VOLTAMMETRIC DETERMINATION OF LEAD IONS USING MODIFIED ELECTRODE BASED ON ZEOLITE IMIDAZOLE FRAMEWORK-8
Mai Thi Thanh1,2, Nguyen Hai Phong1, Tran Thanh Minh1, Phan The Binh3, Nguyen Phi Hung3, Nguyen Thi Vuong Hoan4, Dinh Quang Khieu1
1College of Sciences, Hue University
2Faculty of Physics-Chemistry-Biology, Quang nam University
3College of Pedagogy, Hue University
4Department of Chemistry, Qui Nhon University
*corresponding author: [email protected], Tel: 0935718285
Abstract
Zeolite Imidazole Framework (ZIF-8) composite was prepared by hydrothermal process. ZIF-8 was characterized by XRD, SEM and TG-DSC. The stability of ZIF-8 in different conditions was discussed. ZIF-8 was used as a novel electrode modifier for the determination of trace levels of lead. ZIF-8 shows quite a good capability for the efficient adsorption of lead from aqueous solutions. The parameters affecting the electrochemical process, such as types of electrodes, solvents, the amount of ZIF-8 suspension, electrolyte solution pH were investigated. Under the optimal conditions, the electrochemical sensor exhibited a linear response to the concentration of lead in the range of 2-500 ppb with a detection limit of 8.6 ppb.
Key words: Zeolite Imidazole Framework (ZIF-8); Modified electrode; Differential Pulse Anodic Stripping Voltammetry; Bismuth film (BiF); Determination of Pb(II)
1. INTRODUCTION
Environmental pollution is one of serious problems facing our recent world. Especially heavy metals ( Hg (II), Pb(II), Cd(II), Ni(II), etc. ), 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 upon concentration to very small [1, 2]. Lead may cause brain and kidney irreversible damages. In addition, acetates and phosphates of lead are carcinogens [3] So it is urgently need 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) [4], and inductively coupled plasma mass spectroscopy (ICP-MS) [5] and X-ray fluorescence [6], are employed currently for trace analysis of lead. These methods have excellent sensitivities and good selectivity; but several disadvantages such as time consuming and high cost of instrument limited their applications.
Electrochemical methods including stripping voltammetric techniques 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.
Anodic stripping voltammetry (ASV) have been used extensively to determine heavy metals [7, 8]. Recently, glassy carbon electrodes chemically modified with porous materials such as modified mesoporous materials [9],Clay-mesoporous silica composite [10], multi wall carbon nanotube [11] have receiving considerable attention for ASV because they exhibit significant improvements in terms of, fast response, high selectivity, low detection limit, renewability.
Metal-organic frameworks (MOFs) are gaining significant attention for their potential applications in gas separation and storage, sensors, and catalysis during the last years. Zeolite imidazole frameworks ZIF-8, being a kind of MOFs, consists of Zn atoms linked through
nitrogen atoms by 2 methyl- imidazole (Im) links to create neutral frameworks and to provide tunable nanosized pores formed by four-, six-, eight-, and twelve-membered ring ZnN4 [12].
ZIF-8 has appeared as a novel kind of highly porous materials combining desirable properties from both zeolites and conventional MOFs including high surface areas (up to 1500 m2.g-1), crystallinity, thermal and chemical stability [12, 13]. Due to high surface areas, highly order structure and high reactive site density the using of MOFs based composite for the electrochemical field is becoming increasing. Mao et al [14] reported copper (II)-2,2′-bipyridine- benzene-1,3,5-tricarboxylate (Cu(II) based MOF-199) used as selective electrocatalysts for the reduction of O2 and CO2. MOF modified by Au–SH–SiO2 nanoparticles was utilized to determinate of hydrazine and L-cysteine by electrochemistry analysis [15]. The amino- functionalized MOF-199 material could provide an excellent modifier for developing a sensitive electrode for the determination of lead trace [16].
In the present paper, ZIF-8 was synthesized by hydrothermal process. It has been used to modify glassy carbon electrode for Pb(II) determination by 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 DP-ASV.
2. MATERIAL AND METHODS 2.1. Materials
Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Korea, ≥99%), methanol (CH3OH, Merck) and 2- methylimidazole (C4H6N2, Aldrich, 99%) were utilized in synthesis of ZIF-8.
For electrochemistry analysis all working solutions of reagents were prepared from chemicals for analysis (Merck, Germany). Nafion solution was prepared from nafion solution (5%, d = 0.874 g.mL–1. Aldrich) and ethanol (96%, Merck) with volume ratio Vnafion/Vethanol: 1:4. Britton–
Robinson buffer solution (B-R BS) of pH = 4.7 were prepared from 0.04 M H3BO3, 0.04 M H3PO4
and 0.04 M CH3COOH (Merck). pH of the B-R BS was adjusted by addition of 1.0 M sodium hydroxide solutions into the B-R BS and then checking by pH meter. Working solution of Pb(II) was prepared daily from stock solution (1000 ppm, Merck, Germany).
2.2. Preparation of ZIF-8
ZIF-8 was synthesized as procedure of ZIF-8 [17,18,19]. Briefly, 2.8 mmol of zinc (II) were dissolved in 1.4 mmol of methanol. A solution consisting of 64.4 mmol of 2-methylimidazole (C4H6N2, Aldrich, 99%) and 1.4 mol of methanol was added to the Zn based solution and vigorously stirred for 24 h at ambient temperature. Finally, this solution was centrifuged at 300 rpm, and washed thoroughly with methanol. This washing procedure was repeated 3 times. The resultant crystals were dried overnight at 120 °C.
2.3. Voltammetric procedure
Preparation of working electrode - modified electrodes
The preparation procedure was carried out as reference [15]. In brief, glassy carbon electrode (GCE) was firstly polished with 0.05 m Al2O3 slurries on a polishing cloth, then rinsed ultrasonically with 2 M HNO3, absolute ethanol and double-distilled water. Subsequently, the GCE was electrochemically cleaned by cyclic potential scan between −1.5 V and +1.5 V in 0.5 M ABS pH 4.5 with the scan rate of 0.10 V.s−1. After that, the electrode was rinsed with double- distilled water. The composite of nafion-ZIF-8 was prepared by dispersing 5.0 mg ZIF-8 into 1.0 mL nafion (0.1 wt%) and sonicated. The GCE was coated with 5 L of the nafion-ZIF-8 composite and dried under the room temperature (25oC) (denoted as Naf/ZIF-8 /GCE). For the same procedure, the mixture of bismuth film (BiF) and ZIF-8, BiF and nafion, nafion, nafion and ZIF-8 were prepared, respectively and GCE were coated by these mixtures. The obtained electrodes including ZIF-8-BiF-Nafion/GCE; Nafion/GCE; ZIF-8-Nafion/GCE; BiF/GCE were denoted as BiF/Naf/ZIF-8/GCE; Naf/GCE; Naf/ZIF-8/GCE; BiF/GCE, respectively.
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. Then, voltammetric measurement was conducted as follows:
I) Accumulation of Pb from Pb(II) on the modified GCE: Pb(II) reduced into Pb at a potential of –1.2 V (Eacc) during 60s (Accumulation time tacc) was accumulated on the surface of the modified electrode). During this step, the electrode was rotated at a constant rate of 1000 rpm.
II) Anodic stripping voltammetric determination of Pb on electrode modified GCE by :
Cyclic Voltammetric technique (CV-ASV): After accumulation as dicribed in I), the electrode rotation was off and then, cyclic voltammograms were recorded from –1.2 V to +0.2 V (forward potential scan) and then from +0.2 V to –1.2 V (reverse potential scan) at a scan rate of 0.1 V.s−1.
Differential pulse anodic stripping voltammetry (DP-ASV): After accumulation as dicribed in I),the electrode rotation was off for 10 s and then DP-ASV voltammograms were recorded from –1.2 V to +0.2 V at scan rate of 0.02 V.s−1 and suitable differential voltammetry technique parameters. DP–ASV voltammograms of blank solution (without Pb(II) and prepared from double-distilled water) was similarly recorded before each measurement.
The Pb(II) quantitation was done by standard addition method.
2.4. Apparatus
The phases were monitored by powder X- ray diffraction (XRD), recorded on 8D Advance Bucker, Germany with CuKa radiation. Thermal gravity-differential scanning carlometry (TG- DSC) was conducted by using TG-DSC SETARAM under air atmosphere. Morphology of ZIF-8 was observed by scanning electron microscopy (SEM. Jeole-3432). Particle sizes were analyzed by DLS with Nano Brook 90 plus PANS. A CPA-HH5 Computerized Polarographic Analyzer (produced by Laboratory of Computer Application to Chemistry, Institute of Chemistry, Vietnam Academy of Science and Technology) 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 working electrode, a Ag/AgCl/3M KCl as reference electrode and a platinum wire as auxiliary electrode. All measurements were carried out at room temperature.
3. RESULT AND DISCUSSION 3.1. Characterization
Figure 1a shows XRD pattern of ZIF-8. The XRD pattern of ZIF-8 in this work was agreed well with patterns simulated from references [17,18,19] and no obvious peaks of impurities can be detected in these XRD patterns. There were well defined diffractions (011), (022), (112), (022), (013), (224), (114), (233), (134) and (334) at two theta = 7.2; 10.1; 12.7; 14.9;
16.1; 22.1; 24.9; 25.5 and 26.5 degree in the XRD pattern of ZIF-8, indicating that the crystallinity of ZIF-8 in this work was relatively high.
5 10 15 20 25 30
(a)
(334)(134)(233)
(114)
(022)
(013)(022)
(112)
(002)
500 cps
ZIF-8
(011)
Intensity (abr.)
2 theta (degree)
0 200 400 600 800
0 1 2 3 4 5 6 7 8 9 10 11
(b)
-0.641mg -6.282%
-2.524mg -24.735%
-4.444mg -43.552%
TGA (mg)
Temperature(0C)
Fig. 1. (a) XRD pattern ; (b) TG-DSC diagram of ZIF-8.
TG-DSC of ZIF-8 is presented in Figure 1b. The sample shows a weight loss around 6.3% with endothermic peak at 100 oC was assigned to the removal of water molecules present in the channels and coordinated to the metal centers of ZIF-8. At temperatures between 120 and 400
oC, 24.7% of the weight was lost in ZIF-8 was due to the displacement of the coordinated DMF molecules. A third weight loss of 43.5% corresponding to larger exothermic peak was observed from 400 to 600 oC was ascribed to the decomposition of imidazole ligand, After 600oC no weight loss was observed and this residual material could correspond to the formation of zinc oxides.
Fig. 2. (a) SEM observation and (b) Size distribution of ZIF-8.
SEM observation (Fig. 2a) reveals that morphology of ZIF-8 consists of spherical particle in size around 70-100 nm. The result is similar to mean size particles analyzed by DSL as shown in Fig.2b. The particle size distribution curve is symmetric and mean size is around 90 nm indicating that morphology of ZIF-8 is highly dispersed. Nitrogen sorption study for the ZIF-8 reveals a reversible type I isotherm, characteristic of microporous materials (Fig. 3). The sudden uptake at high relative pressure can be related to physisorbed liquid nitrogen on the crystal surfaces of the nanoparticles[13]. The total volume is about 1.16 cm3.g-1, and the BET and Langmuir surface area, are 1383 and 1909 m2.g-1respectively which was similar with that in the previous literature [21,22.23]
0.0 0.2 0.4 0.6 0.8 1.0 150
200 250 300 350 400 450 500 550 600 650 700 750 800
Adsorbed(cm3.g-1 STP)
Relative pressure(P/P0)
Fig. 3. Nitrogen adsorption/desorption isotherms of ZIF-8.
3.2. Stability of ZIF-8
Stability of ZIF-8 in different conditions is important for its application in catalyst as well as electrode modifiers. In present paper, The effect of time, solvents, pH on ZIF-8 structure was investigated.
5 10 15 20 25 30
Z-3mZ-6mZ-8mZ-12m Z-1m ZIF-8 CpsIntensity(abr)
2theta(Degree)
Fig. 4. XRD patterns of ZIF-8 in ambient atmosphere.
Figure 4 shows XRD patterns of ZIF-8 in ambient atmosphere for 1-12 months. The characteristic diffractions of ZIF-8 were unchangeable indicating that ZIF-8 is stable in ambient condition for a year.
5 10 15 20 25 30
(a)
ZIF-8 Z-3DZ-7D Z-14D
500CpsIntensity(abr)
2 theta(degree) 0 5 10 15 20 25 30
(b)
ZPH-3 ZPH-10ZIF-8ZPH-12
ZPH-6
ZPH-2 500(Cps)Intensity(abr)
2 theta(degree)
Fig. 5. XRD patterns of ZIF-8 submerged in water at room temperature (a) and water with pH different (b).
XRD patterns of ZIF-8 submerged in water at room temperature for 1-14 days are shown in Figure 5a. XRD results showed that ZIF-8 is stable in water at least for 14 days. Figure 5b shows pH effects on ZIF-8 structure. ZIF-8 was soaked in water with pH range of 2-12 for 24 hours. XRD analyses 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 range of pH = 3-12.
5 10 15 20 25 30
Z-H2O Z-C2H5OH Z-C6H
6
ZIF-8 500 CpsIntensity(abr)
2 theta(degree)
Fig. 6. XRD patterns of ZIF-8 submerged in some boiling solvents.
The stability of ZIF-8 in boiling solvent including C2H5OH, H2O and C6H6 were investigated by XRD as shown in Figure 6. The characteristic diffractions of ZIF-8 under boiling solvents seem to be observed clearly. These results implied that ZIF-8 is stable in mentioned conditions.
3.3. Optimization 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 7A, the stripping voltammetry peak varied from -0.624 to -0.586 V indicating that Ep of Pb(II) depended on the kind of working electrode. The current response on the Naf/GCE (curve c) and the bare GCE (curve e) exhibited broad peak, especially on Naf/GCE was almost not detectable. By the addition of ZIF-8 the current response at Naf-ZIF-8/GGE provided a well-defined peak as that at BiF/GCE which is widely used in DP-AVS for the determination of Pb(II).It is worth to note that the current response at BiF/Naf-ZIF-8/GCE exhibited the well-defined peak with highest intensity. The intensity of Ip at BiF/Naf/ZIF-8/GCE was 1.82 fold in compared with that at BiF/GCE as well as Naf/ZIF-8/GCE.
The favorable signal-promoting effect of the ZIF-8 indicate that it could accelerate the rate of electron transfer of Pb(II) and have good electrocatalytic activity for redox reaction of Pb(II). The proposed mechanism of the stripping voltammetric measurement is the following. First, Pb(II) ions were accumulated from the bulk onto the surface of the BiF/Naf/ZIF-8/GCE by selective complexation with N atoms in imidazole of ZIF-8 to form a metal–ligand complex, and then the complexed ions accumulated in electrode surface were reduced by applying a constant voltage of 0.9 V in accumulation. Lead is then electrochemically stripped back into the solution by scanning toward positive potential using the differential pulse voltammetric method. The porous structure of ZIF-8 with the binding properties of nitrogen group provide a high number of reactive sites that readily for accessible the target analyte (Pb(II)). This synergistic combinations of these effects lead to a greater amount of lead accumulation on the surface of BiF/Naf/ZIF-8/GCE . The BiF/Naf/ZIF-8/GCE was selected for further studies.
Solvent using to disperse ZIF-8 effected significantly on the current peak (Ip). Three solvent (e.g. dimethylformamide (DMF), water and ethanol) were used to disperse ZIF-8. The signals Ip
were presented at Figure 7B. The results show that water solvent is favorable for dispersing ZIF-8 because it provided highest Ip with lowest RSDip =0.9. Then water was selected as solvent for further studies.
The effect of ZiF-8 amount was examined using an amount range from 6.25 to 37.5 g corresponding to solution volume from 2.5 to 15 L. The results showed that the oxidation peak current of lead increased quickly through increasing the amount of ZIF-8 suspension loaded on the surface of GCE up to 12.5 g (5.0 L). Further increase, caused a reduction in the anodic
peak current of lead. The oxidation peak current reached maximum at amount of 12.5 g with lowest RSD = 0.89 as shown in Figure 7C. This can arise from larger film thickness, which caused increasing resistance of the film modifier against the electron transfer and sluggish mass transfer process for lead. As a result, 12.5 g (V = 5.0 L) ZIF-8 suspension was selected as optimum amount.
-0.8 -0.7 -0.6 -0.5 -0.4
1.0 1.5 2.0 2.5 3.0 3.5
4.0 U (V)
j (A)
U (V)
(a) BiF/NafZIF-8/GCE
(b) BiF/Naf/GCE (c) Naf/GCE
(d) NafZIF-8/GCE
(e) GCE (f) BiF/GCE
-0.8 -0.7 -0.6 -0.5 -0.4
1.0 1.5 2.0 2.5 3.0 3.5 4.0
B
DMF2 H2O C2H5OH
j (mA)
U (V)
6 13 19 25 31 38
0 1 2 3 4
C
Ip,Pb(mA)
mZIF-8 (mg)
Fig. 7. A) DP-ASVs of Pb(II) (50 ppb) in acetate buffer solution (pH 4.72) at BiF/Naf/ ZIF- 8/GCE (a); BiF/Naf/GCE (b); Naf/GCE (c); Naf/ZIF-8/GCE (d); bare GCE (e); BiF/GCE (f).
B).Anodic stripping current (Ip) of Pb(II) for DMF, H2O and C2H5OH; C) Anodic stripping current (Ip) of Pb(II) for different ZIF-8 amount : Pb(II): 50 ppb; acetate buffet (pH = 4.72), [Bi(III)] = 300
ppb.
The effect of pH on the response of lead has been conducted in the pH range from 2.60 to 5.6 as shown in Figure 8a. It was noted that the anodic peak current increased with increasing pH from 2.60–3.19. A further increase of the pH led to a decline of the current. These 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 decreased, and electrostatic attraction resulted in a higher stripping current of lead. The best signal intensity was reached at pH = 3.2. The relationship between pH and anodic peak potential, Epa is shown in Figure 8b. As can be seen, anodic peak potential shift negatively with increasing pH from 2.7 to 5.6 suggesting that protons involve directly to lead oxidation. The linear regression equation can be expressed as follows:
Epa (mV) = (-0.031 ∓0.010¿ pH – (-0.428 ±0.041¿ ( R = -0.9651) (1)
where Epa is anodic peak potential, R is relation coefficient. The slope of regression is close to theoretical value of
1
2 ∗0.0599
0 (25oC) for two electron and one protons.-1.2 -0.8 -0.4 0 0.4
-30 -20 -10 0 10 20 30
A
i (A)
U (V)
pH = 2.7 pH = 3.2 pH = 3.6 pH = 4.1 pH = 4.6 pH = 4.9 pH = 5.6
2.7 3.2 3.6 4.1 4.6 4.9 5.6 0
5 10 15 20 25
(b)
Ip,Pb(mA)
pH
2 3 4 5 6
-0.60 -0.55 -0.50
(c)
Ep,Pb = (-0,428 ± 0,041) + (-0,031 ± 0,010).pH r = 0,9651
E p,Pb (V)
pH
Fig. 8. a) Anodic stripping current (Ip) of Pb(II) at different pH, condition: [Pb(II)]: 500 ppb;
[Bi(III)] = 300 ppb. (the Ip were average value of four time measurements for all cases); b) Effect of pH on current intensity of anodic peak; c) Linear regression of Epa vs. pH
-1.2 -0.8 -0.4 0 0.4
0 50 100
150 U (V)
i (A)
U (V)
= 20 mV/s = 40 mV/s = 50 mV/s = 75 mV/s = 100 mV/s = 200 mV/s = 300 mV/s = 400 mV/s = 500 mV/s
0 40 80 120 160 200
0 20 40 60 80
(b)
Ip,Pb = (2,1988 ± 3,999) + (0,3287 ± 0,014).n r = 0,9962
I p,Pb (mA)
v (mV/s)
Fig.9. a) CVs of BiF/Naf/ZiF-8/GCE with increasing of scan rate to inner to outer: 20-500 mV.s-1; b) Linear regression of Ip vs. v.
The effect of scan rate on electrochemical behaviors can provide useful information involving electrochemical mechanism. Therefore effect of scan rate on Epa and Ipa were investigated by CV. Figure 9a shows the typical CV of BiF/Naf/ZIF-8/GCE at various scan rates. Peak current enhanced with the increasing of scan rate from 20 – 500 mV/s-1. The linear relation of Ip against v ( r = 0.9962) indicated that the electron transfer reaction involved with a surface-confined process (Figure 9b).
The electron transfer coefficient (s) and the electron transfer rate constant (ks) were calculated based on the Lavion equation [23].
E
pc= E
o+ RT
(1−α ) nF ln ( 1−α ) nF
RT K
s+ RT
(1− α )nF lnv
(2)where n is electron transfer number, R is gas constant (8.314 J.mol-1.K-1), T is the temperature in Kelvin, F is Faraday constant ( F = 96,485 C.mol-1). The plot of anodic peak potential (Eap) against lnv is shown in Figure 10a.The linear regression with high relative coefficient (R = 0.9962) was obtained. From the value of slope and intercept of regression equation provided the electron transfer coefficient = 0.459). The relation between Eap and v could be analyses by model:
y=y0+A1. e−x/t1+A2. e−x/t2 (3) The non-linear regression through this model provided the equation
Eap = -0,4778 + (-0,0504).
e
-ν/29,76 + (-0,0707).e
-ν/324,6 ; r = 0,9975 (4) The high relative coefficient confirmed that this model fixed well experimental data.The standard potential ( E0Pb2+¿Pb=−0.599V ) with reference electrode Ag/AgCl/KCl 1M is could calculated from extrapolating of eq. 4. From eq. 2, the Ks could be calculated as follow:
-0,6410 =
E
o+ RT
( 1−α ) nF ln (1−α)nF RTK
sor -0,6410 = -0,599
−RT
(1 −α ) nF [ ln RT (1−α)nF +ln K
s]
-0,042 = -b
[ lnb + ln K
s]
, với b =RT (1−α ) nF
= 0,0237 Ks = 248,3 s-1.
The large value of Ks confirmed that the surface reaction occurred quickly.
3 4 5 6
-0.56 -0.54 -0.52 -0.50
A
Ep,Pb = (-0,6410 ± 0,003) + (0,0237 ± 0,001).ln
r = 0,9995
E p,Pb (V)
ln 0 100 200 300 400 500
-0.56 -0.54 -0.52 -0.50
(b)
E p,Pb (V)
v (mV/s) Fig.10. a) The linear relation between Eap and lnv; b) Eap and v.
-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 0
20 40 60 80 100
A
i (A)
U (V)
2 ppb 500 ppb
0 100 200 300 400 500 0
25 50 75 100
2 4 8 12 16 20
B
Ip,Pb = (1,783 ± 2,705) + (0,182 ± 0,013).CPb r = 0,993
I p,Pb (A)
CPb (ppb)
Fig. 11. The DP-ASV curves of Pb(II) with increasing lead concentration in the range of 2-500 ppb; b) The linear regression of Ip vs. CPb2+.
Under the optimal conditions, the analytical performance for the determination of lead with a ZIF-8- modified electrode was conducted. The response current peak (Ip) was linear in the concentration range from 2 ppb to 500 ppb (R2 = 0.993) as shown in Figure 11a. Regression equation of the calibration curves was Ip = (1.783
± 2.705¿
+ (0.182±
0.013).C
Pb(Figure 11b). The limit of detection (LOD) calculated based on the concentration from 12 ppb to 100 ppb. As shown Figure. 12.
-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 0
10 20 30 40
U (V)
i (A)
U (V)
12 ppb 100 ppb
Fig. 12. a) The DP-ASV curves of Pb(II) with increasing Pb(II) from 12-100 ppb; b) The linear regression of Ip vs. CPb.
The LOD was calculated from 3Sb/S, where Sb is the standard deviation of measurement (n
=7) and S is the slope of linearity. The LOD was found to be 8.56 ppb.
Figure 13. DPVs of Pb(II) with 20 ppb ; 100 ppb and 200 ppb.
(b)
As for the reproductive of measurement, a series of 7 repetitive measurements of solutions containing 20, 100 and 200 ppb Pb(II) on the same electrode resulted in reproducible stripping peaks (see Figure 13), with a low relative standard deviation of 7.08%; 3.92%; 0.78%, respectively. Moreover, the modified electrode can be repeatedly used up to 7 cycles without loss of analytical performance. These results indicate that ZIF-8 modified electrode has good stability. The GCE modified with ZIF-8 developed in the present work shows good LOD compared with other similar materials/GCE reported previously in the literature [24-27]. This indicates that ZIF-8 is a potential material for electrode modifiers.
4. CONCLUSIONS
Novel electrode material named zeolite imidazole framework-ZIF-8 has been prepared by a hydrothermal process. ZIF-8 is stable in ambient atmosphere and several solvents even in boiling condition. ZIF-8 is also stable in water at pH range of 3-10. We demonstrate that ZIF-8 could be used as a very suitable modifier for constructing an efficient and highly selective electrode for the determination of lead.
Acknowledgements
This work was supported by the project B2016-DHH-20 sponsored by Ministry of Education and Training, Vietnam
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