Multichannel detection of Cu²⁺ based on a rhodamine–ethynylferrocene conjugate

Abstract

A novel multichannel chemosensor DR3 juxtaposed with a rhodamine chromophore and the electrochemical characterization of an ethynylferrocene group was developed. This chemosensor could selectively recognized Cu²⁺ in the presence of other competing ions in a wide pH range, which exhibits multiple responses for UV/vis absorption, fluorescence emission, and electrochemical parameters.

---

Introduction

Recently, due to its innate advantages such as high sensitivity, selectivity and real-time monitoring, luminescent chemosensors for the detection of transition metals have attracted increasing attention. These usually contain reaction sites and display an obvious change in optical characteristics upon host–guest interactions.¹ Among the transition metals, copper is the third most abundant essential heavy metal in the human body, after zinc and iron, and plays a crucial role in biological processes. It is an important catalytic cofactor in redox chemistry for proteins.² However, an abnormal level of Cu²⁺ in living systems may lead to various neurodegenerative diseases including Menkes, Wilson’s, and Alzheimer’s diseases.^3–5 Thus, it is of significant importance to develop a luminescent chemosensor for monitoring Cu²⁺ in biological processes. As far as we know, a large number of Cu²⁺-chemosensors based on fluorescence enhancement have been reported.^6–19 Most of them are based on the changes of UV/vis absorption and fluorescence emission. However, few multichannel Cu²⁺-selective chemosensors have been developed so far through multiple responses such as chromaticity, fluorescence, electrochemistry etc. Compared with single signal detection, multichannel detection has a higher sensitivity, more excellent selectivity and anti-interference ability. More importantly, a multichannel detection system can make self-calibration measurements come true through different analytic methods.

Zade et al. reported a thiophene-based salphen-type chemosensor for the detection of Cu²⁺ and Zn²⁺ with electrochemical properties.²⁰ It is expected to introduce an excellent group to multichannel chemosensors, which have electrochemical characteristics, to establish multichannel analytic systems. Ferrocene has a brilliant ability to store electrons, strengthening the coordination between the chemosensor and the metal ion.²¹ It is well known that ferrocene-containing chemosensors exhibit an electrochemical response upon complexation of a suitable guest ion. Zhang and Zapata et al. also observed a significant potential shift of Fe^III/Fe^II upon coordination of an analyte, which exhibits multi-responsive signaling.²² A further study relying on rhodamine-based multichannel chemosensors for Cr³⁺, Hg²⁺ etc. linked with ferrocene has been reported.²³ However, multichannel Cu²⁺-chemosensors have barely been mentioned to the best of our knowledge.²⁴

Herein, we synthesized a novel multichannel chemosensor DR3 for the detection of Cu²⁺ (Scheme 1). It is expected to achieve a three-channel Cu²⁺-selective chemosensor through introducing a ferrocene group to the rhodamine chromophore. We designed an acetylene group linked with the electrochemical properties of a ferrocene group and chromatic, fluorescent rhodamine with a ring-opening process, increasing the multichannel output signaling upon interaction with Cu²⁺. A hydrazide functional group was introduced into the chemosensor to act as a potential reaction site for Cu²⁺. Upon reaction with the copper ion, the fluorescence and absorption intensity evidently increases due to a process of spirolactam ring-opening and hydrolysis. This multichannel chemosensor DR3 exhibits suitable variations of the absorption spectrum, fluorescence emission and electrochemical parameters.

Scheme 1 Design of Cu²⁺ multichannel chemosensor DR3.

Experimental section

Materials and measurements

All reagents were purchased from commercial suppliers and used without further purification. The ¹H and ¹³C NMR spectra were measured on a Bruker Avance 500 or Brucker Avance 400 spectrometer in CDCl₃. Electrospray ionization mass spectra (ESI-MS) were measured on a Micromass LCTTM system. UV-visible spectra were recorded on a Perkin-Elmer 35 spectrometer and fluorescence measurements were performed on a Perkin-Elmer LS 50B fluorescence spectrophotometer. Electrochemical measurements were performed with an Eco Chemie Autolab. The pH measurements were made with a PHS-3C Precision Ph/mV meter. TLC analysis was conducted on silica gel plates and chromatography was performed over silica gel (mesh 300–400).

UV/vis and fluorescence experiments

A stock solution of 1.0 mM DR3 was prepared in acetonitrile. Cu(NO₃)₂·3H₂O was dissolved in doubly distilled water to form a 5.0 mM stock solution. For competing metal ions, various metal ion solutions of NaNO₃, Co(NO₃)₂, KNO₃, Zn(NO₃)₂·6H₂O, Ni(NO₃)₂·6H₂O, Mg(NO₃)₂·6H₂O, Ca(NO₃)₂·4H₂O, MnCl₂·4H₂O, Pb(NO₃)₂, HgCl₂, AgNO₃, Ba(NO₃)₂ were used. Before fluorescence and UV/vis titration investigations were conducted, the stock solution of DR3 was mixed with the stock solutions of metal salts in a 10 mL volumetric flask and diluted with H₂O and CH₃CN to volume. Spectral data were recorded at 2 min after the addition. For fluorescence measurements, excitation was provided at 530 nm, and emission was collected from 546 to 700 nm. The wide pH was adjusted by HCl or NaOH solutions.

Electrochemical test

Electrochemical characteristics were tested as previously reported in the literature.²³ All measurements for DPV were carried out in a one-compartment cell under N₂ gas, equipped with a glassy-carbon working electrode, a platinum wire counter electrode, and a silver reference electrode. All measurements for CV were carried out in a one-compartment cell under N₂ gas, equipped with a glassy-carbon working electrode, a platinum wire counter electrode, and a saturated calomel electrode reference electrode. The supported electrolyte was a 0.10 mol L⁻¹ CH₃CN solution of tetrabutylammonium hexafluorophosphate (Bu₄NPF₆). The scan rate was 100 mV s⁻¹.

Synthesis of compound DR1

Compound DR1 was synthesized as previously reported in the literature.⁹ JR1: ¹H NMR (400 MHz, CDCl₃) δ: 7.85 (d, J = 8.1 Hz, 1H, Ar–H), 7.70 (dd, J = 8.1, 1.6 Hz, 1H, Ar–H), 7.32 (d, J = 1.5 Hz, H, Ar–H), 6.66–6.57 (m, 2H, Ar–H), 6.48 (d, J = 2.5 Hz, 2H, Ar–H), 6.41 (dd, J = 8.8, 2.6 Hz, 2H, Ar–H), 2.99 (s, 12H, –CH₃); DR1: ¹H NMR (400 MHz, CDCl₃) δ: 8.12 (d, J = 1.7 Hz, 1H, Ar–H), 7.83–7.66 (m, 1H, Ar–H), 7.05 (t, J = 8.8 Hz, 1H, Ar–H), 6.67–6.55 (m, 2H, Ar–H), 6.48 (d, J = 2.5 Hz, 2H, Ar–H), 6.41 (dd, J = 8.9, 2.6 Hz, 2H, Ar–H), 2.99 (s, 12H, –CH₃).

Synthesis of compound DR2

Compound DR2 was synthesized according to the literature with some modifications.⁹ A mixture of DR1 (0.5 mmol, 233 mg), ethynylferrocene (0.5 mmol, 157 mg), 35 mg (0.05 mmol) of PdCl₂(PPh₃)₂, and PPh₃ (26 mg, 0.1 mmol), 4.8 mg (0.025 mmol) of CuI, THF (20 mL), NEt₃ (5 mL) under nitrogen, was heated to 95 °C and refluxed for 12 h after completion of the reaction by TLC. The solvent was evaporated and the crude product was purified by column chromatography with CH₂Cl₂/NEt₃ (v/v = 200/4) to afford a purplish red target product DR2 (247 mg, 83%). ¹H NMR (400 MHz, CDCl₃) δ: 8.08 (d, J = 0.7 Hz, 1H, Ar–H), 7.75–7.66 (m, 1H, Ar–H), 7.13 (d, J = 7.9 Hz, 1H, Ar–H), 6.63 (d, J = 8.8 Hz, 2H, Ar–H), 6.48 (d, J = 2.5 Hz, 2H), 6.40 (dd, J = 8.9, 2.6 Hz, 2H, Ar–H), 4.55 (t, J = 1.8 Hz, 2H, ferrocene-H), 4.30–4.28 (m, 2H, ferrocene-H), 4.27 (d, J = 4.5 Hz, 5H, ferrocene-H), 2.99 (s, 12H, –CH₃); ¹³C NMR (101 MHz, CDCl₃): 169.19, 152.91, 152.09, 151.91, 137.38, 128.75, 127.93, 127.34, 125.55, 124.08, 108.66, 106.40, 98.53, 90.75, 84.31, 71.62, 70.08, 69.19, 64.30, 46.15, 40.26. ESI-MS m/z 595.2 [M]⁺.

Synthesis of compound DR3

A solution of DR2 (0.25 mmol, 160 mg), excess 98% N₂H₄·H₂O (1 mL) was resolved in 10 mL of ethanol and refluxed for 6 h. The solvent was evaporated and the crude product was purified by column chromatography with EtOAc/PE (v/v = 3/2) to get the desired product DR3 (110 mg, 73%). ¹H NMR (400 MHz, CDCl₃) δ: 8.06 (s, 1H, Ar–H), 7.55 (dd, J = 7.9, 1.4 Hz, 1H, Ar–H), 7.03 (d, J = 7.9 Hz, 1H, Ar–H), 6.52 (d, J = 8.8 Hz, 2H, Ar–H), 6.48 (d, J = 2.3 Hz, 2H, Ar–H), 6.39–6.36 (m, 2H, Ar–H), 4.52 (t, J = 1.7 Hz, 2H, ferrocene-H), 4.26 (s, 2H, ferrocene-H), 4.25 (s, 5H, ferrocene-H), 3.63 (s, 2H, –NH₂), 2.97 (s, 12H, –CH₃); ¹³C NMR (101 MHz, CDCl₃): 165.56, 153.48, 151.55, 150.33, 135.50, 130.21, 127.94, 125.94, 124.30, 123.79, 108.81, 105.22, 99.01, 89.69, 84.90, 71.55, 70.05, 69.35, 69.03, 65.89, 40.31. ESI-MS m/z 609.2 [M + H]⁺.

Results and discussion

DR3 was prepared based on a two-step route, shown in Scheme 2. Firstly, compound DR2 was achieved in 83% yield by a Sonogashira reaction catalysed by PdCl₂(PPh₃)₂, PPh₃, and CuI. Then compound DR2 was reacted with hydrazine hydrate to obtain DR3 in 73% yield by refluxing for 6 h. These compounds were characterized and confirmed by ¹H NMR, ¹³C NMR and ESI-MS (ESI, Fig. S1–S6†).

Scheme 2 Synthesis of DR3.

The influence of pH on the fluorescence of DR3 was investigated first, as shown in Fig. 1a. Under acidic conditions (pH < 5), ring opening of rhodamine occurred due to the protonation of the chemosensor.²⁵ When the pH ranged from 5.0 to 9.0, no obvious characteristic fluorescence emission of rhodamine was observed. However, the addition of Cu²⁺ led to a fluorescence enhancement over a comparative range (5.0–9.0), which is attributed to the ring-opening process of rhodamine and hydrolysis. The multichannel chemosensors DR3 and DR3–Cu²⁺ remained unaffected in terms of fluorescence intensity in pH 6.0–8.0, suggesting that it was insensitive to pH around 7.0 and could be suitable for physiological conditions. These results were also the same for the UV/vis spectra in the range of pH 5–9, as illustrated in Fig. 1b.

Fig. 1 (a) The fluorescence response of DR3 (10 μm) at 585 nm before and after the addition of 10 equiv. Cu²⁺ at different pH conditions. (b) The absorbance response of DR3 (10 μm) at 555 nm before and after the addition of 10 equiv. Cu²⁺ at different pH conditions.

The UV/vis spectrum of DR3 in a solvent system of CH₃CN/HEPES 80 : 20 with titration of Cu²⁺ was shown in Fig. 2a. It exhibits almost no absorption in the visible wavelength range, indicating that chemosensor DR3 is predominantly in the form of spirolactam. Upon addition of Cu²⁺, a new absorption peak at 555 nm was observed, and the intensity gradually increased with increasing Cu²⁺ ion concentration. This could be attributed to the formation of a ring-opened amide form of DR3 upon interaction with Cu²⁺. The inset in Fig. 2a shows the absorbance at 555 nm as a function of Cu²⁺ concentrations. The ring-opening mechanism was also confirmed by the color change of 10 μM DR3 upon adding different cations (Fig. 2b). Among the metal ions, only Cu²⁺ can induce an obvious color change from colorless to pink in a solution of DR3, allowing colorimetric detection of Cu²⁺ by the naked eye. Fig. 2c shows the absorption response of DR3 towards various metal ions in HEPES/CH₃CN (2 : 8, v/v, pH 7.0) solutions. DR3 exhibits almost no absorbance enhancement at around 555 nm upon addition of Hg²⁺, Pb²⁺, Ba²⁺, Mn²⁺, Ca²⁺, Mg²⁺, Ni²⁺, Ag⁺, Zn²⁺, K⁺, Cr³⁺, Co²⁺, Na⁺, DR3. These results indicate that chemosensor DR3 has a high selectivity towards Cu²⁺.

Fig. 2 (a) Absorption spectra of DR3 (10 μM) in HEPES/CH₃CN (2 : 8, v/v, pH 7.0) solutions upon addition of increasing concentrations of 0–8 equiv. Cu(NO₃)₂·3H₂O. Inset: the absorbance at 555 nm as a function of Cu²⁺ concentrations. (b) Color changes upon addition of different cations in HEPES/CH₃CN (2 : 8, v/v, pH 7.0) solutions. From left to right: DR3, Cu²⁺, Hg²⁺, Pb²⁺, Ba²⁺, Mn²⁺, Ca²⁺, Mg²⁺, Ni²⁺, Ag⁺, Zn²⁺, K⁺, Cr³⁺, Co²⁺ and Na⁺. (c) Absorption spectra of DR3 in HEPES/CH₃CN (2 : 8, v/v, pH 7.0) solutions with 8 equiv. of metal ions: DR3, Cu²⁺, Hg²⁺, Pb²⁺, Ba²⁺, Mn²⁺, Ca²⁺, Mg²⁺, Ni²⁺, Ag⁺, Zn²⁺, K⁺, Cr³⁺, Co²⁺ and Na⁺.

Fig. 3a shows the fluorescence spectra of DR3 in HEPES/CH₃CN (2 : 8, v/v, pH 7.0) solutions with the addition of Cu²⁺. Chemosensor DR3 shows a very weak emission at around 585 nm upon excitation at 530 nm. When Cu²⁺ was added to the DR3 buffer solution, a fluorescence intensity at 585 nm was observed, attributed to the ring-opening process of rhodamine derivatives. The solution showed an approximately 274-fold enhancement in the fluorescence intensity. This fact means that DR3 could act as an off-on luminescence chemosensor for Cu²⁺. The fluorescence intensity finally levelled off until the amount of added Cu²⁺ was 2.2 × 10⁻⁴ M (Fig. 3a inset). As an excellent luminescence chemosensor, it is important to have high selectivity. As illustrated in Fig. 3c, additions of other metal ions including Hg²⁺, Pb²⁺, Ba²⁺, Mn²⁺, Ca²⁺, Mg²⁺, Ni²⁺, Ag⁺, Zn²⁺, K⁺, Cr³⁺, Co²⁺, Na⁺ and DR3 induced no obvious fluorescence enhancement under the same conditions. These observations indicated that chemosensor DR3 could selectively recognize Cu²⁺ in a HEPES/CH₃CN (2 : 8, v/v, pH 7.0) solution. For practical applications, the detection limit was also calculated as 6.85 × 10⁻⁶ M for Cu²⁺ (3σ/slope), which is sufficiently low for the detection of many chemical systems for Cu²⁺ (Fig. 3b).²⁶

Fig. 3 (a) Fluorescence spectra of DR3 in HEPES/CH₃CN (2 : 8, v/v, pH 7.0) solutions upon addition of increasing concentrations of 0–24 equiv. Cu²⁺. Inset: fluorescence intensity at 585 nm as a function of Cu²⁺ concentrations. (b) Fluorescence intensity at 585 nm of DR3 (10 μM) in HEPES/CH₃CN (2 : 8, v/v, pH 7.0) solutions as a function of Cu²⁺ concentration (0–3.5 μM). (c) Fluorescence intensity of DR3 (10 μm) in HEPES/CH₃CN (2 : 8, v/v, pH 7.0) solutions upon addition of 24 equiv. metal ions (blank, Cu²⁺, Hg²⁺, Pb²⁺, Ba²⁺, Mn²⁺, Ca²⁺, Mg²⁺, Ni²⁺, Ag⁺, Zn²⁺, K⁺, Cr³⁺, Co²⁺, Na⁺).

As designed, DR3 shows an evident change in its reversible ferrocene/ferricinium redox cycles upon complexation. Differential pulse voltammetry (DPV) curves of DR3 were recorded in a CH₃CN solution containing 0.1 M n-tetrabutylammonium hexafluorophosphate (n-Bu₄NPF₆) as a supporting electrolyte in the absence and presence of Cu²⁺.^23a As shown in Fig. 4a, a significant displacement was observed upon addition of Cu²⁺. The oxidation peak was shifted in CH₃CN from 0.728 to 0.54 V (ΔE_1/2 = 188 mV). For cyclic voltammetry (CV), a significant shift of the redox potential of the ferrocenyl group was also observed (Fig. 4b). The CV behavior of chemosensor DR3 was measured in CH₃CN, suggesting a reversible one-electron redox process. The addition of Cu²⁺ induces a positive shift of the ferrocene/ferricinium couple, which is attributed to the redox process of Cu²⁺. This fact indicated that DR3 could be a multi-signal chemosensor for Cu²⁺ for electrochemical measurements.

Fig. 4 (a) DPV of DR3 (100 μM) in a CH₃CN solution in the absence and presence of 1.6 equiv. of Cu²⁺, with n-Bu₄NPF₆ as a supporting electrolyte. (b) CV of DR3 (100 μM) in a CH₃CN solution in the absence and presence of Cu²⁺, with n-Bu₄NPF₆ as a supporting electrolyte.

In order to explore the mechanism, the reaction product of DR3 and Cu²⁺ was detected by ESI spectra analyses (ESI, Fig. S7†). The ion peak was detected at m/z 609.2, which corresponded to [DR3 + H]⁺. In addition, the main ion peak at m/z 595.2, corresponding to intermediate DR2, was detected after the addition of Cu²⁺ to a DR3 aqueous solution. This suggested that Cu²⁺ induces the hydrolysis and spirolactam ring-opening of rhodamine. To further confirm Cu²⁺ mediated hydrolysis, we carried out a chemical reversibility experiment in a CH₃CN–water solution. Upon addition of 500 μM chelating agent, EDTA, to the DR3–Cu²⁺ complex in HEPES/CH₃CN (2 : 8, v/v, pH 7.0) solutions, the color and fluorescence intensity showed no obvious changes (Fig. 5). These findings indicated that DR3 is an irreversible luminescence chemosensor for Cu²⁺. According to the obtained results, we proposed that the reaction process may proceed as per the route depicted in Scheme 3.

Fig. 5 Fluorescence spectra in HEPES/CH₃CN (2 : 8, v/v, pH 7.0) solutions: (a) DR3 (10 μM); (b) DR3 (10 μM) with Cu²⁺ (240 μM); (c) DR3 (10 μM) with Cu²⁺ (240 μM) and EDTA (500 μM).

Scheme 3 The proposed reaction mechanism for DR3 with Cu²⁺.

Conclusions

In summary, we have designed a multichannel chemosensor DR3 for Cu²⁺ with the chromatic, fluorescent rhodamine derivatives and the electrochemical characterization of a ferrocenyl group. The novel chemosensor exhibits a multi-responsive colorimetric, fluorescent and electrochemical detection of Cu²⁺. Multichannel chemosensor DR3 could detect micromole levels of Cu²⁺ in a wide pH range (5.0–9.0). From the results above, we believe that DR3 could be further used for monitoring intracellular Cu²⁺ ions in biological systems.

Acknowledgements

The authors are grateful for the financial support from the open fund of the State Key Laboratory of Materials-Oriented Chemical Engineering (KL13-07), the China Postdoctoral Science Foundation (2014M550287), Specialized Research Fund for the Doctoral Program of Higher Education (20123221110012), Jiangsu Planned Projects for Postdoctoral Research Funds, and the State Key Laboratory of Coordination Chemistry of Nanjing University.

References

1. Y. Yang, Q. Zhao, W. Feng and F. Y. Li, Chem. Rev., 2013, 113, 192.
2. H. Tapiero, D. M. Townsend and K. D. Tew, Biomed. Pharmacother., 2003, 57, 386.
3. D. J. Waggoner, T. B. Bartnikas and J. D. Gitlin, Neurobiol. Dis., 1999, 6, 221.
4. C. Vulpe, B. Levinson, S. Whitney, S. Packman and J. Gitschier, Nat. Genet., 1993, 3, 7.
5. K. J. Barnham, C. L. Masters and A. I. Bush, Nat. Rev. Drug Discovery, 2004, 3, 205.
6. C. W. Yu, W. Ting, K. Xu, J. Zhao, M. H. Li, S. X. Weng and J. Zhang, Dyes Pigm., 2013, 96, 38.
7. A. F. Liu, L. Yang, Z. Y. Zhang, Z. L. Zhang and D. M. Xu, Dyes Pigm., 2013, 99, 472.
8. S. L. Hu, S. S. Zhang, Y. Hu, Q. Tao and A. X. Wu, Dyes Pigm., 2013, 96, 509.
9. J. L. Fan, P. Zhan, M. M. Hu, W. Sun, J. Z. Tang, J. Y. Wang, S. G. Sun, F. L. Song and X. J. Peng, Org. Lett., 2013, 15, 492.
10. M. Kumar, N. Kumar, V. Bhalla, P. R. Sharma and T. Kaur, Org. Lett., 2012, 14, 406.
11. Z. Q. Hua, X. M. Wang, Y. C. Feng, L. Ding and H. Y. Lu, Dyes Pigm., 2011, 88, 257.
12. J. L. Liu, C. Y. Li and F. Y. Li, J. Mater. Chem., 2011, 21, 7175.
13. P. H. Xie, F. Q. Guo, D. Li, X. Y. Liu and L. Liu, J. Lumin., 2011, 131, 104.
14. D. P. Wang, Y. Shiraishi and T. Hirai, Chem. Commun., 2011, 47, 2673.
15. W. Y. Liu, H. Y. Li, B. X. Zhao and J. Y. Miao, Org. Biomol. Chem., 2011, 9, 4802.
16. C. W. Yu, J. Zhang, R. Wang and L. G. Chen, Org. Biomol. Chem., 2010, 8, 5277.
17. V. Dujols, F. Ford and A. W. Czarnik, J. Am. Chem. Soc., 1997, 119, 7386.
18. J. T. Yeh, W. C. Chen, S. R. Liu and S. P. Wu, New J. Chem., 2014, 38, 4434.
19. X. M. Wu, Z. Q. Guo, Y. Z. Wu, S. Q. Zhu, T. D. James and W. H. Zhu, ACS Appl. Mater. Interfaces, 2013, 5, 12215.
20. A. K. Asatkar, S. P. Senanayak, A. Bedi, S. Panda, K. S. Narayan and S. S. Zade, Chem. Commun., 2014, 50, 7036.
21. (a) A. P. D. Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, T. J. M. Huxley, C. P. Mccoy, J. T. Rademacher and T. E. Rice, Chem. Rev., 1997, 97, 1515; (b) W. H. Zhu, L. W. Song, Y. H. Yang and H. Tian, Chem. - Eur. J., 2012, 18, 13388; (c) M. Li, Z. Q. Guo, W. H. Zhu, F. Marken and T. D. James, Chem. Commun., 2015, 51, 1293.
22. (a) B. C. Zhu, C. C. Gao, Y. Z. Zhao, C. Y. Liu, Y. M. Li, Q. Wei, Z. M. Ma, B. Du and X. L. Zhang, Chem. Commun., 2011, 47, 8656; (b) F. Zapata, A. Caballero, A. Espinosa, A. Tarraga and P. Molina, Org. Lett., 2007, 9, 2385.
23. (a) K. W. Huang, H. Yang, Z. G. Zhou, M. X. Yu, F. Y. Li, X. Gao, T. Yi and C. H. Huang, Org. Lett., 2008, 10, 2557; (b) X. Q. Chen, S. W. Nam, M. J. Jou, Y. M. Kim, S. J. Kim, S. S. Park and J. Y. Yoon, Org. Lett., 2008, 10, 5235; (c) H. Yang, Z. G. Zhou, K. W. Huang, M. X. Yu, F. Y. Li, T. Yi and C. H. Huang, Org. Lett., 2007, 9, 4729.
24. Y. S. Mi, Z. Cao, Y. T. Chen, Q. F. Xie, Y. Y. Xu, Y. F. Luo, J. J. Shi and J. N. Xiang, Analyst, 2013, 138, 5274.
25. Y. Xiang and A. J. Tong, Org. Lett., 2006, 8, 1549.
26. D. N. Beatrice, M. Jerome, L. Dominique and F. F. Suzanne, Inorg. Chem., 2006, 45, 5691.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra00152h

---