Boric-Acid-Functional Lanthanide Metal–Organic Frameworks for

Dec 23, 2016 - Zhong-Rui Yang†, Man-Man Wang‡, Xue-Sheng Wang‡, and Xue-Bo Yin†§. † State Key Laboratory of Medicinal Chemical Biology and ...
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Boric Acid-Functional Lanthanide Metal-Organic Frameworks for Selective Ratiometric Fluorescence Detection of fluoride Ions Zhong-Rui Yang, Man-Man Wang, Xue-Sheng Wang, and Xue-Bo Yin Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 23 Dec 2016 Downloaded from http://pubs.acs.org on December 24, 2016

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Analytical Chemistry

Boric Acid-Functional Lanthanide Metal-Organic Frameworks for Selective Ratiometric Fluorescence Detection of Fluoride Ions †

‡

‡

Zhong-Rui Yang, Man-Man Wang, Xue-Sheng Wang, and Xue-Bo Yin

†,§,

*

†

State Key Laboratory of Medicinal Chemical Biology and Tianjin Key Laboratory of Biosensing and Molecular Recognition, College of Chemistry, Nankai University, Tianjin, 300071, China ‡

School of Public Health, North China University of Science and Technology, Tangshan 063000, Hebei, China

§

Collaborative Innovation Center of Chemical Science and -Engineering (Tianjin), Nankai University, Tianjin, 300071, China * E-mail: [email protected]; Fax: +86-22-23503034 ABSTRACT: Here we report that boric acid is used to tune the optical properties of lanthanide metal-organic frameworks (LMOFs) for dual-fluorescence emission and improves the selectivity of LMOFs for the determination of fluoride ions. The LMOFs are prepared with 5-boronoisophthalic acid (5-bop) and Eu3+ ions as the precursors. Emission mechanism study indicates that 5-bop is excited with UV photon to produce its triplet state, which then excites Eu3+ ions for their red emission. This is the general story of antenna effect, but electron-deficient boric acid decreases the energy transfer efficiency from the triplet state of 5-bop to Eu3+ ions, so dual emission from both 5-bop and Eu3+ ions is efficiently excited at the single excitation of 275 nm. Moreover, boric acid is used to identify fluoride specifically as free accessible site. The ratiometric fluorescent detection of fluoride ions is validated with the dual-emission at single excitation. The LMOFs are well monodisperse, so the determination of aqueous fluoride ions is easily achieved with high selectivity and low detection limit (2 µM). For the first time, we reveal that rational selection of functional ligands can improve the sensing efficiency of LMOFs through tuning their optical property and enhancing the selectivity toward targets.

emission profile, such as the simultaneous emission from both the ligand and Ln ions in single LMOFs.

INTRODUCTION As a promising subfamily of metal-organic frameworks (MOFs), lanthanide MOFs (LMOFs) show unique luminescent properties, such as large Stokes shift, bright visible emission, long decay lifetime, and undisturbed emissive energy particularly those with europium and terbium as metal nodes.1,2 Various sensitizing chromophores have been selected to coordinate and excite Ln ions, so a wide spectrum from blue to infrared light is observed.3-6 LMOFs have been regarded as promising sensing matrix, but previous works mainly focused on the single emission from lanthanide ions through the sensitization of ligands.5 The single emission may be interfered by the excitation intensity, emission collection efficiency, probe and analyte concentration for the sensing purpose.7 Ratiometric fluorescence alleviates the problem and possesses excellent quantitative ability by the use of dual-fluorescence emission.811 If single-excitation achieves the dual-fluorescence, perfect inner-reference is obtained.12

Selecting an organic ligand possessing desired functional group is the key to tune the energy gap. If the functional group can recognize a given target simultaneously, the identification selectivity of the LMOFs could be also improved.15-17 So far, the ligands containing -NH2, -OH, -Br, -NO2, and -SO3 have been used to construct MOFs,18-21 but the groups showed little effect on the π-conjugated system of the host to tune the optical property of LMOFs. Moreover, the natural tendency of metal ions to coordinate with amino- and other groups makes the preparation of MOFs containing free functional sites difficult.2,22 Boric acid can identify diol and Lewis base specifically, such as fluoride and cyanide anions.23,24 Moreover, boric acid group is electron-deficient and could control the energy level of π-conjugated ligand obviously. Therefore, it is rational to surmise that boric group could tune the energy level of the ligand for unique fluorescence efficiency of LMOFs, such as the dual-emission from single excitation reported thereafter. Boric acid is also potentially promising to selectively recognize diol and Lewis base. However, no boric acidfunctional ligand has been reported to prepare functional MOFs for the sensing purpose to the best of our knowledge.

The aromatic chromophore of ligands absorbs ultraviolet radiation to excite Ln ions through the energy transfer from the triplet state (T1) of ligand to Ln ions.13 However, if the aromatic ligands are modified with functional groups, the energy level of their singlet state (S1) and/or T1 may be tuned.14 The energy transfer efficiency could be changed for specific 1

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In this work, we synthesize Eu-MOF 1 with Eu3+ ions as metal node and 5-boronoisophthalic acid (5-bop) as functional ligand. The introduction of boric acid group tuned the energy level, so Eu-MOF 1 showed two emissive centers at 320-500 nm (maximum at 366 nm) and 590-750 nm (maximum at 625 nm) at the single excitation of 275 nm. Eu3+ ions and 5-bop played an important role for the dual-emission as validated with its counterparts: Eu-MOF 2 prepared from Eu3+ ions and isophthalic acid (isp) and Tb-MOF 1 and 2 with Tb3+ ions as metal node as well as 5-bop and isp as ligands. All of the counterparts showed single emission from the metal ions. The dual-emission mechanism was proposed in combination of T1 energy level of the ligands and the energy gap between T1 and Ln3+ ions. Fluoride is one of the most toxic and dangerous elements because of its threat to human health with dental and skeletal fluorosis even at its low concentration.25,26 In combination of high affinity of boric group toward fluoride, ratiometric fluorescence for the selective detection of fluoride ions was illustrated with the dual-emission of Eu-MOF 1 at the single excitation of 275 nm. Previous fluoride sensing is generally carried out in organic media.27-31 Our Eu-MOF probe was well dispersed for the determination of aqueous fluoride ions with high selectivity and low detection limit (2 µM). To our knowledge, this is the first example that boric group was used to tune the energy level of the ligand and identify fluoride specially for ratiometric fluorescence determination. With the dual-emission, the bright red luminescence faded away with the decreased intensity gradually, while the color changed from red to blue along with the increased fluoride concentration. The change in color and intensity was easily distinguished by naked eyes compared with the single intensity change from Ln ions. The result validated that the introduction of boric acid is a promising strategy to achieve ratiometric fluorescence and improve the sensitivity and selectivity of LMOFs. The other functional ligands could be stimulated to build functional MOFs to tune the energy level for interesting emission and improve sensing selectivity.

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by Bruker TENSOR 27 Fourier transform infrared spectroscopy. X-ray photoelectron spectroscopy (XPS) analysis was performed by Kratos Axis Ultra DLD spectrometer fitted with a monochromated Al-Kα X-ray source (1486.6 eV), hybrid (magnetic/electrostatic) optics, and a multi-channel plate and delay line detector. Powder X-ray diffraction (PXRD) patterns were recorded at room temperature (298 K) with a D/max-2500 X-ray diffractometer (Rigaku, Japan) using CuKα radiation (λ=1.5418 Å). High resolution transmission electron microscopy (HRTEM) were recorded with Tecnai G2F20, FEI Co. (America) operated at an accelerating voltage of 200 kV. The morphologies of the samples were characterized by scanning electron microscopy (SEM, JEOL JSM7500F). Thermogravimetric analyses were performed on a Netzsch TG 209 TG-DTA analyzer from room temperature to 700 °C with the heating rate of 15 °C min-1. Synthesis of boric acid-functional MOFs of Eu-MOF 1 and Tb-MOF 1. Boric acid-functional Eu-MOF 1 were synthesized by simple solvothermal method. A mixture of EuCl3·6H2O (36.6 mg, 0.1 mmol) and 5-bop (four different reactant concentrations based on at 0.05, 0.5, 10, and 20 mmol L-1 5-bop) were vigorously stirred for 2 h in DMF/H2O (7:3) solution. Then, the mixed solution was transferred into a Teflon vessel in a stainless steel autoclave, heated at 150 °C for 2, 4, 8, 12, and 24 h. The mixture was cooled to room temperature and the products were collected after centrifugation, washed thoroughly with DMF and ethanol, and dried at the room temperature. Eu-MOF 1 was white powder. Tb-MOF 1 and Gd-MOF were synthesized in the similar procedure as Eu-MOF 1, except for the use of TbCl3·6H2O (0.10 mmol, 37.3 mg) and GdCl3·6H2O (0.10 mmol, 37.1 mg) as the metal sources, respectively. Synthesis of Eu-MOF 2 and Tb-MOF 2. Eu-MOF 2 [Eu2(isp)3(H2O)2] was synthesized by a solvothermal method similar to Eu-MOF 1. Briefly, 0.1 mmol (36.6 mg) of EuCl3·6H2O and 0.1 mmol (16.6 mg) of isophthalic acid (isp) were dissolved in 8 mL of DMF/H2O (7:3) mixture. Then, the mixture was stirred for 2 h and transferred into a Teflon reactor autoclave. The mixture was kept at 150 °C in oven for 12 h and cooled to room temperature. The products were collected by centrifugation, washed with DMF and ethanol, dried at the room temperature. Eu-MOF 2 was white powder, the same as Eu-MOF 1. Tb-MOF 2 were synthesized in a similar procedure to Eu-MOF 2, except for the use of TbCl3·6H2O (0.10 mmol, 37.3 mg) as the metal source.

EXPERIMENTAL SECION Reagents and Chemicals. Europium (III) chloride hexahydrate (EuCl3·6H2O), terbium (III) chloride hexahydrate (TbCl3·6H2O), gadolinium (III) chloride hexahydrate (GdCl3·6H2O) was obtained from Sigma–Aldrich, Shanghai, China. Isophthalic acid (isp) was brought from Shanghai Macklin Biochemical Co. Ltd. 5-Boronoisophthalic acid (5bop) was purchased from HWRK Chem Co., Ltd., Beijing, China. Fluoride standard (1000 mg L-1) was obtained from Macklin Biochemical Co., Ltd., China, Shanghai. Sodium chloride, potassium iodide, sodium sulfate, sodium hydrogen carbonate, sodium nitrate, aluminum sulfate, magnesium sulfate, copper sulfate, calcium sulfate, zinc sulfate, ferric chloride were obtained from Municipality kemi’ou Chemical Reagent Co. (Tianjin, China). All the chemicals were obtained at least of analytical grade and used without further purification. Ultrapure water was prepared with an Aquapro system (18.25 MΩ cm).

Ratiometric Detection of Fluoride Ions. The solution of EuMOF 1 (0.2 mg mL-1) was added to a clean plastic centrifuge tube. Then, 0.1 mM freshly prepared NaF solution was added drop-wise to make the final concentration ranged from 0 to 500 µM with the final volume of 4 mL. The mixture solution was mixed well and their fluorescence spectra were recorded under the excitation at 275 nm after 1 min incubation. Determination of Fluoride in River and Underground Water Samples. River water was collected from the Weijin River (Tianjin, China) and filtered with 0.22 µm membranes to remove impurities. Underground water samples (The samples contain fluoride, arsenic, and a large number of chloride and sulfate ions) were obtained from Shanxi, China. A series of water samples containing different concentrations of fluoride were prepared by adding different volumes of 1000

Instrumentation and Characterization. Steady-state fluorescence experiments were performed by a Hitachi FL4500 Fluorescence Spectrometer. Fourier transform infrared (FT-IR) spectra (KBr pellet) were measured (400-4000 cm-1) 2

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mg L-1 fluoride standard solution. The fluorescence spectra were recorded after 1 min incubation for ratiometric detection.

Eu-MOF 1, Eu-MOF 2, and Eu-MOF 1 soaked in water for two weeks; (F) FTIR spectra of ligands, Eu-MOF 1 and 2.

RESULT AND DISCUSSION

Their powder X-ray diffraction (PXRD) patterns showed the same structure from Eu-MOF 1 and 2 (Figure 1E). Moreover, their patterns matched well with the simulated result from single crystal date of Eu-MOF 2. Thus, Eu-MOF 1 and 2 were isostructural and phase-pure with high crystallinity. Boric group therefore did not coordinate to Eu3+ ion, and as free accessible site, did not affect the structure of Eu-MOF 1. Fourier transform infrared spectra (FTIR) spectra of isp, 5-bop, Eu-MOF 1, and 2 were recorded (Figure 1F). The peak at 1695 cm-1 was the C=O stretching vibration (νC=O) of the ligands, but disappeared in Eu-MOFs, so carboxylate groups coordinated to Eu3+ to form Eu-MOFs. B-O absorption peak was observed at 1313 cm–1 in 5-bop and Eu-MOF 1, clearly indicating the successful preparation of Eu-MOF 1 with free boric acid site.

Preparation and Characterization of LMOFs. Boric acidfunctional Eu-MOF 1 was prepared with 5-boronisophthalic acid (5-bop) and EuCl3 by simple solvothermal method. The nanospheres of Eu-MOF 1 were obtained, while their size could be adjusted from 200 nm to 1.30 µm with the concentration of 5-bop ranged from 0.5 to 20 mmol L-1 (Figure S1 and S2). We also investigated the effect of reaction time, but the size of the nanospheres did not change remarkably (Figure S3). However, the yield of Eu-MOF 1 increased along with the increased reaction time. Therefore, the reaction time of 12 h and the 5-bop concentration of 10 mmol L-1 were selected for the further work to prepare Eu-MOF 1 considering the suitable size and high efficiency. As a comparison, Eu-MOF 2 was prepared with Eu ions as metal node and isophthalic acid (isp) as the ligand under the same condition. To validate the emission mechanism, TbMOF 1 or 2 were also prepared with Tb as metal node and 5bop or isp as ligand, respectively. Both Eu-MOF 1 and 2 exhibited spherical structure with almost the same size and uniform distribution as observed in SEM images with the average diameters of about 780 nm and 760 nm, respectively (Figure 1). The Tb-MOFs showed the same structure as EuMOFs (Figure S4). All of the results illustrated that the introduction of boric acid in ligand did not interfere with the size and morphology of the as-prepared LMOFs. Highresolution TEM images illustrated that the MOFs were well dispersed (inset of Figure 1A and 1B). Fast diffusion kinetics is expected for fast response of fluoride ions because of the nanosize and spherical structure of Eu-MOF 1.

Figure 2. (A) The coordination environments of the two kinds of independent Eu3+ ions in the building unit of Eu-MOF 2; (B) Coordination geometries of Eu1 and Eu2 ions; (C) Different coordination modes of isp; (D) Perspective view of the 2-D sheet, which contained from 1D rod-like chain; (E) Ribbonlike structure of Eu-MOF 2 along b-axis. Because Eu-MOF 1 and 2 were isostructural and Eu-MOF 1 formed single-crystal difficultly, single-crystal X-ray diffraction (XRD) pattern of Eu-MOF 2 was selected to reveal the crystallization and composition of the Eu-MOFs. Monoclinic system was observed with space group p21/n (Table S1 and S2). The fundamental building unit of EuMOFs contained two kinds of independent Eu atoms, three deprotonated isp ligands and two coordinated H2O molecules with the formula of Eu2(isp)3(H2O)2 (Figure 2). Surrounding Eu1 atoms, there were seven oxygen atoms from six isp and one H2O molecule with seven-coordinated pentagonal bipyramid geometry. Eu2 atoms adopted nine-coordinated and the coordination polyhedron was described as a distorted tricapped trigonal prism with nine oxygen atoms from six isp and one H2O molecule (Figure 2B). Two kinds of unique isp linkers were observed (Figure 2C). Eu1 and Eu2 were arranged alternately and linked by the discrete carboxylate group of isp, resulting in an infinite 1D rod-like chain (Figure S5). The adjacent 1D chain was connected through the ligands to form 2D layer structure (Figure 2D and 2E). The 2D layers further connected to each other for the 3D extended framework with one-dimensional channel (Figure S6). TGA results revealed that the frameworks of Eu-MOF 1 and 2 are stable until to 427 °C (Figure S7).

Figure 1. Characterization of LMOFs. SEM images of (A) Eu-MOF 1 and (B) 2; Size distribution of (C) Eu-MOF 1 and (D) 2 obtained from the SEM images in (A) and (B); (E) Powder XRD patterns of simulation from Eu2(isp)3(H2O)2 of CIF, 3

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triplet excited-state (T1) of isp and 5-bop were investigated (Table S3 and Figure S10). The S1 of 5-bop was 35335 cm−1 (283 nm), while isp was 35714 cm−1 (280 nm) obtained from their UV–vis absorption spectra (Figure. S10). Thus, almost the same excitation wavelength was observed from isp and 5bop. However, T1 of 5-bop and isp was 23923 cm-1 (418 nm) and 22831 cm-1 (438 nm) based on the phosphorescence of isostructural gadolinium-5-bop or -isp MOFs at 77 K, respectively (Table S3 and Figure S10). The T1 of isp and 5bop matches the energy levels of Eu3+ (5D0, 18674 cm−1) and Tb3+ (5D4, 20500 cm−1) to sensitize Eu3+ and Tb3+ efficiently. The energy gap between T1 of the ligands and Ln ions was listed in Table S3 and that between the T1 of 5-bop and Eu3+ was maximum. Thus, the gap causes incomplete energy transfer for the dual emission from Eu-MOF 1. Tb3+ ions show higher energy than Eu3+ ions, so the T1 of both 5-bop and isp showed higher match degree to Tb3+ ions. Only the characteristic emission of the metal ions was observed in EuMOF 2 and two Tb-MOFs. Thus, the introduction of boric acid group tunes the energy efficiency for incomplete energy transfer in Eu-MOF 1 and the dual emission was therefore observed at the single excitation of 275 nm.

Optical Properties. The absorption spectra of 5-bop and EuMOF 1 were recorded. Both Eu-MOF 1 and 5-bop exhibited similar absorption profile where the broad peak observed around 260-320 nm was derived from the π-π* transition of 5bop (Figure S8). The slight red-shift in the spectrum of EuMOF 1 was attributed to the extensive π-conjugated system formed between Eu3+ and 5-bop. The fluorescence spectra of 5-bop and EuCl3 were investigated to illustrate the emission property of Eu-MOF 1 (Figure S9). Upon excitation at 275 nm, 5-bop exhibited a strong emission at 320 nm with the peak width at half height (PWHH) of 40 nm. Eu3+ ions showed weak emission with four peaks at the excitation of 395 nm. The four peaks were attributed to the 5D0→7F1 (592 nm), 5D0 →7F2 (625 nm), 5D0→7F3 (653 nm), and 5D0→7F4 (700 nm) transitions as the characteristic emission of Eu3+ (Figure S9).3234 The f-f forbidden transition of Eu3+ ions weakens their light absorption, so the direct excitation of Eu3+ ions was less efficient for the weak emission observed from free ions.35,36 LMOFs have been extensively used as sensing platform with the luminescence of lanthanide ions, but few works paid close attention to the fluorescence of ligand.37 As we known, the single emission is generally influenced by the testing conditions.7 Thus, it is necessary to select the appropriate ligands to tune the energy transfer efficiency for specific dualemission profile. We investigated the fluorescence of Eu-MOF 1 and 2, Tb-MOF 1 and 2 (Figure 3). All of the four LMOFs were efficiently excited at 275 nm and the results validated the coordination and sensitization effect of 5-bop and isp to Eu and Tb ions. Interestingly, only Eu-MOF 1 exhibited two strong emission bands at 366 and 590-625 nm as shown in Figure 3A. The band at 366 nm was similar to that of 5-bop. The emission around 590-625 nm showed the characteristic emission of Eu3+ ions with the well-remained four peaks, but the emission intensity was obviously enhanced compared with that of free Eu3+ ions. Different to Eu-MOF 1, Eu-MOF 2, TbMOF 1 and 2 only showed the characteristic emission of Eu3+ and Tb3+ and not the emission from the ligands was observed even they were excited at the optimal wavelength of ligands at 275 nm (Figure 3).

We investigated the luminescence of the products of EuMOF 1 at different reaction time to further explore the luminescence mechanism (Figure S11). When 5-bop and Eu3+ ions were simply mixed, the typical emission of Eu3+ ions was observed with the obviously enhanced intensity. The PWHH of the emission at 360 nm became large, the same as that of Eu-MOF 1. However, no solid product was observed. The phenomenon can be explained by the formation of molecular complex between Eu3+ ions and 5-bop.37,38 With the increase of reaction time, the spherical structure of MOF formed gradually (Figure S3), but the fluorescence profile kept constant (Figure S11). Significantly enhanced emission resulted in when Eu3+ ions were coordinated to 5-bop moieties. Therefore, we hypothesized that luminescence mechanism of Eu-MOF 1 was the same as that of the molecular complex formed with Eu3+ and 5-bop.1,39 Among various energy transfer mechanisms, antenna effect was the most successful to illustrate the emission from Ln complexes to date.35,40,41 The emission occurs from the direct excitation of the ligand to S1 state followed by intersystem crossing to T1 state.42 Emission from Ln ions is therefore observed when non-radiative energy transfer occurs from the ligand triplet state to Ln ions,43 or relaxes via non-radiative processes. Therefore, we proposed the emission process from Eu-MOF 1 as illustrated in Figure 4A because Eu-MOF 1 had the same emission properties to the products at different reaction time. Organic ligand 5-bop absorbed incident radiation to populate S1. According to Reinhoudt's empirical rule,44 intersystem crossing process is effective when the energy gap between S1 and T1 of the ligand is larger than 5000 cm-1,14 while the energy gap of 5-bop was 11412 cm−1 (Table S3). The previous work illustrated that the T1 of the ligand ranged in 22000-27000 cm-1 is appropriate to sensitize Eu3+.44 Therefore, the T1 of 5-bop (23923 cm-1) is efficient for the energy transfer. Once populated, Eu3+ ions showed the characteristic emission. Therefore, the dual-emission of Eu-MOF 1 was attributed to the antenna effect and less transfer efficiency than its counterparts.

Figure 3. Luminescent spectra of (A) Eu-MOF 1, (B) TbMOF 1, (C) Eu-MOF 2, and (D) Tb-MOF 2 nanospheres in water at the excitation of 275 nm. Luminescence Mechanism of LMOFs. To better understand how the introduction of boric acid influences the emission behavior of the LMOFs, the singlet-state energy (S1) and 4

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Analytical Chemistry excitation wavelength of Eu-MOF 1 was 280 nm even different emissions were selected at 366 and 625 nm (Figure 4C). Moreover, the optimal excitation wavelength was almost the same as that of 5-bop although whose maximum emission was observed at 336 nm. Therefore, the single excitation at 275 nm produced two emissions from both 5-bop and Eu ions in Eu-MOF 1 with the procedure illustrated in Figure 4A.

To further validate the energy transfer process, we investigated the absorption and excitation spectra of Eu-MOF 1. The broad emission of 5-bop overlaid partly with the excitation spectrum of Eu-MOF 1 (Figure 4B and Figure S12). So the emission of Eu-MOF 1 at 625 nm was assigned to as the efficient energy transfer to excite Eu3+ ions from the ligand. As direct evidence, we observed that the maximum

Figure 4. (A) Schematic representation of absorption, migration, and emission of Eu-MOF 1 depicting with the antenna effect. Abbreviations: a = absorption at 280 nm; f = fluorescence at 366 nm; p = phosphorescence; l = luminescence around 580–710 nm from Eu3+ in Eu-MOF 1. ISC = intersystem crossing; ET = energy transfer; S = singlet; and T = triplet. (B) Excitation spectrum of Eu-MOF 1 with λEm at 625 nm and emission spectrum of 5-bop with λEx at 275 nm. Overlap indicates the potential of energy transfer for excitation of Eu3+ with the emission from 5-bop within Eu-MOF 1. (C) Emission (Em) spectra of 5-bop (black) and EuMOF 1 (red); excitation (Ex) spectra of 5-bop (black) and Eu-MOF 1 with λEm = 625 nm (red) and λEm = 366 nm (blue). (D) Dayto-day fluorescence stability of 0.2 mg·L-1 Eu-MOF 1 in water. Ratiometric Detection of Fluoride Ions. The chemical stability and day-to-day fluorescence stability of Eu-MOF 1 was essential for real application. The consistent PXRD patterns of Eu-MOF 1 were observed before and after soaked in water for two weeks and were well agreed with the simulated ones from single-crystal XRD data (Figure 1E). The results indicated the high chemical stability of Eu-MOF 1 for its real application in aqueous system. Moreover, their fluorescence profile was well remained within 1 month and illustrated its high day-to-day stability (Figure 4D). Thus, EuMOF 1 remained structure and fluorescence stability as a prominent candidate for fluorescent detection in aqueous system.

intensity ratio and fluoride concentration ranged from 4 µM to 80 µM. Thus, Eu-MOF 1 is interesting for the quantitative determination of fluoride ions with the detection limits of 2 µM or 0.034 mg·L-1 (S/N = 3, Figure 5C). The detection limits were far below the maximum level (1 mg·L-1) of fluoride in drinking water permitted by World Health Organization (WHO) and the American Public Health Association.45 Thus, simple ratiometric fluorescence was developed to determine fluoride ions in water sample. It was noted that when fluoride was added into Eu-MOF 1 solution gradually, the bright red luminescence faded away with the decreased intensity, while the color also changed from red to blue (inset of Figure 5A). The change in color and intensity was easily distinguished by naked eyes compared with the single intensity change from the emission of Ln ions. Thus, naked eyes sensing makes the detection of fluoride ions more convenient and practical. Meanwlile, we explored the single fluorescence at 366 nm or 625 nm for the determination of fluoride ions. The detection limits were 3.5 µM and 8.3 µM, repsectively, higher than that obtained from ratiometric fluorescence strategy. Poor concentration-dependent response was also observed as illustrated in Figure S14 with the two single emissions. Moreover, ratiometric fluorescence strategy showed high concentration tolerance of interference because of its inner-reference effect.

Considering the high affinity between boric acid and fluoride, the dual-emission of Eu-MOF 1 was investigated when fluoride ions were added into Eu-MOF 1 solution. The emission at 366 nm was enhanced but the emission around 625 nm decreased gradually at the single-excitation of 275 nm with increased fluoride concentration (Figure 5A-C, Figure S13 and S14). The results indicated that Eu-MOF 1 can rationally detect fluoride ions through ratiometric fluorescence strategy. The dual emission of Eu-MOF 1 was recorded with the fluoride concentration ranged from 0 to 500 µM (Figure 5A and 5B). The intensity ratio at 625 and 366 nm decreased to 0.26 from the original 0.93 when 100 µM fluoride was tested. Excellent linear relationship was observed between the 5

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showed strong fluorescence at 366 nm from 5-bop and at 625 nm from Eu3+ ions. However, the disrupted pπ–π conjugation of 5-bop decreased the intersystem crossing efficiency with the addition of fluoride. Therefore, the emission of 5-bop increased and the intensity from Eu3+ ions decreased because of the strong electron-withdrawing property of fluoride.

To validate the specificity of sensing system toward fluoride, we tested the luminescence intensity from other species with and without fluoride ions. Under the identical conditions, only fluoride showed the remarkable luminescence change, while negligible change was observed from the other ions (Na+, K+, Mg2+, Ca2+, Al3+, Zn2+, Cu2+, Fe3+, Cl-, I- SO42-, HCO3-, and NO3-, Figure 5D). Moreover, no interference was found to fluoride ions in the presence of the foreign ions (Figure S15). The results revealed that Eu-MOF 1 was highly selective to fluoride ions because of the high affinity between boric acid and fluoride.

The dual emission was used for ratiometric fluorescence detection of fluoride in river and underground water samples based on the outstanding characteristics of the boric acidfunctional Eu-MOF 1. The underground water samples contain various ions (Table S4), but the results obtained from our ratiometric fluorescence method were well consistent to those from ion chromatographic strategy. The recovery of fluoride in the real samples ranged from 95.2 % to 114.5 %, indicating no significant interference encountered for the determination of fluoride in the samples (Table S5). All of the above results strongly illustrated that the proposed method showed great potential for quantitative detection of fluoride in water sample with high accuracy and reliability .

The fluorescence change of boric acid complexes caused by fluoride is generally considered as strong covalent interaction between boron and fluoride and the change from sp2 trigonal boron –B(OH)2 to sp3 hybridized –BF3.46 The boroncontaining ligands possess an empty pπ orbital on the boron center,47 so the binding of fluoride to the boron center perturbs the pπ–π conjugation between the boron and the aromatic chromophore through the OH-/F- exchange. Eu-MOF 1

Figure 5. (A) Fluorescence spectra (λex=275 nm) of 0.2 mg mL-1 Eu-MOF 1 upon the addition of fluoride at different concentration. Inset: the photos of the mixture solution to illustrate the color and intensity change at different concentration of fluoride. (B) The part amplification of the emission profile in (A). (C) Plot of the intensity ratio of I625/I366 vs fluoride concentration. (D) Luminescence ratio of I625/I366 from the responses of Eu-MOF 1 in the presence of 100 µM fluoride and 500 µM interference ions. and high affinity between boric acid and fluoride. The other functional ligands could be stimulated to build functional MOFs to tune the energy level for interesting emission and improve sensing selectivity.

CONCLUUSION In conclusion, we have successfully prepared the dualemission boric acid functional Eu-MOF through tuning the energy transfer efficiency with boric acid-functional ligand and Eu3+ ions. Dual-emission was observed at 366 nm and 625 nm with the antenna effect at the single-excitation of 275 nm. The functional MOFs show nanoscale spherical structure to facilitate the diffusion for fast response. With their good monodisperse, the MOFs are used to determine aqueous fluoride ions. Broad response range and low detection limit are therefore achieved with the ratiometric fluorescence detection

AUTHOR INFORMATION Corresponding Author * To whom correspondence should be addressed. E-mail: [email protected]; Fax: +86-22-23503034.

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All authors have given approval to the final version of the manuscript.

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ACKNOWLEDGEMETS

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This work was supported by the National Basic Research Program of China (973 Program, No. 2015CB932001), Natural Science Foundation of China (No. 21675090, 21435001, and 21375064), and Tianjin Natural Science Foundation (15ZCZDSF00060).

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ASSOCIATED CONTENTS

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Supporting Information

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The characterization of the LMOFs, including SEM images, size distribution, framework structure and thermogravimetric analysis; Optical properties of this system, such as UV-vis, FL and phosphorescence spectrum; The interference ions and real sample analysis with ratiometric detection of fluoride ions. The Supporting Information is available free of charge on the ACS Publications website.

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REFERENCES

(36) (37) (38)

(1) (2)

(39)

(3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23)

(24)

Hu, Z.; Deibert, B. J.; Li, J. Chem. Soc. Rev. 2014, 43, 5815-5840. Cui, Y.; Yue, Y.; Qian, G.; Chen, B. Chem. Rev. 2012, 112 11261162. Allendorf, M. D.; Bauer, C. A.; Bhakta, R. K.; Houk, R. J. Chem. Soc. Rev. 2009, 38, 1330-1352. Duan, T. W.; Yan, B.; Weng, H. Micropor. Mesopor. Mat. 2015, 217, 196-202. Zhou, J.; Li, H.; Zhang, H.; Li, H.; Shi, W.; Cheng, P. Adv. Mater. 2015, 27, 7072-7077. Guo, Y.; Feng, X.; Han, T.; Wang, S.; Lin, Z.; Dong, Y.; Wang, B. J. Am. Chem. Soc. 2014, 136, 15485-15488. Hou, X. F.; Yu, Q. X.; Zeng, F.; Ye, J. H.; Wu, S. Z. J. Mater. Chem. B 2015, 3, 1042-1048. Dai, C.; Yang, C. X.; Yan, X. P. Anal. Chem. 2015, 87, 1145511459. Xu, R.; Wang, Y.; Duan, X.; Lu, K.; Micheroni, D.; Hu, A.; Lin, W. J. Am. Chem. Soc. 2016, 138, 2158-2161. Tan, H.; Li, Q.; Ma, C.; Song, Y.; Xu, F.; Chen, S.; Wang, L. Biosens. Bioelectron. 2015, 63, 566-571. Fan, J.; Hu, M.; Zhan, P.; Peng, X. Chem. Soc. Rev. 2013, 42, 2943. Dong, Y.; Cai, J.; Fang, Q.; You, X.; Chi, Y. Anal. Chem. 2016, 88, 1748-1752. Gao, B.; Zhang, D.; Don, T. J. Phy.l Chem. C 2015, 119, 1640316413. Einkauf, J. D.; Kelley, T. T.; Chan, B. C.; de Lill, D. T. Inorg. Chem. 2016, 55, 7920-7927. Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem. Int. Ed. 2004, 43, 2334-2375. Wang, C.; Liu, D.; Xie, Z.; Lin, W. Inorg. Chem. 2014, 53, 13311338. Wang, S.; Cao, T.; Yan, H.; Li, Y.; Lu, J.; Ma, R.; Li, D.; Dou, J.; Bai, J. Inorg. Chem. 2016, 55, 5139-5151. Choi, K. M.; Na, K.; Somorjai, G. A.; Yaghi, O. M. J. Am. Chem. Soc. 2015, 137, 7810-7816. Lammert, M.; Bernt, S.; Vermoortele, F.; De Vos, D. E.; Stock, N. Inorg. Chem. 2013, 52, 8521-8528. Lu, T.; Zhang, L.; Sun, M.; Deng, D.; Su, Y.; Lv, Y. Anal. Chem. 2016, 88, 3413-3420. Diring, S.; Wang, D. O.; Kim, C.; Kondo, M.; Chen, Y.; Kitagawa, S.; Kamei, K.; Furukawa, S. Nat. Commun. 2013, 4, 2684. Lin, X.; Hong, Y.; Zhang, C.; Huang, R.; Wang, C.; Lin, W. Chem. Commun. (Camb) 2015, 51, 16996-16999. Bull, S. D.; Davidson, M. G.; van den Elsen, J. M.; Fossey, J. S.; Jenkins, A. T.; Jiang, Y. B.; Kubo, Y.; Marken, F.; Sakurai, K.; Zhao, J.; James, T. D. Acc. Chem. Res. 2013, 46, 312-326. Li, D.; Chen, Y.; Liu, Z. Chem. Soc. Rev. 2015, 44, 8097-8123.

(40) (41) (42) (43) (44) (45)

(46) (47)

Bresner, C.; Aldridge, S.; Fallis, I. A.; Jones, C.; Ooi, L. L. Angew. Chem. Int. Ed. 2005, 44, 3606-3609. Sohn, H.; Letant, S.; Sailor, M. J.; Trogler, W. C. J. Am. Chem. Soc. 2000, 122, 5399-5400. Chereddy, N. R.; Nagaraju, P.; Niladri Raju, M. V.; Saranraj, K.; Thennarasu, S.; Rao, V. J. Dyes. Pigments 2015, 112, 201-209. Erdemir, S.; Kocyigit, O. Sens. Actuators B Chem. 2015, 221, 900-905. Alici, O. Spectrochim. Acta A 2016, 167, 78-83. Niu, H.; Shu, Q.; Jin, S.; Li, B.; Zhu, J.; Li, L.; Chen, S. Spectrochim. Acta A 2016, 153, 194-198. Hu, J.; Liu, R.; Cai, X.; Shu, M.; Zhu, H. Tetrahedron 2015, 71, 3838-3843 Zhang, S. Y.; Shi, W.; Cheng, P.; Zaworotko, M. J. J. Am. Chem. Soc. 2015, 137, 12203-12206. Rao, X.; Song, T.; Gao, J.; Cui, Y.; Yang, Y.; Wu, C.; Chen, B.; Qian, G. J. Am. Chem. Soc. 2013, 135, 15559-15564. Lu, L.; Chen, C.; Zhao, D.; Sun, J.; Yang, X. Anal. Chem. 2016, 88, 1238-1245. Liu, X.; Wang, J. Y.; Li, Q. Y.; Jiang, S.; Zhang, T. H.; Ji, S. F. J. Rare Earth 2014, 32, 189-194. Thomas, J.; Ambili, K. S. J. Mol. Struct. 2015, 1098, 167-174. Binnemans, K. Chem. Rev. 2009, 109, 4283-4374. Wang, T.; Li, P.; Li, H. ACS Appl. Mater. Interfaces 2014, 6, 12915-12921. Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T. Chem. Rev. 2012, 112, 1105-1125. Liu, X.; Akerboom, S.; de Jong, M.; Mutikainen, I.; Tanase, S.; Meijerink, A.; Bouwman, E. Inorg. Chem. 2015, 54, 11323-11329. Freslon, S.; Luo, Y.; Daiguebonne, C.; Calvez, G.; Bernot, K.; Guillou, O. Inorg. Chem. 2016, 55, 794-802. Heffern, M. C.; Matosziuk, L. M.; Meade, T. J. Chem. Rev. 2014, 114, 4496-4539. Meng, Q. G.; Xin, X. L.; Zhang, L. L.; Dai, F. N.; Wang, R. M.; Sun, D. F. J. Mater. Chem. 2015, 3, 24016-24021. Steemers, F. J.; Verboom, W.; Reinhoudt, D. N.; Vandertol, E. B.; Verhoeven, J. W. J. Am. Chem. Soc. 1995, 117, 9408-9414. Wu, X.; Chen, X.-X.; Song, B.-N.; Huang, Y.-J.; Ouyang, W.-J.; Li, Z.; James, T. D.; Jiang, Y.-B. Chem. Commun. 2014, 50, 13987-13989. Nishiyabu, R.; Kubo, Y.; James, T. D.; Fossey, J. S. Chem. Commun. (Camb) 2011, 47, 1106-1123. Liu, X. Y.; Bai, D. R.; Wang, S. Angew. Chem. Int. Ed. 2006, 45, 5475-5478.

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