Terbium-Tetracarboxylate Framework as a Luminescent Probe for the Selective Detection of Nitrofurazone

A novel terbium-tetracarboxylate framework with the 5,5’-(diazene-1,2-iyl)diisophthalic acid (H4abtc) ligand, formulated as [Tb(Habtc)(DMSO)(H2O)2]n (ZTU-5), has been synthesized and structurally characterized. ZTU-5 features a 2D-layered structure constructed by the binuclear terbium secondary building units (SBUs) and abtc4– ligand, which can be further expanded into a 3D-supramolecular framework by the hydrogen bond interactions. In addition, the magnetic and fluorescence properties of ZTU-5 are investigated and ZTU-5 exhibits highly selective and sensitive detection of nitrofurazone (NZF).


Introduction
Antibiotics are widely used as the specific drug for treating bacterial infection in humans and animals, while the abuse of antibiotics has caused the high levels of antibiotic residues in surface and groundwater as well as in drinking water [1][2][3]. Owing to the antibiotic wastewaters being highly poisonous and difficult to degrade, monitoring of antibiotic wastewaters was significant, but challenging [4,5]. Compared with the traditional detection method of antibiotics using instrumental methods such as liquid chromatography (LC), capillary electrophoresis (CE), liquid chromatography mass spectrometry (LC-MS), Raman spectroscopy (RS), ion mobility spectrometry (IMS), and so forth, the metal organic frameworks (MOFs) used as luminescent probes for the selective detection of antibiotics has been considered as a very effective and proven technology [6][7][8][9][10][11]. Despite some successes, the design and discovery of new MOFs as luminescent probes for highly selective and sensitive detection of antibiotics is also challenging and of great significance [12][13][14][15].
Hence, we have successfully constructed one novel terbium-tetracarboxylate framework with the H 4 abtc ligand, formulated as [Tb(Habtc)(DMSO)(H 2 O) 2 ] n (ZTU-5), which features a 2D-layered structure constructed by the binuclear terbium secondary building units (SBUs) and abtc 4-ligand, which further expands into a 3D-supramolecular framework by the hydrogen bond interactions. Herein, its syntheses, crystal structures, and magnetic and fluorescence properties are discussed in detail.

Materials and Methods
All the chemical reagents were commercially purchased and used without further purification. The powder X-ray diffraction (XRD) patterns were recorded on crushed single crystals in the 2θ

Crystal Structure Determination
Single-crystal X-ray diffraction data of ZTU-5 were collected on a Bruker with a Mercury CCD area detector (Mo-Kα, λ = 0.71073 Å). Empirical absorption corrections were applied to the data using the Crystal Clear program [16]. The structures of ZTU-5 were solved by direct methods and refined by full-matrix least-squares on F 2 using the SHELXTL-2017 program [17]. Metal cations were located from the E-maps and other non-hydrogen atoms were located in successive difference Fourier syntheses. All non-hydrogen atoms were refined anisotropically except for a few badly disordered atoms and the lattice solvent molecules. The organic hydrogen atoms were positioned geometrically with fixed thermal factors, while the coordinated water molecules were located using the difference Fourier method and refined freely. Crystallographic data and other pertinent information for ZTU-5 are summarized in Table 1,and the selected bond distances and bond angles are listed in Table S1. The CCDC number for ZTU-5 is 1950505.

Synthesis and Structure Description of the Crystal Structures
The solvothermal reaction of the Tb(NO3)3·6H2O and H4abtc ligand in a mixed-solvent of DMSO and H2O (V/V = 1:1) led to one novel terbium tetracarboxylate framework (ZTU-5). ZTU-5 is  Figure S1). The aromatic rings of two Habtc 3− ligands are arranged in an offset face-to-face mode with the parallel distance of 3.4531(26) Å (Figure 1b), which indicates the existence of weak π-π stacking [18]. In addition, the binuclear terbium SBUs are bridged by the Habtc 3-ligands and extended into the 1D lanthanide-carboxylate chain (Figure 1c), which further expands into a 2D and 3D-supramolecular framework by the hydrogen bond interactions ( Figure 1d and Table S2), involving O7-H7···O3 of the carboxylate groups and O10-HB···O3, O11-H11B···O4 between the pairs of water molecules and carboxylate groups.

Hirshfeld Surface Analysis
In order to study the intermolecular interactions in ZTU-5, the Hirshfeld surface analysis and 2D finger-printing were computed by the Crystal Explorer program [19]. As shown in Figure 2, the 3D hirshfeld surface mapped visually shows the interactions of crystal structure in ZTU-5; the red area denotes the strong interactions, which are attributed to the mostly hydrogen bonding including O···H, but the electron density of the blue region is weak interactions [20].The significant interaction distribution mapped on the molecular surface of ZTU-5 was presented by the 2D fingerprint plots ( Figure S2

Hirshfeld Surface Analysis
In order to study the intermolecular interactions in ZTU-5, the Hirshfeld surface analysis and 2D finger-printing were computed by the Crystal Explorer program [19]. As shown in Figure 2, the 3D hirshfeld surface mapped visually shows the interactions of crystal structure in ZTU-5; the red area denotes the strong interactions, which are attributed to the mostly hydrogen bonding including O···H, but the electron density of the blue region is weak interactions [20].The significant interaction distribution mapped on the molecular surface of ZTU-5 was presented by the 2D fingerprint plots ( Figure S2 [21].

XRD Patterns and Thermogravimetric Analyzer Data
The XRD of ZTU-5 was performed to confirm its purity and structure, and all the peak positions on the curves for ZTU-5are well matched with the simulated XRD patterns ( Figure S3). In order to investigate the stability of ZTU-5 in solvent, the samples of ZTU-5 were immersed in DMF solution for 24 h at room temperature, and the XRD patterns of ZTU-5 are still consistent with the simulated ones, suggesting the stability of ZTU-5. In addition, ZTU-5 exhibits a weight loss of 18.03% from 35 to 310 °C, which is attributed to the loss of one coordinated DMSO molecule and two coordinated water molecules (calcd. 18.16%) ( Figure S4).

Magnetic Property
The magnetic susceptibility of ZTU-5 was measured in the temperature range of 2-300 K under 1000 Oe. The χmT product for ZTU-5 is 23.36 cm 3 K mol −1 at 300 K, which is close to the expected theoretical value for two uncoupled Tb(III) ions (23.65 cm 3 K mol -1 and g = 3/2, 7 F6) [18]. Upon further cooling, the value of χmT sequentially decreases, reaching a minimum value of 16.20 cm 3 K mol −1 at 2 K. In addition, the magnetic data were fitted by the Curie-Weiss equation, in order to obtain a Curie constant C = 23.38 cm 3 K mol -1 and Weiss temperature θ = −3.90 K ( Figure  S5). The decrease observed in the χmT value and the negative θ values suggest the presence of the weak anti-ferromagnetic interaction and other effects such as magnetic anisotropy and thermal depopulation of the Tb(III) excited states in ZTU-5 [22][23][24].

Luminescence Property
The solid state luminescence property of ZTU-5 was explored at room temperature, whichexhibits the typical emission peaks at 488.5, 542.5, 588.5, and 622.5 nm when excited at 308 nm

XRD Patterns and Thermogravimetric Analyzer Data
The XRD of ZTU-5 was performed to confirm its purity and structure, and all the peak positions on the curves for ZTU-5 are well matched with the simulated XRD patterns ( Figure S3). In order to investigate the stability of ZTU-5 in solvent, the samples of ZTU-5 were immersed in DMF solution for 24 h at room temperature, and the XRD patterns of ZTU-5 are still consistent with the simulated ones, suggesting the stability of ZTU-5. In addition, ZTU-5 exhibits a weight loss of 18.03% from 35 to 310 • C, which is attributed to the loss of one coordinated DMSO molecule and two coordinated water molecules (calcd. 18.16%) ( Figure S4).

Magnetic Property
The magnetic susceptibility of ZTU-5 was measured in the temperature range of 2-300 K under 1000 Oe. The χ m T product for ZTU-5 is 23.36 cm 3 K mol −1 at 300 K, which is close to the expected theoretical value for two uncoupled Tb(III) ions (23.65 cm 3 K mol −1 and g = 3/2, 7 F 6 ) [18]. Upon further cooling, the value of χ m T sequentially decreases, reaching a minimum value of 16.20 cm 3 K mol −1 at 2 K. In addition, the magnetic data were fitted by the Curie-Weiss equation, in order to obtain a Curie constant C = 23.38 cm 3 K mol −1 and Weiss temperature θ = −3.90 K ( Figure S5). The decrease observed in the χ m T value and the negative θ values suggest the presence of the weak anti-ferromagnetic interaction and other effects such as magnetic anisotropy and thermal depopulation of the Tb(III) excited states in ZTU-5 [22][23][24].

Luminescence Property
The solid state luminescence property of ZTU-5 was explored at room temperature, whichexhibits the typical emission peaks at 488.5, 542.5, 588.5, and 622.5 nm when excited at 308 nm ( Figures S6 and  S7), which are assigned to 5 D 4 → 7 F J (J = 6−3) transitions [25]. The strong luminescent emission band appears at 542.5 nm, which arises from the 5 D 4 → 7 F 5 transition. The band at 488.5 nm is attributed to the 5 D 4 → 7 F 6 transition and the weaker emission bands at 588.5 and 622.5 nm correspond to the 5 D 4 → 7 F 4 and 5 D 4 → 7 F 3 transitions, respectively. In addition, the quantum yield and luminescence lifetime of ZTU-5 were measured at 25°C, and the corresponding quantum yield and lifetime for ZTU-5 are 26.42% and 1.982 ms, respectively.
Considering the good luminescent property of ZTU-5, the sensing of the antibiotics was performed through the luminescent detection. In order to explore the influence of different antibiotics in ZTU-5, metronidazole (MDZ), furazolidone (FZD), nitrofurantoin (NFT), nitrofurazone (NZF), ronidazole (RDZ), dimetridazole (DTZ), ornidazole (ODZ), and chloramphenicol (CAP) with different sizes and configurations were investigated ( Figure S8). In a typical experiment, a 5 mg sample of ZTU-5 was dispersed in 10 mL of different antibiotic in DMF solution (50 ppm) and processed into a suspension solution. Then, the resultant suspensions were monitored and the fluorescence intensity of these antibiotics showed the quench effect compared with the blank control sample, and the quenching efficiency (%) was calculated by the absolute quantum yield ratio (Figure 3a). Particularly, the NZF solution exhibited a drastic quenching effect in ZTU-5, which indicated that ZTU-5 can act as a promising luminescent probe for the detection of NZF among various nitro-antibiotics [12][13][14]. In addition, the possible quenching mechanism was proposed as the collision interaction between the structures of ZTU-5 and nitro-antibiotics, consuming the energy transfer and resonance energy transfer, and leading to a reduced luminescent intensity [12][13][14]. Owing to the hydrogen bond interactions between ZTU-5 and NZF, as well as the conjugative effect of NZF, ZTU-5 exhibits highly selective and sensitive detection of NZF among various nitro-antibiotics.  Considering the good luminescent property of ZTU-5, the sensing of the antibiotics was performed through the luminescent detection. In order to explore the influence of different antibiotics in ZTU-5, metronidazole (MDZ), furazolidone (FZD), nitrofurantoin (NFT), nitrofurazone (NZF), ronidazole (RDZ), dimetridazole (DTZ), ornidazole (ODZ), and chloramphenicol (CAP) with different sizes and configurations were investigated ( Figure S8). In a typical experiment, a 5 mg sample of ZTU-5 was dispersed in 10 mL of different antibiotic in DMF solution (50 ppm) and processed into a suspension solution. Then, the resultant suspensions were monitored and the fluorescence intensity of these antibiotics showed the quench effect compared with the blank control sample, and the quenching efficiency (%) was calculated by the absolute quantum yield ratio (Figure 3a). Particularly, the NZF solution exhibited a drastic quenching effect in ZTU-5, which indicated that ZTU-5 can act as a promising luminescent probe for the detection of For exploring the detection limit of ZTU-5 as the NZF probe, a series of concentrations of NZF solution were prepared (0.5−500 ppm) in DMF solution (Figure 3b). The luminescence intensity of ZTU-5 gradually decreased with the increasing concentration of NZF. The decreased luminescence intensity could be clearly observed when the ZTU-5 samples were immersed in a 0.5 ppm of NZF solution. According to the Stern-Volmer equation, the quenching constants (Ksv) value is 8.12 × 10 3 M −1 (Figure S9), which indicates a strong quenching effect of NZF in ZTU-5 with a good application prospect for the detection of NZF in DMF solution [12][13][14]. These results indicated that ZTU-5 exhibits highly selective and sensitive detection of NZF.

Conclusions
A novel terbium-tetracarboxylate framework (ZTU-5) with H 4 abtc ligand was successfully synthesized and structurally characterized. ZTU-5 features a 2D-layered structure constructed by the binuclear terbium SBUs and abtc 4ligand, which can be further expanded into a 3D-supramolecular framework by the hydrogen bond interactions. In addition, the magnetic and fluorescence properties of ZTU-5 are investigated and ZTU-5 exhibits high sensitivity and selectivity sensing for NZF nitro-antibiotics.