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A robust microporous metal–organic framework as a highly selective and sensitive, instantaneous and colorimetric sensor for Eu3+ ions† Yanfei Gao,a Xueqiong Zhang,a Wei Suna and Zhiliang Liu*a,b An extremely thermostable magnesium metal–organic framework (Mg-MOF) is reported for use as a highly selective and sensitive, instantaneous and colorimetric sensor for Eu3+ ions. There has been extensive interest in the recognition and sensing of ions because of their important roles in biological and environmental systems. However, only a few of these systems have been explored for specific rare earth ion detection. A robust microporous Mg-MOF for the recognition and sensing of Eu3+ ions with high selectivity at low concentrations in aqueous solutions has been synthesized. This stable metal–organic framework (MOF) contains nanoscale holes and non-coordinating nitrogen atoms inside the walls of the
Received 10th September 2014, Accepted 26th November 2014
holes, which makes it a potential host for foreign metal ions. Based on the energy level matching and
DOI: 10.1039/c4dt02752c
efficient energy transfer between the host and the guest, the Mg-MOF sensor is both highly selective and sensitive as well as instantaneous; thus, it is a promising approach for the development of luminescent
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probing materials with unprecedented applications and its use as an Eu3+ ion sensor.
Emerging microporous metal–organic frameworks (MOFs) have attracted significant attention for their superior functional properties and applications in gas storage and separation,1–6 heterogeneous catalysis,7–14 ion exchange and drug delivery,15–17 while their sensing function has rarely been employed, such as in the sensing of cations.18 In particular, a few of these materials have been explored for rare earth ion detection.19 Moreover, to the best of our knowledge, the instantaneous detection of an individual Ln3+ ion in aqueous solution with high selectivity and sensitivity is still a challenge. To date, many compounds have been designed for Ln3+ ion sensing; however, these compounds usually suffer from significant interference from other ions.20,21 The self-assembly of guest Ln3+ ions in porous host MOFs has allowed us to generate unique luminescent platforms for sensing Ln3+ ions. Guest-dependent luminescence sensors based on MOFs have the distinct advantages of fast response and high selectivity and sensitivity.19,22,23 Based on the ability of the coordinating atoms in the organic linkers and terminal organic solvents within MOFs to induce bonding interactions with guest-cations, microporous MOFs are expected to be very
a College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot, P. R. China. E-mail: [email protected] b Inner Mongolia Key Lab of Nanoscience and Nanotechnology, Hohhot, P. R. China † Electronic supplementary information (ESI) available: Experimental details and additional data. CCDC 1000768. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt02752c
This journal is © The Royal Society of Chemistry 2015
promising receptors for guest-cations. By exploiting the energy level matching between the host-MOFs and the guest cations, the host–guest interaction can readily transfer external energy to a given guest Ln3+ ion and enhance its characteristic emission; therefore, recognition and sensing can be achieved. Typically, the precursors of host MOFs can contain any desired functional group (or functional sites), which upon synthesis of the host MOFs, result in the desired functional groups (or functional sites) being exposed within the porous structure. Thus, robust, 3D, microporous metal–organic frameworks [Mg( pdda)(DMF)]n (Mg-MOF) containing functional sites that point inwards were obtained in this study as transparent crystals generated from the rigid, v-shaped carboxylic ligand H2pdda (4,4′-(pyrazine-2,6-diyl)) dibenzoic acid, together with Mg(NO3)2·6H2O under solvothermal conditions in moderate yields (ESI S3†). The structure and formula of the Mg-MOF was established based on the single-crystal X-ray structure, elemental analysis, and thermogravimetric analysis (TGA). As we had expected, a robust porous structure containing functional sites on the walls provided a chance for the Mg-MOF to serve as a “functional lantern”, binding the specific lanthanide cations and enhancing their luminescence. Single-crystal X-ray diffraction analysis revealed that Mg-MOF crystallizes in the monoclinic space group C2/c. In Mg-MOF, each asymmetric unit contains one Mg2+ ion, one pdda2− ligand, and one DMF molecule. As shown in Fig. 1a, the Mg2+ ion is located at an inversion center, and the pdda2− ligand and DMF molecule are in general positions. Mg2+ is
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Fig. 2
Fig. 1 (a) Ball-and-stick representation of the structural unit of the MgMOF. Symmetry codes: A, 2 − x, y, 1.5 − z; B, 0.5 + x, 0.5 + y, 1 + z; C, 1.5 − x, 1.5 − y, 1 − z; D, 0.5 + x, 1.5 − y, 0.5 + z; E, 1.5 − x, 0.5 + y, 0.5 − z. (b) 3D topology of Mg-MOF. (c) 2D plane view of Mg-MOF and 1D chain constituted by binuclear Mg2+ subunit.
six-coordinated with a distorted octahedral geometry, which is completed by four monodentate carboxylic oxygen atoms, one oxygen atom from the DMF molecule and one nitrogen atom from the pdda2− ligand. The Mg–N distance is 2.318(7) Å, and the average Mg–O bond length is 2.054 Å. The coordination modes of the pdda2− ligand are shown in Scheme S1 (ESI†), and the four oxygen atoms of the two carboxyl groups in a single pdda2− ligand are monodentate. The neighboring Mg2+ ions are linked by two-fold monodentate carboxylic oxygen atoms incorporated into a 1D chain along the c direction and further connected to a nitrogen atom from the pdda2− ligand to form an interesting 3D framework. The pdda2− ligand can be considered as a 5-connected node, and the structure can be simplified as a 3,4-connected framework with the point Schlafli symbol {333·454·54}, as shown in Fig. 1b. Interestingly, there are large permanent pores with dimensions of 9.522 Å × 7.344 Å in the 3D framework and uncoordinated nitrogen atoms in the inner walls of the pores. PLATON analysis24 indicates that the solvent accessible volume and porosity are 566.7 Å3 and 29.17%, respectively. To reveal the thermal stability and to further support the molecular formula of the compound, the TGA experiment was performed on a pure single crystal sample of Mg-MOF under an N2 atmosphere with a heating rate of 10 °C min−1 over the range of 25–1200 °C (Fig. 2). The compound shows excellent thermal stability. The TGA curve shows two regions of weight loss. The first weight loss between 230 and 300 °C is 18.24%, which corresponds to the loss of one coordinated DMF molecule, calculated to be 18.25%. The second weight loss,
1846 | Dalton Trans., 2015, 44, 1845–1849
TGA of Mg-MOF.
above 530 °C, results from the decomposition of the compound. On the basis of the above analysis we can conclude that, in the range 300–530 °C, the compounds remain intact after the removal of the coordinated DMF molecules. Among the lightweight metal elements, magnesium is particularly interesting because it bears a number of similarities to the transition metal ions. However, it rarely assembles MOFs, and if does any, they have poor thermal stability.25,26 To the best of our knowledge, the Mg-MOF we obtained is the first sample with good thermal stability. According to the TGA pattern analysis, when the Mg-MOF was heated to 300 °C, the coordinating DMF molecules began to be removed, and the compound retained its stability up to temperatures of 530 °C. After the as-synthesized Mg-MOF was heated at 300 °C under N2 for 0.5 h ( producing Act-MOF), a PXRD analysis of the sample showed that the framework structure remained intact. As a matter of fact, MOFs have several advantages for detecting lanthanide cations.27 In this case, MOFs are expected to be highly promising receptors for guest cations, and energy level matching between the host-MOFs and guest lanthanide ions may produce a synergistic effect for the efficient identification of specific lanthanide ions. Thus, we have analyzed the luminescence properties of Ln3+@Act-MOF [Ln3+ = Eu3+, Dy3+, Tb3+] to explore the ability of Act-MOF to identify lanthanide ions. To introduce lanthanide ions into the pores of the activated sample, the Act-MOF was soaked in aqueous LnCl3 solutions [Ln3+ = Eu3+, Dy3+, Tb3+], yielding Eu3+@Act-MOF, Dy3+@Act-MOF, and Tb3+@Act-MOF (for experimental details see ESI S5†). As confirmed by the powder X-ray diffraction patterns (PXRD) (Fig. 3), Ln3+ loading only minimally impacts the crystalline integrity of the Act-MOF. However, due to the role of the newly introduced salt, the crystal lattice of the Act-MOF may have undergone some distortions and the PXRD noise peaks increased slightly, or the ratio of signal to noise decreased. Photoluminescence studies were performed on solid samples of H2pdda, Act-MOF, and Ln3+@MOF. Fig. S3† shows that the free H2pdda ligand has emission with a
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Fig. 3 PXRD patterns of Mg-MOF, Act-MOF, Eu3+@Act-MOF, Dy3+@Act-MOF, Tb3+@Act-MOF and the simulated pattern of Mg-MOF.
maximum at ca. 467 nm when excited at a wavelength of 396 nm, which may be ascribed with high probability to the π→π* and/or n→π* transitions of the H2pdda. The emission of the Act-MOF displays maxima at ca. 421 nm and 458 nm when excited at a wavelength of 360 nm. The blueshift of the ligand’s emission in the Act-MOF should be attributed to the deprotonation and coordination of the ligand to Mg2+ ions.28 The fluorescence measurements of Eu3+@Act-MOF showed very strong characteristic peaks of Eu3+, while the emission of
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the Act-MOF almost disappeared. This result indicates that a unique luminescent platform has been formed via the introduction of Eu3+ ions into the pores of the Act-MOF. For comparison, we measured the emission spectra of the Eu3+@ActMOF and a thoroughly ground mixture of Act-MOF and EuCl3·6H2O under the same conditions. The Eu3+@Act-MOF displays enhanced characteristic emission of Eu(III), whereas the mechanically ground mixture displays weak emission bands for the Act-MOF and EuCl3·6H2O (Fig. S4†). These results demonstrate that the guest Eu3+ ions were encapsulated in the holes of the host Act-MOF, and the host–guest interaction can achieve effective energy transfer, enhancing the photoluminescence of Eu3+.29 It is noteworthy that the Ln3+ (Eu3+, Dy3+, Tb3+) loaded Act-MOF exhibits a very interesting photoluminescence phenomenon. As shown in Fig. 4a, the emission intensities of the Dy3+@Act-MOF and Tb3+@Act-MOF are almost negligible compared with the Eu3+@Act-MOF under the same test conditions. Furthermore, the emission peaks of the Dy3+@Act-MOF and Tb3+@Act-MOF showed no characteristic emission peaks for the rare earth ions, while the Eu3+@Act-MOF displays rather strong characteristic emission peaks of Eu3+ in the range of 400–750 nm. More interestingly, when the Act-MOF was soaked in aqueous solutions of EuCl3, a distinctive pink colour was readily observed under a standard UV lamp by the unaided eye (as shown in Fig. 4b and Fig. S2†). The response time is approximately 30 s, and the rapid response and easy observation by the naked eye make Act-MOF an unprecedentedly practical and useful material for use as a sensor to detect Eu3+.
Fig. 4 (a) Photoluminescence of Eu3+@Act-MOF, Dy3+@Act-MOF and Tb3+@Act-MOF samples and partial enlarged view. (b) Powder samples illuminated with 254 nm laboratory UV light. (c) Eu3+ concentration-dependent fluorescence spectra.
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The fluorescence quantum yield and lifetime of the Eu3+@Act-MOF solid samples were measured, and the results showed comparatively higher quantum yields (5.06%) (ESI Fig. S6†) and a very long fluorescence lifetime (2.5 ms) (ESI Fig. S7†), which is superior to most of the reported luminescent Eu3+ complexes.30,31 It is well known that the luminescence of lanthanide cations often suffers from weak light absorption and the spin- or parity-forbidden f–f transition. However, the lanthanide-centered emission can overcome this defect via the “antenna effect” or “luminescence sensitization”.32–34 Five characteristic peaks of Eu(III) are shown in the emission spectrum of the Eu3+@Act-MOF (Fig. 4a), which are attributed to 5D0→7F0 (579 nm), 5D0→7F1 (591 nm), 5D0→7F2 (613 nm), 5D0→7F3(650 nm) and 5D0→7F4 (698 nm).35 In this case, the uncoordinated N atoms with conjugated electronic systems within the pores of the frameworks may bind to the Eu3+ ions, providing an energy transfer channel and sensitizing the emission of Eu3+. This frameworkto-Eu3+ energy transfer behavior was confirmed by analyzing the emission spectrum of the Act-MOF and the excitation spectrum of the Eu3+@Act-MOF. As shown in Fig. S5,† the luminescence intensities of the Act-MOF can be systematically modulated by tuning the excitation energy. Upon increasing or reducing the excitation wavelength, the emission intensities of the Eu3+@Act-MOF in the luminescence spectra gradually changed. These results demonstrate that the emission energy of the framework matches well with the excitation energy of the Eu3+ ions. The enhanced luminescence of the Eu3+@ActMOF originates from the efficient framework-to-Eu3+ energy transfer by effectively preventing non-radiative decay.29,36–38 According to the above analysis, the Act-MOF is a good chromophore for the efficient luminescent detection of Eu3+. To explore the material probing ability, we also investigated the sensing features of the above host–guest Eu3+@Act-MOF luminescent platform at different concentrations of Eu3+ ions, various pH values and in mixed-cation systems. Fig. 4c shows the fluorescence response of the Act-MOF to Eu3+ at various concentrations of Eu3+ ions under optimal experimental conditions at pH = 7. As illustrated, when the concentration of Eu3+ ions is in the range of 0.2–3.4 ppm, the luminescence intensity of the experimental system is linear with the concentration of Eu3+ ions. Thus, we can quantitatively use calibration curves to determine the concentration of Eu3+ ions at very low concentrations (ESI Fig. S8†). A wide range of pH value adaptability in aqueous solution is rather important in practical application for use as a sensor system. We investigated the luminescence intensity of Eu3+@MOF under different pH conditions. As shown in Fig. 5, in the pH value range of 3–9, the luminescence intensity of all of the samples remains greater than 50%, which indicates that the Act-MOF can be used as a sensor for Eu3+ ions over a wide pH range. Additionally, we investigated the sensing selectivity of the Act-MOF to Eu3+ ions in the presence of several other cationic species (Tb3+, Dy3+, Mn2+, Cu2+, Co2+, Cd2+, and Fe3+). The luminescence intensity of the sensing system for mixedion samples remains greater than 60% (Fig. 6), which indicates
1848 | Dalton Trans., 2015, 44, 1845–1849
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Fig. 5 The fluorescence intensity of Eu3+@Act-MOFs in aqueous solutions with different pH values.
Fig. 6 The sensing selectivity of the Act-MOFs to Eu3+ ion in the presence of several other coexisted cationic species.
that the other cations only slightly affect the recognition of the Eu3+ ions. Therefore, the sensing selectivity of the Act-MOF for Eu3+ ions is much higher.
Conclusions In summary, we have synthesized a highly thermostable MgMOF material containing nanoscale holes, which is a potential host for foreign metal ions due to the non-coordinating nitrogen atoms in the walls of the holes. The Act-MOF luminescent platform represents significant recognition ability for Eu3+ and we have developed a luminescent Eu3+@Act-MOF approach to create a sensor to detect Eu3+ ions. Based on energy level matching and efficient energy transfer between the Act-MOF and the Eu3+ ions, this promising luminescent platform is a highly selective and sensitive sensor for Eu3+ ions. The robust microporous Act-MOF can recognize Eu3+ ions at very low concentrations in aqueous solutions, and the luminescence intensity of the experimental system is linearly dependent on the concentration of Eu3+ ions. Moreover, the sensitivity is only slightly affected over a wide pH range in the presence of many other cations. This Eu3+@Act-MOF sensor is not only stable and reliable but also exhibits a fast response. Its ability to unambiguously recognize Eu3+ ions highlights that this is a very promising approach to develop MOF luminescent
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platforms with its potential applications for sensing other Ln3+ ions.
Acknowledgements
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We are grateful for the financial support provided by the NSFC of China (21361016) and the Inner Mongolia Autonomous Region Fund for Natural Science (2013ZD09).
Notes and references 1 Y. L. Liu, J. F. Eubank, A. J. Cairns, J. Eckert, V. C. Kravtsov, R. Luebke and M. Eddaoudi, Angew. Chem., Int. Ed., 2007, 46, 3278–3283. 2 L. Hamon, P. L. Llewellyn, T. Devic, A. Ghoufi, G. Clet, V. Guillerm, G. D. Pirngruber, G. Maurin, C. Serre, G. Driver, W. van Beek, E. Jolimaitre, A. Vimont, M. Daturi and G. Ferey, J. Am. Chem. Soc., 2009, 131, 17490–17499. 3 L. Hamon, C. Serre, T. Devic, T. Loiseau, F. Millange, G. Férey and G. D. Weireld, J. Am. Chem. Soc., 2009, 131, 8775–8777. 4 S. S. Kaye, A. Dailly, O. M. Yaghi and J. R. Long, J. Am. Chem. Soc., 2007, 129, 14176–14177. 5 N. L. Rosi, J. Eckert, M. Eddaoudi, D. T. Vodak, J. Kim, M. O’Keeffe and O. M. Yaghi, Science, 2003, 300, 1127– 1129. 6 J. L. C. Rowsell, E. C. Spencer, J. Eckert, J. A. K. Howard and O. M. Yaghi, Science, 2005, 309, 1350–1354. 7 A. Corma, H. Garcia and F. X. L. I. Llabres i Xamena, Chem. Rev., 2010, 110, 4606–4655. 8 G. Ferey, Chem. Soc. Rev., 2008, 37, 191–214. 9 J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen and J. T. Hupp, Chem. Soc. Rev., 2009, 38, 1450–1459. 10 L. Ma, C. Abney and W. Lin, Chem. Soc. Rev., 2009, 38, 1248–1256. 11 C. D. Wu, A. Hu, L. Zhang and W. B. Lin, J. Am. Chem. Soc., 2005, 127, 8940–8941. 12 H. K. Chae, D. Y. Siberio-Perez, J. Kim, Y. Go, M. Eddaoudi, A. J. Matzger, M. O’Keeffe and O. M. Yaghi, Nature, 2004, 427, 523–527. 13 H. Li, M. Eddaoudi, M. O’Keeffe and O. M. Yaghi, Nature, 1999, 402, 276–279. 14 J. S. Seo, D. Whang, H. Lee, S. I. Jun, J. Oh, Y. J. Jeon and K. Kim, Nature, 2000, 404, 982–986. 15 M. Ma, H. Noei, B. Mienert, J. Niesel, E. Bill, M. Muhler, R. A. Fischer, Y. Wang, U. Schatzschneider and N. MetzlerNolte, Chem. – Eur. J., 2013, 19, 6785–6790.
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16 W. G. Lu, C. Y. Su, T. B. Lu, L. Jiang and J. M. Chen, J. Am. Chem. Soc., 2006, 128, 34–35. 17 S. Noro, R. Kitaura, M. Kondo, S. Kitagawa, T. Ishii, H. Matsuzaka and M. Yamashita, J. Am. Chem. Soc., 2002, 124, 2568–2583. 18 C.-Y. Sun, X.-L. Wang, C. Qin, J.-L. Jin, Z.-M. Su, P. Huang and K.-Z. Shao, Chem. – Eur. J., 2013, 19, 3639– 3645. 19 L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf, R. P. Van Duyne and J. T. Hupp, Chem. Rev., 2012, 112, 1105– 1125. 20 F. Luo and S. R. Batten, Dalton Trans., 2010, 39, 4485– 4488. 21 J. An, C. M. Shade, D. A. Chengelis-Czegan, S. Petoud and N. L. Rosi, J. Am. Chem. Soc., 2011, 133, 1220–1223. 22 H. Li, W. Shi, K. Zhao, Z. Niu, H. Li and P. Cheng, Chem. – Eur. J., 2013, 19, 3358–3365. 23 Z.-J. Lin, T.-F. Liu, Y.-B. Huang, J. Lu and R. Cao, Chem. – Eur. J., 2012, 18, 7896–7902. 24 K. L. Mulfort, O. K. Farha, C. L. Stern, A. A. Sarjeant and J. T. Hupp, J. Am. Chem. Soc., 2009, 131, 3866–3868. 25 Q. Zhai, Q. Lin, T. Wu, S.-T. Zheng, X. Bu and P. Feng, Dalton Trans., 2012, 41, 2866–2868. 26 N. B. Shustova, A. F. Cozzolino, S. Reineke, M. Baldo and M. Dinca, J. Am. Chem. Soc., 2013, 135, 13326–13329. 27 J. Cao, Y. Gao, Y. Wang, C. Du and Z. Liu, Chem. Commun., 2013, 49, 6897–6899. 28 J. R. Lakowicz, in Principles of Fluorescence Spectroscopy, Springer, Berlin, 3rd edn, 2006. 29 M. J. Dong, M. Zhao, S. Ou, C. Zou and C. D. Wu, Angew. Chem., Int. Ed., 2014, 53, 1575–1579. 30 M. Y. Berezin and S. Achilefu, Chem. Rev., 2010, 110, 2641– 2684. 31 Y. Cui, Y. Yue, G. Qian and B. Chen, Chem. Rev., 2011, 112, 1126–1162. 32 E. G. Moore, A. P. S. Samuel and K. N. Raymond, Acc. Chem. Res., 2009, 42, 542–552. 33 K. Binnemans, Chem. Rev., 2009, 109, 4283–4374. 34 S. Wherland, Coord. Chem. Rev., 1993, 123, 169–199. 35 G. Vicentini, L. B. Zinner, J. Zukerman-Schpector and K. Zinner, Coord. Chem. Rev., 2000, 196, 353–382. 36 B. L. Chen, C. D. Liang, J. Yang, D. S. Contreras, Y. L. Clancy, E. B. Lobkovsky, O. M. Yaghi and S. Dai, Angew. Chem., Int. Ed., 2006, 45, 1390–1393. 37 D. M. D’Alessandro, B. Smit and J. R. Long, Angew. Chem., Int. Ed., 2010, 49, 6058–6082. 38 R. E. Morris and P. S. Wheatley, Angew. Chem., Int. Ed., 2008, 47, 4966–4981.
Dalton Trans., 2015, 44, 1845–1849 | 1849
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A robust microporous metal–organic framework as a highly selective and sensitive, instantaneous and colorimetric sensor for Eu3+ ions† Yanfei Gao,a Xueqiong Zhang,a Wei Suna and Zhiliang Liu*a,b An extremely thermostable magnesium metal–organic framework (Mg-MOF) is reported for use as a highly selective and sensitive, instantaneous and colorimetric sensor for Eu3+ ions. There has been extensive interest in the recognition and sensing of ions because of their important roles in biological and environmental systems. However, only a few of these systems have been explored for specific rare earth ion detection. A robust microporous Mg-MOF for the recognition and sensing of Eu3+ ions with high selectivity at low concentrations in aqueous solutions has been synthesized. This stable metal–organic framework (MOF) contains nanoscale holes and non-coordinating nitrogen atoms inside the walls of the
Received 10th September 2014, Accepted 26th November 2014
holes, which makes it a potential host for foreign metal ions. Based on the energy level matching and
DOI: 10.1039/c4dt02752c
efficient energy transfer between the host and the guest, the Mg-MOF sensor is both highly selective and sensitive as well as instantaneous; thus, it is a promising approach for the development of luminescent
www.rsc.org/dalton
probing materials with unprecedented applications and its use as an Eu3+ ion sensor.
Emerging microporous metal–organic frameworks (MOFs) have attracted significant attention for their superior functional properties and applications in gas storage and separation,1–6 heterogeneous catalysis,7–14 ion exchange and drug delivery,15–17 while their sensing function has rarely been employed, such as in the sensing of cations.18 In particular, a few of these materials have been explored for rare earth ion detection.19 Moreover, to the best of our knowledge, the instantaneous detection of an individual Ln3+ ion in aqueous solution with high selectivity and sensitivity is still a challenge. To date, many compounds have been designed for Ln3+ ion sensing; however, these compounds usually suffer from significant interference from other ions.20,21 The self-assembly of guest Ln3+ ions in porous host MOFs has allowed us to generate unique luminescent platforms for sensing Ln3+ ions. Guest-dependent luminescence sensors based on MOFs have the distinct advantages of fast response and high selectivity and sensitivity.19,22,23 Based on the ability of the coordinating atoms in the organic linkers and terminal organic solvents within MOFs to induce bonding interactions with guest-cations, microporous MOFs are expected to be very
a College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot, P. R. China. E-mail: [email protected] b Inner Mongolia Key Lab of Nanoscience and Nanotechnology, Hohhot, P. R. China † Electronic supplementary information (ESI) available: Experimental details and additional data. CCDC 1000768. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt02752c
This journal is © The Royal Society of Chemistry 2015
promising receptors for guest-cations. By exploiting the energy level matching between the host-MOFs and the guest cations, the host–guest interaction can readily transfer external energy to a given guest Ln3+ ion and enhance its characteristic emission; therefore, recognition and sensing can be achieved. Typically, the precursors of host MOFs can contain any desired functional group (or functional sites), which upon synthesis of the host MOFs, result in the desired functional groups (or functional sites) being exposed within the porous structure. Thus, robust, 3D, microporous metal–organic frameworks [Mg( pdda)(DMF)]n (Mg-MOF) containing functional sites that point inwards were obtained in this study as transparent crystals generated from the rigid, v-shaped carboxylic ligand H2pdda (4,4′-(pyrazine-2,6-diyl)) dibenzoic acid, together with Mg(NO3)2·6H2O under solvothermal conditions in moderate yields (ESI S3†). The structure and formula of the Mg-MOF was established based on the single-crystal X-ray structure, elemental analysis, and thermogravimetric analysis (TGA). As we had expected, a robust porous structure containing functional sites on the walls provided a chance for the Mg-MOF to serve as a “functional lantern”, binding the specific lanthanide cations and enhancing their luminescence. Single-crystal X-ray diffraction analysis revealed that Mg-MOF crystallizes in the monoclinic space group C2/c. In Mg-MOF, each asymmetric unit contains one Mg2+ ion, one pdda2− ligand, and one DMF molecule. As shown in Fig. 1a, the Mg2+ ion is located at an inversion center, and the pdda2− ligand and DMF molecule are in general positions. Mg2+ is
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Fig. 2
Fig. 1 (a) Ball-and-stick representation of the structural unit of the MgMOF. Symmetry codes: A, 2 − x, y, 1.5 − z; B, 0.5 + x, 0.5 + y, 1 + z; C, 1.5 − x, 1.5 − y, 1 − z; D, 0.5 + x, 1.5 − y, 0.5 + z; E, 1.5 − x, 0.5 + y, 0.5 − z. (b) 3D topology of Mg-MOF. (c) 2D plane view of Mg-MOF and 1D chain constituted by binuclear Mg2+ subunit.
six-coordinated with a distorted octahedral geometry, which is completed by four monodentate carboxylic oxygen atoms, one oxygen atom from the DMF molecule and one nitrogen atom from the pdda2− ligand. The Mg–N distance is 2.318(7) Å, and the average Mg–O bond length is 2.054 Å. The coordination modes of the pdda2− ligand are shown in Scheme S1 (ESI†), and the four oxygen atoms of the two carboxyl groups in a single pdda2− ligand are monodentate. The neighboring Mg2+ ions are linked by two-fold monodentate carboxylic oxygen atoms incorporated into a 1D chain along the c direction and further connected to a nitrogen atom from the pdda2− ligand to form an interesting 3D framework. The pdda2− ligand can be considered as a 5-connected node, and the structure can be simplified as a 3,4-connected framework with the point Schlafli symbol {333·454·54}, as shown in Fig. 1b. Interestingly, there are large permanent pores with dimensions of 9.522 Å × 7.344 Å in the 3D framework and uncoordinated nitrogen atoms in the inner walls of the pores. PLATON analysis24 indicates that the solvent accessible volume and porosity are 566.7 Å3 and 29.17%, respectively. To reveal the thermal stability and to further support the molecular formula of the compound, the TGA experiment was performed on a pure single crystal sample of Mg-MOF under an N2 atmosphere with a heating rate of 10 °C min−1 over the range of 25–1200 °C (Fig. 2). The compound shows excellent thermal stability. The TGA curve shows two regions of weight loss. The first weight loss between 230 and 300 °C is 18.24%, which corresponds to the loss of one coordinated DMF molecule, calculated to be 18.25%. The second weight loss,
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TGA of Mg-MOF.
above 530 °C, results from the decomposition of the compound. On the basis of the above analysis we can conclude that, in the range 300–530 °C, the compounds remain intact after the removal of the coordinated DMF molecules. Among the lightweight metal elements, magnesium is particularly interesting because it bears a number of similarities to the transition metal ions. However, it rarely assembles MOFs, and if does any, they have poor thermal stability.25,26 To the best of our knowledge, the Mg-MOF we obtained is the first sample with good thermal stability. According to the TGA pattern analysis, when the Mg-MOF was heated to 300 °C, the coordinating DMF molecules began to be removed, and the compound retained its stability up to temperatures of 530 °C. After the as-synthesized Mg-MOF was heated at 300 °C under N2 for 0.5 h ( producing Act-MOF), a PXRD analysis of the sample showed that the framework structure remained intact. As a matter of fact, MOFs have several advantages for detecting lanthanide cations.27 In this case, MOFs are expected to be highly promising receptors for guest cations, and energy level matching between the host-MOFs and guest lanthanide ions may produce a synergistic effect for the efficient identification of specific lanthanide ions. Thus, we have analyzed the luminescence properties of Ln3+@Act-MOF [Ln3+ = Eu3+, Dy3+, Tb3+] to explore the ability of Act-MOF to identify lanthanide ions. To introduce lanthanide ions into the pores of the activated sample, the Act-MOF was soaked in aqueous LnCl3 solutions [Ln3+ = Eu3+, Dy3+, Tb3+], yielding Eu3+@Act-MOF, Dy3+@Act-MOF, and Tb3+@Act-MOF (for experimental details see ESI S5†). As confirmed by the powder X-ray diffraction patterns (PXRD) (Fig. 3), Ln3+ loading only minimally impacts the crystalline integrity of the Act-MOF. However, due to the role of the newly introduced salt, the crystal lattice of the Act-MOF may have undergone some distortions and the PXRD noise peaks increased slightly, or the ratio of signal to noise decreased. Photoluminescence studies were performed on solid samples of H2pdda, Act-MOF, and Ln3+@MOF. Fig. S3† shows that the free H2pdda ligand has emission with a
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Fig. 3 PXRD patterns of Mg-MOF, Act-MOF, Eu3+@Act-MOF, Dy3+@Act-MOF, Tb3+@Act-MOF and the simulated pattern of Mg-MOF.
maximum at ca. 467 nm when excited at a wavelength of 396 nm, which may be ascribed with high probability to the π→π* and/or n→π* transitions of the H2pdda. The emission of the Act-MOF displays maxima at ca. 421 nm and 458 nm when excited at a wavelength of 360 nm. The blueshift of the ligand’s emission in the Act-MOF should be attributed to the deprotonation and coordination of the ligand to Mg2+ ions.28 The fluorescence measurements of Eu3+@Act-MOF showed very strong characteristic peaks of Eu3+, while the emission of
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the Act-MOF almost disappeared. This result indicates that a unique luminescent platform has been formed via the introduction of Eu3+ ions into the pores of the Act-MOF. For comparison, we measured the emission spectra of the Eu3+@ActMOF and a thoroughly ground mixture of Act-MOF and EuCl3·6H2O under the same conditions. The Eu3+@Act-MOF displays enhanced characteristic emission of Eu(III), whereas the mechanically ground mixture displays weak emission bands for the Act-MOF and EuCl3·6H2O (Fig. S4†). These results demonstrate that the guest Eu3+ ions were encapsulated in the holes of the host Act-MOF, and the host–guest interaction can achieve effective energy transfer, enhancing the photoluminescence of Eu3+.29 It is noteworthy that the Ln3+ (Eu3+, Dy3+, Tb3+) loaded Act-MOF exhibits a very interesting photoluminescence phenomenon. As shown in Fig. 4a, the emission intensities of the Dy3+@Act-MOF and Tb3+@Act-MOF are almost negligible compared with the Eu3+@Act-MOF under the same test conditions. Furthermore, the emission peaks of the Dy3+@Act-MOF and Tb3+@Act-MOF showed no characteristic emission peaks for the rare earth ions, while the Eu3+@Act-MOF displays rather strong characteristic emission peaks of Eu3+ in the range of 400–750 nm. More interestingly, when the Act-MOF was soaked in aqueous solutions of EuCl3, a distinctive pink colour was readily observed under a standard UV lamp by the unaided eye (as shown in Fig. 4b and Fig. S2†). The response time is approximately 30 s, and the rapid response and easy observation by the naked eye make Act-MOF an unprecedentedly practical and useful material for use as a sensor to detect Eu3+.
Fig. 4 (a) Photoluminescence of Eu3+@Act-MOF, Dy3+@Act-MOF and Tb3+@Act-MOF samples and partial enlarged view. (b) Powder samples illuminated with 254 nm laboratory UV light. (c) Eu3+ concentration-dependent fluorescence spectra.
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The fluorescence quantum yield and lifetime of the Eu3+@Act-MOF solid samples were measured, and the results showed comparatively higher quantum yields (5.06%) (ESI Fig. S6†) and a very long fluorescence lifetime (2.5 ms) (ESI Fig. S7†), which is superior to most of the reported luminescent Eu3+ complexes.30,31 It is well known that the luminescence of lanthanide cations often suffers from weak light absorption and the spin- or parity-forbidden f–f transition. However, the lanthanide-centered emission can overcome this defect via the “antenna effect” or “luminescence sensitization”.32–34 Five characteristic peaks of Eu(III) are shown in the emission spectrum of the Eu3+@Act-MOF (Fig. 4a), which are attributed to 5D0→7F0 (579 nm), 5D0→7F1 (591 nm), 5D0→7F2 (613 nm), 5D0→7F3(650 nm) and 5D0→7F4 (698 nm).35 In this case, the uncoordinated N atoms with conjugated electronic systems within the pores of the frameworks may bind to the Eu3+ ions, providing an energy transfer channel and sensitizing the emission of Eu3+. This frameworkto-Eu3+ energy transfer behavior was confirmed by analyzing the emission spectrum of the Act-MOF and the excitation spectrum of the Eu3+@Act-MOF. As shown in Fig. S5,† the luminescence intensities of the Act-MOF can be systematically modulated by tuning the excitation energy. Upon increasing or reducing the excitation wavelength, the emission intensities of the Eu3+@Act-MOF in the luminescence spectra gradually changed. These results demonstrate that the emission energy of the framework matches well with the excitation energy of the Eu3+ ions. The enhanced luminescence of the Eu3+@ActMOF originates from the efficient framework-to-Eu3+ energy transfer by effectively preventing non-radiative decay.29,36–38 According to the above analysis, the Act-MOF is a good chromophore for the efficient luminescent detection of Eu3+. To explore the material probing ability, we also investigated the sensing features of the above host–guest Eu3+@Act-MOF luminescent platform at different concentrations of Eu3+ ions, various pH values and in mixed-cation systems. Fig. 4c shows the fluorescence response of the Act-MOF to Eu3+ at various concentrations of Eu3+ ions under optimal experimental conditions at pH = 7. As illustrated, when the concentration of Eu3+ ions is in the range of 0.2–3.4 ppm, the luminescence intensity of the experimental system is linear with the concentration of Eu3+ ions. Thus, we can quantitatively use calibration curves to determine the concentration of Eu3+ ions at very low concentrations (ESI Fig. S8†). A wide range of pH value adaptability in aqueous solution is rather important in practical application for use as a sensor system. We investigated the luminescence intensity of Eu3+@MOF under different pH conditions. As shown in Fig. 5, in the pH value range of 3–9, the luminescence intensity of all of the samples remains greater than 50%, which indicates that the Act-MOF can be used as a sensor for Eu3+ ions over a wide pH range. Additionally, we investigated the sensing selectivity of the Act-MOF to Eu3+ ions in the presence of several other cationic species (Tb3+, Dy3+, Mn2+, Cu2+, Co2+, Cd2+, and Fe3+). The luminescence intensity of the sensing system for mixedion samples remains greater than 60% (Fig. 6), which indicates
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Fig. 5 The fluorescence intensity of Eu3+@Act-MOFs in aqueous solutions with different pH values.
Fig. 6 The sensing selectivity of the Act-MOFs to Eu3+ ion in the presence of several other coexisted cationic species.
that the other cations only slightly affect the recognition of the Eu3+ ions. Therefore, the sensing selectivity of the Act-MOF for Eu3+ ions is much higher.
Conclusions In summary, we have synthesized a highly thermostable MgMOF material containing nanoscale holes, which is a potential host for foreign metal ions due to the non-coordinating nitrogen atoms in the walls of the holes. The Act-MOF luminescent platform represents significant recognition ability for Eu3+ and we have developed a luminescent Eu3+@Act-MOF approach to create a sensor to detect Eu3+ ions. Based on energy level matching and efficient energy transfer between the Act-MOF and the Eu3+ ions, this promising luminescent platform is a highly selective and sensitive sensor for Eu3+ ions. The robust microporous Act-MOF can recognize Eu3+ ions at very low concentrations in aqueous solutions, and the luminescence intensity of the experimental system is linearly dependent on the concentration of Eu3+ ions. Moreover, the sensitivity is only slightly affected over a wide pH range in the presence of many other cations. This Eu3+@Act-MOF sensor is not only stable and reliable but also exhibits a fast response. Its ability to unambiguously recognize Eu3+ ions highlights that this is a very promising approach to develop MOF luminescent
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platforms with its potential applications for sensing other Ln3+ ions.
Acknowledgements
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We are grateful for the financial support provided by the NSFC of China (21361016) and the Inner Mongolia Autonomous Region Fund for Natural Science (2013ZD09).
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