Selective electrochemical determination of trace level copper using a salicylaldehyde azine/MWCNTs/Nafion modified pyrolytic graphite electrode by the anodic stripping voltammetric method

Abstract

A novel Cu²⁺ electrochemical sensor based on a salicylaldehyde azine/MWCNTs/Nafion-modified pyrolytic graphite electrode was prepared for anodic stripping analysis of Cu²⁺. Salicylaldehyde azine was synthesized and then used for the selective determination of the heavy metal pollutant Cu²⁺ due to its azine structure, which gave selective complexing ability toward Cu²⁺. The use of MWCNTs with their strong adsorption ability could greatly enhance the sensitivity. Nafion, a proton-exchange polymer, was used as the conductive matrix in which the salicylaldehyde azine and MWCNTs could be strongly fixed to the substrate electrode surface. The as-prepared electrochemical sensor showed remarkably enhanced selectivity and sensitivity towards Cu²⁺. The response current of the sensor was linear with Cu²⁺ concentration ranging from 5 to 300 nM under 15 min accumulation at open-circuit potential, with a very low detection limitation of about 1 nM. The hybrid functionalized electrode also exhibited good selectivity to avoid the interference of other heavy metal ions like Cd²⁺, Pb²⁺ and Hg²⁺ in the mixture solution together with Cu²⁺. Real application towards environmental sample analysis confirmed that the modified electrode could be applied for the selective determination of trace levels of Cu²⁺ in the Yellow River.

---

1. Introduction

The contamination of natural water by heavy metals is a serious problem in the biosystem since heavy metals can cause many disorders in the plant and animal kingdoms and tend to accumulate through food chains.^1,2 The copper ion (Cu²⁺) is an essential heavy metal in the human body and plays an important role in various physiological processes.³ Excess intake of copper from polluted water and copper-rich food has frequently occurred in recent years.⁴ Some serious diseases, such as anemia, Menkes disease and Wilson’s disease, are associated with an abnormal level of copper in the human body.^5,6 Copper-containing waste water indiscriminately pumped into rivers during industrial production is still a serious global problem. Thus, the monitoring of Cu²⁺ in environmental samples has become essential.

There are various techniques which have been used for the determination of copper, such as fluorescence,⁷ atomic absorption spectroscopy,^8,9 and inductively coupled plasma-optical emission spectroscopy.¹⁰ In comparison with other methods, an electrochemical method has more attractive features including simplicity, robustness and inexpensiveness.¹¹ In the past, mercury based electrodes, chemically modified electrodes based on gold coated diamond^12–14 and boron-doped diamond electrodes¹⁵ have been used for trace heavy metals detection. Traditional methods also have some disadvantages like toxicity, low chemical stability and inconvenience, and new electrode materials for trace heavy metals detection are highly sought.¹⁶

Recently, the use of chemically modified electrodes (CME) has been very common for the determination of several metallic species, and the employment of this method could not only enhance selectivity for the analyte but also improve detection limits.^17–19 The choice of modifier could confer special characteristics to the electrode surface. Electrodes modified with synthesis receptors have been used for heavy metal detection, where the receptors extracted particular metal ions by complexation in the analysis.^20–22 In the literature, various receptors such as crown ethers,²³ iminodiacetic acid,²⁴ di-(2-imino-cyclopentylidine mercaptomethyl) disulfide,²⁵ 2,2-biquinoline-Nafion,²⁶ and 1,2-bismethyl (2-aminocyclopentene-carbodithioate) ethane²⁷ have been used for the determination of metal ions by using CMEs. Though these receptors were known to exhibit higher selectivity, their exploitation in electroanalysis areas was very limited owing to their non-conductive properties.

Therefore, it is necessary to improve the matrix conductivity when a receptor is employed in an electrochemical sensor. Nanoscale materials, such as metal nanoparticles, carbon materials, and mesoporous silica, exhibit unique chemical, physical and electronic properties and thus have been applied to fabricate novel modified electrodes for heavy metal ions sensing.^28–35

Herein we report a new method for the synthesis, the characterization, and the metal recognition properties of salicylaldehyde azine (SA) (2,2′-(1E,1′E)-hydrazine-1,2-diylidenebis(methan-1-yl-1-ylidene) diphenol). In this work, a receptor SA has been combined with MWCNTs as a novel modifier for selective determination of heavy metal copper using differential pulse anodic stripping voltammetry (DPASV). The selective complexing ability of SA and the electrical conductivity of MWCNTs synergistically improved the electrochemical performance towards Cu²⁺ sensing. Nafion, a sulfonated cation-exchange polymer, supplied good stability of the functionalized hybrid electrode. It has been confirmed that such a fabricated SA/MWCNTs/Nafion hybrid modified PGE could remarkably enhance the sensitivity and selectivity for stripping measurements of Cu²⁺.

2. Experimental

2.1 Apparatus and chemicals

The electrochemical measurements were carried out on a CHI660C electrochemical workstation (CHI Instrument, Shanghai, China). All measurements were performed in a conventional three-electrode cell; an unmodified PGE (Tianjin Aidahengsheng Technology Co., China) or modified PGE, a saturated calomel electrode (SCE) and a platinum wire were used as the working electrode, reference electrode, and counter electrode, respectively. The morphology of the SA/MWCNTs/Nafion/PGE was determined with a JEOL 2010 TEM (JEOL, Japan) equipped with a LaB6 filament at 200 kV. UV-vis spectra were recorded on a Perkin Elmer Lambda 35 UV-vis Spectrophotometer with a quartz cuvette (pathlength = 1 cm).

All reagents were of analytical grade and used as received. 0.1 M phosphate buffer solution (PBS) at pH 7.0 was prepared from NaH₂PO₄ and Na₂HPO₄ and the pH value was adjusted with 1 M H₂SO₄ or NaOH solution. Nafion was obtained from Aldrich. Stock solutions of Cu²⁺, Cd²⁺, Pb²⁺ and Hg²⁺ were prepared from their nitrate salts in deionised water. Multi-wall carbon nanotubes were synthesized according to the literature.³⁶

2.2 Synthesis of SA

1.22 g salicylaldehyde was dissolved in 50 mL ethanol and 0.25 mL of hydrazine hydrate (99%) was added at room temperature. The reaction mixture was stirred at room temperature overnight. After completion of the reaction the obtained yellow precipitate was filtered and washed several times with cold ethanol with a yield of 85%. EI-MS: (M + H) 241.2. ¹H NMR (400 Hz, DMSO-d6), δ: 11.122 (s, 2H), 8.999 (s, 2H), 7.676–7.699 (m, 2H), 7.372–7.414 (m, 2H), 6.943–6.982 (m, 4H).

2.3 Preparation of the electrode

The modified electrodes were prepared by a casting method. Prior to the surface coating, the pyrolytic graphite electrode (PGE) was carefully polished with 1.0, 0.3, and 0.05 μm alumina powders in series, and then treated with 50% nitric acid, ethanol and water in an ultrasonic bath, respectively. The original 5% w/w Nafion solution was diluted to 0.5% w/w solution with N,N-dimethylformamide (DMF). Then 1 mg of MWCNTs and 1 mg of SA were added into the above solution and sonicated for 20–30 min to form a homogeneous suspension. Finally, 5 μL SA/MWCNTs/Nafion suspension was casted on the pretreated PGE surface and dried under an infrared lamp. For comparison, Nafion/PGE, SA/Nafion/PGE, MWCNTs/Nafion/PGE and SA/MWCNTs/Nafion/PGE were prepared with a similar method.

2.4 Electrochemical measurements

0.1 M PBS at pH 7.0 was used as the supporting electrolyte. The SA/MWCNTs/Nafion/PGE at open-circuit potential was immersed into a buffer solution containing Cu²⁺ or other ions for 15 min without applying any potential and stirring during the accumulation period. As a comparison to the open-circuit accumulation, electroaccumulation was performed by applying a potential at −1.2 V for 15 min in a solution containing Cu²⁺ or other ions. Subsequently, the electrode was rinsed thoroughly with deionized water and transferred to the fresh supporting electrolyte, which had been deoxygenated by nitrogen bubbling for 30 min, while a potential of −0.6 V was applied to the electrode for about 30 s to reduce Cu²⁺. Using differential pulse anodic stripping voltammetry, quantitative determinations of Cu²⁺ were performed at optimized conditions: an amplitude of 0.05; a pulse width of 0.05 s; a sampling width of 0.0167 s; a pulse period of 0.2 s; a quiet time of 2 s. After each measurement, the modified electrode was regenerated by applying 0.3 V for 200 s in fresh supporting electrolyte to remove the previous residual copper from the electrode surface.

3. Results and discussion

3.1 The selective binding properties of SA towards metal ions

Based on the outstanding ability to selectively coordinate with metal ions, bisazine-type receptors have been used as ionophore molecules for ion-selective electrodes and other applications.^37–40 In this work, salicylaldehyde azine (SA) was synthesized by an easy and effective method (ESI, Fig. S1†). The metal ion binding properties of receptor SA have been firstly studied by using UV-vis spectroscopic techniques. SA itself displayed two strong absorption bands at 290 and 350 nm in CH₃CN–H₂O (1 : 1, v/v) mixture at neutral pH 7 buffered with HEPES–NaOH. In the presence of various cations such as Fe³⁺, Mn²⁺, Zn²⁺, Cd²⁺, Pb²⁺, Cu²⁺, Co²⁺, Ni²⁺, Ag⁺, Mg²⁺, Hg²⁺, Fe²⁺, Na⁺, K⁺ and Ca²⁺ (all as their nitrate salts), only Cu²⁺ remarkably changed the absorption spectrum of SA, whereas the other cations tested induced negligible responses (ESI, Fig. S2†). Upon addition of Cu²⁺ (0–20 equiv) to a solution of SA (2.0 × 10⁻⁵ M), as shown in Fig. 1, the absorption band of SA at 290 nm gradually decreased along with a slight blue shift, and the absorption band at 350 nm gradually disappeared while a new red-shift absorption band centered at 405 nm appeared, which was responsible for the mixed solution color change from colorless to yellow-green (inset of Fig. 1). During the titration of Cu²⁺ with the solution of SA, a clear isosbestic point at 380 nm was observed, indicating the existence of a coordination equilibrium between several structures. The ESI-MS analysis of the interaction system of SA with Cu²⁺ exhibited one main peak at m/z = 304.00, assigned to the SA·Cu²⁺ complex (m/z calc. = 304.02).

Fig. 1 Absorption spectra of SA (2.0 × 10⁻⁵ M) in CH₃CN–H₂O (1 : 1, v/v) buffer solution upon addition of Cu²⁺ (0–20 equiv); inset: the color change of the mixed solution of SA (2 × 10⁻⁵ M) upon addition of 1 equiv of Cu²⁺ in CH₃CN–H₂O (1 : 1, v/v) buffer solution.

The above results indicate the highly selective metal ion binding properties of SA, which can be used as a colorimetric probe for the selective detection of Cu²⁺ in aqueous buffer media. Based on the selective complexing ability of SA towards Cu²⁺, SA-modified pyrolytic graphite electrodes were further constructed for the electrochemical determination of Cu²⁺.

3.2 Characterization of SA/MWCNTs/Nafion composite film

Fig. 2 shows scanning electron microscope (SEM) images of MWCNTs/Nafion and SA/MWCNTs/Nafion composite films coated on a PGE. It can be observed that the MWCNTs/Nafion composite film was homogeneously casted on the electrode surface (Fig. 2a). This surface morphology gave an large accessible surface area. When SA was introduced into the MWCNTs/Nafion composite, the homogeneous distribution of MWCNTs on the electrode surface was not affected (Fig. 2b). However, the profile of the MWCNTs became rough and the conductivity decreased because the SA and MWCNTs were miscible throughout the matrix. This result was similar to a former report.²³ Furthermore, the MWCNTs formed a network on the electrode and could be more effective for electron transfer, increasing the sensitivity of the sensors.

Fig. 2 SEM images of the electrode surfaces prepared with different modifiers: (a) Nafion/MWCNTs/PGE and (b) SA/MWCNTs/Nafion/PGE.

3.3 Electrochemical behavior of Cu²⁺ at SA/MWCNTs/Nafion/PGE

The combination of accumulation and reduction prior to the actual stripping detection process can enhance both the sensitivity and the selectivity of the analysis of metal ions.⁴¹ For this work, the structural features of SA facilitated the formation of a SA·Cu²⁺ complex in the course of open-circuit accumulation. The complex Cu²⁺ could be further reduced to Cu⁰, which was deposited on the modified electrode under cathodic potentiostatic conditions for a defined time period. Finally, the deposited Cu⁰ was stripped off the electrode and the stripping current could be measured. A diagrammical illustration is depicted in Scheme 1.

Scheme 1 Schematic illustration of Cu²⁺ detection using a SA/MWCNTs/Nafion modifier.

The typical cyclic voltammetric behaviour of SA/MWCNTs/Nafion/PGE in 0.1 M PBS is shown in Fig. 3. Compared with the cyclic voltammogram of the electrode in pure PBS (dotted line), a well-defined anodic voltammetric peak at −0.1 V, attributed to the oxidation of metallic copper, can be obtained after the accumulation of Cu²⁺ (solid line).

Fig. 3 Cyclic voltammograms of the SA/MWCNTs/Nafion/PGE in 0.1 M pH 7.0 PBS with (solid line) and without (dotted line) open-circuit 15 min accumulation. Accumulation solution: PBS containing 5.0 μM Cu²⁺. Scan rate: 100 mV s⁻¹. Reduction potential −0.6 V for 120 s.

To clarify the function of each component in the electrode coating matrix, further investigation using differential pulse anodic stripping voltammetry was carried out under the same experimental conditions. Fig. 4 depicts the DPASV performance of 5.0 μM Cu²⁺ in 0.1 M PBS (pH 7.0) for various modified PGEs. No peaks were observed for the bare and Nafion film modified electrodes after accumulation at open-circuit potential for 15 min (Fig. 4a and b). A weak stripping peak was observed for SA/Nafion/PGE (Fig. 4c) due to the formation of the SA·Cu²⁺ complex on the modified electrode surface. Compared with SA/Nafion/PGE, a larger stripping peak at −0.17 V was observed for MWCNTs/Nafion/PGE (Fig. 4d), indicating that MWCNTs with a large surface area could provide multiadsorbing sites and increase the stripping currents. Further, the highest peak at −0.15 V was found for SA/MWCNTs/Nafion/PGE under the same conditions (Fig. 4e), and the current response observed for SA/MWCNTs/Nafion/PGE was improved by nearly two times compared to MWCNTs/Nafion/PGE. The most probable reason is that the MWCNTs can improve the accumulation capacity of the modified electrode, and SA is capable of forming a strong complex with Cu²⁺. The combination of MWCNTs with SA on Nafion/PGE could synergistically enhance the accumulation of Cu²⁺ from the sample solution to the modified electrode surface and result in the significant increase of the stripping peak current.

Fig. 4 Differential pulse anodic stripping voltammetry of 5.0 μM Cu²⁺ in 0.1 M PBS pH 7.0: (a) bare PGE electrode; (b) Nafion/PGE; (c) SA/Nafion/PGE; (d) MWCNTs/Nafion/PGE; (e) SA/MWCNTs/Nafion/PGE. Amplitude of 0.05 V; pulse width of 0.05 s; pulse period of 0.2 s; quiet time of 2 s.

3.4 Optimization of experimental parameters

Fig. 5a shows the effect of the pH of the supporting electrolyte on the Cu²⁺ differential pulse anodic stripping peak current at SA/MWCNTs/Nafion/PGE. The peak current increased notably with the pH from 6.0 to 7.0, and then decreased from 7.0 to 8.0, which is due to SA instability at low pH values and the precipitation of Cu²⁺ at high pH values. Therefore, pH 7.0 was selected for subsequent measurements. Furthermore, the effect of accumulation time on peak current is illustrated in Fig. 5b. The peak current increased rapidly with increasing accumulation time from 5 min to 15 min. Further increasing the accumulation time, there was a gradual increase in the current response. A similar process was observed in the influence of reduction time on peak current (Fig. 5c). The current response increased sharply with increasing the reduction time from 30 s to 120 s, and then increased slightly from 120 s to 300 s. Based on the sensitivity and short analysis time, 15 min accumulation time and 120 s reduction time were chosen in subsequent experiments. Finally, the effect of reduction potential was investigated in the potential range of −0.8 V to −0.4 V, as depicted in Fig. 5d. The peak current increased with the reduction potential from −0.8 V to −0.6 V, and decreased from −0.6 V to −0.4 V, which might be attributed to substantial hydrogen evolution at the modified PGE surface. Hence, a potential of −0.6 V was chosen for further experiments.

Fig. 5 Differential pulse anodic stripping voltammetry of 5.0 μM Cu²⁺ in 0.1 M PBS pH 7.0: (a) effect of pH; (b) effect of accumulation time; (c) effect of reduction time; (d) effect of reduction potential. Amplitude of 0.05 V; pulse width of 0.05 s; pulse period of 0.2 s; quiet time of 2 s.

3.5 Determination of Cu²⁺ at SA/MWCNTs/Nafion/PGE

Fig. 6 shows the DPASV of varying concentrations of Cu²⁺ in 0.1 M PBS for SA/MWCNTs/Nafion/PGE with an open-circuit accumulation of 15 min. The stripping peak current increased linearly with Cu²⁺ concentration ranging from 5.0 nM to 300 nM. The correlation coefficient was found to be 0.997 and the calculated limit of detection for 15 min of open-circuit accumulation was 1.0 nM. The performance characteristics of the proposed electrode for Cu²⁺ electroanalysis was comparable or superior to some reported chemically modified electrodes, as summarized in Table 1. The relative standard deviation of 10 determinations for 40 nM is 0.91%, indicating good reproducibility of the modified electrode. The current response for SA/MWCNTs/Nafion/PGE had no significant change after its preparation for three weeks under ambient conditions, showing high stability of SA/MWCNTs/Nafion/PGE.

Fig. 6 Differential pulse anodic stripping voltammetry of 5.0–300 nM Cu²⁺ in 0.1 M PBS pH 7.0 for SA/MWCNTs/Nafion/PGE. Amplitude of 0.05 V; pulse width of 0.05 s; pulse period of 0.2 s; quiet time of 2 s.

Table 1 Comparison of various modified electrodes for the detection of Cu²⁺

Modifier	Method	Linear range (M)	Limit of detection (M)	Reference
2-Aminothiazole	DPASV	7.5 × 10^−8–2.5 × 10^−6	3.1 × 10^−8	42
Natural zeolite	DPASV	5.0 × 10^−8–5.0 × 10^−6	1.5 × 10^−8	43
PCHA	DPASV	1.0 × 10^−8–1.0 × 10^−6	5.0 × 10^−10	44
Mercury film	DPASV	1.0 × 10^−5–5.0 × 10^−9	1.0 × 10^−9	45
Calix[4]arene	DPASV	5.0 × 10^−8–1.6 × 10^−6	1.1 × 10^−8	46
Salicylidine-functionalized polysiloxane	Potentiometry	2.3 × 10^−7–1.0 × 10^−3	2.7 × 10^−8	47
BHAB	Potentiometry	5.0 × 10^−8–1.0 × 10^−2	3.0 × 10^−8	48
SA/MWCNTs/Nafion	DPASV	5.0 × 10^−9–3.0 × 10^−7	1.0 × 10^−9	This work

---

3.6 Interference

The stripping analysis of Cu²⁺ can be affected by interferences in the presence of other heavy metal ions such as Cd²⁺, Pb²⁺ and Hg²⁺, which generate large errors in stripping analysis, especially in the presence of higher concentrations (>0.1 μM).⁴¹ In this work, Cd²⁺, Pb²⁺ and Hg²⁺ were selected to evaluate the anti-interference of the modified electrode. Fig. 7 shows the differential pulse stripping voltammograms obtained for the SA/MWCNTs/Nafion/PGE using electroaccumulation and open-circuit accumulation for Cu²⁺ in the presence of a 10-fold excess of Cd²⁺, Pb²⁺, Hg²⁺, respectively. Four peaks were observed at potentials of −0.82, −0.61, −0.16 and 0.72 V by using 15 min electroaccumulation in the mixture solution, which were assigned to the stripping peaks of Cd²⁺, Pb²⁺, Cu²⁺ and Hg²⁺, respectively. This showed that the interfering metal ions were electrodeposited together with the target ion at the modified electrode during the electroaccumulation procedure, which resulted in poor selectivity. This phenomenon was similar to a former report.⁴¹ Compared to the electroaccumulation, only one stripping peak of Cu²⁺ was obtained at −0.16 V by using 15 min open-circuit accumulation. This might be attributed to SA with an outstanding ability to selectively coordinate with Cu²⁺. These results showed that the modified electrode exhibited high selectivity and sensitivity for Cu²⁺ over other interfering or competing metal ions using the open-circuit accumulation procedure. The current peak of Cu²⁺ observed for the SA/MWCNTs/Nafion/PGE using open-circuit accumulation was less than at the same electrode by electrodeposition, which might be due to less Cu²⁺ adsorption on the electrode surface during the open-circuit accumulation.

Fig. 7 Differential pulse anodic stripping voltammetry obtained for SA/MWCNTs/Nafion/PGE in 0.1 M PBS pH 7.0 containing 0.5 μM Cu²⁺ + 5.0 μM Pb²⁺ + 5.0 μM Hg²⁺ + 5.0 μM Cd²⁺. Solid line: open-circuit 15 min accumulation. Dotted line: electroaccumulation at −1.2 V for 15 min. Amplitude of 0.05 V; pulse width of 0.05 s; pulse period of 0.2 s; quiet time of 2 s.

3.7 Application to real sample analysis

In order to test its accuracy in practical analysis, SA/MWCNTs/Nafion/PGE was used for the determination of Cu²⁺ in the Yellow River. Yellow River samples were filtered through a standard 0.45 μm filter and analyzed using the standard addition method. The results are shown in Table 2. Thus the SA/MWCNTs/Nafion/PGE has a great potential for real sample analysis with a high accuracy and good reliability.

Table 2 Recovery study of Cu²⁺ in Yellow River

Added (Cu^2+ nM)	Found (nM)	Recovery (%)
40	41.7 ± 1.7	104
60	59.3 ± 2.1	98.8
80	81.0 ± 1.4	101

---

4. Conclusion

In this work, a novel chemically modified pyrolytic graphite electrode for selective determination of Cu²⁺ has been developed. Due to the combination of the selective complexing ability of SA with Cu²⁺ and the good conductivity of MWCNTs, SA/MWCNTs/Nafion/PGE has shown a remarkably enhanced sensitivity and selectivity for Cu²⁺ determination in the presence of other metal ions. The as-fabricated electrochemical sensor was successfully applied in determining Cu²⁺ in real water samples with satisfactory results. In addition, the present results suggest that such a designing scheme could provide an excellent platform for electroanalysis and has potential for the fabrication of chemical sensors for heavy metals.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (20972170 and 21275150) and the Funds for Distinguished Young Scientists of Gansu (1210RJDA013).

References

1. C. Tu, Y. Shao, N. Gan, D. Xu and Z. J. Guo, Inorg. Chem., 2004, 43, 4761–4766.
2. E. L. S. Wong, E. Chow and J. J. Gooding, Electrochem. Commun., 2007, 9, 845–849.
3. Y. J. Zheng, Q. Huo, P. Kele, F. M. Andreopoulos, S. M. Pham and R. M. Leblanc, Org. Lett., 2001, 21, 3277–3280.
4. S. A. Reddy, K. J. Reddy, S. L. Narayan and A. V. Reddy, Food Chem., 2008, 109, 654–659.
5. C. Vulpe, B. Levinson, S. Whitney, S. Packman and J. Gitschier, Nat. Genet., 1993, 1, 7–13.
6. L. Zeng, E. W. Miller, A. Pralle, E. Y. Isacoff and C. J. Chang, J. Am. Chem. Soc., 2006, 128, 10–11.
7. M. T. Fernandez-Arguelles, W. J. Jin, J. M. Costa-Fernandez, R. Pereiro and A. Sanz-Medel, Anal. Chim. Acta, 2005, 549, 20–25.
8. M. Yuce, H. Nazir and G. Donmez, Bioelectrochemistry, 2010, 79, 66–70.
9. B. Rezaei, E. Sadeghi and S. Meghdadi, J. Hazard. Mater., 2009, 168, 787–792.
10. M. Bingöl, G. Yentür, B. Erand and A. B. Öktem, Czech J. Food Sci., 2010, 28, 213–216.
11. C. A. Sahin and I. Tokgoz, Anal. Chim. Acta, 2010, 667, 83–87.
12. A. Economou and P. R. Fielden, Analyst, 2003, 128, 205–212.
13. O. A. Farghaly and M. A. Ghandour, Environ. Res., 2005, 97, 229–235.
14. Y. Song and G. M. Swain, Anal. Chim. Acta, 2007, 593, 7–12.
15. O. El Tall, N. Jaffrezic-Renault, M. Sigaud and O. Vittori, Electroanalysis, 2007, 19, 1152–1159.
16. I. Svancara, C. Prior, S. B. Hocever and J. Wang, Electroanalysis, 2010, 22, 1405–1420.
17. M. R. Ganjali, N. Motakef-Kazami, F. Faridbod, S. Khoee and P. Norouzi, J. Hazard. Mater., 2010, 173, 415–419.
18. B. C. Janegitz, L. H. Marcolino-Junior, S. P. Campana-Filho, R. C. Faria and O. Fatibello-Filho, Sens. Actuators, B, 2009, 142, 260–266.
19. P. R. Oliveira, A. A. Tanaka, N. R. Stradiotto and M. F. Bergamini, Curr. Anal. Chem., 2012, 8, 520–527.
20. D. W. M. Arrigan, Analyst, 1994, 119, 1953–1966.
21. K. Kalcher, J. M. Kaumann, J. Wang, I. Svancara, K. Vytras, C. Neuhold and Z. Yang, Electroanalysis, 1995, 7, 5–22.
22. P. Ugo and L. M. Moretto, Electroanalysis, 1995, 7, 1105–1113.
23. S. Anandhakumar and J. Mathiyarasu, Microchim. Acta, 2013, 180, 1065–1071.
24. R. Agraz, M. T. Sevilla and L. Hernandez, Anal. Chim. Acta, 1993, 283, 650–656.
25. M. S. Won, J. H. Park and Y. B. Shim, Electroanalysis, 1993, 5, 421–426.
26. Z. Gao, A. Ivaska and P. Li, Anal. Sci., 1992, 8, 337–343.
27. A. Safavi, M. Pakniat and N. Maleki, Anal. Chim. Acta, 1996, 335, 275–282.
28. P. K. Sahoo, B. Panigrahy, S. Sahoo, A. K. Satpati, D. Li and D. Bahadur, Biosens. Bioelectron., 2013, 43, 293–296.
29. R. Z. Ouyang, S. A. Bragg, J. Q. Chambers and Z. L. Xue, Anal. Chim. Acta, 2012, 722, 1–7.
30. W. Chemnasiry and F. E. Hernandez, Sens. Actuators, B, 2012, 173, 322–328.
31. A. Mohadesi, Z. Motallebi and A. Salmanipour, Analyst, 2010, 135, 1686–1690.
32. J. Chang, G. Zhou, E. R. Christensen, R. Heideman and J. Chen, Anal. Bioanal. Chem., 2014, 1–19.
33. A. Afkhami, F. Soltani-Felehgari, T. Madrakian, H. Ghaedi and M. Rezaeivala, Anal. Chim. Acta, 2013, 771, 21–30.
34. D. P. Wang, Y. Tang and W. D. Zhang, Microchim. Acta, 2013, 180, 1303–1308.
35. M. Sadhukhan and S. Barman, J. Mater. Chem. A, 2013, 1, 2752–2756.
36. L. Chen, H. Liu, K. Yang, J. Wang and X. Wang, Mater. Chem. Phys., 2008, 112, 407–411.
37. M. Suresh, A. K. Mandal, S. Saha, E. Suresh, A. Mandoli, R. D. Liddo, P. P. Parnigotto and A. Das, Org. Lett., 2010, 12, 5406–5409.
38. Y. Y. Fu, H. X. Li and W. P. Hu, Eur. J. Org. Chem., 2007, 15, 2459–2463.
39. A. Caballero, R. Martinez, V. Lloveras, I. Ratera, J. Vidal-Gancedo, K. Wurst, A. Tarraga, P. Molina and J. Veciana, J. Am. Chem. Soc., 2005, 127, 15666–15667.
40. R. Martinez, A. Espinosa, A. Tarraga and P. Molina, Org. Lett., 2005, 7, 5869–5872.
41. D. W. Pan, Y. Wang, Z. P. Chen, T. T. Lou and W. Qin, Anal. Chem., 2009, 81, 5088–5094.
42. R. M. Takeuchi, A. L. Santos, P. M. Padilha and N. R. Stradiotto, Talanta, 2007, 71, 771–777.
43. S. K. Alpat, U. Yuksel and H. Akcay, Electrochem. Commun., 2005, 7, 130–134.
44. H. Alemu and B. S. Chandravanshi, Anal. Chim. Acta, 1998, 368, 165–173.
45. B. S. Sherigara, Y. Shivaraj, R. J. Mascarenhas and A. K. Satpati, Electrochim. Acta, 2007, 52, 3137–3142.
46. E. C. Canpolat, E. Sar, N. Y. Coskun and H. Cankurtaran, Electroanalysis, 2007, 19, 1109–1115.
47. H. M. Abu-Shawish, S. M. Saadeh and A. R. Hussien, Talanta, 2008, 76, 941–948.
48. M. B. Gholivand, M. Rahimi-Nasrabadi, M. R. Ganjali and M. Salavati-Niasari, Talanta, 2007, 73, 553–560.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra12342e

---