English

• With the development of the economy and the improvement of people′s living standards, people have higher and higher requirements for physical health and environmental protection. In recent years, antibiotics, as an important drug, have been widely used in the treatment and prevention of infectious diseases in humans and animals^[1]. However, if a large number of antibiotics are misused and abused, antibiotic residues will bring many problems to humans, animals, and the ecological environment, increase bacterial resistance, and cause tumors, cancer, serious drinking water, or ecological pollution^[2-4]. Therefore, the detection of antibiotic residues is very necessary and urgent. At present, Methods commonly used to detect antibiotic residues include liquid chromatography-tandem mass spectrometry (LC-MS)^[5-6], high-performance liquid chromatography (HPLC)^[7-8], capillary electrophoresis (CE)^[9-10] and so on^[11-12]. However, these methods are limited by long time, complex operation, high cost, and high repair costs. Therefore, it is necessary to develop a simple, efficient, and low-cost method to detect antibiotic residues.

  Recently, coordination polymers (CPs), as a kind of emerging inorganic-organic hybrid crystalline material, have been widely concerned by many researchers because of their varied, rich and adjustable porous crystal structure. CPs have many applications because of their attractive structure, such as gas adsorption and desorption^[13-15], catalysis^[16-18], and sensing^[19-21]. In addition, CPs can also be used as a fluorescence sensor to measure small molecules such as anions^[22-23], cations^[24-25], and antibiotics^[26-27]. Among them, the research on antibiotics is more and more popular. Antibiotic nitrofurantoin (NFT) belongs to the nitrofuran class of antibacterial drugs, which mainly has relatively good antibacterial activity against Escherichia coli and other Enterobacteriaceae. Excessive or insufficient residual levels of NFT in human or animal bodies will produce adverse effects, so it is important to detect the content of antibiotic residue of NFT.

  In this work, a stable and interesting 2D coordination polymer [Cd₂(L)(dbpy)₂(HCOO)(H₂O)]·3H₂O (1), where H₃L=5-((4-carboxybenzyl)amino)isophthalic acid and dbpy=5, 5′-dimethyl-2, 2′-bipyridine, was successfully synthesized using Cd(ClO₄)₂·6H₂O, H₃L, and dbpy under hydrothermal conditions. CP 1 can be used as a fluorescence sensor to selectively and sensitively identify NFT in an aqueous medium among eight antibiotics, namely nitrofurantoin (NFT), ofloxacin (OFX), norfloxacin (NFX), cefixime (CFX), cefradine (CFA), amoxicillin (AMO), gentamycin (GEM), penicillium (PCL) (Scheme S1, Supporting information), with a low detection limit (LOD) of 0.19 μmol·L^-1. The fluorescence quenching mechanism has also been explored and proposed, which may be the fluorescence resonance energy transfer (FRET). These results indicate that CP 1 can be used as a novel fluorescence sensor for the detection of NFT and has a potential application prospect in the detection of antibiotics.

  1.   Experimental

  1.1   Materials and methods

  All chemical reagents and solvents were purchased commercially from reagent companies and used directly without further purification. Powder X-ray diffraction (PXRD) patterns were determined on a Bruker D8 ADVANCE A25X diffractometer equipped (Cu Kα, λ=0.154 056 nm) at a voltage of 40 kV and a current of 40 mA with a scan speed of 8 (°)·min^-1 and from 5° to 50° (2θ). Elemental analysis (C, H, and N) were obtained by an Elementar Vario EL Ⅲ analyzer. The UV-Vis absorption spectrum was attained at ambient temperature by a Beijing Puxi TU-1901 double-beam UV-Vis spectrophotometer. Thermogravimetric analyses (TGA) were performed on a STA449F31 thermogravimetric analyzer from room temperature to 800 ℃ under an N₂ atmosphere with a heating rate of 15 ℃·min^-1. The solid and liquid fluorescence spectra were recorded on a Hitachi F-7000 fluorescence spectrophotometer at room temperature.

  1.2   Synthesis of CP 1

  Cd(ClO₄)₂·6H₂O (63.0 mg, 0.15 mmol), H₃L (16.0 mg, 0.05 mmol), dbpy (9.3 mg, 0.05 mmol), NaOH (6.0 mg, 0.15 mmol), DMF (2.0 mL), and H₂O (4.0 mL) were added in a 15 mL hydrothermal reactor, covered and sonicated for 10 min, and then heated at 150 ℃ for 48 h. After cooling to room temperature, light yellow granular crystals were obtained, with a yield of 17.9% (based on H₃L). Elemental analysis Cacld. for C₄₁H₄₃Cd₂N₅O₁₂(%): C, 48.15; H, 4.24; N, 6.85. Found(%): C, 48.43; H, 4.20; N, 6.89. Main IR peaks(KBr, cm^-1): 3 410, 1 663, 1 552, 1 382, 1 249, 1 162, 1 042, 779.

  1.3   Fluorescence sensing experiments

  The luminescence responses to antibiotics were investigated according to literature approaches^[28]. 4.0 mg finely ground powder of CP 1 was separately added into 4 mL aqueous solutions of 100 μmol·L^-1 antibiotics to obtain uniform suspensions by sonicated for 30 min, including NFT, OFX, NFX, CFX, CFA, AMO, GEM, and PCL. Like the above method, the blank experiments were performed by the suspension of CP 1 in deionized water. The titration tests were conducted by collecting the emission spectra of CP 1 in the presence of NFT with different concentrations, respectively.

  1.4   X-ray structure determination

  Single-crystal X-ray diffraction data of CP 1 were collected on a Bruker APEX-Ⅱ CCD single X-ray diffractometer with Mo Kα radiation (λ=0.071 073 nm) at 293 K. The crystal structure of CP 1 was solved by direct methods and refined by the full-matrix least-squares method based on F² using Olex2 and SHELXTL-2018/3^[29]. Anisotropic thermal parameters were used to refine all the non-hydrogen atoms of CP 1, and all the hydrogens of CP 1 were obtained by geometric calculation and placed in appropriate positions. A part of the solvent molecules was disordered in the pores of the structural framework, and it is difficult to fix and refine solvent peaks. So the disordered guest molecules need to be removed through step SQUEEZE. The final chemical formula was determined by combining the results of the crystal structure, TGA, elemental analysis, and SQUEEZE results. Crystallographic data and structure refinements are summarized in Table 1, and some selected lengths and bond angles are listed in Table 2.

  Table 1

  

  Table 1.  Crystal data and structure refinement of CP 1

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  Parameter	1		Parameter	1
  Empirical formula	C41H43Cd2N5O12		Z	2
  Formula weight	1 022.65		μ / mm-1	0.922
  Crystal system	Triclinic		F(000)	972
  Space group	P1		Reflection collected	22 326
  a / nm	1.250 16(13)		Independent reflection	10 302 (Rint=0.027 6)
  b / nm	1.470 19(15)		Goodness-of-fit on F2	1.071
  c / nm	1.477 81(15)		Data, restraint, parameter	10 302, 0, 518
  α / (°)	66.571 0(10)		Final R indices* [I > 2σ(I)]	R1=0.033 4, wR2=0.086 3
  β / (°)	89.373 0(10)		R indices (all data)	R1=0.046 0, wR2=0.099 6
  γ / (°)	78.957 0(10)
  $* R_1=\sum\left(|| F_0|-| F_{\mathrm{c}}||\right) / \sum\left|F_0\right| ; w R_2=\left[\sum w\left(\left|F_0\right|^2-\left|F_{\mathrm{c}}\right|^2\right)^2 / \sum w\left(\left|F_0\right|^2\right)^2\right]^{1 / 2} . $

  Table 2

  

  Table 2.  Selected bond lengths (nm) and bond angles (°) of CP 1

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  Cd1—O4	0.225 3(2)	Cd2—O1A	0.234 3(2)	N1—C18	0.132 9(4)
  Cd1—O2	0.231 9(3)	Cd2—O6B	0.236 3(2)	C2—C3	0.149 9(4)
  Cd1—N2	0.233 8(3)	Cd2—N3	0.238 7(3)
  Cd2—O7	0.233 6(2)	O1—C1	0.123 9(4)
  O4—Cd1—O9	81.10(9)	O7—Cd2—O1A	91.45(8)	C9—O5—Cd2D	93.72(18)
  O9—Cd1—O2	106.87(11)	N4—Cd2—O5B	106.88(9)	C18—N1—Cd1	124.2(2)
  O4—Cd1—N2	88.75(9)	O7—Cd2—N3	84.88(9)
  N2—Cd1—N1	70.62(10)	C1—O2—Cd1C	125.8(2)
  Symmetry codes: A: x-1, y+1, z; B: x-1, y, z; C: x+1, y-1, z; D: x+1, y, z.

  2.   Results and discussion

  2.1   Crystal structure of CP 1

  Single crystal X-ray diffraction study reveals that CP 1 conforms to the triclinic P1 space group and each asymmetric unit contains two crystallographically independent Cd²⁺ cations, one completely deprotonated L^3- anion, two dbpy ligands, one HCOO^- anion coming from the decompose of DMF, one coordinated water molecule and three lattice water molecules (Fig. 1a). The Cd1 ion is six-coordinated surrounded by one O atom from one μ₁-η¹ carboxylate group in L^3- ligand (O4), two O atoms from one HCOO^- anion (O1 and O2), O atom from the coordinated water molecule (O9), and two N atoms from one dbpy ligand (N1 and N2). Cd2 center is encircled by four O atoms originating from two μ₁-η¹η¹ carboxylate groups of two separated L^3- anions (O7, O8, O5B, and O6B), one O atom coming from HCOO^- (O1A), and two N atoms deriving from dbpy ligand(N3 and N4), presenting a seven-coordinated environment. The Cd—O bond lengths range from 0.225 3(2) to 0.256 6(2) nm, and the minimum and maximum Cd—N bond distances are 0.233 8(3) and 0.238 7(3) nm, respectively, which are within the normal ranges. Two adjacent Cd²⁺ cations share one HCOO^- anion adopting a μ₂-η²η¹ bridging mode to construct a dinuclear secondary building unit (SBU) [Cd₂(HCOO)(dbpy)₂], in which the Cd1…Cd2C non-bonding distance is 0.471 57(7) nm and pyridine rings in the two dbpy molecules are almost parallel (Fig. 1b). Neighboring SBUs are firstly linked by isophthalate groups in L^3- linkers to form a 1D chain along the b-axis (Fig.S1a). Adjacent 1D chains are further connected through the 4-carboxybenzylamino groups to generate the 2D layer of 1 (Fig. 1c). In this 2D network, the bridging L^3- ligands are located in the anti-conformation with the dbpy ligand (Fig.S1b). From the view of topology, the whole structure can be simplified into a 3-connected 2D hcb layer with the point symbol of (6³), in which every L^3- ligand and SBU can be viewed as three-connected nodes (Fig. 1d). Moreover, 2D layers of 1 are held together via abundant intermolecular interactions, such as the π…π stacking interactions existing between the phenyl rings N1-C18-C19-C20-C21-C22 and N2-C23-C24-C25-C26-C27, or between rings N3-C30-C31-C32-C33-C34 and N4-C35-C36-C37-C38-C39 of dbpy ligands (centroid-to-centroid distance falling in a range of 0.365 97(3) to 0.367 97(3) nm, and dihedral angles of π…π planes are 2.273(131)° and 2.315(139)°, respectively), and the N—H…O hydrogen bonds between amino and carboxylate oxygen atoms in two L^3- (Fig.S1c), generating a 3D supramolecular structure (Fig. 1e). The hydrogen bond distance (H5A…O8) of N5—H5A…O8 is 0.291 nm, and N5…O8 distance is 0.302 7(4) nm. Detailed hydrogen bonds are shown in Table S1.

  Figure 1

  

  Figure 1.  Structure of CP 1: (a) asymmetric unit with 30% thermal ellipsoids, showcasing the coordination mode of two Cd²⁺ centers, L^3- and dbpy ligands; (b) dinuclear [Cd₂(HCOO)(dbpy)₂] SBU; (c) 2D network; (d) 2D hcb layer; (e) 3D supramolecular structure

  The guest species are omitted for clarity; Symmetry codes: A: x-1, y+1, z; B: x-1, y, z; C: x+1, y-1, z; D: x+1, y, z.

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  2.2   PXRD and TGA

  To determine the purity and stability of CP 1, PXRD and TGA experiments were explored. As shown in Fig. 2, the diffraction peaks of as-synthesized CP 1 were similar to the simulated one, which indicates that the synthesized CP 1 is relatively pure. Moreover, the thermal stability of CP 1 was also investigated under a nitrogen atmosphere from room temperature to 800 ℃. As shown in Fig. 3, firstly, the actual weight loss of CP 1 from 80 to 164 ℃ was 5.76%, corresponding to the loss of three free water molecules (Calcd. 5.28%). Secondly, the actual weight loss of 6.05% between 214 and 335 ℃ corresponds to the loss of a coordination H₂O molecule and an HCOO^- ion (Calcd. 6.16%). Finally, the rapid weight loss at 346 ℃ indicates that the skeleton of CP 1 has collapsed. The above results indicate that the synthesized CP 1 is a crystal material with high purity and stability.

  Figure 2

  

  Figure 2.  PXRD patterns of CP 1

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  Figure 3

  

  Figure 3.  TGA curve of CP 1

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  2.3   Photoluminescence properties

  To explore the luminescence properties of CP 1. The solid-state luminescence properties of the H₃L ligand and CP 1 were investigated and measured at room temperature. As shown in Fig. 4, the H₃L ligand showed a strong maximum emission peak at 456 nm (λ_ex=276 nm), which is due to the typical and common π-π* transition of ligands^[30-31]. At the same time, CP 1 performed a similar and broad emission peak at 427 nm (λ_ex=305 nm). The maximum emission peak of CP 1 exhibited a slight blue shift compared with the H₃L ligand, which may be due to the ligand-to-metal charge transfer (LMCT)^[32-34]. The aqueous suspensions of CP 1 were found to be highly luminescent, with peak maxima at 416 nm (Fig.S2). The curves remained almost similar to that observed in the case of solid samples. The slight blue shift in the emission maxima may be due to the solvent effect. As shown in Fig.S3 of the supplementary information, to verify the fluorescence stability of the suspension, we tested the fluorescence emission spectra for 0, 0.5, 1, 2, 4, and 6 h, respectively. By comparing the spectra, it was found that the fluorescence emission spectra of the complex suspension were relatively stable within at least 6 h. At the same time, after placing the suspension for 24 h, we recovered the CP 1 material through centrifugation. After drying, PXRD identification showed that the main diffraction peaks did not change (Fig.S4). The results illustrate that the structure of CP 1 is stable in the aqueous phase and is not damaged. The above interesting fluorescent properties of CP 1 arouse our great interest in the study of antibiotic identification with CP 1.

  Figure 4

  

  Figure 4.  Fluorescence emission spectrum of the H3L ligand and CP 1

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  2.4   Fluorescence sensing NFT

  Because CP 1 has strong stability and fluorescence properties, we conducted a series of fluorescence sensing experiments for eight antibiotics (NFT, OFX, NFX, CFX, CFA, AMO, GEM, and PCL). Firstly, the selective sensing ability of CP 1 toward different antibiotics was explored by adding 100 μmol·L^-1 aqueous solutions of NFT, OFX, NFX, CFX, CFA, AMO, GEM, and PCL to the suspensions of CP 1 (Fig. 5a). The fluorescence intensity of CP 1 was the weakest when 100 μmol·L^-1 NFT was added. Meanwhile, the corresponding fluorescence quenching efficiency (QE) was also investigated according to the formula: QE=[(I₀-I)/I₀]×100%, where I₀ and I are the fluorescence intensity of CP 1 suspension without and with antibiotics, respectively. The quenching efficiencies of PCL, AMO, GEM, CFA, NFX, CFX, OFX, and NFT were 1.47%, 1.95%, 1.17%, 2.44%, 4.29%, 4.39%, 7.02%, and 95.12%, respectively (Fig. 5b). These results indicate that CP1 can selectively detect NFT.

  Figure 5

  

  Figure 5.  (a) Fluorescence spectra of CP 1 when eight antibiotics were added; (b) Fluorescence quenching efficiency of CP 1 aqueous suspension when eight antibiotics were added; (c) Fluorescence titration experiment by gradual adding NFT to CP 1 water suspension; (d) Stern-Volmer plots for CP 1 sensing NFT

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  To explore the sensitivity of CP 1 to detect NFT, a fluorescence titration experiment was performed. After adding different concentrations of NFT into the suspension of CP 1, the fluorescence intensity under different concentrations was measured. As shown in Fig. 5c, when the concentration of NFT gradually increased to 80 μmol·L^-1, the fluorescence intensity of CP 1 suspension was quenched by more than 90%. To better describe the results of fluorescence titration, the fluorescence quenching efficiency can be quantitatively analyzed using the Stern-Volmer equation: I₀/I=1+K_svc_NFT, where K_sv is the quenching constant, c_NFT is the molar concentration of NFT, and I₀ and I are the fluorescence intensity of CP 1 suspension before and after adding NFT^[35-36]. According to the Stern-Volmer equation, after adding different concentrations of NFT, the fluorescence intensity showed a good linear relationship at a low concentration of 2 to 30 μmol·L^-1 with a high correlation coefficient (R²=0.994 4), and the K_sv of NFT was 5.576 9×10⁴ L·mol^-1 (Fig. 5d). In addition, according to the detection limit formula: LOD=3σ/K_sv (σ is the standard deviation of ten standard blank samples)^[37-38], the LOD of NFT was calculated to be 0.19 μmol·L^-1, which is better than previously reported CP-based sensors performance (Table 3).

  Table 3

  

  Table 3.  Comparison of K_sv and LOD values for CPs as sensors in the detection of NFT

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  Sensor	Ksv / (L·mol-1)	LOD / (μmol·L-1)	Ref.
  Zn6Yb2L4(OAc)2(DMF)EtOH	5.5×104	1.27	[39]
  [Eu2Na(Hpddb)(pddb)2(CH3COO)2]	4.42×104	0.68	[40]
  Nd18-MOF	2.4×104	0.9	[41]
  [Cd(tptc)0.5(o-bimb)]	3.4×104	0.3	[42]
  {[Eu(H2O)(BTCTB)]·2H2O}	2.10×104	0.6	[43]
  CP 1	5.576 9×104	0.19	This work

  In addition, to explore the possible reason for CP 1 fluorescence quenching caused by NFT, the UV-Vis spectra of eight antibiotics were gained. As shown in Fig. 6, the fluorescence emission spectra of CP 1 overlapped with the UV-Vis absorption spectra of NFT, OFX, CFX, and NFX antibiotics (size order of overlapping areas: NFT > OFX > NFX > CFX), and there was almost no overlap with the other antibiotics. This indicates that CP 1 fluorescence quenching may be caused by the fluorescence resonance energy transfer (FRET), in which NFT can readily absorb the emission light of CP 1^[44-46]. These results indicate that CP 1 can be used as a novel fluorescence sensor to selectively and sensitively identify NFT.

  Figure 6

  

  Figure 6.  UV-Vis absorption spectra of eight antibiotics and emission spectrum of CP 1

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  3.   Conclusions

  In conclusion, a new stable coordination polymer (CP) [Cd₂(L)(dbpy)₂(HCOO)(H₂O)]·3H₂O (1) was successfully synthesized by hydrothermal method using Cd(ClO₄)₂·6H₂O, H₃L, and dbpy. As a new fluorescence sensor, CP 1 performs a strong fluorescence emission peak at 427 nm (λ_ex=305 nm) and can show selectivity and sensitivity for NFT in an aqueous medium with a low detection limit (0.19 μmol·L^-1). In addition, the fluorescence quenching may be caused by the fluorescence resonance energy transfer. These results indicate that CP 1 can be used as a novel and interesting fluorescent probe to detect NFT and it has potential applications in human health, food safety, and environmental protection.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  Structure of CP 1: (a) asymmetric unit with 30% thermal ellipsoids, showcasing the coordination mode of two Cd²⁺ centers, L^3- and dbpy ligands; (b) dinuclear [Cd₂(HCOO)(dbpy)₂] SBU; (c) 2D network; (d) 2D hcb layer; (e) 3D supramolecular structure

  The guest species are omitted for clarity; Symmetry codes: A: x-1, y+1, z; B: x-1, y, z; C: x+1, y-1, z; D: x+1, y, z.

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  Figure 2  PXRD patterns of CP 1

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  Figure 3  TGA curve of CP 1

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  Figure 4  Fluorescence emission spectrum of the H3L ligand and CP 1

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  Figure 5  (a) Fluorescence spectra of CP 1 when eight antibiotics were added; (b) Fluorescence quenching efficiency of CP 1 aqueous suspension when eight antibiotics were added; (c) Fluorescence titration experiment by gradual adding NFT to CP 1 water suspension; (d) Stern-Volmer plots for CP 1 sensing NFT

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  Figure 6  UV-Vis absorption spectra of eight antibiotics and emission spectrum of CP 1

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  Table 1.  Crystal data and structure refinement of CP 1

  Parameter	1		Parameter	1
  Empirical formula	C41H43Cd2N5O12		Z	2
  Formula weight	1 022.65		μ / mm-1	0.922
  Crystal system	Triclinic		F(000)	972
  Space group	P1		Reflection collected	22 326
  a / nm	1.250 16(13)		Independent reflection	10 302 (Rint=0.027 6)
  b / nm	1.470 19(15)		Goodness-of-fit on F2	1.071
  c / nm	1.477 81(15)		Data, restraint, parameter	10 302, 0, 518
  α / (°)	66.571 0(10)		Final R indices* [I > 2σ(I)]	R1=0.033 4, wR2=0.086 3
  β / (°)	89.373 0(10)		R indices (all data)	R1=0.046 0, wR2=0.099 6
  γ / (°)	78.957 0(10)
  $* R_1=\sum\left(|| F_0|-| F_{\mathrm{c}}||\right) / \sum\left|F_0\right| ; w R_2=\left[\sum w\left(\left|F_0\right|^2-\left|F_{\mathrm{c}}\right|^2\right)^2 / \sum w\left(\left|F_0\right|^2\right)^2\right]^{1 / 2} . $

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  Table 2.  Selected bond lengths (nm) and bond angles (°) of CP 1

  Cd1—O4	0.225 3(2)	Cd2—O1A	0.234 3(2)	N1—C18	0.132 9(4)
  Cd1—O2	0.231 9(3)	Cd2—O6B	0.236 3(2)	C2—C3	0.149 9(4)
  Cd1—N2	0.233 8(3)	Cd2—N3	0.238 7(3)
  Cd2—O7	0.233 6(2)	O1—C1	0.123 9(4)
  O4—Cd1—O9	81.10(9)	O7—Cd2—O1A	91.45(8)	C9—O5—Cd2D	93.72(18)
  O9—Cd1—O2	106.87(11)	N4—Cd2—O5B	106.88(9)	C18—N1—Cd1	124.2(2)
  O4—Cd1—N2	88.75(9)	O7—Cd2—N3	84.88(9)
  N2—Cd1—N1	70.62(10)	C1—O2—Cd1C	125.8(2)
  Symmetry codes: A: x-1, y+1, z; B: x-1, y, z; C: x+1, y-1, z; D: x+1, y, z.

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  Table 3.  Comparison of K_sv and LOD values for CPs as sensors in the detection of NFT

  Sensor	Ksv / (L·mol-1)	LOD / (μmol·L-1)	Ref.
  Zn6Yb2L4(OAc)2(DMF)EtOH	5.5×104	1.27	[39]
  [Eu2Na(Hpddb)(pddb)2(CH3COO)2]	4.42×104	0.68	[40]
  Nd18-MOF	2.4×104	0.9	[41]
  [Cd(tptc)0.5(o-bimb)]	3.4×104	0.3	[42]
  {[Eu(H2O)(BTCTB)]·2H2O}	2.10×104	0.6	[43]
  CP 1	5.576 9×104	0.19	This work

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