English

• 0.   Introduction

  Due to its low price and excellent antibacterial activity, the use of tetracycline (TET) is increasing worldwide. However, the heavy use of antibiotics will increase resistance and produce adverse reactions. TET residues can accumulate in the body, leading to gastrointestinal disorders, allergies, and adverse reactions in the central nervous system, as well as environmental pollution^[1-2]. In addition, with the rapid development of industry, many nitroaromatic compounds (NACs) are used to manufacture insecticides, pharmaceuticals, explosives, and more. The discharge of toxic substances is increasing year by year, leading to increasingly serious water and environmental pollution, and p-nitrophenol (4-NP) is also one of the toxic substances. When a large amount of 4-NP waste liquid is discharged, it can cause long-term residue in soil and water, causing serious harm to human health and the environment^[3-5]. Therefore, the effective and rapid detection of TET and 4-NP is of great significance for environmental protection and human health^[6-8].

  Metal-organic frameworks (MOFs) are a class of crystalline porous materials constructed from organic ligands and inorganic metal centers (metal ions or metal clusters)^[9-11]. Through the combination and design of metal ions and organic ligands, MOF materials with different pore sizes and structures can be obtained according to the requirements^[12-15]. They are also easy to synthesize, low in cost, and easy to modify^[16-18]. Due to its many advantages, MOFs have been successfully applied in the fields of sensing, magnetism, optoelectronics, catalysis, and gas storage^[19-23]. MOFs synthesized with transition metal ions with d¹⁰ electronic structure usually have better luminescence properties, possibly because metal ions enhance the emission of organic ligands^[24-26].

  (1, 1′∶4′, 1″-Terphenyl)-2, 2″, 4, 4″-tetracarboxylic acid (H₄L) has four carboxylic groups, and as such, it can construct various structures from different directions. It can be fully or partially deprotonated, providing hydrogen bond donors and acceptors, making it an ideal candidate for constructing supramolecular networks based on the degree of carboxyl deprotonation^[27]. Therefore, we synthesized a novel MOF[Cd(L)_0.5(4, 4′-bpy)_0.5(H₂O)₄]_n (1) (4, 4′-bpy=4, 4′-bipyridine). This paper describes its synthesis, structure, thermal stability, and fluorescence sensing properties. It has good stability and fluorescence properties and can be used as a highly sensitive and selective fluorescent probe for the detection of TET and 4-NP.

  1.   Experimental

  1.1   Reagents and instruments

  All reagents and solvents were commercially available and used directly without further purification. The C, H, and N elemental analyses were conducted with a PerkinElmer PE-2400 elemental analyzer. The crystal data were collected on a Bruker SMART APEX-Ⅱ single-crystal X-ray diffractometer. Powder X-ray diffraction (PXRD) patterns were recorded with a Bruker D8 ADVANCE diffractometer operating at 40 kV and 40 mA using Cu Kα radiation (λ=0.154 18 nm) at a scanning rate of 2 (°)·min^-1 from 5° to 50°. Thermal gravimetric analysis (TGA) was performed with a NETZSCH STA 449F3 thermal gravimetric analyzer in flowing nitrogen at a heating rate of 10 ℃·min^-1. The UV-Vis spectra were measured using a UV-2700 spectrophotometer. Fluorescence experiments were carried out on the Hitachi F-7100 Fluorescence Spectrophotometer.

  1.2   Synthesis of MOF 1

  A mixture of Cd (NO₃)₂·4H₂O (0.1 mmol, 0.030 8 g), H₄L (0.05 mmol, 0.020 3 g), 4, 4′-bpy (0.05 mmol, 0.007 8 g) and a solvent mixture of DMF (5 mL), H₂O (10 mL), and HNO₃ (0.1 mL, 6 mol·L^-1) were added into a 20 mL Teflon-lined stainless autoclave and heated at 95 ℃ for 3 d. After cooling to room temperature, colorless transparent crystals in sheet shape were collected, and dried in air. Yield: 74% (based on Cd). Anal. Calcd. for C₁₆H₁₁NO₅Cd(%): C, 46.91; H, 2.71; N, 3.42. Found(%): C, 45.9; H, 2.53; N, 3.84.

  1.3   Crystal structure determination

  The crystals of MOF 1 with regular shapes, transparent colors, and appropriate sizes were selected, and the intensity data were collected on a Bruker Smart APEX Ⅱ CCD diffractometer with Mo Kα radiation (λ=0.071 073 nm) at room temperature. The crystal structure was solved using direct methods and then refined by the full-matrix least-squares techniques on F² using SHELXL-97. The diffraction data were corrected by semi-empirical absorption using the SADABS program. All non-hydrogen atoms were refined anisotropically and all of the hydrogens were geometrically placed in calculated positions. Crystal data and structural refinement parameters for 1 are summarized in Table 1. Selected bond distances and bond angles are listed in Table 2.

  Table 1

  

  Table 1.  Crystal data and structure refinement parameters for MOF 1

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  Parameter	1		Parameter	1
  Formula	C16H11NO5Cd		V /nm3	2.798 1(5)
  Formula weight	409.67		Z	8
  Crystal system	Monoclinic		Dc/(g·cm-3)	1.945
  Space group	C2/c		F(000)	1 616
  a /nm	2.522 4(3)		Goodness-of-fit on F2	1.011
  b /nm	0.711 08(8)		R1, wR2 [I > 2σ(I)]	0.030 2, 0.057 7
  c /nm	1.7487 6(18)		R1, wR2 (all data)	0.045 0, 0.062 4
  β /(°)	116.864(2)

  Table 2

  

  Table 2.  Selected bond lengths (nm) and angles (°) for MOF 1

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  Cd1—N1	0.223 7(2)	Cd1—O2	0.224 4(2)	Cd1—O4B	0.232 2(2)
  Cd1—O3C	0.233 16(19)	Cd1—O1A	0.238 13(19)	Cd1—O3B	0.240 66(19)
  N1—Cd1—O2	96.52(9)	N1—Cd1—O4B	167.93(8)	O2—Cd1—O4B	93.40(8)
  N1—Cd1—O3C	96.20(8)	O2—Cd1—O3C	84.29(7)	O4B—Cd1—O3C	91.60(7)
  N1—Cd1—O1A	84.90(8)	O2—Cd1—O3B	105.70(7)	O4B—Cd1—O1A	86.21(8)
  O3C—Cd1—O3B	145.52(5)	N1—Cd1—O3B	114.76(8)	O4B—Cd1—O3B	55.52(7)
  Symmetry codes: A: -x+1/2, y-1/2, -z+1/2; B: x, -y+1, z+1/2; C: -x+1/2, -y+1/2, -z.

  2.   Results and discussion

  2.1   Crystal structure

  MOF 1 crystallizes in a monoclinic crystal system with a C2/c space group. The smallest asymmetric structural unit of 1 is composed of one Cd(Ⅱ) ion, a half L^4- ligand, a half 4, 4′-bpy ligand, and one water molecule. As shown in Fig. 1a, each Cd(Ⅱ) ion adopts the six-coordinated mode, where five oxygen atoms (O1A, O2, O3B, O3C, O4B) come from four different L^4- ligands and one N atom (N1) comes from the 4, 4′-bpy ligand, forming an irregular octahedron structure. The bond lengths of Cd—O and Cd—N range from 0.224 4(2) to 0.240 66(19) nm and 0.223 7(2) nm, both within the normal range (Table 2). As shown in Fig. 1b, Cd(Ⅱ) connects L^4- and 4, 4′-bpy to form a 2D plane structure, and the layers are connected by L^4- to form a 3D network structure.

  Figure 1

  

  Figure 1.  (a) Coordination mode of Cd(Ⅱ) in MOF 1; (b) 3D framework of 1

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  2.2   TGA of MOF 1

  The thermal stability of MOF 1 was tested by TGA. 30 mg of the sample was collected and heated at a heating rate of 8 ℃·min^-1 from 30 to 1 000 ℃ under a flowing nitrogen atmosphere to obtain a TGA curve of 1. As shown in Fig.S1, the weight loss of 4.43% (Calcd. 4.39%) before 271 ℃ corresponds to the loss of lattice water. Starting from 340 ℃, the skeleton of 1 began to collapse.

  2.3   PXRD of MOF 1

  To verify the purity of the sample of MOF 1, the PXRD pattern of 1 was measured. As shown in Fig.S2, the diffraction peak position of 1 in the PXRD experiment was consistent with the simulated peak position of crystal structure theory, indicating that the purity of 1 is relatively high.

  2.4   IR analysis

  IR analysis on MOF 1 was performed. As shown in Fig.S3, there was no absorption near 1 700 cm^-1, indicating that all carboxylic acid groups in the organic ligand have removed protons. There were antisymmetric vibrational (ν_as) absorption peaks and symmetric vibrational (ν_s) absorption peaks of carboxyl groups at 1 387, 1 420, 1 550, and 1 609 cm^-1, the Δν (ν_as-ν_s) values of 1 were 222 and 140 cm^-1, respectively, indicating that the carboxyl groups in the ligand have already coordinated with the metal ion. The carboxyl groups in 1 may take a non-single coordination form, that is, both single and bidentate chelation. The peaks within the range of 1 000-1 300 cm^-1 are attributed to the stretching vibration of C—C and C—N bonds. The infrared spectrum analysis of 1 is consistent with the crystal structure analysis of 1.

  2.5   Photoluminescence property of MOF 1

  Due to the excellent luminescent properties of d¹⁰ metal MOFs, the solid-state fluorescence of MOF 1 and H₄L were studied. As shown in Fig.S4, H₄L showed an emission peak at 439 nm when excited at 396 nm, while 1 showed an emission peak at 393 nm when excited at 353 nm. Compared with H₄L, the maximum emission peak of 1 exhibited a certain blue shift, and 1 had higher fluorescence emission than the ligand H₄L. The reason for the blue shift may be that 1 does not form an effective π-conjugated system, which reduces the electron delocalization range of the ligand^[28].

  2.6   Antibiotics sensing

  Antibiotics have been widely used in the treatment of bacterial infections. The abuse and overuse of antibiotics worldwide not only lead to the pollution of water and soil but also lead to the continuous increase of bacterial resistance to antibiotics^[29-30]. Therefore, detecting trace antibiotics in food and water is of great significance. Herein, ten different antibiotics were studied, including lincomycin hydrochloride (LIN), metronidazole (MET), ornidazole (ORN), TET, roxithromycin (ROX), chloramphenicol (CAP), gentamicin sulfate (GEN), azithromycin (AZM), cefixime (CEF) and penicillin sodium (PEN).

  As shown in Fig. 2a, the above ten antibiotics were sequentially added to the suspension of MOF 1. When TET was added, the fluorescence of 1 showed the maximum quenching. In addition, anti-interference experiments were conducted on TET in the presence of other antibiotics. According to the experimental results, when TET was added in the presence of other antibiotics, the fluorescence of 1 was still largely quenched (Fig. 2b), indicating that 1 has good selectivity. Subsequently, quantitative experiments were conducted on TET. As the concentration of TET increased, the fluorescence intensity of 1 gradually decreased. When the TET concentration reached 150 μmol·L^-1, the quenching efficiency of TET was as high as 95.92% (Fig. 2c). The I₀/I value was also linearly correlated with low TET concentration (R²=0.996 0, Fig. 2d). The detection limit calculation (3σ/k; σ: standard deviation, k: slope) gave a detection limit for 1 towards TET of 0.15 μmol·L^-1. This further proves that MOF 1 is very effective for detecting TET.

  Figure 2

  

  Figure 2.  (a) Luminescent intensity of MOF 1 in different antibiotics; (b) Luminescence intensity of 1 in mixed antibiotics; (c) Emission spectra of 1 with different TET concentrations; (d) Plot of I₀/I-1 vs c_TET in low concentration range

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  2.7   NACs sensing

  NACs have high explosive and chemical toxicity, and nitro explosives are used in the synthesis of insecticides as well as in the production of high explosives in many industries^[31]. Because of their explosive performance, they pose great harm to the environment and human health^[32]. Therefore, the effective detection of NACs has attracted widespread attention due to their importance for human health and national security. Herein, ten different NACs were studied, including 2, 4, 6-trinitrophenol (TNP), m-nitroaniline (3-NA), 2, 4-dinitrophenylhydrazine (DNP), 4-NP, p-nitrobenzoic acid (PNBA), p-nitrophenylhydrazine (4-NPH), o-nitrobenzaldehyde (2-NT), o-nitroaniline(2-NA), o-nitrophenol (2-NP), and nitrobenzene (NB).

  When different NACs were added to the suspension of MOF 1, 1 exhibited varying degrees of quenching, and the fluorescence intensity of 1 was maximally quenched when 4-NP was added (Fig. 3a). In addition, anti-interference experiments were conducted on 4-NP in the presence of other NACs. According to the experimental results, when 4-NP was added in the presence of other NACs, the fluorescence of 1 was still largely quenched (Fig. 3b), indicating that 1 has excellent selectivity for 4-NP. This excellent anti-interference ability provides a possibility for the detection of 4-NP in complex systems. Then, quantitative experiments were conducted on 4-NP. As the concentration of 4-NP gradually increased, the fluorescence intensity of 1 also gradually decreased. When the 4-NP concentration reached 20 μmol·L^-1, the quenching efficiency of 4-NP was as high as 95.71% (Fig. 3c). The I₀/I value was also linearly correlated with low 4-NP concentration (R²=0.992 4, Fig. 3d). The detection limit for 1 towards 4-NP was 0.062 μmol·L^-1.

  Figure 3

  

  Figure 3.  (a) Luminescent intensity of MOF 1 in different NACs; (b) Luminescence intensity of 1 in mixed NACs; (c) Emission spectra of 1 with different 4-NP concentrations; (d) Plot of I₀/I-1 vs c_4-NP in low concentration range

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  2.8   Possible sensing mechanism

  According to the literature that has been reported, it is found that most of the causes of fluorescence quenching are as follows: (ⅰ) collapse and decomposition of the crystal skeleton, (ⅱ) energy competition and absorption mechanism, (ⅲ) central metal ion exchange, (ⅳ) resonance energy transfer (RET) mechanism, (ⅴ) photoinduced electron transfer (PET) mechanism^[33-38].

  One possible quenching mechanism is that the collapse and decomposition of the crystal structure can lead to the fluorescence quenching of MOF 1. To verify the possibility of this mechanism, the powder of 1 was soaked in the solution of TET and 4-NP respectively for 24 h. As shown in Fig. 4, the PXRD pattern of the powder was consistent with the main diffraction peak of the previous PXRD pattern, indicating that the crystal was still structurally intact. Therefore, skeleton collapse is not the cause of fluorescence quenching. Secondly, NACs and antibiotics were detected in 1, and there were no metal ions in the whole quenching process, which indicates that there was no exchange of metal ions in the process of fluorescence quenching. There may also be energy distribution and transfer mechanisms or energy competition and absorption mechanisms. When studying the fluorescence quenching mechanism, we usually compare the UV-Vis absorption spectra of various analytes with the excitation or emission spectra of MOFs and then speculate on the possible quenching mechanism. To explain the quenching effect of TET and 4-NP on 1, the UV-Vis absorption spectra of antibiotics and NACs were recorded with a partial overlap of the emission or excitation spectra of 1, respectively. TET had absorption in the 330-600 nm range, which overlapped with the emission spectrum of 1 (Fig. 5), suggesting that the fluorescence quenching of 1 may be caused by RET. 4-NP had absorption in the 250-360 nm range, which overlapped with the excitation spectrum of 1 (Fig. 6). At this time, the fluorescence quenching of 1 may be caused by energy competitive absorption^[39].

  Figure 4

  

  Figure 4.  PXRD patterns of MOF 1 and the as-synthesized samples immersed in various solutions for 24 h

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  Figure 5

  

  Figure 5.  UV-Vis absorption spectra of the antibiotics and emission spectrum of MOF 1

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  Figure 6

  

  Figure 6.  UV-Vis absorption spectra of the NACs and excitation spectrum of MOF 1

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  2.9   Detection of TET and 4-NP in Yanhe River water

  To verify the practicality of MOF 1, the spiked recovery experiments were conducted on TET and 4-NP in Yanhe River water samples. As shown in Table 3 and 4, the spiked recoveries at different concentrations were obtained, ranging from 95% to 97% and from 98% to 102%. The relative standard deviation (RSD) values were 1.7%-2.7% and 1.4%-2.6%, respectively, indicating that 1 can be used to detect TET and 4-NP in real samples.

  Table 3

  

  Table 3.  Recovery test of TET spiked in Yanhe River water samples

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  cTET/(μmol·L-1)		RSD*/%	Recovery/%
  Spiked	Detected
  0	—
  2	1.9	1.7	97
  4	3.8	2.5	95
  6	5.7	2.7	96
  *n=3.

  Table 4

  

  Table 4.  Recovery test of 4-NP spiked in Yanhe River water samples

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  c4-NP/(μmol·L-1)		RSD*/%	Recovery/%
  Spiked	Detected
  0	—
  2	2.0	2.0	101
  3	3.1	1.4	102
  4	3.9	2.6	98
  *n=3.

  3.   Conclusions

  Using H₄L as an organic carboxylic acid ligand, 4, 4′-bpy as a nitrogen-containing ligand, and Cd(Ⅱ) as the central metal ion, Cd-MOF (1) was successfully synthesized by a hydro-solvothermal method. The structure analysis shows that 1 is a 3D network structure. Through many experiments, it is found that 1 has high selectivity and sensitivity to TET and 4-NP, and can also be used for anti-interference testing of TET and 4-NP. The detection limits of TET and 4-NP were 0.15 and 0.062 μmol·L^-1, respectively. In addition, the reason for the fluorescence quenching of 1 was analyzed in detail. The quenching of TET may be caused by resonance energy transfer, and the quenching of 4-NP may be caused by energy competitive absorption. Finally, actual sample testing was conducted on 1, and TET and 4-NP in the Yanhe River were detected through spiked recovery experiments.

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

  
  
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  Figure 1  (a) Coordination mode of Cd(Ⅱ) in MOF 1; (b) 3D framework of 1

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  Figure 2  (a) Luminescent intensity of MOF 1 in different antibiotics; (b) Luminescence intensity of 1 in mixed antibiotics; (c) Emission spectra of 1 with different TET concentrations; (d) Plot of I₀/I-1 vs c_TET in low concentration range

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  Figure 3  (a) Luminescent intensity of MOF 1 in different NACs; (b) Luminescence intensity of 1 in mixed NACs; (c) Emission spectra of 1 with different 4-NP concentrations; (d) Plot of I₀/I-1 vs c_4-NP in low concentration range

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  Figure 4  PXRD patterns of MOF 1 and the as-synthesized samples immersed in various solutions for 24 h

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  Figure 5  UV-Vis absorption spectra of the antibiotics and emission spectrum of MOF 1

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  Figure 6  UV-Vis absorption spectra of the NACs and excitation spectrum of MOF 1

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  Table 1.  Crystal data and structure refinement parameters for MOF 1

  Parameter	1		Parameter	1
  Formula	C16H11NO5Cd		V /nm3	2.798 1(5)
  Formula weight	409.67		Z	8
  Crystal system	Monoclinic		Dc/(g·cm-3)	1.945
  Space group	C2/c		F(000)	1 616
  a /nm	2.522 4(3)		Goodness-of-fit on F2	1.011
  b /nm	0.711 08(8)		R1, wR2 [I > 2σ(I)]	0.030 2, 0.057 7
  c /nm	1.7487 6(18)		R1, wR2 (all data)	0.045 0, 0.062 4
  β /(°)	116.864(2)

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  Table 2.  Selected bond lengths (nm) and angles (°) for MOF 1

  Cd1—N1	0.223 7(2)	Cd1—O2	0.224 4(2)	Cd1—O4B	0.232 2(2)
  Cd1—O3C	0.233 16(19)	Cd1—O1A	0.238 13(19)	Cd1—O3B	0.240 66(19)
  N1—Cd1—O2	96.52(9)	N1—Cd1—O4B	167.93(8)	O2—Cd1—O4B	93.40(8)
  N1—Cd1—O3C	96.20(8)	O2—Cd1—O3C	84.29(7)	O4B—Cd1—O3C	91.60(7)
  N1—Cd1—O1A	84.90(8)	O2—Cd1—O3B	105.70(7)	O4B—Cd1—O1A	86.21(8)
  O3C—Cd1—O3B	145.52(5)	N1—Cd1—O3B	114.76(8)	O4B—Cd1—O3B	55.52(7)
  Symmetry codes: A: -x+1/2, y-1/2, -z+1/2; B: x, -y+1, z+1/2; C: -x+1/2, -y+1/2, -z.

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  Table 3.  Recovery test of TET spiked in Yanhe River water samples

  cTET/(μmol·L-1)		RSD*/%	Recovery/%
  Spiked	Detected
  0	—
  2	1.9	1.7	97
  4	3.8	2.5	95
  6	5.7	2.7	96
  *n=3.

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  Table 4.  Recovery test of 4-NP spiked in Yanhe River water samples

  c4-NP/(μmol·L-1)		RSD*/%	Recovery/%
  Spiked	Detected
  0	—
  2	2.0	2.0	101
  3	3.1	1.4	102
  4	3.9	2.6	98
  *n=3.

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