An Efficient Voltammetric Sensor Based on Graphene Oxide-Decorated Binary Transition Metal Oxides Bi₂O₃/MnO₂ for Trace Determination of Lead Ions

Abstract

Herein we present a facile synthesis of the graphene oxide-decorated binary transition metal oxides of Bi₂O₃ and MnO₂ nanocomposites (Bi₂O₃/MnO₂/GO) and their applications in the voltammetric detection of lead ions (Pb²⁺) in water samples. The surface morphologies, crystal structures, electroactive surface area, and charge transferred resistance of the Bi₂O₃/MnO₂/GO nanocomposites were investigated through the scanning electron microscopy (SEM), power X-ray diffraction (XRD), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) techniques, respectively. The Bi₂O₃/MnO₂/GO nanocomposites were further decorated onto the surface of a glassy carbon electrode (GCE), and Pb²⁺ was quantitatively analyzed by using square-wave anodic stripping voltammetry (SWASV). We explored the effect of the analytical parameters, including deposition potential, deposition time, and solution pH, on the stripping peak current of Pb²⁺. The Bi₂O₃/MnO₂/GO nanocomposites enlarged the electroactive surface area and reduced the charge transferred resistance by significant amounts. Moreover, the synergistic enhancement effect of MnO₂, Bi₂O₃ and GO endowed Bi₂O₃/MnO₂/GO/GCE with extraordinary electrocatalytic activity toward Pb²⁺ stripping. Under optimal conditions, the Bi₂O₃/MnO₂/GO/GCE showed a broad linear detection range (0.01–10 μM) toward Pb²⁺ detection, with a low limit of detection (LOD, 2.0 nM). The proposed Bi₂O₃/MnO₂/GO/GCE electrode achieved an accurate detection of Pb²⁺ in water with good recoveries (95.5–105%).

1. Introduction

As a common heavy metal ion (HMI), Pb²⁺ has adverse effects on our health and the environment because of its high toxicity, even at low concentrations. Pb²⁺ in the aquatic environment barely degrades and is easily enriched in aquatic food [1,2]. Therefore, Pb²⁺ chronically endangers human health via the food chain and gradually induces life-threatening circumstances. Excessive levels of Pb²⁺ in human body can severely destroy our organs and nervous system, which is highly associated with various cancers such as lung, kidney, and brain cancers [3]. Hence, a highly efficient determination of Pb²⁺ in water is quite essential to guarantee our health.

Over the last few decades, conventional analytical techniques have been developed to reliably detect Pb²⁺, including X-ray fluorescence spectrometry [4], UV-Vis spectroscopy [5], atomic absorption spectrophotometry [6], inductively coupled plasma mass spectrometry [7], and inductively coupled plasma-atomic emission spectrometry [8]. These analytical techniques are very robust and accurate, even in complex sample matrixes; however, they often require expensive and bulky equipment, cumbersome and time-consuming operation procedures, and highly skilled personnel. Without a doubt, they are not suitable for an on-field analysis. In recent years, stripping voltammetry, especially SWASV, has emerged as a powerful alternative for the trace determination of HMIs due to its advantages of portability, low cost, rapid response, excellent sensitivity, and feasibility for on-site analyses [9,10]. A voltammetric determination of Pb²⁺ often involves hanging mercury drop electrodes or mercury film electrodes. Owing to their superior stripping characteristics, these mercury-based electrodes are excellent in their sensitivity and reproducibility [11,12]. However, the toxic mercury contaminates samples and poses health risks to analysts. Alternatively, eco-friendly bismuth film electrodes can provide comparable sensing properties for HMI determination [13,14]. Unlike mercury electrodes, bismuth film electrodes usually suffer from surface passivation, which degrades their stripping signals. Therefore, designing novel materials with extraordinary sensing performance toward Pb²⁺ is highly desirable and challenging.

Transition metal oxide nanostructures have been extensively used for the voltammetric detection of HMIs because of their natural abundance, high adsorption capacity, and favorable catalytic activity [15,16]. Among transition metal oxides, MnO₂ has attracted increasing attention due to its earth abundance, low cost, eco-friendliness, favorable electrocatalytic activity, and excellent adsorption capability [17,18]. Nanostructured α-MnO₂ has demonstrated a high affinity for adsorption of Cu²⁺, Pb²⁺, Zn²⁺, Cd²⁺, Hg²⁺, etc. [19,20]. Therefore, MnO₂ nanostructures have recently been used for the voltammetric detection of HMIs [21,22,23]. Owing to its nontoxicity, cost-effectiveness, relatively narrow band gap, high adsorption capacity, and admirable catalytic properties, nanoscale Bi₂O₃ has also found growing interest in various fields such as photocatalysis [24], electroreduction [25], supercapacitors [26,27,28], and voltammetric sensors [29,30]. It has been reported that nanoscale Bi₂O₃ displayed a high affinity to HMIs such as Cd²⁺ [31,32]. The electrochemical reduction of Bi₂O₃ can produce a porous Bi layer and further form a “fused alloy” with the heavy metal, which accumulates more HMIs on its surface and eventually enhances the sensitivity [33]. For these reasons, Bi₂O₃-based electrodes have emerged as promising alternatives to mercury-based electrodes for HMI determination.

In contrast to single transition metal oxides, binary transition metal oxide electrocatalysts generally show a higher electrocatalytic activity [34,35]. However, binary transition metal oxides have rarely been used to detect HMIs [36,37,38]. Fe₂O₃/NiO heterojunctions possess a lower diffusion energy barrier for lead atoms, thus significantly improving the anti-interference ability for detecting Pb²⁺ [37]. Bi₂O₃/Fe₂O₃-decorated graphene oxide (GO) has demonstrated a remarkable electrocatalytic activity toward Cd²⁺ determination, having a low LOD of 1.85 ng L⁻¹ [38]. In our recent work, the synergistic interaction between β-Bi₂O₃ microspheres and shuttle-like α-Fe₂O₃ nanoparticles enabled the concurrent determination of Cd²⁺ and Pb²⁺ in environmental and food samples at the nanomolar levels [30].

Binary transition metal oxides such as Bi₂O₃/MnO₂ have been successfully used in supercapacitors [39,40] and the voltammetric detection of H₂O₂ [41]. In addition, the individual Bi₂O₃ or MnO₂ nanostructures have also been used to detect Pb²⁺. However, to the best of our knowledge, GO-decorated Bi₂O₃/MnO_x composites have not yet been reported. Herein, we fabricated GO-decorated binary transition metal oxides of Bi₂O₃ and MnO₂ nanocomposites (Bi₂O₃/MnO₂/GO) and used them as a delicate electrocatalyst for Pb²⁺ determination. GO nanoflakes are an electron-rich species that can reduce Pb²⁺ into metallic Pb by applying a suitable potential. In addition, abundant oxygen-containing functional groups (OxFGs) such as carboxyl, hydroxy, carbonyl, and epoxide groups in the edge of GO flakes can firmly bind Pb²⁺ onto their surface through electrostatic and coordination interactions, which facilitates the adsorption of Pb²⁺ [42,43]. Generally, the sensing performance for HMIs mainly relies on the adsorption capacity and electrocatalytic activity of the sensing material that is decorated on the electrode [44], which can be readily tailored using morphology engineering [45,46]. In this regard, we synthesized dandelion-like α-MnO₂ and flower-like β-Bi₂O₃ nanocomposites to enhance the Pb²⁺ adsorption and electrocatalytic activity. With the synergistic interaction of both MnO₂ and Bi₂O₃, Bi₂O₃/MnO₂/GO nanocomposites were expected to boost the stripping voltammetric responses of Pb²⁺. The Bi₂O₃/MnO₂/GO-modified glassy carbon electrode (Bi₂O₃/MnO₂/GO/GCE) showed an extraordinary electrocatalytic activity toward the stripping voltammetric behavior of Pb²⁺, with a wide linear detection range (LDR, 0.01–10 μM), low LOD (2.6 nM), and high sensitivity (53.43 μA μM⁻¹). Furthermore, the Bi₂O₃/MnO₂/GO/GCE could reliably determine Pb²⁺ in water with good recoveries.

2. Materials and Methods

2.1. Chemicals and Solutions

GO nanoflakes were purchased from Xianfeng Nanotechnology, Inc. (Nanjing, China). Manganese sulfate monohydrate (MnSO₄·H₂O), bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O), lead nitrate (Pb(NO₃)₂), potassium peroxydisulfate (K₂S₂O₈), potassium sulfate (K₂SO₄), N, N-dimethylformamide (DMF), potassium ferri/ferro-cyanide (K_3/4[Fe(CN)₆]), sodium acetate (NaAc), acetic acid (HAc), concentrated H₂SO₄, and anhydrous alcohol were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All reagents were of analytical grade and directly used as received. Water samples were collected from a local lake and from our laboratory. 0.3312 g of Pb(NO₃)₂ was completely dissolved in 100 mL of 0.1 M HAc-NaAc buffer (pH = 5.5) to prepare a 0.01 M Pb²⁺ stock solution. A series of standard solutions of Pb²⁺ with different concentrations were prepared by appropriately diluting the stock solution with the 0.1 M HAc-NaAc buffer (pH = 5.5). Deionized water (DI water, resistivity of 18.2 MΩ·m) was used for all experiments.

2.2. Preparation of Bi₂O₃/MnO₂/GO Nanocomposites

2.2.1. Preparation of Dandelion-like α-MnO₂ Microspheres

Dandelion-like α-MnO₂ microspheres were prepared via a facile hydrothermal treatment route [47]. Typically, 1.3522 g of MnSO₄·H₂O, 2.1626 g of K₂S₂O₈ and 1.3941 g of K₂SO₄ were sequentially added to 60 mL of 0.6 M H₂SO₄ and magnetically stirred for 30 min to completely dissolve. Then, the mixture solution was decanted into a 100 mL Teflon-lined stainless steel container and heated at a temperature of 140 °C for 12 h. The resultant product was repeatedly rinsed with DI water and dried at 60 °C overnight for further use.

2.2.2. Synthesis of Flower-like β-Bi₂O₃ Microspheres

Flower-like β-Bi₂O₃ microspheres were synthesized by a simple hydrothermal treatment followed by a thermal decomposition at high temperature [30]. In brief, 0.03 mol of Bi(NO₃)₃·5H₂O was dissolved into 11 mL of HAc, and 14 mL of anhydrous ethanol was then added to form a white suspension. The resulting suspension was ceaselessly stirred for 45 min, and 28 mL of DMF was then added to yield a clear solution. Subsequently, the mixture solution was poured into a 100 mL Teflon-lined stainless steel autoclave and reacted at a temperature of 120 °C for 40 min. The precursor was centrifuged at 10,000 rpm for 5 min, where it was alternately rinsed with anhydrous alcohol and DI water and allowed to dry at 80 °C overnight. Finally, the resultant β-Bi₂O₃ precursor was further transferred to a porcelain boat and calcinated at 350 °C for 4 h at a heating rate of 2 °C min⁻¹ in an air atmosphere to yield orange-yellow β-Bi₂O₃ microspheres.

2.2.3. Preparation of Bi₂O₃/MnO₂/GO Nanocomposites

At first, 5 mg of dandelion-like α-MnO₂ microspheres, flower-like β-Bi₂O₃ microspheres, and GO nanoflakes were separately dispersed into 10 mL of DI water under ultrasonication to form their respective uniform dispersions at a concentration of 0.5 mg mL⁻¹. Then, 1 mL of the α-MnO₂, β-Bi₂O₃, and GO dispersions were further mixed and subjected to a 30 min ultrasonication to obtain a uniform Bi₂O₃/MnO₂/GO dispersion. The amount of the three materials were optimized during our preliminary experiments. Bi₂O₃/MnO₂/GO composites containing 5 mg of each of the three materials showed the largest stripping peak current of Pb²⁺. Therefore, we selected the composite with this component content as the sensing material.

2.3. Characterizations of Sensing Materials

The microscopic morphologies of the GO nanoflakes, dandelion-like α-MnO₂ microspheres, flower-like β-Bi₂O₃ microspheres, and Bi₂O₃/MnO₂/GO nanocomposites were observed using field-emission SEM (Sigma HD, Zeiss, Oberkochen, Germany). Before taking the SEM measurements, a few thin layers of Au were coated onto the surface of the samples. The crystalline structures of these materials were studied using a powder XRD (Rigaku Ultima IV, Tokyo, Japan) with monochromatized Cu Kα radiation (λ = 1.54 A).

2.4. Fabrication of Modified Electrodes

Before electrode modification, the GCE was thoroughly polished to a shining mirror-like surface with 0.05 μm of alumina slurry, and it was ultrasonically cleaned with anhydrous alcohol and DI water for three cycles to remove residual contaminants. Then, the polished GCE was exposed to infrared light to allow the material to adequately dry. The Bi₂O₃/MnO₂/GO/GCE electrode was prepared by using a conventional drop-casting method. Specifically, 5 μL of the Bi₂O₃/MnO₂/GO dispersion was cast on the surface of the freshly polished GCE and dried under the exposure of infrared light to form a firm sensing film. For comparison, the MnO₂/GO/GCE, Bi₂O₃/GO/GCE, and GO/GCE were also fabricated using the same procedure, aside from the dispersion used.

2.5. Electrochemical Measurements

All electrochemical measurements were performed on a CHI 660E electrochemical workstation (Chenhua Inc., Shanghai, China) equipped with a classic three-electrode system, which consists of the Bi₂O₃/MnO₂/GO/GCE electrode, a Pt wire, and a saturated calomel electrode (SCE) as the working, counter, and reference electrodes, respectively. A 10 mL electrochemical cell made of glass was used for the electrochemical measurements. Unless otherwise specified, the 0.1 M HAc-NaAc buffer (pH = 5.5) functioned as the supporting electrolyte. To assess the electrochemical performance, the CV curves and Nyquist plots of different modified electrodes were recorded in a solution of 2 mM [Fe(CN)₆]^3−/4− and 0.1 M KCl. To improve the stripping responses, a suitable deposition was employed in the Pb²⁺ standard solutions. After 30 s of rest, the stripping peak currents of Pb²⁺ were recorded between −1.0 V and −0.5 V using the SWASV technique. The frequency, step potential, and pulse amplitude of the SWASV were set at 15 Hz, 4 mV, and 25 mV, respectively. When not in use, the Bi₂O₃/MnO₂/GO/GCE electrode was stored in the air. After each determination, the electrode surface was refreshed by immersing it into a blank solution and applying +0.3 V for 150 s to ensure the complete removal of the residual metals.

3. Results and Discussion

3.1. Physical Characterization

The microscopic morphologies of the GO nanoflakes, dandelion-like α-MnO₂ microspheres, flower-like β-Bi₂O₃ microspheres, and Bi₂O₃/MnO₂/GO nanocomposites were observed using the SEM technique, and their SEM images are shown in Figure 1. The GO nanosheets exhibited a typical lamellar structure with obvious wrinkles (Figure 1A). Dandelion-like nanostructures are found in the image of the α-MnO₂, consisting of many radially distributed nanorods (Figure 1B). Typical flower-like β-Bi₂O₃ microspheres are observed in Figure 1C, which consist of many interconnected thin nanosheets. The unique dandelion-like α-MnO₂ and flower-like Bi₂O₃ structures enlarge the electroactive surface area, thereby improving the sensing performance. In addition, the interconnected porous microstructures are found in the dandelion-like α-MnO₂ and flower-like Bi₂O₃ microspheres, which facilitate the electrolyte infiltration and adsorption of HMIs. As illustrated in Figure 1D, typical microspheres are observed in the SEM image of the Bi₂O₃/MnO₂/GO nanocomposites. In addition, the microspheres are partially wrapped by GO nanosheets. To conform the composition of the microspheres, the energy-dispersive X-ray spectroscopy (EDS) mappings of the Bi₂O₃/MnO₂/GO nanocomposites were also measured (Figure 1E). The uniformly dispersed C, O, Mn, and Bi distribution suggests the presence of C, O, Mn, and Bi elements. In addition, the distributions of Mn and Bi exhibit obvious microsphere structures, indicating that the microsphere consists of both Bi₂O₃ and MnO₂. All of these results indicate the successful synthesis of Bi₂O₃/MnO₂/GO nanocomposites.

Figure 2 displays the XRD patterns of the GO nanoflakes, dandelion-like α-MnO₂ microspheres, flower-like β-Bi₂O₃ microspheres, and Bi₂O₃/MnO₂/GO nanocomposites. A sharp diffraction peak was observed at 2θ of 9.68° in the XRD pattern of the GO, which is attributed to the (001) crystal plane of the GO [48]. α-MnO₂ nanostructures displayed distinct diffraction peaks at 2θ of 12.78°, 18.08°, 28.64°, 37.62°, 50°, 56.14°, 60.18°, and 69.54°, corresponding to the (110), (200), (310), (211), (411), (600), (521), and (541) planes of α-MnO₂ (JCPDS 440141), respectively [49]. Additionally, we detected sharp diffraction peaks without any apparent impurity peaks, demonstrating that the as-prepared α-MnO₂ microspheres were of high purity. Broad diffraction peaks were observed for the flower-like β-Bi₂O₃ at 28.00°, 32.52°, 46.32°, and 55.64°, which can be indexed to the (201), (220), (222), and (213) crystal facets (JCPDS 651209) [24]. The characteristic diffraction peaks of both the GO α-MnO₂ and β-Bi₂O₃ can be observed in the XRD pattern of the Bi₂O₃/MnO₂/GO nanocomposites. However, the intensity of the diffraction peaks of the α-MnO₂ and β-Bi₂O₃ microspheres were significantly weakened, which was mainly due to the presence of a large amount of GO partially masking the diffraction peaks of the α-MnO₂ and β-Bi₂O₃. This further confirmed that the Bi₂O₃/MnO₂/GO nanocomposites were successfully synthesized.

3.2. Electrochemical Properties of Different Electrodes

The CV curves for the different electrodes were scanned in a solution of 2.0 mM [Fe(CN)₆]^3−/4− and 0.1 M KCl to assess their electrochemical properties. As shown in Figure 3A, a pair of sharp and symmetric redox peaks occur at all electrodes, with an almost identical anodic and cathodic peak current (I_pa and I_pc), indicating that the redox of Fe(III)/Fe(II) is a quasi-reversible process. After the modification of the GO, Bi₂O₃/GO, MnO₂/GO and MnO₂/Bi₂O₃/GO, the redox peak currents were sequentially enhanced. The corresponding effective electroactive areas were also estimated based on the Randles–Sevcik equation:

$${\mathrm{I}}_{\mathrm{pc}}=2.69\times {10}^5\times n^{\frac{3}{2}}{v}^{\frac{1}{2}}{D}^{\frac{1}{2}}A{C}^0$$

(1)

$$R_{\mathrm{f}}=\frac{A}{A_{\mathrm{geom}}}$$

(2)

where A is the effective electroactive area, A_geom is the geometric surface area (diameter of 3.0 mm, 7.07 mm²), and the other symbols retain their usual meanings. The effective electroactive area and roughness factor of these electrodes were estimated according to Equations (1) and (2) (

Table 1. A comparison of the electrochemical properties of different electrodes.

Electrode	Ipc	Electroactive Area	Roughness Factor	Rct
GCE	32.83 μA	7.00 mm2	0.990	4126 Ω
GO/GCE	35.86 μA	7.65 mm2	1.082	2815 Ω
Bi2O3/GO/GCE	44.58 μA	9.50 mm2	1.344	2632 Ω
MnO2/GO/GCE	48.94 μA	10.43 mm2	1.475	1898 Ω
Bi2O3/MnO2/GO/GCE	58.94 μA	12.57 mm2	1.778	1761 Ω

). The effective electroactive area of the bare GCE was very close to its actual geometric area. The effective electroactive area of the Bi₂O₃/MnO₂/GO/GCE was 1.8 and 1.6 times greater than that of the bare GCE and GO/GCE, respectively. This indicates that the Bi₂O₃/MnO₂/GO nanocomposites significantly boosted the electroactive surface area, which is closely related to the high specific area of the GO, dandelion-like MnO₂, and flower-like Bi₂O₃ nanostructures. The large electroactive surface area of the Bi₂O₃/MnO₂/GO not only increases the accessible catalytic active sites, but also facilitates the adsorption of more Pb²⁺, which ultimately results in the improvement of the Pb²⁺ sensing performance.

EIS is a useful technique to assess the interfacial properties, mass-transport, and kinetic parameters, in addition to the charge transferred resistance (R_ct) of electrodes by observing the change in a semicircle diameter [50,51,52]. Figure 3B displays the Nyquist plots of the different electrodes. Typically, a Nyquist diagram includes a semicircle at the higher frequency domain and a straight line at the lower frequency region, which is closely related to the electron-transfer-limited and diffusion-controlled processes, respectively [53,54]. Clearly, the bare GCE showed the largest semicircle (R_ct = 4126 Ω), suggesting that the electron transfer was severely retarded in the unmodified bare. When GO was decorated on the GCE, the R_ct reduced to 2815 Ω due to the good electrical conductivity of GO. When the β-Bi₂O₃ and α-MnO₂ microspheres were further introduced into the GO/GCE, the respective R_ct reduced to 2632 Ω and 1898 Ω, respectively. As anticipated, the smallest semicircle diameter was achieved in the Bi₂O₃/MnO₂/GO/GCE (R_ct = 1761 Ω). This indicates that the Bi₂O₃/MnO₂/GO effectively promotes the electron transfer, which ultimately improves the electrochemical sensing performance.

3.3. Stripping Voltammetric Responses of Pb²⁺ on Different Electrodes

The voltammetric behavior of 1.0 μM of Pb²⁺ on the different modified electrodes were studied using the SWASV technique (Figure 4). As a control, we also recorded the SWASV curves of the different electrodes in the absence of Pb²⁺. In the absence of Pb²⁺, no noticeable response peaks were found in any of the electrodes (Figure S1). In unmodified GCE, a weak stripping peak was observed at −0.656 V with an anodic stripping peak current (I_pa) of 4.118 μA, indicating that a sluggish oxidation process occurred in unmodified GCE. When GO nanoflakes were decorated on the GCE surface, the I_pa (Pb²⁺) increased to 6.277 μA because GO, with its large surface area and abundant OXFGs, facilitates the adsorption of Pb²⁺. When the flower-like β-Bi₂O₃ and dandelion-like α-MnO₂ microspheres were introduced into the GO/GCE, their I_pa (Pb²⁺) significantly increased to 9.541 μA and 10.95 μA, respectively, while their respective anodic stripping peak potentials (E_pa) also decreased. This suggests that the decoration of flower-like β-Bi₂O₃ and dandelion-like α-MnO₂ microspheres promotes an efficient electron transfer, which is closely related to high affinity capacity and extraordinary electrocatalytic activity toward Pb²⁺. As expected, the GO-coated binary transition metal oxides of Bi₂O₃/MnO₂ remarkably improved the stripping voltammetric response of Pb²⁺, with the highest I_pa of 58.07 μA and the lowest E_pa (−0.667 V). Notably, the stripping peak current for the GO-coated binary transition metal oxide was about five times higher than that of the GO-coated single metal oxides, suggesting that the synergistic effect between the flower-like β-Bi₂O₃ and dandelion-like α-MnO₂ microspheres is attributed to the enhanced I_pa and reduction in overpotential.

3.4. Optimization of Determination Conditions

3.4.1. Effect of Deposition Parameters

Deposition parameters have a prominent effect on the voltammetric behavior of Pb²⁺. As illustrated in Figure 5A, the I_pa (Pb²⁺) gradually increased when the deposition potential shifted from −1.3 V to −1.0 V, then sharply declined as the deposition potentials shifted further. At an excessively negative deposition potential, hydrogen bubbles would be generated on the surface of the Bi₂O₃/MnO₂/GO/GCE, resulting in the exfoliation of the deposited Pb²⁺. When the deposition potential was higher than –1.0 V, the electrochemical energy was not sufficient to reduce the deposited Pb²⁺. Therefore, the optimal deposition potential was set at −1.0 V. Generally, prolonging the deposition time can enhance the adsorption amount of HMIs on the electrode surface, thereby increasing the stripping peak current. As presented in Figure 5B, the I_pa (Pb²⁺) steadily increased with deposition time until reaching a plateau at 300 s. This was mainly because the surface adsorption sites of the Bi₂O₃/MnO₂/GO/GCE were saturated at 300 s. Thus, the optimum deposition time was set at 300 s.

3.4.2. Effect of Solution pH

It is well-known that a solution’s pH has a significant impact on the I_pa (Pb²⁺). Therefore, the influence of the solution’s pH was also explored. As illustrated in Figure 6, the I_pa (Pb²⁺) slowly increased as the pH increased from 3.0 to 4.5, and then sharply increased until the pH of 5.5. Afterwards, the I_pa (Pb²⁺) dramatically decreased when the pH exceeded 5.5. Therefore, pH = 5.5 was chosen as the optimal solution pH. This phenomenon can be interpreted as follows. At lower pH values, the H⁺ adsorption on the electrode surface neutralizes the negative charge on the electrode surface, which reduces the adsorption of Pb²⁺, resulting in a decrease in the I_pa (Pb²⁺). Pb²⁺ tends to be hydrolyzed in a solution with a higher pH so that the concentration of free Pb²⁺ in the solution decreases and the I_pa (Pb²⁺) decreases.

3.5. Stripping Kinetics of Pb²⁺ on the Bi₂O₃/MnO₂/GO/GCE

In order to study the stripping kinetics of Pb²⁺, the cyclic voltammograms of 1.0 μM of Pb²⁺ were measured by the Bi₂O₃/MnO₂/GO/GCE at different scanning rates (0.05–0.40 V s⁻¹). Figure 7A shows the CV curves of 1.0 μM of Pb²⁺ at various scanning rates. A pair of well-shaped redox peaks occurred at all scanning rates with almost identical I_pa and I_pc (I_pa/I_pc ≈ 1), suggesting that Pb²⁺ stripping is a quasi-reversible process. As the scanning rate increased, the I_pa and I_pc gradually increased. In addition, the anodic peaks shift to more positive potential while the cathodic peaks shift to more negative potential. As illustrated in Figure 7B, both the I_pa and I_pc are linearly correlated to the square root of scanning rates (v^1/2), demonstrating that Pb²⁺ stripping was primarily controlled by the diffusion.

3.6. Calibration Plot, LDR, and LOD

Under optimal determination conditions, the I_pa (Pb²⁺) at various concentrations were measured on the Bi₂O₃/MnO₂/GO/GCE via the SWASV technique. As illustrated in Figure 8A, well-defined stripping peaks of Pb²⁺ occurred at about −0.65 V with a slight positive shift at higher concentrations. As shown in the inset of Figure 8A, the stripping peaks of low concentrations of Pb²⁺ slightly shifted to more negative biases, probably due to the electrode surfaces not being exactly the same. However, the obvious positive shift in the peak potential at higher concentrations is probably due to the occurrence of concentration polarization. Moreover, the I_pa (Pb²⁺) gradually increased with Pb²⁺ concentration. The I_pa (Pb²⁺) are in good proportion to Pb²⁺ concentration from 0.01 to 10 μM (Figure 8B). The corresponding linear regression equation was expressed as I_pa(μA) = 53.45C (μM) + 0.578, with a good correlation coefficient (R²) of 0.998. The LOD was calculated as 2.0 nM (0.41 μg L⁻¹) based on 3σ/s (where σ is the standard deviation in blank solution and s is the slope of the calibration plot). A comparison of the analytical properties for Pb²⁺ was also made between the Bi₂O₃/MnO₂/GO/GCE composite and previously reported ones. As shown in

Table 2. A comparison on the analytical properties for Pb²⁺ determination.

Electrodes	Method	LDR (μg L−1)	LOD (μg L−1)	Refs.
α-Fe2O3/NiO/GCE	SWASV	10.4–186	4.14	[37]
BiF/ERGO/SPE	SWASV	1.00–60.0	0.80	[55]
Fe3O4/Bi2O3/C3N4/GCE	SWASV	2.07–622	0.21	[36]
SnS-Bi2O3/GCE	SWASV	20.7–207	0.29	[16]
L-Cys/GR–CS/GCE	DPASV	1.04–64.1	0.12	[56]
SWCNHs/SPE	SWASV	1.0–60.0	0.40	[57]
Bi2O3/CPE	DPASV	10.0–100	5.00	[32]
Fe3O4@G2-PAD/CPE	SWASV	0.50–80.0	0.17	[58]
g-C3N4/r-GO/GCE	SWASV	1.00–300	0.15	[59]
L-cysine/Au@SiO2 @Fe3O4/NG/GCE	SWASV	5.00–80	0.60	[60]
AuNPs/GCE	DPASV	62.1–290	62.0	[61]
MIL-100(Cr)/GCE	SWASV	207–2070	9.94	[62]
TBA/MCH-Au	SWASV	10.4–207	7.18	[63]
BiNPs/GCE	SWASV	5.00–60	0.80	[64]
Bi2O3/MnO2/GO/GCE	SWASV	2.07–2072	0.41	This work

, the analytical properties of the Bi₂O₃/MnO₂/GO/GCE composite, including the LDR, LOD, and sensitivity, well matches or even exceeds the previously reported electrodes.

3.7. Anti-Interference Ability

Excellent selectivity is essential for trace determination of HMIs in complex sample matrix. Therefore, the anti-interfering ability of the Bi₂O₃/MnO₂/GO/GCE was also studied. To explore the anti-interfering ability, the SWASV responses of 1.0 μM Pb²⁺ were recorded on the Bi₂O₃/MnO₂/GO/GCE in presence of 100-fold concentration interfering species, such as common cations (i.e., Na⁺, K⁺, Ca²⁺, Mg²⁺, Zn²⁺, Fe²⁺, Co²⁺, Cu²⁺, Cd²⁺, Al³⁺) and anions (i.e., Cl⁻, NO₃⁻, SO₄²⁻, PO₄³⁻). The relative errors are less than 5% in presence of these potential interfering species (Table S1), indicating that the Bi₂O₃/MnO₂/GO/GCE possesses excellent selectivity. The extraordinary selectivity of the Bi₂O₃/MnO₂/GO/GCE may due to the higher affinity of Bi₂O₃/MnO₂/GO for Pb²⁺. Interestingly, a sharp stripping peak of Cd²⁺ was also observed at −0.865 V on the Bi₂O₃/MnO₂/GO/GCE. In addition, the stripping peaks of Pb²⁺ and Cd²⁺ did not overlap each other with a broad potential separation of 0.215 V, suggesting the feasibility of simultaneous detection of Pb²⁺ and Cd²⁺.

3.8. Reproducibility, Repeatability and Stability

To assess the practicability of the Bi₂O₃/MnO₂/GO/GCE composite, we also studied their reproducibility, repeatability, and stability. The relative standard deviation (RSD) for parallel detections of 10 μM of Pb²⁺ using five independent Bi₂O₃/MnO₂/GO/GCEs was 4.59% (Figure S2), indicating satisfactory reproducibility. The RSD for five consecutive detections of 10 μM of Pb²⁺ was 5.38% (Figure S3), suggesting admirable repeatability. Moreover, the I_pa of 10 μM of Pb²⁺ retained 92.05% of its initial values after one week (Figure S4), indicating excellent storage stability.

3.9. Determination of Trace Pb²⁺ in Water Samples

Under the optimal determination conditions, the Pb²⁺ concentrations in the water samples were quantitatively determined by the SWASV technique using the Bi₂O₃/MnO₂/GO/GCE composite. As summarized in

Table 3. Determination of Pb²⁺ in the water samples using the Bi₂O₃/MnO₂/GO/GCE composite (n = 3).

Samples	Detected (μM)	Added (μM)	Found (μM)	RSD (%)	Recovery (%)
Lake water	0.121	0.100	0.226	5.89	105%
Lake water	0.121	0.200	0.312	4.72	95.5%
Tap water	ND	0.100	0.104	4.26	104%
Tap water	ND	0.500	0.492	3.83	98.4%

ND: not detected.

, the Pb²⁺ concentration from a local lake was determined to be 0.121 μM, while no Pb²⁺ was determined in the tap water. To further confirm the accuracy and precision, a series of Pb²⁺ standard solutions of known concentrations were separately spiked into the water samples, and recovery assays were then performed. The Bi₂O₃/MnO₂/GO/GCE exhibited acceptable RSD values (3.83–5.89%) and satisfactory recoveries (95.5–105%). The Bi₂O₃/MnO₂/GO/GCE has demonstrated tremendous prospects in the sensitive determination of Pb²⁺ from complex matrixes.

4. Conclusions

In this study, GO-coated binary transition metal oxides of Bi₂O₃/MnO₂ nanocomposites were used to fabricate a sensitive voltammetric sensor for the trace detection of lead ions in water samples. The Bi₂O₃/MnO₂/GO nanocomposites boosted the electroactive surface area and significantly reduced the charge transferred resistance. Moreover, the synergistic enhancement effect from the GO nanoflakes, dandelion-like α-MnO₂ microspheres, and flower-like β-Bi₂O₃ microspheres endowed Bi₂O₃/MnO₂/GO/GCE with extraordinary electrocatalytic activity toward the stripping voltammetric behavior of Pb²⁺. Under optimal detection conditions, the Bi₂O₃/MnO₂/GO/GCE exhibited a relatively wide LDR (0.01–10 μM), low LOD (2.0 nM) and high sensitivity (53.43 μA μM⁻¹). Moreover, the Bi₂O₃/MnO₂/GO/GCE exhibited an anti-interference ability even in presence of a 100-fold concentration of common cations and anions, as well as outstanding reproducibility, repeatability, and stability. The MnO₂/Bi₂O₃/GO/GCE composite realized the sensitive detection of trace Pb²⁺ in water samples with satisfactory recovery. Together with portable and smart electrochemical devices, the proposed Bi₂O₃/MnO₂/GO nanocomposites demonstrate promising prospects in the in situ detection of HMIs.