Sol-Gel Synthesis and Characterization of Highly Selective Poly(N-methyl pyrrole) Stannous(II)Tungstate Nano Composite for Mercury (Hg(II)) Detection

Abstract

The sol-gel process was used to create a new type of polypyrrole-Stannous(II)tungstate nanocomposite by poly(N-methyl pyrrole (PNMPy) sol in Stannous(II)tungstate gel, produced separately using sodium silicotungstic acid and Tn(II)chloride. Tin(II)tungstate (SnWO₃) was made by changing the mixing volume ratios of SnWO₃ and with a constant amount of an organic polymer. The composite was characterized by TGA, XRD, FTIR, and SEM measurements. A commercially available glassy carbon electrode (GCE) was modified with PNMPy/nano-Stannous(II)WO₃ nanocomposites to create a chemical sensor for selective detection of Hg²⁺ ions using an effective electrochemical methodology. In the I-V technique, selectively toxic Hg²⁺ ion was targeted selectively, which shows a rapid reaction toward PNMPy/nano-Stannous(II)WO₃/Nafion/GCE sensor. It also demonstrates long-term stability, an ultra-low detection limit, exceptional sensitivity, and excellent reproducibility and repeatability. For 0.1 mM to 1.0 nM aqueous Hg²⁺ ion solution, a linear calibration plot (r²: 0.9993) was achieved, with a suitable sensitivity value of 2.8241 AM⁻¹ cm⁻² and an extraordinarily low detection limit (LOD) of 3.40.1 pM (S/N = 3). As a result, the cationic sensor modified by PNMPy/nano-Stannous(II)WO₃/GCE could be a promising electrode.

1. Introduction

Despite the widespread potential and everywhere use of conducting polymers research, commercial applications are limited. Only limited practicable applied sciences have progressed from the laboratory perception stage for conducting polymers.

Because of excessive cost, obdurate, limited durability, and thermal stability, the conducting polymers have failed to make large commercialization. To overcome all these shortcomings, efforts by scientists are made by making composites with different initiators [1,2,3,4,5]. Nevertheless, until the early 19th century, nanocomposites did not gain widespread popularity but suddenly gained giant knock when a composite of mica and nylon was reported by Toyota researchers having the strength and yield by five times [6,7].

The growing interest in polymer-nanoparticle composites has been aided by subsequent advances. The appealing applications of conjugated organic polymers are photovoltaics, flexible electronics, and light-emitting displays. Composite of phosphonic acid terminated organic-inorganic hybrid, thiophene quantum dot, and polypyrrole hybrids are great to obtain improved performance with customized ligands and are used in various flexible films supercapacitors and many electronics [8,9,10,11,12]. Mixing of any matrix or nanorods dispersions in the polythiophene cause the increase in device performance. Nanocrystalline materials provide three-times higher power conversion efficiencies as compared to standard blends used in volumes. The improved sensing and catalytic properties offered by the conducting polymer noble metal nanocomposites are as compared to pure conducting polymers [13,14,15].

The most hazardous metal in the environment is mercury (II) [16]. It is the main culprit for the serious nervous system health risk. The trace accumulation of mercury in the kidney, liver, and central nervous system causes serious risk. The immunological system and the human body’s endocrine gland become affected by the pose of mercury in the environment [17,18]. Production of calculators, mercury vapor lamps, batteries, barometers, cameras, medical laboratories chemicals, cathode tubes causes a trace of mercury in industrial waste. Even there is a significant negative impact of monitoring Hg²⁺ in the environment on human health. For environmental protection and health monitoring, it is necessary to monitor and sens Hg²⁺. Colorimetrical fluorescent, [19,20,21,22,23] and electrochemical sensors [24,25] for Hg²⁺ already published.

However, most of these sensors could hardly detect 10 nM, a toxic level of Hg²⁺ designated by the U.S. Environmental Protection Agency (EPA). To improve the LOD here in we develop PNMPY/NANO-Stannous(II)WO₃ composite modified GCE sensor for the Hg²⁺ ions in an aqueous system.

In this study, we used a co-precipitation approach to make a PNMPy/nano-Stannous(II)WO₃ composite for electrochemical detection of the Hg²⁺ ion at room temperature. Various standard techniques were used to examine the structural and morphological properties of the PNMPy/nano-Stannous(II)WO₃ nanocomposite. Following the successful production of the PNMPy/nano-Stannous(II)WO₃ composite, we coated the PNMPy/nano-Stannous(II)WO₃ composite to the GCE for electrochemical testing of Hg²⁺ ion detection. Furthermore, we evaluated the electrochemical response of PNMPy/nano-Stannous(II)WO₃ composite with bare GCE and PNMPy/nano-Stannous(II)WO₃/GCE for the detection of Hg²⁺ ion and found that the PNMPy/nano-Stannous(II)WO₃ composite outperforms the bare GCE.

2. Experimental Section

2.1. Materials and Methods

Gallium nitrate, mercury(II)chloride, manganese (II) sulfate, Nickel(II) chloride, cobalt (II)nitrate, iron (III)chloride, yttrium nitrate, 5% ethanolic Nafion solution, thallium nitrate disodium phosphate, cerium(III)nitrate, antimony(III)chloride, arsenic(III)chloride, were from Sigma-Aldrich and used without further purification.

N-methyl pyrrole monomer buys from Merck’s, CDH’s stannous chloride, and Loba Chemie’s sodiummetatungstate were used as essential reagents. All of the other chemicals and reagents were of analytical purity (lead nitrate, nitric acid, sulfuric acid, sodium nitrate, hydrochloric acid, etc.). During this research, we used the instruments: Orion 720 pH/ion meter (Thermo Fisher Scientific, Waltham, MA, USA) for emf and pH measurement having connected saturated calomel reference electrode. Elcon (Eletech Lab Instrument, Ambala, India) water bath incubator shaker with automatic temperature control was used. Digital, Sartorius-21OS, Tokyo, Japan. An electronic balance, 18.6 Mcm⁻¹ from Millipore, BR, Burlington, MA, USA, was used to make aqueous solutions of various concentrations. Link-Oxford-sis (EDX) with a Jeol JSM 6300 (SEM, Tokyo, Japan) was employed. For functional group analysis, model 2000 (Waltham, MA, USA), a Perkin Elmer FTIR spectrophotometer was employed. At ambient conditions, I-V characteristics of produced PNMPY/NANO-Stannous(II)WO₃/Nafion/GCE composites were measured to detect the Hg²⁺ ion using a Keithley electrometer (6517A, Beaverton, OR, USA).

2.2. Preparation of Poly(N-methyl pyrrole)/Nano-Stannous(II)WO₃ Composite

In the presence of the oxidizing agent (ammonium persulphate), NMPy was polymerized into PNMPy, then mixing it for further sonicating the mixture for 24 h at 40 °C constant temperature. Mixing an inorganic Stannous(II)tungstate precipitate (produced by combining 1 M HCl stannous chloride in an aqueous sodium meta tungstate in various mixing ratios) and agitating it for a further 2 h was performed using sol-gel methods. The reaction mixture was stirred for another 24 h under the same conditions. Using a Buchner funnel, the powder was filtered out before being washed with methanol, 0.1 M hydrochloric acid, and distilled water. In a vacuum dryer, the resulting greenish-black powder was dried for 24 h at room temperature.

By soaking the samples in a 1 M HNO₃ solution for 24 h, shaking periodically, and replenishing the supernatant liquid regularly, the samples were transformed from containing metal ions to H⁺ form. The excess acid was removed by washing with DMW several times (dematerialized water). The samples were finally dried at 40 °C. As a result, several nanocomposite samples were created and selected for further study based on their Na⁺ ion exchange capacity (IEC) (1.2 meq/g) and physical appearance.

2.3. Modification of GCE with PNMPy/Nano-Stannous(II)WO₃ Composite

The PNMPY/NANO-Stannous(II)WO₃ composite was used to fabricate the GCE with the help of 5% ethanolic Nafion solution (95:5, ethanol: Nafion) as well as a 0.4 mm surface (approximately) thick sheet of PNMPY/NANO-Sn(II)WO₃. The formatted film on the GCE was then dried in an oven at 45.0 °C for 2 h. The working electrode (WE) in the electrochemical cell was made of PNMPY/NANO-Sn(II)WO₃ composite manufactured GCE, the counter electrode (CE) was made of platinum wire, and the supporting electrolyte was made of containing 0.1 M phosphate buffer solution (PBS; pH 7.0). The target analyte was aqueous Hg²⁺ ion concentrations ranging from 0.1 M to 0.1 nM. In 5.0 mL of PBS (pH = 7.0), all current-voltage (I-V) measurements were performed for electrode calibration. The sensitivity of the proposed Hg²⁺ ion sensor was calculated using the slope of the calibration plot and the active surface area of the GCE. I-V technique was used to aqueous Hg²⁺ ion using an electrometer (Keithley, 6517A Electrometer, Beaverton, OR, USA) with PNMPY/NANO-Stannous(II)WO₃/GCE as WE.

3. Results and Discussion

3.1. Characterization

To achieve suitable chemical stability, excellent electrical conductivity, a higher surface to volume ratio, well repeatability behavior, and better capacity, a fibrous matrices gel of Sn (II) tungstate and PNMPy sol was mixed by sol-gel method to obtain PNMPY/NANO-Stannous(II)WO₃ nanocomposite.

Figure 1A–D show the SEM pictures of PNMPY/NANO-Stannous(II)WO₃ composites at various magnifications. Based on the morphological evolution of the SEM images, it is observed that the interaction between the inorganic matrices with the conducting polymer and the morphology of composite is entirely changed. The white luster in the SEM images is because of the presence of metal inorganic, and the other thicker part in SEM photographs shows the conducting organic constituents that make the composite have great characteristics.

As shown if Figure 2, the 15% weight loss up to 100 °C is because of the loss of external moisture water molecules present on the surface of the composite [26]. Weight loss of about 20% is maybe because of organic part degradation between 100 and 400 °C. The beginning of pyrophosphate and phosphate of the inorganic part present in the composite takes parts after 400 °C. The peak in the DTA at 700 °C is an indication of exothermic reaction during the material phase change that indicates the exothermic or endothermic behavior of the combustion reaction of PNMPy/nano-Stannous(II)WO₃ during heating.

The presence of C=C and aromatic C-H stretching vibrations in the composite spectra are responsible for high-intensity peaks in the 3100–3200 cm⁻¹ range [27,28]. C-H in-plane deformation mode and ring stretching mode as well as further out of plane deformation mode is the characteristic by the peak presence of at around 786–850 cm⁻¹ in the composite [29,30]. The IR bands positions, as well as the numbers in Figure 3, are the same for the Stannous(II)tungstate that is in the region of around 600–3200 cm⁻¹ range. The peaks at 2800–3200 cm⁻¹ and around 1730 cm⁻¹ are for the C-H stretching vibrations and C=C characteristic peaks, respectively. Ring deformation modes are shifted due to the bulky group of the interaction of the Stannous(II)tungstate and poly(n-methyl pyrrole) backbone. The cause of the shift is bulky groups in the composite that creates the polaron-polaron interaction.

In the current study, X-ray photoelectron spectroscopy (XPS) is used here to detect the elemental composition and electronic stages of the metals present in the composite. It is a quantitative elemental approach for a substance. The materials are irradiated at the top, typically between 1.0 and 10.0 nm, are low case using their kinetic energy. By XPS measurement, here in PNMPY/NANO-Stannous(II)WO₃, C, N, Si, Sn, and O element and their chemical state was analyzed in the composite. The full XPS spectrum of PNMPY/NANO-Stannous(II)WO₃ nanocomposite is presented in Figure 4a. A 35.9 eV binding energy is for the W4f7/2 spin-orbit peak in Figure 4b, which is consistent with the tungsten reference data [31]. The C1s spectrum, shown in Figure 4, has peaked at 285.2eV. (c). The presence of carbon in nanocomposite was shown by the C1s peaks. [32]. N1s is executed at 399.3 eV in the PNMPY/NANO-Stannous(II)WO₃, as clear in Figure 4d, [33]. A 532.6 eV is the indication of the presence of oxygen (i.e., O₂⁻) in Figure 4e in the PNMPY/NANO-Stannous(II)WO₃ nanocomposite [34]. The presence of Si in the form of Si2p in the composite was shifted at roughly 104 eV in Figure 4f. As a result, the PNMPY/NANO-Stannous(II)WO₃ nanocomposite is found to include six distinct elements.

3.2. Applications: Chemical Sensor Development

3.2.1. Detection of Hg²⁺ Ion Using PNMPY/NANO-Stannous(II)WO₃/GCE by the I-V Method

As a chemical sensor, the toxic Hg²⁺ ion in an aqueous medium was detected using a PNMPY/NANO-Stannous(II)WO₃ composite modified GCE. The Hg²⁺ ion strongly responded in the I-V technique when it comes into touch with PNMPY/NANO-Stannous(II)WO₃. The potential application of a Hg²⁺ ion sensor made from the PNMPY/NANO-Stannous(II)WO₃/Nafion/GCE assembly to detect and quantify the designated toxin, Hg²⁺ ion, in a buffer solution was investigated. The development of the Hg²⁺ ion sensor based on the PNMPY/NANO-Stannous(II)WO₃/Nafion/GCE assembly is still in its early stages, and there have been no other reports found. The suggested PNMPY/NANO-Stannous(II)WO₃/GCE sensor has several advantages, including increased air stability, better electrochemical property during the determination, ease of operation, assembly and manufacturing, and safe chemo-characteristic [35]. The potential application of a chemical sensor with PNMPY/NANO-Stannous(II)WO₃/Nafion/GCE for the detection of targeted analytes Hg²⁺ ion in a suitable buffer system was evaluated in detail, with PNMPY/NANO-Stannous(II)WO₃/Nafion/GCE acting as brilliant electron mediator during the sensing operation. The proposed sensor was built using an ordered PNMPY/NANO-Stannous(II)WO₃/GCE assembly as the WE, with the targeted analyte being an Hg²⁺ ion. The applied current vs. potential (I-V) was investigated on a thin layer of PNMPY/NANO-Stannous(II)WO₃ composite on GCE employed as the WE in the sensing operation of Hg²⁺ ion in the phosphate buffer system. In the electrode, a 1.0 s timer was set as the holding period in the electrometer.

Scheme 1 depicts a possible reduction pathway for the Hg²⁺ ion. The electrode fabrication in layer-by-layer is given in Scheme 1a, which followed the drop-coating method [31,32]. The phenomena of two-electrode systems in the detection of target mercury ion is presented in Scheme 1b. The reduction phenomena of mercury with the fabricated electrode are given in Scheme 1c. The result of a two-electrode system in the detection of target mercury ion is found and presented in Scheme 1d. A decrease in an electron from the conduction band of A PNMPY/NANO-Stannous(II)WO₃ composite was identified during the Hg²⁺ ion reduction by the manufactured PNMPY/NANO-Stannous(II)WO₃/Nafion/GCE assembly, which caused the reducing current response during the I-V measurement at ambient condition. Hg²⁺ ion was converted to Hg using the hypothesized Hg²⁺ ion reduction mechanism. The built WE were used to detect Hg²⁺ ions after GCE was coated with a slurry of PNMPY/NANO-Stannous(II)WO₃ composite nanomaterials. Two electrons were removed from the conduction band of the targeted Hg²⁺ ion during the electrochemical reduction process, lowering the I-V response of PNMPY/NANO-Stannous(II)WO₃/Nafion/GCE sensors [36].

The current responses for 13 hazardous heavy metal ions are shown in Figure 5a, where the Hg²⁺ ion solution (red-dotted) in PBS (pH = 7.0) causes a unique I-V response with the PNMPY/NANO-Stannous(II)WO₃/Nafion/GCE surface (1.0 M; 25.0 µL). The electrochemical property of the PNMPY/NANO-Stannous(II)WO₃ COMPOSITE was investigated in PBS with pH values ranging from 5.7 to 8.0, as shown in Figure 5b. The results reveal that the PNMPY/NANO-Stannous(II)WO₃ composite performs different electrochemical responses in various pH levels. The electrocatalytic property of the PNMPY/NANO-Stannous(II)WO₃ composite changes with various pH levels, as seen by the variation in the current response. The highest current response is obtained when pH is optimized by using Hg²⁺ ion (1.0 M; 25.0 µL) in PBS. As a result, throughout the rest of the experiments in this Hg²⁺ ion determination by the PNMPY/NANO -Stannous(II)WO₃/GCE assembly, pH value 7.0 was employed. Current intensities using Hg²⁺ ion (1.0 µM; 25.0 µL) in PBS (5.0 mL, pH = 7.0) for bare GCE (blue-dotted) and PNMPY/NANO-STANNOUS(II)WO₃ COMPOSITE fabricated GCE (red-dotted) are given in Figure 5c. The PNMPY/NANO-Stannous(II)WO₃/GCE provides a considerably higher response than bare GCE, which demonstrates the PNMPY/NANO-STANNOUS(II)WO₃ composite’s exhibited excellent electrochemical property toward the detection of Hg²⁺ ion. In 5.0 mL of PBS, Figure 5d illustrates the current response of the PNMPY/NANO-Stannous(II)WO₃ composite manufactured GCE in the absence of Hg²⁺ ion (blue-dotted) and the presence of Hg²⁺ ion (red-dotted; 1.0 M; 25.0 L). The proposed PNMPY/NANO-Stannous(II)WO₃/GCE sensor’s Hg²⁺ ion sensing capabilities is demonstrated by a significant decrease in current responsiveness when Hg²⁺ ions are present in the electrochemical system [37].

The surface current response variation was examined after each injection of Hg²⁺ ion solutions ranging from low (0.1 nM) to high (0.1 M) concentrations in 5.0 mL PBS. Using an aqueous Hg²⁺ ion solution of various concentrations (0.1 nM to 0.1 M), I-V responses from the PNMPY/NANO-STANNOUS(II)WO₃ composite manufactured GCE surface were determined and presented in Figure 6a. The amplification peak current at 0.4 V is shown in Figure 6b. At ambient temperature, it indicates that as the potential increases, the I-V responses for the PNMPY/NANO-Stannous(II)WO₃ composite manufactured GCE sensor rise as well (25.0 °C). The I-V responses likewise decreased regularly as the concentration of Hg²⁺ ion solution increased from dilute (0.1 nM) to concentrated (0.1 M). The LOD of the devised sensor was determined using aqueous Hg²⁺ ions (0.1 nM to 0.1 M). Figure 6 shows the calibration plot (at +0.4 V) for the entire concentration range (c). The calibration plot revealed a higher sensitivity value of 2.8241 AM-1 cm⁻². From the calibration plot, the linear dynamic range of the developed PNMPY/NANO-Stannous(II)WO₃/GCE sensor was attained as 0.1 nM to 1.0 mM (r² = 0.9993), and the LOD was estimated as 3.4 ± 0.1 pM (3 × noise (N)/slope(S)). I-t responses for Hg²⁺ ion (1.0 µM; 25.0 µL) in 5.0 mL of PBS solutions using the PNMPY/NANO-Stannous(II)WO₃ composite nanostructures fabricated GCE and presented in Figure 6d. Approximately in 10 s, a constant current response was achieved with Hg²⁺ ion (1.0 µM; 25.0 µL) in 5.0 mL of PBS.

Current response repeatability, as shown in Figure 7a, depicts the PNMPY/NANO-Stannous(II)WO₃ composite manufactured by GCE employing 25.0 L of 0.1 M Hg²⁺ ion and seven different WE in runs R1–R6 under identical conditions. The superb repeatability (RSD = 3.28%, n = 6) is confirmed because of the similar current response in all experiments. When the same WE were used, as shown by Figure 7b, in run r1~r6, that is the proof of reproducibility. Outstanding reproducibility confirms for the sensor as we obtain almost similar I-V responses in seven repeated experiments (RSD = 3.34%, n = 6). I-V response drops somewhat when the same WE are used in multiple solutions under the same conditions. This could be because the number of active sites in the PNMPY/NANO-Stannous(II)WO₃ Composite decreases with each run. Furthermore, the long-stability of the fabricated PNMPY/NANO-Stannous(II)WO₃/GCE sensor probe has been performed and presented in Figure 7c. It is measured in every one-hour interval with a similar electrode in the identical solution system (0.1 µM, 25.0 µL Hg²⁺ ion; 5.0 mL, 0.1 M PBS at pH 7.0). It is found that the sensor current responses exhibited the almost remained similar response in every measurement, which concluded that PNMPY/NANO-Stannous(II)WO₃/GCE sensor probe is very stable. From the stability data after 8 h, the current intensity is retained at 98% of its original value, i.e., recorded just after modification of PNMPY/NANO-Stannous(II)WO₃/GCE. The prepared sensor is stable enough to detect the target analyte in the identical conditions of the electrochemical system by the reliable I-V method [38] in a short response time. The possible interfering effect of the common inorganic cations during this mercury detection by using PNMPY/NANO-Stannous(II)WO₃/GCE electrode was examined. The outcome is indicated of 100-fold of Na⁺, Ca²⁺, K⁺, Cu²⁺, Cd²⁺, Mn²⁺, Pb²⁺, Mg²⁺, and Fe³⁺. There has no interference effect in this Hg²⁺ determination (interference < ±5%) in I-V response by PNMPY/NANO-Stannous(II)WO₃/GCE electrode in identical conditions.

3.2.2. Real Sample Analysis by PNMPY/NANO-Stannous(II)WO₃/GCE Sensor

In industrial wastewater as well as from plastic bottled water (inside the care irradiation) after seven days of sunshine, the PNMPY/NANO-Stannous(II)WO₃/GCE sensor (S2) was used to determine Hg²⁺ ion. The accuracy of the aqueous Hg²⁺ ion determination is checked using the standard addition method. A total of 25.0 mL of aqueous Hg²⁺ ion of various concentrations and an equivalent quantity of genuine sample were mixed and examined by PNMPY/NANO-Stannous(II)WO₃/GCE as a WE in PBS (5.0 mL, pH 7.0). Table 1 represents the obtained outcomes, which demonstrated that the PNMPY/NANO-Stannous(II)WO₃/GCE modified sensor exhibited a quantitative (~100%) recovery of Hg²⁺ ion. So, we can conclude that the I-V method is proper and reliable in analyzing real samples by the PNMPY/NANO-Stannous(II)WO₃/Nafion/GCE assembly.

Nano-porosity, size, as well as the shape of PNMPY/NANO-Stannous(II)WO₃ all influence the I-V response in the Hg²⁺ ion detection. A surface-mediated reduction reaction occurs when the PNMPY/NANO-Stannous(II)WO₃ composite surface comes into contact with the target Hg²⁺ ion. Reduction in Hg²⁺ ion removes electrons from the PNMPY/NANO-Stannous(II)WO₃ composite surface, which ultimately decreases the conductance of the PNMPY/NANO-Stannous(II)WO₃/GCE assembly; hence, the current response decreases. Superior consistency as well as stability also displayed by PNMPY/NANO-Stannous(II)WO₃/GCE sensor [39,40]. Despite these developments in this proposed sensor, there are still a few challenges that need to be addressed before it can be commercialized. A comparison of the electrode parameters of the published study [41,42,43,44,45,46] with the current electrode is given in Table 2.

4. Conclusions

PNMPy/Stannous(II)WO₃ ternary nanocomposite was synthesis by chemical oxidative polymerization sol-gel process by initiators of N-methyl pyrrole monomer, tin(II)tungstate. Based on morphological investigations, N-methyl pyrrole molecules were in situ polymerized on separately prepared layered Stannous(II)tungstate. The core in the whole process is the Stannous(II)tungstate. In comparison to the homopolymer, the PNMPy/Stannous(II)WO₃ has better temperature stability. Due to the electronic characteristics of the PNMPy, the composite demonstrated improved durability. The PNMPY/NANO-Stannous(II)WO₃/Nafion/GCE electrode was effectively used as a chemical sensor for different metal ions but found highly selective for aqueous Hg²⁺ ion concentration. The constructed Hg²⁺ ion chemical sensor based on PNMPY/NANO-Stannous(II)WO₃ composite coated on GCE performed exceptionally well as an electron mediator in the reduction in Hg²⁺ ion in the PBS system. The suggested PNMPY/NANO-Stannous(II)WO₃/Nafion/GCE sensor for the Hg²⁺ ion has better sensitivity and ultra-low limit of detection over a broad range of concentrations in a fast reaction time, as well as an excellent linear response. Effective chemical sensors could be produced by employing PNMPY/NANO-Stannous(II)WO₃ for sustainability.

Author Contributions

Conceptualization, A.K. and M.M.R.; methodology, A.K.; validation, A.A.P.K., A.M.A. and I.K.; formal analysis, A.K.; investigation, A.K.; resources, A.K.; funding by A.K. All authors have read and agreed to the published version of the manuscript.

Funding

This project was funded by the deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah, Under Grant Number (G-31-130-1442). The authors, therefore, acknowledge with thanks DSR technical and financial support.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

This project was funded by the deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah, Under Grant Number (G-31-130-1442). The authors, therefore, acknowledge with thanks DSR technical and financial support.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures, Scheme and Tables

Figure 1. SEM image of the composite PNMPy/nano-Stannous(II)WO3 (A,B) at different magnifications while from (C,D) showing the elemental composition (C,D) with the help of SEM-EDX.

Figure 2. Thermogravimetric analysis curve (TGA—black in color) for the PNMPy/nano-Stannous(II)WO3 composite in a nitrogen atmosphere while differential thermal analysis (DTA—blue) is shown for reaction endothermic or exothermic.

Figure 3. FTIR spectra of PNMPy/nano-Stannous(II)WO3 composite in KBr showing different bonding stretching.

Figure 4. X-ray photoelectron spectroscopy (XPS) study of the PNMPy/nano-Stannous(II)SiO3 composite showing different peaks for different metals in (a) for composite, (b) carbon, (c) nitrogen, (d) oxygen, (e) tungsten, and (f) for Tin.

Scheme 1. Schematic representation of mercury detection with PNMPY/NANO-Stannous(II)WO3/Nafion/GCE sensor probe by electrochemical approach. (a) Fabricated electrode with the PNMPY/NANO-Stannous(II)WO3. (b) Detection method, (c) outcome of the result, and (d) detection phenomena.

Figure 5. I-V response of modified electrode in the detection of mercury. (a) Selectivity study using thirteen chemicals, (b) pH optimization, (c) bare and PNMPy/nano-Stannous(II)WO3 modified GCE, and (d) without and with the presence of Hg2+ ion (1.0 µM; 25.0 µL).

Figure 6. (a) Current variations for different concentration (0.1 nM to 0.1 M) of aqueous Hg2+ ion in 0.0 to +1.5 V, (b) magnification of I-V responses at 0.4 V, (c) calibration plot of PNMPy/nano-Stannous(II)WO3/CNTs composite fabricated GCE at +0.4 V, and (d) current variation with time for the PNMPy/nano-Stannous(II)WO3/Nafion/GCE in 1.0 µM; 25.0 µL Hg2+ ion.

Figure 7. (a) Repeatability using different WE (0.1 µM, 25.0 µL Hg2+ ion; 5.0 mL, 0.1 M PBS at pH 7.0), (b) reproducibility using the same WE (0.1 µM, 25.0 µL Hg2+ ion; 5.0 mL, 0.1 M PBS at pH 7.0), and (c) stability test (Every reading is taken after 1 h (0.1 µM, 25.0 µL Hg2+ ion; 5.0 mL, 0.1 M PBS at pH 7.0).

Table 1

Analysis of real environmental samples with PNMPy/nano-Stannous(II)WO₃/Nafion/GCE by electrochemical approach.

Real Sample	Hg2+ Ion Conc. a Added	Hg2+ Ion Conc. a Deter b by the PNMPy/Nano-Stannous(II)WO3/GCE	Recovery b (%)	RSD c (%)(n = 3)
S1	1.000 nM	1.015 nM	101.5	3.4
S1	1.000 µM	1.056 µM	105.6	3.1
S1	1.000 mM	1.021 mM	102.1	2.8
S2	1.000 nM	0.983 nM	98.3	3.3
S2	1.000 µM	1.001 µM	100.1	3.8
S2	1.000 mM	0.998 mM	99.8	2.9

S1 and S2: Real: Real water samples collected from the industrial effluent treatment plant and Safa drinking water bottle, respectively, Jeddah, Saudi Arabia. ^a mean of the three repeated determinations (S/N = 3) with the PNMPy/nano-Stannous(II)WO₃/Nafion/GCE. Concentration of Hg²⁺ ion determined/Concentration of Hg²⁺ ion taken. Relative standard deviation value indicates precision among three repeated determinations.

Table 2

Comparison of Hg(II)-selective PNMPy/nano-Stannous(II)WO₃/GCE modified GCE with other Hg (II)-selective electrodes.

Electrode	Methods	Detection Limit	Linear Range	References
Graphene quantum dot using a thiourea-based imprinted polymer	CV	30.2 nM	5 × 10(−8) M to 2.3 × 10(−5) M	[41]
Sepiolite/FeS2 nanocomposite	SWASV	4.12 nM	10–120 nM	[42]
AuNP-biopolymer carbon screen-printed	SWASV	1.69 ppb	50–500 nM	[43]
SN-rGO modified GCE	SWASV	8.93 nM	0.6–1.7 μM	[44]
Thiosemicarbazone ligands bearing azido groups	ASV	0.5 nM	1 nM–0.8 μM	[45]
GO-chitosan-AuNPs/MTU modified ITO	DPV	0.78 nM	5.0–110.0 nM	[46]
PNMPy/nano-Stannous(II)WO3/GCE	I-V	3.401 pM	1.0 nM to 0.1 mM	Current study

References

1. Khan, A.; Khan, A.A.P.; Rahman, M.M.; Asiri, A.M.; Inamuddin,; Alamry, K.A.; Hameed, S.A. Preparation and characterization of PANI@G/CWO nanocomposite for enhanced 2-nitrophenol sensing. Appl. Surf. Sci.; 2018; 433, pp. 696-704. [DOI: https://dx.doi.org/10.1016/j.apsusc.2017.09.219]

2. Khan, A.; Asiri, A.M.; Khan, A.A.P.; Rub, M.A.; Azum, N.; Rahman, M.M.; Al-Youbi, A.O.; Qusti, A.H. Dual nature, self oxidized poly(o-anisidine) functionalized multiwall carbon nanotubes composite: Preparation, thermal and electrical studies. Compos. Part B Eng.; 2014; 58, pp. 451-456. [DOI: https://dx.doi.org/10.1016/j.compositesb.2013.10.059]

3. Khan, A.; Khan, A.A.P.; Asiri, A.M.; Rahman, M.M.; Alhogbi, B.G. Preparation and properties of novel sol-gel-derived quaternized poly(n-methyl pyrrole)/Sn(II)SiO₃/CNT composites. J. Solid State Electrochem.; 2015; 19, pp. 1479-1489. [DOI: https://dx.doi.org/10.1007/s10008-015-2760-8]

4. Khan, A. Electrical conductivity and cation exchange kinetic studies on poly-o-toluidine Th(IV) phosphate nano-composite cation exchange material. Talanta; 2007; 73, pp. 850-856. [DOI: https://dx.doi.org/10.1016/j.talanta.2007.05.003]

5. Khan, A.A.; Habiba, U.; Khan, A. Synthesis and Characterization of Organic-Inorganic Nanocomposite Poly-o-anisidine Sn(IV) Arsenophosphate: Its Analytical Applications as Pb(II) Ion-Selective Membrane Electrode. Int. J. Anal. Chem.; 2009; 2009, 659215. [DOI: https://dx.doi.org/10.1155/2009/659215] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/20140082]

6. Usuki, A.; Kojima, Y.; Kawasumi, M.; Okada, A.; Fukushima, Y.; Kurauchi, T.; Kamigaito, O. Synthesis of nylon 6-clay hybrid. J. Mater. Res.; 1993; 8, pp. 1179-1184. [DOI: https://dx.doi.org/10.1557/JMR.1993.1179]

7. Kojima, Y.; Usuki, A.; Kawasumi, M.; Okada, A.; Fukushima, Y.; Kurauchi, T.; Kamigaito, O. Mechanical properties of nylon 6-clay hybrid. J. Mater. Res.; 1993; 8, pp. 1185-1189. [DOI: https://dx.doi.org/10.1557/JMR.1993.1185]

8. Liu, J.; Tanaka, T.; Sivula, K.; Alivisatos, A.P.; Fréchet, J.M.J. Employing End-Functional Polythiophene to Control the Morphology of Nanocrystal−Polymer Composites in Hybrid Solar Cells. J. Am. Chem. Soc.; 2004; 126, pp. 6550-6551. [DOI: https://dx.doi.org/10.1021/ja0489184]

9. Zhang, M.; Nautiyal, A.; Du, H.; Li, J.; Liu, Z.; Zhang, X.; Wang, R. Polypyrrole film based flexible supercapacitor: Mechanistic insight into influence of acid dopants on electrochemical performance. Electrochim. Acta; 2020; 357, 136877. [DOI: https://dx.doi.org/10.1016/j.electacta.2020.136877]

10. Shimoga, G.; Palem, R.; Choi, D.-S.; Shin, E.-J.; Ganesh, P.-S.; Saratale, G.; Saratale, R.; Lee, S.-H.; Kim, S.-Y. Polypyrrole-Based Metal Nanocomposite Electrode Materials for High-Performance Supercapacitors. Metals; 2021; 11, 905. [DOI: https://dx.doi.org/10.3390/met11060905]

11. Huang, Y.; Li, H.; Wang, Z.; Zhu, M.; Pei, Z.; Xue, Q.; Zhi, C. Nanostructured Polypyrrole as a flexible electrode material of supercapacitor. Nano Energy; 2016; 22, pp. 422-438. [DOI: https://dx.doi.org/10.1016/j.nanoen.2016.02.047]

12. Jain, R.; Jadon, N.; Pawaiya, A. Polypyrrole based next generation electrochemical sensors and biosensors: A review. TrAC Trends Anal. Chem.; 2017; 97, pp. 363-373. [DOI: https://dx.doi.org/10.1016/j.trac.2017.10.009]

13. Drelinkiewicz, A.; Hasik, M.; Kloc, M. Pd/polyaniline as the catalysts for 2-ethylanthraquinone hydrogenation. The effect of palladium dispersion. Catal. Lett.; 2000; 64, pp. 41-47. [DOI: https://dx.doi.org/10.1023/A:1019078718474]

14. Radford, P.; Creager, S. Dual-stream flow injection method for amplified electrochemical detection of ferrocene derivatives. Anal. Chim. Acta; 2001; 449, pp. 199-209. [DOI: https://dx.doi.org/10.1016/S0003-2670(01)01343-5]

15. Khan, A.; Asiri, A.M.; Khan, A.A.P.; Rub, M.A.; Azum, N.; Rahman, M.M.; Khan, S.B.; Alamry, K.A.; Ab Ghani, S. Sol–gel synthesis and characterization of conducting polythiophene/tin phosphate nano tetrapod composite cation-exchanger and its application as Hg(II) selective membrane electrode. J. Sol-Gel Sci. Technol.; 2013; 65, pp. 160-169. [DOI: https://dx.doi.org/10.1007/s10971-012-2920-6]

16. Tian, Y.; Wang, Y.; Xu, Y.; Liu, Y.; Li, D.; Fan, C. A highly sensitive chemiluminescence sensor for detecting mercury (II) ions: A combination of Exonuclease III-aided signal amplification and graphene oxide-assisted background reduction. Sci. China Ser. B Chem.; 2015; 58, pp. 514-518. [DOI: https://dx.doi.org/10.1007/s11426-014-5258-9]

17. Ke, J.; Li, X.; Zhao, Q.; Hou, Y.; Chen, J. Ultrasensitive Quantum Dot Fluorescence quenching Assay for Selective Detection of Mercury Ions in Drinking Water. Sci. Rep.; 2015; 4, 5624. [DOI: https://dx.doi.org/10.1038/srep05624]

18. Chen, K.; She, S.; Zhang, J.; Bayaguud, A.; Wei, Y. Label-free colorimetric detection of mercury via Hg²⁺ ions-accelerated structural transformation of nanoscale metal-oxo clusters. Sci. Rep.; 2015; 5, 16316. [DOI: https://dx.doi.org/10.1038/srep16316]

19. Moreton, J.A.; Delves, H.T. Simple direct method for the determination of total mercury levels in blood and urine and nitric acid digests of fish by inductively coupled plasma mass spectrometry. J. Anal. At. Spectrom.; 1998; 13, pp. 659-665. [DOI: https://dx.doi.org/10.1039/a802242i]

20. Xiao, X.; Ulstrup, J.; Li, H.; Wang, M.; Zhang, J.; Si, P. Nanoporous gold assembly of glucose oxidase for electrochemical biosensing. Electrochim. Acta; 2014; 130, pp. 559-567. [DOI: https://dx.doi.org/10.1016/j.electacta.2014.02.146]

21. Lang, X.-Y.; Fu, H.-Y.; Hou, C.; Han, G.-F.; Yang, P.; Liu, Y.-B.; Jiang, Q. Nanoporous gold supported cobalt oxide microelectrodes as high-performance electrochemical biosensors. Nat. Commun.; 2013; 4, 2169. [DOI: https://dx.doi.org/10.1038/ncomms3169] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/23851924]

22. Choi, S.; Kim, Y. Gold nanoparticle-based fluorescent “turn-on” sensing system for the selective detection of mercury ions in aqueous solution. RSC Adv.; 2015; 5, pp. 95268-95272. [DOI: https://dx.doi.org/10.1039/C5RA20152G]

23. Wegner, S.V.; Okesli, A.; Chen, A.P.; He, C. Design of an Emission Ratiometric Biosensor from MerR Family Proteins: A Sensitive and Selective Sensor for Hg²⁺. J. Am. Chem. Soc.; 2007; 129, pp. 3474-3475. [DOI: https://dx.doi.org/10.1021/ja068342d] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/17335208]

24. Ono, A.; Togashi, H. Highly Selective Oligonucleotide-Based Sensor for Mercury(II) in Aqueous Solutions. Angew. Chem.; 2004; 116, pp. 4400-4402. [DOI: https://dx.doi.org/10.1002/ange.200454172]

25. Thomas, J.M.; Ting, R.; Perrin, D.M. High affinity DNAzyme-based ligands for transition metal cations? A prototype sensor for Hg²⁺. Org. Biomol. Chem.; 2004; 2, pp. 307-312. [DOI: https://dx.doi.org/10.1039/b310154a]

26. Duval, C. Inorganic Thermogravimetric Analysis; Elsevier: Amsterdam, The Netherlands, 1963.

27. Wang, Y.; Rubner, M. Stability studies of the electrical conductivity of various poly(3-alkylthiophenes). Synth. Met.; 1990; 39, pp. 153-175. [DOI: https://dx.doi.org/10.1016/0379-6779(90)90180-S]

28. Wang, J.; Keene, F. Mechanism of mediation of the electrochemical oxidation of K₄Fe(CN)₆ at poly-[tris(3-{ω-[4-(2,2′-bipyridyl)] alkyl}-thiophene)iron(II)]-film modified electrodes in aqueous solutions. Electrochim. Acta; 1996; 41, pp. 2563-2569. [DOI: https://dx.doi.org/10.1016/0013-4686(96)00070-9]

29. Roncali, J. Conjugated poly(thiophenes): Synthesis, functionalization, and applications. Chem. Rev.; 1992; 92, pp. 711-738. [DOI: https://dx.doi.org/10.1021/cr00012a009]

30. Toshima, N.; Hara, S. Direct synthesis of conducting polymers from simple monomers. Prog. Polym. Sci.; 1995; 20, pp. 155-183. [DOI: https://dx.doi.org/10.1016/0079-6700(94)00029-2]

31. Solonin, Y.M.; Khyzhun, O.Y.; Graivoronskaya, E.A. Nonstoichiometric Tungsten Oxide Based on Hexagonal WO₃. Cryst. Growth Des.; 2001; 1, pp. 473-477. [DOI: https://dx.doi.org/10.1021/cg015545z]

32. Utsumi, S.; Honda, H.; Hattori, Y.; Kanoh, H.; Takahashi, K.; Sakai, H.; Abe, M.; Yudasaka, M.; Iijima, S.; Kaneko, K. Direct Evidence on C−C Single Bonding in Single-Wall Carbon Nanohorn Aggregates. J. Phys. Chem. C; 2007; 111, pp. 5572-5575. [DOI: https://dx.doi.org/10.1021/jp071273k]

33. Siaj, M.; Maltais, C.; Zahidi, E.M.; Oudghiri-Hassani, H.; Wang, J.; Rosei, A.F.; McBreen, P.H. Carbon−Nitrogen Place Exchange on NO Exposed β-Mo₂C. J. Phys. Chem. B; 2005; 109, pp. 15376-15382. [DOI: https://dx.doi.org/10.1021/jp051776v] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/16852950]

34. Rahman, M.M.; Khan, S.B.; Faisal, M.; Rub, M.A.; Al-Youbi, A.O.; Asiri, A.M. Electrochemical determination of olmesartan medoxomil using hydrothermally prepared nanoparticles composed SnO2–Co₃O₄ nanocubes in tablet dosage forms. Talanta; 2012; 99, pp. 924-931. [DOI: https://dx.doi.org/10.1016/j.talanta.2012.07.060]

35. Rahman, M.M.; Alenazi, N.A.; Hussein, M.A.; Alam, M.M.; Alamry, K.A.; Asiri, A.M. Hybride ZnCdCrO embedded aminated polyethersulfone nanocomposites for the development of Hg²⁺ ionic sensor. Mater. Res. Express; 2018; 5, 065019. [DOI: https://dx.doi.org/10.1088/2053-1591/aac681]

36. Rahman, M.M.; Alamry, K.A.; Awual, R.; Mekky, A.E. Efficient Hg(II) ionic probe development based on one-step synthesized diethyl thieno[2,3-b]thiophene-2,5-dicarboxylate (DETTDC2) onto glassy carbon electrode. Microchem. J.; 2020; 152, 104291. [DOI: https://dx.doi.org/10.1016/j.microc.2019.104291]

37. Hussain, M.M.; Rahman, M.M.; Arshad, M.N.; Asiri, A.M. Hg²⁺ Sensor Development Based on (E)-N′-Nitrobenzylidene-Benzenesulfonohydrazide (NBBSH) Derivatives Fabricated on a Glassy Carbon Electrode with a Nafion Matrix. ACS Omega; 2017; 2, pp. 420-431. [DOI: https://dx.doi.org/10.1021/acsomega.6b00359]

38. Arshad, M.N.; Rahman, M.M.; Asiri, A.M.; Sobahi, T.R.; Yu, S.-H. Development of Hg²⁺ sensor based on N′-[1-(pyridin-2-yl)ethylidene]benzenesulfono-hydrazide (PEBSH) fabricated silver electrode for environmental remediation. RSC Adv.; 2015; 5, pp. 81275-81281. [DOI: https://dx.doi.org/10.1039/C5RA09399F]

39. Awual, R.; Hasan, M.; Eldesoky, G.E.; Khaleque, A.; Rahman, M.M.; Naushad, M. Facile mercury detection and removal from aqueous media involving ligand impregnated conjugate nanomaterials. Chem. Eng. J.; 2016; 290, pp. 243-251. [DOI: https://dx.doi.org/10.1016/j.cej.2016.01.038]

40. Katowah, D.F.; Alqarni, S.; Mohammed, G.I.; Al Sheheri, S.Z.; Alam, M.M.; Ismail, S.H.; Asiri, A.M.; Hussein, M.A.; Rahman, M.M. Selective Hg²⁺ sensor performance based various carbon-nanofillers into CuO-PMMA nanocomposites. Polym. Adv. Technol.; 2020; 31, pp. 1946-1962. [DOI: https://dx.doi.org/10.1002/pat.4919]

41. Soman, S.; PV, A. Covalently modified graphene quantum dot using a thiourea based imprinted polymer for the selective electrochemical sensing of Hg(II) ions. J. Polym. Res.; 2021; 28, pp. 1-12. [DOI: https://dx.doi.org/10.1007/s10965-021-02716-6]

42. Koshki, M.-S.; Baghayeri, M.; Fayazi, M. Application of sepiolite/FeS₂ nanocomposite for highly selective detection of mercury(II) based on stripping voltammetric analysis. J. Food Meas. Charact.; 2021; 15, pp. 5318-5325. [DOI: https://dx.doi.org/10.1007/s11694-021-01097-0]

43. Hwang, J.-H.; Fox, D.; Stanberry, J.; Anagnostopoulos, V.; Zhai, L.; Lee, W. Direct Mercury Detection in Landfill Leachate Using a Novel AuNP-Biopolymer Carbon Screen-Printed Electrode Sensor. Micromachines; 2021; 12, 649. [DOI: https://dx.doi.org/10.3390/mi12060649] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/34205934]

44. Bi, C.-C.; Ke, X.-X.; Chen, X.; Weerasooriya, R.; Hong, Z.-Y.; Wang, L.-C.; Wu, Y.-C. Assembling reduced graphene oxide with sulfur/nitrogen-“hooks” for electrochemical determination of Hg(II). Anal. Chim. Acta; 2020; 1100, pp. 31-39. [DOI: https://dx.doi.org/10.1016/j.aca.2019.11.062] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/31987150]

45. Raicopol, M.D.; Chira, N.A.; Pandele, A.M.; Hanganu, A.; Ivanov, A.A.; Tecuceanu, V.; Bugean, I.G.; Buica, G.-O. Electrodes modified with clickable thiosemicarbazone ligands for sensitive voltammetric detection of Hg(II) ions. Sens. Actuators B Chem.; 2020; 313, 128030. [DOI: https://dx.doi.org/10.1016/j.snb.2020.128030]

46. Zhou, N.; Chen, H.; Li, J.; Chen, L. Highly sensitive and selective voltammetric detection of mercury(II) using an ITO electrode modified with 5-methyl-2-thiouracil, graphene oxide and gold nanoparticles. Mikrochim. Acta; 2013; 180, pp. 493-499. [DOI: https://dx.doi.org/10.1007/s00604-013-0956-0]