English

• 0.   Introduction

  With the rapid development of industry, toxic and harmful substances (including heavy metal ions, toxic small molecules, etc.) are increasingly harmful to the environment and human health. Therefore, the detection of toxic and harmful substances plays an indispensable role in life sciences, biomedical and environmental science. Until now, various analytical techniques, such as atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), inductively coupled plasma mass spectrometry (ICPMS), X-ray fluorescence spectroscopy (XRF), have been used for determination of toxic and harmful substances^[1-2]. However, complicated sample preparation, expensive instruments and high cost make these analytical methods inconvenient^[3]. Therefore, the detection of toxic and harmful substances in aqueous solutions by a simple method is significant for human health and ecological environment.

  Compared with transition metals, lanthanide metals have unique properties of 4f orbitals. The electrons in the 4f orbital can undergo f-f or f-d transitions, presenting extremely rich energy level transitions and absorbing or emitting light of a very wide range of wavelengths from the ultraviolet to the infrared region^[4]. So, the luminescent lanthanide MOFs (Ln - MOFs) have especial advantages in the field of fluorescence probes due to their excellent optical properties such as high color purity, large Stokes shift, sharp line emission and relatively long fluorescence lifetime, which can provide very favorable conditions for the design of modern high efficiency luminescent materials^[5-7]. Until now, Ln-MOFs have been widely employed for the detection of inorganic ions and anions, organic (or inorganic) small molecules, antibiotics, neurotransmitters, explosives and temperature sensing^[8-14].

  In this study, we used 3, 5-bis(3', 5'-dicarboxylphe- nyl)-1H-1, 2, 4-triazole (H₄BDT, Scheme 1) to design and construct three isostructural heterometallic MOFs (Ln(Na) - MOFs), namely {[LnNa(BDT) (H₂O)₃]·2H₂O}_n (Ln=Tb (1), Dy (2), Ho (3), by self-assembly under solvothermal conditions. Ln(Na)-MOFs were characterized by single crystal X-ray diffraction, elemental analysis, thermogravimetric analysis (TGA), IR and powder X - ray diffraction (PXRD). Because of the antenna effect of rare earth complexes and the synergetic effect of organic ligands and central metal ions, Tb(Na) - MOF (1) showed excellent luminescent properties, and can detect Fe³⁺, acetaldehyde molecules and Cr₂O₇^2-. In addition, 1 doped carbon paste electrode (1/CPE) had excellent electrochemical activity for sensing and identifying catechol.

  Scheme 1

  

  Scheme 1.  Structure of H₄BDT

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  1.   Experimental

  1.1   Materials and methods

  Lanthanide metal salts (Tb(NO₃)₃·6H₂O, DyCl₃·6H₂O, HoCl₃-6H₂O), H₄BDT and other chemical reagents were commercially purchased and didn't require further purification. Elemental analyses of C, N, and H were performed on an Elementar Vario IE type element analyzer (Elementar Company in Germany). IR (KBr pellet) spectra were recorded on a Shimad- zu IRAFFINITY-1S spectrometer in a range of 4 000500 cm^-1. Thermogravimetric measurements were performed on a ZPY-2P Thermal Analyzer (Shanghai Kaip- ing Instrument Factory) from 30 to 900 ℃ with a heating rate of 10℃•min^-1 under N₂ atmosphere. PXRD patterns were collected on a PANalytical X' Pert PRO instrument with Mo Ka radiation (A =0.154 06 nm, U= 45 kV, I=40 mA, 26 =5°-30°). Fluorescent data were collected on a Hitachi F-7000 fluorescence spectrometer at room temperature. Cyclic voltammetry (CV) measurements were performed on a CHI660D Electrochemical Workstation.

  1.2   Synthesis of Ln(Na)-MOF

  A mixture of lanthanide metal salts (0.05 mmol), H₄BDT (0.05 mmol), 4, 4' - bipyridine (0.05 mmol), (CH₃)₂CHOH (1 mL) and HAc-NaAc buffer solution (5 mL, pH=6) were sealed in a 25 mL Teflon-lined stainless-steel autoclave. Then it was heated in an oven at 160 ℃ for 3 d, and was cooled to room temperature at a descent rate of 4℃•h^-1. Colourless crystals of Tb (Na)-MOF (1) were obtained and washed with water and ethyl alcohol, then dried in air (63% yield based on Tb). Anal. Calcd. for C₁₈H₁₇O₁₃N₃NaTb(%): C, 32.48; H, 2.56; N, 6.32. Found(%): C, 32.88; H, 2.80; N, 6.30. FT-IR (KBr pellet, cm^-1): 3 400(m), 1 629(m), 1 560(s), 1 413(s), 1 362(s), 1 120(w), 1 025(w), 929 (w), 875(w), 667(w), 600(w). The IR spectrum is shown in Fig.S1 (Supporting information).

  Primrose yellow crystals of Dy(Na)-MOF (2) were obtained and washed with water and ethyl alcohol, then dried in air (78% yield based on Dy). Anal. Calcd. for C₁₈H₁₇O₁₃N₃NaDy(%): C, 32.31; H, 2.54; N, 6.28. Found (%): C, 32.82; H, 2.45; N, 6.33. FT-IR (KBr pellet, cm^-1, Fig.S1): 3 391(m), 1 631(m), 1 560(s), 1 418(s), 1 364(s), 1 110(w), 1 024(w), 928(w), 877(w), 670(w), 602(w).

  Light pink crystals of Ho(Na)-MOF (3) were obtained and washed with water and ethyl alcohol, then dried in air (86% yield based on Ho). Anal. Calcd. for C₁₈H₁₇O₁₃N₃NaHo(%): C, 32.19; H, 2.53; N, 6.26.Found(%): C, 32.52; H, 2.40; N, 6.32. FT - IR (KBr pellet, cm^-1, Fig.S1): 3 384(m), 1 627(m), 1 559(s), 1 419(s), 1 363(s), 1 109(w), 1 022(w), 927(w), 878(w), 670(w), 599(w).

  1.3   X-ray crystallography

  Single crystal with regular morphology and high crystallinity were selected by optical microscope. The single crystal diffraction data were collected by Bruker D8 Venture system with Mo Ka radiation (λ=0.071 073 nm) at 296 K. The collected diffraction data were absorption-corrected using SADABS program. The structure were solved by dual methods using SHELXT and refined by full-matrix least-squares methods against F². Crystallographic data and structure refinement for 1 - 3 are displayed in Table 1. The selected bond lengths and bond angles for Ln(Na)-MOF are summarized in Table S1.

  Table 1

  

  Table 1.  Crystallographic data for 1-3

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  Parameter	1	2	3
  Empirical formula	C18H17O13N3TbNa	C18H17O13N3DyNa	C18H17O13N3HoNa
  Formula weight	665.25	668.83	671.26
  Crystal system	Monoclinic	Monoclinic	Monoclinic
  Space group	P21/n	P21/n	P21/n
  a/nm	0.743 72(19)	0.748 82(6)	0.749 78(12)
  b/nm	1.824 1(5)	1.830 81(14)	1.833 1(3)
  c/nm	1.538 4(4)	1.546 83(12)	1.547 3(3)
  β/(°)	93.711(4)	93.596 2(12)	93.473(2)
  Volume/nm3	2.082 7(9)	2.116 4(3)	2.122 8(6)
  Z	4	4	4
  Dc/(g.cm-3)	2.122	2.099	2.100
  Absorption coefficient/mm-1	3.498	3.632	3.828
  F(000)	1 304	1 308	1 312
  θ range for data collection/(°)	1.734-26.540	1.725-27.628	1.724-27.485
  Reflection collected	4 335	4 849	4 844
  Reflection unique (Rinl)	4 335 (0.027 3)	4 849 (0.025 7)	4 844 (0.021 9)
  Data, restraint, parameter	4 335, 0, 326	4 849, 0, 326	4 844, 13, 326
  Goodness-of-fit on F 2	0.863	1.134	1.227
  Final R indices [I > 2σ(I)]a, b	R1=0.022 8, wR2=0.055 7	R1=0.027 1, wR2=0.060 8	R1=0.022 9, wR2=0.063 1
  R indices (all data)	R1=0.026 1, wR2=0.069 3	R1=0.039 4, wR2=0.074 7	R1=0.029 3, wR2=0.074 0
  $ ^{\text{a}}{{R}_{1}}=\Sigma \left( \left| {{F}_{o}} \right|-\left| {{F}_{c}} \right| \right)/\Sigma \left| {{F}_{o}} \right|{{;}^{\text{b}}}w{{R}_{2}}={{\left\{ \Sigma \left[ w{{\left( {{\left| {{F}_{o}} \right|}^{2}}-{{\left| {{F}_{c}} \right|}^{2}} \right)}^{2}} \right]/\Sigma \left[ w{{\left( {{\left| {{F}_{o}} \right|}^{2}} \right)}^{2}} \right] \right\}}^{1/2}}.$

  CCDC: 2077995, 1; 2083696, 2; 2083700, 3.

  1.4   Fluorescence sensing experiment

  3 mg powders of 1 ground with an agate mortar were dispersed into 3 mL different solvents or different nitrate aqueous solution (M(NO₃)_x) or different potassium solution (K_yX) with a concentration of 0.01 mol·L^-1. Among them, the solvents included N, N -dimethylfor- mamide (DMF), dimethyl sulfoxide (DMSO), ethanol (EA), formaldehyde (HCHO), isopropanol (IPA), ethyl acetate (EAC), n - butanol (NBA), methanol (MT), trichloromethane (CHCl₃) glycol ether (GE), normal hexane (Hex), acetic acid (HAc), hexamethylene (CYH), acetaldehyde (AH), acetone (CP), and water(H₂O). The metal ions in M(NO3)_T were Sr³⁺, Zn²⁺, Mg²⁺, K⁺, Ag⁺, Na⁺, Pb²⁺, Bi³⁺, Cd²⁺, Al³⁺, Co²⁺, Hg²⁺, Ni²⁺, Fe³⁺, andCu²⁺; and the anions in K_yX were Br^-, I^-, SCN^-, Cl^-, C₂O₄^2-, h₂po₄^-, so₄^2-, no₂^-, CO₃^2-, ClO₄^-, S^2-, SO₃^2-, IO₃^-, PO4^3-, CH₃COO^- (Ac^-), and Cr₂O₇^2-. Before fluorescence measurement, the suspension was ultrasonically for 30 min and allowed to rest at room temperature for 3 d.

  1.5   Electrochemical sensing experiment

  MOF 1 was ground into fine powder using an agate mortar. Accurately obtained 10 mg powder of 1, 3 g graphite powder, 0.5 g liquid paraffin were thoroughly mixed. Then, the 1/CPE electrode was prepared by filling the polyvinyl chloride (PVC) pipe with an inner diameter of 3 mm with compacting and inserting copper wire as the conductor. It was left at room temperature for a week and the CPE electrode was polished to a mirror surface on the weighing paper before it was used. In phosphate buffered saline (pH=7), the electrochemical activity of 1/CPE was investigated by CV (sweep rate: 0.02 mV·s^-1) to detect the o-dihydroxy- benzene (catechol) in water with different concentrations (c=0.02-0.085 mmol·L^-1).

  2.   Results and discussion

  2.1   Crystal structure descriptions of 1-3

  Crystallographic analyses show that complexes 1-3 crystallize in the monoclinic system with the space group of P2i/n (Table 1). Because the complexes exhibit isomorphic 3D coordination frameworks with heteronuclear bimetallic units, so only the structure of 1 is described herein as a representative. The asymmetric unit of 1 contains one Tb³+ ion, one Na+ ion, one BDT^4- ligand, three coordinated water molecules, and two lattice water molecules with the formula of [TbNa(BDT) (H₂O)₃]-2H₂O. One Tb³+ ion in the complex coordinates with seven oxygen atoms (O1, O3c, O4c, O5a, O6a, O7b, O8b) of four BDT^4- ligands and two oxygen atoms (O9 and O10) from two water molecules to form a nine- coordinated configuration (Fig. 1a). Among them, O4c, O5a and O8b are located at the caps of the three flanks of the distorted triangular prism, respectively, O1, O7b and O9 are the three vertices of the upper surface, and O3c, O6a and O10 are the three vertices of the lower surface of the triangular prism. Six carboxylic oxygens (O3c and O4c, O5a and O6a, O7b and O8b) are the chelating coordination, respectively, while an oxygen atom (O1) is the bridging coordination. At the same time, O5a and O7b also coordinate with Na+ ion as bridging oxygen, connectting Tb³+ and Na+ into a heteronuclear bimetallic unit (the distance of Tb…Na ions is 0.383 77 nm). Na+ ion forms a five-coordinated configuration with an oxygen (O5a) in a water molecule and four atoms in BDT^4- ligands, among them, the carboxyl oxygen O2d, O7b and O11 of three BDT^4- ligands and the N atom of one BDT^4- are all bridging coordination. And O1 on the carboxyl group acts as a bridging carboxyl group to connect the heteronuclear bimetallic units into a 1D chain (Fig. 1b). The 1D chain is connected by the flexible BDT^4- to form a 2D two-layer plane structure (Fig. 1c), then it expands into a 3D microporous structure (Fig. 1d). The bond lengths and bond angles of 1 are shown in Table S1. The Tb—O and Na—O bond lengths are in the ranges of 0.232 5(4)- 0.252 1(3) nm and 0.223 4(4)-0.247 3(6) nm, respectively. The Na —N bond length is 0.251 6(5) nm, and the O—Tb—O and O—Na—N bond angles are in the rangesof 76.11(13)⁰ - 162.57(19)⁰ and89.41(19)⁰- 108.98(16)°, respectively. These bond lengths conform to the reported ranges of Tb—O bond length and O— Tb—O bond angle^[15]. In 1-3, with the increase of atomic number from Tb to Ho, the average Ln—O bond lengths decreased from 0.243 7 to 0.242 9 nm gradually due to the lanthanide contraction effect^[16]. So the average Na—O and Na—N bond lengths increased gradually from 0.234 9 to 0.236 9 nm and from 0.251 6 to 0.252 4 nm, respectively (Fig. 1d-1f).

  Figure 1

  

  Figure 1.  (a) Coordination environment of 1; (b) 1D chains of heteronuclear bimetallic 1; (c) 2D plane of 1; (d-f) 3D structures of 1-3, respectively

  Symmetry codes: a: 1-x, 1-y, 1-z; b: 1.5-x, -0.5+y, 0.5-z; c: 1.5-x, -0.5+y, 1.5-z; d: -0.5+x, 0.5-y, -0.5+z

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  2.2   Thermal stability and PXRD analysis

  The TGA curves of 1-3 have been recorded (Fig.S2-1f). The first weight loss of the MOFs ranging from 30 to 200 ℃ were 13.0% for 1 (Calcd. 13.5%), 13.46% for 2 (Calcd. 13.46%), 13.46% for 3 (Calcd. 13.4%), which should be attributed to the weight loss for all the coordination and lattice water molecules. The second weight loss of MOFs 1 -3 correspond to the slow decomposition of the framework over 470 ℃. PXRD patterns of 1 -3 are shown in Fig.S3. Most peak positions in simulated and experimental patterns were consistent with each other, indicating that 1-3 have good phase purity.

  2.3   Luminescence characteristic in solid state

  The solid state fluorescence spectra of H₄BDT and 1-3 has been measured at room temperature under the excitation of 325 nm (slits: 5 nm, PMT voltage: 350 V) (Fig. 2). Only MOF 1 showed four well resolved characteristic emission peaks at 490.6, 544.6, 584.8, and 621.6 nm, corresponding to typical f - f transitions of Tb³+ ions, namely ⁵D₄→⁷F₆, ⁵D₄→⁷F₅, ⁵D₄→⁷F₄, and ⁵D₄→⁷F₃. Under the same condition, compared with 1, the fluorescence intensity of H₄BDT, 2, and 3 were negligible. Therefore, the intensity change of the strongest emission peak (544.6 nm) was used as the detection index for analysis.

  Figure 2

  

  Figure 2.  Solid-state fluorescence spectra of H4BDT, 1, 2, and 3 at room temperature

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  2.4   Fluorescent sensing of Tb(Na)-MOF

  2.4.1   Sensing experiments for small organic molecules

  The fluorescence spectra of 1 in different solvents showed that the luminescence intensity of 1 was different (Fig.S4), and the fluorescence quenching phenomenon took place in AH (Fig. 3). The UV-Vis absorption of 1 in different solvents was recorded (Fig. S5). Compared to other solvents, the UV-Vis absorption band of AH had an obvious peak at 278 nm. Therefore, the fluorescence quenching of 1 by AH may be caused by competitive absorption.

  Figure 3

  

  Figure 3.  Relative fluorescence intensity of 1 in different solvents

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  To better understand the luminescence responses of AH to 1, the relationships between the concentrations of AH and the luminescence intensities were studied as shown in Fig. 4. The luminescence intensity of 1 gradually decreased with the increase of AH concentration, presenting a first order exponential relationship: y= 3 966.35exp(-x/83.837) + 327.699 (R²=0.993 94). When the concentration of AH was in a range of 5-100 μmol·L^-1, there was a linear relationship (Fig. 4 inset) between the concentration of AH and the luminescence intensity of 1: y= 3 970.5 - 26.57x (R²=0.998 16); at the same time the detection limit (LOD) calculated was 0.125 μmol·L^-1 calculated by formula 3σ/k. Compared with reported documents for detection AH by luminescent MOFs (LOD: 0.000 58%, about 102.7 μmol·L^-1)^[17], MOF 1 showed more excellent characteristics in terms of sensitivity.

  Figure 4

  

  Figure 4.  Nonlinear fitting graph for 1 sensing AH

  Inset: linear fitting graph for 1 sensing AH

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  2.4.2   Sensing experiments for metal ions

  The fluorescence spectra of 1 in the presence of different metal ions are shown in Fig. S6. It is found that different metal ions had different effects on the fluorescence intensity of 1. Especially in the solution of Fe³⁺, the phenomenon of fluorescence quenching was most obvious. Therefore, MOF 1 may be a good sensing material for detecting Fe³+ ion (Fig. 5). To better know the fluorescence quenching mechanism, the UV - Vis absorption spectra of different metal ions was recorded (Fig. S7). Compared to other metal ions, the UV -Vis absorption band of Fe³+ had an obvious peak at 325 nm. Therefore, the fluorescence quenching of 1 by Fe³+ may be caused by competitive absorption^[18].

  Figure 5

  

  Figure 5.  Relative fluorescence intensity of 1 in the presence of different metal ions

  Inset: nonlinear fitting graph for 1 sensing Fe³⁺

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  The luminescence intensity of 1 decreased gradually when the concentration of Fe³+ was changed from 10^-11 to 10^-2 mol·L^-1 (Fig. 5, inset), and there was a good nonlinear relationship between the luminescence intensity of 1 and the concentration of Fe³+, y=2 897.2exp[-x/(3.84x10^-6)] +1 555.7exp[-x/(1.35x 10^-4)]+209.97 (R²=0.994).

  2.4.3   Sensing experiments for anions

  The fluorescence spectra of 1 in different anion aqueous solutions was recorded (Fig. S8). The most interesting feature was that the fluorescence intensity of the inorganic anions incorporated 1 suspension was heavily dependent on the species of inorganic anions. Especially, Cr₂O₇^2- anion showed a drastic quenching effect on the luminescence of 1. While other inorganic anions have no significant quenching effect, indicative of the fact that 1 can be considered as a promising luminescent probe for Cr₂O₇^2- anion (Fig. 6). The UV - Vis spectra of different inorganic anions was recorded (Fig. S9). Compared to other inorganic anions, the UV- Vis absorption band of Cr₂O₇^2- had an obvious peak between 310 and 400 nm. Therefore, this unique quenching effect for 1 possibly results from competitive absorption^[19].

  Figure 6

  

  Figure 6.  Relative fluorescence intensity of 1 in the presence of different anions

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  Quantitatively, the fluorescence spectra of 1 in Cr₂O₇^2- aqueous solutions with different concentrations (0-700 μmol·L^-1) are shown in Fig. 7. There was the first-order derivative relationship between the fluorescence intensity of 1 and the concentration of Cr₂O₇^2-: y=785.2exp(-x/7.7)+3 993.4exp(-x/285.6)-243.6 (R²= 0.998). When the concentration of Cr₂O₇^2- was in a range of 200 - 500 pmol·L^-1, there was a good linear relation (Fig. 7, inset). The linear equation was y= -4.398x+2 559.61 (R²=0.996), and the LOD was 6.72 μmol·L^-1 (3σ/k). It was shown that MOF 1 had a lower LOD and good selectivity for Cr₂O₇^2-.

  Figure 7

  

  Figure 7.  Nonlinear fitting graph for 1 sensing Cr₂O₇^2-

  Inset: linear fitting graph for 1 sensing Cr₂O₇^2-

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  2.5   Electrochemical sensing of Tb(Na)-MOF

  Catechol as an important phenolic compound is widely used in pharmaceutical industry and organic chemical industry, which has been identified as a highly toxic environmental pollutant because of its serious harm to the environment and organisms^[20]. Therefore, how to fastly detect catechol with low cost, high sensitivity, high reproducibility has been widely concerned.

  As an excellent multifunctional material, MOFs have gradually attracted more and more attention in electrochemical fields such as capacitors and catalytic hydrolysis^[21]. However, there are no reports on the direct detection of catechol by electrochemical method.

  The results of catechol detection of 1 by CV are shown in Fig. 8. It can be seen from Fig. 8 that the reduction current gradually increased with the increase of catechol concentration, and they were linearly related (Fig. 8, inset): I=5.28x10^-7-6.10x10^-7c(R²=0.997 0). This indicates that Tb(Na)-MOF has a broad prospect in electrochemical sensing materials.

  Figure 8

  

  Figure 8.  CV curves of catechol detection by 1/CPE

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  3.   Conlusions

  In summary, three novel luminescent 3D microporous heterometallic MOFs (Ln(Na) - MOFs, Ln=Tb (1), Dy (2), Ho (3)) were successfully constructed. Fluorescence analysis revealed that Tb(Na)-MOF could act as a fluorescent probe for highly sensitive induced Fe³⁺ and Cr₂O₇^2- (LOD of Cr₂O₇^2-: 6.72 μmol·L^-1) in aqueous solutions and could detect the environmental pollutants acetaldehyde molecules (LOD: 0.125 μmol·L^-1) efficiently and high sensitively. In addition, 1/CPE for detection of catechol showed a good electrochemical activity. Finally, the multifunctional sensing properties of Tb(Na) - MOF may make it expected to be further developed as a potentially valuable detection material for environmental pollutants.

  Conflicts of interest: The authors declare no competing financial interest.

  Acknowledgments: This work was supported by National Natural Science Foundation of China (Grants No. 22063010, 21763028) and College Students Innovation and Entrepreneurship Programme (Grants No.YCX2021101, D2020037).

  Supporting information is available at http://www.wjhxxb.cn

  
  
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  Scheme 1  Structure of H₄BDT

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  Figure 1  (a) Coordination environment of 1; (b) 1D chains of heteronuclear bimetallic 1; (c) 2D plane of 1; (d-f) 3D structures of 1-3, respectively

  Symmetry codes: a: 1-x, 1-y, 1-z; b: 1.5-x, -0.5+y, 0.5-z; c: 1.5-x, -0.5+y, 1.5-z; d: -0.5+x, 0.5-y, -0.5+z

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  Figure 2  Solid-state fluorescence spectra of H4BDT, 1, 2, and 3 at room temperature

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  Figure 3  Relative fluorescence intensity of 1 in different solvents

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  Figure 4  Nonlinear fitting graph for 1 sensing AH

  Inset: linear fitting graph for 1 sensing AH

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  Figure 5  Relative fluorescence intensity of 1 in the presence of different metal ions

  Inset: nonlinear fitting graph for 1 sensing Fe³⁺

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  Figure 6  Relative fluorescence intensity of 1 in the presence of different anions

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  Figure 7  Nonlinear fitting graph for 1 sensing Cr₂O₇^2-

  Inset: linear fitting graph for 1 sensing Cr₂O₇^2-

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  Figure 8  CV curves of catechol detection by 1/CPE

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  Table 1.  Crystallographic data for 1-3

  Parameter	1	2	3
  Empirical formula	C18H17O13N3TbNa	C18H17O13N3DyNa	C18H17O13N3HoNa
  Formula weight	665.25	668.83	671.26
  Crystal system	Monoclinic	Monoclinic	Monoclinic
  Space group	P21/n	P21/n	P21/n
  a/nm	0.743 72(19)	0.748 82(6)	0.749 78(12)
  b/nm	1.824 1(5)	1.830 81(14)	1.833 1(3)
  c/nm	1.538 4(4)	1.546 83(12)	1.547 3(3)
  β/(°)	93.711(4)	93.596 2(12)	93.473(2)
  Volume/nm3	2.082 7(9)	2.116 4(3)	2.122 8(6)
  Z	4	4	4
  Dc/(g.cm-3)	2.122	2.099	2.100
  Absorption coefficient/mm-1	3.498	3.632	3.828
  F(000)	1 304	1 308	1 312
  θ range for data collection/(°)	1.734-26.540	1.725-27.628	1.724-27.485
  Reflection collected	4 335	4 849	4 844
  Reflection unique (Rinl)	4 335 (0.027 3)	4 849 (0.025 7)	4 844 (0.021 9)
  Data, restraint, parameter	4 335, 0, 326	4 849, 0, 326	4 844, 13, 326
  Goodness-of-fit on F 2	0.863	1.134	1.227
  Final R indices [I > 2σ(I)]a, b	R1=0.022 8, wR2=0.055 7	R1=0.027 1, wR2=0.060 8	R1=0.022 9, wR2=0.063 1
  R indices (all data)	R1=0.026 1, wR2=0.069 3	R1=0.039 4, wR2=0.074 7	R1=0.029 3, wR2=0.074 0
  $ ^{\text{a}}{{R}_{1}}=\Sigma \left( \left| {{F}_{o}} \right|-\left| {{F}_{c}} \right| \right)/\Sigma \left| {{F}_{o}} \right|{{;}^{\text{b}}}w{{R}_{2}}={{\left\{ \Sigma \left[ w{{\left( {{\left| {{F}_{o}} \right|}^{2}}-{{\left| {{F}_{c}} \right|}^{2}} \right)}^{2}} \right]/\Sigma \left[ w{{\left( {{\left| {{F}_{o}} \right|}^{2}} \right)}^{2}} \right] \right\}}^{1/2}}.$

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