Solvothermal Preparation of a Lanthanide Metal-Organic Framework for Highly Sensitive Discrimination of Nitrofurantoin and l-Tyrosine

Metal-organic frameworks (MOFs) have been rapidly developed for their broad applications in many different chemistry and materials fields. In this work, a multi-dentate building block 5-(4-(tetrazol-5-yl)phenyl)-isophthalic acid (H3L) containing tetrazole and carbolxylate moieties was employed for the synthesis of a two-dimensional (2D) lanthanide MOF [La(HL)(DMF)2(NO3)] (DMF = N,N-dimethylformamide) (1) under solvothermal condition. The fluorescent sensing application of 1 was investigated. 1 exhibits high sensitivity recognition for antibiotic nitrofurantoin (Ksv: 3.0 × 103 M−1 and detection limit: 17.0 μM) and amino acid l-tyrosine (Ksv: 1.4 × 104 M−1 and detection limit: 3.6 μM). This work provides a feasible detection platform of 2D MOFs for highly sensitive discrimination of antibiotics and amino acids.


Introduction
Metal-organic frameworks (MOFs) are coordination compounds with open-framework structure. In the past few decades, MOFs have been widely studied in many fields such as gas storage and separation, adsorption and separation of small chemical species, sensing, catalysis and drug delivery [1][2][3][4][5][6]. By improving the synthesis method, adjusting the proportion of metal salts, ligands and solvents, and changing the pore structures or porosities, the catalytic, gas adsorption and separation or fluorescence performance can be optimized [7][8][9]. MOFs can be used as chemical sensors because the interactions between MOFs and analytes could influence their luminescent properties [10][11][12]. Usually, the chemical sensors could have a "turn on" or "turn off" response to small molecules [13][14][15]. The characteristic of a good sensor is usually summarized as "4S": sensitivity, selective, stability and speed of response [16][17][18][19][20][21][22].
Since antibiotics were discovered, these molecules have showed applications for disease cure. However, antibiotic pollution has become more and more serious, which is caused by the abuse of antibiotics [23][24][25]. Nitrofurantoin is an antibiotic that is widely used in the prevention and treatment of animal infectious diseases [26,27], but it has been banned in many countries because of its carcinogenicity and mutagenicity. In 2008, the Ministry of Agriculture of China set the maximum limit of nitrofurantoin in aquatic products at 0.5 µg/kg [28].
On the other hand, L-tyrosine is an essential amino acid. The deficiency of L-tyrosine may cause phenylketonuria (PKU) [29,30]. PKU is an inborn metabolic error that prevents the conversion of L-tyrosine, causing damage to the central nervous system [31,32]. In may cause phenylketonuria (PKU) [29,30]. PKU is an inborn metabolic error that prevents the conversion of L-tyrosine, causing damage to the central nervous system [31,32]. In addition, L-tyrosine is associated with dopamine (associated with Parkinson's disease), norepinephrine and epinephrine synthesis [33].

Structural Description of 1
1 belongs to monoclinic space group C2/c. As shown in Figure 1a, the asymmetric unit of 1 consists of one La III center (La1), one de-protonated HL − , two mono-coordinated DMF and one bi-coordinated NO3 − . The HL − ligand serves as a μ4-bridge to link four La centers by a μ4-η 1 :η 1 :η 1 :η 2 coordination mode through four carboxylate oxygen atoms Scheme 1. Preparation of the lanthanide metal-organic framework for highly sensitive discrimination of nitrofurantoin and L-tyrosine.

Results and Discussion
2.1. Structural Description of 1 1 belongs to monoclinic space group C2/c. As shown in Figure 1a, the asymmetric unit of 1 consists of one La III center (La1), one de-protonated HL − , two mono-coordinated DMF and one bi-coordinated NO 3 − . The HL − ligand serves as a µ 4 -bridge to link four La centers by a µ 4 -η 1 :η 1 :η 1 :η 2 coordination mode through four carboxylate oxygen atoms (O1, O2, O3 and O4). Two DMF molecules are mono-coordinated to La1 through the terminal oxygen atoms (O8 and O9). Besides, two oxygen atoms (O5 and O6) from the nitrate ion are coordinated to La1 forming a bidentate chelating coordination mode [37,38]. The range. Figure 1b shows that two neighboring lanthanide atoms (La1A and La1B) are connected by the carboxylate oxygen atoms (O3 and O4) of HL − forming a binuclear building block. The intermetallic distance of La1A and La1B is 4.1258(4) Å. These binuclear building blocks are further linked through HL − to form a two-dimensional network. It is noted that the tetrazole group of HL − is not coordinated and protruded out of the two-dimensional network. There are hydrogen bonding interactions between the two-dimensional networks: N(1)-H(1)···O(5), 2.9651(1) Å and C(3)-H(3)···O(7), 3.3256(1) Å. The hydrogen-bonding interactions further assemble the two-dimensional networks into a three-dimensional supramolecular structure [39].

PXRD, FT-IR and SEM Characterizations of 1
The powder X-ray diffraction (PXRD) pattern of 1 is shown in Figure 2a. The experimental peaks are consistent with the theoretical one obtained by single-crystal X-ray data [40][41][42]. We also investigated the stability of 1 in different solvents such as ethanol, DMF and DMA. After 24 h soaking 1 in these solvents, the experimental PXRD patterns of 1 were also consistent with theoretical pattern, indicating that 1 was stable in these solvents ( Figure 2b). The slight variation of diffraction intensity may be related to the different crystal orientation or solvent effects [43,44].

PXRD, FT-IR and SEM Characterizations of 1
The powder X-ray diffraction (PXRD) pattern of 1 is shown in Figure 2a. The experimental peaks are consistent with the theoretical one obtained by single-crystal X-ray data [40][41][42]. We also investigated the stability of 1 in different solvents such as ethanol, DMF and DMA. After 24 h soaking 1 in these solvents, the experimental PXRD patterns of 1 were also consistent with theoretical pattern, indicating that 1 was stable in these solvents (Figure 2b). The slight variation of diffraction intensity may be related to the different crystal orientation or solvent effects [43,44].   FT-IR spectrum of 1 in the range of 4000-400 cm −1 was measured ( Figure S1, Supplementary Materials). The strong and wide peak close to 3412 cm −1 can be ascribed to the presence of N-H stretching vibration. The strong peaks of carboxyl groups appear in the region of 1648-1584 cm −1 (antisymmetric stretching vibrations) and 1384-1420 cm −1 (symmetric stretching vibrations) [45][46][47]. No FT-IR peaks around 1700 cm −1 also demonstrated complete deprotonation of carboxyl groups [48][49][50][51][52]. The peaks located at 1499 cm −1 are from the tetrazole group. The band located at 1296 cm −1 can be ascribed to the vibration of C-N [53][54][55]. The vibration bands from 779 cm −1 to 719 cm −1 are from the aromatic benzene rings.

Photo-Luminescent Properties of 1
It is well known that luminescence can be divided into two basic modes according to the spin state of electrons during radiation relaxation: fluorescence and phosphorescence [68][69][70]. The origin of lanthanide MOFs can be mainly ascribed to four kinds of mechanisms listed below: (1) the luminescence based on the organic ligand, (2) the luminescence based on metal center, (3) the luminescence based on the charge transfer, and (4) the luminescence based on guest emission. Lanthanide metal ions contain 4f electrons shielding by 5s 2 5p 6 orbits, which is hardly perturbed by its chemical surroundings, which enable lanthanide ions to have good optical performance [71][72][73].
UV-Vis spectra of H3L and 1 were measured at room temperature ( Figure S3, Supplementary Materials). The maximum absorption peak positions of H3L and 1 were different, which can be ascribed to the coordination between H3L and the metal center. Solid-state fluorescence spectra of H3L and 1 were also measured, as shown in Figure S4, Supplementary Materials. The peak value of 1 appeared at the same wavelength of H3L excited at 300 nm, indicating that the fluorescence emission of 1 is from the ligand [74][75][76].
In order to explore different photo-luminescence responses to antibiotics [77,78], the powder of 1 was evenly distributed in ethanol solution with a concentration of 0.

Photo-Luminescent Properties of 1
It is well known that luminescence can be divided into two basic modes according to the spin state of electrons during radiation relaxation: fluorescence and phosphorescence [68][69][70]. The origin of lanthanide MOFs can be mainly ascribed to four kinds of mechanisms listed below: (1) the luminescence based on the organic ligand, (2) the luminescence based on metal center, (3) the luminescence based on the charge transfer, and (4) the luminescence based on guest emission. Lanthanide metal ions contain 4f electrons shielding by 5s 2 5p 6 orbits, which is hardly perturbed by its chemical surroundings, which enable lanthanide ions to have good optical performance [71][72][73].
UV-Vis spectra of H 3 L and 1 were measured at room temperature ( Figure S3, Supplementary Materials). The maximum absorption peak positions of H 3 L and 1 were different, which can be ascribed to the coordination between H 3 L and the metal center. Solid-state fluorescence spectra of H 3 L and 1 were also measured, as shown in Figure S4, Supplementary Materials. The peak value of 1 appeared at the same wavelength of H 3 L excited at 300 nm, indicating that the fluorescence emission of 1 is from the ligand [74][75][76].
In order to explore different photo-luminescence responses to antibiotics [77,78], the powder of 1 was evenly distributed in ethanol solution with a concentration of 0.   Fluorescence lifetime is an important parameter for judging the mechanism of fluorescence quenching [80]. We investigated the fluorescence lifetimes for the suspensions of 1 with the addition of NFT or L-Tyr at room temperature, as shown in Figure 6 and Table 1. When NFT or L-Tyr was added into the suspensions of 1, the fluorescence lifetimes change from 4.44 ns to 4.77 ns or 2.82 ns, indicating the existence of a dynamic fluorescent quenching mechanism. The results also suggested that NFT and L-Tyr may interact with 1 through weak interactions such as hydrogen bonding interactions. Considering that the photo-luminescent origin of 1 is from ligand-based luminescent emission and the 2D coordination framework of 1 have uncoordinated tetrazole moieties of HL − , these uncoordinated tetrazole moieties could have hydrogen bonding interactions with the analytic species, which can reduce the photo-luminescent emission of 1 and cause the dynamic quenching phenomenon [81].
We also investigated the cycling performance of 1 (Figure 7). After four cycles, the efficiency of NFT or L-Tyr was 62.98 or 76.48%, indicating medium cycling performance. Fluorescence lifetime is an important parameter for judging the mechanism of fluorescence quenching [80]. We investigated the fluorescence lifetimes for the suspensions of 1 with the addition of NFT or L-Tyr at room temperature, as shown in Figure 6 and Table 1. When NFT or L-Tyr was added into the suspensions of 1, the fluorescence lifetimes change from 4.44 ns to 4.77 ns or 2.82 ns, indicating the existence of a dynamic fluorescent quenching mechanism. The results also suggested that NFT and L-Tyr may interact with 1 through weak interactions such as hydrogen bonding interactions. Considering that the photo-luminescent origin of 1 is from ligand-based luminescent emission and the 2D coordination framework of 1 have uncoordinated tetrazole moieties of HL − , these uncoordinated tetrazole moieties could have hydrogen bonding interactions with the analytic species, which can reduce the photo-luminescent emission of 1 and cause the dynamic quenching phenomenon [81].
We also investigated the cycling performance of 1 (Figure 7). After four cycles, the efficiency of NFT or L-Tyr was 62.98 or 76.48%, indicating medium cycling performance.

General Remarks
H3L was purchased from Jinan Henghua Technology Co., Ltd., Shan Dong, China. All other chemicals were purchased commercially and applied directly. Perkin-Elmer 240 element analyzer (PerkinElmer, Dublin, Ireland) is used for microanalysis of C, H and N elements. The ultraviolet-visible spectrophotometer (model UV-2600) manufactured by Shimadzu Company (Shimadzu, Kyoto, Japan), was used to measure the absorption spectra at room temperature, with the test wavelength ranging from 200-800 nm. Powder X-ray diffraction analysis was characterized by Rigaku D/Max-2500 (Rigaku, Tokyo, Japan) X-ray diffractometer equipped with Cu-Kα radiation at a wavelength of 0.154 nm. Photo-luminescence lifetimes and solid-state fluorescence spectra were measured by FS5 fluorescence spectrometer (Edinburgh Instruments, Edinburgh, UK). The RF-5301 fluorescence spectrophotometer (Shimadzu, Kyoto, Japan) was used to carry out the photo-luminescence sensing experiment, equipped with a plotter unit and 1 cm × 1 cm quartz battery in phosphorescent mode.

General Remarks
H3L was purchased from Jinan Henghua Technology Co., Ltd., Shan Dong, China. All other chemicals were purchased commercially and applied directly. Perkin-Elmer 240 element analyzer (PerkinElmer, Dublin, Ireland) is used for microanalysis of C, H and N elements. The ultraviolet-visible spectrophotometer (model UV-2600) manufactured by Shimadzu Company (Shimadzu, Kyoto, Japan), was used to measure the absorption spectra at room temperature, with the test wavelength ranging from 200-800 nm. Powder X-ray diffraction analysis was characterized by Rigaku D/Max-2500 (Rigaku, Tokyo, Japan) X-ray diffractometer equipped with Cu-Kα radiation at a wavelength of 0.154 nm. Photo-luminescence lifetimes and solid-state fluorescence spectra were measured by FS5 fluorescence spectrometer (Edinburgh Instruments, Edinburgh, UK). The RF-5301 fluorescence spectrophotometer (Shimadzu, Kyoto, Japan) was used to carry out the photo-luminescence sensing experiment, equipped with a plotter unit and 1 cm × 1 cm quartz battery in phosphorescent mode.

General Remarks
H 3 L was purchased from Jinan Henghua Technology Co., Ltd., Shan Dong, China. All other chemicals were purchased commercially and applied directly. Perkin-Elmer 240 element analyzer (PerkinElmer, Dublin, Ireland) is used for microanalysis of C, H and N elements. The ultraviolet-visible spectrophotometer (model UV-2600) manufactured by Shimadzu Company (Shimadzu, Kyoto, Japan), was used to measure the absorption spectra at room temperature, with the test wavelength ranging from 200-800 nm. Powder X-ray diffraction analysis was characterized by Rigaku D/Max-2500 (Rigaku, Tokyo, Japan) X-ray diffractometer equipped with Cu-Kα radiation at a wavelength of 0.154 nm. Photo-luminescence lifetimes and solid-state fluorescence spectra were measured by FS5 fluorescence spectrometer (Edinburgh Instruments, Edinburgh, UK). The RF-5301 fluorescence spectrophotometer (Shimadzu, Kyoto, Japan) was used to carry out the photoluminescence sensing experiment, equipped with a plotter unit and 1 cm × 1 cm quartz battery in phosphorescent mode.

Preparation of [La(HL)(DMF) 2 (NO 3 )] (1)
La(NO 3 ) 2 ·6H 2 O (129.9 mg, 0.3 mmol), H 3 L (31.0 mg, 0.1 mmol), ethanol (3 mL) and DMF (1 mL) were added to a beaker and stirred for 0.5 h, and then the mixture was transferred to a steel high-pressure reaction kettle which is heated to 90 • C for 72 h, and cooled to the ambient temperature within 36 h. The resulting pale yellow powder was cleaned with ethanol several times (Scheme 2). At room temperature, the powder of 1 was evenly distributed in ethanol solution with a concentration of 0.1 mg/mL by the ultrasonic method, which was used for further fluorescent measurement. Yield: 36% based on H 3 L. Elemental analysis calculations (%) for C 21

Preparation of [La(HL)(DMF)2(NO3)] (1)
La(NO3)2·6H2O (129.9 mg, 0.3 mmol), H3L (31.0 mg, 0.1 mmol), ethanol (3 mL) and DMF (1 mL) were added to a beaker and stirred for 0.5 h, and then the mixture was transferred to a steel high-pressure reaction kettle which is heated to 90 °C for 72 h, and cooled to the ambient temperature within 36 h. The resulting pale yellow powder was cleaned with ethanol several times (Scheme 2). At room temperature, the powder of 1 was evenly distributed in ethanol solution with a concentration of 0.1 mg/mL by the ultrasonic method, which was used for further fluorescent measurement. Yield: 36% based on H3L. Elemental analysis calculations (%) for C21H22LaN7O9: C 38.

X-ray Crystallography
The Bruker SMART 1000 CCD diffractometer (Bruker, AVANCE, Billerica, MA, USA) was used to measure the diffraction data of a single crystal of 1, equipped with graphite monochromatic Mo-Kα aperture radiation (λ = 0.71073 Å). The ω-φ scanning strategy was applied with Lorentz polarization and empirical absorption correction. By utilizing the SHELXS97 program, the structure of F 2 was improved by utilizing a full matrix least squares calculation. The anisotropic temperature factor was assigned to all atoms except hydrogen atoms, and the isotropic temperature factor was arbitrarily selected to be 1.2 times that of the parent [82,83]. The R(F), WR(F 2 ) and goodness of fit protocol factors, details of data collection and analysis are shown in Table 2. The selected bond lengths, angle hydrogen bonds [Å] and angles [°] are given in Table S1, Supplementary Materials. CCDC-1873742 represents the crystal data for this work. The data is available free of charge through the Cambridge Crystal Data Center.

X-ray Crystallography
The Bruker SMART 1000 CCD diffractometer (Bruker, AVANCE, Billerica, MA, USA) was used to measure the diffraction data of a single crystal of 1, equipped with graphite monochromatic Mo-Kα aperture radiation (λ = 0.71073 Å). The ω-ϕ scanning strategy was applied with Lorentz polarization and empirical absorption correction. By utilizing the SHELXS97 program, the structure of F 2 was improved by utilizing a full matrix least squares calculation. The anisotropic temperature factor was assigned to all atoms except hydrogen atoms, and the isotropic temperature factor was arbitrarily selected to be 1.2 times that of the parent [82,83]. The R(F), WR(F 2 ) and goodness of fit protocol factors, details of data collection and analysis are shown in Table 2. The selected bond lengths, angle hydrogen bonds [Å] and angles [ • ] are given in Table S1, Supplementary Materials. CCDC-1873742 represents the crystal data for this work. The data is available free of charge through the Cambridge Crystal Data Center.

Conclusions
In summary, the preparation, structural characterization and photo-luminescent sensing performance of a 2D lanthanide metal-organic framework was reported. It can be utilized to detect trace nitrofurantoin (K sv : 3.0 × 10 3 M −1 and detection limit: 17.0 µM) and L-tyrosine (K sv : 1.4 × 10 4 M −1 and detection limit: 3.6 µM) with high sensitivity and good recyclability. This work not only enriches the research of lanthanide MOFs, but also provides a theoretical basis for 2D MOF-based chemical sensors. Table S1: Selected bond lengths (Å) and angles ( • ), Figure S1: FT-IR spectrum of 1, Figure S2: (a) The SEM images of H 2 L showed that the powder is needle-like with the length between 0. .99 m at a scale of 5 µm (b) The SEM images of H 2 L showed that the powder is acicular on a 2 µm scale., Figure S3: UV-Vis spectra of H 3 L and 1, Figure S4: Solid-state fluorescence of H 3 L and 1, Figure S5: powder X-ray patterns for 1, 1 + NFT and 1 + L-Tyr.

Data Availability Statement:
The data presented in this study are available with the authors.