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A 3D porous luminescent terbium metal-organic framework for selective sensing of F− in aqueous solution
Inorganic Chemistry Communications 80 (2017) 53–57
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
A 3D porous luminescent terbium metal-organic framework for selective sensing of F− in aqueous solution Yongyan Wan, Wei Sun, Jingjuan Liu, Zhiliang Liu ⁎ College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, Inner Mongolia, China
a r t i c l e
i n f o
Article history: Received 22 March 2017 Received in revised form 12 April 2017 Accepted 14 April 2017 Available online 14 April 2017 Keywords: Metal-organic frameworks Halide and pseudohalide ions Selective sensing Rapid response Thermal and pH stability
a b s t r a c t An extremely stable Tb metal-organic framework (Tb-MOF), namely [Tb(H4btca)∙3H2O]n (H4btca = [1,1′-biphenyl]-2,3,3′,5′-tetracarboxylic acid), was synthesized under hydrothermal condition. The single-crystal X-ray diffraction indicated Tb-MOF exhibits a 3D structure with 1D channel, which crystallizes as an orthorhombic system with a Pccn space group. In addition, Tb-MOF displays bright green luminescence under the irradiation of UV light at 254 nm. Meanwhile, the investigations showed that Tb-MOF possessed excellent thermal and pH stability. More importantly, it can selectively and rapidly sense F− among proprietary halide and pseudohalide ions, and can be used for quantitative detecting F−. © 2017 Published by Elsevier B.V.
Metal-organic frameworks (MOFs), self-assembled from metal ions with organic linkers, have been recently received increasing attention because of its structural controllability and high porosity, and idiosyncratic surface properties [1,2]. Accordingly, MOFs have been widely used in catalysis, gas storage and separation, luminescent sensing, drug delivery and so on [3–10]. Besides, many MOFs display excellent luminescent sensing in especial for lanthanide based metal-organic frameworks (Ln-MOFs) [11–15]. Compared to transition metals, LnMOFs are attracted immense attention owing to many fascinating properties such as unique luminescent properties, high color purity, the extremely sharp emissions, large Stokes' shifts, and relatively long fluorescence lifetime [16–18]. Therefore, Ln-MOFs show a promising application as luminescent sensing materials. In recent years, serious destructions of the environment and biological systems were caused by toxic ions, organic solvents, and heavy metals and so on [19]. For example, F− is a typical pollutant anion and it is widely exists in natural water. Intaking excessive fluoride will cause skeletal fluorosis or dental fluorosis [20–22]. Therefore, fast and efficient sensing F− becomes one of the most hotspots in the field of chemistry. As is known to us all, the traditional analysis techniques for sensing F−, such as gas chromatography, inductively coupled plasma mass spectrometry, synchrotron radiation X-ray spectrometry and atomic absorption and so on. However, these methods not only require sophisticated instrumentations, but also time-consuming and high cost [23–25]. Hence, searching for a convenient, time-saving, high sensitivity and low cost method for sensing F− is very necessary. Therefore, ⁎ Corresponding author. E-mail address: [email protected] (Z. Liu).
http://dx.doi.org/10.1016/j.inoche.2017.04.010 1387-7003/© 2017 Published by Elsevier B.V.
luminescent sensors based on Ln-MOFs have been considered as one of the compelling approaches for sensing F−. For example, Chen et al. [26] synthesized a TbL (HL = 4-(pyrimidin-5-yl) benzoic acid) complex for sensing F− in partial halogen-containing anion. Chen et al. [27] constructed a novel Cu-MOF, namely [Cu4I(TIPE)3 ∙ 3I] (TIPE = tetra(3imidazoylphenyl)ethylene), which can be applied to selectively sense F− in a small-scope of halogen anion. Based on these considerations above, we report a novel Tb-MOF, [Tb(H4bcta)∙ 3H2O]n [28], constructed by Tb(NO3)3 ∙ 6H2O and H4bcta (H4btca = [1,1′-biphenyl]-2,3,3′,5′-tetracarboxylic acid) under solvothermal reaction. Structural analysis by the single-crystal X-ray diffraction revealed that the Tb-MOF exhibited a 3D structure with 1D channel. Interestingly, Tb-MOF can be used as a luminescence prober for sensing F− in aqueous solution, with good selectivity and rapid response. It is noteworthy to mention that this is the first example that Ln-MOF as luminescence prober selectively sensing F− among proprietary halide and pseudohalide anions. The single-crystal X-ray diffraction analyses [29] (crystal data for TbMOF was shown in Table S1) indicated Tb-MOF crystallized as an orthorhombic system with a Pccn space group. As shown in Fig. 1a, the asymmetric unit contains one Tb3 + metal centra, one Hbtca3− ligand, and three free water molecules. Among them, Tb1 atom was coordinated by nine oxygen atoms that six oxygen atoms were from four bidentate carboxyls and three oxygen atoms were from two terdentate carboxyls. According to the SHAPE calculation [30], Tb1 and the surrounding coordination atoms form spherical capped square antiprism geometric configuration. Two adjacent and crystallographic symmetry terbium atoms linked by carboxyl oxygen atoms of the Hbtca3 − ligand to form the [Tb2(COO)7] secondary building unit (SBU). Furthermore, the SBU
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Fig. 1. (a) The coordination environment of Tb3+ in the asymmetric unit; (b) The 3D structure of Tb-MOF containing un-deprotonated carboxyl groups in the pores along the c axis. Bottom: the corresponding channel was proved by the yellow column. (c) The 3D structure of Tb-MOF along the a axis. Bottom: the corresponding channel was proved by the yellow column. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
connected by Hbtca3− ligands to expand into a 3D framework structure. Along the c axis, the Tb-MOF shows two types of 1D channel with the size of 10 × 8 Å2 and 5 × 5 Å2, respectively (Fig. 1b). As shown in Fig. 1c, the Tb-MOF exhibits uniform 1D channels and the size is about 9.8 × 10.4 Å2. The PLATON software is used to calculate porosity of TbMOF, which is 41.8%. The distances of Tb\\O range from 2.320(6) to 2.566(10) Å, and the corresponding angles O\\Tb\\O vary in the range of 50.3(4)–155.2(3)° (selected bond lengths (Å) and angles (°) for Tb-MOF was listed in Table S2). It is notable that there exists a undeprotonated carboxyl group in the pore of Tb-MOF. The IR spectra of H4btca and Tb-MOF were carried out at room temperature (shown in Fig. S1). The peak at 1674 cm−1 for Tb-MOF demonstrated that all the four carboxyls of H4btca ligand formed coordination bond with metal atoms. Besides, the PXRD pattern of as-synthesized TbMOF was shown in the Fig. S2. The diffraction peaks of Tb-MOF were consistent with the simulated pattern, indicating that the as-
synthesized sample possess the excellent phase purity. Meanwhile, the sharp peaks indicate the as-synthesized sample show the excellent crystallinity. Furthermore, the thermal stability of Tb-MOF was observed by the TGA analyses in a nitrogen atmosphere (shown in Fig. S3). As was shown in the TGA curve of Tb-MOF, the weight of Tb-MOF loss 9.82% in the temperature range 50–377 °C due to losing three free water molecules (calculated 10.1%). As temperature continues rising, ligands start to decompose thermally and the structural collapsed. To explore the stability of Tb-MOF in aqueous solution, we observed the luminescence intensities of Tb-MOF at various pH values [31]. The samples of Tb-MOF dispersed in aqueous solution with the pH values from 4 to 10, the luminescence intensities of Tb-MOF are almost unaltered (shown in Fig. 2a). Moreover, the measured corresponding PXRD patterns were almost same with the as-synthesis Tb-MOF (shown in Fig. 2b). Thus, the excellent pH stability evidenced Tb-MOF
Fig. 2. (a) Luminescence intensity of Tb-MOF immersed in different pH aqueous solutions. (b) The PXRD patterns of Tb-MOF after treated with different pH aqueous solutions.
Y. Wan et al. / Inorganic Chemistry Communications 80 (2017) 53–57
Fig. 3. The solid-state luminescence spectrum of Tb-MOF sample; inset: the photograph of Tb-MOF and under the irradiation of UV light at 254 nm.
as a luminescence prober for sensing pollutant in special environment is feasibility. To further explore the property of Tb-MOF, the solid-state luminescent spectra of Tb-MOF and H4bcta were measured at room temperature (shown in Fig. 3 and Fig. S4). Under excitation wavelength of 260 nm, the H4btca ligand exhibits a broad peak emission and with a maximum intensity at 400 nm, which was presumably attributed to the π–π* transitions of the ligand. Tb-MOF has four characteristic emission spectra upon excitation wavelength at 260 nm. The emission peaks of Tb-MOF appeared at 491 nm (5D4 → 7D6), 547 nm (5D4 → 7D5), 586 nm (5D4 → 7D4), and 620 nm (5D4 → 7D3), respectively. Significantly, the strongest emission peak at 547 nm corresponds to the 5D4 → 7F5 transitions for the Tb3+ ion, resulting in the bright green light which can be observed by naked eyes. It is obvious that the luminescence spectrum of Tb-MOF do not contain the ligand-based emission peaks. This phenomenon can be explained by the efficient energy-transfer which happened between the ligand and the Tb3+ ion. We speculated that the enhanced luminescence property can be explained by the antenna effect [32] with an efficient energy transfer between the ligand and the Tb3+ ion. Owing to green luminescence emission, the Tb-MOF was selected to explore its sensing ability for halide and pseudohalide ions [33]. As is shown in Fig. 4a, it can be clearly observed that the luminescence
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intensity changed after different halide and pseudohalide ions X− (NaX, X = F−, Cl−, Br−, I−, SCN− or N− 3 ) added. Interestingly, as was shown in Fig. 4c, there was an obvious quenching effect owing to the existence of F− on the emission of Tb-MOF at 547 nm, whereas other anions showed no or only minor interferences. The above phenomenon can be easily captured by naked eyes under a 254 nm UV lamp irradiation. The Fig. 4b showed that Tb-MOF had excellent sensing behavior towards F− by quenching among all halide and pseudohalide ions and the response speed is quickly. To further explore the interactions between F− concentration and the quenching efficiency, Tb-MOF samples were dispersed into different concentrations aqueous solutions of F− in the range of 10−4–10−2 M. Results showed that the luminescence intensities of Tb-MOF gradually decreased with the increasing concentrations of F−. When the concentration of F− reached at 0.01 M, luminescence of the Tb-MOF was almost entirely quenched. The detection limit of Tb-MOF for F− was determined at 4.97 × 10−4 M (shown in Fig. S5). Furthermore, the relationship between luminescence intensity and the concentration of F− across the whole concentration range has been built, and it can be well fitted as the following formula: I0/I = 1 + 0.0661 exp(C/23.17574) (I = luminescence intensity of Tb-MOF suspensions with different concentrations of F−; I0 = luminescence intensities of Tb-MOF suspensions; C = the concentrations of F−). As was shown in Fig. 5a, the correlation coefficient (R2) is 0.994. Therefore, Tb-MOF has already proved to be a luminescence sensor for quantitative detection of F− in aqueous solution. As far as we known, few luminescent MOFs acting as sensors for F− in water system were observed in literatures [34,35]. Specifically, the comparison of other MOF-based fluorescent sensors for F− sensing was listed in Table S3. Besides, in order to further understand the detail influence of other halide and pseudohalide ions to the selectively quenching response of F−. Selective experiments [36] towards various anions were conducted by adding 1 × 10− 2 M of F−, Cl−, Br−, I−, SCN−, N− 3 , under the established conditions. As was shown in Fig. 5b, the luminescence intensity of Tb-MOF showed a negligible effect after adding other anions, while the luminescence intensity was nearly completely disappeared in seconds when F− was introduced. Results indicate that the Tb-MOF can sense F− with excellent highly selective in all halide and pseudohalide ions. We further investigated the possible sensing mechanism of luminescence quenching by F−. The PXRD pattern was measured after immersing Tb-MOF into the NaF solution. As can be seen in Fig. S6, the PXRD pattern of Tb-MOF handled by F− solution showed a marked difference compared to the as-synthesized Tb-MOF, proving that the crystal
Fig. 4. (a) Luminescence spectra of Tb-MOF in different anions solutions; (b) The picture of Tb-MOF in different anions solutions under 254 nm UV-lamp; (c) Luminescence intensities of Tb-MOF in different anions solutions.
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Fig. 5. (a) Concentration-dependent luminescence quenching of Tb-MOF after dispersed in different concentration of F−; inset: the plot of intensity versus F− concentration. (b) The relative intensities for Tb-MOF in different solutions; inset: the corresponding photographs of Tb-MOF under the irradiation of 254 nm UV light.
structure has been destroyed. We speculate the quenching mechanism is that the presence of F− led to collapse of the crystal framework of Tb-MOF, thus causing the luminescence quenching [37]. In summary, a 3D porous metal-organic framework [Tb(H4btca)∙3H2O]n had been synthesized, which showed bright green luminescence. In addition, Tb-MOF displays excellent thermal and pH stability. More importantly, Tb-MOF can high selectively and rapidly sense F− among all halide and pseudohalide ions. All results indicate that it can be used as a luminescence prober for quantitative detection of F− in aqueous solution.
Acknowledgment This work was supported by the NSFC of China (21361016) and the Inner Mongolia Autonomous Region Fund for Natural Science (2013ZD09). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.inoche.2017.04.010.
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[29]
[30] [31]
[32]
(m), 533 (w), 454 (m). Elemental Anal (%) Calcd: C, 35.55, H, 2.41 Found: C, 35.7, H, 2.2. Structure determination for Tb-MOF was performed on a Bruker ApexII CCD diffractometer equipped with graphite-monochromatic Mo-Kα radiation (λ = 0.71073 Å) at 293K. Empirical absorption corrections were applied using the SADABS program. The suitable structure of Tb-MOF was solved by direct methods and refined anisotropically by full-matrix least squares techniques on F2 values, using the SHELX-97 program. Hydrogen atoms were added on appropriate positions in theory and refined with isotropic thermal parameters riding on those parent atoms. (CCDC reference number is 1535052). A. Ruiz-Martinez, D. Casanova, S. Alvarez, Polyhedral structures with an odd number of vertices: nine-coordinate metal compounds, Chem. Eur. J. 14 (2008) 1291–1303. pH stability experiments: The fine grinding sample of Tb-MOF (1 mg) was dispersed in 3 mL aqueous solution with various pH value (pH = 2–13), respectively. The pH values of the solutions were tuned by adding either HNO3 or NaOH solutions. After ultrasonic treatment for 15 min, the suspension was obtained and used for luminescent measurements. Then the samples were filtrated and used for PXRD measurements. L. Wang, G. Fan, X. Xu, D. Chen, L. Wang, W. Shi, P. Cheng, Detection of polychlorinated benzenes (persistent organic pollutants) by a luminescent sensor based on a lanthanide metal–organic framework, J. Mater. Chem. A 5 (2017) 5541–5549.
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[33] Luminescence sensing experiments. First, the samples of Tb-MOF (1 mg) were dispersed in 1 × 10−2 M different anions solutions (2 mL), such as F−, Cl−, Br−, I−, SCN− and N− 3 . Then, the homogeneous emulsions were formed after ultrasonic treatment for 15 min, which were used for luminescent measurements at room temperature. [34] Z.H. Lin, S.J. Ou, C.Y. Duan, B.G. Zhang, Z.P. Bai, Naked-eye detection of fluoride ion in water: a remarkably selective easy-to-prepare test paper, Chem. Commun. (2006) 624–626. [35] Q.W. Xu, C. Wang, Z.B. Sun, C.H. Zhao, A highly selective ratiometric bifunctional fluorescence probe for Hg(2 +) and F(−) ions, Org. Biomol. Chem. 13 (2015) 3032–3039. [36] Luminescence selective sensing experiments: The samples of Tb-MOF (3 mg) were dispersed in aqueous solution (3 mL), and divided into three parts averagely. The first was added 1 mL water as a blank sample; the second was gradually added 1 × 10−2 M solutions of interfering anions (200 μL). The third was stepwise added 200 μL solutions of all anions, including F−, Cl−, Br−, I−, SCN− and N− 3 . The three samples were used for luminescent measurements after ultrasonic treatment for 15 min. [37] W. Liu, X. Huang, C. Xu, C. Chen, L. Yang, W. Dou, W. Chen, H. Yang, W. Liu, A multiresponsive regenerable europium-organic framework luminescent sensor for Fe3+, CrVI anions, and picric acid, Chem. Eur. J. 22 (2016) 18769–18776.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
A 3D porous luminescent terbium metal-organic framework for selective sensing of F− in aqueous solution Yongyan Wan, Wei Sun, Jingjuan Liu, Zhiliang Liu ⁎ College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, Inner Mongolia, China
a r t i c l e
i n f o
Article history: Received 22 March 2017 Received in revised form 12 April 2017 Accepted 14 April 2017 Available online 14 April 2017 Keywords: Metal-organic frameworks Halide and pseudohalide ions Selective sensing Rapid response Thermal and pH stability
a b s t r a c t An extremely stable Tb metal-organic framework (Tb-MOF), namely [Tb(H4btca)∙3H2O]n (H4btca = [1,1′-biphenyl]-2,3,3′,5′-tetracarboxylic acid), was synthesized under hydrothermal condition. The single-crystal X-ray diffraction indicated Tb-MOF exhibits a 3D structure with 1D channel, which crystallizes as an orthorhombic system with a Pccn space group. In addition, Tb-MOF displays bright green luminescence under the irradiation of UV light at 254 nm. Meanwhile, the investigations showed that Tb-MOF possessed excellent thermal and pH stability. More importantly, it can selectively and rapidly sense F− among proprietary halide and pseudohalide ions, and can be used for quantitative detecting F−. © 2017 Published by Elsevier B.V.
Metal-organic frameworks (MOFs), self-assembled from metal ions with organic linkers, have been recently received increasing attention because of its structural controllability and high porosity, and idiosyncratic surface properties [1,2]. Accordingly, MOFs have been widely used in catalysis, gas storage and separation, luminescent sensing, drug delivery and so on [3–10]. Besides, many MOFs display excellent luminescent sensing in especial for lanthanide based metal-organic frameworks (Ln-MOFs) [11–15]. Compared to transition metals, LnMOFs are attracted immense attention owing to many fascinating properties such as unique luminescent properties, high color purity, the extremely sharp emissions, large Stokes' shifts, and relatively long fluorescence lifetime [16–18]. Therefore, Ln-MOFs show a promising application as luminescent sensing materials. In recent years, serious destructions of the environment and biological systems were caused by toxic ions, organic solvents, and heavy metals and so on [19]. For example, F− is a typical pollutant anion and it is widely exists in natural water. Intaking excessive fluoride will cause skeletal fluorosis or dental fluorosis [20–22]. Therefore, fast and efficient sensing F− becomes one of the most hotspots in the field of chemistry. As is known to us all, the traditional analysis techniques for sensing F−, such as gas chromatography, inductively coupled plasma mass spectrometry, synchrotron radiation X-ray spectrometry and atomic absorption and so on. However, these methods not only require sophisticated instrumentations, but also time-consuming and high cost [23–25]. Hence, searching for a convenient, time-saving, high sensitivity and low cost method for sensing F− is very necessary. Therefore, ⁎ Corresponding author. E-mail address: [email protected] (Z. Liu).
http://dx.doi.org/10.1016/j.inoche.2017.04.010 1387-7003/© 2017 Published by Elsevier B.V.
luminescent sensors based on Ln-MOFs have been considered as one of the compelling approaches for sensing F−. For example, Chen et al. [26] synthesized a TbL (HL = 4-(pyrimidin-5-yl) benzoic acid) complex for sensing F− in partial halogen-containing anion. Chen et al. [27] constructed a novel Cu-MOF, namely [Cu4I(TIPE)3 ∙ 3I] (TIPE = tetra(3imidazoylphenyl)ethylene), which can be applied to selectively sense F− in a small-scope of halogen anion. Based on these considerations above, we report a novel Tb-MOF, [Tb(H4bcta)∙ 3H2O]n [28], constructed by Tb(NO3)3 ∙ 6H2O and H4bcta (H4btca = [1,1′-biphenyl]-2,3,3′,5′-tetracarboxylic acid) under solvothermal reaction. Structural analysis by the single-crystal X-ray diffraction revealed that the Tb-MOF exhibited a 3D structure with 1D channel. Interestingly, Tb-MOF can be used as a luminescence prober for sensing F− in aqueous solution, with good selectivity and rapid response. It is noteworthy to mention that this is the first example that Ln-MOF as luminescence prober selectively sensing F− among proprietary halide and pseudohalide anions. The single-crystal X-ray diffraction analyses [29] (crystal data for TbMOF was shown in Table S1) indicated Tb-MOF crystallized as an orthorhombic system with a Pccn space group. As shown in Fig. 1a, the asymmetric unit contains one Tb3 + metal centra, one Hbtca3− ligand, and three free water molecules. Among them, Tb1 atom was coordinated by nine oxygen atoms that six oxygen atoms were from four bidentate carboxyls and three oxygen atoms were from two terdentate carboxyls. According to the SHAPE calculation [30], Tb1 and the surrounding coordination atoms form spherical capped square antiprism geometric configuration. Two adjacent and crystallographic symmetry terbium atoms linked by carboxyl oxygen atoms of the Hbtca3 − ligand to form the [Tb2(COO)7] secondary building unit (SBU). Furthermore, the SBU
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Fig. 1. (a) The coordination environment of Tb3+ in the asymmetric unit; (b) The 3D structure of Tb-MOF containing un-deprotonated carboxyl groups in the pores along the c axis. Bottom: the corresponding channel was proved by the yellow column. (c) The 3D structure of Tb-MOF along the a axis. Bottom: the corresponding channel was proved by the yellow column. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
connected by Hbtca3− ligands to expand into a 3D framework structure. Along the c axis, the Tb-MOF shows two types of 1D channel with the size of 10 × 8 Å2 and 5 × 5 Å2, respectively (Fig. 1b). As shown in Fig. 1c, the Tb-MOF exhibits uniform 1D channels and the size is about 9.8 × 10.4 Å2. The PLATON software is used to calculate porosity of TbMOF, which is 41.8%. The distances of Tb\\O range from 2.320(6) to 2.566(10) Å, and the corresponding angles O\\Tb\\O vary in the range of 50.3(4)–155.2(3)° (selected bond lengths (Å) and angles (°) for Tb-MOF was listed in Table S2). It is notable that there exists a undeprotonated carboxyl group in the pore of Tb-MOF. The IR spectra of H4btca and Tb-MOF were carried out at room temperature (shown in Fig. S1). The peak at 1674 cm−1 for Tb-MOF demonstrated that all the four carboxyls of H4btca ligand formed coordination bond with metal atoms. Besides, the PXRD pattern of as-synthesized TbMOF was shown in the Fig. S2. The diffraction peaks of Tb-MOF were consistent with the simulated pattern, indicating that the as-
synthesized sample possess the excellent phase purity. Meanwhile, the sharp peaks indicate the as-synthesized sample show the excellent crystallinity. Furthermore, the thermal stability of Tb-MOF was observed by the TGA analyses in a nitrogen atmosphere (shown in Fig. S3). As was shown in the TGA curve of Tb-MOF, the weight of Tb-MOF loss 9.82% in the temperature range 50–377 °C due to losing three free water molecules (calculated 10.1%). As temperature continues rising, ligands start to decompose thermally and the structural collapsed. To explore the stability of Tb-MOF in aqueous solution, we observed the luminescence intensities of Tb-MOF at various pH values [31]. The samples of Tb-MOF dispersed in aqueous solution with the pH values from 4 to 10, the luminescence intensities of Tb-MOF are almost unaltered (shown in Fig. 2a). Moreover, the measured corresponding PXRD patterns were almost same with the as-synthesis Tb-MOF (shown in Fig. 2b). Thus, the excellent pH stability evidenced Tb-MOF
Fig. 2. (a) Luminescence intensity of Tb-MOF immersed in different pH aqueous solutions. (b) The PXRD patterns of Tb-MOF after treated with different pH aqueous solutions.
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Fig. 3. The solid-state luminescence spectrum of Tb-MOF sample; inset: the photograph of Tb-MOF and under the irradiation of UV light at 254 nm.
as a luminescence prober for sensing pollutant in special environment is feasibility. To further explore the property of Tb-MOF, the solid-state luminescent spectra of Tb-MOF and H4bcta were measured at room temperature (shown in Fig. 3 and Fig. S4). Under excitation wavelength of 260 nm, the H4btca ligand exhibits a broad peak emission and with a maximum intensity at 400 nm, which was presumably attributed to the π–π* transitions of the ligand. Tb-MOF has four characteristic emission spectra upon excitation wavelength at 260 nm. The emission peaks of Tb-MOF appeared at 491 nm (5D4 → 7D6), 547 nm (5D4 → 7D5), 586 nm (5D4 → 7D4), and 620 nm (5D4 → 7D3), respectively. Significantly, the strongest emission peak at 547 nm corresponds to the 5D4 → 7F5 transitions for the Tb3+ ion, resulting in the bright green light which can be observed by naked eyes. It is obvious that the luminescence spectrum of Tb-MOF do not contain the ligand-based emission peaks. This phenomenon can be explained by the efficient energy-transfer which happened between the ligand and the Tb3+ ion. We speculated that the enhanced luminescence property can be explained by the antenna effect [32] with an efficient energy transfer between the ligand and the Tb3+ ion. Owing to green luminescence emission, the Tb-MOF was selected to explore its sensing ability for halide and pseudohalide ions [33]. As is shown in Fig. 4a, it can be clearly observed that the luminescence
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intensity changed after different halide and pseudohalide ions X− (NaX, X = F−, Cl−, Br−, I−, SCN− or N− 3 ) added. Interestingly, as was shown in Fig. 4c, there was an obvious quenching effect owing to the existence of F− on the emission of Tb-MOF at 547 nm, whereas other anions showed no or only minor interferences. The above phenomenon can be easily captured by naked eyes under a 254 nm UV lamp irradiation. The Fig. 4b showed that Tb-MOF had excellent sensing behavior towards F− by quenching among all halide and pseudohalide ions and the response speed is quickly. To further explore the interactions between F− concentration and the quenching efficiency, Tb-MOF samples were dispersed into different concentrations aqueous solutions of F− in the range of 10−4–10−2 M. Results showed that the luminescence intensities of Tb-MOF gradually decreased with the increasing concentrations of F−. When the concentration of F− reached at 0.01 M, luminescence of the Tb-MOF was almost entirely quenched. The detection limit of Tb-MOF for F− was determined at 4.97 × 10−4 M (shown in Fig. S5). Furthermore, the relationship between luminescence intensity and the concentration of F− across the whole concentration range has been built, and it can be well fitted as the following formula: I0/I = 1 + 0.0661 exp(C/23.17574) (I = luminescence intensity of Tb-MOF suspensions with different concentrations of F−; I0 = luminescence intensities of Tb-MOF suspensions; C = the concentrations of F−). As was shown in Fig. 5a, the correlation coefficient (R2) is 0.994. Therefore, Tb-MOF has already proved to be a luminescence sensor for quantitative detection of F− in aqueous solution. As far as we known, few luminescent MOFs acting as sensors for F− in water system were observed in literatures [34,35]. Specifically, the comparison of other MOF-based fluorescent sensors for F− sensing was listed in Table S3. Besides, in order to further understand the detail influence of other halide and pseudohalide ions to the selectively quenching response of F−. Selective experiments [36] towards various anions were conducted by adding 1 × 10− 2 M of F−, Cl−, Br−, I−, SCN−, N− 3 , under the established conditions. As was shown in Fig. 5b, the luminescence intensity of Tb-MOF showed a negligible effect after adding other anions, while the luminescence intensity was nearly completely disappeared in seconds when F− was introduced. Results indicate that the Tb-MOF can sense F− with excellent highly selective in all halide and pseudohalide ions. We further investigated the possible sensing mechanism of luminescence quenching by F−. The PXRD pattern was measured after immersing Tb-MOF into the NaF solution. As can be seen in Fig. S6, the PXRD pattern of Tb-MOF handled by F− solution showed a marked difference compared to the as-synthesized Tb-MOF, proving that the crystal
Fig. 4. (a) Luminescence spectra of Tb-MOF in different anions solutions; (b) The picture of Tb-MOF in different anions solutions under 254 nm UV-lamp; (c) Luminescence intensities of Tb-MOF in different anions solutions.
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Fig. 5. (a) Concentration-dependent luminescence quenching of Tb-MOF after dispersed in different concentration of F−; inset: the plot of intensity versus F− concentration. (b) The relative intensities for Tb-MOF in different solutions; inset: the corresponding photographs of Tb-MOF under the irradiation of 254 nm UV light.
structure has been destroyed. We speculate the quenching mechanism is that the presence of F− led to collapse of the crystal framework of Tb-MOF, thus causing the luminescence quenching [37]. In summary, a 3D porous metal-organic framework [Tb(H4btca)∙3H2O]n had been synthesized, which showed bright green luminescence. In addition, Tb-MOF displays excellent thermal and pH stability. More importantly, Tb-MOF can high selectively and rapidly sense F− among all halide and pseudohalide ions. All results indicate that it can be used as a luminescence prober for quantitative detection of F− in aqueous solution.
Acknowledgment This work was supported by the NSFC of China (21361016) and the Inner Mongolia Autonomous Region Fund for Natural Science (2013ZD09). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.inoche.2017.04.010.
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[29]
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(m), 533 (w), 454 (m). Elemental Anal (%) Calcd: C, 35.55, H, 2.41 Found: C, 35.7, H, 2.2. Structure determination for Tb-MOF was performed on a Bruker ApexII CCD diffractometer equipped with graphite-monochromatic Mo-Kα radiation (λ = 0.71073 Å) at 293K. Empirical absorption corrections were applied using the SADABS program. The suitable structure of Tb-MOF was solved by direct methods and refined anisotropically by full-matrix least squares techniques on F2 values, using the SHELX-97 program. Hydrogen atoms were added on appropriate positions in theory and refined with isotropic thermal parameters riding on those parent atoms. (CCDC reference number is 1535052). A. Ruiz-Martinez, D. Casanova, S. Alvarez, Polyhedral structures with an odd number of vertices: nine-coordinate metal compounds, Chem. Eur. J. 14 (2008) 1291–1303. pH stability experiments: The fine grinding sample of Tb-MOF (1 mg) was dispersed in 3 mL aqueous solution with various pH value (pH = 2–13), respectively. The pH values of the solutions were tuned by adding either HNO3 or NaOH solutions. After ultrasonic treatment for 15 min, the suspension was obtained and used for luminescent measurements. Then the samples were filtrated and used for PXRD measurements. L. Wang, G. Fan, X. Xu, D. Chen, L. Wang, W. Shi, P. Cheng, Detection of polychlorinated benzenes (persistent organic pollutants) by a luminescent sensor based on a lanthanide metal–organic framework, J. Mater. Chem. A 5 (2017) 5541–5549.
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[33] Luminescence sensing experiments. First, the samples of Tb-MOF (1 mg) were dispersed in 1 × 10−2 M different anions solutions (2 mL), such as F−, Cl−, Br−, I−, SCN− and N− 3 . Then, the homogeneous emulsions were formed after ultrasonic treatment for 15 min, which were used for luminescent measurements at room temperature. [34] Z.H. Lin, S.J. Ou, C.Y. Duan, B.G. Zhang, Z.P. Bai, Naked-eye detection of fluoride ion in water: a remarkably selective easy-to-prepare test paper, Chem. Commun. (2006) 624–626. [35] Q.W. Xu, C. Wang, Z.B. Sun, C.H. Zhao, A highly selective ratiometric bifunctional fluorescence probe for Hg(2 +) and F(−) ions, Org. Biomol. Chem. 13 (2015) 3032–3039. [36] Luminescence selective sensing experiments: The samples of Tb-MOF (3 mg) were dispersed in aqueous solution (3 mL), and divided into three parts averagely. The first was added 1 mL water as a blank sample; the second was gradually added 1 × 10−2 M solutions of interfering anions (200 μL). The third was stepwise added 200 μL solutions of all anions, including F−, Cl−, Br−, I−, SCN− and N− 3 . The three samples were used for luminescent measurements after ultrasonic treatment for 15 min. [37] W. Liu, X. Huang, C. Xu, C. Chen, L. Yang, W. Dou, W. Chen, H. Yang, W. Liu, A multiresponsive regenerable europium-organic framework luminescent sensor for Fe3+, CrVI anions, and picric acid, Chem. Eur. J. 22 (2016) 18769–18776.