A series of europium-based metal organic frameworks with tuned intrinsic luminescence properties and detection capacities

Abstract

Three new isostructural luminescent complexes [(CH₃)₂NH₂]₂[Eu₆(μ₃-OH)₈(BDC-NH₂)₆(H₂O)₆], [(CH₃)₂NH₂]₂[Eu₆(μ₃-OH)₈(BDC-F)₆(H₂O)₆] and [(CH₃)₂NH₂]₂[Eu₆(μ₃-OH)₈(BDC-NO₂)₆(H₂O)₆] have been synthesized based on [(CH₃)₂NH₂]₂[Eu₆(μ₃-OH)₈(1,4-NDC)₆(H₂O)₆] under solvothermal conditions. In addition, these Eu-MOFs possess tuned intrinsic luminescence properties and detection capacities for different analytes, such as Cr₂O₇²⁻, acetone and Fe³⁺. Here, these Eu-MOFs display distinct intrinsic luminescence properties because of the synergistic effect of donor groups and nonradiative vibrations, which are reflected in the lifetime, fluorescence quantum yield and emission intensity. Meanwhile, the detection experiments show that the one with the higher fluorescence quantum yield displays much more sensitive and efficient detection capacities for different kinds of analytes. Overall, this work indicated that tuning functional groups is vital to gain Eu-MOFs with good intrinsic luminescence properties, and make the detection capacities more sensitive and efficient.

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Introduction

Lanthanide-based metal organic frameworks (Ln-MOFs), one special class of the MOF families, not only inherit the features of high porosity, adjustable pore size, and large internal surface areas of MOFs,^1–4 but also possess unique characteristics, including long fluorescence lifetime, wide emission range from ultraviolet to near-infrared, as well as the large Stokes shift.^5–8 The intrinsic luminescence features of lanthanides together with the advantages of MOFs offer excellent prospects for the design of novel luminescent materials for the detection of ions, small molecules, pH value, and temperature through the purposeful selection of composition in advance.⁹ The organic linkers in MOFs can absorb the light, and energy can be transferred to Ln^III ions from the ligands to generate luminescence, which is called ligand-to-metal charge transfer (LMCT) states.^10–13 Using the “antenna effect”, the drawback of Ln^III caused by Laporte forbidden f–f transitions for weak absorption extinction coefficients (less than 4 M⁻¹ cm⁻¹) could be solved miraculously. This indirect excitation of the lanthanide excited state induces great photophysical properties and makes the Ln-MOFs more selective and sensitive for the detection/monitoring.^14–16

However, enhancement of the efficiency of overall antenna effect in such systems still remains a great challenge. These excited states are vulnerable to be quenched by nonradiative vibration energy transfer processes due to an O–H oscillator, N–H and even C–H oscillators.^16,17 Hence, the replacing of ubiquitous C–H bonds with C–F bonds which possess low energetic vibrations is advisable.^18–20 Meanwhile, the use of donating ligands also enables to exclude such non desirable oscillators. The charge transfer character of the ligands can be tuned by changing the donor group or the conjugated backbone.^19,21 Therefore, the choice of organic ligands with rational functional groups is vital for the enhancement of efficiency in the sensitisation process from the antenna to the lanthanide excited states, which would lead to remarkable intrinsic luminescence properties, and make the detection capacities more efficient as a result. Recently, Mohamed Eddaoudi's group has reported a series of unprecedented highly-connected porous rare-earth (RE) based MOFs, based on highly coordinated hexanuclear and nanonuclear carboxylate based clusters.^22,23 The method to gain control over the assembly of highly porous RE-MOFs inspired us.

In this work, we choose ligands 2-aminoterephthalic acid (BDC-NH₂), 2-fluoroterephthalic acid (BDC-F), and 2-nitroterephthalic acid (BDC-NO₂) with the mere difference in functional groups to synthesize series of new isostructural luminescent Eu-MOFs as fluorescent probe. On the basis of Eu-1,4-NDC (1,4-NDC = 1,4-naphthalenedicarboxylate), three new isostructural luminescent Eu-BDC-R (R = –NH₂, –F, –NO₂) have been designed and synthesized under solvothermal conditions. These four ligands possess different luminescence characteristics affected by the π-conjugated systems, heavy atom effect, electron-donating groups or electron-withdrawing group. As a result, these Eu-MOFs display distinct intrinsic luminescence properties reflected from the lifetimes, fluorescence quantum yields and emission intensities. However, they show significant differences due to the synergistic effect of donor groups and nonradiative vibrations, which lead to a quenching in Eu-BDC-NO₂ and different degrees of luminescent intensities in the others. Further studies indicate that these europium-based metal organic frameworks (Eu-MOFs) could be fluorescence probes for detecting the Cr₂O₇²⁻, acetone and Fe³⁺. And the detecting experiments on the Cr₂O₇²⁻, acetone and Fe³⁺ show that the Eu-MOF with higher fluorescence quantum yield could be more sensitive and efficient to detect the specific analytes. The proof-of-concept demonstrations indicate the tune of appropriate functional groups in ligands would be a facile and effective strategy for the development of superior luminescent Eu-MOFs materials.

Experimental

Materials and instruments

All reagents purchased were analytically pure from commercial sources and used without further purification. Powder X-ray diffraction (PXRD) measurements of the MOFs were performed on a D8 Advance X diffractometer equipped with a Cu anode (λ = 1.5406 Å); and the data were collected within the 2θ range of 3–50°. Scanning electron microscope (SEM) images were measured with a JSM-6701F. Thermogravimetric analysis (TGA) was carried out on a TGA-50 thermogravimetric analyzer with a heating rate of 10 K per minute from 40–800 °C under an air flow. Fourier-transform infrared (FT-IR) spectra were recorded on a Nicolet 6700 FTIR spectrophotometer using KBr pellets, and the spectra data were acquired in the range of 4000–400 cm⁻¹. Fluorescence spectra were recorded using an F-7000 fluorescence spectrometer with a xenon lamp as an excitation source. The luminescence lifetimes and the fluorescence quantum efficiencies were acquired using an integrating sphere from FLS980 phosphorimeter.

Synthesis of Eu-1,4-NDC-MOF [(CH₃)₂NH₂]₂[Eu₆(μ₃-OH)₈(1,4-NDC)₆(H₂O)₆]

Mohamed Eddaoudi's group has reported a method to gain cubic structure Eu-1,4-NDC with Eu₆(OH)₈ cluster by using 2-fluorobenzoic acid (2-FBA) as modulator and structure directing agent, and the chemical formula had been proved by single-crystal X-ray diffraction already.²³ Here we use this method to synthesis the material with minor adjustment. Eu(NO₃)₃·6H₂O (18.4 mg, 0.0413 mmol), 1,4-NDC (10 mg, 0.048 mmol), and 2-FBA (48.7 mg, 0.348 mmol) were added to the mixed solution of N,N′-dimethylformamide (DMF, 2.2 ml), H₂O (0.5 ml) and HNO₃ (0.2 ml, 3.57 M in DMF and the volume ratio of HNO₃ : DMF is 1 : 3.57) in a 3.5 ml scintillation vial, forming a suspension solution using an ultrasound method, sealed, and heated to 120 °C in 3 h, hold the temperature for 14 h and then cooled to room temperature in 3 h. The off-white octahedral crystals of Eu-1,4-NDC were collected and air dried. FT-IR (4000–400 cm⁻¹): 3427(s), 1603(s), 1435(s), 1409(s), 1363(s), 1264(s), 1212(m), 1163(m), 1097(m), 1031(m), 868(m), 824(m), 811(m), 792(m), 783(s), 745(m), 663(m), 636(m), 576(m), 519(m), 474(m). Elemental analysis: C 35.01% (theo: 35.85%), H 3.412% (3.302%), N 0.84% (1.1%).

Synthesis of Eu-BDC-NH₂-MOF [(CH₃)₂NH₂]₂[Eu₆(μ₃-OH)₈(BDC-NH₂)₆(H₂O)₆]

Eu(NO₃)₃·6H₂O (18.4 mg, 0.0413 mmol), BDC-NH₂ (9 mg, 0.048 mmol) and 2-FBA (48.7 mg, 0.348 mmol) were added to the mixed solution of DMF (2.2 ml), H₂O (0.5 ml) and HNO₃ (0.2 ml, 3.57 M in DMF and the volume ratio of HNO₃ : DMF is 1 : 3.57) in a 3.5 ml scintillation vial, forming a suspension solution using an ultrasound method, sealed, and heated to 120 °C in 3 h, hold the temperature for 40 h and then cooled to room temperature in 3 h. The khaki octahedral crystals of Eu-BDC-NH₂ were collected and air dried. FT-IR (4000–400 cm⁻¹): 3438(s), 1660(s), 1574(s), 1433(s), 1383(s), 1257(s), 1098(m), 824(m), 773(s), 663(m), 532(m). Elemental analysis: C 22.61% (theo: 26.73%), H 2.963% (3.342%), N 3.82% (4.79%).

Synthesis of Eu-BDC-F-MOF [(CH₃)₂NH₂]₂[Eu₆(μ₃-OH)₈(BDC-F)₆(H₂O)₆]

Eu(NO₃)₃·6H₂O (18.4 mg, 0.0413 mmol), BDC-F (9 mg, 0.048 mmol) and 2-FBA (48.7 mg, 0.348 mmol) were added to the mixed solution of DMF (2.2 ml), H₂O (0.5 ml) and HNO₃ (0.25 ml, 3.57 M in DMF and the volume ratio of HNO3 : DMF is 1 : 3.57) in a 3.5 ml scintillation vial, forming a suspension solution using an ultrasound method, sealed, and heated to 120 °C in 3 h, hold the temperature for 17 h and then cooled to room temperature in 3 h. The off-white octahedral crystals of Eu-BDC-F were collected and air dried. FT-IR (4000–400 cm⁻¹): 3430(s), 1616(s), 1496(s), 1414(s), 1384(s), 1224(m), 1085(m), 1011(m), 951(m), 899(w), 827(m), 773(m), 677(m), 560(m), 485(m), 443(m). Elemental analysis: C 27.14% (theo: 26.6%), H 2.528% (2.813%), N 2.09% (1.19%).

Synthesis of Eu-BDC-NO₂-MOF [(CH₃)₂NH₂]₂[Eu₆(μ₃-OH)₈(BDC-NO₂)₆(H₂O)₆]

Eu(NO₃)₃·6H₂O (18.4 mg, 0.043 mmol), BDC-NO₂ (10 mg, 0.048 mmol) and 2-FBA (48.7 mg, 0.348 mmol) were added to the mixed solution of DMF (2.2 ml), H₂O (0.5 ml) and HNO₃ (0.2 ml, 3.57 M in DMF and the volume ratio of HNO3 : DMF is 1 : 3.57) in a 3.5 ml scintillation vial, forming a suspension solution using an ultrasound method, sealed, and heated to 120 °C in 3 h, hold the temperature for 17 h and then cooled to room temperature in 3 h. The colorless octahedral crystals of Eu-BDC-NO₂ were collected and air dried. FT-IR (4000–400 cm⁻¹): 3425(s), 1623(s), 1535(s), 1492(s), 1388(s), 1254(m), 1168(m), 1100(m), 1065(m), 919(w), 834(m), 821(m), 808(m), 781(m), 748(m), 717(m), 663(m), 517(m). Elemental analysis: C 23.02% (theo: 24.82%), H 2.193% (2.625%), N 4.15% (4.45%).

Activated protocols

The reaction products were immersed in 40 ml of anhydrous DMF for 3 days (DMF exchanged three times per day). DMF-exchanged materials were quickly washed with anhydrous dichloromethane and immersed in 40 ml anhydrous dichloromethane for 3 days (anhydrous dichloromethane exchanged three times per day). Dichloromethane-exchanged materials were quickly washed with anhydrous methanol and immersed in 40 ml anhydrous methanol for 2 days (anhydrous methanol exchanged three times per day).

Single-crystal X-ray diffraction

The diffraction data of Eu-BDC-NH₂ were collected in a Rigaku Supernova CCD diffractometer equipped with a mirror-monochromatic enhanced Cu-Kα radiation (λ = 1.54184 Å) at 282 K. The data set was corrected by empirical absorption correction using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm.²⁴ The structure was solved by direct methods and refined by full-matrix least-squares on F² with anisotropic displacement by using the SHELXTL software package.²⁵ Non-hydrogen atoms were refined with anisotropic displacement parameters during the final cycles. Hydrogen atoms of ligands were calculated in ideal positions with isotropic displacement parameters. Those H atoms in –OH/H₂O groups of the europium-based clusters were not added but were calculated into molecular formula of the crystal data. There is large solvent accessible pore volume in the structure of Eu-BDC-NH₂, which is occupied by highly disordered solvent molecules. No satisfactory disorder model for these solvent molecules could be achieved, and therefore the SQUEEZE program implemented in PLATON was used to remove these electron densities of these disordered species.²⁶ Thus, all of electron densities from free solvent molecules have been “squeezed” out. The details of crystal data and structural refinement can be found in Table S1 (ESI†), as well as the corresponding CIF files.

Luminescence sensing experiment

The solid-state excitation spectra and emission spectra of the free ligands and three samples were investigated at room temperature. These three Eu-MOFs (2.0 mg) were simply immersed in DMF solutions (2.0 ml) containing 0.01 mol l⁻¹ M⁺ (M⁺ = K⁺, Hg²⁺, Na⁺, Zn²⁺, Al³⁺, Pb²⁺, Ag⁺, Mg²⁺, Mn²⁺, Ni²⁺, Cr³⁺, Fe³⁺) or various anions (anion = F⁻, Cl⁻, NO₂⁻, NO₃⁻, SO₄²⁻, CO₃²⁻, HCO₃⁻, I⁻, Br⁻, Ac⁻, Cr₂O₇²⁻) at room temperature for 20 h. For the experiments of organic solvent molecules, 2 mg Eu-MOFs was immersed in 2 ml pure small organic molecules solvents. The mixtures were then used for luminescent measurements.

Results and discussion

Structural characterization

Single-crystal X-ray diffraction reveals that Eu-BDC-NH₂ crystallizes in the cubic space group Fm3̄m with the lattice parameter a = 21.713 (3) Å (Table S1 in the ESI†). In the structure, all Eu atoms are nine-coordinated in a tetragonal antidipyramid coordination geometry with four μ₃-OH groups and four O atoms from four different carboxylate groups, leaving the ninth coordination site for a water molecule. The adjacent Eu³⁺ ions are bridged via μ₃-OH and deprotonated carboxylate groups to give a [Eu₆(μ₃-OH)₈(O₂C–)₁₂], a 12-connected molecular building block. Each hexanuclear molecular building block is linked by BDC-NH₂ ligands to form a 3D framework with two types of polyhedral cages: octahedral and tetrahedral (Fig. 1). From the topological view point, the Eu₆ cluster serves as a 12-connected node, the 3D structure of Eu-BDC-NH₂ can thus be simplified as a 12-c net with the point symbol of {3²⁴·4³⁶·5⁶}, which corresponds to the fcu topology.

Fig. 1 (a) and (b) 12-connected Eu-hexanuclear cluster. (c) The tetrahedral and (d) octahedral cages with respective watchet and green spheres representing the cavities, (e) the augmented fcu natural tiling. Eu, cyan; C, black; O, red; N, blue; and H, white.

Upon the successful preparation of Eu-BDC-NH₂, we further synthesized other Eu-MOFs with ligands possessing different functional groups. The observed power X-ray diffraction (PXRD) patterns for the as-synthesized samples of Eu-BDC-NH₂ crystal X-ray diffraction data, confirming the phase purity of the as-synthesized products. And the peak patterns of the other three Eu-MOFs are also in good agreement with that of Eu-BDC-NH₂, indicating the successful preparation of all isostructural Eu-MOFs (Fig. 2). Meanwhile, SEM images showed that all Eu-MOFs are perfect octahedral crystals (Fig. S1, ESI†), which is in agreement with the fact that the four Eu-MOFs are isostructural with UiO-66 based on zirconium.²⁷ The existence of functional groups in Eu-MOFs is characterized by FT-IR, as shown in Fig. S2 (ESI†). The peak at 1535 cm⁻¹ can be attributed to asymmetric stretching vibration of nitro groups.²⁸ The presence of amino group in sample is evident through the existence of the vibration at 1660 cm⁻¹ which corresponds to the aromatic N–H stretching modes.²⁹ The presence of the fluorine functional groups is indirectly supported by the vibration at the 1000–1150 cm⁻¹ region.³⁰ This vibration corresponds to the enhanced in-plane C–H bending mode of the aromatic ring with substituted fluorine groups. Furthermore, we investigated thermal stability of the Eu-MOFs and the TGA results are given in Fig. S3 (ESI†). The weight loss before 100 °C was due to the evaporation of the solvents in the pores of MOFs. Due to the different amount of residual solvent in MOFs, the percentage of weight loss before 100 °C was different with each other. The ability of the nitrogen atoms to withdraw electrons inductively weakens the neighboring carbon–carbon bonds, causing thermal breakdown of Eu-MOFs at lower temperatures in Eu-BDC-NH₂ and Eu-BDC-NO₂.³¹ Conversely, the Eu-BDC-F and Eu-1,4-NDC started to decompose above 500 °C.

Fig. 2 PXRD patterns for Eu-MOFs. (a) Eu-1,4-NDC; (b) Eu-BDC-F; (c) Eu-BDC-NH₂; (d) Eu-BDC-NO₂. (e) simulated patterns for Eu-BDC-NH₂.

Luminescence properties

To study the effect of different functional groups in ligands on the intrinsic luminescence properties of Eu-MOFs, the luminescence properties of pure ligands (BDC-NH₂, BDC-F, BDC-NO₂ and 1,4-NDC) were firstly measured. Because of the influence of withdrawing groups and nonradiative vibrations, a quenching behavior was observed in BDC-NO₂. For other three ligands, there are maximum peaks at 478 nm (λ_ex = 278 nm) for 1,4-NDC, 378 nm (λ_ex = 277 nm) for BDC-F, 545 nm (λ_ex = 365 nm) for BDC-NH₂, respectively, which are due to the π → π* transition (Fig. 3). These results indicate that different functional groups have a dramatic effect on the luminescence characteristic of the ligands. Thus the ligands with different groups may cause distinct intrinsic luminescence properties of Eu-MOFs by antenna effect. To verify our speculation, the intrinsic luminescence properties of the Eu-MOFs were investigated in detail. The emission spectra of Eu-1,4-NDC, Eu-BDC-F, and Eu-BDC-NH₂ all exhibit five characteristic emission peaks at 580, 595, 616, 654, and 693 nm, corresponding to the f–f electronic transitions (⁵D₀ → ⁷F_J, J = 0–4) of the Eu³⁺ ion. The excitation spectrums are obtained by monitoring the same emission wavelength at 616 nm, which is dominated mainly by a broad band centered at about 280, 278 and 298 nm, respectively. It indicates the occurrence of the antenna effects, which means that energy absorbed by ligand subsequently transfers to the lanthanide ions, generating f–f emissions of Eu³⁺ ions.^32,33 It is well known that the ⁵D₀ → ⁷F₂ transition is a typical electric dipole transition and is very sensitive to the local symmetry of Eu³⁺, while the parity-allowed magnetic dipole transition ⁵D₀ → ⁷F₁ is practically independent of the ions' surroundings. Hence, the intensity ratios I (⁵D₀ → ⁷F₂)/I (⁵D₀ → ⁷F₁) (I₀₂/I₀₁) can be seen as an indicator for the local environment of ions. According to the calculated intensity ratio I (⁵D₀ → ⁷F₂)/I (⁵D₀ → ⁷F₁) (I₀₂/I₀₁) as 3.45, 3.13 and 3.54, it is considered that the Eu³⁺ ion is in low symmetry,^34–38 which corresponds to the nine-coordinated configuration of Eu atoms in MOFs.

Fig. 3 The excitation (blank) and emission spectra (red) of (a) 1,4-NDC; (b) BDC-F; (c) BDC-NH₂; (d) Eu-1,4-NDC; (e) Eu-BDC-F; (f) Eu-BDC-NH₂.

The fluorescence quantum efficiencies of BDC-NH₂, BDC-F, and 1,4-NDC are 1.24%, 3.40%, 35.77% respectively. In addition, the luminescence quantum efficiencies of Eu-BDC-NH₂, Eu-BDC-F, and Eu-1,4-NDC are 3.21%, 30.83%, 57.01% (Table S2, ESI†), and the lifetimes are as long as 0.54, 0.72 and 1.06 ms (Fig. S4, ESI†), respectively, due to the effective energy transfer from the ligand to Eu³⁺. The nitro groups were electron-withdrawing, which result in a complete quenching of the ligands, further lead to the quenching of Eu-BDC-NO₂, while the electron-donating amino groups and conjugated naphthalene moieties induce the enhancement of the intrinsic luminescence properties. Although the fluorine group is an electron-withdrawing group, the overall antenna effect has also got a beneficial effect due to the promotion of nonradiative deactivation in the system. And the experimental results proved this concept: the Eu-BDC-NO₂ was nonluminous, while the fluorescence quantum efficiencies of BDC-F was 3.40%, and the fluorescence quantum efficiencies of Eu-BDC-F was 30.83%. Because of the influence of different functional groups, the ligands possess different fluorescence characteristics, and as a result, the materials synthesized display distinct intrinsic luminescence properties. Hence, the tune of functional groups in ligands is crucial for the intrinsic luminescence properties of Eu-MOFs.

Sensing of anions

In light of the distinct intrinsic luminescence properties of these three MOFs, we firstly investigated the effect of functional groups on detecting anions. As shown in Fig. 4. The results show that only Cr₂O₇²⁻ anions exhibit extremely quenching effect on the luminescence of three Eu-MOFs. Fluorescence quenching titrations were performed with the Cr₂O₇²⁻ anions. As shown in Fig. S5 (ESI†), the luminescence intensities are largely dependent on the concentration of Cr₂O₇²⁻. With the increase of concentration, the intensity decreases evidently. It should be noted that the luminescence intensities are completely quenched in 500 μM, indicating these Eu-MOFs could be fluorescence probes for detecting the Cr₂O₇²⁻ anions.

Fig. 4 The I/I₀ value of Eu-BDC-NH₂ (blue), Eu-BDC-F (amaranth), and Eu-1,4-NDC (khaki) toward different anions (10⁻² mol l⁻¹) when excited at 298, 278, 280 nm, respectively.

In addition, the quenching effect is treated as a function of Cr₂O₇²⁻ concentration as shown in Fig. 5, this quenching effect could be quantitatively explained by Stern–Volmer equation:

(I₀/I) = 1 + K_SV[M],

where I₀ and I are the luminescent intensity before and after the addition of Cr₂O₇²⁻; M is the concentration of Cr₂O₇²⁻; and the K_SV is the quenching coefficient. On the basis of experimental data, the K_SV values of Eu-BDC-NH₂, Eu-BDC-F, and Eu-1,4-NDC are calculated as 7320, 9690, 11 150, with the corresponding linear correlation coefficient of 0.99639, 0.99548, and 0.99172, respectively. The plot shows that the quenching percentage of Eu-1,4-NDC is near quintuple higher than that of Eu-BDC-NH₂ under the same concentration of Cr₂O₇²⁻ anions solution, and the value of Eu-BDC-F is double higher than that of Eu-BDC-NH₂. All these results indicate that the Eu-MOF with higher fluorescence quantum efficiency possesses higher quenching coefficient in detecting Cr₂O₇²⁻.

Fig. 5 (a) Quenching percentage plot of Eu-MOFs in the Cr₂O₇²⁻ solution (40 μM). (b) K_SV curves of Eu-1,4-NDC (blank), Eu-BDC-F (red) and Eu-BDC-NH₂ (blue) between I₀/I with the concentration of Cr₂O₇²⁻.

To further understand the luminescent quenching effects by Cr₂O₇²⁻, the UV-vis spectra of anions were measured (Fig. S6, ESI†). Among the anions above, only the Cr₂O₇²⁻ anions have a wide absorption band which overlapped with absorption bands of these Eu-MOFs and hence reduce the absorption of light by organic ligands, thereby affecting the energy transformation from ligands to Eu³⁺ ions. This indicates that the influence of the anions on antenna effect definitely plays a key role, since the antenna effect is crucial to determine the emission of lanthanide.^36,38–40

Sensing of organic solvent molecules

We also investigated the potential application for sensing of common organic solvent molecules. As shown in Fig. 6, the luminescence intensities are strongly dependent on the solvent molecules, particularly in the case of acetone, which exhibits the most significant quenching behavior. To further investigate the quenching effect of acetone, these three Eu-MOFs were dispersed in acetone solution with gradually increasing acetone contents to monitor the emissive response. It is obvious that the luminescent intensity decreases with the addition of acetone. The luminescent intensity is almost completely quenched at a concentration of 1.0% vol (Fig. S7, ESI†), implying these Eu-MOFs could be used as candidates for selecting of acetone.

Fig. 6 The I/I₀ value of Eu-BDC-NH₂ (blue), Eu-BDC-F (amaranth), and Eu-1,4-NDC (khaki) toward different pure solvent solutions when excited at 298, 278, 280 nm, respectively.

Meanwhile, the linear Stern–Volmer relationships are observed with K_SV value of 23.71, 18.98, 12.45, and the plot shows that the quenching percentage of Eu-1,4-NDC is 59% higher than that of Eu-BDC-NH₂, and the value of Eu-BDC-F is 26% higher than that of Eu-BDC-NH₂ (Fig. 7). The results above indicate that the Eu-MOF with higher fluorescence quantum efficiency possesses higher quenching coefficient in detecting acetone, which is in good agreement with that of Cr₂O₇²⁻.

Fig. 7 (a) Quenching percentage plot of Eu-MOFs in the acetone solution (0.5% vol). (b) K_SV curves of Eu-1,4-NDC (blue), Eu-BDC-F (blank) and Eu-BDC-NH₂ (red) between I₀/I with the concentration of acetone.

The quenching mechanism was also investigated by UV-vis adsorption spectroscope (Fig. S8, ESI†), which is similar to the Cr₂O₇²⁻ anions. The mechanism is attributed to the competition between the acetone molecules and excited MOFs for the adsorption of the light source energy. Upon illumination, the excitation energy absorbed by the organic ligands is transferred to acetone molecules, resulting in a decrease, or even full quenching.^28,41–43

Sensing of metal ions

As shown in Fig. 8, these Eu-MOFs exhibit markedly different quenching effects responding to the studied metal ions. Impressively, the luminescent intensity is obviously quenched by Fe³⁺ ions. For further investigation, we measured the luminescent spectra in different concentrations of Fe³⁺ ions solutions (Fig. S9, ESI†). It is obvious that the luminescent intensity decreases upon the gradually increase in concentration of Fe³⁺ ions from 0 to 500 μM, and the emissions are almost quenched when the concentration of Fe³⁺ ions is increased to 500 μM. The decrease of fluorescence intensity is nearly proportional to the concentration of Fe³⁺ ions, revealing that it would be a luminescent turn-off sensor for detecting the Fe³⁺ ions. On the basis of experimental data, the K_SV values of Eu-BDC-NH₂, Eu-BDC-F, and Eu-1,4-NDC are calculated as 4930, 7520, 9340, with the corresponding linear correlation coefficient of 0.99178, 0.99879, 0.99617, respectively. The plot shows that the quenching percentage of Eu-1,4-NDC is 63% higher than that of Eu-BDC-NH₂ under the same concentration of Fe³⁺ ions solution, and the value of Eu-BDC-F is 36% higher than that of Eu-BDC-NH₂ (Fig. 9). It is no surprise that the relationship between fluorescence quantum efficiency and quenching coefficient in detecting acetone is consistent with that of Cr₂O₇²⁻ and acetone, the higher the fluorescence quantum efficiency of Eu-MOF is, the higher the quenching efficiency for detection of Fe³⁺ ions is.

Fig. 8 The I/I₀ value of Eu-BDC-NH₂ (blue), Eu-BDC-F (amaranth), and Eu-1,4-NDC (khaki) toward different metal ions (10⁻² mol l⁻¹) when excited at 298, 278, 280 nm, respectively.

Fig. 9 (a) Quenching percentage plot of Eu-MOFs in the Fe³⁺ ions solution (60 μM). (b) K_SV curves of Eu-1,4-NDC (blue), Eu-BDC-F (red) and Eu-BDC-NH₂ (blank) between I₀/I with the concentration of Fe³⁺.

From the literatures, the quenching effect on luminescent MOFs by metal ions may be attributed to the following cases: (1) interaction between the metal ions and organic ligands; (2) collapse of the crystal structure; (3) the ions exchange between the central ions of MOFs and the targeted ions.^44–48 In order to elucidate the possible mechanism for the luminescent quenching phenomena in this work, the detailed data of ICP was measured, the results (Table S3, ESI†) indicated that the Eu³⁺ ions were replaced by Fe³⁺ ions. Moreover, the PXRD was employed to monitor the structural data of the pristine and exchanged compounds. From Fig. S10 (ESI†), it is observed that the Eu-BDC-F and Eu-BDC-NH₂ showed significantly different PXRD patterns compared with the original ones, implying that the crystal structure changed and partially collapsed. Therefore, the quenched luminescence of Fe³⁺@Eu-MOFs could be attributed to the substitution of Eu³⁺ and the collapse of the original crystal structure, thus affecting the overall antenna effect.

Conclusion

In summary, we synthesized four isostructural Eu-MOFs with different groups. These europium-based metal organic frameworks (Eu-MOFs) could be candidates for sensing the Cr₂O₇²⁻, acetone and Fe³⁺. Notably, Eu-BDC-NH₂, Eu-BDC-F, and Eu-1,4-NDC exhibited distinct intrinsic luminescence properties through the antenna effect. The order of fluorescence quantum yield of these Eu-MOFs is Eu-1,4-NDC > Eu-BDC-F > Eu-BDC-NH₂. The results of detection experiments for Cr₂O₇²⁻, acetone and Fe³⁺ showed that the quenching efficiencies for these three analytes are all submitted to the order of Eu-1,4-NDC > Eu-BDC-F > Eu-BDC-NH₂, corresponding to the intrinsic luminescence properties of the probes. Overall this work indicates the tune of functional groups in ligands could be crucial for gaining Eu-MOFs with high intrinsic luminescence properties. Meanwhile, it is also an effective and efficient strategy to enhance the detection capacities of these Eu-MOFs.

Acknowledgements

Financial support by the National Key Basic Research Program of China (“973”) (2013CB733503), the Natural Science Foundation of China (No. 21606007, 21136001 and 21536001) and the Fundamental Research Funds for the Central Universities (No. ZY1509) are greatly appreciated.

Notes and references

1. X.-L. Huang, L. Liu, M.-L. Gao and Z.-B. Han, RSC Adv., 2016, 6, 87945–87949.
2. H.-C. Zhou, J. R. Long and O. M. Yaghi, Chem. Rev., 2012, 112, 673–674.
3. D. Zhao, D. Q. Yuan, D. F. Sun and H.-C. Zhou, J. Am. Chem. Soc., 2009, 26, 9186–9188.
4. Z. C. Hu, B. J. Deibert and J. Li, Chem. Soc. Rev., 2014, 43, 5815–5840.
5. X. S. Lian, D. Zhao, Y. J. Cui, Y. Yang and G. D. Qian, Chem. Commun., 2015, 51, 17676–17679.
6. J. Rocha, L. D. Carlos, F. A. Almeida Paz and D. Ananias, Chem. Soc. Rev., 2011, 40, 926–940.
7. Y. J. Cui, H. Xu, Y. F. Yue, Z. Y. Guo, J. C. Yu, Z. X. Chen, J. K. Gao, Y. Yang, G. D. Qian and B. L. Chen, J. Am. Chem. Soc., 2012, 134, 3979–3982.
8. K.-M. Wang, L. Du, Y.-L. Ma, J.-S. Zhao, Q. Wang, T. Yan and Q.-H. Zhao, CrystEngComm, 2016, 18, 2690–2700.
9. Y. J. Cui, Y. F. Yue, G. D. Qian and B. L. Chen, Chem. Rev., 2012, 112, 1126–1162.
10. L. V. Meyer, F. Schönfeld and K. Müller-Buschbaum, Chem. Commun., 2014, 50, 8093–8108.
11. Y. H. Yao, X. D. Song, J. S. Qiu and C. Hao, J. Phys. Chem. A, 2014, 118, 6191–6196.
12. T.-W. Duan and B. Yan, J. Mater. Chem. C, 2015, 3, 2823–2830.
13. D. P. Yan, R. Gao, M. Wei, S. D. Li, J. Lu, D. G. Evans and X. Duan, J. Mater. Chem. C, 2013, 1, 997–1004.
14. K. Binnemans, Chem. Rev., 2009, 109, 4283–4374.
15. D. E. Barry, D. F. Caffrey and T. Gunnlaugsson, Chem. Soc. Rev., 2016, 45, 3244–3274.
16. T. Gunnlaugsson and J. P. Leonard, Chem. Commun., 2005, 25, 3114–3131.
17. J.-C. G. Bünzli and C. Piguet, Chem. Soc. Rev., 2005, 34, 1048–1077.
18. P. B. Glover, A. P. Bassett, P. Nockemann, B. M. Kariuki, R. Van Deun and Z. Pikramenou, Chem.–Eur. J., 2007, 13, 6308–6320.
19. N. M. Shavaleev, R. Scopelliti, F. Gumy and J.-C. G. Bünzli, Inorg. Chem., 2009, 48, 2908–2918.
20. L. M. Song, J. Hu, J. S. Wang, X. H. Liu and Z. Zhen, Photochem. Photobiol. Sci., 2008, 7, 689–693.
21. E. J. Kyprianidou, T. Lazarides, S. Kaziannis, C. Kosmidis, G. Itskos, M. J. Manos and A. J. Tasiopoulos, J. Mater. Chem. A, 2014, 2, 5258–5266.
22. D. Alezi, A. M. P. Peedikakkal, Ł. J. Weseliński, V. Guillerm, Y. Belmabkhout, A. J. Cairns, Z. J. Chen, Ł. Wojtas and M. Eddaoudi, J. Am. Chem. Soc., 2015, 137, 5421–5430.
23. D.-X. Xue, Y. Belmabkhout, O. Shekhah, H. Jiang, K. Adil, A. J. Cairns and M. Eddaoudi, J. Am. Chem. Soc., 2015, 137, 5034–5040.
24. CrysAlis RED, version 1.171.32.38, SCALE3 ABSPACK scaling algorithm; Oxford Diffraction Ltd, Blacksburg, Virginia, 2013.
25. G. M. Sheldrick, SHELXTL NT Version 5.1, Program for Solution and Refinement of Crystal Structures, University of Göttingen, Germany, 1997.
26. A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7–13.
27. J. H. Cavka, S. Jakobsen, U. Olsbye, N. Guillou, C. Lamberti, S. Bordiga and K. P. Lillerud, J. Am. Chem. Soc., 2008, 130, 13850–13851.
28. H. L. Huang, J.-R. Li, K. K. Wang, T. T. Han, M. M. Tong, L. S. Li, Y. B. Xie, Q. Y. Yang, D. H. Liu and C. L. Zhong, Nat. Commun., 2015, 6, 8847.
29. M. Kandiah, S. Usseglio, S. Svelle, U. Olsbye, K. P. Lillerud and M. Tilset, J. Mater. Chem., 2010, 20, 9848–9851.
30. T. W. Goh, C. X. Xiao, R. V. Maligal-Ganesh, X. L. Li and W. Y. Huang, Chem. Eng. Sci., 2015, 124, 45–51.
31. J. B. DeCoste, G. W. Peterson, H. Jasuja, T. Grant Glover, Y.-G. Huang and K. S. Walton, J. Mater. Chem. A, 2013, 1, 5642–5650.
32. T.-W. Duan, B. Yan and H. Weng, Microporous Mesoporous Mater., 2015, 217, 196–202.
33. X. Shen and B. Yan, RSC Adv., 2015, 5, 6752–6757.
34. Y. Zhou, H. Chen and B. Yan, J. Mater. Chem. A, 2014, 2, 13691–13697.
35. Y.-Y. Liu, R. Decadt, T. Bogaerts, K. Hemelsoet, A. M. Kaczmarek, D. Poelman, M. Waroquier, V. Van Speybroeck, R. Van Deun and P. Van Der Voort, J. Phys. Chem. C, 2013, 117, 11302–11310.
36. X.-Y. Xu and B. Yan, ACS Appl. Mater. Interfaces, 2015, 7, 721–729.
37. J.-N. Hao and B. Yan, J. Mater. Chem. A, 2014, 2, 18018–18025.
38. T.-W. Duan and B. Yan, J. Mater. Chem. C, 2014, 2, 5098–5104.
39. F.-Y. Yi, J.-P. Li, D. Wu and Z.-M. Sun, Chem.–Eur. J., 2015, 21, 11475–11482.
40. B. Wang, X.-L. Lv, D. Feng, L.-H. Xie, J. Zhang, M. Li, Y. Xie, J.-R. Li and H.-C. Zhou, J. Am. Chem. Soc., 2016, 138, 6204–6216.
41. X.-Z. Song, S.-Y. Song, S.-N. Zhao, Z.-M. Hao, M. Zhu, X. Meng, L.-L. Wu and H.-J. Zhang, Adv. Funct. Mater., 2014, 24, 4034–4041.
42. J.-A. Hua, Y. Zhao, Y.-S. Kang, Y. Liu and W.-Y. Sun, Dalton Trans., 2015, 44, 11524–11532.
43. F.-Y. Yi, W. T. Yang and Z.-M. Sun, J. Mater. Chem., 2012, 22, 23201–23209.
44. S. Dang, E. Ma, Z.-M. Sun and H.-J. Zhang, J. Mater. Chem., 2012, 22, 16920–16926.
45. J. Zhao, Y.-N. Wang, W.-W. Dong, Y.-P. Wu, D.-S. Li and Q.-C. Zhang, Inorg. Chem., 2016, 55, 3265–3271.
46. R. Wang, X.-Y. Dong, H. Xu, R.-B. Pei, M.-L. Ma, S.-Q. Zang, H.-W. Hou and T. C. W. Mak, Chem. Commun., 2014, 50, 9153–9156.
47. Q. Tang, S. Liu, Y. Liu, J. Miao, S.-J. Li, L. Zhang, Z. Shi and Z.-P. Zheng, Inorg. Chem., 2013, 52, 2799–2801.
48. J.-M. Zhou, W. Shi, H.-M. Li, H. Li and P. Cheng, J. Phys. Chem. C, 2014, 118, 416–426.

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Footnote

† Electronic supplementary information (ESI) available: The crystallographic data for crystal structure of Eu-BDC-NH₂, SEM images, FTIR spectra, fluorescence lifetime, ICP results, PXRD patterns, TGA curve, UV-vis spectra and luminescence spectra. CCDC 1494828. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra23263a

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