A Ni(II) Coordination Polymer as a Multifunctional Luminescent Sensor for Detection of UO22+, Cr2O72−, CrO42− and Nitrofurantoin

A new Ni coordination polymer [Ni(MIP)(BMIOPE)]n (1) was constructed (BMIOPE = 4,4′-bis(2-methylimidazol-1-yl)diphenyl ether, and H2MIP = 5-methylisophthalic acid), possessing two-dimensional (2D) twofold parallel interwoven net structure with a 44∙62 point symbol. Complex 1 has been successfully obtained based on mixed-ligand strategy. The fluorescence titration experiments revealed that complex 1 could act as multifunctional luminescent sensor to simultaneously detect UO22+, Cr2O72− and CrO42−, and NFT (nitrofurantoin). The limit of detection (LOD) values for complex 1 are 2.86 × 10−5, 4.09 × 10−5, 3.79 × 10−5 and 9.32 × 10−5 M for UO22+, Cr2O72−, CrO42− and NFT. The Ksv values are 6.18 × 103, 1.44 × 104, 1.27 × 104 and 1.51 × 104 M−1 for NFT, CrO42−, Cr2O72− and UO22+. Finally, the mechanism of its luminescence sensing is studied in detail. These results manifest that complex 1 is a multifunctional sensor for sensitive fluorescent UO22+, Cr2O72−, CrO42− and NFT detection.


Introduction
Uranium, a naturally occurring heavy metal element, can be used as nuclear fuel and plays a crucial part in the nuclear power industry. However, uranium contamination is easily caused by nuclear accidents and various nuclear industries such as uranium mining and processing and nuclear power plants. Owing to its high radioactivity and chemical toxicity, it has also aroused worldwide concern about potential risks to human health. The stable form of uranium in aqueous solution is uranyl ion (UO 2 2+ ). Excessive accumulation of UO 2 2+ in the human body may cause irreversible damage to the kidney, liver and other important organs as well as the immune system, leading to an increased risk of cancer [1]. According to the United States Environmental Protection Agency (EPA), the permissible value of UO 2 2+ in drinking water should not exceed 35 µg L −1 [2]. In recent years, the detection of uranium has attracted more attention. As is known to all, chromium (VI) is widely used in the leather tanning, chromium plating, printing and dyeing industries. It also produces a large amount of chromium-containing wastewater. Heavy metal ions are not able to be degraded directly and easily accumulate in ecosystems. For example, CrO 4 2− and Cr 2 O 7 2− , with high toxic in water, will cause serious harm to human health after entering the human body because of their carcinogenicity [3]. On the other hand, Nitrofurantoin (NFT) is a broad-spectrum nitrofuran antibiotic widely used in aquaculture and animal husbandry to treat protozoan and bacterial infections. However, abused or blindly added NFT may result in increased bacterial resistance and environmental pollution; there is no doubt that this is a large threat to ecological environments and human health [4]. Thus, the trace detection of UO 2 2+ , CrO 4 2− , Cr 2 O 7 2− and NFT from natural water or wastewater has been a matter for concern.
Conventional detection methods have been used to detect these pollutants, such as atomic absorption spectrometry (AAS), inductively coupled plasma mass spectrometry (ICP-MS), X-ray fluorescence spectrometry (XRF) and so on. However, these technologies are readily available and costly and cannot be detected in real time and on the spot. Hence, exploring and developing simple, fast and accurate methods is essential to detect toxic organic solvents, metal cations and anions [5].
Coordination polymers (CPs), as a novel class of crystalline porous materials, have been demonstrated to have high potentials in many fields such as fluorescent sensing, delivering biomedicine, gas storage, molecular recognition, catalysis and so on, on account of specific surface area, functional pore structures and adjustable atom-precise guesthost structures [6][7][8][9][10]. Recently, luminescent CPs as a kind of chemical sensors have been widespread used to monitor various environmental pollutants including toxicity metal ions, toxic inorganic anions, nitro explosives and antibiotics on account of their remarkable advantages such as on-site detection, high selectivity, sensitivity and low detection limit [11][12][13][14] [16]. Chen et al. reported a Co-MOF with exposed pyrimidyl Lewis base sites as fluorescent sensor for UO 2 2+ and Al 3+ [17]. These CPs based on Lanthanide metals and transition metals exhibit detection performance for UO 2 2+ . However, it is still a great challenge to improve the fluorescence response and sensitivity of CP-based sensors, which also need to be further researched and developed. At present, few CPs can simultaneously detect UO 2 2+ , Cr 2 O 7 2− , and CrO 4 2− and NFT [18]. Inspired by the aforementioned viewpoints, we report a new Ni(II) CP [Ni(MIP) (BMIOPE)]n (1), while H 2 MIP = 5-methylisophthalic acid, and BMIOPE = 4,4 -bis(2methylimidazol-1-yl)diphenyl ether (Scheme S1). Complex 1 exhibits 2D twofold parallel interwoven net. A series of the fluorescent detection experiments have found that complex 1 could be a high-efficiency multi-responsive sensor by fluorescence quenching to detect low concentrations of UO 2 2+ cation, Cr 2 O 7 2− /CrO 4 2− anions and NFT antibiotic with noteworthy selectivity and high sensitivity. Furthermore, the possible sensing mechanisms were studied in detail.

PXRD and Thermal Analyses
The powder XRD pattern for complex 1 is shown in Supplemental Figure S2. In comparison with the simulated one from crystal structure analysis, the main peak patterns of complex 1 match very well, indicating single phase purity of the bulk sample. In addition, thermal stability was investigated by using thermal analyzer in temperature range of 25-800 °C under the N2 atmosphere. The curve displayed that the skeleton of complex 1 began to collapse after ca. 440 °C and displayed relatively good thermal stability ( Figure S3).

Photoluminescence Properties
Solid-state luminescence properties of complex 1 were tested at room temperature. The center of the emission bands appears at 357 nm (λex = 315 nm) and 363 nm (λex = 303 nm) for H2MIP and BMIOPE, respectively, which might be ascribed to n/π→π* electron transitions [19,20]. Complex 1 displays the emission maxima centered at 346 nm upon excitation at 245 nm, as exhibited in Supplemental Figure S4. Since the fluorescence intensity of carboxylate ligands is very weak compared with that of nitrogen-containing

PXRD and Thermal Analyses
The powder XRD pattern for complex 1 is shown in Supplemental Figure S2. In comparison with the simulated one from crystal structure analysis, the main peak patterns of complex 1 match very well, indicating single phase purity of the bulk sample. In addition, thermal stability was investigated by using thermal analyzer in temperature range of 25-800 • C under the N 2 atmosphere. The curve displayed that the skeleton of complex 1 began to collapse after ca. 440 • C and displayed relatively good thermal stability ( Figure S3).

Photoluminescence Properties
Solid-state luminescence properties of complex 1 were tested at room temperature. The center of the emission bands appears at 357 nm (λ ex = 315 nm) and 363 nm (λ ex = 303 nm) for H 2 MIP and BMIOPE, respectively, which might be ascribed to n/π→π* electron transitions [19,20]. Complex 1 displays the emission maxima centered at 346 nm upon excitation at 245 nm, as exhibited in Supplemental Figure S4. Since the fluorescence intensity of carboxylate ligands is very weak compared with that of nitrogen-containing ligands, the fluorescence of complex 1 may be ascribed to the intra-ligand transitions in BMIOPE ligand [21,22]. To further verify the fluorescence properties of complex 1, the fluorescence quantum yield of complex 1 and the ligand BMIOPE were measured at room temperature. The fluorescence quantum yield of the ligand BMIOPE was 3.58%. The fluorescence quantum yield of complex 1 was measured under the same conditions, and the results showed that the fluorescence quantum yield of complex 1 was 8.12%. The fluorescence quantum yield is improved, mainly because the ligand is better fixed to form a more stable rigid structure after the metal ion connected with the ligand, which hinders its torsion and leads to the improvement of fluorescence yield.

Selective Detection of UO 2 2+ Cations
Considering the excellent luminescence performance, sensing properties of complex 1 towards metal ions were explored. The powder sample of 1 (5 mg) was added into different aqueous solution containing M(NO3)n (M = Na + , Ag + , Li + , Cd 2+ , Zn 2+ , Pb 2+ , Co 2+ , Ca 2+ , Cu 2+ , Ni 2+ , Mg 2+ , Al 3+ , Cr 3+ and UO 2 2+ , 10 mM), respectively. The emission spectra of complex 1 is the metal ion-dependent, and UO 2 2+ ion shows the significant fluorescence quenching effects on the emission intensities of complex 1 (Figure 2a). So, complex 1 could be used to selectively detect UO 2 2+ in aqueous solutions. Thus, to explore the sensing sensitivity to detect UO 2 2+ , titration experiments were implemented to explore the intensities of complex 1 with increasing amounts of UO 2 2+ ionic solutions. With increasing solution volume of UO 2 2+ , the intensities of 1 continuously decrease (Figure 2b). Stern-Volmer (SV) formula I 0 /I = 1 + K SV [Q] was used to analyze the quenching efficiency (I 0 and I mean the emission intensities prior and after the addition of analytes; [Q] means the concentration of the analytes; K SV refers to the quenching coefficient). Figure 2c presents the Stern-Volmer plots: the I 0 /I is linearly proportional to UO 2 2+ concentration (R 2 = 0.9937) at low concentrations, and the slope K SV for UO 2 2+ is quantified to be 1.51 × 10 4 M −1 , confirming high sensitivity of complex 1 toward UO 2 2+ . The limit of detection (LOD) could be estimated by the formula 3σ/K SV (σ means the standard deviation). From Table S2, the LOD value for UO 2 2+ ion is calculated to be 2.86 × 10 −5 M, which is higher than those reported by CP-based sensors. In order to measure the influence of other metal ions on the detection of UO 2 2+ , interference experiments were also performed, as depicted in Figure 2d. It is found that the addition of other competing ions only reduced the emission intensity of complex 1 to some extent, but the intensity was almost quenched after the addition of UO 2 2+ ions. The results indicate that the presence of other competing ions does not disturb the detection of complex 1 toward UO 2 2+ . In comparison with other CPs-based sensors for detecting UO 2 2+ ions, complex 1 has good detection and analysis performance [16,17,23].  (Table S2). Furthermore, we also carried out the interference experiments to evaluate the selectivity of complex 1 against Cr VI (Cr 2 O 7 2− /CrO 4 2− ). As shown in Figures 3d and 4c, the capability of complex 1 to detect Cr 2 O 7 2− and CrO 4 2− anions is not readily interfered by other anions. The sensing capabilities are commensurate with previously reported CP-based luminescent sensors for detection of hexavalent chromate [24][25][26][27]. Molecules 2023, 28, x FOR PEER REVIEW 5 of 12

Selective Detection of Cr2O7 2− /CrO4 2− Anions
The detection ability of complex 1 towards different anions was explored under the same experimental procedure as UO2 2+ (Table S2). Furthermore, we also carried out the interference experiments to evaluate the selectivity of complex 1 against Cr VI (Cr2O7 2-/CrO4 2− ). As shown in Figures 3d and 4c, the capability of complex 1 to detect Cr2O7 2-and CrO4 2− anions is not readily interfered by other anions. The sensing capabilities are commensurate with previously reported CP-based luminescent sensors for detection of hexavalent chromate [24][25][26][27].

Selective Detection of NFT
The excellent fluorescence response of complex 1 towards UO2 2+ , Cr2O7 2-and CrO4 2− ions drove us to explore its sensing abilities towards various antibiotics such as PCL (penicillin), NFT (nitrofurantoin), CAP (chloramphenicol), SDZ (sulfadiazine) and DTZ (dimetridazole). From Figure 5a, the intensities of complex 1 were reduced to almost zero only with the addition of NFT, and the results show that complex 1 could be used as possible fluorescence probe to detect NFT with high selectivity. The final quenching efficiency order is as follows: PCL ˂ SDZ ˂ CAP ˂ DTZ ˂ NFT. A series of titration experiments were executed to explore the sensitivity for NFT, which revealed that the emission intensity decreased gradually with adding nitrofurantoin (Figure 5b). As shown in Figure  5c, the curve between the concentrations of NFT and I0/I can be well-fitted by employing the S-V formula (R 2 = 0.9975). The KSV and LOD values of complex 1 towards NFT are 6.18 × 10 3 M −1 and 9.32 × 10 −5 M, respectively, as exhibited in Figure 5c and Supplemental Table  S3. Otherwise, the interference experiments manifested that the presence of other antibiotics does not make any significant change in the sensing of NFT, confirming that complex 1 has good anti-interference ability (Figure 5d). The sensing performances of com-  ions drove us to explore its sensing abilities towards various antibiotics such as PCL (penicillin), NFT (nitrofurantoin), CAP (chloramphenicol), SDZ (sulfadiazine) and DTZ (dimetridazole). From Figure 5a, the intensities of complex 1 were reduced to almost zero only with the addition of NFT, and the results show that complex 1 could be used as possible fluorescence probe to detect NFT with high selectivity. The final quenching efficiency order is as follows: PCL < SDZ < CAP < DTZ < NFT. A series of titration experiments were executed to explore the sensitivity for NFT, which revealed that the emission intensity decreased gradually with adding nitrofurantoin (Figure 5b). As shown in Figure 5c, the curve between the concentrations of NFT and I 0 /I can be well-fitted by employing the S-V formula (R 2 = 0.9975). The K SV and LOD values of complex 1 towards NFT are 6.18 × 10 3 M −1 and 9.32 × 10 −5 M, respectively, as exhibited in Figure 5c and Supplemental Table S3. Otherwise, the interference experiments manifested that the presence of other antibiotics does not make any significant change in the sensing of NFT, confirming that complex 1 has good anti-interference ability (Figure 5d). The sensing performances of complex 1 are superior to most reported CPs-based luminescent sensing materials for detection of nitrofurantoin [28][29][30][31].

Detection Mechanism
To better understand the strong quenching effect of UO2 2+ , Cr2O7 2-/CrO4 2− and NFT on the fluorescence of complex 1, the quenching mechanism was explored. The fluorescence quenching UO2 2+ , Cr2O7 2-/CrO4 2− and NFT caused by the structural collapse was

Detection Mechanism
To better understand the strong quenching effect of UO 2 2+ , Cr 2 O 7 2− /CrO 4 2− and NFT on the fluorescence of complex 1, the quenching mechanism was explored. The fluorescence quenching UO 2 2+ , Cr 2 O 7 2− /CrO 4 2− and NFT caused by the structural collapse was first excluded because the patterns of complex 1 after sensing tests are well matched with the original samples ( Figure S2) [32,33]. Additionally, the most significantly overlaps are between the emission spectra of complex 1 and the UV-vis absorption band of the analytes (UO 2 2+ , Cr 2 O 7 2− /CrO 4 2− and NFT), which indicated that there was energy competitive absorption between complex 1 and the analytes, finally causing fluorescence quenching [34][35][36] (Figure 6). According to the density functional theory (DFT), by employing Gaussian 09 with B3LYP/6-31G* method, the HOMO and LUMO energy of the antibiotics was calculated. It is well known that the lower LUMO energy level of antibiotics means the much easier electron transfer from the fluorescent material to antibiotics, leading to the higher fluorescence quenching effects. The order of LUMO energy is PCL >SDZ >CAP >DTZ >NFT, which is in accordance with the quenching efficiency (Supplemental Figure S5 and Table S3). The results indicate that the photo-induced electron transfer (PET) process is pivotal in detecting antibiotics [37][38][39][40]. Thus, the fluorescence quenching of complex 1 toward UO 2 2+ , Cr 2 O 7 2− /CrO 4 2− and NFT is caused by the competitive absorption and PET mechanism.
Molecules 2023, 28, x FOR PEER REVIEW Additionally, fluorescence lifetime measurements were performed to further tigate the kinetic property of complex 1 toward UO2 2+ , Cr2O7 2-, CrO4 2− and NFT. T orescence lifetime studies exhibit similar triexponential function-fitted decay curve varying average lifetimes for the different samples ( Figures S6-S10). The fluore lifetime of the suspension of complex 1 is 120 ns. With the addition of UO2 2+ , C CrO4 2− and NFT, the average lifetimes of complex 1 are 109 ns, 111 ns, 115 ns and respectively. The fluorescence lifetimes of complex 1 show very little change befo after detection of different quenchers, suggesting that the luminescence quenchin cess should be a static quenching mechanism [41].

Materials and Physical Measurement
All reagents are analytically pure grade in the experiment and are not furthe fied for use. Thermogravimetric analysis was measured on a NETZSCH STA 449 piter TGA analyzer (Selb, Germany) using an empty Al2O3 crucible as the sta Measuring temperature ranges from 25 to 800 °C with a heating rate of 10 °C min −1 N2 atmosphere. Powder X-ray diffraction patterns of the title complex were ob using a Shimadzu XRD-6000 X-ray diffractometer (Kyoto, Japan) with Cu-Kα (λ = Å) radiation at room temperature and 2θ ranging from 5° to 50°. Perkin Elmer (Wa MA, USA) 240C elemental analyzer was used to obtain elemental analyses of com Infrared spectra were obtained on a Bruker VERTEX 80 spectrometer (Billerica USA) in the 4000-400 cm −1 region. Fluorescence data were collected on the Perkin LS55 Fluorescence Spectrophotometer.

Materials and Physical Measurement
All reagents are analytically pure grade in the experiment and are not further purified for use. Thermogravimetric analysis was measured on a NETZSCH STA 449 F5 Jupiter TGA analyzer (Selb, Germany) using an empty Al 2 O 3 crucible as the standard. Measuring temperature ranges from 25 to 800 • C with a heating rate of 10 • C min −1 under N 2 atmosphere. Powder X-ray diffraction patterns of the title complex were obtained using a Shimadzu XRD-6000 X-ray diffractometer (Kyoto, Japan) with Cu-Kα (λ = 1.5418 Å) radiation at room temperature and 2θ ranging from 5 • to 50 • . Perkin Elmer (Waltham, MA, USA) 240C elemental analyzer was used to obtain elemental analyses of complex 1. Infrared spectra were obtained on a Bruker VERTEX 80 spectrometer (Billerica, MA, USA) in the 4000-400 cm −1 region. Fluorescence data were collected on the Perkin Elmer LS55 Fluorescence Spectrophotometer.

X-ray Crystallography
Crystal X-ray diffraction data for the titled complex was performed on Bruker APEX D8 QUEST diffractometer with a Photon 100 CMOS detector (Mo-Kα radiation, λ = 0.71073 Å). The absorption correction and data processing were conducted through the SADABS and SAINT programs [42]. The structure of the titled complex was solved by direct methods and further refined on F 2 with full-matrix least-squares procedure using the SHELXTL-2014 software package [43]. The non-hydrogen atoms were anisotropically refined, while all hydrogen atoms on carbon atoms were yielded geometrically and refined in the riding mode. Details of the crystallographic data for complex 1 are listed in Table 1. Selected bond lengths and angles for complex 1 are provided in Table S1.

Luminescent Sensing Experiments
The ground powder samples (5 mg) of complex 1 were immersed in 3 mL of water and then ultrasonicated for 1 h to obtain a suspension for luminescence detection. The aqueous solutions of Na n X (HCO 3 − , CO 3  ) or different antibiotics (PCL (penicillin), DTZ (dimetridazole), SDZ (sulfadiazine), NFT (nitrofurantoin) and CAP (chloramphenicol)) were employed at a concentration of 10 mM for the qualitative and anti-interference experiments. Titration tests of different concentrations were performed by gradually addition of analyte (UO 2 2+ , Cr 2 O 7 2− , CrO 4 2− , NFT) in the aqueous solutions, and then emission spectra were collected.

Conclusions
In this work, a novel Ni-CP [Ni(MIP)(BMIOPE)] n (1) was synthesized using H 2 MIP and BMIOPE ligands. Complex 1 is a 2D twofold parallel interwoven net structure with a 4 4 ·6 2 point symbol. Complex 1 could be used as multifunctional luminescent probe toward UO 2 2+ , Cr 2 O 7 2− /CrO 4 2− and NFT with high selectivity and sensitivity in aqueous media. The detection limits of complex 1 were found to be 2.86 × 10 −5 , 4.09 × 10 −5 , 3.79 × 10 −5 and 9.32 × 10 −5 M for UO 2 2+ , Cr 2 O 7 2− , CrO 4 2− and NFT, respectively. The quenching mechanism proved that the quenching involved in sensing UO 2 2+ cations and Cr 2 O 7 2− /CrO 4 2− anions was caused by the energy competitive absorption, while the quenching processes towards NFT might be ascribed to the synergistic effect of the competitive absorption and PET mechanism. The fluorescence lifetime characterization shows that the quenching effect of complex 1 towards UO 2 2+ , Cr 2 O 7 2− /CrO 4 2− and NFT was attributed to static quenching mechanism.
Supplementary Materials: The following supporting information can be downloaded at: https://www. mdpi.com/article/10.3390/molecules28124673/s1, Table S1. Selected bond lengths (Å) and angles ( • ) for complex 1. Table S2. Standard deviation and detection limit calculation of complex 1 for UO 2 2+ , Cr 2 O 7 2− , CrO 4 2− and NFT. Table S3. HOMO and LUMO energy levels of selected antibiotics calculated by density functional theory (DFT) at B 3 LYP/6-31G** level; Scheme S1. Schematic drawing of the ligands BMIOPE and H 2 MIP; Figure S1.  Figure S2. The PXRD patterns for complex 1 under different conditions; Figure S3. The TG profile of complex 1; Figure S4. Solid-state excitation and emission spectra of complex 1; Figure S5. The HOMO and LUMO energy levels for different antibiotics. Figure S6. Fluorescence decay curve for complex 1. Figure S7. Fluorescence decay curve for complex 1 in the presence of UO 2 2+ . Figure S8. Fluorescence decay curve for complex 1 in the presence of