Combining the PeT and ICT mechanisms into one chemosensor for the highly sensitive and selective detection of zinc

Abstract

A novel fluorescent sensor (ZS1) based on the dual-mechanism of PeT/ICT for the highly sensitive and selective detection of Zn²⁺ was designed and synthesized. ZS1 displays remarkable selectivity for Zn²⁺ with an enhanced red-shift in both absorption and emission, which results from the Zn²⁺-triggered deprotonation of its amide group. ZS1 could detect as low as 7.2 × 10⁻⁹ M Zn²⁺ with an association constant value of 6.27 × 10⁴ M⁻¹. More importantly, it displays specific and sensitive recognition of Zn²⁺ and especially avoids the interference of Cd²⁺ in aqueous solution. The probe was also demonstrated to detect Zn²⁺ in living cells.

---

Introduction

Zinc, which is widely distributed in air, water, and solids, is the second most abundant transition metal ion in organisms.¹ Zinc plays crucial roles in many important biological processes such as structural and catalytic cofactors, neural signal transmitters, and gene expression regulators. The normal concentration range for zinc ions in biological systems is narrow, and both a deficiency and excess cause many pathological states such as Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), epilepsy, Parkinson's disease, ischemic stroke, and infantile diarrhea.² Accordingly, it is desirable to develop new analytical methods for the detection and monitoring of zinc ions in vitro and in vivo.

In fact, numerous fluorescent sensors have been developed to detect and analyze zinc ions due to their simplicity, high sensitivity, and real-time detection.³ Unfortunately, only a few zinc ion fluorescent sensors that can distinguish Zn²⁺ from Cd²⁺ with high selectivity have been reported.⁴ Zn²⁺ and Cd²⁺ are in the same group of the periodic table and have similar properties, which cause similar spectral changes when they are coordinated with fluorescent sensors. Therefore, for practical applications, it is still strongly desirable to develop fluorescent chemosensors with excellent performance for Zn²⁺ under physiological conditions.

It should be noted that compared with a conventional one mechanism method such as photo-induced electron transfer (PeT),⁵ excited-state intramolecular proton transfer (ESIPT),⁶ intramolecular charge transfer (ICT),⁷ and aggregation-induced emission (AIE)⁸ based sensors, which usually give one single signaling response, the combination of two or more mechanisms based sensors would be more attractive since such a multi-mechanism will usually produce multiple signals to amplify recognition events to a greater extent and finally improve selectivity and sensitivity. To date, several fluorescent sensors have been developed on the basis of multi-mechanism. For example, Akkaya et al. reported BODIPY-derived probes for the sensing of Hg²⁺, GSH, and biological thiols based on the PeT/ICT dual-mechanism.⁹ Wang and Zhang et al. applied the PeT/ESIPT dual-mechanism to design a 3-hydroxyflavone based probe for thiols.¹⁰ However, the combination of PeT/ICT dual-mechanism based zinc sensors have been scarcely explored. Recently, Yoon, Shin, and Xu et al. developed a novel PeT/ICT dual-mechanism based fluorescent sensor by Zn²⁺-triggered amide tautomerization for the high selectivity of zinc,^4a which is difficult to accomplish via a single mechanism.

Inspired by all of these works, we report here a novel PeT/ICT dual-mechanism based fluorescence turn-on zinc sensor, ZS1, which is a 2-picolylamine (PA) derivative of 4-aminonaphthalimide, for the detection of Zn²⁺. In ZS1, an amide group has been inserted to link 1,8-naphthalimide fluorophore and PA chelator. We envision that the fluorescence of ZS1 would be quenched not only by the PeT effect from the PA chelator but also by the ICT effect from the 4-amide group. However, the capture of Zn²⁺ by the amide-PA receptor resulted in red shifts in both the absorption and fluorescence spectra due to the repressed PeT process from the N atoms of PA to the fluorophore and the enhanced ICT process from the N atom in the 4 position of ZS1 to 1,8-naphthalimide by the deprotonation of NH in the 4-amide group. This type of binary effect of the PeT/ICT mechanisms of Zn²⁺ to ZS1 exhibits high binding affinity and selectivity towards Zn²⁺ (Scheme 1).

Scheme 1 The PeT/ICT dual-mechanism response strategy for the design of ZS1.

Results and discussion

ZS1 can be readily prepared in two convenient steps under facile reaction conditions with a high yield starting with 4-amino-N-butyl-1,8-naphthalimide (1). The product (ZS1) was well characterized by ¹H, ¹³C NMR, and HR-MS (Scheme 2).

Scheme 2 Synthesis of ZS1: (a) 2-chloroacetyl chloride/NEt₃, DCM, reflux, 1 h, 89%; (b) 2-picolylamine/DIPEA/KI, CH₃CN, reflux, 10 h, 48%.

In its UV-vis absorption spectrum (Fig. 1), ZS1 exhibits a broad band from 300 to 450 nm with its maximum centered at 360 nm, which is assigned to the π–π* transitions of the 1,8-naphthalimide. Upon the addition of Zn²⁺ (0–10.0 equiv.), this band red-shifts to 366 nm, and is accompanied by three clear isosbestic points at 266, 342, and 407 nm, respectively, which are due to the deprotonation of the amide NH group, and this deprotonation process strengthens the electron-donating ability from the nitrogen atom of the 4-amide group to 1,8-naphthalimide. Furthermore, a good linear relationship (R² = 0.994) was observed between the changes in the absorbance at 366 nm with Zn²⁺ in the range of 0–120.0 μM (Fig. S1, ESI†).

Fig. 1 Absorption spectra of ZS1 (20.0 μM) in Tris-HCl buffer (10 mM, pH 7.2, containing 1% CH₃CN) in the presence of different concentrations of Zn²⁺ (0–10.0 equiv.).

As expected, ZS1 alone is weak blue fluorescent (ε = 11 598 M⁻¹ cm⁻¹, Φ₀ = 0.033, λ_ex = 360 nm, λ_em = 465 nm, Table S1, ESI†) in neutral aqueous solution (10 mM Tris-HCl buffer, pH 7.2, containing 1% CH₃CN). While the addition of 10.0 equiv. of Zn²⁺ induced a red-shift in the emission of ZS1 to 522 nm and triggered a ca. 5.3-fold (green fluorescence, ε = 12 145 M⁻¹ cm⁻¹, Φ₀ = 0.175, λ_ex = 360 nm, λ_em = 522 nm, Table S1, ESI†) increase in the integrated emission for ZS1 (Fig. 2a). These fluorescence behaviors were attributed to the coordination of the PA chelator and the deprotonated amide nitrogen of ZS1 with Zn²⁺, which simultaneously repressed the PeT effect from the PA chelator and enhanced the ICT process from the deprotonated 4-amide group. The titration of ZS1 with Zn²⁺ was followed by fluorescence to determine the ZS1/Zn²⁺ binding ratio and association constant (K_a). The K_a of ZS1/Zn²⁺ was determined to be 6.27 × 10⁴ M⁻¹ by the Hill plot analysis (Fig. 3a). Moreover, the Job's plot, which exhibits a maximum at 0.34 M fraction of Zn²⁺, indicated that a 2 : 1 complex is formed between ZS1 and Zn²⁺ (Fig. 3b). We also carried out HPLC-MS measurements for the ZS1-Zn²⁺ solution (Fig. S3, ESI†). All the results agree well with the proposed structure of the ZS1–Zn²⁺ complex (Fig. S4, ESI†).

Fig. 2 (a) Fluorescence spectra of ZS1 (10.0 μM) in Tris-HCl buffer (10 mM, pH 7.2, containing 1% CH₃CN) in the presence of different concentrations of Zn²⁺ (0–80.0 equiv.) (λ_ex = 360 nm). Inset: fluorescence intensity (λ_em = 522 nm) changes as a function of Zn²⁺ concentration. (b) Emission spectra of ZS1 (10.0 μM) in Tris-HCl buffer (10 mM, pH 7.2, containing 1% CH₃CN) in the presence of various metal ions (λ_ex = 360 nm, 10.0 eq. of Ag⁺, Ca²⁺, Cd²⁺, Cr³⁺, Cu²⁺, Fe²⁺, Fe³⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, Pb²⁺, and Zn²⁺, respectively).

Fig. 3 (a) Hill plot of sensor ZS1. Fluorescence intensity at 522 nm response as a function of Zn²⁺ concentration (10 mM Tris-HCl buffer, pH 7.2, containing 1% CH₃CN, λ_ex = 360 nm). The solid line represents a linear fit to the experimental data. (b) Job's plot of sensor ZS1, the total concentration of the sensor and Zn²⁺ is 100.0 μM (10 mM Tris-HCl buffer, pH 7.2, containing 1% CH₃CN, λ_ex = 360 nm).

To clarify the actual ZS1/Zn²⁺ interaction, an ¹H NMR titration experiment was conducted. As shown in Fig. 4, the two methylene protons at 3.97 and 3.56 ppm, which are attributed to the H-7′ and 8′, respectively, together with the pyridine protons (H-3′, 4′, 5′, 6′) were dramatically shifted downfield after the addition of Zn²⁺ due to the chelation of the lone-pair electrons of the two nitrogen atoms of PA by Zn²⁺. Moreover, the chemical shifts of five aromatic resonances, which are attributed to the 1,8-naphthalimide protons, experienced an opposite shift, thus indicating that the deprotonation of 4-amide group of ZS1 occurred in the presence of Zn²⁺. These results agree well with the optical responses.

Fig. 4 ¹H NMR titration experiment of ZS1 in the presence of different concentrations of Zn²⁺ (0–10.0 equiv. of Zn²⁺, in d₆-DMSO).

Furthermore, the fluorescence titration of ZS1 with various metal ions was conducted to examine its selectivity (Fig. 2b). Much to our delight, the examined alkali, alkaline-earth metal ions, and transition metal ions showed nominal changes in the fluorescence of spectra of ZS1. Competing experiments were then carried out in the presence of Zn²⁺ mixed with other competing metal ions (Fig. 5a and b). Except for Cu²⁺, other background metal ions had no obvious interference with the detection of Zn²⁺ ions. It should be noted that ZS1 displayed considerable ability to distinguish Zn²⁺ from Cd²⁺, which have properties similar to Zn²⁺ and generally cause strong interference.

Fig. 5 (a) The fluorescence responses of ZS1 (10.0 μM) with 10.0 equiv. of the competing metal ions in 10 mM Tris-HCl buffer, pH 7.2, containing 1% CH₃CN, followed by 10 equiv. of Zn²⁺. Metal ions include Ag⁺, Ca²⁺, Cd²⁺, Cr³⁺, Cu²⁺, Fe²⁺, Fe³⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, and Pb²⁺, respectively, λ_ex = 360 nm. (b) Fluorescence responses of ZS1 to various metal ions. Black bars represent the addition of 10.0 equiv. of the appropriate metal ion to a 10.0 μM solution of ZS1 in 10 mM Tris-HCl buffer, pH 7.2, containing 1% CH₃CN, λ_ex = 360 nm; red bars represent the addition of 10 equiv. of Zn²⁺ to the solutions containing ZS1 (10.0 μM, in 10 mM Tris-HCl buffer, pH 7.2, containing 1% CH₃CN) and the appropriate metals (10.0 equiv.), λ_ex = 360 nm.

pH effects on the fluorescence of ZS1 and the ZS1–Zn²⁺ system were also investigated. As depicted in Fig. 6, ZS1 alone is inert to pH in the range of 12.0–6.0, but the fluorescent intensity dramatically increased from pH 6.0 to 3.0 due to the inhibited PeT process by the protonation of the two nitrogen atoms of the PA chelator. However, satisfactory Zn²⁺-sensing abilities were exhibited in a range of pH from 6.0 to 9.0, which indicate that ZS1 could be used in neutral natural systems or in a mildly acidic or basic environment.

Fig. 6 Effect of the pH on the fluorescence emission of ZS1 (10.0 μM) alone and ZS1–Zn²⁺ system (10.0 μM of ZS1 in the presence of 10.0 equiv. of Zn²⁺), slit = 1.5/3.

For practical purposes, the detection limit of ZS1 for the analysis of Zn²⁺ is also an important parameter. The fluorescence titration curve revealed that the fluorescence intensity of ZS1 at 522 nm increased linearly with the amount of Zn²⁺ in the range of 0–5.0 μM (R² = 0.99) (Fig. S2, ESI†). Thus, the detection limit of ZS1 for Zn²⁺ was calculated to be 7.2 × 10⁻⁹ M, which reveals the high sensitivity for the analysis of zinc ions using ZS1.

Due to the favorable properties of ZS1 in vitro, the potential utility of ZS1 in living cells was studied. HeLa cells that were incubated with 5.0 μM of ZS1 for 0.5 h at 37 °C exhibited weak blue fluorescence (Fig. 7a). The cells were then treated with ZnCl₂ (5.0 μM) for 0.5 h at 37 °C and this resulted in a dramatic increase of intracellular green fluorescence (Fig. 7d), which indicated that ZS1 was cell membrane permeable and capable of imaging Zn²⁺ in living cells.

Fig. 7 Fluorescence image of HeLa cells incubated with ZS1 (5.0 μM) for 0.5 h, and then washed quickly with PBS for imaging (a). (b) Bright-field images of live cells in (a). (c) The overlay images of live cells in (a) and (b). The cells were then treated with ZnCl₂ (5.0 μM) for 0.5 h, which resulted in a dramatic increase in intracellular green fluorescence (d). (e) Bright-field images of live cells in (d). (f) The overlay images of live cells in (d) and (e).

Conclusion

In conclusion, we successfully developed a novel PeT/ICT dual-mechanism based fluorescent probe, ZS1, for the selective detection of Zn²⁺ in an aqueous solution. ZS1 displays excellent fluorescent selectivity for Zn²⁺ with an enhanced red-shift in both absorption and emission, which results from the Zn²⁺-triggered deprotonation of its amide group. Moreover, based-on this PeT/ICT dual-mechanism, ZS1 can easily distinguish Zn²⁺ from Cd²⁺ in an aqueous solution, which is usually a technical problem for other related probes. Furthermore, the fluorescence imaging of Zn²⁺ in living cells indicated that this probe might be favorable for biological applications. We anticipate that the experimental results of this study will inspire the future designing of metal-ion sensors in water for a variety of chemical and biological applications.

Experimental section

Materials and measurements

All solvents were of analytical grade. NMR experiments were carried out on a Bruker AV-400 NMR spectrometer with chemical shifts reported in ppm (in CDCl₃, d₆-DMSO or TMS as an internal standard). The mass spectrum (MS) was obtained on a SHIMADZU LCMS-2020 spectrometer. All pH measurements were made with a Sartorius basic pH-Meter PB-10. Fluorescence spectra were obtained on a PerkinElmer LS55 Fluorescence spectrophotometer. Absorption spectra were obtained on a Shimadzu UV 2501(PC)S UV-Visible spectrophotometer. Unless otherwise noted, the excitation and emission widths for ZS1 were all 3.

Synthesis

4-Amino-N-butyl-1,8-naphthalimide (1). Compound 1 was obtained according to a published procedure.^{11 1}H NMR (400 MHz, CDCl₃) δ 8.59 (d, J = 7.2 Hz, 1H), 8.41 (d, J = 8.1 Hz, 1H), 8.11 (d, J = 8.4 Hz, 1H), 7.64 (t, J = 7.9 Hz, 1H), 6.88 (d, J = 8.2 Hz, 1H), 5.00 (s, 2H), 4.16 (t, J = 7.2 Hz, 2H), 1.71 (m, 2H), 1.44 (m, 2H), 0.97 (t, J = 7.3 Hz, 3H).

4-(2-Chloroacetyl) amino-N-butyl-1,8-naphthalimide (2). 4-Amino-N-butyl-1,8-naphthalimide (1) (500 mg, 1.9 mmol) was dissolved in dry dichloromethane (30 mL), then triethylamine (0.4 mL, 2.9 mmol) and 2-chloroacetyl chloride (0.2 mL, 2.5 mmol) were added at 0 °C and the solution was refluxed for 1 h until all the starting material was consumed, which was monitored by TLC analysis. The reaction mixture was washed with water (100 mL), and extracted with dichloromethane (3 × 50 mL). The extract was dried over sodium sulfate and then concentrated under vacuum. The product was purified by flash chromatography using petroleum ether/dichloromethane (1 : 2, v/v) as the eluant to give 2 as a pale yellow solid (580 mg, 89%); ¹H NMR (400 MHz, CDCl₃) δ 9.14 (brs, 1H), 8.64 (m, 2H), 8.48 (d, J = 7.9 Hz, 1H), 8.19 (d, J = 8.4 Hz, 1H), 7.83 (t, J = 7.7 Hz, 1H), 4.40 (s, 2H), 4.18 (t, J = 7.2 Hz, 2H), 1.71 (m, 2H), 1.45 (m, 2H), 0.98 (t, J = 7.1 Hz, 3H).

4-(2-(Picolylamino)acetyl)amino-N-butyl-1,8-naphthalimide (3, ZS1). 4-(2-Chloroacetyl) amino-N-butyl-1,8-naphthalimide (2) (300 mg, 0.87 mmol), 2-picolylamine (0.17 mL, 1.65 mmol), N,N-diisopropylethylamine (DIPEA) (1.50 mL), and potassium iodide (90 mg) were added to acetonitrile (50 mL). The mixture solution was then refluxed for 10 h until all the starting material was consumed, which was monitored by TLC analysis. The solvent was then removed under reduced pressure to obtain a dark oil, which was purified by flash chromatography using ethyl acetate as the eluant to give 3 as a pale yellow solid (173 mg, 48%); R_f = 0.60 (12 : 1 dichloromethane : methanol); m.p. = 131–132 °C; ¹H NMR (400 MHz, CDCl₃) δ 10.99 (brs, 1H), 8.68 (d, J = 8.2 Hz, 1H), 8.61 (m, 2H), 8.55 (d, J = 4.7 Hz, 1H), 8.43 (d, J = 8.5 Hz, 1H), 7.75 (t, J = 7.9 Hz, 1H), 7.69 (td, J = 7.7, 1.6 Hz, 1H), 7.27–7.18 (m, 2H), 4.17 (t, J = 7.2 Hz, 2H), 4.10 (s, 2H), 3.64 (s, 2H), 1.72 (m, 2H), 1.45 (m, 2H), 0.98 (t, J = 7.3 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 170.40, 164.25, 163.69, 157.81, 149.73, 138.78, 136.85, 132.67, 131.02, 128.94, 126.53, 123.25, 122.72, 122.52, 118.02, 117.03, 54.99, 53.13, 40.20, 30.22, 20.38, 13.82; HR-MS (ESI-TOF): m/z 417.1960 [M + H]⁺, calc'd 417.1927.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (Grant No. 20902082), the Zhejiang Provincial Natural Science Foundation of China (Grant No. Y3100416) and the Program for Innovative Research Team of Zhejiang Sci-Tech University (13060052-Y).

Notes and references

1. (a) P. Jiang and Z. Guo, Coord. Chem. Rev., 2004, 248, 205; (b) J. M. Berg and Y. Shi, Science, 1996, 271, 1081; (c) X. Xie and T. G. Smart, Nature, 1991, 349, 521; (d) B. L. Vallee and K. H. Falchuk, Physiol. Rev., 1993, 73, 79; (e) K. H. Falchuk, Mol. Cell. Biochem., 1998, 188, 41.
2. (a) A. I. Bush, Curr. Opin. Chem. Biol., 2000, 4, 184; (b) A. I. Bush, Trends Neurosci., 2003, 26, 207; (c) M. P. Cuajungco and G. J. Lees, Brain Res. Rev., 1997, 23, 219.
3. (a) Z. Guo, S. Park, J. Yoon and I. Shin, Chem. Soc. Rev., 2014, 43, 16; (b) Z. Liu, W. He and Z. Guo, Chem. Soc. Rev., 2013, 42, 1568; (c) L. Yuan, W. Lin, K. Zheng, L. He and W. Huang, Chem. Soc. Rev., 2013, 42, 622; (d) G. Zhang, H. Li, S. Bi, L. Song, Y. Lu, L. Zhang, J. Yu and L. Wang, Analyst, 2013, 138, 6163; (e) S. Jiao, L. Peng, K. Li, Y. Xie, M. Ao, X. Wang and X. Yu, Analyst, 2013, 138, 5762; (f) P. Li, X. Zhou, R. Huang, L. Yang, X. Tang, W. Dou, Q. Zhao and W. Liu, Dalton Trans., 2014, 706; (g) Y. Zhang, X. F. Guo, W. X. Si, L. H. Jia and X. H. Qian, Org. Lett., 2008, 10, 473; (h) A. Ajayaghosh, P. Carol and S. Sreejith, J. Am. Chem. Soc., 2005, 127, 14962; (i) D. A. Pearce, N. Jotterand, I. S. Carrico and B. Imperiali, J. Am. Chem. Soc., 2001, 123, 5160; (j) S. C. Burdette, C. J. Frederickson, W. Bu and S. J. Lippard, J. Am. Chem. Soc., 2003, 125, 1778; (k) Z. Xu, X. Liu, J. Pan and D. R. Spring, Chem. Commun., 2012, 48, 4764; (l) Y. Ding, Y. Xie, X. Li, J. P. Hill, W. Zhang and W. Zhu, Chem. Commun., 2011, 47, 5431.
4. (a) Z. Xu, K.-H. Baek, H. N. Kim, J. Cui, X. Qian, D. R. Spring, I. Shin and J. Yoon, J. Am. Chem. Soc., 2010, 132, 601–610; (b) Z. Xu, X. Qian, J. Cui and R. Zhang, Tetrahedron, 2006, 62, 10117–10122; (c) L. Xue, C. Liu and H. Jiang, Org. Lett., 2009, 11, 1655–1658; (d) Y. Tan, J. Gao, J. Yu, Z. wang, Y. Cui, Y. Yang and G. Qian, Dalton Trans., 2013, 11465–11470; (e) P. Li, X. Zhou, R. huang, L. Yang, X. Tang, W. Dou, Q. Zhao and W. Liu, Dalton Trans., 2014, 706–713; (f) K. Wu, Y. Gao, Z. Yu, F. Yu, J. Jiang, J. Guo and Y. Han, Anal. Methods, 2014, 6, 3560–3563; (g) M. Khan, C. R. Goldsmith, Z. Huang, J. Georgiou, T. T. Luyben, J. C. Roder, S. J. Lippard and K. Okamoto, Proc. Natl. Acad. Sci. U. S. A., 2014, 111, 6786–6791; (h) T. Mistri, M. Dolai, D. Chakraborty, A. R. Khuda-Bukhsh, K. K. Das and M. Ali, Org. Biomol. Chem., 2012, 10, 2380–2384; (i) G. Sivaraman, T. Anand and D. Chellappa, Analyst, 2012, 137, 5881–5884; (j) J. F. Zhu, W. H. Chan and A. W. M. Lee, Tetrahedron Lett., 2012, 53, 2001–2004; (k) L. Praveen, C. H. Suresh, M. L. P. Reddy and R. L. Varma, Tetrahedron Lett., 2011, 52, 4730–4733; (l) B. K. Datta, S. Mukherjee, C. Kar, A. Ramesh and G. Das, Anal. Chem., 2013, 85, 8369–8375.
5. (a) T. Matsumoto, Y. Urano, T. Shoda, H. Kojima and T. Nagano, Org. Lett., 2007, 9, 3375; (b) L.-Y. Lin, X.-Y. Lin, F. Lin and K.-T. Wong, Org. Lett., 2011, 13, 2216; (c) Y. Gabe, Y. Urano, K. Kikuchi, H. Kojima and T. Nagano, J. Am. Chem. Soc., 2004, 126, 3357; (d) J. Wang and X. Qian, Chem. Commun., 2006, 109; (e) J. Wang, X. Qian and J. Cui, J. Org. Chem., 2006, 71, 4308; (f) J. Wang and X. Qian, Org. Lett., 2006, 8, 3721; (g) J. Liu, K. Wu, S. Li, T. Song, Y. Han and X. Li, Dalton Trans., 2013, 3854.
6. (a) T. I. Kim, H. J. Kang, G. Han, S. J. Chung and Y. Kim, Chem. Commun., 2009, 5895; (b) R. Hu, J. A. Feng, D. H. Hu, S. Q. Wang, S. Y. Li, Y. Li and G. Q. Yang, Angew. Chem., Int. Ed., 2010, 49, 4915; (c) M. Santra, B. Roy and K. H. Ahn, Org. Lett., 2011, 13, 3422; (d) Z. Xu, L. Xu, J. Zhou, Y. Xu, W. Zhu and X. Qian, Chem. Commun., 2012, 48, 10871; (e) S. Chen, P. Hou, J. Wang and X. Song, RSC Adv., 2012, 2, 10869; (f) M. Kumar, N. Kumar and V. Bhalla, RSC Adv., 2013, 3, 1097.
7. (a) D. Srikun, E. W. Miller, D. W. Domaille and C. J. Chang, J. Am. Chem. Soc., 2008, 130, 4596; (b) L. Yuan, W. Lin, J. Song and Y. Yang, Chem. Commun., 2011, 47, 12691; (c) F. Yu, P. Li, P. Song, B. Wang, J. Zhao and K. Han, Chem. Commun., 2012, 48, 2852; (d) X. Li, S. Zhang, J. Cao, N. Xie, T. Liu, B. Yang, Q. He and Y. Hu, Chem. Commun., 2013, 49, 8656; (e) J. Liu, K. Wu, X. Li, Y. Han and M. Xia, RSC Adv., 2013, 3, 8924.
8. (a) L. Peng, Z. Zhou, R. Wei, K. Li, P. Song and A. Tong, Dyes Pigm., 2014, 108, 24; (b) X. Li, C. Yang, K. Wu, Y. Hu, Y. Han and S. H. Liang, Theranostics, 2014, 4, 1233; (c) H. Wang, Y. Huang, X. Zhao, W. Gong, Y. Wang and Y. Cheng, Chem. Commun., 2014, 50, 15075; (d) G. N. Zhao, B. Tang, Y. Q. Dong, W. H. Xie and B. Z. Tang, J. Mater. Chem. B, 2014, 2, 5093; (e) R. T. K. Kwok, C. W. T. Leung, J. W. Y. Lam and B. Z. Tang, Chem. Soc. Rev., 2015, 44, 4228.
9. (a) O. A. Bozdemir, R. Guliyev, O. Buyukcakir, S. Selcuk, S. Kolemen, G. Gulseren, T. Nalbantoglu, H. Boyaci and E. U. Akkaya, J. Am. Chem. Soc., 2010, 132, 8029; (b) M. Isik, R. Guliyev, S. Kolemen, Y. Altay, B. Senturk, T. Tekinay and E. U. Akkaya, Org. Lett., 2014, 16, 3260; (c) M. Isik, T. Ozdemir, I. S. Turan, S. Kolemen and E. U. Akkaya, Org. Lett., 2013, 15, 216.
10. M. Lan, J. Wu, W. Liu, H. Zhang, W. Zhang, X. Zhuang and P. Wang, Sens. Actuators, B, 2011, 156, 332.
11. B. Liu and H. Tian, Chem. Commun., 2005, 3156.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental details, synthetic details of MS1, additional spectroscopic data, and copies of NMR spectra. See DOI: 10.1039/c5ra11194c

---