 3.51(±0.03)Cr(III) 2.13(±0.02)Al(III) 2.21(±0.3)Na(I) 2.98(±0.06)

280

330

380

430

480

530

0 2 4 6 8 10 12

E (m

V)

pM

Hg Ag

Cu Cd

Co Zn

Pb Al

Cr Ca

Fe Na

Mg NH4

Fig. 3. Potential response of the MWCNTs–BEPT modified carbon paste electrode towardsvarious cations.

276 M.H. Mashhadizadeh et al. / Materials Science and Engineering C 47 (2015) 273–280

3.1. Electrode composition and modification

Definitely, optimal selection of carbon powder, pasting liquid, andmodifier additives can undoubtedly be ranked among the mostimportant factors influencing the resultant behavior of carbon pastes.In order to avoid the choice – and later testing – of evidently unsuitablecarbon paste mixtures, it seems to be necessary to point out some basicprinciples of the preparation of carbon pastes. Some importantcharacteristics of the carbon paste components, such as properties ofthe binder, binder/graphite powder weight ratio, nature and amountof ionophore, and especially nature and amount of the nano-materials,which considerably affect the sensitivity and selectivity of theion-selective electrodes, have been reported [32,33]. Therefore, in thissection, the effect of the absence, presence, and amount of ionophoreand CNTs, amount of binder (paraffin oil) on potentiometric responseof the proposed sensor is investigated and the results are shown inTable 2. As expected, the bare electrode (No. 1) has no remarkablesensitivity, linearity, and LOD values towards Hg(II) ion reflecting inability of the unmodified electrode to sense mercury ions. However,by adding 3.5 mg of the ionophore (BEPT) to the carbon paste composi-tion, the modified electrode (No. 3) shows the promoted analyticalcharacteristics, signifying substantial effect of the BEPT ligand on thepotentiometric response of the CPE. The sensitivity of the electroderesponse increased with increasing amount of the modifier (electrodeNos. 4–6). However, further addition of the ionophore (sensor Nos. 6and 7) displays somewhat super-Nernstian slopes and small sensitivityand linearity, most probably due to some inhomogeneities and possiblesaturation of the paste, thereupon, decrease conductivity of the carbonpaste [34].

Furthermore, as mentioned earlier, the presence of MWCNTs in thecarbon paste composition, improves significantly the electricalconductivity and surface to volume ratio of the carbon paste bulk ofthe electrode and in consequence, the dynamic working range andspecially sensitivity of the sensor were improved conspicuously(Nos. 9–12). Nevertheless, excessive addition of MWCNTs lessenssurprisingly the uniformity and adhesion of the modified carbonpaste. However, by increasing the PO/GP weight ratio this problemcan be largely obviated. In this case, because of non-conductivity ofthe binder, electrical conductivity of the paste may be diminishedpartially but so low that it can be neglected.

Concisely, the modified carbon paste electrode with the GP/PO/BEPT/MWCNT weight percentage ratio of 52.5/32/5.5/10 was selected asthe one with the optimal paste ingredient composition (No. 12)and was used in further studies. The finally introduced electrodedisplayed a Nernstian slope of 28.9 (±0.4) mV/decade in a wide Hg(II)concentration range of 4.0 (±0.3) × 10−9–2.2 (±0.2) × 10−3 mol L−1.The detection limit of the sensor, as elicited from the intersectionof the two extrapolated segments of the calibration graph was 3.1

Table 2Optimization of the carbon paste ingredients and their analytical characteristics.

No. Composition

BEPT (mg) GP (mg) PO (mg) MWCNT (mg) Slop

1 66 34 18.02 – 63 32 5.0 21.43 3.5 65 31.5 – 25.44 4.0 64 32 26.35 5.0 64 31 27.16 5.5 63.5 31 – 28.07 6.5 62.5 31 – 33.28 7.0 62 31 – 40.09 5.5 58.5 31 5.0 28.510 5.5 57.5 31 6.0 29.311 6.0 54 32 8.0 32.512 5.5 52.5 32 10.0 28.9

GP: graphite powder, PO: paraffin oil, MWCNT: multi-walled carbon nanotube, LR: linear ranga Values in parenthesis is SDs based on three replicate analyses.

(±0.3) × 10−9 mol L−1 (Fig. 4). These results indicate that theBEPT compound was deservedly adopted as a natural ion carrierin modification of a CPE to selective potentiometric determinationof mercury(II) ion concentration in aqueous media. Besides, usingMWCNTs in the paste composition amplify considerably the electrodesignal.

3.2. Effect of pH on the electrode performance

Effect of pH on the electrode potentiometric response in theproposed concentrations of 1.0 × 10−5 and 1.0 × 10−6 mol L−1 of theHg(NO3)2 test solution was investigated and the results were shownin Fig. 5. The pH values were changed in the range of 1.0 to 8.0using HNO3 and NaOH 0.1 M solutions. As can be seen, the potentio-metric response of the proposed electrode in the pH range of 3.0–4.5(in 1.0 × 10−5 mol L−1 of Hg2+) and 3.0–5.2 (in 1.0 × 10−6 mol L−1

of Hg2+) did not change by varying pH, indicating satisfactoryperformance of the electrode in this range. However, the curves showa drastic drift in the potential values beyond this range. In the valuesless than 3.0, by increasing acidity of the solutions, a significant growthin the potential values can be seen. At such acidic solution, the observedgrowing in potential values indicates that, the protonated ionophorepossesses a poor response to Hg(II) ion, but a strong response toH3O+ ions that are in the solution in high quantity. Therefore, in suchmanner, a sharp increase in the potential values is not too far-fetched.

Electrode characteristics

e (mV/decade) LR (M) LOD (M) R2

8.1 × 10−5–2.2 × 10−2 4.5 × 10−5 0.962.1 × 10−6–1.0 × 10−2 1.0 × 10−6 0.973.2 × 10−7–2.2 × 10−3 2.0 × 10−7 0.972.0 × 10−7–2.2 × 10−3 1.0 × 10−7 0.967.4 × 10−8–2.2 × 10−2 5.4 × 10−8 0.981.5 × 10−8–2.2 × 10−3 1.0 × 10−8 0.994.0 × 10−7–2.2 × 10−3 2.1 × 10−7 0.976.3 × 10−7–2.0 × 10−3 4.5 × 10−7 0.983.2 × 10−8–2.5 × 10−2 2.5 × 10−8 0.978.5 × 10−9–3.0 × 10−3 6.2 × 10−9 0.966.3 × 10−9–2.4 × 10−2 4.5 × 10−9 0.97

(±0.4)a 4.0 × 10−9–2.2 × 10−3 3.1(±0.2) × 10−9 0.99

e, and LOD: limit of detection.

320

370

420

470

520

570

2 4 6 8 10 12

E (m

V)

pHg

LOD

Fig. 4. Calibration curve for Hg(II)-selective CPE based on BEPT–MWCNTs.

330

350

370

390

410

430

450

0 20 40 60 80 100

E (m

V)

Time (s)

2.2×10−4 M

2.2×10−5 M

2.2×10−6 M

2.2×10−7 M

Fig. 6. Dynamic response time of the proposed carbon paste electrode (No. 12) for stepchanges in Hg(NO3)2 concentration.

277M.H. Mashhadizadeh et al. / Materials Science and Engineering C 47 (2015) 273–280

Also, at high pH values (more than 4.5 and 5.2), due to the formation ofthe Hg(OH)2 species and subsequent decrease in the activity of Hg2+

species in the bulk of the solution, the observed potential values, willbe sensibly decreased. These results can be expoundedwith the followingreaction:

Hg2+ + 2OH− → Hg(OH)2 Ksp = 3.0 × 10−26.

According to the mercury ion initial concentrations, the requisite pHvalues for mercury hydroxide formation introduced by the Hg(OH)2 Ksp

equation, were accurately consistent withwhatwas observed practically.

3.3. Response time and lifetime of the Hg(II) electrode

Normally, dynamic response time is one of the most imperativefactors for any chemical sensor. The critical emf response of theelectrode was evaluated according to IUPAC suggestions [35]. Accord-ingly, about all electrodes, the average response time was defined as

250

270

290

310

330

350

370

390

410

430

450

0 1 2 3 4 5 6 7 8 9

E (m

V)

pH

1.0×10−5 M

1.0×10−6 M

Fig. 5. Effect of pH of test solutions on the response of the proposed Hg2+ carbon pasteelectrodes for 1.0 × 10−5 and 1.0 × 10−6 M of Hg(NO3)2 solutions.

the required time for the electrodes to reach a cell potential of 90%(t90%) of the final equilibrium values, after successive immersions in aseries of solutions, each having a 10-fold concentration difference.The measurements of potential versus time were executed with theHg(II) nitrate solutions from lower (1.0 × 10−7 mol L−1) to higher(1.0 × 10−4mol L−1) concentrations. As depicted in Fig. 6, the electrodesurface processes reach to equilibriumwithin about 10 s. Such relativelyshort response time signifies the rapid interaction kinetics of the BEPTligand with Hg(II) ions and fast exchange kinetics of the metal–ligandcomplexation–decomplexation procedure.

Importantly, the lifetime of the MWCNTs–BEPT–CPE was examinedwithin one month by weekly recalibrating of the sensor utilized in thepotentiometric analysis of 22 μM of mercury(II) standard solution. Itwas found that, no momentous change in the electrode sensitivity(LOD)was observed after about onemonth storage (four replicate trials,RSD = 3.4%). It should be noted that, for long time non-usage period,the electrode must be kept in double distilled water and in a cool darkplace. Nevertheless, after onemonth storage, the electrode performancebegins to decline most likely because of the fact that; triazenecompounds, which were here used as an ionophore, are not very stablecompounds at the ambient temperature and may be decayed after along period [21].

It is noteworthy that, to reuse the electrode after a long timenot-usage period, a fresh surface of it could be produced by scrapingout the old surface by a smooth graph or weighing paper.

3.4. Interference studies

Ion-selective electrodes can be characterized by many differentparameters such as the slope of the potential response, detection limit,and response time. Among them, one of the most important factors isselectivity coefficient of the electrode over different interference ionsin a given system. Therefore, in this work, selectivity coefficients of theproposed electrode towards different cationic species (Mn+) wereevaluated by using matched potential method (MPM) and fixedinterference method (FIM) [36]. According to the MPM, the potentio-metric selectivity coefficient is defined as the activity ratio of primaryand interfering ions that give the same potential change under identicalconditions. At first, a known activity (a′Hg) of the primary ion solutionwas added into the reference solution containing a fixed activity(aHg = 1.0 × 10−4 mol L−1) of primary ions, and the correspondingpotential changes were recorded. Then, a certain concentration of theinterfering ion (j) was added to the reference solution until the same

Table 3Selectivity coefficients of various interfering ions for proposed sensor concluded by FIMand MPM methods.

Interfering ions KHg,jFIM kHg,j

MPM

Ag+ 5.0 × 10−4 3.0 × 10−4

Pb2+ 7.5 × 10−4 5.4 × 10−4

Cu2+ 6.2 × 10−4 3.5 × 10−4

Cd2+ 2.5 × 10−4 8.2 × 10−5

Co2+ 4.5 × 10−5 4.0 × 10−5

Zn2+ 6.4 × 10−5 3.0 × 10−5

Al3+ 1.2 × 10−5 2.0 × 10−5

Cr3+ 3.5 × 10−5 2.8 × 10−5

Ca2+ 2.1 × 10−5 8.1 × 10−6

Mg2+ 2.5 × 10−5 1.4 × 10−5

Na+ 1.3 × 10−5 1.0 × 10−5

NH4+ 2.4 × 10−5 2.1 × 10−5

K+ 1.6 × 10−5 8.5 × 10−6

Table 5Potentiometric recovery determination ofmercury(II) quantity in different water samplesby standard addition method.

Samples Hg(II) (mg L−1)a Recovery %

Added (SAM) Found (MCPE)

Laboratory water 0 0 10210 10.2 (±0.3)

Mineral water 0 0 10110 10.1 (±0.2)

Distilled water 0 0 9910 9.9 (±0.1)

a Values in parentheses are RSDs based on three replicate analyses.

278 M.H. Mashhadizadeh et al. / Materials Science and Engineering C 47 (2015) 273–280

potential changewas obtained. According to thismethod, the selectivitycoefficient (kHg,jMPM) is defined as:

kMPMHg; j ¼ a0Hg−aHg

� �=aj:

The selectivity coefficients of interfering specieswere also calculatedusing the fixed interference method (FIM). In this way, the emf of a cellcomprising the modified CPE and a reference electrode (ISE cell) ismeasured for solutions of constant activity of interfering ion (aj), andvarying activity of the primary ion. The obtained emf values are plottedvs. log (aHg) and then the intersection of the extrapolated linear portionsof this plot indicates the value of aHg, which is to be used to calculate KHg,jpot

from the Nikolsky–Eisenman equation as follows:

K FIMHg; j ¼ a DLð Þ= aj

� �2=Z j; aj ¼ 2:0� 10−2M

where, (aHg (DL)), is the primary ion activity in detection limit, aj is theinterference ion activity (2.0 × 10−2 mol L−1), and ZHg and Zj are theprimary and interference ions' charge, respectively. All results are givenin Table 3. Fortunately, as seen, the majority of the interfering ions don'taffect significantly the response of the modified electrode, and most ofthem show low values of selectivity coefficients, indicating no interfer-ence in the performance of themodified carbonpaste electrode assembly.As the data in Table 1 suggest, the formation constants of Mn+–BEPTcomplexes with various interfering metal ions compared withHg(II)–BEPT are very low indicating high tendency of the BEPTcompound as ionophore to Hg(II) ion among all of the analyzed alkali,alkaline earth and transition metal ions. As well, there is a good rapportbetween the formation constants and the selectivity coefficient order,because the selectivity of an ion-selective sensor is mainly related tothe stability of the complex between ion and ionophore [21]. It wasfinally concluded that, based on the obtained results, the proposedelectrode has a high performance to selective potentiometric assay ofHg(II) in aqueous samples infected by some interfering ions.

Table 4Comparison of the response characteristics and selectivity coefficients of different Hg(II)-selec

Ref. Slope (mV/decade) LOD (M) LR (M) pH

[15] 48.5 1.0 × 10−7 3.0 × 10−7–3.1 × 10−2 1.0[36] 29.0 1.0 × 10−6 1.7 × 10−6–1.0 × 10−1 1.0[37] 55.5 4.0 × 10−6 6.0 × 10−6–1.0 × 10−3 4.0[38] 69.5 2.4 × 10−6 1.0 × 10−5–1.0 × 10−2 1.5[39] 28.4 7.0 × 10−10 1.0 × 10−9–1.0 × 10−3 2.0[38] 58.8 1.5 × 10−7 3.2 × 10−7–3.2 × 10−4 3.8[40] 25.0 8.9 × 10−6 1.2 × 10−5–1.0 × 10−1 6.6[18] 30.0 2.0 × 10−7 7.0 × 10−7–5.0 × 10−2 1.0This work 28.9 3.1 × 10−9 4.0 × 10−9–2.2 × 10−3 3.0

3.5. Comparison of proposed method with literature methods

In order to well perceive the effectiveness of the ongoing electrode,some comparative parameters between the analytical performance ofthe present method and previously reported literature methods forthe determination of Hg(II) have been listed in Table 4 [15,18,37–42].As shown in Table 4,most of thepropoundedHg(II)-selective electrodesendure disturbing effects from some common interfering species suchas Cu2+, Cd2+, Ag+, and Pb2+ ions. In the contrary, most of the foreignions have no salient effect on the response of the proposed electrodeand the majority show low values of selectivity coefficients, signifyingnegligible interference in the performance of the carbon paste elec-trodes. These results ascertain that the proposed MWCNTs–BEPT–CPEhas a large advantage over other reported methods in terms ofNernstian slope, linear working range and limit of detection becauseof extensive melioration with respect to those of the lately publishedHg(II)-selective electrodes.

3.6. Analytical applications

In order to appraise the developed method's performance inpractical analytical situations, the present method was employed forthe determination of Hg(II) content in some water samples usingstandard addition method (Table 5). This table approves that theamount of Hg(II) obtained by the proposed method agreed well withthe added amount in the recovery range of %(99–102) demonstratingthe acceptable performance of the supplied electrode for the recoveryof mercury(II) content from some water samples with differentmatrices. As well, the mercury content in the aqueous samples wasalso estimated by the traditional volumetric method using EDTA astitrant. In this manner, the reported electrode was used satisfactorilyas an indicator electrode for potentiometric titration of 25.0 mL ofHg2+ solution (1.0 × 10−4 mol L−1) by EDTA solution (0.01 mol L−1).Titration curve and its first derivative are shown in Fig. 7. As can beseen, a significant jump in the curve truly shows the equivalent pointindicating successful determination of mercury ion concentration bypotentiometric titration method using Hg(II) selective CPE as indicatorelectrode. Thus, determination of Hg(II) can be carried out in variousmatrices by the proposed method.

tive electrodes.

range t90% (s) − log KHg,jpot

–4.0 5 Cu(1.5), Ag(3.0), Cd(1.2), Co(2.5), Al(NR), Pb(1.8), K(2.8)–3.0 30 Cu(2.3), Ag(−3.1), Cd(1.5), Co(3.5), Al(NR), Pb(3.2), K(0.3)–9.0 30–50 Cu(3.3), Ag(1.7), Cd(1.7), Co(NR), Al(NR), Pb(2.2), K(NR)–7.5 10–20 Cu(2.6), Ag(1.0), Cd(2.1), Co(3.2), Al(2.4), Pb(2.4), K(NR)–3.5 35 Cu(3.2), Ag(2.4), Cd(NR), Co(3.8), Al(NR), Pb(2.9), K(4.6)–7.8 – Cu(0.8), Ag(1), Cd(2.6), Co(2.3), Al(3.3), Pb(1.9), K(1.4)–9.3 10 Cu(1.4), Ag(0.5), Cd(1.1), Co(1.0), Al(1.6), Pb(1.3), K(NR)–3.5 b10 Cu(4.5), Ag(4.1), Cd(3.2), Co(4.4), Al(NR), Pb(4.3), K(4.7)–5.2 5 Cu(3.5), Ag(3.3), Cd(3.6), Co(3.3), Al(4.9), Pb(3.0), K(4.8)

60

110

160

210

260

310

360

0 100 200 300 400 500 600

E (m

V)

VEDTA (μL)

0

0.5

1

1.5

2

0 100 200 300 400 500

ΔE

/ΔV

VEDTA (μL)

Fig. 7. Potentiometric titration curve of 25.0 mL of 1.0 × 10−4 M Hg(NO3)2 solution with0.01 M of EDTA and its first order derivative at pH 4.0 (inset).

279M.H. Mashhadizadeh et al. / Materials Science and Engineering C 47 (2015) 273–280

3.7. Reproducibility and repeatability of the sensor

For validation of the proposed method, various parameters, such asrepeatability, reproducibility, precision, and accuracy of analysis wereevaluated by accomplishment five replicate measurements for 22 μMof standard Hg(II) solutions by one paste over a single day (intradayassay, n = 5, RSD = 2.4%). The results obtained confirmed both thehigh precision and accuracy of the proposed procedure.

In order to study the reproducibility of the electrode preparationprocedure, six modified electrodes with optimum compositions (No.12) based on the same fabrication procedure were identically preparedand used to assay 22 μMof Hg(II) standard solution. The results showedthe average of slopes, detection limits and linear dynamic ranges thatwere 28.9 (±0.4) mV/decade, 3.1 (±0.5) × 10−9 mol L−1 and (4.0(±0.3) × 10−9–2.2 (±0.4) × 10−3 mol L−1) respectively. The relativestandard deviation of measurements of 22 μM of Hg2+ solutionwas ±2.0 mV with these six electrodes. The results indicate that themodified electrode has high reproducibility and repeatability in boththe preparation procedure and the potentiometric determinations.

4. Conclusions

In this approach, a novel CPE developed chemically using MWCNTsand a triazene compound (BEPT) as an ionophore has been reportedfor selective and sensitive potentiometric quantification of Hg(II) ionin the aqueous medium. The contrived sensor demonstrated aNernstian slope of 28.9 (±0.4) mV/decade over a wide concentrationrange of 4.0 × 10−9–2.2 × 10−3 mol L−1 with a lower detection limitof 3.1 × 10−9 mol L−1. Also, the potentiometric response time of theelectrode was about 10 s over the pH range of 3–5.2. Owing to utilizinga loftier ionophore besides MWCNTs in the carbon paste composition,the propounded sensor compared with previously reported Hg(II)selective sensors offers preponderantly attractive advantageous,especially, in terms of the selectivity over some foreign metal ions.The proposed method is promising for Hg(II) quantification in someaqueous samples having various matrices with satisfactory results.

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