Highly sensitive simultaneous electrochemical determination of trace amounts of Pb(II) and Cd(II) using a carbon paste electrode modified with multi-walled carbon nanotubes and a newly synthesized Schiff base

Electrochimica Acta 89 (2013) 377–386

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Highly sensitive simultaneous electrochemical determination of trace amounts of Pb(II) and Cd(II) using a carbon paste electrode modified with multi-walled carbon nanotubes and a newly synthesized Schiff base Abbas Afkhami a,∗ , Hamed Ghaedi a , Tayyebeh Madrakian a , Majid Rezaeivala b a b

Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran Department of Chemical Engineering, Hamedan University of Technology, Hamedan, Iran

a r t i c l e

i n f o

Article history: Received 27 August 2012 Received in revised form 5 November 2012 Accepted 12 November 2012 Available online 23 November 2012 Keywords: Simultaneous determination Pb2+ and Cd2+ determination Modified electrodes Square wave stripping voltammetry

a b s t r a c t A new chemically modified electrode was constructed for rapid, simple, accurate, selective and highly sensitive simultaneous determination of lead and cadmium using square wave anodic stripping voltammetry (SWASV). The electrode was prepared by incorporation of new synthesized Schiff base and multi-walled carbon nanotubes (MWCNT) in carbon paste electrode. The limit of detection was found to be 0.25 ng mL−1 and 0.74 ng mL−1 for Pb2+ and Cd2+ , respectively. The stability constants of the complexes of the ligand with several metal cations in ethanol medium were determined. The effects of different cations and anions on the simultaneous determination of metal ions were studied and it was found that the electrode is highly selective. The proposed chemically modified electrode was used for the determination of lead and cadmium in several foodstuffs and water samples. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Lead and cadmium are two of the most serious environmental contaminants [1], which are highly toxic to nervous, immune, reproductive and gastrointestinal systems of both humans and animals [2,3]. Additionally, heavy metals are toxic and hazardous pollutants in the environment due to their non-biodegradability and persistence, which can cause serious threat to living organisms [4]. For example, the accumulation of Pb or Cd in the human body exhibits severe deleterious effects on neurobehavioral development in children, increases blood pressure, and causes kidney injury and anemia [5]. So the development of a highly sensitive method for the determination of trace amounts of lead and cadmium has received a considerable attention. Over the past decades, many techniques have been employed for the determination of heavy metals. Several analytical techniques such as spectrophotometry, inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma optical emission spectrometry (ICP-OES), atomic absorption spectrometry (AAS), atomic fluorescence spectrometry and neutron activation analysis have been developed for the determination of these elements [6–11]. However, most of the above methods require several time consuming manipulation steps, sophisticated instruments and

∗ Corresponding author. Tel.: +98 811 8272404; fax: +98 811 8272404. E-mail address: [email protected] (A. Afkhami). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.11.050

special trainings. Particularly, electrothermal atomic absorption spectrometry (ET-AAS) has shown satisfactory detection limit for the determination of lead at nano-molar levels in water samples. Unfortunately, influence of the salt content for the determination of metals in samples with saline matrices, as well as the lack of multi elemental analysis and the incapacity for speciation studies has reduced their application to water samples [10]. Electrochemical stripping voltammetry has been widely recognized as a powerful technique for the determination of trace metal ions due to its unique ability to preconcentrate target metal ions during the accumulation step [12]. Over the past decades, the mercury electrode has long been widely used as the working electrode. However, the toxicity of mercury restricts its use [13]. Thus it is very important to develop a sensitive, selective and non-toxic electrode for the determination of heavy metals by stripping methods. Over the past five decades, carbon paste, i.e. a mixture of carbon (graphite) powder and a binder (pasting liquid), has become one of the most popular electrode materials used for the laboratory preparation of various electrodes, sensors, and detectors [14]. In recent years, to improve the sensitivity, selectivity, detection limit and other features of carbon paste electrode (CPE), chemically modified carbon paste electrodes (CMCPEs) have been used. For example, within the electrode structure, various materials such as appropriate ligands, ion exchangers, functionalized nanoparticles (gold nano-particles and others), . . .have been used as modifiers [15–18]. The operation mechanism of such CMCPEs depends on the properties of the modifier materials used to improve selectivity

378

A. Afkhami et al. / Electrochimica Acta 89 (2013) 377–386

Scheme 1. The structure of the synthesized Schiff base Z-BHPBP.

and sensitivity toward the target species. Initially, non-conductive reagents, such as mineral oil or paraffin oil were used as binders [14]. A review of literature revealed that the Schiff bases are the excellent choice as modifier for the fabrication of ion sensors due to their peculiar properties [19,20]. Room temperature Ionic liquids RTILs are good choices as binder in carbon paste electrodes owing to their interesting properties, such as stability, low vapor pressure, low toxicity, low melting temperature, high ionic conductivity and good electrochemical and thermal stability [21–23]. Combination of the above mentioned characteristics makes modified CPEs as a good electrode for the detection of metal ions. In this work we applied multi-walled carbon nanotubes (MWCNTs), the air and water stable ionic liquid (1-butyl1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP]Tf2 N)) and a newly synthesized Schiff base for the construction of a modified CPE. The electrode was used to the simultaneous determination of trace amounts of Cd2+ and Pb2+ by square wave anodic stripping voltammetry (SWASV). Using the Schiff base caused an improvement in selectivity and sensitivity of the determination of these metal ions because of their interactions with the applied ligand [24] and MWCNTs improved the sensitivity of the determination and mechanical stability of the electrode [25,26]. compound (Z)-2-((3-(4-(3-(5-bromo-2-hydroxybenThe zylideneamino)propyl)piperazin-1-yl)propylimino) methyl)-4bromophenol (Z-BHPBP) (Scheme 1), (L) is a newly synthesized Schiff base capable to form complexes with target ions. It is successfully applied as a selective agent for the voltammetric determination of lead and cadmium at a carbon paste electrode modified with MWCNT and a RTIL as the binder. Such properties caused obtaining low detection limits in the voltammetric determinations with modified CPEs as working electrode. The created selectivity in this method makes the electrode very suitable for the detection of trace amounts of these metal ions in various real samples. To the best of our knowledge, this electrode represents the better limit of detection, selectivity and wider linear range rather than other reported electrodes for determination of lead and cadmium simultaneously. Besides this producer, a wider range of real samples have been investigated including tobacco, human hair, soya, sugar, rice, tap water, chemical waste water, shrimp and fish. 2. Experiment 2.1. Apparatus and chemicals All electrochemical experiments including cyclic voltammetry (CV) and SWASV were performed using a Metrohm model 797 VA Computrac polarograph. A conventional three-electrode system was used with a carbon paste working electrode (unmodified or modified), a saturated Ag/AgCl reference electrode and a Pt wire as the counter electrode. In addition, impedance spectroscopy was performed in an analytical system using an Autolab Model 302N, potentiostat/galvanostat connected to a three-electrode cell and linked with a computer (Pentium IV, 1200 MHz). The system was

run on a PC using Nova 1.7 software. For impedance measurements, a frequency range of 100 kHz to 1.0 Hz was employed. The AC voltage amplitude was 5 mV, and the equilibrium time was 10 min. A magnetic stirrer (PAR-305) with a Teflon-coated magnet was used to provide the convective transport during the preconcentration step. An Agilent 8453 diode array UV–vis spectrophotometer (Agilent, USA) equipped with a 1.0 cm path length quartz cell was used to obtain the absorbance spectra and absorbance curves. Infrared spectra were recorded with a Fourier transform infrared (FTIR) spectrometer (FT-IR, PerkinElmer, spectrum 100). A Mettler balance (Toledo-AB104, Greifensee, Switzerland) was used for weighing the solid materials. A micropipetter (Eppendorf – Multipette® plus) was used for transferring the solutions throughout the present experimental work. A pH-meter, Model 713, with a glass electrode (Metrohm, Swiss), was used to determine pH values of the solutions. All the chemicals were of analytical grade purchased from Merck or Aldrich. Unless otherwise stated, all the solutions were prepared with doubly distilled water (DDW). The 100.0 mg L−1 cadmium and lead standard solutions were prepared by the dissolution of adequate amounts of nitrate salts in DDW. MWCNT (purity more than 95%) with outer diameter between 5 and 20 nm, inner diameter between 2 and 6 nm and tube length from 1 to 10 ␮m was from Plasma-Chem GmbH (Germany). Paraffin oil and graphite powder were obtained from Merck Company and used as received. Britton–Robinson (B–R) universal buffer, acetate buffer, NH4 Cl buffer solution, KNO3 solution and phosphate buffer were prepared in DDW and were tested as supporting electrolytes. pH adjustments were performed with 0.01–1.0 mol L−1 HCl or NaOH solutions. 2.2. Preparation of modified carbon paste electrode The modified carbon paste electrode was prepared by mixing 75% (w/w) Z-BHPBP (L), MWCNT and graphite powder (L: MWCNT: graphite powder 15:18:44%) with 25% (w/w) [BMP]Tf2 N (as the binder) in a mortar and pestle. The mixture amount of 0.25 g was homogenized in a mortar for 30 min and the resulted composite was dispersed in dichloromethane (for more homogeneity of the electrode composite components leading to an increase in the reproducibility after each electrode surface polishing). The homogenized composite was stirred by a magnetic stirrer till the solvent evaporated completely. Then, the prepared modified composite was air dried for 24 h. Finally the homogenized paste was then inserted into a plastic needle-type capillary tube with a 1.5 mm diameter and a 5 cm length, using a 0.5 mm diameter copper wire connected to the measurement system. 2.3. Voltammetric measurements The analysis of Cd2+ and Pb2+ using SWASV was carried out in a 10.0 mL aliquot (pH 3.5) using the following steps after purging with nitrogen for at least 5 min: (a) in pre-conditioning step a potential of 0.900 V vs. Ag/AgCl was applied for 30 s before each measurement to ensure dissolution of the remaining deposits on the surface of the modified electrode; (b) the preconcentration step proceeded at −1.100 V vs. Ag/AgCl for 190 s; at the end of the preconcentration time, stirring was stopped and a 8 s rest period was allowed for the solution to become quiescent; (c) the anodic square wave voltammograms were recorded when the potential was swept from −1.250 to −0.220 V vs. Ag/AgCl. In the pre-conditioning and preconcentration processes, the detection solutions were stirred with a magnetic stirrer. The peak currents at about −1.000 and −0.430 V vs. Ag/AgCl for Cd2+ and Pb2+ were measured, respectively. All measurements were carried out at room temperature (25.0 ± 0.1 ◦ C). Calibration graphs were prepared for the peak current against Cd2+ and Pb2+ concentrations. Calibration

A. Afkhami et al. / Electrochimica Acta 89 (2013) 377–386

379

Fig. 1. UV–vis absorption spectra for the titration of L with (A) Pb2+ , (B) Cd2+ , (C) Cu2+ and (D) Hg2+ in ethanol at 25 ◦ C. The arrows indicate the direction of the absorbance changes with the increase in molar ratio of the metals (M:L).

graphs were prepared by plotting the net anodic peak currents vs. Cd2+ and Pb2+ concentrations in the solutions. 2.4. Synthesis of (Z)-2-((3-(4-(3-(5-bromo-2hydroxybenzylideneamino)propyl)piperazin-1yl)propylimino)methyl)-4-bromophenol (Z-BHPBP) (Scheme 1) 5-Bromosalicylaldehyde (1 mmol, 0.201 g), N,N -bis(3-aminopropyl)piperazine (0.5 mmol, 0.1 g) and silica gel (0.5 g) were mixed together in a tube and irradiated in a microwave oven. The progress of the reaction was monitored by gas chromatography. Upon the completion of the reaction, the crude product was recrystallized from ethanol and then dried over sodium sulfate. The solvent was evaporated and the product was washed with diethyl ether and dried [27]. The product was identified by melting point, mass spectrum, elemental analysis and IR and 1 H and 13 C NMR spectra. Anal. Calcd. for C24 H30 Br2 N4 O2 (MW: 566.33): C, 50.90; H, 5.34; N, 9.89. Found: C, 50.92; H, 5.30; N, 9.90%. Yield: 0.25 g (88%). M.p.107.0–109.0 1C. IR (Nujol, cm−1 ): 1635 [(C N)], 1163(s) [(C O)]. MS (EI): m/z: 566 [Schiff base]+ . 1 H NMR (400 MHz, CDCl3 , ppm) ıH: 1.81(m, 4H, 9-H), 2.33–2.40 (m, 12H, 10-H and 11H), 3.58 (t(3J = 8.0 Hz), 4H, 8-H), 6.79–7.31 (m, 6H, aromaticring), 8.20 (s, 2H,7-H, C N), 13.49 (bs, 2H, OH). 13 C NMR (400 MHz, CDCl3 , ppm) ıC: 27.8(t, C-9), 53.2(t, C-11), 55.8(t, C-10), 57.4(t, C-8), 109.8(s, C-6), 119.1(d, C-5), 120.1(s, C-2), 153.2, 154.8(d, C-3orC-4) 160.5(s, C-5) (aromatic ring), 163.8(d, C-7, C N).

The proposed method was applied to determine lead and cadmium in tuna fish, shrimp, tobacco, rice, hair, sugar and soya samples. An amount of approximately 100–300 mg of hair was sampled for each subject from the occipital zone of the head at 1 cm from the scalp by using surgical scissors with tungsten carbide covered cutting edges, in order to avoid sample contamination from metals released through the friction exerted during sampling. All other samples were purchased from local markets in Hamedan and were then prepared as in the reported procedure [28–33]. 2.6. Spectrophotometric titrations Standard stock solutions of ligands (1.0 × 10−3 mol L−1 ) and metal ions (1.0 × 10−2 mol L−1 ) were prepared by dissolving appropriate and exactly weighed (with an accuracy of 0.0001 g) pure solid compounds in pre-calibrated 25.0 mL volumetric flasks and diluted to the mark with ethanol. Working solutions were prepared by appropriate dilution of the stock solutions. According to the spectra reported in Fig. 1, titration of the ligand solution was carried out by the addition of microliter amounts of a concentrated standard solution of the metal ion in ethanol (1.0 × 10−2 mol L−1 ) using a precalibrated micropipette, followed by absorbance intensity reading at room temperature (25.0 ± 0.1 ◦ C) at the related max . 3. Results and discussion 3.1. Spectrophotometric study of the interaction between Schiff base and metal ions

2.5. Preparation of real samples In order to demonstrate the applicability and reliability of the method for real world samples, several samples, including tap water, chemical waste water, fish, shrimp, rice, tobacco, soya, sugar and hair samples were prepared and analyzed by the method. Tap water samples were taken from our research laboratory (BuAli Sina University, Hamedan, Iran) without pretreatment before determination, the pH value was adjusted at 3.5 with B–R buffer.

The Z-BHPBP with two oxygens and four nitrogens donating Schiff base is insoluble in water. Consequently, we studied the complexation of this ligand with several metal ions in ethanol. Typical spectra are shown in Fig. 1. As it can be observed from Figs. 1A–C, significant apparent changes in the spectrum of ligand, on addition of metal ions indicate that among the investigated cations the ligand has stronger interaction with Pb2+ and Cd2+ and Cu2+ ions. Also, Fig 1D is useful to compare the absorbance changes of the

380

A. Afkhami et al. / Electrochimica Acta 89 (2013) 377–386

Table 1 The overall stability constants of complexation between investigated metal ions with L in ethanol at 25 ◦ C and (N = 6). Metal ion

Cd2+

Cu2+

Hg2+

Co2+

Pb2+

Ni2+

M:L Log ␤

1:1 6.40 ± 0.11

1:2 5.90 ± 0.18

– –

1:1 3.81 ± 0.09

1:1 6.23 ± 0.21

2:1 4.23 ± 0.07

ligand after the addition of target metal ions (Cd2+ and Pb2+ ) with a metal, e.g. Hg2+ , which has lower affinity to ligand. It is clear due to the formation of a very weak complex, the spectral changes of the L after addition of Hg2+ ions is negligible while in Fig. 1A–C the spectral changes are significant due to the formation of stable complexes. Further analyzing the absorbance at maximum absorbance wavelength using a nonlinear least-squares curve-fitting program (KINFIT program) the stoichiometry and stability constants of L complexes with the investigated metal ions were obtained [34]. The results are given in Table 1. From the results observed for analytes at a mole ratio of 1.0, it can be immediately concluded that a 1:1 complex of ML is formed in ethanol solution. The unique nature of Schiff base donor is enhanced by the existence of widely spread ␲-conjugation system. Therefore, the Schiff base may be used as an efficient modifier in the electrodes for the preconcentration of metal ions specially Cd2+ , Pb2+ and Cu2+ ions (Fig. 1A–C). We decided to use L as a suitable modifier for the selective determination of Cd2+ , and Pb2+ using carbon nanotube paste electrode. 3.2. Voltammetricbehavior of different electrodes

Fig. 3. Impedance plots for (a) CPE, (b) CPEIL and (c) MWCNT-CPEIL at optimum composition, for 1.0 × 10−3 mol L−1 K3 [Fe(CN)6 ] in 1.0 mol L−1 KCl.

CV was used to characterize the different electrodes to explain the effect of RTIL and MWCNTs. Cyclic voltammograms for 1.0 mmol L−1 Fe(CN)6 3−/4− in 0.1 mol L−1 KCl solution on CPE, CPEIL , MWCNT-CPE and MWCNT-CPEIL are shown in Fig. 2. The quasireversible one-electron redox behavior of ferricyanide ions was observed on the bare CPE with a peak separation (Ep ) of 0.920 V at the scan rate of 80 mV s−1 . After the preparation of the electrode using RTIL and MWCNTs, the peak currents for Fe(CN)6 3−/4− increased, while the Ep decreased. Ep was found to be0.760, 0.710 and 0.560 V for CPE with the ionic liquid [BMP]Tf2 N as the binder (CPEIL ), MWCNTs modified CPE with paraffin as the binder (MWCNT/CPE) and MWCNT/CPE with the ionic liquid as the binder (MWCNT/CPEIL )respectively. Comparison of the Ep obtained for different electrodes indicates that the modification of electrodes with MWCNTs and using RTIL as the binder instead of paraffin caused easier and faster charge transfer at the electrode surface. In an attempt to clarify the differences among the electrochemical performance of the CPE, CPEIL and MWCNT/CPEIL , electrochemical impedance spectroscopy (EIS) was used as a

procedure for the characterization of each electrode surface. As such, the Nyquist plots for 1.0 × 10−3 mol L−1 K3 [Fe(CN)6 ] in 1.0 mol L−1 KCl showed a significant difference in responses for electrodes (Fig. 3). The semicircular elements correspond to the charge transfer resistances (Rct ) at the electrode surface with a large diameter, was observed for the bar CPE in the frequency range 10−1 to 106 Hz. However, the diameter of the semicircle diminished when CPEIL and MWCNT/CPEIL were employed. Additionally, the charge transfer resistance (Rct ) values obtained from this observation implied that the charge transfer resistance of the electrode surface decreased and the charge transfer rate increased upon using MWCNT/CPEIL . A Warburg at 45◦ was also observed for all the electrodes of interest. The Rct value for the MWCNT-CPEIL electrode was less than that for CPE and CPEIL . Fig. 4 shows the SWASVs of 500.0 ng mL−1 of Pb2+ and 400 ng mL−1 Cd2+ at CPE, CPEIL , MWCNT/CPE, MWCNT/CPEIL and Z-BHPBP modified MWCNT/CPEIL (Z-BHPBP/MWCNT/CPEIL ) in the positive going scan in the potential range −1.250 to −0.220 V vs.

Fig. 2. Cyclic voltammograms for 1.0 × 10−3 mol L−1 K3 [Fe(CN)6 ] in 1.0 mol L−1 KCl obtained at different electrodes and scan rate of 80 mV s−1 .

Fig. 4. Square wave voltammograms for (a) CPE, (b) CPEIL , (c) MWCNT/CPE, (d) MWCNT/CPEIL and (e) Z-BHPBP/MWCNT/CPEIL in 500 ng mL−1 of Pb2+ and 400 ng mL−1 Cd2+ in pH 3.5 B-R buffer solution. Conditions: deposition potential, −1.100 V vs. Ag/AgCl; deposition time, 190 s; resting time, 8 s; SW frequency, 40 Hz; pulse amplitude, 65.0 mV.

A. Afkhami et al. / Electrochimica Acta 89 (2013) 377–386

381

Fig. 5. SEM images for CPE (A) and MWCNTs paste electrode (B).

Ag/AgCl. The response of the CPE with paraffin as the binder (curve a) does not show any well-defined peaks in the potential range −1.250 to −0.220 V vs. Ag/AgCl. Curve b refers to the signal at CPE with the ionic liquid as binder (CPEIL ), shows two peaks at about −1.000 and −0.430 V vs. Ag/AgCl corresponding to the oxidation of Cd and Pb at the electrode surface, respectively. The difference between curves a and b can be due to the higher conductivity of CPEIL . In the deposition time Cd2+ and Pb2+ accumulate by reduction at the electrode surface at deposition potential of −1.100 V vs. Ag/AgCl. The reduced metal ions are then oxidized in the stripping step by scanning the potential in the range −1.250 to −0.220 V vs. Ag/AgCl and the SWASV peaks are observed. However, the signals of the MWCNT/CPE (curve c) are higher than those for CPE but smaller than those for CPEIL , thus it can be concluded that MWCNTs can successfully increase the electrochemical signals. The observed increase in the anodic peak current, using MWCNT, represents the larger microscopic area of the modified electrode (resulted by the presence of MWCNT). As can be seen from curve d, the peak currents of both metal ions at the MWCNT/CPEIL are more intense than those at the MWCNT/CPE. The electrochemical signals for both metal ions have a dramatic increase at the Z-BHPBP/MWCNT/CPEIL (curve e) as compared with those at MWCNT/CPEIL . This can be attributed to the L for wellorganized accumulation of Pb2+ and Cd2+ close to the electrode surface of the electrode by the complexation reaction (as described in Section 3.1). So this modified electrode was chosen for the determination of lead and cadmium simultaneously in the synthetic and several real samples. Scheme S1 shows the suggested mechanism for the complexation, reduction and accumulation, and stripping of the analytes.

electrode. So the best ratio of the L in carbon nanotube paste composition was 15% (w/w). Also, the effect of the amount of MWCNT on the voltammetric response was also studied between 0.0 and 35%. The peak current was increased by increasing the amount of MWCNT up to 18%. Higher amounts of MWCNT in the matrix of the modified electrode did not show a considerable change in the peak current. As it is shown in SEM images for carbon paste electrode and MWCNTs paste electrode (Fig. 5), at the surface of CPE (Fig. 5A), the layer of irregular flakes of graphite powder was present and isolated with each other. By addition of MWCNTs to the carbon paste, most of the MWCNTs were in the form of small bundles or single tubes. It can be seen that MWCNTs were distributed on the surface of electrode with special three dimensional structures (Fig. 5B)

3.3.2. Effect of pH The effect of pH of the buffer solution on the electrochemical responses of Pb2+ and Cd2+ was studied. Fig. 6 shows the effect of pH in the range 2.0–8.0 on the determination of the lead and cadmium ions. For both the ions, the peak current increased by increasing pH in the range 2.0 to about 3.5 and decreased at higher pHs. At pH 3.5 the peak current reached a maximum for both ions. Therefore, B–R buffer of pH 3.5 was used for further studies. The decrease in the currents at pHs lower than 3.50 can be due to the competition between proton ion and the metal ions for binding to the donating atoms of the Schiff base at the surface of Z-BHPBP/MWCNT/CPEIL . On the other hand, when the pH value was below 3.5 ligand can slowly dissolve in acidic solution (Schiff base is decomposed) and lose its ability in deposing of Pb2+ and Cd2+ ions. At pHs higher than 3.5, the decrease in the anodic peak current may be due to the hydrolysis of cations.

3.3. Effects of the variables The experimental variables were optimized as below. 3.3.1. Carbon paste electrode composition Some important features of the carbon paste electrodes, such as the properties of the binder, the binder/graphite powder ratio and especially, the nature and amount of the nano-material used have been reported to significantly influence the sensitivity and selectivity of the electrodes [17]. The effect of the amount of L within the carbon nanotube paste electrode was investigated. The use of L as a modifier can greatly improve the sensitivity and selectivity of determinations, which is due to its functional sites. The peak current intensity was increased by increasing the amount of modifier, because the concentration of L on the surface of the modified electrode increased correspondingly. At 15% (w/w) of L, the largest peak current was obtained. However, the continuous increase of the amount of modifier caused a decrease in the peak current, because excessive L may result in a decrease in the conductivity of the

Fig. 6. Effect of pH on the stripping peak current for 450 ng mL−1 of (a) Cd2+ and (b) Pb2+ solution. Conditions: deposition potential, −1.100 V vs. Ag/AgCl; deposition time, 190 s; resting time, 8 s; SW frequency, 40 Hz; pulse amplitude, 65.0 mV.

382

A. Afkhami et al. / Electrochimica Acta 89 (2013) 377–386

In the beginning the increase in the frequency caused a relatively fast increase in the peak heights of metal ions owing to the increase in the effective scan rate up to 40 Hz. The SW frequency together with the step potential defines an effective scan rate. However, above the frequency of 40 Hz the peak currents started to level off and the peaks began to distort. The increase in the step potential did not significantly enhance the peak heights of Pb2+ and Cd2+ despite of the dramatic increase in the effective scan rate. Therefore, the well-shaped SWASV current peaks for quantitative measurements at low concentrations of metal ions were obtained at 65 mV potential amplitude, 40 Hz SW frequency and 6 mV step potential. 3.4. Simultaneously determination of lead and cadmium −1

Fig. 7. Effect of deposition potential on the stripping peak current for 450 ng mL of (a) Cd2+ and (b) Pb2+ solution. Conditions: pH 3.5; deposition time, 190 s; resting time, 8 s; SW frequency, 40 Hz; pulse amplitude, 65.0 mV.

3.3.3. Deposition potential The effect of the deposition potentials on signals was investigated. As shown in Fig. 7, the deposition potential changed from −0.600 to −1.400 V for 450 ng mL−1 of Pb2+ and Cd2+ , the negative shifts of deposition potential can clearly increase the reduction of Pb2+ and Cd2+ on the surface of electrode and increase the peak current. But the deposition potentials more negative than −1.100 V caused poor reproducibility of the stripping currents of Pb and Cd, because hydrogen evolution is significant at such negative potentials [35]. The hydrogen bubbles might damage the metal alloys deposition at the electrode surface and lead to a decrease in current signals at very negative potentials. Therefore, −1.10 V was applied for the next experiments. 3.3.4. Deposition time The effect of deposition time on the response of the solution of 450 ng mL−1 of Cd2+ andPb2+ , in pH 3.5, B–R buffer, in the range 30–250 s was studied (Fig. 8). The results are presented in Fig. 8. The anodic stripping peak current increased by increasing the deposition time up to 180 s, above which it remained nearly constant. Therefore, a deposition time of 190 s was selected for further works. 3.3.5. Voltammetric parameters In order to obtain well-defined SWASV signals, the voltammetric parameters (potential amplitude, SW frequency and step potential) have also been optimized. The electrochemical signals of Pb2+ and Cd2+ increased linearly by increasing pulse amplitude, but the resolution and the shape of the peaks also failed above 65 mV.

As mentioned before, lead and cadmium coexist in various real samples. Therefore, the next attempt was reserved to simultaneous determination of the target metal ions. The proposed SWASV method has been thus employed for simultaneous determination of Pb2+ and Cd2+ in synthetic and several real samples by using ZBHPBP/MWCNT/CPEIL . In this respect, two cases were studied. In the first case, the concentration of lead increased in the presence of a fixed concentration of cadmium (Fig. 9A) and vice versa (Fig. 9B). In the second case, both the ions were determined by simultaneous increase in their concentrations (Fig. 10). The LOD and linear range achieved from both cases have good conformity together. Therefore, it can be concluded that, by employing the proposed SWASV method, the simultaneous determination of both ions is as possible as their individual determinations. 3.5. Analytical parameters Calibration graphs were constructed under the optimum conditions described above using Z-BHPBP-MWCNT-CPEIL : SW frequency of 40 Hz, the pulse amplitude of 65.0 mV, deposition potential of −1.100 V, resting time of 8 s. Fig. 10 shows the net square wave voltammograms for different concentrations of Pb2+ and Cd2+ obtained under the optimum conditions. The characteristics of the calibration graphs are given in Table 2. The limit of detection, defined as LOD = 3Sb /m, where LOD, Sb and m are the limit of detection, standard deviation of the blank and the slope of the calibration graph, respectively. Sb was estimated by 8 replicate determinations of the blank signals. The repeatability of the electrode in the determination of Pb2+ and Cd2+ was evaluated by performing six determinations with the same standard solutions of Pb2+ and Cd2+ . The relative standard deviation (RSD) for the response of the electrode toward a 35 ng mL−1 of Cd2+ and 25 ng mL−1 of Pb2+ solutions was 2.30% and 3.56%, respectively. The reproducibility of the response of the electrode was also studied. Six electrodes were prepared from the same batch in six different days and they were evaluated by performing the determination of 50 ng mL−1 of Cd2+ and Pb2+ solutions. The RSD for the response of between electrodes was 2.85% and 3.45% for Cd2+ and Pb2+ , respectively. The results show that the repeatability and reproducibility of the sensor for the simultaneous determination of Cd2+ and Pb2+ is acceptable. The electrode and the graphite–Z-BHPBP–binder mixture were stable for 8 weeks. After that the response of the electrode decreased and the noise in the responses increased (analytical parameter were shown in Table 2). 3.6. Selectivity

Fig. 8. Effect of deposition time on the stripping peak current for 450 ng mL−1 of (a) Cd2+ and (b) Pb2+ solution. Conditions: pH 3.5; deposition potential,−1.100 V vs. Ag/AgCl; resting time, 8 s; SW frequency, 40 Hz; pulse amplitude, 65.0 mV.

To study the selectivity of the proposed method, the effects of various cations and anions on the simultaneous determination of 500 ng mL−1 of Pb2+ and Cd2+ was studied. An error of

A. Afkhami et al. / Electrochimica Acta 89 (2013) 377–386

383

Fig. 9. Net square wave voltammograms for Z-BHPBP/MWCNT/CPEIL in the solutions with different concentrations (bottom to top: 1.0, 10.0, 100.0, 180.0, 250.0, 300.0, 400.0, 550.0, 700.0, 900.0, 1100.0 ng mL−1 ) of Pb2+ in the presence of constant concentration of 1000 ng mL−1 of cadmium (A) and different concentrations (bottom to top: 0.40,10.0, 100.0, 180.0, 250.0, 300.0, 400.0, 550.0, 700.0, 900.0, 1200.0 ng mL−1 ) of Cd2+ in the presence of the constant concentration of 250 ng mL−1 of Pb2+ (B). Conditions: pH 3.5; deposition potential, −1.100 V vs. Ag/AgCl; deposition time, 190 s; resting time, 8 s; SW frequency, 40 Hz; pulse amplitude, 65.0 mV.

Fig. 10. Net square wave voltammograms for Z-BHPBP/MWCNT/CPEIL in the solutions with different concentrations (from bottom to top, 0.40, 10.0, 100.0, 180.0, 250.0, 300.0, 400.0, 550.0, 700.0, 900.0, 1200.0 ng mL−1 ) of Cd2+ and (from bottom to top, 1.0,10.0, 100.0, 180.0, 250.0, 300.0, 400.0, 550.0, 700.0, 900.0, 1100.0 ng mL−1 of) Pb2+ . Conditions: pH 3.5; deposition potential, −1.100 V vs. Ag/AgCl; deposition time, 190 s; resting time, 8 s; SW frequency, 40 Hz; pulse amplitude, 65.0 mV. Table 2 Analytical parameter of the proposed electrode for the determination of Pb2+ and Cd2+ . LRa (cm−3 )

Metal ion 2+

Pb Cd2+ a b

0.40–1100.00 1.00–1200.00

LR; Linear range. CLE, Calibration line equation.

CLEb −3

I (␮A) = 0.0757CPb (ng cm ) + 8.3318 I (␮A) = 0.1166CCd (ng cm−3 ) + 14.102

R2

LOD (ng cm−3 )

0.9988 0.9996

0.25 0.74

384

A. Afkhami et al. / Electrochimica Acta 89 (2013) 377–386

Table 3 Simultaneous determination of Cd2+ and Pb2+ in several real samples by the proposed method (N = 6). Sample Tobacco

Added (ng g−1 )

Analyte 2+

Pb

Cd2+ Human hair

Pb2+ Cd2+

Shrimp

Pb2+ Cd2+

Fish

Pb2+ Cd2+

Rice

Pb2+ Cd2+

Sugar

Pb2+ Cd2+

Soya

Pb2+ Cd2+

Found (ng g−1 )

ICP-OES method (ng g−1 )

Recovery (%)

0 20 0 20

2.36 21.64 5.07 25.62

± ± ± ±

0.07 0.55 0.17 0.70

– 96.70 – 102.20

2.16 21.58 5.12 25.73

± ± ± ±

0.05 0.43 0.10 0.64

0 20 0 20

28.37 47.30 82.30 104.60

± ± ± ±

0.95 1.25 2.34 3.23

– 97.80 – 102.24

28.14 48.00 82.07 104.11

± ± ± ±

1.04 1.42 2.20 2.90

0 20 0 20

34.48 52.80 17.08 37.02

± ± ± ±

1.13 1.50 0.56 1.10

– 96.90 – 99.80

34.57 53.10 17.20 36.84

± ± ± ±

0.97 1.23 0.70 1.30

0 20 0 20

9.12 28.27 5.60 26.70

± ± ± ±

0.30 0.83 0.18 0.78

– 97.10 – 104.30

9.04 28.42 5.42 27.10

± ± ± ±

0.18 0.91 0.24 1.02

0 20 0 20

34.12 53.73 1.68 22.08

± ± ± ±

1.21 1.60 0.06 0.66

– 99.20 – 101.18

34.32 54.12 1.80 21.72

± ± ± ±

1.13 1.23 0.10 0.82

0 20 0 20

14.23 34.00 6.87 26.04

± ± ± ±

0.47 1.02 0.22 0.85

– 99.30 – 96.90

14.53 33.82 7.12 25.74

± ± ± ±

0.28 1.14 0.45 0.72

0 20 0 20

9.16 28.43 11.86 30.84

± ± ± ±

0.42 0.86 0.39 1.04

– 97.40 – 96.70

9.32 28.20 11.31 31.27

± ± ± ±

0.63 0.94 0.23 0.94

Table 4 Results of metal ions determination in various water samples using proposed method (N = 6). Analyte (ng cm−3 )

Sample Tap water

2+

Pb

Cd2+ Pb2+

Chemical wastewater

Cd2+

Added (ng cm−3 )

Found (ng cm−3 )

Recovery (%)

ICP-OES method (ng cm−3 )

0 25 0 25

4.69 ± 0.15 29.18 ± 0.78 1.23 ± 0.09 26.53 ± 0.80

– 98.30 – 101.1

4.63 29.06 1.19 26.16

0 25 0 25

29.17 55. 21 19.38 43.19

– 101.90 – 97.30

29.24 ± 0.86 55. 09 ± 1.23 19.47 ± 0.42 43.74 ± 1.10

±3% was considered tolerable. The ions Li+ , Na+ , NH4+ , Ag+ , Mg2+ , Ba2+ , Sr2+ ,Mn2+ , Hg2+ , Cu2+ , Ni2+ , Ca2+ , Al3+ , SO4 2− , ClO4 − , I− , Br− , NO2 − and NO3 − did not interfere on the determination of Cd2+ and Pb2+ at a 2.5:1 interferent:analyte mass ratio. But EDTA interfered on the determination of the analytes by significant suppression of their signals, because it forms stable complexes with the

± ± ± ±

0.95 1.18 0.68 1.24

± ± ± ±

0.09 0.81 0.06 0.69

analytes and prevents their accumulation on the electrode. As the results showed, the electrode is highly selective for the simultaneous determination of lead and cadmium in the presence of different cations and anions especially Cu2+ . Due to the strong interaction of Cu2+ with this ligand, it was expected that copper would have the ability to create disturbance

Table 5 Comparison of some characteristics of the different modified electrodes reported for the determination of Cd2+ and Pb2+ . Electrode* CPE CPE CPE Pt Pt CILE BINMSDE CPE BFE CMCPEIL

Modifier* SiO2 –Al2 O3 mixed-oxide Diacetyldioxime Antimony film Kaolin Kaolin Hydroxyapatite Bismuth–Nafion-medical stone Montmorillonite-bismuth GNNC MWCNT-Schiff base

Method* DPASV DPASV SWASV SWASV SWASV SWASV SWASV SWASV DPASV SWASV

LOD for Cd (␮mol dm−3 ) – 4.0 × 10−2 ≈7.1 × 10−3 0.0054 – 5.0 × 10−4 4.2 × 10−3 3.5 × 10−1 9.0 × 10−2 6.6 × 10−4

LOD for Pb (␮mol dm−3 ) −3

≈1.1 × 10 1.0 × 10−2 ≈9.7 × 10−4 – 3.6 × 10−3 2.0 × 10−4 3.4 × 10−4 2.0 × 10−1 2.0 × 10−2 1.2 × 10−4

Ref [36] [37] [38] [39] [40] [41] [42] [43] [44] This work

* CPE; carbon paste electrode, DPASV; differential pulse anodic stripping voltammetry, SWASV; square-wave anodic stripping voltammetry, CILE; carbon ionic liquid electrode, BINMSDE; bismuth–nafion-medical stone doped disposable electrode, BFE; bismuth film electrode, GNNC; graphite nanofibers–nafion composite.

A. Afkhami et al. / Electrochimica Acta 89 (2013) 377–386

in the measurement of target ions. But given that the complexes formed between Cu2+ and the ligand has a molar ratio of 1–2, ML2 , it can be concluded that this ligand as a modifier on the electrode surface collects smaller amounts of copper ions compared with target ions. 3.7. Applications The applicability and feasibility of the proposed electrode for the analysis of real samples with different matrices was assessed by its application to the simultaneous determination of Pb2+ and Cd2+ ions in various real samples including water, fish tissue, human hair and several foodstuff samples. The results are collected in Tables 3 and 4. The results obtained by this method show a good agreement with those obtained by ICP-OES. The results confirm applicability of the proposed method for precise and accurate determination of the Cd2+ and Pb2+ ions in a wide variety of real samples with different complex matrices. Fig. S1(A) shows the electro chemical signal of the metal ions in fish samples and the corresponding standard addition plots. As seen in Fig. S1 the proposed sensor is capable to determine the target metal ions the real samples without any significant interferes or matrix problems. 4. Conclusions This work demonstrates that new modified MWCNT-carbon paste electrode fabricated from Z-BHPBP can be used for quantification of lead and cadmium ions in different samples. The proposed electrode exhibits excellent voltametric performance. It responds to lead and cadmium ions, presents a good selectivity over most of the cations and showed a low detection limit. Z-BHPBP formed stable complexes with Cd2+ and Pb2+ ions in the solution and also at the solution/electrode interface and the complexed ions were reduced at the electrode surface in the deposition step. The reduced ions were then monitored by SWASV. By a simple polishing of the electrode on a paper, the renewable active surface of the CMCPE could be obtained easily which permits to eliminate the irreversible contamination of the surface and to minimize the memory effects, especially in real sample analysis. The sensor is characterized by a relatively fast response, long term and responsive potential stability and was successfully applied to the determination of Pb2+ and Cd2+ in different real samples. The comparison results of the proposed sensor with some reported sensors for the determination of Pb2+ and Cd2+ are given in Table 5. The proposed sensor provides higher sensitivity, lower limit of detection, wider linear range and higher selectivity over most of the reported electrodes. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.electacta.2012.11.050. References [1] D.D. Runnells, T.A. Shepherd, E.E. Angino, Determining natural background concentrations in mineralized areas, Environmental Science and Technology 26 (1992) 2316. [2] T. Kemper, S. Sommer, Estimate of heavy metal contamination in soils after a mining accident using reflectance spectroscopy, Environmental Science and Technology 36 (2002) 2742. [3] A. Jang, Y. Seo, P.L. Bishop, The removal of heavy metals in urban run off by sorption on mulch, Environmental Pollution 133 (2005) 117. [4] J.O. Nriagu, J.M. Pacyna, Quantitative assessment of worldwide contamination ofair, water and soils by trace metals, Nature 333 (1988) 134. [5] E. Shams, R. Torabi, Determination of nanomolar concentrations of cadmium by anodic-stripping voltammetry at a carbon paste electrode modified with zirconium phosphated amorphous silica, Sensors and Actuators B 117 (2006) 86.

385

[6] K. Leopold, M. Foulkes, P.J. Worsfold, Preconcentration techniques for the determination of mercury species in natural waters, TrAC – Trends in Analytical Chemistry 28 (2009) 426. [7] N. Pourreza, H. Parham, A.R. Kiasat, K. Ghanemi, N. Abdollahi, Solid phase extraction of mercury on sulfur loaded with N-(2-chloro benzoyl)-N phenylthiourea as a new adsorbent and determination by cold vapor atomic absorption spectrometry, Talanta 78 (2009) 1293. [8] H. Bagheri, A. Afkhami, M. Saber-Tehrani, H. Khoshsafar, Preparation and characterization of magnetic nanocomposite of Schiff base/silica/magnetite as a preconcentration phase for the trace determination of heavy metal ions in water, food and biological samples using atomic absorption spectrometry, Talanta 97 (2012) 87. [9] A. Afkhami, T. Madrakian, H. Siampour, Highly selective determination of trace quantities of mercury in water samples after preconcentration by the cloud-point extraction method, International Journal of Environment Analytical Chemistry 86 (2006) 1165. [10] A. Afkhami, M. Saber-Tehrani, H. Bagheri, Simultaneous removal of heavymetal ions in wastewater samples using nano-alumina modified with 2,4-dinitrophenylhydrazine, Journal of Hazardous Materials 181 (2010) 836. [11] Y. Li, Y. Jiang, X.P. Yan, Z.M. Ni, Determination of trace mercury in environmental and foods samples by online coupling of flow injection displacement sorption preconcentration to electrothermal atomic absorption spectrometry, Environmental Science and Technology 36 (2002) 4886. [12] K. Kalcher, J.M. Kauffmann, J. Wang, I. Svancara, K. Vytras, C. Neuhold, Z. Yang, Sensors based on carbon paste in electrochemical analysis: a review with particular emphasis on the period 1990–1993, Electroanalysis 79 (1995) 5. [13] W. Kemula, Z. Kublik, S. Godowski, Analytical application of the hanging mercury drop electrode: determination of impurities in high purity zinc, Journal of Electroanalytical Chemistry 1 (1959) 91. [14] M.Mzloum-Ardakani, H. Beitollah, M.K. Amini, F. Mirkhalaf, M.A. Alibeik, New strategy for simultaneous and selective voltammetric determination of norepinephrine, acetaminophen and folic acid using ZrO2 nanoparticles-modified carbon paste electrode, Sensors and Actuators B 151 (2010) 243. [15] A. Afkhami, H. Ghaedi, Multiwalled carbon nanotube paste electrode as an easy, inexpensive and highly selective sensor for voltammetric determination of Risperidone, Analytical Methods 4 (2012) 1415. [16] A. Afkhami, T. Madrakian, S.J. Sabounchei, M. Rezaei, S. Samiee, M. Pourshahbaz, Construction of a modified carbon paste electrode for the highly selective simultaneous electrochemical determination of trace amounts of mercury(II) and cadmium(II), Sensors and Actuators B 161 (2012) 542. [17] A. Afkhami, T. Madrakian, H. Ghaedi, Construction of a chemically modified electrode for the selective determination of nitrite and nitrate ions based on a new nanocomposite, Electrochimica Acta 66 (2012) 255. [18] A.A. Ensafi, H. Karimi-Maleh, S. Mallakpour, Simultaneous determination of ascorbic acid, acetaminophen, and tryptophan by square wave voltammetry using N-(3,4-dihydroxyphenethyl)-3,5-dinitrobenzamide-modified carbon nanotubes paste electrode, Electroanalysis 24 (2012) 666. [19] H.A.T. Rihai, Complexes of zinc, cadmium and mercury(II) with the zwitterionic form of NN -ethylenebis (salicylideneimine), Polyhedron 2 (1983) 723. [20] D.A. Atwood, Cationic group 13 complexes, Coordination Chemistry Reviews 176 (1998) 407. [21] M.R. Ganjali, H. Khoshsafar, F. Faridbod, A. Shirzadmehr, M. Javanbakht, P. Norouzi, Room temperature ionic liquids (RTILs) and multiwalled carbon nanotubes (MWCNTs) as modifiers for improvement of carbon paste ion selective electrode response; A comparisonstudy with PVC membrane, Electroanalysis 21 (2009) 2175. [22] M.R. Ganjali, H. Khoshsafar, A. Shirzadmehr, M. Javanbakht, F. Faridbod, Improvement of carbon paste ion selective electrode response by using room temperature ionic liquids (RTILs) and multi- walled carbon nanotubes (MWCNTs), International Journal of Electrochemical Science 4 (2009) 435. [23] A. Afkhami, T. Madrakian, A. Shirzadmehr, H. Bagheri, M. Tabatabaee, A selective sensor for nano level detection of lead (II) in hazardous wastes using ionicliquid/Schiffbase/MWCNTs/nanosilica as a highly sensitive composite, Ionics 4 (2009) 435. [24] H.J. Wang, Stripping Analysis: Principles, Instrumentation and Application, VCH, Deerfield Beach, FL, 1985. [25] A. Jorio, M.S. Dresselhaus, G. Dresselhaus (Eds.), Carbon Nanotubes: Advanced Topics in the Synthesis, Structure, Properties, and Applications, Topics in Applied Physics, vol. 111, Springer-Verlag, Berlin, Heidelberg, 2008. [26] Z. Wang, Q. Liang, Y. Wang, G. Luo, Carbon nanotube-intercalated graphite electrodes for simultaneous determination of dopamine and serotonin in the presence of ascorbic acid, Journal of Electroanalytical Chemistry 540 (2003) 129. [27] H. Keypour, M. Rezaeivala, Y. Fall, A.A. Dehghani-Firouzabadi, Solvent-free synthesis of some N4O2, N 4S2 and N6 schiff base ligands assisted by microwave irradiation, Arkivoc 10 (2009) 292. [28] K. Helrich, Official Methods of Analysis, Association of Official Analytical Chemists (AOAC), Arlington, VA, 1990, p. 1. [29] F.A. Silva, I.L. Alcantara, P.S. Roldan, Florentino, P.M. Padilha, Determination of Hg in water by CVAAS using 2-aminothiazole modified silica, Eclética Química 30 (2005) 47. [30] W.Z. Yang, Q. Hu, J. Ma, L. Wang, G.G. Yang, G. Xie, Solid phase extraction and spectrophotometric determination of mercury in tobacco and tobacco additives with 5-(p-aminobenzylidene)-thiothiorhodanine, Journal of the Brazilian Chemical Society 17 (2006) 1039.

386

A. Afkhami et al. / Electrochimica Acta 89 (2013) 377–386

[31] S. Abbasi, A. Farmany, M.B. Gholivand, A. Naghipour, F. Abbasi, H. Khani, Kinetic-spectrophotometry method for determination of ultra-trace amounts of aluminum in food samples, Food Chemistry 116 (2009) 1019. [32] International Atomic Energy Agency (IAEA), Report on the Second Research Co-Ordination Meeting of IAEA, Neuherberg, Germany, 1985. [33] L. Hosseinzadeh, S. Abbasi, F. Ahmadi, Adsorptive cathodic stripping voltammetry determination of ultra-trace of lead in different real samples, Analytical Letters 40 (2007) 2693. [34] A. Afkhami, T. Madrakian, H. Tahmasebi, H. Keypour, H. Khanmohammadi, Interaction of new polyamine ligand N,N,N’,N’-tetrakis(2-salicylideneaminoethyl)butane-1,4-diamine with iodine in chloroform and dichloromethane solutions, Physics and Chemistry of Liquids 46 (2008) 372. [35] A. Krolicka, A. Bobrowski, A. Kowal, Effects of electroplating variables on the voltammetric properties of bismuth deposits plated potentiostaticall, Electroanalysis 18 (2006) 1649. [36] M. Ghiaci, B. Rezaei, R.J. Kalbasi, High selective SiO2 –Al2 O3 mixed-oxide modified carbon paste electrode for anodic stripping voltammetric determination of Pb(II), Talanta 73 (2007) 37. [37] C.G. Hu, K.B. Wu, X. Dai, S.S. Hu, Simultaneous determination of lead(II) and cadmium(II) at a diacetyldioxime modified carbon paste electrode by differential pulse stripping voltammetry, Talanta 60 (2003) 17. [38] E. Tesarova, L. Baldrianova, S.B. Hocevar, I. Svancara, K. Vytras, B. Ogorevc, Anodic stripping voltammetric measurement of trace heavy metals at

[39]

[40]

[41]

[42]

[43]

[44]

antimony film carbon paste electrode, Electrochimica Acta 54 (2009) 1506. M.A. El Mhammedi, M. Achak, M. Hbid, M. Bakasse, T. Hbid, A. Chtaini, Electrochemical determination of cadmium(II) at platinum electrode modified with kaolin by square wave voltammetry, Journal of Hazardous Materials 170 (2009) 590. M.A. El Mhammedi, M. Achak, M. Bakasse, A. Chtaini, Electroanalytical method for determination of lead(II) in orange and apple using kaolin modified platinum electrode, Chemosphere 76 (2009) 1130. Y.H. Li, X.Y. Liu, X.D. Zeng, Y. Liu, X.T. Liu, W.Z. Wei, S.L. Luo, Simultaneous determination of ultra-trace lead and cadmium at a hydroxyapatite-modified carbon ionic liquid electrode by square-wave stripping voltammetry, Sensors and Actuators B 139 (2009) 604. H. Li, J. Li, Z. Yang, Q. Xu, C. Hou, J. Peng, X. Hu, Simultaneous determination of ultratrace lead and cadmium by square wave stripping voltammetry with in situ depositing bismuth at Nafion-medical stone doped disposable electrode, Journal of Hazardous Materials 191 (2011) 26. L. Luo, X. Wang, Y. Ding, Q. Li, L. Jia, D. Deng, Voltammetric determination of Pb2+ and Cd2+ with montmorillonite-bismuth-carbon electrodes, Applied Clay Science 50 (2010) 154. D. Li, J. Jia, J. Wang, Simultaneous determination of Cd(II) and Pb(II) by differential pulse anodic stripping voltammetry based on graphite nanofibers–Nafion composite modified bismuth film, Talanta 83 (2010) 332.