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Two Schiff-base fluorescent sensors for selective sensing of aluminum (III): Experimental and computational studies
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 352–357
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Two Schiff-base fluorescent sensors for selective sensing of aluminum (III): Experimental and computational studies Jing-Can Qin a, Xiao-ying Cheng a, Ran Fang a, Ming-fang Wang a, Zheng-yin Yang a,⇑, Tian-rong Li a, Yong Li b a b
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The sensors show selectively
response toward Al3+. The sensors exhibit a large
fluorescence enhancement in the presence of Al3+. The detection limits reached at 108 M.
a r t i c l e
i n f o
Article history: Received 3 March 2015 Received in revised form 13 June 2015 Accepted 23 July 2015
a b s t r a c t Two Schiff-base fluorescent sensors have been synthesized, which both can act as fluorescent probes for Al3+, upon addition of Al3+, they exhibit a large fluorescence enhancement which might be attributed to the formation of 1:1 ligand-Al complexes which inhibit photoinduced electron transfer (PET) progress, and that the proposed binding modes of the sensors and Al3+ are identified by theoretical calculations. Ó 2015 Elsevier B.V. All rights reserved.
Keywords: Fluorescent sensor Al3+ PET Theoretical calculations
1. Introduction Following oxygen and silicon, aluminum is the third most abundant of all elements and is the most widely existing metal ion in the environment because of acidic rain and human activities [1,2]. Meanwhile, as a non-essential element for the human body, the wide use of aluminum made people expose to it and the intake of aluminum can lead to many diseases, such as microcytic hypochromic anemia, Al-related bone disease (ARBD), encephalopathy, dementia, myopathy and Alzheimer’s disease [3–5,1]. According ⇑ Corresponding author. E-mail address: [email protected] (Z.-y. Yang). http://dx.doi.org/10.1016/j.saa.2015.07.095 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
to a WHO report, the average daily human intake of aluminum is approx. 3–10 mg, the tolerable weekly aluminum intake in the human body is estimated to be 7 mg/kg of body weight [6,7]. Therefore, it is important to monitor the concentration levels of aluminum in the environment and many scientific fields. In the past few years, fluorescent probes have currently attracted significant interest because of its high sensitivity, selectivity, rapidity and easy operational procedure, therefore, the design and synthesis of fluorescent chemosensors remains an important endeavor in chemistry [8–19]. Additionally, compared with other metal cations, the detection of Al3+ has always been problematic because of its poor coordination ability, strong hydration ability and the lack of spectroscopic
J.-C. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 352–357
characteristics [20–22]. In general, as a hard-acid, it has been found that Al3+ prefers a coordination sphere containing N and O as hard-base donor sites, on the other hand, the Schiff-base could provide a nitrogen–oxygen-rich coordination environment for the hard-acid Al3+ [23–26]. Therefore, it can be conceived that the Schiff-base would develop a fluorescent probe for detection of Al3+. Out of consideration of these circumstances, we report two Al3+ chemosensors, 1-phenyl-3-methyl-5-hydroxypyrazole-4-aceton e-(isonicotinoyl)Hydrazone(A),1-phenyl-3-methyl-5-hydroxypyra zole-4-acetone-(nicotinoyl)Hydrazone (B) which are prepared by condensing PMAP and the corresponding hydrozones (Scheme 1), they can form the similar structures with aluminum and are identified by theoretical calculations. The spectroscopic studies also demonstrate that they have excellent selectivity toward Al3+ over other metal ions, More importantly, the detection limits for Al3+ are 4 108, 8 108 M respectively.
2. Experimental 2.1. Reagents and apparatus Unless otherwise stated, all solvents and reagents were obtained from commercial suppliers and used without further purification. The solutions of metal ions were prepared by dissolving the desired amount of the corresponding metal nitrate in ethanol. 1H NMR spectra of A was recorded on a Varian VR 200 MHz spectrometer in DMSO-d6 and the rest 1H NMR spectra were recorded with a Bruker Avance Drx 300-MHz spectrometer. ESI–MS were determined on a Bruker esquire 6000 spectrometer. UV–Vis absorption spectra were measured on a Perkin Elmer Lambda 35 UV–vis spectrophotometer. Fluorescence spectra were generated on a Hitachi RF-5301 spectrophotometer equipped with
353
quartz cuvettes of 1 cm path length. Melting points were determined on a Beijing XT4-100x microscopic melting point apparatus. 2.2. Synthesis 2.2.1. Synthesis of 1-phenyl-3-methyl-4-acetyl-pyrazolone-5 (PMAP) [27] 15 g of 1-phenyl-3-methyl-pyrazolone-5 (PMP) were placed in a flask equipped with a stirrer, separatory funnel and a reflux condenser and dissolved in 60–80 mL dioxane by application heat, 12 g of Ca(OH)2 were added, and 9.9 mL of acetylchloride were added dropwise within 1 min, the mixture became a thick paste and the temperature increased during the first few minutes and reflux for 30 min. The calcium complex in the flask was decomposed by pouring the mixture into HCl (200 mL, 2 N) which caused crystal to separate. The final product was filtered and recrystallized from methanol and water, yield: 56%. m. p: 58 °C. 2.2.2. Synthesis of the sensors The synthetic route of the sensors is shown in Scheme 1. An ethanol solution of Isonicotinohydrazide (1.37 g, 0.01 mol) was added to the ethanol solution which contained PMAP (2.17 g, 0.01 mol). With refluxing at 80–85 °C for 12 h, the final product was collected then washed with ethanol twice. The sensor A: Red precipitates, m. p. 228–229 °C, yield: 89.3%. 1H NMR of A (300 MHz in DMSO-d6) (Fig. S1) d: 12.402 (s, 1H, H15), 11.377 (s, 1H, H9), 8.967 (s, 1H, H14), 8.668 (d, J = 3.6 Hz 1H, H12), 8.148 (d, J = 7.5 Hz, 1H, H10), 7.876 (d, J = 7.8 Hz, 2H, H1,5), 7.473 (m, 1H, H11), 7.269 (m, 2H, H2,4), 7.002 (m, 2H, H3), 2.367 (s, 3H, CH3, H8), and 2.219 (s, 3H, CH3, H6). IR (KBr, cm1) (Fig. S2): 3437, 1617, 1589. ESI–MS (Fig. S3): [M + 1]+: 336.04. The synthetic method of B is similar to that of A.: Light yellow precipitates, m. p. 228–229 °C yield: 87.5%. 1H NMR of B (300 MHz
Scheme 1. Synthesis of the sensors.
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J.-C. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 352–357
in DMSO-d6) (Fig. S4) d: 12.516 (s, 1H, H15), 11.589 (s, 1H, H9), 8.822 (s, 2H, H11,13), 7.997 (d, J = 7.5 Hz, 2H, H1,5), 7.848 (d, J = 6.0 Hz, 2H, H10,14), 7.402 (m, 2H, H2,4), 7.111 (m, 1H, H3), 2.461 (s, 3H, CH3, H8), and 2.377 (s, 3H, CH3, H6). IR (KBr, cm1) (Fig. S5): 3427, 1633, 1594. ESI–MS (Fig. S6): [M + 1]+: 336.01.
3. Results and discussion 3.1. UV–Vis analysis The sensors properties were initially investigated as a function of the concentration of Al3+ by UV–Vis analysis in ethanol. In the presence of Al3+, the changes of the absorption spectra were similar between the sensor A and the sensor B, the the extent of variation were different as shown in Fig. 1. The maximum absorption wavelength was observed at about 380 nm in the absence of Al3+. Upon titration of Al3+, the absorption band at about 380 nm gradually disappeared and a new absorption bands appeared at 300 nm with increasing intensity. Moreover, a clear isosbestic points at 350 nm appeared which clearly indicated the presence of new complex in equilibrium with the receptor. All of these might be attributed to the interaction the sensors with Al3+ which were further confirmed by ESI–MS, IR.
Fig. 1. (a) Changes in the absorption spectra of A (10 lM) in ethanol at room temperature as a function of added Al(NO3)3 (0,1, 2, 3, 4, 5, 6, 7, 8, 9, 10 lM). (b) Changes in the absorption spectra of B (10 lM) in ethanol at room temperature as a function of added Al(NO3)3 (0,1, 2, 3, 4, 5, 6, 7, 8, 9, 10 lM).
3.2. Fluorescence study The effect of Al3+ on the fluorescence properties of the sensors was investigated in ethanol. As shown in Fig. 2, the free sensors A, B displayed weak fluorescence intensity upon excitation at 369, 375 nm. Upon addition of several metal ions such as Na+, K+, Ag+, Ca2+, In3+, Cd2+, Co2+,Ga3+, Ni2+, Fe3+, Mn2+, Mg2+, Pb2+, Cu2+, Cr3+, Ba2+, Zn2+ and Al3+ , the fluorescence of A, B were only significantly enhanced with fluorescence emission wavelength change from 557 to 450 nm in the presence of Al3+, the changes of fluorescence intensity could be better exhibited through fluorescence titrations. As shown in Fig. 3, with addition of increasing concentration of Al3+, the sensor showed maximum fluorescence emission at about 450 nm which are because the addition of Al3+ resulted in inhibiting photo-induced electron transfer (PET) process (Scheme 2). More specifically, the fluorescence bands of the sensors without Al3+ at about 450 nm were with low intensity due to the quenching mechanism by photo-induced electron transfer (PET) which was induced by lone pair electrons from the nitrogen atom of –C@N. After addition of Al3+, because of the chelation of
Fig. 2. (a) Fluorescence spectra of A (10 lM) upon the addition of metal salts (9.0 equiv.) of Na+, K+, Ag+, Ca2+, In3+, Cd2+, Co2+,Ga3+, Ni2+, Fe3+, Mn2+, Mg2+, Pb2+, Cu2+, Cr3+, Ba2+, Zn2+, Al3+ in ethanol (slit widths: 3 nm/3 nm). (b) Fluorescence spectra of B (10 lM) upon the addition of metal salts (6.0 equiv.) of Na+, K+, Ag+, Ca2+, In3+, Cd2+, Co2+,Ga3+, Ni2+, Fe3+, Mn2+, Mg2+, Pb2+, Cu2+, Cr3+, Ba2+, Zn2+, Al3+ in ethanol (slit widths: 3 nm/3 nm).
J.-C. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 352–357
Fig. 3. (a) Fluorescence spectra of A (10 lM) in ethanol upon the addition of Al3+ (0–9.0 equiv.) (b) fluorescence spectra of A (10 lM) in ethanol upon the addition of Al3+ (0–6.0 equiv.).
Scheme 2. Proposed mechanism for detection of Al3+ by A/B.
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Fig. 4. (a) Fluorescence responses of A (10 lM) to various metal ions (9.0 equiv.) in ethanol. Black bars represent fluorescence intensity of [A–Al] ([A] = 10 lM and [Al] = 90 lM). Red bars represent emission intensity of a mixture of A (10 lM) with the metal ions written below the bars (90 lM) followed by addition of 90 lM Al3+ to the mixed solutions. (b) Fluorescence responses of B (10 lM) to various metal ions (6.0 equiv.) in ethanol solution. Black bars represent fluorescence intensity of [B–Al] ([B] = 10 lM and [Al] = 60 lM). Red bars represent emission intensity of a mixture of B (10 lM) with the metal ions written below the bars (60 lM) followed by addition of 60 lM Al3+ to the mixed solutions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the nitrogen atom of –C@N with Al3+ inhibiting the PET process of the lone pair electrons on N of –C@N– [28–31], as a result, the fluorescence intensity increased significantly. To understand the recognition abilities of the sensors toward Al3+, the competition experiments were carried out. As shown in Fig. 4, the systems of other metal ions and Al3+ coexisted were examined in ethanol, the result suggested that all the coexistent metal ions had no obvious interference with the detection of Al3+ except in the case of Fe3+, which were attributed to theirs inherent to the magnetic property. In addition, from the fluorescence titration data, the binding constants (log K) for the sensors and Al3+ in ethanol were 11.484 (Fig. S7) and 8.6234 (Fig. S8) respectively according to the following equation.
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J.-C. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 352–357 Table 1 Major IR data of the receptor and its complexes. Compound
m (pyrazoloneOH) cm1
m (benzoylC@O) cm1
m (–C@N) cm1
m (M–O) cm1
A A–Al Dm (cm1) B B–Al Dm (cm1)
3437 3424 13 3437 3427 10
1617 1601 16 1633 1615 18
1589 1556 33 1594 1553 41
– 578 – 577
of 0.5, which indicated that it was a 1:1 stoichiometry of the binding mode of the sensor A and Al3+. The result was further confirmed by the appearance of a peak at m/z 423.3 assignable to + [HL + Al3++NO and a peak at m/z 491.2 assignable to 3 H] 3+ + + [HL + Al +C2H5OH + NO 3 + Na 2H] in the ESI/MS (Fig. S11). As shown in Fig. 5b, the maximum point also appeared at a mole fraction of 0.5, the result indicated that it was a 1:1 stoichiometry of the binding mode of the sensor B and Al3+ was further confirmed by the appearance of a peak at m/z 423.3 assignable to + [HL + Al3++NO and a peak at m/z 468.2 assignable to 3 H] 3+ + [HL + Al + C2H5OH + NO 3 H] in the ESI/MS (Fig. S12). IR spectra (Figs. S13 and S14) further proved formation of complexes between the sensors and Al3+ as shown Table 1, the large differences of the IR spectra of the sensors in the absence and presence of Al3+ indicate that Al3+ indeed directly interact with nitrogen atom of C@N, oxygen atom in the –C@O of benzoyl and –OH in pyrazolone.
Fig. 5. (a) Job’s plot for determining the stoichiometry of A and Al3+ in ethanol (XAl = [Al3+]/([Al3+] + [A]), the total concentration of A and Al3+ was 10 lM). (b) Job’s plot for determining the stoichiometry of B and Al3+ in ethanol (XAl = [Al3+]/([Al3+] + [B]), the total concentration of B and Al3+ was 10 lM).
log
F F min ¼ log K þ nlog ½M F max F
The binding constant K was obtained from the plot of linear regression of log [(F Fmin)/(Fmax F)] vs log [M] in Eq. (1), where the intercept was log K. In the equation, Fmin, Fmax, and F are the fluorescence intensity in the absence of Al3+, presence of saturated Al3+, and the fluorescence intensity of the [L-Al] complex at time intervals. [M] was the concentration of free metal ions which could be assumed equal to its total concentration [32]. More importantly, the lowest detection limits of the sensors for Al3+ were determined as 4 108 M (Fig. S9) and 8 108 M (Fig. S10). Thus, the sensors could be used as a selective fluorescent sensor for the detection of micromolar concentrations of Al3+ in many chemical and biological systems. 3.3. The complexation of the sensors with Al3+ In order to further validate the stoichiometry of the sensors and Al3+, we also carried out Job’s plot, the total concentration of the sensors and Al3+ was 10 lM. XAl = ([Al3+]/(Al3+] + [sensors]. As shown in Fig. 5a, the maximum point appeared at a mole fraction
Fig. 6. (a) Optimized structure of [Al(A)(C2H5OH)(NO3)]. (b) Optimized structure of [Al(B)(C2H5OH)(NO3)].
J.-C. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 352–357 Table 2 Major the bond length of the receptor and its complexes. Compound
The bond length (Al–O) Å
The bond length (Al–O=) Å
The bond length (Al–N) Å
A–Al B–Al
1.8321 1.8305
1.8784 1.8856
2.1180 2.1170
3.4. The optimized configuration To get insight into the proposed binding mode, the binding mode of the sensors and Al3+ carried out density functional theory (DFT) calculations with B3LYP/6-31G(d) method [33–35] According to the above discussion and the relevant literature reports [36–40], we proposed a rational coordinated mode that Al3+ is hexa-coordinated with one tridentate ligand, one bidentate nitrate and one monodentate ethanol. As depicted in Fig. 6, the sensors could chelate Al3+ through interactions with carboxylate oxygen of benzoyl, imine nitrogen and oxygen of pyrazolone group and forms 1:1 complexes, the optimized configurations of the sensors and Al3+ were shown Table 2, the donors of the ligand were almost on the same plane with Al3+, which was used to explain the sensing mechanism. 4. Conclusion In summary, we have successfully developed two simple chemosensors which exhibited high selectivity for Al3+ over other metal ions. Upon addition of Al3+, the sensors showed remarkable fluorescence enhancement in the presence of Al3+. More importantly, the detection limits reached at 108 M. The results suggested that the receptors could serve as an excellent fluorescent chemosensor for Al3+ and might accelerate the development of new efficient chemosensors. Acknowledgments This work is supported by the National Natural Science Foundation of China (81171337), Gansu NSF (1308RJZA115). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2015.07.095. References [1] D. Jeyanthi, M. Iniya, K. Krishnaveni, D. Chellappa, RSC Adv. 3 (2013) 20984–20989.
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Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Two Schiff-base fluorescent sensors for selective sensing of aluminum (III): Experimental and computational studies Jing-Can Qin a, Xiao-ying Cheng a, Ran Fang a, Ming-fang Wang a, Zheng-yin Yang a,⇑, Tian-rong Li a, Yong Li b a b
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The sensors show selectively
response toward Al3+. The sensors exhibit a large
fluorescence enhancement in the presence of Al3+. The detection limits reached at 108 M.
a r t i c l e
i n f o
Article history: Received 3 March 2015 Received in revised form 13 June 2015 Accepted 23 July 2015
a b s t r a c t Two Schiff-base fluorescent sensors have been synthesized, which both can act as fluorescent probes for Al3+, upon addition of Al3+, they exhibit a large fluorescence enhancement which might be attributed to the formation of 1:1 ligand-Al complexes which inhibit photoinduced electron transfer (PET) progress, and that the proposed binding modes of the sensors and Al3+ are identified by theoretical calculations. Ó 2015 Elsevier B.V. All rights reserved.
Keywords: Fluorescent sensor Al3+ PET Theoretical calculations
1. Introduction Following oxygen and silicon, aluminum is the third most abundant of all elements and is the most widely existing metal ion in the environment because of acidic rain and human activities [1,2]. Meanwhile, as a non-essential element for the human body, the wide use of aluminum made people expose to it and the intake of aluminum can lead to many diseases, such as microcytic hypochromic anemia, Al-related bone disease (ARBD), encephalopathy, dementia, myopathy and Alzheimer’s disease [3–5,1]. According ⇑ Corresponding author. E-mail address: [email protected] (Z.-y. Yang). http://dx.doi.org/10.1016/j.saa.2015.07.095 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
to a WHO report, the average daily human intake of aluminum is approx. 3–10 mg, the tolerable weekly aluminum intake in the human body is estimated to be 7 mg/kg of body weight [6,7]. Therefore, it is important to monitor the concentration levels of aluminum in the environment and many scientific fields. In the past few years, fluorescent probes have currently attracted significant interest because of its high sensitivity, selectivity, rapidity and easy operational procedure, therefore, the design and synthesis of fluorescent chemosensors remains an important endeavor in chemistry [8–19]. Additionally, compared with other metal cations, the detection of Al3+ has always been problematic because of its poor coordination ability, strong hydration ability and the lack of spectroscopic
J.-C. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 352–357
characteristics [20–22]. In general, as a hard-acid, it has been found that Al3+ prefers a coordination sphere containing N and O as hard-base donor sites, on the other hand, the Schiff-base could provide a nitrogen–oxygen-rich coordination environment for the hard-acid Al3+ [23–26]. Therefore, it can be conceived that the Schiff-base would develop a fluorescent probe for detection of Al3+. Out of consideration of these circumstances, we report two Al3+ chemosensors, 1-phenyl-3-methyl-5-hydroxypyrazole-4-aceton e-(isonicotinoyl)Hydrazone(A),1-phenyl-3-methyl-5-hydroxypyra zole-4-acetone-(nicotinoyl)Hydrazone (B) which are prepared by condensing PMAP and the corresponding hydrozones (Scheme 1), they can form the similar structures with aluminum and are identified by theoretical calculations. The spectroscopic studies also demonstrate that they have excellent selectivity toward Al3+ over other metal ions, More importantly, the detection limits for Al3+ are 4 108, 8 108 M respectively.
2. Experimental 2.1. Reagents and apparatus Unless otherwise stated, all solvents and reagents were obtained from commercial suppliers and used without further purification. The solutions of metal ions were prepared by dissolving the desired amount of the corresponding metal nitrate in ethanol. 1H NMR spectra of A was recorded on a Varian VR 200 MHz spectrometer in DMSO-d6 and the rest 1H NMR spectra were recorded with a Bruker Avance Drx 300-MHz spectrometer. ESI–MS were determined on a Bruker esquire 6000 spectrometer. UV–Vis absorption spectra were measured on a Perkin Elmer Lambda 35 UV–vis spectrophotometer. Fluorescence spectra were generated on a Hitachi RF-5301 spectrophotometer equipped with
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quartz cuvettes of 1 cm path length. Melting points were determined on a Beijing XT4-100x microscopic melting point apparatus. 2.2. Synthesis 2.2.1. Synthesis of 1-phenyl-3-methyl-4-acetyl-pyrazolone-5 (PMAP) [27] 15 g of 1-phenyl-3-methyl-pyrazolone-5 (PMP) were placed in a flask equipped with a stirrer, separatory funnel and a reflux condenser and dissolved in 60–80 mL dioxane by application heat, 12 g of Ca(OH)2 were added, and 9.9 mL of acetylchloride were added dropwise within 1 min, the mixture became a thick paste and the temperature increased during the first few minutes and reflux for 30 min. The calcium complex in the flask was decomposed by pouring the mixture into HCl (200 mL, 2 N) which caused crystal to separate. The final product was filtered and recrystallized from methanol and water, yield: 56%. m. p: 58 °C. 2.2.2. Synthesis of the sensors The synthetic route of the sensors is shown in Scheme 1. An ethanol solution of Isonicotinohydrazide (1.37 g, 0.01 mol) was added to the ethanol solution which contained PMAP (2.17 g, 0.01 mol). With refluxing at 80–85 °C for 12 h, the final product was collected then washed with ethanol twice. The sensor A: Red precipitates, m. p. 228–229 °C, yield: 89.3%. 1H NMR of A (300 MHz in DMSO-d6) (Fig. S1) d: 12.402 (s, 1H, H15), 11.377 (s, 1H, H9), 8.967 (s, 1H, H14), 8.668 (d, J = 3.6 Hz 1H, H12), 8.148 (d, J = 7.5 Hz, 1H, H10), 7.876 (d, J = 7.8 Hz, 2H, H1,5), 7.473 (m, 1H, H11), 7.269 (m, 2H, H2,4), 7.002 (m, 2H, H3), 2.367 (s, 3H, CH3, H8), and 2.219 (s, 3H, CH3, H6). IR (KBr, cm1) (Fig. S2): 3437, 1617, 1589. ESI–MS (Fig. S3): [M + 1]+: 336.04. The synthetic method of B is similar to that of A.: Light yellow precipitates, m. p. 228–229 °C yield: 87.5%. 1H NMR of B (300 MHz
Scheme 1. Synthesis of the sensors.
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in DMSO-d6) (Fig. S4) d: 12.516 (s, 1H, H15), 11.589 (s, 1H, H9), 8.822 (s, 2H, H11,13), 7.997 (d, J = 7.5 Hz, 2H, H1,5), 7.848 (d, J = 6.0 Hz, 2H, H10,14), 7.402 (m, 2H, H2,4), 7.111 (m, 1H, H3), 2.461 (s, 3H, CH3, H8), and 2.377 (s, 3H, CH3, H6). IR (KBr, cm1) (Fig. S5): 3427, 1633, 1594. ESI–MS (Fig. S6): [M + 1]+: 336.01.
3. Results and discussion 3.1. UV–Vis analysis The sensors properties were initially investigated as a function of the concentration of Al3+ by UV–Vis analysis in ethanol. In the presence of Al3+, the changes of the absorption spectra were similar between the sensor A and the sensor B, the the extent of variation were different as shown in Fig. 1. The maximum absorption wavelength was observed at about 380 nm in the absence of Al3+. Upon titration of Al3+, the absorption band at about 380 nm gradually disappeared and a new absorption bands appeared at 300 nm with increasing intensity. Moreover, a clear isosbestic points at 350 nm appeared which clearly indicated the presence of new complex in equilibrium with the receptor. All of these might be attributed to the interaction the sensors with Al3+ which were further confirmed by ESI–MS, IR.
Fig. 1. (a) Changes in the absorption spectra of A (10 lM) in ethanol at room temperature as a function of added Al(NO3)3 (0,1, 2, 3, 4, 5, 6, 7, 8, 9, 10 lM). (b) Changes in the absorption spectra of B (10 lM) in ethanol at room temperature as a function of added Al(NO3)3 (0,1, 2, 3, 4, 5, 6, 7, 8, 9, 10 lM).
3.2. Fluorescence study The effect of Al3+ on the fluorescence properties of the sensors was investigated in ethanol. As shown in Fig. 2, the free sensors A, B displayed weak fluorescence intensity upon excitation at 369, 375 nm. Upon addition of several metal ions such as Na+, K+, Ag+, Ca2+, In3+, Cd2+, Co2+,Ga3+, Ni2+, Fe3+, Mn2+, Mg2+, Pb2+, Cu2+, Cr3+, Ba2+, Zn2+ and Al3+ , the fluorescence of A, B were only significantly enhanced with fluorescence emission wavelength change from 557 to 450 nm in the presence of Al3+, the changes of fluorescence intensity could be better exhibited through fluorescence titrations. As shown in Fig. 3, with addition of increasing concentration of Al3+, the sensor showed maximum fluorescence emission at about 450 nm which are because the addition of Al3+ resulted in inhibiting photo-induced electron transfer (PET) process (Scheme 2). More specifically, the fluorescence bands of the sensors without Al3+ at about 450 nm were with low intensity due to the quenching mechanism by photo-induced electron transfer (PET) which was induced by lone pair electrons from the nitrogen atom of –C@N. After addition of Al3+, because of the chelation of
Fig. 2. (a) Fluorescence spectra of A (10 lM) upon the addition of metal salts (9.0 equiv.) of Na+, K+, Ag+, Ca2+, In3+, Cd2+, Co2+,Ga3+, Ni2+, Fe3+, Mn2+, Mg2+, Pb2+, Cu2+, Cr3+, Ba2+, Zn2+, Al3+ in ethanol (slit widths: 3 nm/3 nm). (b) Fluorescence spectra of B (10 lM) upon the addition of metal salts (6.0 equiv.) of Na+, K+, Ag+, Ca2+, In3+, Cd2+, Co2+,Ga3+, Ni2+, Fe3+, Mn2+, Mg2+, Pb2+, Cu2+, Cr3+, Ba2+, Zn2+, Al3+ in ethanol (slit widths: 3 nm/3 nm).
J.-C. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 352–357
Fig. 3. (a) Fluorescence spectra of A (10 lM) in ethanol upon the addition of Al3+ (0–9.0 equiv.) (b) fluorescence spectra of A (10 lM) in ethanol upon the addition of Al3+ (0–6.0 equiv.).
Scheme 2. Proposed mechanism for detection of Al3+ by A/B.
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Fig. 4. (a) Fluorescence responses of A (10 lM) to various metal ions (9.0 equiv.) in ethanol. Black bars represent fluorescence intensity of [A–Al] ([A] = 10 lM and [Al] = 90 lM). Red bars represent emission intensity of a mixture of A (10 lM) with the metal ions written below the bars (90 lM) followed by addition of 90 lM Al3+ to the mixed solutions. (b) Fluorescence responses of B (10 lM) to various metal ions (6.0 equiv.) in ethanol solution. Black bars represent fluorescence intensity of [B–Al] ([B] = 10 lM and [Al] = 60 lM). Red bars represent emission intensity of a mixture of B (10 lM) with the metal ions written below the bars (60 lM) followed by addition of 60 lM Al3+ to the mixed solutions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the nitrogen atom of –C@N with Al3+ inhibiting the PET process of the lone pair electrons on N of –C@N– [28–31], as a result, the fluorescence intensity increased significantly. To understand the recognition abilities of the sensors toward Al3+, the competition experiments were carried out. As shown in Fig. 4, the systems of other metal ions and Al3+ coexisted were examined in ethanol, the result suggested that all the coexistent metal ions had no obvious interference with the detection of Al3+ except in the case of Fe3+, which were attributed to theirs inherent to the magnetic property. In addition, from the fluorescence titration data, the binding constants (log K) for the sensors and Al3+ in ethanol were 11.484 (Fig. S7) and 8.6234 (Fig. S8) respectively according to the following equation.
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J.-C. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 352–357 Table 1 Major IR data of the receptor and its complexes. Compound
m (pyrazoloneOH) cm1
m (benzoylC@O) cm1
m (–C@N) cm1
m (M–O) cm1
A A–Al Dm (cm1) B B–Al Dm (cm1)
3437 3424 13 3437 3427 10
1617 1601 16 1633 1615 18
1589 1556 33 1594 1553 41
– 578 – 577
of 0.5, which indicated that it was a 1:1 stoichiometry of the binding mode of the sensor A and Al3+. The result was further confirmed by the appearance of a peak at m/z 423.3 assignable to + [HL + Al3++NO and a peak at m/z 491.2 assignable to 3 H] 3+ + + [HL + Al +C2H5OH + NO 3 + Na 2H] in the ESI/MS (Fig. S11). As shown in Fig. 5b, the maximum point also appeared at a mole fraction of 0.5, the result indicated that it was a 1:1 stoichiometry of the binding mode of the sensor B and Al3+ was further confirmed by the appearance of a peak at m/z 423.3 assignable to + [HL + Al3++NO and a peak at m/z 468.2 assignable to 3 H] 3+ + [HL + Al + C2H5OH + NO 3 H] in the ESI/MS (Fig. S12). IR spectra (Figs. S13 and S14) further proved formation of complexes between the sensors and Al3+ as shown Table 1, the large differences of the IR spectra of the sensors in the absence and presence of Al3+ indicate that Al3+ indeed directly interact with nitrogen atom of C@N, oxygen atom in the –C@O of benzoyl and –OH in pyrazolone.
Fig. 5. (a) Job’s plot for determining the stoichiometry of A and Al3+ in ethanol (XAl = [Al3+]/([Al3+] + [A]), the total concentration of A and Al3+ was 10 lM). (b) Job’s plot for determining the stoichiometry of B and Al3+ in ethanol (XAl = [Al3+]/([Al3+] + [B]), the total concentration of B and Al3+ was 10 lM).
log
F F min ¼ log K þ nlog ½M F max F
The binding constant K was obtained from the plot of linear regression of log [(F Fmin)/(Fmax F)] vs log [M] in Eq. (1), where the intercept was log K. In the equation, Fmin, Fmax, and F are the fluorescence intensity in the absence of Al3+, presence of saturated Al3+, and the fluorescence intensity of the [L-Al] complex at time intervals. [M] was the concentration of free metal ions which could be assumed equal to its total concentration [32]. More importantly, the lowest detection limits of the sensors for Al3+ were determined as 4 108 M (Fig. S9) and 8 108 M (Fig. S10). Thus, the sensors could be used as a selective fluorescent sensor for the detection of micromolar concentrations of Al3+ in many chemical and biological systems. 3.3. The complexation of the sensors with Al3+ In order to further validate the stoichiometry of the sensors and Al3+, we also carried out Job’s plot, the total concentration of the sensors and Al3+ was 10 lM. XAl = ([Al3+]/(Al3+] + [sensors]. As shown in Fig. 5a, the maximum point appeared at a mole fraction
Fig. 6. (a) Optimized structure of [Al(A)(C2H5OH)(NO3)]. (b) Optimized structure of [Al(B)(C2H5OH)(NO3)].
J.-C. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 352–357 Table 2 Major the bond length of the receptor and its complexes. Compound
The bond length (Al–O) Å
The bond length (Al–O=) Å
The bond length (Al–N) Å
A–Al B–Al
1.8321 1.8305
1.8784 1.8856
2.1180 2.1170
3.4. The optimized configuration To get insight into the proposed binding mode, the binding mode of the sensors and Al3+ carried out density functional theory (DFT) calculations with B3LYP/6-31G(d) method [33–35] According to the above discussion and the relevant literature reports [36–40], we proposed a rational coordinated mode that Al3+ is hexa-coordinated with one tridentate ligand, one bidentate nitrate and one monodentate ethanol. As depicted in Fig. 6, the sensors could chelate Al3+ through interactions with carboxylate oxygen of benzoyl, imine nitrogen and oxygen of pyrazolone group and forms 1:1 complexes, the optimized configurations of the sensors and Al3+ were shown Table 2, the donors of the ligand were almost on the same plane with Al3+, which was used to explain the sensing mechanism. 4. Conclusion In summary, we have successfully developed two simple chemosensors which exhibited high selectivity for Al3+ over other metal ions. Upon addition of Al3+, the sensors showed remarkable fluorescence enhancement in the presence of Al3+. More importantly, the detection limits reached at 108 M. The results suggested that the receptors could serve as an excellent fluorescent chemosensor for Al3+ and might accelerate the development of new efficient chemosensors. Acknowledgments This work is supported by the National Natural Science Foundation of China (81171337), Gansu NSF (1308RJZA115). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2015.07.095. References [1] D. Jeyanthi, M. Iniya, K. Krishnaveni, D. Chellappa, RSC Adv. 3 (2013) 20984–20989.
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