Facile one-pot synthesis of novel water-soluble fluorescent hyperbranched poly(amino esters)

Abstract

Two kinds of water-soluble fluorescent hyperbranched poly(amino esters) were first synthesized by a convenient one-pot approach via the A₂ + B₃ Michael addition reaction of trimethylolpropane triacrylate and aliphatic diamines. Their structures and optical properties were determined by FTIR, NMR, GPC, UV-vis, and fluorescence spectra. The hyperbranched poly(amino esters) in aqueous solutions and PVA films state showed strong blue fluorescence when these polymers were excited by a UV lamp. In addition, it was found that their fluorescence exhibited a pH-dependent behavior. It was also noteworthy that the fluorescence could be changed by adding metal ions. The fluorescence of the polymer could be quenched when the concentration of Hg²⁺ and Fe³⁺ reached at 10⁻² mol L⁻¹. Therefore, the prepared hyperbranched poly(amino ester) is a promising ion probe for detecting Hg²⁺ and Fe³⁺.

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1 Introduction

Since Allen^'s group¹ first found that a polyamide-amine (PAMAM) dendrimer can emit strong fluorescence under proper conditions, both dendrimers and polymers containing unconventional luminescent groups have attracted a great deal of interest.^2–10 Unconventional luminescent polymers are composed by aliphatic tertiary amines, amides, C=O, C=S, C=N, N=O, and N=N in hyperbranched polymers.^10–12 In contrast with conventional luminescent polymers, unconventional luminescent polymers have a lot of advantages such as environmentally friendly, easy preparation, biocompatibility, hydrophilic, etc. As a result, those polymers can be applied widely in various fields, including bio-imaging sciences,¹³ ion detection¹⁴ and drug delivery.¹⁵

To date, there have been a few reports of the photoluminescent dendrimers and polymers carrying no typical chromophore, for example poly(amido amines) (PAMAM),^16,17 polyethylenimine,¹⁸ poly(N-vinylpyrrolidone),¹⁹ poly(propyl ether imine) (PPEI),²⁰ polylurea,²¹ hyperbranched poly(amido amine)s²² and poly(ether amide)s.²³ The strong fluorescence of all these dendrimers and hyperbranched polymers have been attributed to the aliphatic tertiary amine in the unique branching structure. Very recently, shu et al.²⁴ produced a hyperbranched poly(amido acids) (HBPAAs) through direct self-condensation of N-(3-aminopropyl) diethano succinate amine, the obtained HBPAAs shows high pH-dependent photoluminescence. Hang et al.²⁵ synthesized the siloxane-poly(amidoamine) dendrimers via aza-Michael reaction. It was proposed that the blue photoluminescence of Si-PAMAM is ascribed to the aggregation of carbonyl groups caused by the N–Si coordination bonds of the dendrimers structure. Zhao et al.¹¹ have reported poly[(maleic anhydride)-alt-(vinyl acetate)] (PMV), a pure oxygenic nonconjugated polymer which can emit light. And the emission of PMV was associated with the clustering of the locked carbonyl groups. Xue et al.²⁶ prepared a chromophore-free aliphatic hyperbranched polyether by one-pot approach through the proton transfer polymerization. The synthesized hyperbranched polyether shows a bright blue-green fluorescence in its ethanol solution and solid state. However, the synthetic approaches of the fluorescent dendrimers usually require multistep reaction process to obtain finally products. Moreover, there are lots of disadvantages of the synthetic methods of luminescent polymers, such as preparation of monomers, addition of catalysts or initiating agents. And the reported polymers generally dissolve in the organic solvent, like ethanol, methanol, DMSO, THF. It is well known that the solubility of the photoluminescent polymers plays a crucial role in their application area. Consequently, it is of great academic importance and industrial significance to develop water-soluble photoluminescent hyperbranched polymers via a one-pot convenient synthetic approach.

In this paper, two kinds of novel water-soluble fluorescent hyperbranched poly(amino ester) which containing both amines and carbonyl groups were firstly synthesized via a one-pot A₂ + B₃ polycondensation–addition reaction of trimethylolpropane triacrylate and aliphatic diamines. The fluorescence of the hyperbranched poly(amino ester) in aqueous solutions and PVA films state has been studied. Furthermore, the effect of pH and metal ions on the fluorescence properties of polymers was also investigated.

2 Material

FeCl₃·6H₂O and FeCl₂·4H₂O were purchased from Tianjin Fuchen Chemical Reagents Factory. CoCl₂·6H₂O, NaCl, HgCl₂, MgSO₄, ZnSO₄·7H₂O, CuSO₄·5H₂O, Al(NO₃)₃·9H₂O and Zr(NO₃)₄·5H₂O were purchased from Sinopharm Chemical Reagent Co., Ltd, China. 1,6-hexamethylenediamine and ethylenediamine were supplied by Zhengzhou Paini Chemical reagents Factory and Tianjin Tianli Chemical Reagents Co., Ltd, respectively. Trimethylolpropane triacrylate was purchased from Aladdin Reagents (Shanghai) Co. Ltd, China. Polyvinylalcohol (PVA, degree of polymerization and alcoholysis degree is 1700 and 99%, respectively) was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. Dimethyl formamide and acetone were purchased from Guangdong Guanghua Sci-Tech Co., Ltd. All materials were used without further purification.

3 Characterization

Proton nuclear magnetic resonance (¹H NMR, ¹³C NMR) spectra were recorded on a Bruker Advance 400 spectrometer at 25 °C using DMSO as the solvent. Molecular weights were determined by a Dionex Gel Permeation Chromatography (GPC) UltiMate3000 equipped with an Agilent RID using a column system (Shodex OH-pak SB-803 HQ, Showa Denko, Japen). H₂O was used as an eluent, and the measurement was carried out at a flow rate of 1 mL min⁻¹. Ultraviolet absorption (UV) spectra in aqueous solution were detected using a HITACHI U-3900H UV-vis spectrophotometer. Fourier transform infrared spectra (FT-IR) were recorded on a Bruker TENSOR 27 infrared spectrophotometer using the KBr pellet technique within the 4000–400 cm⁻¹ region. The luminescence (excitation and emission) spectra of the samples were determined with a Hitachi F-4600 fluorescence spectrophotometer using a monochromated Xe lamp as an excitation source, the excitation and emission slits were both set at 5 nm. Fluorescent excitation/emission spectra, fluorescence lifetime and absolute quantum yield (QY) of the pure hyperbranched poly(amino ester) were recorded on a steady/transient-state fluorescence spectrometer equipped with an integrating sphere (FLS980, Edinburgh Instruments).

4 Synthesis of hyperbranched poly(amino ester) by polycondensation–addition

Synthesis of the polymer P1

6.97 g (0.02 mol) of trimethylolpropane triacrylate was added into a solution of 6.97 g (0.06 mol) of 1,6-hexamethylenediamine in 60 mL DMF. Then the mixture was stirred vigorously at 60 °C for 60 h under N₂ protection. After cooling to room temperature, the solvent was removed by rotary evaporation. The acquired crude production was precipitated into 200 mL acetone under stirring. The product was purified by re-precipitation from distilled water into acetone, then dried under vacuum at 50 °C for 1 day (yield: 41.3%).

Synthesis of the polymer P2

P2 was produced by using 6.97 g (0.02 mol) trimethylolpropane triacrylate and 3.64 g (0.06 mol) ethylenediamine on the basis of the synthesis procedure of P1 (yield: 39.7%).

The synthetic route is showed in Scheme 1.

Scheme 1 Synthetic route of hyperbranched poly(amino ester).

5 Synthesis of PVA film

A typical formulation prepared for PVA films: 5 mL of P1 solution (20 mg mL⁻¹ in distilled water) were mixed with 5 mL of PVA solution (0.05 g in 10 mL distilled water).

PVA films were produced based on the literature.²⁷ 4 mL of as-obtained formulations were dropped on a cleaned glass substrate (7 cm of diameter) and allowed drying for overnight under ambient circumstances to from freestanding films. Then, the resulting films were peeled off from the glass substrate.

6 Results and discussions

The resulting polymers were characterized by FTIR measurements, the FTIR spectra of P1 and P2 are presented in Fig. 1. The bands at about 3489 cm⁻¹ and 3417 cm⁻¹ are attributed to the bending vibration of the amino groups.²⁸ Two characteristic bands at 1633 cm⁻¹ and 1406 cm⁻¹ are assigned to the C=O stretching and N–H bending vibration,²⁹ respectively. The band at 2820 cm⁻¹ is related to the stretching vibration of C–H groups. And the absorption peak of C–O is clearly observed at 1126 cm⁻¹. The similar FTIR spectra profile is observed for P1 and P2, suggesting that two kinds of polymer have the identical function groups in their molecular structures. FTIR spectra shows P1 and P2 are successfully synthesized.

Fig. 1 FTIR spectra of P1 and P2.

In order to gain more information about the exact structure of hyperbranched poly(amino ester), the NMR was used to further confirm the construction of hyperbranched poly(amino ester). The ¹H and ¹³C NMR spectra of P1 are showed in Fig. 2. In the ¹H NMR spectrum of P1 (Fig. 2(a)), the signal at δ = 3.3 ppm, corresponding to the methylene groups that adjacent to the tertiary amine groups. While the peak at δ = 3.0 ppm (fg) comes from the protons of methylene groups that next to the secondary amine, other signals are marked in this figure. In Fig. 2(b), we can see the characteristic C=O signal at 161.4 ppm (i), indicating the existence of carbonyl group on the surface of P1. The two carbon signals of the two types of methylene groups next to the tertiary amine at δ = 56.5 ppm (o) and 50.0 ppm (n) are attributed to transformation of part of the secondary amine formed into a tertiary amine, and the ascription of other carbons in P1 is marked (Fig. 2).

Fig. 2 (a) ¹H; (b) ¹³C NMR spectra of P1.

The P1 and P2 were dissolved in distilled water, and the optical properties of the solutions were measured. The UV-vis absorption spectra of P1 and P2 (Fig. 3) shows an evident absorption band centered at about 232 nm, which is assigned to π–π* transitions of carbonyl groups, and the absorption peak at around 285 nm is ascribed to n–π* transition of carbonyl groups. A shoulder peak at 339 nm is contributed to n–π transition of the tertiary amine groups.³⁰ The similar UV-vis absorption bands are observed both P1 and P2.

Fig. 3 UV-vis spectra of P1 and P2.

Fig. 4(a) illustrates the excitation and emission spectra of P1 in aqueous solution at 10 mg mL⁻¹. The maximum excitation and emission wavelengths are at about 363 and 440 nm, respectively. Fig. 4(b) shows the emission fluorescence spectra of P1 and P2 excited at 363 nm. It can be seen that the luminescent intensity of P1 is slightly stronger than that of P2, whose the emission wavelength shifts to about 451 nm when excited the same wavelength, may be due to the more rigid structure of P2. The reason is that the structure of P2 is more easily achieve congestion environment during the reaction process. Both P1 and P2 aqueous solutions show strong blue emission under 365 nm UV irradiation (see inset). It is noticed that the P1 and P2 without typical chromophores could show strong intrinsic fluorescence. Thus, exploring the source of the fluorescence of these polymers should be interesting. In the earlier reports, the tertiary amine or its oxidation has been considered as the emitting source.^14–21 More recently, a few of communications have demonstrated that the aggregation of carbonyl groups emit bright light. So the aggregation of carbonyl groups has been also regarded as the fluorescence center.^11,25 But, if there are exist both tertiary amines as well as carbonyl groups in the molecular structure, how about the mechanism of the fluorescence? The paper reported by D. Wu. et al. present that a coexistence of tertiary amines/carbonyl groups of hyperbranched polymer was the key to the fluorescence.² In this work, the synthesized hyperbranched poly(amino ester) (P1 and P2) not only have the tertiary amines but also the carbonyl groups in their structures. As we know, when the molecular structure of polymer becomes more crowded, the carbonyl groups tend to be gathered to a certain extent. And the aggregate carbonyl groups could be conducive to luminescence of polymer. Besides, we also mentioned that the tertiary amines are assigned to the fluorescence center for the luminescent polymers. In summary, a proposal was presented that the co-existence of tertiary amines and carbonyl groups play a critical role in producing the blue luminescent species.

Fig. 4 (a) Excitation and emission spectra of the P1 in aqueous solution at 10 mg mL⁻¹; (b) fluorescence spectra of the P1 and P2 aqueous solution (10 mg mL⁻¹); (c) fluorescence spectra of P1 at different concentration; (d) fluorescence spectra of the P1 at different pH, λ_ex = 363 nm.

In order to study the effect of P1 and P2 solution concentrations on the fluorescence, the fluorescence of the P1 and P2 with various concentrations were tested. Fig. 4(c) demonstrates that the fluorescence intensity of P1 increases quickly as increasing polymer concentrations. Because the more fluorescence groups of a luminophor is, the stronger its light emission will be. The P1 at high concentration possesses more tertiary amine units and C=O groups, thus the strong fluorescence for the P1 solution was observed. In the meantime, the relationship between fluorescence and concentration of P2 solutions was also investigated. It is found that the fluorescence intensity of P2 solutions enhances with the increase of concentrations of P2 solution (Fig. S3†).

To gain more insights into fluorescence properties, the effect of different pH on the fluorescence of polymers was studied. Fig. 4(d) displays that the fluorescence intensity of P1 increases gradually from pH 10 to 4, where the highest fluorescence intensity was achieved. However, as the pH was lowered further from 2 to 1, there appeared a decrease of fluorescence intensity. These results suggested that the fluorescence intensity of P1 is sensitive to the pH environment and shows the strong pH-dependence. The protonation of the tertiary amine groups is contributed to the increase of the fluorescence intensity of P1. On the contrary, the fluorescence intensity of P1 solution decreases under stronger acidic may be owing to the hydrolysis of ester bonds.

To check whether the hyperbranched poly(amino ester) shows fluorescence in the solid state, PVA films added P1 or P2 were studied. The strong blue fluorescence for PVA film with addition of P1 was also observed even with the naked eye under UV light, as displayed in Fig. 5. The PVA emission film with addition of P2 emits yellow-blue photoluminescence, whereas the as-produced pure PVA film did not emit light under UV lamp, as shown in Fig. S4.† To the best of our knowledge, non-radiative pathways can be hindered due to the restriction of intramolecular free rotations (RIR) in the solid or aggregated state,³¹ hence development of the radiative channels which enhance the fluorescence of PVA films. This phenomenon is very similar to the RIR process in aggregation-induced emission (AIE) system. Practically, intramolecular and intermolecular highly dense the clusters of carbonyl groups may be responsible for the AIE-active.

Fig. 5 PVA film with addition of P1 (a) UV light at 365 nm; (b) natural light.

The absolute quantum yield and fluorescence lifetimes of the pure P1 were measured on a steady/transient-state fluorescence spectrometer equipped with an integrating sphere (FLS980, Edinburgh Instruments), as presented in Fig. 6. As seen in Fig. 6(a), the excitation and emission bands of the pure P1 are, separately, centered at 461 and 563 nm wavelength, which was obviously different from those of the P1 aqueous solution may be owing to the solvent effect and concentration of P1. For light-emitting materials, the fluorescence lifetime is an inherent characteristic that depends on the nature of the fluorescent sites and the environment. The transient photoluminescence decay curve of the pure P1 was record at 563 nm after excitation at 461 nm (Fig. 6(b)), and the fluorescence lifetime (τ) is 6.83 ns. Importantly, the strongest luminescent was surveyed from the polymer P1, which the absolute quantum yield with a value of 8.66% under the excitation at 461 nm using multi scans (Fig. 6(c)).

Fig. 6 (a) Excitation spectra of the pure P1; (b) the transient photoluminescence decay curve of the pure P1 at 566 nm after excitation at 461 nm; (c) the absolute fluorescence quantum yield of the pure P1 excited at 461 nm.

The effect of external metal ions on the fluorescence intensity of the P1 was investigated with different metal cations, including FeCl₃·6H₂O, FeCl₂·4H₂O, CoCl₂·6H₂O NaCl, HgCl₂, MgSO₄, ZnSO₄·7H₂O, CuSO₄·5H₂O, Al(NO₃)₃·9H₂O, Zr(NO₃)₄·5H₂O. The fluorescence of the P1 solutions with different metal cations is showed in Fig. 7(a). Interestingly, Hg²⁺ and Fe³⁺ make the fluorescence intensity of the P1 solutions decrease greatly. Nevertheless, other cations have little effect on the fluorescence of the P1 solutions. It indicated that the fluorescence of the P1 solutions is extremely sensitive to Fe³⁺ and Hg²⁺. Besides, the effect of Hg²⁺ and Fe³⁺ concentration on the fluorescence of P1 was also tested. From the Fig. 7(b) and (c), it is found that the fluorescence of the P1 solutions decreases dramatically with increasing Hg²⁺ and Fe³⁺ concentration. When both Hg²⁺ and Fe³⁺ concentration reached at 10⁻² mol L⁻¹, the fluorescence of the P1 solutions decreases to almost zero. That is to say, Hg²⁺ and Fe³⁺ are strong fluorescence quenchers in the P1 solution. Based on the literatures,^19,32,33 different fluorescent quenching mechanisms were reported for Hg²⁺ and Fe³⁺. The reason of fluorescence quenching for Hg²⁺ is probably owing to the heavy atom effect. Generally, heavy atoms could quench the fluorescence because of the spin–orbit coupling effect of the solute induced by the high nuclear charge of heavy atoms. While the fluorescence quenching aroused by Fe³⁺ can be deciphered that the formation of the metal complex by virtue of Fe³⁺ having the biggest charge/radius ratio among the metal ions. As a result, Fe³⁺ readily take part in energy transfer or electron transfer processes, which causing a non-radiative channel. It should be pointed out that the P1–Fe³⁺ complex involves more non-radiative decay channels which remarkably led to the fluorescence quenching than Hg²⁺. Through the above experiments, we can draw a conclusion that the water-soluble fluorescent P1 as an effective metal probe for the Hg²⁺ and Fe³⁺ (Fig. 7).

Fig. 7 (a) Effects of metal ions (at a concentration of 10⁻⁴ mol L⁻¹) on the fluorescence spectra of P1 aqueous solutions at a concentration of 10 mg mL⁻¹; (b) Fe³⁺ on the fluorescence spectra of P1 aqueous solutions (at a concentration of 4 mg mL⁻¹); (c) Hg²⁺ on the fluorescence spectra of P1 aqueous solutions (at a concentration of 4 mg mL⁻¹).

7 Conclusions

In summary, we have successfully synthesized two novel water-soluble photoluminescent hyperbranched poly(amino ester) through one-pot A₂ + B₃ Michael addition reaction. The hyperbranched poly(amino ester) can emit strong blue fluorescence in their aqueous solution, and the fluorescent intensity increase with the increase of solution concentrations. Furthermore, the PVA films with addition of P1 or P2 show intense blue and yellow-blue fluorescence, respectively. Particularly, it was found that the fluorescence of P1 shows the pH-dependent as the increase of pH values. The absolute quantum yield and fluorescence lifetimes of the pure P1 were respectively 8.66% and 6.83 ns. Intriguingly, the fluorescence of P1 is extremely sensitive to Hg²⁺ and Fe³⁺, because of the heavy atom effect and the big charge/radius ratio, respectively. As a result, P1 could be employed as a potential and novel fluorescence probe for Hg²⁺ and Fe³⁺.

Acknowledgements

This work is supported by Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University (CX201626) and College students' innovative experimental project of China (201610699226).

References

1. W. I. Lee, Y. Bae and A. J. Bard, J. Am. Chem. Soc., 2004, 126, 8358–8359.
2. D. Wu, Y. Liu, C. He and H. G. Suat, Macromolecules, 2005, 38, 9906–9909.
3. S. Niu, H. X. Yan, Z. Y. Chen, L. X. Yuan, T. Y. Liu and C. Liu, Macromol. Rapid Commun., 2016, 37, 136–142.
4. S. Niu, H. X. Yan, S. Li, C. Tang, Z. Y. Chen, X. L. Zhi and P. L. Xu, J. Mater. Chem. C, 2016, 4, 6881–6893.
5. S. Niu, H. X. Yan, S. Li, P. Xu, X. L. Zhi and Z. Z. Li, Macromol. Chem. Phys., 2016, 217, 1185–1190.
6. S. Niu, H. X. Yan, Z. Y. Chen, S. Li, P. L. Xu and X. L. Zhi, Polym. Chem., 2016, 7, 3747–3755.
7. Y. Ji and Y. Qian, RSC Adv., 2014, 4, 58788–58794.
8. W. Yang, C. Y. Pan, M. D. Luo and H. B. Zhang, Biomacromolecules, 2010, 11, 1840–1846.
9. J. Mei, N. L. C. Leung, R. T. K. Kwok, W. Y. L. Jacky and B. Z. Tang, RSC Adv., 2015, 115, 11718–11940.
10. W. B. Wu, R. L. Tang, Q. Q. Li and Z. Li, Chem. Soc. Rev., 2015, 44, 3997–4022.
11. E. Zhao, W. Y. L. Jacky, L. Meng, Y. Hong, H. Q. Deng, G. X. Bai, X. H. Huang, J. H. Hao and B. Z. Tang, Macromolecules, 2015, 48, 64–71.
12. Y. Chen, A. J. H. Spiering, S. Karthikeyan, G. W. M. Peters, E. W. Meijer and P. P. Sijbesma, Nat. Chem., 2012, 4, 559–562.
13. M. Sun, C. Y. Hong and C. Y. Pan, J. Am. Chem. Soc., 2012, 134, 20581–20584.
14. K. Xiang, L. J. He, Y. N. Li, C. H. Xu and S. H. Li, RSC Adv., 2015, 5, 97224–97230.
15. B. Kang, M. M. Afifi, L. A. Austin and M. A. El-Sayed, ACS Nano, 2013, 7, 7420–7427.
16. D. G. Wang and T. Imae, J. Am. Chem. Soc., 2004, 126, 13204–13205.
17. M. J. Jasmine, M. Kavitha and E. Prasad, J. Lumin., 2009, 129, 506–513.
18. L. Pastor-Pérez, Y. Chen, Z. Shen, A. Lahoz and S. Stiriba, Macromol. Chem. Phys., 2007, 28, 1404–1409.
19. G. S. Song, Y. N. Lin, Z. C. Zhu, H. Y. Zheng, J. P. Qiao, C. C. He and H. L. Wang, Macromol. Rapid Commun., 2015, 36, 278–285.
20. G. Jayamurugan, C. P. Umesh and N. Jayamurugan, Organic Letters, 2008, 10, 9–12.
21. R. B. Restani, P. I. Morgado, M. P. Ribeiro, L. J. Correia, A. Aguiar-Ricardo and V. D. B. Bonifácio, Angew. Chem., Int. Ed., 2012, 51, 5162–5165.
22. W. Yang and C. Y. Pan, Macromol. Rapid Commun., 2009, 30, 2096–2101.
23. Y. Lin, J. W. Gao, H. W. Liu and Y. S. Li, Macromolecules, 2009, 42, 3237–3246.
24. S. F. Shiau, T. Y. Juang, H. W. Chou and M. Liang, Polymer, 2013, 54, 623–630.
25. H. Lu, L. L. Feng, S. S. Li, J. Zhang, H. F. Lu and S. Y. Feng, Macromolecules, 2015, 48, 476–482.
26. X. P. Miao, T. Liu, C. Zhang, X. X. Geng and Y. Meng, Phys. Chem. Chem. Phys., 2016, 18, 4295–4299.
27. K. Jiang, S. Sun and L. Zhang, Angew. Chem., Int. Ed., 2015, 54(18), 5360–5363.
28. Y. Chem, L. Z. Zhou, Y. Pang, W. Huang, F. Qiu, X. L. Jiang, X. Y. Zhu, D. Y. Yan and Q. Chen, Bioconjugate Chem., 2011, 22, 1162–1170.
29. M. Tajabadi, M. E. Khosroshahi and S. Bonakdar, Colloids Surf., A, 2013, 431, 18–26.
30. K. Jiang, S. Sun, L. Zhang, Y. H. Wang, C. Z. Cai and H. W. Lin, ACS Appl. Mater. Interfaces, 2015, 7, 23231–23238.
31. R. Hu, N. L. C. Leung and B. Z. Tang, Chem. Soc. Rev., 2014, 43, 4494–4562.
32. Y. Zhang, L. Wang, H. Zhang, Y. Liu, H. Wang, Z. Kang and S. Lee, RSC Adv., 2013, 3, 3733–3738.
33. K. Seehafer, M. Bender and S. T. Schwaebel, Macromolecules, 2014, 47, 7014–7020.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19062f

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