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Inorganica Chimica Acta 455 (2017) 247–253
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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Research paper
Two 5,50 -methylenebis(salicylaldehyde)-based Schiff base fluorescent sensors for selective sensing of Al3+ in DMSO/H2O solution Wang Ruo b, Jiang Guang-Qi a,b,⇑, Li Xiao-Hong b a State Key Laboratory Breeding Base of Green Pesticide and Agricultural Bioengineering, Key Laboratory of Green Pesticide and Agricultural Bioengineering, Ministry of Education, Guizhou University, Guiyang 550025, China b College of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China
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Article history: Received 6 September 2016 Received in revised form 3 November 2016 Accepted 10 November 2016 Available online 11 November 2016 Keywords: Fluorescent sensor Schiff base Aluminum
a b s t r a c t Two 5,50 -methylenebis(salicylaldehyde)-based Schiff base fluorescent sensors, 1 and 2, were synthesized. Sensors 1 and 2 both showed selective fluorescent sensing for Al3+ with low detection limits of 1.49 108 and 1.51 108 M, respectively, in DMSO/H2O (19:1, v/v) solution at the pH value of 5.2. The 1:1 stoichiometries of the Al complexes were determined by Job plots and further verified with LC–MS data and 1H NMR studies. The sensing mechanisms of sensors 1 and 2 to Al3+ were proposed. The fluorescence color changes could be easily detected by the naked eye under a UV lamp. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction After oxygen and silicon, aluminum is the third most abundant element in the Earth’s crust and is the most common metal ion in the environment because of acidic rain and human activities [1,2]. Studies have shown that the deposition of aluminum in bone and the nervous system in the human body can cause neurotoxicity in high dosages in the form of microcytic hypochromic anemia, aluminum-related bone disease, encephalopathy, dementia, myopathy and Alzheimer’s disease, in addition, high concentrations of aluminum ions can hamper plant growth [3–12]. According to a WHO report, the average daily human intake of aluminum is around 3–10 mg. The tolerable weekly aluminum dietary intake in the human body is estimated to be 7 mg/kg of body weight [13,14]. Thus, the design of chemosensors for detecting aluminum in aqueous medium is of great significance. Schiff bases are good ligands for metal ions, with which they can form coordinate bonds. This characteristic has resulted in their extensive use in the production of sensors [15–25]. Many analytical methods have played vital roles in the detection of metal ions, including ion selective electrodes [26–28], voltammetric [29–31] and colorimetric sensors [32–37]. Recent research ⇑ Corresponding author at: College of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China. E-mail address: [email protected] (J. Guang-Qi). http://dx.doi.org/10.1016/j.ica.2016.11.008 0020-1693/Ó 2016 Elsevier B.V. All rights reserved.
has highlighted a growing number of fluorescent sensors that have been designed and synthesized based on bis-Schiff bases, owing to their nitrogen-oxygen-rich coordination environments for metal ions [38–41]. In the present work, two fluorescent sensors, 1 and 2, based on 5,50 -methylenebis(salicylaldehyde) have been investigated for their sensing abilities toward common metal cations. Sensors 1 and 2 both show selective fluorescent sensing abilities for Al3+ in DMSO/H2O (19:1, v/v) solution. The detection limits of sensors 1 (1.49 108 M) and 2 (1.51 108 M) are lower than the WHO guideline (7.4 lM) for drinking water [42].
2. Experimental 2.1. Materials and instrumentation Metal salts were purchased commercially (Aldrich and Alfa Aesar Chemical Co., Ltd.). The solutions of metal ions were prepared from chlorides of K+, Ca2+, Mn2+, Cr3+ and nitrate salts of Al3+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Hg2+, Ga3+, In3+ and Mg2+. All fluorescence measurements were made on a Cary Eclipse Fluorescence Spectrometer (Varian) in a 1 cm quartz cell. 1H NMR (solvent CDCl3 or DMSO-d6) spectral analyses were performed on a JEOL-ECX 500 NMR spectrometer at room temperature using TMS as an internal standard. LC–MS was determined on a Agilent LC/ MSD Trap based on infusion methods. UV–Vis absorption spectra
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Fig. 1. Structures of the Schiff base fluorescent sensors 1 and 2.
were recorded on a UV-1800 spectrophotometer (Beijing General Instrument Co., China) in a 1 cm quartz cell. 2.2. Preparation of Schiff base The Schiff base fluorescent sensors 1 and 2 were prepared according to previously described procedures [43] (see Fig. 1).
Fig. 3. Effects of pH towards the fluorescence intensity of sensors 1 and 2 (0.1 mM) for Al3+ (5.0 eq.) at 491 and 473 nm, respectively.
3.2. Fluorescence response of sensors 1 and 2 to Al3+ 3. Results and discussion 3.1. General information Solutions of metal ions were prepared from the corresponding metal chloride or nitrate salts. Stock solutions of metal ions (1 mM) were prepared by dissolving the desired amount of metal salts in distilled water. Stock solutions of sensors 1 and 2 (0.1 mM) were prepared in DMSO solution. The pH values were adjusted by a hexamethylenetetramine-HCl buffer solution. The detection limit was evaluated based on fluorescence titration and then calculated with the equation 3r/K, where r is the standard deviation of blank measurements and K is the slope of intensity versus sample concentration [44]. The binding constant values were determined from the emission intensity data, following the modified Benesi–Hildebrand equations [45].
1 1 1 þ ¼ F F min KðF max F min Þ½Al3þ F max F min where Fmin, F and Fmax are the emission intensities of the organic moiety considered in the absence of aluminum ions, at an intermediate aluminum concentration and at a concentration of complete interaction, respectively, and where K is the binding constant concentration.
The fluorescence responses of sensors 1 and 2 (10 lM) to several metal ions, such as Al3+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Mn2+, Cr3+, Cd2+, Pb2+, Hg2+, K+, Ca2+, Ga3+, In3+ and Mg2+, in DMSO/H2O (19:1, v/v) solution were investigated. As shown in Fig. 2, the free sensors 1 and 2 showed only weak fluorescence emissions at 491 and 473 nm, respectively. With the addition of various metal ions, there are no remarkably changes in their fluorescence spectra, except in the case of Al3+. The fluorescence intensity showed an enhancement of 70- and 40-fold for sensors 1 and 2, respectively, in the presence of Al3+, relative to that of the free sensors. 3.3. Effect of pH towards the fluorescence intensity The sensitivity of the fluorescent sensors complexes (1 + Al3+ or 2 + Al3+) at different pH values was studied in a DMSO/H2O (1/9, v/v) solution. Experiments were conducted by adjusting pH using NaOH and HCl solutions. As depicted in Fig. 3, both sensors were found to be quite effective near the pH of 5.2. Further studies were performed under the condition of pH value equal to 5.2. 3.4. The spectroscopic titration experiments In DMSO/H2O (19:1, v/v) solution at the pH value of 5.2, the absorption spectrum of sensor 1 shows two absorption bands centered at 339 and 268 nm (Fig. 4a). Upon addition of Al3+ to
Fig. 2. Fluorescence intensity of sensors 1 (a) and 2 (b) (1 mM) upon addition of various metal ions (5 equiv.) in DMSO/H2O (19:1, v/v). Inset: visual fluorescence changes of sensors 1 and 2 + Al3+ under illumination with a 365 nm UV lamp.
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Fig. 4. UV–vis absorption spectra of sensors 1 (a) and 2 (b) (0.1 mM) with gradual addition of Al3+ (0–1 equiv.) in DMSO/H2O (19:1, v/v), pH = 5.2.
Fig. 5. Fluorescence spectra of sensors 1 (a) and 2 (b) (0.1 mM) with gradual addition of Al3+ (0–1 equiv.) in DMSO/H2O (19:1, v/v), pH = 5.2.
the solution of sensor 1, the absorption band at 339 nm significantly decreased, a new band centered at 391 nm appeared with increasing intensity and a well-defined isosbestic point at 360 nm was observed, which clearly indicated the presence of a new complex in equilibrium [46,47]. For sensor 2 (Fig. 4b), the absorption spectrum is characterized by two bands centered at 301 and 336 nm. Upon addition of Al3+ to the solution of sensor 2, the UV–vis response showed an obvious concentration-dependent blue shift of the maximum absorption peak from 336 to 327 nm, and a new band centered at 396 nm appeared with a well-defined isosbestic point at 366 nm, also clearly indicating the presence of a new complex in equilibrium. In the fluorescence titration experiments, the emission peaks of sensors 1 and 2 at 493 and 476 nm, respectively, were significantly enhanced and reached a maximum when 1.0 equiv. of Al3+ was added (Fig. 5a and b). 3.5. Competitive selectivity experiments To check the practical applicability of sensors 1 and 2 as selective fluorescent sensors for Al3+, competitive selectivity experiments of sensors 1 and 2 toward various metal ions were examined in DMSO/H2O (19:1, v/v) solution at the pH value of 5.2. As shown in Fig. 6a and b, sensors 1 and 2 were treated with 1.0 equiv. of Al3+ in the presence of other metal ions. Relatively low interferences were observed for the detection of Al3+ in the presence of other metal ions, except for Cu2+ and Fe3+, which could be attributed to their inherent magnetic properties [48–50]. The major competing ions, In and Ga, also showed relatively low interferences for the detection of Al3+. 3.6. Stoichiometry measurements In order to validate the stoichiometry of sensors 1 or 2 to Al3+, we performed Job plots in DMSO/H2O (19:1, v/v) solution. As
Fig. 6. Competitive selectivity experiments of sensors 1 (a) and 2 (b) (0.1 mM) toward various metal ions (0.1 mM), pH = 5.2. The yellow bars represent the fluorescence intensity of sensors 1 and 2 in the presence of 1 equiv. Al3+ and the blue bars represent the fluorescence intensity of sensors 1 and 2 + Al3+ after subsequent addition of other metal ions (kmax = 491 and 473 nm for sensors 1 and 2, respectively). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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shown in Fig. 7a and b, the maximum points appeared at a mole fraction of 0.5 for sensors 1 and 2, respectively, which indicated that the stoichiometry of sensors 1 and 2 to Al3+ was 1:1. These results were further confirmed by the appearance of a peak at m/z 1035.8, assignable to [2sensor 1 or 2 + 2Al3+-4H] in the LC–MS (Fig. 8a and b). More importantly, from titration profiles, the binding constants of sensors 1 and 2 for Al3+ were estimated as
2.01 104 (R2 = 0.984) and 5.46 105 (R2 = 0.995), respectively, based on the modified Benesi–Hildebrand equation (Fig. 9a and b). In addition, from the fluorescence titration experiments, the detection limits of sensors 1 and 2 for Al3+ were found to be 1.49 108 and 1.51 108 M, respectively (Fig. 10a and b), which are below the WHO acceptable limit (7.4 lM) in drinking water.
Fig. 7. Job plots for determining the stoichiometry of sensors 1 (a) and 2 (b) to Al3+ in DMSO/H2O (19:1, v/v) (kmax = 491 and 473 nm for sensors 1 and 2, respectively) (XAl = [Al3+]/([Al3+] + [sensor 1 or 2]), the total concentration of sensor 1 or 2 and Al3+ was 10 mM).
Fig. 8. LC–MS of [2sensor 1 + 2Al3+-4H] (a) and [2sensor 2 + 2Al3+-4H] (b).
Fig. 9. Hill Plots of sensors 1 (a) and 2 (b).
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Fig. 10. The detection limits of sensors 1 (a) and 2 (b) for Al3+ based on fluorescence titration experiments.
3.6.1. 1H NMR titration In order to further validate the conjugations of sensors 1 and 2 to Al3+, 1H NMR titrations were used to supplement the above experiments. Increasing concentrations of Al3+ (as its nitrate salt) were added to the DMSO-d6 solutions of sensors 1 and 2, respectively. Significant spectral changes were observed, as shown in Fig. 11a and b. In the 1H NMR of the free sensors 1 and 2, the peaks at 12.21 or 12.16 ppm and 10.86 or 10.79 ppm correspond to the
protons of ACOANH and AOH, respectively. Upon addition of Al3+, the proton peaks of the AOH at 10.86 or 10.79 were weakened significantly, this phenomena could be attributed to the loss of AOH protons. The peaks of the ACOANH protons at 12.21 or 12.16 ppm became double peaks and downfield shifted, and the signals of the ACH@N protons at 8.63 or 8.74 ppm downfield shifted in the presence of Al3+, these results could also be attributed to the binding of Al3+, which could strengthen the electronwithdrawing ability of the carbonyl group oxygen and the imine nitrogen. The adjacent protons of the ACOANH group downfield shifted due to the reduction of electron density. 3.7. Proposed sensing mechanisms of sensors 1 and 2 to Al3+ From the differences in the 1H NMR spectra of sensors 1 and 2 in the absence and presence of Al3+, coupled with LC–MS and the Job plot analysis, we have demonstrated that sensors 1 and 2 can chelate Al3+ through interactions with the carbonyl group oxygen, imine nitrogen and oxygen of the phenolic hydroxyl group, leading to 1:1 complexes (Scheme 1) [51,52]. In DMSO/H2O (19:1, v/v) solution, sensors 1 and 2 exhibited weak fluorescence, probably because of the combination of non-radiative processes, including photo-induced electron transfer from the lone pair electrons of the imine moiety to the 5,50 -methylenebis(salicylaldehyde) fluorophore and isomerizations of the ACH@N bonds in the excited state. Upon addition of Al3+, fluorescence enhancements were observed as a result of the coordinations of sensors 1 and 2 to Al3+, which resulted in an increase of its structural rigidity and the inhibition of the photo-induced electron transfer and ACH@N isomerization processes [53–56].
Fig. 11. 1H NMR spectra of sensors 1 (a) and 2 (b) to Al3+ in DMSO-d6/D2O (19:1, v/ v).
Scheme 1. Proposed sensing mechanisms of sensors 1 or 2 to Al3+.
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4. Conclusions Two highly selective and sensitive fluorescent sensors based on 5,50 -methylenebis (salicylaldehyde) for Al3+ detection were synthesized. The 1:1 stoichiometries of the Al complexes were determined by Job plot analysis, LC–MS data and 1H NMR studies. The sensing mechanisms of these two sensors to Al3+ were proposed. The detection limits of these two sensors for Al3+ can reach the 108 M level, far below the WHO guideline for drinking water (7.4 lM). Further efforts towards the construction of other Al3+ fluorescent sensors are currently underway in our laboratory.
Acknowledgements We are thankful for financial support from the foundation of Guizhou Province, China (No. [2014] 7619), the teaching reform and research project of the Guizhou University, China (No. JG 2013014), and the opening foundation of the Key Laboratory of Green Pesticide and Agricultural Bioengineering, Ministry of Education, Guizhou University, (No. 2016GDGP0102), the Science and Technology Innovation Foundation of the Guizhou University (No. 2016009).
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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Research paper
Two 5,50 -methylenebis(salicylaldehyde)-based Schiff base fluorescent sensors for selective sensing of Al3+ in DMSO/H2O solution Wang Ruo b, Jiang Guang-Qi a,b,⇑, Li Xiao-Hong b a State Key Laboratory Breeding Base of Green Pesticide and Agricultural Bioengineering, Key Laboratory of Green Pesticide and Agricultural Bioengineering, Ministry of Education, Guizhou University, Guiyang 550025, China b College of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China
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Article history: Received 6 September 2016 Received in revised form 3 November 2016 Accepted 10 November 2016 Available online 11 November 2016 Keywords: Fluorescent sensor Schiff base Aluminum
a b s t r a c t Two 5,50 -methylenebis(salicylaldehyde)-based Schiff base fluorescent sensors, 1 and 2, were synthesized. Sensors 1 and 2 both showed selective fluorescent sensing for Al3+ with low detection limits of 1.49 108 and 1.51 108 M, respectively, in DMSO/H2O (19:1, v/v) solution at the pH value of 5.2. The 1:1 stoichiometries of the Al complexes were determined by Job plots and further verified with LC–MS data and 1H NMR studies. The sensing mechanisms of sensors 1 and 2 to Al3+ were proposed. The fluorescence color changes could be easily detected by the naked eye under a UV lamp. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction After oxygen and silicon, aluminum is the third most abundant element in the Earth’s crust and is the most common metal ion in the environment because of acidic rain and human activities [1,2]. Studies have shown that the deposition of aluminum in bone and the nervous system in the human body can cause neurotoxicity in high dosages in the form of microcytic hypochromic anemia, aluminum-related bone disease, encephalopathy, dementia, myopathy and Alzheimer’s disease, in addition, high concentrations of aluminum ions can hamper plant growth [3–12]. According to a WHO report, the average daily human intake of aluminum is around 3–10 mg. The tolerable weekly aluminum dietary intake in the human body is estimated to be 7 mg/kg of body weight [13,14]. Thus, the design of chemosensors for detecting aluminum in aqueous medium is of great significance. Schiff bases are good ligands for metal ions, with which they can form coordinate bonds. This characteristic has resulted in their extensive use in the production of sensors [15–25]. Many analytical methods have played vital roles in the detection of metal ions, including ion selective electrodes [26–28], voltammetric [29–31] and colorimetric sensors [32–37]. Recent research ⇑ Corresponding author at: College of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China. E-mail address: [email protected] (J. Guang-Qi). http://dx.doi.org/10.1016/j.ica.2016.11.008 0020-1693/Ó 2016 Elsevier B.V. All rights reserved.
has highlighted a growing number of fluorescent sensors that have been designed and synthesized based on bis-Schiff bases, owing to their nitrogen-oxygen-rich coordination environments for metal ions [38–41]. In the present work, two fluorescent sensors, 1 and 2, based on 5,50 -methylenebis(salicylaldehyde) have been investigated for their sensing abilities toward common metal cations. Sensors 1 and 2 both show selective fluorescent sensing abilities for Al3+ in DMSO/H2O (19:1, v/v) solution. The detection limits of sensors 1 (1.49 108 M) and 2 (1.51 108 M) are lower than the WHO guideline (7.4 lM) for drinking water [42].
2. Experimental 2.1. Materials and instrumentation Metal salts were purchased commercially (Aldrich and Alfa Aesar Chemical Co., Ltd.). The solutions of metal ions were prepared from chlorides of K+, Ca2+, Mn2+, Cr3+ and nitrate salts of Al3+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Hg2+, Ga3+, In3+ and Mg2+. All fluorescence measurements were made on a Cary Eclipse Fluorescence Spectrometer (Varian) in a 1 cm quartz cell. 1H NMR (solvent CDCl3 or DMSO-d6) spectral analyses were performed on a JEOL-ECX 500 NMR spectrometer at room temperature using TMS as an internal standard. LC–MS was determined on a Agilent LC/ MSD Trap based on infusion methods. UV–Vis absorption spectra
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Fig. 1. Structures of the Schiff base fluorescent sensors 1 and 2.
were recorded on a UV-1800 spectrophotometer (Beijing General Instrument Co., China) in a 1 cm quartz cell. 2.2. Preparation of Schiff base The Schiff base fluorescent sensors 1 and 2 were prepared according to previously described procedures [43] (see Fig. 1).
Fig. 3. Effects of pH towards the fluorescence intensity of sensors 1 and 2 (0.1 mM) for Al3+ (5.0 eq.) at 491 and 473 nm, respectively.
3.2. Fluorescence response of sensors 1 and 2 to Al3+ 3. Results and discussion 3.1. General information Solutions of metal ions were prepared from the corresponding metal chloride or nitrate salts. Stock solutions of metal ions (1 mM) were prepared by dissolving the desired amount of metal salts in distilled water. Stock solutions of sensors 1 and 2 (0.1 mM) were prepared in DMSO solution. The pH values were adjusted by a hexamethylenetetramine-HCl buffer solution. The detection limit was evaluated based on fluorescence titration and then calculated with the equation 3r/K, where r is the standard deviation of blank measurements and K is the slope of intensity versus sample concentration [44]. The binding constant values were determined from the emission intensity data, following the modified Benesi–Hildebrand equations [45].
1 1 1 þ ¼ F F min KðF max F min Þ½Al3þ F max F min where Fmin, F and Fmax are the emission intensities of the organic moiety considered in the absence of aluminum ions, at an intermediate aluminum concentration and at a concentration of complete interaction, respectively, and where K is the binding constant concentration.
The fluorescence responses of sensors 1 and 2 (10 lM) to several metal ions, such as Al3+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Mn2+, Cr3+, Cd2+, Pb2+, Hg2+, K+, Ca2+, Ga3+, In3+ and Mg2+, in DMSO/H2O (19:1, v/v) solution were investigated. As shown in Fig. 2, the free sensors 1 and 2 showed only weak fluorescence emissions at 491 and 473 nm, respectively. With the addition of various metal ions, there are no remarkably changes in their fluorescence spectra, except in the case of Al3+. The fluorescence intensity showed an enhancement of 70- and 40-fold for sensors 1 and 2, respectively, in the presence of Al3+, relative to that of the free sensors. 3.3. Effect of pH towards the fluorescence intensity The sensitivity of the fluorescent sensors complexes (1 + Al3+ or 2 + Al3+) at different pH values was studied in a DMSO/H2O (1/9, v/v) solution. Experiments were conducted by adjusting pH using NaOH and HCl solutions. As depicted in Fig. 3, both sensors were found to be quite effective near the pH of 5.2. Further studies were performed under the condition of pH value equal to 5.2. 3.4. The spectroscopic titration experiments In DMSO/H2O (19:1, v/v) solution at the pH value of 5.2, the absorption spectrum of sensor 1 shows two absorption bands centered at 339 and 268 nm (Fig. 4a). Upon addition of Al3+ to
Fig. 2. Fluorescence intensity of sensors 1 (a) and 2 (b) (1 mM) upon addition of various metal ions (5 equiv.) in DMSO/H2O (19:1, v/v). Inset: visual fluorescence changes of sensors 1 and 2 + Al3+ under illumination with a 365 nm UV lamp.
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Fig. 4. UV–vis absorption spectra of sensors 1 (a) and 2 (b) (0.1 mM) with gradual addition of Al3+ (0–1 equiv.) in DMSO/H2O (19:1, v/v), pH = 5.2.
Fig. 5. Fluorescence spectra of sensors 1 (a) and 2 (b) (0.1 mM) with gradual addition of Al3+ (0–1 equiv.) in DMSO/H2O (19:1, v/v), pH = 5.2.
the solution of sensor 1, the absorption band at 339 nm significantly decreased, a new band centered at 391 nm appeared with increasing intensity and a well-defined isosbestic point at 360 nm was observed, which clearly indicated the presence of a new complex in equilibrium [46,47]. For sensor 2 (Fig. 4b), the absorption spectrum is characterized by two bands centered at 301 and 336 nm. Upon addition of Al3+ to the solution of sensor 2, the UV–vis response showed an obvious concentration-dependent blue shift of the maximum absorption peak from 336 to 327 nm, and a new band centered at 396 nm appeared with a well-defined isosbestic point at 366 nm, also clearly indicating the presence of a new complex in equilibrium. In the fluorescence titration experiments, the emission peaks of sensors 1 and 2 at 493 and 476 nm, respectively, were significantly enhanced and reached a maximum when 1.0 equiv. of Al3+ was added (Fig. 5a and b). 3.5. Competitive selectivity experiments To check the practical applicability of sensors 1 and 2 as selective fluorescent sensors for Al3+, competitive selectivity experiments of sensors 1 and 2 toward various metal ions were examined in DMSO/H2O (19:1, v/v) solution at the pH value of 5.2. As shown in Fig. 6a and b, sensors 1 and 2 were treated with 1.0 equiv. of Al3+ in the presence of other metal ions. Relatively low interferences were observed for the detection of Al3+ in the presence of other metal ions, except for Cu2+ and Fe3+, which could be attributed to their inherent magnetic properties [48–50]. The major competing ions, In and Ga, also showed relatively low interferences for the detection of Al3+. 3.6. Stoichiometry measurements In order to validate the stoichiometry of sensors 1 or 2 to Al3+, we performed Job plots in DMSO/H2O (19:1, v/v) solution. As
Fig. 6. Competitive selectivity experiments of sensors 1 (a) and 2 (b) (0.1 mM) toward various metal ions (0.1 mM), pH = 5.2. The yellow bars represent the fluorescence intensity of sensors 1 and 2 in the presence of 1 equiv. Al3+ and the blue bars represent the fluorescence intensity of sensors 1 and 2 + Al3+ after subsequent addition of other metal ions (kmax = 491 and 473 nm for sensors 1 and 2, respectively). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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shown in Fig. 7a and b, the maximum points appeared at a mole fraction of 0.5 for sensors 1 and 2, respectively, which indicated that the stoichiometry of sensors 1 and 2 to Al3+ was 1:1. These results were further confirmed by the appearance of a peak at m/z 1035.8, assignable to [2sensor 1 or 2 + 2Al3+-4H] in the LC–MS (Fig. 8a and b). More importantly, from titration profiles, the binding constants of sensors 1 and 2 for Al3+ were estimated as
2.01 104 (R2 = 0.984) and 5.46 105 (R2 = 0.995), respectively, based on the modified Benesi–Hildebrand equation (Fig. 9a and b). In addition, from the fluorescence titration experiments, the detection limits of sensors 1 and 2 for Al3+ were found to be 1.49 108 and 1.51 108 M, respectively (Fig. 10a and b), which are below the WHO acceptable limit (7.4 lM) in drinking water.
Fig. 7. Job plots for determining the stoichiometry of sensors 1 (a) and 2 (b) to Al3+ in DMSO/H2O (19:1, v/v) (kmax = 491 and 473 nm for sensors 1 and 2, respectively) (XAl = [Al3+]/([Al3+] + [sensor 1 or 2]), the total concentration of sensor 1 or 2 and Al3+ was 10 mM).
Fig. 8. LC–MS of [2sensor 1 + 2Al3+-4H] (a) and [2sensor 2 + 2Al3+-4H] (b).
Fig. 9. Hill Plots of sensors 1 (a) and 2 (b).
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Fig. 10. The detection limits of sensors 1 (a) and 2 (b) for Al3+ based on fluorescence titration experiments.
3.6.1. 1H NMR titration In order to further validate the conjugations of sensors 1 and 2 to Al3+, 1H NMR titrations were used to supplement the above experiments. Increasing concentrations of Al3+ (as its nitrate salt) were added to the DMSO-d6 solutions of sensors 1 and 2, respectively. Significant spectral changes were observed, as shown in Fig. 11a and b. In the 1H NMR of the free sensors 1 and 2, the peaks at 12.21 or 12.16 ppm and 10.86 or 10.79 ppm correspond to the
protons of ACOANH and AOH, respectively. Upon addition of Al3+, the proton peaks of the AOH at 10.86 or 10.79 were weakened significantly, this phenomena could be attributed to the loss of AOH protons. The peaks of the ACOANH protons at 12.21 or 12.16 ppm became double peaks and downfield shifted, and the signals of the ACH@N protons at 8.63 or 8.74 ppm downfield shifted in the presence of Al3+, these results could also be attributed to the binding of Al3+, which could strengthen the electronwithdrawing ability of the carbonyl group oxygen and the imine nitrogen. The adjacent protons of the ACOANH group downfield shifted due to the reduction of electron density. 3.7. Proposed sensing mechanisms of sensors 1 and 2 to Al3+ From the differences in the 1H NMR spectra of sensors 1 and 2 in the absence and presence of Al3+, coupled with LC–MS and the Job plot analysis, we have demonstrated that sensors 1 and 2 can chelate Al3+ through interactions with the carbonyl group oxygen, imine nitrogen and oxygen of the phenolic hydroxyl group, leading to 1:1 complexes (Scheme 1) [51,52]. In DMSO/H2O (19:1, v/v) solution, sensors 1 and 2 exhibited weak fluorescence, probably because of the combination of non-radiative processes, including photo-induced electron transfer from the lone pair electrons of the imine moiety to the 5,50 -methylenebis(salicylaldehyde) fluorophore and isomerizations of the ACH@N bonds in the excited state. Upon addition of Al3+, fluorescence enhancements were observed as a result of the coordinations of sensors 1 and 2 to Al3+, which resulted in an increase of its structural rigidity and the inhibition of the photo-induced electron transfer and ACH@N isomerization processes [53–56].
Fig. 11. 1H NMR spectra of sensors 1 (a) and 2 (b) to Al3+ in DMSO-d6/D2O (19:1, v/ v).
Scheme 1. Proposed sensing mechanisms of sensors 1 or 2 to Al3+.
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4. Conclusions Two highly selective and sensitive fluorescent sensors based on 5,50 -methylenebis (salicylaldehyde) for Al3+ detection were synthesized. The 1:1 stoichiometries of the Al complexes were determined by Job plot analysis, LC–MS data and 1H NMR studies. The sensing mechanisms of these two sensors to Al3+ were proposed. The detection limits of these two sensors for Al3+ can reach the 108 M level, far below the WHO guideline for drinking water (7.4 lM). Further efforts towards the construction of other Al3+ fluorescent sensors are currently underway in our laboratory.
Acknowledgements We are thankful for financial support from the foundation of Guizhou Province, China (No. [2014] 7619), the teaching reform and research project of the Guizhou University, China (No. JG 2013014), and the opening foundation of the Key Laboratory of Green Pesticide and Agricultural Bioengineering, Ministry of Education, Guizhou University, (No. 2016GDGP0102), the Science and Technology Innovation Foundation of the Guizhou University (No. 2016009).
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