Multi-Responsive Luminescent Sensors Based on Two-Dimensional

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Multi-responsive luminescent sensors based on 2D lanthanide-metal organic frameworks for highly selective and sensitive detection of Cr(III), Cr(VI) ions and benzaldehyde Zan Sun, Meng Yang, Yue Ma, and Li-Cun Li Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00638 • Publication Date (Web): 03 Jul 2017 Downloaded from http://pubs.acs.org on July 4, 2017

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Crystal Growth & Design

Multi−responsive

luminescent

sensors

based

on

2D

lanthanide−metal organic frameworks for highly selective and sensitive detection of Cr(III), Cr(VI) ions and benzaldehyde Zan Sun, Meng Yang, Yue Ma, Licun Li* Department of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry and Tianjin Key Laboratory of Metal and Molecule−based Material Chemistry, Nankai University, Tianjin 300071, China

Abstract: Four new isostructural 2D lanthanide(III)−metal organic frameworks, namely [Ln(L)(HCOO)(H2O)]n (Ln = Eu 1, Gd 2, Ho 3 and Tb 4, H2L = 5−((2'−cyano−[1,1'−biphenyl]−4−yl)methoxy)isophthalic acid), with a uninodal {44.62}−sql topology have been successfully isolated. Compounds 1 and 4 exhibit excellent applications as luminescent sensors for sensing benzaldehyde in methanol and Cr3+, CrO42− and Cr2O72− in water with high sensitivity and selectivity based on luminescence quenching effects. Interestingly, 1 and 4 display excellent recyclable behaviors and can be recycled at least 5 times for sensing benzaldehyde, Cr(III) and Cr(VI) ions. These two compounds are the first multi−functional Ln−MOFs sensors for detecting benzaldehyde, Cr(III) and Cr(VI) ions, simultaneously. Therefore, these two materials may be excellent multi−functional recyclable luminescent sensors.

Introduction Recently, lanthanide(III)−metal organic frameworks (Ln−MOFs) have attracted extensive attention due to their fascinating architectures and diverse applications such as magnetism, drug delivery, gas storage/separation, light−emitting devices and chemical sensing.1−9 Among many fields mentioned above, luminescent sensing is very meaningful and a large number of luminescent Ln−MOFs, especially Eu− and Tb−MOFs, are considered to be excellent candidates for sensing anions,10−13 1

ACS Paragon Plus Environment


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cations,14−20 small organic molecules21−27 because they display unique, easily separable, line−like emission spectrum and exhibit the strong red and green light in the visible regions.28−31 Chromium is widely applied in industrial production and plays an important role in the human body. The existence of chromium in nature is two common oxidation states, Cr(III) and Cr(VI) ions. The toxicity of Cr(III) and Cr(VI) ions is extremely different. Cr(III) ion is an essential biological trace element for public health and excessive Cr(III) ions probably lead to some serious diseases, such as mutations or malignant cells, due to integrating with DNA in the body.32−37 On the other hand, Cr(VI) ions are carcinogenic species and important oxidant in industry. Due to massive utilization in the daily life of industry and agriculture, they have brought severe environmental problems and health risks.34 Therefore, it is of great significance to detect chromium ions quickly and sensitively in water medium.38−41 In addition, benzaldehyde is an important chemical raw material and can be used as a solvent, lubricant, plasticizer, condiment and fragrance. The excessive benzaldehyde beyond the normal range will not only have a seriously effect on the human health, but also can severely contaminate the environment.42−45 Therefore, the development of methods for the detection of benzaldehyde is urgent with respect to environmental protection and safety considerations. Up to now, many analysis methods,46−49 including inductively coupled plasma mass spectrometry, atomic absorption spectrometry and chromatography, have been employed to detect Cr(III), Cr(VI) ions as well as benzaldehyde; however, these methods are frequently limited by high cost, time−spending, sample preconditioning and complicated instruments. Therefore, it is very essential to exploit the mobile, time−saving and low−cost methods for the detection of Cr3+, CrO42−, Cr2O72− and benzaldehyde. Recently, luminescence sensing based on the quenching effect of luminescent Ln−MOFs as chemical sensors offers an alternative that has already been confirmed to be a much easier, sensitive and facile method,50−54 which mainly relies on the electronic interactions between host coordination frameworks and guest analytes. However, to the best of our knowledge, reports about Ln−MOFs as 2


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luminescent sensors for sensing Cr(III), Cr(VI) ions as well as benzaldehyde are very rare.

For

example,

Liu

et

[Eu2(tpbpc)4·CO3·4H2O]·DMF·solvent

al.

reported

a

which

exhibits

high

1D

compound

selectivity,

high

sensitivity, and low detection limit for the detection of Cr3+, CrO42− and Cr2O72− ions in

aqueous

solution.33

Du

et

al.

synthesized

a

Tb−MOF

[Tb(TBOT)(H2O)](H2O)4(DMF)(NMP)0.5 displaying recyclable selective sensing of Cr2O72−.41 Cheng et al. reported a Tb−MOF {[Tb2(TATAB)2]·4H2O·6DMF}n which can trace amounts of benzaldehyde in benzyl alcohol.75 Li et al. synthesized two novel Ln−MOFs [Ln(H2DMPhIDC)3(H3DMPhIDC)]n (Ln = Eu and Tb), which are selectively

sensitive

to

benzaldehyde−based

derivatives

(benzaldehyde,

m−methylbenzaldehydes, m−carboxylbenzaldehyde and m−hydroxybenzaldehyde).42 To date, Ln−MOFs which can detect Cr(III), Cr(VI) and benzaldehyde simultaneously have not been reported. Inspired by the aspects discussed above, we choose an unexplored ligand H2L, 5−((2'−cyano−[1,1'−biphenyl]−4−yl)methoxy)isophthalic acid, as a linking ligand to construct Ln−MOFs due to the following reasons: 1) The bridging ligand could promote the formation of fascinating architectures because of the versatile coordination modes of carboxylate groups; 2) The H2L ligand possessing a largely electronic conjugate system can transfer energy to Ln3+ centers easily; 3) The “soft base” N atoms of cyano group have weak affinity to the “hard acid” Ln3+ ions, so they often remain uncoordinated. These free N atoms may interact with some transition metal ions and play a key role in the sensing process through causing the luminescence intensity changes of Ln−MOFs. In this work, a series of novel Ln−MOFs, namely [Ln(L)(HCOO)(H2O)]n (Ln = Eu 1, Gd 2, Ho 3 and Tb 4) have been obtained under the solvothermal conditions. Compounds 1 − 4 are 2D layer structure which exhibits a uninodal sql topology net with the point symbol of {44.62}. Compounds 1 and 4 exhibit excellent recyclable, selective and sensitive detection of Cr3+, CrO42−, Cr2O72− and benzaldehyde, which represent the first examples of Ln−MOFs that can be used to detect Cr(III), Cr(VI) ions as well as benzaldehyde. The 3



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possible mechanisms for selective luminescence quenching responses towards different analytes are also discussed in detail.

Experimental Materials and methods All of the chemicals and solvents are commercially available and were used without any further purification. The H2L ligand was purchased from Jinan Camolai Trading Company. Powder X−ray diffraction (PXRD) patterns were recorded on a Rigaku MiniFlex 600 diffractometer with Cu Kα radiation (λ = 1.5406 Å). The infrared spectra were measured on a Bruker Tensor 27 IR spectrometer using KBr pellets in the range 4000 – 400 cm−1. Elemental analyses (C, H, and N) were carried out on a Perkin–Elmer 240 elemental analyzer. Thermogravimetric analyses (TGA) were recorded on a Netzsch TG 209 TG−DTA analyzer at a heating rate of 10 °C / min from the room temperature to 800 °C under flowing nitrogen atmosphere. UV−vis spectra were measured on a JASCO V−570 spectrophotometer. The fluorescent spectra were recorded on an F−4500 FL Spectrophotometer. X−ray photoelectron spectra (XPS) were recorded on a Thermo Scientific ESCALAB 250Xi X−ray photoelectron spectroscopy. Synthesis of [Eu(L)(HCOO)(H2O)]n (1). A mixture of Eu2O3 (17.5 mg, 0.05 mmol) and H2L (18.6 mg, 0.05 mmol) was mixed in 7 mL of DMF−H2O (2:5 in v/v) with three drops of HClO4. The final mixture was put into a Teflon−lined stainless steel vessel (23 mL) and heated at 180 °C for 3 days, followed by cooling to room temperature at a rate of 10 °C h−1. Light−yellow crystals were collected (yield: 13.5 mg, 46% based on H2L). Anal. Calcd for C23H16EuNO8 (1): C 47.11, H 2.75, N 2.39 %. Found: C 46.75, H 2.57, N 2.19 %. FT−IR (KBr pellets, cm−1): 3649m, 3485m, 2232m, 1632s, 1603s, 1534s, 1448s, 1416s, 1366s, 1264m, 1131w, 1033m, 804m, 783m, 762w, 704m, 417w. Synthesis of [Gd(L)(HCOO)(H2O)]n (2). The synthetic method for the preparation of 2 is almost identical to that of 1 except that Gd2O3 (18.1 mg, 0.05 mmol) was used instead of Eu2O3. Light−yellow crystals were collected (yield: 16.6 mg, 56% based on 4


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H2L). Anal. Calcd for C23H16GdNO8 (2): C 46.69, H 2.73, N 2.37 %. Found: C 46.43, H 2.96, N 2.51 %. FT−IR (KBr pellets, cm−1): 3650m, 3486m, 2232m, 1632s, 1604s, 1537s, 1449s, 1417s, 1365s, 1265m, 1131w, 1034m, 807m, 784m, 763w, 704m, 418w. Synthesis of [Ho(L)(HCOO)(H2O)]n (3). The synthetic method for the preparation of 3 is almost identical to that of 1 except that Ho2O3 (18.9 mg, 0.05 mmol) was used instead of Eu2O3. Light−yellow crystals were collected (yield: 11.4 mg, 38% based on H2L). Anal. Calcd for C23H16HoNO8 (3): C 46.09, H 2.69, N 2.34 %. Found: C 46.03, H 2.25, N 2.21 %. FT−IR (KBr pellets, cm−1): 3652m, 3486m, 2230m, 1635s, 1605s, 1540s, 1452s, 1418s, 1335s, 1265m, 1132w, 1034m, 810m, 784m, 763w, 704m, 419w. Synthesis of [Tb(L)(HCOO)(H2O)]n (4). The synthetic method for the preparation of 4 is almost identical to that of 1 except that Tb4O7 (18.3 mg, 0.024 mmol) was used instead of Eu2O3. Light−yellow crystals were collected (yield: 9.8 mg, 33% based on H2L). Anal. Calcd for C23H16TbNO8 (4): C 46.56, H 2.72, N 2.36 %. Found: C 46.42, H 3.09, N 2.33 %. FT−IR (KBr pellets, cm−1): 3649m, 3487m, 2231m, 1634s, 1602s, 1535s, 1448s, 1416s, 1367s, 1265m, 1132w, 1033m, 807m, 784m, 763w, 705m, 424w. X−ray crystallography The crystallographic data for complexes 1 − 4 were performed on a Rigaku Saturn CCD diffractometer equipped with graphite−monochromated Mo Kα radiation (λ = 0.71073 Å). The structures were solved by the direct methods and refined by the full−matrix least−squares method on F2 (SHELXTL−2014).52 All non−hydrogen atoms were located from the Fourier maps and were refined with anisotropic displacement parameters. Hydrogen atoms of water molecules were found from different Fourier maps and then refined with isotropic temperature factors. Hydrogen atoms attached to carbon atoms were placed on calculated positions, and their positions were refined using a riding model. The pertinent crystallographic data and structure refinement parameters for complexes 1 − 4 are listed in Table 1. The 5



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selected bond lengths and angles of 1 − 4 are given in Table S1. The CCDC numbers of 1 − 4 are 1546699 − 1546702.

Table 1 Crystallographic data and structure refinement details for 1 − 4 formula Mr T, K Crystal system space group a, Å b, Å c, Å β, deg V, Å3 Z Dc, gcm−3 µ, mm−1 θ / (deg) Unique reflns / Rint GOF R1, wR2 [I > 2σ(I)] R1,wR2(all data)

1

2

3

4

C23H16EuNO8 586.33 113 Monoclinic P21/c 14.1521(13) 17.0952(15) 8.5606(7) 98.060(2) 2050.6(3) 4 1.899 3.112 3.142 to 27.535

C23H16GdNO8 591.62 113 Monoclinic P21/c 14.1577(13) 17.0844(15) 8.5332(7) 98.116(2) 2043.3(3) 4 1.923 3.299 3.142 to 27.518

C23H16HoNO8 599.30 113 Monoclinic P21/c 14.1672(13) 17.0614(15) 8.5282(7) 98.047(2) 2041.1(3) 4 1.950 3.930 3.140 to 27.514

C23H16TbNO8 593.30 113 Monoclinic P21/c 14.1744(13) 17.0343(15) 8.5026(7) 97.878(2) 2033.6(3) 4 1.938 3.532 3.139 to 27.532

4669 / 0.0265

4626 / 0.0317

4649 / 0.0357

4667 / 0.0270

1.013

1.048

1.003

1.025

0.0151, 0.0577

0.0151, 0.0389

0.0207, 0.0667

0.0142, / 0.0492

0.0158, 0.0582

0.0162, 0.0392

0.0230, 0.0686

0.0149, 0.0495

2

2 2

2 2 1/2

R1 = Σ(||Fo| – |Fc||)/Σ|Fo|; wR2 = {Σw(|Fo| – |Fc| ) /Σw(|Fo| ) } .

Luminescence sensing experiments The luminescence properties of 1 and 4 were investigated in the solid state and in various solvent emulsions at room temperature. The 1 (or 4) –solvent emulsions were prepared by introducing 2 mg of 1 (or 4) crystalline powder into 2.0 mL of different solvents, including H2O, methanol (MeOH), ethanol (EtOH), propanol (PrOH), isopropanol (i−PrOH), toluene, CH2Cl2, CHCl3, CH3CN, benzene and benzaldehyde, and ultrasonicated for 30 min, and then formed stable suspensions for fluorescence study. For the sensing of ions and anions, 2 mg powder sample of 1 (or 4) was dispersed in an aqueous solution (2 mL) of MClx (Mx+ = Na+, K+, Mg2+, Ba2+, Co2+, Ni2+, Cu2+, Fe3+ and Cr3+, 1 × 10−2 mol L−1) or KyX (X = Cl−, Br−, I−, NO3−, CO32−, SO42−, CrO42−, and Cr2O72−, 1 × 10−2 mol L−1) at room temperature. 6


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Typical procedure for fluorescence titration studies For sensing properties with respect to benzaldehyde, different amounts of 0.01 M benzaldehyde were introduced into a 1 (or 4) emulsion in MeOH solution. For the sensing of ions and anions, the powder sample of 1 (or 4) was dispersed in 2 mL solution of Cr(III) and Cr(VI) ions with different concentrations and subsequently transferred to a quartz cell of 1 cm width to carry out the quenching tests. Recyclable luminescence experiments The reproducibility of 1 and 4 towards sensing Cr3+, CrO4−, Cr2O72− or benzaldehyde was studied. After the first quenching experiment, the powder of 1 (or 4) was recovered by centrifugation and washed with water (Cr3+, CrO42− and Cr2O72−) or MeOH (benzaldehyde). The recovered solid was collected and subsequently used in the successive quenching experiments.

Results and discussion Description of the crystal structure of 1 and 4 The single crystal X−ray diffraction studies performed on 1 − 4 reveal that all four Ln–MOFs are isostructural 2D coordination frameworks and crystallizes in monoclinic P21/c space group. Compound 4 is described as a representative example. The asymmetric unit of 4 contains one terbium(III) ion, one L2− ligand, one coordinated H2O molecule and one formate anion which may be obtained in situ during the solvothermal reaction due to the decomposition of DMF.26,56,57 The Tb(III) center is eight−coordinated with a distorted triangular dodecahedron geometry defined by four O atoms from four different L2− carboxylate ligands, three O atoms from two formate anions and one O atom from an individual water molecular (Figure 1a). Tb−Ocarboxylate bond lengths vary from 2.3393(12) to 2.5448(12) Å and Tb−Owater bond length is 2.4834(12) Å, which are comparable to the reported Tb3+ complexes with carboxylic acids ligands and water molecules.58−62 L2− and formate ligand adopt µ4−η1:η1:η1:η1 (Figure 1b) and µ2−η1:η2 (Figure 1c) coordination fashion, respectively. The dihedral angle between benzene ring containing cyano group and the central phenyl ring is 40.421° and the carboxylate−containing phenyl ring is twisted by 7



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8.511° from the central benzene ring. Furthermore, a 1D chain is formed by connecting Tb3+ ions via the bridging carboxylate of L2− ligand (Figure S1a) and these chains are further linked by the µ4−bridging ligands to generate a 2D layer structure (Figure 2a). Finally, 3D supramolecular structure is formed through Owater–H···NCN hydrogen bonds interactions (O(1W)−H(1WB) 0.83 Å, H(1WB)···N(1) 2.04 Å, O(1W)···N(1) 2.866 Å, and O(1W)−H(1WB)···N(1) 173.1°) and intermolecular π···π interactions between phenyl moieties of the adjacent layers. The distances between the centroids of adjacent phenyl rings are 3.6702 and 3.6773 Å (Figure S1b and Table S2). To further explore the crystal structure of 4, TOPOS software was used to analysis the topology of the 2D framework.63,64 The structure of compound 4 can be simplified as a single nodal sql network with the point symbol of {44·62} (Figure 2b).

Figure 1 (a) Representation of the coordination environments of the Tb3+ centre with hydrogen atoms omitted for clarity (Symmetry code: #1 x, −y+1/2, z+1/2; #2 −x+1, −y+1, −z+3; #3 −x+1, y−1/2, −z+5/2; #4 x, −y+1/2, z−1/2.); (b) the coordination mode of L2− ligand; (c) the coordination mode of formate anion.

8


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Figure 2 (a) the overall 2D layered network of complex 4; (b) Topology of complex 4.

PXRD and thermogravimetric analysis of complexes 1 – 4 The PXRD patterns for compounds 1 – 4 are consistent with those simulated from single crystal structure data, illustrating that bulk samples of compounds 1 – 4 are pure phases (Figure S2). The thermogravimetric analysis (TGA) data of 1 – 4 show a resembling thermal behavior owing to their structural similarity (Figure S3). Herein, only the weight loss process of compound 4 is described in detail. With the increasing temperature, compound 4 is stable up to ca. 128 °C and then displays a weight loss of approximately 2.82% (calc. 3.03%) in the 128 – 165 °C range corresponding to the loss of one coordinated aqua ligand. The coordination framework of 4 remained intact in the range of 165 – 395 °C. With further increase of temperature, the decomposition of coordination framework occurred in the range of 395 – 600 °C. The residual 31.26% after 600 °C is attributed to Tb2O3 (calc. 30.83%). Luminescent Properties The solid−state luminescent properties of compounds 1 and 4 were measured at room temperature. Compound 1 exhibits an intense red light when excited at 348 nm. The characteristic emission peaks centered at 591, 613, 651, and 698 nm are ascribed to the transitions 5D0 → 7FJ (J = 1, 2, 3 and 4) of the Eu3+ ion (Figure S4a). Among these emission peaks, the emission at 613 nm from the 5D0 → 7F2 induced by electronic dipole transition is the strongest. As shown in Figure S4b, the emission spectrum of compound 4 shows the characteristic emission and emits a bright green light when excited at a wavelength of 359 nm. The emission bands at 490, 546, 583 and 620 nm 9



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are assignable to the transitions 5D4 → 7F6, 5D4 → 7F5, 5D4 → 7F4 and 5D4 → 7F3, respectively. The 5D4 → 7F5 transition is the most prominent in all emission peaks. Sensing of organic small molecules Because compounds 1 and 4 displayed the representative emissions from the 5D0 → 7

FJ (J = 1, 2, 3 and 4) transitions of Eu3+ ions and the 5D4 → 7FJ (J = 6, 5, 4 and 3)

transitions of Tb3+ ions, compounds 1 and 4 may be potential luminescent MOFs materials for sensing small organic molecules. The PXRD patterns of both compounds after being immersed in water and other common solvents demonstrated that the coordination frameworks were almost unchanged compared to the simulated ones (Figure S5).

Figure 3 Emission spectra of 1 (a) and 4 (b) dispersed in different solvents.

As shown in Figure 3 and Figure S6, the suspensions of compounds 1 and 4 still display the characteristic emission peaks of corresponding Ln3+ ions in various solvents while the emission intensities of the emulsions are largely dependent on the types of solvents. Compounds 1 and 4 have the same order of luminescence intensities 10


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in different solvents: H2O > MeOH > EtOH > PrOH > i−PrOH > Toluene > CH2Cl2 > CHCl3 > CH3CN > Benzene > Benzaldehyde. Interestingly, the luminescence signals of 1 and 4 almost disappeared when dispersed in benzaldehyde. These results reveal that compounds 1 and 4 can be served as luminescent sensors for selectively sensing benzaldehyde molecules. To examine the relationship between the luminescence intensity of 1 and 4 and the concentration of benzaldehyde in detail, the suspensions of 1 or 4 in MeOH with gradually increasing benzaldehyde concentrations were prepared, and the corresponding emission spectra were recorded. It can be clearly seen that the luminescence intensity slowly decreases with increasing benzaldehyde concentration (Figure 4a and 4b). To further understand the results, the luminescence quenching efficiency can be calculated by using the Stern–Volmer (SV) equation: I0/I = 1 + Ksv[A], in which Ksv is the quenching constant (M−1), I0 and I are the luminescence intensities before and after the addition of benzaldehyde and [A] is the molar concentration of benzaldehyde (mM), respectively.65,66 The Stern–Volmer plots display almost linear relationship for benzaldehyde at low concentrations, with the Ksv values of 1294.6 and 988.6 M−1 for 1 and 4, respectively (Figure 4c and 4d). The Stern−Volmer curves deviate from the linear correlation with the increasing concentration (Figure S7), illustrating the occurrence of both static and dynamic quenching simultaneously. Further detailed analysis denotes that the detection limits for benzaldehyde are 11 and 13 µmol L–1 for 1 and 4, respectively, according to 3δ/k (δ: standard error; k: slope) (Table S3, Figure S8 and S9).67−70 The recyclable performance of compounds 1 and 4 for detection of benzaldehyde was studied. After 5 times recycling, the luminescence intensity and quenching ability of compounds 1 and 4 exhibited negligible changes (Figure S10).

11



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Figure 4 Emission spectra of 1 (a) and 4 (b) dispersed in MeOH solutions with different concentrations of benzaldehyde; The Stern–Volmer plot of I0/I versus the concentration of benzaldehyde for compounds 1 (c) and 4 (d) at low concentration.

Up to now, the mechanism for such quenching effects of small solvent molecules has already been studied.71,72 The possible reasons of luminescence quenching caused by benzaldehyde are discussed as follows: 1) The stability of 1 and 4 in different solvents was confirmed by PXRD patterns (Figure S5). These results proved that the frameworks of 1 and 4 remained unchangeable indicating that the luminescence quenching was not caused by the collapse of coordination framework; 2) Benzaldehyde has no absorption band in the range of 300 to 750 nm in the UV/Vis spectra (Figure S11), indicating that the competing absorption is not the reason for quenching; 3) The high quenching efficiency of 1 and 4 towards benzaldehyde may be due to the weak hydrogen bonding interaction between benzaldehyde and coordination frameworks, which will decrease the ligand–metal energy transformation (LMET) efficiency and lead to a quenching effect on the luminescent intensity, according to the reported literature.73−75 Thus, we can speculate that H−bond interactions between guest benzaldehyde molecules and host coordination frameworks have an effect on the energy transfer efficiency from the ligands to Ln3+ ions, leading to fluorescence quenching. 12


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Metal Ions Detection Considering that 1 and 4 possess excellent stability in water system (Figure S5), both compounds have been employed to investigate the sensing ability for metal ions in aqueous solution as potential fluorescent sensors. A series of luminescent response experiments were carried out in aqueous solution of various metal ions as shown in Figure 5. It was found that Na+, K+, Mg2+, Ba2+, Co2+, Ni2+, Cu2+ and Fe3+ ions decreased luminescence intensity to a different extent and only Cr3+ caused a significant emission quenching effect on luminescence of compounds 1 and 4, implying that the 1 and 4 can be regarded as hopeful candidates for selective recognizing of Cr3+ ion. In order to examine the sensitivity of 1 and 4 toward Cr3+ ion, 1 and 4 were dispersing in different concentrations of Cr3+ ions to form a series of suspensions and the luminescence spectra were recorded (Figure 6). The luminescence intensities gradually decreased upon the increasing concentration of Cr3+ ions. Furthermore, the quenching effect can be analyzed by the Stern−Volmer equation: I0/I = 1 + KSV[A].65,66 As shown in Figure 6c and 6d, the Stern−Volmer quenching curve for Cr3+ ion is nearly linear at low concentrations and the values of KSV are 1356.9 and 999.5 M−1 for 1 and 4, respectively. The Stern−Volmer curves deviate from the linear correlation with the increase of concentration (Figure S12), illustrating the existence of both static and dynamic quenching processes. The calculated detection limit of Cr3+ ion are 15 and 19 µmol L–1 for 1 and 4, respectively (Table S3), according to 3δ/k.67−70 Moreover, the interference experiment from common metal cations was carried out and the results indicate that other cations have no or minor effect on the detection of Cr3+, further confirming that 1 and 4 can selectively and sensitively sense Cr3+ ion in water (Figure S15). In addition, 1 and 4 can be recycled at least five times by centrifuging the suspension and used after washing several times with water. The luminescence intensity and quenching ability of 1 and 4 exhibited negligible changes (Figure S16).

13



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Figure 5 Emission spectra of 1 (a) and 4 (b) dispersed in various metal ions.

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Figure 6 Emission spectra of 1 (a) and 4 (b) dispersed in aqua solutions with different concentrations of Cr3+; The Stern–Volmer plot of I0/I versus the concentration of Cr3+ for compounds 1 (c) and 4 (d) at low concentration.

The possible quenching mechanism of 1 and 4 in the presence of Cr3+ ion is analyzed. After immersing 1 or 4 in water solution of Cr3+, the PXRD patterns were measured. As shown in Figure S17, the PXRD pattern of Cr3+−1 (or 4) remained largely consistent with the simulated one. Thus, the collapse of the crystal structure is not the reason for the luminescence quenching. As shown in Figure S11, there is the partial overlap between the absorption plot of Cr3+ and the excitation bands of 1 or 4, consequently, the solution of Cr3+ ions may absorb the energy of the excitation wavelength, resulting in the luminescence quenching of 1 and 4 to a certain degree. In addition, because the cyano group of L2− ligand remains uncoordinated, we speculate that the weak coordination interaction between Cr3+ and uncoordinated cyano group may lead to the energy migration and luminescence quenching.76−78 To confirm this speculation, X-ray photoelectron spectroscopy (XPS) measurement was carried out. The N1s peak from free cyano group nitrogen atoms at 399.9 eV in 4 is shifted to 400.3 eV induced by addition of Cr3+ in 4, suggesting the weak interaction between Cr3+ cations and cyano group basic sites in 4/Cr3+ (Figure S18). In a word, energy absorption and weak coordination interaction may have a synergistic effect on the quenching of fluorescence intensity. Anions Detection We also investigated the effects of different water solutions of potassium salts KyX (X = Cl−, Br−, I−, NO3−, CO32−, SO42−, CrO42− and Cr2O72−) on the fluorescence intensities of 1 and 4. Interestingly, CrO42− and Cr2O72− anions have a striking quenching effect on the fluorescence of 1 and 4 whereas other anions have no or only minor effects on the fluorescence intensity (Figure 7). To examine the selectivity of 1 and 4, the interference experiment is carried out (Figure S19 and S20). The fluorescence intensity decreased immediately after adding CrO42− or Cr2O72− anions into the solution of other anions, further confirming that 1 or 4 can be excellent candidates for sensing CrO42− or Cr2O72− anions in water. 15



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Figure 7 Emission spectra of 1 (a) and 4 (b) dispersed in various anions.

Furthermore, the titration experiments were carried out to investigate sensitivity by adding the finely grounded powder 1 or 4 into water solution of CrO42− or Cr2O72− ions with varying concentrations. The emissive curves with different concentrations were recorded; as expected, significant fluorescence quenching were observed with the increasing concentration of Cr(VI) (CrO42− and Cr2O72−) anions in aqueous solution (Figure 8 and S21). Moreover, the Stern−Volmer plots for Cr(VI) anions exist a good linear correlation at low concentration (Figure 8 and S21). The values of KSV are calculated to be 1537.4 (1 for CrO42−), 2762.6 (1 for Cr2O72−), 1307.0 (4 for CrO42−) and 2133.5 (4 for Cr2O72−) M−1, respectively. The plots of all titration experiments deviate from linearity with the concentration increasing (Figure S22 and S23), demonstrating that both static and dynamic quenching happened simultaneously. The detection limits which are calculated according to a ratio of 3δ/k67−70 reach as low as 12 (1 for CrO42−), 10 (1 for Cr2O72−), 18 (4 for CrO42−) and 21 (4 for Cr2O72−) µmol 16


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L–1, respectively (Figure S24−S27). Specifically, a comparison of other MOFs−based fluorescent sensors for Cr(III) and Cr(VI) ions sensing is listed in Table S3. Additionally, recyclability experiments on the detection of CrO42− and Cr2O72− for 1 and 4 were carried out. Fortunately, the tested samples can be easily recovered for at least five times by centrifuging the prepared suspensions and washing with deionized water several times (Figure S28). These results reveal that both 1 and 4 are very intelligent sensors for CrO42− and Cr2O72− ions. The PXRD patterns after soaking in the solutions of Cr(VI) anions were in agreement well with the as−synthesized sample, illustrating that the crystal frameworks of 1 and 4 remained intact after sensing experiments (Figure S29).

Figure 8 (a) and (b) Emission spectra of 1 dispersed in aqueous solutions with different concentrations of CrO42− and Cr2O72−; (c) and (d) The Stern–Volmer plot of I0/I versus the concentration of CrO42− and Cr2O72− for compound 1 at low concentration.

The possible sensing mechanism of luminescence quenching by CrO42− and Cr2O72− anions was further investigated. The PXRD patterns of 1 or 4 after immersing in the aqueous solution of CrO42− or Cr2O72− were in agreement well with the simulated one (Figure S29), implying that the framework of 1 or 4 remained unchanged. Thus, the luminescence quenching resulted from the collapse of the crystal structure can be excluded. On the other hand, the UV−vis spectra of Cr(VI) anions in aqueous solution 17



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were recorded (Figure S11). As shown in Figure S11, the absorption bands of CrO42− or Cr2O72− shows extensive overlap with the excitation bands of 1 or 4. Thus, the energy of the excitation light will be strongly absorbed by CrO42− or Cr2O72−, reducing the efficiency of energy transfer from the ligand to the lanthanide ions, ultimately leading to luminescence quenching.79−82

CONCLUSIONS In summary, a series of novel 2D Ln–MOFs have been successfully obtained, which further expands to a 3D supramolecular network via H−bond and π···π interactions. Compounds 1 and 4 not only possess excellent solvents stability and thermostability but also can be considered as excellent luminescent sensors for the selective and sensitive detection of benzaldehyde, Cr(III) and Cr(VI) ions. Moreover, 1 and 4 can be regenerated easily and quickly, exhibiting excellent recyclable performance for the detection of benzaldehyde, Cr3+, CrO42− or Cr2O72− ions. To the best of our knowledge, compounds 1 and 4 are the first Ln–MOFs materials which can detect chromium ions in different chemical valences in aqueous solution and benzaldehyde in methanol solution at the same time. The low detection limits and high quenching constants KSV indicate

that

1

and

4

may

potentially

be

acted

as

multi−responsive

luminescence−based sensors for quantitative and highly sensitive detection of benzaldehyde, Cr(III) and Cr(VI) ions.

ASSOCIATED CONTENT Supporting Information X-ray crystallographic files (CIF) of Ln–MOFs 1−4, tables of selected bond lengths and angles, Powder X-ray diffraction patterns, TGA curves, additional crystal structure figures, absorption and emission spectra, quenching-efficiency plot, Stern−Völmer plots. This material is available free of charge via the Internet at http://pubs.acs.org. Accession Codes CCDC 1546699−1546702 contains the supplementary crystallographic data for this paper.

These

data

can

be

obtained

free

18


of

charge

via

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www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION Corresponding Author *E−mail: [email protected] ORCID Licun Li: 0000-0001-8380-2946 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (No. 21471083) and the MOE Innovation Team (IRT13022) of China.

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For Table of Contents Use Only

Multi−responsive

luminescent

sensors

based

on

2D

lanthanide−metal organic frameworks for highly selective and sensitive detection of Cr(III), Cr(VI) ions and benzaldehyde Zan Sun, Meng Yang, Yue Ma, Licun Li*

Two novel lanthanide(III)−matal organic frameworks (Ln = Eu and Tb) exhibit multi−responsive luminescent sensors for the highly selective and sensitive detection of Cr3+, CrO42−, Cr2O72− ions and benzaldehyde through luminescent quenching effect.

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Multi-Responsive Luminescent Sensors Based on Two-Dimensional

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