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polypyrrole nanocomposite functionalized electrode
Sensors & Actuators: B. Chemical 304 (2020) 127286
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Sensitive and selective detection of Pb (II) and Cu (II) using a metal-organic framework/polypyrrole nanocomposite functionalized electrode Nan Wanga,1, Wei Zhaoa,1, Ziyang Shena, Shengjun Sunb, Hongxiu Daic, Houyi Maa, Meng Lina,
T
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a Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China b Shandong Provincial Key Laboratory of Oral Biomedicine, College of Stomatology, Shandong University, Jinan 250021, China c Department of Chemistry, Liaocheng University, Liaocheng 252059, China
A R T I C LE I N FO
A B S T R A C T
Keywords: NH2-MIL-53(Al) Polypyrrole Nanocomposite Electrochemical Analysis Heavy m Etal ions
An electrochemical sensor based on a NH2-MIL-53(Al)/polypyrrole (PPy) nanocomposite modified gold electrode for determination of ultra-trace Pb (II) and Cu (II) was developed. The PPy nanowires were synthesized through a chemical polymerization process, and the NH2-MIL-53(Al) was deposited on the PPy nanosubstrates by an in-situ electrochemical method. Field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) spectroscopy and attenuated total reflection infrared (ATRIR) spectroscopy were used to study the structural features of the nanocomposite. The electrochemical behavior of the NH2-MIL-53(Al)/PPy nanocomposite toward Pb (II) and Cu (II) was investigated by differential pulse voltammetry (DPV) technique. Due to the synergistic effect of the PPy and NH2-MIL-53(Al), the nanocomposite modified electrode exhibited good sensing performance to Pb (II) and Cu (II) in the range of 1–400 μg L−1 with detection limit of 0.315 μg L−1 and 0.244 μg L−1, respectively. The limit of detection and the sensitivity of the NH2-MIL-53(Al)/PPy nanocomposite modified electrode were significantly enhanced than those of the individual modified electrodes. The effective electrochemical property of the nanomaterial for the Pb (II) and Cu (II) could provide a new direction for the heavy metal ions determination.
1. Introduction Heavy metal pollution, including lead ion (Pb(II)) and copper ion (Cu(II)), has attracted widespread attention due to their high toxicity and enrichment in living organisms. Even a small amount of heavy metal ions could cause enormous damages to human and animal physiological systems and nerve central systems [1–3]. Therefore, effective detection and removal of heavy metal ions are very necessary and urgent [4]. So far, various analytical methods have been applied to the detection of heavy metal ions [5–7]. Owing to the simple operation, high sensitivity and low detection limit, electrochemical strategy is considered to be the commonly analytical method for detecting trace heavy metal ions [8–11]. The electrochemical analysis approach based on the differential pulse voltammetry (DPV) is regarded as a powerful analytical testing technique, which has greatly improved the detection limit of heavy metal ions through pre-concentration and stripping processes [12]. During the stage of pre-concentration, heavy metal ions are reduced or chelated onto the modified electrode from the bulk
solution for ion enrichment. In order to construct electrochemical sensors with good sensitivity, selectivity and low limit of detection, it is very important to design and prepare ideal electrode materials for the combination of heavy metal ions [13,14]. In view of the extremely high specific surface area and specific pore size, metal-organic frameworks (MOFs) have made significant progress in the fields of adsorption/separation, catalysis and contaminant removal [15,16]. Recently, the application of MOFs with electrochemical sensors also has been reported. For example, Guo et al. synthesized a petal-like NH2-MIL-53(Cr) to realize excellent electrochemical performance for the detection of heavy metal ions [17]. In addition, MOFs can be used as sensor materials for identifying small molecules such as glucose, hydrogen peroxide and nucleic acid molecules [15,18,19]. However, most of the MOFs suffer from poor conductivity and stability during the electrochemical applications. In this regard, the coupling of MOFs with conducting polymers is considered to be a solution to the above limitation. As a commonly studied conducting polymer, polypyrrole (PPy) is often used for metal
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Corresponding author. E-mail address: [email protected] (M. Lin). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.snb.2019.127286 Received 6 August 2019; Received in revised form 10 October 2019; Accepted 14 October 2019 Available online 17 October 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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nitrogen. 10 mg of the synthetic nanomaterials, PPy nanowires, NH2MIL-53(Al) and NH2-MIL-53(Al)/PPy nanocomposites, were dispersed in 5 mL aqueous solution by sonication for 30 min, and then, 20 μL dispersion was coated on the surface of the gold electrode, dried under an infrared lamp to form a modified electrode. DPV was employed to assess the determination performance to Pb (II) and Cu (II). The NH2-MIL-53(Al)/PPy modified the electrode was immersed in 0.1 M acetic acid/sodium acetate buffer solution (pH = 5.0) containing different concentrations of Pb (II) and Cu (II) at first. After the pre-concentration process reached equilibrium (about 20 min), the electrode was taken out, washed and transferred into a 0.1 M metal ions-free acetic acid/sodium acetate buffer solution (pH = 5.0) to reduce the enriched metal ions onto the surface of modified electrode at -0.4 V for 120 s. Subsequently, the stripping reoxidation currents of were recorded by DPV at an amplitude of 20 mV, a pulse width of 0.05 s, a pulse period of 0.2 s and quiet time of 2 s.
ions detection and removal based on its good conductivity, high stability, and excellent ion exchange performance [7,20]. Herein, we develop a new type of electrochemical sensor based on NH2-MIL-53(Al)/ PPy nanocomposite to achieve high sensitivity and selectivity detection of Pb (II) and Cu (II). The NH2-MIL-53(Al) crystals were fabricated through an in-situ electrochemical method using PPy nanowires as substrate. The combination of NH2-MIL-53(Al) and PPy nanowires provided a platform for electrochemical detection of heavy metal ions attributed to the large specific surface area of the NH2-MIL-53(Al) and the excellent conductivity of the PPy nanowires. The fabricated NH2MIL-53(Al)/PPy nanocomposite modified gold electrode exhibited good electrochemical response toward Pb (II) and Cu (II) by using differential pulse voltammetric measurements. Operating parameters such as pH, pre-concentration time and anti-interference ability and stability of the modified electrode were also studied. 2. Experimental
3. Results and discussion 2.1. Reagents and materials 3.1. Morphological and structural characterizations Aluminum plate (99.5%), 2-aminoterephthalic acid, acetic acid, sodium acetate, N,N-dimethylformamide (DMF), potassium chloride (KCl) and pyrrole were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Metal ions standard solutions were offered by Aladdin. All chemicals used were of analytical grade without further purification, and deionized water (18.25 MΩ cm, UHK/UPT ultrapure water system) was used to prepare all aqueous solutions.
The morphologies of the PPy nanowires, NH2-MIL-53(Al) and NH2MIL-53(Al)/PPy nanocomposites were investigated by SEM and TEM. The PPy nanowires with an average diameter about 40 nm are shown in Fig. 1A, and Fig. 1B displays a nanosheet structure of NH2-MIL-53(Al). Typical SEM and TEM images of the fabricated NH2-MIL-53(Al)/PPy nanocomposites are illustrated in Fig. 1C. As expected, the PPy nanowires acted as supported skeleton with the NH2-MIL-53(Al) synthesized along the nanowires during the in-situ electrochemical process. Fig. 2A shows the XRD patterns of the PPy nanowires, NH2-MIL53(Al) and NH2-MIL-53(Al)/PPy nanocomposites. A broad characteristic peak exhibits at 2θ = 21° corresponding to the amorphous behavior of the X-ray scattering of the PPy chain [23], and the XRD peaks attributing to (110), (031), (112), (250) reflection of the crystal of the NH2-MIL-53(Al) emerge at 2θ = 9.2°, 17.5°, 28.26°, and 40.64°, respectively [24]. After the combination of the NH2-MIL-53(Al) and PPy nanowires, the typical X-ray scanning patterns of the NH2-MIL-53(Al) is revealed in the blue line of Fig. 2A. The ATR-IR spectra are shown in Fig. 2B. As the spectrum of the PPy nanowires presented, the characteristic peaks at 1448 cm−1 and 1540 cm−1 are ascribed to the vibration of the pyrrole ring [25]. The peaks at 1301 cm−1, 1049 cm−1 and 792 cm−1 correspond to C–H in plane vibration, deformation vibration and swing vibration, respectively. For the NH2-MIL-53(Al), two obvious absorption peaks at 1503 cm−1 and 1402 cm−1 are attributed to the carbonyl asymmetric and symmetric vibrations, and the band around 1602 cm−1 and 1660 cm−1 can be assigned to the vibration of the benzene ring and the NeH bending vibration of NH2 group, respectively [24,26]. Due to the π-π interaction between the PPy and the NH2-MIL-53(Al) backbones, the peak at 1584 cm−1 corresponding to the vibration of the aromatic ring is presented in the spectrum of NH2MIL-53(Al)/PPy nanocomposites [27]. Both of the XRD patterns and ATR-IR spectra effectively prove the successful preparation of the NH2MIL-53(Al)/PPy composites, which are in good agreement with the SEM and TEM investigations.
2.2. Apparatus The morphological characterizations of PPy nanowires, NH2-MIL-53 (Al) and NH2-MIL-53(Al)/PPy nanocomposite were characterized using field-emission scanning electron microscopy (FE-SEM, JSM-7600 F) and transmission electron microscopy (TEM, JEM-1011). X-ray diffraction (XRD) spectra of the powder samples were collected using a D8 Advance (Bruker) X-ray diffractometer with CuKα radiation (λ = 1.5418 Å). Attenuated total reflection infrared (ATR-IR) spectra were recorded by an infrared spectrophotometry (Bruker Tensor II, Germany) with a resolution of 2 cm−1. Differential pulse voltammetry (DPV) was performed on a CHI 750E workstation (C. H. Instruments Co., Shanghai, China) in a conventional three-electrode system with modified electrode, platinum plate, saturated calomel electrode (SCE, with a saturated KCl solution) were used as working electrode, counter electrode and reference electrode, respectively. 2.3. Synthesis of NH2-MIL-53(Al)/PPy nanocomposite The PPy nanowires were prepared according to the previous literature [21]. The NH2-MIL-53(Al)/PPy nanocomposite was obtained by in-situ electrochemical synthesis approach [22], and the detailed processes are described as follows. At beginning, the PPy nanowires (0.075 g) was well-dispersed in 100 mL 10% DMF aqueous solution containing 1.50 g 2-aminoterephthalic acid and 0.75 g KCl. Subsequently, two parallel aluminum plate electrodes (1 cm × 5 cm) separated by 3.0 cm were applied a current density of 20 mA cm−1 in the above solution at 90 °C for 1 h. Then, the gray precipitate was filtered and washed by DMF to dissolve the residual 2-aminoterephthalic acid. After centrifugation, the resulting powder was dried overnight at 100 °C. The NH2-MIL-53(Al) was produced without the addition of PPy nanowires.
3.2. Electrochemical responses of Pb (II) and Cu (II) on different electrodes The electrochemical performances of three different types of modified gold electrodes were evaluated by determining of Pb (II) and Cu (II) in acetic acid/sodium acetate buffer solution (0.1 M, pH = 5.0) with a certain concentration of the metal ions. After the pre-concentration step for Pb (II) and Cu (II), the PPy, the NH2-MIL-5 3(Al) and the NH2MIL-53(Al)/PPy modified electrodes were placed in an acetic acid/sodium acetate buffer solution with an applied potential of -1.0 V for 2 min to reduce the heavy metal ions. As Fig. 3 shown, two stripping peak were observed around -0.185 V for Pb (II) and 0.20 V for Cu (II) at
2.4. Electrochemical detection of Pb (II) and Cu (II) The bare gold electrode (diameter: 4 mm) was polished with 1 μm and 50 nm alumina powder, washed with nitric acid (6 mol L−1), deionized water and ethanol, respectively, and dried in a stream of 2
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Fig. 1. Typical SEM and TEM images of PPy nanowires (A), NH2-MIL-53(Al) (B) and NH2-MIL-53(Al)/PPy nanocomposites (C), respectively.
the PPy nanowires modified electrode, because of the coordination of metal ions with the nitrogen on the pyrrole unit [28]. The current signal of the stripping peak has increased at the NH2-MIL-53(Al) modified electrode. That could be ascribed to the metal ions chelation properties of the amino and carboxylic groups from NH2-MIL-53(Al) toward Pb (II) and Cu (II). Compared with the PPy nanowires and NH2-MIL-53(Al) modified electrodes, significantly enhanced electrochemical signal was measured by the NH2-MIL-53(Al)/PPy modified electrode. The obtained current peaks are over 3-fold than those of recorded by the other two modified electrodes, which are due to the synergistic amplification effect of the NH2-MIL-53(Al) and the PPy. 3.3. Optimization of experimental conditions The pH of the solution and the pre-concentration time are the important factors in terms of sensitivity and selectivity. pH significantly affects the structural characterization of the functional materials by protonation or deprotonation reaction. To study the influence of the solution acidity during the enrichment of Pb (II) and Cu (II), various pH ranged from 3.5 to 6.5 were investigated. As displayed in Fig. 4A, the peak currents increase with the pH increased from 3.5 to 5.0. The result could be proposed that the amino groups of NH2-MIL-53 (Al) were
Fig. 3. DPV curves of the bare gold, PPy nanowires, NH2-MIL-53(Al) and NH2MIL-53(Al)/PPy nanocomposites modified electrodes in the presence of 400 μg L−1 of Pb (II) and Cu (II).
protonated at lower pH [29], and the protonated amino group (-NH3+) repelled the cations by electrostatic reaction. In contrast, the reduction of peak currents might be led to the formation of metal ion hydroxide
Fig. 2. XRD patterns (A) and ATR-IR spectra (B) of the PPy nanowires, NH2-MIL-53(Al) and NH2-MIL-53(Al)/PPy nanocomposites. 3
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Fig. 4. Optimization of experimental conditions. Effect of pH (A), and influence of pre-concentration time (B). DPV data obtained in a solution containing 400 μg L−1 of Pb (II) and Cu (II).
electrochemical performance between NH2-MIL-53(Al)/PPy modified gold electrodes and other modified electrodes for heavy metal ion detection. It can be seen that a sufficiently low detection limit and a reasonable linear range can be obtained from our proposed method. Simultaneous determination of Pb (II) and Cu (II) was conducted with the same concentration levels under the pre-concentration time of 20 min, and the results are depicted in Fig. 7A. In the case of the Pb (II) and Cu (II) coexisted, two distinct stripping peaks at -0.185 V and 0.20 V corresponding to the oxidation of Pb and Cu arise, and the oxidation peaks are well distinguished. Therefore, it is feasible that the NH2-MIL-53(Al)/PPy modified electrode can simultaneously detect the two target ions. Fig. 7B displays the fitting curves in the range of 1–400 μg L−1 for the Pb (II) and Cu (II), and the correlation coefficients are 0.9669 and 0.9876, respectively. According to the above experimental phenomenon, with the concentrations of the heavy metal ions increasing, the increased electrochemical peak currents indicating the NH2-MIL-53(Al)/PPy nanocomposites can be used as an electrochemically active material for the Pb (II) and Cu (II) determination [35].
complexes at higher pH [30,31], which could inhibit the capture of Pb (II) and Cu (II). Fig. 4B illustrates the effect of pre-concentration time on the electrochemical responses of Cu (II) and Pb (II). The re-oxidation currents curves by the pre-concentration time are continuously rising in the range of 0–20 min, while the time exceeded 20 min, the curves are only slightly increased, probably due to the saturation of the surface active sites [32]. 3.4. Analytical performance Analysis of individual Pb (II) and Cu (II) at various concentrations with the NH2-MIL-53(Al)/PPy modified gold electrodes were performed using DPV. Prior to the DPV measurements, a 20 min pre-concentration process was carried out in various concentrations of Pb (II) and Cu (II) acetic acid/sodium acetate buffer solution (0.1 M, pH 5.0). As Figs. 5a and 6 a shown, well-defined DPV peaks of Pb (0)/Pb (II) and Cu (0)/Cu (II) are clearly observed around -0.185 V and 0.20 V. Due to the enrichment of Pb (II) and Cu (II) from the bulk metal ions solution, the curves of the DPV responses recorded by the NH2-MIL-53(Al)/PPy modified gold electrode against the different concentrations of Pb (II) and Cu (II) are nonlinear (Fig. 5b and 6 b) [33]. The fitting equations are y = 0.2143x0.78/(1+(0.026x)0.78) for Pb (II) and y = 0.1009x0·99/ (1 + (0.0068x)0·99) for Cu (II) in the range of 1–400 μg L−1, and the correlation coefficients as 0.9985 and 0.9978, respectively. The calibration curves (Fig. 5b inset and 6 b inset) show nonlinear increase in peak currents at a low concentration range from 1 to 20 μg L−1, and the detection limit of Pb (II) and Cu (II) are calculated as 0.315 μg L−1 and 0.244 μg L−1 by Pividori and co-workers reported method [33]. On account of the synergistic effect of different active materials, the detection ability to Pb (II) and Cu (II) are greatly promoted, which is resulted in the limit of detection calculated by the use of the NH2-MIL53(Al)/PPy nanocomposites were much lower than the other reports using various electrode materials. Table 1 shows a comparison of
3.5. Interference study In order to evaluate the selectivity of the proposed modified electrode, the test by adding 400 μg L−1 of interfering cations (such as Cd (II), Hg (II), Ca (II), Ni (II) and Zn (II)) with the coexisting of the same concentration of Pb (II) and Cu (II) was implemented. As shown in Fig. 8, most of these metal cations do not interfere during the detection of Pb (II) and Cu (II) from the perspective of the DPV response. The result confirms that the NH2-MIL-53(Al)/PPy nanocomposite modified electrode cannot be affected by other metal species. The selective behavior of the prepared nanocomposite could be attributed to the high affinity of amino and carboxyl groups, and the adsorption capacity of the chelating groups decreases in the sequence of Cu ≫ Pb ≫ Cd ≫ Zn
Fig. 5. DPV response (A) and calibration curve (B) toward Pb (II) in the concentration range of 1–400 μg L−1. 4
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Fig. 6. DPV stripping signals (A) and DPV peak current (B) for the NH2-MIL-53(Al)/PPy nanocomposites modified electrode under various the concentration of Cu (II). Table 1 Comparison of different materials to Pb (II) and Cu (II) determination. Electrode
Analytical methods
NH2-MIL-53(Cr) NH2-MIL-88(Fe)-rGO
SWASV DPASV
Ti3C2
SWASV
MIL-100(Cr)
SWASV
NH2-MIL-53(Al)/PPy
DPV
Detection range (μg L−1)
Analytes
4
Pb(II) Pb(II) Cu(II) Pb(II) Cu(II) Pb(II) Cu(II) Pb(II) Cu(II)
80-1.6 × 10 2-62 0.32-3.2 20-310 6.4-96 20-2.07 × 103 6.4-640 1-400 1-400
Detection limit (μg L−1)
Refs
6.3 0.8 0.32 8.5 2.05 10 0.7 0.315 0.244
17 29 30 34 This work
Fig. 7. Simultaneous detection of Pb (II) and Cu (II) in a concentration range of 1–400 μg L−1 (A) the NH2-MIL-53(Al)/PPy nanocomposites modified electrode, and the corresponding calibration curves for Pb (II) and Cu (II), respectively (B).
Fig. 8. DPV curve on the NH2-MIL-53(Al)/PPy modified gold electrode after a pre-concentration process in a solution containing 400 μg L−1 of Cd (II), Hg (II), Ca (II), Ni (II), Zn (II), Pb (II) and Cu (II) (A), and interference investigation of Pb (II) and Cu (II) in the presence of other common cations (B).
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Acknowledgments
Table 2 Detection results of the spiked Pb (II) and Cu (II) in tap water samples. Analytes
Spiked (μg L−1)
Found (μg L−1, n = 3)
Recovery (%)
Pb (II)
0 50 100 0 50 100
– 48.4 ± 0.7 101.2 ± 0.4 – 49.6 ± 0.2 100.7 ± 0.8
– 96.8 101.2 – 99.2 100.7
Cu (II)
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≫ Ni, Hg [36,37]. Therefore, we can conclude that the fabricated material has good selectivity for Pb (II) and Cu (II). 3.6. Reproducibility and stability Reproducibility and stability are of great important performance indicators for measuring the prepared electrochemical sensors. Five different electrodes were prepared in parallel to assess the reproducibility of the NH2-MIL-53(Al)/PPy coated gold electrode. The measurements revealed that the relative standard deviation of the peak current as 3.52% demonstrating the nanomaterial modified electrode has good reproducibility. In addition, the stability of the fabricated electrode was also investigated. The NH2-MIL-53(Al)/PPy nanocomposite modified electrode was immersed in 0.1 M acetic acid/sodium acetate buffer solution (pH 5.0) for 2 weeks, and the peak current of the same concentration of metal ions was again determined to be about 90% of the initial value. These results prove that the NH2-MIL-53(Al)/PPy nanocomposite modified electrodes exhibit good reproducibility and stability. 3.7. Tested of Pb (II) and Cu (II) in real samples The practical performance of the NH2-MIL-53(Al)/PPy nanocomposite modified electrode was investigated by the determination of Pb (II) and Cu (II) in tap water samples. The recovery results obtained by incorporating a certain concentration of heavy metal ion solution in water samples are given in Table 2. It can be seen that the nanocomposite modified electrode shows good recovery indicating the constructed sensor can be used for the determination of heavy metal ions in environmental samples. 4. Conclusion In this study, a nanocomposite of metal-organic framework (NH2MIL-53(Al)) supported by PPy nanowires was prepared by a simple insitu electrochemical deposition method. The morphological and structural characterizations of the synthesized nanocomposites were carried out by SEM, TEM, XRD and ATR-IR techniques. The NH2-MIL-53(Al)/ PPy nanocomposite was used as a sensing material for individual and simultaneous determination of Pb (II) and Cu (II). In comparison with the PPy nanowires and the NH2-MIL-53(Al) modified electrodes, the performance of the NH2-MIL-53(Al)/PPy nanocomposite modified electrode proved that the developed sensor is highly sensitive to detect Pb (II) and Cu (II) in the range of 1–400 μg L−1. The NH2-MIL-53(Al)/ PPy nanocomposite modified electrodes also demonstrate strong antiinterference ability and good stability. Based on the synergistic effect of PPy and NH2-MIL-53(Al), the effective electrochemical sensor provides a new strategy for detection of the heavy metal ions. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 6
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X.W. Guo, Size- and morphology-controlled NH2-MIL-53(Al) prepared in DMFwater mixed solvents, Dalton Trans. 42 (2013) 13698–13705. S.T. Navale, A.T. Mane, A.A. Ghanwat, A.R. Mulik, V.B. Patil, Camphor sulfonic acid (CSA) doped polypyrrole (PPy) films: measurement of microstructural and optoelectronic properties, Measurement 50 (2014) 363–369. M. Sharma, G. Madras, S. Bose, Contrasting effects of graphene oxide and poly (ethylenimine) on the polymorphism in poly(vinylidene fluoride), Cryst. Growth Des. 15 (2015) 3345–3355. S.Y. Wang, L. Ma, M.Y. Gan, S.N. Fu, W.Q. Dai, T. Zhou, X.W. Sun, H.H. Wang, H.N. Wang, Free-standing 3D graphene/polyaniline composite film electrodes for high-performance supercapacitors, J. Power Sources 299 (2015) 347–355. Y. Wei, R. Yang, J.H. Liu, X.J. Huang, Selective detection toward Hg (II) and Pb (II) using polypyrrole/carbonaceous nanospheres modified screen-printed electrode, Electrochim. Acta 105 (2013) 218–223. S. Duan, Y.M. Huang, Electrochemical sensor using NH2-MIL-88(Fe)-rGO composite for trace Cd2+, Pb2+, and Cu2+ detection, J. Electroanal. Chem. Lausanne (Lausanne) 807 (2017) 253–260. X.L. Zhu, B.C. Liu, H.J. Hou, Z.Y. Huang, K.M. Zeinu, L. Huang, X.Q. Yuan, D.B. Guo, J.P. Hu, J.K. Yang, Alkaline intercalation of Ti3C2 MXene for simultaneous electrochemical detection of Cd (II), Pb(II), Cu(II) and Hg(II), Electrochim. Acta 248 (2017) 46–57. Y. Wang, L. Wang, W. Huang, T. Zhang, X.Y. Hu, J.A. Perman, S.Q. Ma, A metalorganic framework and conducting polymer based electrochemical sensor for high performance cadmium ion detection, J. Mater. Chem. A Mater. Energy Sustain. 5 (2017) 8385–8393. M.R. Awual, Solid phase sensitive palladium (II) ions detection and recovery using ligand based efficient conjugate nanomaterials, Chem. Eng. J. 300 (2016) 264–272. A.H.A. Hassan, L. Sappia, S.L. Moura, F.H.M. Ali, W.A. Moselhy, Md.P.T. Sotomayor, M.I. Pividori, Biomimetic magnetic sensor for electrochemical determination of scombrotoxin in fish, Talanta 194 (2019) 997–1004. D.F. Wang, Y.C. Ke, D. Guo, H.X. Guo, J.H. Chen, W. Weng, Facile fabrication of cauliflower-like MIL-100(Cr) and its simultaneous determination of Cd2+, Pb2+, Cu2+ and Hg2+ from aqueous solution, Sens. Actuators B Chem. 216 (2015) 504–510. Lalchhingpuii, D. Tiwari, Lalhmunsiama, S.M. Lee, Chitosan templated synthesis of
Nan Wang is a PhD candidate in School of Chemistry and Chemical Engineering, Shandong University. Her research focuses on electrochemical sensors. Wei Zhao is an undergraduate student in School of Chemistry and Chemical Engineering, Shandong University. Ziyang Shen is an undergraduate student in School of Chemistry and Chemical Engineering, Shandong University. Shengjun Sun is a lecturer in School of Stomatology, Shandong University. His main research interest focuses on the biological properties of nanomaterials. Hongxiu Dai is a lecturer in Department of Chemistry, Liaocheng University. Her research focuses on electrochemical sensors. Houyi Ma is a professor in School of Chemistry and Chemical Engineering, Shandong University. His current research interests include electrochemical corrosion and electrocatalysis. Meng Lin is an associate professor in School of Chemistry and Chemical Engineering, Shandong University. His main research interests focus on electrochemical sensors and preparation of novel nanomaterials.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Sensitive and selective detection of Pb (II) and Cu (II) using a metal-organic framework/polypyrrole nanocomposite functionalized electrode Nan Wanga,1, Wei Zhaoa,1, Ziyang Shena, Shengjun Sunb, Hongxiu Daic, Houyi Maa, Meng Lina,
T
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a Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China b Shandong Provincial Key Laboratory of Oral Biomedicine, College of Stomatology, Shandong University, Jinan 250021, China c Department of Chemistry, Liaocheng University, Liaocheng 252059, China
A R T I C LE I N FO
A B S T R A C T
Keywords: NH2-MIL-53(Al) Polypyrrole Nanocomposite Electrochemical Analysis Heavy m Etal ions
An electrochemical sensor based on a NH2-MIL-53(Al)/polypyrrole (PPy) nanocomposite modified gold electrode for determination of ultra-trace Pb (II) and Cu (II) was developed. The PPy nanowires were synthesized through a chemical polymerization process, and the NH2-MIL-53(Al) was deposited on the PPy nanosubstrates by an in-situ electrochemical method. Field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) spectroscopy and attenuated total reflection infrared (ATRIR) spectroscopy were used to study the structural features of the nanocomposite. The electrochemical behavior of the NH2-MIL-53(Al)/PPy nanocomposite toward Pb (II) and Cu (II) was investigated by differential pulse voltammetry (DPV) technique. Due to the synergistic effect of the PPy and NH2-MIL-53(Al), the nanocomposite modified electrode exhibited good sensing performance to Pb (II) and Cu (II) in the range of 1–400 μg L−1 with detection limit of 0.315 μg L−1 and 0.244 μg L−1, respectively. The limit of detection and the sensitivity of the NH2-MIL-53(Al)/PPy nanocomposite modified electrode were significantly enhanced than those of the individual modified electrodes. The effective electrochemical property of the nanomaterial for the Pb (II) and Cu (II) could provide a new direction for the heavy metal ions determination.
1. Introduction Heavy metal pollution, including lead ion (Pb(II)) and copper ion (Cu(II)), has attracted widespread attention due to their high toxicity and enrichment in living organisms. Even a small amount of heavy metal ions could cause enormous damages to human and animal physiological systems and nerve central systems [1–3]. Therefore, effective detection and removal of heavy metal ions are very necessary and urgent [4]. So far, various analytical methods have been applied to the detection of heavy metal ions [5–7]. Owing to the simple operation, high sensitivity and low detection limit, electrochemical strategy is considered to be the commonly analytical method for detecting trace heavy metal ions [8–11]. The electrochemical analysis approach based on the differential pulse voltammetry (DPV) is regarded as a powerful analytical testing technique, which has greatly improved the detection limit of heavy metal ions through pre-concentration and stripping processes [12]. During the stage of pre-concentration, heavy metal ions are reduced or chelated onto the modified electrode from the bulk
solution for ion enrichment. In order to construct electrochemical sensors with good sensitivity, selectivity and low limit of detection, it is very important to design and prepare ideal electrode materials for the combination of heavy metal ions [13,14]. In view of the extremely high specific surface area and specific pore size, metal-organic frameworks (MOFs) have made significant progress in the fields of adsorption/separation, catalysis and contaminant removal [15,16]. Recently, the application of MOFs with electrochemical sensors also has been reported. For example, Guo et al. synthesized a petal-like NH2-MIL-53(Cr) to realize excellent electrochemical performance for the detection of heavy metal ions [17]. In addition, MOFs can be used as sensor materials for identifying small molecules such as glucose, hydrogen peroxide and nucleic acid molecules [15,18,19]. However, most of the MOFs suffer from poor conductivity and stability during the electrochemical applications. In this regard, the coupling of MOFs with conducting polymers is considered to be a solution to the above limitation. As a commonly studied conducting polymer, polypyrrole (PPy) is often used for metal
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Corresponding author. E-mail address: [email protected] (M. Lin). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.snb.2019.127286 Received 6 August 2019; Received in revised form 10 October 2019; Accepted 14 October 2019 Available online 17 October 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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nitrogen. 10 mg of the synthetic nanomaterials, PPy nanowires, NH2MIL-53(Al) and NH2-MIL-53(Al)/PPy nanocomposites, were dispersed in 5 mL aqueous solution by sonication for 30 min, and then, 20 μL dispersion was coated on the surface of the gold electrode, dried under an infrared lamp to form a modified electrode. DPV was employed to assess the determination performance to Pb (II) and Cu (II). The NH2-MIL-53(Al)/PPy modified the electrode was immersed in 0.1 M acetic acid/sodium acetate buffer solution (pH = 5.0) containing different concentrations of Pb (II) and Cu (II) at first. After the pre-concentration process reached equilibrium (about 20 min), the electrode was taken out, washed and transferred into a 0.1 M metal ions-free acetic acid/sodium acetate buffer solution (pH = 5.0) to reduce the enriched metal ions onto the surface of modified electrode at -0.4 V for 120 s. Subsequently, the stripping reoxidation currents of were recorded by DPV at an amplitude of 20 mV, a pulse width of 0.05 s, a pulse period of 0.2 s and quiet time of 2 s.
ions detection and removal based on its good conductivity, high stability, and excellent ion exchange performance [7,20]. Herein, we develop a new type of electrochemical sensor based on NH2-MIL-53(Al)/ PPy nanocomposite to achieve high sensitivity and selectivity detection of Pb (II) and Cu (II). The NH2-MIL-53(Al) crystals were fabricated through an in-situ electrochemical method using PPy nanowires as substrate. The combination of NH2-MIL-53(Al) and PPy nanowires provided a platform for electrochemical detection of heavy metal ions attributed to the large specific surface area of the NH2-MIL-53(Al) and the excellent conductivity of the PPy nanowires. The fabricated NH2MIL-53(Al)/PPy nanocomposite modified gold electrode exhibited good electrochemical response toward Pb (II) and Cu (II) by using differential pulse voltammetric measurements. Operating parameters such as pH, pre-concentration time and anti-interference ability and stability of the modified electrode were also studied. 2. Experimental
3. Results and discussion 2.1. Reagents and materials 3.1. Morphological and structural characterizations Aluminum plate (99.5%), 2-aminoterephthalic acid, acetic acid, sodium acetate, N,N-dimethylformamide (DMF), potassium chloride (KCl) and pyrrole were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Metal ions standard solutions were offered by Aladdin. All chemicals used were of analytical grade without further purification, and deionized water (18.25 MΩ cm, UHK/UPT ultrapure water system) was used to prepare all aqueous solutions.
The morphologies of the PPy nanowires, NH2-MIL-53(Al) and NH2MIL-53(Al)/PPy nanocomposites were investigated by SEM and TEM. The PPy nanowires with an average diameter about 40 nm are shown in Fig. 1A, and Fig. 1B displays a nanosheet structure of NH2-MIL-53(Al). Typical SEM and TEM images of the fabricated NH2-MIL-53(Al)/PPy nanocomposites are illustrated in Fig. 1C. As expected, the PPy nanowires acted as supported skeleton with the NH2-MIL-53(Al) synthesized along the nanowires during the in-situ electrochemical process. Fig. 2A shows the XRD patterns of the PPy nanowires, NH2-MIL53(Al) and NH2-MIL-53(Al)/PPy nanocomposites. A broad characteristic peak exhibits at 2θ = 21° corresponding to the amorphous behavior of the X-ray scattering of the PPy chain [23], and the XRD peaks attributing to (110), (031), (112), (250) reflection of the crystal of the NH2-MIL-53(Al) emerge at 2θ = 9.2°, 17.5°, 28.26°, and 40.64°, respectively [24]. After the combination of the NH2-MIL-53(Al) and PPy nanowires, the typical X-ray scanning patterns of the NH2-MIL-53(Al) is revealed in the blue line of Fig. 2A. The ATR-IR spectra are shown in Fig. 2B. As the spectrum of the PPy nanowires presented, the characteristic peaks at 1448 cm−1 and 1540 cm−1 are ascribed to the vibration of the pyrrole ring [25]. The peaks at 1301 cm−1, 1049 cm−1 and 792 cm−1 correspond to C–H in plane vibration, deformation vibration and swing vibration, respectively. For the NH2-MIL-53(Al), two obvious absorption peaks at 1503 cm−1 and 1402 cm−1 are attributed to the carbonyl asymmetric and symmetric vibrations, and the band around 1602 cm−1 and 1660 cm−1 can be assigned to the vibration of the benzene ring and the NeH bending vibration of NH2 group, respectively [24,26]. Due to the π-π interaction between the PPy and the NH2-MIL-53(Al) backbones, the peak at 1584 cm−1 corresponding to the vibration of the aromatic ring is presented in the spectrum of NH2MIL-53(Al)/PPy nanocomposites [27]. Both of the XRD patterns and ATR-IR spectra effectively prove the successful preparation of the NH2MIL-53(Al)/PPy composites, which are in good agreement with the SEM and TEM investigations.
2.2. Apparatus The morphological characterizations of PPy nanowires, NH2-MIL-53 (Al) and NH2-MIL-53(Al)/PPy nanocomposite were characterized using field-emission scanning electron microscopy (FE-SEM, JSM-7600 F) and transmission electron microscopy (TEM, JEM-1011). X-ray diffraction (XRD) spectra of the powder samples were collected using a D8 Advance (Bruker) X-ray diffractometer with CuKα radiation (λ = 1.5418 Å). Attenuated total reflection infrared (ATR-IR) spectra were recorded by an infrared spectrophotometry (Bruker Tensor II, Germany) with a resolution of 2 cm−1. Differential pulse voltammetry (DPV) was performed on a CHI 750E workstation (C. H. Instruments Co., Shanghai, China) in a conventional three-electrode system with modified electrode, platinum plate, saturated calomel electrode (SCE, with a saturated KCl solution) were used as working electrode, counter electrode and reference electrode, respectively. 2.3. Synthesis of NH2-MIL-53(Al)/PPy nanocomposite The PPy nanowires were prepared according to the previous literature [21]. The NH2-MIL-53(Al)/PPy nanocomposite was obtained by in-situ electrochemical synthesis approach [22], and the detailed processes are described as follows. At beginning, the PPy nanowires (0.075 g) was well-dispersed in 100 mL 10% DMF aqueous solution containing 1.50 g 2-aminoterephthalic acid and 0.75 g KCl. Subsequently, two parallel aluminum plate electrodes (1 cm × 5 cm) separated by 3.0 cm were applied a current density of 20 mA cm−1 in the above solution at 90 °C for 1 h. Then, the gray precipitate was filtered and washed by DMF to dissolve the residual 2-aminoterephthalic acid. After centrifugation, the resulting powder was dried overnight at 100 °C. The NH2-MIL-53(Al) was produced without the addition of PPy nanowires.
3.2. Electrochemical responses of Pb (II) and Cu (II) on different electrodes The electrochemical performances of three different types of modified gold electrodes were evaluated by determining of Pb (II) and Cu (II) in acetic acid/sodium acetate buffer solution (0.1 M, pH = 5.0) with a certain concentration of the metal ions. After the pre-concentration step for Pb (II) and Cu (II), the PPy, the NH2-MIL-5 3(Al) and the NH2MIL-53(Al)/PPy modified electrodes were placed in an acetic acid/sodium acetate buffer solution with an applied potential of -1.0 V for 2 min to reduce the heavy metal ions. As Fig. 3 shown, two stripping peak were observed around -0.185 V for Pb (II) and 0.20 V for Cu (II) at
2.4. Electrochemical detection of Pb (II) and Cu (II) The bare gold electrode (diameter: 4 mm) was polished with 1 μm and 50 nm alumina powder, washed with nitric acid (6 mol L−1), deionized water and ethanol, respectively, and dried in a stream of 2
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Fig. 1. Typical SEM and TEM images of PPy nanowires (A), NH2-MIL-53(Al) (B) and NH2-MIL-53(Al)/PPy nanocomposites (C), respectively.
the PPy nanowires modified electrode, because of the coordination of metal ions with the nitrogen on the pyrrole unit [28]. The current signal of the stripping peak has increased at the NH2-MIL-53(Al) modified electrode. That could be ascribed to the metal ions chelation properties of the amino and carboxylic groups from NH2-MIL-53(Al) toward Pb (II) and Cu (II). Compared with the PPy nanowires and NH2-MIL-53(Al) modified electrodes, significantly enhanced electrochemical signal was measured by the NH2-MIL-53(Al)/PPy modified electrode. The obtained current peaks are over 3-fold than those of recorded by the other two modified electrodes, which are due to the synergistic amplification effect of the NH2-MIL-53(Al) and the PPy. 3.3. Optimization of experimental conditions The pH of the solution and the pre-concentration time are the important factors in terms of sensitivity and selectivity. pH significantly affects the structural characterization of the functional materials by protonation or deprotonation reaction. To study the influence of the solution acidity during the enrichment of Pb (II) and Cu (II), various pH ranged from 3.5 to 6.5 were investigated. As displayed in Fig. 4A, the peak currents increase with the pH increased from 3.5 to 5.0. The result could be proposed that the amino groups of NH2-MIL-53 (Al) were
Fig. 3. DPV curves of the bare gold, PPy nanowires, NH2-MIL-53(Al) and NH2MIL-53(Al)/PPy nanocomposites modified electrodes in the presence of 400 μg L−1 of Pb (II) and Cu (II).
protonated at lower pH [29], and the protonated amino group (-NH3+) repelled the cations by electrostatic reaction. In contrast, the reduction of peak currents might be led to the formation of metal ion hydroxide
Fig. 2. XRD patterns (A) and ATR-IR spectra (B) of the PPy nanowires, NH2-MIL-53(Al) and NH2-MIL-53(Al)/PPy nanocomposites. 3
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Fig. 4. Optimization of experimental conditions. Effect of pH (A), and influence of pre-concentration time (B). DPV data obtained in a solution containing 400 μg L−1 of Pb (II) and Cu (II).
electrochemical performance between NH2-MIL-53(Al)/PPy modified gold electrodes and other modified electrodes for heavy metal ion detection. It can be seen that a sufficiently low detection limit and a reasonable linear range can be obtained from our proposed method. Simultaneous determination of Pb (II) and Cu (II) was conducted with the same concentration levels under the pre-concentration time of 20 min, and the results are depicted in Fig. 7A. In the case of the Pb (II) and Cu (II) coexisted, two distinct stripping peaks at -0.185 V and 0.20 V corresponding to the oxidation of Pb and Cu arise, and the oxidation peaks are well distinguished. Therefore, it is feasible that the NH2-MIL-53(Al)/PPy modified electrode can simultaneously detect the two target ions. Fig. 7B displays the fitting curves in the range of 1–400 μg L−1 for the Pb (II) and Cu (II), and the correlation coefficients are 0.9669 and 0.9876, respectively. According to the above experimental phenomenon, with the concentrations of the heavy metal ions increasing, the increased electrochemical peak currents indicating the NH2-MIL-53(Al)/PPy nanocomposites can be used as an electrochemically active material for the Pb (II) and Cu (II) determination [35].
complexes at higher pH [30,31], which could inhibit the capture of Pb (II) and Cu (II). Fig. 4B illustrates the effect of pre-concentration time on the electrochemical responses of Cu (II) and Pb (II). The re-oxidation currents curves by the pre-concentration time are continuously rising in the range of 0–20 min, while the time exceeded 20 min, the curves are only slightly increased, probably due to the saturation of the surface active sites [32]. 3.4. Analytical performance Analysis of individual Pb (II) and Cu (II) at various concentrations with the NH2-MIL-53(Al)/PPy modified gold electrodes were performed using DPV. Prior to the DPV measurements, a 20 min pre-concentration process was carried out in various concentrations of Pb (II) and Cu (II) acetic acid/sodium acetate buffer solution (0.1 M, pH 5.0). As Figs. 5a and 6 a shown, well-defined DPV peaks of Pb (0)/Pb (II) and Cu (0)/Cu (II) are clearly observed around -0.185 V and 0.20 V. Due to the enrichment of Pb (II) and Cu (II) from the bulk metal ions solution, the curves of the DPV responses recorded by the NH2-MIL-53(Al)/PPy modified gold electrode against the different concentrations of Pb (II) and Cu (II) are nonlinear (Fig. 5b and 6 b) [33]. The fitting equations are y = 0.2143x0.78/(1+(0.026x)0.78) for Pb (II) and y = 0.1009x0·99/ (1 + (0.0068x)0·99) for Cu (II) in the range of 1–400 μg L−1, and the correlation coefficients as 0.9985 and 0.9978, respectively. The calibration curves (Fig. 5b inset and 6 b inset) show nonlinear increase in peak currents at a low concentration range from 1 to 20 μg L−1, and the detection limit of Pb (II) and Cu (II) are calculated as 0.315 μg L−1 and 0.244 μg L−1 by Pividori and co-workers reported method [33]. On account of the synergistic effect of different active materials, the detection ability to Pb (II) and Cu (II) are greatly promoted, which is resulted in the limit of detection calculated by the use of the NH2-MIL53(Al)/PPy nanocomposites were much lower than the other reports using various electrode materials. Table 1 shows a comparison of
3.5. Interference study In order to evaluate the selectivity of the proposed modified electrode, the test by adding 400 μg L−1 of interfering cations (such as Cd (II), Hg (II), Ca (II), Ni (II) and Zn (II)) with the coexisting of the same concentration of Pb (II) and Cu (II) was implemented. As shown in Fig. 8, most of these metal cations do not interfere during the detection of Pb (II) and Cu (II) from the perspective of the DPV response. The result confirms that the NH2-MIL-53(Al)/PPy nanocomposite modified electrode cannot be affected by other metal species. The selective behavior of the prepared nanocomposite could be attributed to the high affinity of amino and carboxyl groups, and the adsorption capacity of the chelating groups decreases in the sequence of Cu ≫ Pb ≫ Cd ≫ Zn
Fig. 5. DPV response (A) and calibration curve (B) toward Pb (II) in the concentration range of 1–400 μg L−1. 4
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Fig. 6. DPV stripping signals (A) and DPV peak current (B) for the NH2-MIL-53(Al)/PPy nanocomposites modified electrode under various the concentration of Cu (II). Table 1 Comparison of different materials to Pb (II) and Cu (II) determination. Electrode
Analytical methods
NH2-MIL-53(Cr) NH2-MIL-88(Fe)-rGO
SWASV DPASV
Ti3C2
SWASV
MIL-100(Cr)
SWASV
NH2-MIL-53(Al)/PPy
DPV
Detection range (μg L−1)
Analytes
4
Pb(II) Pb(II) Cu(II) Pb(II) Cu(II) Pb(II) Cu(II) Pb(II) Cu(II)
80-1.6 × 10 2-62 0.32-3.2 20-310 6.4-96 20-2.07 × 103 6.4-640 1-400 1-400
Detection limit (μg L−1)
Refs
6.3 0.8 0.32 8.5 2.05 10 0.7 0.315 0.244
17 29 30 34 This work
Fig. 7. Simultaneous detection of Pb (II) and Cu (II) in a concentration range of 1–400 μg L−1 (A) the NH2-MIL-53(Al)/PPy nanocomposites modified electrode, and the corresponding calibration curves for Pb (II) and Cu (II), respectively (B).
Fig. 8. DPV curve on the NH2-MIL-53(Al)/PPy modified gold electrode after a pre-concentration process in a solution containing 400 μg L−1 of Cd (II), Hg (II), Ca (II), Ni (II), Zn (II), Pb (II) and Cu (II) (A), and interference investigation of Pb (II) and Cu (II) in the presence of other common cations (B).
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Acknowledgments
Table 2 Detection results of the spiked Pb (II) and Cu (II) in tap water samples. Analytes
Spiked (μg L−1)
Found (μg L−1, n = 3)
Recovery (%)
Pb (II)
0 50 100 0 50 100
– 48.4 ± 0.7 101.2 ± 0.4 – 49.6 ± 0.2 100.7 ± 0.8
– 96.8 101.2 – 99.2 100.7
Cu (II)
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≫ Ni, Hg [36,37]. Therefore, we can conclude that the fabricated material has good selectivity for Pb (II) and Cu (II). 3.6. Reproducibility and stability Reproducibility and stability are of great important performance indicators for measuring the prepared electrochemical sensors. Five different electrodes were prepared in parallel to assess the reproducibility of the NH2-MIL-53(Al)/PPy coated gold electrode. The measurements revealed that the relative standard deviation of the peak current as 3.52% demonstrating the nanomaterial modified electrode has good reproducibility. In addition, the stability of the fabricated electrode was also investigated. The NH2-MIL-53(Al)/PPy nanocomposite modified electrode was immersed in 0.1 M acetic acid/sodium acetate buffer solution (pH 5.0) for 2 weeks, and the peak current of the same concentration of metal ions was again determined to be about 90% of the initial value. These results prove that the NH2-MIL-53(Al)/PPy nanocomposite modified electrodes exhibit good reproducibility and stability. 3.7. Tested of Pb (II) and Cu (II) in real samples The practical performance of the NH2-MIL-53(Al)/PPy nanocomposite modified electrode was investigated by the determination of Pb (II) and Cu (II) in tap water samples. The recovery results obtained by incorporating a certain concentration of heavy metal ion solution in water samples are given in Table 2. It can be seen that the nanocomposite modified electrode shows good recovery indicating the constructed sensor can be used for the determination of heavy metal ions in environmental samples. 4. Conclusion In this study, a nanocomposite of metal-organic framework (NH2MIL-53(Al)) supported by PPy nanowires was prepared by a simple insitu electrochemical deposition method. The morphological and structural characterizations of the synthesized nanocomposites were carried out by SEM, TEM, XRD and ATR-IR techniques. The NH2-MIL-53(Al)/ PPy nanocomposite was used as a sensing material for individual and simultaneous determination of Pb (II) and Cu (II). In comparison with the PPy nanowires and the NH2-MIL-53(Al) modified electrodes, the performance of the NH2-MIL-53(Al)/PPy nanocomposite modified electrode proved that the developed sensor is highly sensitive to detect Pb (II) and Cu (II) in the range of 1–400 μg L−1. The NH2-MIL-53(Al)/ PPy nanocomposite modified electrodes also demonstrate strong antiinterference ability and good stability. Based on the synergistic effect of PPy and NH2-MIL-53(Al), the effective electrochemical sensor provides a new strategy for detection of the heavy metal ions. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 6
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Nan Wang is a PhD candidate in School of Chemistry and Chemical Engineering, Shandong University. Her research focuses on electrochemical sensors. Wei Zhao is an undergraduate student in School of Chemistry and Chemical Engineering, Shandong University. Ziyang Shen is an undergraduate student in School of Chemistry and Chemical Engineering, Shandong University. Shengjun Sun is a lecturer in School of Stomatology, Shandong University. His main research interest focuses on the biological properties of nanomaterials. Hongxiu Dai is a lecturer in Department of Chemistry, Liaocheng University. Her research focuses on electrochemical sensors. Houyi Ma is a professor in School of Chemistry and Chemical Engineering, Shandong University. His current research interests include electrochemical corrosion and electrocatalysis. Meng Lin is an associate professor in School of Chemistry and Chemical Engineering, Shandong University. His main research interests focus on electrochemical sensors and preparation of novel nanomaterials.
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