Epoxy-based polymer bearing triphenylamine units: a highly selective fluorescent chemosensor for Hg²⁺ ions

Abstract

A simple epoxy based polymer 1 bearing a triphenylamine unit is shown to be an on–off type fluorescent sensor for Hg²⁺ ions in THF–water (4 : 1, v/v; 10 mM HEPES, pH ∼ 7.0) with high sensitivity and selectivity. In the sensing process, other metal ions such as Cu²⁺, Zn²⁺, Co²⁺, Ni²⁺, Cd²⁺, Pb²⁺, Mg²⁺, Ca²⁺, Sr²⁺ and Ag⁺ did not interfere.

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Introduction

Mercury is an important environmental contaminant which causes serious problems for both human health and ecology. The uptake of mercury containing compounds can have serious adverse impacts on the brain, kidney, and central nervous system.¹ It is therefore highly desirable to design and develop effective fluorescent chemosensors for mercury ions. In the last few years, considerable efforts have been devoted to developing small molecular fluorescent sensors that selectively recognize such metal ions.² However, there are very few reports on polymeric sensors for the fluorometric recognition of Hg²⁺ ions.^3,4 In this context, epoxy-based polymers are particularly attractive due to their easy preparation and processability, good chemical resistance, dimensional stability and their potential for reuse. In addition, epoxy resins contain reactive epoxy groups which are capable of undergoing reactions with a variety of functional groups such as –COOH, –NH₂ etc. and can therefore be used as excellent precursors for the synthesis of functionalized polymers. Nevertheless, the application of epoxy-based polymers in the design of fluorescent sensors for Hg²⁺ ions remains unused. Furthermore, due to its strong luminescence properties, the triphenylamine motif is an established fluorophore motif which has been explored in the construction of fluorogenic chemosensors for a variety of important chemical species.⁵

Our interest is centered on the synthesis of designed compounds which have material properties combining the advantages of both small molecules and polymers. In this regard, as part of our continued interest in the design and synthesis of epoxy-based fluorescent chemosensors,⁶ we herein report the simple development of epoxy-based polymer 1 appended with triphenylamine signalling units for the selective recognition of Hg²⁺ ions over other metal ions. To the best of our knowledge, mercury ion selective epoxy-based macromolecular probes, bearing triphenylamine units, have not been reported yet. Our design is particularly interesting because it avoids migration or leaching through covalent immobilization of the triphenylamine moiety in the polymer matrix.

Results and discussion

As depicted in Scheme 1, the designed epoxy based polymer 1 was prepared by a simple condensation of the diglycidyl ether of bisphenol-A (DGEBA) and 4-aminotriphenylamine. Polymer 1 is soluble in common organic solvents such as THF, 1,4-dioxane, CH₂Cl₂, CHCl₃, DMF and DMSO. The structure of 1 was confirmed by FT-IR, UV-vis, NMR, GPC and elemental analyses (ESI, Fig. S1–S3†). The peaks due to the aromatic protons were observed between 7.17 and 6.87 ppm in the ¹H NMR spectrum (ESI, Fig. S1†). To compare the recognition properties, model compound 2 was synthesized in moderate yield following the method outlined in Scheme 2. The structure of this compound was established by FT-IR, ¹H NMR and elemental analysis (ESI, Fig. S4†).

Scheme 1 Synthesis of polymer 1.

Scheme 2 Synthesis of model compound 2.

The thermal properties of 1 were investigated by DSC and TGA (ESI, Table S1†). It is evident from the DSC thermogram that the polymer is amorphous. The polymer was found to be stable up to 290 °C as evidenced from the TGA analysis. Therefore, the resulting polymer can provide the desirable thermal properties required for its practical application as a fluorescent sensor.

Prior to the sensing studies, we investigated the photophysical properties of 1. The spectroscopic properties were evaluated in HEPES buffered (10 mM) THF–water (4 : 1, v/v) at 25 °C. A summary of the photophysical data is gathered in Table 1. As shown in Fig. 1, polymer 1 did not show any absorption band shifting but gave a slightly blue-shifted emission (Δλ_max = 3–5 nm) compared to the non-polymeric model compound 2. This is attributed to the greater perturbation of the excited state energy levels in polymer 1. It is significant to note that polymer 1 in the same solvent mixture gave a comparatively more intense emission band with a change in Stokes shift. In Fig. 2, the emission spectra of 1 in the solid state and in solution are shown and they reveal a bathochromic shift of 15 nm. Taken together, these observations reflect a kind of macromolecular effect, probably inducing a complex interplay between intra chain or inter chain interactions and the extent of the intramolecular charge transfer (ICT)^7,8 associated with the planarization of the triphenylamine moieties. However, a substantial decrease in the fluorescence intensity for 1, on moving from solution to a solid state (Fig. 2), is probably associated with a greater perturbation of the chromophoric system within the ensemble of polymer chains.⁹

Table 1 Photophysical properties of 1 and 2 in THF–water (4 : 1, v/v; HEPES 10 mM, pH 7.0) (λ_exc = 300 nm; [1] = 1.29 × 10⁻⁵ M; [2] = 8.0 × 10⁻⁶ M)

Compound	λ^absmax (nm)	λ^emmax (nm)	Stokes shift/cm^−1
1	300	416	9294
2	300	422	9636

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Fig. 1 UV-vis and fluorescence spectra of 1 (c = 1.29 × 10⁻⁵ M) and 2 (c = 8.0 × 10⁻⁶ M) in THF–water (4 : 1, v/v; 10 mM HEPES, pH ∼ 7.0).

Fig. 2 Fluorescence spectra of 1 (c = 1.29 × 10⁻⁵ M) in (a) the solid state, (b) THF–water (4 : 1, v/v; 10 mM HEPES, pH ∼ 7.0) and (c) THF–water (4 : 1, v/v).

Molecular modelling was further applied to gain an insight into the gross structural conformation of the triphenylamine moiety in 1 and 2. The structures of 1 (considering the smallest chain with –OMe end groups for clarity) and 2 were optimized separately using the molecular mechanics technique, as implemented in HyperChem Lite 3.0 (Fig. 3). Inspection of the optimized geometries of 1 and 2 indicates that under the influence of the polymer chain, the triphenylamine moiety was found to adopt a propeller-shape with twist angles around the bond connecting the central nitrogen atom to the phenyl rings. This is probably reflected in the difference of the spectral appearances.

Fig. 3 Optimized geometries of 1 and 2 (HyperChem Lite 3.0).

The fine tuning of the fluorescence of the triphenylamine moieties within the epoxy-based polymer matrix prompted us to examine its sensitivity and selectivity to specific metal ions. Titration experiments were carried out using a set of representative metal ions, such as Cu²⁺, Zn²⁺, Co²⁺, Ni²⁺, Pb²⁺, Mg²⁺, Ca²⁺, Sr²⁺ and Ag⁺ ions (taken as their nitrate salts), and Cd²⁺ and Hg²⁺ ions (as perchlorate salts) to evaluate the metal ion binding properties of 1. Upon initial addition of the surveyed metal ions (c = ∼10⁻⁵ M) a bathochromic-shifted emission band of increased intensity (Δλ_max = 3–5 nm) (ON-state) was observed when excited at 300 nm. The corresponding absorption profile of 1 was virtually invariant to such metal ions (Fig. 4 and 5). Interestingly, in contrast to 1, the emission wavelength and intensity of 2 remained constant up to this concentration range of metal ions. The combination of these results suggests that the metal ions influence the polymer chain conformations that mutually enforce the ICT process, and partially suppress the non-radiative deactivation channels.

Fig. 4 UV-vis and fluorescence spectral changes of 1 (c = 1.29 × 10⁻⁵ M) with different concentrations of Hg²⁺ ions (up to c = 8.5 × 10⁻⁴ M) in THF–water (4 : 1, v/v; 10 mM HEPES, pH ∼ 7.0; λ_exc = 300 nm).

Fig. 5 UV-vis and fluorescence spectral changes of 1 (c = 1.29 × 10⁻⁵ M) with different concentrations of Cu²⁺ ions (up to c = 8.4 × 10⁻⁵ M) in THF–water (4 : 1, v/v; 10 mM HEPES, pH ∼ 7.0; λ_exc = 300 nm).

However, increasing the addition of such metal ions, except Hg²⁺ ions, resulted in only negligible quenching of the fluorescence. The effect of Hg²⁺ cations on the spectral properties of 1 is observed to be significant. Inspection of Fig. 4 reveals that successive addition of Hg²⁺ ions resulted in an on–off type quenching of the fluorescence emission.

Unlike the metal ions surveyed, strong ligation of Hg²⁺, at the nitrogen atom of the triphenylamine moieties of 1, was evidenced from the blue shift with a hyperchromic effect of the absorption profile (Fig. 4) ascribed to the perturbation of the ground state. Furthermore, Fe³⁺ also resulted in strong quenching however the addition of fluoride ions, that form the complex FeF₆³⁻, eliminated the interference of the Fe³⁺ ion.¹⁰

The ability of the metal ions to quench the fluorescence emission of 1 has been estimated by the Stern–Volmer plot (Fig. 6). The plot shows more marked quenching with Hg²⁺ ions, compared with the other metal ions studied, demonstrating the good chemosensitivity of 1 for Hg²⁺ ions.

Fig. 6 Stern–Volmer plot with different metal ions in THF–water (4 : 1, v/v) (λ_exc = 300 nm; 10 mM HEPES; [1] = 1.29 × 10⁻⁵ M).

Fig. 7 shows the comparative Stern–Volmer plot of 1 and 2 as a function of Hg²⁺ ion concentrations. It is noticed that 1 shows better sensitivity for Hg²⁺ ions compared to 2. To explore the selectivity of 1 for Hg²⁺ ions, the experiment was also carried out in the presence of Hg²⁺ mixed with a series of potentially competing metal ions, such as Cu²⁺, Zn²⁺, Co²⁺, Ni²⁺, Pb²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ag⁺ and Cd²⁺ (Fig. 8 and 9). It was observed that the Hg²⁺-induced fluorescence response of 1 was unaffected by the background of coexistent metal ions, demonstrating its ability to discriminate the Hg²⁺ ion.

Fig. 7 Stern–Volmer plot for 1 and 2 upon addition of Hg²⁺ (up to c = 6.7 × 10⁻⁴ M) in THF–water (4 : 1, v/v; 10 mM HEPES, pH ∼ 7.0; λ_exc = 300 nm); inset: fluorescence spectral changes of 2 upon addition of Hg²⁺ ions.

Fig. 8 Hg²⁺ ion selectivity of 1 (c = 1.29 × 10⁻⁵ M) in THF–water (4 : 1, v/v; 10 mM HEPES, pH ∼ 7.0) in the presence of various metal ions: Mⁿ⁺ = Cu²⁺, Zn²⁺, Co²⁺, Ni²⁺, Cd²⁺, Pb²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ag⁺; c = ∼10⁻⁶ M); [Hg²⁺] = ∼2.5 × 10⁻⁶ M (λ_exc = 300 nm).

Fig. 9 Bar graph of Fig. 8 (A) 1 only; (B) 1 + other metal ions (Cu²⁺, Zn²⁺, Co²⁺, Ni²⁺, Cd²⁺, Pb²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ag⁺; c = ∼10⁻⁶ M); (C) B + Hg²⁺ (c = ∼2.5 × 10⁻⁶ M).

In our opinion, the quenching of the emission in the presence of Hg²⁺ ions, in the present case, is a consequence of a heavy atom effect coupled with the proximal approach of Hg²⁺, through the formation of a 1–Hg²⁺ complex that modulates ICT between the nitrogen atom and the triphenylamine moiety (Scheme 3).¹¹

Scheme 3 Suggested coordination induced sensing mechanism of 1 for Hg²⁺.

A DFT study on the model compound 2 was performed to understand the nature of the interaction between the triphenylamine unit and the Hg²⁺ ion (ESI, Fig. S5†). As can be seen from Fig. S5,† the –OH groups and the amine nitrogen are intimately involved in the complexation of the Hg²⁺ ions. It is our belief that a similar situation occurs in the polymeric matrix.

Conclusion

In summary, a simple one-step procedure for synthesizing epoxy-based polymer 1 with triphenylamine units has been described. Fine tuning of the fluorescence of 1 is largely due to the polymer chain conformations around the triphenylamine fluorophore moieties. The polymer 1 displays excellent turn-OFF fluorescence in THF–water (4 : 1, v/v; 10 mM HEPES, pH ∼ 7.0) with high selectivity and sensitivity towards Hg²⁺ ions even in the presence of other physiologically and environmentally important metal ions. The greater processing ability and modular nature of the polymers (i.e., into particles, films, etc.), compared to that of small molecules, are also attractive features of this system. In our opinion, a fluorescent macromolecule of this type would be of great importance to a new generation of chemosensory materials for Hg²⁺ detection. Further studies are currently progressing in our laboratory.

Experimental

Materials and characterization

For the synthesis of 1 and 2, 4-aminotriphenylamine was prepared according to the reported procedure.¹² Other chemicals were purchased and used without purification. The melting point was determined on VEEGO melting point apparatus. NMR spectra were recorded on a Brucker NMR spectrometer. IR spectra were recorded on a Perkin Elmer spectrophotometer using KBr discs. The molecular weights and molecular weight distribution of the polymer were determined by gel permeation chromatography (GPC) (Series 200, Perkin Elmer) on a Mixed PL gel [(300 × 7.5) mm, 5 μm], in THF as an eluent at a flow rate of 1.0 mL min⁻¹, calibrated by polystyrene standards at 30 °C. Elemental analyses were performed on a 2400 Series II CHN analyser (Perkin Elmer) using helium as the driving gas and oxygen as the combustion gas. TGA/DTG and DSC analyses were conducted with Perkin Elmer Diamond TG/DTA and Perkin Elmer Diamond DSC instruments, respectively, under a nitrogen atmosphere. Fluorescence and UV-vis spectra were recorded on a Hitachi F-4600 FL spectrophotometer and a Shimadzu UV-1800 spectrophotometer, respectively.

Syntheses

Polymer 1. 4-Aminotriphenylamine (0.2 g, 0.79 mmol) was mixed homogeneously with the diglycidyl ether of bisphenol-A (0.3 g, 0.79 mmol) under slow heating and polymerized at 95–100 °C for 10 h. The product was dissolved in 1,4-dioxan and precipitated with plenty of water. The precipitate was collected and vacuum dried at 60 °C for 24 h. FTIR (KBr): ν_max = 3401, 2924, 1607, 1586, 1510, 1491, 1237, 1180, 1036, 828, 751, 694 cm⁻¹; ¹H NMR (DMSO-d₆, 400 MHz) δ 7.17 (br s, H^b), 7.04 (br s, Hⁱ), 6.70–6.87 (br m, H^a,c,d,e,j), 4.10 (br s, –OH), 3.87 (br s, –OH), 3.57–3.27 (br m, polymer chain –CH₂–, –CH– and –OH), 1.50 (s, H^k); ¹³C NMR (DMSO-d₆, 100 MHz) δ: 156.72, 148.29, 145.90, 143.11, 129.57 (unresolved), 128.32, 127.85, 122.04, 121.69, 114.36, 113.42, 70.53, 67.04, 62.01, 59.90, 57.80, 55.87, 41.01, 31.21. GPC (using polystyrene and THF): M_w = 16 991, PDI = 7.47; anal. calcd for (C₅₇H₅₉N₂O₇)_n: C, 77.46; H, 6.68; N, 3.17%. Found: C, 76.89; H, 6.32; N, 2.97%.

Model compound 2. A suspension of 4-aminotriphenylamine (0.4 g, 1.54 mmol), 2-chloroethanol (1.24 g, 3.38 mmol) and calcium carbonate (0.77 g, 7.69 mmol) in 30 mL water was refluxed with vigorous stirring for 36 h, and the filtrate was extracted with dichloromethane. The organic phase was dried over anhyd. Na₂SO₄ and filtered. The solvent was removed in vacuo to get the crude product, which was column chromatographed (silica gel 60–120) and eluted with a mixture of petroleum ether–EtOAc 2 : 1 to obtain the pure product (60–65%), mp: 109–110 °C. FTIR (KBr): ν_max = 3350, 2929, 1586, 1485, 1312, 1274, 1219, 1071, 816, 733, 694 cm⁻¹; ¹H NMR (DMSO-d₆, 400 MHz) δ 7.21 (t, 4H, J = 4 Hz, H^d), 6.92–6.87 (m, 8H, H^a,c,d), 6.68 (d, J = 8 Hz, H^b), 4.77 (br s, 1H, –OH), 3.53 (br m, 4H, H^g), 3.38 (br m, 5H, H^f, –OH). Anal. calcd for C₂₂H₂₄N₂O₂: C, 41.50; H, 3.77; N, 4.40%. Found: C, 41.42; H, 3.68; N, 6.03%.

General procedure of UV-vis and fluorescence titrations

Stock solutions of 1 and 2 were prepared in THF–water (4 : 1, v/v; 10 mM HEPES, pH 7.0) and 2 mL of the solutions was taken in a 10 mm path length quartz cuvette. The THF was of spectroscopic grade. Stock solutions of the cations were prepared in double distilled water, and were individually added in appropriate amounts to the host solutions. Upon addition of each metal ion, the changes in absorbance and fluorescence of the receptor were noted. The excitation and emission slits were 5 nm and 2.5 nm respectively. All of the titration experiments were recorded at room temperature.

Acknowledgements

Funding from Department of Science and Technology (grant no. SR/FTP/CS-88/2007), Government of India, is gratefully acknowledged. We thank the NMR Research Centre, IISc, Bangalore for the ¹³C NMR spectrum and SICART-CVM, Gujarat for the GPC analysis.

References

1. (a) B. R. J. Von, J. Appl. Toxicol., 1995, 15, 483; (b) H. H. Harris, I. J. Pickering and G. N. George, Science, 2003, 301, 1203; (c) V. Amendola, L. Fabbrizzi, M. Licchelli, C. Mangano and P. Pallavicini, Acc. Chem. Res., 2001, 34, 488; (d) E. M. Nolan and S. J. Lippard, Acc. Chem. Res., 2009, 42, 193; (e) R. A. Løvstad, BioMetals, 2004, 17, 111; (f) D. G. Barceloux and D. Barceloux, J. Toxicol., Clin. Toxicol., 1999, 37, 217; (g) B. Sarkar, in Metal ions in biological systems, ed. H. Siegel and A. Siegel, Marcel Dekker, New York, 1981, vol. 12, p. 233.
2. (a) M. Suresh, A. Shrivastav, S. Mishra, E. Sureh and A. Das, Org. Lett., 2008, 10, 3013–3016; (b) H. N. Kim, W. X. Ren, J. S. Kim and J. Yoon, Chem. Soc. Rev., 2012, 41, 3210–3244 and references cited therein; (c) M. Yuan, Y. Li, J. Li, C. Li, X. Liu, J. Lv, J. Xu, H. Liu, S. Wang and D. Zhu, Org. Lett., 2007, 9, 2313; (d) H.-K. Chen, C.-Y. Lu, H.-J. Cheng, S.-J. Chen, C.-H. Hu and A.-T. Wu, Carbohydr. Res., 2010, 345, 2557; (e) J. Hu, M. Zhang, B. L. Yu and Y. Ju, Bioorg. Med. Chem. Lett., 2010, 20, 4342; (f) A. Thakur, S. Sardar and S. Ghosh, Inorg. Chem., 2011, 50, 7066; (g) S.-M. Cheung and W.-H. Chan, Tetrahedron, 2006, 62, 8379; (h) H. N. Kim, M. H. Lee, H. J. Kim, J. S. Kim and J. Yoon, Chem. Soc. Rev., 2008, 37, 1465; (i) J. Wang and B. Liu, Chem. Commun., 2008, 4759; (j) Y. Zhao, B. Zheng, J. Du, D. Xiao and L. Yang, Talanta, 2011, 85, 2194; (k) J. S. Kim, S. Y. Lee, J. Yoon and J. Vicens, Chem. Commun., 2009, 32, 4791; (l) H. Liu, P. Yu, D. Du, C. He, B. Qiu, X. Chen and G. Chen, Talanta, 2010, 81, 433; (m) G. He, X. Zhang, C. He, X. Zhao and C. Duan, Tetrahedron, 2010, 66, 9762; (n) K. Ghosh, T. Sarkar and A. Samadder, Org. Biomol. Chem., 2012, 10, 3236; (o) A. Jiménez-Sánchez, N. Farfán and R. Santillan, Tetrahedron Lett., 2013, 25, 5279.
3. (a) C. H. Li, C. J. Zhou, H. Y. Zheng, X. D. Yin, Z. C. Zuo, H. B. Liu and Y. L. Li, J. Polym. Sci., Part A: Polym. Chem., 2008, 46, 1998; (b) J. Hu, L. Dai and S. Liu, Macromolecules, 2011, 44, 4699.
4. J. Li, Y. Wu, F. Song, G. Wei, Y. Cheng and C. Zhu, J. Mater. Chem., 2012, 22, 478 and references cited therein.
5. (a) K. Ghosh, G. Masanta and A. P. Chattopadhyay, Eur. J. Org. Chem., 2009, 26, 4515; (b) J. Cremer and C. A. Briehn, Chem. Mater., 2007, 19, 4155.
6. (a) S. Ghosh, C. K. Dey and R. Manna, Tetrahedron Lett., 2010, 51, 3177; (b) S. Ghosh and R. Manna, Supramol. Chem., 2011, 23, 558.
7. (a) A. Bhaskar, G. Ramakrishna, Z. Lu, R. Twieg, J. M. Hales, D. J. Hagan, E. V. Stryland and T. Goodson, J. Am. Chem. Soc., 2006, 128, 11840; (b) J. Cremer and C. A. Briehn, Chem. Mater., 2007, 19, 4155.
8. A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. M. Huxley, C. P. McCoy, J. T. Radmacher and T. E. Rice, Chem. Rev., 1997, 97, 1515.
9. I. Grabchev and V. Bojinov, J. Photochem. Photobiol., A, 2001, 139, 157.
10. H. Y. Xie, J. G. Liang, Z. L. Zhang, Y. Liu, Z. K. He and D. W. Pang, Spectrochim. Acta, Part A, 2004, 60, 2527.
11. (a) A. W. Czarnik, Fluorescent Chemosensors for Ion and Molecule Recognition, American Chemical Society, Washington DC, 1992; (b) Y. Yang, X. Gou, J. Blecha and H. Cao, Tetrahedron Lett., 2010, 51, 3422.
12. B. T. Mayers and A. J. Fry, Org. Lett., 2006, 8, 411.

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Footnote

† Electronic supplementary information (ESI) available: NMR, GPC, DFT and thermal data. See DOI: 10.1039/c3ra45620j

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