English

• 0.   Introduction

  Copper, as one of the first metals utilized by mankind, has been extensively utilized in manufacturing, industry, and agriculture^[1-3]. Additionally, copper is a vital trace metal that, at low concentrations, is crucial to the healthy operation of organs and metabolic processes^[4]. However, copper, as a heavy metal element, is enriched through the food chain and is difficult to degrade^[5-7]. A significant body of research has demonstrated that Cu²⁺ may be associated with the development of hereditary or neurodegenerative diseases when present in high concentrations in the body for extended periods. These conditions, such as Wilson's disease, Alzheimer's disease, and Parkinson's disease, seriously threaten human health^{[5, 8-10]}. In recent years, the extensive use of copper in various industries has resulted in a significant release of Cu²⁺ into the environment through wastewater, causing severe water pollution^{[6, 11]}. The U.S. Environmental Protection Agency (EPA) and the World Health Organization (WHO) have set limits for copper levels in drinking water at 1.3 and 2 mg·L^-1, respectively^[4-5]. Given the significance of Cu²⁺ in human health and the environment, there is a critical need for the development of a feasible, effective, and simple method for detecting Cu²⁺ in environmental water samples^[5-6].

  Current methods for detecting Cu²⁺ include electrochemical methods^[12], inductively coupled plasma - mass spectrometry (ICP - MS) ^[12-13], atomic emission/ absorption spectrometry (AES/AAS) ^[14], and graphite furnace atomic absorption spectrometry (GFAAS) ^[15]. While these methods are known for their sensitivity, accuracy, and reliability, the widespread use of rapid testing is hindered by the need for specialized laboratories. This limitation is due to the costly equipment, intricate sample preparation, and lengthy testing procedures associated with copper detection^{[6, 16]}. Compared with the traditional detection techniques mentioned above, fluorescence sensing technology has become increasingly popular in recent years^[5]. It has been widely used for the detection of heavy metal ions including Cu²⁺ in the environment due to its simple operation process, high sensitivity, fast and accurate response, and inexpensive equipment^[17-19]. Among the many reported luminescent materials, fluorescent metal-organic frameworks (FMOFs) are promising sensing materials due to their simple synthesis, tunable fluorescence properties, structural and functional diversity, and multiple detection mechanisms^{[6, 20]}. Moreover, its fluorescence sensing properties have also been intensively studied for use in environmental detection, medicine, and industrial production. Research has made significant progress in the detection of a wide range of substances and parameters, such as metal ions, anions, antibiotics, volatile organic compounds (VOCs), nitro explosives, amino acids, pH, and temperature^{[4, 21-25]}. The development of FMOF sensing materials with high sensitivity, high stability, and simple preparation methods is now imperative for human health^[22].

  The fluorescence properties of MOFs are highly dependent on the nature of the organic ligands and metal centers used^[21]. In terms of metal centers, there are numerous reports on the improved luminescence properties of FMOF materials synthesized with transition metal ions (Zn²⁺ and Cd²⁺) with d¹⁰ electronic configurations, which may be attributed to the enhancement of the emission of organic ligands by metal ions^[23]. In terms of organic ligands, MOFs based on ligands with large π-conjugated systems usually have good fluorescence properties according to previous studies. Moreover, the mixed - ligand strategy has been proven to be an effective way to construct novel MOFs compared with the monoligand system due to the reasonable variation in the secondary structural units of MOFs and the greater tunability of their structural skeletons^[21]. The ligand 1, 4 - bis(3, 5 - dicarboxyphenoxy)benzene (H₄L) reported in this paper is a semirigid ether-bonded aromatic polycarboxylic acid. It has abundant coordination modes that enhance the stability of the skeleton and the tunability of the structure. The ether bonds between the benzene rings are free to rotate and can effectively reduce the spatial site resistance^[26]. In addition, the pyridine nitrogen atom of the di - ligand provides more coordination possibilities, increasing the structural flexibility of the MOFs. Therefore, a novel Zn-MOF ({[Zn₆(L)₃(2, 2'-bpy) ₄]·2H₂O}_n) was synthesized by a hydrothermal method using H₄L as the primary ligand and 2, 2'-bpy (2, 2'-bipyridine) as the secondary ligand. The Zn-MOFs were comprehensively and thoroughly characterized by infrared spectroscopy (IR), powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and fluorescence tests, and their luminescence sensing properties and detection mechanisms were investigated.

  1.   Experimental

  1.1   Materials and measurements

  The analytically pure H₄L ligand purchased from Shandong Jinan Henghua Reagent Co., Ltd., was used directly. Other reagents and solvents were purchased from commercial sources and used without further purification. The sample for elemental analysis (EA) was dried under vacuum and tested with a CHN - O - Rapid instrument. The IR spectra in a range of 4 000 - 400 cm^-1 were obtained on a Bruker TENSOR27 spectrometer. The PXRD patterns were recorded on a Bruker D8 Advance X-ray diffractometer with Mo Kα radiation (λ= 0.071 069 nm) in a range of 5°-50° in 2θ at a rate of 5 (°) ·min^-1. The operating voltage and current were 40 kV and 25 mA, respectively. TGA was carried out on a DuPont thermal analyzer at a heating rate of 10 ℃ · min^-1 under N₂ flow. The luminescence spectra were characterized on an FS5 Spectrofluorometer with a xenon arc lamp as the light source. A pass width of 5 nm was used for the emission measurement, and a pass width of 5 nm was used for the excitation spectra; all the measurements were performed under the same experimental conditions. The UV - visible spectra were obtained from a JASCO V- 570 spectrophotometer. The accurate elemental ratio of zinc/copper was determined by a PerkinElmer NexION 300X inductively coupled plasma atomic emission spectrometer. XPS data were obtained from a Thermo ESCALAB 250XL electronic spectrometer.

  1.2   Synthesis of Zn-MOF

  ZnSO₄·7H₂O (0.028 7 g, 0.1 mmol), H₄L (0.043 8 g, 0.1 mmol), and 2, 2'-bpy (0.0156 g, 0.1 mmol) were added to a mixed solution of 6.0 mL distilled water and 3.0 mL CH₃CN. The reaction mixture was sealed in a reaction kettle and kept at a constant temperature of 160 ℃ for 72 h after stirring for 30 min at room temperature (Scheme 1). After cooling naturally to room temperature, a large amount of colorless strip crystals were obtained by rinsing with sufficient water, with a yield of approximately 72.8% (based on H₄L). EA (%) for C₁₀₆H₆₆Zn₆N₈O₃₂ (calculated values in parentheses): C 53.59 (54.04); H 2.94 (2.82); N 4.69 (4.76). IR (KBr, cm^-1): 3 460w, 3 080w, 1 722w, 1 618s, 1 580s, 1 496s, 1 473m, 1 445s, 1 401s, 1 366s, 1 317s, 1 243m, 1 194s, 1 126m, 1 090w, 1 054w, 1 040w, 1 024w, 1 012w, 978 m, 937w, 911w, 862w, 818m, 804m, 780s, 766s, 736m, 718s, 653w, 629w, 608w, 567w, 518w, 451w, 419w.

  Scheme1 1

  

  Scheme1 1.  Synthesis route of Zn-MOF

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  1.3   X-ray crystallography

  Crystallographic diffraction data for Zn-MOF were collected on a Bruker SMART APEX Ⅱ diffractometer with a CCD area detector and Mo Kα radiation (λ = 0.071 073 nm) at 296 (2) K, and crystal cell parameters were determined using SMART. Absorption corrections were applied by using the program SADABS. The asymmetric unitary structure of the crystal was resolved via direct methods and refined by the full-matrix least-squares method on F² employing the SHELXS- 97 program. All non - H atoms were anisotropically refined and theoretically H - added to C and O atoms^[27]. The data were collected using the MARCCD program and processed using the HKL2000 program^[28]. After all non - H atoms were refined anisotropically, H atoms attached to C atoms were placed geometrically and refined using a riding model approximation, with the length of C—H being 0.093 nm and U_iso(H)=1.2 U_eq(C). H atoms attached to O atoms were located from difference Fourier maps, and their bond lengths were restrained in a range from 0.084 to 0.096 nm; then they were refined using a riding model, with U_iso(H)=1.5U_eq (O). The crystal data and structure refinement details of Zn- MOF are shown in Table 1 and Table S1 (Supporting information).

  Table 1

  

  Table 1.  Crystal data and structure refinement of Zn-MOF

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  Parameter	Zn-MOF
  Formula	C53H33Zn3N4O16
  Formula weight	1 177.94
  Crystal system	Triclinic
  Space group	P1
  a / nm	0.975 56(7)
  b / nm	1.364 94(9)
  c / nm	1.979 02(12)
  α/(°)	108.314(2)
  β/(°)	90.635(2)
  γ/(°)	107.070(2)
  V / nm3	2.3760(3)
  Z	2
  F(000)	1 194
  Dc / (Mg·m-3)	1.646
  μ / mm-1	1.58
  Reflection collected	25 341
  Independent reflection	11 561
  Rint	0.049
  GOF	0.93
  R1, wR2 [I > 2σ(I)]	0.051 3, 0.150 5
  R1, wR2 (all data)	0.078 6, 0.178 0

  1.4   Fluorescence detection experiment of Zn-MOF

  To investigate the ability of Zn- MOF to detect metal cations, a powder sample of Zn - MOF (5.0 mg) was dispersed in distilled water (50 mL) after ultrasonic treatment, and the upper suspension was removed for fluorescence detection after standing for three days. Different M(NO₃)_n solutions (200 µL, 0.1 mmol·L^-1, Mⁿ⁺=Na⁺, K⁺, Mn²⁺, Ca²⁺, Ba²⁺, Co²⁺, Ni²⁺, Fe²⁺, Fe³⁺, Cu²⁺, Zn²⁺, Cd²⁺, Ag⁺, Cr³⁺) were added to the Zn-MOF suspension (2.0 mL) to monitor the changes in the fluorescence spectra before and after the reaction.

  2.   Results and discussion

  2.1   IR spectra analyses of Zn-MOF

  According to the IR spectra (Fig. S1), the wide absorption peak at about 3 413 cm^-1 is attributed to the strong stretching vibrations of the ν(OH) of the carboxyl and water molecules in the ligands H₄L and Zn - MOF, respectively. The strong expansion vibration peak of ν(C=O) in the carboxyl group of the ligand H₄L appeared at about 1 713 cm^-1, ν_as (COO) and ν_s (COO) at 1 631 and 1 499 cm^-1, respectively, and ν_as (C—O) at 1 267 cm^{-1 [29-30]}. The expansion vibrations of ν(C—O) and ν(C=O) of Zn - MOF were about 1 246 and about 1 615 cm^-1, and the expansion vibrations of ν_as (COO) and ν_s (COO) were 1 575 and 1 495 cm^-1 respectively. Compared with the ligands, the ν values of Zn-MOF all were shifted to lower frequencies, indicating the presence of monodentate or bidentate coordinating carboxyl groups in Zn - MOF^{[29, 31]}. These structural features are consistent with the results of the single - crystal X - ray diffraction analysis.

  2.2   Crystal structure description

  Zn-MOF with a 3D structure crystallizes in the P1 space group of the trigonal crystal system. Its asymmetric structural unit contains three Zn²⁺ ions (both Zn1 and Zn4 occupy 100%, and Zn2 and Zn3 occupy 50%, respectively), one and a half L^4- ligands, two 2, 2'- bpy molecules, and a free H₂O molecule. Zn1 is coordinated to six carboxylate oxygen atoms (O2, O14, O2^ⅰ, O14^ⅰ, O4^ⅱ, O4^ⅲ) from six L^4- ligands, forming a six-ligand centre. Zn2 in a five - coordinated tetragonal cone configuration with two N atoms (N1, N2) from the same 2, 2'-bpy and three carboxylate oxygen atoms (O1, O13, O4^ⅱ) on three L^4- ligands. The coordination pattern of Zn3 is similar to that of Zn2, and the coordination pattern of Zn4 is the same as that of Zn1 (Fig. 1a). The Zn—O bond lengths in the Zn - MOF range from 0.199 6(3) to 0.220 3(3) nm and the Zn—N bond lengths range from 0.208 1(4) to 0.215 7(4) nm (Table S1). These results are consistent with the reported correlation values for Zn - MOF^{[4, 32-34]}. As shown in Fig. 1a, Zn1, Zn2, and Zn2^ⅰ together form a trinuclear structure, while Zn3, Zn4, and Zn3^ⅵ form another trinuclear structure in this Zn-MOF. The two different trinuclear structures are linked to each other by two carboxyl groups on the same L^4- ligand to form a 1D chain. This 1D chain is then linked by different ligand carboxyl groups, ultimately forming a 3D network structure (Fig. 1b).

  Figure 1

  

  Figure 1.  (a) Single-crystal structure, (b) 1D chain, (c) 3D framework, (d) topological diagram of Zn-MOF

  Symmetry codes: ^ⅰ -x, -y+1, -z; ^ⅱ x-1, y, z; ^ⅲ -x+1, -y+1, -z; ^ⅳ x+1, y+1, z+1; ^ⅴ x, y+1, z+1; ^ⅵ -x+1, -y+2, -z+1; ^ⅶ -x+1, -y+2, -z.

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  Topologically, since both different trinuclear structures can be simplified to six nodes (light and dark green) and both L^4- ligands with different coordination modes can be reduced to four nodes (pink and gray), the Zn - MOF framework can thus be reasonably simplified to a reticulated topology of {4²·8⁴} {4⁴·6²}₂ {4⁸·6⁶·8}₂ (Fig. 1c).

  2.3   PXRD analyses and thermal stability

  PXRD patterns of Zn-MOF were obtained at room temperature to determine the sample purity of the crystalline powders. The Zn - MOF crystal powder diffraction pattern was in general agreement with the peak positions in the single- crystal structure simulation pattern in Fig. S2, further demonstrating that the powder sample of Zn-MOF is a pure phase and can be used for subsequent experiments. To investigate the thermal stability of the Zn - MOF structure, TGA curves of Zn - MOF were obtained from room temperature to 800 ℃. As shown in Fig. S3, Zn - MOF lost approximately 2.04% of its weight in a range of 28 -282 ℃ (Calcd. 1.53%), which can be attributed to the loss of two water molecules. The fact that its primary structure could be stabilized up to approximately 440 ℃ indicates that this Zn - MOF has good thermal stability, which may be attributed to its strong Zn—O bonding and 3D network structure. The remarkable stability of Zn-MOF thus lays a foundation for the exploration of its functionality in aqueous solutions and for further applications.

  2.4   Photoluminescence property

  MOFs, consisting of a variety of π - conjugated organic ligands and metal ions with d¹⁰ electron configurations, have been investigated by a wide range of scholars for use in photochemical sensing^{[4, 35]}. The solidstate fluorescence spectra of the ligands H₄ L and Zn -MOF were obtained at room temperature. As shown in Fig. 2, H₄L showed a broad emission peak at 360 - 480 nm (λ_ex=337 nm), which can be attributed to the n-π* or π-π* electron leap between the ligands^[36-37]. The emission peak of Zn - MOF can be seen in the fluorescence spectrum at 446 nm (λ_ex=365 nm). Because Zn(Ⅱ) is in group ⅡB and possesses a d¹⁰ full electron configuration, the main reason for the luminescence of Zn-MOF is the electron transfer that occurs within the ligand^[38-39]. The intensity of the emission peak of Zn-MOF was stronger than that of the free ligand, mainly due to the addition of auxiliary ligands and metal ions to form an ordered MOF structure, which enhances the structural rigidity and thus reduces the energy loss of the H₄L ligand due to radiation-free decay^[40-41].

  Figure 2

  

  Figure 2.  Luminescence spectra of the H₄L ligand and Zn-MOF at 298 K in the solid-state

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  2.5   Luminescence sensing for Cu²⁺

  As shown in Fig. 3, the fluorescence intensity of the Zn-MOF supernatant decreased to varying degrees when Co²⁺, Ni²⁺, Fe²⁺, Fe³⁺, and Cu²⁺ ions were added. However, the fluorescence emission of the Zn - MOF supernatant was completely quenched when Cu²⁺ was added only. Thus Zn - MOF has high selectivity potential as a fluorescent sensor for Cu²⁺ ions.

  Figure 3

  

  Figure 3.  Luminescence intensities of the Zn-MOF supernatant in the presence of different metal cations at a concentration of 10 µmol·L^-1

  λ_ex=244 nm.

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  In addition, the effect of Cu²⁺ on the fluorescence emission intensity of Zn - MOF when coexisting with other metal ions was investigated by anti - interference experiments. The quenching effect of Cu²⁺ on this fluorescent probe was comparable to that in the presence of coexisting ions, indicating that the presence or absence of coexisting ions does not interfere with the sensing performance of Zn-MOF for the fluorescence detection of Cu²⁺ ions (Fig. S4). The experimental results show that Zn - MOF has excellent anti - interference performance for the fluorescence detection of Cu²⁺ ions.

  Sensitivity is an important indicator of fluorescent probes. The ability of Zn - MOF to detect Cu²⁺ was further investigated by monitoring the change in the maximum fluorescence intensity of the Zn-MOF supernatant when Cu²⁺ was added drop by drop. The intensity of the strongest emission peak of Zn-MOF at 325 nm gradually decreased with the continuous addition of Cu²⁺ ions (Fig. 4a). When the concentration of Cu²⁺ ions was 9.0 µmol·L^-1, the fluorescence intensity of Zn - MOF was completely quenched. The quenching efficiency was quantitatively assessed according to the Stern - Volmer equation^[42]: (I₀/I)=K_SVc_M+1, where I₀/ I is used to analyze the fluorescence detection response of Zn-MOF to Cu²⁺, and I₀, I, K_SV and c_M denote the fluorescence emission intensity at 325 nm of the Zn-MOF supernatant before and after the addition of Cu²⁺, the Stern-Volmer quenching constant and the concentration of Cu²⁺, respectively. The concentration of Cu²⁺ ions showed a good linear relationship with I₀/I in a range of 0 - 1.2 µmol·L^-1 with a correlation coefficient (R²) 0.998 8. The limit of detection (LOD) of Zn - MOF for Cu²⁺ ions was 67.1 nmol·L^-1 according to the formula: LOD=3σ/ k^[43-44], where σ is the standard deviation of five measurements of blank samples, and k is the slope of the linear fit between fluorescence intensity and Cu²⁺ concentration (Fig. 4b). The LOD for copper ions in drinking water is well below the EPA and WHO standards of 2.0×10⁴ and 3.1×10⁴ nmol·L^-1, respectively^[45]. Zn-MOF has a lower LOD for Cu²⁺ than the existing fluorescent probes for detecting Cu²⁺ ions (Table 2). The Zn - MOF can be considered to have some potential in the field of research for the detection of environmental contaminants in water.

  Figure 4

  

  Figure 4.  (a) Effect of the concentration of Cu²⁺ on the fluorescence intensity of Zn-MOF; (b) Variation of I₀/I of Zn-MOF with Cu²⁺ concentration

  λ_ex=244 nm; Inset: linear relationship between I₀/I and c_Cu²⁺ in low-concentration range.

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  Table 2

  

  Table 2.  Comparison of Cu²⁺ detection limits of some fluorescent probes

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  Probe	Solvent	KSV /(L·mol-1)	LOD /(mol·L-1)	Detection range/(µmol·L-1)	Ref.
  MOF-525	HEPES (pH=7.4)	2.56×107	2.2×10-10	0.01-0.2	[46]
  Eu-BTB	H2O		1.0×10-8	0.05-10	[5]
  [Zn3(μ3-Hbptc)2(μ2-4, 4'-bpy)2(H2O)4]n·2nH2O	HEPES (pH=7.4)	1.641×105	3.24×10-8	0-0.7	[4]
  [Ca(H2tcbpe-F)(H2O)2]	H2O	4.39×105	1.3×10-7	0.5-4.0	[47]
  Eu-DATA/BDC	H2O		1.5×10-7	1-40	[16]
  [Tb2(DCSAL)3(H2O)11]·3DCSAL·4H2O	C2H3N	4.8×104	1.7×10-7	3-50	[48]
  CuNCs@Tb@UiO-66-(COOH)2	HEPES (pH=8.0)		1.78×10-7	0-32	[6]
  {[Zn2Na(L)2(H2O)2][OAc]·2H2O}n	H2O	7.75×104	6.5×10-7	0.5-1.2×103	[50]
  MOF-525	H2O	4.5×105	6.7×10-7	1.6-19.0	[51]
  Ce(1, 5-NDS)1.5(H2O)5	H2O	7.668×103	3.0×10-6	10-100	[52]
  MIL-53-L	H2O	6.15×103	1.0×10-5	0-500	[53]
  [Eu(L)(H2O)2]·2H2O	H2O	1.163×103	1.0×10-5	0.1-105	[54]
  Zn-MOF	H2O	2.396×105	6.71×10-8	0-1.2	This work

  Since cycling performance is a key factor in determining the usefulness of fluorescent probes, the cycling ability of Zn - MOF was investigated. The fluorescence emission intensity of the recovered Zn - MOF sample was restored as before, after five elution cycles of 0.1 mmol·L^-1 EDTA and H₂O solution (Fig. 5). This experimental result demonstrates the potential practical application of Zn -MOF as a recoverable fluorescent probe that can be repeatedly used for the efficient detection of Cu²⁺ in aqueous solutions.

  Figure 5

  

  Figure 5.  Changes in the fluorescence intensity of Zn-MOF supernatant in five cycles of Cu²⁺ detection

  λ_ex=244 nm, λ_em=325 nm.

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  2.6   Recognition mechanism

  The fluorescence mechanism of selective quenching of MOFs by Cu²⁺ ions in the current literature can be summarized as follows: (a) the collapse of the MOF skeleton due to the introduction of Cu²⁺ ions^[55-56], (b) the replacement of metal nodes in the MOFs skeleton by Cu²⁺ ions as a guest metal and the formation of a new structure^[57], (c) the existence of a competing excitation energy uptake between the MOFs and the Cu ions^[58-59], (d) interactions between Cu²⁺ ions and the MOFs framework to form new chemical bonds, thereby reducing the original fluorescence intensity^{[47, 60]}, and (e) weak interactions between Cu²⁺ ions and complexes^[61]. The following series of experiments were conducted to explore the underlying mechanism involved.

  There are no significant changes in the IR spectra and PXRD patterns of Zn-MOF before and after immersion in a Cu²⁺ aqueous solution, indicating that the original backbone structure of Zn - MOFs is still intact (Fig. 6 and 7). Therefore, it is proved that the fluorescence quenching process is not caused by the collapse of the skeleton of the MOFs, and the possibility that Cu²⁺ ions replace Zn²⁺ ions in the skeleton and that new bonds are created between the Cu²⁺ ions and the Zn - MOFs is also ruled out.

  Figure 6

  

  Figure 6.  Infrared spectra of Zn-MOF before and after soaking in a Cu²⁺ aqueous solution

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  Figure 7

  

  Figure 7.  PXRD patterns of Zn-MOF before and after soaking in a Cu²⁺ aqueous solution

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  As shown in Fig. 8, the UV - Vis absorption spectrum of Zn - MOF supernatant (266 - 335 nm) and the UV - Vis absorption spectrum of Cu²⁺ aqueous solution (190-250 nm) did not overlap significantly, so there is no competitive absorption of excitation energy between Zn-MOF and Cu²⁺ ions^[62]. After several elutions with an EDTA solution (0.1 mmol·L^-1) and water, there was no significant change in the color of Zn - MOF powder before and after immersion in a Cu²⁺ ion solution (1.0 mmol·L^-1). This phenomenon indicates that the central ions of Zn - MOF are not displaced with Cu²⁺ ions and that there is no adsorption of Cu²⁺ ions. To verify this speculation, further analysis was performed using ICP-MS. As shown in Table S2, the content of Zn²⁺ ions in the skeleton before and after immersion of Zn-MOF in Cu²⁺ ions was unchanged, and there were almost no Cu²⁺ ions in the Zn-MOF after immersion. This further indicates that Zn²⁺ ions in Zn - MOF are not displaced by Cu²⁺ ions, and there is no adsorption of Cu²⁺ ions^[63].

  Figure 8

  

  Figure 8.  UV-Vis spectra of Zn-MOF and Cu²⁺ ions in water

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  The bonding elements present in Zn - MOF and Zn- MOF@Cu²⁺ were analyzed using XPS (Fig. 9a). The new Cu2 p_1/2 and Cu2p_3/2 photoelectron spectral lines appeared at 954.07 and 934.27 eV, respectively, after immersion of Zn - MOF in Cu²⁺ ions (Fig. 9b) ^[64-65]. The Cu2p_3/2 peak at 934.27 eV was very close to the value of Cu(OAc)₂ (935.0 eV), suggesting that Cu²⁺ may interact with Zn - MOF via uncoordinated carboxylate O. However, the binding energy of O1s changed from 531.85 to 531.76 eV (Fig. 9c and 9d), indicating the existence of a weak interaction between the Cu²⁺ ion and the Zn-MOF^[66-67].

  Figure 9

  

  Figure 9.  XPS spectra of Zn-MOF samples before and after soaking in a Cu²⁺ aqueous solution: (a) full spectra and high-resolution spectra of (b) Cu2p and (c, d) O1s

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  3.   Conclusions

  In conclusion, a 3D Zn - MOF with a novel structure has been obtained by a hydro- solvothermal method with the ether - bonded aromatic tetracarboxylic acid ligand 1, 4-bis(3, 5- dicarboxyphenoxy)benzene (H₄L) as the primary ligand and 2, 2' - bpy (2, 2' - bipyridine) as the secondary ligand, and this material exhibited good thermal stability. In addition, the fluorescence analysis experiments show that the luminescent Zn - MOF exhibits high efficiency and specificity for the detection of Cu²⁺ in water compared with the reported detection systems. The linear range of Cu²⁺ (0 - 1.2 µmol·L^-1) is wide, and the detection limit (67.1 nmol·L^-1) is much lower than that of the EPA and WHO standards. In addition, it can be used as a'turn - off'fluorescent probe for Cu²⁺ detection in aqueous solution due to its good resistance to interference and recyclability. Relevant experiments and spectral analyses have revealed that the weak interaction between Cu²⁺ and the luminescent Zn-MOF can achieve stable and ultrahigh sensitivity detection in aqueous environments.

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

  Scheme1 1  Synthesis route of Zn-MOF

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  Figure 1  (a) Single-crystal structure, (b) 1D chain, (c) 3D framework, (d) topological diagram of Zn-MOF

  Symmetry codes: ^ⅰ -x, -y+1, -z; ^ⅱ x-1, y, z; ^ⅲ -x+1, -y+1, -z; ^ⅳ x+1, y+1, z+1; ^ⅴ x, y+1, z+1; ^ⅵ -x+1, -y+2, -z+1; ^ⅶ -x+1, -y+2, -z.

  下载: 全尺寸图片 幻灯片
  

  Figure 2  Luminescence spectra of the H₄L ligand and Zn-MOF at 298 K in the solid-state

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  Figure 3  Luminescence intensities of the Zn-MOF supernatant in the presence of different metal cations at a concentration of 10 µmol·L^-1

  λ_ex=244 nm.

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  Figure 4  (a) Effect of the concentration of Cu²⁺ on the fluorescence intensity of Zn-MOF; (b) Variation of I₀/I of Zn-MOF with Cu²⁺ concentration

  λ_ex=244 nm; Inset: linear relationship between I₀/I and c_Cu²⁺ in low-concentration range.

  下载: 全尺寸图片 幻灯片
  

  Figure 5  Changes in the fluorescence intensity of Zn-MOF supernatant in five cycles of Cu²⁺ detection

  λ_ex=244 nm, λ_em=325 nm.

  下载: 全尺寸图片 幻灯片
  

  Figure 6  Infrared spectra of Zn-MOF before and after soaking in a Cu²⁺ aqueous solution

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  Figure 7  PXRD patterns of Zn-MOF before and after soaking in a Cu²⁺ aqueous solution

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  Figure 8  UV-Vis spectra of Zn-MOF and Cu²⁺ ions in water

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  Figure 9  XPS spectra of Zn-MOF samples before and after soaking in a Cu²⁺ aqueous solution: (a) full spectra and high-resolution spectra of (b) Cu2p and (c, d) O1s

  下载: 全尺寸图片 幻灯片

  Table 1.  Crystal data and structure refinement of Zn-MOF

  Parameter	Zn-MOF
  Formula	C53H33Zn3N4O16
  Formula weight	1 177.94
  Crystal system	Triclinic
  Space group	P1
  a / nm	0.975 56(7)
  b / nm	1.364 94(9)
  c / nm	1.979 02(12)
  α/(°)	108.314(2)
  β/(°)	90.635(2)
  γ/(°)	107.070(2)
  V / nm3	2.3760(3)
  Z	2
  F(000)	1 194
  Dc / (Mg·m-3)	1.646
  μ / mm-1	1.58
  Reflection collected	25 341
  Independent reflection	11 561
  Rint	0.049
  GOF	0.93
  R1, wR2 [I > 2σ(I)]	0.051 3, 0.150 5
  R1, wR2 (all data)	0.078 6, 0.178 0

  下载: 导出CSV

  Table 2.  Comparison of Cu²⁺ detection limits of some fluorescent probes

  Probe	Solvent	KSV /(L·mol-1)	LOD /(mol·L-1)	Detection range/(µmol·L-1)	Ref.
  MOF-525	HEPES (pH=7.4)	2.56×107	2.2×10-10	0.01-0.2	[46]
  Eu-BTB	H2O		1.0×10-8	0.05-10	[5]
  [Zn3(μ3-Hbptc)2(μ2-4, 4'-bpy)2(H2O)4]n·2nH2O	HEPES (pH=7.4)	1.641×105	3.24×10-8	0-0.7	[4]
  [Ca(H2tcbpe-F)(H2O)2]	H2O	4.39×105	1.3×10-7	0.5-4.0	[47]
  Eu-DATA/BDC	H2O		1.5×10-7	1-40	[16]
  [Tb2(DCSAL)3(H2O)11]·3DCSAL·4H2O	C2H3N	4.8×104	1.7×10-7	3-50	[48]
  CuNCs@Tb@UiO-66-(COOH)2	HEPES (pH=8.0)		1.78×10-7	0-32	[6]
  {[Zn2Na(L)2(H2O)2][OAc]·2H2O}n	H2O	7.75×104	6.5×10-7	0.5-1.2×103	[50]
  MOF-525	H2O	4.5×105	6.7×10-7	1.6-19.0	[51]
  Ce(1, 5-NDS)1.5(H2O)5	H2O	7.668×103	3.0×10-6	10-100	[52]
  MIL-53-L	H2O	6.15×103	1.0×10-5	0-500	[53]
  [Eu(L)(H2O)2]·2H2O	H2O	1.163×103	1.0×10-5	0.1-105	[54]
  Zn-MOF	H2O	2.396×105	6.71×10-8	0-1.2	This work

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