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

Abstract

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 K_sv 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.

1. 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 (UO₂²⁺). Excessive accumulation of UO₂²⁺ 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 UO₂²⁺ 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, CrO₄²⁻ and Cr₂O₇²⁻, 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 UO₂²⁺, CrO₄²⁻, Cr₂O₇²⁻ 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 guest-host 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]. Many researchers reported the use of LCPs to detect Fe³⁺, Pb²⁺, Al³⁺, Hg²⁺, Cr₂O₇²⁻, CrO₄²⁻ and other small organic molecules with high sensitivity. However, there are only few reports of LCPs to detect UO₂²⁺ by fluorescence quenching. Ye et al. reported an anionic Tb-MOF as dual-channel luminescence probe for UO₂²⁺ ion [15]. Hou et al. reported a carboxyl-functionalized Zn-MOF as bi-functional chemical probe for sensing trace amounts of Pb²⁺ and UO₂²⁺ ions in aqueous solution [16]. Chen et al. reported a Co-MOF with exposed pyrimidyl Lewis base sites as fluorescent sensor for UO₂²⁺ and Al³⁺ [17]. These CPs based on Lanthanide metals and transition metals exhibit detection performance for UO₂²⁺. 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 UO₂²⁺, Cr₂O₇²⁻, and CrO₄²⁻ and NFT [18].

Inspired by the aforementioned viewpoints, we report a new Ni(II) CP [Ni(MIP)(BMIOPE)]n (1), while H₂MIP = 5-methylisophthalic acid, and BMIOPE = 4,4′-bis(2-methylimidazol-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 UO₂²⁺ cation, Cr₂O₇²⁻/CrO₄²⁻ anions and NFT antibiotic with noteworthy selectivity and high sensitivity. Furthermore, the possible sensing mechanisms were studied in detail.

2. Results and Discussion

2.1. Structure Description of [Ni(MIP)(BMIOPE)]_n (1)

Structure analysis shows that complex 1 crystallizes in monoclinic crystal system with space group C2/c by employing X-ray single crystal diffraction. The asymmetric unit consists of two independent Ni²⁺ ions (Ni1, Ni2), one MIP²⁻ organic linker and one BMIOPE organic linker. From the view of Figure 1a, Ni1 ions display six-coordinated octahedral configuration, consisting of four O atoms (O1, O1A, O2, O2A) from two MIP²⁻ linkers and two N atoms (N1 and N1A) from two BMIOPE linkers (Ni1-O1 = 2.0775(15) Å, Ni-O2 = 2.200(2) Å, Ni1-O1A = 2.0775(15) Å, Ni-O2A = 2.200(2) Å, Ni1-N1 = 2.048(2) Å, Ni1-N1B = 2.048(2) Å, symmetry code: A 1 − x, +y, 1.5 − z). Ni2 ions are six-coordinated by four O atoms (O3, O4, O3B, O4B) from two MIP²⁻ organic linkers and two N atoms (N4C and N4D) from two BMIOPE organic linkers, forming an octahedral configuration (Ni2-O3 = 2.0759(14) Å, Ni2-O3B = 2.0759(14) Å, Ni2-O4 = 2.1521(16) Å, Ni2-O4B = 2.1521(16) Å, Ni2-N4C = 2.1208(19) Å, Ni2-N4D = 2.1208(19) Å, symmetry code: B 0.5 − x, −0.5 − y, 1 − z, C 0.5 − x, 1.5 − y, 1 − z, D +x, −2 + y, +z). In complex 1, neighbor Ni1 and Ni2 ions are connected to MIP²⁻ ligands to from zigzag one-dimensional (1D) chains [Ni(MIP)]_n with Ni1∙∙∙Ni2 range of 9.065 Å (Figure S1a), while BMIOPE ligands connect adjacent Ni1 and Ni2 ions to generate zigzag 1D chains [Ni(BMIOPE)]_n with a Ni1∙∙∙ Ni2 separation of 16.522 Å (Figure S1b). These two zigzag chains are interconnected by Ni1 and Ni2 ions to generate a 2D layer arranged parallel along the [1 0 −1] direction. Two kinds of large cavities have the maximum dimensions of 15.622 Å × 17.795 Å (type A) and 15.622 Å × 21.568 Å (type B) in each net calculated on the diagonal Ni∙∙∙Ni separation (Figure 1b). Such large cavities allow the other net to penetrate and form twofold parallel interwoven net (Figure 1c). Topologically, Ni1 and Ni2 ions act as four connecting nodes, and complex 1 is a 4⁴∙6² 4-connected uni-nodal network (Figure 1d). The calculated free volume in complex 1 is 2.5% (per unit crystal volume of 5336.1 ų) calculated using PLATON. The framework of complex 1 is further stabilized by weak but extensive inter- and intra-molecular C–H∙∙∙O hydrogen bonds connecting the 2D layers into a 3D hydrogen-bond structure.

2.2. 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).

2.3. 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 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 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.

2.4. Selective Detection of UO₂²⁺ 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⁺, Cd²⁺, Zn²⁺, Pb²⁺, Co²⁺, Ca²⁺, Cu²⁺, Ni²⁺, Mg²⁺, Al³⁺, Cr³⁺ and UO₂²⁺, 10 mM), respectively. The emission spectra of complex 1 is the metal ion-dependent, and UO₂²⁺ 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 UO₂²⁺ in aqueous solutions. Thus, to explore the sensing sensitivity to detect UO₂²⁺, titration experiments were implemented to explore the intensities of complex 1 with increasing amounts of UO₂²⁺ ionic solutions. With increasing solution volume of UO₂²⁺, the intensities of 1 continuously decrease (Figure 2b). Stern–Volmer (SV) formula I₀/I = 1 + K_SV[Q] was used to analyze the quenching efficiency (I₀ 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 I₀/I is linearly proportional to UO₂²⁺ concentration (R² = 0.9937) at low concentrations, and the slope K_SV for UO₂²⁺ is quantified to be 1.51 × 10⁴ M⁻¹, confirming high sensitivity of complex 1 toward UO₂²⁺. 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 UO₂²⁺ 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 UO₂²⁺, 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 UO₂²⁺ ions. The results indicate that the presence of other competing ions does not disturb the detection of complex 1 toward UO₂²⁺. In comparison with other CPs-based sensors for detecting UO₂²⁺ ions, complex 1 has good detection and analysis performance [16,17,23].

2.5. Selective Detection of Cr₂O₇²⁻/CrO₄²⁻ Anions

The detection ability of complex 1 towards different anions was explored under the same experimental procedure as UO₂²⁺. The powder sample of complex 1 (5 mg) was evenly dispersed in 10 mM aqueous solution of Na_nX (CO₃²⁻, NO₃⁻, Br⁻, HCO₃⁻, SCN⁻, C₂O₄²⁻, HPO₄²⁻, PO₄³⁻, SO₄²⁻, H₂PO₄⁻, CrO₄²⁻ and Cr₂O₇²⁻) to explore its sensing abilities toward various anions. As demonstrated in Figure 3a, the emission intensities of complex 1 are obviously quenched by Cr₂O₇²⁻ and CrO₄²⁻ ions while other anions show no obvious quenching effect, indicating that Cr₂O₇²⁻ and CrO₄²⁻ ions could be detected by complex 1. To further explore the fluorescent sensitivities of complex 1 toward Cr₂O₇²⁻ and CrO₄²⁻ anions, titration experiments were performed in aqueous solution. The luminescent intensities of complex 1 gradually decreased with the increasing concentration of Cr₂O₇²⁻ and CrO₄²⁻ anions (Figure 3b and Figure 4a). The Ksv values are 1.27 × 10⁴ M⁻¹ (R² = 0.9980) for Cr₂O₇²⁻ and 1.44 × 10⁴ M⁻¹ (R² = 0.9858) for CrO₄²⁻, as depicted in Figure 3c and Figure 4b. The LOD values for Cr₂O₇²⁻ and CrO₄²⁻ anions are 4.09 × 10⁻⁵ M and 3.79 × 10⁻⁵ M (Table S2). Furthermore, we also carried out the interference experiments to evaluate the selectivity of complex 1 against Cr^VI (Cr₂O₇²⁻/CrO₄²⁻). As shown in Figure 3d and Figure 4c, the capability of complex 1 to detect Cr₂O₇²⁻ and CrO₄²⁻ 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].

2.6. Selective Detection of NFT

The excellent fluorescence response of complex 1 towards UO₂²⁺, Cr₂O₇²⁻ and CrO₄²⁻ 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 (R² = 0.9975). The K_SV and LOD values of complex 1 towards NFT are 6.18 × 10³ 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].

2.7. Detection Mechanism

To better understand the strong quenching effect of UO₂²⁺, Cr₂O₇²⁻/CrO₄²⁻ and NFT on the fluorescence of complex 1, the quenching mechanism was explored. The fluorescence quenching UO₂²⁺, Cr₂O₇²⁻/CrO₄²⁻ 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 (UO₂²⁺, Cr₂O₇²⁻/CrO₄²⁻ 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 UO₂²⁺, Cr₂O₇²⁻/CrO₄²⁻ and NFT is caused by the competitive absorption and PET mechanism.

Additionally, fluorescence lifetime measurements were performed to further investigate the kinetic property of complex 1 toward UO₂²⁺, Cr₂O₇²⁻, CrO₄²⁻ and NFT. The fluorescence lifetime studies exhibit similar triexponential function-fitted decay curves with varying average lifetimes for the different samples (Figures S6–S10). The fluorescence lifetime of the suspension of complex 1 is 120 ns. With the addition of UO₂²⁺, Cr₂O₇²⁻, CrO₄²⁻ and NFT, the average lifetimes of complex 1 are 109 ns, 111 ns, 115 ns and 110 ns, respectively. The fluorescence lifetimes of complex 1 show very little change before and after detection of different quenchers, suggesting that the luminescence quenching process should be a static quenching mechanism [41].

3. Materials and Methods

3.1. 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 Al₂O₃ crucible as the standard. Measuring temperature ranges from 25 to 800 °C with a heating rate of 10 °C min⁻¹ under N₂ 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.

3.2. Preparation of [Ni(MIP)(BMIOPE)]_n (1)

Ni(NO₃)₂·6H₂O (14.5 mg, 0.05 mmol), 5-methylisophthalic acid (H₂MIP) (9.1 mg, 0.05 mmol) and 4,4′-bis(2-methylimidazol-1-yl)diphenyl ether (BMIOPE) (16.5 mg, 0.05 mmol) were dissolved in 2.5 mL mixed solvent of water and N,N-dimenthylformamide (V_H2O:V_DMF = 1:1.5). The solution was charged into a Teflon-lined stainless steel vessel, heated from room temperature to 100 °C in 2 h for 3 days under autogenous pressure. The product was then cooled to 40 °C in 2 days and finally cooled to room temperature (yield 56.9%, calculated on Ni(NO₃)₂·6H₂O). Elemental analysis (%) for C₂₉H₂₄N₄NiO₅: C, 61.41; H, 4.26; N, 9.88. Found: C, 61.54; H, 4.27; N, 9.91. IR (KBr, cm⁻¹): 3130 w, 1606 m, 1548 s, 1508 s, 1440 m, 1382 m, 1302 w, 1238 s, 1173 w, 1107 w, 1005 w, 841 m, 811 w, 774 w, 738 m, 548 w, 443w.

3.3. 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 F² 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. Crystal data and structure refinement parameters of complex 1.

Complex	1
Formula	C29H24N4NiO5
Formula weight	567.23
Crystal system	monoclinic
Space group	C2/c
a (Å)	20.4678(11)
b (Å)	7.8281(4)
c (Å)	34.2314(18)
α (o)	90
β (o)	103.369(2)
γ (o)	90
Volume (Å3)	5336.1(5)
Z	8
T (K)	273(2)
Dcalcd (mg·m−3)	1.412
μ (mm−1)	0.774
Rint	0.0341
F(000)	2352.0
θ range (°)	2.769 ≤ θ ≤ 25.998
Reflns. collected	49,207
Data/restraints/parameters	5254/1/357
Goodness of fit on F2	1.038
Final R indices [I > 2σ (I)]	R1 = 0.0394, wR2 = 0.0891
R indices (all data)	R1 = 0.0474, wR2 = 0.0919
Largest diff. peak and hole (e Å−3)	0.38 and −0.62

R₁ = Σ| |F_o| − |F_c| |/Σ|F_o|. ωR₂ = Σ[w(F_o² − F_c²)²]/Σ[w(F_o²)²]^1/2.

. Selected bond lengths and angles for complex 1 are provided in Table S1.

3.4. 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_nX (HCO₃⁻, CO₃²⁻, NO₃⁻, Br⁻, C₂O₄²⁻, SCN⁻, HPO₄²⁻, PO₄³⁻, SO₄²⁻, H₂PO₄⁻, CrO₄²⁻ and Cr₂O₇²⁻), M(NO₃)_n (M = Li⁺, Na⁺, Ag⁺, Ca²⁺, Cd²⁺, Co²⁺, Zn²⁺, Pb²⁺, Ni²⁺, Mg²⁺, Cr³⁺, Al³⁺, Cu²⁺ and UO₂²⁺) 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 (UO₂²⁺, Cr₂O₇²⁻, CrO₄²⁻, NFT) in the aqueous solutions, and then emission spectra were collected.

4. Conclusions

In this work, a novel Ni-CP [Ni(MIP)(BMIOPE)]_n (1) was synthesized using H₂MIP and BMIOPE ligands. Complex 1 is a 2D twofold parallel interwoven net structure with a 4⁴∙6² point symbol. Complex 1 could be used as multifunctional luminescent probe toward UO₂²⁺, Cr₂O₇²⁻/CrO₄²⁻ 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 UO₂²⁺, Cr₂O₇²⁻, CrO₄²⁻ and NFT, respectively. The quenching mechanism proved that the quenching involved in sensing UO₂²⁺ cations and Cr₂O₇²⁻/CrO₄²⁻ 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 UO₂²⁺, Cr₂O₇²⁻/CrO₄²⁻ and NFT was attributed to static quenching mechanism.