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A highly selective colorimetric and fluorescent chemosensor for Al(III) based-on simple naphthol in aqueous solution
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 157 (2016) 6–10
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A highly selective colorimetric and fluorescent chemosensor for Al(III) based-on simple naphthol in aqueous solution Zhaodi Liu ⁎, Huajie Xu, Liangquan Sheng ⁎, Shuisheng Chen, Deqian Huang, Jie Liu Department of Chemistry and Materials Engineering, Fuyang Normal College, Fuyang, China
a r t i c l e
i n f o
Article history: Received 4 May 2015 Received in revised form 30 November 2015 Accepted 5 December 2015 Available online 9 December 2015 Keywords: Al(III) Colorimetric Fluorescent chemosensor
a b s t r a c t A colorimetric and fluorescent chemosensor (L) for Al(III) was synthesized and fully characterized. L could be both used as a colorimetric and fluorescent chemosensor for the detection of Al3+ ions with low detection limit (8.87 × 10−7 M) in CH3CN–H2O (1:1, v/v) solution. The binding ratio of L–Al3+ was determined from the Job plot (absorption and fluorescence spectra) and MALDI-TOF MS data to be 1:1. The binding constant (Ka) of Al3+ binding to L was calculated to be 4.8 × 105 M−1 from a Benesi–Hildebrand plot. Moreover, the binding site of L with Al3+ was determined by 1H NMR titration experiment. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Aluminum is the most prevalent (8.3% by weight) metallic element and the third most abundant of all elements (after oxygen and silicon) in the earth's crust [1]. It is well known that aluminum has widespread prevalence in food additives, aluminum-based pharmaceuticals, and storage/cooking utensils. This would inevitably lead to environmental pollution and accumulation in human body [2,3]. Especially, excess accumulation of Al3+ would cause human illnesses in virtue of its toxicological effects on the central nervous system, such as dementia and encephalopathy, Parkinson's disease, and Alzheimer's disease [4–7]. In plants, higher concentration of aluminum affects the growth of root and seed [8,9]. Thus, the monitoring of aluminum is essential in environment, medicine, foodstuff, etc. Recently, the fluorescent method has become popular as compared to the other traditional detection methods viz., AMS, AAS, GFAAS, ICP– AAS, and IC–AES, due to its operational simplicity, high selectivity and sensitivity, real-time response and naked eye detection [10–13]. However, it has always been troublesome to detect Al3 + due to its poor coordination ability, strong hydration ability, and the lack of spectroscopic characteristics, and only a few fluorescent sensors have been reported [14–19], on the other hand, most of the Al3 + sensors are difficult to synthesize and insoluble in aqueous solvents. Therefore, there is a great demand for developing low cost and real-time monitoring systems that can effectively determine Al3+ ion in natural environment and living organisms. Naphthalene ring has been proved as an ideal fluorophore due to its characteristic photophysical properties and the competitive stability in ⁎ Corresponding authors. E-mail addresses: [email protected] (Z. Liu), [email protected] (L. Sheng).
http://dx.doi.org/10.1016/j.saa.2015.12.004 1386-1425/© 2015 Elsevier B.V. All rights reserved.
the environment, and its correlative derivates have been synthesized as effective fluorescent probes in determination of some metal ions [20–24]. Especially, some highly selective fluorescent probes for Al3+ using naphthalene derivates have been reported in recent years, they exhibited high signal response toward Al3 + and showed good application prospect [25–31]. Herein, a simple and highly selective colorimetric/fluorescent sensor, (E)-N′-((2-hydroxynaphthalen-1yl)methylene)benzohydrazide (L), was synthesized and fully characterized. The chemosensor showed effective colorimetric/fluorescent single selectivity and high sensitivity for Al3+ in CH3CN–H2O solution, which will enhance the application prospect of the chemosensor for Al3+ in environment monitoring and biological analysis.
2. Experimental 2.1. General information Absorbance spectra measurements were measured on a Purkinje general UV-1901 spectrophotometer. Fluorescence spectra measurements were performed on a Cary Eclipse fluorescence spectrophotometer. 1H NMR spectra were recorded on a Varian INOVA-400 MHz spectrometer with tetramethylsilane (TMS) as internal standard. All reagents were of analytic grade and used without further purification. The solutions of cations were prepared from their chloride salts. Double distilled water was used throughout the experiment. Stock solution (5.0 × 10−3 mol/L) of L was prepared by dissolving the requisite amount of it in different solvents. Stock solutions of various ions were prepared by dissolving their chlorides salts in double distilled water. The excitation wavelength was 402 nm and slit widths were set to 2.5 nm for both excitation and emission.
Z. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 157 (2016) 6–10
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Scheme 1. The synthetic route of L.
2.2. Synthesis The synthetic route of (E)-N′-((2-hydroxynaphthalen-1yl)methylene)benzohydrazide (L) is shown in Scheme 1. A solution of 2-hydroxy-1-naphthaldehyde (0.34 g, 2 mmol) and benzoyl hydrazine (0.27 g, 2 mmol) in 30 mL ethanol was stirred at 60 °C for 2 h. After completion of the reaction, the obtained yellow precipitate was filtered and washed several times with cold ethanol. After drying under reduced pressure, the reaction afforded 0.52 g (89%) as a yellow solid. 1H NMR (400 MHz, CD3CN) δ 12.91 (s, 1H), 10.54 (s, 1H), 9.40 (s, 1H), 8.10 (d, J = 8.6 Hz, 1H), 7.98 (d, J = 7.5 Hz, 2H), 7.95–7.86 (m, 2H), 7.70–7.54 (m, 4H), 7.44 (t, J = 7.5 Hz, 1H), 7.25 (d, J = 9.0 Hz, 1H). 13C NMR (100 MHz, DMSO) δ 162.50, 157.98, 146.82, 132.74, 132.60, 132.10, 131.58, 128.97, 128.65, 127.78, 127.56, 123.54, 120.58, 118.89, 108.48. MALDI-TOF MS: 290.79.
3. Results and discussion 3.1. Absorption spectra studies UV–vis experiments were carried out to obtain the detailed absorption properties. The UV–vis titration of the Al3+ was conducted by using 50 μM of L in CH3CN–H2O (1:1, v/v) solution, as shown in Fig. 1. Upon binding with Al3+, a new absorption peak appears at 402 nm and its absorption intensity gradually increases with the addition of Al3+ ion with the color changing from colorless to blue, while the absorbance of L at 359 nm gradually decreases. At the same time, an isosbestic point appears at 377 nm between them. Since Al3 + concentration increased up to 50 μM, the absorbance at 359 nm and 402 nm changed slowly at even higher Al3+ concentration, which implied 1:1 binding stoichiometry between L and Al3+ (Fig. 2). To gain insight into the stoichiometry of the sensor L–Al3 +, the method of continuous variations (Job's plot) was used, as shown in Fig. 3. As expected, when the molar fraction of sensor L was 0.5, the
Fig. 1. UV–vis absorption spectra of L (50 μM) in CH3CN–H2O(1:1, v/v) solution with Al3+ ions (0–100 μM). Inset: color change of the solution upon Al3+ addition. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Absorbance at 359 nm and 402 nm of L as a function of Al3+ concentration.
absorbance value approached a maximum, which demonstrated the formation of a 1:1 complex between sensor Al3+ and L. UV–vis spectroscopy analysis of L toward various metal ions such as Li+, Na+, K+, Cr3+, Cu2+, Hg2+, Pb2+, Ba2+, Fe3+, Sr2+, Mn2+, Co2+, Ca2 +, Cd2 +, Ni2 +, Mg2 +, Zn2 +, and Al3 + was conducted (Fig. 4). Upon the addition of 5 equiv. of Al3+ to the L solution, a remarkable change was observed by visual inspection and UV–vis absorption spectroscopy. The significant change to wavelength resulted in a color change from colorless to blue, which can be easily observed by the “naked-eye”. The addition of other representative metal ions (5.0 equiv.) such as Li+, Na+, K+, Cr3 +, Cu2 +, Hg2 +, Pb2 +, Ba2 +, Fe3 +, Sr2 +, Mn2 +, Co2 +, Ca2 +, Cd2 +, Ni2 +, Mg2 +, and Zn2 + did not give rise to significant color and UV–vis absorption spectroscopy changes. These results suggest that L could be served as a potential colorimetric sensor selective for Al3+. 3.2. Fluorescence study The fluorescence titration was carried out using 50 μM L in the presence of different concentrations of Al3 + from 0 to 2 equiv. in CH3CN–H2O (1:1, v/v) solution. As shown in Fig. 5, Al3 + ion was
Fig. 3. Job's plots according to the method for continuous variations. The total concentration of L and Al3+ is 100 μM.
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Z. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 157 (2016) 6–10
Fig. 4. (a) Change in UV–vis spectrum of L in CH3CN–H2O (1:1, v/v) upon addition of different metal ions and (b) color changes of L upon additions of different metal ions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
gradually titrated, the fluorescence intensity of L exhibited a remarkable fluorescence enhancement at 468 nm, and when the amount of Al3+ ion added was about 50 μM, the fluorescence intensity almost reached maximum accompanied with the color changing from colorless to blue. Fluorescence quantum yield (Фf) of free L is 0.0028, whereas it reaches 0.53 (quinine sulfate in 0.5 M H2SO4 as a standard, Фf = 0.55) when L binds with Al3+ with an approximately 189-fold enhancement in the fluorescence intensity at 468 nm. These spectral changes indicate the formation of a new complex between L and Al3+. The causes of fluorescence enhancement can be reasonably expected that CN isomerization may be inhibited upon the coordination of L
Fig. 5. Fluorescence spectra of L (50 μM) upon the addition of increasing amounts of Al3+ ions (0–100 μM) in CH3CN–H2O (1:1, v/v). Inset: Changes in fluorescent intensity of L at 468 nm upon the addition of increasing [Al3+]; change in fluorescence color of L with addition of Al3+ under 365 nm UV light. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
to Al3+ [11,32]. In that case, the imine with an unfixed CN structure is weakly fluorescent emission because the CN isomerization is the predominant decay process in the excited states. In contrast, the complexation with Al3+ ions restricts the rotation of the CN bond and results in the suppression of the CN isomerization so that its fluorescence increases drastically. In order to gain insight into the CN isomerizationbased decay process, the fluorescence lifetime (τ) was measured by single photon counting and it shows a good single-exponential decay. The lifetime of free L is 2.56 ns and the radiative decay rate constant (kr) and nonradiative decay rate constant (knr) are calculated to be about 1.09 × 106 s− 1 and 3.78 × 108 s− 1, indicating that the nonradiative decay is the predominant process in the excited states of L. When Al3 + was added, the lifetime of L increases to 3.98 ns, the radiative
Fig. 6. Job's plots according to the method for continuous variations. The total concentration of L and Al3+ is 100 μM.
Z. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 157 (2016) 6–10
Fig. 7. Benesi–Hildebrand plot of L with Al3+ in CH3CN–H2O (1:1, v/v) solution.
and nonradiative decay rate constants were changed to 1.33 × 108 s−1 and 1.18 × 108 s−1, respectively. The radiative decay was larger than that of free L, so its fluorescence increases drastically [26,33]. A Job plot indicated a 1:1 stoichiometric complexation of L with Al3+ (Fig. 6). In addition, the formation of a 1:1 complex between L and Al3+ was further conformed by the appearance of a peak at m/z 733.767, assignable to 2[L− + Al3 + − H+]+ in the MALDI-TOF MS (Fig. S2), confirming the formation of a 1:1 L–Al complex species. From the fluorescence titration profiles, the association constant for L–Al3 + in CH3CN–H2O (1:1, v/v) was determined as 4.8 × 105 M−1 by a Hill plot (Fig. 7, R2 = 0.98116). By using the above-mentioned fluorescence titration results (Fig. S1), the detection limit for Al3+ was determined as 8.87 × 10− 7 M. The detection limit was sufficiently low to detect the micromolar.
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An important feature of L is its high selectivity toward the analyte compared with other species. Sensing properties of fluorescent sensor L for detecting Al3 + and competitive experiments were also verified. The other metal ions tested did not incur significant fluorescence changes, as shown in Fig. 8. Furthermore, to validate the high selectivity of L toward Al3 +, the fluorescence competitive experiments of other various metal ions were also investigated (Fig. 9). The L was treated with 1.0 equiv. of Al3 + in the presence of other metal ions (5.0 eq.). Their fluorescence intensities were recorded, respectively. As shown in Fig. 10, the fluorescence was quenched by Fe3+ and Cu2+, which were commonly considered as quenchers due to their inherent magnetic property. The Al3+ ion could still maintained its fluorescence properties in the presence of other metal ions (Li+, Na+, K+, Cr3+, Cu2+, Hg2+, Pb2+, Ba2+, Fe3+, Sr2+, Mn2+, Co2+, Ca2+, Cd2+, Ni2+, Mg2+, Zn2+). So the competition experiment means that L can be used as a selective fluorescent sensor for Al3+ in the presence of most competing metal ions. The binding pattern between L and Al3+ was examined by 1H NMR titration experiments in deuterated dimethyl sulfoxide (DMSO-d6), as shown in Fig. 10. The phenolic OH peak (proton Ha) and the imine group NH (proton Hb) of L were observed at δ = 12.91 ppm and δ = 10.54 ppm, respectively. The addition of Al3+ in L brought a significant change in the 1H NMR spectrum of L. The phenolic OH proton Ha peak disappeared indicating the deprotonation of the ligand in the presence of Al3 +. The imine group proton Hb shifted to 10.65 ppm because of the CN group and carbonyl group to coordinate with Al3+, which changes the electron distribution in the sensor. In summary, we have successfully developed a novel chemosensor L which exhibited effective colorimetric/fluorescent single selectivity and high sensitivity for Al3+ in aqueous solution, and fluorescence enhancement due to CN isomerization was inhibited upon the coordination of L to Al3 +. More importantly, because of its good water-solubility and lower detection limit, the colorimetric/fluorescent chemosensor will enhance the application prospect of the chemosensor for Al3+ in environment monitoring and biological analysis.
Fig. 8. Fluorescence spectra of L (50 μM) upon the addition of various metal ions in CH3CN–H2O (1:1, v/v) solution. Inset: Change in fluorescence color of L with addition of various metal ions under 365 nm UV light. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Z. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 157 (2016) 6–10
Fig. 9. Relative fluorescence of L and its complexation with Al3+ in the presence of various metal ions. Black bars: L (50 μM) and L with 5 equiv. of Li+, Na+, K+, Cr3+, Cu2+, Hg2+, Pb2+, Ba2+, Fe3+, Sr2+, Mn2+, Co2+, Ca2+, Cd2+, Ni2+, Mg2+, and Zn2+ stated. Red bars: 50 μM of L and 1 equiv. of Al3+ with 5 equiv. of metal ions stated. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 10. Partial 1H NMR spectra of L and L–Al3+ in DMSO-d6.
Acknowledgments This project is supported by the National Natural Science Foundation of China (21171040), the International Sea Area Resources Survey and Development of the 12th Five-year Plan of China (DY125-15-E-01), the Educational Commission of Anhui Province of China (2014KJ016, 2014KJ024, 2015KJ002), and the Natural Science Foundation of Fuyang Normal College (2015FSKJ02ZD, 2014FSKJ06, FS201402001B).
Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.saa.2015.12.004.
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Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A highly selective colorimetric and fluorescent chemosensor for Al(III) based-on simple naphthol in aqueous solution Zhaodi Liu ⁎, Huajie Xu, Liangquan Sheng ⁎, Shuisheng Chen, Deqian Huang, Jie Liu Department of Chemistry and Materials Engineering, Fuyang Normal College, Fuyang, China
a r t i c l e
i n f o
Article history: Received 4 May 2015 Received in revised form 30 November 2015 Accepted 5 December 2015 Available online 9 December 2015 Keywords: Al(III) Colorimetric Fluorescent chemosensor
a b s t r a c t A colorimetric and fluorescent chemosensor (L) for Al(III) was synthesized and fully characterized. L could be both used as a colorimetric and fluorescent chemosensor for the detection of Al3+ ions with low detection limit (8.87 × 10−7 M) in CH3CN–H2O (1:1, v/v) solution. The binding ratio of L–Al3+ was determined from the Job plot (absorption and fluorescence spectra) and MALDI-TOF MS data to be 1:1. The binding constant (Ka) of Al3+ binding to L was calculated to be 4.8 × 105 M−1 from a Benesi–Hildebrand plot. Moreover, the binding site of L with Al3+ was determined by 1H NMR titration experiment. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Aluminum is the most prevalent (8.3% by weight) metallic element and the third most abundant of all elements (after oxygen and silicon) in the earth's crust [1]. It is well known that aluminum has widespread prevalence in food additives, aluminum-based pharmaceuticals, and storage/cooking utensils. This would inevitably lead to environmental pollution and accumulation in human body [2,3]. Especially, excess accumulation of Al3+ would cause human illnesses in virtue of its toxicological effects on the central nervous system, such as dementia and encephalopathy, Parkinson's disease, and Alzheimer's disease [4–7]. In plants, higher concentration of aluminum affects the growth of root and seed [8,9]. Thus, the monitoring of aluminum is essential in environment, medicine, foodstuff, etc. Recently, the fluorescent method has become popular as compared to the other traditional detection methods viz., AMS, AAS, GFAAS, ICP– AAS, and IC–AES, due to its operational simplicity, high selectivity and sensitivity, real-time response and naked eye detection [10–13]. However, it has always been troublesome to detect Al3 + due to its poor coordination ability, strong hydration ability, and the lack of spectroscopic characteristics, and only a few fluorescent sensors have been reported [14–19], on the other hand, most of the Al3 + sensors are difficult to synthesize and insoluble in aqueous solvents. Therefore, there is a great demand for developing low cost and real-time monitoring systems that can effectively determine Al3+ ion in natural environment and living organisms. Naphthalene ring has been proved as an ideal fluorophore due to its characteristic photophysical properties and the competitive stability in ⁎ Corresponding authors. E-mail addresses: [email protected] (Z. Liu), [email protected] (L. Sheng).
http://dx.doi.org/10.1016/j.saa.2015.12.004 1386-1425/© 2015 Elsevier B.V. All rights reserved.
the environment, and its correlative derivates have been synthesized as effective fluorescent probes in determination of some metal ions [20–24]. Especially, some highly selective fluorescent probes for Al3+ using naphthalene derivates have been reported in recent years, they exhibited high signal response toward Al3 + and showed good application prospect [25–31]. Herein, a simple and highly selective colorimetric/fluorescent sensor, (E)-N′-((2-hydroxynaphthalen-1yl)methylene)benzohydrazide (L), was synthesized and fully characterized. The chemosensor showed effective colorimetric/fluorescent single selectivity and high sensitivity for Al3+ in CH3CN–H2O solution, which will enhance the application prospect of the chemosensor for Al3+ in environment monitoring and biological analysis.
2. Experimental 2.1. General information Absorbance spectra measurements were measured on a Purkinje general UV-1901 spectrophotometer. Fluorescence spectra measurements were performed on a Cary Eclipse fluorescence spectrophotometer. 1H NMR spectra were recorded on a Varian INOVA-400 MHz spectrometer with tetramethylsilane (TMS) as internal standard. All reagents were of analytic grade and used without further purification. The solutions of cations were prepared from their chloride salts. Double distilled water was used throughout the experiment. Stock solution (5.0 × 10−3 mol/L) of L was prepared by dissolving the requisite amount of it in different solvents. Stock solutions of various ions were prepared by dissolving their chlorides salts in double distilled water. The excitation wavelength was 402 nm and slit widths were set to 2.5 nm for both excitation and emission.
Z. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 157 (2016) 6–10
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Scheme 1. The synthetic route of L.
2.2. Synthesis The synthetic route of (E)-N′-((2-hydroxynaphthalen-1yl)methylene)benzohydrazide (L) is shown in Scheme 1. A solution of 2-hydroxy-1-naphthaldehyde (0.34 g, 2 mmol) and benzoyl hydrazine (0.27 g, 2 mmol) in 30 mL ethanol was stirred at 60 °C for 2 h. After completion of the reaction, the obtained yellow precipitate was filtered and washed several times with cold ethanol. After drying under reduced pressure, the reaction afforded 0.52 g (89%) as a yellow solid. 1H NMR (400 MHz, CD3CN) δ 12.91 (s, 1H), 10.54 (s, 1H), 9.40 (s, 1H), 8.10 (d, J = 8.6 Hz, 1H), 7.98 (d, J = 7.5 Hz, 2H), 7.95–7.86 (m, 2H), 7.70–7.54 (m, 4H), 7.44 (t, J = 7.5 Hz, 1H), 7.25 (d, J = 9.0 Hz, 1H). 13C NMR (100 MHz, DMSO) δ 162.50, 157.98, 146.82, 132.74, 132.60, 132.10, 131.58, 128.97, 128.65, 127.78, 127.56, 123.54, 120.58, 118.89, 108.48. MALDI-TOF MS: 290.79.
3. Results and discussion 3.1. Absorption spectra studies UV–vis experiments were carried out to obtain the detailed absorption properties. The UV–vis titration of the Al3+ was conducted by using 50 μM of L in CH3CN–H2O (1:1, v/v) solution, as shown in Fig. 1. Upon binding with Al3+, a new absorption peak appears at 402 nm and its absorption intensity gradually increases with the addition of Al3+ ion with the color changing from colorless to blue, while the absorbance of L at 359 nm gradually decreases. At the same time, an isosbestic point appears at 377 nm between them. Since Al3 + concentration increased up to 50 μM, the absorbance at 359 nm and 402 nm changed slowly at even higher Al3+ concentration, which implied 1:1 binding stoichiometry between L and Al3+ (Fig. 2). To gain insight into the stoichiometry of the sensor L–Al3 +, the method of continuous variations (Job's plot) was used, as shown in Fig. 3. As expected, when the molar fraction of sensor L was 0.5, the
Fig. 1. UV–vis absorption spectra of L (50 μM) in CH3CN–H2O(1:1, v/v) solution with Al3+ ions (0–100 μM). Inset: color change of the solution upon Al3+ addition. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Absorbance at 359 nm and 402 nm of L as a function of Al3+ concentration.
absorbance value approached a maximum, which demonstrated the formation of a 1:1 complex between sensor Al3+ and L. UV–vis spectroscopy analysis of L toward various metal ions such as Li+, Na+, K+, Cr3+, Cu2+, Hg2+, Pb2+, Ba2+, Fe3+, Sr2+, Mn2+, Co2+, Ca2 +, Cd2 +, Ni2 +, Mg2 +, Zn2 +, and Al3 + was conducted (Fig. 4). Upon the addition of 5 equiv. of Al3+ to the L solution, a remarkable change was observed by visual inspection and UV–vis absorption spectroscopy. The significant change to wavelength resulted in a color change from colorless to blue, which can be easily observed by the “naked-eye”. The addition of other representative metal ions (5.0 equiv.) such as Li+, Na+, K+, Cr3 +, Cu2 +, Hg2 +, Pb2 +, Ba2 +, Fe3 +, Sr2 +, Mn2 +, Co2 +, Ca2 +, Cd2 +, Ni2 +, Mg2 +, and Zn2 + did not give rise to significant color and UV–vis absorption spectroscopy changes. These results suggest that L could be served as a potential colorimetric sensor selective for Al3+. 3.2. Fluorescence study The fluorescence titration was carried out using 50 μM L in the presence of different concentrations of Al3 + from 0 to 2 equiv. in CH3CN–H2O (1:1, v/v) solution. As shown in Fig. 5, Al3 + ion was
Fig. 3. Job's plots according to the method for continuous variations. The total concentration of L and Al3+ is 100 μM.
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Fig. 4. (a) Change in UV–vis spectrum of L in CH3CN–H2O (1:1, v/v) upon addition of different metal ions and (b) color changes of L upon additions of different metal ions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
gradually titrated, the fluorescence intensity of L exhibited a remarkable fluorescence enhancement at 468 nm, and when the amount of Al3+ ion added was about 50 μM, the fluorescence intensity almost reached maximum accompanied with the color changing from colorless to blue. Fluorescence quantum yield (Фf) of free L is 0.0028, whereas it reaches 0.53 (quinine sulfate in 0.5 M H2SO4 as a standard, Фf = 0.55) when L binds with Al3+ with an approximately 189-fold enhancement in the fluorescence intensity at 468 nm. These spectral changes indicate the formation of a new complex between L and Al3+. The causes of fluorescence enhancement can be reasonably expected that CN isomerization may be inhibited upon the coordination of L
Fig. 5. Fluorescence spectra of L (50 μM) upon the addition of increasing amounts of Al3+ ions (0–100 μM) in CH3CN–H2O (1:1, v/v). Inset: Changes in fluorescent intensity of L at 468 nm upon the addition of increasing [Al3+]; change in fluorescence color of L with addition of Al3+ under 365 nm UV light. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
to Al3+ [11,32]. In that case, the imine with an unfixed CN structure is weakly fluorescent emission because the CN isomerization is the predominant decay process in the excited states. In contrast, the complexation with Al3+ ions restricts the rotation of the CN bond and results in the suppression of the CN isomerization so that its fluorescence increases drastically. In order to gain insight into the CN isomerizationbased decay process, the fluorescence lifetime (τ) was measured by single photon counting and it shows a good single-exponential decay. The lifetime of free L is 2.56 ns and the radiative decay rate constant (kr) and nonradiative decay rate constant (knr) are calculated to be about 1.09 × 106 s− 1 and 3.78 × 108 s− 1, indicating that the nonradiative decay is the predominant process in the excited states of L. When Al3 + was added, the lifetime of L increases to 3.98 ns, the radiative
Fig. 6. Job's plots according to the method for continuous variations. The total concentration of L and Al3+ is 100 μM.
Z. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 157 (2016) 6–10
Fig. 7. Benesi–Hildebrand plot of L with Al3+ in CH3CN–H2O (1:1, v/v) solution.
and nonradiative decay rate constants were changed to 1.33 × 108 s−1 and 1.18 × 108 s−1, respectively. The radiative decay was larger than that of free L, so its fluorescence increases drastically [26,33]. A Job plot indicated a 1:1 stoichiometric complexation of L with Al3+ (Fig. 6). In addition, the formation of a 1:1 complex between L and Al3+ was further conformed by the appearance of a peak at m/z 733.767, assignable to 2[L− + Al3 + − H+]+ in the MALDI-TOF MS (Fig. S2), confirming the formation of a 1:1 L–Al complex species. From the fluorescence titration profiles, the association constant for L–Al3 + in CH3CN–H2O (1:1, v/v) was determined as 4.8 × 105 M−1 by a Hill plot (Fig. 7, R2 = 0.98116). By using the above-mentioned fluorescence titration results (Fig. S1), the detection limit for Al3+ was determined as 8.87 × 10− 7 M. The detection limit was sufficiently low to detect the micromolar.
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An important feature of L is its high selectivity toward the analyte compared with other species. Sensing properties of fluorescent sensor L for detecting Al3 + and competitive experiments were also verified. The other metal ions tested did not incur significant fluorescence changes, as shown in Fig. 8. Furthermore, to validate the high selectivity of L toward Al3 +, the fluorescence competitive experiments of other various metal ions were also investigated (Fig. 9). The L was treated with 1.0 equiv. of Al3 + in the presence of other metal ions (5.0 eq.). Their fluorescence intensities were recorded, respectively. As shown in Fig. 10, the fluorescence was quenched by Fe3+ and Cu2+, which were commonly considered as quenchers due to their inherent magnetic property. The Al3+ ion could still maintained its fluorescence properties in the presence of other metal ions (Li+, Na+, K+, Cr3+, Cu2+, Hg2+, Pb2+, Ba2+, Fe3+, Sr2+, Mn2+, Co2+, Ca2+, Cd2+, Ni2+, Mg2+, Zn2+). So the competition experiment means that L can be used as a selective fluorescent sensor for Al3+ in the presence of most competing metal ions. The binding pattern between L and Al3+ was examined by 1H NMR titration experiments in deuterated dimethyl sulfoxide (DMSO-d6), as shown in Fig. 10. The phenolic OH peak (proton Ha) and the imine group NH (proton Hb) of L were observed at δ = 12.91 ppm and δ = 10.54 ppm, respectively. The addition of Al3+ in L brought a significant change in the 1H NMR spectrum of L. The phenolic OH proton Ha peak disappeared indicating the deprotonation of the ligand in the presence of Al3 +. The imine group proton Hb shifted to 10.65 ppm because of the CN group and carbonyl group to coordinate with Al3+, which changes the electron distribution in the sensor. In summary, we have successfully developed a novel chemosensor L which exhibited effective colorimetric/fluorescent single selectivity and high sensitivity for Al3+ in aqueous solution, and fluorescence enhancement due to CN isomerization was inhibited upon the coordination of L to Al3 +. More importantly, because of its good water-solubility and lower detection limit, the colorimetric/fluorescent chemosensor will enhance the application prospect of the chemosensor for Al3+ in environment monitoring and biological analysis.
Fig. 8. Fluorescence spectra of L (50 μM) upon the addition of various metal ions in CH3CN–H2O (1:1, v/v) solution. Inset: Change in fluorescence color of L with addition of various metal ions under 365 nm UV light. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 9. Relative fluorescence of L and its complexation with Al3+ in the presence of various metal ions. Black bars: L (50 μM) and L with 5 equiv. of Li+, Na+, K+, Cr3+, Cu2+, Hg2+, Pb2+, Ba2+, Fe3+, Sr2+, Mn2+, Co2+, Ca2+, Cd2+, Ni2+, Mg2+, and Zn2+ stated. Red bars: 50 μM of L and 1 equiv. of Al3+ with 5 equiv. of metal ions stated. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 10. Partial 1H NMR spectra of L and L–Al3+ in DMSO-d6.
Acknowledgments This project is supported by the National Natural Science Foundation of China (21171040), the International Sea Area Resources Survey and Development of the 12th Five-year Plan of China (DY125-15-E-01), the Educational Commission of Anhui Province of China (2014KJ016, 2014KJ024, 2015KJ002), and the Natural Science Foundation of Fuyang Normal College (2015FSKJ02ZD, 2014FSKJ06, FS201402001B).
Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.saa.2015.12.004.
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