b State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, China ;
c College of Overseas Education, Nanjing Tech University, Nanjing 211816, China
The design of fluorescent chemosensors that can selectively recognize and sense specific cations has attracted increasing attention due to their significance in environmental settings [1, 2]. Compared with traditional methods that detect metal ions such as AAS (atomic absorption spectrometer), ICP (inductively coupled plasma emission spectrometer) [3, 4], fluorescent chemosensor method has a number of advantages such as high sensitivity, selectivity, versatility, rapidity, direct visual perception and relatively simple handling .
Aluminum is the most abundant metal element in the earth crust and is extensively used in daily life [6, 7]. The general population is exposed to aluminum from its widespread use in water treatment, food additive, aluminum-based pharmaceuticals, occupational dusts, aluminum containers and cooking utensils. The World Health Organization (WHO) prescribed the average human intake of aluminum as around 3-10 mg/day with a weekly dietary intake of 7 mg/kg [8-10]. Aluminum in the human body is accumulated slowly and the toxicity is chronic and imperceptible. Its toxicity may cause damage of the central nervous system, and is suspected to play a role in neurodegenerative diseases such as Alzheimer’s and Parkinson’s diseases [11, 12]. Therefore, highly selective and sensitive chemosensors for Al3+ are particularly needed. However, the detection of Al3+ has always been challenging due to its lack of spectroscopic characteristics and poor coordination ability compared to transition metals . Some Al3+ fluorescent sensors based on small molecules have been reported so far . Most of the Al3+ probes are found to coordinate with atoms like N and O [15-18]. For example, Yu-Ping Dong et al. synthesized a water-soluble probe for the real-time and "turn-on" detection of Al3+ by the introduction of three carboxylate functional groups into anaryl-substituted pyrrole derivative. It exhibits rapid response, excellent selectivity, and sensitivity to Al3+. However, most of these reported Al3+ sensors suffer from interference of Zn2+ or Cu2+ ions, and require complicated synthetic procedures [19-21].
Most of the fluorescent chemosensors for cations are composed of a cation recognition unit (ionophore) together with a fluorogenic unit (fluorophore) . Among fluorophores, pyrene is known as one of the most useful fluorogenic units since it displays not only a well-defined monomer emission at 375 nm but also an efficient excimer emission around 475 nm [22-24]. Xin Sun et al. reported a bifunctional probe based on a pyrene-amino acid conjugate for the differential response of Al3+ and H+ . Two solvent-dependent sensing mechanisms for Al3+, which feature a ratiometric change from excimer to monomer in CH3OH and a turn-on response in water, are also disclosed.
Herein, we designed a novel ratiometric fluorescent chemosensors (Z)-N-((pyren-4-yl)methylene)(furan-2-yl)-methanamine(BF)for the detection of Al3+ based on a Monomer/Excimer Switch mechanism. The BF was obtained through the condensation of pyrene-1-carboxaldehyde with furfurylamine. Photophysical properties of this new derivative were examined and we demonstrated that it could selectively detect Al3+ via colorimetric and ratiometricfluorescent channels.2. Experimental
Pyrene-1-carboxaldehyde (Aldrich, USA), furfurylamine (Aldrich, USA) were used as received. All other chemicals and solvents were of analytical grade and used without further purification. The 1H and 13C NMR spectra were measured on a BruckerAvance 400 spectrometer in CDCl3. Electrospray ionization mass spectra (ESI-MS) were measured on a Micromass LCTTM system. UV-visible spectra were performed on a Perkin-Elmer 35 spectrometer and fluorescent measurements were recorded on a Perkin-Elmer LS 50B fluorescence spectrophotometer.
Synthesis of compound BF: To a methanol solution of 2-(aminomethyl) furan 99% (3 mmol, 0.291 g in 5 mL dry methanol) was added a solution of 1-pyrenecarboxaldehyde (3 mmol, 0.690 g in 25 mL methanol). The reaction mixture was refluxed for 4 h. Then the mixture was allowed to cool to room temperature to produce a yellow precipitate. The precipitate was filtered on a Buchner funnel, washed several times with cold methanol and dried in air to form yellow crystals of BF. Yield, 85.2%; 1H NMR (400 MHz, CDCl3):δ 9.36 (s, 1H), 8.85 (d, 1H, J=9.3 Hz), 8.56 (d, 1H, J=8.1 Hz), 8.38-7.82 (m, 9H), 7.45 (dd, 1H, J=1.7, 0.7 Hz), 5.00 (s, 2H); 13C NMR (101 MHz, CDCl3):δ 161.69, 142.29, 133.03, 131.15, 130.53, 130.02, 128.74, 128.66, 128.38, 127.45, 126.43, 126.13, 125.94, 125.70, 124.93, 124.81, 124.63, 122.50, 110.49, 58.17. Elemental analysis as calculated for C22H15NO (%): C, 85.41; H, 4.89; N, 4.53. Found (%): C, 85.52; H, 4.80; N, 4.46. TOF-MS: m/z 310.1 [M+H]+, 332.0 [M+Na]+.
For the UV-vis and fluorescence titrations, a stock solution of 1.0 mmol/L BF was prepared in ethanol. Al(NO3)3·9H2O was dissolved in water to form a 1.0 mmol/L stock solution. For competing metal ions, various metal ion solutions of NaNO3, Co(NO3)2, KNO3, Zn(NO3)2·6H2O, Ni(NO3)2·6H2O, Mg(NO3)2·6H2O, Ca(NO3)2·4H2O, MnCl2·4H2O, Pb(NO3)2, HgCl2, AgNO3, Ba(NO3)2, FeCl2·4H2O, CdNO3·4H2O, CrCl3·6H2O, Cu(NO3)2·3H2O were used. Working solutions of BF, Al3+ and other ions were prepared from their respective stock solutions. The totally mixed resulting solutions were detected in a quartz optical cell with a 1 cm optical path length. Before fluorescence and UV/vis titration investigations were conducted, the stock solution of BF was mixed with the stock solutions of metal salts in a 10 mL volumetric flask and diluted with ethanol-H2O (9:1, v/v, pH 7.2, HEPES buffer) to volume. Spectral data were recorded immediately after the addition. For fluorescence measurements, excitation wavelength was provided at 357 nm, and emission wavelength was collected from 375 nm to 600 nm.3. Results and discussion
BF was prepared based on a simple Schiff base formation between 1-pyrenecarboxaldehyde and 2-(aminomethyl) furan in methanol as shown in Scheme 1. The compound was characterized and confirmed by 1H NMR, 13C NMR, ESI-MS (Figs. S1-S3 in Supporting information) and EA.
We first investigated the influences of pH to the "BFAl3+"system. The pH dependence experiment revealed that the ratio of fluorescence intensity at 503 nm and 407 nm (Ⅰ503/Ⅰ407) for the solutions BF and (BF+Al3+) could remain stable in a wide pH range from 4.0 to 10.0 (Fig. S4 in Supporting Information). So we select ethanol-H2O (9:1, v/v, pH 7.2, HEPES buffer) as the preferential solution in the following photophysical characterization studies.
The UV/vis spectrum of BF in a solvent system of ethanol-H2O (9:1, v/v, pH 7.2, HEPES buffer) in Al3+ titration was shown in Fig. 1a. The peak at 360 nm in the UV-vis spectra decreased gradually upon the addition of Al3+, while new bands developed at 446 nm and there is an isosbestic point at 375 nm. The inset in Fig. 1a shows the absorbance at 446 nm as a function of Al3+ concentrations. These spectral changes could also be observed visually (Fig. 1c). The colorless solution of BF turned to green as soon as Al3+ was added. It clearly demonstrates the potential of BF as a colorimetric probe for Al3+. In addition, the addition of one equivalent of transition and post-transition metal ions (Na+, K+, Ca2+, Mg2+, Ni2+, Ba2+, Ag+, Zn2+, Mn2+, Cu2+, Cd2+, Pb2+, Co2+, Hg2+ and Fe2+) did not induce spectral changes in the long wavelength absorption band with the sole exception of Cr3+ (the absorption of BF-Cr3+ is about 1/3 of BF-Al3+, as shown in Fig. 1b). These results indicate that chemosensor BF has a high selectivity for Al3+.
|Figure 1. (a) Changes of the UV-vis spectra of BF (10 μmol/L) in ethanol-H2O (9:1, v/v, pH 7.2, HEPES buffer) with externally adding Al3+ (0-75 μmol/L). (Inset) Plot of absorbance of BF as a function of [Al3+] at 446 nm. (b) UV visible spectra changes observed upon the addition of various ions (1 equiv.) to the solutions of BF (10 μmol/L). (c) The naked eye color changes of BF upon the addition of Al3+ and other ions.|
Fig. 2a shows the fluorescence spectra of BF with the addition of Al3+. BF exhibited a strong monomer emission at 407 nm, while the addition of Al3+ induced a dramatic change of its emission spectrum where the monomer emission diminished significantly at the cost of excimer emission enhancement at 503 nm with a clear isoemissive point at 475 nm. The emission color changed from light blue to bright green, which could be observed by a naked eye (Fig. 2b inset). An approximately linear relationship (R2=0.985) was observed with Al3+ concentrations ranging from 1 μmol/L to 35 μmol/L, and the fluorescence intensity ratio at Ⅰ503/Ⅰ407 increased from 0.2 to 2.0 (Fig. 2a inset), suggesting that BF would be useful for quantification of Al3+ concentrations over a wide dynamic range. We propose that two BF units interact with Al3+ resulting in an intermolecular stacking of two pyrene fragments. For practical applications, we calculated the detection limit on the basis of 3σ/K , where σ is the fluorescence intensity ratio of BF in the absence of Al3+ and K is the slope of the intensity ratio versus sample concentration plot (Fig. 2b). The detection limit is 1.04 μmol/L for Al3+, which is superior/comparable to the majority of other reported probes (Table 1). Hence, the BF can be applied to detect Al3+ in micron range, which could fulfill the requirement as a fluorescent chemosensor for Al3+.
|Figure 2. (a) Changes in the fluorescent spectra of BF (1 μmol/L, ethanol-H2O (9:1, v/v, pH 7.2, HEPES buffer)) upon gradual addition of Al3+ (0-42.5 μmol/L). (Inset) The ratio of fluorescence intensity at 503 nm and 407 nm (Ⅰ503/Ⅰ407) as function of Al3+ concentrations. (b) The ratio of fluorescence intensity at 503 nm and 407 nm (Ⅰ503/Ⅰ407) as function of Al3+ concentrations (1-5.5 μmol/L). (Inset) The fluorescence color of only BF (left) and BF with Al3+ (right).|
An important feature of the chemosensor is its high selectivity toward the analyte over other competitive species. The effect of common metal ions on the fluorescence spectra of BF was studied (Fig. 3). The introduction of 10 μmol/L Al3+ into a BF ethanol-H2O (9:1, v/v, pH 7.2, HEPES buffer) solution (1 μmol/L) induced a remarkable change in the fluorescence intensity, while the addition of other metal ions caused very weak alterations. With Cr3+ ions, however, a slightly response was observed. The discrimination between Al3+ and Cr3+ ions could be realized ratiometrically using the large changes in Al3+ to trigger the monomer-excimer switching of BF. Thus, BF could be used as selective Al3+ probe.
|Figure 3. Fluorescence responses of 1 μmol/LBF to various 10 μmol/L transition metal ions. Spectra were acquired in ethanol-H2O (9:1, v/v, pH 7.2, HEPES buffer). Bars represent the ratio of fluorescence intensity at 503 nm (Ⅰ503) over that at 407 nm (Ⅰ407). The black bars represent the fluorescence emission of 1 μmol/L BF solution with different competing metal ions. The red bars represent the change of the emission that occurs on the subsequent addition of 10 μmol/L Al3+ to the above solutions.|
To test whether the proposed complex could be reversed, we used a similar method to that of Bing-Qin Yang et al. . We added EDTA (10 μmol/L) to the solutions of BF-Al3+ species. The addition of EDTA could restore the initial value of free probe (Fig. 4a), and the color green faded. Thus, the probes could be revived upon the addition of EDTA and reused up to 4 cycles as demonstrated in Fig. 4b. These indicate that BF has a good reversibility.
|Figure 4. (a) Reversibility of Al3+ coordination to probe BF by EDTA. Black line: free probe BF (1 μmol/L), red line: probe BF + 10 equiv. of Al3+, Blue line: probe BF + 10 equiv. of Al3+ + 10 equiv. of EDTA. (b) Fluorescence intensity at 503 nm for four cycles of the switching process.|
To quantify the complexation ratio between BF and the Al3+, the Job’s plot was drawn using absorption of the probes and Al3+ at the maximum absorption wavelength plotted against continuous variation with a total concentration of 100 μmol/L ([BF]+[Al3+]) (Fig. 5a). The maximum point appears at the mole fraction of 0.35, indicating a 2:1 stoichiometry of the [BF-Al3+] system.
|Figure 5. (a) The Job’s plot of chemosensor BF, with a total concentration of ([BF] + [Al3+])=100 μmol/L. The detection wavelength was 446 nm. (b) The Benesi-Hildebrand plot for the determination of the association constant of BF with Al3+ (λem=446 nm).|
where A is the obtained absorbance of the probe at the maximum absorption wavelength after the addition of Al3+, and A0 stands for the initial absorbance of free probe. Also, K is the binding constant and n is the number of Al3+ atoms bound per BF (here, n=1/2). When the 1/(A -A0) was plotted as a function of 1/[Al3+]1/2 (y=A + Bx), a linear relationship was obtained (R2=0.974), further proving the 2:1 stoichiometry. The binding constant value was calculated as 1.1 × 103 (mol/L)1/2 (BF/Al3+).
In order to support the binding of Al3+with the receptor BF, the 1H NMR titration was performed in methanol-d (Fig. 6). The 1H NMR spectrum of BF contained signals for the HC5 5N (azomethine, H) at 9.44 ppm and furan protons appeared at 7.54, 8.72 and 8.40 ppm. The pyrene protons appeared in the region of 8.00-8.24 ppm. The chemical shifts for other protons appeared in the usual positions. Addition of 1 equiv. of Al(NO3)3·9H2O led to a down field shift of the azomethine proton (10.17 ppm) and the proton on the furan ring (7.70 ppm). The methylene proton (-CH2) that connected with the azomethine shifted about 0.39 ppm down-field compared to that in the 1H NMR spectrum of the parent ligand BF (4.97 ppm). All other protons remained almost invariant in the presence of Al3+ ion. This clearly indicated that Al3+ was coordinately bound with both furan oxygen and imine nitrogen atom.
|Figure 6. 1HNMR spectraof BFand Al3+-BFcomplexin methanol-d on400 MHz Bruker Instrument.|
Based on the binding stoichiometry along with the alterations in the 1H NMR spectroscopy, the proposed reaction mechanism of BF and Al3+ was predicted in Scheme 2. The transformation into folded conformationmight be partly attributed to the participation of the imine group in the Schiff base and the O atoms in furan in the complex formation with the aluminum ions, which forced the two pyrene moieties to become relatively folded at face to face orientations. The conformational change resulted in the weak pyrene monomer emission to strong pyrene excimer emission switch.
In order to investigate the practical applications of the chemosensor BF, a paper strip test was performed by immersing filter papers into an ethanol-H2O (9:1, v/v, pH 7.2, HEPES buffer) solution of BF (10-4 mol/L) and then drying in air according to the method reported by Xin-Dong Jiang et al. . The color changes under sunlight and fluorescence image changes under 365 nm UV light in various concentrations of Al3+ are illustrated in Fig. 7. After adding Al3+ on the test paper, the color changing to yellow green could be observed clearly compared with the fluorescence image changing to blue green under the 365 nm UV lamp. Furthermore, the test paper strip was capable to detect Al3+ ion up to 10 μmol/L. Therefore, the BF-based test strips can conveniently detect Al3+ withoutanyadditionalequipment.They wereeasilyfabricatedand low-cost, useful in practical and efficient Al3+ test kits.
|Figure 7. Color changes (up) and fluorescence images (down) of test papers after adding different concentrations of Al3+ (a) 0, (b) 10-5 mol/L, (c) 10-4 mol/L, and (d) 10-3 mol/L.|
In summary, we report a one-step synthesis and characterization of a new fluorescent chemosensor for the distinct detection of Al3+ in ethanol-H2O (9:1, v/v, pH 7.2, HEPES buffer) solution. This simple molecule has ratiometric and colorimetric properties to detect the Al3+ with a color change from colorless to brilliant green. Probe BF can detect Al3+ without any interference from other metal ions. The UV-vis and fluorescence titrations help us to determine the Ka values as 1.1 × 103(mol/L)1/2 and the detection limit is 1.04 μmol/L. The reversible binding of Al3+ to BF was confirmed by reacting with EDTA. The complexation of BF to Al3+ was confirmed by 1H NMR and Job’s Plot studies. An Al3+ driven monomer to intermolecular excimer formation of BF has been observed. Furthermore, test papers have established the utility of the probes in monitoring Al3+, indicating its potential application to detect and estimate the concentration of Al3+ in environment.Acknowledgments
We are grateful for the financial support from the open fund of the State Key Laboratory of Materials-Oriented Chemical Engineering (No. KL13-07), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20123221110012), the China Postdoctoral Science Foundation (No. 2014M550287), and the Postgraduate Education and Innovation Project of Jiangsu Province (No. KYLX-0772).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.cclet.2016.04.011.
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