Electrochemical Sensor for Ultra-Sensitive Detection of Lead (II) Ions in Water Using Na₃BiO₄-Bi₂O₃ Mixed Oxide Nanostructures ^†

Abstract

This study aimed to detect trace amounts of lead using Na₃BiO₄-Bi₂O₃ mixed oxide nanostructures. Scanning electron microscopy (SEM) showed the presence of nanoplates with an average thickness of 90 nm. X-ray diffraction (XRD indicated the presence of poly-crystalline Na₃BiO₄ and Bi₂O₃ in the ratio 1:4. The chemical structure of the prepared samples was also studied through X-ray photoelectron spectroscopy. These nanostructured electrodes are highly sensitive to Pb²⁺ ions with a limit of detection of 68 ppt (0.32 nM).

1. Introduction

Fast industrial growth and manufacturing have led to the contamination of water resources by heavy metal ions [1,2,3]. Lead is one of the most toxic heavy metals capable of deeply affecting the health and well-being of humans. Even trace concentrations of lead can be fatal as it has a tendency to attach itself to the vital processes in our body and severely affect their performance [4,5,6,7]. Thus, there is a pressing need to develop portable and cheap methods for the detection of ultra-trace levels of lead, even in the remote corners of the globe.

A number of techniques like anodic stripping voltammetry [8,9], chromatography [10], atomic emission spectrometry [11], inductively coupled plasma mass spectrometry [12], etc., have been developed for the detection of heavy metal ions. Anodic stripping voltammetry is inexpensive, relatively simple, and portable. It provides a low detection limit and multi-element detection capability. However, the detection capability of this technique is dependent on the electrodes employed. Huge research efforts are being made to develop cheap, environmentally friendly, and highly sensitive electrodes.

Many new materials are being explored for the development of high-sensitivity sensors to detect lead. They include metals, metal oxides, and carbon-based composites. Metal oxides are particularly interesting as they are not toxic and offer good electron transfer kinetics and higher adsorption [13,14]. Transition metal oxides, including Fe₃O₄ [15], MnO₂ [16], MnFe₂O₄ [17], and Co₃O₄ [18], have been reported to show significant activity towards lead (II) ion sensing.

In this work, we have proposed ultrasensitive and cheap electrodes using Na₃BiO₄-Bi₂O₃ mixed oxide nanostructures. These electrodes have been synthesized using electrodeposition, as it is relatively economical and offers unique advantages over other methods. The deposition rate and deposit morphologies can be controlled easily by varying deposition potential, deposition time, and electrolyte concentration [19,20]. A limit of detection of 68 ppt (0.32 nM) has been observed for Pb²⁺ ions using these electrodes.

2. Heavy Metal Ion Detection Procedure

SWASV with cathodic preconcentration [21,22] was used for the detection of Pb²⁺ ions. Distilled water containing 0.1 M perchloric acid (HClO₄) was mixed with PbCl₂ to prepare the electrolyte (for simultaneous detection, PbCl₂ and HgCl₂ were added). A potential of −1 V was applied at the working electrode for 10 min in the preconcentration step. In the second step, stripping was performed. The step potential, amplitude, and frequency of the square wave were optimized to obtain the best results. The used optimized values are 2 mV, 20 mV, and 15 Hz, respectively.

3. Results and Discussion

3.1. Morphological, Structural and Compositional Studies

SEM was used to study the surface morphology of the obtained samples. The presence of nanoplates (thickness ≈ 90 nm) is shown in Figure 1. These nanoplates appear to have sharp and well-defined edges.

XRD pattern indicates the presence of polycrystalline monoclinic structures (Figure 2). Semiquantitative concentration analysis (PANalytical X’Pert HighScore) indicates that the ratio of Na₃BiO₄ to Bi₂O₃ is 1:4.

XPS spectroscopy was used in order to study the chemical bonding of Bi with Na and O. High-resolution XPS of Bi 4f, C 1s, and O 1s is shown in Figure 3. Elemental scan of Bi 4f shows prominent peaks at 164 eV (4f_5/2) and 158.7 eV (4f_7/2). These peaks correspond to the Bi³⁺ state in Bi₂O₃ [23]. Two small shoulders at 165.1 (4f_5/2) and 159.7 (4f_7/2) eV closely resemble the values reported for Bi⁵⁺ in Na₃BiO₄ [23,24]. Na 1s scan shows three peaks at 1071.7, 1070, and 1069.4 eV.

3.2. Lead (II) Ion Detection

Maximum current density and area under the peak have been plotted as a function of voltage (Figure 4) and Pb²⁺ ion concentration (Figure 5). Analysis shows that the maximum peak current and peak area are linearly dependent on concentration with (R² = 0.99). The per unit area sensitivity of these electrodes towards lead detection can be estimated from the slope of linear fit obtained for maximum current density (y = 0.0024x − 0.0362). Thus, a sensitivity of 2.4 μA/ppt is seen. An almost linear dependence of peak area on the concentration of Pb²⁺ ions further enhances the reliability of these measurements as estimation of accurate peak intensity becomes difficult at very low concentrations (i.e., 68 ppt). This is mainly due to the appearance of broad peaks at such concentration, which limits the accurate identification of peak position. However, the determination of peak area still remains unaffected due to the presence of broad peaks. Similar broad peaks at very low concentrations have been reported for lead sensing on transition metal oxides. However, this issue has not been addressed in these studies.

Table 1. Detection limit comparison for Pb²⁺ ions.

Electrode	Limit of Detection (nM)	Reference
Fe3O4	119	[22]
MnO2	75	[23]
MnFe2O4	54	[24]
Co3O4	2.5	[21]
Na3BiO4-Bi2O3	0.32	This Work

shows the limit of detection towards Pb²⁺ for various transition metal oxides. Pb²⁺ ion detection on Co₃O₄ is particularly interesting, as nanostructures similar to the present work have been reported. Compared with our results, Co₃O₄ nanostructures show a significantly low sensitivity towards Pb²⁺ detection. Moreover, their limit of detection is also 2.5 nM as against the value of 0.32 nM reported in the present study. Other transition metal oxides like Fe₃O₄, MnO₂, and MnFe₂O₄ also showed poor detection limits. MgO nanoflowers have been reported to show better detection limits towards Pb²⁺ ions [25]. However, the reported synthesis method for MgO nanoflowers is quite complex and requires calcination at a high temperature of 973 K. Moreover, the use of a nafion polymer membrane layer over the MgO nanoflowers electrode raises concerns regarding the stability of the MgO nanoflowers for electrochemical detection of Pb²⁺ ions.

3.3. Simultaneous Detection of Lead (II) and Mercury (II) Ions

The simultaneous detection of Pb²⁺ and Hg²⁺ was also performed (Figure 6). The results indicate that the reported electrodes are selective towards the detection of Pb²⁺ ions as the separation of peak potentials is seen. The position and intensities of the stripping peak of Pb²⁺ at 680 ppt concentration are found to be effectively the same as reported in Figure 4. Thus, the reported electrodes for the detection of Pb²⁺ ions in water are not affected by the presence of other ions in water. Thus, the proposed sensor can be used for sensing lead in naturally contaminated waters with high sensitivity.

4. Conclusions

Na₃BiO₄-Bi₂O₃ mixed oxide nanoplates (thickness ≈ 90 nm) were synthesized using potentiostatic electrodeposition. XRD studies indicate the presence of polycrystalline monoclinic structures. A high sensitivity of 2.4 μA/ppt and a limit of detection of 68 ppt towards the detection of Pb²⁺ ions is seen. The reported electrodes are selective toward Pb²⁺ ions detection, as indicated by clear separate peaks for Pb²⁺ and Hg²⁺ ions during simultaneous detection. The presence of Hg²⁺ ions has no effect on the peak current density and peak position of Pb²⁺ stripping peaks. This proposed sensor for the detection of Pb²⁺ ions is an ideal candidate for commercial deployment.