Electrochemistry Communications 34 (2013) 270–273
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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.
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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).
Acknowledgements
This work was supported by the National Key Scientiﬁc Program–
Nanoscience and Nanotechnology (2011CB933700) and National Natural Science Foundation of China (21073197). X.J.H. acknowledges the
CAS Institute of Physical Science, University of Science and Technology
of China (2012FXCX008), for ﬁnancial support.
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