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Letters to Analytical Chemistry Clicking Fluoroionophores onto Mesoporous Silicas: A Universal Strategy toward Efficient Fluorescent Surface Sensors for Metal Ions Zhen Jin, Xiao-Bing Zhang,* De-Xun Xie, Yi-Jun Gong, Jing Zhang, Xin Chen, Guo-Li Shen, and Ru-Qin Yu State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, P. R. China Mesoporous SBA-15 silica is an excellent support for constructing fluorescent surface sensors. In this letter, we reported a two-step surface reaction involved strategy to construct efficient fluorescent surface sensors for metal ions by clicking fluoroionophores onto azide-functionalized SBA-15. Our experimental results indicate that such a strategy exhibits an obviously higher loading efficiency within commercial SBA-15 than a previously reported strategy. As a proof-of-concept, a newly designed alkynefunctionalized Hg2+ fluoroionophore was grafted onto SBA-15 to form a fluorescent Hg2+ surface sensor. It shows improved sensitivity and selectivity than the fluoroionophore itself working in the solution phase with a detection limit of 2.0 × 10-8 M for Hg2+. The development of fluorescent molecular sensors for metal ions in homogeneous systems has become a hot research topic of current analytical chemistry.1 However, the investigation of sensors based on fluoroionophores modified on a solid surface has been rarely reported.2 In a point of practicability, a fluorescent surface sensor owns more favorable unique features than a molecular probe used only in homogeneous solution in terms of real-time and online detection and reusability. Moreover, molecular interaction on a surface often exhibits remarkable divergence from that in solution, which may endow alternative strategies for developing sensors with new or improved functionalities. Recently, the modification of different fluoroionophores on the surface of nanostructured materials, including silica nanoparticles,3 Si nanowires,4 and magnetic nanoparticles,5 have been reported for * To whom correspondence should be addressed. E-mail:
[email protected]. (1) (a) Nolan, E. M.; Lippard, S. J. Chem. Rev. 2008, 108, 3443–3480. (b) McRae, R.; Bagchi, P.; Sumalekshmy, S.; Fahrni, C. J. Chem. Rev. 2009, 109, 4780–4827. (c) Xu, Z.; Yoon, J.; Spring, D. R. Chem. Soc. Rev. 2010, 39, 1996–2006. (2) (a) Van Der Veen, N. J.; Flink, S.; Deij, M. A.; Egberink, R. J. M.; Van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Am. Chem. Soc. 2000, 122, 6112– 6113. (b) Basabe-Desmonts, L.; Beld, J.; Zimmerman, R. S.; Hernando, J.; Mela, P.; Garcia Parajo, M. F.; Van Hulst, N. F.; Van Den Berg, A.; Reinhoudt, D. N.; Crego-Calama, M. J. J. Am. Chem. Soc. 2004, 126, 7293– 7299. (c) Kim, Y. R.; Kim, H. J.; Kim, J. S.; Kim, H. Adv. Mater. 2008, 20, 4428–4432. 10.1021/ac101305e 2010 American Chemical Society Published on Web 06/30/2010
detection of various metal ions. Most of these sensors are performed undergoing a fluorescence quenching mechanism, and fluorescence enhancement response in heterogeneous system seems to be more difficult. Mesoporous SBA-15 silica is an ideal inorganic scaffold for building organic-inorganic hybrid materials with considerable potential for many applications, including heterogeneous catalysis,6 and as an excellent support for constructing chemical sensors.7 Though conceptually simple, highly efficient loadings of functional organic molecules within SBA-15 has been declared the bottleneck for building such hybrid materials. Click reaction has attracted increasing attention in the field of surface sciences in recent years.8 As such a cycloaddition reaction could afford a thermally and hydrolytically stable triazole based conjugated linkage with high efficiency (yields are often above 95%) under mild reaction conditions even in aqueous media. The unique properties of this reaction can be expected to provide a robust strategy for uniform, high-density covalent immobilization of functional molecules on a solid surface such as SBA-15. Bein and coauthors first reported the high-density graft of the trypsin onto SBA-15 by an azide/alkyne-based click chemistry.9 Although their strategy shows a high loading level for trypsin, the necessary three steps involved surface reaction is expected to exhibit a limited loading level for other functional molecules. (3) (a) Brasola, E.; Mancin, F.; Rampazzo, E.; Tecilla, P.; Tonellato, U. Chem. Commun. 2003, 3026–3027. (b) Teolato, P.; Rampazzo, E.; Arduini, M.; Mancin, F.; Tecilla, P.; Tonellato, U. Chem.sEur. J. 2007, 13, 2238–2245. (4) Mu, L.; Shi, W.; Chang, J. C.; Lee, S. T. Nano Lett. 2008, 8, 104–108. (5) (a) Lee, H. Y.; Bae, D. R.; Park, J. C.; Song, H.; Han, W. S.; Jung, J. H. Angew. Chem., Int. Ed. 2009, 48, 1239–1243. (b) Park, M.; Seo, S.; Lee, I. S.; Jung, J. H. Chem. Commun. 2010, 46, 4478–4480. (6) (a) Wight, A. P.; Davis, M. E. Chem. Rev. 2002, 102, 3589–3614. (b) Yiu, H. H. P.; Wright, P. A. J. Mater. Chem. 2005, 15, 3690–3700. (7) (a) Gao, L.; Wang, Y.; Wang, J.; Huang, L.; Shi, L.; Fan, X.; Zou, Z.; Yu, T.; Zhu, M.; Li, Z. Inorg. Chem. 2006, 45, 6844–6850. (b) Wang, J. Q.; Huang, L.; Xue, M.; Wang, Y.; Gao, L.; Zhu, J. H.; Zou, Z. J. Phys. Chem. C 2008, 112, 5014–5022. (8) (a) Wu, P.; Feldman, A. K.; Nugent, A. K.; Hawker, C. J.; Scheel, A.; Voit, B.; Pyun, J.; Frechet, J. M. J.; Sharpless, K. B.; Fokin, V. V. Angew. Chem., Int. Ed. 2004, 43, 3928–3932. (b) Lutz, J. F. Angew. Chem., Int. Ed. 2007, 46, 1018–1025. (9) Schlossbauer, A.; Schaffert, D.; Kecht, J.; Wagner, E.; Bein, T. J. Am. Chem. Soc. 2008, 130, 12558–12559.
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Scheme 1. (a) Covalent Graft of Fluoroionophore into the Mesoporous SBA-15 Silica by a Two-Step Surface Reaction Involved Click Strategy to Construct Fluorescent Surface Sensor with a PET Response Mechanism and (b) Chemical Structures of Compounds 1 and 2
Subsequently, Stack and coauthors proposed a click-based “direct” synthetic route for controlled loadings of several small molecules in SBA-15,10 while it is limited to self-prepared SBA-15 and not suitable for commercial SBA-15. Herein we proposed a two-step surface reaction involved strategy to graft fluoroionophore onto azide-functionalized SBA-15 (SBA-15-N3) by cycloaddition. Different from the Bein’s strategy,9 the functional group conversion from chloride to azide was carried out in a homogeneous system. The experimental results indicate that such a two-step strategy indeed exhibits an obviously improved loading efficiency within commercial SBA-15. As a proof-of-concept, a new designed alkyne-functionalized Hg2+ fluoroionophore was reacted with SBA-15-N3 to form a conjugated triazole linkage (Scheme 1a). The as-prepared fluorescent surface sensor shows a fluorescence enhancement response toward Hg2+ by suppressing its fluorescent PET process (Scheme 1a) and exhibits improved sensitivity and selectivity than the fluoroionophore itself in solution phase. Loading efficiency of our proposed two-step strategy was first compared with that of reported three-step strategy,9 by using 1-ethynylpyrene as a model functional molecule (see the Supporting Information and Figure S1 in the Supporting Information). Experimental results indicate that our proposed two-step strategy could indeed provide higher loading efficiency than the threestep strategy. As naphthalimide derivatives containing piperazine moiety linked with one pyridine group have been well proved to show a highly selective fluorescence response toward Hg2+ by several different groups,5b,11 compounds 1 and 2 containing one naphthalimide moiety, one piperazine group, and an alkyne group with linkages of different length were designed and synthesized with an 2-(aminomethyl)pyridine moiety serving as both the Hg2+ (10) Nakazawa, J.; Stack, T. D. P. J. Am. Chem. Soc. 2008, 130, 14360–14361. (11) (a) Guo, X. F.; Qian, X. H.; Jia, L. H. J. Am. Chem. Soc. 2004, 126, 2272– 2273. (b) Li, C. Y.; Zhang, X. B.; Qiao, L.; Zhao, Y.; He, C. M.; Huan, S. Y.; Lu, L. M.; Jian, L. X.; Shen, G. L.; Yu, R. Q. Anal. Chem. 2009, 81, 9993– 10001.
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receptor and the quencher of PET (Scheme 1b). Compound 2 shows a poor fluorescence enhancement in solution than that of compound 1 induced by 10 µM of Hg2+ (see Figure S2 in the Supporting Information). Therefore, compound 1 was chosen for further investigation. It was grafted on the as-prepared SBA15-N3 via Cu(I)-catalyzed 1,3-dipolar cycloaddition to afford SBA15-1. The functionalization density of compound 1 on SBA-15 was estimated to be 0.101 mmol g-1 by TGA (see the Supporting Information and Figure S3 in the Supporting Information). Its structure was characterized using IR spectra and UV-vis spectra (see Figures S4 and S5 in the Supporting Information). Figure 1 shows the fluorescence spectra of SBA-15-1 suspended in buffered (Tris-HNO3, pH ) 7.2) aqueous solutions containing different concentrations of Hg2+. The introduction
Figure 1. Fluorescence emission spectra of the fluorescence surface sensor (0.15 mg/mL; corresponding to 1.5 × 10-5 M of 1) exposed to various concentration of Hg2+: 0, 2 × 10-8, 5 × 10-8, 8 × 10-8, 2 × 10-7, 5 × 10-7, 8 × 10-7, 2 × 10-6, 5 × 10-6, 1 × 10-5, 2 × 10-5, 3 × 10-5 mol/L from bottom to top. These spectra were measured in buffered (Tris-HNO3, pH ) 7.2) water/ethanol (1: 1, v/v) solution. The excitation wavelength was 420 nm.
Figure 2. Fluorescence titration curve of (a) SBA-15-1 (0.15 mg/ mL; corresponding to 1.5 × 10-5 M of 1); (b) 1 itself (1 × 10-5 M) with Hg2+ in buffered (Tris-HNO3, pH ) 7.2) water/ethanol (1: 1, v/v) solution. Inset shows the fluorescence responses at low Hg2+ concentrations; λex ) 420 nm, λem ) 520 nm.
of Hg2+ could induce remarkable enhancement of its fluorescence at 520.0 nm, and the emission intensity was gradually increased with increasing Hg2+ concentration (Figures 1 and 2a). The corresponding titration experiment using compound 1 itself was also carried out, with results shown in Figure 2b. One can find that the fluorescent increment for SBA-15-1 induced by Hg2+ is smaller than that recorded using 1 itself, and the background fluorescent intensity of SBA-15-1 is obviously larger than that of free 1 (Figure 1 and Figure S2 in the Supporting Information). It is probable that part of the fluoroionophore on the surface of SBA-15 is protonated by the acidic silanol groups of the silica network, which could partly suppress the PET from the N of piperazine to the naphthalimide moiety within the compound 1 framework3b and induce an increased background fluorescent intensity. The detection limit for Hg2+ is established at 2.0 × 10-8 M (inset of Figure 2a). If the same criterion (5% fluorescence enhancement) was employed to define the sensitivity of 1 itself in solution, a value of only 1.0 × 10-6 M was achieved (inset of Figure 2b), indicating a remarkably improved low detection limit is achieved by the proposed surface sensor. This result might be ascribed to the high surface-to-volume ratio of SBA-15, which is beneficial for a large loading amount of the receptors on its surface, as well as the large and straight channel
Figure 3. Fluorescence response of (a) SBA-15-1 (1.5 × 10-5 M of 1); (b) 1 itself (1 × 10-5 M) to 10 µM of Hg2+ or 100 µM of other metal ions (the black bar portion) and to the mixture of 100 µM of other metal ions with 10 µM of Hg2+ (the gray bar portion).
of SBA-15 which is favorable for facilitating the entering and diffusion of Hg2+. The effects of pH on the fluorescence characteristics of the new surface sensor were also investigated. (see Figure S6 in the Supporting Information). It was obtained from the fluorescence titration curve that the pKa′ of SBA-15-1 is about 5.5, and in a range of pH from 6.0 to 8.0, acidity does not affect the fluorescent intensity of SBA-15-1 in a mixed ethanol-water solution of (1:1, v/v). Therefore, all of the detections of metal ions were operated in the mixed solution containing Tris-HNO3 (0.05 M, pH ) 7.2). The stoichiometry of the SBA-15-1-Hg2+ complex was also estimated to be 1:1 using Job’s method of continuous variation for the fluorescence data (see Figure S7 in the Supporting Information). The selectivity experiments of the surface sensor were extended to a number of common metal ions. Figure 3a (the black bar portion) showed the fluorescence change of SBA-15-1 induced by different metal ions of interest; no obvious fluorescence intensity changes was observed with common interferences, indicating that our surface sensor exhibits high selectivity to Hg2+ over other competing metal ions. To test practical applicability of our fluorescent surface for Hg2+, competition experiments were also carried out. Ten times the concentration of the abovementioned metal ions (100 µM) are added to 10 µM of Hg2+ in Tris-HNO3 buffered water/C2H5OH, and the fluorescence Analytical Chemistry, Vol. 82, No. 15, August 1, 2010
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response of the surface sensor (Figure 3a, the gray bar portion) is detected. Our sensor showed almost unchanged responses to Hg2+ before and after addition of other interfering metal ions. These experimental results show that the response of the sensor to Hg2+ is unaffected by the presence of the other possible contaminating metal ions. Corresponding experiments using compound 1 itself as a fluoroionophore was also carried out, with results shown in Figure 3b. In this case, Ag+ and Fe3+ also triggered slight fluorescence enhancement of 1, while Cu2+ showed slight fluorescence quenching interference to 1. All these results indicate that our proposed surface sensor shows improved selectivity than that of the fluoroionophore itself working in the solution phase. The practical application of the proposed fluorescent surface sensor was evaluated by determination of recovery of spiked Hg2+ in tap and river water samples (see the Supporting Information and Table S2 in the Supporting Information). The results obtained in the real water samples show good recovery values, which confirmed that the proposed sensor was applicable for practical Hg2+ detection in real samples with other potentially competing species coexisting. To investigate the reusability of the present surface sensor, the chemical reversibility behavior of the binding of SBA-15-1 with Hg2+ was then studied in the buffered ethanol-water solution (see the Supporting Information and Figure S8 in the Supporting Information). The experimental result indicates that the present SBA-15-1 based fluorescence surface sensor could be easily regenerated for repeated use. Moreover, considering its stability and ease to use as solid, such a SBA1511 based surface sensor may be used for many applications, such as application in removal of toxic heavy metal ions from environmental aqueous systems.
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In summary, we have proposed a two-step surface reaction involved strategy to construct efficient fluorescent surface sensors by clicking alkyne-functionalized fluoroionophores onto azidefunctionalized SBA-15. Following this strategy, a fluorescence enhancement-based Hg2+ surface sensor was prepared by grafting a new designed alkyne-functionalized Hg2+ fluoroionophore onto SBA-15. It shows improved analytical performance characteristics toward Hg2+ in terms of sensitivity and selectivity than the fluoroionophore itself working in the solution phase. Since numerous fluoroionophores have been developed for a wide range of targets, our strategy provides a new general platform for constructing fluorescent surface sensors for various targets including metal ions, anions, and even neutral molecules. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grants J0830415, 20975034), “973” National Key Basic Research Program of China (Grant 2007CB310500), Program for Changjiang Scholars and Innovative Research Team in University, and Ministry of Education of China (Grant NCET07-0272). SUPPORTING INFORMATION AVAILABLE Apparatus and experimental procedures, supplementary spectra data and tables, and synthesis and characteristic data. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review May 19, 2010. Accepted June 25, 2010. AC101305E
