Two Bisligand-Coordinated Luminescent Zn(II)-Coordination Polymers for Sensing of Ions and Pesticides in Aqueous Solutions

INTRODUCTION

Coordinated polymers (CPs), which are built up by metal centers and organic linkers through coordination bonds, have become a popular research topic during the past decade.^[1-3] The exchangeable metal sites, diversified organic components and tunable architectures enable CPs to possess specific intricate chemical and physical properties like a high specific surface area, tailorable porosity, high stability and excellent functionality. These superior properties make CPs the rapidly expanding materials of interest with a wide range of applications, such as gas storage^[4] and separation, ^[5] adsorption, ^[6] catalysis, ^{[7, 8]} magnetic materials, ^{[9, 10]} luminophores, ^[11] electronics, ^[12] and photoluminescence.^[13] Luminescent CPs are an important branch of CPs. The exploration of luminescent CPs has primarily been focused on the chemical sensing, including cations, ^{[14, 15]} anions, ^{[16, 17]} antibiotics, ^{[18, 19]} pesticides, ^[20] bioactive molecules, ^[21] amino acids, ^[22] pH values, ^[23] and temperatures.^[24] Typically, the luminescent emission of CPs can arise from organic ligands and/or metal cores.^[11] For organic ligands-based emission, the photoluminescence was emitted by π-electron-rich aromatic ligands or neighboring ligands via an intra- or inter-ligand charge transfer process.^{[11, 25, 26]} Another case is emissive metal ion-based CPs, for instance, the most commonly used lanthanide ions based luminescent CPs, ^[25] which can transport the absorbed energy via a ligand-to-metal charge transfer process. Therefore, organic ligands play fundamental roles not only in the excitation and emission of photoluminescence but also in the diverse architectures of a luminescent CP. Typically, organic ligands feature N-donor moieties and/or carboxylate groups.^[27] To benefit the advantage of both types of functional groups, one strategy is to construct luminescent CPs by a bis-group-containing ligand, and another one is to build luminescent CPs by using two different types of ligands bearing different functional groups.^[28]

With the fast growth of economics and booming development of societies, environmental pollution seriously threatens human health and natural life.^[29] An overdose or deficiency of Fe³⁺ may cause serious health problems.^[30] The inorganic Cr₂O₇^2- anion is a mutagenic and carcinogenic species that may damage DNA to induce cancers and other detrimental genetic defects.^[31] Besides inorganic ions, the organic contaminants also need to be considered. Pesticides are widely used to improve the agricultural production.^[32] Due to the vast types and huge usage amounts of persistent or easily degradable pesticides, the detection of residuals in the environment or agricultural products is a crucial issue.^[33] Until now, pollutants have been commonly detected by use of the techniques such as atomic absorption spectrometry (AAS), inductively coupled plasma-mass spectrometry (ICP-MS), gas chromatography-mass spectrometry (GC-MS), and high-performance liquid chromatography (HPLC).^[34-37] Unfortunately, these methods are often expensive, complicated, area-demanding and time-consuming. Therefore, the exploration of new techniques under easy manipulation with the advantages of short response time, low cost, and real-time monitoring capabilities is highly demand. The fluorescent detection techniques meeting such requirements have been extensively investigated.^[11]

In the present work, based on the above consideration, two new 1D and 2D mixed ligand-coordinated luminescent Zn(II)-MOFs coordination polymers namely: [Zn(PTA)(DTP)(H₂O)₂]·(DMF) (CP-1) (H₂PTA = terephthalic acid, 3, 5-di(1, 2, 4-triazol-1-yl) pyridine (DTP) and DMF = dimethylformamide) and [Zn(BTC)-(DTP)]·(CH₃CN)_1.5·(H₂O)₄ (CP-2) (H₃BTC = benzene-1, 3, 5-tricarboxylate), were designed and successfully synthesized under solvothermal conditions. CPs with excellent fluorescent properties were employed as multi-responsive sensors. These CPs showed effective, sensitive and selective fluorescent sensing abilities toward inorganic ions like Fe³⁺ and Cr₂O₇^2- as well as organic pesticide pollutants like imidacloprid (MMT) and IMI (full name). The mechanistic investigation revealed that both the inner filter effect (IFE) and the fluorescence resonance energy transfer (FRET) effect are involved in the luminescent detection of inorganic ions and pesticides.

RESULTS AND DISCUSSION

Synthesis of CPs. CP-1 with 61% yield based on DTP ligand was prepared from Zn²⁺ and mixed organic linkers like DTP and H₂PTA in a mixed solvent of DMF and water (v/v = 1:1). CP-2 was obtained from the reaction between Zn²⁺ and the mixture of DTP/H₃BTC ligands in acetonitrile and water (v/v = 1:1) solution (71% yield based on DTP ligand). The air-stable colorless crystalline solid materials CP-1 and CP-2 were characterized by single-crystal X-ray diffraction analysis, FTIR, PXRD, TGA and elemental analysis.

X-ray Crystal Structure. The structures of CP-1 and CP-2 were determined by X-ray single-crystal diffraction analysis and the selected bond lengths and bond angles for CPs are listed in Table S1. The results show that CP-1 and CP-2 crystallize in monoclinic C2/c (No. 15) and triclinic Pī (No. 2) space group, respectively. The asymmetric unit of CP-1 is composed of half-Zn(II) ion, one coordinated water molecule, half lattice DMF molecule, half DTP and half PTA ligands (Figure 1a). The unique Zn(II) center in CP-1 is five-coordinated in a distorted hexahedral geometry by two carboxylic oxygen atoms from two PTA^2- ligands, one nitrogen from the DTP ligand and two oxygen atoms from two terminal water molecules. The fully deprotonated PTA^2- ligand serves as a two-connected linker that coordinates with two Zn(II) ions, thereby generating a "V"-shaped one-dimensional chain (Figure 1b-c).

Figure 1

Figure 1.  (a) The coordination environment of Zn(II) ions and the ligands in CP-1. Symmetry codes: #1: 1 - x, y, 2.5 - z; #2: 0.5 - x, -1.5 - y, 3 - z; (b) A view of the 1D chain along the a-axis; (c) A view of the 1D chain along the b-axis.

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The asymmetric unit of CP-2 (Figure 2a) includes one Zn(II) ion, one DTP and one partially deprotonated HBTC^2- ligand, two lattice water molecules and two lattice acetonitrile molecules. The Zn(1) ion is four-coordinated with two carboxylic oxygen atoms from two BTC^2- ligands and two nitrogen atoms from two DTP ligands. This coordination environment enables the Zn(1) ion to display a distorted tetrahedral geometry. Both of HBTC^2- and DTP ligands connect two Zn(II) ions to form 1D chains along the a- and c-axis, respectively. Furthermore, the two chains cross each other by the Zn(II) ions to construct a double-layer 2D network (Figure 2b-c).

Figure 2

Figure 2.  (a) The coordination environment of Zn(II) ions and the ligands in CP-2. Symmetry codes: #1: x, y, -1+z; #2: -1+x, y, z; #3: 1+x, y, 1+z; #4: 1+x, y, z. (b) A view of the 2D layer along the a-axis. (c) A view of the 2D layer along the b-axis.

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Characterizations. The FTIR spectra of CP-1 and CP-2 were an alyzed and displayed in Figure S1. The characteristic stretching vibration peak of C=O in the protonated carboxyl group appears at approximately 1680-1700 cm^-1, ^[38] which completely disappears from the profiles of the metal-containing compound CP-1.^{[39, 40]} However, the related peak is found in the FTIR spectrum of CP-2 at 1700 cm^-1 (Figure S1b) ascribed to the incomplete deprotonation of HBTC^2- in the process of CP-2 formation. In addition, the asymmetric and symmetric stretching vibrations of C=O in the Zn(II)-coordinated carbonyl groups of the ligands occur at 1591 and 1370 cm^-1 for CP-1 and 1621 and 1347 cm^-1 for CP-2. Besides FTIR analysis, the phase purities of CP-1 and CP-2 were evaluated by PXRD analysis. Figure S2 illustrates that the main peaks from the experiment are consistent with those of the simulated data, indicating good phase purity of the synthesized materials. In addition, the thermal stabilities of the two CPs were determined by TGA under a N₂ atmosphere from ambient temperature to 800 ℃ (10 ℃ min^-1). As shown in Figure S3, the first weight loss of CP-1 at 20.5% (calcd. 19.8%) from room temperature to 200 ℃ corresponds to the loss of coordinated water and lattice DMF mole- cules. The temperature for sharp weight loss occurred at 270 ℃ due to the collapse of the skeleton of CP-1. For CP-2, an initial weight loss of 13.5% (calcd. 14.1%) was observed from room temperature to approximately 190 ℃, which corresponds to the departure of coordinated water and lattice DMF molecules. As Figure S2 shows, for CP-2, a rapid mass loss was observed from the curve caused by skeleton collapse at approximately 300 ℃. Considering that the stability of as-synthesized CPs would be important parameter during the sensing of analysts in aqueous media, the CPs materials in aqueous solution under different pHs (2, 4, 6, 8, 10, 12) were evaluated at room temperature for 24 hours. Figure S4 shows that the immersed CPs exhibit PXRD patterns similar to that of the intact ones, indicating strong pH stability of the CPs materials.

Fluorescent Properties of CPs and Fluorescent Sensing Solid-State Fluorescence. CPs conjugated with d¹⁰-metals and organic linkers usually exhibit excellent luminescence properties. Hence, they can be used as potential luminescent sensors. The luminescence characterizations of CP-1 and CP-2 were evaluated, as depicted in Figure S5, where the apexes of their emission curves are located at 322 nm. The wavelength is similar to that of the free DTP ligand, while a difference in excited wavelength is found for the two CPs, with the maxima (λ_max) at 302 nm for CP-1 and 277 nm for CP-2 nm. The emission of the two CPs can be ascribed to the electronic transition of intra-ligand π→π* and n→π* orbitals^[41] of the DTP ligand within CPs.

Fluorescent Sensing. Based on the excellent fluorescence ability of the two CPs, their sensing abilities towards different metal ions and pesticides were investigated. Before the sensing process, the finely grounded sensors (2.0 mg) were ultrasonically dispersed in water (10.0 mL) for 30 minutes. Then the sensing experiments were performed by mixing the sensors suspension (0.2 mg/mL) and analytes solution (2 mM for the ions and 0.2 mM for the pesticides).

Sensing of the Cations and Anions in Water. Figure 3 shows that most of the tested ions caused negligible fluorescence quenching of the sensors; however, when introducing Fe³⁺ and Cr₂O₇^2-, significant fluorescence quenching was observed. The quenching efficiencies (defined as 1 - I/I₀) caused by Fe³⁺ and Cr₂O₇^2- are 97.4% and 96.3% for CP-1 and 97.6% and 98.2% for CP-2, respectively.

Figure 3

Figure 3.  Quenching efficiencies of CP-1 and CP-2 dispersed in aqueous solutions and treated with various ions (1 mM).

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Besides the fluorescence sensing analyzed at a unique concentration of different targeted pollutants, a kinetic exploration of the fluorescence quenching was further measured by titration experiments to evaluate the effect of concentration of the quenchers on fluorescence intensity of the sensors. Hence, a quenching constant value of K_sv was introduced by the equation Stern Volmer (SV): I₀/I - 1 = K_sv[Q], where [Q] is the molar concentration of the quencher, and I₀ and I represent the fluorescence intensities of the sensors in aqueous solutions without and with the presence of quencher.^{[42, 43]} Figures S6-S9 show that fluorescence intensities of the sensors decreased gradually as the concentrations of the quenchers increased. In addition, at low analyte concentrations, a linear correlation was exhibited from kinetic plots (0-0.1 mM for both Fe³⁺ and Cr₂O₇^2-). However, with continuous introduction of the quencher to the suspension of the sensor, the linear curve bent upward. The K_sv values calculated from the SV equation from the linear ranges of the curves are 6.98×10³ M^-1 for Fe³⁺ and 7.80×10³ M^-1 for Cr₂O₇^2- when CP-1 was used, and 8.10×10³ M^-1 for Fe³⁺ and 1.60×10⁴ M^-1 for Cr₂O₇^2- when CP-2 was explored as the sensor.

In addition, another important parameter of the limit of detection (LOD) was also introduced to evaluate the sensors. The LOD equation is defined as 3σ/K_sv (here, σ is the relative standard error counted from ten repeated measurements of the sensor in aqueous suspensions). During fluorescent evaluation of CP-1, the calculated LOD values are 1.10×10^-6 M for Fe³⁺ and 9.80×10^-7 M for Cr₂O₇^2- (Table S2); when using CP-2 as the sensor, the corresponding values are 9.44×10^-7 M for Fe³⁺ and 4.77×10^-7 M for Cr₂O₇^2-.

Sensing of the Pesticides in Water. The excellent performance of CPs in fluorescent sensing of the ions encouraged us to further explore their applications in the detection of pesticides. Therefore, pesticides (Table S3) DIP, PCNB, IMZ, GLY, TPN, CAR, 2, 4-D, IMI, and MMT in aqueous solutions were used. Figure 4 shows that IMZ, 2, 4-D, GLY, DIP, PCNB, TPN, and CAR have no significant effect on the fluorescence intensity of CP-1. Moderate quenching was observed when IMI was used and MMT caused the most significant drop in fluorescence intensity for CP-1. When employing CP-2 as the sensor, IMZ, GLY, DIP, PCNB and TPN contributed no obvious effect on the fluorescent intensity; 2, 4-D, CAR and MMT contributed a moderate quenching; and IMI gave rise to the most significant quenching of the fluorescence of CP-2.

Figure 4

Figure 4.  Quenching efficiencies of CP-1 and CP-2 dispersed in aqueous solutions and treated with various pesticides (0.1 mM).

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Furthermore, the best sensing effects toward pesticides of MMT for CP-1 and IMI for CP-2 were further analyzed by kinetic titration experiments. As seen from Figures S10-S11, both CP-1 and CP-2 shared similarities in the process of sensing of ions, i.e. as the concentration of pesticides increased, the intensities of the fluore- scence dropped significantly. The plots of Ksv values illustrate a satisfactory linear correlation in the low analyte concentration range, and the curves bent upward at higher concentrations. The calculated Ksv and LOD values are 4.76×10⁴ M^-1 and 1.61×10^-7 M for MMT when CP-1 was used and 3.05×10⁴ M^-1 and 2.51×10^-7 M for IMI when CP-2 was employed as the senor. The calculated values as compared with the reported values in the fluorescent sensing of IMI or MMT are listed in Table S4.

Recyclability and Fluorescence Stability. The recyclability and regeneration of a fluorescent sensor are considered for real applications; therefore, we further studied the recycling experiments. After recording fluorescence with or without the presence of analyte, the sensing materials were regenerated by simple centrifugation and washing with deionized water. As shown in Figure S12, there was no significant drop in the initial intensity even after five cycles of application, indicating good reusability of the sensing materials. Furthermore, fluorescence stability of the sensors under different pH conditions was also checked. Figure S13 exhibits a fluorescence summit at pH around 7. However, a slight drop of initial fluorescence intensity of the sensor was observed in case of increase or decrease of pH of the suspension. In addition, the sensors' stability with the presence of different cations and anions was also evaluated. After the sensing process of CPs for the salts, the suspensions were kept for additional 24 hours. There was no obvious appearance of transformation for both CPs under microscope. And, the single crystals of CPs under different conditions were also evaluated by using X-ray diffraction analysis (5 single crystals were analyzed for each sample), finding no change of the unit cells for the two CPs. Thereby, both CPs are stable under the tested conditions.

Possible Mechanisms for Luminescent Sensing. Regarding the exploration of possible mechanisms involved in luminescent quenching during sensing of the analytes, attention was firstly focused on the structural transformation of the sensors.^[44] Accordingly, FTIR (Figure S14) and PXRD (Figure S15) of CP-1 and CP-2 were investigated and evaluated before and after the fluorescent sensing processes, and the well-matched curves in both spectra indicated that their frameworks remained intact. Next, considering energy transformation, ^{[45, 46]} the UV-Vis absorption spectra of the analytes were evaluated and illustrated together with the excitation and emission of CP-1 and CP-2. As shown from Figure 5, there is strong absorption at wavelength of 277, 302, and 322 nm by both Fe³⁺ and Cr₂O₇^2-, and the UV-Vis absorption of the quenchers overlapped with both the excitation and emission light of CP-1 and CP-2. The same result occurred when pesticides MMT and IMI were used. An obvious superposition between the UV-Vis absorption of the quenchers and the excitation and emission spectra of the related sensors was observed, indicating that both IFE and FRET mechanisms played key roles in reducing the fluorescent intensity of the sensors in dynamic process of the low analyte concentration range. When raising the concentration of analytes, the dynamic curves bent upward, indicating the involvement of a static mechanism occurred, which showed an interaction between analyte and the sensor.^[47]

Figure 5

Figure 5.  UV-Vis absorption spectra of different ions (a) and pesticides (b).

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CONCLUSION

Two mixed ligand-coordinated 1D and 2D Zn-based CPs were fabricated and characterized. The configuration of these CPs was confirmed by using single-crystal X-ray diffraction analysis. The CPs exhibited excellent fluorescence properties. Both sensors displayed extra sensitive and selective properties toward Fe³⁺ and Cr₂O₇^2- during sensing, but CP-1 outperformed the sensing of IMI, and CP-2 was preferably sensitive toward MMT during sensing. Considering the involved mechanisms of the fluorescent quenching processes, IFE and FRET mechanisms were believed to play dominant roles. This work demonstrated and provided fluorescent CPs that could be used as potential fluorescent sensors to detect environmental pollutants.

EXPERIMENTAL

Materials and Methods. All the materials and solvents were obtained commercially and used without further purification. ZnSO₄·7H₂O, H₂PTA, H₃BTC, DMF and acetonitrile were commercially available from Aladdin. DTP was synthesized following a reported method.^[48] Powder X-ray diffraction (PXRD) measurements were performed using a Bruker-Avance X-ray diffractometer equipped with a Cu-target tube and a graphite monochromator scanning over the range of 5-50° at the rate of 0.2 °/s. The simulated X-ray diffraction patterns were generated from properly treated cif files of the related CPs crystals by using the Mercury software. Elemental analyses for C, H, and N were carried out using a Perkin-Elmer 240 CHN elemental analyzer. The Fourier transform infrared (FTIR) spectrum was obtained using an Agilent Cary630 spectrophotometer in the range of 4000 to 500 cm^-1. UV-Vis spectroscopic studies were carried out using a Varian UV50 Conc spectrophotometer. All luminescence measurements were performed using an Agilent Cary Eclipse fluorescence spectro-photometer at room temperature. A Mettler Toledo 1600TH ther-mal analyzer was used to record TG curves at a heating rate of 10℃/min in a flowing nitrogen atmosphere of 10 mL/min using platinum crucibles over the temperature range from room temperature to 800 ℃.

X-ray Structure Determination. Single crystals suitable for X-ray analysis of CPs were selected and mounted on a Bruker APEX-II CCD diffractometer at 150.0(1) K during data collection using a Mo-Kα radiation (λ = 0.71073 Å). Integration and scaling of intensity data were performed by using SAINT program.^[49] Data were corrected for the effects of absorption using SADABS.^{[50, 51]} Using Olex 2, ^[52] the structures were solved with the SIR 2004 structure solution program^[53] by direct methods. Further, the structures were refined with ShelXL refinement package^[54] using least-squares minimization. Non-hydrogen atoms were anisotropically refined and the hydrogen atoms in the riding mode and isotropic temperature factors were fixed at 1.2 times U(eq) of the parent atoms (1.5 times for methyl groups). For C₂₀H₂₂N₈O₇Zn (CP-1) (CCDC 2082190) (M_r = 551.82 g/mol): monoclinic C2/c space group, a = 12.9809(19), b = 14.6111(19), c = 12.3777(19) Å, β = 101.852(4)°, V = 2297.6(6) ų, Z = 4, T = 150.0(1) K, μ(MoKα) = 1.130 mm^-1, D_c = 1.595 g/cm³, 11147 reflections measured (4.25≤ 2θ≤49.43°), 1962 unique (R_int = 0.1190, R_sigma = 0.0768) which were used in all calculations. The final R₁ = 0.1746 (I > 2σ(I)) and wR₂ = 0.3401 (all data). For C₄₂H₂₉N₁₇O₁₄Zn₂ (CP-2) (CCDC 2131062) (M_r = 1126.56 g/mol): triclinic system, space group Pī, a = 10.1370(13), b = 10.9499(14), c = 11.7439(16) Å, β = 82.503(4)°, V = 1152.0(3) ų, Z = 1, T = 150.0(1) K, μ(MoKα) = 1.129 mm^-1, D_c = 1.624 g/cm³, 12177 reflections measured (3.91≤2θ≤52.92°), 4684 unique (R_int = 0.0843, R_sigma = 0.1162) which were used in all calculations. The final R₁ = 0.1511 (I > 2σ(I)) and wR₂ = 0.2552 (all data).

Fluorescence Sensing Analyses. Quenching test with unique concentration: Before the fluorescence sensing analysis, a stock solution of well-dispersed Zn-CP suspension (0.2 mg/mL) and different analyte solutions (2 mmol/mL) were prepared. Thereafter, equal volumes of the suspension and the analyte solution were mixed in a cuvette and ultrasonicated for 5 minutes. Afterwards, the resulting mixture was excited by UV light with wavelength of 302 nm for CP-1 and 277 nm for CP-2, and the intensities of the emitted light were recorded at a wavelength of 322 nm for both CPs. The used analytes included the following cations (2 mM): MCl_1-3 (M = K⁺, Na⁺, Mg²⁺, Ca²⁺, Ni²⁺, Co²⁺, Mn²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺, Ba²⁺, Al³⁺, Cr³⁺ and Fe³⁺); anions (2 mM): K_1-2X (X = F^-, Cl^-, Br^-, I^-, Ac^-, SCN^-, NO₃^-, ClO₃^-, ClO₄^-, HPO₄^2-, H₂PO₄^-, CO₃^2-, B₄O₇^2-, SO₃^2-, SO₄^2- and Cr₂O₇^2-); and pesticides (0.2 mM): (dipterex, DIP; pentachloro-nitrobenzene, PCNB; imazalil, IMZ; glyphosate, GLY; chlorothalonil, TPN; carbendazim, CAR; 2.4-dichlorophenoxyacetic acid, 2, 4-D; imidacloprid, IMI and metamitron, MMT). The used time for each measurement during the fluorescent sensing process is less than 30 seconds.

Luminescence kinetic titration: Before the kinetic quenching analysis, the well-mashed CP (1 mg) was added to water (10 mL), and the mixtures were stirred under ultrasonic conditions for 30 minutes. Before each fluorescent sensing measurement, aliquots (2 to 10 µL) of analytes' stock solution (20 mM for the ions and 5 mM for the pesticide) were injected into and well vortexed with the sensor suspension (4 mL).

Recyclability of Luminescence Experiments. The suspended solid CP material was centrifuged after recording the fluorescence of the blank sample in aqueous solution. Thereafter, the sediment was mixed with the analyst solution under ultrasonic conditions for 5 minutes, and the resulting mixture was subjected to a fluorescent sensing test. Then, the suspension was centrifuged again and rinsed several times with deionized water. Following the typical procedure, the experiments were repeated for an additional 4 cycles.

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