Zinc(II)-based coordination polymer encapsulated Tb³⁺ as a multi-responsive luminescent sensor for Ru³⁺, Fe³⁺, CrO₄²⁻, Cr₂O₇²⁻ and MnO₄⁻

Abstract

A zinc(II)-based coordination polymer (CP), namely [Zn(modbc)₂]_n (Zn-CP) (modbc = 2-methyl-6-oxygen-1,6-dihydro-3,4′-bipyridine-5-carbonitrile), has been synthesized and characterized. Single-crystal structural determination reveals that Zn-CP is a two-dimensional framework structure with tetranuclear homometallic Zn₄(modbc)₄ units cross-linked by modbc. The excellent luminescence as well as good stability of Zn-CP do not enable it to have selective sensing capability for different ions. After encapsulation of Tb³⁺ in Zn-CP, the as-obtained fluorescent functionalized Tb³⁺@Zn-CP maintained excellent luminescence as well as stability, which made it a highly selective and sensitive multiresponsive luminescent sensor for Ru³⁺, Fe³⁺, CrO₄²⁻, Cr₂O₇²⁻, and MnO₄⁻ with high sensitivity, good anti-interference performance, and quick response time (∼10 s). The detection limits are 0.27 μM, 0.57 μM, 0.10 μM, 0.43 μM and 0.15 μM, respectively. A possible sensing mechanism was discussed in detail.

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1. Introduction

At present, hexavalent chromium (CrO₄²⁻, Cr₂O₇²⁻) as a potential carcinogen mainly exists in natural water and various industrial processes, which can cause serious harm to the environment and human health.^1,2 In addition, the extensive application of MnO₄⁻ in research experiments and industry has caused serious water pollution.³ On the other hand, Fe³⁺ affects the activity of hemoglobin and transferrin in organisms. Both excess and deficiency of the normal permissible limit will lead to serious diseases.^4,5 The purification and trace detection of ruthenium have been paid attention to,⁶ and the olefin reaction catalyzed by ruthenium also won the 2005 Nobel Prize in chemistry.⁷ The development of ion selective sensing is very important in life and environmental science, so new materials for detecting Ru³⁺, Fe³⁺, CrO₄²⁻, Cr₂O₇²⁻, and MnO₄⁻ have attracted great attention.^8–11 To date, various methods and techniques for ion detection have been reported.^12–14 Among them, fluorescent chemosensors have attracted extensive research interest due to their remarkable advantages such as fast response, high sensitivity and simplicity.^15–17

3D microporous CPs can produce significant fluorescence signals and visible emission, and has become the most reported chemosensors.^18–22 At present, Ln-CPs, compared with transition metal-based CPs, has attracted great interest due to its unique optical characteristics, such as large Stokes shift and long fluorescence lifetime obtained by “antenna effect” of 4f–4f electron transition.^19,20,23,24 Recently, lanthanide ions were doped into CPs by post-synthesis method (PSM), and an alternative strategy to construct Ln-CPs and optimize photoluminescence was proposed.^25,26 Any desired fluorescent probe can be obtained by changing the molar ratio of reactants.^27,28

Some luminescent CPs have been developed in recent years to effectively simultaneously sense Fe³⁺ and Cr₂O₇²⁻.^29,30 The reported polymer as a fluorescent probe for the specific detection of MnO₄⁻ contaminants is still rare to date.^31,32 However, there is no multifunctional ion sensor for sensing Ru³⁺, Fe³⁺, CrO₄²⁻, Cr₂O₇²⁻ and MnO₄⁻ at the same time. Moreover, low-dimensional chemosensors with one-dimensional or two-dimensional structures are rarely reported as ion sensors.³³ Herein, asymmetric unflexible 2-methyl-6-oxygen-1,6-dihydro-3,4′-bipyridine-5-carbonitrile (modbc) containing coordination N and O atoms, was utilized as an anionic ligand (Scheme 1). A new compound with the formula, [Zn(modbc)₂]_n (Zn-CP) has been obtained via solvothermal reaction showing a new type of two-dimensional open framework structure. To improve the selectivity of Zn-CP, Tb³⁺@Zn-CP was obtained via PSM. Tb³⁺@Zn-CP detect a series of metal ions and anions by taking the advantage of its excellent luminescence and good stability. The results demonstrate that Tb³⁺@Zn-CP is an efficient multiresponsive sensor for optical detection of Ru³⁺, Fe³⁺, CrO₄²⁻, Cr₂O₇²⁻ and MnO₄⁻ with a fast response of 10 s and lower limit of detection.

Scheme 1 The structure of modbc.

2. Experimental section

2.1 General methods and materials

Materials. Chloride or nitrate salt of Ag⁺, Pb²⁺, Mn²⁺, Hg²⁺, Ni²⁺, Ca²⁺, Cd²⁺, Zn²⁺, Co²⁺, Cu²⁺, Fe³⁺, Ru³⁺ were purchased from Sigma-Aldrich and used as received. Potassium salt of I⁻, C₂O₄²⁻, CrO₄²⁻, Cr₂O₇²⁻, SO₄²⁻, CO₃²⁻, HCO₃⁻, PO₄³⁻, MnO₄⁻ and (NH₄)₂Fe(SO₄)₂ were analytical grade and used without purification. Water purified in a Milli-Q water filtration system to a resistance of 18 mΩ cm⁻¹, was used in all measurement experiments. Diethylether and dimethyl formamide (DMF) were purified by standard methods.

Apparatus. Element analysis (C, H and N), electronic and infrared spectra were measured with PerkinElmer analyzer model 240 spectrometer, JASCO V-570 spectrometer and Bruker Vector 22 FT-IR spectrometer, respectively. Thermogravimetric analysis (TGA) was performed by a heating rate of 10 K min⁻¹ with Netzsch STA409PC Thermal Analyzer. Powder X-ray diffraction (PXRD) was investigated by Bruker D8 diffractometer at 40 KV, 40 mA, copper target tube and graphite monochrometer. The X-ray diffraction data were collected with a Bruker APEX-II CCD diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) at 296 K. The structure was solved by the standard crystallographic program SHELXS-97 and SHELXL-97 and refined with full-matrix least-squares on F².^34,35 The emission spectra were detected by Spex Fluorog-2 spectrofluorimeter (model F111, Spex Industries, Edison, NJ, USA). The spectrometer uses 450 W xenon lamp (XBO 450 W/1, Osram, Germany) as excitation light source and 950 V photomultiplier tube (R928, Hamamatsu, Japan) as detector.

2.2 Synthesis of [Zn(modbc)₂]_n (Zn-CP)

A mixture of ZnSO₄ (0.05 mmol, 0.008 g) and modbc (0.1 mmol, 0.021 g) was dissolved in 8 mL of DMF/H₂O (1 : 3), and stirred at room temperature for 15 min. Then the mixture was transferred into a 25 mL Teflon-lined stainless steel vessel. The reactor was heated to 120 °C for 72 h under self-generated pressure, then cooled to ambient temperature at a rate of 2.0 °C h⁻¹. Finally, light yellow bulk-shaped crystals were collected, filtered, washed with H₂O (3 × 1 mL), diethylether (3 × 1 mL). The total yields of is ca. 68% based on the metal ions. Elem. anal. calcd for C₁₂H₈Zn_0.50N₃O: C, 59.33; H, 3.32; N, 17.30%. Found: C, 58.63; H, 4.02; N, 16.87%. IR (KBr pellet, cm⁻¹): ν = 3370.13 (vs), 2984.28 (s), 2340.06 (m), 1602.97 (s), 1385.90 (s), 1108.44 (vs), 982.48 (vs), 857.44 (s), 538.28 (vs) (Fig. S1†).

2.3 Preparation of Tb³⁺@Zn-CP

Tb³⁺@Zn-CP was prepared by soaking 200 mg powder of Zn-CP in 1.0 mM Tb(NO₃)₃ aqueous solution for 24 h, centrifuged, washed with Milli-Q water to remove the remaining Tb³⁺, and dried under 60 °C vacuum conditions for tonight.

2.4 General fluorescence measurements

Zn-CP and Tb³⁺@Zn-CP (100 μM) were well-dispersed in DMF by sonicating for 30 min to obtain stock solution and stored in a 4 °C refrigerator. The aqueous solution of nitrate salt or chloride of Ag⁺, Pb²⁺, Mn²⁺, Hg²⁺, Ni²⁺, Ca²⁺, Cd²⁺, Zn²⁺, Co²⁺, Cu²⁺, Fe³⁺, Ru³⁺ (1.0 mM) were prepared for luminescent experiments. Generally, ferrous salt is easy to be oxidized in the air, but it is relatively stable and not easy to be oxidized after forming double salt, so we chose double salt (NH₄)₂Fe(SO₄)₂ to prepare the aqueous solution of Fe²⁺. In addition, the same concentrations (1.0 mM) of aqueous solution containing potassium salt of I⁻, C₂O₄²⁻, CrO₄²⁻, Cr₂O₇²⁻, SO₄²⁻, CO₃²⁻, HCO₃⁻, PO₄³⁻, MnO₄⁻ were also prepared.

Fluorescence emission spectra of Zn-CP and Tb³⁺@Zn-CP in 1 cm path length quartz cuvette were measured by a Spex Fluorog-2 spectrofluorimeter. 1.0 mM above ions were added to a quartz cuvette for luminescence detection immediately by excitation with 330 nm. The slit size for both excitation and emission was 4 nm. The fluorescence stability of Zn-CP and Tb³⁺@Zn-CP with different pH values were investigated in 5 mM Tris–HCl/NaCl buffer. The Stern–Volmer equation: I₀/I = 1 + K_sv[Q] was applied to judge the quenching effect.³⁶ The detection limit was calculated according to 3σ/k recommended by IUPAC, where the standard deviation, σ value was estimated by 15 repeated fluorescent measurements of Zn-CP and Tb³⁺@Zn-CP, and k value was obtained using a calibration curve of I vs. [Q].³⁷

3. Results and discussion

3.1 Description of structure

A 2D CP was synthesized by self-assemble of ZnSO₄ and modbc by one-pot solvothermal method with a yield of ∼68%. X-ray crystallographic analysis shows that Zn-CP crystallizes in the tetragonal space group P4₃2₁2, Z = 28 (Table S1†). The symmetric unit consists of one Zn²⁺ center and two modbc anions. In Zn-CP, each Zn(II) center has a distorted octahedral geometry and it is connected to two O atoms from two modbc ligands and four N atoms from four modbc ligands, as shown in Fig. 1a. Fig. 1b displays that each modbc molecule adopts tridentate coordination mode. The Zn–O distance is 2.390 Å, while the Zn–N length is in the range of 2.089–2.104 Å. The L–Zn–L (L = O, N) bond angle is in the scope of 59.10(8)–173.81(11)° (Table S2†). In addition, the structure of Zn-CP is characterized in that each zinc chain serves as a secondary construction unit and is further connected into a 2D framework through modbc ligands, and the framework comprises the tetranuclear homometallic Zn₄(modbc)₄ units with different Zn1⋯Zn1 distances of 10.741 and 15.187 Å (Fig. 1c), which is larger than Cd1⋯Cd1 distances of 10.434 and 14.756 Å.³⁸

Fig. 1 (a) Coordination environment of Zn²⁺ in Zn-CP. (b) Coordination mode of modbc molecule. (c) Two-dimensional reticulated structure of Zn-CP along the c axis; red, O; gray, C; green, Zn1; blue, N. All H atoms are omitted for clarity.

3.2 Property characterization

TGA analysis of Zn-CP was measured to evaluate its thermal stability at ≤500 °C, as shown in Fig. S2.† PXRD confirmed that Zn-CP has phase purity and excellent stability in H₂O, common organic solvents and metal salt aqueous system (Fig. S3†). In addition, the corresponding PXRD after the sample was heated at different temperatures for 8 h showed that Zn-CP skeleton still had excellent chemical stability at ≤180 °C, as shown in Fig. S4.† High stability of Zn-CP may be due to the synergy between N/O atoms and Zn²⁺ according to Pearson's hard/soft acid–base principle,^39,40 or relatively high density 2D framework, and the synergy effect of these factors.⁴¹ Obviously, the Zn-CP framework with excellent chemical stability provides the possibility for its practical application as an ion sensor.

Then, Zn-CP was soaked in Tb(NO₃)₃ aqueous solution to obtain Tb³⁺@Zn-CP. The crystal integrity of Zn-CP remained unchanged after doping Tb³⁺, which was confirmed by PXRD (Fig. S5†). PXRD of Tb³⁺@Zn-CP after soaking in FeCl₃, RuCl₃, KMnO₄, K₂CrO₄ and K₂Cr₂O₇ solutions for 12 h, respectively, also shows that it has excellent crystal integrity and chemical stability. In order to obtain the amount of Tb³⁺ doped in Zn-CP, ICP analysis on Tb³⁺@Zn-CP was performed (Table S3†). The result shows that the ratio of Zn²⁺ and Tb³⁺ is approximately 80 : 1. Tb³⁺@Zn-CP and Zn-CP were further analyzed by X-ray photoelectron spectroscopy (XPS), as shown in Fig. S6.† After being treated with Tb³⁺, three new peaks corresponding to Tb 3d_3/2, Tb 3d_5/2, Tb 4d appeared at 1277.5, 1243.0 and 153.0 eV, thus the existence of Tb³⁺ in the composite material can be determined.⁴²

3.3 Fluorescence properties

Considering that Zn-CP composed of d¹⁰ ions and aromatic ligand may be a promising luminescent material,⁴³ the fluorescence properties of Zn-CP were investigated in 5 mM Tris–HCl/NaCl buffer (pH 7.0) at room temperature. Under 312 and 330 nm excitation, the luminescence of Zn-CP and Tb³⁺@Zn-CP in aqueous solution show the intense emission centered at 395 and 390 nm, respectively, while the luminescence of modbc exhibits a similar emission centered at 475 nm at 415 nm excitation. The fluorescence emission of Zn-CP and Tb³⁺@Zn-CP also show good stability within 12 h (Fig. S7†). In addition, the fluorescence stability of Zn-CP and Tb³⁺@Zn-CP dispersed in different pH 1.0–13.0 solution were performed, as shown in Fig. S8.† Obviously, Tb³⁺@Zn-CP framework with excellent chemical stability than Zn-CP offers the possibility for its practical application based on the fact that industrial effluent and polluted rivers are usually acidic or alkaline.⁴⁴

3.4 Detection of ions

To detect water pollution, we explored the potential detection of Zn-CP to various ions. 1 mM aqueous solution of Ag⁺, Pb²⁺, Mn²⁺, Hg²⁺, Ni²⁺, Ca²⁺, Cd²⁺, Zn²⁺, Co²⁺, Cu²⁺, Fe³⁺, Fe²⁺, Ru³⁺, I⁻, C₂O₄²⁻, CrO₄²⁻, Cr₂O₇²⁻, SO₄²⁻, CO₃²⁻, HCO₃⁻, PO₄³⁻ and MnO₄⁻ were prepared, and we investigated their effects on the fluorescence intensity of Zn-CP. All experimental cations have a remarkable effect on the luminescence intensity of Zn-CP (Fig. S9†), and all experimental inions show negligible effect on the luminescence intensity of Zn-CP (Fig. S10†) indicating that Zn-CP shows no selective sensing ability toward different both cations and anions.

To improve the selectivity of Zn-CP, Tb³⁺@Zn-CP was obtained via PSM. A series of luminescence experiments were carried out by the addition of different metal ions to Tb³⁺@Zn-CP. Other metal ions have a negligible effect on the luminescence intensity of Tb³⁺@Zn-CP except Ru³⁺ and Fe³⁺, indicating that Tb³⁺@Zn-CP can detect Ru³⁺ and Fe³⁺ selectively among coexisting ions (Fig. 2). Furthermore, the titration of Tb³⁺@Zn-CP (10 μM) with Ru³⁺ or Fe³⁺ (0.0–90 μM) were performed, as shown in Fig. S11 and S12.† With the gradual addition of Ru³⁺ or Fe³⁺, the fluorescence of Tb³⁺@Zn-CP solution was obviously quenched.

Fig. 2 Luminous intensity of Tb³⁺@Zn-CP upon different ions (Ru³⁺, Ag⁺, Cd²⁺, Zn²⁺, Pb²⁺, Ca²⁺, Mn²⁺, Cu²⁺, Co²⁺, Ni²⁺, Fe²⁺, Hg²⁺ and Fe³⁺) in 5 mM Tris–HCl/NaCl buffer (pH 7.0). [Tb³⁺@Zn-CP] = 10 μM and [cations] = 50 μM. λ_ex: 330 nm, λ_F: 390 nm, slit width: 4 nm.

Compared with Ru³⁺, the influence of Fe³⁺ on the luminescence intensity of Tb³⁺@Zn-CP is relatively smaller (Fig. 3a and b). To better analyze, the quenching sensitivity was quantified by the Stern–Volmer equation: I₀/I = 1 + K_sv[Q].⁴⁵ At low concentrations (0–20 μM), a good linearity plot was obtained with correlation coefficient of 0.997 for Fe³⁺ and 0.994 for Ru³⁺, respectively. Also, the quenching constant (K_sv) were determined to be 7.2 ± 0.56 × 10³ M⁻¹ for Fe³⁺ and 1.72 ± 0.19 × 10⁴ M⁻¹ for Ru³⁺, respectively. However, the Stern–Volmer plot deviated from the straight line at higher concentrations due to self-absorption or energy transfer processes.^46,47 In addition, the LODs based on Tb³⁺@Zn-CP were also obtained to be 0.57 ± 0.06 μM for Fe³⁺ and 0.27 ± 0.01 μM for Ru³⁺, respectively (Fig. S13†). The calculated LOD value is far below MCL for Fe³⁺ in drinking water as required by U.S. EPA standard (5.36 μM).⁴⁸ The performance of Tb³⁺@Zn-CP composite for sensing Ru³⁺ and Fe³⁺ is comparable with some recently reported MOF composites (Table S4†).

Fig. 3 (a) Relative fluorescence intensity and (b) the corresponding quenching efficiency of Tb³⁺@Zn-CP vs. different Ru³⁺ or Fe³⁺ ion concentrations. λ_ex: 330 nm, λ_F: 390 nm for Tb³⁺@Zn-CP, slit width: 4 nm. [Tb³⁺@Zn-CP] = 10 μM.

To corroborate the practical applicability of Tb³⁺@Zn-CP as a sensor for Fe³⁺, we performed a series of anti-jamming experiments (Fig. 4 and S14†). The coexisting ions above mentioned showed negligible influence on the detection of Fe³⁺. The results confirmed that Tb³⁺@Zn-CP as a sensor can detect Fe³⁺ even in the presence of such potential interfering ions. Similarly, Tb³⁺@Zn-CP as a sensor can detect Ru³⁺ even in the presence of such potentially interfering ions, as shown in Fig. 4 and S15.† Satisfyingly, the fluorescence response is quite quick, and it is controlled in ≤10 s for detecting both Ru³⁺ and Fe³⁺ (Fig. S16†).

Fig. 4 Comparison of the luminescence intensity of Tb³⁺@Zn-CP: after addition of mixed ions (Ag⁺, Cd²⁺, Zn²⁺, Pb²⁺, Cr³⁺, Cu²⁺, Co²⁺, Ni²⁺, Mg²⁺; total concentration of mixed metal ions is 45 μM), and followed by addition of Fe³⁺ or Ru³⁺ ([Fe³⁺] = 5 or 45 μM); ([Ru³⁺] = 5 or 45 μM). λ_ex: 330 nm, λ_F: 390 nm, slit width: 4 nm.

To explore the potential selectivity of Tb³⁺@Zn-CP for various anions, potassium salts of I⁻, C₂O₄²⁻, CrO₄²⁻, Cr₂O₇²⁻, SO₄²⁻, CO₃²⁻, HCO₃⁻, PO₄³⁻ and MnO₄⁻ were added to an aqueous of Tb³⁺@Zn-CP (10 μM) and the fluorescence of Tb³⁺@Zn-CP was observed (Fig. 5). Notably, the fluorescence intensity of Tb³⁺@Zn-CP shows a irregular quenching with addition of CrO₄²⁻, Cr₂O₇²⁻ and MnO₄⁻, respectively, however other anions show a negligible influence, which indicates that Tb³⁺@Zn-CP can detect CrO₄²⁻, Cr₂O₇²⁻ and MnO₄⁻ selectively.

Fig. 5 Luminous intensity of Tb³⁺@Zn-CP upon different ions (I⁻, C₂O₄²⁻, CrO₄²⁻, Cr₂O₇²⁻, SO₄²⁻, CO₃²⁻, HCO₃⁻, PO₄³⁻ and MnO₄⁻) in 5 mM Tris–HCl/NaCl buffer (pH 7.0) except CrO₄²⁻ (pH 8.0), Cr₂O₇²⁻ (pH 6.0). [Tb³⁺@Zn-CP] = 10 μM and [ions] = 40 μM. λ_ex: 330 nm, λ_F: 390 nm, slit width: 4 nm.

To investigate the fluorescence sensitivity ability of Tb³⁺@Zn-CP for detecting CrO₄²⁻, Cr₂O₇²⁻ and MnO₄⁻, the corresponding luminescence spectra of Tb³⁺@Zn-CP (10 μM) were recorded by ion concentration titration (0.0–70 μM), as shown in Fig. S17–S19.† With the gradual addition of CrO₄²⁻, Cr₂O₇²⁻ and MnO₄⁻, respectively, the fluorescence of Tb³⁺@Zn-CP were significant quenched. The influence order of three ions on luminous intensity of Tb³⁺@Zn-CP is as follows: CrO₄²⁻ > MnO₄⁻ > Cr₂O₇²⁻, as shown in Fig. 6a and b. Quantitatively, the quenching sensitivity was quantified by the Stern–Volmer equation.⁴⁵ At low concentrations for CrO₄²⁻ (0–20 μM) and for MnO₄⁻ (0–30 μM), a good linearity plot was obtained with correlation coefficient of ∼0.99. Also, the quenching constant (K_sv) were determined to be 1.18 ± 0.16 × 10⁵ M⁻¹ for CrO₄²⁻ and 7.44 ± 0.77 × 10⁴ M⁻¹ for MnO₄⁻, respectively. However, the Stern–Volmer plot deviated from the straight line at higher concentrations due to self-absorption or energy transfer processes.^46,47 In addition, the LODs based on Tb³⁺@Zn-CP were also obtained to be 0.10 ± 0.02 μM for CrO₄²⁻ and 0.15 ± 0.03 μM for MnO₄⁻, respectively (Fig. S20a and b†). However, Fig. S20c† show that the Stern–Volmer plot for Cr₂O₇²⁻ presents a good linear relationship with a correlation coefficient of 0.995. The quenching constant (K_sv) and the value of the detection limit (3σ/k) were evaluated to be 2.49 ± 0.1 × 10⁴ M⁻¹ and 0.43 ± 0.04 μM (S/N = 3), respectively, which also lies well below several MOF fluorescent sensors for detecting CrO₄²⁻, Cr₂O₇²⁻ and MnO₄⁻ for a specific comparison (Table S5†).

Fig. 6 (a) Relative fluorescence intensity and (b) the corresponding quenching efficiency of Tb³⁺@Zn-CP vs. different CrO₄²⁻ (pH 8.0), Cr₂O₇²⁻ (pH 6.0) and MnO₄⁻ (pH 7.0) ion concentrations in 5 mM Tris–HCl/NaCl buffer. λ_ex: 330 nm, λ_F: 390 nm for Tb³⁺@Zn-CP, slit width: 4 nm. [Tb³⁺@Zn-CP] = 10 μM.

Furthermore, we measured the fluorescence response of Tb³⁺@Zn-CP to CrO₄²⁻, Cr₂O₇²⁻ and MnO₄⁻, respectively, in the presence of other anions, as shown in Fig. 7 and S21–23.† The CrO₄²⁻, Cr₂O₇²⁻ and MnO₄⁻-dependent fluorescence intensity of Tb³⁺@Zn-CP was not affected by the presence of coexisting anions such as I⁻, C₂O₄²⁻, SO₄²⁻, CO₃²⁻, HCO₃⁻ and PO₄³⁻. Therefore, Tb³⁺@Zn-CP has desirable anti-interference ability for detecting CrO₄²⁻, Cr₂O₇²⁻ and MnO₄⁻. Satisfyingly, the response time is quite short, and it can control in ∼10 s (Fig. S24†).

Fig. 7 Comparison of the luminescence intensity of Tb³⁺@Zn-CP: after addition of mixed ions (I⁻, C₂O₄²⁻, SO₄²⁻, CO₃²⁻, HCO₃⁻ and PO₄³⁻; total concentration of mixed ions is 30 μM), and followed by addition of CrO₄²⁻, Cr₂O₇²⁻ and MnO₄⁻ (5 or 30 μM). λ_ex: 330 nm, λ_F: 390 nm, slit width: 4 nm.

3.5 The underlying mechanism of luminescence quenching

The crystal integrity of Tb³⁺@Zn-CP remained unchanged after soaking in aqueous solutions of FeCl₃, RuCl₃, KMnO₄, K₂CrO₄ and K₂Cr₂O₇, respectively, which was confirmed by PXRD, indicating that the luminescence quenching was not caused by the collapse of the main framework structure (Fig. S5†).⁴⁹ The possible sensing mechanism of fluorescence quenching by ions were determined by further analysis.

For MnO₄⁻ ion detection, the result of ICP analysis show that the ratio of Zn²⁺/Tb³⁺/Mn²⁺ is approximately 108 : 1 : 12 comparing with Zn²⁺/Tb³⁺ (80 : 1) in Tb³⁺@Zn-CP, which ion exchange is involved in the detection of MnO₄⁻ by Tb³⁺@Zn-CP (Table S3†).⁵⁰ In addition, the color of the solid sample after immersion in KMnO₄ aqueous solution was observed to have change in both ultraviolet and ordinary light, which also confirmed MnO₄⁻ was diffused into 2D structure of Tb³⁺@Zn-CP (Fig. S25†). We further measured X-ray photoelectron spectroscopy (XPS) analysis on Tb³⁺@Zn-CP after soaked in KMnO₄, as shown in Fig. S26.† New peak at 642.5 eV appears corresponding to Mn 2p, by which the existence of MnO₄⁻ in the composites can be ascertained. The O 1s spectrum of Tb³⁺@Zn-CP + MnO₄⁻ can be fitted into two peaks at 530.0 eV and 531.5 eV, which correspond to carboxyl group oxygen atoms and –OH species, respectively (Fig. S27†). The result shows that carboxyl oxygen atoms of Zn-CP and hydroxyl oxygen atoms of H₂O participate in the formation of hydrogen bonds of MnO₄⁻.²⁸ In order to examine the sensing behavior of Tb³⁺@Zn-CP to MnO₄⁻, titration experiment of Tb³⁺@Zn-CP (10 μM) with MnO₄⁻ was carried out (Fig. S28†). On gradual addition of MnO₄⁻ (0–300 μM), the absence intensity of Tb³⁺@Zn-CP solution is obviously increasing. MnO₄⁻ solution shows obvious absorption in the range 300–400 nm, as shown in Fig. S29.† This means that the excitation wavelength of Tb³⁺@Zn-CP is totally overlapped by the absorption spectra of MnO₄⁻, which demonstrates that transfer of the excitation energy to the emissive center is effectively suppressed by MnO₄⁻ ions. In a word, luminescence quenching of Tb³⁺@Zn-CP by MnO₄⁻ originates from three possible approaches: hydrogen bond interaction between MnO₄⁻ and the frameworks, ion exchange, and competitive absorption of excitation energy by MnO₄⁻.^49,50

For CrO₄²⁻ and Cr₂O₇²⁻ anion detection, we performed XPS analysis on Tb³⁺@Zn-CP after soaked in K₂CrO₄ and K₂Cr₂O₇, respectively, as shown in Fig. S30 and S31.† New peak at 586.0 eV appears corresponding to Cr 2p, by which the existence of Cr₂O₇²⁻, CrO₄²⁻ in the composites can be ascertained. To examine the sensing behavior of Tb³⁺@Zn-CP to CrO₄²⁻ and Cr₂O₇²⁻, titration experiment of Tb³⁺@Zn-CP (10 μM) with CrO₄²⁻ and Cr₂O₇²⁻ were carried out. On gradual addition of Cr₂O₇²⁻ (0–300 μM), the absence intensity of Tb³⁺@Zn-CP solution is obviously increasing (Fig. S32†). With gradual addition of CrO₄²⁻ (0–60 μM), new absorption bands centered at 369 nm appeared with increasing intensity (Fig. S33†). From Fig. S29,† we can see that CrO₄²⁻ and Cr₂O₇²⁻ solution show obvious absorption in the range 300–400 nm. The luminescence quenching of Tb³⁺@Zn-CP originates from the competitive absorption of excitation energy of CrO₄²⁻ and Cr₂O₇²⁻.⁵¹

For the Fe³⁺ detection process, the mechanism involving an Fe³⁺–framework interaction, or Fe³⁺–Zn²⁺ exchange cannot be excluded. In order to prove our hypothesis, ICP analysis of Tb³⁺@Zn-CP after doping into Fe³⁺ was performed (Table S3†). The ratio of Zn²⁺/Tb³⁺/Fe³⁺ is approximately 295 : 1 : 12 comparing with Zn²⁺/Tb³⁺ (80 : 1) in Tb³⁺@Zn-CP, which ion exchange is involved in the detection of Fe³⁺ by Tb³⁺@Zn-CP. Further, XPS analysis on Tb³⁺@Zn-CP after soaked in FeCl₃ was performed, as shown in Fig. S34.† New peak at 711.3 eV appears corresponding to Fe 2p, by which the existence of Fe³⁺ ions in the composites can be ascertained. To examine the sensing behavior of Tb³⁺@Zn-CP to Fe³⁺, titration experiment of Tb³⁺@Zn-CP (10 μM) with Fe³⁺ was carried out. On gradual addition of Fe³⁺ (0–60 μM), the absence intensity of Tb³⁺@Zn-CP solution is obviously increasing (Fig. S35†). From Fig. S29,† we can see that Fe³⁺ also show significant absorption in the range 300–400 nm. In short, the fluorescence quenching of Tb³⁺@Zn-CP by Fe³⁺ originates from two possible approaches: ion exchange and from the competitive absorption of excitation energy by Fe³⁺.

For Ru³⁺ ion detection, we investigated XPS analysis on Tb³⁺@Zn-CP after soaked in RuCl₃, as shown in Fig. S36.† New peak at 281.8 eV appears corresponding to Ru 3d, by which the existence of Ru³⁺ in the composites can be ascertained. In addition, the color of the solid sample after immersion in RuCl₃ aqueous solution was observed to have change in ordinary light, which also confirmed Ru³⁺ was diffused into two-dimensional structure of Tb³⁺@Zn-CP (Fig. S25†). To examine the sensing behavior of Tb³⁺@Zn-CP to Ru³⁺, titration experiment of Tb³⁺@Zn-CP (10 μM) with Ru³⁺ was carried out. With gradual addition of Ru³⁺ (0–60 μM), the absence intensity of Tb³⁺@Zn-CP solution is obviously increasing (Fig. S37†). From Fig. S29,† we can see that the solution of Ru³⁺ show obvious absorption in the range 300–400 nm. The luminescence quenching of Tb³⁺@Zn-CP originates from the competitive absorption of excitation energy by Ru³⁺.

Although Tb³⁺@Zn-CP as a luminescent sensor for Ru³⁺, Fe³⁺, CrO₄²⁻, Cr₂O₇²⁻ and MnO₄⁻, the selectivity of it is not good, because these ions might exist simultaneously in environmental samples and complicated samples such as surface water or wastewater. However, compared with Zn-CP, Tb³⁺@Zn-CP can obviously improve the selectivity. The framework of Zn-CP contains uncoordinated nitrogen atoms and oxygen atoms possessing unshared pair electrons, which may coordinate with most metal ions or form hydrogen bonds with anions. Zn-CP is used as a parent coordination compound to encapsulate Tb³⁺ to obtain Tb³⁺@Zn-CP. Therefore, compared with Zn-CP, there is an obvious decrease in the adsorption capacity of Tb³⁺@Zn-CP to metal ions and anions. In addition, compared with Zn-CP, the excitation spectrum of Tb³⁺@Zn-CP also changed (Fig. S7†). Generally speaking, if the excitation spectrum of MOF overlaps with the ultraviolet-visible absorption spectrum of analyte to a certain extent, there is likely to be competitive energy absorption between MOF and analyte.⁵² So the competitive absorption of Ru³⁺, Fe³⁺, CrO₄²⁻, Cr₂O₇²⁻ and MnO₄⁻ is the main factor for Zn-CP fluorescence quenching.

As shown in Fig. S38 and Table S6,† the lifetime is shortened from 3.17 to 0.79 ns after Zn-CP treated with Tb³⁺. However, the lifetimes of Tb³⁺@Zn-CP remain in the presence and absence of Ru³⁺, Fe³⁺, CrO₄²⁻, Cr₂O₇²⁻ and MnO₄⁻, which suggest that there are the static quenching mechanisms in selectively differentiate these ions by Tb³⁺@Zn-CP.⁵³

4. Conclusion

We have successfully synthesized a zinc(II)-containing coordination polymer, namely [Zn(modbc)₂]_n (Zn-CP), using modbc as the linker. Zn-CP exhibited high stability in aqueous solution of metal salts, organic solvents and different temperatures. After encapsulation of Tb³⁺ ions in Zn-CP, the as-obtained fluorescent functionalized Tb³⁺@Zn-CP was obtained. An investigation of sensing properties reveals that Tb³⁺@Zn-CP as a luminescent sensor exhibits instant and selective luminescence quenching properties toward Ru³⁺, Fe³⁺, CrO₄²⁻, Cr₂O₇²⁻ and MnO₄⁻ ions in DMF/H₂O media. It should be mentioned that the two-dimensional luminescent probes for detecting a trace amount ions (μM), well antiinterference performance, and quick response time (∼10 s) are still few reports.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21571141) and Tianjin Natural Science Foundation, China (No. 15JCYBJC20300).

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Footnote

† Electronic supplementary information (ESI) available: Crystallographic data, tables, IR, UV, TGA, PXRD, ICP, XPS, and luminescent spectra. CCDC 1882227. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9ra09541a

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