Mesoporous MgO nanosheets: 1,6-hexanediamin-assisted synthesis and their applications on electrochemical detection of toxic metal ions

Journal of Physics and Chemistry of Solids 74 (2013) 1032–1038

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Mesoporous MgO nanosheets: 1,6-hexanediamin-assisted synthesis and their applications on electrochemical detection of toxic metal ions Zhengcui Wu a,b,n, Chengrong Xu a,b, Huamao Chen a,b, Yaqin Wu a,b, Hao Yu a,b, Yin Ye a,b, Feng Gao a,b,n a b

Anhui Key Laboratory of Molecule-Based Materials, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, People's Republic of China Anhui Key Laboratory of Functional Molecular Solids, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, People's Republic of China

art ic l e i nf o

a b s t r a c t

Article history: Received 18 October 2012 Received in revised form 15 February 2013 Accepted 26 February 2013 Available online 15 March 2013

The mesoporous MgO nanosheets with uniformly distributed mesoporosity and high specific surface area of 102.8 m2/g were simply synthesized on a large scale by calcination of hexagonal Mg(OH)2 nanosheet precursor, which was prepared using 1,6-hexanediamin-assisted solution approach. The as-prepared mesoporous MgO nanosheets were used to construct a cheap, easy and environmentally-friendly electrochemical sensor on glassy carbon electrode for the simultaneous and selective electrochemical determination of four toxic metal ions of Hg(II), Cu(II), Pb(II) and Cd(II) in an aqueous solution, which exhibits high sensitivity and selectivity. The DPV responses of the sensor toward separate measurements of Hg(II), Cu(II), Pb(II) and Cd(II) at different concentrations show the linear detection range was 0.005– 1.71, 0.01–2.13, 0.01–2 and 0.01–0.21 μM. The simultaneous and selective determination of these species in the quaternary mixtures presents the linear responses in the range of 0.005–1.71, 0.01–1.92, 0.01–1.76 and 0.01–0.2 μM. The favorable performance makes this sensor extremely attractive for onsite environmental monitoring of heavy metal ions. & 2013 Elsevier Ltd. All rights reserved.

Keywords: A. Nanostructures A. Oxides B. Chemical synthesis D. Electrochemical properties

1. Introduction The detection and measurement of heavy metal ions present in aquatic streams are of great importance in assessing environmental damage and possible strategies for remediation. Simple and efficient ways of detection of heavy metal ions are of continuing interest. There have been numerous reports on optical detection of heavy metal ions by using heavy metal ions-sensitive fluorophores or chromophores. However, most of these methods rely on optical techniques, such as colorimetry [1–3], fluorescence [4–6] and fluorescence polarization [7]. Compared with an optical instrument, electrochemical devices are relatively cost-effective and miniaturizable, so they are very attractive for trace analysis of heavy metal ions with high sensitivity, selectivity, short analysis time and low power consumption, making electrochemical method being emerged as a preferable alternative [8–17]. However, most of the recent research papers on electrochemical sensors are focused on the detection of individual heavy metal ions. Considering many toxic metal ions are often coexisted in the real samples, there are increasing demands for adequately sensitive and selective analytical techniques with multielement

n Corresponding authors at: Anhui Key Laboratory of Molecule-Based Materials, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, People's Republic of China. Tel./fax: þ86 553 3869303. E-mail addresses: [email protected] (Z. Wu), [email protected] (F. Gao).

0022-3697/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2013.02.029

capabilities. Although it has made some achievements on the simultaneous and selective electrochemical detection of several toxic metal ions in low concentrations, a large part of the reports are focused on using carbon nanotubes, complex or organic molecule as the electrode's modified materials. For example, some recent research papers reported the simultaneous and selective electrochemical detection of toxic metal ions using carbon nanotubes-based composite film electrode, such as a multiwalled carbon nanotubes-sodium dodecyl benzene sulfonate (MWCNTsNaDBS) modified stannum film electrode for the determination of Cd2 þ and Zn2 þ [9], carbon nanotubes/poly (1,2-diaminobenzene) nanoporous composite film electrode for simultaneous determination of trace amounts of Cd2 þ and Cu2 þ [10], a 2 2′-azinobis (3-ethylbenzothiazoline-6-sulfonate) diammonium salt (ABTS)multiwalled carbon nanotubes (MWCNTs) nanocomposite/Bi film modified glassy carbon electrode for determination of trace Pb2 þ and Cd2 þ [11], the bismuth modified carbon nanotubes (CNTs)poly (sodium 4-styrenesulfonate) composite film electrode (CNTsPSS/Bi) for Pb2 þ and Cd2 þ [12], a bimetallic Hg–Bi/single-walled carbon nanotubes composite electrode for the simultaneous detection of Zn2 þ , Cd2 þ and Pb2 þ [13]. Other research papers include the detection of the heavy metal ions of Pb2 þ , Cd2 þ , Cu2 þ and Zn2 þ at highly aligned multi-wall carbon nanotube tower electrodes [14], a nafion-modified glassy carbon electrode and CuDPABA complex (DPABA is methyl 3, 5-bis{bis-[(pyridin-2-yl) methyl] amino}methyl-benzoate) for the determination of Pb2 þ and Cd2 þ [15], a bismuth/poly(p-aminobenzene sulfonic acid) film

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electrode for the simultaneous measurement of Pb2 þ , Cd2 þ and Zn2 þ [16]. Only a few examples of metal oxide micro-/nanostructures for simultaneous detection of toxic metal ions have been reported, such as γ-AlOOH(boehmite)@SiO2/Fe3O4 porous magnetic microspheres for detection of ultratrace Zn2 þ , Cd2 þ , Pb2 þ , Cu2 þ and Hg2 þ in drinking water [8], porous magnesium oxide flowers for the detection of Pb2 þ and Cd2 þ [17]. Considering their potentially good electrochemical properties with easily controlled sizes and morphologies at low cost, it is of great significance to develop electrochemical sensors based on metal oxide micro-/nanostructures for the simultaneous and selective detection of toxic metal ions in low concentrations. Magnesium oxide, with a nontoxic and environmentally friendly nature, represents an important class of functional metal oxides, has been adopted as absorbents to remove toxic ions and organic pollutants from water [18–20], and the high adsorption abilities toward metal ions made it have good electrochemical responses [17]. The mesoporous MgO nanostructure developing larger surface area enables larger adsorption capacity on metal ions, which could improve the corresponding sensing performance and extend the electrochemical determination species of metal ions. Generally, MgO can be obtained by the high temperature thermal evaporation method [21] or via calcining its various precursors [17–20,22–27], the latter provides the opportunity to obtain the porosity in its micro-/nanostructures. It has been shown that the electrochemical performance of nanocrystals highly depends on its microstructure, surface area, and the presence of dopant, suggesting it is paramountly important to tailor MgO micro-/nanostructures with the desired features, e.g., high electronic conductivity, low diffusion resistance to protons/cations, and high electroactive area. In the developed methods for generating inorganic semiconductor nanostructures, the usage of different kinds of ligands for acquiring various nanostructured units and their assembly has been generally employed. For example, short-chain liquid watersoluble alkylamines such as ethylenediamine, n-butylamine and diethylenetriamine have often been used as the solvent and structure-directing coordination molecular template [28], and long-chain solid water-insoluble alkylamines such as hexadecylamine, octadecylamine and dodecylamine, are also reported as coordinate agent, surfactant or the organic template [29]. Moreover, presently, most of the used water-soluble amine compounds are liquid; the usage of abundant liquid amine will limit the extension of the method due to the indissolubility of some reactants and its irritative odor. The development of simple and effective methods for creating novel inorganic semiconductor micro-/nanoarchitectures using a suitable water-soluble solid amine compound is important to technology and remains an attractive, but elusive goal. Encouragingly, we have successfully introduced a water-soluble solid macrocycle polyamine-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene (CT) for the morphogenesis of complex inorganic semiconductor architectures [30]. The successes led us to further expand other water-soluble solid amines to direct the growth of inorganic crystals with novel morphologies and architectures. Recently, we demonstrate new examples of constructing CuO and NiO micro-/nanostructures by a simple, cost-effective low-melting-point water-soluble solid amine of 1,6-hexanediamin [31], opening a new route for the construction of inorganic semiconductor micro-/nanomaterials with a new kind of water-soluble solid amine. In this paper, we introduced 1,6-hexanediamin to direct the growth of hexagonal Mg(OH)2 nanosheet precursor, which was further calcined to obtain mesoporous MgO nanosheets. The as-prepared mesoporous MgO nanosheets were used to construct an electrochemical sensor on glassy carbon electrode for the simultaneous and selective detection of toxic metal ions of Hg2 þ , Cu2 þ , Pb2 þ and Cd2 þ in an aqueous solution, which exhibits high sensitivity and selectivity.

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2. Experimental 2.1. Chemicals and materials All of the chemical reagents were of analytical grade and used as received without any further purification. In a typical synthesis, 1 mmol of MgCl2  6H2O was dissolved in 40 mL of distilled water under constant stirring, followed by the addition of 1 mL of 1,6hexanediamine which was preliminarily melted at 50 1C. Then the solution was loaded into a 50 mL teflon-lined stainless steel autoclave and maintained at 150 1C for 5 h, and allowed to cool to room temperature naturally. The white precipitate was collected by centrifugation and washed with distilled water and ethanol for several times, then dried in a vacuum at 50 1C for 12 h. Afterward, the product was placed in a crucible and carefully heated up from room temperature to 500 1C at the rate of 5 1C/min and then maintained at 500 1C for 1 h in air. Finally, the product was collected. To study the electrochemical property of the product, stock solution of Hg(II), Cu(II), Pb(II) and Cd(II), was prepared by directly dissolving HgCl2, CuCl2, PbCl2 and CdCl2, in doubly distilled water. Acetate buffer solutions (0.10 M) with various pH values were prepared with NaAc and HAc. 2.2. Apparatus X-ray powder diffraction (XRD) patterns of the products were recorded on a Shimadzu XRD-6000 X-ray diffractometer at a scanning rate of 0.05 s−1 with a 2θ range from 10 to 801, with high-intensity Cu Kα radiation (λ¼ 0.154060 nm). Field emission scanning electron microscope (FESEM) images were obtained on a Hitachi S-4800 fieldemission scanning electron microscope operated at an accelerating voltage of 5.0 kV. Transmission electron microscopy (TEM) analysis was used JEOL 2010 with an accelerating voltage of 200 kV. Brunauer–Emmett–Teller (BET) nitrogen adsorption–desorption was measured using a Micromeritics ASAP 2020 accelerated surface area and porosimetry system. Thermal gravimetric analysis (TGA) of the assynthesized sample was carried out on a Shimadzu DTG-60A thermal analyzer at a heating rate of 5 1C/min from room temperature to 500 1C in air. Electrochemical experiments were performed by CHI660B electrochemical analyzer (ChenHua Instruments Co. Ltd., Shanghai, China) with a conventional three-electrode system in acetate buffer solutions. The working electrode was as-prepared mesoporous MgO nanosheets–nafion modified glassy carbon electrode (MgO–Nafion/GCE). A saturated calomel electrode and a platinum wire electrode were used as the reference and the auxiliary electrode, respectively. 2.3. Electrode fabrication MgO–Nafion/GCE was prepared as following: 5% of the nafion solution was diluted to 0.5% by N,N-dimethylformamide. Ten miligram of the mesoporous MgO nanosheets was dispersed in above solution by ultrasonic treatment for 30 min. Next, 5 µL of the suspension was dropped onto the surface of cleaned GCE and dried in air. The other modified electrodes were similarly prepared as above. Prior to each electrochemical experiment, solutions were purged with purified nitrogen for 15 min to remove oxygen, and maintained under a nitrogen atmosphere during the course of experiments.

3. Results and discussion 3.1. Characterizations of mesoporous MgO nanosheets The morphology and size of as-prepared precursor were first characterized with field-emission scanning electron microscopes

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Fig. 1. FESEM and TEM images of as-prepared Mg(OH)2 nanosheets and mesoporous MgO nanosheets. (a) and (b) FESEM and TEM images of the Mg(OH)2 nanosheets. (c and d) FESEM and TEM images of the mesoporous MgO nanosheets.

(FESEM). Fig. 1a displays a panoramic FESEM image of the precursor prepared by 1,6-hexanediamin assisted solution approach, which shows the sample is large-scale hexagonal nanosheets with average length of 100–120 nm. Fig. 1a not only shows that the product consists entirely of such hexagonal nanosheet structure but also gives the information that the hexagonal nanosheet structure of high yield and good uniformity can be easily achieved with this simple and easily controlled approach. The corresponding TEM image in Fig. 1b further clearly identified the hexagonal nanosheet structure of the sample. The X-ray diffraction (XRD) pattern in Fig. 2a shows the diffraction peaks of the hexagonal phase of Mg(OH)2 (JCPDS card, no. 84-2163) with lattice parameters of a¼3.154 Å and c¼4.764 Å, suggesting high crystallinity and purity of the product. A simple calcination process has been applied to obtain the porosity on the nanosheets from the corresponding Mg(OH)2 precursor by the release of small molecules at elevated temperature. As expected, mesoporous MgO nanosheets have been prepared via calcining Mg(OH)2 nanosheets precursor. Fig. 2b shows the XRD pattern of the as-obtained product after calcination and the diffraction peaks are in good agreement with the standard XRD pattern of the pure cubic phase of MgO (JCPDS card, no. 78-0430) with lattice parameter of a ¼4.220 Å, indicating that the pure phase of MgO can be obtained by calcining Mg(OH)2 precursor. FESEM and TEM images of the calcination product are shown in Fig. 1c and d, respectively. The FESEM image in Fig. 1c indicates that the as-obtained MgO sample basically remains the sheetlike morphology with a smaller size about average length of 90–115 nm, indicating a little shrinkage of the product after calcination. As seen from the TEM image in Fig. 1d, the uniform distributed mesoporous nanostructures on the nanosheets were

Fig. 2. XRD patterns of (a) the Mg(OH)2 nanosheets and (b) the mesoporous MgO nanosheets.

clearly presented and the diameter of the mesoporosity was in the range of 3.5–8.2 nm. Such MgO nanosheets possess mesoporous structures also evidenced by the nitrogen sorption experiment (Fig. 3). The isotherm of the sample can be categorized as type IV, with a distinct hysteresis loop observed in the range of 0.4–1.0 P/P0. The measurement shows that the Brunauer–Emmett–Teller (BET) surface area is 102.8 m2/g, which is higher than that of MgO flowers (32.97 m2/g) [17], nanocubes (33 m2/g) [26], and nanoplates

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Fig. 3. Nitrogen adsorption–desorption isotherm of the mesoporous MgO nanosheets sample. The inset is its BJH pore-size distribution curve.

Fig. 4. TGA curve of the as-prepared Mg(OH)2 nanosheet precursor.

(92 m2/g) [32], and there are bimodel pore size distribution, one at 3.7 nm, the other at 53.5 nm (Fig. 3, inset), which are due to the mesoporosity in MgO nanosheets and the irregular packing of nanosheets during the measurement process. Such porous structure provides efficient transport pathways, leading to high accessibility and adsorptivity of toxic metal ions, facilitated electron transfer process and enhanced the sensing functionality in terms of the sensitivity. Thermogravimetric analysis (TGA) was carried out in air to analyze the dehydration process of the Mg(OH)2 precursor. The TGA curve of the sample shows a weight loss of 28.6% related to the procedure of dehydration (Fig. 4), that is believed to correspond to the release of water in the crystals, close to the theoretical calculations (31.0%) according to the stoichiometry. Due to the gradual removal of water by calcination of Mg(OH)2 precursor, it generates mesoporous structure on the nanosheets, forming MgO mesoporous nanosheets. Thanks to their mesoporous nanostructures, the electrochemical sensing performance was significantly improved, compared with that of Mg(OH)2 precursor. 3.2. Electrochemical impedance spectra and cyclic voltammograms of the different electrodes The fabricated MgO–Nafion/GCE was firstly electrochemically characterized using electrochemical impedance spectra (EIS) and cyclic voltammograms. In electrochemical impedance spectroscopy, it includes a semicircle portion at the high frequencies

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Fig. 5. Electrochemical impedance spectra of the naked GCE, Mg(OH)2–Nafion/GCE and MgO–Nafion/GCE in the solution of 5.0 mM Fe(CN)3−/4− and 0.2 M KCl. 6

Fig. 6. Electrochemical cyclic voltammograms of the naked GCE, MgO–Nafion/GCE and Mg(OH)2–Nafion/GCE in the solution of 5.0 mM Fe(CN)3−/4− and 0.2 M KCl. The 6 scan rate is 100 mV s−1.

corresponding to the electron transfer limited process and a linear part at the low frequencies resulting from the diffusion limited electrochemical process. The semicircle diameter of EIS equals the electron transfer resistance (Ret). The increase or decrease in its value exactly characterized the modification of the electrode surface. Fig. 5 exhibits the impedance spectroscopies of different electrodes. After the Mg(OH)2 nanosheets or the mesoporous MgO nanosheets and nafion were modified onto the GCE (curve a and b), the semicircle diameter of EIS, Ret, increased comparing with the naked glassy carbon electrode (curve c), indicating that the Mg(OH)2 and MgO coatings provide larger ohmic drop with respect to the naked glassy carbon electrode. At the same time, the Ret of the Mg (OH)2 nanosheets is larger than that of the mesoporous MgO nanosheets, which meant that the electron transfer ability of the mesoporous MgO nanosheets is stronger than that of the Mg(OH)2 nanosheets. The modification using different materials could be 4− further confirmed using cyclic voltammograms of Fe(CN)3− 6 / (Fig. 6). It is clear that the larger in the cathodic–anodic peak separation and the smaller peak current in the amperometric response waves of the redox probe compared to that of naked GCE after modification using MgO–Nafion and Mg(OH)2–Nafion, account4− ing for the electron-transfer kinetics of Fe(CN)3− 6 / , are gradually obstructed, which is probably related to the higher ohmic drop and smaller surface area available, respectively, in the electrode process. The results were consisted with the EIS of different modified electrodes.

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3.3. Individual determination of Hg(II), Cu(II), Pb(II) and Cd(II) at MgO–Nafion/GCE The experimental factors, including electrode conditioning, supporting electrolyte and pH value, were optimized and the results were shown in Fig. S1. The optimized experimental conditions used for the analytical determination of Cu(II) were next extended to detect Cu(II), Hg(II), Pb(II) and Cd(II). Fig. 7 shows the DPV responses of MgO–Nafion/GCE toward separate measurements of Hg(II), Cu(II), Pb(II) and Cd(II) at different concentrations. Individually, Hg(II), Cu(II), Pb(II) and Cd(II) are detected at potentials of 0.151, −0.021, −0.556 and −0.776 V with a series of voltammograms at concentration of 0.005–1.71, 0.01–2.13, 0.01–2 and 0.01–0.21 μM. And the sensitivities of the modified electrode were 15.29 μA/μM for Hg(II), 20.42 μA/μM for Cu(II), 5.29 μA/μM for Pb(II) and 50.76 μA/μM for Cd(II), respectively. Moreover, the peak currents exhibit excellent linear dependence on the concentration of the corresponding metal ions (insets in Fig. 7). The linear response of metal ions was wider than or compares favorably with that of corresponding metal ions at other reported electrodes [8,9,17]. It is clear that the analytical performances of the asprepared mesoporous MgO nanosheets in this study are better or comparable to those with some other materials. The high sensitivity of MgO–Nafion/GCE can be explained by the porous behavior of the nanostructured platform. It is well known that Mg(OH)2 and MgO have been adopted as absorbents to remove toxic ions and organic pollutants from water [18–20,33] due to their favorable electrostatic attraction mechanism [33], and the high adsorption abilities toward metal ions made them have good electrochemical responses. The MgO nanosheets with

mesoporous structure developing larger surface area enable larger adsorption capacity and more reactive-sites for metal ions, improving the corresponding sensing performance [17], which is similar with our recent report about β-Ni(OH)2 nanosheets and mesoporous NiO nanosheets for the detection of mercury (II) [31]b.

3.4. Simultaneous determination of Hg(II), Cu(II), Pb(II) and Cd(II) at MgO–Nafion/GCE Thanks to the enough large separation between peak potentials of Hg(II), Cu(II), Pb(II) and Cd(II) with 130–535 mV (Fig. 7), the simultaneous and selective determination of these species in the quaternary mixtures is feasible. Simultaneous determination of these metal ions was performed according to the same parameters determined for the individual experiments. As shown in Fig. 8, the modified electrode shows clearly separated individual peaks at 0.189, −0.0389, −0.551 and −0.747 V for Hg(II), Cu(II), Pb(II) and Cd (II), respectively, in their coexistence, and the peak currents of these four metal ions exhibit excellent linear responses with the corresponding metal ion concentration in the range of 0.005–1.71, 0.01–1.92, 0.01–1.76 and 0.01–0.2 μM, respectively (Fig. S2). However, a slight shift (∼5–38 mV) in the peak corresponding to the individual metal ion was observed when simultaneously increased the concentration of Hg(II), Cu(II), Pb(II) and Cd(II). Encouragingly, the sensitivities of the electrode remained roughly the same as compared to the individual determination at the concentration tested in this investigation, implying that simultaneous measurement of above four metal ions is feasible.

Fig. 7. (a)–(d) Individual DPV responses of MgO–Nafion/GCE toward Hg(II), Cu(II), Pb(II), and Cd(II) at different concentrations in 0.1 M NaAc–HAc of pH 5.0. The insets are the corresponding plots of current as a function of the concentration of Hg(II), Cu(II), Pb(II) and Cd(II), respectively, with linear trendlines.

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Fig. 8. DPV responses of MgO–Nafion/GCE for the simultaneous detection of Hg(II), Cu(II), Pb(II), and Cd(II) in 0.1 M NaAc–HAc of pH 5.0. (Concentrations: Hg(II): 0.005–1.71 μM, Cu(II): 0.01–1.92 μM, Pb(II): 0.01–1.76 μM, and Cd(II): 0.01–0.2 μM.)

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by calcination of Mg(OH)2 nanosheet precursor prepared using 1,6-hexanediamin-assisted solution approach and developed a cheap, easy and environmentally-friendly MgO–Nafion/GCE for the simultaneous and selective electrochemical determination of Hg(II), Cu(II), Pb(II) and Cd(II) in an aqueous solution. The voltammetric peak of Hg(II), Cu(II), Pb(II) and Cd(II) toward separate measurements at MgO–Nafion/GCE appears at different potentials with a separation of 130–535 mV between peak potentials, providing enough large separation for the simultaneous and selective determination of these species in the quaternary mixtures. The simultaneous and selective determination of Hg(II), Cu(II), Pb(II) and Cd(II) presents the linear responses in the range of 0.005–1.71, 0.01–1.92, 0.01–1.76 and 0.01–0.2 μM, in their coexistence. The results demonstrated MgO–Nafion/GCE exhibited the prominent activities for determination of Hg(II), Cu(II), Pb(II) and Cd(II), which could be used as a desirable alternative amperometric sensor for the simultaneous monitoring of low concentrations of toxic metal ions in real water samples.

Acknowledgments This work was financially supported by the Natural Science Foundation of China (21175002 and 21201007), the Natural Science Foundation for Distinguished Youth of Anhui Province (1108085J09), the Natural Science Foundation of Anhui Province (11040606M32), the Natural Science Key Foundation of Anhui Province Education Administration (KJ2010A144), and the Innovation Foundation of Anhui Normal University (2011cxjj10 and 2011cxjj11).

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jpcs.2013.02.029. Fig. 9. DPV responses of MgO–Nafion/GCE at different concentrations of Cu(II) from 0.06 μM to 0.96 μM in 0.1 M NaAc–HAc of pH 5.0 while keeping the concentrations of Cd(II), Pb(II) and Hg(II) constant at 0.16 μM, 1.6 μM and 0.48 μM, respectively.

3.5. Selectivity of the electrode Selective detection is always a challenging work in sensing field, as the other metal ions commonly present in real sample under the experimental conditions used for the individual detection of metal ions. Fig. 9 shows the DPV responses obtained at MgO–Nafion/GCE in the different concentrations of Cu(II) while keeping the concentrations of Cd(II), Pb(II) and Hg(II) constant at 0.16 μM, 1.6 μM and 0.48 μM, respectively. The peak current of Cu(II) is positively proportional to its concentration from 0.06 to 0.96 μM while the responses of the other three coexisting ions are practically unaltered. The sensitivity response of the electrode toward Cu(II) was found to be 19.78 μA/μM, which is slightly smaller (reduced by 3.1%) than the value of individual detection. The decrease in sensitivity might be attributed to competition between analytes and interferent metal ions for the active deposition sites of MgO–Nafion/GCE, or might be attributed to that the reduction potential of Hg(II) is in close proximity to that of Cu(II), which is about 150 mV of the peak separation. It is worth noting that 3.1% slight drop of the sensitivity toward Cu(II) is very reasonable and tolerable.

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