Microchimica Acta

, Volume 170, Issue 3, pp 307–312

A sensitive mercury (II) sensor based on CuO nanoshuttles/poly(thionine) modified glassy carbon electrode

Authors

  • Zhaojing Yin
    • Anhui Key Laboratory of chemo-Biosening, College of Chemistry and Materials ScienceAnhui Normal University
  • Jiajia Wu
    • Anhui Key Laboratory of chemo-Biosening, College of Chemistry and Materials ScienceAnhui Normal University
    • Anhui Key Laboratory of chemo-Biosening, College of Chemistry and Materials ScienceAnhui Normal University
Original Paper

DOI: 10.1007/s00604-010-0359-4

Cite this article as:
Yin, Z., Wu, J. & Yang, Z. Microchim Acta (2010) 170: 307. doi:10.1007/s00604-010-0359-4

Abstract

Shuttle-like copper oxide (CuO) was prepared by a hydrothermal decomposition process. The resulting material was characterized by scanning electron microscopy and X-ray diffraction. It was then immobilized on the surface of a glassy carbon electrode modified with a film of poly(thionine). A pair of well-defined and reversible redox peaks for Hg(II) was observed with the resulting electrode in pH 7.0 solutions. The anodic and cathodic peak potentials occurred at 0.260 V and 0.220 V (vs. Ag/AgCl), respectively. The modified electrode displayed excellent amperometric response to Hg(II), with a linear range from 40 nM to 5.0 mM and a detection limit of 8.5 nM at a signal-to-noise ratio of 3. The sensor exhibited high selectivity and reproducibility and was successfully applied to the determination of Hg(II) in water samples.

Keywords

CuO nanoshuttleHg2+Poly(thionine)Sensor

As the development of industry, heavy metal ions (Hg2+, Cu2+, Pb2+, Cd2+ etc.) become the ones of the most toxic species of superficial and underground water. Mercury is one of the few metals which strongly bioconcentrates and biomagnifies. It has only harmful effects with no useful physiological functions when presents in living organisms and easily transformed from a less toxic inorganic form to a more toxic organic form especially in fish [1]. It remains in industrial wastes because of its growing area in production of some batteries, thermometers, cameras, cathode tubes, calculators, medical laboratory chemicals, a catalyst in production of urethane polymers for plastics, a cathode in electronic production of chlorine and caustic soda, mercury vapor lamps and barometers [2]. Owing to the growing awareness of environmental pollution and toxicity, it is very important to determine mercury (II) at lower and lower levels in biological material, natural waters, soil and air.

Classical methods and techniques are used for mercury (II) quantification such as cold vapor atomic absorption spectrometry (CV-AAS) [3], cold vapor atomic fluorescence spectrometry (CV-AFS) [4], X-ray fluorescence spectrometry [5] and neutron activation analysis [6]. Although the classical techniques are very powerful for monitoring toxic analysis such as heavy metals, they are known to be expensive, time-consuming and not adapted for in situ and real time detection and requires highly trained personnel [7]. The application of electrochemical methods has become a well-established routine analytical technique. Chemically modified electrodes have received increasing attentions for enhancing the sensitivity and selectivity of electrochemical analysis techniques in the past decades [8]. Numerous voltammetric methods especially using chemically modified electrodes (CME) have been developed. Ju and Leech have prepared a protein monolayer modified electrode by the self-assembly of metallothionein at a gold disk. They determined trace Hg2+ with cathodic stripping differential pulse voltammetry through pre-concentrating method and the detection limit was 0.08 μM [9]. Berchmans et al. have fabricated a self-assembled monolayer modification of a gold electrode using 2-mercaptobenzimidazole. They pre-concentrated Hg2+ chemically and determined the Hg2+ with stripping voltammetry after reduction of pre-concentrated Hg2+ prior to determination by anodic stripping voltammetry. The detection limit can come to ppm level [10]. Also, the determine of Hg2+ has been carried out by using impregnated graphite electrodes [11] and graphite electrodes modified with a PDC/Au (III) complex [12].

Considerable work has proved the fact that S atom contained in compounds promote the coordination of compounds with heavy metal ions. The binding capabilities play an important biological role in the storage and transport of essential metal ions, and the detoxification or sequestration of toxic metal ions [13]. Thionine (TH) is a phenothiazine redox dye which can be easily dissolved in water and ethanol [14]. The chemical structure of TH is a small planar molecule with two −NH2 groups symmetrically distributed on each side and an S atom in the centre [15]. In addition, poly(thionine) as an excellent electron transfer mediator between the electrode and the redox centre has been widely used in many sensors such as NADH biosensor [16], H2O2 biosensor [17]. Recently, composite nanomaterials are well known and used for the fabrication of various electrometric sensors for analytical purposes [18]. The combination of inorganic nanomaterials and organic polymers to modified electrode is attractive for the purpose of creating high performance or high functional. Copper oxide nanoparticles are of particular interest due to their use in heterogeneous catalysts for the oxidation of hydrocarbons [19], carbon monoxide [20]. Zhuang et al. used CuO nanorods previously for the detection of glucose by supporting the CuO nanorods on a copper electrode substrate [21]. Le et al. fabricated a H2O2 sensor by electrodepositing nano-copper oxide on a glassy carbon electrode [22].

In this paper, CuO nanoshuttles were prepared and the PTH film was obtained with electrochemically polymerizing thionine on glassy carbon electrode. An amperometric sensor was fabricated by immobilizing CuO nanoshuttles on the surface of the modified PTH film GCE. The prepared sensor gave an excellent amperometric response to Hg2+ in a 0.10 M pH 7.0 phosphate buffer solution and possessed several advantages such as high sensitivity, fast response time, long-time stability, reproducibility and relative low detection limit. The constructed sensor could be applied to determine low concentration Hg2+ in sample.

Experimental

Reagents and instrument

All reagents were purchased from Shanghai Chemical Reagent Company (http://www.reagent.com.cn). Stock solution of HgCl2 (0.01 M) was prepared by directly dissolving HgCl2 in water. The thionine solution (0.01 M) was prepared with dissolving thionine in alcohol completely and then diluting with water. Phosphate buffer solutions (0.10 M) with various pH values from 5.5 to 8.0 were prepared with Na2HPO4 and NaH2PO4. All solutions were prepared with doubly distilled water.

Electrochemical experiments were performed with a CHI660A electrochemical analyzer (Shanghai Chenhua Apparatus, China) with conventional three-electrode cells. The working electrode was a glassy carbon electrode (Φ = 3 mm) (GCE), poly(thionine) (PTH) modified electrode (PTH/GCE), nano-CuO immobilized PTH/GCE (nano-CuO /PTH/GCE), respectively. The reference electrode was an Ag/AgCl (saturated KCl) electrode and platinum electrode was used as the auxiliary electrode. Prior to each experiment, solutions were purged with purified nitrogen for 15 min to remove oxygen. All the measurements were performed at room temperature unless otherwise specified. Scanning electron microscopy (SEM) was obtained on S-4800 field emission scanning electron microanalyser (Hitachi, Japan). X-ray diffraction (XRD) were performed with an X-ray diffractometer (Shimadzu, Japan) using a Cu Kα source (l = 0.154060 nm) at 40 kV, 30 mA in the range of 20° < 2θ < 80° at a scan rate of 6.0° min−1.

Synthesis of CuO nanoshuttles

The CuO nanoshuttles were synthesized firstly with a simple process according to the literature [23] with a slight modification. 10 mM NaOH was dissolved in 20 mL distilled water under stirring, then 5 mM CuCl2·2H2O was added into the NaOH solution. The bluish Cu(OH)2 precipitates appeared, immediately. This solution was further stirred until the reaction completed; centrifuged and washed with distilled water several times, the bluish precipitates were re-dispersed in distilled water and stirred for another 1 h. Then the solution was loaded into a Teflon-lined autoclave of 50 mL capacity and filled with distilled water up to 80% of the total volume. The autoclave was maintained at 120 °C for 12 h. When cooling to room temperature, naturally, the dark precipitates were filtered and washed with distilled water several times. Finally, the product was dried in vacuum at 60 °C for 4 h.

Fabrication of the modified electrode

Prior to the modification, the GCE was polished to a mirror-like surface with 0.05 μm α-Al2O3, thoroughly rinsed with water and sonicated in absolute ethanol and water (each for 5 min). The PTH/GCE was prepared by electropolymerizing thionine on the GCE by cyclic voltammetry between −0.5 V and 0.5 V at 50 mV s−1 in a 0.10 M phosphate buffer solutions (pH = 6.0) containing 5 mM thionine for 40 cycles. The obtained PTH/GCE electrode was washed with doubly distilled water and dried at room temperature. CuO nanoshuttles (5 mg) were suspended in 1.0 mL water and sonicated for 10 min to form a suspension. A 10 µL of suspension dispersion was coated onto the surface of PTH /GCE and the electrode was dried in air to form CuO/PTH/GCE.

Results and discussion

Characterization of the CuO/PTH/GCE

The morphology and size of the prepared product were characterized with SEM. Figure 1(a) showed SEM images of the CuO nanoshuttles. The SEM photograph illustrated that the prepared CuO nano-particles were a well-defined shuttlelike morphology with almost uniform size and shape. The SEM image of the PTH film on the surface of GCE was showed in Fig. 1(b). From this image, the configuration of PTH film was netlike. This netlike PTH film has advantage to immobilize the CuO nanoparticles onto the surface of PTH/GCE. Figure 1(c) showed the SEM image of the CuO nanoshuttle/PTH/GCE. From Fig. 1(c), the SEM images indicated that nano-CuO had been successfully immobilized on the surface of the modified PTH electrode. Furthermore, it has reported that mercapto-possess a selective capacity to bind metal ions. PTH molecule contains mercapto group. The S atom in mercapto group could bind with Cu to form Cu-S bond. Therefore, the nano-CuO particles could stably exist on the surface of the PTH netlike film.
https://static-content.springer.com/image/art%3A10.1007%2Fs00604-010-0359-4/MediaObjects/604_2010_359_Fig1_HTML.gif
Fig. 1

SEM images of the as-prepared sample CuO, scale bar is 2 μm (a), CuO nanoshuttles on GCE, scale bar is 1 μm (b) and the film of PTH/nanoshuttles, scale bar is 2 μm (c)

Figure 2 showed the XRD profiles taken from the as-prepared nano-CuO. All the reflections on the pattern could be indexed to the monoclinic CuO phase with lattice constants comparable to the reported data (JCPDS Card. No. 89-5899). By means of XRD procedure, no obvious peaks of impurity were found. This result indicates that a single phase of CuO could be obtained by the reaction of CuCl2 and NaOH at 120 °C for 12 h.
https://static-content.springer.com/image/art%3A10.1007%2Fs00604-010-0359-4/MediaObjects/604_2010_359_Fig2_HTML.gif
Fig. 2

XRD patterns of the CuO nanoshuttles

Electrochemical activity of the Hg2+ on CuO/PTH/GCE

In order to explore the electrochemical behavior of Hg2+ on the nano- CuO/PTH/GCE, the cyclic voltammetric experiments were performed in 0.1 M phosphate buffer solutions (pH = 7.0) in the presence of 6 μM Hg2+ with different modified electrode. The obtained cyclic voltammetric curves were shown in Fig. 3. There was a faintness oxidation peak occurring at voltammetric curves using bare GCE (see Fig. 3 curve a) but not had homologous reduction peak scanning between 0.5 ∼ 0.1 V. This result indicated the electrochemical process of Hg2+ on bare GCE was irreversible. While on the PTH/GCE, the oxidation current increased obviously (Fig. 3b). Thus, on the CuO/PTH/GCE, not only the peak current enhanced and the peak potential shifted negatively to 0.26 V but also the cathodic peak of Hg2+ was present at 0.22 V (Fig. 3d). The electrochemical process of Hg2+ on CuO/PTH/GCE was reversible which demonstrated that compound materials had great catalysis to Hg2+. When the GCE was only modified with nano- CuO, the oxidation peak current increased and peak potential has a degree of negatively shifting compared to the bare GCE (Fig. 3c). Figure S3 showed the CVs of the modified electrode in 0.1 M phosphate buffer solutions (pH = 7.0) upon successive additions of 2 μM Hg2+.
https://static-content.springer.com/image/art%3A10.1007%2Fs00604-010-0359-4/MediaObjects/604_2010_359_Fig3_HTML.gif
Fig. 3

Cyclic voltammograms of 6 μM Hg2+ on (a) bare GCE, (b) PTH/GCE, (c) CuO/GCE and (d) CuO/PTH /GCE electrode in phosphate buffer solutions (pH 7.0). Scan rate: 100 mV s−1

Effect of pH

To obtain optimal electrochemical behaviors of Hg2+, the influences of the buffer solutions pH on electrochemical behaviors of Hg2+on nano-CuO/PTH/GCE were examined in 0.1 M phosphate buffer solutions at various pH values ranging from 5.5 to 8.0. Figure 4 showed the dependence of anodic peak current on solution pH. The anodic peak current enhanced with increasing the pH value of solutions and reached the highest at pH = 7.0. Afterward, the anodic peak current decreased with increasing the pH value of solution. Under lower pH condition, the nano-CuO particles could not exist steadily in the acidic medium, resulting in anodic peak current of Hg2+ decreasing. When pH value of solution was in excess of 7.0, the PTH was unsteady because the NH3+ in the structure can convert to NH2 group, and the structure of polymer was destroyed [24, 25]. The anodic peak current of Hg2+ decreased also. Therefore, pH 7.0 was the optimal for Hg2+ redox on nano-CuO/PTH/GCE. On the other hand, the pH value of solution was changed from 5.5 to 6.5, a negative shift of anodic peak potential was observed (with the slope of −30 mV) and remained constant afterward. According to Nernst equation, this reveals that the proportion of the electron and proton involved in the reactions is 1:1 and the reduction is a two-electron process. Therefore, according to a previous report [26], a possible mechanism for the electrochemical reaction of Hg2+ can be given as following:
https://static-content.springer.com/image/art%3A10.1007%2Fs00604-010-0359-4/MediaObjects/604_2010_359_Fig4_HTML.gif
Fig. 4

The relationship between solution pH and the cathodic peak current Ipc (containing 5 μM Hg2+)

$$ 2HgCl_4^{2 - } + { }2{e^{-} } \to H{g_2}C{l_2} + { }6C{l^{-} } $$
(1)
$$ H{g_2}C{l_2} + { }2C{l^{-} } \to H{g^0} + { }2HgCl_4^{2 - } $$
(2)
$$ H{g^0} - 2{e^{-} } + 4C{l^{-} } \to HgCl_4^{2 - } $$
(3)

Amperometric response to Hg2+

Figure 5 displayed a typical amperometric response of Hg2+ on nano-CuO /PTH /GCE sensor with the addition of successive aliquots of 0.4 μ M Hg2+ in phosphate buffer solutions (pH = 7.0) under the constant potential at +0.25 V. The fabricated sensor yielded a rapid and sensitive response to each injection of Hg2+, a sharp rise in current followed by a steady-state value (Iss) within 3 s. The plot of Iss versus CHg2+ (inset, Fig. 5) depicted a linear relationship over the range from 0.4 nM to 5.0 mM. The correlation coefficient of the line is 0.999, and the linear equation is \( y = 0.0930 + 0.0218x \) (x, the concentration of the Hg2+, μM; y, the cathodic peak current, μA). Based on signal to noise ratio (S/N) of 3, the detection limit was estimated to be 8.5 nM, which was lower than other modified electrodes [2729].
https://static-content.springer.com/image/art%3A10.1007%2Fs00604-010-0359-4/MediaObjects/604_2010_359_Fig5_HTML.gif
Fig. 5

Amperometric responses of the CuO/PTH/GCE at 0.26 V upon successive additions of 0.4 μM Hg2+ into 5 mL phosphate buffer solutions (pH = 7.0). Inset A shows plots of the peak currents as a function of concentration

The reproducibility and stability of sensor and anti- interferences

To examine the reproducibility and stability of the prepared sensor, electrochemical experiments were repeatedly performed 10 times with the same sensor in the solution containing 10 μM Hg2+. The relative standard deviation was calculated to be 2.4%, which revealed that the reproducibility of the prepared sensor was excellent. The sensor also showed an excellent stability. The same sensor was used for approximately 80 times during 40 days and a small decrease of peak current (about 6.5%) for 10 μM Hg2+ was observed, which can be attributed to the excellent stability of the nano-CuO/PTH film on surface of GCE.

The interferences of some heavy metal ions, such as Ag+, pb2+, Cd2+, Cr3+, Cu2+ on sensor were also examined with the nano-CuO/PTH /GCE. The obtained results were showed in Fig. S4. According to the experimental result, these heavy metal ions did not interfere to determine Hg2+ within 50 times of Hg2+ concentration. Other probable coexisting ions such as Na+, K+, Ca2+, Mg2+, Cl, Br, I, and NO3 ions also did not interfere on determination. It indicated that the prepared sensor possessed high selectivity for Hg2+ ion.

Application for sample analysis

In order to evaluate the validity of the constructed sensor for the determination of Hg2+, real water samples were spiked with different concentrations of Hg2+ and were analyzed under optimized conditions using the above technique. The data given in Table 1 shows the satisfactory results.
Table 1

Quantitative determination of Hg2+ from distilled water sources spiked with known quantity of Hg2+

Sample (Distilled water)

Hg2+ added (µM)

Hg2+ Found (µM)a

Recovery (%)

1

10.0

9.8 ± 0.4

98.0

2

20.0

19.8 ± 0.2

99.0

3

30.0

29.7 ± 0.4

99.0

4

40.0

40.9 ± 0.6

102.0

5

50.0

50.4 ± 0.6

101.0

aAverage of three determinations

Conclusion

We have successfully fabricated a new, high sensitivity, fast response and highly selective amperometric Hg2+ sensor based on CuO nanoshuttles. The PTH was used as a binder to stabilize the CuO nanoshuttles onto the surface of a GCE and was used to enhance the stability of the sensor. The nano-CuO/PTH/GCE exhibited the prominent activity for redox of Hg2+ ion and the fabricated sensor gave a good stability and reproducibility, which could be used as an amperometric sensor for determination of low concentration Hg2+ in sample.

Acknowledgments

We thank the National Natural Science Foundation of China (Grant No. 20775002) for financial support. The work was supported by Program for Innovative Research Team in Anhui Normal University.

Supplementary material

604_2010_359_MOESM1_ESM.doc (746 kb)
ESM 1(DOC 746 kb)

Copyright information

© Springer-Verlag 2010