An aggregation-induced emission-based fluorescent chemosensor of aluminium ions

Abstract

Here we used an aggregation-induced emission (AIE) molecule, 9,10-bis[4-(3-sulfonatopropoxyl)-styryl]anthracene sodium salt (BSPSA), to act as a highly selective and sensitive fluorescent chemosensor for the detection of aluminium ions. This would widen the applications of AIE luminogens, and may show potentials for aluminium ion detection.

---

As the most widely spread metal and the third most abundant element in the Earth's crust (8.8%) after oxygen and silicon, aluminium has been widely exposed to people due to its wide use in food additives, aluminum-based pharmaceuticals, and cooking utensils. However, the toxicity of aluminium ion (Al³⁺) is becoming recognized and paid attention to because it is indicated to be involved as a causative factor in the etiology of neurological disorders, such as Parkinson's disease (PD),¹ Alzheimer's disease (AD)² and dialysis encephalopathy.³ Thus, the monitoring of Al³⁺ is crucial. It has been reported that Al³⁺ can be detected by various methods including atomic absorption spectrometry,⁴ inductively coupled plasma-atomic emission spectrometry,⁵ inductively coupled plasma-mass spectrometry⁶ and some electrochemical methods.⁷ However, these methods are relatively expensive and the interferences generated from the matrix or the overlapped spectra are still confused. There is therefore a need to develop a simple, selective and highly sensitive method to detect Al³⁺.

Recently, several optical methods based on chemiluminescence⁸ have been used for monitoring Al³⁺ combining with Au(I)⁹ or gold nanoparticles.¹⁰ Fluorescence-based methods have also been used for the detection of Al³⁺ according to the turn-on fluorescence signals by using some molecules, such as naphthol and naphthalene-based molecule,¹¹ molecularly imprinted polymer,¹² pyrrolidine constrained bipyridyl-dansyl click fluoroionophore,¹³ pyrimidine-based fluorescent receptor,¹⁴ 8-hydroxyquinoline-5-carbaldehyde Schiff-base,¹⁵ derivatives of oxazoline and imidazoline,¹⁶ or a hybrid ligand from BINOL scaffold and β-amino alcohol.¹⁷ However, the detection is commonly hard to extend to the higher concentration, because the luminescence will be weakened or quenched at high concentrations. This phenomenon is widely known as “concentration quenching”, which is mechanistically associated with the “aggregation-caused quenching (ACQ)”. To extend the practical applications, the further studies are still desirable.

Opposite to ACQ, Tang's group discovered an uncommon light emission induced by aggregate formation, termed as “aggregation-induced emission (AIE)”.¹⁸ The AIE process offers a platform to look into light emissions from luminogenic aggregates,¹⁹ and develops the efficient luminogens for the new applications in optoelectronic and sensing systems.²⁰ As reported, AIE luminogens have been utilized as sensitive and selective chemosensors and bioprobes for detection, such as detecting protein,²¹ amino groups,²² lectins,²³ CO₂ and Cl₂,²⁴ explosives, Hg²⁺,²⁵ Ag⁺,²⁶ CN⁻ and Zn²⁺.²⁷ In addition, a fluorescent sensor of a carboxylic salt was prepared for Al³⁺ sensing, while the linear range still need to be widened.²⁸ Recently, we developed a new AIE molecule to act as a highly sensitive “switch-on” AIE fluorescent probe for protein imaging.²⁹ Hence, it is possible to find a new AIE luminogen for the detection of Al³⁺.

In our studies, we developed a simple Al³⁺ chemosensor with highly sensitivity and selectivity based on aggregation induced emission of a molecule of 9,10-bis[4-(3-sulfonatopropoxyl)-styryl]anthracene sodium salt (BSPSA). According to our previous works, the BSPSA was synthesized^29,30 (the synthesis is shown in Experimental in ESI†). The optical properties of BSPSA were first examined by recording FL signals of BSPSA in different THF–water solution. THF was added into different value of water to obtain different THF fractions (0% to 98%). As shown in Fig. 1A, no remarkable fluorescence was recorded when BSPSA was dissolved in water (0%). However, when the THF fraction increased to 10%, a quite weak emission at about 510 nm appeared. With the further increase of THF fractions (10%–98%), the intensity of emission increased gradually. When the THF fraction increased to 80%, the fluorescence intensity was 10.9 times higher than the intensity in water. At 98%, the dramatically increased emission with a 36.8 times higher intensity than the one in water was resulted. In addition, an obviously red shift was observed (Fig. 1A), which might be generated from the different states of aggregation at different fractions of poor solvents.

Fig. 1 Fluorescence characteristics of BSPSA. (A) Fluorescence spectra of BSPSA in different THF fractions. (B) FL intensity of BSPSA (0.40 μM) at 510 nm versus THF fraction (λ_ex = 404 nm). Inset: the fluorescent imaging of BSPSA in the solutions with different THF fractions.

The fluorescence intensity of BSPSA versus THF fraction is shown in Fig. 1B, and the inset is the imaging of BSPSA in different solutions. By naked eyes, we can observe the fluorescence increase with the increase of THF fraction, and find the most dramatically increase from 80% to 98%. In the pure water (THF fraction = 0%), a quite weak blue emission was observed according to the weak emission at 432 nm, which might be generated from the anthracene ring. The blue-green emission was resulted at the THF fraction of 30%, which was mainly generated from the emissions at about 510 nm. With the further increase of THF, the intensity of emission at 510 nm increased gradually, and showed the green fluorescence at 80% and 98% of THF (Fig. 1A). Therefore, BSPSA proved to be an AIE luminogen with THF as the poor solvent.

The responses of BSPSA to Al³⁺ were recorded to demonstrate the potential application of BSPSA for Al³⁺ detection. THF–water (9 : 1, v/v) was selected as the solvent because the THF–water (9 : 1, v/v) can dissolve BSPSA well and has little effect on the optical properties of BSPSA monomer (Fig. 1B). In addition, it can also dissolve metal ions very well, which is highly desirable for the aqueous sample analysis. In the experiment, we added different amount of Al³⁺ stock solution (10 mM) to the 3 mL of BSPSA solution (4.75 μM), and then recorded the fluorescence spectra. Fig. 2 shows the spectra of BSPSA in the presence of different concentrations of Al³⁺ in THF–water (9 : 1, v/v). As resulted, 4.75 μM of BSPSA shows rather weak emission. However, with the addition of Al³⁺ to the BSPSA solution, the fluorescence intensity increases gradually and the emission band moves from 510 nm to 550 nm. This might be generated from the aggregation of BSPSA in the presence of Al³⁺.

Fig. 2 Fluorescence spectra of BSPSA (4.75 μM) in the presence of different concentrations of Al³⁺ in THF–water (9/1, v/v). [Al³⁺] = 0, 0.33, 0.67, 1.33, 2.0, 2.67, 4.0, 5.33, 7.0, 8.67, 10.67, 12.67, 15.33, 18.00, 21.33, 24.67, 29.33, 36.00, 42.67, 52.67, 72.67, 92.67 μM.

Fig. 3 shows the SEM images of BSPSA with and without addition of Al³⁺. As demonstrated, no obvious particle is observed in BSPSA solution (Fig. 3A), while the small particles of 1–3 μm are recorded with the addition of Al³⁺ (Fig. 3B). In addition, with the increase of THF faction for Al³⁺ added BSPSA solution, the diameters of small particles increased gradually, which could demonstrate the particles are generated from the aggregation (Fig. S1†). Therefore, the fluorescence increased along with the aggregation of BSPSA in the presence of Al³⁺. The soluble BSPSA shows weak luminescence in aqueous medium but it is emissive in the aggregate sate, which is in accordance with our previous work.²⁹

Fig. 3 SEM images of BSPSA (0.71 μM) in THF–water (9/1, v/v) without (A) and with addition of Al³⁺ (50 μM) (B).

The obtained BSPSA with two potential terminal-oxygen groups might be useful for the sensing of Al³⁺. As shown in Scheme S2,† BSPSA shows weak fluorescence in THF–water, while the strong fluorescence would be obtained in the presence of Al³⁺. This might be generated from the aggregation with intermolecular coordination of BSPSA with Al³⁺ by its terminal sulfonate groups. The coordination interaction could be preliminary concluded by the data of IR, UV/vis, and isothermal titration calorimetry (ITC) (see ESI†). When the sulfonate group in BSPSA forms the complex with Al³⁺ ion, the anthracene ring easily constructs the coplanar conformation with the benzyl ring, which could further favour the aggregation and suppressed intramolecular-rotation to cause non-radiative decay.^31,32 Thus, the FL signal is enhanced with the addition of Al³⁺ based on the AIE process. This is in accordance with the reported sensing of metal ions by AIE FL emissions generated from the binding of metal ions with –O or –N of AIE molecules.^25,26

The selectivity of the response of BSPSA to Al³⁺ was examined by recording fluorescence responses of BSPSA to 17 kinds of metal ions. These metal ions included Al³⁺, Ba²⁺, Ca²⁺, Cd²⁺, Co²⁺, Cr³⁺, Cu²⁺, Fe³⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺, Pb²⁺, Sr²⁺, Hg²⁺ and Zn²⁺. Compared with other metal ions, the extremely increased FL intensity was recorded with the addition of Al³⁺ to the BSPSA solution. As shown in Fig. 4, the fluorescence band with a maximum peak at about 550 nm was observed in THF–water (9 : 1, v/v) when Al³⁺ was added in the BSPSA solution. In addition, the fluorescence quenching was observed for Fe³⁺, showing a quite small band at about 510 nm. This quenching effect might be generated from the electron transfer between the BSPSA and metal ions.^33,34 The metal ion of Fe³⁺ possesses the relative high electron mobility with BSPSA due to a relative high standard electrode potential and small diameter comparing with other ions,³³ which facilitates electron transporting to lead to the structural relaxation of BSPSA molecules.³⁵ Thus, with the relaxation of BSPSA by Fe³⁺, the AIE process might be inhibited and lead to the FL quenching.²⁰ However, there was no obvious difference between the signals of the blank and other metal ions (showing the similar emission at about 510 nm). In addition, the red shift from 510 nm to 550 nm with the addition of Al³⁺ might be due to the aggregation of AIE luminogens. The inset of Fig. 4 is the fluorescent imaging of BSPSA after the addition of metal ions. As demonstrated, a yellowgreen emission is shown when adding Al³⁺ to the BSPSA solution, while no obvious fluorescence is observed with the presence of other metal ions. This selectivity of Al³⁺ with the enhanced FL signal might be generated from the relative low standard electrode potential, which would inhibit the electron transfer from BSPSA molecules to metal ions for FL quenching.^33,34 This is in accordance with the spectra data in Fig. 4. Furthermore, the SEM images of BSPSA in THF–water (9/1, v/v) with the addition of other metal ions demonstrated no obvious particles, which also confirmed well selectivity of the present method (shown in Fig. S4†).

Fig. 4 Fluorescent responses of BSPSA (0.71 μM) upon addition of different metal ions (50 μM) in THF–water (9/1, v/v). Inset: the fluorescent imaging of BSPSA with the addition of different metal ions.

To further study the sensing of Al³⁺ by this AIE chemosensor, the effects of THF fraction and pH were examined. As demonstrated in Fig. 1, the fluorescence intensity of BSPSA increased with the increase of THF fraction, which might decrease the sensitivity of Al³⁺ detection: in the BSPSA solution with high THF fraction, the high background would result in the low value of I/I₀; in the solution with low THF fraction, the low fluorescence intensity would obtain the low signal of Al³⁺. Therefore, it is important to examine the effects of THF fraction on fluorescence intensity.

In the experiment, the solutions with different THF fractions were used for the sensing of 50 μM Al³⁺. As shown in Fig. S5-A,† in the presence of Al³⁺, the fluorescence intensity increases with the increase of THF fraction when the THF fraction is lower than 20%. The further increase to 50% of THF does not result obvious changes of fluorescence signal. However, fluorescence intensity increases when the THF fraction is higher than 60%, and the highest signal at 90% of THF is resulted. While for the solution without addition of Al³⁺, the emission signal increases slowly when the THF is lower than 30%, and no obvious change is recorded with the increase of THF fraction till 90%. Although we got the dramatically increased background at 98% of THF solution, the background signal is still lower than the FL response with the addition of Al³⁺. As resulted, we selected 90% of THF for the Al³⁺ detection due to its high fluorescence emission with the low background.

Considering Al³⁺ is an amphoteric ion, the detection might be disturbed by pH value of the solution. Therefore, the effect of pH on the detection of Al³⁺ was also studied. Fig. S5-B† shows the change of fluorescence intensity with the change of pH value of BSPSA solution, which demonstrates no obvious signal changes with the change of pH value. This might be attributed to the low pK_a1 value of H₂SO₃ (1.89), which made the terminal sulfonate groups could not be protonated to affect the binding with Al³⁺. Thus, pH has little effect on the detection of Al³⁺, which will enlarge the application field of this sensing system.

For application, the BSPSA chemosensor was used for detecting Al³⁺ in deep-fried dough sticks. The deep-fried dough stick is a traditional food for breakfast, which is made from wheat. However, in the traditional preparing process, the alum was always acted as the swelling agent to make the deep-fried dough sticks more delicious. Therefore, due to the toxicity of Al³⁺, the monitoring of Al³⁺ in the deep-fried dough sticks is crucial.

In the experiment, we firstly determined the linearity of the BSPSA chemosensor for Al³⁺ detection. As shown in Fig. S6,† the fluorescence intensity increases with increasing Al³⁺ concentration from 0.3300 to 52.67 μM, while no obvious increase is recorded when the concentration is higher than 52.67 μM. The linear range is 5.330–52.67 μM (143.9–1442 μg L⁻¹) (R = 0.9965), and the detection limitation is 0.33 μM. This linear range was higher and wider than the electrochemical technique,⁷ spectrofluorimetric detection,³⁶ spectrophotometric determination,³⁷ and even electrothermal atomization atomic absorption spectrometry method.³⁸ Furthermore, this linearity was also wider than the reported AIE-based method using sodium 4-(2,5-diphenyl-1H-pyrrol-1-yl)benzoate (TriPP–COONa).²⁸ The reproducibility of Al³⁺ sensing by BSPSA has also been confirmed by the repeated sensing of Al³⁺ for 7 times (Fig. S7†). Then, 10 μL of the prepared deep-fried dough sticks sample was added into 3 mL of BSPSA solution (4.75 μM), and fluorescence spectrum was recorded. According to the linearity, we obtained 7.635 μM of Al³⁺ in the sample–BSPSA solution. With calculation, 3.092 mg of Al³⁺ was resulted in 1.20 g of original sample, giving the concentration of 2577 mg kg⁻¹ in the deep-fried dough sticks. This is much higher than national standards of 100 mg kg⁻¹.³⁹ In addition, the same prepared deep-fried dough sticks sample was analyzed by the conventional method of ICP-AES, which resulted 2694 mg kg⁻¹ Al³⁺ in the deep-fried dough sticks. Thus, our result is close to the data obtained by the standard method of ICP-AES, giving the relative error of 4.34%. Furthermore, it should be noted that there might be a limitation for reaching a higher sensitivity due to the use of 90% THF for sensing. Therefore, the improvement is still needed in the future studies.

Conclusions

In summary, an AIE molecule of BSPSA has been used to act as a highly selective and sensitive fluorescent chemosensor for the detection of Al³⁺. The sensing is based on the increased fluorescence emission with the aggregation of BSPSA induced by the coordination of BSPSA with Al³⁺ in THF–water. In addition, pH of the solution had little effect on the detection, which enlarged the application field of this chemosensor. The application of this chemosensor for detecting Al³⁺ in deep-fried dough sticks demonstrated the potentials for the Al³⁺ detection in real samples.

Acknowledgements

The authors gratefully acknowledge the support from the National Nature Science Foundation of China (21175014), National Grant of Basic Research Program of China (no. 2011CB915504), and A Foundation for the Author of National Excellent Doctoral Dissertation of PR China (201221).

Notes and references

1. E. Altschuler, Med. Hypotheses, 1999, 53, 22–23.
2. L. Tomljenovic, J. Alzheimer's Dis., 2011, 23, 567–598.
3. C. Exley and E. R. House, Monatsh. Chem., 2011, 142, 357–363.
4. M. Sun and Q. H. Wu, J. Hazard. Mater., 2010, 176, 901–905.
5. G. Tangen, T. Wickstrom, S. Lierhagen, R. Vogt and W. Lund, Environ. Sci. Technol., 2002, 36, 5421–5425.
6. A. G. Coedo, T. Dorado, I. Padilla and J. C. Farinas, Talanta, 2007, 71, 2108–2120.
7. H. Wang, Z. Yu, Z. Wang, H. Hao, Y. Chen and P. Wan, Electroanalysis, 2011, 23, 1095–1099.
8. F. Nie and H. Lu, Spectrochim. Acta, Part A, 2008, 71, 350–354.
9. F. K.-W. Hau, X. He, W. H. Lam and V. W.-W. Yam, Chem. Commun., 2011, 47, 8778–8780.
10. X. K. Li, J. N. Wang, L. L. Sun and Z. X. Wang, Chem. Commun., 2010, 46, 988–990.
11. H. M. Park, B. N. Oh, J. H. Kim, W. Qiong, I. H. Hwang, K. D. Jung, C. Kim and J. Kim, Tetrahedron Lett., 2011, 52, 5581–5584.
12. S. Ng and R. Narayanaswamy, Anal. Bioanal. Chem., 2006, 386, 1235–1244.
13. D. Maity and T. Govindaraju, Chem. Commun., 2010, 46, 4499–4501.
14. K. K. Upadhyay and A. Kumar, Org. Biomol. Chem., 2010, 8, 4892–4897.
15. X. H. Jiang, B. D. Wang, Z. Y. Yang, Y. C. Liu, T. R. Li and Z. C. Liu, Inorg. Chem. Commun., 2011, 14, 1224–1227.
16. A. Jeanson and V. Bereau, Inorg. Chem. Commun., 2006, 9, 13–17.
17. M. Dong, Y. M. Dong, T. H. Ma, Y. W. Wang and Y. Peng, Inorg. Chim. Acta, 2012, 381, 137–142.
18. J. D. Luo, Z. L. Xie, J. W. Y. Lam, L. Cheng, H. Y. Chen, C. F. Qiu, H. S. Kwok, X. W. Zhan, Y. Q. Liu, D. B. Zhu and B. Z. Tang, Chem. Commun., 2001, 1740–1741.
19. B. Xu, Z. Chi, H. Li, X. Zhang, X. Li, S. Liu, Y. Zhang and J. Xu, J. Phys. Chem. C, 2011, 115, 17574–17581.
20. Y. N. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Soc. Rev., 2011, 40, 5361–5388.
21. C.-X. Yuan, X.-T. Tao, L. Wang, J.-X. Yang and M.-H. Jiang, J. Phys. Chem. C, 2009, 113, 6809–6814.
22. Y.-S. Zheng, Y.-J. Hu, D.-M. Li and Y.-C. Chen, Talanta, 2010, 80, 1470–1474.
23. T. Sanji, K. Shiraishi, M. Nakamura and M. Tanaka, Chem.–Asian J., 2010, 5, 817–824.
24. Z. Ning, Z. Chen, Q. Zhang, Y. Yan, S. Qian, Y. Cao and H. Tian, Adv. Funct. Mater., 2007, 17, 3799–3807.
25. X. J. Liu, C. Qi, T. Bing, X. H. Cheng and D. H. Shangguan, Anal. Chem., 2009, 81, 3699–3704.
26. J. H. Ye, L. J. Duan, C. C. Yan, W. C. Zhang and W. J. He, Tetrahedron Lett., 2012, 53, 593–596.
27. F. Sun, G. X. Zhang, D. Q. Zhang, L. Xue and H. Jiang, Org. Lett., 2011, 13, 6378–6381.
28. T. Han, X. Feng, B. Tong, J. Shi, L. Chen, J. Zhi and Y. Dong, Chem. Commun., 2012, 48, 416–418.
29. F. F. Wang, J. Y. Wen, L. Y. Huang, J. J. Huang and J. Ouyang, Chem. Commun., 2012, 48, 7395–7397.
30. B. J. Sun, X. J. Yang, L. Ma, C. X. Niu, F. F. Wang, N. Na, J. Y. Wen and J. Ouyang, Langmuir, 2013, 29, 1956–1962.
31. T. Y. Han, X. Feng, B. Tong, J. B. Shi, L. Chen, J. G. Zhi and Y. P. Dong, Chem. Commun., 2012, 48, 416–418.
32. X. Y. Shi, H. Wang, T. Y. Han, X. Feng, B. Tong, J. B. Shi, J. G. Zhi and Y. P. Dong, J. Mater. Chem., 2012, 22, 19296–19302.
33. Y. Q. Zhang, X. D. Li, L. J. Gao, J. H. Qiu, L. P. Heng, B. Z. Tang and L. Jiang, ChemPhysChem, 2014, 15, 507–513.
34. L. Heng, X. Wang, Y. Dong, J. Zhai, B. Z. Tang, T. Wei and L. Jiang, Chem.–Asian J., 2008, 3, 1041–1045.
35. G. Yu, S. Yin, Y. Liu, J. Chen, X. Xu, X. Sun, D. Ma, X. Zhan, Q. Peng, Z. Shuai, B. Tang, D. Zhu, W. Fang and Y. Luo, J. Am. Chem. Soc., 2005, 127, 6335–6346.
36. D. Kara, A. Fisher and S. J. Hill, Anal. Chim. Acta, 2008, 611, 62–67.
37. M. Bahram, T. Madrakian, E. Bozorgzadeh and A. Afkhami, Talanta, 2007, 72, 408–414.
38. L. Correia, M. E. Soares and M. D. Bastos, J. Agric. Food Chem., 2006, 54, 9312–9316.
39. Maximum Levels of Contaminants in Foods (GB 2762-2005), 2005.

---

Footnote

† Electronic supplementary information (ESI) available. See: DOI: 10.1039/c4ra05095a

---