Two luminescent lanthanide(III) metal–organic frameworks as chemosensors for high-efficiency recognition of Cr(VI) anions in aqueous solution

Ji-Yong Zou *a, Ling Li a, Sheng-Yong You a, Yue-Wei Liu a, Hong-Min Cui a, Jian-Zhong Cui *b and Shao-Wei Zhang c
aInstitute of applied chemistry, Jiangxi academy of sciences, Nanchang, 330096, P.R. China. E-mail: zoujiyong@jxas.ac.cn; Fax: (+86)-0791-88133587
bDepartment of Chemistry, Tianjin University, Tianjin, 300354, P.R. China
cKey Laboratory of Theoretical Organic Chemistry and Functional Molecule of the Ministry of Education, School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan, Hunan 411201, P. R. China

Received 26th July 2018 , Accepted 30th August 2018

First published on 30th August 2018


Two new lanthanide(III) metal–organic frameworks (MOFs) {[(CH3)2NH2]2[Ln4(FDA)7(DMF)2]·0.5DMF}n [Ln = Eu (1), and Tb (2)] based on furan-2,5-dicarboxylic acid (H2FDA) have been successfully assembled and well characterized in detail. These MOFs are isostructural and demonstrate 12-connected sqc15 topologies, which are rarely observed in MOF chemistry, especially in lanthanide(III) MOFs. Moreover, these two MOFs could show a tolerance towards moisture and organic solvents and satisfactory chemical stabilities. More importantly, they exhibit sensitive and selective luminescence quenching response towards Cr2O72− and CrO42− anions in aqueous solution with the average quenching Ksv values of 1.25 × 104 L mol−1 (Cr2O72−) and 3.56 × 103 L mol−1 (CrO42−) for 1 and 1.46 × 104 L mol−1 (Cr2O72−) and 4.35 × 103 L mol−1 (CrO42−) for 2 and the detection limits of 1.14 × 10−4 mol L−1 (Cr2O72−) and 1.12 × 10−4 mol L−1 (CrO42−) for 1 and 7.42 × 10−5 mol L−1 (Cr2O72−) and 1.27 × 10−4 mol L−1 (CrO42−) for 2. The high quenching Ksv values and low detection limits make them more feasible in sensing Cr(VI) anions in aqueous solution. The possible detection mechanism has been discussed in detail.


Introduction

Hexavalent chromium ions, especially Cr2O72− and CrO42− anions, have been widely employed in diverse industrial productions, such as pigment printing, leather tanning, electroplating and other relevant fields.1 Conversely, they have also been confirmed to be two of the most severe environmental nonbiodegradable pollutants and can accumulate in living organisms, severely causing several adverse human health problems such as cancer, deformity and gene mutation.2 In particular, excessive Cr2O72− and CrO42− anions have lethal effects on organisms and the maximum contamination standard of hexavalent chromium ions in drinking water has been defined by the United States Environmental Protection Agency (EPA).3 Therefore, it is highly necessary to explore an effective method for detecting these hexavalent chromium ions sensitively and rapidly in aqueous solution, which is strikingly significant for environmental protection and human health. Indeed, many sophisticated analytical techniques such as atomic absorption spectroscopy, chromatography, inductively coupled plasma mass spectrometry and electrochemical analysis have been available to detect them efficiently.4 However, the drawbacks of these methods such as being time consuming, high cost, complicated pretreatment and the need for trained personnel are conspicuous as well. Therefore, seeking a new convenient, practically feasible, inexpensive and quantitative method for hexavalent chromium ion detection is imperative.

Recently, luminescent chemosensors have proven to be promising and compelling alternatives for detection because of their advantages in providing the opportunity of online monitoring with instantaneous and quantitative detection while the samples do not need redundant pretreatment.5 As a new class of chemosensors, luminescent metal–organic frameworks (MOFs) have recently attracted intensive interest because of their luminescence and accessible porosity, endowing them with the capability of luminescence changes via the host–guest interactions, and providing an opportunity of their superior performance in chemical sensing applications.6 For example, in 2017, our group reported two zinc(II)-lanthanide(III) MOFs to detect aniline and the corresponding detection limit could reach 7.5 μmol L−1 and 5.2 μmol L−1, respectively.7 Subsequently, we synthesized a zinc(II) MOF to be used as a luminescent probe to detect acetylacetone with a low detection limit of 0.647 μmol L−1.8 Recently, we explored luminescent probes based on a zinc(II) MOF and its lanthanide(III) post-functionalized materials to recognize aniline and benzaldehyde in DMF and Cr2O72− and CrO42− in aqueous solution with low detection limits.9 These results further indicate that luminescent MOFs are fascinating in chemical sensing applications. Compared with transition metal based MOFs, lanthanide(III) MOFs have superior properties in detection owing to their unique characteristics, such as a long luminescence lifetime, a large Stokes shift, sharp line emissions and bright luminescent colors of lanthanide(III) ions, particularly of europium(III) ions and terbium(III) ions.10

Herein, two luminescent lanthanide(III) MOFs, {[(CH3)2NH2]2[Ln4(FDA)7(DMF)2]·0.5DMF}n [Ln = Eu (1) and Tb (2)] have been successfully isolated, which can be viewed as rare 12-connected sqc15 topological nets. Remarkably, 1 and 2 could show a tolerance towards moisture and organic solvents. Furthermore, these two MOFs exhibit perfect sensitive and selective luminescence quenching response towards Cr2O72− and CrO42− anions in aqueous solution with high Ksv values and low detection limits, which are rarely observed in lanthanide MOFs for simultaneously sensing Cr2O72− and CrO42− anions in aqueous solution.

Experimental section

General methods and materials

All reagents and solvents were purchased from commercial sources and used as received without further purification. Elemental analyses of C, H and N were performed using a PerkinElmer 240 CHN elemental analyzer. IR spectra were recorded in the range 400–4000 cm−1 with a Bruker Tensor 27 spectrometer on KBr disks. Powder X-ray diffraction measurements were carried out using a D/Max-2500 X-ray diffractometer with Cu/Kα radiation. Thermogravimetric analysis (TGA) was carried out using a Delta Series TA-SDTQ 600 in a nitrogen atmosphere from room temperature to 800 °C (10 °C min−1) using aluminum crucibles. The luminescence spectrum was obtained on a Varian Cary Eclipse Fluorescence spectrophotometer. UV/vis spectra were recorded on an Agilent Cary100 spectrophotometer.
Synthesis of {[(CH3)2NH2]2[Eu4(FDA)7(DMF)2]·0.5DMF}n (1). A mixture of H2FDA (0.2 mmol, 0.0312 g), 4,4′-H2BPDC (0.1 mmol, 0.0244 g) (4,4′-H2BPDC = 4,4′-biphenyl dicarboxylic acid), Eu(NO3)3·6H2O (0.1 mmol, 0.0393 g) and 0.5 mL HNO3 in DMF solution (2 mL concentrated HNO3 dissolved in 10 mL DMF) in 6 mL DMF was sealed in a 23 mL Teflon-lined stainless autoclave and heated at 160 °C for 3 days and subsequently cooled to room temperature at a rate of 5.5 °C h−1. Colorless block crystals of 1 were collected by washing with DMF several times and dried in air. Yield: 78% based on Eu. Anal. calcd for C53.5H47.5N4.5Eu4O37.5: C 32.76; N 3.21; H 2.44. Found: C 33.26; N 3.57; H 2.65%. IR (KBr, cm−1): 3369w, 3041s, 2807s, 1641vs, 1563vs, 1382vs, 1306vs, 1162s, 1018m, 780s, 561m.
Synthesis of {[(CH3)2NH2]2[Tb4(FDA)7(DMF)2]·0.5DMF}n (2). 2 was synthesized with the same procedure as that for 1 except that Eu(NO3)3·6H2O was replaced by Tb(NO3)3·6H2O. Colorless block crystals of 2 were collected by washing with DMF several times and dried in air. Yield: 74% based on Tb. Anal. calcd for C53.5H47.5N4.5Tb4O37.5: C 32.30; N 3.17; H 2.41. Found: C 32.51; N 3.65; H 2.58%. IR (KBr, cm−1): 3369w, 3041s, 2807s, 1641vs, 1563vs, 1382vs, 1306vs, 1162s, 1018m, 780s, 561m.
Preparations of the suspensions 1-anions and 2-anions. The suspensions 1-anions and 2-anions were prepared by adding 5 mg crystalline powders 1 and 2 to 4 mL potassium salt aqueous solutions (F, Cl, Br, I, NO3, OAc, SO42−, Cr2O72− and CrO42−) with a concentration of 10−2 mol L−1, respectively, and ultrasonicated for 30 min, and then stable suspensions for fluorescence investigation were formed. For sensing Cr2O72− and CrO42−, 5 mg crystalline powders 1 and 2 were dispersed in 3 mL H2O, forming stable suspensions, and then 10−2 mol L−1 K2Cr2O7 and K2CrO4 solutions were added slowly. For testing the anti-interference abilities of anions for 1 and 2, 5 mg crystalline powders 1 and 2 were dispersed in 3 mL H2O, forming stable suspensions, and then 50 μL of 0.1 mol L−1 various potassium salt aqueous solutions were added to the suspension of 1 and 2 and the corresponding emission spectra were monitored. Subsequently, 50 μL 0.1 mol L−1 K2CrO4 and K2Cr2O7 was added into the suspension of 1 and 2, respectively, and the corresponding emission spectra were monitored.

Crystallographic study

Data collection of 1 and 2 was performed using an Oxford Supernova diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The temperature was brought down with a cryojet controller attached to an Oxford Supernova diffractometer. The structures were solved by direct methods. All non-hydrogen atoms were refined by full-matrix least-squares techniques on F2 using the SHELXS-97 and SHELXL-97 programs.11 Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The hydrogen atoms were placed in idealized positions and located in the difference Fourier map. The compositions of the as-synthesized 1 and 2 were calculated based on the elemental analysis, TGA and single crystal structure analysis results. CCDC 1835672 for 1 and 1835674 for 2 contain the supplementary crystallographic data of this paper. The crystallographic data for 1 and 2 are summarized in Table 1.
Table 1 Crystal data and structure refinement details for 1 and 2
Compound 1 2
a R 1 = ∑(|Fo| − |Fc|)/∑|Fo|. b wR2 = [∑{w(|Fo|2 − |Fc|2)2}/∑[w(|Fo|2)2]]1/2.
Formula C53.5H47.5N4.5Eu4O37.5 C53.5H47.5N4.5Tb4O37.5
Formula weight 1961.31 1989.16
Temperature (K) 150(2) 150(2)
Crystal system Monoclinic Monoclinic
Space group P21/n P21/n
a (Å) 9.07320(10) 9.0774(2)
b (Å) 26.0989(3) 25.9711(5)
c (Å) 13.9595(2) 13.9493(3)
α (°) 90 90
β (°) 103.8210(10) 103.844(2)
γ (°) 90 90
V3) 3209.91(7) 3193.01(12)
Z 2 2
D c (g cm−3) 1.989 2.031
μ (mm−1) 28.452 22.264
F(000) 1860.0 1880.0
GOF on F2 1.214 1.027
R int 0.0637 0.0529
R 1,a wR2[thin space (1/6-em)]b [I > 2σ(I)] 0.1068, 0.2875 0.0610, 0.1576
R 1, wR2 (all data) 0.1170, 0.3072 0.0698, 0.1701


Results and discussion

Description of the crystal structures

The solvothermal reactions of lanthanide nitrates with H2FDA and 4,4′-H2BPDC in DMF, in the presence of nitric acid at 160 °C afforded two homometallic lanthanide(III) MOFs, in which 4,4′-H2BPDC is absent in the final products but assists in the formation of MOFs. In contrast, H2FDA was completely deprotonated, resulting in three forms of FDA2− ligands (Scheme 1) and the counterions are protonated dimethylammonium cations, formed from the decomposition of N,N-dimethylformamide. A single-crystal X-ray diffraction study revealed that 1 and 2 are isostructural and crystallize in the monoclinic system with the space group P21/n (Table 1). Therefore, the structure of 1, as a representative, is described in detail. As shown in Fig. 1a, there are two crystallographically independent Eu(III) ions, three and a half FDA2− ligands, one coordinated DMF molecule, a quarter of free DMF molecule and one dimethylammonium cation in the asymmetric unit. Eu1 is eight-coordinated with a distorted dodecahedral {EuO8} geometry (Fig. 1b) by eight oxygen atoms (O2A, O5A, O6A, O7A, O9, O11, O14A and O17A) from seven FDA2− ligands. The coordination environment of Eu2 is similar to that of Eu1, adopting a distorted dodecahedral {EuO8} geometry (Fig. 1c) with seven oxygen atoms (O1, O4A, O10, O12, O15B, O16 and O17) from six FDA2− ligands and one oxygen atom (O18) from one DMF molecule. The Eu–O bond distances range from 2.292(8) to 2.822(9) Å and the O–Eu–O bond angles vary from 51.1(2) to 169.4(3). In 1, H2FDA ligands were completely deprotonated, generating three coordination modes of FDA2−: (η2–η1–μ2)–(η2–η1–μ2)–μ4, η1–η1–η1–η1–μ4 and η1–η1–η2–μ3 (Scheme 1). In the (η2–η1–μ2)–(η2–η1–μ2)–μ4 coordination mode of FDA2−, two carboxylates adopt a similar coordinated fashion with two carboxylic oxygen atoms coordinating to two Eu(III) ions via the tridentate chelating-bridging mode; in the η1–η1–η1–η1–μ4 coordination mode, two carboxylates adopt a similar coordinated fashion with two carboxylic oxygen atoms coordinating to two Eu(III) ions separately with the monodentate mode; while in the η1–η1–η2–μ3 coordination mode, one carboxylate adopts a similar coordinated fashion to the η1–η1–η1–η1–μ4 mode with two carboxylic oxygen atoms coordinating to two Eu(III) ions separately, while the oxygen atoms of the other carboxylate coordinate to one Eu(III) ion. Interestingly, each Eu(III) ion is connected by FDA2− ligands with the aforementioned coordination modes to generate a 1D europium ion chain (Fig. 1d). The 1D europium ion chains can be further interconnected to each other into a 3D framework (Fig. 1e). The 3D anionic framework is composed of voids of 620.1 Å3, presenting a solvent-accessible volume of 19.3%, calculated by PLATON12 with the removal of dimethylammonium cations and the free DMF molecules.
image file: c8dt03050b-s1.tif
Scheme 1 Coordination modes of H2FDA in 1.

image file: c8dt03050b-f1.tif
Fig. 1 (a) The asymmetric unit in 1 (thermal ellipsoids are drawn at the 30% probability level). [Symmetry code: (A) 1 + X, Y, Z; (B) 1/2 + X, 1/2 − Y, 1/2 + Z]. All hydrogen atoms and dimethylammonium cations were omitted for clarity; (b) the distorted dodecahedral coordination environment of Eu1 in 1; (c) the distorted dodecahedral coordination environment of Eu2 in 1; (d) the 1D chain in 1; (e) 3D framework of 1; green: Eu, red: O, blue: N, black: C.

A further investigation on the intricate framework of 1 was conducted by topological analysis using the freely available TOPOS software package.13 As shown in Fig. 2a, if the [Eu4(COO)4(FDA)] tetrauclear units are only treated as 12-connected nodes, then the structure of 1 can be simplified as a uninodal 12-connected net with the Schläfli symbol of (318·442·56) (Fig. 2b), corresponding to the sqc15 topology.14 The 12-membered polyhedron in 1 is defined by the close and six further distant nodes with a highly distorted cube-octahedron in which the two triangles above and below the central hexagon are displaced in opposite directions. Alternatively, 1 can also be viewed as the [Eu4(COO)4(FDA)] tetrauclear nodes link to the ligand FDA2− nodes in which the tetrauclear nodes act as highly novel 9-connected nodes of tricapped trigonal prismatic geometry and the ligand FDA2− nodes act as 3-connected nodes with trigonal geometry. Consequently, 1 is a binodal (3,9)-connected (42·6)3(46·621·89) topology. To further investigate the topology of 1, a systematic survey on various uninodal 12-connected nets has been performed using RCSR. As reported in the literature, 14 different types of 12-connected nets have been enumerated, in which the majority of them are based on a more regular cubeoctahedral structural matrix. Nevertheless, the sqc15 topology of 1 is clearly different from other 12-connected reported network topologies and is rarely observed in MOFs. Furthermore, only a few examples of transition metal-based MOFs with M3(OH)(CO2)6 clusters having sqc15 topology are observed,15 while such sqc15 topology has never been observed in lanthanide MOFs. Therefore, 1 and 2 represent the first examples of a class of lanthanide frameworks that possess a sqc15 topology.


image file: c8dt03050b-f2.tif
Fig. 2 (a) The simplified 12-connected node in 1; (2) the sqc15 topology of 1.

Stability characterization of 1 and 2

To confirm their crystalline phase purity, PXRD experiments have been carried out at room temperature. The diffraction peaks of bulk samples are consistent with the simulated patterns in terms of the single crystal data, indicating the presence of mainly one crystalline phase in the corresponding samples of the two isostructural MOFs (Fig. S1, ESI). Additionally, their chemical stability was investigated. 1 and 2 were immersed in water, CH3OH, C2H5OH, CH3CN, THF, DMF, DMA, CH2Cl2, 1,4-dioxane, and glycol, demonstrating their tolerance towards moisture and organic solvents and showing their satisfactory chemical stabilities, which were confirmed by PXRD (Fig. S2, ESI). The TGA of 1 and 2 was also carried out under an N2 atmosphere to confirm their thermal stability (Fig S5, ESI). The TGA curves show that the weight losses of 1 and 2 before 210 °C are analogous, corresponding to the removal of half of the uncoordinated DMF molecules (found: 2.02% for 1 and 1.86% for 2, calculated: 1.86% for 1 and 1.84% for 2). After the losses of all uncoordinated DMF molecules, the two coordinated DMF molecules (found: 8.42% for 1 and 7.66% for 2, calculated: 7.45% for 1 and 7.35% for 2) leave when increasing the temperature up to 320 °C, and then the frameworks begin to decompose upon further heating.

Luminescence properties and Cr(VI) anion recognition

The lanthanide MOFs have shown good luminescence properties in terms of the intrinsic nature of 4f ions, such as sharp and characteristic emissions, a large Stokes shift, relatively long luminescence lifetimes, and bright luminescent colors of Ln(III) ions, especially for Eu3+ and Tb3+ ions. Hence, the solid-state luminescence properties of powder samples 1 and 2 were measured at room temperature (Fig. S6, ESI). Upon excitation at 350 nm, 1 shows a characteristic emission of Eu3+ ions with the emission peaks of 591, 615, 652 and 698 nm, ascribed to the transitions 5D07FJ (J = 1, 2, 3 and 4) of Eu3+ ions. The strongest emission peak is at 615 nm from the 5D0 → 7F2 induced by the electronic dipole transition. Similarly to 1, 2 exhibits a characteristic emission of Tb3+ ions with the emission peaks of 489, 544, 590 and 620 nm when excited at a wavelength of 358 nm. The peaks can be ascribed to the transitions 5D47FJ (J = 6, 5, 4 and 3) of Tb3+ ions and the strongest emission peak at 544 nm is from the 5D47F5 transitions.

Recently, much effort has been devoted to exploiting luminescent MOFs due to their excellent performance in the rapid recognition of cations, anions and small molecules.16 Lanthanide(III) MOF-based luminescence chemosensors are particularly important due to their favorable advantages in the detection field. In this work, 1 and 2 were explored as luminescence chemosensors to recognize Cr(VI) anions via the luminescence quenching effects. Several potassium salt aqueous solutions (F, Cl, Br, I, NO3, OAc SO42−, Cr2O72− and CrO42−) with a concentration of 10−2 mol L−1 were separately added to the aqueous suspension of 1 and 2. The luminescence intensities of 1 and 2 are prominently dependent on the intrinsic features of anions (Fig. 3a and b). Particularly, Cr2O72− and CrO42− anions have a predominant influence on the intensities of 1 and 2, tremendously weakening their luminescence intensities and dramatically quenching them. In contrast, the other anions display slight or no discernible effect on the luminescence intensities of 1 and 2. The aforementioned results suggest that 1 and 2 can be regarded as promising luminescence turn-off chemosensors to selectively recognize Cr2O72− and CrO42− anions among other common anions. This fact was also supported by the competing experiments for the anti-interference sensing ability of 1 and 2: the intensities were acutely quenched in the presence of Cr2O72− and CrO42− anions, while no obvious changes were observed when the competitive anions were added into the corresponding solutions (Fig. 4a–d). Consequently, the above-demonstrated results strongly confirm that 1 and 2 qualify as chemosensors for Cr2O72− and CrO42− anions in aqueous solution with high selectivity and outstanding anti-interference sensing ability.


image file: c8dt03050b-f3.tif
Fig. 3 (a) The luminescence of 1 dispersed in 10−2 mol L−1 anions; (b) the luminescence of 2 dispersed in 10−2 mol L−1 anions.

image file: c8dt03050b-f4.tif
Fig. 4 (a) The luminescence of 1 (5D07F2) dispersed in H2O with the addition of different corresponding anions (pink) and subsequent addition of Cr2O72− (green); (b) the luminescence of 1 (5D07F2) dispersed in H2O with the addition of different corresponding anions (pink) and subsequent addition of CrO42− (green); (c) the luminescence of 2 (5D47F5) dispersed in H2O with the addition of different corresponding anions (pink) and subsequent addition of Cr2O72− (green); (d) the luminescence of 2 (5D47F5) dispersed in H2O with the addition of different corresponding anions (pink) and subsequent addition of CrO42− (green).

To further assess the sensing ability of 1 and 2 against Cr2O72− and CrO42− anions, the relationship between the changes of luminescence for them and the concentration of Cr(VI) anions in aqueous solution has been established by titration experiments. As shown in Fig. 5a–d, the emission intensity of 1 and 2 in aqueous solution gradually decreased with the increasing concentration of Cr2O72− or CrO42− anions, respectively, further indicative of the existence of luminescence quenching effects. Moreover, the luminescence intensity of 1 decreased sharply to 48.8% at the concentration of 8.33 × 10−5 mol L−1 Cr2O72− anions and 51.77% at the concentration of 3.33 × 10−4 mol L−1 CrO42− anions. Similar to 1, the luminescence intensity of 2 decreased sharply to 51.0% at the concentration of 6.67 × 10−5 mol L−1 Cr2O72− anions and 53.1% at the concentration of 2.0 × 10−4 mol L−1 CrO42− anions. When the concentration of Cr(VI) anions approached 6.67 × 10−4 mol L−1 (Cr2O72−), 1.0 × 10−4 mol L−1 (CrO42−) for 1 and 5 × 10−4 mol L−1 (Cr2O72−), 9.33 × 10−4 mol L−1 (CrO42−) for 2, the luminescence signals of 1 and 2 were almost completely quenched. The quenching efficiency of 1 and 2 can be quantitatively evaluated by Ksv using the Stern–Volmer (SV) equation: I0/I = 1 + Ksv[Cr(VI)].7–9,17 Herein, Ksv stands for the quenching constant (L mol−1) and I0 and I are the luminescence intensities initially and after the addition of Cr(VI) anions, while [Cr(VI)] represents the molar concentration of Cr(VI) anions (mmol L−1). As shown in Fig. 6a–d, the Stern–Volmer quenching curves for Cr(VI) anions are nearly linear at low concentrations with the average Ksv values of 1.25 × 104 L mol−1 (Cr2O72−) and 3.56 × 103 L mol−1 (CrO42−) for 1, and 1.46 × 104 L mol−1 (Cr2O72−) and 4.35 × 103 L mol−1 (CrO42−) for 2 and the linear fit correlation coefficient of 0.996 (Cr2O72−) and 0.995 (CrO42−) for 1, and 0.998 (Cr2O72−) and 0.994 (CrO42−) for 2. Comparatively, the average Ksv values of 1 and 2 for sensing Cr2O72− anions are slightly superior to the value of functional fluorescent aramids (1.16 × 104 L mol−1) for Cr(VI) anion sensing in dimethyl sulfoxide/water but significantly inferior to the values of Eu3+@MIL-124 (60340 L mol−1) and the fluorescent carbon dot nanosensor (6.90 × 104 L mol−1) for Cr(VI) anion sensing in aqueous solution.18 The plot deviates from linearity with increase in concentration, demonstrating the concurrence of only one type of quenching process of 1 and 2. Thereby, the quenching process can be quantitatively controlled by the concentration of Cr(VI) anions. The detection limits of Cr2O72− and CrO42− anions for 1 and 2 can be evaluated based on the well-established formula 3δ/k,19 in which δ is standard error and k is the slope according to the Stern–Volmer equation with values of 1.14 × 10−4 mol L−1 (Cr2O72−) and 1.12 × 10−4 mol L−1 (CrO42−) for 1, and 7.42 × 10−5 mol L−1 (Cr2O72−) and 1.27 × 10−4 mol L−1 (CrO42−) for 2, further indicative of an excellent sensitivity for sensing Cr(VI) anions in water.


image file: c8dt03050b-f5.tif
Fig. 5 (a) The luminescence of 1 dispersed in H2O with different concentrations of 10−2 mol L−1 K2Cr2O7 solution; (b) the luminescence of 1 dispersed in H2O with different concentrations of 10−2 mol L−1 K2CrO4 solution; (c) the luminescence of 2 dispersed in H2O with different concentrations of 10−2 mol L−1 K2Cr2O7 solution; (d) the luminescence of 2 dispersed in H2O with different concentrations of 10−2 mol L−1 K2CrO4 solution.

image file: c8dt03050b-f6.tif
Fig. 6 (a) The correlation between the luminescence of 1 (5D07F2) and the concentration of K2Cr2O7 solution (Inset: The linear correlation between the luminescence of 1 (5D07F2) and the low concentration of K2Cr2O7 solution); (b) the correlation between the luminescence of 1 (5D07F2) and the concentration of K2CrO4 solution (Inset: The linear correlation between the luminescence of 1 (5D07F2) and the low concentration of K2CrO4 solution); (c) the correlation between the luminescence of 2 (5D47F5) and the concentration of K2Cr2O7 solution (Inset: The linear correlation between the luminescence of 2 (5D47F5) and the low concentration of K2Cr2O7 solution); (d) the correlation between the luminescence of 2 (5D47F5) and the concentration of K2CrO4 solution (Inset: The linear correlation between the luminescence of 2 (5D47F5) and the low concentration of K2CrO4 solution).

Actually, it is noteworthy that there have been only a small number of MOF-based luminescent chemosensors for simultaneously sensing Cr2O72− and CrO42− anions in aqueous solution.9,20 Nevertheless, these known chemosensors are based on transition metal MOFs, but lanthanide MOFs have been rarely reported. Specifically, a comparison of other MOF-based luminescent chemosensors for simultaneously sensing Cr2O72− and CrO42− anions in aqueous solution is given in Table S1. Significantly, 1 and 2 possess high quenching constants and low detection limits, making them luminescent chemosensors for simultaneously sensing Cr2O72− and CrO42− anions in aqueous solution with high sensitivity and selectivity and providing more possibilities for luminescent chemosensors to be feasible in practical applications.

Quenching mechanism

The possible mechanism of the luminescence quenching by Cr2O72− and CrO42− anions in aqueous solution was further explored. The PXRD patterns of 1 and 2 after dispersing in aqueous solution in the presence of Cr2O72− and CrO42− anions were in good agreement with the simulated patterns of the single crystal data, demonstrating that the 3D frameworks of 1 and 2 still remained stable (Fig. S3, ESI). Consequently, the luminescence quenching effect derived from the collapse of the framework could be excluded. Generally, the luminescence of lanthanide(III) MOFs mainly suffers from the “antenna effect” process: the ligands efficiently absorb and transfer light to the luminescent central lanthanide ions.21 Therefore, the UV-Vis spectra of Cr2O72− and CrO42− anions in aqueous solution were obtained (Fig. S7, ESI). The excitation bands are located in the range of 320–389 nm for 1 and 320–405 nm for 2, which are extensively overlapped by the absorption bands of Cr2O72− (298–410 nm) and CrO42−(296–394 nm) anions. This fact indicates that Cr2O72− and CrO42− anions could drastically absorb the energy of the excitation light of 1 and 2, reducing the efficiency of energy transformation from the ligand to the lanthanide ions, which definitely led to the luminescence quenching effect of 1 and 2.

Conclusions

In summary, two novel 3D 12-connected sqc15 topological lanthanide(III) MOFs based on furan-2,5-dicarboxylic acid have been rationally constructed, showing the satisfied solvent- stabilities. The results of luminescence titration and anti-interference demonstrate that 1 and 2 are versatile turn-off luminescent chemosensors for simultaneously sensing Cr2O72− and CrO42− anions in aqueous solution with high sensitivity, selectivity and anti-interference ability. Further detailed explorations reveal that the luminescence quenching response of 1 and 2 originates from the competitive absorption between Cr(VI) anions and 1 or 2. Notably, the detection limits of 1 and 2 for sensing Cr(VI) anions could reach 1.14 × 10−4 mol L−1 (Cr2O72−) and 1.12 × 10−4 mol L−1 (CrO42−) for 1, and 7.42 × 10−5 mol L−1 (Cr2O72−) and 1.27 × 10−4 mol L−1 (CrO42−) for 2. All results indicate that 1 and 2, as high-efficiency luminescent chemosensors, show the possibility of practical application in sensing Cr2O72− and CrO42− anions in aqueous solution.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the National Natural Science Foundation of China (No. 21473121, 21561014 and 21562023), the Key Research Project of Jiangxi Province (No. 20171BBF60074), the Key Research Project of Jiangxi Academy of Sciences (No. 2018-YZD2-11) and the Project on the Integration of Industry, Education and Research of Jiangxi Academy of Sciences (No. 2016-YCXY-05) is greatly acknowledged.

Notes and references

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Footnote

Electronic supplementary information (ESI) available: PXRD spectra, TG spectra, IR spectra, and luminescence measurement. CCDC 1835672 and 1835674. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8dt03050b

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