A Facile Electrochemical Sensor Based on PyTS–CNTs for Simultaneous Determination of Cadmium and Lead Ions

A simple and easy method was implemented for the contemporary detection of cadmium (Cd2+) and lead (Pb2+) ions using 1,3,6,8-pyrenetetrasulfonic acid sodium salt-functionalized carbon nanotubes nanocomposites (PyTS–CNTs). The morphology and composition of the obtained PyTS–CNTs were characterized using scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), and X-ray photoelectron spectroscopy (XPS). The experimental results confirmed that the fabricated PyTS–CNTs exhibited good selectivity and sensitivity for metal ion-sensing owing to the insertion of sulfonic acid groups. For Cd2+ and Pb2+, some of the electrochemical sensing parameters were evaluated by varying data such as the PyTS–CNT quantity loaded on the pyrolytic graphite electrode (PGE), pH of the acetate buffer, deposition time, and deposition potential. These parameters were optimized with differential pulse anodic sweeping voltammetry (DPASV). Under the optimal condition, the stripping peak current of the PyTS–CNTs/Nafion/PGE varies linearly with the heavy metal ion concentration, ranging from 1.0 μg L−1 to 90 μg L−1 for Cd2+ and from 1.0 μg L−1 to 110 μg L−1 for Pb2+. The limits of detection were estimated to be approximately 0.8 μg L−1 for Cd2+ and 0.02 μg L−1 for Pb2+. The proposed PyTS–CNTs/Nafion/PGE can be used as a rapid, simple, and controllable electrochemical sensor for the determination of toxic Cd2+ and Pb2+.


Introduction
Toxic heavy metals are one of the most serious threats to the environment and have drawn more attention because of their non-biodegradable and persistent nature [1][2][3]. Many industries, such as those for battery outputs, metal plating apparatus, mining activities, electroplating, fossil combustibles, pesticides and paper, tanneries, manure, diverse plastics, and metallurgies, are major sources of the toxic heavy metals [4,5]. In the past decade, the rapid increase in industrial production has resulted numerous heavy metals invading the ecosystem [6,7]. These heavy metal ions, such as

Reagents and Materials
The experimental reagents were analytical grade and used as received without any further purification. PyTS (Haite Plastic Pigment Co., Ltd., Kunshan, China) and CNTs (Cangji Nanometer Technology Co., Ltd., Nanjing, China) were purchased for preparing the PyTS-CNT nanocomposite. Nafion solution (5 wt %, Sigma-Aldrich, Shanghai, China) and 2-propanol (Titan Scientific Co., Ltd., Shanghai, China) were used for preparing the chemically modified electrode. Standard cadmium solution (Cd 2+ , 1000 μg mL −1 ) and standard lead solution (Pb 2+ , 1000 μg mL −1 ) were supplied by Alfa Aesar (Shanghai, China). For the pH adjustment, a series of HAc-NaAc buffers containing acetic acid (Beilian Chemical Co., Ltd., Tianjin, China) and sodium acetate (Tianjin Baishi Chemical Co., Ltd.) at a concentration of 0.1 mol L −1 were prepared by mixing the two salts at different ratios. All the test solutions were prepared with deionized water (≥18.2 Ω). All the measurements were obtained at room temperature (18-25 °C).

Instruments
All electrochemical measurements were recorded on the CHI1040C electrochemical workstation (CHI Instrument, Shanghai, China) with a three-electrode setup consisting of a pyrolytic graphite electrode (d = 3 mm, 0.07 cm 2 geometric area) as the working electrode, a Pt wire as the counter electrode (counter electrode), and a silver/silver chloride electrode as the reference electrode (Ag/AgCl). The solution pH was measured using a digital pH meter (Mettler-Toledo, Shanghai, China). The morphology of PyTS-CNTs was characterized using scanning electron microscopy (SEM, Carl Zeiss, Germany) in conjunction with energy dispersive spectroscopy (EDS, BRUKER, Germany). X-ray photoelectron spectroscopy (XPS) analysis was performed on the Thermo Scientific ESCALAB 250XI X-ray photoelectron spectrometer.

Preparation of PyTS-CNTs
PyTS-CNTs were prepared by sonicating the mixture of PyTS and CNTs and then centrifuging the mixture suspension. The CNTs (4 mg) and PyTS (80 mg) were dispersed separately in deionized water and then mixed together [31]. The mixture solution was ultrasonicated, followed by vigorous stirring at room temperature for 2 h. Finally, the well-dispersed CNT suspension was centrifuged and rinsed with distilled water twice. The preparation of the PyTS-CNTs is illustrated in Scheme 1.

Preparation of PyTS-CNT-Modified PGE
PyTS-CNTs were redispersed into a mixture of deionized water and 2-propanol (1:4, v:v) containing 0.1 wt % Nafion as a binder to form a 2 mg mL −1 slurry. The slurry was then sonicated for 30 min. Prior to electrode modification, the pyrolytic graphitic electrode (PGE, Φ = 3 mm) was carefully polished with alumina slurry (0.05, 0.3, and 1.0 µm in particle size), and cleaned with ethanol and distilled water, sequentially. The pretreated PGE was dried by flowing N 2 . The PyTS-CNTs/Nafion/PGE was prepared by casting 4 µL of homogeneous suspensions onto the PGE surface and then dried at room temperature.

DPASV Analysis
Cyclic voltammetry (CV) and DPASV measurements were performed on the CHI 1040C electrochemical workstation in 0.1 M of fresh HAc-NaAc (pH = 5.0) solution containing a certain concentration of the target heavy metal ions (Cd 2+ and Pb 2+ ). The target metals ions were reduced and then electrodeposited on the surface of the PyTS-CNTs/Nafion/PCE at a negative potential of −1.2 V for 270 s under stirring. After an equilibration of 10 s, the DPASV measurements were performed in a potential window ranging from −0.6 V to −1.0 V with an amplitude of 50 mV, pulsation of 50 ms, time interval of 0.5 s, and a potential step accretion of 4 mV. A "cleaning" step was performed by maintaining the conditioning potential (E cond ) at 0.6 V for 60 s after each DPASV measurement to remove the residual heavy metals on the PyTS-CNT hybrid film.

PyTS-CNT Characterization
The structural and elementary distribution of the PyTS-CNTs were characterized using SEM and EDS mapping. Their surface morphology was investigated using SEM. As shown in Figure 1a, the abundant PyTS-CNTs interweaved to form a netlike structure. The PyTS-CNT nanocomposites show better membrane-forming ability compared to pure CNTs, which show weak electron conductivity and more disconnections. The EDS spectrum of the PyTS-CNTs, as shown in Figure 1b, confirms the existence of sulfur element within the PyTS-CNT nanocomposites. In addition, the selected SEM image in Figure 1c and the corresponding EDS mappings (Figure 1d-f) indicate that the primary elements (C, O, and S) were uniformly distributed on the PyTS-CNTs surface. The S element mapping confirms that sulfur-containing groups were successfully functionalized onto the CNT surface.
The chemical valence of the elements on the surface of the CNTs can be identified by XPS spectra. As shown in Figure 2a, the contents of carbon, oxygen, and sulfur on the surface of the PyTS-CNTs have an atom % of 96.85%, 2.8%, and 0.35%, respectively. The wide-scan XPS survey of the PyTS-CNTs exhibits two powerful curves representing graphitic C1s (284.79 eV) and O1s (532.14 eV), and a weak S2p peak around 168.16 eV. The high-resolution C1s spectrum consists of four deconvoluted peaks, as shown in Figure 2b [34]. Notably, the PyTS peak is the most prominent in the spectrum of the PyTS-CNT material, which indicates that PyTS has been successfully decorated onto the CNT surface.

Electrochemical Behaviors of Various Modified Electrodes
To investigate their electrochemical response towards heavy metal ions, the results for different modified electrodes, including bare PGE, Nafion/PGE, CNTs/Nafion/PGE, and PyTS-CNTs/Nafion/PGE, were carefully compared. As shown in Figure 3, stripping peaks appear for both Cd 2+ and Pb 2+ for these three electrodes. The weakest stripping peaks were observed at bare PGE in the voltage range from −1.0 V to −0.4 V (vs. Ag/AgCl) after accumulating at −1.2 V for 270 s. The stripping peak currents were 0.24 µA for Cd 2+ and 0.55 µA for Pb 2+ .
Slightly increased stripping peak currents were observed for Nafion/PGE, with the peak current of 1.29 μA for Cd 2+ and 0.70 μA for Pb 2+ . Meanwhile, the stripping peak currents of both Cd 2+ and Pb 2+ are higher than those of bare and Nafion/PGE. Two well-defined stripping peaks were obtained with the CNTs/Nafion/PGE (5.39 μA for Cd 2+ and 5.48 μA for Pb 2+ ) and PyTS-CNTs/Nafion/PGE (7.27 μA for Cd 2+ and 6.43 μA for Pb 2+ ). Compared with bare PGE, Nafion/PGE, and CNTs/Nafion/PGE, the PyTS-CNTs/Nafion/PGE shows better sensing performance towards heavy metal ions, which can be attributed to the following: (1) The oxygen-containing groups, such as C=O, -OH, -COOH, and -SO3 2− , are critical for heavy metal accumulation; (2) the PyTS-CNTs provide excellent affinity for Cd 2+ and Pb 2+ , and the interlaced PyTS-CNTs provide a three-dimensional network structure, which is beneficial for the heavy metal ion diffusion. The results above indicate that PyTS-CNTs/Nafion/PGE has the best sensing properties for the simultaneous determination of Cd 2+ and Pb 2+ , with a peak potential separation of 340 mV.

Optimization of Experimental Conditions
Some stripping parameters affected the peak currents of Cd 2+ and Pb 2+ during the DPASV measurements. Batch studies were performed to evaluate the effects of various tests parameters, such as the mass loading of PyTS-CNTs onto the PGE, deposition time, deposition voltage, and acetate buffer pH.
The thickness of the PyTS-CNT membrane also affects the stripping peak currents of Cd 2+ and Pb 2+ . The thickness was defined by the mass loading of the PyTS-CNT slurry. Figure 4a shows the mass loading effect of the PyTS-CNT slurry towards the stripping peak currents of Cd 2+ and Pb 2+ . For Cd 2+ and Pb 2+ , the maximum stripping peak currents were obtained when 3 μL of slurry was cast onto the PGE. This phenomenon was likely caused by the increasing PyTS-CNTs that introduced more active sites on the electrode surface, which can improve the electron transportation and accumulate more heavy metal ions at the PyTS-CNTs/PGE. However, excessively increasing the volume of the PyTS-CNT slurry will result in a thicker film, which slows down mass transport and Slightly increased stripping peak currents were observed for Nafion/PGE, with the peak current of 1.29 µA for Cd 2+ and 0.70 µA for Pb 2+ . Meanwhile, the stripping peak currents of both Cd 2+ and Pb 2+ are higher than those of bare and Nafion/PGE. Two well-defined stripping peaks were obtained with the CNTs/Nafion/PGE (5.39 µA for Cd 2+ and 5.48 µA for Pb 2+ ) and PyTS-CNTs/Nafion/PGE (7.27 µA for Cd 2+ and 6.43 µA for Pb 2+ ). Compared with bare PGE, Nafion/PGE, and CNTs/Nafion/PGE, the PyTS-CNTs/Nafion/PGE shows better sensing performance towards heavy metal ions, which can be attributed to the following: (1) The oxygen-containing groups, such as C=O, -OH, -COOH, and -SO 3 2− , are critical for heavy metal accumulation; (2) the PyTS-CNTs provide excellent affinity for Cd 2+ and Pb 2+ , and the interlaced PyTS-CNTs provide a three-dimensional network structure, which is beneficial for the heavy metal ion diffusion. The results above indicate that PyTS-CNTs/Nafion/PGE has the best sensing properties for the simultaneous determination of Cd 2+ and Pb 2+ , with a peak potential separation of 340 mV.

Optimization of Experimental Conditions
Some stripping parameters affected the peak currents of Cd 2+ and Pb 2+ during the DPASV measurements. Batch studies were performed to evaluate the effects of various tests parameters, such as the mass loading of PyTS-CNTs onto the PGE, deposition time, deposition voltage, and acetate buffer pH.
The thickness of the PyTS-CNT membrane also affects the stripping peak currents of Cd 2+ and Pb 2+ . The thickness was defined by the mass loading of the PyTS-CNT slurry. Figure 4a shows the mass loading effect of the PyTS-CNT slurry towards the stripping peak currents of Cd 2+ and Pb 2+ . For Cd 2+ and Pb 2+ , the maximum stripping peak currents were obtained when 3 µL of slurry was cast onto the PGE. This phenomenon was likely caused by the increasing PyTS-CNTs that introduced more active sites on the electrode surface, which can improve the electron transportation and accumulate more heavy metal ions at the PyTS-CNTs/PGE. However, excessively increasing the volume of the PyTS-CNT slurry will result in a thicker film, which slows down mass transport and prevents the diffusion of the target analytes at the modified electrode. Therefore, 3 µL of PyTS-CNTs slurry was chosen as the optimal mass loading volume. The effect of solution acidity (pH), a significant factor in the DPASV process, was also carefully investigated, because the solution pH affects sorption behavior of Cd 2+ and Pb 2+ . As shown in Figure 4b, the electrochemical responses of both Cd 2+ and Pb 2+ increased when the solution pH increased from 4.0 to 5.0. This is likely due to the competitive adsorption between the positive protons and target ions. When the pH is above 5.0, the stripping peak currents of Cd 2+ and Pb 2+ decreased rapidly, owing to the formation of heavy metal hydroxides in a weak acid solution; therefore, the pH of 5.0 was selected as the optimized solution acidity for future measurements. Figure 4c shows the effect of deposition potential on the stripping peak currents at a potential window ranging from −1.0 V to −1.4 V. When the deposition voltage negatively shifted from −1.0 V to −1.2 V, an obvious increase in the stripping peak current occurred for both Cd 2+ and Pb 2+ . While the accumulation potential was lower than −1.2 V, the stripping peak currents were dramatically reduced because of the serious effect of the hydrogen evolution reaction (HER). Therefore, the optimal deposition potential was selected as −1.2 V for both Cd 2+ and Pb 2+ detections. Figure 4d shows the effect of deposition time, from 90 s to 300 s. The stripping currents of Cd 2+ and Pb 2+ increased from 90 s to 270 s, indicating that a longer deposition time caused more Cd 2+ and Pb 2+ to be deposited onto the sensing electrode surface to form alloys. However, the peak currents of Cd 2+ and Pb 2+ increased slowly when the deposition time was above 270 s. This can be attributed to the adsorption saturation of the active site on the modified electrode. Therefore, 270 s was selected as the optimal deposition time. prevents the diffusion of the target analytes at the modified electrode. Therefore, 3 μL of PyTS-CNTs slurry was chosen as the optimal mass loading volume. The effect of solution acidity (pH), a significant factor in the DPASV process, was also carefully investigated, because the solution pH affects sorption behavior of Cd 2+ and Pb 2+ . As shown in Figure  4b, the electrochemical responses of both Cd 2+ and Pb 2+ increased when the solution pH increased from 4.0 to 5.0. This is likely due to the competitive adsorption between the positive protons and target ions. When the pH is above 5.0, the stripping peak currents of Cd 2+ and Pb 2+ decreased rapidly, owing to the formation of heavy metal hydroxides in a weak acid solution; therefore, the pH of 5.0 was selected as the optimized solution acidity for future measurements. Figure 4c shows the effect of deposition potential on the stripping peak currents at a potential window ranging from −1.0 V to −1.4 V. When the deposition voltage negatively shifted from −1.0 V to −1.2 V, an obvious increase in the stripping peak current occurred for both Cd 2+ and Pb 2+ . While the accumulation potential was lower than −1.2 V, the stripping peak currents were dramatically reduced because of the serious effect of the hydrogen evolution reaction (HER). Therefore, the optimal deposition potential was selected as −1.2 V for both Cd 2+ and Pb 2+ detections. Figure 4d shows the effect of deposition time, from 90 s to 300 s. The stripping currents of Cd 2+ and Pb 2+ increased from 90 s to 270 s, indicating that a longer deposition time caused more Cd 2+ and Pb 2+ to be deposited onto the sensing electrode surface to form alloys. However, the peak currents of Cd 2+ and Pb 2+ increased slowly when the deposition time was above 270 s. This can be attributed to the adsorption saturation of the active site on the modified electrode. Therefore, 270 s was selected as the optimal deposition time.

Analyses for Determination of Cd(II) and Pb(II)
The PyTS-CNTs/Nafion/PGE was applied to determine the concentration of Cd 2+ and Pb 2+ with the DPASV method under the optimized conditions. The investigation was performed by maintaining one species' concentration constant and changing the other heavy metal ion's concentration. As shown in Figure 5a,c, two separated stripping peaks were observed at

Analyses for Determination of Cd(II) and Pb(II)
The PyTS-CNTs/Nafion/PGE was applied to determine the concentration of Cd 2+ and Pb 2+ with the DPASV method under the optimized conditions. The investigation was performed by maintaining one species' concentration constant and changing the other heavy metal ion's concentration. As shown in Figure 5a,c, two separated stripping peaks were observed at approximately −0.82 V and −0.55 V, attributed to the stripping process of Cd 2+ and Pb 2+ , respectively. Figure 5a,b show that the stripping current is linear with the Pb 2+ concentration, ranging from 1 µg L −1 to 110 µg L −1 , for a constant Cd 2+ concentration of 50 µg L −1 . The linear regression equation was obtained as y = 0.195x − 0.139, (R 2 = 0.996, y: current µA, x: concentration µg L −1 ). This indicates that the Cd 2+ concentration had an excellent linear correlation with the stripping current, while that of its counterpart, Pb 2+ , remained unchanged. Similar results were also observed, as shown in Figure 5c,d; the stripping peak current of Cd 2+ is also linear with its concentration, ranging from 1 µg L −1 to 90 µg L −1 , in the presence of 50 µg L −1 of Pd 2+ . The correlation equation can be defined by y = 0.10x + 0.332 (R 2 = 0.991, y: current µA, x: concentration µg L −1 ). These results indicate that the obtained PyTS-CNT-modified electrode can be used for the simultaneous determination of Cd 2+ and Pb 2+ . The limits of detection were estimated to be 0.8 µg L −1 for Cd 2+ and 0.02 µg L −1 for Pb 2+ (S/N = 3), which are approximately 6 and 500 times lower than the World Health Organization standard for drinking water.
counterpart, Pb 2+ , remained unchanged. Similar results were also observed, as shown in Figure 5c,d; the stripping peak current of Cd 2+ is also linear with its concentration, ranging from 1 μg L −1 to 90 μg L −1 , in the presence of 50 μg L −1 of Pd 2+ . The correlation equation can be defined by y = 0.10x + 0.332 (R 2 = 0.991, y: current μA, x: concentration μg L −1 ). These results indicate that the obtained PyTS-CNT-modified electrode can be used for the simultaneous determination of Cd 2+ and Pb 2+ . The limits of detection were estimated to be 0.8 μg L −1 for Cd 2+ and 0.02 μg L −1 for Pb 2+ (S/N = 3), which are approximately 6 and 500 times lower than the World Health Organization standard for drinking water.
The results of some previous studies were compared with those of the proposed electrochemical sensor electrode in terms of the sensing performance, including the limits of detection and linear ranges ( Table 1). As shown, it can be seen that the detection limits of the fabricated PyTS-CNTs/Nafion/PGE sensor are lower than those in most previous reports using carbon nanotubes, graphene, and other core-shell nanoparticles. It is clear that the PyTS-functionalized CNTs possess a large surface area, fast electron transfer, and rich active sites for toxic metal ions. Additionally, the PyTS-CNTs/Nafion/PGE shows an improved analytical performance towards heavy metal ions in terms of a wider linear range and low detection limits; hence, its potential applications are wider, and it can be used as an effective electrochemical sensing platform for the determination of toxic metal ions owing to its superior analytical productivity.  The results of some previous studies were compared with those of the proposed electrochemical sensor electrode in terms of the sensing performance, including the limits of detection and linear ranges ( Table 1). As shown, it can be seen that the detection limits of the fabricated PyTS-CNTs/Nafion/PGE sensor are lower than those in most previous reports using carbon nanotubes, graphene, and other core-shell nanoparticles. It is clear that the PyTS-functionalized CNTs possess a large surface area, fast electron transfer, and rich active sites for toxic metal ions. Additionally, the PyTS-CNTs/Nafion/PGE shows an improved analytical performance towards heavy metal ions in terms of a wider linear range and low detection limits; hence, its potential applications are wider, and it can be used as an effective electrochemical sensing platform for the determination of toxic metal ions owing to its superior analytical productivity.

Interferences
The anti-interference effect of the proposed sensor was also evaluated by adding some common interfering ions into the mixture solution containing 50 µg L −1 Cd 2+ and 50 µg L −1 Pb 2+ . By allowing a stripping current change of 10% for simultaneously detecting 50 µg L −1 Cd 2+ and 50µg L −1 Pb 2+ , 500-fold Ca 2+ , 300-fold Mg 2+ , 100-fold Al 3+ , 10-fold Cr 3+ , 5-fold Mn 2+ , 1-fold Zn 2+ , and Ni 2+ can be effectively tolerated. As shown, the significant amounts of NO 3 2− , SO 4 2− , and PO 4 3− ions had a negligible effect on the stripping currents of Cd 2+ and Pb 2+ [45]. The results in Figure 6 indicate that these metal ions do not interfere with the detection of target heavy metal ions. However, high concentration of Co 2+ , Fe 2+ , and Fe 3+ had a negative influence on the stripping peak currents of Cd 2+ and Pb 2+ , owing to their competing adsorption of Cd 2+ and Pb 2+ on the PyTS-CNT electrode surface during the pre-accumulation step. We found that the interference of these ions is negligible on our proposed electrode by using a pretreatment process, such as a masking agent.

Interferences
The anti-interference effect of the proposed sensor was also evaluated by adding some common interfering ions into the mixture solution containing 50 μg L −1 Cd 2+ and 50 μg L −1 Pb 2+ . By allowing a stripping current change of 10% for simultaneously detecting 50 μg L −1 Cd 2+ and 50μg L −1 Pb 2+ , 500-fold Ca 2+ , 300-fold Mg 2+ , 100-fold Al 3+ , 10-fold Cr 3+ , 5-fold Mn 2+ , 1-fold Zn 2+ , and Ni 2+ can be effectively tolerated. As shown, the significant amounts of NO3 2− , SO4 2− , and PO4 3− ions had a negligible effect on the stripping currents of Cd 2+ and Pb 2+ [45]. The results in Figure 6 indicate that these metal ions do not interfere with the detection of target heavy metal ions. However, high concentration of Co 2+ , Fe 2+ , and Fe 3+ had a negative influence on the stripping peak currents of Cd 2+ and Pb 2+ , owing to their competing adsorption of Cd 2+ and Pb 2+ on the PyTS-CNT electrode surface during the pre-accumulation step. We found that the interference of these ions is negligible on our proposed electrode by using a pretreatment process, such as a masking agent.

Repeatability and Reproducibility
As shown in Table 2, the reproducibility and repeatability of the PyTS-CNTs were investigated by repetitive measurements of 50 µg L −1 Cd 2+ and 50 µg L −1 Pb 2+ solutions under the optimal conditions. Repeatability of the proposed electrochemical sensor was examined using one modified electrode for five tests, and the relative standard derivation (RSD) was 2.8% for Cd 2+ and 2.4% for Pb 2+ in five calculations. Meanwhile, the five fabricated electrodes exhibited similar electrochemical responses with RSDs of 6.4% for Cd 2+ and 4.2% for Pb 2+ , indicating the satisfactory reproducibility of the proposed electrode. These results demonstrate the good repeatability and reproducibility of the PyTS-CNTs/PGE.

Conclusions
In summary, PyTS-CNTs were prepared using a one-pot and facile sonochemical synthesis method. The as-prepared PyTS-CNTs were used to prepare the PyTS-CNTs/Nafion/PGE electrochemical sensor to evaluate its sensing performance towards heavy metal ions of Cd 2+ and Pb 2+ . Experimental results confirm that the PyTS-CNTs possess good electrochemical response and sensitivity to detect toxic metal ions. The fabrication of the PyTS-CNT electrode as an electrochemical sensor was fast, simple, and controllable. More importantly, the facile and environmentally friendly electrochemical sensor may provide a cost-effective platform for the green, facile, and sensitive analysis of Cd 2+ and Pb 2+ in environmental samples.