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Using Electrode Made of Carbon Nanotubes and Bismuth Oxide for the Determination of Metal Concentration by Anodic Stripping Voltammetry
We have successfully manufactured a new electrode modified with bismuth oxide (Bi2O3) using carbon nanotubes (CNTs). The electrode was fabricated to detect cadmium (Cd), lead (Pb), and indium (In) by differential pulse anodic stripping voltammetry (DP-ASV). The electrode surface was studied by scanning electron microscopy (SEM), and the reduction and oxidation processes were studied by cyclic voltammetry (CV) techniques. Operational parameters such as electrode size, bismuth concentration, and electrolytic background were optimized. The DP-ASV method used fabricated electrodes with a linear response range from 1.5–20 μg·L−1 with Cd(II) and Pb(II) and 2.5–20 μg·L−1 with In(III); low detection limit (LOD) of 0.22 μg·L−1 with Cd(II), 0.65 μg·L−1 with In(III), and 0.26 μg·L−1 with Pb(II); and good repeatability with relative standard deviations (RSD) of 2.65%, 2.51%, and 3.34% with Cd(II), Pb(II), and In(III), respectively (n = 8). The electrode can be used to test the content of Cd(II), In(III), and Pb(II) in water.
The Cd and Pb determination is very important because of their toxic effects to environment and humans . Several compounds of In can cause cancer and are toxic . Some analytical methods such as atomic absorption spectrometry (AAS) [3–8], inductively coupled plasma-mass spectrometry (ICP-MS) [9–11], and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) [12–16] have been applied to analyse Cd(II), In(III), and Pb(II). These techniques have high selectivity and sensitivity, but they are very expensive and time-consuming. The electroanalytical method not only has high accuracy and sensitivity but also is low cost, has good repeatability, and allows in situ measurement . Carbon paste electrodes (CPEs) have been widely applied in analytical chemistry in some recent years [18–21]. Nowadays, CNTs are also used in CPEs because they have high mechanical strength, electrical conductivity, and surface area [22–25]. The electrochemical analysis methods using various working electrodes (WEs) have been widely used, in which mercury electrodes have been most commonly used. However, mercury is very toxic. Bismuth is less toxic than mercury, and it has some similar electrochemical characteristics of mercury, so it has been used a lot to replace mercury in electrochemical analysis[26–35]. In this study, we manufactured a new modified CNT electrode with Bi2O3 as replacement for mercury electrodes, and this electrode is used as a WE in electrochemical analysis equipment for the simultaneous analysis of Cd(II), In(III), and Pb(II). The method was tested successfully on water samples. In addition, the results were compared with those of graphite furnace atomic absorption spectroscopy (GF-AAS). The accuracy of the method is evaluated by using the sediment certified reference material (CRM) and graphite furnace atomic absorption spectrometry (GF-AAS).
2.1. Chemicals and Reagents
CH3COONa, KI, KCl, NaNO3, paraffin oil, CH3COOH (100%, m/v), and HNO3 (65%, m/v) were purchased from Merck (Germany). Multiwalled carbon nanotubes (MWCNTs 95%, diameter × length: 10–35 nm × 1–10 μm). Bismuth oxide (Bi2O3, grain size < 10 μm) was provided by Sigma-Aldrich. Working standards for Pb, Cd, and In were prepared using standard solutions supplied by Merck (standard solutions of 1000 ppm for Cd, Pb, and In).
The Metrohm 797 VA Computrace (Switzerland) with the working electrode (WE) was the CPE modified with Bi2O3 using CNTs. For AAS measurements, a PerkinElmer 3300, USA, was used. For SEM measurements, a field emission SEM S-4800, Hitachi, Japan, was used.
2.3. Fabrication of Electrodes
CNTs (heated at 700°C for 15 min) and parafﬁn oil (6 : 4, w/w) were mixed with an agate mortar and pestle, and then they were transferred into test tubes with the help of ultrasonic agitation for 2 h. By continuous mixing of carbon paste with Bi2O3 by mixing similar to the abovementioned method, the modified carbon nanotube paste was obtained. This modified carbon nanotube stuff was packed into a Teflon tube. Electrode surface was cleaned with a filter paper.
The analytical solution was added into the electrolyte beaker. The measurement conditions were as follows: the deposition potential (Edep) −1.2 V, deposition time (tdep) 120 s, speed 15 mV per second, and pulse amplitude (ΔE) 50 mV·s−1. After a rest time, 20 s, the DP-ASV was saved. The DP-ASV of blank solution was saved with the similar measurement conditions. Before the test, oxygen was removed from the analytical solution by exposing to pure nitrogen gas for 5 minutes.
3. Results and Discussions
3.1. Influence of Electrode Diameters
In stripping voltammetry analysis, all preconcentration and stripping processes happen on the surface of the WE, so the electrode diameter has a great influence on the limit diffusion line. So, we study the influence of different electrode diameters (diameters are 1.8, 2.2, 2.5, 3, 3.5, and 4.0 mm) on the peak (Ip) of metal ions. The results are shown in Figure 1. At 3 mm electrode diameter, Ip of all 3 ions are high, the peak is balanced, and the repeatability is good. Therefore, we chose 3 mm as the optimum electrode diameter.
3.2. Influence of Bi2O3
The simultaneous analysis of Cd(II), Pb(II), and In(III) by DP-ASV using a CPE modified with 1%, 3%, 5%, and 8% (w/w) Bi2O3 was researched. The CPE modified with 5% (w/w) Bi2O3 produced the highest Cd, In, and Pb peaks; resolution is high, and repeatability is good. Therefore, the CPE modified with 5% (w/w) Bi2O3 was used for the further studies.
3.3. Influence of Edep and tdep
Influence of Edep on the Ip of Cd(II), Pb(II), and In(III) has been investigated with Edep from −0.9 to −1.4 V (Figure 2). The effect of tdep in the range from 30 s to 300 s was also researched (Figure 3). The best stripping signal was obtained at Edep −1.2 V and tdep 120s. From these results, Edep and tdep of −1.2 V and 120 s were selected for the further studies.
3.4. Cyclic Voltammetry (CV)
The reduction and oxidation processes were studied by potential scanning from −1.2 to 0.3 V and continuous scanning from 0.3 V to −1.2 V at a speed of 15 mV per second. The results are shown in Figure 4. In Figure 4, the anodic scan resulted in a peak potential of Bi which was −0.12 V that reflects the oxidation of the metallic bismuth.
3.5. Morphological Surface Characterization
Figure 5 shows the scanning electron micrograph (SEM) surface image of the CPE containing 5% Bi2O3 (w/w) before and after reduction at Edep −1.2 V with tdep of 120 s. After the electrochemical reduction, the electrode surface was changed because Bi2O3 was converted to Bi at −1.2 V for 120 s following the reaction 
According to the SEM image, the electrode surface has got a porous structure, and the surface is relatively uniform. Therefore, it is supposed to be beneficial for the stripping analysis.
3.6. Influence of Supporting Electrolytes
The influence of the background electrolytes, acetate buffer, mixture of NaNO3 and acetate buffer, KCl and acetate buffer, and KI and acetate buffer, on the Ip of metals were studied. The results are shown in Figure 6. According to , simultaneous measurement of Cd(II), Pb(II), and In(III) is possible if there is a difference of peak potential of at least 100 mV. In Figure 6(a), the resolution between Cd and In signals in acetate buffer as well as the mixture of NaNO3 and acetate buffer and KCl and acetate buffer solution is not good. Amongst these mentioned electrolytes, the supporting electrolyte mixture of 0.1 M KI and acetate buffer is the best choice with the best resolution, and largest peak current is for Cd(II), Pb(II), and In(III) (Figure 6(b)). Therefore, a medium containing 0.1 M KI and acetate buffer was selected.
3.7. Linear Response Range, LOD, and Reproducibility
The DP-ASV at different concentrations was recorded. The results of linear range are shown in Figure 7(a), and the calibration curves are shown in Figures 7(b)–7(d). The linear response ranges were 1.5–20 μg·L−1 with Cd(II) and Pb(II) and 2.5–20 μg ·L−1 with In(III).
The CPE modified with Bi2O3 using MWCNTs also demonstrated LOD (S/N = 3) of 0.22 μg·L−1 with Cd(II), 0.26 μg·L−1 with Pb(II), and 0.65 μg·L−1 with In(III) (at tdep = 120 s). The method has a fine reproducibility with RSD of 2.65%, 3.34%, and 2.51%, respectively (n = 8) (with the concentration of Cd(II), Pb(II)), and In(III) to be 10 μg·L−1).
Various Bi precursor-modified and Bi2O3-modified carbon electrodes for analysis of Cd(II), Pb(II), In(III) are shown in Table 1.
SPE: screen printed electrode; GCE: glassy carbon electrode; SW-ASV: square wave anodic stripping voltammetry; BONP: bismuth oxide nanoparticle; IL: ionic liquid; BiOCl: bismuth-oxychloride.
3.8. Influence of Ions
The influence of several ions K+, Na+, Ca2+, Mg2+, Zn2+, Fe3+, and Cu2+ at concentration range from 0.1 to 100 mg·L−1 on the peak of Cd(II), Pb(II), and In(III) was examined. No signal changes were observed on the Ip of Cd(II), Pb(II), and In(III) (changes in peak currents < 10%). The influence of cetyltrimethylammonium bromide and Triton X-100 surfactants in range from 0.1 to 10 mg·L−1 concentrations was also studied. A decrease in the Ip value was found for Cd(II), Pb(II), and In(III) determination on increasing the amount of the surfactant. Nevertheless, this surfactant interference was reduced by the UV irradiation with a UV lamp at 254 nm wavelength in 90 min.
3.9. Determination of CRM
The accuracy of the method was studied by analysis of sediment CRM (MESS-2). The results are shown in Table 2. The results of these five trials were not significantly different from the certified values following Student’s t test (texp = 2.52 greater than t (0.05, 4)).
Note. is the average value of the DP-ASV method (5 measurements).
3.10. Determination of River Water Samples
Water pollution in some rivers of Hanoi, Vietnam, has been a serious problem, with high concentration of metal ions and other ions [38, 39]. So, we took some river samples in Hanoi to analyse Cd(II), In(III), and Pb(II). The samples were acidified by 10% HNO3, and then the solution was filtered using the 0.45 μm membrane filter. The sample was treated with UV light for 90 minutes at wavelength 254 nm.
After filtering, 10.0 mL of the solution was taken, and the mixture of 0.1 M KI and acetate buffer was added. Content analysis of Cd, In, and Pb in 3 different river samples by DP-ASV was performed using the manufactured CPE. The content of Pb(II) in the samples ranged from 1.45 ± 0.01 μg·L−1 to 1.93 ± 0.20 μg·L−1. The content of Cd(II) and In(III) in samples was smaller than the limit of quantitative. The results of Pb(II) in this sample using the DP-AAS method with CPE modified with Bi2O3 using MWCNTs were compared with GF-AAS and are shown in Table 3. The results of these five measurements were not significantly different from the certified values.
Note. All concentrations are expressed in μg·L−1; the result of the DP-ASV method is the average value of 5 measurements.
The CPE modified with 5% (w/w) Bi2O3, 3 mm diameter, using CNTs was applied for simultaneous analysis of the concentration of Cd(II), Pb(II), and In(III) by the DP-ASV method. The CPE modified with 5% (w/w) Bi2O3 are not only easy to make and easy to use, but also environmentally friendly. The method has good accuracy, low detection limit, and good repeatability.
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conﬂicts of interest regarding the publication of this paper.
The authors gratefully acknowledge the Hanoi University of Industry and Chemical Analysis Laboratory, Institute of Chemistry, Vietnam Academy of Science and Technology, for providing support to this work.
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