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Mesoporous MnFe2O4 nanocrystal clusters for electrochemistry detection of lead by stripping voltammetry
Journal of Electroanalytical Chemistry 755 (2015) 203–209
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Mesoporous MnFe2O4 nanocrystal clusters for electrochemistry detection of lead by stripping voltammetry Xiao-Juan Han a, Shao-Feng Zhou a,⁎, Hong-Lei Fan b, Qiao-Xin Zhang c, Ya-Qing Liu a,⁎ a b c
School of Materials Science and Engineering, North University of China, Taiyuan 030051, PR China School of Chemical Engineering and Environment, North University of China, Taiyuan 030051, PR China School of Mechanical and Electronic Engineering, Wuhan University of Technology, Wuhan 430070, PR China
a r t i c l e
i n f o
Article history: Received 9 April 2015 Received in revised form 30 July 2015 Accepted 31 July 2015 Available online 1 August 2015 Keywords: Lead MnFe2O4 Stripping voltammetry
a b s t r a c t High-adsorption MnFe2O4 nanocrystal clusters (MnFe2O4 NC) with mesoporous structure were used for selective analysis of Pb2+ by square wave anodic stripping voltammetry (SWASV) in this work. The as-prepared MnFe2O4 NCs with diameter of 200–400 nm and mesoporous structure composed of nanocrystals with a size of about 8–12 nm were characterized using SEM, HRTEM and XRD. Electrochemical properties were characterized by cyclic voltammetry and electrochemical impedance spectroscopy. The chemical and electrochemical parameters influencing on deposition and stripping of metal ions, such as supporting electrolytes, pH value, deposition potential, and deposition time, were also studied. The MnFe2O4 NC modified GCE has a high selectivity toward Pb2+ with a favorable sensitivity (19.9 μA μM−1) and LOD (0.054 μM) for Pb2+ under the optimized conditions while the response to Cd2+, Hg2+, Cu2+ and Zn2+ is poor. No interference from Cd2+, Zn2+ and Hg2+, favorable stability and potential practical applicability were recognized in the electrochemical determination of Pb2+. The above results provided a potential material for the design of new sensing materials in the application field of electrochemical detection toward toxic metal ions. © 2015 Elsevier B.V. All rights reserved.
1. Introduction It is well known that environmental contamination by heavy metals has been a serious and complex problem and has remained a focus of attention all over the world [1,2]. Heavy metals, unlike organic contaminants, were not biodegradable and tended to accumulate in living organisms and most of them were known to be poisonous and carcinogenic [3]. Effective determination of heavy metals in low concentration was important due to their hazards to the physical health of human being [4–6]. The electrochemical technique, as an alternative to the conventional spectroscopic techniques, has been recognized as a promising method for trace and on-site analysis of toxic heavy metal ions due to portability, high sensitivity, good selectivity, low cost, and suitability [7–12]. It is known that the electrochemical performance is highly dependent on the sensing materials, which is reasonable when presumed that the materials with high adsorption capacity can improve the efficiency for accumulating analytes [13]. Materials with high adsorbability are leading a new opportunity for the sensing performance in electrochemical detection. Very recently, based on the remarkable adsorption ability, nanomaterials have been developed as a detection strategy toward heavy metal ions [14–18]. Due to the adsorption capacity, O2-plasma oxidized multi-walled ⁎ Corresponding authors. E-mail addresses: [email protected] (S.-F. Zhou), [email protected] (Y.-Q. Liu).
http://dx.doi.org/10.1016/j.jelechem.2015.07.054 1572-6657/© 2015 Elsevier B.V. All rights reserved.
carbon nanotubes [15] and γ-AlOOH(boehmite)@SiO2/Fe3O4 porous magnetic microspheres [19] were found to be useful for the electrochemical detection of heavy metal ions. Polypyrrole/reduced graphene oxide nanocomposites were used for identifying Hg2 + by means of their highly specific adsorption ability toward Hg2 + [17]. Porous Co3O4 microsheets presented a high sensitivity and a quite nice low detection limit toward Pb2+ because of their high adsorption capacities [13]. As ideal adsorbents, ferrite (MFe2O4; M = Fe, Mn, Zn, or Co) nanoparticles have attracted great attention for the characteristic of high adsorption capacity and superparamagnetism. With the high adsorption capacity of nanostructure, the superparamagnetic materials can disperse well in the solution due to the absence of any residual magnetization without the external applied magnetic field, avoiding the typical aggregation problems of ferromagnetic materials [20,21]. With a high adsorption performance, the MnFe2O4 NCs combined with uniform mesoporous structure and small constituent nanocrystals (about 8–12 nm in size) even provide a synergistic effect for enhanced adsorption performance toward As(III) [22]. However, to the best of our knowledge, the high-adsorption ferrite (MFe2O4; M = Fe, Mn, Zn, or Co) materials used as sensing materials for the detection of heavy metal ions have rarely been reported. In this work, we modified a glass carbon electrode (GCE) using mesoporous MnFe2O4 nanocrystal clusters (MnFe2O4 NC) for selective analysis of Pb2 + by square wave anodic stripping voltammetry
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(SWASV), and the selective detection toward Pb2+ was achieved. The optimizing of experimental conditions, such as supporting electrolytes, pH values, deposition potential, and deposition time were investigated. Other heavy Metal ions, such as Cd2+, Hg2+, Cu2+ and Zn2+, were chosen as potential interfering ions for investigating the electrochemical selective behavior of mesoporous MnFe2O4 NC toward Pb2+. Furthermore, the stability of mesoporous MnFe2O4 NC modified electrode was also studied.
2. Experimental 2.1. Chemical reagents All chemicals are of analytical grade, which are commercially available from Shanghai Chemical Reagent Co. Ltd and without further purification in this study. Acetate buffer solution of 0.1 M with different pH was prepared by mixing stock solutions of 0.1 M NaAc, HAc and NaOH. Phosphate buffer solutions (PBS) of 0.1 M were prepared by mixing stock solutions of 0.1 M H3PO4, KH2PO4, K2HPO4, and NaOH. NH4Cl-NH3·H2O (0.1 M) solution was prepared by mixing stock solutions of 0.1 M NH4Cl and NH3·H2O in different proportions. The water (18.2 MΩ cm) that was used to prepare all solutions was purified with the NANO pure Diamond UV water system.
2.2. Apparatus All electrochemical measurements were performed using a CHI 660D computer-controlled potentiostat (ChenHua Instruments Co., Shanghai, China). A conventional three-electrode configuration was employed, consisting of a bare or modified glassy carbon electrode (GCE) as a working electrode, an Ag/AgCl/saturated KCl electrode as a reference electrode, and a platinum wire as a counter electrode. The morphology of MnFe2O4 NCs was investigated by scanning electron microscopy (SEM, Quanta 200 FEG, FEI Company, USA). High Resolution Transmission electron microscopy (HRTEM) was carried out on a JEM2010 microscope. X-ray diffraction (XRD) was carried out using a PW1710 instrument with Cu Kα radiation λ = 0.15406 Å.
2.3. Synthesis of MnFe2O4 NCs MnFe2O4 NCs were synthesized according to a previous report [23]. In brief, 5 mmol of FeCl3 · 6H2O (1.35 g) and 2.5 mmol MnCl2·4H2O (0.50 g) were dissolved in ethylene glycol (40 mL) to form a clear solution, followed by the addition of NaAc (3.6 g) and polyethylene glycol (1.0 g). The mixture was stirred vigorously for 30 min and then sealed in a Teflon-lined autoclave (50 mL capacity), heated at 200 °C in an electric oven for 8 h, and then cooled to room temperature naturally. The black products were centrifuged and washed with deionized water and absolute alcohol for several times and finally dried at 60 °C for 6 h.
2.4. Preparation of MnFe2O4 NC modified electrode Prior to modification, the bare glassy carbon electrode was sequentially polished with 0.3 μm and 0.05 μm alumina power slurries to a mirror-shiny surface, and then sonicated with 1:1 HNO3 solution, absolute ethanol and deionized water for 1 min, respectively. The MnFe2O4 film on the surface of glassy carbon electrode was performed in the following manner: 2 mg of MnFe2O4 NCs was added into 2 mL of Milli-Q water, and sonicated for 3 min to obtain a uniform dispersion. A drop of the above solution was pipetted onto the fresh surface of GCE and dried in air. After evaporation, a thin MnFe2O4 film was formed on the electrode surface.
2.5. Electrochemical measurements Square wave anodic stripping voltammetry (SWASV) was used for the observation of electrochemical behavior under optimized conditions. Pb, Cd, Hg, Cu, and Zn were deposited at the potential of 1.0 V for 150 s by the reduction of Pb2 +, Cd2 +, Hg2 +, Cu2 +, and Zn2 + in 0.1 M HAc-NaAc (pH = 5.0). The anodic stripping (reoxidation of metal to metal ions) of electrodeposited metal was performed in the potential range of −1.0 to −0.2 V at the following condition: frequency, 25 Hz; amplitude, 25 mV; increment potential, 4 mV; vs Ag/AgCl. A desorption potential of −0.1 V for 150 s was performed to remove the residual metals under stirring condition. The same experiment conditions were applied in the interference and stability studies. Cyclic voltammograms (CV) and electrochemical impedance spectra (EIS) were performed in mixing solution of 5 mM Fe(CN)36 −/4 − with 0.1 M KCl and the scanning rate was 100 mV s−1. 3. Results and discussion 3.1. Characterization of MnFe2O4 NCs Fig. 1a shows monodisperse, uniform and clean spherical shape of MnFe2O4 microspheres with diameter of 200–400 nm at a large scale. Fig. 1b indicates that the typical individual ferrite microsphere is a loose cluster, and clearly, the cluster is composed by small nanocrystals with the size of about 8–12 nm. Between the small nanocrystals, the structure of mesoporous can be found in Fig. 1b. This architecture might render them with a large surface area for adsorption of guest molecules [24–26]. The selected-area electron diffraction (SAED) pattern showed that the diffraction spots are widened into narrow arcs (insets of Fig. 1b), indicating the clusters are made up of many misaligned ferrite nanocrystals [24,27], in agreement with the analysis of HRTEM (Fig. 1b). The d value of crystal plane (311) in Fig. 1c and characteristic peaks in Fig. 1d verify the crystalline structure of MnFe2O4 NCs. And the broad diffraction peaks shown in Fig. 1d suggest the small size of ferrite NCs formed. Based on the calculations with the Scherrer formula for the strongest (311) diffraction peak, the average primary grain size is estimated to be 11 nm for MnFe2O4 NCs, which is consistent with the analysis of Fig. 1b. Taking the analyses of SEM, HRTEM, and XRD mentioned above, the as-prepared MnFe2O4 NCs with diameter of 200–400 nm and mesoporous structure are composed of nanocrystals with a size of about 8–12 nm, might render them with large surface area for adsorption and detection of heavy metal ions. 3.2. CV and EIS characterization of MnFe2O4 NC modified GCE The fabricated MnFe2O4 NC modified GCE was firstly electrochemically characterized using cyclic voltammograms (CV) and electrochemical impedance spectra (EIS) (Fig. 2). Fig. 2a displays the CV response of bare GCE and MnFe2O4 NC modified GCE electrodes. As compared with the bare GCE, the anodic and cathodic peak currents were decreased at MnFe2O4 NC modified GCE. This was due to the poor conductivity of MnFe2O4 NCs. Fig. 2b shows that the impedance spectra of MnFe2O4 NC modified GCE electrodes include a semicircle portion and a linear portion. The semicircle diameter at higher frequencies corresponds to the electron transfer resistance (Ret), and the linear part at lower frequencies corresponds to the diffusion process [28]. As seen in Fig. 2b, it was observed that the Ret of the MnFe2O4 NC modified GCE was about 1.8 kΩ, while the EIS of the bare GCE displays an almost straight line. It means that the Ret of the MnFe2O4 NC modified GCE is much higher than the bare GCE, showing that the modification of MnFe2O4 NCs can hinder the electron-transporting on surface of the electrode. The nanocrystal cluster structure of MnFe2O4 could create a further barrier and hinder the access of the redox probe to the electrode surface, resulting in a large electron transfer resistance of the MnFe2O4 NC modified GCE, which is consistent with the CV results.
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Fig. 1. SEM image (a), HRTEM image (b, c) and XRD pattern of the MnFe2O4 NCs. Inset in panel b is the corresponding SAED pattern.
Fig. 2c presents the SWASV analytical characteristic of bare GCE and MnFe2O4 NC modified GCE toward Pb2+. When the accumulation process was carried out for 150 s at − 1.0 V in a solution containing
0.5 μM Pb2 + in 0.1 M HAc–NaAc (pH 5.0) without deaeration, weak peaks with the peak current about 5.0 μA were observed at the bare GCE in the potential range of − 1.0 to − 2.0 V. For the MnFe2O4 NC
Fig. 2. Cyclic voltammograms (a) and Nyquist diagram of electrochemical impedance spectra (b) for bare GCE and MnFe2O4 NC modified GCE in the solution of 5 mM Fe(CN)3−/4− 6 containing 0.1 M KCl. Potential scan rate: 100 mV s−1. (c) SWASVs for 0.5 μM Pb2+ on the bare GCE and MnFe2O4 NC modified GCE in 0.1 M HAc–NaAc (pH 5.0). Deposition potential: −1.0 V, step potential: 5 mV, amplitude: 20 mV, and frequency: 25 Hz.
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modified GCE, the peak current increases to about 11.0 μA, which is much higher than the bare GCE. It shows that the MnFe2O4 NCs could provide more remarkable electrochemical performances toward Pb2+ detection. It might be attributed to the unique mesoporous structure of MnFe2O4 NCs composed of nanocrystals, which might render them with large contact area of the electrode/electrolyte interface and provided more active sites for the electrochemical reactions in the detection process of Pb2 +. Besides, the Fe and Mn species in the MnFe2O4 NCs could provide a synergistic effect for the detection of Pb2+ contributed by the higher electrochemical reactivity.
3.3. Optimum experimental conditions of electrochemical detection of Pb2+ The SWASV was applied in this work due to the better sensitivity and lower background than the CV technology [29]. In order to check the performance of MnFe2O4 NC modified electrode toward Pb2+, we analyzed the SWASV response under different experimental conditions of supporting electrolytes, pH value, deposition potential and deposition time.
3.3.1. Supporting electrolytes The stripping voltammetric response of 0.5 μM Pb2 +was also assessed by varying the supporting electrolytes. The 0.1 M pH 5.0 solutions of PBS, NH4Cl–NH4OH, and NaAc–HAc were compared in respect to the response toward Pb2 + on the MnFe2O4 NC modified electrode using SWASV (Fig. 3a). No signal was obtained in PBS solution. Lower peak intensity (contrast to NaAc–HAc solution) was observed in NH4Cl–NH4OH. When in NaAc–HAc, the stripping peak height became much greater, showing that the effectivity and sensitivity of the MnFe2O4 NC modified electrode toward Pb2+ were greater than in the electrolyte of NaAc–HAc.
3.3.2. pH value Fig. 3b shows the effect of solution pH on the lead stripping current in the presence of 0.5 μM Pb2+ in 0.1 M NaAc–HAc. The pH value was adjusted by the ratio of NaAc to HAc. It was found that the optimized pH value should be 4.0–6.0. And the stripping current became poorer when the pH is too low or too high. It is consistent with the results reported previously that the adsorption capacity of the adsorbent (porous magnesium oxide nanoflowers) almost reaches maximum as the pH values are about 4.0–5.0 [14]. It was previously reported that the voltammetric signal of metal, such as Pb, was controlled by how well the electrode materials can capture Pb and is subsequently collected on the electrode surface [15]. So the stripping current of the modified electrode is affected by its own maximum adsorption capacity toward metal ions under different pH conditions. When the pH is too low, the stability of MnFe2O4 NCs will be destroyed. When the pH value is higher than 6, there will be a decrease in adsorption due to precipitation of Pb(II) in the form of Pb(OH)2 [30]. Therefore, the pH 5.0 was used throughout. 3.3.3. Deposition potential Different deposition potential in the range of −1.2 to −0.8 V were examined using SWASV with standard additions of 0.5 μM Pb2 + in 0.1 M NaAc–HAc (pH 5.0). As shown in Fig. 3c, the resulting stripping current decreased when the deposition potential shifted from −1.0 V to −0.8 V (vs. Ag/AgCl). However, when the potential was lower than − 1.0 V, the stripping current kept steady basically. Small bubbles were observed on electrode surface when the deposition potential was less than − 1.0 V. One possibility was that H2 was generated on the surface of the electrode under such negative potentials [31], which blocked the deposition of Pb(0). The hydrogen bubbles might damage the metal alloy deposition at the electrode surface and lead to a decrease in current signals at very negative potentials [32]. Therefore, a deposition potential of −1.0 V was selected for further studies.
Fig. 3. Optimum experimental conditions. Influence of (a) supporting electrolytes; (b) pH value; (c) deposition potential; and (d) deposition time on the voltammetric response of the MnFe2O4 NC modified GCE. Data were evaluated by SWASV of 0.5 μM Pb2+.
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3.3.4. Deposition time Different deposition times of 30, 60, 90, 120, 150, 180, and 210 s were examined using SWASV with standard additions of 0.5 μM Pb2+ in 0.1 M NaAc–HAc (pH 5.0). As seen in Fig. 3d, when the deposition time is less than 150 s, the stripping current increases linearly with the increase of deposition time. The case after 180 s deposition suggests that the electrode surface is saturated by the Pb2+. It is worthwhile to note that as contacted with Pb2 +, the adsorption equilibrium of MnFe2O4 NCs is strongly related to initial Pb2+ concentration. For the detection of low concentrations of Pb2+, longer deposition time could be applied. For high concentrations of Pb2 +, short deposition time could be used to avoid the saturation of the electrode surface. Considering the time consumed, an optimized deposition time of 150 s was used throughout. Finally, the optimum experimental conditions of electrochemical detection of Pb2 + were determined in the presence of 0.5 μM Pb2 + in 0.1 M NaAc–HAc (pH 5.0) by depositing for 150 s, under the deposition potential of −1.0 V. Other parameters such as step potential, amplitude and frequency were set under the fixed value 5 mV, 20 mV and 25 Hz, respectively. 3.4. Electrochemical detection of Pb2+ with MnFe2O4 NC modified GCE Under the optimal experimental conditions, Pb2+ was determined on the MnFe2O4 NC modified GCE using SWASV. Fig. 4a presents the SWASV response toward Pb2 + over the concentration range of 0.2– 1.1 μM. The linearization equation was i/μA = − 2.81 + 19.9 μA/μM, with the correlation coefficients of 0.998 (Fig. 4b). The limit of detection (LOD, calculated by 3σ method) was 0.054 μM. The sensitivity and detection limit of the present study, together with previous determined values for Pb2+ electrochemical sensing in various other modified electrodes, are summarized in Table 1. In contrast to other electrode systems, it can be observed that the preferable sensitivity (19.9 μA/μM) and LOD (0.054 μM) can be obtained at the MnFe2O4 NC modified GCE. 3.5. Selectivity and interference measurements The SWASV responses of MnFe2O4 NC modified GCE toward the Pb2+, Cd2+, Hg2+ and Cu2+ at different concentrations are shown in Fig. 4. Individually, Pb2+, Cd2+, Hg2+ and Cu2+ were detected at potentials of −0.6, −0.8, +0.28 and −0.08 V, respectively. It is showed that the voltammetric peak for the stripping of Pb2+, Cd2+, Hg2+ and Cu2+ on the MnFe2O4 NC modified GCE appears at different potentials with a separation of 190–506 mV between the stripping peaks, such a separation between the voltammetric peaks is large enough, and hence the selective detection using the MnFe2O4 NC modified GCE is feasible. Fig. 5 shows that the sensitivity toward Pb2+, Cd2+, Hg2+ and Cu2+ is 19.9, 4.01, 3.70 and 6.50 μA/μM, respectively. It can be found that the sensitivity toward Pb2+ is about 4 times higher than that of Cd2+ and Hg2+, 3
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Table 1 Comparison of current sensitivity and LOD with previously reported values of different electrodes for electrochemical detection of Pb2+. Electrodes
Sensitivity (μA/μM)
LOD
Ref.
MgO–Nafi GCE on/GCE Band Fe3O4/rGO GCE Fe3O4/rGO GCE poMWCNTs GCE MnO2 nanoparticles GCE MgSiO3/Nafion GCE MnFe2O4/ GCE
5.89 13.6 5.8 3.55 4.42 9.44 19.9
– 0.169 μM – 0.057 nM 0.075 μM 0.274 nM 0.054 μM
[33] [34] [35] [15] [36] [37] This work
times higher than that of Cu2+. Besides, the stripping signals of Zn2+ even were not emerged under the electrochemical stripping conditions identical to those in Fig. 4. It demonstrates that the MnFe2O4 NC modified GCE has a high selectivity toward Pb2+. In order to investigate the interference of the other heavy metal ions (Cd2+, Hg2+, Cu2+ and Zn2+) to the electrochemical detection of Pb2+, a series of interference measurements were studied. Fig. 6 presents the SWASV response of 0.5 μM Pb2+ on MnFe2O4 NC modified electrode in 0.1 M HAc–NaAc (pH 5.0) in the presence of 5.0 μM Cd2+, Zn2+, Hg2+ and Cu2 +, respectively. The interference results of Cd2+, Zn2 +, Hg2+ and Cu2+ toward Pb2+ collected from SWASV responses were summarized and showed in Fig. 7. According to Fig. 7, the peak currents of Pb2+ were almost the same in the presence of 5.0 μM Cd2+, Zn2+ and Hg2+, respectively. It means Cd2+, Zn2+ and Hg2+ do not interfere the detection of Pb2+ obviously for the possible reason on the adsorption ability of MnFe2O4 toward Pb2+. On the contrary, the peak current of Pb2+ was decreased by 60% after adding 5.0 μM Cu2+ into the solution in which 0.5 μM Pb2 + existed, which is due to the competition of adsorption sites between Pb2+ and Cu2+ on the electrode surface. 3.6. Stability measurement Repetitive deposition by the reduction of Pb2 + anodic stripping (reoxidation of Pb0 to Pb2 +) of electrodeposited Pb 0 cycles was then performed to characterize the reproducibility of the electrode performance. Fig. 8 shows the SWASV results of 0.5 μM Pb2 + on MnFe2 O 4 NC modified electrode after continuous cycling for 20 times. We found that the stripping current of the electrode was nearly constant. The relative standard deviation (RSD) in the peak currents was 4.6%. It demonstrated that the MnFe2O4 NC modified GCE showed favorable stability under the optimized condition. 3.7. Real sample analysis For the evaluation of the practical application of the modified electrode, water samples were taken from Fen River water. The real sample
Fig. 4. SWASV responses (a) and the corresponding calibration plot (b) of the MnFe2O4 NC modified GCEs toward Pb2+ derived from triplicate experiments over a concentration range of 0.2 to 1.1 μM by depositing for 150 s in 0.1 M HAc–NaAc (pH 5.0). Deposition potential: −1.0 V, step potential: 5 mV, amplitude: 20 mV, and frequency: 25 Hz.
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Fig. 5. (a–d) SWASV responses of the MnFe2O4 NC modified GCEs toward the individual Pb2+, Cd2+, Hg2+ and Cu2+ at different concentrations in 0.1 M HAc–NaAc (pH 5.0). Electrochemical stripping conditions are identical to those in Fig. 4.
Fig. 6. Interference studies of typical SWASV responses of 0.5 μM Pb2+ on MnFe2O4 NC modified electrode in HAc–NaAc (pH 5.0) in the presence of (a) Cd2+, (b) Zn2+, (c) Hg2+ and (d) Cu2+, respectively. SWASV conditions are identical to those in Fig. 4.
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Table 2 Determination of Pb2+ in real samples (number of samples assayed = 3). Sample
Pb2+ added (μM)
Pb2+ found (μM)
Recovery (%)
Fen River water
0.100
0.0921
92.1
a potential material for the design of new sensing materials in the application field of electrochemical detection of toxic metal ions. References
Fig. 7. Interference studies of the MnFe2O4 NC modified GCEs in 0.1 M HAc–NaAc (pH 5.0), containing 0.5 μM Pb2+ in the presence of 5.0 μM Cd2+,Zn2+, Hg2+ and Cu2+, respectively. Electrochemical stripping conditions are identical to those in Fig. 4.
was diluted with 0.1 M HAc–NaAc buffer solution (pH 5.0) in a ratio of 1:9, and no further sample treatment was done. No obvious signals for Pb2 + were observed in the samples. In order to evaluate the validity of the proposed method for the detection, recovery studies were carried out on the real samples to which known amount of Pb2+ was added. The results are shown in Table 2 and the recovery obtained was calculated to be 92.1%, which revealed that the proposed method has potential for practical application.
4. Conclusions In summary, the as-prepared MnFe2O4 NCs with diameter of 200–400 nm and mesoporous structure composed of nanocrystals with a size of about 8–12 nm have been used for the selective electrochemical detection of Pb2+ based on SWASV. The MnFe2O4 NC modified GCE has a high selectivity toward Pb2 + with a favorable sensitivity (19.9 μA μM−1) and LOD (0.054 μM) for Pb2+ under the optimized conditions while the response to Cd2+, Hg2+, Cu2+ and Zn2+ is poor. In addition, no interference from Cd2 +, Zn2 + and Hg2 + was recognized during the detection of Pb2 +. Moreover, the MnFe2O4 NC modified GCE offered favorable stability and potential practical applicability in the electrochemical determination of Pb2+. The above results provided
Fig. 8. Stability study of the MnFe2O4 NC modified GCEs toward Pb2+.
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Mesoporous MnFe2O4 nanocrystal clusters for electrochemistry detection of lead by stripping voltammetry Xiao-Juan Han a, Shao-Feng Zhou a,⁎, Hong-Lei Fan b, Qiao-Xin Zhang c, Ya-Qing Liu a,⁎ a b c
School of Materials Science and Engineering, North University of China, Taiyuan 030051, PR China School of Chemical Engineering and Environment, North University of China, Taiyuan 030051, PR China School of Mechanical and Electronic Engineering, Wuhan University of Technology, Wuhan 430070, PR China
a r t i c l e
i n f o
Article history: Received 9 April 2015 Received in revised form 30 July 2015 Accepted 31 July 2015 Available online 1 August 2015 Keywords: Lead MnFe2O4 Stripping voltammetry
a b s t r a c t High-adsorption MnFe2O4 nanocrystal clusters (MnFe2O4 NC) with mesoporous structure were used for selective analysis of Pb2+ by square wave anodic stripping voltammetry (SWASV) in this work. The as-prepared MnFe2O4 NCs with diameter of 200–400 nm and mesoporous structure composed of nanocrystals with a size of about 8–12 nm were characterized using SEM, HRTEM and XRD. Electrochemical properties were characterized by cyclic voltammetry and electrochemical impedance spectroscopy. The chemical and electrochemical parameters influencing on deposition and stripping of metal ions, such as supporting electrolytes, pH value, deposition potential, and deposition time, were also studied. The MnFe2O4 NC modified GCE has a high selectivity toward Pb2+ with a favorable sensitivity (19.9 μA μM−1) and LOD (0.054 μM) for Pb2+ under the optimized conditions while the response to Cd2+, Hg2+, Cu2+ and Zn2+ is poor. No interference from Cd2+, Zn2+ and Hg2+, favorable stability and potential practical applicability were recognized in the electrochemical determination of Pb2+. The above results provided a potential material for the design of new sensing materials in the application field of electrochemical detection toward toxic metal ions. © 2015 Elsevier B.V. All rights reserved.
1. Introduction It is well known that environmental contamination by heavy metals has been a serious and complex problem and has remained a focus of attention all over the world [1,2]. Heavy metals, unlike organic contaminants, were not biodegradable and tended to accumulate in living organisms and most of them were known to be poisonous and carcinogenic [3]. Effective determination of heavy metals in low concentration was important due to their hazards to the physical health of human being [4–6]. The electrochemical technique, as an alternative to the conventional spectroscopic techniques, has been recognized as a promising method for trace and on-site analysis of toxic heavy metal ions due to portability, high sensitivity, good selectivity, low cost, and suitability [7–12]. It is known that the electrochemical performance is highly dependent on the sensing materials, which is reasonable when presumed that the materials with high adsorption capacity can improve the efficiency for accumulating analytes [13]. Materials with high adsorbability are leading a new opportunity for the sensing performance in electrochemical detection. Very recently, based on the remarkable adsorption ability, nanomaterials have been developed as a detection strategy toward heavy metal ions [14–18]. Due to the adsorption capacity, O2-plasma oxidized multi-walled ⁎ Corresponding authors. E-mail addresses: [email protected] (S.-F. Zhou), [email protected] (Y.-Q. Liu).
http://dx.doi.org/10.1016/j.jelechem.2015.07.054 1572-6657/© 2015 Elsevier B.V. All rights reserved.
carbon nanotubes [15] and γ-AlOOH(boehmite)@SiO2/Fe3O4 porous magnetic microspheres [19] were found to be useful for the electrochemical detection of heavy metal ions. Polypyrrole/reduced graphene oxide nanocomposites were used for identifying Hg2 + by means of their highly specific adsorption ability toward Hg2 + [17]. Porous Co3O4 microsheets presented a high sensitivity and a quite nice low detection limit toward Pb2+ because of their high adsorption capacities [13]. As ideal adsorbents, ferrite (MFe2O4; M = Fe, Mn, Zn, or Co) nanoparticles have attracted great attention for the characteristic of high adsorption capacity and superparamagnetism. With the high adsorption capacity of nanostructure, the superparamagnetic materials can disperse well in the solution due to the absence of any residual magnetization without the external applied magnetic field, avoiding the typical aggregation problems of ferromagnetic materials [20,21]. With a high adsorption performance, the MnFe2O4 NCs combined with uniform mesoporous structure and small constituent nanocrystals (about 8–12 nm in size) even provide a synergistic effect for enhanced adsorption performance toward As(III) [22]. However, to the best of our knowledge, the high-adsorption ferrite (MFe2O4; M = Fe, Mn, Zn, or Co) materials used as sensing materials for the detection of heavy metal ions have rarely been reported. In this work, we modified a glass carbon electrode (GCE) using mesoporous MnFe2O4 nanocrystal clusters (MnFe2O4 NC) for selective analysis of Pb2 + by square wave anodic stripping voltammetry
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(SWASV), and the selective detection toward Pb2+ was achieved. The optimizing of experimental conditions, such as supporting electrolytes, pH values, deposition potential, and deposition time were investigated. Other heavy Metal ions, such as Cd2+, Hg2+, Cu2+ and Zn2+, were chosen as potential interfering ions for investigating the electrochemical selective behavior of mesoporous MnFe2O4 NC toward Pb2+. Furthermore, the stability of mesoporous MnFe2O4 NC modified electrode was also studied.
2. Experimental 2.1. Chemical reagents All chemicals are of analytical grade, which are commercially available from Shanghai Chemical Reagent Co. Ltd and without further purification in this study. Acetate buffer solution of 0.1 M with different pH was prepared by mixing stock solutions of 0.1 M NaAc, HAc and NaOH. Phosphate buffer solutions (PBS) of 0.1 M were prepared by mixing stock solutions of 0.1 M H3PO4, KH2PO4, K2HPO4, and NaOH. NH4Cl-NH3·H2O (0.1 M) solution was prepared by mixing stock solutions of 0.1 M NH4Cl and NH3·H2O in different proportions. The water (18.2 MΩ cm) that was used to prepare all solutions was purified with the NANO pure Diamond UV water system.
2.2. Apparatus All electrochemical measurements were performed using a CHI 660D computer-controlled potentiostat (ChenHua Instruments Co., Shanghai, China). A conventional three-electrode configuration was employed, consisting of a bare or modified glassy carbon electrode (GCE) as a working electrode, an Ag/AgCl/saturated KCl electrode as a reference electrode, and a platinum wire as a counter electrode. The morphology of MnFe2O4 NCs was investigated by scanning electron microscopy (SEM, Quanta 200 FEG, FEI Company, USA). High Resolution Transmission electron microscopy (HRTEM) was carried out on a JEM2010 microscope. X-ray diffraction (XRD) was carried out using a PW1710 instrument with Cu Kα radiation λ = 0.15406 Å.
2.3. Synthesis of MnFe2O4 NCs MnFe2O4 NCs were synthesized according to a previous report [23]. In brief, 5 mmol of FeCl3 · 6H2O (1.35 g) and 2.5 mmol MnCl2·4H2O (0.50 g) were dissolved in ethylene glycol (40 mL) to form a clear solution, followed by the addition of NaAc (3.6 g) and polyethylene glycol (1.0 g). The mixture was stirred vigorously for 30 min and then sealed in a Teflon-lined autoclave (50 mL capacity), heated at 200 °C in an electric oven for 8 h, and then cooled to room temperature naturally. The black products were centrifuged and washed with deionized water and absolute alcohol for several times and finally dried at 60 °C for 6 h.
2.4. Preparation of MnFe2O4 NC modified electrode Prior to modification, the bare glassy carbon electrode was sequentially polished with 0.3 μm and 0.05 μm alumina power slurries to a mirror-shiny surface, and then sonicated with 1:1 HNO3 solution, absolute ethanol and deionized water for 1 min, respectively. The MnFe2O4 film on the surface of glassy carbon electrode was performed in the following manner: 2 mg of MnFe2O4 NCs was added into 2 mL of Milli-Q water, and sonicated for 3 min to obtain a uniform dispersion. A drop of the above solution was pipetted onto the fresh surface of GCE and dried in air. After evaporation, a thin MnFe2O4 film was formed on the electrode surface.
2.5. Electrochemical measurements Square wave anodic stripping voltammetry (SWASV) was used for the observation of electrochemical behavior under optimized conditions. Pb, Cd, Hg, Cu, and Zn were deposited at the potential of 1.0 V for 150 s by the reduction of Pb2 +, Cd2 +, Hg2 +, Cu2 +, and Zn2 + in 0.1 M HAc-NaAc (pH = 5.0). The anodic stripping (reoxidation of metal to metal ions) of electrodeposited metal was performed in the potential range of −1.0 to −0.2 V at the following condition: frequency, 25 Hz; amplitude, 25 mV; increment potential, 4 mV; vs Ag/AgCl. A desorption potential of −0.1 V for 150 s was performed to remove the residual metals under stirring condition. The same experiment conditions were applied in the interference and stability studies. Cyclic voltammograms (CV) and electrochemical impedance spectra (EIS) were performed in mixing solution of 5 mM Fe(CN)36 −/4 − with 0.1 M KCl and the scanning rate was 100 mV s−1. 3. Results and discussion 3.1. Characterization of MnFe2O4 NCs Fig. 1a shows monodisperse, uniform and clean spherical shape of MnFe2O4 microspheres with diameter of 200–400 nm at a large scale. Fig. 1b indicates that the typical individual ferrite microsphere is a loose cluster, and clearly, the cluster is composed by small nanocrystals with the size of about 8–12 nm. Between the small nanocrystals, the structure of mesoporous can be found in Fig. 1b. This architecture might render them with a large surface area for adsorption of guest molecules [24–26]. The selected-area electron diffraction (SAED) pattern showed that the diffraction spots are widened into narrow arcs (insets of Fig. 1b), indicating the clusters are made up of many misaligned ferrite nanocrystals [24,27], in agreement with the analysis of HRTEM (Fig. 1b). The d value of crystal plane (311) in Fig. 1c and characteristic peaks in Fig. 1d verify the crystalline structure of MnFe2O4 NCs. And the broad diffraction peaks shown in Fig. 1d suggest the small size of ferrite NCs formed. Based on the calculations with the Scherrer formula for the strongest (311) diffraction peak, the average primary grain size is estimated to be 11 nm for MnFe2O4 NCs, which is consistent with the analysis of Fig. 1b. Taking the analyses of SEM, HRTEM, and XRD mentioned above, the as-prepared MnFe2O4 NCs with diameter of 200–400 nm and mesoporous structure are composed of nanocrystals with a size of about 8–12 nm, might render them with large surface area for adsorption and detection of heavy metal ions. 3.2. CV and EIS characterization of MnFe2O4 NC modified GCE The fabricated MnFe2O4 NC modified GCE was firstly electrochemically characterized using cyclic voltammograms (CV) and electrochemical impedance spectra (EIS) (Fig. 2). Fig. 2a displays the CV response of bare GCE and MnFe2O4 NC modified GCE electrodes. As compared with the bare GCE, the anodic and cathodic peak currents were decreased at MnFe2O4 NC modified GCE. This was due to the poor conductivity of MnFe2O4 NCs. Fig. 2b shows that the impedance spectra of MnFe2O4 NC modified GCE electrodes include a semicircle portion and a linear portion. The semicircle diameter at higher frequencies corresponds to the electron transfer resistance (Ret), and the linear part at lower frequencies corresponds to the diffusion process [28]. As seen in Fig. 2b, it was observed that the Ret of the MnFe2O4 NC modified GCE was about 1.8 kΩ, while the EIS of the bare GCE displays an almost straight line. It means that the Ret of the MnFe2O4 NC modified GCE is much higher than the bare GCE, showing that the modification of MnFe2O4 NCs can hinder the electron-transporting on surface of the electrode. The nanocrystal cluster structure of MnFe2O4 could create a further barrier and hinder the access of the redox probe to the electrode surface, resulting in a large electron transfer resistance of the MnFe2O4 NC modified GCE, which is consistent with the CV results.
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Fig. 1. SEM image (a), HRTEM image (b, c) and XRD pattern of the MnFe2O4 NCs. Inset in panel b is the corresponding SAED pattern.
Fig. 2c presents the SWASV analytical characteristic of bare GCE and MnFe2O4 NC modified GCE toward Pb2+. When the accumulation process was carried out for 150 s at − 1.0 V in a solution containing
0.5 μM Pb2 + in 0.1 M HAc–NaAc (pH 5.0) without deaeration, weak peaks with the peak current about 5.0 μA were observed at the bare GCE in the potential range of − 1.0 to − 2.0 V. For the MnFe2O4 NC
Fig. 2. Cyclic voltammograms (a) and Nyquist diagram of electrochemical impedance spectra (b) for bare GCE and MnFe2O4 NC modified GCE in the solution of 5 mM Fe(CN)3−/4− 6 containing 0.1 M KCl. Potential scan rate: 100 mV s−1. (c) SWASVs for 0.5 μM Pb2+ on the bare GCE and MnFe2O4 NC modified GCE in 0.1 M HAc–NaAc (pH 5.0). Deposition potential: −1.0 V, step potential: 5 mV, amplitude: 20 mV, and frequency: 25 Hz.
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modified GCE, the peak current increases to about 11.0 μA, which is much higher than the bare GCE. It shows that the MnFe2O4 NCs could provide more remarkable electrochemical performances toward Pb2+ detection. It might be attributed to the unique mesoporous structure of MnFe2O4 NCs composed of nanocrystals, which might render them with large contact area of the electrode/electrolyte interface and provided more active sites for the electrochemical reactions in the detection process of Pb2 +. Besides, the Fe and Mn species in the MnFe2O4 NCs could provide a synergistic effect for the detection of Pb2+ contributed by the higher electrochemical reactivity.
3.3. Optimum experimental conditions of electrochemical detection of Pb2+ The SWASV was applied in this work due to the better sensitivity and lower background than the CV technology [29]. In order to check the performance of MnFe2O4 NC modified electrode toward Pb2+, we analyzed the SWASV response under different experimental conditions of supporting electrolytes, pH value, deposition potential and deposition time.
3.3.1. Supporting electrolytes The stripping voltammetric response of 0.5 μM Pb2 +was also assessed by varying the supporting electrolytes. The 0.1 M pH 5.0 solutions of PBS, NH4Cl–NH4OH, and NaAc–HAc were compared in respect to the response toward Pb2 + on the MnFe2O4 NC modified electrode using SWASV (Fig. 3a). No signal was obtained in PBS solution. Lower peak intensity (contrast to NaAc–HAc solution) was observed in NH4Cl–NH4OH. When in NaAc–HAc, the stripping peak height became much greater, showing that the effectivity and sensitivity of the MnFe2O4 NC modified electrode toward Pb2+ were greater than in the electrolyte of NaAc–HAc.
3.3.2. pH value Fig. 3b shows the effect of solution pH on the lead stripping current in the presence of 0.5 μM Pb2+ in 0.1 M NaAc–HAc. The pH value was adjusted by the ratio of NaAc to HAc. It was found that the optimized pH value should be 4.0–6.0. And the stripping current became poorer when the pH is too low or too high. It is consistent with the results reported previously that the adsorption capacity of the adsorbent (porous magnesium oxide nanoflowers) almost reaches maximum as the pH values are about 4.0–5.0 [14]. It was previously reported that the voltammetric signal of metal, such as Pb, was controlled by how well the electrode materials can capture Pb and is subsequently collected on the electrode surface [15]. So the stripping current of the modified electrode is affected by its own maximum adsorption capacity toward metal ions under different pH conditions. When the pH is too low, the stability of MnFe2O4 NCs will be destroyed. When the pH value is higher than 6, there will be a decrease in adsorption due to precipitation of Pb(II) in the form of Pb(OH)2 [30]. Therefore, the pH 5.0 was used throughout. 3.3.3. Deposition potential Different deposition potential in the range of −1.2 to −0.8 V were examined using SWASV with standard additions of 0.5 μM Pb2 + in 0.1 M NaAc–HAc (pH 5.0). As shown in Fig. 3c, the resulting stripping current decreased when the deposition potential shifted from −1.0 V to −0.8 V (vs. Ag/AgCl). However, when the potential was lower than − 1.0 V, the stripping current kept steady basically. Small bubbles were observed on electrode surface when the deposition potential was less than − 1.0 V. One possibility was that H2 was generated on the surface of the electrode under such negative potentials [31], which blocked the deposition of Pb(0). The hydrogen bubbles might damage the metal alloy deposition at the electrode surface and lead to a decrease in current signals at very negative potentials [32]. Therefore, a deposition potential of −1.0 V was selected for further studies.
Fig. 3. Optimum experimental conditions. Influence of (a) supporting electrolytes; (b) pH value; (c) deposition potential; and (d) deposition time on the voltammetric response of the MnFe2O4 NC modified GCE. Data were evaluated by SWASV of 0.5 μM Pb2+.
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3.3.4. Deposition time Different deposition times of 30, 60, 90, 120, 150, 180, and 210 s were examined using SWASV with standard additions of 0.5 μM Pb2+ in 0.1 M NaAc–HAc (pH 5.0). As seen in Fig. 3d, when the deposition time is less than 150 s, the stripping current increases linearly with the increase of deposition time. The case after 180 s deposition suggests that the electrode surface is saturated by the Pb2+. It is worthwhile to note that as contacted with Pb2 +, the adsorption equilibrium of MnFe2O4 NCs is strongly related to initial Pb2+ concentration. For the detection of low concentrations of Pb2+, longer deposition time could be applied. For high concentrations of Pb2 +, short deposition time could be used to avoid the saturation of the electrode surface. Considering the time consumed, an optimized deposition time of 150 s was used throughout. Finally, the optimum experimental conditions of electrochemical detection of Pb2 + were determined in the presence of 0.5 μM Pb2 + in 0.1 M NaAc–HAc (pH 5.0) by depositing for 150 s, under the deposition potential of −1.0 V. Other parameters such as step potential, amplitude and frequency were set under the fixed value 5 mV, 20 mV and 25 Hz, respectively. 3.4. Electrochemical detection of Pb2+ with MnFe2O4 NC modified GCE Under the optimal experimental conditions, Pb2+ was determined on the MnFe2O4 NC modified GCE using SWASV. Fig. 4a presents the SWASV response toward Pb2 + over the concentration range of 0.2– 1.1 μM. The linearization equation was i/μA = − 2.81 + 19.9 μA/μM, with the correlation coefficients of 0.998 (Fig. 4b). The limit of detection (LOD, calculated by 3σ method) was 0.054 μM. The sensitivity and detection limit of the present study, together with previous determined values for Pb2+ electrochemical sensing in various other modified electrodes, are summarized in Table 1. In contrast to other electrode systems, it can be observed that the preferable sensitivity (19.9 μA/μM) and LOD (0.054 μM) can be obtained at the MnFe2O4 NC modified GCE. 3.5. Selectivity and interference measurements The SWASV responses of MnFe2O4 NC modified GCE toward the Pb2+, Cd2+, Hg2+ and Cu2+ at different concentrations are shown in Fig. 4. Individually, Pb2+, Cd2+, Hg2+ and Cu2+ were detected at potentials of −0.6, −0.8, +0.28 and −0.08 V, respectively. It is showed that the voltammetric peak for the stripping of Pb2+, Cd2+, Hg2+ and Cu2+ on the MnFe2O4 NC modified GCE appears at different potentials with a separation of 190–506 mV between the stripping peaks, such a separation between the voltammetric peaks is large enough, and hence the selective detection using the MnFe2O4 NC modified GCE is feasible. Fig. 5 shows that the sensitivity toward Pb2+, Cd2+, Hg2+ and Cu2+ is 19.9, 4.01, 3.70 and 6.50 μA/μM, respectively. It can be found that the sensitivity toward Pb2+ is about 4 times higher than that of Cd2+ and Hg2+, 3
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Table 1 Comparison of current sensitivity and LOD with previously reported values of different electrodes for electrochemical detection of Pb2+. Electrodes
Sensitivity (μA/μM)
LOD
Ref.
MgO–Nafi GCE on/GCE Band Fe3O4/rGO GCE Fe3O4/rGO GCE poMWCNTs GCE MnO2 nanoparticles GCE MgSiO3/Nafion GCE MnFe2O4/ GCE
5.89 13.6 5.8 3.55 4.42 9.44 19.9
– 0.169 μM – 0.057 nM 0.075 μM 0.274 nM 0.054 μM
[33] [34] [35] [15] [36] [37] This work
times higher than that of Cu2+. Besides, the stripping signals of Zn2+ even were not emerged under the electrochemical stripping conditions identical to those in Fig. 4. It demonstrates that the MnFe2O4 NC modified GCE has a high selectivity toward Pb2+. In order to investigate the interference of the other heavy metal ions (Cd2+, Hg2+, Cu2+ and Zn2+) to the electrochemical detection of Pb2+, a series of interference measurements were studied. Fig. 6 presents the SWASV response of 0.5 μM Pb2+ on MnFe2O4 NC modified electrode in 0.1 M HAc–NaAc (pH 5.0) in the presence of 5.0 μM Cd2+, Zn2+, Hg2+ and Cu2 +, respectively. The interference results of Cd2+, Zn2 +, Hg2+ and Cu2+ toward Pb2+ collected from SWASV responses were summarized and showed in Fig. 7. According to Fig. 7, the peak currents of Pb2+ were almost the same in the presence of 5.0 μM Cd2+, Zn2+ and Hg2+, respectively. It means Cd2+, Zn2+ and Hg2+ do not interfere the detection of Pb2+ obviously for the possible reason on the adsorption ability of MnFe2O4 toward Pb2+. On the contrary, the peak current of Pb2+ was decreased by 60% after adding 5.0 μM Cu2+ into the solution in which 0.5 μM Pb2 + existed, which is due to the competition of adsorption sites between Pb2+ and Cu2+ on the electrode surface. 3.6. Stability measurement Repetitive deposition by the reduction of Pb2 + anodic stripping (reoxidation of Pb0 to Pb2 +) of electrodeposited Pb 0 cycles was then performed to characterize the reproducibility of the electrode performance. Fig. 8 shows the SWASV results of 0.5 μM Pb2 + on MnFe2 O 4 NC modified electrode after continuous cycling for 20 times. We found that the stripping current of the electrode was nearly constant. The relative standard deviation (RSD) in the peak currents was 4.6%. It demonstrated that the MnFe2O4 NC modified GCE showed favorable stability under the optimized condition. 3.7. Real sample analysis For the evaluation of the practical application of the modified electrode, water samples were taken from Fen River water. The real sample
Fig. 4. SWASV responses (a) and the corresponding calibration plot (b) of the MnFe2O4 NC modified GCEs toward Pb2+ derived from triplicate experiments over a concentration range of 0.2 to 1.1 μM by depositing for 150 s in 0.1 M HAc–NaAc (pH 5.0). Deposition potential: −1.0 V, step potential: 5 mV, amplitude: 20 mV, and frequency: 25 Hz.
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Fig. 5. (a–d) SWASV responses of the MnFe2O4 NC modified GCEs toward the individual Pb2+, Cd2+, Hg2+ and Cu2+ at different concentrations in 0.1 M HAc–NaAc (pH 5.0). Electrochemical stripping conditions are identical to those in Fig. 4.
Fig. 6. Interference studies of typical SWASV responses of 0.5 μM Pb2+ on MnFe2O4 NC modified electrode in HAc–NaAc (pH 5.0) in the presence of (a) Cd2+, (b) Zn2+, (c) Hg2+ and (d) Cu2+, respectively. SWASV conditions are identical to those in Fig. 4.
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Table 2 Determination of Pb2+ in real samples (number of samples assayed = 3). Sample
Pb2+ added (μM)
Pb2+ found (μM)
Recovery (%)
Fen River water
0.100
0.0921
92.1
a potential material for the design of new sensing materials in the application field of electrochemical detection of toxic metal ions. References
Fig. 7. Interference studies of the MnFe2O4 NC modified GCEs in 0.1 M HAc–NaAc (pH 5.0), containing 0.5 μM Pb2+ in the presence of 5.0 μM Cd2+,Zn2+, Hg2+ and Cu2+, respectively. Electrochemical stripping conditions are identical to those in Fig. 4.
was diluted with 0.1 M HAc–NaAc buffer solution (pH 5.0) in a ratio of 1:9, and no further sample treatment was done. No obvious signals for Pb2 + were observed in the samples. In order to evaluate the validity of the proposed method for the detection, recovery studies were carried out on the real samples to which known amount of Pb2+ was added. The results are shown in Table 2 and the recovery obtained was calculated to be 92.1%, which revealed that the proposed method has potential for practical application.
4. Conclusions In summary, the as-prepared MnFe2O4 NCs with diameter of 200–400 nm and mesoporous structure composed of nanocrystals with a size of about 8–12 nm have been used for the selective electrochemical detection of Pb2+ based on SWASV. The MnFe2O4 NC modified GCE has a high selectivity toward Pb2 + with a favorable sensitivity (19.9 μA μM−1) and LOD (0.054 μM) for Pb2+ under the optimized conditions while the response to Cd2+, Hg2+, Cu2+ and Zn2+ is poor. In addition, no interference from Cd2 +, Zn2 + and Hg2 + was recognized during the detection of Pb2 +. Moreover, the MnFe2O4 NC modified GCE offered favorable stability and potential practical applicability in the electrochemical determination of Pb2+. The above results provided
Fig. 8. Stability study of the MnFe2O4 NC modified GCEs toward Pb2+.
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