Fabrication of a new carbon paste electrode modified with multi-walled carbon nanotube for stripping voltammetric determination of bismuth(III)

Electrochimica Acta 103 (2013) 206–210

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Fabrication of a new carbon paste electrode modified with multi-walled carbon nanotube for stripping voltammetric determination of bismuth(III) Fariba Fathirad a,b , Daryoush Afzali c,d,∗ , Ali Mostafavi a , Tayebeh Shamspur a , Samieh Fozooni e a

Chemistry Department, Shahid Bahonar University of Kerman, Kerman, Iran Young Research Society, Shahid Bahonar University of Kerman, Kerman, Iran c Department of Environment, Institute of Science and High Technology and Environmental Sciences, Graduate University of Advanced Technology, Kerman, Iran d Department of Chemistry, Graduate University of Advanced Technology, Kerman, Iran e Zarand High Education Center, Mining Engineering Department, Shahid Bahonar University of Kerman, Kerman, Iran b

a r t i c l e

i n f o

Article history: Received 12 January 2013 Received in revised form 29 March 2013 Accepted 30 March 2013 Available online 18 April 2013 Keywords: Modified carbon paste electrode Carbon nanotubes Stripping voltammetry Bismuth determination

a b s t r a c t A highly selective and sensitive carbon paste electrode modified with multi-walled carbon nanotubes and 4-[1-(4-methoxyphenyl) methylidene]-3-methyl-5-isoxazolone was used for accumulation and determination of trace amounts of bismuth using the differential pulse anodic stripping voltammetric method. The analytical procedure consisted of a closed-circuit accumulation step onto the modified carbon paste electrode. An anodic peak related to the oxidation of accumulated Bi(0) on the electrode surface was observed at about −0.05 V. The calibration curve was linear in the range of 1–400 ␮g L−1 . The limit of detection was 0.2 ␮g L−1 and the relative standard deviations for seven replicated determinations at 30, 100 and 300 ␮g L−1 of bismuth were 3.6, 2.4 and 1.6%, respectively. The modified electrode was applied for the determination of bismuth in pharmaceutical, biological and several water samples. The accuracy and precision of results were comparable to those obtained by the graphite furnace atomic absorption spectroscopy method. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Bismuth compounds are used in semiconductors, cosmetic preparation, metallurgy and alloy industry, iron castings, electronics, lubricating oils and greases, pigments, medicines, nuclear reactor cooling fluids and also as a reagent for the purification of sugar [1–3]. The use of bismuth and its compounds in different areas of life is increasing and therefore it is not surprising that numerous electroanalysis techniques such as potentiometric stripping analysis (PSA) [4], cathodic stripping voltammetry (CSV) [5] and anodic stripping voltammetry [6–9] have been developed to determine the quantity of bismuth in a compound. In the stripping voltammetric method, the analyte (metal ion) is adsorbed from the sample solution to the electrode surface according to a complexation between the metal ion and the ligand which is immobilized on the electrode surface. In the electroanalytical determination of trace elements, carbon paste electrodes (CPEs) [10] have been

∗ Corresponding author at: Department of Chemistry, Graduate University of Advanced Technology, Kerman, Iran. Tel.: +983426226611. E-mail address: daryoush [email protected] (D. Afzali). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.03.162

extensively used due to their wide anodic potential range, low residual current, ease of fabrication, easy surface renewal and low cost. The detection limit of CPE can be further lowered by chemically modifying the active electrode surface with suitable modifiers, such as nanomaterials [11,12], surfactants [13], complexes [14,15] and macrocycles [16,17]. The main reason for any modification is to improve the selectivity and sensitivity of the electrochemical measurements by preconcentrating the target analyte from a dilute solution onto the electrode surface. Nowadays, carbon nanotubes (CNTs) are also used in carbon paste electrodes [18–20]. CNTs have very interesting physicochemical properties, such as ordered structure with high aspect ratio, ultra-light weight, high mechanical strength, high electrical and thermal conductivity, metallic or semi metallic behavior and high surface area [21]. Transfer of electrons between electroactive species and electrodes increase with CNTs [22,23]. In the present work, the performance of a CPE modified with multi-walled carbon nanotubes (MWCNTs) and a new ligand of 4-[1-(4-methoxyphenyl) methylidene]-3-methyl-5-isoxazolone (MMMI) has been investigated for detecting trace levels of Bi(III) using differential pulse anodic stripping voltammetry (DPASV). The superior performance of the MMMI multi-walled

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nanotubes-modified carbon paste electrode (MMMI/MWNT/CPE) has been demonstrated by the determination of Bi(III) in several real samples. 2. Experimental 2.1. Apparatus All voltammetric experiments were performed using Autolab electroanalyzer Model PG-STAT-101 from Metrohm (Switzerland). The electrochemical cell consisted of a carbon paste modified electrode as a working electrode, Ag/AgCl/KCl (saturated) as a reference electrode and a platinum wire as an auxiliary electrode. A RH BKT/C (Germany) magnetic stirrer was employed in the deposition step. A Metrohm 827 pH meter (Switzerland) with a combined glass electrode for adjusting the pH of solutions was also used.

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2.5. Experimental procedure 2.5.1. Closed-circuit method A cell containing the desired concentration of Bi(III) in 0.1 mol L−1 HCl solution was subjected to the deposition voltage (−0.5 V) under stirring. Following the electrochemical deposition step and a short equilibration period (10 s), differential pulse (DP) voltammograms were recorded by potential scanning from −0.5 to 0.5 V. Each scan was repeated three times with a new surface of electrode for each analyzed solution and the mean of these voltammograms was obtained. The resulting oxidation peak at about −0.05 V was registered and its currents used as a measure of bismuth concentration. All experiments were carried out at ambient temperature (23 ± 1 ◦ C).

Standard solution of Bi(III) (1000.0 ␮g L−1 ) was purchased from Merck (Darmstadt, Germany) and working solutions were prepared daily by appropriate dilution. 4-[1-(4-Methoxyphenyl) methylidene]-3-methyl-5-isoxazolone was synthesized in the organic chemistry laboratory of Shahid Bahonar University of Kerman (Fig. 1) as described in the literature [24]. Multi-walled carbon nanotubes of 95% purity and length 1–10 ␮m and with the number of walls varying from 3 to 15 were purchased from Plasma Chem. GmbH (Germany). Pure graphite powder, nitric acid, sulfuric acid, acetic acid and hydrochloric acid were purchased from Merck. Metal ions solutions were prepared by dissolving appropriate amounts of their salts (Merck) in distilled water.

2.5.2. Open-circuit method The electrochemical method was based on open-circuit accumulation of bismuth ions onto a surface of modified carbon paste electrode. In the first step MCPE was immersed in a 10 mL of sample solution containing 100 ␮g L−1 of Bi(III) and the solution was stirred for 9 min. In the second step the medium was replaced with 0.1 mol L−1 HCl solution as electrolyte, where the accumulated bismuth(III) was reduced for 20 s in −0.5 V and finally in the third step, the differential pulse waveform was recorded from −0.5 to +0.5 V to electrochemically strip the Bi0 back into Bi3+ (with 20 mV s−1 scan rate, 100 mV pulse amplitude, and 4 ms pulse period). The resulting oxidation peak constitutes the analytical signal. These two methods work well, but the open- circuit experiment have its drawback, since more times and steps are required in comparison to the closed-circuit experiment. Therefore the closedcircuit method was applied as an experiment method.

2.3. Functionalization of multi-walled carbon nanotubes

2.6. Procedure of real samples preparation

Prior to use, MWCNTs were oxidized with concentrated HNO3 as described in the literature but with minor modification [25]. In order to create binding sites onto the surface, 3.0 g of MWCNTs were first soaked to a stirred solution of 30 mL concentrated HNO3 for 12 h at an ambient temperature [26]. The solution was then filtered through a 0.45 ␮m membrane filter and the MWCNTs were washed several times with distilled water to obtain neutral pH.

2.6.1. Bismuth subcitrate tablet First, ten bismuth tablets were mixed in mortar. Then 0.1 g of mixture was digested in 5 mL of concentrate H2 SO4 and 3 mL of HClO4 mixture in a closed vessel microwave digestion system. The acidic solution was evaporated almost to dryness and the residue was dissolved in 20 mL 5 mol L−1 HNO3 and evaporated to remove vapors of nitric oxides. The solution was transferred to a 100 mL volumetric flask and diluted with distilled water to the mark.

2.2. Reagents and solutions

2.4. Preparation of the electrode The unmodified carbon paste was made by hand-mixing 70 mg of graphite powder and 30 ␮L of paraffin oil with a mortar and pestle. The CPE was constructed by packing this paste into a glass tube (3.4 mm inner diameter) and providing it with a copper contact. A small amount of carbon paste was forced from the tube and cut off with a scalpel. The electrode surface was smoothed by polishing on a piece of graph paper while a slight manual pressure was applied to the piston. For the preparation of the modified carbon paste electrode (MCPE) 45 mg of graphite powder, 5 mg of chelating agent of MMMI, 25 mg of multi-walled carbon nanotubes and 25 ␮L of paraffin oil were mixed and prepared as for the unmodified electrode.

OMe

N H3C

O

O

Fig. 1. Structure of 4-[1-(4-methoxyphenyl) methylidene]-3-methyl-5-isoxazolone (MMMI).

2.6.2. Biological sample A 10 mL urine sample was treated with 10 mL of concentrated HNO3 (63%) and an HClO4 (70%) mixture of 2:1 in a 50 mL beaker covered with a watch glass and the content of the beaker was heated on a hot plate (100 ◦ C for 15 min, 150 ◦ C for 10 min). The watch glass was now removed and the acidic solution was evaporated to dryness at 150 ◦ C. 3 mL HClO4 was added to the resulting white residue and the mixture was heated at 160 ◦ C to dryness. Finally 5 mL of 1 mol L−1 H2 SO4 was added to the sample. The mixture was heated at 150 ◦ C for 1 min and the volume was made up to the mark in a 25 mL volumetric flask. All of the above experiments were carried out under a hood with necessary precautions. 2.6.3. Water samples The water samples were collected from Kerman and Shahdad. These included samples of tap water (Kerman drinking water, Iran), well water (Shahid Bahonar University of Kerman, Iran) and river water (Shoor, Shahdad, Iran). pH of all the samples was adjusted to 1.0 and the samples were filtered to remove any suspended materials.

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3. Results and discussion 3.1. Voltammetric behavior of Bi(III) at MMMI/MWCNT/CPE The ability of the MCPE to pre-concentrate Bi(III) was investigated. Fig. 2(a)–(e) show the differential pulse stripping voltammetry (DPSV) of MCPE (with nanotube and MMMI), MCPE (without nanotube and with MMMI), MCPE (with nanotube and without MMMI), unmodified CPE and MCPE (with nanotube and MMMI, no Bi(III)) in 0.1 mol L−1 HCl containing 100.0 ng mL−1 Bi(III) respectively. A small anodic peak was found when the unmodified carbon paste electrode (CPE) was applied. This peak is an evidence for the absence of a significant pre-concentration of Bi(III) at the unmodified electrode, whereas MMMI–MCPE (without nanotube) showed a well-defined anodic stripping peak at −0.05 V in a 0.1 mol L−1 of HCl solution containing 100.0 ng mL−1 of Bi(III). This peak is due to the oxidation of elemental bismuth, produced by the reduction of accumulated Bi(III) at the negative potentials. Bi(III) is adsorbed from the sample solution to the electrode surface according to a complexation between the metal ion and the MMMI immobilized on the electrode surface. Three modifiers act as ligands and the metal ion is the central atom. Hence the surface concentrations of Bi(III) are much larger than those of the unmodified electrode, and the sensitivity is greatly increased. Fig. 2 shows that the anodic peak current of bismuth at MMMI–MCPE is several times greater than the unmodified electrode. When MMMI–MCPE (with nanotube) was applied, the peak current showed a significant increase in a solution of 0.1 mol L−1 of HCl containing 100.0 ng mL−1 of Bi(III). The data in Fig. 2(e) was recorded when MCPE (with nanotube and MMMI) was applied and in the absence of Bi(III). 3.2. Effect of supporting electrolyte The effect of the supporting solution on the electrode response in concentration of 100.0 ␮g L−1 of bismuth was tested for four different solutions (HCl, HNO3 , CH3 COOH and H3 PO4 ). Fig. 3 shows that the best solution, which achieved the most ideal shape and the highest current, was HCl. The effect of HCl concentration on voltammograms was examined over the range from 0.01 to 0.7 mol L−1 . The current increased in the interval 0.01 mol L−1 to 0.08 mol L−1

Fig. 3. Effect of the supporting solution on the electrode response in concentration of 100.0 ␮g L−1 of Bi(III): (a) 0.1 mol L−1 HCl, (b) 0.1 mol L−1 HNO3 , (c) 0.1 mol L−1 H3 PO4 and (d) 0.1 mol L−1 CH3 COOH. Optimal condition: deposition potential: −0.5 V, deposition time: 120 s, scan rate: 20 mV s−1 , pulse amplitude: 100 mV s−1 and pulse time: 4 s.

HCl, then was constant up to 0.2 mol L−1 and decreased at higher concentrations. So the concentration of 0.1 mol L−1 HCl was used as stripping medium in all of the measurements. 3.3. Effect of deposition potential and time The dependence of the bismuth stripping peak current on the deposition potential was studied and the results showed that the peak current increases as the potential becomes more negative between −0.1 and −0.5 V and starts to level off as the potential becomes lower than −0.5 V. Since a lower deposition potential value would increase the possibility of co-deposition of interfering species, therefore a potential of −0.5 V was chosen for subsequent experiments. The effect of deposition time on the DPSV response of bismuth was also studied. The peak current increased with increasing time up to 120 s, and remains constant at longer times. Thus, the time of 120 s was selected in subsequent experiments. 3.4. Calibration curve, limit of detection and reproducibility Standard solutions containing different concentrations of Bi(III) were prepared in 0.1 mol L−1 HCl and adjusted to the optimized anodic stripping voltammetric procedure. Voltammograms at these concentrations are shown in Fig. 4. The calibration curve was linear in the range of 1.0–400.0 ␮g L−1 according to the equation y = 0.8835x + 1.0737, where y and x are the peak current (␮A) and Bi(III) concentration (␮g L−1 ), respectively. The linear correlation coefficient was 0.9994. The limit of detection was calculated by taking replicate current measurements at −0.5 V for a blank solution; the detection limit based on the mean of seven measurements gave a value of 0.2 ng mL−1 Bi(III). The relative standard deviations from seven determinations of bismuth at each of three concentrations, 30.0, 100.0 and 300.0 ␮g L−1 , were 3.6, 2.4 and 1.6% respectively. 3.5. Co-existing ions effect

Fig. 2. Differential pulse anodic stripping voltammograms in a hydrochloric acid solution of 0.1 mol L−1 after closed circuit accumulation (120 s at −0.5 V): (a) MMMI/MWCNT/CPE, (b) MMMI/CPE, (c) MWCNT/CPE, (d) CPE; with 100 ␮g L−1 Bi(III) in solution and (e) MMMI/MWCNT/CPE, no Bi(III) in solution. Optimal condition: stripping medium: 0.1 mol L−1 HCl, deposition potential: −0.5 V, deposition time: 120 s, scan rate: 20 mV s−1 , pulse amplitude: 100 mV s−1 and pulse time: 4 s.

In order to check the selectivity of the proposed stripping voltammetric method for the bismuth ions, versus the other ions, various metal ions, as potential interferents, were tested by adding the interfering ions to a solution containing 100.0 ␮g L−1 of bismuth under the optimized conditions. The criterion for interference of each ion was set at ±5.0% in the obtained recovery for a solution containing Bi(III), without any interfering. The results showed that

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Table 1 Determination of bismuth in pharmaceutical and biological samples. Founda

Recovery (%)

2.56 × 10 ␮g g

2.46 × 105 ± (0.05 × 105 ) ␮g g−1

96.0

– 20.0 ng mL−1 (added)

2.20 ± 0.06 ng mL−1 21.9 ± 0.3 ng mL−1

– 98.6

Sample

Original content

Bismuth subcitrate Urine a

5

−1

Mean ± standard deviation (n = 4).

Table 2 Determination of bismuth in water samples. Sample

Spiked (ng mL−1 )

Present methoda (ng mL−1 )

Recovery (%)

GFAAS method (ng mL−1 )

Tap water (Kerman)

0.0 5.0 30.0

N.D.b 5.1 ± 0.2 28.9 ± 0.9

– 102.0 96.3

N.D. – 29.9 ± 0.4

– – 99.7

Well water (Kerman University)

0.0 5.0 30.0

N.D. 4.7 ± 0.2 30.3 ± 0.9

– 95.2 101.0

N.D. – 29.5 ± 0.9

– – 98.3

River water (Shoor, Shahdad)

0.0 5.0 30.0

N.D. 5.2 ± 0.2 29.6 ± 0.8

– 104.0 98.7

N.D. – 30.5 ± 0.9

– – 101.7

a b

Recovery (%)

Mean ± standard deviation (n = 4). Not detected.

4. Conclusions This paper has demonstrated that a CPE modified with 4[1-(4-methoxyphenyl) methylidene]-3-methyl-5-isoxazolone and multi-walled carbon nanotubes can be applied to trace level determinations of Bi(III). The recommended method possesses lower detection limit and wider linear range. Due to high selectivity for bismuth ions shown by this new modifier, the method has been successfully employed for the determination of Bi(III) in real samples. Moreover the modified electrode is low cost, easy to prepare and has fine characteristics such as selectivity, sensitivity, stability, reproducibility and surface renewal. References Fig. 4. Differential pulse anodic stripping voltammograms in a solution containing bismuth; (a) 1.0 ng mL−1 , (b) 5.0 ng mL−1 , (c) 10.0 ng mL−1 , (d) 30.0 ng mL−1 , (e) 50.0 ng mL−1 , (f) 100.0 ng mL−1 , (g) 150.0 ng mL−1 , (h) 200.0 ng mL−1 , (i) 250.0 ␮g L−1 , (j) 300.0 ng mL−1 , (k) 350.0 ng mL−1 and (l) 400.0 ng mL−1 respectively. Optimal condition: stripping medium: 0.1 mol L−1 HCl, deposition potential: −0.5 V, deposition time: 120 s, scan rate: 20 mV s−1 , pulse amplitude: 100 mV s−1 and pulse time: 4 s.

less than 500-fold molar excess of Ca2+ , Mg2+ , Al3+ , SO4 2− and Ba2+ or 300-fold molar excess of Cd2+ , Co3+ , Pb2+ , CN− , ClO4 − and Cr3+ or 200-fold molar excess of Mn2+ or 150-fold molar excess of Ni2+ or 100-fold molar excess of Zn2+ , Te4+ , Se4+ , Fe3+ , Hg2+ and Sn2+ or 80-folder molar excess of Sb3+ and 20-folder molar excess of Cu2+ , did not interfere with the determination of Bi(III). This indicates that the MMMI/MWCNT/CPE electrode is selective for Bi3+ in comparison the tested potential interfering ions. 3.6. Real samples analysis The proposed method was applied to the determination of Bi(III) in bismuth subcitrate tablet and urine sample and the analytical results are shown in Table 1. The proposed method was also checked by spiking bismuth in different water samples. Finally the results which were obtained by the proposed method were compared with those obtained by the graphite furnace atomic absorption spectroscopy (GFAAS) method (Table 2).

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