English

• 0.   Introduction

  Metal-organic frameworks (MOFs), as a class of burgeoning porous materials, have aroused great attention due to their structural diversities and the potential application in fields of gas separation, heterogeneous catalysis, fluorescence, magnetism, and electrochemistry, etc^[1-8]. In these fields, luminescent detection, as an essential method in the domain of biotechnology and environmental monitoring, has attracted considerable attention from chemists at present. A large number of functional luminescent detection materials have been reported for detecting ions^[9-10], molecules^[11-13], and even biomacromolecules^[14-15]. We all know the structure is an essential factor, while the formation of MOFs is often self-assembly driven by a coordination process^[16-17]. Even though there is still a big challenge for the prediction and control of the final structure, the importance of choice for ligands and metal ions has been a known fact. During the self-assembly process, the diverse and possible coordination between organic ligands and inorganic metal ions greatly affects the final structure. From the view of performance, MOFs composed of full-electronic d¹⁰ configuration metal ions and aromatic carboxylic acids are probably expected to be potential fluorescent probes. The electron-rich aromatic rings usually endow the structures with excellent fluorescence. And, possible porous structure bridged by long ligands can facilitate the enhancement of hostguest interactions between MOFs and guest analytes. So, Cd-based MOFs with unique photophysical properties can enable them to be functional materials for luminescent sensing.

  Given the above-mentioned background, we used 5-(((4-carboxyphenyl)oxy)methyl)benzene-1,3-dicarboxylic acid (H₃L, Scheme 1) as ligand and Cd^Ⅱ as coordination ions to construct a 2D microporous MOF [Cd₃(L)₂ (H₂O)₉]·9H₂O(1). Herein, we report the synthesis, characterization, and luminescence detection properties of MOF 1. The excellent fluorescent characteristic makes it a fluorescent turn-off sensor for Fe³⁺, CrO₄^2-, and Cr₂O₇^2- ions. The quenching mechanism was also studied in detail.

  Scheme 1

  

  Scheme 1.  Structure of H₃L

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  1.   Experimental

  1.1   Materials and methods

  All reagents and solvents were commercially available and used without further purification. IR spectra were recorded from a KBr pellet in the 4 000-400 cm^-1 region on a BRUKER TENSOR27 spectrometer. Elemental analyses (C, H, and N) were obtained on a CHNO-Rapid instrument. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 Advance X-ray diffractometer (Cu Kα, λ=0.154 18 nm) at 40 kV and 30 mA in a 2θ range of 5°-50° with a scan rate of 10 (°)·min^-1. Thermogravimetric analysis (TGA) were carried out in a nitrogen stream on a Dupont thermal analyzer from room temperature to 800 ℃ under N₂ flow. Fluorescence spectra were characterized at room temperature by a Varian-Cray Eclipse fluorescence spectrophotometer. The UV-Vis absorption spectra were recorded on a JASCO V-570 spectrophotometer.

  1.2   Synthesis of MOF 1

  A mixture of H₃L (31.6 mg, 0.1 mmol), Cd(NO₃)₂·4H₂O (92.5 mg, 0.3 mmol), H₂O (10 mL), and NaOH (0.5 mL, 0.5 mol·L^-1) was placed in a 15 mL Teflonlined stainless steel vessel with stirring for 30 min and then heated under autogenous pressure at 120 ℃ for 72 h. Followed by slow cooling to room temperature, colorless block-shaped crystals of MOF 1 were obtained by filtration, washed with H₂O, and dried in air. Yield: ca. 87%. Elemental analysis Calcd. for C₃₂H₅₄O₃₂Cd₃(%): C, 29.82; H, 4.19. Found(%): C, 30.90; H, 3.97. FT-IR (KBr pellet, cm^-1): 3 414 (s), 2 924 (m), 2 853 (w), 1 694 (m), 1 608 (s), 1 529 (s), 1 449 (m), 1 384 (s), 1 251 (s), 1 161 (m), 1 110 (w), 1 044 (m), 950 (w), 897 (w), 870 (w), 841 (w), 771 (s), 728 (s), 677 (w), 582 (w) (Fig.S1, Supporting information).

  1.3   X-ray crystallography

  Single crystal X-ray diffraction data for MOF 1 were collected in the Beijing Synchrotron Radiation Facility (BSRF) beamline 3W1A, which were mounted on a MARCCD-165 detector (λ=0.071 00 nm) with the storage rings working at 2.5 GeV. In the process, the crystals were protected by liquid nitrogen at 100(2) K. Structure was solved via direct methods employed in the SHELXS-2018, and refined based on F² with fullmatrix least-squares techniques. Hydrogen atoms were refined with isotropic displacement factors and included in the final refinement with geometrical restraints. The disordered free water molecules were found. Thus, the SQUEEZE (PLATON) procedure was used and the generated data were used for further refinement. Four solvent-accessible voids (about 22.4%) were identified in the unit cell, each with a volume of 0.256 nm³ and 90 electrons. So the total volume and electrons corresponded reasonably to 36 H₂O molecules with Z=4. The contribution to the overall formula of disordered water molecules was deduced through a combination of elemental analysis and TGA characterizations. The basic information for the crystal and structural refinement data is listed in Table 1. Some selected bond lengths and angles are listed in Table S1.

  Table 1

  

  Table 1.  Crystal data and structure refinement details for MOF 1

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  Parameter	MOF 1
  Empirical formula	C32H54Cd3O32
  Formula weight	1 287.95
  Crystal system	Monoclinic
  Space group	C2/c
  a / nm	1.909 1(5)
  b / nm	0.731 40(15)
  c / nm	3.325 4(9)
  β/(°)	100.13(3)
  V / nm3	4.571(2)
  Z	4
  Dc / (g·cm-3)	1.872
  μ / mm-1	1.462
  F(000)	2 584
  GOF	1.076
  θ range for data collection / (°)	3.0-31.2
  Reflection collected, independent	12 677, 7 321 (Rint=0.034)
  Reflection observed [I > 2σ(I)]	7 208
  R1 [I > 2σ(I)]	0.034
  wR2 [I > 2σ(I)]	0.091

  2.   Results and discussion

  2.1   Structural description for MOF 1

  MOF 1 crystallizes in the monoclinic crystal system with the C2/c space group. The asymmetric unit of MOF 1 contains one and a half independent Cd^Ⅱ cations, one L^3- anion, four and a half coordination water molecules, and four and a half lattice water molecules (Fig. 1a). Both Cd1 and Cd2 atoms are seven-coordinated with the [CdO₇] coordination environment, where Cd2 is located on the 2-fold axis with half occupancy. The coordinated Cd1 displays a distorted pentagonal bipyramidal configuration, while a distorted mono-capped triangular-prism geometry is observed for Cd2. The seven O atoms of each Cd atom are derived from two chelating carboxylate anions of L^3- and three coordinated water molecules (Fig. 1a). Even though Cd1 and Cd2 are all coordinated with two L^3- ions and three H₂O molecules, the L^3- ions display different cis and trans coordination. The Cd—O bond lengths fall in a range of 0.225 0(3) to 0.260 89(16) nm, and bond angles of O—Cd—O vary from 52.70(5)° to 170.71(6)° (Table S1).

  Figure 1

  

  Figure 1.  (a) Displacement ellipsoid plot (30% probability) of the coordination environments of Cd^Ⅱ ions in MOF 1; (b)Two 1D chains constructed by L^3- ions extending along two directions, respectively (the angle between two directions is 33° and the polyhedrons of Cd1 and Cd1A are displayed with different colors); (c) 2D layer structure of MOF 1 (the polyhedron of Cd2 is represented with green color); (d) Space fill of MOF 1 viewed along the b-axis with the 1D channels

  Symmetry codes: i x+1/2, y+1/2, z; ii-x, y, -z-1/2

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  As shown in Fig. 1b, Cd1 and Cd1A (symmetry related) are connected by L^3- ions respectively, forming the 1D infinite chains along two different directions with an angle of 33°. Then, the second Cd2 atoms link two 1D chains to give a 2D layer (Fig. 1c). The adjacent layers are further stacked with each other through hydrogen bonding (Table S2) and π… π weak interactions of phenyl rings, forming a 3D supramolecular framework containing 1D channels along the b-axis (Fig. 1d). And the symmetrical inversion is located through the center of the channel. A PLATON calculation indicates that the solvent volume of the accessible void is 1.018 7 nm³, amounting to 22.4% of the total unit-cell volume. The two phenyl rings in L^3- are almost coplanar with a small dihedral angle of 5.001(67)° (Fig.S2).

  2.2   PXRD and TGA

  The PXRD patterns of the bulk samples were recorded at room temperature to prove their crystalline phase purity. The good purity of MOF 1 can be confirmed by excellent agreement patterns (Fig. 2a).

  Figure 2

  

  Figure 2.  PXRD patterns (a) and TGA curve (b) of MOF 1

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  TGA was carried out to check the thermal stability under the N₂ atmosphere in a temperature range of 25-800 ℃ at a heating rate of 5 ℃·min^-1. The first weight loss is attributed to nine free H₂O molecules from 25 to 140 ℃, with a total loss of 12% (Calcd. 12.5%). Then, after the release of some coordinated H₂O, the main structure starts to decompose (Fig. 2b).

  2.3   Fluorescence detection of Fe³⁺ and Cr₂O₇^2-/ CrO₄^2- ions

  At room temperature, the solid-state luminescence property of free ligand and MOF 1 were investigated under excitation at 340 nm (H₃L) and 320 nm (MOF 1), showing emission peaks at 391 and 400 nm, respectively (Fig. 3). It is speculated that the fluorescence may be attributed to the charge transfer from ligand to ligand (LLCT) or intra-ligand (n-π^* or π-π^*) emission in the complicated network. By measuring the luminescence of the upper uniform suspension of MOF 1 immersed in common solvents (H₂O, CH₃OH, C₂H₅OH, DMF, DMA, and CH₃CN), MOF 1 showed a strong and stable luminescence emission in DMF (Fig. S3a). The measured PXRD patterns were in agreement with the simulated patterns, indicating excellent solvent stability (Fig. S3b). The excellent luminescent behavior and special solvent stability encourage us to explore luminescent sensing properties.

  Figure 3

  

  Figure 3.  Solid-state luminescence spectra of H₃L and MOF 1 at room temperature

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  At room temperature, 15 nitrate solutions containing different cations with the concentration of 1 mmol·L^-1 (Mⁿ⁺=K⁺, Na⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Pb²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Fe²⁺, and Fe³⁺) were added into the DMF suspension of MOF 1. Then the luminescent intensities of these suspensions were recorded (λ_ex=320 nm). As shown in Fig. 4a, the significant luminescence quenching effect was discovered only in the presence of Fe³⁺ ion, and the quenching efficiency (η_q) was 95.9% (η_q=(I₀-I)/I₀ ×100%, where I₀ and I are the maximum luminescent intensities before and after addition of the targeted species). To speculate the as-prepared MOF possesses considerable sensitivity and selectivity toward Fe³⁺ ion, the detection capability of MOF 1 for Fe³⁺ cation was further investigated by luminescence titration experiments. As the concentration of Fe³⁺ ion gradually increased, the luminescent intensity of MOF 1 decreased gradually (Fig. 5a). When the concentration was up to 400 µmol·L^-1, the quenching efficiency reached about 93.3%. According to the Stern-Volmer (S-V) equation^[18]: I₀/I=K_svc_Fe³⁺+1, the quenching efficiency was quantitatively analyzed. There existed a nearly linear correlation (R² =0.998 3) between the relative fluorescence intensity and the Fe³⁺ concentration in a range of 0-70 µmol·L^-1, and the quenching constant K_sv in DMF solution was 6.4×10³ L·mol^-1 (Fig. 5b). The limit of detection (LOD) for Fe³⁺ was calculated to be 0.76 µmol·L^-1 by the equation^[19]: LOD=3σ/K_sv (σ: standard deviation). The sensing ability of MOF 1 was comparable to those of previously reported compounds for the detection of Fe³⁺ ions (Table S3). Further, an antiinterference experiment on Fe³⁺ ions by other ions was completed to study the selectivity for Fe³⁺ ions. Keeping the concentration of other metal ions at 400 µmol·L^-1 in DMF solution, then the Fe³⁺ ion was introduced into the above system with a final concentration of 400 µmol·L^-1. Then, the emissions were recorded at room temperature. As shown in Fig. 6a, the luminescent intensities with other anions just decreased slightly in the absence of Fe³⁺, but the luminescence was almost completely quenched after the addition of the Fe³⁺ ion. The results indicate the quenching efficiency of Fe³⁺ ion toward MOF 1 is hardly affected by other disturbing ions.

  Figure 4

  

  Figure 4.  Luminescence quenching efficiencies of MOF 1 toward Fe³⁺ cation (a) and Cr₂O₇^2-/CrO₄^2- anions (b) in the presence of different ions with a concentration of 0.5 mmol·L^-1

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  Figure 5

  

  Figure 5.  Emission spectra (a, c) and S-V plots (b, d) of MOF 1 in DMF solution sensing Fe³⁺ ions (a, b) and Cr₂O₇^2- anions (c, d)

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  Figure 6

  

  Figure 6.  Interference plots for the decline in fluorescence intensities upon the addition of different ions (400 µmol·L^-1) followed by Fe³⁺ (a) and Cr₂O₇^2- (b) ions

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  Further, the same procedure as metal cationdetecting, a series of X^n- anions (n=1: X=F, Cl, Br, I, CH₃COO, NO₃, HSO₃, HCO₃, SCN, H₂PO₄; n=2: X=C₂O₄, SO₄, HPO₄, CrO₄, Cr₂O₇; n=3: X=PO₄) was added to the DMF suspension of MOF 1 with a final concentration of 1 mmol·L^-1. Then the fluorescence intensity of MOF 1 was recorded. All the anions showed little obvious fluorescence quenching, but the hexavalent chromium ions (Cr₂O₇^2-/CrO₄^2-) provided maximum luminescence quenching efficiencies of 74.2% and 55.2% (Fig. 4b). Ion titration experiment was carried out to assess the quenching effect. As shown in Fig. 5c, the fluorescence emission intensity of MOF 1 decreased significantly with the addition of Cr₂O₇^2-. When the concentration of Cr₂O₇^2- was up to 400 µmol·L^-1, the quenching efficiency could be up to 70.6%. An excellent linear S-V plot for Cr₂O₇^2- was fitted at low concentration (Fig. 5d), providing a correlation coefficient of 0.989 2. The quenching constant K_sv was calculated to be 1.81×10³ L·mol^-1 according to the S-V equation. Based on the detection limit formula, the LOD was calculated to be 2.05 µmol·L^-1. The related parameters of MOF 1 for sensing Cr₂O₇^2- ion were listed in Table S4, and also compared with some known reports. To eliminate the interference ability of other anions, the competition experiment is absolutely essential. Based on the presence of other anions in the DMF solution, the same amount of Cr₂O₇^2- was introduced. As shown in Fig. 6b, the luminescence spectra of MOF 1 before and after the addition of Cr₂O₇^2- were measured. The luminescence intensity declined rapidly once Cr₂O₇^2- was added, dedicating other anions would not disturb the selective sensing of Cr₂O₇^2- by MOF 1.

  2.4   Luminescent quenching mechanism

  The possible mechanism for the luminescence quenching by Fe³⁺ and Cr₂O₇^2- ions was explored in detail. First, the consistent PXRD patterns of MOF 1 before and after detection experiments suggested the integrity of the structure (Fig.S4a). Second, the competition and transfer of energy absorption between MOF 1 and analyte ions were supposed to be the main reason for the luminescence quenching^[20-22]. To prove this viewpoint, UV-Vis absorption spectra of both the Fe³⁺, Cr₂O₇^2-/CrO₄^2- ions and MOF 1 were recorded (Fig. S4b). There was one 250-425 nm absorption band for Fe³⁺ and also two broad bands in a range of 200-450 nm for CrO₄^2-/Cr₂O₇^2- were observed. These absorption bands overlapped with the absorption and emission peaks of MOF 1. The effective overlap result in the luminescence quenching of MOF 1 firmly. Therefore, the reasonable mechanism for sensing Fe³⁺ and Cr₂O₇^2-/ CrO₄^2- was proved firmly. That is the competitive absorption of excitation light between analyte ions and MOF 1, as well as intermolecular energy transfer from MOF 1 to analyte ions.

  3.   Conclusions

  We have used 5-(((4-carboxyphenyl)oxy)methyl) benzene-1,3-dicarboxylic acid (H₃L) as ligand for the divalent Cd(Ⅱ) cation to form MOF 1 [Cd₃(L)₂(H₂O)₉]·9H₂O. MOF 1 exhibits a 2D microporous structure with accessible voids of 22.4%. The luminescent property study reveals that MOF 1 would be an outstanding luminescent sensor for the highly selective and sensitive detection of Fe³⁺ and Cr₂O₇^2-/CrO₄^2- ions. The K_sv values of MOF 1 on Fe³⁺ and Cr₂O₇^2- ions were 6.4×10³ and 1.81×10³ L·mol^-1, leading to the detection limitations of 0.76 and 2.05 µmol·L^-1, respectively. By UV-Vis and PXRD experiments, a reasonable detection mechanism was proved to be the competitive absorption upon ultraviolet excitation as well as energy transfer between the host MOF 1 and guest Fe³⁺/Cr₂O₇^2-/ CrO₄^2- ions.

  Supporting information is available at http://www.wjhxxb.cn

  
  Acknowledgments: This work was financially supported by the National Natural Science Foundation of China (Grant No. 21671124) and the young project of Taiyuan University (Grant No.21TYKQ20). Competing interests: The authors declare no competing interests.
  
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• 

  Scheme 1  Structure of H₃L

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  Figure 1  (a) Displacement ellipsoid plot (30% probability) of the coordination environments of Cd^Ⅱ ions in MOF 1; (b)Two 1D chains constructed by L^3- ions extending along two directions, respectively (the angle between two directions is 33° and the polyhedrons of Cd1 and Cd1A are displayed with different colors); (c) 2D layer structure of MOF 1 (the polyhedron of Cd2 is represented with green color); (d) Space fill of MOF 1 viewed along the b-axis with the 1D channels

  Symmetry codes: i x+1/2, y+1/2, z; ii-x, y, -z-1/2

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  Figure 2  PXRD patterns (a) and TGA curve (b) of MOF 1

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  Figure 3  Solid-state luminescence spectra of H₃L and MOF 1 at room temperature

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  Figure 4  Luminescence quenching efficiencies of MOF 1 toward Fe³⁺ cation (a) and Cr₂O₇^2-/CrO₄^2- anions (b) in the presence of different ions with a concentration of 0.5 mmol·L^-1

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  Figure 5  Emission spectra (a, c) and S-V plots (b, d) of MOF 1 in DMF solution sensing Fe³⁺ ions (a, b) and Cr₂O₇^2- anions (c, d)

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  Figure 6  Interference plots for the decline in fluorescence intensities upon the addition of different ions (400 µmol·L^-1) followed by Fe³⁺ (a) and Cr₂O₇^2- (b) ions

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  Table 1.  Crystal data and structure refinement details for MOF 1

  Parameter	MOF 1
  Empirical formula	C32H54Cd3O32
  Formula weight	1 287.95
  Crystal system	Monoclinic
  Space group	C2/c
  a / nm	1.909 1(5)
  b / nm	0.731 40(15)
  c / nm	3.325 4(9)
  β/(°)	100.13(3)
  V / nm3	4.571(2)
  Z	4
  Dc / (g·cm-3)	1.872
  μ / mm-1	1.462
  F(000)	2 584
  GOF	1.076
  θ range for data collection / (°)	3.0-31.2
  Reflection collected, independent	12 677, 7 321 (Rint=0.034)
  Reflection observed [I > 2σ(I)]	7 208
  R1 [I > 2σ(I)]	0.034
  wR2 [I > 2σ(I)]	0.091

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•