Electrochemistry Communications 34 (2013) 270–273 Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom Short communication Effect of morphology of α-MnO2 nanocrystals on electrochemical detection of toxic metal ions Qiao-Xin Zhang a, Dai Peng b, Xing-Jiu Huang a,c,⁎ a b c School of Mechanical and Electronic Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, PR China School of Materials Science and Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, PR China Research Center for Biomimetic Functional Materials and Sensing Devices, Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, PR China a r t i c l e i n f o Article history: Received 28 May 2013 Received in revised form 29 June 2013 Accepted 2 July 2013 Available online 8 July 2013 Keywords: MnO2 Nanoparticles Nanobowls Nanotubes Heavy metal ions Electrochemical detection a b s t r a c t Three different morphologies of α-MnO2 (nanoparticles, nanobowls and nanotubes) have been prepared for the electrochemical determination of Zn(II), Cd(II), Pb(II), Cu(II) and Hg(II). The three different morphologies of MnO2-modified electrodes offered an obvious regularity in individual electrochemical determination of Zn(II), Pb(II), Cu(II) and Hg(II): MnO2 nanobowls N MnO2 nanotubes N MnO2 nanoparticles. This study indicates the effect of morphology of MnO2 on its electrochemical detection of toxic metal ions. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Electrochemical technique has been recognized as a promising method for trace and on-site analysis of toxic heavy metal ions due to its portability, high sensitivity, good selectivity, low cost and suitability [1–4]. In the past few years, mercury had been widely used for stripping detection of trace heavy metal ions [5–8]. However, the extremely toxicity and environmentally hazardous nature of mercury restricted the use of mercury electrode for disposal and on-site analysis [9]. Extensive research efforts have therefore been devoted to find appropriate electrode materials to meet the growing demands for determination of heavy metal. Bismuth electrode and the composite materials with bismuth are most attractive alternatives to mercury electrode [10–15]. Besides, carbon nanotubes (CNTs) and chemically functional with multi-walled carbon nanotubes (MWCNTs)-modified electrodes are highly beneficial for electrochemical detection [16–18]. What's more, novel modifiers, such as organic, inorganic compounds, with characteristic functional groups constantly provoke increasing research interest [19–23]. Especially, metal oxide, as an environmentally friendly material, has been widely used for the electrochemical stripping analysis of heavy metals due to its low cost and adsorption capacity (e.g., MgO, γ-AlOOH(boehmite)@SiO2/Fe3O4, SnO2 and Co3O4) [24–27]. ⁎ Corresponding author at: Research Center for Biomimetic Functional Materials and Sensing Devices, Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, PR China. Tel.: +86 551 65591142; fax: +86 551 65592420. E-mail address: [email protected] (X.-J. Huang). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.07.005 However, to the best of our knowledge, the effect of different morphologies on electrochemical detection of toxic metal ions has rarely been reported before. MnO2 has been widely used as an absorbent to remove toxic ions from water due to its stability and favorable sorption characteristics [28–30]. As a number of studies have demonstrated that appropriate materials are highly beneficial for electrochemical detection because of their selective adsorption properties [23,24,27]. However, the use of electrodes modified with MnO2 in the stripping voltammetry is, to the best of our knowledge, unexplored. In this study, we evaluated electrochemical stripping properties of the three typical morphologies of MnO2 nanocrystals (i.e., nanoparticles, nanobowls and nanotubes) towards metal ions. This work shows the effect of different morphologies of MnO2 on electrochemical detection of toxic metal ions. 2. Experimental 2.1. Chemical reagents All reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and were of analytical grade. Acetate buffer solution (NaAc–HAc) of 0.1 M was prepared by mixing stock solutions of 0.1 M NaAc and HAc. Ultrapure fresh water was obtained from a Millipore water purification system (MilliQ, specific resistivity N18 MΩ cm, S.A., Molsheim, France) and used in all runs. Q.-X. Zhang et al. / Electrochemistry Communications 34 (2013) 270–273 2.2. Synthesis of three different morphologies of MnO2 The MnO2 nanoparticles were prepared according to that previously reported [31]. Briefly, 0.003 mol KMnO4 was dissolved in 30 mL C2H5OH (1 M) and then stirred for 24 h. The resulting product was washed with deionized H2O and dried at 70 °C for 24 h. The MnO2 nanobowls were synthesized by a hydrothermal method as reported elsewhere [32]. In a typical synthesis, 0.08 mol MnSO4 · H2O and 0.08 mol (NH4)2S2O8 were dissolved in 150 mL deionized water. The mixture was stirred for 10 min to form a homogeneous pink solution and then was heated in a stainless steel reactor at 90 °C for 24 h. The resulting product was washed with deionized H2O and dried at 60 °C for 24 h. The MnO2 nanotubes were synthesized using a modified strategy that has been used previously [33]. In our work, 0.006 mol MnSO4 · H2O and 0.016 mol KMnO4 were dissolved in 150 mL deionized water. The mixture was heated in a stainless steel reactor at 160 °C for 12 h. The resulting product was washed with deionized H2O and dried at 60 °C for 24 h. 271 electrode (GCE, 3 mm diameter), an Ag/AgCl reference electrode and a platinum wire counter electrode. 2.4. Preparation of 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, respectively. The MnO2 nanobowls/ nafion film on the surface of glassy carbon electrode was performed in the following manner: 5.0 μL of MnO2 ultrapure water solution was dripped onto the surface of a freshly polished glassy carbon electrode. The electrode was allowed to dry, and then 2.0 μL of 0.5% w/w nafion solution was pipetted onto it to increase conductivity and to prevent the MnO2 nanocrystals from falling off. The electrode was then allowed to air-dry at room temperature. MnO2 nanoparticles/nafion electrode and MnO2 nanotubes/nafion electrode were prepared in the same way. 2.5. Electrochemical measurements 2.3. Apparatus Scanning electron microscopy (SEM) images were taken by an FEI Quanta 200 FEG field emission scanning electron microscope. X-ray diffraction (XRD) analysis was carried out with a D/MAX-RB diffractometer with Cu-Kα radiation. Transmission electron microscopy (TEM) was carried out on a JEM-2010 microscope. Electrochemical experiments were recorded using a CHI 660D computer-controlled potentiostat (ChenHua Instruments Co., Shanghai, China). A conventional three-electrode system consisted of a glassy carbon working Square wave anodic stripping voltammetry (SWASV) was used for the detection under optimized conditions. Cadmium, lead, copper and mercury were deposited at the potential of − 1.2 V for 150 s by the reduction of Cd(II), Pb(II), Cu(II) and Hg(II) in 0.1 M NaAc–HAc (pH 5.0), and a deposition potential of − 1.3 V for 150 s in 0.1 M NaAc–HAc (pH 5.0) was applied for Zn(II) detection. The anodic stripping (reoxidation of metal to metal ions) of electrodeposited metal was performed at the following optimized parameters: frequency, 50 Hz; amplitude, 25 mV; and amplitude 0.05 V. Fig. 1. SEM images of (a) MnO2 nanoparticles, (b) MnO2 nanobowls and (c) MnO2 nanotubes. The insets show HRTEM and the corresponding electron SAED patterns. (d) XRD patterns of three different morphologies of MnO2. 272 Q.-X. Zhang et al. / Electrochemistry Communications 34 (2013) 270–273 3. Results and discussion Fig. 1a shows the SEM image of MnO2 nanoparticles. The morphology reveals well uniformly distributed nanoparticles with nanorods. The insets in Fig. 1a are HRTEM image and SAED pattern for the MnO2 nanoparticles. The SAED pattern indicates the nanoparticle is amorphous. Fig. 1b shows the SEM image of MnO2 bowls. It can be observed that this product consists of microsphere/nanorod hierarchical nanostructures. An HRTEM image and an SAED pattern were based on a single nanorod indicating the single-crystalline nature (Inset in Fig. 1b). An average d-spacing of about 0.5 nm is observed from the HRTEM. This d-spacing is consistent with the spacing between (200) planes. The SAED pattern and the HRTEM analysis suggest that the nanorods grow along the [001] direction. The nanotubes were homogenously distributed as shown in Fig. 1c. HRTEM and SAED analysis (Inset in Fig. 1c) suggest that the nanotubes possess the same growth direction as the nanobowls. Three different morphologies of MnO2 were further characterized by XRD and the corresponding patterns are shown in Fig. 1d. All the diffraction peaks observed are consistent with the face-centered cubic pattern for α-MnO2 (JCPDS no. 44-0141). Fig. 2a shows the SWASV responses of the MnO2 nanoparticles/ nafion toward Pb(II) over the concentration range of 0.1 to 0.9 μM in 0.1 M NaAc–HAc (pH 5.0). As seen from the calibration plot of Pb(II) (Inset in Fig. 2a), the peak currents increased linearly versus the Pb(II) concentrations with the sensitivity of 4.42 μA μM−1, the limit of detection (LOD) was calculated to be 0.075 μM (3σ method). Similarly, Fig. 2b presents the SWASV responses of the MnO2 nanobowls/nafion toward Pb(II) over the concentration range of 0.3 to 1.2 μM in 0.1 M NaAc–HAc (pH 5.0). A sensitivity of 15.43 μA μM−1 with a detection limit of 0.072 μM (3σ method) was obtained. Fig. 2c shows the SWASV responses of the MnO2 nanotubes/nafion toward Pb(II) over the concentration range of 0.3 to 1.9 μM in 0.1 M NaAc–HAc (pH 5.0). The LOD was calculated to be 0.105 μM (3σ method) with a sensitivity of 12.25 μA μM−1. It could be observed that the sensitivity for analysis of Pb(II) is as follows: MnO2 nanobowls/nafion N MnO2 nanotubes/nafion N MnO2 nanoparticles/nafion, and the LOD for analysis of Pb(II) has no significant differences. Some other common heavy metal ions were tested to evaluate the selectivity of three different morphologies of MnO2, and we found that Zn(II), Cd(II), Cu(II) and Hg(II) could also be detected. Fig. 3 shows the comparison of sensitivity and error bars for individual analysis of Zn(II), Cd(II), Cu(II), Hg(II) and Pb(II) at MnO2 nanoparticles/nafion, MnO2 nanobowls/nafion and MnO2 nanotubes/nafion composite-modified GCE, respectively. It is clear that the sensitivity for individual analysis of Zn(II), Cu(II) and Hg(II) is as follows: MnO2 nanobowls N MnO2 nanotubes N MnO2 nanoparticles, and the sensitivity for individual analysis of Cd(II) has no obvious regularity. What's more, the result shows that the selectivity in individual electrochemical determination of MnO2 nanotubes/nafion composite-modified GCE is as follows: Cd(II) N Pb(II) N Cu(II) N Zn(II), Hg(II). Finally, it is very important to point out that the different electroanalysis properties might be due to the different facets of MnO2 nanocrystals. 4. Conclusion We have realized three designs of the electrode surface for the electrochemical determination of five toxic metal ions using MnO2 nanoparticles, MnO2 nanobowls and MnO2 nanotubes. Moreover, the three different morphologies of MnO2-modified electrodes Fig. 2. SWASV responses and the corresponding calibration plots of (a) MnO2 nanoparticles/nafion, (b) MnO2 nanobowls/nafion and (c) MnO2 nanotubes/nafion electrode towards Pb(II) at different concentrations in 0.1 M NaAc–HAc solution (pH 5.0). Fig. 3. Comparison of sensitivity for Zn(II), Cd(II), Cu(II), Hg(II) and Pb (II) at MnO2 nanoparticles/nafion, MnO2 nanobowls/nafion and MnO2 nanotubes/nafion compositemodified GCE. The error bars are evaluated by the standard deviation of the sensitivity, which are acquired from the electrochemical experiments of each toxic metal ions for three times. Q.-X. Zhang et al. / Electrochemistry Communications 34 (2013) 270–273 offered an obvious regularity in individual electrochemical determination of Zn(II), Pb(II), Cu(II) and Hg(II): MnO2 nanobowls N MnO2 nanotubes N MnO2 nanoparticles and displays that the selectivity in individual electrochemical determination of MnO2 modified electrode is as follows: Cd(II) N Pb(II) N Cu(II) N Zn(II), Hg(II). 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