Synthesis of 3,4-diaminobenzenethiol and its application in gold nanoparticle-based colorimetric determination of copper ions

Abstract

An improved synthetic method for the synthesis of 3,4-diaminobenzenethiol is presented. The practical application of this reagent as a new ligand for gold nanoparticles for the rapid detection of Cu²⁺ in water is demonstrated. The method is shown to have high sensitivity and selectivity. Well-defined peaks, proportional to the concentration of the corresponding Cu²⁺, were observed from 0.5 μM to 2 μM, and the recovery was in the range of 92–109%. This method provides a facile route for Cu²⁺ analysis.

---

Introduction

In recent years, the development of the economy has led to serious environmental pollution. In particular, a large quantity of waste water that contains a huge number of heavy metal ions has been poured into rivers, lakes and other water sources. Heavy metal contaminants are known to be very difficult to degrade and easily accumulated. Additionally, heavy metals can readily enter the food chain, thereby harming the health of animals, plants and humans via biological enrichment.

Copper is an essential nutrient for organisms, but a high Cu²⁺ concentration can cause metabolic disorders, developmental stagnation and even death.^1–3 It is therefore important to monitor the concentration of Cu²⁺ in the environment.

At present, the methods for the detection of Cu²⁺ mainly include atomic absorption spectrometry,^4,5 inductively coupled plasma emission spectrometry,^6,7 inductively coupled plasma mass spectrometry⁸ and spectrophotometry.⁹ However, some of these methods may not be highly sensitive, and some of them are too complex and expensive. Therefore, a simple, sensitive and rapid Cu²⁺ detection method would be of great benefit to the wastewater monitoring industry.

In recent years, gold nanoparticles (AuNPs) have been widely used in various research fields because of their high extinction coefficient and unique optical properties.^10–17 Gold nanoparticle-based colorimetric assay is one of these applications and has recently gained much interest because of its low cost, simple sample processing procedures, and sufficient sensitivity.^18–27 Recently, a colorimetric approach for sensing Cu²⁺ was reported using bare AuNPs.²⁸ This method can be used without complicated pre-treatment but it has relatively high limit of detection. Moreover, some rapid visual methods were reported for the detection of Cu²⁺ based on modified AuNPs^29,30 or gold nanorods (AuNRs).^31,32 These methods could be sensitive to Cu²⁺ . However, as the reagents required can be expensive or difficult to obtain, it is necessary to find ligands that are inexpensive and technically easy to synthesize.

3,4-Diaminobenzenethiol, which has two amino groups and a thiol group, was first reported by Schmidt et al.³³ Its mercapto functional group can react with AuNPs via a strong covalent Au–S bond. The exposed amino functional groups on the benzene ring from neighboring AuNPs can bind to copper ions, resulting in AuNP aggregation (Scheme 1). 3,4-Diaminobenzenethiol can therefore be used as a ligand to modify gold nanoparticles for the colorimetric detection of copper ions that can be quantitatively monitored by UV/Vis spectrophotometry.

Scheme 1 Schematic representation of the sensing mechanism of the AuNP-based colorimetric determination of Cu²⁺.

In this paper, a simple and optimized approach for the synthesis of 3,4-diaminobenzenethiol using a three-step synthetic process with o-nitroaniline as a starting material was developed. The intermediates and final products were characterized using FTIR, NMR and GC-MS. A gold nanoparticle-based colorimetric method was established to detect copper ions using 3,4-diaminobenzenethiol as the functioning ligand. The experimental conditions were optimized, the test results were compared with ICP-OES, and the feasibility of this method was verified.

Experimental section

Chemicals and materials

(1) Ligand. 3,4-Diaminobenzenethiol was synthesized in a three-step procedure, and was sealed in a brown reagent bottle at 5 °C.

(2) 10 mM Cu²⁺ solution. 0.4262 g of copper(II) chloride dihydrate (CuCl₂·2H₂O) was accurately weighed and dissolved with ddH₂O, and then stored at 5 °C. The stock solution was then diluted to the desired concentration with ddH₂O.

(3) Buffers. Phthalate buffer, mixed phosphate buffer and sodium tetraborate buffer were purchased from Shanghai Leici Instrument Co., Ltd. All of the buffers were stored at 5 °C. Prior to use, the pH of the buffers were adjusted again.

Sodium chloride, tri-sodium citrate and other chemical agents were of analytical reagent grade (Sinopharm Chemical Reagent Co., Ltd.). All of the solutions were prepared using ddH₂O (18 MOhm cm resistance), unless noted specially. All of the glassware was cleaned thoroughly with aqua regia (3 : 1 (v/v) HCl–HNO₃) and rinsed with ddH₂O prior to use.

Apparatus

The following apparatus was used: an ACY-600-U ultra-pure water system (Chongqing Yi Yang Enterprise Development Co., Ltd., China); a TU-1901 double beam UV-Vis spectrophotometer (Beijing Purkinje General Instrument Co., Ltd., China); a CM12 transmission electron microscope (Philips Ltd., Netherlands); a UB-7 digital pH meter (Denver Instrument, USA); a ME204E analytical balance (Mettler Toledo Ltd., Switzerland); an ICP-5000 inductively coupled plasma emission spectrometer (Hangzhou Juguang Technology Co., Ltd., China); an Agilent 7890-5975C gas chromatography-mass spectrometer (Agilent Co., Ltd., USA); a Nicolet 6700 Fourier transform infrared spectrometer (Thermo Fisher Co., Ltd., USA); and a VNMR 400 nuclear magnetic resonance spectrometer (Varian Co., Ltd., USA).

Synthesis of AuNPs

Citrate-capped AuNPs were prepared using a method of chemical reduction of HAuCl₄ in the liquid phase.³⁴ Briefly, 35 ml of deionized water and 0.6 ml of HAuCl₄ (25 mM) were mixed in a conical flask. After boiling for several minutes, 1 ml of tri-sodium citrate solution (38.8 mM) was added quickly with vigorous stirring. The color of the solution changed to red in a few seconds. After boiling for 5 min, the solution was cooled to room temperature and then stored at 5 °C.

Synthesis and purification of 3,4-diaminobenzenethiol^33,35–39

(1) Synthesis of 2-nitro-4-thiocyanoaniline. O-Nitroaniline (8.25 g, 60 mmol) and sodium thiocyanate (18 g, 0.22 mol) were dissolved in 100 ml of glacial acetic acid (98%) and then cooled to 5 °C. Bromine (9.6 g, 60 mmol) was dropped into 10 ml of glacial acetic acid (98%) and then cooled to 5 °C. The above two kinds of solution were mixed together and reacted for 72 h, then the mixture was warmed to 15 °C and poured into 500 ml ddH₂O, resulting in an orange turbid solution. The solution was filtered, and the solid material acquired was washed with ddH₂O, and then dissolved in 100 ml of acetone. The resulting solution was filtered and evaporated. A yellow solid product was gained.

(2) Synthesis of 4-amino-3-nitrobenzenethiol. The first step product was added multiple times to 100 ml of potassium hydroxide (6 g) ethanol solution, and stirred for 1 h at 5 °C. Sulfuric acid ethanol solution (5%) was added slowly until the color of the mixture changed from dark violet to orange. The orange mixture was then poured into 400 ml of ddH₂O and extracted three times using 100 ml of ethyl acetate. The combined organic extracts were washed with brine, dried over magnesium sulfate and evaporated. In the end, a red solid was obtained.

(3) Synthesis of 3,4-diaminobenzenethiol. The product of the second step was dissolved in an ethanol–water solution (300 ml, 1 : 1) and then sodium dithionite (13.4 g, 80 mmol) was added over a period of 20 min. After that, the pH was adjusted to 3.0 by adding sodium hydroxide. The stirred solution was refluxed for 1 h at 105 °C and then extracted using chloroform after being cooled to room temperature. A crude solid resulted from vacuum drying and was subjected to silica gel column chromatography using MeOH–EtOAc (1 : 5) to acquire a yellow solid product.

Colorimetric detection of Cu²⁺

1 ml of 3,4-diaminobenzenethiol solution (0.2 mg L⁻¹) and 0.1 ml of Cu²⁺ of different concentrations were added to 0.9 ml of phosphate buffer solution (pH 6.0). The reaction proceeded at room temperature for 5 min, then 1 ml of AuNP suspension was added and the mixture was incubated for 15 min. The UV measurements were carried out using a UV/Vis photometer (TU-1901) in the Faculty of Materials Science and Chemistry, China University of Geosciences.

Fourier transform infrared spectroscopy (FTIR)

After vacuum drying, the sample was ground with dry KBr. The FTIR measurements were carried out using a Nicolet 6700 FTIR spectrometer in the Faculty of Materials Science and Chemistry, China University of Geosciences.

Nuclear magnetic resonance spectroscopy (NMR)

These samples were dissolved with chloroform-d solution, and the nuclear magnetic resonance measurements were carried out using a VNMR 400 NMR spectrometer at Central China Normal University.

Gas chromatographic mass spectrometry (GC-MS)

The samples were dissolved with ethyl acetate, and the GC-MS measurements were carried out by full scanning with an Agilent 7890-5975C gas chromatography-mass spectrometer at the Hubei Environmental Monitoring Center.

Transmission electron microscope characterization (TEM)

The samples were prepared on copper grids. The TEM characterization was carried out in Faculty of Materials Science and Chemistry, China University of Geosciences.

Results and discussion

Chemical structure confirmation of the synthetic products

Fig. S1† shows the FTIR spectra of the synthetic products. In the spectrum of 2-nitro-4-thiocyanoaniline (Fig. S1A†), the characteristic bands of the primary amine (NH₂) are 3475 cm⁻¹ and 3357 cm⁻¹ for asymmetric and symmetric stretching, respectively, the thiocyano moiety’s symmetric stretching appears as an absorption band at 2150 cm⁻¹ (C≡N), and the nitro aromatic moiety’s symmetric stretching band appears at 1552 cm⁻¹ (N–O), confirming the presence of the expected functional groups in the synthesized 2-nitro-4-thiocyanoaniline. The ¹H NMR spectrum of 2-nitro-4-thiocyanoaniline (Fig. S2A†) also indicates the presence of –NH, where the signal at δ = 6.2 ppm is attributed to the protons of the –NH.

In the FTIR spectrum of 4-amino-3-nitrobenzenethiol (Fig. S1B†), while the NH₂ asymmetric and symmetric stretching bands (3463 cm⁻¹ and 3350 cm⁻¹, respectively) and the N–O symmetric stretching band (1554 cm⁻¹) appear, there is no absorption band at 2150 cm⁻¹, indicating that the thiocyano moiety has been hydrolyzed. The ¹H NMR spectrum of 4-amino-3-nitrobenzenethiol (Fig. S2B†) confirms the presence of –SH, where the signal at δ = 3.4 ppm is attributed to the protons of the –SH.

The FTIR spectrum of 3,4-diaminobenzenethiol presented in Fig. S1C† shows that the characteristic bands of NH₂ are present at 3476 cm⁻¹ and 3361 cm⁻¹ for asymmetric and symmetric stretching, respectively, and at 1619 cm⁻¹ for the N–H bond, and the primary aromatic amine symmetric stretching band appears at 1247 cm⁻¹ (C–N). These results show that a portion of the nitro groups in the sample was reduced to amino groups.

GC-MS measurements were further performed to confirm the chemical structure of the synthetic products. The total ion flow chromatogram of 2-nitro-4-thiocyanoaniline (Fig. S3A†) shows that the 2-nitro-4-thiocyanoaniline (molecular weight 195) is eluted at 39.75 min. The other peaks present were from fragment ions whose corresponding molecular weights are 149, 122, 105, 95, 63 and 52. These characteristic peaks were consistent with those of 2-nitro-4-thiocyanoaniline in the NIST library.

Fig. S3B† shows the total ion flow chromatogram of 4-amino-3-nitrobenzenethiol. The NIST library does not contain this material, and therefore there is no direct standard. Despite this, some thiol compound is detected at 34.61 min, and its m/z is close to the molecular weight of 4-amino-3-nitrobenzenethiol. Fragment ions with an m/z of 149, 122, 105, 95, 63 and 52 were also detected. The characteristic peaks indicate that the thiocyano group has been hydrolyzed to the thiol group, which is consistent with reported 4-amino-3-nitrobenzenethiol.³³

The total ion flow chromatogram of 3,4-diaminobenzenethiol is depicted in Fig. S3C.† Because the NIST library does not contain this material, it cannot be directly matched. However, some compound with the same relative molecular weight as the target product is detected at 38.46 min. Besides the fragment ions with an m/z of 95, 80 and 52, a fragment ion with an m/z of 107 that could be –C₆H₃NH₂NH₂ is detected, suggesting that the nitro group has been reduced to an amino group. This result is consistent with reported 3,4-diaminobenzenethiol.³³

Detection mechanism

The AuNPs could be capped by 3,4-diaminobenzenethiol via thiol groups due to their strong binding ability to sulphur. The Au–S bond is stable in high concentration salt solutions and can resist the attack of amino groups. The amino groups on the 3,4-diaminobenzenethiol have a strong binding ability to copper ions, and the stability constant of the amino–Cu complex is higher than most of the other interfering ions. Therefore, the 3,4-diaminobenzenethiol-capped AuNPs could bind to copper ions to form stable complexes, and subsequently aggregate, resulting in a color change. This mechanism is consistent with some previous reports.^40–42 This color change can be quantitatively analyzed using UV-Vis spectrophotometry.

As shown in Fig. 1, the AuNPs show a strong SPR peak at 520 nm (black line). When the ligand 3,4-diaminobenzenethiol was added, this characteristic peak only slightly decreased (red line) and the color of the solution remained red (left inset). However, upon the addition of Cu²⁺, the color of the solution changed from red to blue (right inset). While the original SPR peak at 520 nm significantly decreased, a new distinct SPR peak at 700 nm appeared (blue line), indicating the Cu²⁺-induced aggregation of 3,4-diaminobenzenethiol-capped AuNPs.

Fig. 1 UV-Vis spectra of AuNPs in the absence and presence of the ligand 3,4-diaminobenzenethiol and Cu²⁺. The insets show the optical photographs of the AuNP solutions in the absence (left) and presence (right) of Cu²⁺.

The dispersed and aggregated AuNPs were also confirmed by TEM images (Fig. 2). While Fig. 2A shows that the AuNPs have nearly spherical shapes and are uniformly dispersed, Fig. 2B displays significant aggregation of the AuNPs after the addition of the ligand and Cu²⁺.

Fig. 2 TEM images of AuNPs in the (A) absence and (B) presence of 3,4-diaminobenzenethiol and Cu²⁺.

Optimum concentration of the ligand for the aggregation of AuNPs

Ligand concentration has a high impact on the efficacy of this system. The citrate-stabilized surface of the AuNPs is negatively charged and a high concentration of the positively charged ligand can cause aggregation of the AuNPs. Alternatively, a low concentration of the ligand can lead to isolated AuNPs, but this latter condition may not be conducive to the highly sensitive detection of Cu²⁺. The ligand was therefore diluted to different concentrations and the absorption spectra were recorded.

As shown in Fig. 3, when the ligand concentration was 2 mg L⁻¹, the absorption peak of the AuNPs at 520 nm was significantly lower than the absorption peak with no ligand added (0 mg L⁻¹) and a new absorption peak at 670 nm appeared, demonstrating that the high concentration of the ligand causes AuNP aggregation. When the concentration of the ligands was 0.2 mg L⁻¹, the ligands did not induce significant aggregation of the AuNPs as evidenced by the absorption peak intensity being slightly decreased and the absence of a AuNP solution color change. Therefore, to gain the highest sensitivity, 0.2 mg L⁻¹ of 3,4-diaminobenzenethiol was chosen for subsequent experiments.

Fig. 3 Effect of ligand concentration on the aggregation of the AuNPs.

Optimum reaction time

A concentration of 10 μM of Cu²⁺ was reacted with ligand modified AuNPs at room temperature and the absorbance of the solution was determined at different times. The effect of the reaction time is shown in Fig. 4, where the ordinate denotes A₆₅₀/A₅₂₀, the ratio of the extinction coefficients at 650 nm and 520 nm respectively, which is a proxy for the molar ratio of the aggregated to the dispersed AuNPs. It is shown that A₆₅₀/A₅₂₀ was stable when reacting after 15 min, and therefore in order to ensure the ease of experimental operation, these experimental parameters were chosen as the optimum for analysis.

Fig. 4 Effect of reaction time on the determination.

Optimum pH

The pH value of the solution not only has an influence on the stability of the AuNPs probes, it also affects the stability of the amine–Cu complex. According to previous experimental methods, a range of pH values from 4 to 9 of buffer solution was tested without changing the other conditions, and the absorbance ratio (A₆₅₀/A₅₂₀) was determined. Fig. 5 depicts how the pH of solution influences the aggregation of the AuNPs. At pH values ranging from 4 to 8, A₆₅₀/A₅₂₀ remained stable, indicating that this pH region can be used for subsequent experiments. When the pH was greater than 8, A₆₅₀/A₅₂₀ decreased dramatically. This indicates that the pH value influences the stability of the complexes, likely through the formation of metal hydroxides at higher pH. The formation pH of copper hydroxide for the micromolarity of Cu²⁺ estimated from the corresponding solubility product (K_sp = 2.2 × 10⁻²⁰) is about 6.8. Considering the coordination of the ligand, the actual formation pH would be higher than 6.8. Based on these results, a pH of 6 was chosen as the optimum pH condition in our experiments.

Fig. 5 Effect of pH on the determination.

Detection of Cu²⁺

Under the optimized conditions, different concentrations of Cu²⁺ were mixed with 0.2 mg L⁻¹ of the 3,4-diaminobenzenethiol ligand and the AuNPs for 15 min, and then UV-Vis spectroscopy was used to quantitatively determine the content of Cu²⁺ in aqueous solution. As shown in Fig. 6A, when the Cu²⁺ concentration increased, more ligands bonded to Cu²⁺, and the AuNPs were aggregated. The concentration of the Cu²⁺ was determined by measuring A₆₅₀/A₅₂₀. A linear equation, A = 0.296C − 0.127 (R² = 0.998), was generated over the range of 0.5–2 μM of Cu²⁺ (Fig. 6B).

Fig. 6 (A) UV-Vis spectra of the different concentration gradients of Cu²⁺ and (B) the working curve.

Selectivity of the method

To evaluate the selectivity of the colorimetric detection of Cu²⁺, some other environmentally related metal ions were chosen for investigation, including Na⁺, K⁺, Mg²⁺, Al³⁺, Ca²⁺, Zn²⁺, Fe³⁺, Cr⁶⁺, Mn²⁺, Pb²⁺ and Cd²⁺. Fig. 7 shows that only Cu²⁺ led to a significant increase in the value of A₆₅₀/A₅₂₀ compared to the blank, while that of the other metal ions exhibited no significant change. Additionally, the color of the solution in the presence of Cu²⁺ changed to blue (inset), while that of the solutions containing the other metal ions remained red, also indicating that this colorimetric assay method exhibits high selectivity for Cu²⁺ detection.

Fig. 7 Influence of various cations on the determination of Cu²⁺. Cu²⁺ (2 μM); K⁺, Ca²⁺, Na⁺, Mg²⁺, Al³⁺, and Fe³⁺ (100 μM); Mn²⁺ and Cr⁶⁺ (40 μM); Pb²⁺ and Cd²⁺ (20 μM); Zn²⁺(10 μM). The insets are the corresponding optical photographs.

Real water samples analysis

To demonstrate the viability of the assay in measuring Cu²⁺ in real water samples, we measured the amount of Cu²⁺ in electroplating wastewater. As shown in Table 1, the method provided recoveries of 92 to 109% of Cu²⁺ in electroplating wastewater samples. The concentration of Cu²⁺ in electroplating wastewater was found to be 49 μM. To compare the data with well-established techniques, we also measured the Cu²⁺ content using ICP-OES. The amount of Cu²⁺ in the electroplating wastewater was found to be 52 μM.

Table 1 Results of the detection and recovery of Cu²⁺ in the electroplating wastewater (n = 3)

Sample	Added (μg L^−1)	Detected (μg L^−1)	Recovery (%)	RSD (%, n = 3)
1	3.2	13.18	92	3.2
2	6.4	16.38	95	2.8
3	12.8	22.78	109	2.3

---

Conclusions

In summary, we adopted an improved three-step synthesis method for the synthesis of 3,4-diaminobenzenethiol, a novel ligand for modifying AuNPs. Furthermore, we established a sensitive and selective colorimetric assay for the detection of Cu²⁺ in water, and satisfactory results were obtained from the detection in the actual samples. The operational simplicity of the method may facilitate the analysis of Cu²⁺ in industrial waste water. This method would be valuable for colorimetric detection in environmental monitoring.

Acknowledgements

This work was supported by geological survey project of China Geological Survey (12120113015300), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (NSFC, No. 41521001) and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. 2014029).

Notes and references

1. C. Yin, M. Zhao, W. Jin and Z. Lan, Hydrobiologia, 1993, 251, 321–329.
2. H. Xiang and X. Y. Yu, Toxic Effect of Copper Pollution on Water and Hydrophyte, Hunan Agricultural Sciences, 2009, vol. 11, pp. 54–56.
3. K. Yin, B. Li, X. Wang and W. Zhang, Biosens. Bioelectron., 2015, 64, 81–87.
4. GB/T 15337-2008, General rules for atomic absorption spectrometric analysis, Standard Press, Beijing, China, 2008.
5. R. Kunkel and S. E. Manahan, Anal. Chem., 1973, 45, 1465–1468.
6. H. Wang, Chin. J. Spectrosc. Lab., 2002, 19, 219–223.
7. G. J. Zheng, Metall. Anal., 2001, 21, 36–43.
8. A. T. Townsend, K. A. Miller, S. Mclean and S. Aldous, J. Anal. At. Spectrom., 1998, 13, 1213–1219.
9. X. L. Mo, Soil and Fertilizer Sciences in China, 2011, 88–91.
10. D. C. Marie and A. Didier, Chem. Rev., 2004, 35, 293–346.
11. E. Katz and I. Willner, Angew. Chem., Int. Ed., 2004, 43, 6042–6108.
12. N. Xia, A. Y. Shi, B. R. Zhang, B. F. Zhao, B. F. Liu and L. Liu, Anal. Methods, 2012, 4, 3937–3941.
13. H. Chi, B. Liu, G. Guan, Z. Zhang and M. Y. Han, Analyst, 2010, 135, 1070–1075.
14. J. H. Lee, Z. Wang, J. Liu and Y. Lu, J. Am. Chem. Soc., 2008, 130, 14217–14226.
15. H. Li and L. Rothberg, Proc. Natl. Acad. Sci. U. S. A., 2004, 101, 14036–14039.
16. W. Li, Z. Nie and K. He, et al., Chem. Commun., 2011, 47, 4412–4414.
17. X. Bai, C. Shao, X. Han, Y. Li, Y. Guan and Z. Deng, Biosens. Bioelectron., 2010, 25, 1984–1988.
18. M. R. Hormozi-Nezhad and S. Abbasi-Moayed, Talanta, 2014, 129, 227–232.
19. R. Liu, Z. Chen, S. Wang, C. Qu, L. Chen and W. Zhuo, Talanta, 2013, 112, 37–42.
20. D. Maity, A. Kumar, R. Gunupuru and P. Paul, Colloids Surf., A, 2014, 455, 122–128.
21. A. Safavi and E. Farjami, Anal. Chim. Acta, 2011, 688, 43–48.
22. H. C. Chan, J. H. Kim, J. Jung and B. H. Chung, Biosens. Bioelectron., 2012, 41, 827–832.
23. K. W. Huang, C. J. Yu and W. L. Tseng, Biosens. Bioelectron., 2009, 25, 984–989.
24. Y. M. Sung and S. P. Wu, Sens. Actuators, B, 2014, 201, 86–91.
25. B. Leng, L. Zou, J. Jiang and H. Tian, Sens. Actuators, B, 2009, 140, 162–169.
26. R. Domínguez-González, L. G. Varela and P. Bermejo-Barrera, Talanta, 2014, 118, 262–269.
27. K. D. Bhatt, D. J. Vyas, B. A. Makwana, S. M. Darjee and V. K. Jain, Spectrochim. Acta, Part A, 2013, 121, 94–100.
28. H. H. Deng, G. W. Li, A. L. Liu, W. Chen, X. H. Lin and X. H. Xia, Microchim. Acta, 2014, 181, 911–916.
29. T. Lou, L. Chen, Z. Chen, Y. Wang, L. Chen and J. Li, ACS Appl. Mater. Interfaces, 2011, 3, 4215–4220.
30. Q. Gao, Y. Zheng, C. Song, L. Q. Lu, X. K. Tian and A. W. Xu, RSC Adv., 2013, 3, 21424–21430.
31. Z. Zhang, Z. Chen, C. Qu and L. Chen, Langmuir, 2014, 30, 3625–3630.
32. X. Wang, L. Chen and L. Chen, Microchim. Acta, 2014, 181, 105–110.
33. A. Schmidt, A. G. Shilabin and M. Nieger, Org. Biomol. Chem., 2004, 1, 4342–4350.
34. M. Faraday, Philos. Trans. R. Soc., 1857, 147, 145–181.
35. S. E. Dalton, W. G. Gingell and D. C. Jenkins, et al., US Pat., 4071528, 1978.
36. B. L. Yin, J. Y. Shi and H. Q. Wang, J. Wuhan Inst. Chem. Technol., 1987, 2, 68–69.
37. G. W. A. Wheland, J. Am. Chem. Soc., 1942, 64, 900–909.
38. J. K. Kochi, J. Am. Chem. Soc., 1957, 79, 2942–2948.
39. W. Zhang, K. F. Koehler, B. Harris, P. Skolnick and J. M. Cook, J. Med. Chem., 1994, 37, 745–757.
40. A. Alizadeh, M. M. Khodaei, Z. Hamidi and M. Shamsuddin, Sens. Actuators, B, 2014, 190, 782–791.
41. L. Li, Z. Yuan, X. Peng, L. Li, J. He and Y. Zhang, J. Chin. Chem. Soc., 2014, 61, 1371–1376.
42. Y. Guo, Z. Wang, W. Qu, H. Shao and X. Jiang, Biosens. Bioelectron., 2011, 26, 4064–4069.

---

Footnote

† Electronic supplementary information (ESI) available: FTIR, ¹H NMR and GC-MS spectra. See DOI: 10.1039/c6ra13681h

---