Synthesis and Voltammetric Determination of Pb(II) Using a ZIF-8-Based Electrode

University of Sciences, Hue University, Hue 530000, Vietnam Faculty of Physics-Chemistry-Biology, Quang Nam University, Tam Ky 560000, Vietnam Faculty of Natural Science, Phu Yen University, Phu Yen 620000, Vietnam University of Education, Hue University, Hue 530000, Vietnam Faculty of Natural Sciences, ,u Dau Mot University, ,u Dau Mot 820000, Vietnam Department of Chemistry, Quy Nhon University, Quy Nhon 590000, Vietnam

Heavy metals (Hg(II), Pb(II), Cd(II), Ni(II), and so on) are considered to be one of the main sources of pollution in the environment.Lead is one of the most dangerous environmental pollutants as it has toxic chemical influence even at very small concentration [14,15].So it is urgently needed to find a very sensitive method for the determination of lead trace in the environment.Several analytical techniques, such as spectroscopic methods, especially graphite furnace atomic adsorption spectroscopy (GF-AAS) [16], inductively coupled plasma mass spectroscopy (ICP-MS) [17], and X-ray fluorescence [18], are employed currently for trace analysis of lead.
ese methods have excellent sensitivities and good selectivity; but several disadvantages such as time-consuming and high cost of instrument limit their applications.
Electrochemical methods including anodic stripping voltammetric techniques (ASV) have been recognized as robust tools for trace analysis because of different advantages such as faster analysis, higher selectivity and sensitivity, low cost, easy operation, and possibility to perform analysis in situ [19,20].Differential pulse anodic stripping voltammetry (DP-ASV), as one kind of the ASV method, has been applied for the determination of trace heavy metal ions because of its remarkably high sensitivity.Glassy carbon electrodes chemically modified with porous materials such as modified mesoporous materials [21], clay-mesoporous silica composite [22], multiwall carbon nanotubes [14], reduced graphene oxide/chitosan [23], pretreated graphite pencil [24], graphene films [25], and polyaniline and polypyrrole film [26] have received considerable attention for ASV because they exhibit significant improvements in terms of fast response, high selectivity, low detection limit, and renewability.
Due to high surface areas, highly order structure, and high reactive site density, the usage of MOFs-based composite for the electrochemical field is becoming increasing.Mao et al. [27] reported the usage of copper (II)-2,2′bipyridine-benzene-1,3,5-tricarboxylate (Cu(II) based MOF-199) as selective electrocatalysts for the reduction of O 2 and CO 2 .MOF modified by Au-SH-SiO2 nanoparticles was utilized to determinate hydrazine and L-cysteine by electrochemistry analysis [28].
e amino-functionalized MOF-199 material could provide an excellent modifier for developing a sensitive electrode for the determination of lead trace [29].Ag-loaded ZIF-8 nanocrystals were used as an electrode modifier to detect hydrazine.Ag/ZIF-8/CPE (carbon paste electrode) performed a good catalytic performance toward hydrazine oxidation [30].Xiao et al. [20] reported the nitrogen-doped microporous carbon prepared from pyrolysis of ZIF-8 as an excellent electrode modifier to determine Pb(II) and Cd(II) in aqueous solution.
In the previous paper [31], we demonstrated the using of ZIF-8-modified electrode to determine Pb(II) in aqueous solution.In the present work, we continue to refine the study on the use of ZIF-8 as an electrode modifier for the determination of Pb(II) by the voltammetric technique.e stability of ZIF-8 over time in several solvents and in different pH solutions was studied.ZIF-8 has been used to modify the glassy carbon electrode (GCE) for Pb(II) determination using DP-ASV methods.Several parameters related to the electrode performance were studied and optimized for using in the analysis.To our knowledge, this is the first report on using modified electrode-based ZIF-8 material for the determination of lead by the DP-ASV.O, Sigma-Aldrich, USA) was used to prepare the bismuth film.Nafion solution was prepared from stock solution (5%, d � 0.874 g•mL −1 , Aldrich, USA) and ethanol (96%, Merck, Germany) with a volume ratio V nafion /V ethanol : 1/4.Britton-Robinson buffer solution (B-R BS) of pH � 4.7 was prepared from 0.04 M H 3 BO 3 , 0.04 M H 3 PO 4 , and 0.04 M CH 3 COOH (Merck, Germany).pH of the B-R BS was adjusted by the addition of 1.0 M sodium hydroxide solutions into the B-R BS and then checked by the pH meter.e working solution of Pb(II) was prepared daily from stock solution (1000 ppm, Merck, Germany).

Synthesis of ZIF-8
. ZIF-8 was synthesized according to [32][33][34].Briefly, zinc nitrate (2.8 mmol) was dissolved in methanol (1.4 mL).2-Methylimidazole (64.4 mmol) was dissolved in methanol (1.4 mL), and this solution was added to the zinc solution under sonication condition for 30 minutes and then vigorously stirred for 24 h at room temperature.Finally, this solution was centrifuged at 300 rpm and washed thoroughly with methanol.is washing procedure was repeated 5 times.
e resulting crystals were dried overnight at 120 °C.

Preparation of Working Electrode-Modified Electrodes.
e preparation procedure was carried out as in [28].In brief, glassy carbon electrode (GCE) was first polished with 0.05 μm Al 2 O 3 slurry on a polishing cloth and then rinsed ultrasonically with 2 M HNO 3 , absolute ethanol, and doubledistilled water.Subsequently, the GCE was electrochemically cleaned by a cyclic potential scan between −1.1 V and +0.3 V in 0.5 M acetate buffer solution pH 4.5 with the scan rate of 0.10 V•s −1 .After that, the electrode was rinsed with doubledistilled water.e nafion/ZIF-8 composite was prepared by dispersing 2.5 mg ZIF-8 into 1.0 mL nafion (0.1 wt.%) and sonicated.e GCE was coated with 5 μL of the nafion/ZIF-8 composite and dried under the room temperature (∼25 °C) (denoted as Naf/ZIF-8/GCE).For the same procedure, the GCE modified by nafion was denoted as Naf/GCE.Bismuth film (BiF) was formed simultaneously with Pb(II) preconcentration.e bismuth deposition was conducted at −1.2 V potential under stirring for 120 s.

Voltammetric
Procedure.Solution under study (final volume of 10 mL) containing 0.04 M B-R BS (pH 4.7) and 200 ppb Pb(II) was transferred into the electrochemical cell with the 3 electrodes.en, voltammetric measurement was conducted as follows: (i) For surveying voltammetric characteristics of Pb(II) on the modified GCE: Pb(II) was accumulated on the surface of the modified electrode at a potential of −1.2 V (E acc ) for an accumulated time of 120 s (t acc ).During this step, the electrode was rotated at a constant rate of 1000 rpm.After that, the electrode rotation was off, and then, cyclic voltammograms (CVs) were recorded from -1.1 V to +0.3 V (forward potential scan) and then from +0.3 V to −1.1 V (reverse potential scan) at a scan rate of 0.1 V•s −1 .(ii) For differential pulse anodic stripping voltammetric (DP-ASV) determination of Pb(II) on modified GCE: during the DP-ASV procedure, Pb(II) was accumulated on the surface of the rotating modified 2 Journal of Chemistry electrode as done for the above procedure.After that, the electrode rotation was off for 10 s, and then, DP-ASV voltammograms were recorded from −1.1 V to +0.3 V at a scan rate of 0.02 V•s −1 .DP-ASV voltammograms of blank solution (without Pb(II) and prepared from double-distilled water) were similarly recorded before each measurement.
2.4.Apparatus.XRD was performed by powder X-ray diffraction (XRD), recorded on 8D Advance Bucker, Germany, with CuK α radiation.ermal behavior of ZIF-8 was analyzed by thermal analysis (TG-DTA) using Labsys TG Setaram (France) under air atmosphere.Morphology was observed by scanning electron microscopy using Jeol Jem-2100F TEM (Japan).Particle sizes were analyzed by DLS with Nanobrook 90plus PALS.A CPA-HH5 Computerized Polarography Analyzer (Vietnam) was used for voltammetry experiments.All measurements were done in the cell with three electrodes: a GCE with a diameter of 2.8 ± 0.1 mm used for formatting the modified electrode as a working electrode, a Ag/AgCl/3 M KCl as a reference electrode, and a platinum wire as an auxiliary electrode.All measurements were carried out at room temperature.
TEM observation of ZIF-8 is presented in Figure 2(a).e morphology of ZIF-8 consisted of nanospherical particles around 30-40 nm in diameter.e crystallite size was evaluated by Sherrer's equation from the peak (011).e crystallite size of ZIF-8 was 49.4 nm. e particle size was also analyzed by DLS as shown in Figure 2(b).e distribution curve exhibited the symmetric bell-shape indicating that the particle size had normal distributions.
e agglomerate mean size of ZIF-8 estimated by DLS was 70.7 nm.
e size calculated by DLS is the agglomerate size (grain size) which is formed from single particles.e single particles observed by the TEM consisted of crystallites.e crystallite size was calculated by XRD. e fact the mean size calculated by XRD is similar to that calculated by TEM indicates that the single phase of ZIF-8 with high crystallinity was obtained.Since the agglomerate sizes are only approximately 1.5-2 times the size of particle or crystallite size the agglomerates observed by the TEM are loosen and highly dispersible.
e nitrogen sorption study for the ZIF-8 reveals a reversible type I isotherm, a characteristic of microporous materials (not shown here).e sample of ZIF-8 possessed the high specific surface area of 1484 m 2 •g −1 and the total volume of 1.16 cm 3 •g −1 , which was similar or higher with that in the previous literatures [35][36][37].

Stability of ZIF-8.
e stability of ZIF-8 in different conditions is important for its application in catalyst as well as an electrode modifier.In present paper, the effects of time, solvents, and pH on the ZIF-8 structure were investigated.
Figure 3(a) shows XRD patterns of ZIF-8 exposed to ambient atmosphere for 1-12 months.
e characteristic diffractions of ZIF-8 were unchangeable indicating that ZIF-8 was stable in ambient condition for a year.e chemical stability of ZIF-8 was investigated in refluxing C 2 H 5 OH, H 2 O, and C 6 H 6 at boiling temperature as shown in Figure 3(b).XRD patterns showed that ZIF-8 maintained full crystallinity for 8 hours at boiling conditions.
XRD patterns of ZIF-8 submerged in water at room temperature for 1-14 days are shown in Figure 4(a).XRD patterns collected at designated intervals showed that ZIF-8 was stable in water for at least 14 days.ZIF-8 was soaked in water with pH ranging from 2 to 12 for 24 hours.XRD analyses (Figure 4(b)) showed that characteristic peaks of ZIF-8 submerged in pH 2 solution were not observed implying that ZIF-8 was unstable in this condition.However, it was stable in the pH ranging from 2.7 to 12.

Electrochemical Performance of ZIF-8-Based
Modified Electrodes

Selection of Experimental Conditions.
In order to verify the electrochemical activity of ZIF-8 in the modified GCE for detection of Pb(II), the electrochemical experiments in GCE modified with and without ZIF-8 were performed by DP-ASVs.As can be seen in Figure 5(a), the stripping voltammetry peak varied from −0.624 to −0.586 V indicating that E p of Pb(II) depended on the kind of working electrodes.e nafion was added because nafion membrane can be used as a coating to limit the interference of surfaceactive compounds as well as a stabilizer to improve the mechanical stability of the bismuth film electrode [38].e Journal of Chemistry current response on the Naf/GCE (curve c) and the bare GCE (curve e) exhibited the broad peaks, especially on Naf/GCE was almost not detectable.By the addition of ZIF-8, the current response at Naf/ZIF-8/GGE provided a welldefined peak as that at BiF/GCE which is widely used in DP-ASV for the determination of Pb(II).It is worth noting that adding ZIF-8 and incorporating the bismuth film exhibited the well-defined stripping signal with highest intensity.e intensity of I p (peak current) at BiF/Naf/ZIF-8/GCE was 1.82-fold when compared with that at BiF/GCE as well as Naf/ZIF-8/GCE.e favorable signal-promoting effect of the ZIF-8 indicated that it could accelerate the rate of electron transfer of Pb(II) and had good electrocatalytic activity for the redox reaction of Pb(II).e porous structure of ZIF-8 with the binding properties of the nitrogen group provides a high number of reactive sites that are ready for accessing the target analyte (Pb(II)).is synergistic combination of these effects leads to a greater amount of lead accumulation on the surface of BiF/Naf/ZIF-8/GCE, greatly improving its stripping peak current.e solvent for dispersing ZIF-8 often effects significantly on the current peak (I p ). ree solvents (e.g., dimethylformamide (DMF), water, and ethanol) were used to disperse ZIF-8.
e signals I p were presented at Figure 5(b).e results showed that water was favorable for dispersing ZIF-8 because it provided highest I p with lowest (relative standard deviation) RSD ip � 0.9.erefore, water was selected as a dispersion solvent for further studies.e concentration of Bi(III) in analytical solution may influence the properties of the in situ bismuth film on GCE and effect on the accumulation efficiency and DP-ASV method.As seen from Figure 5(c), the stripping peak currents increased with the Bi(III) concentration from 0 to 300 ppb and decreased in the range 300 to 600 ppb with the peak at 300 ppb.e decreasing of the stripping peak currents after peak being reached was due to hindering mass transfer caused by too thick bismuth film.erefore, the optimized concentration of bismuth was chosen as 300 ppb of Bi(III) for further study.
e effect of pH on the response of Pb(II) has been conducted in the pH ranging from 2.6 to 5.6 as shown in Figure 6(a).It was noted that the anodic peak current increases with increasing pH from 2.6 to 3.3.A further increase in the pH leads to a decline of the current.ese observations might be explained as follows: at low pH, the signal intensity of lead was low, which was due to the protonated amino groups repulsing with the cations via electrostatic repulsion.With the increase of pH, the protonated amino groups decrease, and electrostatic attraction results in a higher stripping current of lead.e best signal intensity with low RSD Ep (4.4%) is reached at pH � 3.3.e pH � 3.3 was chosen for the optimal pH. e relationship between pH and anodic peak potential, E p , is shown in Figure 6(b).As can be seen, anodic peak potential shifts negatively with increasing pH from 2.7 to 5.6 suggesting that protons involved directly to lead oxidation or the oxidation was hindered at low concentrations of protons e linear regression equation can be expressed as follows: where E p is the anodic peak potential and R is the relation coefficient.e linear relation between E p and pH is significant (R � −0.9651, p ≤ 0.001) (Figure 6(c)).e slope of regression is close to the theoretical value of 1/2 × 0.0599 (25 °C).It is possible that the electrochemical process has the involvement of a proton and two electrons.

Effects of Scan
Rate.Important information about the electrochemical mechanism can usually be obtained from the relationship between peak current and scan rate.erefore, the effect of scan rate on E p and I p was investigated by CV as shown in Figure 7.If the electrooxidation reaction is irreversible, then E p is dependent on v.As can be seen from Figure 7(a), peak potential shifts to higher potential as scan rate increases, and then it is concluded that electron transfer in Pb(II) electrooxidation is irreversible.Peak current increases with an increase in the scan rate from 20-500 mV•s −1 (Figure 7(b)) suggesting that the electron transfer reaction involved with a surface-confined process [39].
In order to determine if the electrooxidation reaction is adsorption or diffusion controlled, the plots of peak current (I p ) against square root of the scan rate (v 1/2 ) and ln I p against ln v are performed in Figure 8.If a plot of I p versus v 1/2 is linear and intercepting the origin, this process is controlled by diffusion [40].In the range 20 to 500 mV•s −1 ,

6
Journal of Chemistry I p of lead electrooxidation varied lineally on v 1/2 and was expressed by the following: Even the plot of I p on v 1/2 as shown in Figure 8(a) was linear (R � 0.999; p ≤ 0.001), the intercept does not cross the origin because 95% confidence interval ((−0.645) to (−0.637)) for intercept does not contain zero.It means that the electrode process of lead electrooxidation was not controlled by the diffusion process.
On the other hand, the slope of linear regression line for ln I p and ln v can provide information about the diffusion-or adsorption-controlled process.A slope close to 1 is expected for the adsorption-controlled electrode process while close to 0.5 for the diffusion-controlled process [40], a linear relation with high relative coefficient (R � 0.985, p ≤ 0.001) was obtained as shown in Figure 8(b) and is expressed as follows: ln I p � (0.883 ± 0.099)ln v +(−0.577± 0.477). ( en, its slope of 0.883 is close to 1. en, it was concluded that the oxidation of Pb(II) on the modified electrode was an adsorption-controlled process.
Effective surface area of the modified electrode could be obtained based on the relation between current-potential characteristics.According to the Bard and Faulkner equation [41], the relation between current and potential of the oxidation irreversible process is described by the following equation: where I p is the current peak (A), E p is the potential peak (V), A is the effective surface area (cm 2 ), C is the bulk concentration of Pb(II) (ppb), α is the electron transfer coefficient, F is equal to 96,493 (C mol −1 ), R is the gas constant Journal of Chemistry to 8.314 (J•K −1 mol −1 ), and T is the temperature in Kelvin (298 K).Natural logarithm (4) obtained the dependence below: Plot of the linear relation between ln I p and E p provides a linear regression equation as follows: Effective surface area of the modified electrode which was obtained for the intercept of linear regression line was 48.34 cm 2 .
e geometry surface area of the electrode is 0.065 cm 2 (diameter of the electrode � 2.8 mm).en, effective surface area was 780 times the geometry surface area of the electrode.e modified electrode with increased surface is due to the presence of ZIF-8.e larger effective surface area results in more active sites and leads to a higher signal-to-nose ratio [42].
e electron transfer rate constant (k s ) was calculated based on the Laviron equation [43]: where n is the electron transfer number.e plot of anodic peak potential (E p ) against ln v gives the linear regression equation as follows: E p (V) � −0.641 + 0.024 ln v. (8) e linear regression with high relative coefficient (R � 0.996) was obtained.e value of nα is 1.07 which was obtained from the slope of the regression equation.
e relation between E ap and v could be analyzed by the model: e nonlinear regression through this model provided the following equation: e standard potential (E 0 Pb 2+ /Pb � −0.599 V) with reference electrode Ag/AgCl/KCl 1 M can be the deduced intercept of E p versus v on the ordinate by extrapolating the line to v � 0. e value K s � 0.27 s −1 could be obtained from the value of intercept and nα � 1.07.Since the value of α is assumed to be equal to 0.5, which is commonly employed for a totally irreversible system [44], the value of n was calculated to be 2.14.erefore, the average number of electrons transferred, n, was 2 which confirmed again that two electrons and one proton were involved in the oxidation of Pb(II) on the ZIF-8-based modified electrode.
e imime groups of imidazole in ZIF-8 bind Pb(II) to surface complexes because of its high affinity to Pb(II) ions [45].e Pb(II) were accumulated in the electrode due to the reduction reaction and then dissolved in solution through the oxidation reaction.e electrochemical reactions could occur as follows:  of Chemistry e electrochemistry process at the ZIF-8-based modified electrode was illustrated in Figure 9.

Repeatability, Reproducibility, Accuracy, Linear Range, and Limit of Detection (LOD).
e repeatability of Naf/BiF/ ZIF-8/GCE for DP-ASV was checked with the solution of 20 and 100 ppb Pb (II).e detection of 20 and 100 ppb Pb (II) in each signal was estimated by successive measurements for seven times.e obtained RSD for 20 and 100 ppb were 7.08 and 3.92%, respectively, that were lower than the 1/2 RSD Horwitz predicted [46].Such reasonable RSD of successive measurements demonstrated that the Naf/BiF/ZIF-8/GCE could be repeatedly used for the detection of Pb(II) in either low concentration range or high concentration range.e reproducibility was also estimated with five Naf/BiF/ZIF-8/GCEs in detection of 20 ppb Pb(II) under the optimized conditions.e results exhibited a good reproducibility with the RSD of the current responses as 7.0%.e expectable RSD for five independent electrodes confirmed the good reproducibility of the obtained Naf/BiF/ZIF-8/GCE.e accuracy of the method was estimated through the recovery of a spike of Pb(II).
e solution with known concentration was prepared from stock solution.e 100 μL of 10 ppm Pb(II), 30 mL of 100 ppm Bi(III), and 1 mL of 0.5 M B-R buffet (pH � 3.2) were mixed together and diluted to 100 mL. is solution was split into two portions.e concentration of one portion evaluated by DP-ASV under optimal condition was 9.59 ppb.e other was spiked for increasing the concentration by 10 ppb.e concentration of Pb(II) in spiked solution determined by DP-ASV under optimum conditions was 20.69 ppb.Recovery (% Rev) was 110.1%. is result suggested that the determination of Pb (II) employing the DP-ASV with BiF/Naf-ZiF-8/GCE possessed acceptable error [35].
Under the optimal conditions, the linear range of lead detection with a ZIF-8-modified electrode was conducted.
e response current peak (I p ) was linear in the concentration range 12 ppb to 100 ppb (R � 0.999) as shown in Figure 10(a).Linear regression equation of the calibration curves was I p � (−2.601 ± 0.697) + (0.290 ± 0.012) C Pb(II) (Figure 10(b)).Sensitivity obtained from the slope of the calibration curve was 0.290 μA/ppb.e limit of detection (LOD) was calculated based on the concentration from 12 ppb to 100 ppb.As shown in Figure 10(b) the LOD was calculated from 3 S y /b, where S y is the standard deviation of y-residuals and b is the slope of linearity.
e LOD was found to be 4.16 ppb.
e limit of quantitation (LOQ) calculated from 10 S y /b was 13.9 ppb.e obtained detection limits and the linear ranges in this work were compared with the results reported previously, in which the electrode was modified with gold nanoparticle-graphene-cysteine composite, chitosan, MIL-100, polyaniline, bismuth nanoparticles, and mercaptodiatomite composite [47][48][49][50][51][52].A comparison was listed in Table 1.It could be noticed that the detection limit of Pb(II) from the Nafion/BiF/ZIF-8/GCE electrode was lower or  comparable with those results based on modified electrodes in previous papers.e ZIF-8-modified electrode exhibited better than some of the electrodes based on kaolinite, MPTMS-diatomite, or gold nanoparticles but failed to some others.Overall, the ZIF-8 was proved to be an effective electrode modifier for the detection of Pb(II) in aqueous solution.

Conclusions
A novel electrode modifier named zeolite imidazole framework-ZIF-8 has been prepared by a hydrothermal process.ZIF-8 was stable in ambient atmosphere and water.It is stable in pH ranging from 3 to 12 and exhibits the remarkable chemical resistance to some boiling organic solvents.e obtained ZIF-8 was applicable to the fabrication of an BiF-Naf/ZIF-8/GCE composite electrode, which gave a good performance in the determination of Pb(II) using the DP-ASV technique due to a combination of the advantages of ZIF-8 with high surface area, hierarchical porous structure, and thermal and chemical stabilities.e peak current of Pb(II) was linearly proportional to its concentration over the range of 12 to 100 ppb.e limit of detection and the limit of quantitation were as low as 4.12 ppb and 12.50 ppb, respectively.ese results showed that ZIF-8 is a potential electrode modifier for lead determination in aqueous solution.

Figure 6 :
Figure 6: (a) Anodic stripping current (I p ) of Pb(II) at different pHs and condition: [Pb(II)]: 500 ppb; [Bi(III)] � 300 ppb.(I p was the mean value for successive measurements for four times); (b) effect of pH on I p of anodic peak; (c) linear regression of E p versus pH.

Figure 9 :
Figure 9: Proposed mechanisms of electrochemistry process for Pb(II) determination at the ZiF-8-based modified electrode by DP-ASV.

Figure 10 :
Figure 10: (a) e DP-ASV curves of Pb(II) with increasing Pb(II) from 12 to 100 ppb; (b) the linear regression of I p versus C Pb(II) . 2

Table 1 :
A comparison of Pb(II) detection using modified electrodes.