Selective colorimetric sensing of Co²⁺ and Cu²⁺ using 1-(2-pyridylazo)-2-naphthol derivative immobilized polyvinyl alcohol microspheres

Abstract

An optical analytical naked-eye sensor was proposed based on the complexation reactions of the 1-(2-pyridylazo)-2-naphthol derivative with heavy metals in aqueous solutions. The prepared colorimetric sensor was characterized by scanning electron microscopy, energy-dispersive X-ray fluorescence spectrometry, and Fourier transform infrared spectroscopy, which indicated a spherical sensor and its molecular composition. In the sensing experiment, notable changes in color and absorbance intensity of the colorimetric sensor for Co²⁺ and Cu²⁺ were observed at pH 5 and 4, respectively. The proposed method provided consistent data reliability, with a detection limit of 1 and 5 μM by the naked eye and 0.23 and 0.95 μM by UV-vis spectrometry for Co²⁺ and Cu²⁺, respectively. The prepared chemosensor can be used as a colorimetric sensor to monitor Co²⁺ and Cu²⁺ with satisfactory results, which validate its value in practical applications in environmental systems.

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1. Introduction

Heavy metal contamination in aqueous solutions originates from many sources, such as metal plating facilities, mining operations, pigment manufacturing, petroleum refining, and tanneries.^1–3 Inductively coupled plasma atomic emission spectroscopy, electrochemistry, atomic absorption spectroscopy, atomic fluorescence spectrometry, and UV-vis spectroscopy are the most routinely used methods to detect heavy metals.^4–8 These methods are well established, but many require preconcentration or separation steps prior to detection, cannot be easily deployed directly in the field, lack portability, and rely on trained personnel because of their complexity.⁹ As such, these methods are costly, time consuming, and labor intensive. Therefore, easy to use and rapid monitoring tools for the determination of heavy metals in aqueous solutions are of essential relevance, particularly in sensitive environments, such as drinking water and industrial wastewater effluents. Simple colorimetric methods are extremely attractive because they can eliminate the need for analytical instruments and can be easily read with the naked eye.^10–13

Colorimetric techniques have been commonly used to determine trace heavy metals based on the use of reagents for decades.^14,15 Increasing number of colorimetric sensors has been developed using available chromophores immobilized surfaces of matrix materials.^16–19 Azo compounds are well-known aromatic dyes that have the ability to form colored complexes with heavy metals.^20–23 Taking advantage of this ability, we employed 4-(2-pyridylazo)-1,3-benzenediol for the detection of Cu²⁺ and developed a sensor with excellent detection performance.²⁴ Sedghi et al.²⁵ prepared a colorimetric sensor based on TiO₂/poly(acrylamide-co-methylenebisacrylamide) nanocomposites, which could simultaneously detect Hg²⁺ and Pb²⁺. In the present work, 1-(2-pyridylazo)-2-naphthol (PAN) was selected as a chromophore because it is a heterocyclic azo compound and has been extensively used as a highly sensitive photometric reagent for the determination of metal ions.^26,27 PAN can coordinate through the azo group and the nitrogen atom of pyridyl with metal ions to form a chelate, which results in a significant change in the absorbance intensity and color of the solution.

Metal nanoparticles and organic polymers are selected as matrix materials of colorimetric sensors. Gold and silver nanoparticles are often used as matrix materials of colorimetric sensors because of their excellent optical properties.^28–30 These sensors have advantageous selectivity and low detection limit (L_D), but expensive metal nanoparticles are unstable and have poor repeatability.^31,32 In comparison, organic materials are stable and possess an adequate number of groups by nature, which have the advantage of modification of other functional groups.^33–35 Polyvinyl alcohol (PVA), which is common, inexpensive, nontoxic, and biodegradable and has high mechanical strength, physical and chemical properties of stability, and large specific surface area, can provide sufficient sites for immobilization of functional groups. PVA also contains a large number of hydroxyl groups that can be easily modified with various functional groups.^36–38

In this study, we proposed a method for developing colorimetric sensors for the determination of low concentrations of heavy metals based on PAN derivative immobilized PVA microspheres. The synthesis route of the sensor was simple, and the reaction conditions of the experiment were mild. The approach was based on the creation of heavy metal complexes and detection of their optical properties. The prepared colorimetric sensor can simultaneously detect Co²⁺ and Cu²⁺ and has significant potential as a novel method for practical application.

2. Experimental

2.1. Materials

All reagents and chemicals used in the probe synthesis process and subsequent analytical studies were of analytical grade and used without further purification. PVA (1750 ± 50), paraffin, Span 80, ethanol, epichlorohydrin, NaNO₃, AgNO₃, MgSO₄·7H₂O, Pb(NO₃)₂, CuSO₄·5H₂O, Zn(NO₃)₂·6H₂O, CdCl₂·2.5H₂O, MnSO₄·H₂O, CoCl₂·6H₂O, HgCl₂, NiSO₄·6H₂O, FeSO₄·7H₂O, Fe(NO₃)₃·9H₂O, and NaHSO₃ were obtained from Sinopharm Chemical Reagent Co., Ltd (Beijing, China). Tetrabutylammonium bromide (TBAB), glutaraldehyde (GA), and PAN were purchased from Sigma-Aldrich Reagent Company (Shanghai, China).

2.2. Instruments

Scanning electron microscopy (SEM) was performed using a Hitachi S-4800 microscope (Hitachi Co., Tokyo, Japan), with an accelerating voltage of 5 kV. Energy-dispersive X-ray (EDX) analysis was conducted using an Octane Plus EDX fluorescence spectrometer (Ametek Co., Berwyn, PA, USA) attached to a scanning electron microscope. Fourier transform infrared spectroscopy (FTIR) spectra were recorded with a Nicolet iS10 FTIR spectrometer (Nicolet Co., Madison, WI, USA). ¹H NMR measurement was performed using JNM-ECS 400 spectrometer (JEOL Ltd, Japan) with TMS as an internal standard and CD₃SOCD₃ as solvent. Mass spectra were recorded on Bruker-MicrOTOF II (USA). The absorption spectra of the colorimetric sensor were recorded using a DR 5000 spectrophotometer (HACH Co., Loveland, CO, USA). Absorbance can directly affect the perceived color of the involved chemicals in the visible range.

2.3. Sensor fabrication

Scheme 1 shows that the fabrication process of the colorimetric sensor (PVA–PAN) involved the following steps. Step 1: the PAN derivative was prepared by adding sodium bisulfite (3 g) and ammonium hydroxide (40 mL) to PAN (3 g) and heating the stirred mixture to 60 °C for at least 8 h. The extract was washed with distilled water and left to dry completely. Then the crude product was added into hydrochloric acid (0.3 M, 60 mL) with stirring. After about 10 min, the filtrate was collected by suction filtration and NaOH (1 M, 20 mL) was added into the filtrate under stirring for 10 min. The precipitate was collected by suction filtration, washed with distilled water and left to dry completely. Step 2: PVA (4 g) was dissolved in distilled water (40 mL) with mechanical stirring in a water bath at 90 °C for 1 h. After cooling to 60 °C, liquid paraffin (80 mL) and Span 80 (2 g) were added into the flask with continuous stirring for 2 h. HCl (10 mM, 2 mL) and GA (50%, 4 mL) were then dropped into the flask rapidly with continuous stirring at 80 °C for 30 min. Finally, the PVA microspheres were rinsed repeatedly with petroleum ether and left to dry completely. Step 3: the chlorine-functionalized PVA microspheres (PVA–Cl) were prepared by mixing PVA (2 g), epichlorohydrin (8 mL), and HCl (1 mM, 100 mL) in the flask and heating the stirred mixture to 80 °C for 2 h. Subsequently, PVA–Cl was washed with distilled water and left to dry completely. Step 4: PVA–Cl (1 g), the prepared PAN derivative (2 g), TBAB (0.1 g), and NaOH (125 mM, 40 mL) were heated at 80 °C in the flask for 6 h. The PVA–PAN extract was rinsed with ethanol and left to dry.

Scheme 1 Synthesis routes of the colorimetric sensor (PVA–PAN).

2.4. Detection procedures

The tested water was distilled water spiked with heavy metals at the desired concentrations. The desired solution pH was adjusted by adding NaOH or HCl. The pH was rechecked at the end of the measurement and observed to be unchanged. The prepared PVA–PAN was added to the tested water. We can qualitatively identify the metal ions in the tested water with color changes. Absorbance intensity was detected quantitatively using UV-vis spectrometry.

3. Results and discussion

3.1. Sensor characterization

The SEM images of the PVA and PVA–PAN microspheres are shown in Fig. 1. The microspheres had an approximately spherical shape. Compared with the SEM image of PVA microspheres shown in Fig. 1a, the SEM images shown in Fig. 1b and c clearly revealed that the reaction of PAN and PVA microspheres resulted in PVA-microsphere-loaded PAN with a rougher surface.

Fig. 1 (a) SEM image of the PVA microsphere, (b and c) SEM image of the PVA–PAN microsphere, and (d) EDX analysis of the PVA–PAN microsphere.

EDX analysis was also performed to investigate the surface characteristic of the PVA–PAN microsphere further. The results are shown in Fig. 1d. The elemental composition of the PVA–PAN microsphere was identified through EDX elemental mapping. The elements carbon and oxygen were mainly attributed to the PVA components. EDX analysis showed that the N content of the microsphere surfaces was 1.72% on the PVA–PAN microsphere. EDX analysis also showed that PAN was distributed in the PVA microsphere because the PVA microsphere did not contain nitrogen.

The FTIR spectra of the PVA and PVA–PAN microspheres are shown in Fig. 2. The presence of a band at approximately 3395 cm⁻¹ was attributed to the –OH groups of the PVA microsphere. The presence of bands at 2918 and 2857 cm⁻¹ was attributed to the –CH₂– groups. The peak at 1502 cm⁻¹, which was a characteristic of the PVA–PAN microsphere, correlated with the C=C groups of the benzene ring. Fig. 2b shows a broad band at approximately 3360 cm⁻¹, which was attributed to the –NH– groups and indicated the successful linkage of PAN to the surface of the PVA microsphere.

Fig. 2 FTIR spectra of (a) PVA and (b) PVA–PAN microspheres.

The receptor was characterized by NMR and MS spectra. The ¹H NMR peaks (Fig. S1†) were found at (CD₃SOCD₃, 400 MHz) (δ, ppm (J, Hz)): 8.33–8.38 (m, 2H), 7.92–7.93 (m, 2H), 7.87 (d, J = 9.6, 1H), 7.65 (dd, J = 7.7, 1.1, 1H), 7.51–7.55 (m, 1H), 7.43 (td, J = 7.5, 1.3, 1H), 7.20–7.23 (m, 1H), 6.64 (d, J = 9.7, 1H) and supported the structure of the receptor as shown in Scheme 1. Further, HRMS spectra (Fig. S2†) showing a prominent peak at 271.0762 due to L + Na⁺ also supported the alleged structure of the receptor as given in Scheme 1.

3.2. Evaluation of selectivity

The main aim associated with the fabrication of the proposed sensor was to investigate its sensing ability for heavy metal ions. The selectivity of PVA–PAN toward various metal ions (100 μM), for example, Na⁺, Ag⁺, Mg²⁺, Pb²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Mn²⁺, Co²⁺, Hg²⁺, Ni²⁺, Fe²⁺ and Fe³⁺, was investigated at pH 6.0 by UV-vis absorption studies. Fig. 3 shows that Co²⁺ and Cu²⁺ were easily observed by naked-eye inspection. Thus, PVA–PAN had a high selectivity toward Co²⁺ and Cu²⁺, which can be used to detect Co²⁺ and Cu²⁺ in wastewater. The UV-vis absorption spectra of PVA–PAN with different metal ions (100 μM) at pH 6.0 were shown in Fig. 4a and as shown in Fig. 4b, PVA–PAN exhibited an absorption peak at 480 nm. Upon adding Co²⁺ ions, two new and strong absorption bands appeared at 450 and 575 nm, which were responsible for the change of the solution color from orange to dark green. Upon adding Cu²⁺ ions, the low-energy band was Δλ = 70 nm redshifted (Fig. 4c), resulting in the change of the solution color from orange to purple.

Fig. 3 Visible color change of PVA–PAN to various metal ions (100 μM) at pH 6.0.

Fig. 4 UV-vis spectra of PVA–PAN with (a) different metal ions, (b) Co²⁺ ions and (c) Cu²⁺ ions.

3.3. Effect of pH

The pH response of PVA–PAN with Co²⁺ (100 μM) and Cu²⁺ (100 μM) ions was evaluated. When the pH value ranged from 5 to 14, the absorbance intensity of PVA–PAN with Co²⁺ ions significantly decreased at pH 5 to 10 and stabilized at pH 10 to 14. When the pH value varied from 5 to 1, the absorbance intensity significantly decreased (Fig. 5a). As such, the optimal pH was 5. A similar phenomenon was used to describe the effect of pH on the absorbance intensity of PVA–PAN with Cu²⁺ ions. Fig. 5b shows that the optimal pH was 4 in detecting Cu²⁺ ions with PVA–PAN. In acidic solutions (pH < 3), the PAN molecules were protonated (Scheme 2). The protonated pyridyl exhibited a decreased electron-donating ability. The electric charge repulsion between protonated pyridyl and positively charged Co²⁺ or Cu²⁺ ions resulted in decreased interaction between PVA–PAN and Co²⁺ or Cu²⁺ ions, which indeed prevented the receptor from capturing Co²⁺ or Cu²⁺ ions. In alkaline solution, the precipitates of cobalt hydroxide and copper hydroxide also hindered the interaction between PVA–PAN and Co²⁺ ions and between PVA–PAN and Cu²⁺ ions, respectively.

Fig. 5 Effect of pH on the absorbance intensity of PVA–PAN (a) with Co²⁺ ions at 450 nm and (b) Cu²⁺ ions at 550 nm.

Scheme 2 PVA–PAN species at various pH ranges.

3.4. Sensitivity and calibration graph

The experiments were conducted at optimum pH for each metal, and the sensing signal responses of Co²⁺ and Cu²⁺ ions were obtained after 10 min of contact. Fig. 6a shows two new peaks that appeared at 450 and 575 nm. With the increase in Co²⁺ ion concentration from 0 μM to 100 μM, the two new absorption bands that appeared at 450 and 575 nm were gradually enhanced. In addition, the percentage of the absorbance intensity at 575 nm gradually increased compared with that of the absorbance intensity at 450 nm, resulting in the change of the solution color from orange to dark green. However, at low Co²⁺ ion concentrations, the absorption spectra were characterized by a weak band at 575 nm and a strong band at 450 nm, resulting in the change of the solution color to brown (Fig. 6b). Fig. 6b also shows that the L_D of PVA–PAN for Co²⁺ ions is 1 μM by naked-eye inspection. With the increase in Cu²⁺ ion concentration from 0 μM to 100 μM, the intensity of the new peak that appeared at 550 nm increased (Fig. 6c). The results indicated that a stable complex was formed between PVA–PAN and Cu²⁺ ions. As shown in Fig. 6d, the naked-eye L_D of PVA–PAN for Cu²⁺ ions was 5 μM.

Fig. 6 Absorbance spectra of PVA–PAN at different concentrations of (a) Co²⁺ and (c) Cu²⁺ ions and sequential color response for the sensor at different concentrations of (b) Co²⁺ and (d) Cu²⁺ ions.

The absorbance spectra were recorded individually for various concentrations of Co²⁺ and Cu²⁺ ions. The relationship was linear at low concentrations, which indicated the potential for analyte detection over an extensive range of concentrations. The calibration graph (Fig. 7) showed that a linear response was maintained in the range of 0 μM to 10 μM of Co²⁺ and Cu²⁺ ions, with R² values of 0.9977 and 0.9942, respectively. From the linear graph, L_D was calculated as 0.23 and 0.95 μM for Co²⁺ and Cu²⁺ ions, respectively, using eqn (1):

L_D = 3.3S_b/K (1)

where S_b is the standard deviation of the blank and K is the slope of the linear calibration range. PVA–PAN can be used to detect the low concentration of Co²⁺ and Cu²⁺ ions by UV-vis spectroscopy.

Fig. 7 Calibration plots of the absorbance spectra of PVA–PAN for various concentrations of (a) Co²⁺ and (b) Cu²⁺ ions. Measurements were made at a specific wavelength for each metal.

The obtained detection limit was found to be good in simultaneous determination Co²⁺ and Cu²⁺ ions. For comparison, the experimental results for the determination of Co²⁺ and Cu²⁺ ions obtained by some other methods were listed in Table 1.

Table 1 Comparison of PVA–PAN as a colorimetric sensor for the detection of Co²⁺ and Cu²⁺ ions with previously reported methods

Reagent	LOD		References
	Co^2+	Cu^2+
PVA–PAN	2.3 × 10^−7 M	9.5 × 10^−7 M	Present work
Triazolyl monoazo derivative	—	1.36 × 10^−5 M	17
Ninhydrin–quinoxaline derivative	—	3.43 × 10^−7 M	39
Dicarboxylate-1,5-diphenyl-3-thiocarbazone functionalized core/multi-shell silica nanoparticles	2.8 × 10^−8 M	2.9 × 10^−8 M	18
PAR–MPVA	—	1.6 × 10^−7 M	24
Anthraquinone derivative/2,6-dimethoxyl anthraquinone derivative	—	4.0 × 10^−7 M/6.0 × 10^−8 M	13

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3.5. Sensing mechanism

The Job plot experiment was carried out (Fig. 8) to determine the sensing mechanism of the receptor for Co²⁺ and Cu²⁺. The absorbance at 450 nm shown in Fig. 8a and at 550 nm shown in Fig. 8b reached its maxima when the molar fractions of Co²⁺ and Cu²⁺ were 0.3 and 0.5, respectively. The results indicated that a 1 : 2 complex was formed between the receptor and Co²⁺ and a 1 : 1 complex was formed between the receptor and Cu²⁺. The most significant spectroscopic and visual response was observed upon the addition of Co²⁺ and Cu²⁺. This significant response was attributed to the high thermodynamic affinity of Co²⁺ and Cu²⁺ for typical N-donor ligands and rapid metal-to-ligand binding kinetics. The binding mode of the receptor with Co²⁺ and Cu²⁺ is shown in Scheme 3.

Fig. 8 Job plot for the receptor versus (a) Co²⁺ and (b) Cu²⁺ ions. The total concentration of the receptor and Co²⁺ (Cu²⁺) is 50 μM. We put the amount of PVA–PAN equivalent to the concentration of receptor and the amount of PVA–PAN is indirectly represented by the concentration of receptor.

Scheme 3 Binding mode of the receptor with Co²⁺ and Cu²⁺.

The binding constant K was calculated by Benesi–Hildebrand eqn (2):^40,41

where A is the absorbance measured with different concentration of the metal ions, A₀ is the absorbance of the free receptor, A_max is the maximum absorbance of the receptor and metal ions, K is the binding constant, [M^m+] is the concentration of the metal ions added and n is the binding stoichiometry for the receptor and metal ion. According to the colorimetric titration and the Job plot experiment, the binding constant calculated by Benesi–Hildebrand method was 1.21 × 10⁸ M⁻² for the receptor and Co²⁺ and 1.66 × 10⁴ M⁻¹ for the receptor and Cu²⁺.

3.6. Effect of coexisting ions

Na⁺, Mg²⁺, Pb²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Mn²⁺, Hg²⁺, Ni²⁺, Fe²⁺ and Fe³⁺ (100 μM) were added to the PVA–PAN/Co²⁺ (100 μM) solution and Na⁺, Mg²⁺, Zn²⁺, Cd²⁺, Mn²⁺, Co²⁺, Ni²⁺, Fe²⁺ and Fe³⁺ (100 μM) were added to the PVA–PAN/Cu²⁺ (100 μM) solution to analyze the influence of coexisting ions on the detection of Co²⁺ and Cu²⁺ further. UV-vis absorption spectra were recorded, and the results were shown in Fig. 9. The coexisting ions exhibited negligible interference to the detection of Co²⁺ and Cu²⁺.

Fig. 9 Effects of coexisting ions on the absorbance of (a) PVA–PAN/Co²⁺ at 450 nm and (b) PVA–PAN/Cu²⁺ at 550 nm.

Calibration graphs of interference of Co²⁺ and Cu²⁺ (Fig. S3†) were made in the presence of Cu²⁺ and Co²⁺ ions, respectively. Fig. S3† showed that a linear response was maintained in the range of 0 μM to 10 μM of Co²⁺ and Cu²⁺ ions, with R² values of 0.9976 and 0.9929, respectively. And the limit of detection was calculated as 0.22 and 0.91 μM for Co²⁺ and Cu²⁺ ions from the linear graph, respectively. The results indicated that Cu²⁺ and Co²⁺ ions had negligible interference to the detection of Co²⁺ and Cu²⁺, respectively.

3.7. Reusability of the colorimetric sensor

The PVA–PAN/Co²⁺ and PVA–PAN/Cu²⁺ solutions were decomplexed by adding ethylenediaminetetraacetic acid (EDTA) to analyze the reusability of the sensing system. Upon adding EDTA to the PVA–PAN/Co²⁺ and PVA–PAN/Cu²⁺ solutions, the colors of the solutions changed from dark green to orange and from purple to orange, respectively. These results confirmed that EDTA can adequately regenerate PVA–PAN, and this process was repeated six times. This slight change in absorbance intensity (Fig. 10) indicated the reusability of the sensor.

Fig. 10 Absorption spectra changes of (a) PVA–PAN/Co²⁺ at 450 nm and (b) PVA–PAN/Cu²⁺ at 550 nm at different times.

3.8. Responses to heavy metals in real samples

To determine the efficiency of PVA–PAN in practical application, we collected an untreated water sample from Qinhuai River in Nanjing, China. As shown in Fig. 11a and b, the significant change of the solution color from orange to brown with the addition of Co²⁺ (10 μM) could be observed by the naked eye. When the pH level was adjusted to 5, the color change became increasingly distinct (Fig. 11c). Fig. 11d and e showed that the solution changed in color from orange to purple when Cu²⁺ (10 μM) was added into the aqueous solution. The color change became significant when the pH level was adjusted to 4 (Fig. 11f). The results illustrated that the PVA–PAN has the potential to be an excellent practical sensor for heavy metal detection.

Fig. 11 Absorbance and color changes of PVA–PAN in river water ((a) pH 6, no addition of Co²⁺; (b) pH 6, addition of 10 μM Co²⁺; (c) pH 5, addition of 10 μM Co²⁺; (d) pH 6, no addition of Cu²⁺; (e) pH 6, addition of 10 μM Cu²⁺; and (f) pH 4, addition of 10 μM Cu²⁺).

4. Conclusion

In summary, a highly selective and sensitive PVA–PAN was developed based on absorption changes, which can detect Co²⁺ and Cu²⁺ over other metal ions. The detection of Co²⁺ and Cu²⁺ led to color changes from orange to dark green and from orange to purple, respectively, which were clearly visible to the naked eye. And the PVA–PAN was stable and can be reused. In this sensing system, the effect of pH was considered a key factor to achieve flexibility by controlling the electron-donating ability of pyridyl and the developed precipitates of cobalt hydroxide and copper hydroxide. The design strategy of the sensor could extend the development of colorimetric sensors for common heavy metal ions. The developed sensor has significant potential application in industrial monitoring because of its selectivity, reusability, and sensitivity.

Acknowledgements

We are grateful for grants from National Natural Science Foundation of China (Grant No. 51308183, 51379060), Natural Science Foundation of Jiangsu Province of China (Grant No. BK20130828), Major Science and the Fundamental Research Funds for the Central Universities (Grant No. 2013B32214, 2014B07414) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

References

1. S. E. Bailey, T. J. Olin, R. M. Bricka and D. D. Adrian, Water Res., 1999, 33, 2469–2479.
2. S. Rose, S. Melnyk, A. Savenka, A. Hubanks, S. Jernigan, M. Cleves and S. J. James, Am. J. Biochem. Biotechnol., 2008, 4, 85.
3. S. A. El-Safty, M. A. Shenashen and M. Khairy, Talanta, 2012, 98, 69–78.
4. L. Zhang, Z. Li, X. Du, R. Li and X. Chang, Spectrochim. Acta, Part A, 2012, 86, 443–448.
5. M. A. Alvarez and G. Carrillo, Talanta, 2012, 97, 505–512.
6. D. Sanchez-Rodas, W. Corns, B. Chen and P. Stockwell, J. Anal. At. Spectrom., 2010, 25, 933–946.
7. E. Santoyo, S. Santoyo-Gutiérrez and S. P. Verma, J. Chromatogr. A, 2000, 884, 229–241.
8. S. Tatay, P. Gaviña, E. Coronado and E. Palomares, Org. Lett., 2006, 8, 3857–3860.
9. F. Meziane, V. Raimbault, H. Hallil, S. Joly, V. Conédéra, J. Lachaud, L. Béchou, D. Rebière and C. Dejous, Sens. Actuators, B, 2014, 209, 1049–1056.
10. M. R. Awual, M. Ismael and T. Yaita, Sens. Actuators, B, 2014, 191, 9–18.
11. D. Kim and B. F. Chmelka, RSC Adv., 2015, 5, 16549–16553.
12. Y. Zhou, F. Wang, Y. Kim, S.-J. Kim and J. Yoon, Org. Lett., 2009, 11, 4442–4445.
13. R.-L. Liu, H.-Y. Lu, M. Li, S.-Z. Hu and C.-F. Chen, RSC Adv., 2012, 2, 4415–4420.
14. E. B. Sandell, Soil Sci., 1959, 59, 481.
15. S. PhilipáAnthony, RSC Adv., 2013, 3, 16765–16774.
16. M. R. Awual, T. Yaita, S. A. El-Safty, H. Shiwaku, S. Suzuki and Y. Okamoto, Chem. Eng. J., 2013, 221, 322–330.
17. P. Kaur, D. Sareen and K. Singh, Talanta, 2011, 83, 1695–1700.
18. M. Shenashen, S. El-Safty and E. Elshehy, J. Hazard. Mater., 2013, 260, 833–843.
19. A. K. Mahapatra, G. Hazra, N. K. Das and S. Goswami, Sens. Actuators, B, 2011, 156, 456–462.
20. Y. Sumi and T. Suzuki, Microsc. Res. Tech., 2002, 56, 332–340.
21. M. Omar and G. G. Mohamed, Spectrochim. Acta, Part A, 2005, 61, 929–936.
22. C. R. T. Tarley, W. N. L. dos Santos, C. M. dos Santos, M. A. Z. Arruda and S. L. C. Ferreira, Anal. Lett., 2004, 37, 1437–1455.
23. H. A. Hadar, V. Bulatov, B. Dolgin and I. Schechter, J. Hazard. Mater., 2013, 260, 652–659.
24. Z. Hua, B. Yang, W. Chen, X. Bai, Q. Xu and H. Gu, Appl. Surf. Sci., 2014, 317, 226–235.
25. R. Sedghi, B. Heidari and M. Behbahani, J. Hazard. Mater., 2015, 285, 109–116.
26. X.-m. Kou and H.-l. Yin, Chem. Res. Appl., 2001, 13, 180–181.
27. D. Wang, H. Q. Luo and N. B. Li, Instrum. Sci. Technol., 2005, 33, 427–436.
28. M. Annadhasan, T. Muthukumarasamyvel, V. R. S. Babu and N. Rajendiran, ACS Sustainable Chem. Eng., 2014, 2, 887–896.
29. H. K. Sung, S. Y. Oh, C. Park and Y. Kim, Langmuir, 2013, 29, 8978–8982.
30. S. M. Taghdisi, N. M. Danesh, P. Lavaee, A. S. Emrani, M. Ramezani and K. Abnous, RSC Adv., 2015, 5, 43508–43514.
31. M. S. Wong, P. J. J. Alvarez, Y.-l. Fang, N. Akcin, M. O. Nutt, J. T. Miller and K. N. Heck, J. Chem. Technol. Biotechnol., 2009, 84, 158–166.
32. Y.-L. Hung, T.-M. Hsiung, Y.-Y. Chen and C.-C. Huang, Talanta, 2010, 82, 516–522.
33. D. Prabhakaran, H. Nanjo and H. Matsunaga, Anal. Chim. Acta, 2007, 601, 108–117.
34. M. Kumar, B. P. Tripathi and V. K. Shahi, J. Hazard. Mater., 2009, 172, 1041–1048.
35. A. R. Fajardo, L. C. Lopes, A. F. Rubira and E. C. Muniz, Chem. Eng. J., 2012, 183, 253–260.
36. X. Bai, Z.-f. Ye, Y.-f. Li, L.-c. Zhou and L.-q. Yang, Process Biochem., 2010, 45, 60–66.
37. X. Bai, H. Gu, W. Chen, H. Shi, B. Yang, X. Huang and Q. Zhang, Appl. Biochem. Biotechnol., 2014, 173, 1097–1107.
38. S. J. Sarma, K. Pakshirajan and B. Mahanty, J. Chem. Technol. Biotechnol., 2011, 86, 266–272.
39. A. Kumar, V. Kumar, U. Diwan and K. Upadhyay, Sens. Actuators, B, 2013, 176, 420–427.
40. H. A. Benesi and J. Hildebrand, J. Am. Chem. Soc., 1949, 71, 2703–2707.
41. M. Hariharan, S. C. Karunakaran and D. Ramaiah, Org. Lett., 2007, 9, 417–420.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12765c

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