A new fluorescent sensor for detecting p-nitrophenol based on β-cyclodextrin-capped ZnO quantum dots

Abstract

In this work, the fluorescent β-cyclodextrin-capped ZnO quantum dots (β-CD@ZnO QDs) with a good water-solubility have been synthesized via a facile method. And the synthesized β-CD@ZnO QDs with a diameter ∼3.64 nm and a yellow fluorescence have been characterized by high resolution transmission electron microscope, UV-visible absorption, photoluminescence, and Fourier transform infrared spectroscopy. The β-CD@ZnO QDs were used as a fluorescent sensor for the detection of p-nitrophenol. A linear relationship to the concentration of the p-nitrophenol over the range of 1.0–40 μM with the detection limit of 0.34 μM was obtained. The sensing property of the β-CD-capped ZnO QDs has been successfully examined in real water samples. And the quenching mechanism was proposed, which showed that the quenching effect may be caused by the inclusion complexation between β-CD and p-nitrophenol and electron transfer.

---

Introduction

Quantum dots (QDs) are appealing fluorophores in a variety of biological applications for the long term and simultaneous detection of multiple signals in imaging and labeling because of their unique optical properties. Various research groups have used QDs as a new class of fluorescent sensors.^1,2 As semiconductor QDs, ZnO QDs have gained a lot of attention in various fields because of their nontoxicity, low expense, biocompatibility and the same properties of traditional QDs.^3–6 Recently ZnO QDs were used as fluorescent sensors for the determination of Co²⁺ ions, Cu²⁺ ions, Mg²⁺ ions, free chlorine, ammonia gas, various aromatic aldehydes, dopamine, and picric acid.^7–12

Aromatic nitro-compounds have many toxic influences not only on the human body but also on the animals, plants and aquatic life, therefore, lots of compounds have been listed as hazardous chemicals by environmental protection agency (EPA).^13–15 Among the lots of aromatic nitro-compounds, p-nitrophenol (p-NP), a toxic hydrolysis product of the insecticides parathion and paraoxon, which exists in wastewater, freshwater and marine environments due to its high stability and solubility in water, is one of the toxic substances that can cause significant damages to the environment and the health of living beings.^14,16–21 For example, p-NP can irritate and inflame in human eyes, nose and respiratory tract, and interact with blood to form methaemoglobin which is responsible for the metahaemoglobinemia causing cyanosis confusion and unconsciousness.²² According to its toxicity, the allowed limit of p-NP in the environment by different agents like United States EPA and the European Commission is 0.43 and 0.72 μM, respectively.²³ A variety of techniques have been used to detection p-NP such as spectrophotometry, high performance liquid chromatography, fluorescence, electrochemical methods and electrophoresis.^24–38 Nevertheless, these aforementioned detection methods involve complex sample preparation procedures and generally have expensive instrumentation. Thus, a simple, environment-friendly, effective and reliable sensor for the detection of p-NP in the water is needed.

In this article, water-stable β-CD@ZnO QDs were synthesized in a facile route. And an attempt has been made to utilize the β-CD@ZnO QDs for the determination of p-NP in aqueous solution. A linear relationship to the concentration of the p-nitrophenol over the range of 1.0–40 μM with the detection limit of 0.34 μM was obtained in this work. The mechanism of the system has been discussed and could be attributed to the inclusion complexation between β-CD and p-NP and electron transfer. The β-CD@ZnO QDs synthesized route and the schematic of fluorescence quenching of the β-CD@ZnO QDs by p-NP are shown in Fig. 1. This method shows several advantages such as simplicity, high efficiency, low expense and nontoxicity.

Fig. 1 Synthesized route and the schematic of quenching of the β-CD@ZnO QDs fluorescence by p-NP.

Experimental

Reagents and apparatus

Zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O), potassium hydroxide (KOH), ethanol absolute, and β-cyclodextrin (β-CD) were obtained from Chengdu Kelong Chemical Reagent Co., Ltd., Chengdu, China. p-Nitrophenol (p-NP), o-nitrophenol (o-NP), m-nitrophenol (m-NP), o-dihydroxybenzene (catechol, CC), p-cresol, phenol, phenylamine (P-A), p-chlorophenol (p-CP), benzoic acid (BA), hydroquinone (HQ), methylbenzene (MB), p-nitrotoluene (p-NT), m-dinitrobenzene (m-DNB), 2,4,6-trinitrotoluene (TNT), and nitrobenzene (NB) were purchased from Tianjin Kermel Chemical Reagents Co., Ltd., Tianjin, China. Britton–Robinson (BR) buffer solution (10 mM, pH 7.96) was used for the experiments. Other reagents were of analytical reagents grade and used as received without further purification. In this study, reagents were dissolved in ultrapure water and diluted into different normal concentrations. Ultrapure water (18.2 MΩ cm) was obtained from a water purification system.

All the fluorescence spectra were recorded with a Hitachi F-2700 fluorescence spectrophotometer (Hitachi Ltd., Japan) with excitation and emission slits of 10 nm. UV-visible (UV-vis) absorption spectra were obtained using a Shimadzu UV-2450 spectrophotometer (Suzhou Shimadzu Instrument Co., Ltd., China). Fourier transform infrared (FTIR) spectra were obtained with a Bruker IFS 113v spectrometer (Bruker, Germany) after pelleting the fine powder with KBr. High resolution transmission electron microscopy (HRTEM) images were recorded using a JEM-2100 (JEOL Ltd., Japan) transmission electron microscope. And the size was measured using a software named Nano Measurer. Zn(Ac)₂·2H₂O and KOH dissolved in ethanol completely using a KQ-400KB ultrasonic bath (400 W, Kunshan Ultrasonic Instruments Co., Ltd., China). An 85-2 constant temperature magnetic mixer (Zongda Instrument Plant, Jiangsu, China) was used to mix solution completely. Centrifugation was carried out in a TGL-16M high-speed refrigerated centrifuge (Xiangyi, China).

Preparation of the β-CD-capped ZnO QDs

A simple one-step method was used to synthesize the β-CD-capped ZnO QDs (β-CD@ZnO QDs) according to previous works with a little modification.^3,39–42 First, the ZnO QDs were synthesized by the dropwise addition of 0.1098 g (1 mmol) of Zn(CH₃COO)₂·2H₂O dissolved in 25 mL of ethanol to 0.0842 g (3 mmol) of KOH dissolved in 5 mL of ethanol, which both of the two solutions were dissolved completely by keeping in an ultrasonic bath for 60 min at room temperature. The above mixture was then stirred continuously at room temperature for 60 min. Next to, 0.5 mL of ultrapure water and 0.05 mmol of β-CD, which was dissolved in 10 mL ethanol, were added to the reaction system and kept constant stirring at room temperature for 120 min. After centrifugation, the precipitate was obtained and washed with absolute ethanol and ultrapure water for three times to remove unreacted materials. Then the precipitate was redispersed in 35 mL of water under ultrasonic and agitation for further use.

Detection of p-nitrophenol

In a 1.5 mL polypropylene microcentrifuge tube, 100 μL of the as prepared β-CD@ZnO QDs solution and 250 μL of p-nitrophenol solution (80 μM) were added and the mixture was diluted to 0.5 mL with BR buffer solution (10 mM, pH 7.96). The mixture was thoroughly mixed and incubated for 2 min at room temperature. Then the sample solution was transferred into 1.0 cm standard quartz cuvettes to record the fluorescence spectrum. All samples were excited at 351 nm and the fluorescence emission signal was monitored at the maximum emission wavelength of 540 nm. Three trials for each sample were performed and the results were expressed as the average of three measurements. F₀ − F was used to present the fluorescence quenching efficiency of the β-CD@ZnO QDs by the analyte, where F₀ and F indicate the fluorescence intensity of the β-CD@ZnO QDs in the absence and presence of p-nitrophenol, respectively.

Results and discussion

Characterization of the β-CD@ZnO QDs

UV-vis absorption, fluorescence, FTIR, and high resolution transmission electron microscopy (HRTEM) were used to characterize the as-prepared β-CD@ZnO QDs. The β-CD@ZnO QDs produced an absorption shoulder centered at 335 nm in water (Fig. 2(A)). As shown in Fig. 2(A), the fluorescence excitation and emission spectra of the β-CD@ZnO QDs show the peak position at 351 and 540 nm, respectively. The β-CD@ZnO QDs were almost colorless under the irradiation of the daylight which indicated that the β-CD@ZnO QDs were dispersed well in water, and emitted a strong yellow fluorescence under excited with UV light at 365 nm (inset of Fig. 2(A)). The TEM image (Fig. 2(B)) shows that the β-CD@ZnO QDs can be dispersed well in water and the diameter range of the β-CD@ZnO QDs is 2–5.6 nm with an average size of ∼3.64 nm (Fig. 2(C)).

Fig. 2 (A) UV-vis spectrum of the as synthesized β-CD@ZnO QDs solution without dilution and fluorescence excitation and emission spectra of the β-CD@ZnO QDs with diluted 5 times with ultrapure water (solid line). And the fluorescence of the as synthesized β-CD@ZnO QDs solution with diluted 5 times with 40 μM of p-NP (dashed). Inset: photograph (left) of the β-CD@ZnO QDs solution without dilution under the irradiation of the daylight and photograph (right) of the β-CD@ZnO QDs solution without dilution under excited with UV light at 365 nm. (B) TEM image of the β-CD@ZnO QDs. Inset: HRTEM image of the β-CD@ZnO QDs. (C) The particle size distribution map of the β-CD@ZnO QDs.

The β-CD@ZnO QDs can be stored in a refrigerator at room temperature for 1 month, which kept stable with negligible decrease in fluorescence. The stability of the β-CD@ZnO QDs can be strongly influenced by the solution pH values. As shown in the FTIR spectra (Fig. 3(a)), after β-CD was conjugated into ZnO QDs, the peaks at 3403, 2928, 1642, 1414, and 1031 cm⁻¹ can be seen compared with those of the β-CD (Fig. 3(b)). Compared with FTIR spectrum of β-CD, the peak at 3384 cm⁻¹ moves to 3403 cm⁻¹, the peak at 1642 cm⁻¹ shifts into 1638 cm⁻¹ in that of the β-CD@ZnO QDs. Moreover, an absorption peak was observed at 421 cm⁻¹ corresponding to the Zn–O stretching vibration,^25,40,43 indicating that the functional groups of β-CD were not destroyed and the β-CD was successfully capped on ZnO QDs surface.

Fig. 3 FTIR spectra of β-CD and the β-CD@ZnO QDs.

Detection of the p-NP based on the fluorescence of the β-CD@ZnO QDs

It was found that the β-CD@ZnO QDs fluorescence can be quenched by p-NP (Fig. 2(A), dashed), therefore, a p-NP probe was developed based on this phenomenon. And to achieve the highest fluorescence quenching efficiency, a series of analytical experimental parameters were performed. As shown in Fig. 4(A), the pH of BR buffer solution which varied from 5 to 12 was used to investigate the change of the fluorescence intensity of the β-CD@ZnO ODs. And the β-CD@ZnO QDs solution showed stable fluorescence intensity in the pH range of 6–8 (curve a). When pH is lower than 6, the fluorescence of the β-CD@ZnO QDs quenched well. This is probably caused by the increasing H⁺ ions which can resolve ZnO effectively. And when pH is over 8, the decrease in fluorescence intensity of the β-CD@ZnO QDs appeared due to the competitive binding to the crystals between hydroxyl ions and the ligands which leads to an increase of defects on the surface of the β-CD@ZnO QDs.²⁴

Fig. 4 (A) FL spectra of the β-CD@ZnO QDs solution in BR buffer solution (10 mM) with different pH values (a) and FL spectra of the β-CD@ZnO QDs with p-NP solution (40 μM) in BR buffer solution (10 mM) with different pH values (b). And the effect of pH on the relative fluorescence intensity (F₀ − F) of the β-CD@ZnO QDs + p-NP system (c) (condition: 100 μL of the β-CD@ZnO QDs solution, 40 μM p-NP and 150 μL of 10 mM BR buffer solution with different pH). (B) Effect of buffer solution on the relative fluorescence intensity (F₀ − F) of the β-CD@ZnO QDs + p-NP. (1) Water; (2) BR buffer solution (10 mM); (3) Na₂HPO₄–NaH₂PO₄ buffer solution (0.2 M); (4) KH₂PO₄–NaOH buffer solution (0.1 M) (condition: 100 μL of the β-CD@ZnO QDs solution, 40 μM p-NP and 150 μL of different buffer solutions at pH 7.96). (C) The effect of reaction time on the fluorescence intensity of the β-CD@ZnO QDs + p-NP (40 μM) at pH 7.96.

While the pH value and reaction time can influence the quenching efficiency of p-NP. The fluorescence intensities of β-CD@ZnO QDs with the addition of 40 μM p-NP in BR buffer solution at different pH (5.02–10.88) were recorded, as shown in curve b of Fig. 4(A). The relative fluorescence intensity which was described as (F₀ − F) was also recorded (curve c), which shows that the optimized pH value of quenching efficiency is at 7.96. Therefore, we chose the pH 7.96 as the experimental condition. Furthermore, the effect of different buffer solutions on the quenching efficiency was explored. As shown in Fig. 4(B), the relative fluorescence intensity of the β-CD@ZnO QDs in solution with and without different buffers for detection of 40 μM p-NP indicate that the existence of buffer is conductive to the determination of p-NP. And the quenching efficiency of p-NP in BR buffer solution (pH 7.96, 10 mM) higher than that in other buffer solution which can be seen from Fig. 4(B). Fig. 4(C) shows that the quenching efficiency of p-NP with increasing reaction time. Upon addition of 40 μM p-NP into the β-CD@ZnO QDs aqueous solution and diluted with BR buffer solution at pH 7.96, the fluorescence quenched rapidly, and the fluorescence intensity almost reached its minimum value after 1 min. On the basis of this result, the reaction time of 2 min appears to be selected for the detection of p-NP.

Under the optimum experimental conditions, the effect of the p-NP on the fluorescence of the β-CD@ZnO QDs has been evaluated. As shown in Fig. 5, the fluorescence intensity of the β-CD@ZnO QDs decreased with increasing concentration of p-NP from 0 to 200 μM. And a linear calibration curve to the concentration of p-NP over the range of 1.0–40 μM with the regression equation: F₀ − F = 5.3032C + 32.3944, (R² = 0.9967, where C presents the concentration of p-NP) was obtained, which the relative fluorescence intensity (F₀ − F) of the β-CD@ZnO QDs in the absence (F₀) and presence (F) of p-NP was defined the fluorescence quenching efficiency. The detection limit for p-NP was 0.34 μM based on 3σ/slope (where σ is the standard deviation of the blank measures). And the value is below 0.43 μM which is the allowed limit in drinking water given by the US Environmental Protection Agency (EPA). The performance of the β-CD@ZnO QDs as a fluorescent sensor for p-NP was compared to those of other analytical methods reported previously (as shown in Table 1). And a similar consequence with repeating the experiment three times at the p-NP concentration of 40 μM was obtained and the RSD was 2.12%, which showed a good repeatability of the method. The possible interference has been examined to prove the method with a good selectivity. As shown in Fig. 6, o-NP, m-NP, CC, p-cresol, phenol, P-A, p-CP, BA, HQ, MB, p-NT, m-DNB, TNT, and NB at concentration (50 μM) almost do not quench the fluorescence of the β-CD@ZnO QDs while p-NP (40 μM) quenched the fluorescence effectively, implying that the proposed method has good selectivity for detection of p-NP.

Fig. 5 Effect of the p-NP concentration (at concentration range 0–200 μM) on the fluorescence of the β-CD@ZnO QDs under excitation at 351 nm. Inset: the linear calibration curve of the fluorescence quenching efficiency (F₀ − F) of the β-CD@ZnO QDs with adding the concentrations of p-NP from 1 to 40 μM.

Table 1 Comparison of different analytical methods for p-NP detection

Method	Limit of detection (μM)	Linear range (μM)	Ref.
a Cyclic voltammetry.b Differential pulse voltammetry.c High performance liquid chromatography.
CV^a	1.163	—	15
Amperometry	0.02	0–20	22
Spectrophotometry	—	7.14–71.4	26
DPV^b	0.02	0.025–1	34
HPLC^c	0.001	0.003–1.43	30
Electrophoresis	0.11	—	29
Fluorescence	0.7	0.7–143	31
Fluorescence	0.05	20–100	24
Fluorescence	0.34	1–40	This work

---

Fig. 6 Effect of possible interferents on the fluorescence quenching of the β-CD@ZnO QDs (condition: 100 μL of the β-CD@ZnO QDs solution, 40 μM p-NP, 50 μM possible interferent and 150 μL with BR buffer solution (10 mM, pH 7.96)).

Analytical application of the β-CD@ZnO QDs in analysis of real water samples

To improve that the β-CD@ZnO QDs probe can be used to detect p-NP in real samples, the proposed method was used to analyze the p-NP in the water samples of tap water and Chongde Lake. First, 100 μL of β-CD@ZnO QDs solution mixed with 250 μL of water samples was diluted to 500 μL with BR buffer solution (10 mM, pH 7.96), and the mixture was incubated at room temperature for 2 min, then the fluorescence intensity (540 nm) of the above mixture solution was measured using the fluorescence spectrophotometer at an excitation wavelength of 351 nm. And the fluorescence intensity was not decreased, indicating that no p-NP was detected. For testing the accuracy of this new method, the application of the method was further evaluated with recovery experiments. First of all, tap water samples and Chongde Lake samples were spiked with different concentrations (10.0, 20.0, 25.0 and 35.0 μM) of standard p-NP solution. Then 100 μL of the β-CD@ZnO QDs solution mixed with 250 μL of the above sample solutions was diluted to 500 μL with BR buffer solution (10 mM, pH 7.96). After incubated at room temperature for 2 min, the fluorescence intensity (540 nm) of the above mixture solution was measured using the fluorescence spectrophotometer at an excitation wavelength of 351 nm. The recoveries of the water samples are listed in Table 2.

Table 2 Detection of p-NP in spiked tap water (sample 1) and Chongde Lake (sample 2) samples by this proposed method (n = 3)

Sample	Added (μM)	Found (μM)	Recovery (%)	RSD (%)
a Not detected.
1	0.00	ND^a	—	—
	10.00	10.57	105.70	1.57
	20.00	20.73	103.65	2.43
	25.00	25.46	101.84	2.74
	30.00	34.68	99.09	1.94
2	0.00	ND^a	—	—
	10.00	10.63	106.30	3.29
	20.00	20.35	101.75	2.38
	25.00	24.67	98.68	2.78
	30.00	34.32	98.06	1.76

---

Possible quenching mechanism of the fluorescence system

The quenching mechanism of p-NP on the fluorescence of the β-CD@ZnO QDs has been discussed in this work. According to precious study, the PL quenching of quantum dots by analyte can be attributed to the analyte binding on the surface of quantum dots which traps the excited electron by electron transfer mechanism.^44,45 In this work, the fluorescence quenching of β-CD@ZnO QDs by p-NP can be attributed to the p-NP complexes with β-CD and the electron transfer between the β-CD@ZnO QDs and p-NP. The binding constant for β-CD/p-NP complexation is reported as 90.1 M⁻¹, which is higher than those of β-CD/o-NP complexation (37.8 M⁻¹) and β-CD/m-NP complexation (44.2 M⁻¹),^46,47 indicating that the p-NP can be encapsulated into the β-CD@ZnO QDs. We discussed the quenching process based on the Stern–Volmer equation F₀/F = 1 + K_sv[Q] (where F₀ and F are the β-CD@ZnO QDs fluorescence intensity in the absence and presence of the p-NP, respectively, the [Q] is the p-NP concentration, and K_sv is the quenching constant). The Stern–Volmer plot of the β-CD@ZnO QDs for p-NP is shown in Fig. 7(C) and the linear regression equation is F₀/F = 1.0286 + 10 660[Q] (R² = 0.9895). This result infers that the binding is much stronger and the quenching occurs due to the p-NP complexes with the β-CD@ZnO QDs.

Fig. 7 (A) UV-vis absorption spectra of the β-CD@ZnO QDs, p-NP and their mixture solution with p-NP at different concentrations (1, 10 and 50 μM). (B) UV-vis absorption spectra of the β-CD@ZnO QDs and p-NP (100 μM), and the fluorescence excitation and emission spectra of the β-CD@ZnO QDs with diluted 5 times with ultrapure water. (C) Stern–Volmer plot of p-NP.

The UV-vis absorption spectra of the β-CD@ZnO QDs, p-NP, and the mixture of the β-CD@ZnO QDs with p-NP at different concentrations were recorded. As shown in Fig. 7(A), the absorbance of the β-CD@ZnO QDs solution has an increase with adding p-NP. And as shown in Fig. 7(B), the significant overlap between the p-NP absorption spectrum and the exaction spectrum may cause the fluorescence quenched in some extent. In addition, according to previous studies, the formation of non-fluorescent QDs ions by electron transfer between NPs and electron/hole traps of TeO₂ QDs after addition of NPs may also account for the fluorescence quenching.⁴⁴ Energy transfer should be considered because of the significant contribution for fluorescence quenching. As can be seen in Fig. 7(B), there is no the spectral overlap between the absorption of p-NP and the β-CD@ZnO QDs emission, which hinders the energy transfer from the β-CD@ZnO QDs to p-NP, indicating that the Förster resonance energy transfer (FRET) cannot play a role in the quenching process. In conclusion, the fluorescence quenching can be attributed to the electron transfer mechanism when the host–guest inclusion complexes of the β-CD and p-NP formed.

Conclusions

A new fluorescence method used to detect p-NP in water based on the β-CD@ZnO QDs has been proposed. The mechanism of the system may be attributed to the inclusion complexation between β-CD and p-NP and electron transfer. And the linear response ranges from 1 to 40 μM with the detection limit of 0.67 μM. This method shows several advantages such as good selectivity, simplicity, rapidness, low expense and nontoxicity. This work might provide a potential application in real water quality monitoring.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 21273174) and the Municipal Science Foundation of Chongqing City (No. CSTC-2013jjB00002, CSTC-2015jcyjB50001).

References

1. D. Bera, L. Qian, T. K. Tseng and P. H. Holloway, Materials, 2010, 3, 2260–2345.
2. I. L. Medintz, H. T. Uyeda, E. R. Goldman and H. Mattoussi, Nat. Mater., 2005, 4, 435–446.
3. J. Zhang, R. Zhang, L. H. Zhao and S. Q. Sun, CrystEngComm, 2012, 14, 613–619.
4. Y. S. Fu, X. W. Du, S. A. Kulinich, J. S. Qiu, W. J. Qin, R. Li, J. Sun and J. Liu, J. Am. Chem. Soc., 2007, 129, 16029–16033.
5. H. M. Xiong, Y. Xu, Q. G. Ren and Y. Y. Xia, J. Am. Chem. Soc., 2008, 130, 7522–7523.
6. Y. Z. Zhang, H. P. Wang, H. Jiang and X. M. Wang, Nanoscale, 2012, 4, 3530–3535.
7. H. Sharma, A. Singh, N. Kaur and N. Singh, ACS Sustainable Chem. Eng., 2013, 1, 1600–1608.
8. Z. Chen and D. D. Wu, Sens. Actuators, B, 2014, 192, 83–91.
9. S. M. Ng, D. S. N. Wong, J. H. C. Phung and H. S. Chua, Talanta, 2013, 116, 514–519.
10. H. Sharma, N. Kaur, T. Pandiyan and N. Singh, Sens. Actuators, B, 2012, 166–167, 467–472.
11. N. R. Jana, H. H. Yu, E. M. Ali, Y. G. Zheng and J. Y. Ying, Chem. Commun., 2007, 14, 1406–1408.
12. G. Singh, A. Choudhary, D. Haranath, A. G. Joshi, N. Singh, S. Singh and R. Pasricha, Carbon, 2012, 50, 385–394.
13. F. W. Kutz, B. T. Cook, O. D. Carter-Pokras, D. Brody and R. S. Murphy, J. Toxicol. Environ. Health, Part A, 1992, 37, 277–291.
14. E. L. Kochany, Chemosphere, 1992, 22, 529–536.
15. K. Singh, A. A. Ibrahim, A. Umar, A. Kumar, G. R. Chaudhary, S. Singh and S. K. Mehta, Sens. Actuators, B, 2014, 202, 1044–1050.
16. C. Huber, B. Bartha and P. Schroder, J. Hazard. Mater., 2012, 243, 250–256.
17. R. X. Chen, Q. P. Zhang, Y. Gu, L. Tang, C. Li and Z. Q. Zhang, Anal. Chim. Acta, 2015, 853, 579–587.
18. K. Lanouette, Treatment of phenolic wastes, Chem. Eng., 1977, 84, 99–106.
19. A. L. Buikema, M. J. McGimres and J. Cairns, Mar. Environ. Res., 1979, 2, 87–181.
20. P. S. Wang, J. Y. Xiao, A. Liao, P. Li, M. M. Guo, Y. Xia, Z. L. Li, X. C. Jiang and W. Huang, Electrochim. Acta, 2015, 176, 448–455.
21. L. Yang, H. Zhao, Y. Li and C. P. Li, Sens. Actuators, B, 2015, 207, 1–8.
22. P. Mulchandani, C. M. Hangarter, Y. Lei and W. Chen, Biosens. Bioelectron., 2005, 21, 523–527.
23. Y. Wei, L. T. Kong, R. Yang, L. Wang, J. H. Liu and X. J. Huang, Langmuir, 2011, 27, 10295–10301.
24. Z. X. Zhang, J. Zhou, Y. Liu, J. Tang and W. H. Tang, Nanoscale, 2015, 7, 19540–19546.
25. H. B. Li and C. P. Han, Chem. Mater., 2008, 20, 6053–6059.
26. A. Niazi and A. Yazdanipour, J. Hazard. Mater., 2007, 146, 421–427.
27. G. Norwitz, N. Nataro and P. N. Keliher, Anal. Chem., 1986, 58, 639–641.
28. X. J. Huang, N. N. Qiu and D. X. Yuan, J. Chromatogr. A, 2008, 1194, 134–138.
29. B. Horstkotte, O. Elsholz and V. C. Martin, Talanta, 2008, 76, 72–79.
30. M. Mei, X. J. Huang, J. Yu and D. X. Yuan, Talanta, 2015, 134, 89–97.
31. S. Paliwal, M. Wales, T. Good, J. Grimsley, J. Wild and A. Simonian, Anal. Chim. Acta, 2007, 596, 9–15.
32. C. Nistor, A. Oubina, M. P. Marco, D. Barceló and J. Emnéus, Anal. Chim. Acta, 2001, 426, 185–195.
33. X. F. Guo, Z. H. Wang and S. P. Zhou, Talanta, 2004, 64, 135–139.
34. X. M. Guo, H. Zhou, T. X. Fan and D. Zhang, Sens. Actuators, B, 2015, 220, 33–39.
35. J. Fischer, J. Barek and J. Wang, Electroanalysis, 2006, 18, 195–199.
36. D. Peng, J. X. Zhang, D. G. Qin, J. Chen, D. L. Shan and X. Q. Lu, J. Electroanal. Chem., 2014, 734, 1–6.
37. A. D. Arulraj, M. Vijayan and V. S. Vasantha, Anal. Chim. Acta, 2015, 899, 66–74.
38. T. Wei, X. Huang, Q. Zeng and L. Wang, J. Electroanal. Chem., 2015, 743, 105–111.
39. A. Abdolmaleki, S. Mallakpour and S. Borandeh, Carbohydr. Polym., 2014, 103, 32–37.
40. B. Zhao and H. L. Chen, Mater. Lett., 2007, 61, 4890–4893.
41. H. Peng, B. Cui, G. Li, Y. Wang, N. Li, Z. Chang and Y. Wang, Mater. Sci. Eng., C, 2015, 46, 253–263.
42. H. Q. Shi, W. N. Li, L. W. Sun, Y. Liu, H. M. Xiao and S. Y. Fu, Chem. Commun., 2011, 47, 11921–11923.
43. A. Aboulaich, C. M. Tilmaciu, C. Merlin, C. Mercier, H. Guilloteau, G. Medjahdi and R. Schneider, Nanotechnology, 2012, 23, 335101.
44. X. L. Ren, Z. Z. Chen, X. Y. Chen, J. Liu and F. Q. Tang, J. Lumin., 2014, 145, 330–334.
45. L. Jia, J. P. Xu, D. Li, S. P. Pang, Y. Fang, Z. G. Song and J. Ji, Chem. Commun., 2010, 46, 7166–7168.
46. E. D. A. Yudiarto and T. Kokugan, Sep. Purif. Technol., 2000, 19, 103–112.
47. Y. Inoue, Y. Liu, L. H. Tong, B. J. Shen and D. S. Jin, J. Am. Chem. Soc., 1993, 115, 10637–10644.

---