Scheme 1.
Schematic of the energy adsorption, transfer and emission processes (S: singlet state; T: triplet state).
Citation:
Yan Yang, Shuting Xu, Yanli Gai, Bo Zhang, Lian Chen. Recent Progresses in Lanthanide Metal-Organic Frameworks (Ln-MOFs) as Chemical Sensors for Ions, Antibiotics and Amino Acids[J]. Chinese Journal of Structural Chemistry,
2022, 41(11): 221104.
doi:
10.14102/j.cnki.0254-5861.2022-0138
Recent Progresses in Lanthanide Metal-Organic Frameworks (Ln-MOFs) as Chemical Sensors for Ions, Antibiotics and Amino Acids
English
Recent Progresses in Lanthanide Metal-Organic Frameworks (Ln-MOFs) as Chemical Sensors for Ions, Antibiotics and Amino Acids
Abstract:
Various ions and antibiotics, widely used in industry and clinical medicine, respectively, are massively discharged to atmosphere and water, resulting in severe pollutions on environment and potential threats to human health. Besides, amino acids, the primary substances for the establishment of proteins, cells and tissues, are crucial to human health. Therefore, seeking effective and practicable materials to detect aforesaid analytes is vitally meaningful. Metal-organic frameworks centered with lanthanide ions (Ln-MOFs), also known as lanthanide coordination polymers, are considered as a charming category of multi-functional hybrid crystalline materials with fascinating structures and incomparable luminescent characteristics. Benefited from their unique merits, Ln-MOFs have been largely developed as excellent luminescent sensors for fast and efficient sensing various analytes. In this review, we aim to introduce some of the recent researches between 2018 to 2022 on Ln-MOFs applied as chemical sensors for ions, antibiotics and amino acids based on luminescent quenching and enhancing effects, and provide an update and summary for the latest progresses in this field.
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INTRODUNTION
Metal-organic frameworks (MOFs), a burgeoning class of organic-inorganic hybrid crystalline materials, have exerted a strong momentum of growth in the past decades.[1-7] Self-assembly of various inorganic metal ions/clusters and organic linkers readily generates fascinating MOFs featured with diverse structures, lager porosity and surface areas, variable pore sizes, modifiable pore surface, et al.[8-15] As a special class of MOFs, lanthanide MOFs assembled by Ln3+ and organic ligands have more advantages than MOFs centered with transition metal ions, thanks to the unique luminescent properties of lanthanide ions, such as characteristic sharp emissions, high quantum yields, millisecond lifetimes, large Stocks shifts, and so on.[16-19] Besides, Ln3+ ions possess high coordination numbers from 6 to 13 and multiple coordination modes. When all lanthanide sites are coordinated with organic ligands, the structures can be stabilized. Thus, Ln-MOFs can be applied as a unique class of luminescent sensors compared with other traditional materials. On one hand, the porous structures of Ln-MOFs may provide more opportunities for host-guest interactions, and then increase the sensitivity of these luminescent sensors. On the other hand, the presence of open lanthanide sites or other functionalized sites, such as Lewis basic sites and Lewis acid sites supplied by organic linkers, is beneficial to forming interactions between these open active sites and various guest substances.[20-22] Besides, lanthanide ions, especially Eu3+ and Tb3+, exhibit sharp typical emissions. The luminescent response can be evaluated by monitoring the variations of emission intensities. Moreover, changes of the light colors for most Ln-MOFs can be distinguished by the naked eyes. In virtue of functionalized structures and characteristic luminescence, a great number of Ln-MOFs have been developed and utilized as luminescent chemical sensors for metal ions, [23-32] anions, [33-41] small organic solvent molecules, [42-50] nitro-aromatic explosives, [51-59] temperature, [60-66] antibiotics, [67-72] amino acids[73-75] and biomarkers[76-86], and so forth.
LUMINESCENCE of Ln-MOFS
For Ln3+ ions, the last electrons are successively filled in the 4f orbitals, giving the characteristic electron configuration of Ln3+ ions from [Xe]4f 0 to [Xe]4f14, corresponding to La3+ ions to Lu3+ ions. The shielding effects on the 4f electrons from the outer filled 5s and 5p subshells result in the Laporte forbidden of f-f traditions, giving the long-lived narrow emissions and characteristic colors for Ln3+ ions. Due to the Laporte-forbidden transitions, only weak luminescent emissions are observed when Ln3+ ions are direct excited.[87-89] Fortunately, this conundrum can be solved by introducing sensitizers, which can transfer the adsorbed energy to Ln3+ ions and then induce the strong luminescence of Ln3+ ions. Such energy transfer processes, known as "antenna effect", consist of four steps: light adsorption of the organic ligand, exciting the luminophore from its ground state (S0) to the singlet excited state (S1); energy transition from S1 to the triplet states (T1) through intersystem crossing process (ISC); intermolecular energy transfer from the T1 of ligands to the excited states of Ln3+ ions; radiative transition of excited Ln3+ ion to the ground state, resulting in the luminescence of Ln3+ ions (Scheme 1).[90-92] When the energy transfer from the ligand to the centered Ln3+ ion is thoroughly efficient, the emission of the ligand is completely quenched. If the efficiency of energy transfer is not high enough, the dual emissions of the ligand and Ln3+ ion co-exist.
Scheme 1
As discussed above, the energy matching between the triplet states T1 of ligands and the lowest excited states of Ln3+ ions plays a vital role in the luminescent properties of Ln-MOFs. The excitation energy will be released through non-radiative transition processes if Ln3+ ions are excited to non-emissive levels. The lowest excited states of Eu3+ and Tb3+ are 5D0 and 5D4 levels, located at 17267 and 20500 cm-1, respectively. According to Latva's empirical rule, for Eu3+ and Tb3+ ions, the energy gaps (ΔE = T1 - 5DJ) for optimal energy transfer processes from the ligands to Ln3+ ions should fall in the ranges of 2500-4000 and 2500-4500 cm-1, respectively. Thermal-activated energy back-transfer process will occur when the energy gap between ligands and Ln3+ ions is too small, while too big energy gaps may result in the reduction of energy transfer rates.[93-94] Therefore, selecting ligands with suitable triplet state energies is quite important for the design and synthesis of Ln-MOFs featuring with excellent luminescent performance.
In terms of the unique luminescent properties and diverse structures, the applications of Ln-MOFs on the detection for various harmful substances have been summarized in recent years.[95-102] Herein, we have researched and summarized recent progress in regard to the luminescent sensors based on Ln-MOFs towards ions, antibiotics and amino acids (Table 1 and Table 2).
Table 1
Table 1. Summaries of Mentioned Ln-MOFs, Relative Ligands and Sensing Targets
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Sensing target Ln-MOFs Ligands Refs. Fe3+ H3O[Tb(H2O)2(L)] 1 H4L = 4', 4''', 4''''', 4'''''''-(ethene-1, 1, 2, 2-tetrayl)tetrakis(([1, 10-biphenyl]-4-carboxylic acid)) 112 {[Eu(L)(H2O)]∙4H2O}n 2 H3L = 5-(2′, 5′-dicarboxylphenyl)picolinic acid 113 Tb2(L)3(DMF)4 3 H2L = 2-hydroxyterephthalic acid 114 {[Me2NH2][TbL]∙2H2O}n 10 H4L = 1-(3, 5-dicarboxylatobenzyl)-3, 5-pyrazole dicarboxylic acid 121 [Zn3Eu2(L)2(H2O)6]⋅6H2O 11 H6L = 1, 3, 5-triazine-2, 4, 6-triamine hexaacetic acid 122 Cu2+ {[Eu2(L)2(H2O)5]·3H2O}n 4 H3L = 5-(3′, 5′-dicarboxylphenyl)picolinic acid 115 {[Eu(L)(2H2O)]·(Hbibp)0.5}n 5 H4L = 2-(3′, 4′-dicarboxylphenoxy) isophthalic acid; bibp = 4, 4′-bis(imidazolyl)biphenyl 116 {[Eu2K(L)4(C2H5OH)]∙(H3O)∙(H2O)x}n 12 H2L = 2, 2′-bipyridine-6, 6′-dicarboxylic acid 123 {[Eu(HL)]·3DMF·3H2O}n 13 H4L = 1, 4-bis(2′, 2′′, 6′, 6′′-tetracarboxy-1, 4': 4, 4''-pyridyl)benzene 124 Cd2+ {(Me2H2N)[Eu(L)]·DMF·H2O}n 6 H4L = 2′-amino-[1, 1′: 4′, 1″-terphenyl]-3, 3″, 5, 5″-tetracarboxylic acid 117 [Tb2(L)4(phen)2(NO3)2] 14 HL = phenylacetic acid, phen = 1, 10-phenanthroline 125 Ag+ [La0.88Eu0.02Tb0.10(L)(DMF)2]n·H2O·0.5DMF 7 H3L = 4-(3, 5-dicarboxylatobenzyloxy)-benzoic acid 118 Hg2+ {[Eu4(L)6(phen)4]·m(H2O)(phen)}n 8 H2L = thiobis(4-methylene-benzoic acid); phen = 1, 10-phenanthroline 119 Mg2+ {[Eu2(L)1.5(DMA)3(H2O)2]·2DMA·2H2O}n 9 H4L = benzo-imide-phenanthroline tetracarboxylic acid 120 Cr2O72- {[Eu3(L)3(NO3)7]·NO3·ClO4}n 15 H2LCl2 = 1, 1′-bis(4-carboxyphenyl) (4, 4′-bipyridinium) dichloride 135 CrO42-/Cr2O72- {[Tb(L)(H2O)2]·2H2O}n 16 H3L = 1, 3, 5-tris-(carboxymethoxy)benzene 136 CrO42-/Cr2O72-/MnO4- {[Eu(L)(H2O)2]·5H2O}n 17 H4L+Cl- = 1, 3-bis(3, 5-dicarboxyphenyl)imidazolium chloride 137 Cr2O72- {[Eu2(L)2(H2O)2]·5H2O·6DMAC}n 18 H3L = 4, 4′-(((5-carboxy-1, 3-phenylene)-bis(azanediyl))bis(carbonyl)) dibenzoic acid 138 MnO4- {[Tb2(L)2(H2O)2]·5H2O·6DMAC}n 19 H3L = 4, 4′-(((5-carboxy-1, 3-phenylene)bis-(azanediyl))bis(carbonyl)) dibenzoic acid 138 MnO4- [Eu0.06Tb0.04Gd0.9(HL)1.5(H2O)(DMF)]·2H2O 20 H3L = 5-(3′, 5′-dicarboxylphenyl) nicotinic acid) 139 CO32- [Eu2(Hhpip)2(OAc)6] 21 Hhpip = 2-(2-hydroxyphenyl)imidazo[4, 5-f]-[1, 10]phenanthroline) 140 NO2- {[Tb(L)(OA)0.5(H2O)2]·H2O}n 22 H2L = chelidonic acid, H2OA = oxalic acid 141 C2O42 [Ln(L)(DMF)(H2O)(COO)]n 23 H2L = 4, 4-(9, 9-dimethyl-9H-fluorene-2, 7-diyl) dibenzoic acid 142 ODZ/NFT [Eu(L)(OH)]·xS 24 H2L = 5-(4-carboxyphenyl)picolinic acid, S = solvent molecule 150 NFT/SOM/DTZ {[Tb-(HL)(H2O)2]·x(solv)}n 25 H4L = 5, 5′-(((1, 4-phenylenebis-(azanediyl)) bis(carbonyl))bis(azanediyl))diisophthalic acid 151 NFT/NZF {[Eu2L2(DMF)4]·xDMF·yH2O}n 26 H3L = 5-(4-carboxybenzyloxy)isophthalic acid 152 TCH {[Eu(L)(H2O)]}n 27 H3L = 4, 4′, 4′′-s-triazine-2, 4, 6-tribenzoic acid 153 NFZ/FZD [Dy(L)(DMF)3]n 28 H3L = 1, 3, 5-tris(1-(2-carboxyphenyl)-1H-pyrazol-3-yl) 154 NZF/NFT RhB@[Me2NH2][Tb3(L)3(HCOO)]·DMF·15H2O 29 H3L = 3-(3, 5-dicarboxylphenyl)-5-(4-carboxylphenl)-1H-1, 2, 4-triazole 155 NIF [Tb(HL)(C2O4)]·3H2O 30 H2L = 2-(4-pyridyl)-terephthalic acid, C2O4 = oxalic acid 156 SMZ {Eu(L)DMF}n 31 H3L = 1, 3, 5-tris(4-carboxyphenyl)benzene 157 Glu [(CH3)2NH2]2[Tb6(μ3-OH)8(L)6(H2O)6] 32 H2L = 2-hydroxyterephthalic acid 159 Asp [[Tb(μ6-H2L)(μ2-OH2)2]·xH2O]n 33 H5L = 5, 5'-((5-carboxy-1, 3-phenylene)-bis(oxy))diisophthalic acid 160 Asp {[(CH3)2NH2]5[Tb5(L)5]·solvent}n 34 H4L = 1, 3, 5, 7-tetra(4-carboxybenzene)pyrene 161 Trp [Sm2(L)1.5(H2O)8]·6H2O 35 H4L = 1, 2, 4, 5-benzenetetracarboxylic acid 162 Tyr [La(HL)(DMF)2(NO3)] 36 H3L = 5-(4-(tetrazol-5-yl)phenyl)-isophthalic acid 163 DCN [Tb3(HL)(L)(H2O)6]·NMP·3H2O 37 H5L = 3, 5-di(2′, 4′-dicarboxylphenyl)benzoic acid 164 3, 5-DCA/3, 4-DCA Tb3+@{[NH2(CH3)2]4·[Cd6(L)4(HTz)1.5(H2O)6]·xS}n 38 H4L = 3, 5-di(2, 4-dicarboxylphenyl)pyridine, HTz = 1H-tetrazole 166 temperature ([(CH3)2NH2]Eu0.036Tb0.964L) 39 H4L = biphenyl-3, 30, 5, 50-tetracarboxylic acid 60 temperature [Yb0.05Nd0.95Cl(L)·(DMF)] 40 H2L = 2, 6-naphthalenedicarboxylic acid 61 humidity [DMA]3[Eu4L4·3DMA·7H2O] 41 H3L = [1, 1′-biphenyl]-3, 4′, 5-tricarboxylic acid 167 Table 2
Table 2. Comparison of Quenching Coefficient (Ksv), Limit of Detection (LOD), Response Time and Sensing Mechanism of Selected Ln-MOFsDownLoad:
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Ln-MOFs Sensing target Medium Quenching coefficient (Ksv, M-1) Limit of detection (LOD, M) Response time Sensing mechanism Refs. 1 Fe3+ H2O 3.50 × 103 138.8 ppm - competitive absorption and aFRET 112 2 Fe3+ H2O 1.88 × 104 5.70 × 10-7 10 s competitive absorption and energy transfer 113 3 Fe3+ DMF 1.62 × 105 3.50 × 10-7 < 60 s competitive absorption and electronic interaction 114 4 Cu2+ H2O 3.95 × 103 3.30 × 10-6 - energy absorption and weak coordination interaction 115 5 Cu2+ DMF 4.62 × 103 2.53 × 10-5 - weak interaction 116 Fe3+ DMF 4.84 × 103 1.32 × 10-5 - competitive absorption and weak interaction 6 Cd2+ H2O b23-fold - - coordination interaction between Cd2+ and COOH group 117 8 Hg2+ H2O 9.11 × 104 1.00 × 10-6 - profound interaction between Hg2+ and O from COOH 119 9 Mg2+ EtOH b10.4-fold 1.53 × 10-10 15 s weak coordination of Mg2+ with O atoms in the ligand 120 10 Fe3+ H2O 6.80 × 103 - < 40 s competitive absorption and weak interaction 121 11 Fe3+ EtOH/H2O 1.96 × 103 1.68 × 10-5 - competitive absorption and dynamic quenching 122 13 Cu2+ H2O 4.90 × 106 1.35 × 10-9 15 s coordination between the framework and Cu2+ 124 14 Cd2+ DMF/H2O - 5.0 × 10-7 - decomposition of the framework 125 15 Cr2O72- DMF 1.40 × 104 5.6 × 10-6 < 60 s competitive absorption 135 16 CrO42- H2O 1.11 × 104 6.5 × 10-7 10 s competitive absorption 136 Cr2O72- H2O 1.55 × 104 8.9 × 10-7 10 s competitive absorption 17 CrO42- H2O 1.74 × 103 - - competitive absorption and weak interaction 137 Cr2O72- H2O 1.36 × 103 competitive absorption and weak interaction MnO4- H2O 0.51 × 103 competitive absorption and weak interaction 18 Cr2O72- H2O 1.05 × 103 8.94 × 10-5 - competitive absorption 138 19 MnO4- H2O 1.20 × 103 4.48 × 10-8 competitive absorption 20 MnO4- H2O - 1.97 × 10-8 - competitive absorption and FRET 139 21 CO32- DMSO - c1.23 × 10-5 hydrogen bonding interaction 140 d7.80 × 10-5 hydrogen bonding interaction 22 NO2- H2O 4.74 × 105 2.82 × 10-8 - dynamic quenching behaviour 141 23 C2O42 DMA 2.07 × 103 1.65 × 10-5 - electrostatic interaction and energy-transfer 142 24 ODZ H2O 3.52 × 104 5.20 × 10-7 - competitive absorption 150 NFT H2O 2.33 × 104 4.30 × 10-7 competitive absorption 25 SOM H2O e1.10 × 103 - 10 s photo-induced electron transfer (PET) 151 NFT H2O 1.90 × 104 4.10 × 10-7 10 s PET and competitive absorption DTZ H2O 1.00 × 104 1.39 × 10-6 10 s PET and competitive absorption 26 NFT H2O 5.29 × 104 9.92 × 10-6 - PET 152 NZF H2O 4.38 × 104 1.14 × 10-5 PET 27 TCH H2O 3.09 × 105 4.88 × 10-6 - PET and competitive absorption 153 28 NFZ H2O - 4.76 × 10-8 - competitive absorption and PET 154 FZD H2O 4.82 × 10-8 competitive absorption and PET 29 NZF H2O 5.98 × 104 5.02 × 10-7 - PET and inner-filter effect (IFE) 155 NFT H2O 6.69 × 104 4.48 × 10-7 PET and inner-filter effect (IFE) 30 NIF DMF 1.10 × 104 8.10 × 10-5 - competitive absorption and collision interaction 156 31 SMZ H2O 4.60 × 104 6.55 × 10-7 - inner-filter effect (IFE) 157 32 Glu H2O - 3.60 × 10-6 - PET 159 33 Asp H2O 9.90 × 103 7.95 × 10-6 - collision interaction 160 34 Asp EtOH - 0.025 ppm - energy transfer from Asp to ligand 161 35 Trp H2O - 3.30 × 10-7 < 60 s coordination between Trp and Sm3+ 162 36 Tyr EtOH 1.40 × 104 3.60 × 10-6 - hydrogen bonding interaction 163 NFT EtOH 3.00 × 103 1.70 × 10-5 hydrogen bonding interaction 37 DCN H2O 6.42 × 104 1.4 × 10-7 30 s PET 164 38 3, 4-DCA DMF 25.11 mL∙mg-1 0.0033 mg∙mL-1 15 s IFE at low concentration and dynamic and static quenching at high concentration 166 3, 5-DCA DMF 30.21 mL∙mg-1 0.0026 mg∙mL-1 15 s 39 temperature solid-state f9.42% K-1 g73.9% - - 60 40 temperature solid-state f0.1 and 0.2% K-1 - - - 61 41 humidity solid-state - 0.0003% (v/v) 30 s coordination interaction between open Eu3+ sites and H2O 167 Ln-MOFS APPLIED AS LUMINESCENT SENSORS
Sensors for Ions
Sensors for Metal Cations. Metal cations, especially hazardous metal cations, such as Pb(Ⅱ), Cu(Ⅱ) and Fe(Ⅲ), are commonly utilized in industrial manufacture and play an important role in the biological system of human body. However, most metal cations are poisonous and bio-toxic, and can bring about protein denaturation, enzyme inactivation and even carcinogenesis if they are the beyond dangerous threshold.[4, 22] For instance, the overdose and deficiency of Fe3+ ions may cause a certain degree of lesion to the body function and then result in various diseases, such as liver cirrhosis, cardiac failure, diabetes mellitus and iorn-deficiency anemia.[103-104] Excessive amounts of Cd2+ ions in body can cause gastrointestinal dysfunction and bone softening, and even aggravate the burden of liver and kidney owing to the fact of difficult excretion. In addition, long-term exposure to the environment containing Cd2+ ions can result in anosmia, gingival spots or yellow rings.[4, 105] Therefore, developing effective, convenient and fast-responsive sensors for trace metal cations is extremely important. In recent years, numerous Ln-MOFs are developed and utilized to detect various in trace amounts.[106-112]
Bu et al. employed an aggregation-induced emission (AIE) linker to fabricate a new Tb-MOF (1) with super chemical stabilities in various organic solvents and aqueous solutions of pH = 1-14 for Fe3+ detection in aqueous solution.[113] Different from other reported Ln-MOFs, compound 1 displayed fluorescence derived from the ligand, rather than Tb3+ ions, because the energy gap between the triplet-state energy T1 of the ligand (20055 cm-1) and the lowest excited states of Tb3+ (5D4: 20500 cm-1) fell outside the energy gaps for efficient "antenna effect" (2500-4500 cm-1 for Tb3+). Featuring the outstanding stability and strong emission, 1 exhibited a remarkable response to Fe3+ based on luminescent "turn-off" effect with anti-interference ability and renewability, the Ksv and detection limit of which were 3.50 × 103 M-1 and 138.8 ppm, respectively. The presence of the large overlap between the UV-vis absorption band of Fe3+ and the excitation and emission spectra of 1 indicated that the mechanism of luminescent "turn-off" effect for 1 towards Fe3+ was the combination of competitive absorption between Fe3+ and 1, and fluorescence resonance energy transfer (FRET).
In another work, a new water-stable Eu-MOF (2) with an open 1D channel with the size of 8.9 × 8.9 Å2 was developed by Zang and co-workers.[114] Compound 2 displayed typical red emissions of Eu3+ ions. And luminescent sensing measurements revealed that the luminescent intensity of 2 was quenched by Fe3+ with the quenching efficiency of 97.7% (Figure 1a). Under 254 nm UV lamp, compared with other analytes, the color of suspension with Fe3+ changed from original red to blue, revealing that compound 2 can selectively distinguish Fe3+ distinguished by the naked eye (Figure 1b). Further, the fluorescence titration experiments indicated that compound 2 was a potential probe of Fe3+ with high sensitivity, giving the limit of detection (LOD) and Ksv of 0.57 mM and 3.50 × 103 M-1, respectively. Moreover, the exploration of response time was performed, showing the luminescent intensity of 2 gave a fast response just in 10 seconds (Figure 1c). In addition, to explore the quenching mechanism, a series of verified experiments were executed. As shown in Figure 2, when the suspension of 2 and the solution of Fe3+ were placed in position A and B, respectively, the luminescent intensity was obviously decreased. Whereas, negligible quenching effect was observed when the solution of Fe3+ was transferred to position C (blue and green curves in Figure 2b), indicating that such luminescent quenching phenomenon was caused by the overlap of the adsorption of between compound 2 and Fe3+. Notably, when the suspension of 2 was replaced by the suspension of the mixture of 2 and Fe3+, the quenching behavior became more efficient (red curve in Figure 2b), revealing the existence of strong interaction between compound 2 and Fe3+. Besides, the UV-vis absorption spectrum of Fe3+ had a large overlap with the excitation spectrum of 2, indicating that the competitive absorption might be another reason for such quenching phenomenon. That is, such quenching behavior was ascribed by the synergetic effect of the competitive absorption and the energy-transfer between Fe3+ and the framework of 2.
Figure 1
Figure 1. (a) Quenching efficiency of 2 in aqueous solutions containing various cations (1 mM). (b) Photographs of 2 in aqueous solutions containing various cations (1 mM) under 254 nm UV irradiation. (c) Luminescent intensities of 2 at different response time in the presence of Fe3+ at different concentrations. Reproduced with permission from Ref.[114]Figure 2
Figure 2. (a) Scheme of the luminescent quenching mechanism experiment. (b) The emission spectra of 2 (excited at 280 nm) with Fe3+ (Black curves: 2 solely in position A; red curves: mixture of 2 and Fe3+ in position A; blue curves: 2 in position A while Fe3+ in position B; green curves: 2 in position A while Fe3+ in position C). Reproduced with permission from Ref.[114]Recently, Fu' group have synthesized a novel Tb-MOF (3) with an ultra-high quantum yield (94.91%).[115] Compound 3 featured a 3D interpenetrating framework, where plentiful naked phenolic hydroxyls were located. Because of the presence of such exposed phenolic hydroxyls, compound 3 was considered to be a promising luminescent sensor for Fe3+ equipped with high sensitivity (Ksv = 162 570 M-1) and low LOD (0.35 μM). For simple and convenient detection of Fe3+, luminescent test papers were prepared and the changes in color with the addition of Fe3+ were easily distinguished with naked eyes. The binding energy at 532.9 eV of hydroxyl oxygen (C-OH) was found to move to 533.1 eV, indicating the presence of electronic interaction between hydroxyl supplied by phenolic hydroxyl groups and Fe3+. The excitation spectrum of 3 displayed an obvious overlap with the UV-vis absorption spectrum of Fe3+, implying the existence of competitive energy absorption between 3 and Fe3+ ions. In conclusion, affluent active sites combined with the commonly competitive adsorption were confirmed to be responsible for such charming luminescent quenching phenomenon.
A tri-carboxylate ligand stemmed from in situ decarboxylation reaction of another tetra-carboxylate ligand was selected by Li and co-workers to fabricate a novel 3D Eu-MOF (4) for sensing Cu2+.[116] Interestingly, compound 4 can maintain the integrity of the skeleton in water and common organic solutions (Figure 3a). Inspired by its excellent stability, the luminescent sensing experiments of metal cations were investigated. As displayed in Figure 3b, except for Cu2+, other metal cations showed ignorable influence on the emission intensity of 4. Compound 4 exhibited promising luminescent sensing ability for Cu2+ based on the luminescent quenching effect with the values of Ksv and the detection limit of 3951.5 M-1 and 3.3 μM through the titration experiments (Figure 3c and 3d). Also, as a potential luminescent sensor for Cu2+, 4 possessed high selectivity, anti-interference ability and recyclability (Figure 3e and 3f). The presence of weak coordination interaction between Cu2+ atoms and the free oxygen atoms from uncoordinated carboxylate groups might impede the energy transfer from the ligand to the central Eu3+ ion. In addition, the partial overlap between the absorption spectrum of Cu2+ and the excitation spectrum of 4 might be another possible reason for the quenching effect. Therefore, the possible quenching mechanism may be the combination of the competitive adsorption and weak interaction between Cu2+ and the uncoordinated oxygen atoms from carboxylate groups.
Figure 3
Figure 3. (a) The PXRD patterns of 4 soaked in different organic solvents. (b) Luminescent intensity at 615 nm of 4 immersed in aqueous solutions with different metal ions. (c) Luminescent spectra of 4 in aqueous solutions containing various Cu2+ concentrations. (d) The S-V plot of 4. (e) Anti-interference experiment of 4. (f) The recyclable experiments of 4 for sensing Cu2+. Reproduced with permission from Ref.[116]A 3D supramolecular framework (Eu-MOF, 5) connected by mixed ligands (H4dpc and bibp) for sensitive detection Cu2+ and Fe3+ was fabricated by Yang et al.[117] In this structure, binuclear clusters ([Eu2(COO)2]) were ligated to form a 1D metal chain. The neighboring chains are further linked to a 2D layer (type A) by the dpc4- ligands. Interestingly, another layer (type B) was composed by uncoordinated bibp ligands. Finally, a sandwich-shaped 3D supramolecular framework was further formed by the alternation of two kinds of layers (⋯A-B-A-B⋯)n through hydrogen bonding interactions. It is worth mentioning that compound 5 exhibited stable emission intensities in the pH range of 2-12 (Figure 4a). Furthermore, the emission intensity of compound 5 was obviously quenched by Cu2+ and Fe3+ without interference (Figure 4b). And the Ksv and LOD values of 5 were calculated to be 4.84 × 103 M-1 and 1.32 × 10-5 M for Fe3+ and 4.62 × 103 M-1 and 2.53 × 10-5 M for Cu2+, respectively. Multiple methods, such as power XRD, UV-vis spectroscopy, XPS and ICP-AES, were introduced to explore the quenching mechanism (Figure 4c). When introducing Cu2+, just the N 1s peaks were observed to shift from 398.88 and 401.18 eV to 399.48 and 401.08 eV, while the O 1s peaks kept unchanged, confirming the weak interaction between the N atoms and Cu2+. Whereas, when Fe3+ was introduced, not only did the N 1s peaks shift from 398.88 and 401.18 eV to 399.58 and 401.48 eV, but also the O1s peaks shifted from 531.38 and 532.78 eV to 531.78 and 532.98 eV, respectively, indicating weak interactions of Fe3+ with the N and O atoms from the bibp and dpc4- ligands. That is, the weak interactions between Cu2+, Fe3+ and the Lewis active sites (N atoms) from dpc4- or bibp ligands were considered as the main reason for such quenching behavior.
Figure 4
Figure 4. (a) Luminescence intensities and PXRD patterns of the 5 dispersed in aqueous solutions in the pH range of 2-12. (b) The Luminescence spectra and quenching percentage of 5 dispersed in DMF solutions with different metal cations. (c) N 1s and O 1s XPS spectra of compound 5, Cu2+@5 and Fe3+@5. Reproduced with permission from Ref.[117]For efficient detection of Cd2+ ion, a porous Eu-MOF (6) built from a tetra-carboxylate ligand functionalized with amino group was synthesized by Su and co-workers.[105] In the structure of 6, two kinds of 1D channels were found in bc layer, giving the porosity of 21% calculated by PLATON software. To explore the stability of compound 6, the fresh samples were dispersed in water, boiling water and common organic solvents, and then the PXRD was collected. The results indicated compound 6 can retain structural integrality in such various media. Solid-state luminescent measurements of 6 displayed typical red emission of Eu3+ with the lifetime and quantum yield of 597 μs and 4.2%, respectively. Considering the porous stable structure and excellent luminescent properties, the exploration for metal ions was arranged. When the concentrations of metal ions were 10 mM, all five metal ions including Cd2+, Al3+, Cr3+, Zn2+ and Hg2+ ions show non-negligible luminescent enhancing effects. Especially for Cd2+, the emission intensity was increased to 23-fold compared with the original level. Whereas, the selectivity of these metal ions was barely satisfactory owing to the mutual interference among these five target ions. As the concentration of metal ions decreased to 1 mM, the mutual interference was almost eliminate and compound 6 exerted selective luminescent detection of Cd2+ ion based on the luminescent enhancing effect. The appearance of Cd 3d peaks in Cd2+@Eu-MOF as well as the shift of O 1s peak of Cd2+@Eu-MOF from 531.5 to 531.7 Ev indicated that the interaction between Cd2+ and the carboxylate groups might occur, which amplified the energy transfer from the ligand to the Eu3+ and then enhanced the luminescent intensity of compound 6.
In another work, a luminescence-color change (LCC) sensor for sensing Ag+ ion was reported by Li and co-authors.[118] In the strategy of in-situ synthesis, a ternary co-doped Ln-MOF (La0.88Eu0.02Tb0.10-MOF, 7), in which the ratios of La/Eu/Tb were determined by the ICP, was obtained. Intriguingly, three isomorphic compounds displayed blue-green, red and green colors distinct by naked eyes, respectively. Moreover, as depicted in Figure 5a and 5b, when the excitation wavelengths were located at 303, 305 and 350 nm, compound 7 presented pure orange, white, and blue light, respectively. Benefited from the multi-luminescence, compound 7 might be a candidate for luminescence-color change (LCC) sensor. As expected, under the excitation at 350 nm, as the concentration of Ag+ ion gradually increased, the blue emission tuned by the ligand was continually weaken and the typical green emission of Tb3+ was constantly enhanced, while no obvious variations were observed of the emission intensity of Eu3+ (Figure 5c). That is, as Ag+ ions were introduced to the system, the efficiency of energy transfer process from Tb3+ to Eu3+ was hindered excited at 350 nm, resulting in the color change of compound 7 from blue to green. Additionally, as a LCC sensor for Ag+, compound 7 was recyclable, confirmed by the consistent PXRD patterns before and after immersing in Ag+ ions. The exploration of such LCC sensing mechanism was performed. When Ag+ ions were introduced in the suspension of compound 7, the emission of ligand was discovered to some degree of red shift, indicating the energy of π* orbits of the ligand was heightened and then became more matched with the resonance energy level of the Ln3+ ions (Figure 5d). In other words, the emission spectra of 7 in the presence of Ag+ consist of stronger characteristic emissions of Ln3+ ions and weaker ligand-center emission, causing the color of 7 to turn from blue to green.
Figure 5
Figure 5. (a) Luminescent spectra of compound 7 as excitation wavelengths change from 300 to 350 nm. (b) The corresponding CIE chromaticity diagram. (c) Luminescent spectra of 7 with the gradual addition of Ag+ ions. (d) Scheme for the effect of Ag+ on the energy transfer process from ligand to central metal ions. Reproduced with permission from Ref.[118]In addition, Sun et al. reported a new Eu-MOF (8) based on mixed organic ligands with good water stability for fast sensing and removing Hg2+ in water solutions.[119] Compound 8 displayed characterized red emission of center Eu3+ under 335 nm excitation. Featuring outstanding luminescent properties and super stabilities, the sensing experiments were executed. As described in Figure 6a, compound 8 exhibited fast and remarkable luminescent response towards Hg2+ based on luminescent quenching. As the concentration of Hg2+ increased, the luminescent intensity of 8 decreased gradually and the quenching efficiency reached up to its maximum value at the Hg2+ concentration of 4 × 10-2 mM, giving the Ksv and LOD values of 9.112 × 104 M-1 and 1.00 × 10-6 M, respectively (Figure 6c). Also, the presence of other metal ions had almost no influence on the quenching efficiency, indicating the excellent anti-interference of compound 8 on the application as luminescent probe for Hg2+ (Figure 6b). In addition, compound 8 had the capacity on the adsorption of Hg2+ with the adsorption quantity of 1.8160 mg of Hg per 100 mg. The peak of O 1s before and after immersion in Hg2+ solutions shifted from 531.79 to 532.43 eV, indicating the presence of strong interaction between Hg2+ and the O atoms from carboxylate groups of the aromatic rings, which might be responsible for such quenching behavior to Hg2+ (Figure 6d).
Figure 6
Figure 6. (a) Relative luminescent intensities of compound 8 in aqueous solutions with various metal cations. (b) Anti-interference experiments of 8. (c) Luminescent spectra of compound 8 in aqueous solutions with various concentrations of Hg2+. (d) XPS spectrum for the O 1s region of 8 before and after immersion in Hg2+. Reproduced with permission from Ref.[119]In 2020, Dai and co-workers introduced a conjugated tetracarboxylate ligand to develop a 3D Eu-MOF (9) constructed from a conjugated tetra-carboxylate ligand with a new 4, 4, 6-connected wxk1 topology via a solvent regulation strategy (Figure 7a).[120] Room-temperature luminescent spectrum in solid state of 9 showed no typical emission of Eu3+, probably due to the presence of highly disordered guest solvent molecules. The suspensions of compound 9 containing diverse metal ions were prepared and the luminescent spectra were collected and compared. Different from most metal ions which show no obvious influence on the luminescence intensity of compound 9, Mg2+ ions gave rise to a 10.4-fold amplification of the emission intensity with the detection limit of 1.53 × 10-10 mol/L (Figure 7b). Also, the time-dependent response of Mg2+ towards the emission intensity was researched, and the results indicated that when Mg2+ ions were added in the suspension of compound 9 just for 15 seconds, an apparent luminescent enhancing effect was observed, and the enhancing efficiency was magnified to the maximum at 600 seconds with the emission intensity by 10.4 times compared to the original value (Figure 7c). After five runs, the emission intensity and enhancing efficiency were restored (Figure 7d). The mechanism of such luminescent enhancing behavior was attributed to electron transfer interaction between the Mg2+ ions and the ligand, confirmed by XPS spectra of Mg2+-incorporated 9, in which a new peak at 532.1 eV assigned to O-Mg appeared, suggesting the existence of weak coordination of Mg2+ with O atoms in the ligand.
Figure 7
Figure 7. (a) Scheme of the synthetic strategy for compound 9. (b) Relative luminescent intensities of 9 in solutions containing different metal cations (0.25 mmol L-1). (c) Time-dependent luminescent intensities of the suspension for 9 after adding Mg2+ solution (200 μL). (d) The recyclable experiments of 9 for sensing Mg2+. Reproduced with permission from Ref.[120]In addition, some other Ln-MOFs were reported and employed to detect metal cations in recent years. A Tb-MOF (10) with high air and hydrolytic stability was reported by Hong and co-workers.[121] Numerous Lewis basic pyridyl active sites from the ligand were uncovered inside the structure and made compound 10 a potential sensor for Fe3+ with high sensitivity, fast response and anti-jamming performance. Recently, a hetero-metallic luminescent probe for Fe3+ based on a mixed-metal organic framework (MM-MOF), ZnII-EuⅢ-MOF (11), was prepared by Bai' group.[122] The dynamic quenching mechanism was verified to be the main inducement by the fact that the luminescent lifetime of 11 decreased as the concentrations of Fe3+ increased, proving some interactions occurred between Fe3+ and compound 11. And the Ksv and LOD values were 1.96 × 103 M-1 and 1.68 × 10-5 M, respectively. Another hetero-metallic anionic metal organic framework, EuⅢ-KI-MOF (12), as a sensitive luminescent sensor for Cu2+, was reported by Yang' group.[123] Profited by its anionic framework, 12 exhibited significant adsorption capacity for Cu2+ (143.88 mg/g) combined with luminescence quenching. Recently, Liu and co-workers fabricated a highly water-stable Tb-MOF, (13), with two kinds of 1D channels, applied as luminescent sensors for Cu2+ based on obvious luminescent quenching effects.[124] Notably, as a selective and sensitive sensor for Cu2+, compound 13 exhibited stable quenching efficiency in various media, including deionized water, tap water, and river water, with the Ksv and LOD in water of 4.90 × 106 M-1 and 1.35 × 10-9 M, respectively. In another work, two kinds of ligands (phenylmalonic acid and phenanthroline) were introduced to construct a dinuclear Tb-MOF (14).[125] Compound 14 possessed arresting stability in aqueous solutions in different acid and basic conditions (pH = 4, 7 or 10). Notably, luminescent sensing tests revealed that compound 14 was a promising luminescent probe for Cd2+ based on "turn off" effect with the LOD of 5 × 10-7 M, caused by the decomposition of Tb-MOF framework.
Sensors for Anions. Anions, essentially for biochemical processes, are commonly involved in the fields of industries and agricultures. Excessive anions discharged to environment can have hazardous influence on human health and ecology.[33] For instance, hexavalent chromium (Cr(Ⅵ)), designated as a human cancerogen by International Cancer Research Center and American Toxicology Organization, has serious carcinogenicity and potential teratogenicity, which mostly existed in the form of Cr2O72- and CrO42-. Cr(Ⅵ) can intrude into the body through the digestive tract, respiratory tract, skin and mucous membrane, leading to a series of extremely serious damage.[126-127] Thus, comprehensive attention should be paid to the development of functional sensors for effective and quantificational detecting various anions. In recent years, numerous Ln-MOFs are developed and utilized to detect anions in trace amounts.[128-134]
Gai's group reported a cationic Eu-MOF (15) using a viologen-based zwitterionic ligand for selective detecting Cr2O72-.[135] In compound 15, a trinuclear cluster [Tb3(L)6] as a secondary building unit was linked with other six neighboring trinuclear clusters through six protonated ligands. When excited at 376 nm, strong typical emission peaks of central Eu3+ were observed in the emission spectrum of compound 15, revealing that europium ions can be effectively sensitized by the ligand. The detecting potential of compound 15 to anions was explored through introducing various anions to the DMF suspension of compound 15. As shown in Figure 8a, when most anions, such as ClO4-, MnO4-, F- and NO3-, were added to the suspension for compound 15, ignorable or slight luminescent quenching effects occurred on the emission intensity of 15 by monitoring the strongest emission for Eu3+ at 622 nm. Notably, remarkable luminescent quenching efficiency of 97.8% was discovered as Cr2O72- ion was injected into the prepared suspension. Further quantitative experiments were arranged to estimate the sensitivity of 15 towards Cr2O72- ion. The results indicated that with the gradual increase of Cr2O72- concentration from 0 to 2.5 mM, the luminescent intensity of 15 decreased gradually, giving the values for Ksv and LOD of 1.40 × 104 M-1 and the LOD of 5.6 × 10-6 M (Figure 8b). The UV-vis spectrum of Cr2O72- displayed a broad overlap at 310-410 nm with the absorption of 15, implying competitive absorption between compound 15 and Cr2O72- was considered as the primary factor for such pronounced luminescent quenching phenomenon.
Figure 8
Figure 8. (a) Relative luminescent intensities of 15 soaked in various anions (1.5 mM; before (blue column) and after adding Cr2O72- (red column)). (b) Luminescent spectra of 15 soaked in solutions with different concentrations of Cr2O72- (λex = 376 nm). Reproduced with permission from Ref.[135]Recently, another sensor for Cr2O72- was developed by Wang's group in 2022.[136] By the reaction of Tb(NO3)3∙6H2O with a tricarboxylate ligand, namely 1, 3, 5-tris-(carboxymethoxy)benzene, a super stable Tb-MOF (16) was successfully synthesized. Stability experiments indicated that the structural integrity and luminescent performance of compound 16 can keep their originallevels in aqueous solution with the pH range of 3 to 12, confirming the high stability of 16 on the two aspects of structure and luminescent intensity. Encouraged by the super stability and outstanding luminescent properties, the performance of 16 on the detection for anions was explored. As shown in Figure 9a, the luminescent intensity of 16 was obviously quenched by CrO42-/Cr2O72- ions and the quenching efficiencies were calculated to be 99.11% and 98.81% for CrO42- and Cr2O72-, respectively (Figure 9b). Therefore, compound 16 had the potential as a luminescent probe for CrO42-/Cr2O72- ions. Based on the titration tests, the quenching constants (Ksv) were calculated to be 1.552 × 10-4 M and 1.1134 × 10-4 M for CrO42-/Cr2O72- ions, respectively (Figure 9c and 9d). And the LODs were lower to be 0.65 and 0.89 μΜ for CrO42-/Cr2O72- ions, respectively (Figure 10e and 10f). Moreover, the recycling and competitive measurements manifested that compound 16 was equipped with good reproducibility and antiinterference in the application on detecting CrO42-/Cr2O72- ions. The quenching mechanism was also attributed to the competitive absorption between compound 16 and CrO42-/Cr2O72- ions.
Figure 9
Figure 9. (a) Relative luminescent intensities of compound 16 in aqueous solutions with various anions. (b) Quenching efficiencies of compound 16 in aqueous solutions with various anions. Luminescent spectra of compound 16 in aqueous solutions with different concentrations of (c) Cr2O72- and (d) CrO42- ions. The S-V plots of 16 towards (e) Cr2O72- and (f) CrO42- ions. Reproduced with permission from Ref.[136]Figure 10
Figure 10. (a) CIE chromaticity diagram of 20 with excitation wavelengths varying from 295 to 385 nm. (b) Relative luminescent intensities of 20 immersed in the aqueous solutions containing different anions (1 mM) (λex = 365 nm, red: 613 nm; green: 544 nm; blue: 462 nm). (c) Luminescent spectra of 20 in aqueous solution with different concentrations of MnO4- excited at 365 nm. (d) The S-V plots of 20. (e) Anti-interference experiments of 20. Reproduced with permission from Ref.[139]In 2019, Wang and co-workers reported a multi-responsive luminescent sensor for CrO42-, Cr2O72- and MnO4- ions based on a water-stable Eu-MOF (17).[137] Compound 17 exhibited a 3D porous framework with a 1D open channel, in which uncoordinated O atoms from carboxylate groups were located as open O active sites. In order to evaluate the potential of compound 17 on sensing various anions, luminescent spectra of ground samples of 17 in aqueous suspensions containing various anions (0.01 M) were monitored and analyzed. The results displayed that luminescent intensities of the suspensions with CrO42-/Cr2O72-/MnO4- ions were decreased dramatically to almost zero. Furthermore, as the concentrations of CrO42-/Cr2O72-/MnO4- anions gradually increased, the luminescent quenching efficiencies of 17 visibly enhanced, and reached their maximum values at CrO42-/Cr2O72-/MnO4- concentrations of 3.182/3.75/8.628 mM, respectively. And the Ksv values were 1.74 × 103/1.36 × 103/0.51 × 103 M-1 for CrO42-/Cr2O72-/MnO4-, respectively. Therefore, compound 17 can act as a multi-responsive luminescent probe for CrO42-/Cr2O72-/MnO4- anions.
In another report, Liu et al. developed two isostructural lanthanide metal-organic frameworks, Eu-MOF (18) and Tb-MOF (19).[138] Single-crystal X-ray diffraction revealed compounds 18 and 19 can be simplified as a (5, 7)-connected 3D framework with the point symbol of {32∙44∙54} {34∙46∙56∙65}. Solid-state luminescent spectra in solid state of compounds 18 and 19 were collected. The results show that 18 exhibited typical red emission of Eu3+ ion peaked at 590 and 619 nm, and characteristic green emissions of Tb3+ ion centered at 494, 548, 588 and 624 nm were also observed in the luminescent spectrum of compound 19. Considering the excellent luminescent properties of 18 and 19, the luminescent sensing measurements of them towards anions were performed. Interestingly, the responses of 18 and 19 towards various anions were different. For compound 18, the luminescent intensity was markedly quenched by Cr2O72- ion (1 mM) with the values of Ksv and the detection limit of 1052 M-1 and 8.94 × 10-5 M, respectively; whereas, for compound 19, a remarkable luminescent quenching effect occurred when the sample was immersed in suspension with MnO4- ion (1 mM), and the quenching constant (Ksv) and detection limit (LOD) were calculated to be 1200 M-1 and 4.48 × 10-5 mM. Similar luminescent quenching mechanism with other reported Ln-MOFs was confirmed: competitive adsorption between Cr2O72-/MnO4- anions and compounds 18 and 19.
Different from aforesaid luminescent sensor for MnO4- based on single-emission lanthanide ion, a tri-emission lanthanide metal-organic framework, Eu0.06Tb0.04Gd0.9-MOF (20), for sensing MnO4- was prepared by Wang and co-workers.[139] As displayed in Figure 10a, when the excitation wavelength was adjusted from 295 to 385 nm, the color of compound 20 changed from yellow to white. That is, compound 20 was proved to be a white-light-emitting material. Besides, when dispersed in aqueous solutions with different pH, no changes were found both in the luminescent intensities and PXRD curves, demonstrating the luminescent performance and structure stability of 20. In view of the precious white-light emission property and outstanding stability of compound 20, the luminescent sensing tests for sensing anions were executed. The results illustrated that among all the anions, just MnO4- ion generated an amazing quenching effect on the luminescent intensity of 20 (Figure 10b). The high sensitivity of 20 towards MnO4- was verified by the luminescent titration study. With the stepwise addition of MnO4- concentration, the luminescent quenching phenomenon became increasingly apparent and the quenching efficiency increased rapidly, giving the Ksv value of 1.97 × 10-8 M (Figure 10c). Furthermore, the anti-jamming capability and recyclability of 20 applied as a luminescent sensor for ion were also researched, and the results stated that compound 20 can be regarded as a tri-emission luminescent sensor for MnO4- decorated with virtues of high selectivity, good recyclability and satisfying interference immunity (Figure 10d). The overlap between the UV-vis absorption spectrum of MnO4- and the excitation/emission spectra of 20 suggested that the energy can be competitively adsorbed by MnO4-, leading to the luminescent quenching effect. Besides, the luminescent lifetime researches of 20 before and after being immersed in MnO4- indicated the presence of static quenching mechanism. Thus, dual function of competitive adsorption and the static quenching mechanism resulted in such luminescent quenching effect.
Compared with CrO42-/Cr2O72-/MnO4-, the luminescent sensors for CO32- were rarely reported. Carbonate, such as Na2CO3, K2CO3 and (NH4)2CO3, are usually used as food additives. Excessive intake of carbonate may disrupt the hormone signaling for development and reproduction. Therefore, for detecting CO32-, Cai et al. selected an organic ligand with a hydroxyl group, 2-(2-hydroxyphenyl)imidazo[4, 5-f][1, 10]phenanthroline. By the reaction of europium nitrate and the ligand, a 0D Eu-MOF (21) was successfully prepared.[140] In the structure of compound 21, the hydroxyl group was failed to coordinate with the central Eu3+ ion. The solid-state luminescent spectrum of 21 exhibited characteristic emissions of Eu3+ and very weak emission of ligand was observed, implying that the selected ligand can efficiently sensitize the central Eu3+ ion. Interestingly, when soaked in DMSO solution (10 μM), the emissions of Eu3+ ion for 5D0 → 7F2 transition at 624 nm and 5D0 → 7F5 transition at 704 nm became very weak, and the emission of ligand centered at 470 nm became quite strong, causing the light color to turn from red to blue (Figure 11a). This phenomenon was due to the reflected solvent effect of DMSO on compound 21, hindering the energy transfer from the ligand to Eu3+ ion. When introducing CO32- ion into the DMSO solution of compound 21, an apparent red-shift of the ligand emission centered at 470 nm appeared, and the emission color changed from blue to green distinguished by naked eyes readily (Figure 11a). Further quantitative experiments displayed that as the concentration of CO32- ion increased, the luminescent intensity at 470 nm decreased gradually and a new emission band appeared centered at 542 nm as follows. And the LOD at 542 and 469 nm was calculated to be 12.3 or 7.8 μM, respectively. The quenching mechanism might be ascribed to the hydrogen bonding interactions: the OH groups form the phenyl groups or the imidazole NH groups can act as potential hydrogen donors to form hydrogen bonds with the CO32- ion.
Figure 11
Figure 11. (a) Luminescent spectra of 21 dispersed in DMSO solution (10 μM) with various anions (1 mM) (λex = 350 nm). (b) Luminescent spectra of 21 in DMSO solution with different concentrations of CO32- ion (λex = 350 nm). Reproduced with permission from Ref.[140]In another work, a water-stable 3D Tb-MOF (22) with high hydrolysis-stability for sensing nitrite ion, widely used in pickled products and ham sausages, was reported by Cheng and co-authors.[141] The typical emission peaks for Tb3+ centered at 489, 545, 585 and 621 nm were observed in the luminescent spectrum of compound 22 in solid-state at room temperature. The triplet state energy level T1 of the ligand was calculated to be 22831 cm-1 based on the phosphorescence spectrum and the UV-vis spectrum of Gd-MOF at 77 K. And the energy gap △E between T1 of the ligand and the lowest emissive level of Tb3+ (20500 cm-1) was 2331 cm-1, which fell perfectly in the range of 2500-4500 cm-1, an optimal energy transfer process from the ligand to Tb3+ needed. The result suggested that Tb3+ ion can be sensitized by the ligand effectively. Luminescent sensing experiments for various salts were performed, and the results revealed different from other salts, nitrite induced an obvious luminescent quenching effect on the emission of 22 with the quenching efficiency of 83.2%, indicating it can be applied as a luminescent sensor for NO2- ion. Further titration tests with different volumes of NO2- (3, 5, and 10 μL) confirmed compound 22 can detect NO2- ion sensitively with the values of Ksv and the detection limit to be 4.82 × 105 M-1 and 28.25 nm, respectively. Additionally, anti-interference and recycling measurements verified that as a promising luminescent sensor for NO2-, compound 22 was recyclable at least 5 times and anti-interference. Furthermore, in order to evaluate the ability of 22 on detecting NO2- in practical conditions, tap water was introduced. The recoveries were calculated to be 97.7-100.9%, indicating excellent performance on sensing NO2- in actual conditions. The well-matched PXRD patterns before and after being immersed in NO2- and no obvious overlap between the UV-vis spectra of the ligand and NaNO2 excluded the possible mechanism aroused by the collapse of the skeleton or competitive absorption. The diminishing lifetime with the increase of NO2- concentration revealed the presence of dynamic quenching progress. The triplet state energy level T1 of NaNO2 was calculated to be 19084 cm-1 with the energy gap between the lowest emissive level of Tb3+ (20500 cm-1) being 1416 cm-1, indicating the existence of energy transfer between 22 and NaNO2. Thus, dynamic quenching behavior occurred.
In addition, a new ligand was synthesized to construct a 3D Eu-MOF (23) for sensing C2O42-.[142] Luminescent sensing measurements were performed and the results indicated that compound 23 can detect C2O42- ion with high quenching efficiency and the Ksv and LOD values were calculated to be 2.07×103 M-1 and 16.5 μM, respectively.
As we all know, the fast and accurate detection of ions is of great importance in human health and environmental protection. Herein, we list various luminescent sensors based on Ln-MOFs for detecting different ions, including Fe3+, Cd2+, Cu2+, Zn2+, Ag+, Hg2+, Mg2+, Cr2O72-/CrO42-, MnO4-, CO32-, C2O42- and NO2-. In the short run, the perspective of functionalized Ln-MOFs for rapid detecting various ions looks bright based on brilliant designs and strategies.
Sensors for Antibiotics
Antibiotics, a total of 20 classes containing 260 species, have been widely used in treating diseases caused by bacterial and fungal infections.[67] Whereas, only a small fraction of consumed antibiotics can be absorbed and metabolized by the body, and a large amount of the excessive antibiotics are excreted via urine and feces, being one of the dominating pollutants in water. The terrible thing is that varieties of antibiotics have been detected in subsidiary agricultural products, subsoil water, river water and domestic water, which can penetrate into living organisms through the food chain. Unfortunately, it is difficult to remove antibiotics and their metabolites from the environment through traditional water purification methods.[143-144] Based on the above discussion, it is very important to seek an efficient and accurate method for antibiotic detection. Recently, some luminescent sensors based on lanthanide MOFs have been explored and applied to fast and satisfactory detection for antibiotics.[145-149]
Li et al. selected a di-carboxylate ligand, 5-(4-carboxyphenyl)-picolinic acid, as an organic linker to design and construct a porous 3D Eu-MOF (24), containing a 1D open channel with the void ratio of 40.3%.[150] The PXRD patterns before and after dispersion in aqueous solutions with various pH (pH = 4-10) and solutions containing diverse antibiotics were found to be well matched, indicating the super stability of compound 24. The Eu-MOF in solid state exhibited significant emission of center Eu(Ⅲ) ions, peaked at 596, 619, 655 and 703 nm, assigned to the transition of 5D0 → 7FJ (J = 1, 2, 3 and 4). Moreover, studies on the detection for antibiotics based on the stable and highly luminescent Eu-MOF were carried out. As shown in Figure 12a, ornidazole (ODZ) caused a remarkable luminescent quenching behavior excited at 320 nm with the Ksv and LOD values being 3.52 × 104 M-1 and 5.2 × 10-7 M, respectively. Under 359 nm excitation, the emission intensity of compound 24 was notably quenched by nitrofurantoin (NFT), giving the Ksv and LOD values of 2.33 × 104 M-1 and 4.3 × 10-7 M, respectively (Figure 12b and 12d). That is, compound 24 can be used as an effective luminescent sensor for ODZ and NFT with highly sensitive, excellent stable and anti-interference abilities. The competition adsorption between compound 24 and ODZ and NFT might be responsible for such luminescent quenching phenomenon.
Figure 12
Figure 12. Relative luminescent intensities of 24 in aqueous solutions with different anions (2.0 × 10-4 M) (a) excited at 320 nm and (b) 359 nm. Luminescent spectra of 24 in solutions with various concentrations of (c) ODZ excited at 320 nm and (d) NFT excited at 359 nm. Reproduced with permission from Ref.[150]A new tetra-carboxylate ligand functioned with urea groups, 5, 5′-(((1, 4-phenylenebis(azanediyl))bis(carbonyl))bis(azanediyl))-diisophthalic acid, was elaborately designed and introduced to synthesize a 3D porous framework, Tb-MOF (25), for detecting antibiotics by Zheng' group.[151] Beneficially, compound 25 possessed an unusually stable structure, since the PXRD patterns after being soaked with water at acid, neutral and basic conditions as well as the aqueous solutions containing various antibiotics were well-matched with that of the as-synthesized one. When excited at 336 nm, luminescent spectrum of 25 at room temperature consisted of two main kinds of emissions: one broad emission band centered at 400 nm was ascribed to the ligand and the others were attributed to the 5D4 → 7FJ (J = 6-2) transitions of Tb3+ ion peaked at 488, 544, 585, 620, and 650 nm. Inspired by the stable structure and specific luminescent properties, the capacities of compound 25 on the detection for antibiotics were performed. Three different classes of antibiotics, including nitrofurans (NFs), nitroimidazoles (NMs) and sulfonamides (SAs), displayed distinct influence on the emission intensity of compound 25 based on the "turn on" or "turn off" effects (Figure 13a). As shown in Figure 13b, for nitrofurans (NFs), the emission for ligand at 400 nm was slightly quenched and the emissions for Tb3+ ions were markedly quenched. And the ratio of luminescent intensity (I544/I400) was discovered to stepwise increase as the concentrations of NFs were gradually added (Figure 13d). For nitroimidazoles (NMs), the variation trend of the emission intensities of ligand and Tb3+ ions was similar to that of nitrofurans (NFs) while the values of (I544/I400) decreased on the contrary with the addition of NMs; whereas, for sulfonamides (SAs), the emission intensities of ligand as well as the centered Tb3+ were enhanced at different degrees (Figure 13c), the I544/I400 values of which were scarcely changed after immersion in solutions containing SAs. Further researches proved that compound 25 might be applied as a first-reported dual-emission self-calibrating luminescent sensor for differentiating such three different classes of antibiotics featuring high sensitivity, fast-response and recycle abilities (Figure 13e and 13f). To investigate the sensing mechanisms, a series of experiments were arranged. The luminescent lifetime remained unchanged before and after soaking in antibiotics, revealing that the luminescent quenching might be due to the interaction between the antibiotics and ligand instead of Tb3+ ion. In addition, the lower energies of LUMOs for nitroantibiotics compared with that of the ligand gave the chance for energy transferring from the ligand to nitro-antibiotics (photoinduced electron transfer (PET)), leading to the luminescent quenching behavior. While the energy of LUMO for SOM was higher than that of the ligand, suggesting that SOM was able to be regarded as an electron donor, giving the luminescent enhancing effect. In addition, competitive absorption between nitroantibiotics and 25 was considered as another reason for such luminescent quenching effect. Therefore, the combination of PET and the competitive absorption jointly caused the luminescence quenching effect towards the nitro-antibiotics.
Figure 13
Figure 13. (a) Related luminescent intensities of 25 with different antibiotics. Luminescent spectra of 25 with different concentrations of (b) NFT and (c) SOM. (d) The IR/I0R of 25 after adding different antibiotics. (e) Luminescent response time of 25 towards NFT. (f) The recyclable experiments of 25 for NFT. Reproduced with permission from Ref.[151]Another efficient luminescent sensor for antibiotics in aqueous solution was reported by Wang and co-authors.[152] In this work, a tripodal-shaped semi-rigid ligand, 5-(4-carboxybenzyloxy)-isophthalic acid, was utilized and reacted with europium nitrate under solvothermal conditions to generate a 3D Eu-MOF (26) simplified to a (3, 6)-connected flu-3, 6-C2/c net through the TOPOS analysis. Luminescent spectrum at room temperature of compound 26 was composed of four characteristic emission peaks of Eu3+ ions centered at 590, 614, 650, and 698 nm decorated with red light. No broad emission of the ligand was observed, indicating the effective "antenna effect" of the ligand to Eu3+ ions. Seven common antibiotics were selected to explore the potential of compound 26 on the detection for antibiotics. Most antibiotics displayed almost ignorable influence on the luminescent spectra of compound 26. Significantly, remarkable luminescent quenching effects on the emission intensity were discovered when NFT and NZF were introduced with the quenching efficiencies of 99.65% and 99.69%, respectively. The values of Ksv were calculated to be 5.29 × 104 M-1 for NFT and 4.38 × 104 M-1 for NZF. And the detection limits for NFT and NZF were 9.92 and 11.4 μM, respectively. Lower LUMO energy of NFT and NZF made the energy transfer process from the ligand to NFT/NZF, resulting in the promising sensing for NFT/NZF based on the luminescent quenching behavior.
Recently, Xu and co-workers have reported a three-dimensional Eu-MOF (27) constructed by a tri-carboxylate ligand, 4, 4′, 4′′-s-triazine-2, 4, 6-tribenzoic acid.[153] Interestingly, compound 27 possessed excellent structural stability and luminescent stability in acid and basic aqueous solutions with pH range of 2-14 as well as in diverse common organic solvents. Therefore, the luminescent sensing abilities of compound 27 towards antibiotics were estimated and the results revealed that tetracycline hydrochloride (TCH) had a dramatically quenching effect on the emission intensity of compound 27. The values of Ksv and LOD (the detection limit) were calculated to be 3.09 × 105 M-1 and 7.35 ppm, respectively. Additionally, the luminescent intensity and quenching efficiency can restore their original levels after five cycles, indicating compound 27 might be applied as a promising luminescent probe for TCH with satisfactory regeneration and stability. The lower LUMO energy as well as the overlap between the UV-vis absorption spectra of TCH and the excitation spectrum of compound 27 led to the luminescent quenching effect. That is, the combined action of photo-induced electron transfer (PET) and competitive absorption between 27 and TCH was regarded as the main inducement for such quenching phenomenon.
In addition, some other Ln-MOFs were reported and engaged in the application as luminescent sensors for detecting antibiotics in recent years. In 2019, Gao et al. reported a water-stable 2D Dy-MOF (28) decorated with two different emission bands at 385 nm for the ligand and 481 and 573 nm from the centered Dy(Ⅲ).[154] Therefore, compound 28 was utilized as an effective ratio-metric luminescent probe for FZD and NFZ with the detection limit of 0.0476 μM for FZD and 0.0482 μM for NFZ (Figure 14). At the same time, Li's group synthesized an anionic Tb-MOF by the reaction of terbium nitrate and a tri-carboxylate ligand, 3-(3, 5-dicarboxylphenyl)-5-(4-carboxylphenl)-1H-1, 2, 4-triazole).And a host-guest composite, RhB@Tb-MOF (29), was obtained by incorporating a cationic dye of rhodamine B (RhB) via cation exchange process.[155] The solid-state emission spectrum of compound 29 was composed of relatively weak emission of Tb3+ ions peaked at 545 nm and quite strong emission at 630 nm derived from RhB. Notably, nitrofuran antibiotics (NZF and NFT) represented remarkable luminescent quenching behavior on the luminescent intensity of 29 by monitoring the 5D4 → 7FJ transition of Tb3+ ion at 545 nm, and the quenching efficiency was gradually enhanced as the concentrations of NZF and NFT slowly increased, giving the detection limit of 0.502 and 0.448 μM for NZF and NFT monitored at 545 nm, respectively. Additionally, when quinolone antibiotics such as CPFX and NFX were added to the system of 29, distinguishing changes of the luminescent colors were observed by naked eyes from yellow to white to blue with the addition of CPFX and NFX concentrations. In 2020, Liu and co-authors employed an organic ligand, 2-(4-pyridyl)-terephthalic acid, to design and form a 3D Tb-MOF (30) with excellent stability in acid and basic water and common organic solvents.[156] Thanks to the abnormal stability and promising luminescent performance, compound 30 exhibited adequate luminescent quenching effects for nitrofuran (NIF) with the detection limit and Ksv of 1.1 × 104 M-1 and 8.1 × 10-5 M, respectively. In another research, Wang and co-workers reported a 2D Eu-MOF (31) taken on two different forms, needle-shaped colorless crystals (20-80 μm) and colorless power (2 μm).[157] The Eu-MOF power displayed super structure stability and luminescence stability in water. Luminescent detection on sensing antibiotics disclosed compound 31 had the capacity on effectually sensing for sulfamethzine (SMZ) with high sensitivity and anti-interference, giving the values of Ksv and detection limit to be 4.598 × 104 M-1 and 0.6554 μM, respectively. Cooperative effect of the electron transfer and inner-filter effect (IFE) was considered as the predominant impetus for the luminescent sensing effect towards SMZ.
Figure 14
Figure 14. (a) Luminescent spectra of 28 for different antibiotics (1 mM) in water solutions (λex = 315 nm). (b) Decoded map for different antibiotics (0.1 mm) based on the ratios of I385/I481 and I385/I573. Luminescent spectra of 28 soaked in water with different concentrations of (c) NFZ and (d) FZD. Reproduced with permission from Ref.[154]Herein, we summarize various Ln-MOFs for sensing antibiotics, including ODZ, NFT, SOM, NZF, TCH, FZD, NFZ, NIF and SMZ. Luminescent sensors for antibiotics based on Ln-MOFs exhibit lots of advantages, such as fast-response, high sensitivity, realtime detection, and so on. It is worthy of trust that Ln-MOFs applied as potential luminescent sensors for antibiotics will have a promising future.
Sensors for Amino Acids
As we know, there are more than 20 kinds of amino acids in human body, including eight essential amino acids, which are unable to synthesize by self and have to uptake from daily diet. Importantly, amino acids are primary substances for the establishment of proteins, cells and tissues.[73, 158] The balance of various amino acids is a prerequisite for human health since amino acids are involved in almost all physiological activities in body. Notably, any deficiency of amino acid can impact the normal functions of immune system.[74-75] In recent years, the contents of amino acids or their metabolites in serum have become an important indicator for disease screening.
A new microporous terbium-based metal-organic framework, Tb-MOF (32), was designed and prepared for detecting glutamic acid (Glu) by Zhang and co-authors.[159] In the structure of compound 32, two kinds of cages, tetrahedral and octahedral, were constructed: the tetrahedral one was linked by four hexa-nuclear [Tb6(μ3-OH)8(CO2)12] clusters (hexa-nuclear SBUs) and six ligands with the size of 5.3 Å, and the octahedral one was formed through six hexa-nuclear SBUs and twelve ligands with the size of 7.5 Å. The porous properties of 32 were confirmed by the N2 adsorption tests, giving the BET surface area and pore volume of 613.6 m2∙g-1 and 0.244 cm3∙g-1, respectively. The room-temperature luminescent spectrum of the suspension of compound 32 displayed dual emissions originated from the ligand centered at 430 nm and central Tb3+ ions peaked at 488, 544, 583 and 621 nm. Benefited from the high linkage of hexa-nuclear SBUs and ligands, compound 32 possessed extraordinary chemical stability and light resistance. By virtue of the super stability and dual emissions, the potential of compound 32 as a self-calibrated sensor for Glu was explored. As the solutions of Glu were dropwise added to the suspension of 32, no significant changes were found in the emission intensity of Tb3+ ions, while the emission of the ligand at 430 nm was obviously enhanced (Figure 15a). Thus, compound 32 might be utilized as self-calibrated luminescent sensor relying on the self-regulation through the combination of the detected signal (ligand-related emission) and the reference (Tb3+ centered emission). The ratio of emission intensity at 430 and 544 nm (I430/I544) was equated to a well-fitted linear relationship with the concentrations of Glu, giving the value of the detection limit of 3.6 μM (Figure 15b). Moreover, the color of compound 32 was observed to gradually change from green to greenish-blue to blue by naked eyes, indicating the capacity of compound 32 on colorimetric luminescent sensing for Glu. Additionally, the toxicology and selectivity measurement of compound 32 indicated that as a potential luminescent sensor, compound 32 is highly selective in biological environment with anti-jamming capability (Figure 15c and 15d). Finally, a simple and convenient one-to-two decoder logic gate was fabricated based on the concentration of Glu as the input signal and the ratio of I430/I544 as output signals (Figure 16). Because the dangerous threshold limit value for Glu was 19.2 × 10-5 M, the emission at this concentration was selected as a danger threshold. When the input concentration of Glu was below 19.2 × 10-5 M, the normal gate was triggered. While the input concentration overstepped the critical value, the Danger gate was activated, suggesting the simple and real-time detection for Glu.
Figure 15
Figure 15. (a) Luminescent spectra of 32 with different concentrations of Glu (0-5 mM). (b) The linear relationship between I430/I544 of 32 and Glu concentrations. (c) The recyclable experiments of 32 for Glu. (d) Antiinterference experiment of 32 to Glu. Reproduced with permission from Ref.[159]Figure 16
Figure 16. (a) One-to-two decoder logic gate (Input signals: Glu concentrations; Output signals: the luminescent intensities of 32 at 430 nm and 544 nm. (b) Relative luminescent intensities of 32 as the concentrations of Glu at none, normal, dangerous threshold limit value, and danger. (c) Truth table and output color of the logic analytical device for Glu monitoring. Reproduced with permission from Ref.[159]Zhu et al. developed a 3D Tb-MOF (33) with a (413∙62)(48∙66∙8) topological net.[160] When excited at 335 nm, 33 presented a typical green emission of Tb3+ with the emission peaks centered at 489, 543, 587 and 620 nm coming from the characteristic transitions of 5D4 → 7FJ (J = 6 to 3) for central terbium ions. In view of the outstanding luminescent properties of compound 33, the potential application of 33 on sensing amino acids was explored. Different aqueous suspensions of 33 containing 11 kinds of amino acids at 1.0 × 10-3 M were prepared and the luminescent intensities at 543 nm of them were monitored and analyzed. Different from other suspensions, the suspension containing aspartic acid exhibited an arresting quenching effect on the luminescent intensity of 33. Additionally, with the gradual addition of aspartic acid, the luminescent intensities decreased gradually. And the values of Ksv and the detection limit were calculated to be 9.80 × 103 M-1 and 7.95 × 10-6 M, respectively. Thus, compound 33 can be applied as a promising luminescent sensor for aspartic acid with high selectivity and freedom from jamming. The linear relationship between the emission intensity and the concentration of aspartic acid as well as the decrease of the luminescent lifetime with aspartic acid indicated that such luminescent quenching behavior may be resulted from the presence of the interaction between aspartic acid and the framework of 33.
Another sensor for aspartic acid based on a Tb-MOF (34) constructed from a tetra-carboxylate ligand functionalized with a pyrene ring was covered by Wen' group.[161] Compound 34 featured a water-stable 3D microporous structure with the void ratio of 20.40% through PLATON software. Only a broad emission band centered at 490 nm was observed in the solid state, owing to the strong coordination between Tb3+ ions and the conjugated ligand, thus increasing the electron delocalization. The higher lifetime of Tb-MOF than that of the ligand verified the presence of energy transfer between Tb3+ and the ligand. Eleven kinds of amino acids were selected to investigate the potential of compound 34 as a luminescent sensor for amino acids. It was seen that the emission intensity was significantly enhanced by aspartic acid (Asp), and the luminescent enhancing effects can be observed by naked eyes under 365 nm UV irradiation. Quantitative and competitive experiments indicated that Asp can be selectively detected by compound 34 without disturbance by most amino acids, with a relative lower LOD of 0.025 ppm. Additionally, the singlet-state (S1) of Asp (46512 cm-1) was calculated to be higher than that of the ligand (25641 cm-1) through the UV adsorption spectra, indicating the energy transfer could occur from Asp to the ligand, which might be the major mechanism for such luminescent enhancing effect for Asp.
Except for Eu- and Tb-functionalized MOFs, at another work, Jiang and co-workers reported a 3D Sm-centered lanthanide MOF, Sm-MOF (35), by the reaction of samarium nitrate and a tetra-carboxylate ligand, 1, 2, 4, 5-benzenetetracarboxylic acid.[162] PXRD patterns were collected to confirm the stability of samples immersed in acid and basic water solutions. The results indicated compound 35 can maintain the integrity of the framework in water with pH range of 3-11. The emission spectrum of compound 35 was collected with three strong peaks centered at 562, 598 and 646 nm corresponding to the typical transitions of 4D5/2 → 6HJ (J = 5/2, 7/2 and 9/2) for Sm(Ⅲ) ions, revealing the effective "antenna effect" of the ligand to transfer the absorbed energy to central Sm(Ⅲ) ions. The luminescent sensing potential of 35 on amino acids was investigated: several analysts including common ions, proteins and amino acids were selected to prepare suspensions through ultrasonic treatment, and then the luminescent response of compound 35 towards various analysts was recorded. The results displayed in Figure 17a indicated that most analysts show almost ignorable influence on the luminescent intensity of 35, while a remarkable enhancement occurred in the presence of L-tryptophan (Trp), implying the promising potential of 35 on the detection for Trp. The selectivity tests of 35 towards Trp were arranged, and the luminescent intensity exhibited a well-fitted linear relationship with the concentration of Trp at 0-100 mM with the detection limit of 330 nM (Figure 17c and 17d). Moreover, the luminescent enhancing effects were also observed when the pH of the reaction system fell in the range of 2-12, revealing that compound 35 can detect Trp in wide acid and basic conditions (Figure 17e). Furthermore, the enhancing effect was obviously observed when the Trp was introduced just for 1 minute and the enhancing efficiency reached up to the maximum after immersing for 5 minutes. As discussed above, compound 35 can be considered as a sensitive, stable and fast-responsive luminescent sensor for Trp. Additionally, milk was introduced to estimate the performance of 35 on sensing Trp in actual conditions. The recoveries were calculated to be between 93.8% and 102.9% with the relative standard deviations (RSD) lower than 4.3%, confirming compound 35 can detect Trp in actual conditions. The well matched relationship between the triplet energy (T1) of Trp (21 260 cm-1) and the emissive level of Sm3+ (17 900 cm-1) indicated that Trp molecule could be able to act as the second "antenna molecule" to effectively transfer more energies to Sm3+, resulting in such luminescent enhancing effect on Trp. In addition, the rigid indole group in the structure of Trp could efficiently decrease the occurrence of energy loss processes, such as molecular vibration and intermolecular interaction, and then cause the high-efficient energy transfer from Trp to Sm3+.
Figure 17
Figure 17. (a) Luminescent spectra of 35 in different amino acids. (b) Luminescent spectra of 35 after introducing Trp with different concentrations. (c) The S-V plot of 35. (d) Anti-interference experiment of 35. (e) Luminescent intensities of 35 for sensing Trp at different pH values. (f) The XPS spectra in the O 1s region of 35, Trp@35 and Trp. Reproduced with permission from Ref.[162]In addition, a 2D La-MOF (36) built from an organic linker containing tetrazole and carbolxylate units was prepared under solvothermal conditions for detecting L-Tyr.[163] As-synthesized samples of 36 were dispersed in diverse solvents and the PXRD curves were collected to assess the stability of compound 36. PXRD patterns after soaking in these solvents were well-fitted with the original pattern of the as-synthesized samples, displaying the high stability of 36. Luminescent sensing researches were arranged and the result displayed that the luminescent intensity of compound 36 was completely quenched when L-Tyr was introduced. And the quenching constant (Ksv) and limit of detection (LOD) were found to be 1.40 × 104 M-1 and 3.60 × 10-6 M, respectively. Additionally, compound 36 was also discovered to be able to sense antibiotic nitrofurantoin (NFT) sensitively with the quenching constant (Ksv) and limit of detection (LOD) found to be 3.00 × 103 M-1 and 1.70 × 10-5 M, respectively. The luminescent lifetime decreased as the concentration of L-Tyr was increased, indicating the possible presence of hydrogen bonding interaction, that is, such luminescent quenching effect was ascribed to the dynamic quenching mechanism.
In conclusion, several Ln-MOFs were used as luminescent probes on the detection of amino acids, including Asp, Glu, Trp and Tyr. Compared with ions and antibiotics, Ln-MOFs as luminescent sensors for amino acids have not been widely developed and reported until now. Therefore, most attentions should be focused on the preparation and exploration of functionalized Ln-MOFs with fast and precise detection for amino acids.
Sensors for Other Analytes
Apart from the application on the detection of inorganic ions, antibiotics and amino acids, luminescent Ln-MOFs have been widely developed and utilized as potential chemical sensors for pesticide, [164-166] temperature[60-63] and humidity[167-168].
For efficiently detecting pesticide 2, 6-dichloro-4-nitroaniline (DCN), Jiao and co-authors introduced a rigid penta-carboxylate ligand 3, 5-di(2′, 4′-dicarboxylphenyl)benzoic acid to design and synthesize a 3D Tb-MOF (37).[164] Compound 37 displayed good water stability and pH stability. When excited at 311 nm, compound 37 exhibited strong characteristic green-emission of Tb3+ peaked at 495, 551, 590, and 627 nm. Based on further exploration on the capacity of 37 on luminescent sensing pesticides, a series of tests were arranged and the results indicated that compound 37 presented great potential on sensing pesticide DCN in aqueous solution, giving the quenching constant (Ksv) and the detection limit (LOD) of 6.42 × 104 M-1 and 1.4 × 10-7 M. In addition, Yan' group reported a Tb3+-functionalized Cd-MOF, Tb3+@Cd-MOF (38), for detecting the biomarkers of Fungicides (3, 5-dichloroaniline, 3, 5-DCA) and herbicides (3, 4-dichloroaniline, 3, 4-DCA).[166] It is worth to mention that 38 was obtained via cation-exchange process: the [(CH3)2NH2]+ cations occupied in the open channel of Cd-MOF were replaced by Tb3+ ions through immersing Cd-MOF in the DMF solution of Tb3+ (10-2 M). Compound 38 was proven to be able to sensitively detect 3, 4-DCA and 3, 5-DCA in DMF with the Ksv and LOD of 25.11 and 0.0033 mg∙mL-1 for 3, 4-DCA and 30.21 and 0.0026 mg∙mL-1 for 3, 5-DCA, respectively. Furthermore, when urine samples were introduced, the values of Ksv and LOD for 3, 4-DCA and 3, 5-DCA were similar to those in DMF, indicating 38 can be applied as a candidate for sensing 3, 4-DCA and 3, 5-DCA in practical conditions.
A Eu3+/Tb3+-mixed MOF, Eu0.036Tb0.964-MOF (39), based on the thermally and chemically stable Eu-MOF and Tb-MOF, was reported by Su and co-workers.[60] Compound 39 was proved to act as a ratiometric luminescent thermometer at a wide temperature range from 77 to 377 K. Notably, the temperature and the ratio (I545/I614) of the luminescent intensity at 545 nm for Tb3+ and 614 nm for Eu3+ exhibited a good linear relationship at the range of 220-310 K, and the maximum relative sensitivity (Sm) was calculated to be 9.42% K-1 at 310 K. Moreover, as the temperature increased from 77 to 377 K, the luminescent colors of 39 can be distinguished by naked eyes from green to yellow and to red at last with increasing temperature, indicating the potential of the mixed compound on the application as a sensitive ratiometric temperature sensor. Besides, different from the Eu3+/Tb3+-mixed luminescent thermometer, Murugesu' groups prepared a 5%Yb3+-doped Nd-MOF, Yb3+/Nd3+-MOF (40), as a near-infrared luminescence thermometry.[61] Interestingly, when excited at 365 nm, as the temperature increased from 15 to 300 K, the luminescent intensity of the 4F3/2 → 4I11/2 transition for Nd3+ was observed to manifold: the luminescent intensity at 1066 nm decreased, while the luminescent intensity at 1082 nm increased in the temperature range of 15 to 300 K. A well-fitted linear correlation was observed between the temperature and the luminescent intensity ratio (I1082/I1066). However, under 808 nm excitation, as the temperature increased from 15 to 300 K, the luminescent intensity of the 4F5/2 → 4I7/2 transition at 980 nm for Yb3+ and the 4F3/2 → 4I11/2 transition at 10666 nm for Nd3+ were both increased and the temperature and the luminescent intensity ratio (I980/I1066) were fitted linearly. And the relative sensitivities were varied between approximately 0.1 and 0.2% K-1. All the above discussion revealed that this Yb3+/Nd3+-MOF 40 can be considered as an excellent near-infrared luminescence thermometry with high sensitivity.
Wang et al. prepared a Eu-MOF (41) with plentiful weak Eu-O bonds, possibly generating affluent open metal sites (OMSs).[167] Profited from the potential open metal site, water molecules were observed to bind with the open Eu3+ sites by X-ray single-crystal diffraction. Luminescent spectrum of 41 excited at 365 nm exhibited two main emission bands: the emission band centered at 439 nm from the ligand and the typical emission peaks of Eu3+ located at 593, 617 and 653 nm, indicating that compound 41 possessed a dual-emissive ratiometric luminescence behavior. Thus, the performance of 41 as a ratiometric luminescent probe was explored. Interestingly, as the relative humidity (RH) of the air gradually increased from ~0% to ~40%, the luminescent intensity at 617 nm decreased obviously, while that at 439 nm was found to increase continuously, leading to the luminescence color changing gradually from red to blue-purple, revealing that compound 41 can act as a ratiometric luminescent sensor for humidity by naked eyes.
CONCLUSION AND OUTLOOK
The purposes of this review are aimed to make a classification and summary for recent researches of Ln-MOFs as chemical sensors for ions, antibiotics and amino acids based on "turn on" or "turn off" effects (Table 1), especially the applications of waterstable Ln-MOFs, as well as the probable sensing mechanism. With the continuous development of industry and agriculture, pollutants discharged from industry, agriculture, and daily life to environment have accentuated burden on the environment and became hidden threats to the health of humans, which can enter to the organism through bio-enrichment. In recent years, great and numerous efforts have been executed on the exploration for the efficient methods and materials to detecting environmental pollutions or vital substances of human body. Compared with other detection methods or materials, it is well known that luminescent sensors based on lanthanide metal-organic frameworks (Ln-MOFs) have emerging, benefited from their incomparable advantages of structural diversity, typical emissions and easy operation. Up to now, tens of thousands of Ln-MOFs have been meticulously designed and exploited, and can be applied as luminescent probers for varieties of analytes, such as metal cations, anions, antibiotics, biomarkers, amino acids, and so on.
In order to improve the abilities of luminescent chemical sensors in practical application, developing Ln-MOFs with high stability in water, outlet water or human urine is still a huge challenge since stability is the first step for luminescent sensors towards practical application. On the other hand, the performance for many reported luminescent sensors has been just investigated in aqueous solutions, unable to demonstrate the potential for practical application. Thus, the detection capacity of developed Ln-MOFs in outlet water, domestic water and human serum or urine should be researched as an important project. Besides, for fast, convenient and real-time detection, more attention is deserved to focus on the exploitation of diverse functionalized Ln-MOFs devices, such as luminescent test papers, Ln-MOFs thin films and luminescent encodes. In addition, some other issues, such as the stability of the sensing performance, the sensing mechanism and the relationship between structural features and luminescent properties, are still challenging problems needed to be solved in practical application of luminescent sensors based on Ln-MOFs materials. As we know, Ln3+ ions are inclined to coordinate with the oxygen atoms from carboxylate groups. Therefore, ligands with multi-carboxylate groups and large conjugated systems can improve the coordinated numbers of lanthanide ions and reduce the presence of H2O molecules in the coordination sphere, making the structures stable. That is, introducing ligands with abundant coordinated sites is beneficial to heighten the stability of Ln-MOFs and broad their application in practical conditions. Besides, ligands with well-matched triplet energy with central Ln3+ ion can efficiently sensitize Ln3+ through "antenna effect", thus making the design of high luminescent Ln-MOFs reliable. As a consequence, more efforts should continue to be made on the functionalized design and synthesis of Ln-MOFs.
ACKNOWLEDGEMENTS: This work was financially supported by the National Natural Science Foundation of China (21801107), the Natural Science Foundation of Shandong Province (ZR2018BB005), and the Youth Innovation Team of Shandong Colleges and Universities (2019KJC027). COMPETING INTERESTS
The authors declare no competing interests.
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Full paper can be accessed via http://manu30.magtech.com.cn/jghx/EN/10.14102/j.cnki.0254-5861.2022-0138
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[1]
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Scheme 1 Schematic of the energy adsorption, transfer and emission processes (S: singlet state; T: triplet state).
Figure 1 (a) Quenching efficiency of 2 in aqueous solutions containing various cations (1 mM). (b) Photographs of 2 in aqueous solutions containing various cations (1 mM) under 254 nm UV irradiation. (c) Luminescent intensities of 2 at different response time in the presence of Fe3+ at different concentrations. Reproduced with permission from Ref.[114]
Figure 2 (a) Scheme of the luminescent quenching mechanism experiment. (b) The emission spectra of 2 (excited at 280 nm) with Fe3+ (Black curves: 2 solely in position A; red curves: mixture of 2 and Fe3+ in position A; blue curves: 2 in position A while Fe3+ in position B; green curves: 2 in position A while Fe3+ in position C). Reproduced with permission from Ref.[114]
Figure 3 (a) The PXRD patterns of 4 soaked in different organic solvents. (b) Luminescent intensity at 615 nm of 4 immersed in aqueous solutions with different metal ions. (c) Luminescent spectra of 4 in aqueous solutions containing various Cu2+ concentrations. (d) The S-V plot of 4. (e) Anti-interference experiment of 4. (f) The recyclable experiments of 4 for sensing Cu2+. Reproduced with permission from Ref.[116]
Figure 4 (a) Luminescence intensities and PXRD patterns of the 5 dispersed in aqueous solutions in the pH range of 2-12. (b) The Luminescence spectra and quenching percentage of 5 dispersed in DMF solutions with different metal cations. (c) N 1s and O 1s XPS spectra of compound 5, Cu2+@5 and Fe3+@5. Reproduced with permission from Ref.[117]
Figure 5 (a) Luminescent spectra of compound 7 as excitation wavelengths change from 300 to 350 nm. (b) The corresponding CIE chromaticity diagram. (c) Luminescent spectra of 7 with the gradual addition of Ag+ ions. (d) Scheme for the effect of Ag+ on the energy transfer process from ligand to central metal ions. Reproduced with permission from Ref.[118]
Figure 6 (a) Relative luminescent intensities of compound 8 in aqueous solutions with various metal cations. (b) Anti-interference experiments of 8. (c) Luminescent spectra of compound 8 in aqueous solutions with various concentrations of Hg2+. (d) XPS spectrum for the O 1s region of 8 before and after immersion in Hg2+. Reproduced with permission from Ref.[119]
Figure 7 (a) Scheme of the synthetic strategy for compound 9. (b) Relative luminescent intensities of 9 in solutions containing different metal cations (0.25 mmol L-1). (c) Time-dependent luminescent intensities of the suspension for 9 after adding Mg2+ solution (200 μL). (d) The recyclable experiments of 9 for sensing Mg2+. Reproduced with permission from Ref.[120]
Figure 8 (a) Relative luminescent intensities of 15 soaked in various anions (1.5 mM; before (blue column) and after adding Cr2O72- (red column)). (b) Luminescent spectra of 15 soaked in solutions with different concentrations of Cr2O72- (λex = 376 nm). Reproduced with permission from Ref.[135]
Figure 9 (a) Relative luminescent intensities of compound 16 in aqueous solutions with various anions. (b) Quenching efficiencies of compound 16 in aqueous solutions with various anions. Luminescent spectra of compound 16 in aqueous solutions with different concentrations of (c) Cr2O72- and (d) CrO42- ions. The S-V plots of 16 towards (e) Cr2O72- and (f) CrO42- ions. Reproduced with permission from Ref.[136]
Figure 10 (a) CIE chromaticity diagram of 20 with excitation wavelengths varying from 295 to 385 nm. (b) Relative luminescent intensities of 20 immersed in the aqueous solutions containing different anions (1 mM) (λex = 365 nm, red: 613 nm; green: 544 nm; blue: 462 nm). (c) Luminescent spectra of 20 in aqueous solution with different concentrations of MnO4- excited at 365 nm. (d) The S-V plots of 20. (e) Anti-interference experiments of 20. Reproduced with permission from Ref.[139]
Figure 11 (a) Luminescent spectra of 21 dispersed in DMSO solution (10 μM) with various anions (1 mM) (λex = 350 nm). (b) Luminescent spectra of 21 in DMSO solution with different concentrations of CO32- ion (λex = 350 nm). Reproduced with permission from Ref.[140]
Figure 12 Relative luminescent intensities of 24 in aqueous solutions with different anions (2.0 × 10-4 M) (a) excited at 320 nm and (b) 359 nm. Luminescent spectra of 24 in solutions with various concentrations of (c) ODZ excited at 320 nm and (d) NFT excited at 359 nm. Reproduced with permission from Ref.[150]
Figure 13 (a) Related luminescent intensities of 25 with different antibiotics. Luminescent spectra of 25 with different concentrations of (b) NFT and (c) SOM. (d) The IR/I0R of 25 after adding different antibiotics. (e) Luminescent response time of 25 towards NFT. (f) The recyclable experiments of 25 for NFT. Reproduced with permission from Ref.[151]
Figure 14 (a) Luminescent spectra of 28 for different antibiotics (1 mM) in water solutions (λex = 315 nm). (b) Decoded map for different antibiotics (0.1 mm) based on the ratios of I385/I481 and I385/I573. Luminescent spectra of 28 soaked in water with different concentrations of (c) NFZ and (d) FZD. Reproduced with permission from Ref.[154]
Figure 15 (a) Luminescent spectra of 32 with different concentrations of Glu (0-5 mM). (b) The linear relationship between I430/I544 of 32 and Glu concentrations. (c) The recyclable experiments of 32 for Glu. (d) Antiinterference experiment of 32 to Glu. Reproduced with permission from Ref.[159]
Figure 16 (a) One-to-two decoder logic gate (Input signals: Glu concentrations; Output signals: the luminescent intensities of 32 at 430 nm and 544 nm. (b) Relative luminescent intensities of 32 as the concentrations of Glu at none, normal, dangerous threshold limit value, and danger. (c) Truth table and output color of the logic analytical device for Glu monitoring. Reproduced with permission from Ref.[159]
Figure 17 (a) Luminescent spectra of 35 in different amino acids. (b) Luminescent spectra of 35 after introducing Trp with different concentrations. (c) The S-V plot of 35. (d) Anti-interference experiment of 35. (e) Luminescent intensities of 35 for sensing Trp at different pH values. (f) The XPS spectra in the O 1s region of 35, Trp@35 and Trp. Reproduced with permission from Ref.[162]
Table 1. Summaries of Mentioned Ln-MOFs, Relative Ligands and Sensing Targets
Sensing target Ln-MOFs Ligands Refs. Fe3+ H3O[Tb(H2O)2(L)] 1 H4L = 4', 4''', 4''''', 4'''''''-(ethene-1, 1, 2, 2-tetrayl)tetrakis(([1, 10-biphenyl]-4-carboxylic acid)) 112 {[Eu(L)(H2O)]∙4H2O}n 2 H3L = 5-(2′, 5′-dicarboxylphenyl)picolinic acid 113 Tb2(L)3(DMF)4 3 H2L = 2-hydroxyterephthalic acid 114 {[Me2NH2][TbL]∙2H2O}n 10 H4L = 1-(3, 5-dicarboxylatobenzyl)-3, 5-pyrazole dicarboxylic acid 121 [Zn3Eu2(L)2(H2O)6]⋅6H2O 11 H6L = 1, 3, 5-triazine-2, 4, 6-triamine hexaacetic acid 122 Cu2+ {[Eu2(L)2(H2O)5]·3H2O}n 4 H3L = 5-(3′, 5′-dicarboxylphenyl)picolinic acid 115 {[Eu(L)(2H2O)]·(Hbibp)0.5}n 5 H4L = 2-(3′, 4′-dicarboxylphenoxy) isophthalic acid; bibp = 4, 4′-bis(imidazolyl)biphenyl 116 {[Eu2K(L)4(C2H5OH)]∙(H3O)∙(H2O)x}n 12 H2L = 2, 2′-bipyridine-6, 6′-dicarboxylic acid 123 {[Eu(HL)]·3DMF·3H2O}n 13 H4L = 1, 4-bis(2′, 2′′, 6′, 6′′-tetracarboxy-1, 4': 4, 4''-pyridyl)benzene 124 Cd2+ {(Me2H2N)[Eu(L)]·DMF·H2O}n 6 H4L = 2′-amino-[1, 1′: 4′, 1″-terphenyl]-3, 3″, 5, 5″-tetracarboxylic acid 117 [Tb2(L)4(phen)2(NO3)2] 14 HL = phenylacetic acid, phen = 1, 10-phenanthroline 125 Ag+ [La0.88Eu0.02Tb0.10(L)(DMF)2]n·H2O·0.5DMF 7 H3L = 4-(3, 5-dicarboxylatobenzyloxy)-benzoic acid 118 Hg2+ {[Eu4(L)6(phen)4]·m(H2O)(phen)}n 8 H2L = thiobis(4-methylene-benzoic acid); phen = 1, 10-phenanthroline 119 Mg2+ {[Eu2(L)1.5(DMA)3(H2O)2]·2DMA·2H2O}n 9 H4L = benzo-imide-phenanthroline tetracarboxylic acid 120 Cr2O72- {[Eu3(L)3(NO3)7]·NO3·ClO4}n 15 H2LCl2 = 1, 1′-bis(4-carboxyphenyl) (4, 4′-bipyridinium) dichloride 135 CrO42-/Cr2O72- {[Tb(L)(H2O)2]·2H2O}n 16 H3L = 1, 3, 5-tris-(carboxymethoxy)benzene 136 CrO42-/Cr2O72-/MnO4- {[Eu(L)(H2O)2]·5H2O}n 17 H4L+Cl- = 1, 3-bis(3, 5-dicarboxyphenyl)imidazolium chloride 137 Cr2O72- {[Eu2(L)2(H2O)2]·5H2O·6DMAC}n 18 H3L = 4, 4′-(((5-carboxy-1, 3-phenylene)-bis(azanediyl))bis(carbonyl)) dibenzoic acid 138 MnO4- {[Tb2(L)2(H2O)2]·5H2O·6DMAC}n 19 H3L = 4, 4′-(((5-carboxy-1, 3-phenylene)bis-(azanediyl))bis(carbonyl)) dibenzoic acid 138 MnO4- [Eu0.06Tb0.04Gd0.9(HL)1.5(H2O)(DMF)]·2H2O 20 H3L = 5-(3′, 5′-dicarboxylphenyl) nicotinic acid) 139 CO32- [Eu2(Hhpip)2(OAc)6] 21 Hhpip = 2-(2-hydroxyphenyl)imidazo[4, 5-f]-[1, 10]phenanthroline) 140 NO2- {[Tb(L)(OA)0.5(H2O)2]·H2O}n 22 H2L = chelidonic acid, H2OA = oxalic acid 141 C2O42 [Ln(L)(DMF)(H2O)(COO)]n 23 H2L = 4, 4-(9, 9-dimethyl-9H-fluorene-2, 7-diyl) dibenzoic acid 142 ODZ/NFT [Eu(L)(OH)]·xS 24 H2L = 5-(4-carboxyphenyl)picolinic acid, S = solvent molecule 150 NFT/SOM/DTZ {[Tb-(HL)(H2O)2]·x(solv)}n 25 H4L = 5, 5′-(((1, 4-phenylenebis-(azanediyl)) bis(carbonyl))bis(azanediyl))diisophthalic acid 151 NFT/NZF {[Eu2L2(DMF)4]·xDMF·yH2O}n 26 H3L = 5-(4-carboxybenzyloxy)isophthalic acid 152 TCH {[Eu(L)(H2O)]}n 27 H3L = 4, 4′, 4′′-s-triazine-2, 4, 6-tribenzoic acid 153 NFZ/FZD [Dy(L)(DMF)3]n 28 H3L = 1, 3, 5-tris(1-(2-carboxyphenyl)-1H-pyrazol-3-yl) 154 NZF/NFT RhB@[Me2NH2][Tb3(L)3(HCOO)]·DMF·15H2O 29 H3L = 3-(3, 5-dicarboxylphenyl)-5-(4-carboxylphenl)-1H-1, 2, 4-triazole 155 NIF [Tb(HL)(C2O4)]·3H2O 30 H2L = 2-(4-pyridyl)-terephthalic acid, C2O4 = oxalic acid 156 SMZ {Eu(L)DMF}n 31 H3L = 1, 3, 5-tris(4-carboxyphenyl)benzene 157 Glu [(CH3)2NH2]2[Tb6(μ3-OH)8(L)6(H2O)6] 32 H2L = 2-hydroxyterephthalic acid 159 Asp [[Tb(μ6-H2L)(μ2-OH2)2]·xH2O]n 33 H5L = 5, 5'-((5-carboxy-1, 3-phenylene)-bis(oxy))diisophthalic acid 160 Asp {[(CH3)2NH2]5[Tb5(L)5]·solvent}n 34 H4L = 1, 3, 5, 7-tetra(4-carboxybenzene)pyrene 161 Trp [Sm2(L)1.5(H2O)8]·6H2O 35 H4L = 1, 2, 4, 5-benzenetetracarboxylic acid 162 Tyr [La(HL)(DMF)2(NO3)] 36 H3L = 5-(4-(tetrazol-5-yl)phenyl)-isophthalic acid 163 DCN [Tb3(HL)(L)(H2O)6]·NMP·3H2O 37 H5L = 3, 5-di(2′, 4′-dicarboxylphenyl)benzoic acid 164 3, 5-DCA/3, 4-DCA Tb3+@{[NH2(CH3)2]4·[Cd6(L)4(HTz)1.5(H2O)6]·xS}n 38 H4L = 3, 5-di(2, 4-dicarboxylphenyl)pyridine, HTz = 1H-tetrazole 166 temperature ([(CH3)2NH2]Eu0.036Tb0.964L) 39 H4L = biphenyl-3, 30, 5, 50-tetracarboxylic acid 60 temperature [Yb0.05Nd0.95Cl(L)·(DMF)] 40 H2L = 2, 6-naphthalenedicarboxylic acid 61 humidity [DMA]3[Eu4L4·3DMA·7H2O] 41 H3L = [1, 1′-biphenyl]-3, 4′, 5-tricarboxylic acid 167 
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Table 2. Comparison of Quenching Coefficient (Ksv), Limit of Detection (LOD), Response Time and Sensing Mechanism of Selected Ln-MOFs
Ln-MOFs Sensing target Medium Quenching coefficient (Ksv, M-1) Limit of detection (LOD, M) Response time Sensing mechanism Refs. 1 Fe3+ H2O 3.50 × 103 138.8 ppm - competitive absorption and aFRET 112 2 Fe3+ H2O 1.88 × 104 5.70 × 10-7 10 s competitive absorption and energy transfer 113 3 Fe3+ DMF 1.62 × 105 3.50 × 10-7 < 60 s competitive absorption and electronic interaction 114 4 Cu2+ H2O 3.95 × 103 3.30 × 10-6 - energy absorption and weak coordination interaction 115 5 Cu2+ DMF 4.62 × 103 2.53 × 10-5 - weak interaction 116 Fe3+ DMF 4.84 × 103 1.32 × 10-5 - competitive absorption and weak interaction 6 Cd2+ H2O b23-fold - - coordination interaction between Cd2+ and COOH group 117 8 Hg2+ H2O 9.11 × 104 1.00 × 10-6 - profound interaction between Hg2+ and O from COOH 119 9 Mg2+ EtOH b10.4-fold 1.53 × 10-10 15 s weak coordination of Mg2+ with O atoms in the ligand 120 10 Fe3+ H2O 6.80 × 103 - < 40 s competitive absorption and weak interaction 121 11 Fe3+ EtOH/H2O 1.96 × 103 1.68 × 10-5 - competitive absorption and dynamic quenching 122 13 Cu2+ H2O 4.90 × 106 1.35 × 10-9 15 s coordination between the framework and Cu2+ 124 14 Cd2+ DMF/H2O - 5.0 × 10-7 - decomposition of the framework 125 15 Cr2O72- DMF 1.40 × 104 5.6 × 10-6 < 60 s competitive absorption 135 16 CrO42- H2O 1.11 × 104 6.5 × 10-7 10 s competitive absorption 136 Cr2O72- H2O 1.55 × 104 8.9 × 10-7 10 s competitive absorption 17 CrO42- H2O 1.74 × 103 - - competitive absorption and weak interaction 137 Cr2O72- H2O 1.36 × 103 competitive absorption and weak interaction MnO4- H2O 0.51 × 103 competitive absorption and weak interaction 18 Cr2O72- H2O 1.05 × 103 8.94 × 10-5 - competitive absorption 138 19 MnO4- H2O 1.20 × 103 4.48 × 10-8 competitive absorption 20 MnO4- H2O - 1.97 × 10-8 - competitive absorption and FRET 139 21 CO32- DMSO - c1.23 × 10-5 hydrogen bonding interaction 140 d7.80 × 10-5 hydrogen bonding interaction 22 NO2- H2O 4.74 × 105 2.82 × 10-8 - dynamic quenching behaviour 141 23 C2O42 DMA 2.07 × 103 1.65 × 10-5 - electrostatic interaction and energy-transfer 142 24 ODZ H2O 3.52 × 104 5.20 × 10-7 - competitive absorption 150 NFT H2O 2.33 × 104 4.30 × 10-7 competitive absorption 25 SOM H2O e1.10 × 103 - 10 s photo-induced electron transfer (PET) 151 NFT H2O 1.90 × 104 4.10 × 10-7 10 s PET and competitive absorption DTZ H2O 1.00 × 104 1.39 × 10-6 10 s PET and competitive absorption 26 NFT H2O 5.29 × 104 9.92 × 10-6 - PET 152 NZF H2O 4.38 × 104 1.14 × 10-5 PET 27 TCH H2O 3.09 × 105 4.88 × 10-6 - PET and competitive absorption 153 28 NFZ H2O - 4.76 × 10-8 - competitive absorption and PET 154 FZD H2O 4.82 × 10-8 competitive absorption and PET 29 NZF H2O 5.98 × 104 5.02 × 10-7 - PET and inner-filter effect (IFE) 155 NFT H2O 6.69 × 104 4.48 × 10-7 PET and inner-filter effect (IFE) 30 NIF DMF 1.10 × 104 8.10 × 10-5 - competitive absorption and collision interaction 156 31 SMZ H2O 4.60 × 104 6.55 × 10-7 - inner-filter effect (IFE) 157 32 Glu H2O - 3.60 × 10-6 - PET 159 33 Asp H2O 9.90 × 103 7.95 × 10-6 - collision interaction 160 34 Asp EtOH - 0.025 ppm - energy transfer from Asp to ligand 161 35 Trp H2O - 3.30 × 10-7 < 60 s coordination between Trp and Sm3+ 162 36 Tyr EtOH 1.40 × 104 3.60 × 10-6 - hydrogen bonding interaction 163 NFT EtOH 3.00 × 103 1.70 × 10-5 hydrogen bonding interaction 37 DCN H2O 6.42 × 104 1.4 × 10-7 30 s PET 164 38 3, 4-DCA DMF 25.11 mL∙mg-1 0.0033 mg∙mL-1 15 s IFE at low concentration and dynamic and static quenching at high concentration 166 3, 5-DCA DMF 30.21 mL∙mg-1 0.0026 mg∙mL-1 15 s 39 temperature solid-state f9.42% K-1 g73.9% - - 60 40 temperature solid-state f0.1 and 0.2% K-1 - - - 61 41 humidity solid-state - 0.0003% (v/v) 30 s coordination interaction between open Eu3+ sites and H2O 167
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