Journal of Nanomaterials
Volume 2021 (2021), Article ID 4702995, 12 pages
https://doi.org/10.1155/2021/4702995

Research Article

Preparation of Electrode Material Based to Bismuth Oxide-Attached Multiwalled Carbon Nanotubes for Lead (II) Ion Determination

Hai D. Tran,¹ Anh H. Q. Le,¹ and Uyen P. N. Tran²

¹Ho Chi Minh City University of Natural Resources and Environment, Ho Chi Minh City 700000, Vietnam
²Van Hien University, Ho Chi Minh City 700000, Vietnam

Correspondence should be addressed to Hai D. Tran; tdhai@hcmunre.edu.vn and Uyen P. N. Tran; uyentpn@vhu.edu.vn

Received 11 May 2021; Revised 5 August 2021; Accepted 31 August 2021; Published 25 October 2021

Academic Editor: Mazeyar Parvinzadeh Gashti

Copyright © 2021 Hai D. Tran et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Bi₂O₃ was proven an attractive compound for electrode modification in heavy metal electrochemical analysis. A novel method for synthesizing Bi₂O₃-attached multiwalled carbon nanotubes (Bi₂O₃@CNTs) in solution is successfully developed in this study. Characteristics of the obtained Bi₂O₃@CNTs were proven by modern techniques such as X-ray diffraction, Raman spectroscopy, scanning electronic microscopy, transmission electron microscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and anodic stripping voltammetry. Microscopy images and spectra results reveal that Bi₂O₃ particles are mainly attached at defect points on multiwalled carbon nanotubes (MWCNTs) walls. Paste electrodes based on the MWCNTs and synthesized Bi₂O₃@CNTs were applied for electrochemical measurements. The redox mechanism of Bi₂O₃ on the electrode surface was also made clear by the cyclic voltammetric tests. The recorded cyclic voltammograms and electrochemical impedance spectroscopy demonstrate that the Bi₂O₃@CNTs electrode was in lower charge transfer resistance than the CNTs one and in the controlled diffusion region. Investigation on the electrochemical behavior of Pb²⁺ at the Bi₂O₃@CNTs electrodes found a significant improvement of analytical response, resulting in 3.44 μg/L of the detection limit and 2.842 μA/(μg/L) of the sensitivity with linear sweep anodic stripping voltammetry technique at optimized conditions.

1. Introduction

Currently, researchers are focusing on controlling the level of heavy metals in waterways through developing novel sensors for detecting heavy metals such as lead. As low concentrations of lead (II) can cause severe ecological damage and harm human health (such as anemia, weakness, and kidney and brain damage) [1], developing a fast and highly efficient method for accurately detecting lead in wastewater is extremely necessary. Anodic stripping voltammetry (ASV) is a powerful, sensitive and selective electrochemical technique for determining heavy metal ions such as Pb [1]. These ionic species are first deposited on the working electrode (WE) in the accumulation step under a reduction potential. Followed by a potential scan, the analyte is stripped back into the solution in the stripping step, resulting in a current response proportional to its concentration [2, 3]. These redox processes occur on the WE surface, where mass and electrons are transported, and these processes depend significantly on the properties of the electrode material.

Existing literature provides an overview of several suitable electrochemical materials for WE construction [4]. Although the traditional mercury electrode has advantages, replacing it with other suitable materials is necessary owing to its toxicity to humans and the environment [5, 6]. Advanced carbon forms have been considered promising electrode replacements as they are ecofriendly and exhibit excellent electrical conductivity, chemical stability, structural versatility, and broad potential windows [3]; moreover, they require simple modifications for their application as electrodes. Carbon nanotubes (CNTs) [7], graphene [8], graphene oxide [9], and carbon nanofibers [10] are extremely useful as carbon paste electrodes to trace heavy metals from different sources. The sensing applications of MWCNTs are being developed due to several useful properties such as large surface area, fast electric signal transport, and functionability [11, 12]. The highly porous structure of nanocarbons generates a thick double layer and stable film on the WE surface [13] that are detrimental to metal detection [13, 14]. Therefore, these carbon materials must be modified to overcome these drawbacks.

In the presence of Bi on the WE surface, multicomponent alloys of Bi⁰ and heavy metals are formed during the accumulation step [15], resulting in improved efficiency and sensitivity [16]. Most recent methods to prepare Bi-film-coated electrodes are based on ex situ, in situ, or bulk modification with different Bi compounds [17]. However, these approaches have certain disadvantages [18]. The pH of the solution on the WE surface (interface) is an important parameter affecting the existing forms of bismuth and heavy metal ions [17, 18]. However, proton changes (release or consumption) in the cathodic and anodic processes can vary the pH [19], thereby influencing the repeatability and reproducibility of the Bi-film-coated electrode [20]. In another study, Bi₂O₃ was applied as a bulk modifier, mixed with carbon for electrode fabrication to improve heavy metal detection [17]. In a weakly acidic electrolyte, the Bi film on the WE surface is formed owing to the reduction of Bi³⁺ in Bi₂O₃ to Bi⁰ under an appropriate potential.

This study is aimed at developing a new method for preparing Bi₂O₃-attached multiwalled carbon nanotubes (Bi₂O₃@CNTs) for paste electrode construction. The Bi₂O₃@CNTs were prepared using ultrasonication to effectively disperse Bi on the outer walls of CNTs, resulting in a unique material for lead ion determination. The performance of the fabricated electrodes in detecting Pb²⁺ was analyzed using the ASV technique. This work paves the way to produce cost-effective and environment-friendly electrochemical sensors for monitoring heavy metal contamination in aquatic environments.

2. Experimental

2.1. Materials and Apparatus

Multiwalled carbon nanotubes synthesized through chemical vapor deposition were supplied by Vinanotech (Vietnam). Analytical grade Bi (NO₃)₃·5H₂O (>98%), Pb(NO₃)₂ (>99.95%), CH₃COOH (>99.7%), HNO₃ (~68%), NaOH (>99.9%), and K₃Fe(CN)₆·3H₂O (>99%) were purchased from Sigma-Aldrich (US). Paraffin oil and n-hexane were purchased from Merck (Germany). Double distilled water was used as the medium for preparing solutions.

X-ray diffraction (XRD) patterns of the materials were obtained via a Bruker D2 PHASER spectrometer using Cu Kα radiation of wavelength ( Å). Raman spectra were collected using a LabRAM HR Evolution spectroscopy with a 532 nm laser. The morphologies of the materials were examined using a field emission scanning electron microscope (FESEM S4800, Hitachi, Japan) and a transmission electron microscope (TEM, JEM-1400, Jeol, USA). Electrochemical experiments using a three-electrode cell at 25°C were performed with an MPG2 Biologic system (Biologic Sci. Ins., India) and controlled using the ECLab® software. A platinum grid, Ag/AgCl, and Bi₂O₃@CNTs electrodes were set up as the counter, reference, and working electrodes, respectively. Inductively coupled plasma-mass spectroscopy (ICP-MS) technique applying the US EPA method 200.8 for the validation of the ASV method was conducted by a 7800 ICP-MS instrument (Agilent, US).

2.2. Preparation of the Bi₂O₃@CNTs

Approximately 20 g CNTs was added to 1500 mL of HNO₃ (68%) in a flask, which was placed in an ultrasonic tank. The CNTs were dispersed and oxidized for 2 h at room temperature. The solid material was then collected by centrifugal separation. The obtained sediment was redispersed in water and transferred into a vacuum filtration system for filtration and washing until a neutral solution was obtained. These oxidized CNTs were dried overnight at 110°C and used for Bi₂O₃ attachment.

Next, 3 g of the oxidized CNTs was mixed with 50 mL of Bi(NO₃)₃ solution (20, 50, and 70 mmol/L), which was acidified using 5 mL of HNO₃ solution (0.1 M). Interactions between CNTs and Bi³⁺ occurred for 30 min under stirring. Next, NaOH solution (0.1 M) was slowly dropped into the mixture (at intervals of ~10 s) until the pH reached 6.5. Subsequently, the mixture was sonicated for 10 min. The solid was then filtrated and washed several times. The obtained materials were heated to 270°C and held for 2 h. The final products were labelled as Bi₂O₃@CNTs-20, Bi₂O₃@CNTs-50, and Bi₂O₃@CNTs-70, corresponding to concentrations of 20, 50, and 70 mmol/L of the used Bi(NO₃)₃ solution.

2.3. Electrode Fabrication

Mixtures of CNTs (or Bi₂O₃@CNTs) and paraffin oil (80 : 20 wt.%, respectively) were dispersed in n-hexane under ultrasonication until pastes were formed. The obtained fresh pastes were packed and pressed into the end of the cavity with a 3 mm inner diameter in the polytetrafluoroethylene (PTFE) electrode body. A copper wire was used to conduct electrical signals. The electrode surface was polished on a weigh paper before each electrochemical measurement.

2.4. Electrochemical Measurements

A solution mixture of 0.1 M KCl and 0.1 M acetate buffer (pH 4.0) was used as the electrolytic support for all electrochemical tests. The cyclic voltammetry (CV) measurements with the potential ranging from –0.2 V to 1.0 V at different scan rates (10, 20, 30, 40, 50, 70, 100, 150, and 200 mV/s) and electrochemical impedance spectroscopy (EIS) with a 5 mV amplitude over a frequency range of 1–10⁵ Hz were conducted on the electrodes in the electrolyte containing 50 mM K₃Fe(CN)₆.

The Pb²⁺ concentrations in the electrolyte were quantified using linear sweep anodic stripping voltammetry (LSASV). WE was first subjected to an accumulation potential () for an accumulation time () while the solution was stirred. Following the accumulation step, stirring was stopped to reach an equilibrium period of 10 s, and stripping voltammograms were recorded by sweeping the potential from to +0.2 V at a rate of 50 mV/s. The , , and pH of the electrolyte were optimized.

3. Results and Discussion

3.1. Characterization of Electrode Materials

Figure 1(a) presents the XRD patterns of the CNTs and Bi₂O₃@CNTs. The characteristic diffraction peaks of CNTs at ° and 43.2° were assigned to the C(100) and C(002) planes [21]. These two peaks indicate the existence of amorphous phases in the CNTs owing to their low intensity and large width [22]. The diffraction peaks for α-Bi₂O₃ are observed at ° (021), 32.3° (220), 46.3° (222), and 54.7° (023) in the XRD patterns of the Bi₂O₃@CNTs, corresponding to the JCPDS file no. 76-1730. These results indicate that the synthesized Bi₂O₃ exhibits monoclinic symmetry, as observed in an earlier study [23, 24]. The XRD patterns of Bi₂O₃@CNTs-20, Bi₂O₃@CNTs-50, and Bi₂O₃@CNTs-70 show increased peak intensities at °, resulting from the increased Bi₂O₃ loading [25]. Average crystallite size of Bi₂O₃ particles was found to be 15.8, 16.8, and 14.9 nm for Bi₂O₃@CNTs-20, Bi₂O₃@CNTs-50, and Bi₂O₃@CNTs-70, respectively, by applying the Debey-Scherer equation: , where λ is wavelength, is full width at half maximum, and θ is Bragg’s angle [26].

(a)

(b)

Figure 1: (a) XRD patterns and (b) Raman spectra of CNTs and Bi₂O₃@CNTs.

Differences in the microstructures and photon and electron quantum confinements of the electrode materials could be identified from their Raman spectra, as shown in Figure 1(b). Three prominent Raman bands of the CNTs appear at 1325 cm⁻¹ (D-band), 1582 cm⁻¹ (G-band), and 2656 cm⁻¹ (G’-band). The D-band confirms the presence of amorphous carbon owing to structural defects, with strong dispersion leading to double resonance peaks [27, 28]. The G- and G’-bands indicate graphitic rings with sp² hybridized C=C bonds [27, 29]. Increased intensity ratios of the D- and G-bands () [27] indicate increased microstructural disorder, which is higher for the Bi₂O₃@CNTs than the CNTs. The intensity ratios, , of the CNTs, Bi₂O₃@CNTs-20, Bi₂O₃@CNTs-50, and Bi₂O₃@CNTs-70 (0.75, 0.70, 0.66, and 0.40, respectively) decreased, demonstrating the increased formation of Bi₂O₃ according to the used Bi³⁺ concentrations [27]. The above results could probably be attributed to the attachment of the Bi₂O₃ particles to the CNTs sidewall defects. Finally, the minor D’-band at 1614 cm⁻¹ corresponds to the defective graphite structure in the CNTs, typically amorphous hydrogenated carbon (α:C-H) [30]. For Bi₂O₃@CNTs, the dominant vibration band emerged at a low frequency (181 cm⁻¹), ascribed to the displacements of the O atoms in Bi₂O₃ and the overtone vibrations of Bi [31]. This would have probably originated from CNTs. Furthermore, we could not confirm the origin of the relatively weak band at 277 cm⁻¹; however, it is close to the band at 288 cm⁻¹ corresponding to multiple regions in the Brillouin zone with a number of photon branches [31].

The SEM micrographs of the electrode materials are shown in Figure 2. Figure 2(a) clearly shows that the individual CNTs exist as tubular fibers with diameters less than 50 nm and are randomly twisted together, generating a three-dimensional network. The presence of Bi₂O₃ particles in the Bi₂O₃@CNTs is obvious owing to the increased utilization of Bi³⁺ in the solutions, as observed in Figures 2(b)–2(d). The Bi₂O₃@CNTs-20 sample presents a sparse distribution of Bi₂O₃ particles (Figure 2(b)). In contrast, Bi₂O₃@CNTs-50 and Bi₂O₃@CNTs-70 indicate widespread, discrete, and relatively homogeneous distributions of Bi₂O₃ particles attached to the CNTs sidewalls. When the used Bi³⁺ quantity exceeds the necessary amount, Bi₂O₃ clusters are formed, as shown in Figure 2(d). This may be disadvantageous to the sensitivity of the Bi₂O₃@CNTs-70 electrode owing to the low electrical conductivity of Bi₂O₃ (~1 S/cm) [32] compared to that of the bulk CNTs (~100 S/cm) [33]. The CNTs serve as skeletons for attaching Bi₂O₃ particles.

(a)

(b)

(c)

(d)

Figure 2: SEM images of (a) CNTs, (b) Bi₂O₃@CNTs-20, (c) Bi₂O₃@CNTs-50, and (d) Bi₂O₃@CNTs-70.

TEM analyses of the CNTs and Bi₂O₃@CNTs-50 samples were performed in order to characterize the particle size distribution, and the results are shown in Figure 3. Most of the CNTs are tubular porous structures with nonsmoothed surfaces caused by the defective CNTs sidewalls. Impurities in CNTs did not present through the TEM images (Figure 3(a)) as well as XRD pattern for CNTs (Figure 1(a)). The morphology of Bi₂O₃@CNTs-50 (Figures 3(b) and 3(c)) shows that the Bi₂O₃ particles are discretely attached to the CNTs. The arrow in Figure 3(b) points to a smooth CNT with little or no Bi₂O₃ attached to it. Figure 3(c) shows that Bi₂O₃ nanoparticles are present on the outer walls of the CNTs at defect points but do not enter the hollow channels inside the CNTs, indicating excellent agreement with the Raman spectra analyses. A recent study by Dighole et al. [23] also proved the formation of Bi₂O₃-decorated multiwalled carbon nanotubes via TEM imaging. However, the TEM studies conducted by Dighole et al. did not show the presence of Bi₂O₃ on the outer walls of the CNTs.

(a)

(b)

(c)

Figure 3: TEM images of (a) CNTs, (b) Bi₂O₃@CNTs-50 at 500 nm scale bar, and (c) Bi₂O₃@CNTs-50 at 20 nm scale bar.

3.2. Electrochemical Properties of the Electrodes

The electrochemical characteristics of the electrodes were investigated from CV and EIS measurements, as illustrated in Figures 4–6. The area of the CV curve (Figure 4(a)) of the CNTs electrode is 1.48-folds higher than that of the Bi₂O₃@CNTs-50 electrode, indicating the reduced specific capacitance of the CNTs with attached Bi₂O₃ [34]. This characteristic of the Bi₂O₃@CNTs-50 electrode is advantageous for distinguishing the Faradic current in the analytical signals [35]. The current curves in Figure 4(a) appear as a knob hill at +0.25 V. This may be attributed to the multistep redox reactions between the O atoms in the C–O or/and C=O bond(s) present in the CNTs, although the CNTs were treated at a high temperature [36]. In the CV curve of the Bi₂O₃@CNTs-50 electrode (Figure 4(a)), the small peak C1 at +0.15 V demonstrates the partial reduction of Bi₂O₃ into ions; peak C2 at −0.61 V corresponds to the successive reduction of into and subsequently to Bi⁰ [37]. The anodic potential scan results showed that the Bi⁰ on the surface Bi₂O₃@CNTs-50 electrode was oxidized to BiO⁺ through an active dissolution process [37, 38], resulting in the appearance of peak A1 (Figure 4(a)) at −0.44 V. Peak A2 (+0.48 V) corresponds to the oxidation of Bi⁰ to Bi³⁺. The potentials of the anodic peaks (A1 and A2) are higher than those of the cathodic peaks (C2 and C1), as shown in Figure 4(a). This trend, resulting from the change in surface coverage and thickness of the electrode/electrolyte interface (electrical double layer (EDL)), was also observed earlier [39]. The peak () close to 0 V in the CV curve for the Bi₂O₃@CNTs-50 electrode (Figure 4(a)) is similar to that observed in a previous study [40], but it did not appear for Bi₂O₃/Pt [37] and Bi₂O₃/Au [39] electrodes. Notably, BiO⁺ was formed and captured on the double layer until saturation under controlled charge transfer and mass diffusion [38]. For porous electrode materials such as CNTs, the double layer is thick and has a high ionic capacity [41], particularly for BiO⁺ in the present study, resulting in the peak () because of the discharge in the anodic process.

(a)

(b)

Figure 4: Cyclic voltammograms of (a) CNTs and Bi₂O₃@CNTs-50 electrodes in electrolyte without K₃Fe(CN)₆ and (b) electrode in electrolyte containing 50 mM K₃Fe(CN)₆. Conditions: pH 4 and scan rate of 50 mV/s.

(a)

(b)

Figure 5: Influence of scan rate on peak current determined from CV measurements. Conditions: 50 mM K₃Fe(CN)₆ in the electrolyte at pH 4.

(a)

(b)

Figure 6: (a) Nyquist and (b) Bode plots developed from EIS data of the electrodes. Conditions: 50 mM K₃Fe(CN)₆ in the electrolyte, pH 4.

The CV curve of the ferricyanide system (Fe²⁺/Fe³⁺) was used to characterize the influence of Bi₂O₃ on the electrochemical kinetics at the electrodes. CV’s wave shapes and sharp redox peaks are evident for the Bi₂O₃@CNTs electrodes as expected (Figure 4(b)). The CV curve of the CNTs electrode (in Figure 4(b)) indicates a higher capacitive current and a more significant peak-to-peak separation () compared to that of the others, demonstrating a lower electron transfer rate. The Bi₂O₃@CNTs-50 electrode exhibits the lowest anodic peak potential () and the highest anodic peak current () among the fabricated electrodes, as observed in Table 1. The peak potential and current are affected by the electrode/electrolyte interface and electrical conductivity of the electrode material [42]. The double layer effects on the interface could have decreased in the presence of Bi₂O₃. According to Figure 4(b) and Table 1, the maximum and minimum are 0.677 mA and 0.153 V, respectively, for the Bi₂O₃@CNTs-50 electrode, indicating optimal electrode performance with contributions from the CNTs and Bi₂O₃ for sensing applications. Additionally, the redox electrochemical reactions of the Fe²⁺/Fe³⁺ couple on the Bi₂O₃@CNTs-50 electrode exhibited reversibility as the ratio was close to 1.

Table 1: Fe²⁺/Fe³⁺ peak potentials and currents obtained from CV curves.

Electrodes

CNTs
Bi₂O₃@CNTs-20
Bi₂O₃@CNTs-50
Bi₂O₃@CNTs-70

(mA)
0.613
0.634
0.677
0.649

(V)
0.384
0.373
0.301
0.321

− (mA)
0.736
0.672
0.658
0.675

(V)
0.067
0.143
0.148
0.132

The anodic and cathodic peak currents of the Fe²⁺/Fe³⁺ couple determined from the CV curves of the electrodes as a function of the scan rate () from 10 to 200 mV/s are shown in Figure 5. Good linear correlations between the peak currents and square roots of the scan rates () are observed for the Bi₂O₃@CNTs-50 and Bi₂O₃@CNTs-70 electrodes (Figure 5(a)), indicating that the redox process involves controlled diffusion [43]. However, this behavior is not observed for the CNTs and Bi₂O₃@CNTs-20 electrodes; at low scan rates, the peak current increased with the increasing scan rate for these electrodes, the opposite of what occurred at high scan rates. This trend was also observed in an earlier study [44]. The porous electrode surface generates a complex and thick EDL [45], resulting in a heterogeneous electron transfer rate; therefore, the intensity of the peak current decreases at a high scan rate [46]. However, understanding of this influence is limited in the literature. Consequently, Bi₂O₃ enhanced the electrochemical performance of the electrode, thus proving its potential for analytical applications. Moreover, the poor linear correlations between the peak responses and scan rates are evident in Figure 5(b), verifying that the adsorptive resistance can be neglected [47].

The EIS spectra of the electrodes in Figure 6(a) indicate that the interfacial process is a combination of controlled charge transfer (indicated by semicircles) and controlled mass diffusion (indicated by arcs).

The “knee” in the EIS spectrum of the CNTs electrode (Figure 6(a)) shows the existence of a transition region between charge transfer and mass diffusion [48]. Moreover, the radius of the semicircle is large in the EIS spectrum of the CNTs electrode, indicating the dominance of charge transfer, thereby resulting in the nonlinear relationship between the peak current and the square root of the scan rate (Figure 5(a)). For the Bi₂O₃@CNTs electrodes, the contribution of the charge transfer decreased and that of mass diffusion increased. Steep arcs exceeding 45° at low frequencies indicate the pore narrowing phenomenon on the electrode surface with depth penetration [49]. For porous electrodes, the complex electron transfers occur through the electrolyte, EDL, electrode material itself, and diffusion of the electrolyte onto the electrode [50, 51], represented by the , , , and elements in the equilibrium circuit shown in Figure 6(a). Table 2 presents the parameters obtained from fitting the EIS data to this circuit. The EDL capacitance () of the Bi₂O₃@CNTs electrode is significantly lower than that of the CNTs one, indicating the planar surface of the Bi₂O₃@CNTs electrode owing to the attachment of Bi₂O₃ particles on the CNTs sidewalls at the defect points. Moreover, the Warburg element () of the electrode decreased in the presence of Bi₂O₃, proving that Bi₂O₃ contributed toward decreasing the porosity of CNTs, thereby limiting the diffusion of the electrolyte onto the electrode [52]. The enhanced charge transfer in Bi₂O₃@CNTs electrodes is demonstrated by the lower electrical resistance () when compared to that of the CNTs because of the hybridization reactions between the Bi-6p³ and CNT-p² orbitals, leading to reduced bandgaps in Bi₂O₃ and CNTs [53]. A similar trend was also observed earlier [44]. However, the capacitance () of Bi₂O₃@CNTs is higher than that of the CNTs owing to the high permittivity of Bi₂O₃ [54]. The multistep mechanism of the electrochemical process on the electrode surface in the presence of Bi [40] affects the charge transfer resistance () of the EDL [55] that may have led to a higher of the Bi₂O₃@CNTs electrode than that observed for the CNTs electrode. However, the values of () for the Bi₂O₃@CNTs electrodes are generally lower than that for the CNTs electrode, indicating the positive effect of Bi₂O₃ on electrical signal conductivity. These results agree with the CV measurements shown in Figure 4(b).

Table 2: Parameters obtained from EIS data fitting.

Electrodes

CNTs
Bi₂O₃@CNTs-20
Bi₂O₃@CNTs-50
Bi₂O₃@CNTs-70

(Ω)
4.3
14.0
19.3
24.4

(F)

(Ω)
6.6
211.2
171.7
156.6

(F)

(Ω)
281.7
81.4
58.5
45.4

(Ω)
387.0
266.0
78.4
87.8

Figure 6(b) shows the frequency-dependent phase angles (Bode plots). The phase angles of the electrodes are higher than −90° at low frequencies, indicating the pseudocapacitive behavior of the electrolyte/electrode interface [56]. Moreover, the broad peaks at high frequencies confirm diffusion resistance in the interfacial process. The higher the peak frequency, the stronger is the effect of controlled diffusion [57]. Consequently, controlled diffusion dominates the redox process of the Fe²⁺/Fe³⁺ couple at the Bi₂O₃@CNTs electrodes.

3.3. ASV Studies

The electrochemical responses of the different electrodes to Pb²⁺ (Figure 7(a)) were examined via ASV measurements in 0.1 M acetate buffer solution containing 30 μg/L of Pb²⁺. At an of −0.1 V, Bi₂O₃ was reduced to Bi⁰ (indicated in Figure 4(a)), which further enriched Pb by forming Bi-Pb alloys [15]. Consequently, the Bi₂O₃@CNTs electrodes exhibited enhanced Pb²⁺ sensing properties compared to those of the CNTs electrode, as indicated by their improved anodic peak currents. The peak potential was approximately −0.40 V for the CNTs electrode, whereas it was approximately −3.83 V for the Bi₂O₃@CNTs electrodes. This positive shift reveals a more complex reduction of Pb⁰ because of Bi-Pb alloy formation [15] on the Bi₂O₃@CNTs electrodes than on the CNTs electrode, indicating differences in the interaction mechanism between Pb²⁺ and the CNTs and between Pb²⁺ and Bi₂O₃@CNTs electrode surfaces [58]. The sensitivity for Pb²⁺ detection increases in the following order: CNTs, Bi₂O₃@CNTs-20, Bi₂O₃@CNTs-70, and Bi₂O₃@CNTs-50 electrodes. Therefore, the Bi₂O₃@CNTs-50 electrode is a favorable candidate for Pb²⁺ detection applications.

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(b)

(c)

(d)

Figure 7: (a) Anodic stripping voltammograms for different electrodes and effects of (b) accumulation potential, (c) accumulation time, and (d) pH on Pb²⁺ detection observed for the Bi₂O₃@CNTs-50 electrode. Solution contains 30 μg/L Pb²⁺ and 0.1 M acetate buffer.

Important parameters of the LSASV technique (, , and pH) were considered to optimize Pb²⁺ detection using the Bi₂O₃@CNTs-50 electrode. Figures 7(b)–7(d) show the peak currents obtained from the ASV measurements under different conditions.

The CV measurements presented in Figure 4(a) indicate that Bi³⁺ in Bi₂O₃ is reduced to Bi⁰ at −0.61 V, revealing the upper limit for electrochemical analyses involving Pb²⁺. Figure 7(b) shows that the stripping response of Pb²⁺ depends on the accumulation potential in the interval from −1.4 V to −0.6 V. The Pb oxidative peak signal increases with a negative shift of the from −0.6 V to −1.0 V resulting from the enhanced microstructural parameters of the deposited Bi⁰ particles such as their size and distribution [59], thus improving the Pb-Bi alloy formation [60]. However, the hydrogen evolution reaction on the electrode surface begins to occur at −1.2 V in an acidic electrolyte [16], which could have prevented Pb²⁺ deposition and damaged the Bi⁰ film. This led to a substantial decrease in the stripping response at a high negative . Consequently, was considered the optimum accumulation potential and applied in the accumulation time () studies. Figure 7(c) presents the effects of different values, namely, 30, 60, 90, 120, and 150 s, on the stripping signals. With a prolonged , the amount of Pb deposited on the electrode surface increased until saturation, resulting in a rapidly increasing peak current with up to 120 s and then tending to remain constant at a longer . Thus, was chosen as the suitable accumulation time for subsequent experiments. As depicted in Figure 7(d), the pH has a significant influence on the peak signal. At low pH, the stripping current decreases due to protonation of the CNTs structure with the increasing bandgap [61]. Moreover, the potential for hydrogen evolution shifts to more positive values at low pH; thus, the peak current is low at low pH. In contrast, at high pH, Pb²⁺ and intermediate forms of Bi could be converted to insoluble hydroxide complexes [62], leading to decreasing peak currents with increasing pH. Figure 7(d) shows that the optimum sensitivity of the electrodes for Pb²⁺ is at .

To evaluate the applicability of the Bi₂O₃@CNTs-50 electrode for Pb²⁺ detection, the relationships of current responses with Pb²⁺ concentrations were determined from the electrochemical measurements using the LSASV technique using the following optimal parameters: , , and . The obtained voltammograms are presented in Figure 8, along with the calibration plot of Pb²⁺ in the range of 2–40 μg/L (in the insert). The peak current is directly proportional to the Pb²⁺ concentration according to the following linear regression equation: (correlation coefficient ). The limit of detection (LOD) was 3.44 μg/L according to the 3σ criteria (). Table 3 compares the obtained LOD and sensitivity with those in previous reports, proving the applicability of the fabricated Bi₂O₃@CNTs electrodes for Pb²⁺ detection.

Figure 8: Anodic stripping voltammograms of the Bi₂O₃@CNTs-50 electrode with Pb²⁺ concentration gradient in 0.1 M acetate buffer solution (pH 5.0). Insert: calibration plot. Conditions: and .

Table 3: Performance comparison of Bi₂O₃-modified electrodes for Pb²⁺ detection.

Electrode
Modification method
LOD (μg/L)
Sensitivity (μA/(μg/L))
Method
Ref.

Bi₂O₃-CS/GCPE
Surface dropping
30.1
0.045
DPASV
[63]

Bi₂O₃/SPE
Bulk modification
20.0
0.011
DPASV
[64]

Bi₂O₃/SPE
Bulk modification
10.0
—
DPASV
[65]

Bi₂O₃/SPE
Bulk modification
8.0
1.050
CCSCP
[66]

Bi₂O₃/SPE
Bulk modification
5.0
0.074
SWASV
[67]

Bi₂O₃/SPE
Surface modification
2.3
2.960
SWASV
[16]

Bi₂O₃@CNTs
Attachment
3.4
2.842
LSASV
This work

GCPE: glassy carbon paste electrode; CS: chitosan; SPE: screen-printed electrode; SWASV: square wave anodic stripping voltammetry; DPASV: different pulse anodic stripping voltammetry; CCSCP: constant current stripping chronopotentiometry.

3.4. Application to River Water Media

Water from the Saigon River was collected, filtered through a 0.45 μm membrane, and sonicated for ICP-MS and ASV measurements. Because the Pb²⁺ concentration in the sample was lower than the LOD of the ICP-MS method, this natural water was used as the medium for preparing the ASV analytical solution containing 5 μg/L Pb²⁺, 0.1 M KCl, and 0.1 M acetate buffer (pH 4.0). Under the optimal conditions, the ASV measurement using the Bi₂O₃@CNTs-50 electrode was repeated five times (shown in Table 4), revealing a Pb²⁺ concentration of with a 9.7% standard error. These results prove the excellent applicability of the electrode for Pb²⁺ determination at the trace level.

Table 4: Concentrations of Pb²⁺ determined in river water media.

No.
1
2
3
4
5

Pb²⁺ (μg/L)
4.3
4.7
4.7
4.4
4.8

4. Conclusion

The successful attachment of Bi₂O₃ particles at the defect points on the outer walls of the CNTs produced the Bi₂O₃@CNTs materials with advantageous properties for electrochemical analytical applications. The interfacial process on the Bi₂O₃@CNTs electrodes was predominantly controlled mass diffusion instead of controlled charge transfer observed for the CNTs electrode. The ASV studies using the fabricated electrodes indicated the superior sensitivity of the Bi₂O₃@CNTs electrodes for Pb²⁺ detection compared to that of the CNTs electrode. The optimal parameters for Pb²⁺ detection using the LSASV technique, namely, the accumulation potential, accumulation time, and pH of the solution, were determined. The obtained detection limit and sensitivity from the Pb²⁺ calibration curve revealed the excellent applicability of the Bi₂O₃@CNTs electrodes for detecting heavy metal ions. In the future, the interaction mechanisms between Bi₂O₃ and CNTs and other applications of the Bi₂O₃@CNTs electrodes will be investigated.

Data Availability

The experimental data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

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