Polypyrrole functionalized MoS₂ for sensitive and simultaneous determination of heavy metal ions in water

Abstract

Assessing heavy metal ion (HMI) contamination to sustain drinking water hygiene is a challenge. Conventional approaches are appealing for the detection of HMIs but electrochemical approaches can resolve the limitations of these approaches, such as tedious sample preparation, high cost, time consuming and the need for trained professionals. Here, an electrochemical approach is developed using a nano-sphered polypyrrole (PPy) functionalized with MoS₂ (PPy/MoS₂) by square wave anodic stripping voltammetry for the detection of HMIs. The developed sensor can detect Pb²⁺ with a limit of detection of 0.03 nM and a sensitivity of 36.42 μA nM⁻¹. Additionally, the PPy/MoS₂ sensor was employed for the simultaneous detection of HMIs of Cd²⁺, Pb²⁺, Cu²⁺ and Hg²⁺. The reproducibility, stability and anti-interference studies confirm that the sensor can be used to monitor HMI contamination of water.

Assessing heavy metal ion (HMI) contamination to sustain drinking water hygiene is a challenge.

1. Introduction

The accessibility of essential, consumable water for life is limited, despite the abundance of water. Globally, 1.1 billion people consume contaminated and unhygienic drinking water, which causes various afflictions. Hence, it is imperative to monitor the quality of water. Emerging industries typically release more environmentally hazardous pollutants, particularly heavy metal ions (HMIs).¹ HMIs are ubiquitous and their hazardous characteristics, such as chemical stability, non-degradability, bio-accumulation and adverse effects on the environment and human health, have piqued public interest.² Lead ions (Pb²⁺) are a prevalent HMI pollutant with high toxicity even at very low concentrations. Pb²⁺ contamination leads to serious effects on human health.³ For instance, Pb²⁺ accumulation in young children may lead to long-term brain and nervous system damage, whereas in adults exposure can result in long-term consequences such as hypertension and renal damage. Prolonged exposure to such pollutants may culminate in carcinogenic illness.⁴ Consequently, HMI contamination must be detected and nullified in drinking water before it is supplied. Worldwide, pollution caused by the HMI is regulated by national and international water authorities.^5,6 HMI detection is still a major challenge at the trace level, with complex sampling procedures, and hence there is a need to develop sophisticated, simple, inexpensive as well as sensitive devices.

Conventional methods have drawbacks such as tedious sample preparation, pre-concentration steps, the involvement of specialists for instrument operation and complex evaluation procedures.⁷ This can be overcome using electrochemical methods, such as potentiometry, amperometry and impedance techniques, which are promising for detecting HMIs as these are simple to operate in devices and less expensive.⁸ Square wave anodic stripping voltammetry (SWASV) is one of the effective potentiometric methods used to detect HMIs because it offers high sensitivity, high precision, broad measuring range, ease of use and is suitable for field applications.⁹ SWASV mainly depends on the nature of the catalyst, hence the catalyst surface and components have a prevailing role in SWASV detection of HMIs.

Over the last decades, nanomaterials based on metal oxides,¹⁰ metal sulfides,¹¹ layered double hydroxides,¹² metal/carbonaceous¹³ and organo–inorganic composites¹⁴ combined with conducting polymers (CPs), such as polyaniline (PANI),^15,16 polypyrrole (PPy) and poly(3,4-ethylene-dioxy-thiophene) (PEDOT),¹⁷ have been developed as electrode materials for the detection of HMIs. Molybdenum disulfide (MoS₂) is one of the most widely used electrode materials in energy conversion,^18,19 electrochemical sensors and photocatalysis applications, due to its remarkable stability and the different nanostructures that can be synthesized. For instance, Dharmender Singh Rana et al. synthesized a 2D nanocomposite of MoS₂/rGO by a single-step hydrothermal method for the electrochemical detection of mercury (Hg). The sensor exhibited an excellent limit of detection (LOD) of 1.6 μM. This low LOD may be attributed to the availability of sulfur atoms as active sites to increase the sensitivity of the catalyst.²⁰ Xi Chen et al. prepared rGO/MoS₂ composite for Hg²⁺ detection by an ultrasonic method with MoS₂ and rGO by thermal annealing treatment. The rGO/MoS₂ sensor displayed a low LOD of 1 nM.²¹ Further, by utilizing the synergistic interaction of reduced graphene oxide, molybdenum disulfide and chitosan (rGO/MoS₂/Cs), Chuanen Guo et al. designed a sensitive sensor for Pb²⁺ detection.²² SWASV was employed to study the stripping behavior of Pb²⁺. Arya Nair J. S. et al. fabricated an electrochemical sensor of partially reduced graphene oxide-nano molybdenum disulfide-modified glassy carbon electrode (GCE) (prGO-MoS₂/GCE) for the simultaneous detection of Pb²⁺ and Cd²⁺ in drinking water. The study revealed an LOD for Pb²⁺ and Cd²⁺ of approximately 0.0002 ppt and 0.1 ppt, respectively, highlighting the efficacy of the prGO-MoS₂/GCE in complying with regulatory standards.²³ Saisree S. et al. investigated an electrochemical sensor utilizing sulfur co-doped nitrogen-graphene quantum dots (S,N-GQD) synthesized from polyaniline, with H₂SO₄ serving as both the acid catalyst and S-doping agent, through a straightforward hydrothermal synthesis method. The S,N-GQDs demonstrated simultaneous sensing capabilities for three toxic metal ions: Cd²⁺, Pb²⁺ and Hg²⁺. The detection limits achieved for Cd²⁺, Pb²⁺ and Hg²⁺ were notably low, at 1 pM, 10 pM and 1 pM, respectively, marking a significant advancement in the simultaneous detection of these metal ions. The sensitivity values obtained from the respective linear dynamic ranges were 12, 13 and 5 μA μM⁻¹ cm⁻². The LOD values for Cd²⁺, Pb²⁺ and Hg²⁺ were found to be 10⁻¹², 10⁻¹¹ and 10⁻¹² M, respectively.²⁴

As mentioned, nanomaterials and CP composites are promising for the detection and removal of HMIs. Functionalization of the PPy matrix with MoS₂: (a) increases the surface area to volume ratio; (b) provides a high number of catalytic sites and strong adsorption ability; (c) the sulfur atoms on the surrounding MoS₂ act as soft a Lewis base with a high affinity towards soft metal ions; and (d) the MoS₂ may reduce the agglomeration of PPy. In this regard, the functionalization of conducting PPy with MoS₂ is a way to avoid the limitations for constructing electrochemical sensors.

Herein, we have developed a nano-sphered PPy-functionalized MoS₂ electrode material to achieve highly sensitive detection of Pb²⁺. MoS₂ was synthesized by a simple hydrothermal process and PPy/MoS₂ was synthesized by oxidative polymerization of pyrrole monomer in the presence of MoS₂. The PPy/MoS₂ combination provided a platform for the electrochemical detection of HMIs, attributable to the synergistic effect of the PPy and MoS₂ towards Pb²⁺. The PPy provides a high surface area and the free sulfur groups of MoS₂ provide active sites of adsorption in the PPy/MoS₂ sensor, and necessary conductivity. Through the synergistic effect, the PPy/MoS₂ exhibited a good electrochemical response towards Pb²⁺ by SWASV, with an LOD of 0.03 nM and a sensitivity of 36.42 μA nM⁻¹. The primary advantage of our composite material lies in the surface modification of MoS₂ with the PPy. This modification enhances the number of functional groups available for chelating HMIs through electrostatic interactions. Additionally, polymers are known for their excellent responsiveness to stimuli and provide the requisite conductivity for effective electrochemical sensing. This combination significantly enhances the sensitivity of the sensor.

2. Materials and methods

2.1. Materials

Ammonium hepta-molybdate ((NH₄)₆Mo₇O₂₄·4H₂O) was obtained from Thermo Fisher Scientific India Pvt. Ltd. Thiourea (CH₄N₂S) was purchased from S D Fine-Chem Ltd (SDFCL). Ammonium persulphate ((NH)₄S₂O₆) was obtained from Isochem Laboratories. Pyrrole (99%) was purchased from Sigma-Aldrich. Perchloric acid was procured from Thermo Fisher Scientific India Pvt. Ltd. All chemicals were utilized without any further purification and the solutions were prepared with distilled water.

2.2. Synthesis of MoS₂, and PPy/MoS₂

MoS₂ NPs were synthesized through a simple hydrothermal process.²⁵ Briefly, a known amount of ammonium hepta-molybdate and thiourea (1 : 1 M/M) was dissolved in 40 mL distilled water. The reaction mixture was transferred to a Teflon-lined autoclave and heated at 180 °C for about 24 hours.^25,26 The reaction in the autoclave resulted in a black-coloured precipitate. The precipitate obtained was centrifuged, washed with distilled water and ethanol (1 : 1) and allowed to dry overnight at 80 °C.

PPy and PPy/MoS₂ were synthesized by the oxidative polymerization method. In brief, 10 μL pyrrole was dissolved in 20 mL 0.1 M HClO₄ and kept in an ice bath (below 5 °C) for about 1 hour. To the reaction mixture, 20 mL of 0.1 M (NH₄)₂S₂O₈ in 0.1 M HClO₄ was added dropwise with constant stirring in the ice bath for about 1 hour. After completion of the polymerization, a dark green-coloured precipitate was obtained, washed with distilled water several times and dried overnight at 60 °C.²⁷ Similarly, PPy/MoS₂ was synthesized by oxidative polymerization, by mixing 10 mL pre-dispersed MoS₂ and 10 μL pyrrole dissolved in 10 mL of 0.1 M HClO₄ in 1 : 2, 1 : 1 and 2 : 1 ratios (w/w), designated as PPy/MoS₂-1, PPy/MoS₂-2 and PPy/MoS₂-3, respectively, and kept in an ice bath (below 5 °C) for about 1 hour. To this mixture, 0.1 M (NH₄)₂S₂O₈ in 0.1 M HClO₄ was added dropwise with constant stirring in the ice bath for about 1 hour.²⁸ After completion of the reaction, a cleaning and drying process were followed. The possible mechanism for the functionalization of PPy with MoS₂ is illustrated in Fig. 1a.

Fig. 1. (a) Possible functionalization mechanism of MoS₂ into the PPy polymer skeleton. (b) Schematic illustration of SWASV for detection of Pb²⁺ by PPy/MoS₂ sensor.

2.3. Measurements and electrode preparation

Following the synthesis, electrochemical measurements (SWASV) were performed with a traditional three-electrode system, in which Ag/AgCl was used as the reference electrode, a modified GCE as the working electrode and Pt wire as the counter electrode. SWASV was performed in 0.1 M sodium acetate (Na–Ac) buffer under optimized parameters at a scan rate of 100 mV s⁻¹. Before the electrochemical measurements, the GCE was pre-polished using alumina slurry of different particle sizes on appropriate padding, followed by electrochemical cleaning by cyclic voltammetry in 0.5 M H₂SO₄. A 10 μL portion of 1 mg of PPy/MoS₂ dispersed in 1 mL water : ethanol (1 : 1) solution was drop-cast onto the clean GCE. Structural confirmation of the PPy/MoS₂ sensor was obtained by characterization by X-ray diffraction (XRD) with a Rigaku SmartLab instrument scanning at 3° min⁻¹ in the range 5–80°, and by Raman spectroscopy using an Olympus BX microscope and HORIBA Scientific CCD detector. The surface morphology was analysed by field-emission scanning electron microscopy (FESEM) with a Tescan-Mira 3 LMH model, and FESEM equipped with energy dispersive X-ray (EDAX) and elemental analysis was obtained using a QUANTAX 200 with XFlash BRUKER.

2.4. Mechanism of functionalization of MoS₂ into PPy and HMI detection by PPy/MoS₂ sensor

Functionalization is a process of oxidizing or reducing the polymer (PPy) simultaneously and introducing counter anions or cations, respectively. Depending on the nature of the materials, three significant interactions influence the functionalization process: electrostatic interactions, hydrogen bonding and π–π interactions.²⁹ The conductivity of PPy arises from electron transfer along the conjugated polymer, as well as the motion of charge carriers. Oxidation of PPy results in a positive net charge over the surface of the PPy, resulting in the bipolaron form of PPy, which can be counterbalanced by the anions and become a part of the PPy backbone. The anions might be released back during reduction of the polymer.^30,31 With this scenario, MoS₂ was functionalized into the PPy skeleton aided by electrostatic interactions between the positive charge on PPy and the negative sulfur charge of MoS₂, as shown in Fig. 1a. Considering MoS₂ is bulky by nature, release back into the electrolyte by reduction is not desirable.³² Sensitivity towards HMIs can be achieved by electro-hopping in PPy/MoS₂. The conductivity of PPy is driven by conjugation, with functionalization of PPy developing electron-rich centres that can be used as electro-hopping sites in the PPy skeleton, potentially increasing the conductivity.

PPy, PANI and PEDOT are well-recognized for the availability of numerous N–H functional groups, whereas³³ MoS₂ is recognized for its readily available free sulfur groups. The combined accessibility of the functional groups on individual PPy and MoS₂ of the PPy/MoS₂ positively influences the detection of Pb²⁺. Conversely, sulfur and amine groups, being negatively charged, tend to attract positively charged Pb²⁺ effectively onto the electrode surface by electrostatic interaction. During electrochemical detection of Pb²⁺, the SWASV mechanism typically consists of three basic steps of pre-concentration and stripping of HMIs, as illustrated in Fig. 1b. The first step involves the amine and sulfur groups of the catalyst to form a metal complex on the surface of the working electrode. In step 2, Pb²⁺ is reduced to Pb⁰ during pre-concentration under the influence of the deposition potential. Finally, in step 3, anodic stripping of Pb occurs, where Pb⁰ is oxidized to Pb²⁺, releasing the ions back into the electrolyte solution, which results in a stripping voltammogram, where the stripping peak current is directly proportional to the concentration of Pb²⁺ in the electrolyte.³⁴ In the case of the detection of Pb²⁺ using MoS₂, the process benefits from both a strong adsorption affinity and an electrochemical (EC) reduction process. The sulfur (S⁻²) in MoS₂ acts as an electron donor, facilitating the reduction of Pb²⁺ to Pb⁰. Consequently, the removal of Pb²⁺ is largely driven by the affinity of MoS₂ for the metal ion rather than solely by a strong EC mechanism.

Further enhancing the performance, the oxidative polymerization of pyrrole to form PPy functionalizes the surface of MoS₂. This functionalization increases the number of nitrogen- and oxygen-containing functional groups, improving interactions with HMIs. In addition, PPy provides a higher surface area, which also facilitates better mass transfer of the analyte to the electrode surface and reduces the thickness of the diffusion layer. This ensures that more metal ions can reach the electrode quickly, resulting in a faster electrochemical process.

3. Results and discussion

3.1. Powder XRD and Raman spectroscopy

The structure and purity of PPy, MoS₂ and functionalized PPy/MoS₂ were investigated through XRD, as demonstrated in Fig. 2a. The XRD pattern of bare PPy exhibits a weak and broad peak at 21.5°, which is correlated to the short-range arrangement of the chains and amorphous nature of PPy.³⁵ The diffraction peaks at 2θ values of 13.79°, 33.00° and 57.19° can be indexed as the 002, 100 and 110 crystal diffraction of MoS₂ (JCPDS No. 37-1492).^36,37 Meanwhile, the broad diffraction peak at 2θ = 21.5° corresponds to the PPy of PPy/MoS₂. The diffraction peaks offer favourable evidence for the functionalization of PPy with MoS₂. Furthermore, the Raman spectra were used to examine the characteristics of PPy and PPy/MoS₂, as illustrated in Fig. 2b. The C C backbone stretching and ring stretching modes are indicated by the two broad peaks at 1354.91 and 1595.18 cm⁻¹, respectively, in the PPy Raman spectrum.³⁸ The PPy/MoS₂ spectrum shows two characteristic peaks at 376.83 and 409.91 cm⁻¹, which correspond to the in-plane E_2g and out-of-plane A_1g vibration modes of MoS₂, respectively.³⁹ The appearance of the C C backbone stretching and the ring stretching modes of PPy confirmed the successful functionalization of MoS₂ into the PPy backbone.

Fig. 2. (a) XRD of PPy, MoS₂ and PPy/MoS₂. (b) Raman spectra of PPy and PPy/MoS₂.

3.2. FESEM, EDAX and elemental mapping

Fig. 3a–c shows low- to high-magnification FESEM images of the PPy/MoS₂ sample. The morphology obtained was found to be irregular-sized nanospheres. Moreover, the MoS₂ nanospheres were agglomerated in combination with PPy. The polymer matrix plays an important role in inducing the spherical morphology of MoS₂, by interacting with it.^40,41 FESEM equipped with EDAX confirmed the presence of the elements Mo, S, C, O and N in the PPy/MoS₂, with elemental weight ratios as indicated in Fig. 3d. The elemental mapping verified the uniform distribution of the elements in the material, as shown in Fig. 3e.

Fig. 3. (a–c) Low- to high-magnification FESEM images of PPy/MoS₂. (d and e) EDAX and elemental mapping of PPy/MoS₂.

3.3. Electrochemical detection of HMIs

The SWASV response is sensitive to the deposition potential, deposition time and pH of the electrolyte. Therefore, we investigated the optimal experimental conditions for the detection capability of the PPy/MoS₂ sensor. The effective deposition potential, time and pH for the electrolyte of Pb²⁺ was determined by changing each of these parameters in turn while detecting Pb²⁺ under the same conditions.⁴² As shown in Fig. 4(a and b), as the deposition potential increases the peak current also increases, with a maximum peak current at −0.9 V vs. Ag/AgCl, and then decreases with further increase in the potential. When the potential exceeds −0.9 V, the peak current decreases mainly due to the hydrogen evolution reaction by water splitting. H₂ bubbles at the electrode surface hinder the active site, which is initially available for Pb²⁺ adsorption, resulting in the reduction of the peak current.^8,43 Hence, −0.9 V vs. Ag/AgCl was employed as the optimal deposition potential for the further detection processes. The effect of deposition time on the peak current responses for Pb²⁺ is illustrated in Fig. 4(c and d). The maximum peak current was achieved with a deposition time of 9 min. With further increase in the deposition time, the peak current remains almost constant, which could be due to saturation of the active sites on the sensor surface. Hence, 9 min of deposition time was employed as the optimal deposition time for further detection.

Fig. 4. (a) Optimization of deposition potential on SWASV detection of Pb²⁺ by PPy/MoS₂/GCE (pH = 5.0 (0.1 M Na–Ac), deposition time (min) = 10 and [M]²⁺ = 200 nM) and (b) respective bar graph of optimization of deposition potential from −0.7 to −1.0. (c) Optimization of deposition time on SWASV detection of Pb²⁺ by PPy/MoS₂/GCE (pH = 5.0 (0.1 M Na–Ac), deposition potential (V) = −0.9 and [M]²⁺ = 200 nM) and (d) respective line graph of optimization time from 6 to 10 min. (e) Optimization of pH (0.1 M Na–Ac) for SWASV sensitivity of PPy/MoS₂/GCE (deposition potential (V) = −0.9, deposition time (min) = 10 and [M]²⁺ = 200 nM) and (f) respective bar graph of optimization of pH from 3 to 6.

The pH of the buffer (Na–Ac) solution has been shown to have a significant impact on the sensitivity of SWASV for targeted metal ions. Therefore, the influence of the pH of the Na–Ac buffer on stripping of Pb²⁺ was examined, as illustrated in Fig. 4(e and f). As the pH increased in the range from 3 to 6, the peak current increased, up to a pH of 5. When the pH was greater than 5.0 (basic), the peak current decreased, which may be due to possible reduction of H⁺ to H₂ occurring at the electrode surface. H⁺ reduction may influence the reduction of Pb²⁺ ions.⁴⁴ Hence, pH 5 was employed as the optimized pH for the Na–Ac buffer for further detection of Pb²⁺.

To evaluate the square wave response of modified electrodes to Pb²⁺, experiments were conducted on electrodes modified with different materials by the SWASV method. SWASV sensing behavior of Pb²⁺ at bare GCE, PPy, MoS₂ and PPy/MoS₂ electrodes for the detection of 300 nM Pb²⁺ concentration in a potential range −1.0 to 0.0 V vs. Ag/AgCl is shown Fig. 5. The peak current generated by PPy/MoS₂/GCE significantly improved, indicating that PPy had a good effect on improving the detection of Pb²⁺. In particular, with the synergistic effect with MoS₂, the peak current signal increased significantly compared to that of bare GCE, PPy and MoS₂/GCE. This was mainly because PPy had abundant functional groups to facilitate enrichment of Pb²⁺, thereby further improving the sensitivity of the Pb²⁺ detection.

Fig. 5. SWASV comparison of GCE, PPy, MoS₂ and PPy/MoS₂ electrodes for the detection of 300 nM Pb²⁺ concentration.

Individual ion detection with various concentrations of Pb²⁺ was carried out under optimal conditions. Fig. 6 illustrates the SWASV response of Pb²⁺ concentrations from 0.02 to 300 nM, which demonstrates that as the concentration increases the peak current increases. The linear fit between the peak current and concentration of Pb²⁺ is shown in Fig. 6, with a linear equation of Y = 44.270X + 175.72 (R² = 0.9771) at concentrations from 0.02 to 0.09 nM of Pb²⁺ (Fig. 6a and b), linear equation of Y = 79.920X + 3.314 (R² = 0.9835) at concentrations from 0.1 to 10 nM of Pb²⁺ (Fig. 6c and d) and linear equation of Y = 122.668X + 0.4091 (R² = 0.9751) at concentrations from 20 to 300 nM of Pb²⁺ (Fig. 6e and f). Y corresponds to the peak current and X is the concentration. LOD (S/N = 3) was calculated to be 0.033 nM (S = 1.975 and N = 175.72) with a sensitivity (ΔI/ΔC) of 36.42 μA nM⁻¹.

Fig. 6. (a) SWASV detection of Pb²⁺ with a range of 0.02 to 0.09 nM and (b) its respective calibration plot. (c) SWASV detection of Pb²⁺ with a range of 1 to 10 nM and (d) its respective calibration plot. (e) SWASV detection of Pb²⁺ with a range of 20 to 300 nM and (f) its respective calibration plot, under optimal SWASV experimental conditions.

Simultaneous detection of different concentrations of Cd²⁺, Pb²⁺, Cu²⁺ and Hg²⁺ was carried out using the optimal parameters. Fig. 7a illustrates the PPy/MoS₂/GCE response from 100 to 400 nM of Cd²⁺, Pb²⁺, Cu²⁺ and Hg²⁺ by SWASV. The positive or negative shift in the potential response at different concentrations of HMIs may be attributed to the competitive deposition between the HMIs during simultaneous detection. The stripping peaks for Cd²⁺, Pb²⁺, Cu²⁺ and Hg²⁺ appeared at −0.617, −0.446, −0.019 and 0.255 V vs. Ag/AgCl, respectively. The difference between the Cd²⁺ and Pb²⁺ peak currents was observed to be −0.173 V, between Pb²⁺ and Cu²⁺ it was −0.424 V and between Cu²⁺ and Hg²⁺ it was found to be 0.274 V vs. Ag/AgCl. The peak separation between the HMIs during this simultaneous detection indicates that PPy/MoS₂/GCE is a potential electrochemical sensing catalyst for the simultaneous detection of HMI in drinking water.

Fig. 7. (a) SWASV for simultaneous detection of Cd²⁺, Pb²⁺, Cu²⁺ and Hg²⁺ under optimized SWASV conditions (100 to 350 nM). (b) Interference study of PPy/MoS₂/GCE sensor with addition of equal concentrations (100 nM) of Pb²⁺, Cr³⁺, Fe²⁺, Fe³⁺, Ni²⁺, Co²⁺ and K⁺ for each trial. (c) SWASV reproducibility of the PPy/MoS₂/GCE sensor under self-storage conditions. (d) Stability of the PPy/MoS₂/GCE sensor for SWASV detection of 300 nM of Pb²⁺ in 0.1 M Na–Ac buffer solution, pH = 5.0, deposition potential = −0.9 V and deposition time = 9 min.

To further evaluate the interference of possible metal ions on the SWASV response to Pb²⁺ a 100 nM concentration of Pb²⁺ and equal concentrations of Cr³⁺, Fe²⁺, Fe³⁺, Ni²⁺, Co²⁺ and K⁺ were tested each per trial (Fig. 7b). The relative standard deviation of Pb²⁺ was calculated as 14.324%. The reproducibility of the PPy/MoS₂/GCE sensor was evaluated for ten consistent SWASV readings under a constant concentration of Pb²⁺ (300 nM) (Fig. 7c). The relative standard deviation of Pb²⁺ was calculated to be 2.30%, indicating the very reliable reproducibility of the fabricated sensor. The stability of the PPy/MoS₂ was estimated under the self-storage environment. Fig. 7d shows the decrease in the peak current with day-wise stability. After 14 days, the peak current response was evaluated as 44.44%, which might be due to the deposition of a metal ion layer at the electrode surface or the exfoliation of the catalyst.

The above results indicate that functionalized PPy/MoS₂ possesses excellent stability, reproducibility, simultaneous and individual detection of HMIs. A comparison of other reported catalysts and the PPy/MoS₂ catalyst is given in Table 1. This shows that the developed PPy/MoS₂ displays a much better/comparable performance for the detection of Pb²⁺.

Table 1. Comparison of PPy/MoS₂ sensor with reported electrochemical catalysts for the detection of Pb²⁺.

Sl. no.	Catalyst	Method of detection	LOD	Linear range	Ref.
1	Gr/PANI/SPE	SWASV	0.1 μg L−1	1–300 μg L−1	45
2	Pa/PPy/GO	DPV	0.4 μg L−1	5–150 μg L−1	46
3	F3O4 @ PANI	DPV	0.03 nM L−1	0.1 to 104 nM L−1	47
4	G/PANI/PS	SWV	3.30 μg L−1	10–550 μg L−1	48
5	PANI-MC	ASV	4 nM L−1	20–1000 nM L−1	49
6	PANI-GO	FAAS	0.04 μg L−1	0.5–10 μg L−1	16
7	G/PANI	SWASV	0.1 μg L−1	1–300 μg L−1	15
8	PANA/CPE	DPV	7.12 × 10−14 M	1 × 10−6 to 10 × 10−10 M	50
9	Ag-PANI	SWV	0.04 μM	0.1–0.2 μM	17
10	PEDOT/NT	CV	2.33 μg L−1	5–100 μg L−1	51
11	PEDOT/Sb	SWV	1.8 μg L−1	—	52
12	rGO@CNT@Fe2O3/PPy	ASV	0.1 nM	0.02 to 0.26 μM	53
13	PA/PPy/ZIF-8@ZIF-67	ASV	2.9 nM	0.02 to 200 μM
14	PPy-rGO	SWASV	4.7 × 10−11 M L−1	5 × 10−9 to 7.5 × 10−7 M L−1	54
15	PPy/UIO-66-NH2/GCE	DPASV	0.05 μg L−1	0.5 to 10 μg L−1	55
16	NiCo2O4@PPy/3D graphene	SWASV	0.2 nM	0.0125–0.709 μM	56
17	PA-doped PPy/MoS2	DPASV	1.78 μg L−1	10 to 300 μg L−1	57
18	PPy/Fe3O4 NBs	CV	0.4715 μM L−1	10 to 50 nM L−1	58
19	rGO/MWCNT/AuNP	DPV	7.1 × 10−6 nM m L−1	5 × 10−5–0.2 nM m L−1	59
20	Graphene sensors	ASV	0.5 μg L−1	30–100 μg L−1	60
21	Sn/SnO2	SWASV	2.15 μg L−1	6.2 and 20.7 μg L−1	61
22	Bi/GDY	DPASV	0.146 nM	10.0 nM to 100.0 μM	62
23	CoCu-MOF/PANI	ASV	0.39 μg L−1	—	63
24	MoS2/rGO-GCE	SWASV	0.005 μM	0.05 μM to 0.8 μM	64
25	2D-MoS2	SWASV	0.3 ppb	0 to 20 ppb	65
26	AuNPs/MoS2/GN	CV	1.0 μM	5.0 μM to 5.0 mM	66
27	CNT post-electrode	SWASV	2 nM	9.64 nM to 168.7 nM	67
28	Fe3O4@SiO2@IIP	DPV	0.05 ng mL−1	0.1 to 80 ng mL−1	68
29	Copper-based sensor	ASV	21 nM	10 μM to 25 nM	69
30	PEDOT:PSS/rGO	DPASV	0.09 ppb	1 ppb to 70 ppb	70
31	RGO–MNP	SWV	8.13 × 10−10 M	1.0 × 10−9 to 1.0 × 10−3 M	71
32	PPy/MoS2	SWASV	0.03 nM	0.02 to 300 nM	This work

4. Conclusion

In this work, an efficient electrochemical sensor based on the functionalization of CPs was fabricated for SWASV detection of HMIs. The nano-sphered PPy-functionalized MoS₂ showed excellent sensitivity and selectivity. The developed electrochemical system was effectively analysed for individual and simultaneous detection of Pb²⁺ at trace levels using SWASV. Under optimized conditions, the PPy/MoS₂/GCE system detected Pb²⁺ in the linear range of 0.02 to 300 nM, with an LOD of 0.03 nM and a sensitivity of 36.42 μA nM⁻¹. The stripping peaks for Cd²⁺, Pb²⁺, Cu²⁺ and Hg²⁺ appeared at −0.617, −0.446, −0.019 and 0.255 V vs. Ag/AgCl, respectively. Furthermore, an anti-interference study with additional metal ions, such as Cr³⁺, Fe²⁺, Fe³⁺, Ni²⁺, Co²⁺ and K⁺, was studied and calculated to be 14.324% (RDS) with 44.44% retention in stability after 14 days. We believe that the functionalization of PPy with MoS₂ may offer a potential candidate for the detection and analysis of heavy metals in drinking water.

Data availability

The data supporting the findings of this study are available within the article files. Additional data can be accessed upon request from the corresponding author.

Conflicts of interest

There is no conflict of interest to declare.