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Anionic Lanthanide MOFs as a Platform for Iron-Selective Sensing, Systematic Color Tuning, and Efficient Nanoparticle Catalysis Ya-Pan Wu,†,∥ Guo-Wang Xu,†,∥ Wen-Wen Dong,† Jun Zhao,† Dong-Sheng Li,*,†,‡ Jian Zhang,‡ and Xianhui Bu*,§ †

College of Materials and Chemical Engineering, Hubei Provincial Collaborative Innovation Center for New Energy Microgrid, Key Laboratory of Inorganic Nonmetallic Crystalline and Energy Conversion Materials, China Three Gorges University, Yichang, Hubei 443002, China ‡ State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China § Department of Chemistry and Biochemistry, California State University, Long Beach, 1250 Bellflower Boulevard, Long Beach, California 90840, United States S Supporting Information *

ABSTRACT: New porous anionic Ln-MOFs, namely, [Me2NH2][Ln(CPA)2(H2O)2] (Ln = Eu, Gd), have been prepared through the self-assembly of 5-(4-carboxy phenyl)picolinic acid (H2CPA) and lanthanide ions. They feature open anionic frameworks with 1-D hydrophilic channels and exchangeable dimethylamine ions. The Eu phase could detect Fe3+ ions with high selectivity and sensitivity in either aqueous solution or biological condition. The ratios of lanthanide ions on this structure platform could be rationally tuned to not only achieve dichromatic emission colors with linear correlation but also attain three primary colors (RGB) and even white light with favorable correlated color temperature. Furthermore, the Ag(I)-exchanged phases can be readily reduced to afford Ag nanoparticles. The as-prepared Ag@Ln-MOFs composite shows highly efficient catalytic performance for the reduction of 4nitrophenol.

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INTRODUCTION The design and synthesis of porous metal−organic frameworks (MOFs) are attracting increasing attention due to their diverse compositions and topologies,1 together with various useful properties2 such as fluorescence,3 magnetism,4 gas sorption,5 and catalysis.6 Their tunable topological structures and properties provide an important advantage for use as chemosensory and carrier materials.7 Ln-MOFs are of special interest because of their rich coordination geometry, relatively high stability, and unique luminescent and magnetic properties.8,9 Owing to their versatile sharp emissions (such as red-Eu, green-Tb, and blue-Ce), long lifetimes of the excited states, and narrow band characters, the intriguing luminescence of Ln3+ ions can lead to applications in fluorescent probes as well as multicolor tunable photoluminescence.10 Ln-MOFs have been studied as a luminescence sensor for detection of metal ions (e.g., Fe3+, Cu2+, Al3+), Cr2O72− and PO43− anions, and volatile organic compounds such as acetone.11 However, they usually show luminescent quenching for two or more metal ions simultaneously. To achieve high selectivity and sensitivity for a specific ion or molecule remains an ongoing pursuit. © XXXX American Chemical Society

Ln-based complexes are also capable of efficiently generating multicolored photoluminescent emission.12 A number of mixed Ln-MOFs have been fabricated to fine-tune the photoluminescent emission through dichromatic or trichromatic composite approaches.13 Although trichromatic tuning of photoluminescence was achieved by mixed two or three Ln3+ ions into a single-phase material capable of providing dual functions both as photosensitizer and as chromophore, it is still a great challenge to fine-tune white light emitting MOFs materials through rational control of individual proportion of each color or linear correlation of color tunability. Recently, MOFs with charged frameworks have attracted great interest due to the exchangeability between the extraframework ions and various external guest species.14 By applying appropriate strategies, it is possible to synthesize anionic MOFs for use as host to make metal@MOFs composites by immobilizing noble metal ions through ionexchange.15 However, the ion size and charge have an important effect on feasibility; thus, appropriate functionalizaReceived: October 12, 2016

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DOI: 10.1021/acs.inorgchem.6b02476 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

3.74; N, 5.85%. IR (KBr, cm−1): 1595(w), 1540(m), 1479(w), 1363(s), 1249 (m), 1126 (w), 1101(w), 1059(w), 1008(m), 926(w), 906(w), 837(w), 776(s). Preparation of Bimetallic and Trimetallic Materials. To study the luminescence property of mixed heteronuclear Ln-MOFs, a similar synthetic strategy was used to prepare EuxGd1−x-Ln-MOFs (concentration: x = 0.5, 5, 15 mol %), EuxTb1−x-Ln-MOFs (x = 0.4, 0.6 mol %), TbxGd1−x-Ln-MOFs (x = 0.5, 5, 15 mol %), EuxTbxGd1−2x-LnMOFs, EuxTb3xGd1−4x-Ln-MOFs, and TbxEu3xGd1−4x-Ln-MOFs (x = 0.5 mol %) by adding specific stoichiometric ratios of lanthanide ions of Eu(NO3)3·6H2O, Tb(NO3)3·6H2O, and/or Gd(NO3)3·6H2O. The as-synthesized samples were identified as isomorphism to the homonuclear crystalline compounds, which could be confirmed by their powder X-ray diffraction curves (Figures S10−S13). Catalytic Degradation Characterizations. The catalytic performance of Ag@Ln-MOFs composites for degradation of 4-nitrophenol (4-NP) were carried out at room temperature. The detailed process was as follows: 1.0 mg of Ag@Ln-MOFs sample was quickly dispersed into 3.0 mL of 1.8 × 10−4 mol·L−1 4-NP water solution, followed by the dropwise addition of a small amount of KBH4. At different time intervals, the absorbance of the reaction system was monitored by UVvis absorption spectroscopy. X-ray Crystallography. The crystal diffraction data for Ln-MOFs (Ln = Eu, Gd) were collected on a Rigaku XtaLAB mini diffractometer at 293(2)K using Mo Kα radiation (λ = 0.71073 Å). The program CrystalClear was used for integration of diffraction data, and the program SADABS was applied for an empirical absorption correction.20 The final structure was solved using SHELXS by direct methods and refined by the full-matrix least-squares technique using SHELXL. All non-H atoms were refined anisotropic thermal parameters.21,22 Hydrogen atoms were generated geometrically and further refined isotropically with specific thermal factors. Additionally, the [(CH3)2NH2]+ cations of the cavity were formed through the decomposition of DMF, and the [(CH3)2NH2]+ cations in the pores could be confirmed by X-ray diffraction. The crystallographic data and corresponding partial bond lengths and angles for Eu-MOFs and GdMOFs are given in Tables S1−S3.

tion of the pore surface is crucial. Some excellent works by Fischer et al.16 indicate that noble metals such as Pd, Pt, Au, and Ru could be infiltrated into neutral MOFs via vapor-phase or solution methods and the resulting composites show special properties. However, until now, spontaneous room-temperature in situ reduction of ion-exchanged Ag(I) to Ag nanoparticle in anionic Ln-MOFs, which possess special catalytic performance, has not been reported yet. We have been particularly interested in various types of MOFs for their possible applications in gas adsorption, photocatalysis, and ion detection.17,18 Herein, two new porous anionic Ln-MOFs, [Me2NH2][Ln(CPA)2(H2O)2] (Ln = Eu, Gd), have been synthesized, and they can serve as a multifucntional platform for highly selective detection of metal ions, bright multicolored emission, and carrier of metal nanoparticles for catalysis. Ln-Eu has been demonstrated as a highly senstive luminescent probe for Fe3+ ion in aqueous solution and biological systems with a very low concentration limit. More interestingly, modulating luminescent emission from two or three Ln3+ ions by simple control of reactant stoichiometry could generate bright dichromatic and white light emission. Furthermore, our results demonstrate that the mixed Eu3+, Tb3+, or Gd3+ ions can fine-tune the emission color to approach white light emission. Finally, the ion-exchange between Ag+ ions and Me2NH2+ cations in Ln-MOFs leads to spontaneous reduction to Ag particles to form a Ag@LnMOFs composite, which possesses high catalytic performance for the reduction of 4-NP.

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EXPERIMENTAL SECTION

Materials and Methods. All chemicals were analytically pure grade and purchased from the commercial companies and directly used during the experiment. Elemental analysis such as C, H, and N were tested on a PerkinElmer 2400 Series II analyzer. FT-IR spectra (KBr pellets) were recorded on a Thermo Electron NEXUS 670 FTIR spectrometer in the range of 4000−400 cm−1. Power X-ray diffraction (PXRD) analyses were taken on a Rigaku Ultima IV diffractometer. Thermogravimetric (TG) analyses were performed on a NETZSCH 449C thermal analyzer in the temperature range 25−800 °C with a heating rate of 10 °C min−1 under an air atmosphere. The Commission International de I’Eclairage (CIE) color coordinates were calculated by the standardized CIE method.19 Photoluminescent excited and emission spectra of samples were measured on an Edinburgh FLS55 luminescence spectrometer at room temperature. The UV−vis spectra were performed on a Shimadzu UV 2550 spectrometer. The morphologies of as-synthesized samples were observed using a JEOL-JSM-7500F emission scanning electron microscopy at 10 kV. X-ray photoelectron spectroscopic (XPS) was carried out using an ESCALAB 250 equipment with a monochromatic Al Kα X-ray source. Inductively coupled plasma (ICP) spectroscopy was obtained on a Dual-view Optima 5300 DV ICP-OEM system. Experimental Details. Preparation of [Me2NH2][Ln(CPA)2(H2O)2] (Eu-MOFs, Gd-MOFs). A mixture of Eu(NO3)3·6H2O (0.05 mmol, 16.90 mg) and 5-(4-carboxy phenyl)picolinic acid (H2CPA, Scheme S1 0.05 mmol, 12.15 mg), three drops of HNO3 (63%, aq.), and 5 mL of DMF/H2O (the volume ratio is 3:2) were placed in a 23 mL stainless autoclave at 140 °C for 3 days. Finally, the mixed system was cooled slowly to room temperature; colorless crystals were generated (yield 85.0% based on Eu(NO3)3·6H2O). Elemental analysis: calcd for C28H26N3O10Eu: C, 46.94; H, 3.66; N, 5.86%. Found: C, 47.13; H, 3.85; N, 5.93%. IR data (KBr, cm−1): 1592(w), 1540(m), 1472(w), 1365(s), 1252(m), 1145(w), 1109(w), 1078(w), 1008(m), 927(w), 905(w), 821(w), 776(s). Besides, Gd-MOFs was synthesized in a similar method to that of Eu-MOFs (Table S4). Colorless block crystals were obtained in 78.0% yield. Elemental analysis: calcd for C28H26N3O10Gd: C, 46.59; H, 3.63; N, 5.82%. Found: C, 46.93; H,

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RESULTS AND DISCUSSION Since Eu-MOFs and Gd-MOFs are isostructural and show a similar 2D network, only the structure of Eu-MOFs will be discussed here. Crystal structure analysis revealed that EuMOFs crystallizes in the monoclinic C2/c space group. Its asymmetric unit contains one Eu(III) cation, two CPA2− anions, and two coordinated water molecules. As shown in Figure S1, the Eu(1) cation adopts a distorted bicapped triangle prism geometry coordinated by four carboxylate oxygen atoms, two nitrogen atoms, and two oxygen atoms from water. The lengths of the Eu−O bonds fall in the range of 2.367(3)− 2.413(3) Å and the Eu−N distance is about 2.595(3) Å, which are similar to those reported for Eu(III) complexes based on pyridyl-carboxylate ligands.23 Each CPA2− anion links to two Eu(III) cations through two carboxylate groups and one pyridyl N atom adopting μ1-η1:η0, μ2-η1:η1 coordination modes to form a 2D rhombus grid layer, which is parallel to the [001] direction. The adjacent layers were bridged together through strong hydrogen bonding interaction between carboxylate (O(1), O(1)#4) and coordinated water molecules (O(5),O(5)#3) (Figure S2) to form a pseudo 3D porous framework with a 1D rhombus channel along the c axis (Figure 1). Noteworthy is that the inner surface of the rhombus channel (a dimension of 7.280(3) Å based on inner carboxylate O···O separation) is decorated with uncoordinated carboxylate oxygen atoms, suggesting a hydrophilic character. When the Me2NH2+ counter cations in Eu-MOFs are removed with the SQUEEZE program,24 the PLATON/VOID calculation result B


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Inorganic Chemistry

Figure 1. View of the 3D framework of Eu-MOFs containing 1D rhombus channel along the c axis. The Me2NH2+ cations were encapsulated in 1D channels. Figure 2. Solid-state PL spectrum of Eu-MOFs (excited and monitored at 398 and 617 nm, repectively).

indicates that the empty volume of the anionic Eu-MOFs is 558.8 Å3, corresponding to 20.0% of the total crystal volume (2795.0 Å3). In Eu-MOFs, each rhombus channel is occupied by two Me2NH2+ cations to balance the negative charges of the host framework. Thus, these guest Me2NH2+ cations could be spontaneously replaced with appropriate metal ions, which suggest that the porous anionic framework could be beneficial for the impregnation and immobilization of metal ions. PXRD and TGA Measurements. To confirm the phase purity of the bulk materials, the as-prepared crystalline samples were taken on powder X-ray diffraction (PXRD). The experimental patterns are in agreement with the simulated patterns from crystal diffraction data, which confirm that both Eu-MOFs and Gd-MOFs have high phase purity (Figures S3 and S4). Besides, the thermogravimetric analyses were performed under an air atmosphere (Figures S5 and S6). The TGA curves of the two compounds are similar and both of them exhibit two steps of weight loss, which are attributed to loss of coordinated solvents and the decomposition of the organic groups. Eu-MOFs show the first loss of 5.34% below 200 °C due to the release of two coordinated water molecules (calcd: 5.02%), and the second step occurs from 200 to 500 °C, where the Me2NH2+ cations and the ligands began to decompose. For Gd-MOFs, the weight loss appears between 80 and 200 °C, resulting from the release of two coordinated water molecules (obsd: 5.18% and calcd: 4.99%). Finally, the removal of organic components starts at about 275 °C. Photoluminescent Properties. The luminescent spectra measured at room temperature for the H2CPA ligand and EuMOFs are showed in Figure S7 and Figure 2. The free H2CPA molecule possesses a weak emission at 415 nm, which was presumably attributed to π → π* and n → π* electronic transitions excited at 346 nm. Under 398 nm excitation, the characteristic transitions of the Eu3+ cation in Eu-MOFs can be seen at about 597, 617, 655, and 704 nm, which are contributed from the 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F3, and 5D0 → 7F4 f−f transitions, respectively. The strongest 5D0 → 7F2 transition indicates that Eu3+ ions do not occupy sites of inversion center, which agrees with the structural analyses. The emission of EuMOFs indicates the well-resolved enhanced luminescence of the f−f transitions, which is contributed from the favorable energy transfer from the H2CPA molecule to metal centers. For Gd-MOFs, it shows a broad blue emission band centered at 410 nm under the excitation at about 340 nm (Figure S8). Since the characteristic 4f−4f transition of the Gd3+ cation at about 310 nm is hardly visible because the high excited state 6P7/2 of the

Gd3+ does not accept energy from H2CPA molecule, the broad blue emission could be tentatively assigned to the luminescent emission of the ligand.25 Sensing of Metal Ions. As mentioned above, two Me2NH2+ cations are encapsulated in the cavity of the anionic host lattice, which provides us a chance to explore the ionexchange behavior and luminescent sensing properties. The assynthesized crystalline samples were ground and suspended in aqueous solutions containing different metal ions (Na+, Ca2+, Zn2+, Cd2+, Cu2+, Fe2+, Co2+, Ni2+, Mn2+, Pb2+, Hg2+, In3+, Al3+, Fe3+, Sm3+, Dy3+, Tb3+). The PXRD patterns of the metal ionincorporated Eu-MOFs indicate that Eu-MOF retains its original framework after being immersed in various metal ions solutions (Figure 3). Further, the luminescent emissions

Figure 3. PXRD patterns of Eu-MOFs treated by aqueous solutions containing different metal cations.

are recorded in Figure 4. Noteworthy is that most metal ions possess varying degrees of luminescent quenching effects. For example, the luminescence intensity at 617 nm is about half of the original one when Cu2+ ions are involved, and Fe3+ ions could almost quench the luminescence of Eu-MOFs (Figure 4a,b).The different effects on the emission between Fe3+ and other cations are obviously observed, which indicates that EuMOFs could be considered as a promising luminescent sensor C


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Inorganic Chemistry

Figure 4. (a, b) Comparison of the luminescence intensity of the 5D0 → 7F2 transitions (617 nm) of Eu-MOFs in 10−2 M different metal ions in water solutions. (c) Liquid luminescent spectra of Eu-MOFs as a result of different concentrations of Fe3+. (d) The linear relationship between the enhanced efficiency and the concentrations of Fe3+ in the range of 10−7 to 10−2 mol/L.

Figure 5. (a) The solid-state PL spectra of EuxGd1−x−Ln-MOFs excited at 398 nm. (b) CIE chromaticity diagram for the EuxGd1−x−Ln-MOFs under 398 nm.

for the Fe3+ cation in aqueous phase. This strong quenching effect in Eu-MOFs might possibly result from the interaction between Fe3+ ions (exchanged with Me2NH2+) and the uncoordinated carboxylate groups in the channel, which

could reduce energy transfer from CPA2− ligands to Eu3+ ions. Quantitatively, the luminescence intensity vs Fe 3+ concentration plot was made (Figure 4c). When increasing the concentration of Fe3+, the luminescence of Eu-MOFs was D


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Inorganic Chemistry

Figure 6. (a) The solid-state PL spectra of EuxTb1−x−Ln-MOFs excited at 398 nm. (b) CIE chromaticity diagram for the EuxTb1−x−Ln-MOFs under 398 nm.

Figure 7. (a) The solid-state PL spectra of TbxGd1−x−Ln-MOFs excited at 398 nm. (b) CIE chromaticity diagram for the TbxGd1−x−Ln-MOFs under 398 nm.

enlarging concentration of Fe3+ such that the detection can reach as low as at least 10−6 mol/L. The result indicates that the sensitivity of Eu-MOFs for Fe3+ ion in the physiological solutions is comparable to that of Eu-MOFs observed in a pure aqueous system, further proving the possibility of Eu-MOFs for detection of Fe3+ ions in biosystem. Luminescence of Mixed Bimetallic and Trimetallic Compounds. Considering that powder samples of Eu-MOFs and Gd-MOFs emit red and blue light (two of the three primary colors (RGB)) and that both are isostructural, it is expected that integrating different Ln3+ ions into a single crystal of Eu-MOFs could emit multicolored photoluminescence. Therefore, a series of mixed metallic compounds with variable molar ratios of lanthanide ions can be systematically incorporated into the host frameworks. Herein, we studied six heteronuclear series, four of them based on Eu3+ (EuxGd1−x− Ln-MOFs, EuxTb1−x−Ln-MOFs, EuxTbxGd1−2x−Ln-MOFs, EuxTb3xGd1−4x−Ln-MOFs, with 0 < x < 1) and two of them based on Tb3+ (TbxGd1−x−Ln-MOFs, TbxEu3xGd1−4x−Ln-

gradually quenched, and the luminescent intensity obeys the equation I0/I = 0.00003*exp(c/0.49) + 0.97. As shown in Figure 4d, the luminescence emission of Eu-MOFs gradually decreases with the increase of Fe3+ concentration (as low as 10−7 mol/L). Meanwhile, the Ksv value is calculated to be 10411.1 M−1, proving the high quenching efficiency of Fe3+ on the emission of Eu-MOFs. The detection limit of 10−7 M for Fe3+ ion in aqueous solutions is greatly enhanced compared with that of the reported Eu-MOFs sensors in different systems (Table S5). The Ksv value of Eu-MOFs is approximately 3 times than that of Eu(L1)3.26 Moreover, the sensing function of Eu-MOFs under simulated physiological conditions has been investigated. The living cell environment was simulated by dissolving 4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid (HEPES) in water as a physiological buffer solution. The as-synthesized sample was dispersed in the above-prepared solution with different amounts of Fe(NO 3 ) 3 . As shown in Figure S9, the luminescence emission of Eu-MOFs gradually decreases with E


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Inorganic Chemistry MOFs, with 0 < x < 1). The luminescence spectra of a series of bimetallic compounds are reported in Figures 5−7. The EuxGd1−x, EuxTb1−x, and TbxGd1−x samples display sharp characteristic emission peaks from both Eu3+ and Tb3+ centers. As expected, by increasing the amount of Gd3+/Tb3+ while proportionally decreasing the amount of Eu3+/Tb3+ in heterodinuclear crystals, the red/green emission intensity was decreased, while those of green/blue emission increased accordingly. As seen from Figures 5b, 6b, and 7b, analysis of the chromaticity coordinates (x, y) from various emissions of EuxGd1−x, EuxTb1−x, and TbxGd1−x complexes, including those of between Eu, Tb, and Gd ions, establishes a perfect linear relationship expressed by the equations y = 0.002 + 0.527x, y = −0.310 + 2.785x, and y = 0.823 − 0.856x, respectively, which indicate that correlation of CIE coordinates for these three series of bimetallic complexes is possible (Table 1).

diagram can be systematically tuned with precise correlation to bimetallic ratios in the single crystal. These deliberated linear color changes are comparable to those of reported heteronuclear Ln-MOFs.27 Additionally, the color-tuning in dichromatic spectral regions was rationally finely tuned by the preparation of isomorphous heterodinuclear crystals. Therefore, in order to produce white light emission, a rational stoichiometric ratio of Eu3+- and Tb3+incorporated Gd-MOFs is necessary. It has been found that nearly pure white light emission was generated when the molar ratios of Gd3+:Tb3+:Eu3+ are 0.99:0.005:0.005, 0.98:0.005:0.015, and 0.98:0.015:0.005, respectively (Figure 8a). The CIE chromaticity coordinates of the trimetallic emission are (0.2921, 0.2815), (0.3526, 0.2843), and (0.3315, 0.3317) (Figure 8b), which are very close to those of the pure white light (0.33, 0.33) using the 1931 CIE coordinate diagram.28 The results indicate that, when mixing 0.5−1.5% molar amount of Tb3+ into Eu−Gd system, the calculated chromaticity coordinates would present to the white light region. The suitable correlated color temperature (CCT) could reach 5531 K, which shows excellent full color display ability for its practical application in solid-state lighting.29 This result could help to predict the chromaticity coordinates and guide the preparation of complexes with desired color. Catalytic Activities of 4-Nitrophenol (4-NP). As mentioned above, Ln-MOFs have an open anionic framework with the 1D rhombus channels occupied by Me2NH2+ cations. Due to this interesting structural feature, we focus on the possibility of ion exchange of Ln-MOFs. First, Ln-MOFs (20 mg) were immersed in an aqueous solution of AgNO3 (2.5 mg/ 3 mL) and vacuum-dried at 273 K for 24 h; then, the fresh aqueous solution of AgNO3 was replaced every 24 h. After three times, the samples were rinsed and soaked in distilled water to remove residual free AgNO3. Compared to the IR spectrum of the pristine sample with the Ag+-exchanged products, the peaks of Me2NH2+ cations in the range of 2930− 2830 cm−1 disappeared (Figure S14), which indicates that Me2NH2+ cations are exchanged by Ag+ ions. Second, the Ag+exchanged products (20 mg) were dispersed into 3 mL of ethanol and the resultant mixture was oscillated in the waterbathing constant temperature vibrator for 24 h. Then, the light

Table 1. Color Coordinates of Different Bimetallic and Trimetallic Materials According to CIE 1931 categories Eu-Gd

Eu-Tb

Tb-Gd

Gd-Tb-Eu

label

Ln-MOFs

x

y

CCT/K

A B C D E A B C D A B C D E F G H

Eu1.00 Eu0.15Gd0.85 Eu0.05Gd0.95 Eu0.005Gd0.995 Gd1.00 Eu1.00 Eu0.6Tb0.4 Eu0.4Tb0.6 Tb1.00 Tb1.00 Tb0.15Gd0.85 Tb0.05Gd0.95 Tb0.005Gd0.995 Gd1.00 Gd0.99Tb0.005Eu0.005 Gd0.98Tb0.005Eu0.015 Gd0.98Tb0.015Eu0.005

0.5968 0.5543 0.4456 0.3147 0.1547 0.5968 0.4122 0.5562 0.3222 0.3222 0.2566 0.2157 0.1916 0.1547 0.2921 0.3526 0.3315

0.3217 0.2962 0.2374 0.1465 0.0968 0.3217 0.4962 0.3962 0.5562 0.5562 0.4457 0.3146 0.2157 0.0968 0.2815 0.2843 0.3317

9253 10696 16851 3559 8288 9253 3987 1895 5783 5783 7909 15520 553312 8288 8978 4132 5531

Interestingly, the dichromatic emission colors (RB, RG, and GB spectra) and the chromaticity coordinates in the 1931 CIE

Figure 8. (a) The solid-state PL spectra of EuxTbxGd1−2x−Ln-MOFs, EuxTb3xGd1−4x−Ln-MOFs, TbxEu3xGd1−4x−Ln-MOFs excited at 398 nm. (b) CIE chromaticity diagram for the EuxTbxGd1−2x−Ln-MOFs, TbxEu3xGd1−4x−Ln-MOFs, EuxTb3xGd1−4x−Ln-MOFs under 398 nm. F


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Inorganic Chemistry

Figure 9. (a) TEM image of Ag@Eu-MOFs composite particles, (b) XPS spectra of Ag@Eu-MOFs and Ag@Gd-MOFs, (c) UV−vis spectra of the reduction of 4-NP over Ag@Eu-MOFs, (d) the kinetics study of the catalytic reaction on reduction of 4-NP over Ag@Eu-MOFs.

gray crystalline samples were filtrated, successively washed with water and ethanol, and finally dried in air. The color change of Ln-MOFs from colorless to gray can be attributed to the surface plasma of silver nanoparticles (Ag-NPs). Besides, ICPAES confirmed that the weight percentages of Ag NPs in EuMOFs and Gd-MOFs are 3.74% and 4.98%, respectively. Finally, the composite was denoted as Ag@Ln-MOFs, confirmed by XPS measurements. As shown in Figure 9b, there are two specific peaks with binding energies of 368 and 374 eV due to the 3d5/2 and 3d3/2 electrons of Ag0.30 TEM images show that Ag nanoparticles exhibit variable diameter distribution with an average of about 10 nm for Ag@Eu-MOFs and about 6 nm for Ag@Gd-MOFs, respectively (Figure 9a, Figure S15, and Figure 10a), which could partially immobilize in the rhombus channels. As shown in Figure 10b, the image clearly exhibits a lattice fringe with an interplanar distance of 0.240 nm corresponding to the (111) lattice plane of facecentered cubic Ag. Due to too small particle size and little amount of silver, the silver peaks are hardly seen in the PXRD pattern. Catalytic activity of the Ag@Ln-MOFs (Ln = Eu, Gd) composite was tested through KBH4-catalyzed reduction of 4-

NP to 4-AP. The absorption of 4-NP shows significant reduction at 400 nm, accompanied with a successive increase of that of 4-aminophenol (4-AP) at about 300 nm (Figures 9c and 10c). The UV−vis spectra show an obvious isosbestic point at 318 nm, which sufficiently demonstrates that the 4nitrophenol is only reduced to 4-aminophenol without a byproduct.31 Compared to the Ag@Ln-MOFs composite, the Ag-nanoparticle and pure Ln-MOFs nearly show catalytic degration on 4-NP (Figure S16). The results prove the excellent catalytic activities of the Ag@Ln-MOFs composite due to the highly dispersed and smaller size of the Ag-NPs. Additionally, ln(Ct/C0) and reaction time conform to a linear relationship, and the rate constant k was calculated to be 1.03 × 10−2 s−1 for Eu and 2.13 × 10−2 s−1 for Gd, respectively (Figures 9d and 10d). Noteworthy is that Ag@Gd-MOF has much higher catalytic efficiency toward the degradation of 4-NP than Ag@Eu-MOF, which is also superior to that of Ag-based other catalysts under a similar system (Table S6). Moreover, five recycles of the catalytic activity of Ag@Ln-MOFs show almost equal catalytic performance for the same reaction time (Figures S17 and S18), revealing good catalytic recurrence and the stability of the catalysts. G


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Inorganic Chemistry

Figure 10. (a, b) The TEM images of Ag@Gd-MOFs at different magnification, (c) UV−vis spectra showing the gradual reduction of 4-NP over Ag@Gd-MOFs. (d) The kinetics study of the catalytic reaction on reduction of 4-NP over Ag@Gd-MOFs.

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CONCLUSION

multicolored photoluminescent emission and load metal nanoparticles for catalysis.

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In summary, two new porous isostructural Ln-MOFs (Ln = Eu, Gd) have been demonstrated as a multifunctional platform to sense metal cations, tune multicolored photoluminescence, and load metal nanoparticles. Two compounds possess anionic frameworks with one-dimensional hydrophilic channels encapsulating dimethylamine ions. Clearly, porous anionic Eu-MOFs containing exchangeble dimethylamine ions are very promising as luminescent probes to sense Fe3+ ions with high sensitivity. Moreover, on the basis of the isostructural monometallic red Eu-MOFs and blue Gd-MOFs (two of the three primary colors), the combination of any two of these ions in bimetallic complexes allows for dichromatic emission in a linear fashion. The luminescence investigations demonstrate that mixing of Eu3+, Tb3+, and Gd3+ ions can rationally modulate the emission and obtain white emission, which prove its potential applications in lighting and displays. Finally, spontaneous in situ reduction of ion-exchanged Ag(I) to Ag nanoparticle in an anionic framework results in the Ag@Ln-MOFs composite with a specific size and distribution, which possesses high catalytic reduction activity for the 4-nitrophenol. The studies would not only provide successful examples to prepare porous anionic LnMOFs but also demonstrate an effective strategy to develop

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02476. The X-ray crystallographic CIF data, relevant bond lengths and bond angles, TGA, PXRD curves, and other additional figures for Eu-MOFs and Gd-MOFs (PDF) CCDC 1496495 crystallographic data for Eu-MOFs (CIF) CCDC 1496496 crystallographic data for Gd-MOFs (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.-S.L.). *E-mail: [email protected] (X.B.). ORCID

Dong-Sheng Li: 0000-0003-1283-6334 Jian Zhang: 0000-0003-3373-9621 H


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Inorganic Chemistry

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Xianhui Bu: 0000-0002-2994-4051 Author Contributions ∥

Y.-P.W. and G.-W.X. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the NSF of China (Nos.: 21373122, 21301106, 21673127, 21671119, 51572152, 51502155), the NSF of Hubei Province of China (No. 2014CFB277).

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