A luminescent terbium MOF containing uncoordinated carboxyl groups exhibits highly selective sensing for Fe³⁺ ions

Abstract

A novel double-paned extended 2D network (TbMOF–COOH) containing large volume lantern-like open pores was prepared. With active uncoordinated carboxyl groups pointing to the interior of the pores, TbMOF–COOH has a high selectivity and sensitivity for Fe³⁺ ions via a luminescence quenching mechanism.

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Metal–organic frameworks (MOFs) have attracted great interest not only due to their intriguing architectures rendered by a variety of bridging ligands and metal ions¹ but also because of the potential applications in gas storage and separation,² catalysis,³ drug delivery⁴ and chemical sensing.⁵ Among the above application areas, chemical sensing based on luminescent MOFs is becoming a hot research field in recent years owing to their permanent porosity, tunable intriguing structures and optical properties. The luminescent MOFs containing desired functional groups in the interior of the frameworks can provide an important alternative for exploiting and expanding their unique optical sensing properties. So far, an increasing number of luminescent MOFs with chemical sensing properties have been reported.⁶ However, to date, the luminescent MOFs assembled by Ln³⁺ ions and containing uncoordinated carboxyl groups within the frameworks have been rarely reported.^6c

Detecting Fe³⁺ ions in aqueous solution with high sensitivity and selectivity is much important since iron is an indispensible and vital kind of elements no matter in environment or in the biosphere.⁷ Therefore, there is an urgent need to develop fluorescent chemosensors that are capable of detecting Fe³⁺ ions. Meanwhile, the design of new fluorescent chemosensors for Fe³⁺ ions remains a significant subject in the field of chemistry. Lanthanide metal–organic frameworks (LnMOFs) are well-known for their unique optical properties which make them especially attractive for potential applications such as fluorescence probes. Since the first luminescent EuMOFs for probing Fe³⁺ was reported by Dang et al.,⁸ only a few luminescence LnMOFs have been found for the recognition and sensing of Fe³⁺ ions,⁹ moreover, most of them are exploited as a fluorescence sensor of Fe³⁺ in DMF solution because of their poor stability in the aqueous solution, which limits their practical sensing application in environmental and biological systems.^9a–c Considering the advantages of the MOFs containing desired functional groups in the interior of the frameworks, together with the excellent optical properties of LnMOFs and the practical application in environmental and biological systems, we were motivated to design a novel LnMOFs containing functional sites for the sensing of Fe³⁺ ions in the aqueous environment with high sensitivity and selectivity.

In this communication, we report a novel 2D microporous compound [Tb(Hbtca)(H₂O)₂]·H₂O (TbMOF–COOH) containing uncoordinated carboxyl groups that point to the inner of pores. Because of the reaction feature of the uncoordinated carboxyl groups, TbMOF–COOH can be applied to receiving foreign metal ions and has the chance to realize the sensing properties. TbMOF–COOH was obtained as flaxen block-like crystals generated from a carboxyl-rich ligand of 1,1′-biphenyl-2,3,3′,5′-tetracarboxylic acid (H₄btca) and Tb(NO₃)₃·6H₂O under hydrothermal conditions in moderate yields (ESI†). Single crystal X-ray diffraction analysis reveals that TbMOF–COOH crystallizes in the triclinic P1̄ space group and features as a 2D network based on cage-like Tb₂(OCO)₄ subunits connected by the Hbtca³⁻ ligand. As shown in Fig. 1a, each asymmetric unit of TbMOF–COOH contains one crystallographically independent Tb³⁺ ion, one anion Hbtca³⁻ ligand, two coordinated water molecules and one lattice water molecule. The Tb³⁺ is coordinated by eight oxygen atoms to furnish a distorted dodecahedron configuration, including six carboxylic oxygen atoms of the Hbtca³⁻ ligands and two oxygen atoms of the coordinated water molecules. The Tb–O distances change from 2.287(5) Å to 2.503(6) Å and the O′–Tb–O are in the range of 53.4(2)–144.1(2)° both within the reported results.¹⁰ Two adjacent crystallographically equivalent Tb(III) atoms are joined by four bidentate bridged carboxylic groups from four independent Hbtca³⁻ ligands to give a cage-like Tb₂(OCO)₄ subunit with the Tb(III)⋯Tb(III) separation of 4.1243(9) Å. Each Tb₂(OCO)₄ subunit is further connected by six Hbtca³⁻ ligands to fabricate a double-paned extended 2D network structure which contains lantern-like open pores. The open pore is represented by a blue sphere with diameter of 5 Å (Fig. 1b and c). It is noteworthy that there are uncoordinated carboxyl groups pointing to the interior of pores which can be used to explore the sensing properties of metal ions. As observed from the TGA curve of TbMOF–COOH shown in Fig. S1, ESI,† TbMOF–COOH shows good thermal stability. In the temperature range of 100–250 °C, there is loss of three water molecules (lattice and coordinated) (found = 10.41%, calcd = 9.99%), then its ligands start to decompose thermally above 250 °C.

Fig. 1 (a) The coordination environment of central metal atom in TbMOF–COOH (the hydrogen atoms are omitted). (Symmetry codes: A: 2 − x, 1 − y, −z; B: x, 1 + y, z; C: 1 − x, 1 − y, 1 − z; D: 1 + x, 1 + y, z − 1.) (b) 2D layered structure of TbMOF–COOH (the hydrogen atoms and water molecules are omitted for clarity). (c) The lantern-like open pore included in TbMOF–COOH.

TbMOF–COOH displays strong and distinctive green fluorescence of Tb³⁺ which can be readily observed with the naked eye under a standard UV lamp (254 nm). The solid-state emission spectrum of TbMOF–COOH was shown in Fig. S2, ESI.† The emission spectra of TbMOF–COOH is dominated by four characteristic bands at 488 nm, 545 nm, 584 nm and 621 nm upon excitation at 368 nm, corresponding to the ⁵D₄ → ⁷F_n transitions (n = 6, 5, 4 and 3) for the Tb³⁺ ion. It is well-known that the luminescence of lanthanide cations are often suffers from weak light absorption and the spin- or parity-forbidden of f–f transition. However, owing to the efficient energy-transfer between electron-conjugated ligands and lanthanide, the defect of lanthanide emission can be overcomed via the “antenna effect” or “luminescence sensitization”.¹¹

There are uncoordinated carboxyl groups pointing to the interior pores of luminescent TbMOF–COOH, which provides a possibility to use them as a luminescence sensor to detect specific metal ions. To investigate the potential luminescence sensing properties of metal ions, TbMOF–COOH samples (5.3 mg) were dispersed in the individual aqueous solutions of M(NO₃)_n (5 mL, 0.01 M, M^z+ = Ni²⁺, Cu²⁺, Cr³⁺, Ca²⁺, Zn²⁺, Co²⁺, Mg²⁺, Cd²⁺, Al³⁺ and Fe³⁺, respectively) and MCl₂ (M = Mn²⁺, Fe²⁺, for detailed experiments see ESI†). Afterward, the luminescence properties for the samples were measured and the results were shown in Fig. 2a and S3.† Interestingly, after immersed in various metal ions, TbMOF–COOH displays markedly different luminescence. Especially, when TbMOF–COOH samples were immersed in the aqueous solution containing Fe³⁺ ions, the luminescence of the sample is almost quenched (Fig. 2b), while other metal ions only slightly affect the luminescence intensity of the samples. Therefore, TbMOF–COOH is a selective luminescent sensor for Fe³⁺ ions. Both Fe²⁺ and Fe³⁺ ions are essential for many biological processes, however, they plays different major role during the biological processes.¹² So it is important to detect the one from the other. Our luminescence sensor in this paper has shown excellent specificity for Fe³⁺ over Fe²⁺, overcoming the usual luminescence quenching nature caused by the paramagnetic behavior of iron.

Fig. 2 (a) Responses of the luminescence intensities of the ⁵D₄ → ⁷F₅ transition of TbMOF–COOH treated with different metal ions (0.01 M) in aqueous solution. (b) The corresponding optical images.

As confirmed by the powder X-ray diffraction patterns (PXRD) of some selected samples (Fig. 3), the solid TbMOF–COOH samples after immersed in different metal ion aqueous solution during the above sensing process only minimally impacts the crystalline integrity of TbMOF–COOH. However, due to the role of the newly introduced salt, the crystal lattice of the TbMOF–COOH may afford somewhat distortion and the PXRD noise peaks increased slightly or the ratio of signal to noise decreased.

Fig. 3 PXRD patterns for TbMOF–COOH and the TbMOF–COOH immersed in M^z+ ions (M^z+ = Cu²⁺, Fe²⁺, Fe³⁺, Mn²⁺, respectively).

To explore the sensitivity, we investigated the sensing feature of TbMOF–COOH at different concentrations of Fe³⁺ ions and different reaction time. The TbMOF–COOH samples (3 mg) were immersed in gradually increased concentration of Fe³⁺ aqueous solutions (5 mL) and the luminescence spectra for the individual samples were measured. Fig. 4, shows that the luminescence intensity was strongly dependent on the concentration of Fe³⁺ ions. When the concentration of Fe³⁺ increased, the emission intensities of samples gradually decreased. The luminescence intensities of the TbMOF–COOH were almost quenched when the concentration of Fe³⁺ reached 0.01 mol L⁻¹. A good linear relationship has appeared between luminescence intensity and the Fe³⁺ concentration (R = 0.994) in range of 10–1000 μmol L⁻¹. This linear relationship makes it a reality to detect Fe³⁺ quantitatively. Furthermore, we also studied the time scale for the sensing effect of TbMOF–COOH powder (3 mg) immersed in 0.01 M Fe³⁺ aqueous solution (5 mL). About 5 min, the significant quenching effect occurs and almost keeps the same level until 6.5 h (see Fig. S4, ESI†).

Fig. 4 Concentration-dependent luminescence quenching of TbMOF–COOH after being added different concentrations of Fe³⁺ ions (Ex = 368 nm).

Additionally, we also investigated the sensing selectivity of the TbMOF–COOH to Fe³⁺ ions in the presence of several other co-existed cationic species (5 mL, 0.01 M for each metal ions, M^z+ = Ni²⁺, Ca²⁺, Zn²⁺, Cd²⁺, Fe²⁺ and Mg²⁺). As it is described in Fig. 5, the luminescence intensities of TbMOF–COOH is almost not affected by the mixed cations, but the luminescence nearly disappeared upon the addition of Fe³⁺ (0.01 mol L⁻¹ in the aqueous solution). Compared to the reported Fe³⁺ sensor based on LnMOFs,^9c TbMOF–COOH has high selectivity for the sensing of Fe³⁺ ions in the presence of several other co-existed cationic species.

Fig. 5 Comparison of the luminescence intensities of TbMOF–COOH immersed in M^z+ ions (M^z+ = Ca²⁺, Ni²⁺, Mg²⁺, Zn²⁺, Cd²⁺, Fe²⁺) in the absence and presence of 0.01 M Fe³⁺. Insert: photograph of the corresponding images under the UV lamp (Ex at 254 nm).

According to the UV-Vis absorption spectrum (see Fig. S5, ESI†), the strong absorption of the Fe³⁺ aqueous solution is in the range of 245–550 nm, while the other cations aqueous solution did not overlap the absorption range, thus, the following two possible mechanisms may account for the origin of the high selectivity and sensitivity towards Fe³⁺. Firstly, there exists the competition of absorption of excitation wavelength (368 nm) energy between the Fe³⁺ aqueous solution and the carboxylate ligand.^9b,13 Such competitive adsorption will significantly decrease the transfer of excitation energy to Tb³⁺ from the carboxylate ligand. Secondly, there exists a small overlap between the absorption spectrum of the Fe³⁺ aqueous solution and the prominent emission band at 545 nm of TbMOF–COOH.^6a,14 As a result, both of them lead to the quenching of the luminescence of TbMOF–COOH.

In summary, we have synthesized a novel double-paned extended 2D network (TbMOF–COOH) containing large volume lantern-like open pores. Owing to the existing of uncoordinated carboxyl groups pointing to the interior of pores, TbMOF–COOH can serve as a potential host to bind foreign metal ions. The luminescent TbMOF–COOH represents significant recognition ability for Fe³⁺ ions with high-selectivity and sensitivity. This microporous metal–organic framework (TbMOF–COOH) can recognize Fe³⁺ ions in aqueous environment and the luminescent intensities of the experimental system is linear dependence to the concentration of Fe³⁺ ions. Moreover, in the presence of many other cations, the sensing feature of Fe³⁺ is slightly affected. The power to unambiguously recognize Fe³⁺ ions indicates that this approach is a highly promising strategy to develop luminescent materials with unprecedented practical applications for the sensing of Fe³⁺ ions in environmental or biological systems.

Acknowledgements

We would like to kindly acknowledge the NSFC (21361016) and Inner Mongolia Foundation for Natural Science (2013ZD09) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental and crystallography details, TGA and additional figures. CCDC 1005400 (for TbMOF–COOH). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra10153g

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