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FRET-based rhodamine–coumarin conjugate as a Fe3+ selective ratiometric fluorescent sensor in aqueous media
Tetrahedron Letters xxx (2015) xxx–xxx
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FRET-based rhodamine–coumarin conjugate as a Fe3+ selective ratiometric fluorescent sensor in aqueous media Jing-can Qin, Zheng-yin Yang ⇑, Guan-qun Wang, Chao-rui Li College of Chemistry and Chemical Engineering, State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e
i n f o
Article history: Received 14 May 2015 Revised 5 July 2015 Accepted 6 July 2015 Available online xxxx Keywords: Fluorescent sensor Ratiometric Rhodamine/coumarin Fe3+ FRET
a b s t r a c t In this study, a novel ratiometric fluorescent sensor (HL) for Fe3+ based on the conjugation of rhodamine and coumarin has been designed and synthesized. The free sensor displays fluorescence emission at 475 nm, on the addition of Fe3+ to an aqueous solution of HL, the sensor shows significant fluorescence enhancement at 550 nm which should be attributed to an intramolecular fluorescence resonance energy transfer (FRET) mechanism from coumarin to Rhodamine 6G. Ó 2015 Elsevier Ltd. All rights reserved.
Introduction The development of the fluorescent probe with high selectivity, sensitivity for metal ions has attracted more and more attention because of their potential applications in medicinal and environmental research.1–5 Fe3+ is one of the most essential metal ions and plays a pivotal role for many living organisms by participating in a wide range of metabolic processes such as cellular metabolism, enzyme catalysis, and as an oxygen carrier in hemoglobin.6–8 Less iron in the body can cause diabetes, anemia, liver and kidney damages, and heart diseases.9,10 On the other hand, as a potentially toxic element, the superfluous ingestion of Fe3+ is suspected to be associated with an increased incidence of certain cancers and organ dysfunction.11–14 For the reason, in order to protect the environment and human health, it is necessary to develop an effective fluorescent probe for detection of excessive amounts of Fe3+. To date, there are several fluorescent probes for detection of Fe3+ reported, however, the design of them is mainly based on the change of a single emission intensity such as PET (photoinduced electron transfer) in which the simple ‘off–on’ fluorescent response is easy to be affected by instrumental efficiency and environmental condition.15–18 Ratiometric probes which can eliminate those deficiencies through
⇑ Corresponding author. Tel.: +86 931 8913515; fax: +86 931 8912582. E-mail address: [email protected] (Z.-y. Yang).
simultaneous recording ratio signals of two emissions at different wavelengths are still few in number. In general, there are mostly two mechanisms which can be applied to the design of ratiometric fluorescent probes: intramolecular charge transfer (ICT) and fluorescence (or Forster) resonance energy transfer (FRET), compared with the former, the latter will be able to better determine to the ratio of the two fluorescence emissions.19–23 Fluorescence resonance energy transfer (FRET) is a distance-dependent interaction between the electronic excited states of two different fluorescent groups in which excitation is transferred from a donor moiety to an acceptor moiety without photoemission. The efficiency of FRET is primarily controlled by the scope of spectral overlap between donor emission with acceptor absorption and the distance between donor and acceptor.24–26 Therefore, it can be conceived that Schiff-base containing coumarin (energy donor) and rhodamine (energy acceptor) moieties show extraordinary fluorescence properties and will satisfy a prerequisite for the design of the efficient FRET-based fluorescent ratiometric probe. Out of consideration of these circumstances, we havedesigned and synthesized a FRET-based fluorescent ratiometric chemosensor for Fe3+ in which rhodamine and coumarin moieties are linked by glyoxal. Upon the addition of Fe3+, the sensor exhibits a strong, increasing fluorescent emission centered at 550 nm at the expense of the fluorescent emission of HL centered at 475 nm in ethanol– water solutions (9:1, v/v, Tris–HCl, pH = 7.4). In addition, the presence of other relevant metal ions has almost no influence on the
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fluorescence enhancement. More importantly, the recognition process of HL is reversible by adding a Fe3+ bonding agent Na2EDTA. Experimental Apparatus and reagents All chemicals were obtained from commercial suppliers and used without further purification. 1H NMR spectra were measured on the JNM-ECS 400 MHz instruments using TMS as an internal standard. Bruker esquire 6000 spectrometer, Perkin Elmer Lambda 35 UV–Vis spectrophotometer, Hitachi RF-5301 spectrophotometer equipped with quartz cuvettes of 1 cm path length, Beijing XT4-100 microscopic melting point apparatus. IR spectra were obtained in KBr disks on a Therrno Mattson FT-IR spectrometer in the 4000–400 cm1 region. Analysis The solutions of metal ions (5 mM) were prepared from NaNO3, KNO3, Ca(NO3)2, Mg(NO3)2, Al(NO3)3, CrCl3, FeCl3, FeCl2, CoCl2, HgCl2, Ni(NO3)2, Cu(NO3)2, Pb(NO3)2, Zn(NO3)2, LiNO3, Cd(NO3)2, Mn(NO3)2, BaCl2 respectively, and were dissolved in distilled water. Tris–HCl buffer stock solution prepared using 10 mM, 10 mM Tris and proper amount of HCl, A stock solution of HL (1 mM) was prepared in DMSO. The solution of HL was then diluted to 10 lM under test. For fluorescence measurements, both the excitation and emission slit widths were 3 nm. Synthesis Rhodamine 6G hydrazide (Fig. S1) was synthesized according to the method reported.27 The sensor was synthesized according to the route as shown in Scheme 1.
Synthesis of compound RG128,29 Rhodamine hydrazide (0.86 g, 2 mmol) was dispersed in 20 mL ethanol, excess of glyoxal (40%, 1.16 g, 8 mmol) was added, and the mixture was stirred for 8 h at room temperature. The precipitate produced was filtered and washed 3 times with 10 mL cold ethanol. After drying under reduced pressure, the reaction yielded 0.77 g RG1 as yellow solid. Yield: 76.3%, 1HNMR (400 MHz; CDCl3) (Fig. S2) d (ppm) = 9.40 (d, 1H, J = 7.5 Hz, CH@O), 8.05 (dd, 1H, Ar-H J = 6.9 Hz, J = 7.4 Hz), 7.54 (m, 2H, Ar-H), 7.27 (d, 1H, J = 7.5 Hz, CH@N), 7.03 (m, 1H, Ar-H), 6. 37 (s, 2H, xanthene-H), 6.24 (s, 2H, xanthene-H), 3.54 (s, NHCH2CH3), 3.24–3.29 (q, J = 7.1 Hz, 4H, NCH2CH3), 1.87 (s, 6H, xanthene-CH3) 1.29–1.32 (t, J = 7.1 Hz, 6H, NCH2CH3). Synthesis of 7-diethylaminocoumarin-3-carbohydrazide30,31 4-diethylaminosalicylaldehyde (1.93 g, 10 mmol), diethylmalonate (3.2 g, 20 mmol), and piperidine (1 mL) were stirred for 6 h under reflux in ethanol (30 mL). The solution was poured into 100 ml ice water at room temperature. After filtration, Hydrazine Monohydrate (80%, 2 mL) was added to an ethanol (30 mL) solution of the residues (2.5 g, 8.64 mmol) and the reaction mixture was stirred at ambient temperature for 25 min, The resulting precipitate was filtered and washed 3 times with ethanol (3 20 mL) and further dried in vacuo to give the final product as yellow powder 1. Yield: 45%. Mp: 173–174 °C, 1H NMR (400 MHz; CDCl3) (Fig. S3) d (ppm) = 9.71 (s, 1H), 8.86 (s, 1H), 7.41 (d, J = 8.8 Hz 1H), 6.63 (d, J = 8.8 Hz 1H), 6.47 (s, 1H), 4.06 (s, 2H), 3.44 (q, J = 7.1 Hz, 4H), 1.22 (t, J = 7.1 Hz, 6H). Synthesis of HL An ethanol solution (20 mL) of 7-diethylaminocoumarin-3-carbohydrazide (1 mmol, 0.28 g) was added to another ethanol (20 mL) containing RG1 (1 mmol, 0.46 g). Then the solution was refluxed for 8 h under stirring, the precipitate produced was
Scheme 1. Reagents and conditions: (a) EtOH, diethylmalonate, piperidine, reflux, 6 h; (b) EtOH, N2H4H2O, at room temperature,0.5 h; (c) EtOH, N2H4H2O, reflux, 20 h; (d) EtOH, glyoxal, 40%, at room temperature, 8 h; (e) EtOH, reflux, 8 h.
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filtered, and washed 3 times with 10 mL hot ethanol. After drying under reduced pressure, the reaction afforded HL as a yellow solid. Yield: 60.4%. 1H NMR (400 MHz; CDCl3) (Fig. S4) d (ppm) = 11.83 (s, 1H), 8.72 (s, 1H), 7.99 (d, J = 7.2 Hz 1H), 7.76–7.82 (m, 2H), 7.37– 7.45 (m, 3H), 7.01 (d, J = 7.1 Hz 1H), 6.62 (dd, J = 9.0 Hz, J = 2.1 Hz, 1H), 6.45 (s, 1H), 6.38 (s, 2H), 6.32 (s, 2H), 3.54 (s, 2H), 3.44 (q, J = 7.1 Hz, 4H), 3.24–3.29 (q, J = 7.1 Hz, 4H), 1.87 (s, 6H), 1.29–1.32 (t, J = 7.1 Hz, 6H), 1.22 (t, J = 7.1 Hz, 6H). 13C NMR (CDCl3, 100 MHz) (Fig. S5): 206.68, 165.85, 162.52, 160.13, 157.99, 153.68, 153.22, 150.93, 149.10, 148.65, 147.75, 142.73, 134.09, 131.38, 128.19, 123.81, 123.37, 118.16, 110.37, 108.69, 105.39, 97.67, 96.57, 65.92, 45.13, 38.48, 16.85, 14.78, 12.51. IR (KBr, cm1) (Fig. S6): 3415, 1702, 1619, 1591, ESI-MS: [M+1]+: 726.27. [M+Na]+: 748.25. (Fig. S7) Elemental analysis: C42H43N7O5, Found: C, 69.84.; H, 5.79; N, 13.76. Calcd: C, 69.50; H, 5.97; N, 13.51. Results and discussion Absorption spectroscopic studies on metal ions The optical behavior of HL was initially studied using UV–Vis as a function of the concentration of Fe3+ in ethanol–water solutions (9:1, v/v, Tris–HCl, pH = 7.4). As shown in Figure 1, in the absence of Fe3+, the receptor only exhibited a maximum absorption wavelength at 439 nm which corresponded to the absorption band of coumarin moiety.32–34 Moreover, the characteristic absorption band of Rhodamine 6G at about 530 nm was not observed, demonstrating its existence in spirolactam form.35,36 However, upon addition of Fe3+ to an aqueous solution of HL, the absorption band at 425 nm slightly decreased while the absorption intensity exhibited significantly increased in the range of 385–475 nm. Besides the change of coumarin’s absorption band, a new absorption band appeared at 525 nm with increasing intensity which was ascribed to ring opened rhodamine moieties. This indicated that the interaction of HL with Fe3+ could trigger the formation of the ring-opened form of HL from the spirolactam form. Selectivity studies and effects of metal ions The effect of Fe3+ on the fluorescence properties of HL was also investigated in ethanol–water solutions (9:1, v/v, Tris–HCl,
Figure 2. Fluorescence spectra of HL (10 lM) upon the addition of metal salts (20.0 equiv) of Li+, Na+, K+, Ca2+, Mg2+, Cu2+, Co2+, Mn2+, Ni2+, Zn2+, Ba2+, Fe2+, Cd2+, Hg2+, Pb2+, Cr3+, and Fe3+ in ethanol–water solutions (9:1, v/v, Tris–HCl, pH = 7.4). Excitation wavelength was 450 nm, slit = 3 nm/3 nm.
pH = 7.4). As shown in Figure 2, the free sensor displayed fluorescence emission at 475 nm, attributable to the characteristic signals of coumarin and the characteristic emission of Rhodamine 6G at 550 nm not appeared, which also indicated the rhodamine core was in the ring closed isomeric form (Ex = 450 nm). On addition of various metal ions including Li+, Na+, K+, Ca2+, Mg2+, Cu2+, Co2+, Mn2+, Ni2+, Zn2+, Ba2+, Fe2+, Cd2+, Hg2+, Pb2+, Cr3+ and Fe3+ to aqueous solution of HL, there was no significant change in its fluorescence spectrum except in the presence of Fe3+. Upon the addition of Fe3+, the receptor exhibited remarkable fluorescence enhancement at 550 nm at the expense of the fluorescent emission at 475 nm. The change reflected on the fluorescent colors, one was green, the other was yellow. These phenomena further confirmed the addition of Fe3+ promoted the ring-opened reaction of the Rhodamine 6G spirolactam.
The complexation of HL with Fe3+ In order to further validate the stoichiometry of HL and Fe3+ Job’s method for absorbance measurement was carried out. The total concentration of HL and Fe3+ was 400 lM. XL = ([HL]/([Mn+] + [HL]. As shown in Figure 3, the maximum point appeared at a mole fraction of 0.5. The result indicated that they are a 1:1 stoichiometry of the binding mode of HL with Fe3+ which was further confirmed by the appearance of a peak at m/z 815.81 assignable to [HL+Fe3++ClH+]+ (Fig. S8) in the ESI/MS. Since the formation of 1:1 ligand–metal complex was confirmed by Job’s plot analysis and ESI/MS, in combination with the fluorescence titration (Fig. 4), the binding constant values (KL–Fe = 5.2 104) were determined based on the modified Benesi–Hildebrand equation (Fig. S9).37,38
1=ðF x F 0 Þ ¼ 1=ðF max F 0 Þ þ ð1=K½CÞð1=ðF max F 0 ÞÞ
Figure 1. Changes in the absorption spectra of HL (20 lM) in ethanol–water solutions (9:1, v/v, Tris–HCl, pH = 7.2) at room temperature upon addition of different amounts of Fe3+ ions.
where F0, Fx, and Fmax are the emission intensities (at 550 nm) of the organic moiety considered in the absence of metal ion, at an intermediate metal ion concentration, and at a concentration of complete interaction, respectively, and where K is the binding constant concentration.
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Figure 3. Job’s plot for determining the stoichiometry of HL and Fe3+ in ethanol– water solutions (9:1, v/v, Tris–HCl, pH = 7.4) (XFe = [Fe3+]/([Fe3+] + [HL]), the total concentration of HL and Fe3+ was 400 lM).
Figure 4. Fluorescence spectra of HL (10 lM) in ethanol–water solutions (9:1, v/v, Tris-HCl, pH = 7.4) upon the addition of Fe3+ (0–40 equiv) (kex = 450 nm, slit = 3 nm/ 3 nm).
Figure 5. Fluorescence intensity (at 550 nm) of HL and its complexation with Fe3+ in the presence of various metal ions in ethanol–water solutions (9:1, v/v, Tris–HCl, pH = 7.2), Red bar: HL (10.0 lM) and HL with 20.0 equiv of Li+, Na+, K+, Ca2+, Mg2+, Cu2+, Co2+, Mn2+, Ni2+, Zn2+, Ba2+, Fe2+, Cd2+, Hg2+, Pb2+, and Cr3+ stated. Green bar: 10.0 lM of HL and 20.0 equiv of Fe3+ with 20.0 equiv of metal ions stated (kex = 450 nm, slit = 3 nm/3 nm).
Figure 6. Fluorescence intensity (at 550 nm) of HL (10.0 lM) as a function of Fe3+ concentration in ethanol–water solutions (9:1, v/v, Tris–HCl, pH = 7.4) solutions (1) HL, (2) HL+Fe3+, (3) HL + Fe3+ + EDTANa2, (4) HL + Fe3+ + EDTANa2 + Fe3+ (Ex = 450 nm, slit = 3 nm).
The practical applicability of HL To check the practical applicability of HL as a selective fluorescent sensor for Fe3+, two experiments should be carried out, one was competition experiment, as shown in Figure 5, we found that all the coexistent metal ions had no obvious interference with the detection of Fe3+, the other was the investigation of reversibility which was also a prerequisite in developing fluorescent probe for practical application. The reversibility of the recognition process of HL was performed by adding a bonding agent, Na2EDTA. As shown in Figure 6, the addition of Na2EDTA to a mixture of HL and Fe3+ resulted in diminution of the fluorescence intensity at 550 nm, which indicated the regeneration of the free sensor HL. Therefore, it meant that the receptor HL could be used as a selective fluorescent sensor for detection and recognition of Fe3+ in such fields of environmental analysis.
In addition, the theoretical detection limit was calculated from the fluorescence titration, according to the following equation: detection limit 3r/k, the detection limit reached at 4.05 106 M (Fig. S10) where r was the standard deviation of blank measurements, and K was the slope between intensity versus sample concentration.39 The proposed mechanism According to the above experiments, the Fluorescence (or Forster) resonance energy transfer (FRET) was proposed to explain the fluorescence responses of the sensor to Fe3+ as shown in Scheme 2.40–44 More specifically, before addition of Fe3+, the
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Scheme 2. Proposed mechanism for detection of Fe3+ and by HL.
receptor only showed an emission band of the coumarin at 475 nm, upon addition of Fe3+, a new emission band with a maximum at 550 nm appeared ascribed to Fe3++ induced opening of the spirocyclic ring of the Rhodamine moiety. Thus, the excitation energy is passed from coumarin moiety to the ‘opened-up’ Rhodamine moiety, namely, the FRET process of HL is triggered by Fe3+, as a result, the emission at 475 nm decreased, and a significant enhancement of the characteristic fluorescence of rhodamine at 550 nm emerged quickly. Conclusion In summary, we have successfully developed a novel FRETbased fluorescent ratiometric chemosensor for recognition of Fe3+ based on an asymmetric bis-Schiff in ethanol–water solutions (9:1, v/v, Tris–HCl, pH = 7.4). Moreover, the lower detection limit for Fe3+ reached the 106 M level which is sufficiently low to enable the detection of micromolar concentrations of Fe3+ in many chemical and biological systems. Acknowledgments This work is supported by the National Natural Science Foundation of China (81171337), Gansu NSF (1308RJZA115). Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.07. 023. References and notes 1. Upadhyay, K. K.; Kumar, A. Org. Biomol. Chem. 2010, 8, 4892–4897. 2. Han, T. Y.; Feng, X.; Tong, B.; Shi, J. B.; Chen, L.; Zhi, J. G.; Dong, Y. P. Chem. Commun. 2012, 416–418. 3. Lin, H. Y.; Cheng, P. Y.; Wan, C. F.; Wu, A. T. Analyst 2012, 137, 4415–4417. 4. Shi, X. Y.; Wang, H.; Han, T. Y.; Feng, X.; Tong, B.; Shi, J. B.; Zhi, J.; Dong, Y. P. J. Mater. Chem. 2013, 98, 42–50. 5. He, B. Chem. Sci. 2015, 6, 3180–3186. 6. Lee, J. W.; Helmann, J. D. Nature 2006, 440, 363–367.
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FRET-based rhodamine–coumarin conjugate as a Fe3+ selective ratiometric fluorescent sensor in aqueous media Jing-can Qin, Zheng-yin Yang ⇑, Guan-qun Wang, Chao-rui Li College of Chemistry and Chemical Engineering, State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e
i n f o
Article history: Received 14 May 2015 Revised 5 July 2015 Accepted 6 July 2015 Available online xxxx Keywords: Fluorescent sensor Ratiometric Rhodamine/coumarin Fe3+ FRET
a b s t r a c t In this study, a novel ratiometric fluorescent sensor (HL) for Fe3+ based on the conjugation of rhodamine and coumarin has been designed and synthesized. The free sensor displays fluorescence emission at 475 nm, on the addition of Fe3+ to an aqueous solution of HL, the sensor shows significant fluorescence enhancement at 550 nm which should be attributed to an intramolecular fluorescence resonance energy transfer (FRET) mechanism from coumarin to Rhodamine 6G. Ó 2015 Elsevier Ltd. All rights reserved.
Introduction The development of the fluorescent probe with high selectivity, sensitivity for metal ions has attracted more and more attention because of their potential applications in medicinal and environmental research.1–5 Fe3+ is one of the most essential metal ions and plays a pivotal role for many living organisms by participating in a wide range of metabolic processes such as cellular metabolism, enzyme catalysis, and as an oxygen carrier in hemoglobin.6–8 Less iron in the body can cause diabetes, anemia, liver and kidney damages, and heart diseases.9,10 On the other hand, as a potentially toxic element, the superfluous ingestion of Fe3+ is suspected to be associated with an increased incidence of certain cancers and organ dysfunction.11–14 For the reason, in order to protect the environment and human health, it is necessary to develop an effective fluorescent probe for detection of excessive amounts of Fe3+. To date, there are several fluorescent probes for detection of Fe3+ reported, however, the design of them is mainly based on the change of a single emission intensity such as PET (photoinduced electron transfer) in which the simple ‘off–on’ fluorescent response is easy to be affected by instrumental efficiency and environmental condition.15–18 Ratiometric probes which can eliminate those deficiencies through
⇑ Corresponding author. Tel.: +86 931 8913515; fax: +86 931 8912582. E-mail address: [email protected] (Z.-y. Yang).
simultaneous recording ratio signals of two emissions at different wavelengths are still few in number. In general, there are mostly two mechanisms which can be applied to the design of ratiometric fluorescent probes: intramolecular charge transfer (ICT) and fluorescence (or Forster) resonance energy transfer (FRET), compared with the former, the latter will be able to better determine to the ratio of the two fluorescence emissions.19–23 Fluorescence resonance energy transfer (FRET) is a distance-dependent interaction between the electronic excited states of two different fluorescent groups in which excitation is transferred from a donor moiety to an acceptor moiety without photoemission. The efficiency of FRET is primarily controlled by the scope of spectral overlap between donor emission with acceptor absorption and the distance between donor and acceptor.24–26 Therefore, it can be conceived that Schiff-base containing coumarin (energy donor) and rhodamine (energy acceptor) moieties show extraordinary fluorescence properties and will satisfy a prerequisite for the design of the efficient FRET-based fluorescent ratiometric probe. Out of consideration of these circumstances, we havedesigned and synthesized a FRET-based fluorescent ratiometric chemosensor for Fe3+ in which rhodamine and coumarin moieties are linked by glyoxal. Upon the addition of Fe3+, the sensor exhibits a strong, increasing fluorescent emission centered at 550 nm at the expense of the fluorescent emission of HL centered at 475 nm in ethanol– water solutions (9:1, v/v, Tris–HCl, pH = 7.4). In addition, the presence of other relevant metal ions has almost no influence on the
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fluorescence enhancement. More importantly, the recognition process of HL is reversible by adding a Fe3+ bonding agent Na2EDTA. Experimental Apparatus and reagents All chemicals were obtained from commercial suppliers and used without further purification. 1H NMR spectra were measured on the JNM-ECS 400 MHz instruments using TMS as an internal standard. Bruker esquire 6000 spectrometer, Perkin Elmer Lambda 35 UV–Vis spectrophotometer, Hitachi RF-5301 spectrophotometer equipped with quartz cuvettes of 1 cm path length, Beijing XT4-100 microscopic melting point apparatus. IR spectra were obtained in KBr disks on a Therrno Mattson FT-IR spectrometer in the 4000–400 cm1 region. Analysis The solutions of metal ions (5 mM) were prepared from NaNO3, KNO3, Ca(NO3)2, Mg(NO3)2, Al(NO3)3, CrCl3, FeCl3, FeCl2, CoCl2, HgCl2, Ni(NO3)2, Cu(NO3)2, Pb(NO3)2, Zn(NO3)2, LiNO3, Cd(NO3)2, Mn(NO3)2, BaCl2 respectively, and were dissolved in distilled water. Tris–HCl buffer stock solution prepared using 10 mM, 10 mM Tris and proper amount of HCl, A stock solution of HL (1 mM) was prepared in DMSO. The solution of HL was then diluted to 10 lM under test. For fluorescence measurements, both the excitation and emission slit widths were 3 nm. Synthesis Rhodamine 6G hydrazide (Fig. S1) was synthesized according to the method reported.27 The sensor was synthesized according to the route as shown in Scheme 1.
Synthesis of compound RG128,29 Rhodamine hydrazide (0.86 g, 2 mmol) was dispersed in 20 mL ethanol, excess of glyoxal (40%, 1.16 g, 8 mmol) was added, and the mixture was stirred for 8 h at room temperature. The precipitate produced was filtered and washed 3 times with 10 mL cold ethanol. After drying under reduced pressure, the reaction yielded 0.77 g RG1 as yellow solid. Yield: 76.3%, 1HNMR (400 MHz; CDCl3) (Fig. S2) d (ppm) = 9.40 (d, 1H, J = 7.5 Hz, CH@O), 8.05 (dd, 1H, Ar-H J = 6.9 Hz, J = 7.4 Hz), 7.54 (m, 2H, Ar-H), 7.27 (d, 1H, J = 7.5 Hz, CH@N), 7.03 (m, 1H, Ar-H), 6. 37 (s, 2H, xanthene-H), 6.24 (s, 2H, xanthene-H), 3.54 (s, NHCH2CH3), 3.24–3.29 (q, J = 7.1 Hz, 4H, NCH2CH3), 1.87 (s, 6H, xanthene-CH3) 1.29–1.32 (t, J = 7.1 Hz, 6H, NCH2CH3). Synthesis of 7-diethylaminocoumarin-3-carbohydrazide30,31 4-diethylaminosalicylaldehyde (1.93 g, 10 mmol), diethylmalonate (3.2 g, 20 mmol), and piperidine (1 mL) were stirred for 6 h under reflux in ethanol (30 mL). The solution was poured into 100 ml ice water at room temperature. After filtration, Hydrazine Monohydrate (80%, 2 mL) was added to an ethanol (30 mL) solution of the residues (2.5 g, 8.64 mmol) and the reaction mixture was stirred at ambient temperature for 25 min, The resulting precipitate was filtered and washed 3 times with ethanol (3 20 mL) and further dried in vacuo to give the final product as yellow powder 1. Yield: 45%. Mp: 173–174 °C, 1H NMR (400 MHz; CDCl3) (Fig. S3) d (ppm) = 9.71 (s, 1H), 8.86 (s, 1H), 7.41 (d, J = 8.8 Hz 1H), 6.63 (d, J = 8.8 Hz 1H), 6.47 (s, 1H), 4.06 (s, 2H), 3.44 (q, J = 7.1 Hz, 4H), 1.22 (t, J = 7.1 Hz, 6H). Synthesis of HL An ethanol solution (20 mL) of 7-diethylaminocoumarin-3-carbohydrazide (1 mmol, 0.28 g) was added to another ethanol (20 mL) containing RG1 (1 mmol, 0.46 g). Then the solution was refluxed for 8 h under stirring, the precipitate produced was
Scheme 1. Reagents and conditions: (a) EtOH, diethylmalonate, piperidine, reflux, 6 h; (b) EtOH, N2H4H2O, at room temperature,0.5 h; (c) EtOH, N2H4H2O, reflux, 20 h; (d) EtOH, glyoxal, 40%, at room temperature, 8 h; (e) EtOH, reflux, 8 h.
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filtered, and washed 3 times with 10 mL hot ethanol. After drying under reduced pressure, the reaction afforded HL as a yellow solid. Yield: 60.4%. 1H NMR (400 MHz; CDCl3) (Fig. S4) d (ppm) = 11.83 (s, 1H), 8.72 (s, 1H), 7.99 (d, J = 7.2 Hz 1H), 7.76–7.82 (m, 2H), 7.37– 7.45 (m, 3H), 7.01 (d, J = 7.1 Hz 1H), 6.62 (dd, J = 9.0 Hz, J = 2.1 Hz, 1H), 6.45 (s, 1H), 6.38 (s, 2H), 6.32 (s, 2H), 3.54 (s, 2H), 3.44 (q, J = 7.1 Hz, 4H), 3.24–3.29 (q, J = 7.1 Hz, 4H), 1.87 (s, 6H), 1.29–1.32 (t, J = 7.1 Hz, 6H), 1.22 (t, J = 7.1 Hz, 6H). 13C NMR (CDCl3, 100 MHz) (Fig. S5): 206.68, 165.85, 162.52, 160.13, 157.99, 153.68, 153.22, 150.93, 149.10, 148.65, 147.75, 142.73, 134.09, 131.38, 128.19, 123.81, 123.37, 118.16, 110.37, 108.69, 105.39, 97.67, 96.57, 65.92, 45.13, 38.48, 16.85, 14.78, 12.51. IR (KBr, cm1) (Fig. S6): 3415, 1702, 1619, 1591, ESI-MS: [M+1]+: 726.27. [M+Na]+: 748.25. (Fig. S7) Elemental analysis: C42H43N7O5, Found: C, 69.84.; H, 5.79; N, 13.76. Calcd: C, 69.50; H, 5.97; N, 13.51. Results and discussion Absorption spectroscopic studies on metal ions The optical behavior of HL was initially studied using UV–Vis as a function of the concentration of Fe3+ in ethanol–water solutions (9:1, v/v, Tris–HCl, pH = 7.4). As shown in Figure 1, in the absence of Fe3+, the receptor only exhibited a maximum absorption wavelength at 439 nm which corresponded to the absorption band of coumarin moiety.32–34 Moreover, the characteristic absorption band of Rhodamine 6G at about 530 nm was not observed, demonstrating its existence in spirolactam form.35,36 However, upon addition of Fe3+ to an aqueous solution of HL, the absorption band at 425 nm slightly decreased while the absorption intensity exhibited significantly increased in the range of 385–475 nm. Besides the change of coumarin’s absorption band, a new absorption band appeared at 525 nm with increasing intensity which was ascribed to ring opened rhodamine moieties. This indicated that the interaction of HL with Fe3+ could trigger the formation of the ring-opened form of HL from the spirolactam form. Selectivity studies and effects of metal ions The effect of Fe3+ on the fluorescence properties of HL was also investigated in ethanol–water solutions (9:1, v/v, Tris–HCl,
Figure 2. Fluorescence spectra of HL (10 lM) upon the addition of metal salts (20.0 equiv) of Li+, Na+, K+, Ca2+, Mg2+, Cu2+, Co2+, Mn2+, Ni2+, Zn2+, Ba2+, Fe2+, Cd2+, Hg2+, Pb2+, Cr3+, and Fe3+ in ethanol–water solutions (9:1, v/v, Tris–HCl, pH = 7.4). Excitation wavelength was 450 nm, slit = 3 nm/3 nm.
pH = 7.4). As shown in Figure 2, the free sensor displayed fluorescence emission at 475 nm, attributable to the characteristic signals of coumarin and the characteristic emission of Rhodamine 6G at 550 nm not appeared, which also indicated the rhodamine core was in the ring closed isomeric form (Ex = 450 nm). On addition of various metal ions including Li+, Na+, K+, Ca2+, Mg2+, Cu2+, Co2+, Mn2+, Ni2+, Zn2+, Ba2+, Fe2+, Cd2+, Hg2+, Pb2+, Cr3+ and Fe3+ to aqueous solution of HL, there was no significant change in its fluorescence spectrum except in the presence of Fe3+. Upon the addition of Fe3+, the receptor exhibited remarkable fluorescence enhancement at 550 nm at the expense of the fluorescent emission at 475 nm. The change reflected on the fluorescent colors, one was green, the other was yellow. These phenomena further confirmed the addition of Fe3+ promoted the ring-opened reaction of the Rhodamine 6G spirolactam.
The complexation of HL with Fe3+ In order to further validate the stoichiometry of HL and Fe3+ Job’s method for absorbance measurement was carried out. The total concentration of HL and Fe3+ was 400 lM. XL = ([HL]/([Mn+] + [HL]. As shown in Figure 3, the maximum point appeared at a mole fraction of 0.5. The result indicated that they are a 1:1 stoichiometry of the binding mode of HL with Fe3+ which was further confirmed by the appearance of a peak at m/z 815.81 assignable to [HL+Fe3++ClH+]+ (Fig. S8) in the ESI/MS. Since the formation of 1:1 ligand–metal complex was confirmed by Job’s plot analysis and ESI/MS, in combination with the fluorescence titration (Fig. 4), the binding constant values (KL–Fe = 5.2 104) were determined based on the modified Benesi–Hildebrand equation (Fig. S9).37,38
1=ðF x F 0 Þ ¼ 1=ðF max F 0 Þ þ ð1=K½CÞð1=ðF max F 0 ÞÞ
Figure 1. Changes in the absorption spectra of HL (20 lM) in ethanol–water solutions (9:1, v/v, Tris–HCl, pH = 7.2) at room temperature upon addition of different amounts of Fe3+ ions.
where F0, Fx, and Fmax are the emission intensities (at 550 nm) of the organic moiety considered in the absence of metal ion, at an intermediate metal ion concentration, and at a concentration of complete interaction, respectively, and where K is the binding constant concentration.
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Figure 3. Job’s plot for determining the stoichiometry of HL and Fe3+ in ethanol– water solutions (9:1, v/v, Tris–HCl, pH = 7.4) (XFe = [Fe3+]/([Fe3+] + [HL]), the total concentration of HL and Fe3+ was 400 lM).
Figure 4. Fluorescence spectra of HL (10 lM) in ethanol–water solutions (9:1, v/v, Tris-HCl, pH = 7.4) upon the addition of Fe3+ (0–40 equiv) (kex = 450 nm, slit = 3 nm/ 3 nm).
Figure 5. Fluorescence intensity (at 550 nm) of HL and its complexation with Fe3+ in the presence of various metal ions in ethanol–water solutions (9:1, v/v, Tris–HCl, pH = 7.2), Red bar: HL (10.0 lM) and HL with 20.0 equiv of Li+, Na+, K+, Ca2+, Mg2+, Cu2+, Co2+, Mn2+, Ni2+, Zn2+, Ba2+, Fe2+, Cd2+, Hg2+, Pb2+, and Cr3+ stated. Green bar: 10.0 lM of HL and 20.0 equiv of Fe3+ with 20.0 equiv of metal ions stated (kex = 450 nm, slit = 3 nm/3 nm).
Figure 6. Fluorescence intensity (at 550 nm) of HL (10.0 lM) as a function of Fe3+ concentration in ethanol–water solutions (9:1, v/v, Tris–HCl, pH = 7.4) solutions (1) HL, (2) HL+Fe3+, (3) HL + Fe3+ + EDTANa2, (4) HL + Fe3+ + EDTANa2 + Fe3+ (Ex = 450 nm, slit = 3 nm).
The practical applicability of HL To check the practical applicability of HL as a selective fluorescent sensor for Fe3+, two experiments should be carried out, one was competition experiment, as shown in Figure 5, we found that all the coexistent metal ions had no obvious interference with the detection of Fe3+, the other was the investigation of reversibility which was also a prerequisite in developing fluorescent probe for practical application. The reversibility of the recognition process of HL was performed by adding a bonding agent, Na2EDTA. As shown in Figure 6, the addition of Na2EDTA to a mixture of HL and Fe3+ resulted in diminution of the fluorescence intensity at 550 nm, which indicated the regeneration of the free sensor HL. Therefore, it meant that the receptor HL could be used as a selective fluorescent sensor for detection and recognition of Fe3+ in such fields of environmental analysis.
In addition, the theoretical detection limit was calculated from the fluorescence titration, according to the following equation: detection limit 3r/k, the detection limit reached at 4.05 106 M (Fig. S10) where r was the standard deviation of blank measurements, and K was the slope between intensity versus sample concentration.39 The proposed mechanism According to the above experiments, the Fluorescence (or Forster) resonance energy transfer (FRET) was proposed to explain the fluorescence responses of the sensor to Fe3+ as shown in Scheme 2.40–44 More specifically, before addition of Fe3+, the
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Scheme 2. Proposed mechanism for detection of Fe3+ and by HL.
receptor only showed an emission band of the coumarin at 475 nm, upon addition of Fe3+, a new emission band with a maximum at 550 nm appeared ascribed to Fe3++ induced opening of the spirocyclic ring of the Rhodamine moiety. Thus, the excitation energy is passed from coumarin moiety to the ‘opened-up’ Rhodamine moiety, namely, the FRET process of HL is triggered by Fe3+, as a result, the emission at 475 nm decreased, and a significant enhancement of the characteristic fluorescence of rhodamine at 550 nm emerged quickly. Conclusion In summary, we have successfully developed a novel FRETbased fluorescent ratiometric chemosensor for recognition of Fe3+ based on an asymmetric bis-Schiff in ethanol–water solutions (9:1, v/v, Tris–HCl, pH = 7.4). Moreover, the lower detection limit for Fe3+ reached the 106 M level which is sufficiently low to enable the detection of micromolar concentrations of Fe3+ in many chemical and biological systems. Acknowledgments This work is supported by the National Natural Science Foundation of China (81171337), Gansu NSF (1308RJZA115). Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.07. 023. References and notes 1. Upadhyay, K. K.; Kumar, A. Org. Biomol. Chem. 2010, 8, 4892–4897. 2. Han, T. Y.; Feng, X.; Tong, B.; Shi, J. B.; Chen, L.; Zhi, J. G.; Dong, Y. P. Chem. Commun. 2012, 416–418. 3. Lin, H. Y.; Cheng, P. Y.; Wan, C. F.; Wu, A. T. Analyst 2012, 137, 4415–4417. 4. Shi, X. Y.; Wang, H.; Han, T. Y.; Feng, X.; Tong, B.; Shi, J. B.; Zhi, J.; Dong, Y. P. J. Mater. Chem. 2013, 98, 42–50. 5. He, B. Chem. Sci. 2015, 6, 3180–3186. 6. Lee, J. W.; Helmann, J. D. Nature 2006, 440, 363–367.
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