English

• 1. INTRODUCTION

  Iron(III) is an indispensable trace element in human body as it plays key roles in the formation of hemoglobin and other normal metabolic processes^{[1, 2]}. However, Fe³⁺ cannot be metabolized normally in human body and its excessive or deficient quantities cause serious diseases^[3-5]. Compared with other detection methods, fluorescence sensing method has been widely concerned by researchers attributed to its high selectivity, operational convenience and sensitivity^{[6, 7]}. Therefore, it is important to develop a fluorescence sensor which can be used to detect low concentration of iron(III) ions.

  Coordination polymers (CPs), as an inorganic-organic hybrid materials^[8-11], have attracted extensive attention in gas adsorption^[12], catalysis^{[13, 14]}, drug delivery^[15] and photochromic processes^[16]. In addition to these, CPs as a new kind of fluorescence sensing materials have also attracted the interest of researchers^[17] attributed to their high sensitivity and selectivity for specific type of metals^[18]. Compared with transition metal coordination polymers (such as nickel and zinc CPs)^{[19, 20]}, lanthanide coordination polymers (Ln-CPs) have more abundant structures, large stokes shift and a long luminescent lifetime due to the unique 4f electron layer of lanthanide metals^[21-27], which significantly boost their performance. Based on the antenna effect, organic ligands can efficiently sensitize lanthanide ions through energy-transfer, and finally the lanthanide ions produce the luminescence^[28]. These unique properties make Ln-CPs have different properties and applications compared with transition metal coordination polymers^[29].

  Herein, Ln-CP based on europium [KEu₂(FDCA)₃-(H₂O)₉·0.5(FDCA)] (1) was prepared by 2,5-furandicarboxylic acid, KNO₃ and Eu(NO₃)₃·6H₂O. It was confirmed experimentally that 1 can emit intense fluorescence at 613 nm and possess high selectivity for Fe³⁺ through fluorescence quenching.

  2. EXPERIMENTAL

  2.1 Materials and methods

  2,5-Furandicarboxylic acid, KNO₃, Eu(NO₃)₃·6H₂O and other chemicals were supplied by Hwrkchemical Co, . Ltd. Powder X-ray diffraction patterns were performed on a Rigaku D/Max-2500 diffractometer with a Cu-target tube. Infrared (IR) spectra were recorded on a Nicolet IS10 infrared spectrometer using KBr pellets. The fluorescence spectra, fluorescence lifetimes and sensing properties were measured with the FS5 fluorescence spectrometer (Edinburgh Instruments).

  2.2 Synthesis of [KEu₂(FDCA)₃(H₂O)₉·0.5(FDCA)] (1)

  H₂FDCA (0.2 mmol, 31.2 mg), KNO₃ (0.1 mmol, 10.1 mg) and Eu(NO₃)₃·6H₂O (0.1 mmol, 44.6 mg) were added to 5 mL mixed solvent (V_{ethylene glycol} : V_ethanol: V_H2O = 1:1:1) in a 15 mL Teflon-lined reactor, and heated at 120 ℃ for 48 h. The reactor was then gradually cooled to 30 ℃ in 12 h and the products were filtered and washed thrice with ethanol. Finally, plenty of white crystals were air dried and collected under ambient conditions (yield: 40% based on Eu). FT-IR (KBr, cm^-1): 3124 (b), 2979 (w), 2322 (w), 1577 (s), 1371 (s), 1222 (w), 1168 (w), 1071 (w), 1047 (w), 1028 (w), 1016 (m), 970 (m), 882 (w), 777 (s), 647 (m), 614 (m).

  2.3 X-ray structure determination

  Single-crystal X-ray diffraction data for compound 1 were acquired on a Bruker-AXS SMART APEX2 CCD diffractometer at 293(2) K with MoKα radiation. The crystal data were analyzed using direct methods by the SHELXS program of SHELXTL-2014. In order to reduce the effect caused by disordered solvent molecules, SQUEEZE was applied. More details on the crystallographic studies of 1 are given in the CIF file while the main bond lengths and bond angles are offered in Table 1.

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°)

  DownLoad: CSV

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  K(1)-O(1)	2.987(4)		Eu(1)-O(1)	2.405(4)		Eu(2)-O(5)	2.398(4)
  K(1)-O(1)W	2.983(7)		Eu(1)-O(2)W	2.497(5)		Eu(2)-O(6)W	2.464(5)
  K(1)-O(15)	2.990(4)		Eu(1)-O(3)W	2.473(4)		Eu(2)-O(7)W	2.468(5)
  K(1)-O(5)	2.993(4)		Eu(1)-O(4)W	2.390(4)		Eu(2)-O(8)W	2.458(4)
  K(1)-O(6)	2.995(4)		Eu(1)-O(5)W	2.403(4)		Eu(2)-O(9)W	2.410(4)
  K(1)-O(10)	2.976(4)		Eu(1)-O(6)	2.438(4)		Eu(2)-O(10)	2.442(4)
  K(1)-O(12)	2.902(4)		Eu(1)-O(12)	2.389(4)		Eu(2)-O(11)	2.328(4)
  			Eu(1)-O(14)	2.324(4)		Eu(2)-O(15)	2.359(4)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)-K(1)-O(5)	109.42(12)		O(4)W-Eu(1)-O(2)W	129.71(14)		O(15)-Eu(2)-O(5)	88.43(14)
  O(1)W-K(1)-O(15)	94.72(15)		O(12)-Eu(1)-O(1)	87.75(13)		O(5)-Eu(2)-O(10)	68.95(13)
  O(10)-K(1)-O(5)	54.65(11)		O(4)W-Eu(1)-O(1)	142.44(14)		O(15)-Eu(2)-O(10)	73.38(13)
  O(12)-K(1)-O(5)	161.43(13)		O(5)W-Eu(1)-O(1)	96.76(14)		O(9)W-Eu(2)-O(10)	76.15(13)
  O(15)-K(1)-O(5)	67.34(11)		O(1)-Eu(1)-O(6)	68.75(13)		O(5)-Eu(2)-O(6)W	74.74(16)
  O(1)W-K(1)-O(5)	90.93(15)		O(12)-Eu(1)-O(6)	71.98(12)		O(10)-Eu(2)-O(6)W	129.95(15)
  O(1)-K(1)-O(6)	54.41(10)		O(4)W-Eu(1)-O(6)	73.76(13)		O(15)-Eu(2)-O(6)W	72.35(17)
  O(5)-K(1)-O(6)	137.54(11)		O(5)W-Eu(1)-O(6)	76.18(14)		O(8)W-Eu(2)-O(6)W	130.74(15)
  O(10)-K(1)-O(6)	109.02(11)		O(1)-Eu(1)-O(3)W	75.47(14)		O(9)W-Eu(2)-O(6)W	143.30(16)
  O(12)-K(1)-O(6)	57.49(10)		O(6)-Eu(1)-O(3)W	127.70(14)		O(5)-Eu(2)-O(7)W	76.52(15)
  O(15)-K(1)-O(6)	142.47(12)		O(12)-Eu(1)-O(3)W	143.56(15)		O(10)-Eu(2)-O(7)W	128.19(15)
  O(1)W-K(1)-O(6)	109.31(15)		O(4)W-Eu(1)-O(3)W	133.06(15)		O(15)-Eu(2)-O(7)W	143.57(18)
  O(1)-K(1)-O(15)	158.46(13)		O(5)W-Eu(1)-O(3)W	71.48(15)		O(6)W-Eu(2)-O(7)W	71.74(19)
  O(10)-K(1)-O(15)	57.50(11)		O(1)-Eu(1)-O(2)W	74.99(14)		O(8)W-Eu(2)-O(7)W	132.35(15)
  O(12)-K(1)-O(15)	107.22(11)		O(6)-Eu(1)-O(2)W	127.55(14)		O(9)W-Eu(2)-O(7)W	71.58(18)
  O(12)-Eu(1)-O(4)W	78.39(14)		O(12)-Eu(1)-O(2)W	69.96(14)		O(5)-Eu(2)-O(8)W	142.83(13)
  O(12)-Eu(1)-O(5)W	143.67(14)		O(3)W-Eu(1)-O(2)W	74.49(16)		O(10)-Eu(2)-O(8)W	74.03(13)
  O(4)W-Eu(1)-O(5)W	76.28(14)		O(5)-Eu(2)-O(9)W	97.84(14)		O(15)-Eu(2)-O(8)W	77.82(15)
  O(5)W-Eu(1)-O(2)W	145.97(15)		O(15)-Eu(2)-O(9)W	144.20(15)		O(9)W-Eu(2)-O(8)W	76.09(13)

  2.4 Fluorescence experiment

  The solid sample was ground into powder for luminescence experiment. 3 mg well-grounded powder was immersed in 3 mL ethanol solution of MNO_x (M^m+ = K⁺, Cd²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Ca²⁺, Zn²⁺, Cu²⁺, Fe³⁺, 1 mmol/L) and treated ultrasonicated for 5 min. Then, the fluorescence response in the range of 550~650 nm was monitored under 310 nm excitation.

  3. RESULTS AND DISCUSSION

  3.1 Characterization of [KEu₃(FDCA)₃ (H₂O)₉·0.5(FDCA)] (1)

  Crystallographic analysis exhibits that 1 crystallizes in the P2₁/n space group of monoclinic system. The asymmetric unit contains two Eu³⁺ ions, one K⁺ ion, three FDCA^2–, a-half free FDCA^2– and nine coordinated H₂O molecules (Fig. 1a). The Eu(1) ion locates in a distorted bicapped trigonal prismatic coordination environment completed by four carboxylato oxygens from four FDCA^2– ligands and four oxygens from four H₂O molecules (Fig. 1b). The Eu(2) ion is coordinated by four carboxylato oxygens (O(5), O(10), O(11), O(15)) from four FDCA^2– ligands and four oxygens from four H₂O molecules in a bicapped triangular prism, similar to Eu(1). The bond lengths of Eu–O are in the 2.328(4)~2.497(5) Å range. Interestingly, three FDCA^2– ligands form a "23-crown-9-like" and the K⁺ ion locates in the hole of the "crown ether" (Fig. 1c). The "crown-like" unit connects four Eu ions, and neighboring Eu ions constitute a binuclear unit through -O–C–O- from two FDCA^2– ligands (Fig. 1d). Such binuclear units are further bridged by "crown-like" unit into an infinite 1D chain (Fig. 1e).

  Figure 1

  

  Figure 1.  (a) Asymmetric unit of 1 (The free FDCA^2– ligand is omitted for clarity). (b) Coordination environment of Eu1. (c) "23-crown-9-like" structure. (d) "23-crown-9-like" structure bridges four Eu ions. (e) 1D chain of 1

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  3.2 PXRD and IR spectra

  The PXRD was performed at room temperature to demonstrate the phase purity of the obtained samples. Fig. 2 shows that the peak positions of compound 1 in PXRD pattern have good coincidence with the simulated pattern, indicating that the purity of samples was up to requirement. In left of Fig. 2, the peak at 882 cm^-1 is attributed to γ(C–H) of the furan rings. The ν(C–O–C) on the furan rings results in a moderate peak at 1071 and 1222 cm^-1. Furthermore, the strong and sharp bands at 1375, 1571 cm^-1 correspond to the ν_s(COO^-) and ν_as(COO^-), respectively. The broad absorption band around 3124 cm^-1 is ascribed to characteristic ν(O−H) of coordinated water molecules.

  Figure 2

  

  Figure 2.  PXRD patterns (left) and IR spectra (right) for 1

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  3.3 Thermal stability

  Thermogravimetric analysis (TGA) study was performed under N₂ atmosphere at 25~700 ℃ to evaluate the thermal stability of 1. As depicted in Fig. 3, the initial weight loss of about 15.69% from 27 to 175 ℃ (calcd.: 15.51%) attributes to the detachment of coordinated water molecules.

  Figure 3

  

  Figure 3.  TG curve of 1

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  3.4 Luminescent properties

  At room temperature the solid state luminescent properties of 1 are investigated. As shown in Fig. 4a, Eu(III) can be significantly sensitized and exhibit four sharp fluorescence peaks with λ_Ex = 310, attributed to the transitions of ⁵D₀ → ⁷F₁ (592 nm), ⁵D₀ → ⁷F₂ (613 and 616 nm), and ⁵D₀ → ⁷F₃ (652 nm). The intense bands are observed at 613 and 616 nm, which produced intense red luminescence. The fluorescence lifetime of solid state 1 is investigated, which fit the single exponential decay function I = I₀exp(–t/τ) (τ = fluorescence lifetime) (Fig. 4b)^[30], where I₀ and I are the fluorescence intensity at time 0 and t. The fitting result of 1 could conclude that the fluorescence lifetime is τ = 281 μs.

  Figure 4

  

  Figure 4.  (a) Emission spectra of 1 in solid state; (b) Fluorescence lifetime of 1

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  3.5 Sensing of metal ions

  With the fore-idea that Eu-complex has luminescent properties, its luminescent abilities towards metal ions are investigated. Fig. 5a shows that 1 displays differentiated luminescence response for various metal ions. When adding K⁺ and Cd²⁺, the luminescence intensity of 1 is slightly enhanced, while the addition of other metal ions produced varying degrees of quenching effects, among which Fe³⁺ ion has the most drastic effect. These results suggest that compound 1 shows remarkable selectivity to Fe³⁺. The quenching effect of Fe³⁺ on compound 1 can be seen more significantly (Fig. 5b).

  Figure 5

  

  Figure 5.  (a) Luminescence spectra of 1 dispersed in ethanol solutions of different metal ions λ_{Ex = 310}; (b) the ⁵D₀ → ⁷F₂ transition (613 nm) intensities of 1 when dispersed in various metal ions aqueous

  (insert, photographs of 1 under UV lamp after immersed in different metal ions solution)

  DownLoad: Full-Size Img PowerPoint

  The anti-interference ability is also necessary for sensor, so a series of anti-interference experiments are demonstrated. As shown in Fig. 6, the powder of 1 is well dispersed to the other metal solution, and the luminescence intensity of 1 has rapidly decreased after introducing Fe³⁺ ions. Therefore, 1 has a remarkable anti-interference ability to Fe³⁺ ions even if there are other coexisting metal ions.

  Figure 6

  

  Figure 6.  Fluorescence variation of 1 to different metal ions (green bars, luminescence intensities with different metal ions; red bars, luminescent intensities after the addition of Fe³⁺ ion)

  DownLoad: Full-Size Img PowerPoint

  Further, luminescent quantitative titration experiments were performed to better understand the sensitivity of 1 for Fe³⁺. The fluorescence intensities of 1 are weakened step by step with a corresponding increase in the fluorescence quenching efficiency when increasing the contents of Fe³⁺ in the ethanol solution (Fig. 7 and Fig. 8a). The quenching phenomenon can be explained by the Stern-Volmer (S-V) formula: I₀/I = K_sv[M] + 1 (K_sv = quenching effect constant), where I₀ and I are the luminescent intensities of 1 and 1-Fe³⁺, respectively, and [M] is the concentration of Fe³⁺ in solution. The quenching curves can be fitted by the S-V equation, and show good linearity at lower concentrations from 0 to 0.08 mM (Fig. 8b). K_sv values of Fe³⁺ ions are calculated to be 4.35 × 10⁴ M⁻¹, which are close to that of reported Ln-CPs for detecting Fe³⁺ ions (Table 2)^[31-38]. The equation I₀/I = K_sv[Fe³⁺] + 0.73 (R² = 0.97), which is similar to the ideal S-V equation, shows that the interaction of Fe³⁺ with compound 1 is controlled by the concentration diffusion^{[3, 4]}. The detection limit (LOD) of compound 1 for detecting Fe³⁺ calculated by the ratio of 3δ/slope is 7.44 × 10^-6 M, and the sensing ability of compound 1 is comparable with that of some previously reported Fe³⁺ fluorescent sensors (Table 2)^[31-38]. Such high K_sv value and low detection limits indicate that Fe³⁺ has strong quenching effect for compound 1, thereby confirming its highly applicable nature for the detection of Fe³⁺ in practical applications.

  Figure 7

  

  Figure 7.  Luminescence spectra of 1 suspension with various concentrations of Fe³⁺

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  Figure 8

  

  Figure 8.  (a) Quenching efficiency curve of 1 for Fe³⁺-EtOH, (b) S-V plot of 1

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  Table 2

  

  Table 2.  Comparison of the Sensing Performance of Compound 1 with the Reported Ln-CPs for the Sensing of Fe³⁺

  DownLoad: CSV

  Lanthanide CPs and related materials		Ksv (M-1)		Detection limit (mol/L)		Reference
  CTGU-1 (Tb)		1.883×106		1×10-9		31
  [[Tb2(pyznpy)6·4H2O]·2H2O]n		7.99×104		1.5×10−3		32
  [[Tb(μ6-H2cpboda)(μ2-OH2)2]·H2O]n		6.50×104		8.40×10-7		33
  [KEu2(FDCA)3(H2O)9·0.5(FDCA)]		4.35×104		7.44×10−6		This work
  Eu3+@MIL-124		3.874×104		2.8×10-7		34
  Eu-BPDA		1.25×104		9×10-7		35
  [Eu(atpt)1.5(phen)(H2O)]n		7.60×103		4.5×10-5		36
  BUT-14		2.17×103		3.8×10−6		37
  [Ln(µ3-cpta)(phen)(H2O)2]n		2.138×103		2.0×10-4		38

  The recycling performance is very important for a good sensor. Thus, we investigate the recycling performance of compound 1, which is simply obtained by soaking in ethanol for 24 h. The quenching efficiency of 1 to Fe³⁺ is not significantly reduced after five cycles (Fig. 9), which indicates that 1 can be used as a repeated and stable probe to detect Fe³⁺ in ethanol solution. It also proves that the framework remains stable after recognition Fe³⁺ ions of 1 and the quenching is not due to the collapse of the framework.

  Figure 9

  

  Figure 9.  Recycling experiments for recognition of Fe³⁺

  (blue bars, the luminescence intensity of 1 before added Fe³⁺; red bars, the luminescence intensity of 1-Fe³⁺)

  DownLoad: Full-Size Img PowerPoint

  4. CONCLUSION

  In summary, polymer [KEu₂(FDCA)₃(H₂O)₉·0.5(FDCA)] (1) is synthesized via solvothermal approach. Compound 1 exhibits a one-dimensional chain with "23-crown-9-like" SBUs and possesses high selectivity and sensitivity for the sensing of Fe³⁺ by luminescence quenching mechanism with K_SV = 4.35 × 10⁴ M⁻¹ and LOD = 7.44 × 10^-6 M.

  
  
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  Figure 1  (a) Asymmetric unit of 1 (The free FDCA^2– ligand is omitted for clarity). (b) Coordination environment of Eu1. (c) "23-crown-9-like" structure. (d) "23-crown-9-like" structure bridges four Eu ions. (e) 1D chain of 1

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  Figure 2  PXRD patterns (left) and IR spectra (right) for 1

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  Figure 3  TG curve of 1

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  Figure 4  (a) Emission spectra of 1 in solid state; (b) Fluorescence lifetime of 1

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  Figure 5  (a) Luminescence spectra of 1 dispersed in ethanol solutions of different metal ions λ_{Ex = 310}; (b) the ⁵D₀ → ⁷F₂ transition (613 nm) intensities of 1 when dispersed in various metal ions aqueous

  (insert, photographs of 1 under UV lamp after immersed in different metal ions solution)

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  Figure 6  Fluorescence variation of 1 to different metal ions (green bars, luminescence intensities with different metal ions; red bars, luminescent intensities after the addition of Fe³⁺ ion)

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  Figure 7  Luminescence spectra of 1 suspension with various concentrations of Fe³⁺

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  Figure 8  (a) Quenching efficiency curve of 1 for Fe³⁺-EtOH, (b) S-V plot of 1

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  Figure 9  Recycling experiments for recognition of Fe³⁺

  (blue bars, the luminescence intensity of 1 before added Fe³⁺; red bars, the luminescence intensity of 1-Fe³⁺)

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°)

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  K(1)-O(1)	2.987(4)		Eu(1)-O(1)	2.405(4)		Eu(2)-O(5)	2.398(4)
  K(1)-O(1)W	2.983(7)		Eu(1)-O(2)W	2.497(5)		Eu(2)-O(6)W	2.464(5)
  K(1)-O(15)	2.990(4)		Eu(1)-O(3)W	2.473(4)		Eu(2)-O(7)W	2.468(5)
  K(1)-O(5)	2.993(4)		Eu(1)-O(4)W	2.390(4)		Eu(2)-O(8)W	2.458(4)
  K(1)-O(6)	2.995(4)		Eu(1)-O(5)W	2.403(4)		Eu(2)-O(9)W	2.410(4)
  K(1)-O(10)	2.976(4)		Eu(1)-O(6)	2.438(4)		Eu(2)-O(10)	2.442(4)
  K(1)-O(12)	2.902(4)		Eu(1)-O(12)	2.389(4)		Eu(2)-O(11)	2.328(4)
  			Eu(1)-O(14)	2.324(4)		Eu(2)-O(15)	2.359(4)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)-K(1)-O(5)	109.42(12)		O(4)W-Eu(1)-O(2)W	129.71(14)		O(15)-Eu(2)-O(5)	88.43(14)
  O(1)W-K(1)-O(15)	94.72(15)		O(12)-Eu(1)-O(1)	87.75(13)		O(5)-Eu(2)-O(10)	68.95(13)
  O(10)-K(1)-O(5)	54.65(11)		O(4)W-Eu(1)-O(1)	142.44(14)		O(15)-Eu(2)-O(10)	73.38(13)
  O(12)-K(1)-O(5)	161.43(13)		O(5)W-Eu(1)-O(1)	96.76(14)		O(9)W-Eu(2)-O(10)	76.15(13)
  O(15)-K(1)-O(5)	67.34(11)		O(1)-Eu(1)-O(6)	68.75(13)		O(5)-Eu(2)-O(6)W	74.74(16)
  O(1)W-K(1)-O(5)	90.93(15)		O(12)-Eu(1)-O(6)	71.98(12)		O(10)-Eu(2)-O(6)W	129.95(15)
  O(1)-K(1)-O(6)	54.41(10)		O(4)W-Eu(1)-O(6)	73.76(13)		O(15)-Eu(2)-O(6)W	72.35(17)
  O(5)-K(1)-O(6)	137.54(11)		O(5)W-Eu(1)-O(6)	76.18(14)		O(8)W-Eu(2)-O(6)W	130.74(15)
  O(10)-K(1)-O(6)	109.02(11)		O(1)-Eu(1)-O(3)W	75.47(14)		O(9)W-Eu(2)-O(6)W	143.30(16)
  O(12)-K(1)-O(6)	57.49(10)		O(6)-Eu(1)-O(3)W	127.70(14)		O(5)-Eu(2)-O(7)W	76.52(15)
  O(15)-K(1)-O(6)	142.47(12)		O(12)-Eu(1)-O(3)W	143.56(15)		O(10)-Eu(2)-O(7)W	128.19(15)
  O(1)W-K(1)-O(6)	109.31(15)		O(4)W-Eu(1)-O(3)W	133.06(15)		O(15)-Eu(2)-O(7)W	143.57(18)
  O(1)-K(1)-O(15)	158.46(13)		O(5)W-Eu(1)-O(3)W	71.48(15)		O(6)W-Eu(2)-O(7)W	71.74(19)
  O(10)-K(1)-O(15)	57.50(11)		O(1)-Eu(1)-O(2)W	74.99(14)		O(8)W-Eu(2)-O(7)W	132.35(15)
  O(12)-K(1)-O(15)	107.22(11)		O(6)-Eu(1)-O(2)W	127.55(14)		O(9)W-Eu(2)-O(7)W	71.58(18)
  O(12)-Eu(1)-O(4)W	78.39(14)		O(12)-Eu(1)-O(2)W	69.96(14)		O(5)-Eu(2)-O(8)W	142.83(13)
  O(12)-Eu(1)-O(5)W	143.67(14)		O(3)W-Eu(1)-O(2)W	74.49(16)		O(10)-Eu(2)-O(8)W	74.03(13)
  O(4)W-Eu(1)-O(5)W	76.28(14)		O(5)-Eu(2)-O(9)W	97.84(14)		O(15)-Eu(2)-O(8)W	77.82(15)
  O(5)W-Eu(1)-O(2)W	145.97(15)		O(15)-Eu(2)-O(9)W	144.20(15)		O(9)W-Eu(2)-O(8)W	76.09(13)

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  Table 2.  Comparison of the Sensing Performance of Compound 1 with the Reported Ln-CPs for the Sensing of Fe³⁺

  Lanthanide CPs and related materials		Ksv (M-1)		Detection limit (mol/L)		Reference
  CTGU-1 (Tb)		1.883×106		1×10-9		31
  [[Tb2(pyznpy)6·4H2O]·2H2O]n		7.99×104		1.5×10−3		32
  [[Tb(μ6-H2cpboda)(μ2-OH2)2]·H2O]n		6.50×104		8.40×10-7		33
  [KEu2(FDCA)3(H2O)9·0.5(FDCA)]		4.35×104		7.44×10−6		This work
  Eu3+@MIL-124		3.874×104		2.8×10-7		34
  Eu-BPDA		1.25×104		9×10-7		35
  [Eu(atpt)1.5(phen)(H2O)]n		7.60×103		4.5×10-5		36
  BUT-14		2.17×103		3.8×10−6		37
  [Ln(µ3-cpta)(phen)(H2O)2]n		2.138×103		2.0×10-4		38

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