English

• 1.   Introduction

  Metal-organic frameworks (MOFs) have excellent crystalline, porous, tailorable structures, ultrahigh surface areas, various topologies and fascinating potential applications, including catalysis, ^[1] drug delivery, ^[2] gas storage and separation, ^[3-4] etc. Of course, MOFs have also attracted more and more attention for fluorescent sensing material, especially, the stability, sensitivity and functional selectivity of fluorescent MOFs are the key to promote its application in water.^[5-14] Currently, MOFs can be used as fluorescence probe to detect Fe³⁺, ^[15-19] Fe²⁺, ^[20-21] Al³⁺, ^[22] Cu²⁺, ^[23-24] Mg²⁺, ^[25] Cd²⁺, ^[26-27] Cr³⁺, ^[28-29] Pb²⁺, ^[30] Ce³⁺, ^[31] Hg²⁺, ^[32] etc, And it can also be used to detect anions, such as $ {\rm{PO}}_{\rm{4}}^{3-}$, ^[33-35] $ {\rm{H}}_{\rm{2}}{\rm{PO}}_{\rm{4}}^{\rm{2}-}$, ^[36-37] $ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$, ^[38] $ {\rm{MnO}}_{\rm{4}}^{-}$, ^[39] $ {\rm{CO}}_{\rm{3}}^{2-}$, ^[40] ClO^－/SCN^－, ^[41] $ {\rm{SiF}}_{6}^{2-}, $^[42] adenosine diphosphate (ADP^2－), ^[43] etc. In addition, many MOFs are used as fluorescent sensors to identify samll molecules, such as CO₂, ^[44] H₂O₂/glucose, ^[45] CHCl₃, ^[46] acetone, ^[47-48] antibiotics, ^[49-51] dopamine, ^[52] nicotinamide, ^[53] nitrobenzene, ^[54-55] pentachlorophenol, ^[56] etc. The above MOFs show excellent ability in ions and molecular detection, such as fast response time, high sensitivity, simple operation and low detection limit. However, each MOF can only detect one or two particles as a fluorescent probe.

  Zr⁴⁺ is not one of the essential elements of human body and has no toxic effect on the environment. It is a rare metal and very important atomic energy elements which is widely used in military industry, smelting, atomic energy industry and battery. Among them, Zr-MOF and functional Zr-MOFs are also widely used as fluorescent probes to detect F^－, ^[57] Hg²⁺, ^[58] Bi³⁺, ^[59] H₂S, ^{[60, 61]} picric acid, ^[62] bilirubin, ^[63] etc. Of course, multiple recognition detection for MOFs has also been studied, but relatively few, which is also the development direction of MOFs as fluorescent recognition materials in the future. Therefore, there is an urgent need to develop some new chemosensors for the rapid selective and sensitive detection of metal cations, anions and organic small molecules as the replacement.

  In this work, a new complex, Zr-MOF [L＝2, 2', 2''-([1, 3, 5]-triazine-2, 4, 6-triimino)tribenzoic acid] has been prepared by reaction of L with Zr(Ⅳ) salt in N, N-dimethylformamide (DMF)/H₂O solution (Scheme 1). The ligand L is synthesized by "one pot"' reaction in glacial acetic acid solution, and the complexation ratio of ligand L to Zr⁴⁺ was also determined according to the previous work.^[64] The Zr-MOF was investigated for the fluorescence response to the metal cations, anions and organic small molecules, especially it is reported that Zr-MOF can detect CCl₄ and xylene in DMF solution.

  Scheme 1

  

  Scheme 1.  Synthesis route of Zr-MOF

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  2.   Results and discussion

  2.1   Structural characterization of Zr-MOF

  The FT-IR analysis reveals that the stretching vibration of C＝O shifts from 1680 to 1657 cm^－1, and the stretching vibration reduces, which show that COOH is coordinated with Zr⁴⁺ and exists free COOH in Zr-MOF. The stretching vibration of C—O reduction at 1253 cm^－1 is attributed to the formation of Zr—O bond. The thermogravimetric analyses (TGA) of Zr-MOF were investigated. To further investigate the stability of Zr-MOF, the PXRD patterns of Zr-MOF refluxed in boiling water for 72 h and immersed in acidic and basic aqueous solutions with different pH for 72 h were recorded. It indicated that Zr-MOF has the high water stability and pH stability. It is possibly attributed to the formation of six rings of Zr, N, O atoms and the intramolecular hydrogen bonds.

  2.2   Metal ion detection

  Considering the excellent water stability and fluorescence properties of Zr-MOF, the potential selective sensing of metal cations was investigated. 0.5 mg of freshly prepared samples of Zr-MOF is ground and suspended in 50 mL of DMF solution by ultrasound. Subsequently, it was diluted to 0.001 mg/mL DMF solution. 0.1 mL of different metal chloride aqueous solution (10^－4 mol/L) was add to 2.9 mL diluted solution. The fluorescence properties were recorded and listed in Figure 1a (λ_ex＝275 nm). Specifically, the fluorescence intensity of Zr-MOF at 425 nm decreased slightly when Li⁺, Na⁺, K⁺, Cu⁺, Mg²⁺, Ca²⁺, Hg²⁺ were added, respectively, and decreased obviously when Zn²⁺, Ba²⁺, Mn²⁺, Cu²⁺, Cd²⁺, Fe²⁺, Pb²⁺, Ni²⁺, Co²⁺, Sn²⁺ were added. Most of the metal ions had quenching effect on the Zr-MOF, especially Fe³⁺ ion. Further, the quenching efficiency can be rationalized by the Stern-Volmer (SV) equation, F₀/F＝1+K_SV[M], where F₀ and F are the relative fluorescence intensity before and after adding metal ions, [M] is the molar concentration of the metal ions, and K_SV (L•mol^－1) is the Stern-Volmer quenching constant. The K_SV(Fe³⁺) value was calculated as 9.54×10⁴ L•mol^－1, suggesting a strong quenching effect on fluorescence. The values of K_SV of Zr-MOF with other metal ions were calculated. In addition, the antijamming experiments of Fe³⁺ specifically demonstrated that Zr-MOF could detect Fe³⁺ in the presence of other metal ions (Figure 1b).

  Figure 1

  

  Figure 1.  (a) Fluorescence intensity of different metal ions in DMF of Zr-MOF (Zr-MOF＝2.9 mL, 0.001 mg/mL), and (b) effect of the disturbing metal ions on fluorescence recognition of Zr-MOF to Fe³⁺

  [metal ions]＝0.1 mL, 1×10^－4 mol/L, λ_ex＝275 nm

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  The experiment of Fe³⁺ concentration showed that the fluorescence intensity of Zr-MOF decreased with the increase of Fe³⁺ concentration (Figure 2a). According to the fluorescence intensity of Zr-MOF, different concentrations of Fe³⁺ and Benesi-Hildebrand equation, the detection limit of Fe³⁺ in Zr-MOF/DMF solution was calculated to be 3.78×10^－8 mol/L (Figure 2b).

  Figure 2

  

  Figure 2.  (a) Fluorescence change of Zr-MOF (DMF) in different concentrations of Fe³⁺ ion (Zr-MOF＝2.9 mL, 0.001 mg/mL), and (b) detection limit of Fe³⁺

  [Fe³⁺]＝0.1 mL, 1×10^－6~1×10^－5 mol/L, λ_ex＝275 nm

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  2.3   Anion detection

  The good recognition performance and selectivity of Zr-MOF were also investigated for anions. 0.1 mL of different anionic sodium salts (10^－4 mol/L) were added to the DMF solution of 2.9 mL of diluent Zr-MOF, except KMnO₄. The fluorescence properties were recorded and listed in Figure 3 (λ_ex＝275 nm). There are three changes in the fluorescence intensity of Zr-MOF to anions. The fluorescence intensity of Zr-MOF at 425 nm increased obviously when $ {\rm{PO}}_{\rm{4}}^{3-}$, $ {\rm{HPO}}_{\rm{4}}^{\rm{2}-}$, $ {\rm{H}}_{\rm{2}}{\rm{PO}}_{\rm{4}}^{-}$, $ {\rm{P}}_{2}{\rm{O}}_{7}^{2-}$, S^2－, $ {\rm{SO}}_{3}^{2-}$, $ {\rm{SO}}_{\rm{4}}^{2-}$, $ {\rm{S}}_{2}{\rm{O}}_{3}^{2-}$, $ {\rm{S}}_{2}{\rm{O}}_{5}^{2-}$, F^－, $ {\rm{CO}}_{\rm{3}}^{2-}$, $ {\rm{HCO}}_{\rm{3}}^{-}$, $ {\rm{C}}_{4}{\rm{H}}_{4}{\rm{O}}_{6}^{-}$, $ {\rm{C}}_{\rm{2}}{\rm{O}}_{2}^{2-}$, CH₃COO^－, $ {\rm{SiO}}_{3}^{2-}$, $ {\rm{SeO}}_{\rm{4}}^{2-}$, $ {\rm{MoO}}_{4}^{2-}$, $ \rm{{B}_{4}{O}_{7}^{2-}}$, $ \rm{B{O}_{2}^{-}}$, $ \rm{Sn{O}_{3}^{-}}$, $ {\rm{WO}}_{\rm{4}}^{2-}$ were added, respectively. The fluorescence intensity changed slightly when Cl^－, Br^－, I^－, $ \rm{I{O}}_{3}^{-}$, $ {\rm{NO}}_{\rm{2}}^{-}$, $ {\rm{NO}}_{\rm{3}}^{-}$, $ {\rm{BiO}}_{\rm{3}}^{-}$, SCN^－, $ {\rm{CrO}}_{\rm{4}}^{-}$, HCOO^－, C₆H₅COO^－ were added, respectively. Further, the quenching efficiency can be rationalized by the Stern-Volmer (SV) equation. The K_SV ($ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$, $ {\rm{MnO}}_{\rm{4}}^{-}$) values were calculated as 1.75×10⁴, 784.9 L•mol^－1, respectively. The K_SV values of Zr-MOF with other anions were calculated.

  Figure 3

  

  Figure 3.  Fluorescence intensity of different anions in the DMF of Zr-MOF

  Zr-MOF＝2.9 mL, 0.001 mg/mL, [anions]＝0.1 mL, 1×10^－4 mol/L, λ_ex＝275 nm

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  The experiment of $ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$ concentration showed that the fluorescence intensity of Zr-MOF decreased with increase of the concentration of $ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$ (Figure 4a). According to the fluorescence intensity of Zr-MOFs, different concentration of $ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$ and Benesi-Hildebrand equation, the detection limit of $ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$in Zr-MOF solution was calculated to be 9.12×10^－7 mol/L (Figure 4b). Meantime the interference of fluorescence intensity could be ignored when $ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$ coexisted with fourteen anions $ {\rm{CrO}}_{\rm{4}}^{-}$, Cl^－, Br^－, I^－, $ \rm{I{O}}_{3}^{-}$, $ {\rm{NO}}_{\rm{2}}^{-}$, $ {\rm{NO}}_{\rm{3}}^{-}$, $ {\rm{BiO}}_{\rm{3}}^{-}$, $ \text{BO}_{2}^{-}$, SCN^－, HCOO^－, CH₃COO^－, C₆H₅COO^－, $ {\rm{C}}_{4}{\rm{H}}_{4}{\rm{O}}_{6}^{-}$, respectively (Figure 5a). On the contrary, the fluorescence interference increased significantly when $ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$ coexisted with nineteen anions$ {\rm{PO}}_{\rm{4}}^{3-}$, $ {\rm{HPO}}_{\rm{4}}^{\rm{2}-}$, $ {\rm{H}}_{\rm{2}}{\rm{PO}}_{\rm{4}}^{-}$, $ {\rm{P}}_{2}{\rm{O}}_{7}^{2-}$, S^2－, $ {\rm{SO}}_{3}^{2-}$, $ {\rm{SO}}_{\rm{4}}^{2-}$, $ {\rm{S}}_{2}{\rm{O}}_{3}^{2-}$, $ {\rm{S}}_{2}{\rm{O}}_{5}^{2-}$, F^－, $ {\rm{CO}}_{\rm{3}}^{2-}$, $ {\rm{HCO}}_{\rm{3}}^{-}$, $ {\rm{B}}_{4}{\rm{O}}_{7}^{2-}$, $ {\rm{SiO}}_{3}^{2-}$, $ {\rm{SeO}}_{\rm{4}}^{2-}$, $ {\rm{MoO}}_{4}^{2-}$, $ \rm{Sn}{O}_{3}^{-}$, $ {\rm{WO}}_{\rm{4}}^{2-}$, $ {\rm{C}}_{\rm{2}}{\rm{O}}_{\rm{4}}^{2-}$, respectively. Obviously, the fluorescence decreased obviously when $ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$ and $ {\rm{MnO}}_{\rm{4}}^{-}$ were added to the Zr-MOF at the same time (Figures 5a, 5b). However, the anti-interference ability of $ {\rm{MnO}}_{\rm{4}}^{-}$ was poor when $ {\rm{MnO}}_{\rm{4}}^{-}$ coexisted with other anions (Figure 5b).

  Figure 4

  

  Figure 4.  (a) Fluorescence change of Zr-MOF (DMF) in different concentrations of $ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$ion (Zr-MOF＝2.9 mL, 0.001 mg/ mL), and (b) detection limit of $ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$

  [$ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$]＝0.1 mL, 1×10^－5~1×10^－4 mol/L, λ_ex＝275 nm

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  Figure 5

  

  Figure 5.  (a) Effect of the disturbing anions on fluorescence recognition of Zr-MOF to $ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$, and (b) effect of the disturbing metal ions on fluorescence recognition of Zr-MOF to $ {\rm{MnO}}_{\rm{4}}^{-}$

  Zr-MOF＝2.9 mL, 0.001 mg/mL, [anions]＝0.1 mL, 1×10^－4 mol/L, λ_ex＝275 nm

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  It should be noted that for anions formed by the same elements, the fluorescence of the Zr-MOF also shows a certain change rule (Figure 6). Anions containing P and S elements generally enhance the fluorescence of Zr-MOF. The order of strength is $ {\rm{HPO}}_{\rm{4}}^{\rm{2}-}$ > $ {\rm{PO}}_{\rm{4}}^{3-}$ > $ {\rm{H}}_{\rm{2}}{\rm{PO}}_{\rm{4}}^{-}$ > $ {\rm{P}}_{2}{\rm{O}}_{7}^{2-}$ > H₂O, S^2－ > $ {\rm{S}}_{2}{\rm{O}}_{3}^{2-}$ > $ {\rm{SO}}_{3}^{2-}$ > $ {\rm{S}}_{2}{\rm{O}}_{5}^{2-}$ > H₂O (Figures 6a and 6b). Only F^－ can enhance the fluorescence of the Zr-MOF, while other halogen anions have little effect (Figure 6c). In organic carboxylic acids, $ {\rm{C}}_{4}{\rm{H}}_{4}{\rm{O}}_{6}^{-}$ (malic acid) > $ {\rm{C}}_{\rm{2}}{\rm{O}}_{\rm{4}}^{2-}$ (oxalic acid) > CH₃COO^－ > H₂O≈HCO- O^－≈C₆H₅COO^－ (Figure 6d). It is worth mentioning that F^－ has a significant fluorescence enhancement effect on Zr-MOF in halide ions, and it should be used as a fluorescence probe to detect F^－ in halide ions.

  Figure 6

  

  Figure 6.  (a) Change of fluorescence intensity of Zr-MOF with P-containing anions, and (b) the change of fluorescence intensity of Zr-MOF with S-containing anions, (c) the change of fluorescence intensity of Zr-MOF with halogens, and (d) the change of fluorescence intensity of Zr-MOF with organic acid ions

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  2.4   Organic solvent detection

  The fluorescent recognition ability of Zr-MOF to organic solvents has also been studied. 0.1 mL of different organic solvents were add to DMF solution of 2.9 mL diluent Zr-MOF. The fluorescence properties were recorded and listed in Figure 7a (λ_ex＝275 nm). Compared with DMF, the fluorescences of the Zr-MOF at 425 nm have enhancement effects in methanol, ethanol, isopropanol and DMSO, respectively. The fluorescence intensities of the Zr-MOF were almost the same in acetonitrile, cyclohexane and CH₂Cl₂, respectively. The fluorescence intensity of the Zr-MOF decreased in ethyl acetate, chloroform, benzene, 1, 4-dioxane and THF, respectively. The quenching effect of Zr-MOF is very significant in acetone, CCl₄ and xylene, respectively, especially acetone. Further, the quenching efficiency can be rationalized by Stern-Volmer (SV) equation, F₀/F＝1+K_SV[V], where F₀ and F are the relative fluorescence intensity before and after adding organic solvents, [V] is the molar volume of the organic solvents, and K_SV (V^－1) is the Stern-Volmer quenching constant. The K_SV(acetone, CCl₄ and xylene) values were calculated as 6534.3, 4325.7, 4025.1 mL^－1, respectively. The K_SV values of Zr-MOF with other organic solvents were calculated. The anti-interference experiments showed that the fluorescence intensity of Zr-MOF containing acetone, CCl₄ and xylene remained unchanged when other organic solvents were added, respectively (Figures 7 b~7d).

  Figure 7

  

  Figure 7.  (a) Fluorescence intensity of different organic solvents in the DMF of Zr-MOF, (b) the effect of the disturbing different organic solvents on fluorescence recognition of Zr-MOF to acetone, (c) the effect of the disturbing different organic solvents on fluorescence recognition of Zr-MOF to CCl₄, and (d) the effect of the disturbing different organic solvents on fluorescence recognition of Zr-MOF to xylene

  Zr-MOF＝2.9 mL, 0.001 mg/mL, [organic solvents]＝0.1 mL, λ_ex＝275 nm.

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  The experiment of acetone, CCl₄ and xylene volumes showed that the fluorescence intensity of Zr-MOF decreased with the increase of the volumes of acetone, CCl₄ and xylene, respectively (Figures 8a, 9a and 10a). According to the fluorescence intensity of Zr-MOF, different volumes of acetone, CCl₄, xylene, and Benesi-Hildebrand equation, the detection limit of acetone, CCl₄, xylene in Zr-MOF solution were calculated to be 1.09×10^－6, 2.73×10^－5, 3.17×10^－5 mL, respectively (Figures 8b, 9b, and 10b).

  Figure 8

  

  Figure 8.  (a) Fuorescence change of Zr-MOF (DMF) in different volumes of acetone (Zr-MOF＝2.9 mL, 0.001 mg/mL), and (b) detection limit of acetone

  V(acetone)＝1×10^－5~1×10^－4 mL, λ_ex＝275 nm

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  Figure 9

  

  Figure 9.  (a) Fluorescence change of Zr-MOF (DMF) in different volumes of CCl₄ (Zr-MOF＝2.9 mL, 0.001 mg/mL), and (b) detection limit of acetone

  V(CCl₄)＝1×10^－4~1×10^－3 mL, λ_ex＝275 nm

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  Figure 10

  

  Figure 10.  (a) Fluorescence change of Zr-MOF (DMF) in different volumes of xylene (Zr-MOF＝2.9 mL, 0.001 mg/mL), and (b) detection limit of acetone

  V(xylene)＝1×10^－4~1×10^－3 mL, λ_ex＝275 nm

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  2.5   Effect of pH on fluorescence intensity of Zr-MOF

  The effect of acidity and basicity on the fluorescence of Zr-MOF has also been investigated. 0.1 mL of different concentrations of HCl and NaOH were added to the DMF solution of 2.9 mL of diluent Zr-MOF. The fluorescence properties were recorded and listed in Figure 11 (λ_ex＝275 nm). The fluorescence of Zr-MOF shifted blue (from 433 to 383 nm) when pH was 0~3 (Figure 11a). The fluorescence intensity of the Zr-MOF increased with the concentration of HCl and NaOH. It also showed a symmetrical parabola structure at pH from 0 to 14 (Figure 11b). This is mainly attributed to the combination of H⁺ with NH or the combination of OH^－ with COOH when H⁺ and OH^－ concentrations increase, which makes the rigid framework of Zr-MOF more stable and fluorescence enhanced. The results show that the concentration of H⁺ and OH^－ can affect the fluorescence intensity of Zr-MOF. Therefore, the acidity and basicity of the solution must be considered when Zr-MOF is used as a fluorescence sensor to recognize ions and molecules in the solution. The results showed that the accuracy of the test results was good when the pH of the solution is in the range of 4~10.

  Figure 11

  

  Figure 11.  (a) Fluorescence change of Zr-MOF (DMF) in different pH (Zr-MOF＝2.9 mL, 0.001 mg/mL, pH＝1~14, λ_ex＝275 nm), and (b) effect of pH value for fluorescence properties of Zr-MOF

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  2.6   Quenching mechanism of Zr-MOF fluorescence sensor

  Further experiments were conducted to explore the mechanism of fluorescence quenching. Powder X-ray diffraction (PXRD) spectra of Zr-MOF after sensing each analyte have good spectral consistency with the original of Zr-MOF, which excludes the reason for fluorescence quenching caused by structural collapse. Moreover, The competition between absorption and emission spectra is also one of the causes of fluorescence quenching.^[65] There is partial overlap between the absorption spectrum of each analyte (including Fe³⁺, $ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$, acetone, CCl₄, xylene and Zr-MOF) and the emission spectrum of Zr-MOF. Therefore, there is clear evidence for competitive adsorption between analyte and Zr-MOF. In addition, it shows a larger overlap of the absorption spectra of Fe³⁺, $ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$with the emission spectra of Zr-MOF compared to acetone, CCl₄ and xylene, which may result in more sensitivity towards Fe³⁺, $ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$than acetone, CCl₄ and xylene for Zr-MOF. In addition, for Fe³⁺, it is also possible that the electron transfers from Zr⁴⁺ to Fe³⁺ in the Zr-MOF framework, which results in the decrease of the density of Zr-MOF electron cloud and the fluorescence quenching. Interestingly, only $ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$and $ {\rm{MnO}}_{\rm{4}}^{-}$ have strong quenching effect on the fluorescence of Zr-MOF. On the contrary, the previous study of our group found that $ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$had a significant fluorescence enhancement effect on ligand L. This may be also due to the electron transfer to ligand L via O—Cr—O or O—Cr—N, resulting in enhanced fluorescence. In contrast, in Zr-MOF, electrons may be transferred via O—Zr—O—Cr—O or N—Zr—O—Cr—O, resulting in the absence of electrons in Zr-MOF. For solvents, this may be also attributed to the interaction of solvents acetone, CCl₄, and xylene with Zr-MOF, respectively, which blocked the energy transfer between Zr⁴⁺ and triazine rings and led to fluorescence quenching.

  2.7   Detection of Fe³⁺ in human urine and water samples

  Tens human urine samples were donated by healthy volunteers. The urine samples were centrifuged at 5000 r/min for 10 min. 0.1 mL of urine was added directly to DMF solution of Zr-MOF without any other treatment before test analysis. The concentrations of Fe³⁺ in urine were calculated by Stern-Volmer (SV) equation and the values of y (y＝1.02613×10⁵x+0.0054) were shown in Table 1. The range of Fe³⁺ in tens urines samples was about 1.20~5.67 μmol•L^－1 by calculation. In a sense, complex can be used as a fluorescence probe to measure the molarity of Fe³⁺ conveniently. Of course, the flow injection ICPMS (inductively coupled plasma mass spectrometry) method can also be used to accurately analyze Fe³⁺ in human urine.

  Table 1

  

  Table 1.  Detection of Fe³⁺ concentration in human urines by fluorescence of Zr-MOF^a

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  Entry	F0	F	[Fe3+]/(μmol•L－1)		Error
  			F0/Fb	yc
  1	772.57	584.77	3.37	2.32	±1.05
  2	772.57	693.16	1.20	0.95	±0.25
  3	772.57	590.45	3.23	2.24	±0.79
  4	772.57	574.63	3.61	2.44	±1.17
  5	772.57	550.78	4.22	2.75	±1.47
  6	772.57	501.24	5.67	3.37	±2.30
  7	772.57	645.25	1.90	1.55	±0.35
  8	772.57	650.98	1.96	1.48	±0.48
  9	772.57	569.75	3.73	2.51	±1.22
  10	772.57	615.56	2.67	1.93	±0.74
  a Zr-MOF/DMF＝2.9 mL, 0.001 mg/mL; sample＝0.1 mL; λex＝275 nm; bF0/F＝1+KSV[Fe3+]; c y＝1.02613—105x+0.0054.

  Ten mineral waters were purchased from supermarkets. 0.1 mL of water samples were added directly to DMF solution of Zr-MOF without any other treatment before test analysis. The concentrations of Fe³⁺ in urine were calculated by Stern-Volmer (SV) equation and the values of y (y＝1.02613×10⁵x+0.0054) were shown in Table 2. It should be mentioned that the Environmental Protection Ministry (P. R. China) established the standard (5.4 μmol• L^－1) for the presence of Fe³⁺ in drinking H₂O. The range of Fe³⁺ in tens urines samples was about 1.02~10.91 μmol• L^－1 by calculation. In a sense, Zr-MOF can be used as a fluorescence probe to measure the molarity of Fe³⁺ in water samples conveniently.

  Table 2

  

  Table 2.  Detection of Fe³⁺ concentration in water samples by fluorescence of Zr-MOF^a

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  Entry	F0	F	[Fe3+]/(μmol•L－1)		Error
  			F0/Fb	yc
  1	815.93	570.07	4.52	2.88	±1.64
  2	815.93	399.74	10.91	4.92	±5.99
  3	815.93	692.14	1.87	1.43	±0.44
  4	815.93	653.98	2.60	1.88	±0.72
  5	815.93	726.91	1.28	1.02	±3.26
  6	815.93	525.49	5.79	3.42	±2.37
  7	815.93	558.47	4.83	3.02	±1.81
  8	815.93	725.21	1.31	1.03	±0.28
  9	815.93	653.84	2.60	1.88	±0.72
  10	815.93	572.65	4.45	2.85	±1.60
  a Zr-MOF/DMF＝2.9 mL, 0.001 mg/mL; sample＝0.1 mL; λex＝275 nm. b F0/F＝1+KSV[Fe3+]. c y＝1.02613×105x+0.0054.

  2.8   Simulated detection of acetone, CCl₄ and xylene

  It is necessary to detect acetone, CCl₄ and xylene in mixed organic solvents by using Zr-MOF fluorescence. 0.1 mL of solutions of methanol, ethanol, isopropanol, DMSO, acetonitrile, cyclohexane, CH₂Cl₂, CH₃Cl, ethyl acetate, benzol, Doix, THF were added respectively to 0.1 mL of acetone solution, and the mixture was diluted with DMF to 10 nL acetone per 0.1 mL mixture. 0.1 mL of solutions of methanol, ethanol, isopropanol, DMSO, acetonitrile, cyclohexane, CH₂Cl₂, CH₃Cl, ethyl acetate, benzol, Doix, THF were added respectively to 0.5 mL of CCl₄ solution, and the mixture was diluted with DMF to 50 nL of CCl₄ per 0.1 mL mixture. 0.1 mL of solutions of methanol, ethanol, isopropanol, DMSO, acetonitrile, cyclohexane, CH₂Cl₂, CH₃Cl, ethyl acetate, benzol, Doix, THF were added respectively to 0.1 mL of xylene solution, and the mixture was diluted with DMF to 0.5 μL of xylene per 0.1 mL of mixture.

  Generally speaking, acetone is very volatile and can be almost ignored in organic waste liquid. However, CCl₄ and xylene remain a long time in the organic waste liquid, and have strong carcinogenic effect. The simulation results show that acetone, CCl₄ and xylene can be detected in common mixed organic solvents. This provides a convenient method for the treatment of organic waste liquid. The results of simulated detection are listed in Table 3.

  Table 3

  

  Table 3.  Analog detection of acetone, CCl₄, and xylene by Zr-MOF^a

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  Sample	[Fe3+]			Error
  	F0/Fb	Y	Standard value
  Acetone	12 nL	29 nLc	10 nL	2~19 nL
  CCl4	31 nL	49 nLd	50 nL	1~19 nL
  Xylene	0.4 μL	0.7 μLe	0.50 μL	0.1~0.2 μL
  a Zr-MOF/DMF＝2.9 mL, 0.001 mg/mL; mixed solution＝0.1 mL; λex＝275 nm. b F0/F＝1+KSV[Fe3+]. cy＝2177.8x+0.0069. d y＝461.55x+0.1055. e y＝784.9x+0.0275.

  3.   Conclusions

  In conclusion, a multiple fluorescent senor was developed for the sensitive and selective probe detection of Fe³⁺ and $ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$in aqueous solution, and probe detection of acetone, CCl₄ and xylene in DMF solution. Meanwhile, it also provides an idea to detect F^－ in halide ions. This probe has high sensitivity to acid and base, and the pH of the solution can work well in the range of 4~10. It has been applied to the detection of Fe³⁺ in human urine and water samples, and it has been reported for the first time the detection of CCl₄ and xylene in organic solution.

  4.   Experimental section

  4.1   Instruments and reagents

  Fluorescence spectra were recorded with an LS 55 Fluorescence spectrometer (Perkin Elmer, U.K.). The crystalline phases of the products were determined from powder X-ray diffraction patterns (XRD) using an Empyrean Panalytical diffractometer equipped with Cu-Kα radiation and crushed single crystals over the range 0~50 ℃ at 298 K (Rigaku D/Max-2500 PC, Japan). Fourier transform-in- frared spectra (FT-IR) were performed with a IRPrestige-21 FT-IR spectrophotometer (400~4000 cm^-1) using the KBr pellet technique (Shimadzu, Japan). The thermal stability was investigated by a thermo-gravimetric analysis (TGA) experiment from 25 ℃ to 700 ℃ under nitrogen gas (TGA/DSC3+, Switzerland). Elemental analyses of C, H, N in the solid samples were tested with a Vario EL analyzer (Elementar Analysensysteme GmbH, Germany). UV-vis absorption spectra was recorded with a 950 UV-Vis spectrophotometer (Perkin Elmer Lambda, U.K.). Nuclear magnetic resonance (NMR) experiments were conducted with a 500 MHz/AVANCE Ⅲ (Bruker, Germany).

  Cyanuric chloride (99%, AR) was purchased from J & K Chemical (Beijing, China), DMSO-d₆ was obtained from Macklin (Shanghai, China), o-aminobenzoic acid (99.5%, AR) was purchased from Beijing Chemical (Beijing, China), the transition metal ion salts were purchased from Beilian and Yongda Chemical (Tianjin and Beijing, China), and the anionic sodium salts were obtained from Beilian and Jingke Chemical (Tianjin and Beijing, China);

  4.2   Experimental method

  4.2.1   Synthesis of Zr-MOF

  1 mmol of ligand L and 10 mL of DMF were added to a 50 mL round bottom flask to dissolve it. 1 mmol of ZrO(NO₃)₂ was added into a 20 mL beaker, and the mixture of 10 mL DMF/H₂O [V(H₂O):V(DMF)＝1:9] was added into the beaker. The mixture was dissolved after 5 min by ultrasound, and then slowly dropped into a round bottom flask under stirring conditions. A small amount of white solid is formed during the dropping process. The reaction continued at room temperature for 12 h, and a large number of white solids were formed. Then, the white solids were collected and washed three times with water and EtOH by centrifugation, respectively. The upper emulsion was collected, boiled and centrifuged, and the white solids were collected and washed. The product Zr-MOF was combined and dried for 1 d in 80 ℃ under vacuum. Yield: 92% [based on Zr(Ⅵ)]. ¹H NMR (500 MHz, DMF-d₇) δ: 10.43 (s, 3H, COOH), 8.10 (d, J＝10.0 Hz, 3H, ArH), 7.38 (dd, J＝8.2, 2.5 Hz, 3H, ArH), 6.92~6.95 (m, 3H, ArH), 6.42 (t, J＝7.5 Hz, 3H, ArH), 2.74 [s, 3H(HN) and 4H(2H₂O)]; IR (KBr) v: 3302, 3106, 1657, 1588, 1567, 1523, 1380, 1161 cm^－1. Anal. calcd for C₂₄H₂₂N₆O₈Zr (M_r＝613.69): C 46.97, N 13.69, H 3.61; found C 46.09, N 14.38, H 4.28.

  4.2.2   Cations sensing

  Add 18 different 0.1 mL of metal chloride (1×10^－4 mol/L) to 2.9 mL of solution of Zr-MOFs/DMF (0.001 mg/L) at room temperature, respectively. After mixing for 10 s, the resultant suspensions were then monitored by fluorescence spectroscopy and the fluorescence data were collected under an excitation wavelength of 275 nm. Additionally, to confirm its selective identification ability of Fe³⁺ among other ions, equal amounts of Fe³⁺ (1×10^－6~ 1×10^－5 mol/L) solution were added to the suspensions of Zr-MOFs pretreated with other ions, respectively, and then the fluorescence measurements were carried out. Respectively, different amounts of Fe³⁺ solution were added into Zr-MOFs aqueous dispersion to determine the relationship between the fluorescence intensity and the concentration of Zr-MOFs.

  4.2.3   Anions sensing

  Add 35 different 0.1 mL of sodium salts (1×10^－4 mol/L) to 2.9 mL of solution of Zr-MOFs/DMF (0.001 mg/L) at room temperature, respectively. After mixing for 10 s, the resultant suspensions were then monitored by fluorescence spectroscopy and the fluorescence data were collected under an excitation wavelength of 275 nm. And then, to confirm its selective identification ability of Cr₂O₇^2－ among other anions, equal amounts of Cr₂O₇^2－ (1×10^－5~1×10^－4 mol/L) solution was added to the suspensions of Zr-MOFs pretreated with other anions respectively, and then the fluorescence measurements were carried out. Respectively different amounts of Cr₂O₇^2－ solution were added into Zr-MOFs aqueous dispersion to determine the relationship between the fluorescence intensity and the concentration of Zr-MOFs.

  4.2.4   Organic solvent sensing

  Add 15 different 0.1 mL of organic solvents to 2.9 mL of Zr-MOFs/DMF (0.001 mg/L) solution at room temperature respectively. After mixing for 10 s, the resultant suspensions were then monitored by fluorescence spectroscopy and the fluorescence data were collected under an excitation wavelength of 275 nm. After that, to confirm Zr-MOF selective identification ability of acetone, CCl₄ and xylene among other organic solvents, equal amounts of acetone (1×10^－5~1×10^－4 mL), CCl₄ (1×10^－4~1×10^－3 mL) and xylene (1×10^－4~1×10^－3 mL) solution was added to the suspensions of Zr-MOFs pretreated with other organic solvents, respectively, and then the fluorescence measurements were carried out. Respectively, different amounts of toluene solutions were added into Zr-MOFs aqueous dispersion to determine the relationship between the fluorescence intensity and the concentration of Zr-MOFs.

  4.2.5   pH sensing

  Different pH values 0.1 mL of aqueous solutions were added to 2.9 mL of Zr-MOFs/DMF (0.001mg/L) solution at room temperature, respectively. After mixing for 10 s, the resultant suspensions were then monitored by fluorescence spectroscopy and the fluorescence data which were collected under an excitation wavelength of 275 nm.

  Supporting Information  The FT-IR, PXRD, TAG and ¹H NMR spectra of Zr-MOF. The quenching constants of other metal ions, anions and organic solvents in Zr-MOF. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn/.

  
  
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• 

  Scheme 1  Synthesis route of Zr-MOF

  下载: 全尺寸图片 幻灯片
  

  Figure 1  (a) Fluorescence intensity of different metal ions in DMF of Zr-MOF (Zr-MOF＝2.9 mL, 0.001 mg/mL), and (b) effect of the disturbing metal ions on fluorescence recognition of Zr-MOF to Fe³⁺

  [metal ions]＝0.1 mL, 1×10^－4 mol/L, λ_ex＝275 nm

  下载: 全尺寸图片 幻灯片
  

  Figure 2  (a) Fluorescence change of Zr-MOF (DMF) in different concentrations of Fe³⁺ ion (Zr-MOF＝2.9 mL, 0.001 mg/mL), and (b) detection limit of Fe³⁺

  [Fe³⁺]＝0.1 mL, 1×10^－6~1×10^－5 mol/L, λ_ex＝275 nm

  下载: 全尺寸图片 幻灯片
  

  Figure 3  Fluorescence intensity of different anions in the DMF of Zr-MOF

  Zr-MOF＝2.9 mL, 0.001 mg/mL, [anions]＝0.1 mL, 1×10^－4 mol/L, λ_ex＝275 nm

  下载: 全尺寸图片 幻灯片
  

  Figure 4  (a) Fluorescence change of Zr-MOF (DMF) in different concentrations of $ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$ion (Zr-MOF＝2.9 mL, 0.001 mg/ mL), and (b) detection limit of $ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$

  [$ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$]＝0.1 mL, 1×10^－5~1×10^－4 mol/L, λ_ex＝275 nm

  下载: 全尺寸图片 幻灯片
  

  Figure 5  (a) Effect of the disturbing anions on fluorescence recognition of Zr-MOF to $ {\rm{Cr}}_{2}{\rm{O}}_{7}^{2-}$, and (b) effect of the disturbing metal ions on fluorescence recognition of Zr-MOF to $ {\rm{MnO}}_{\rm{4}}^{-}$

  Zr-MOF＝2.9 mL, 0.001 mg/mL, [anions]＝0.1 mL, 1×10^－4 mol/L, λ_ex＝275 nm

  下载: 全尺寸图片 幻灯片
  

  Figure 6  (a) Change of fluorescence intensity of Zr-MOF with P-containing anions, and (b) the change of fluorescence intensity of Zr-MOF with S-containing anions, (c) the change of fluorescence intensity of Zr-MOF with halogens, and (d) the change of fluorescence intensity of Zr-MOF with organic acid ions

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  Figure 7  (a) Fluorescence intensity of different organic solvents in the DMF of Zr-MOF, (b) the effect of the disturbing different organic solvents on fluorescence recognition of Zr-MOF to acetone, (c) the effect of the disturbing different organic solvents on fluorescence recognition of Zr-MOF to CCl₄, and (d) the effect of the disturbing different organic solvents on fluorescence recognition of Zr-MOF to xylene

  Zr-MOF＝2.9 mL, 0.001 mg/mL, [organic solvents]＝0.1 mL, λ_ex＝275 nm.

  下载: 全尺寸图片 幻灯片
  

  Figure 8  (a) Fuorescence change of Zr-MOF (DMF) in different volumes of acetone (Zr-MOF＝2.9 mL, 0.001 mg/mL), and (b) detection limit of acetone

  V(acetone)＝1×10^－5~1×10^－4 mL, λ_ex＝275 nm

  下载: 全尺寸图片 幻灯片
  

  Figure 9  (a) Fluorescence change of Zr-MOF (DMF) in different volumes of CCl₄ (Zr-MOF＝2.9 mL, 0.001 mg/mL), and (b) detection limit of acetone

  V(CCl₄)＝1×10^－4~1×10^－3 mL, λ_ex＝275 nm

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  Figure 10  (a) Fluorescence change of Zr-MOF (DMF) in different volumes of xylene (Zr-MOF＝2.9 mL, 0.001 mg/mL), and (b) detection limit of acetone

  V(xylene)＝1×10^－4~1×10^－3 mL, λ_ex＝275 nm

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  Figure 11  (a) Fluorescence change of Zr-MOF (DMF) in different pH (Zr-MOF＝2.9 mL, 0.001 mg/mL, pH＝1~14, λ_ex＝275 nm), and (b) effect of pH value for fluorescence properties of Zr-MOF

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  Table 1.  Detection of Fe³⁺ concentration in human urines by fluorescence of Zr-MOF^a

  Entry	F0	F	[Fe3+]/(μmol•L－1)		Error
  			F0/Fb	yc
  1	772.57	584.77	3.37	2.32	±1.05
  2	772.57	693.16	1.20	0.95	±0.25
  3	772.57	590.45	3.23	2.24	±0.79
  4	772.57	574.63	3.61	2.44	±1.17
  5	772.57	550.78	4.22	2.75	±1.47
  6	772.57	501.24	5.67	3.37	±2.30
  7	772.57	645.25	1.90	1.55	±0.35
  8	772.57	650.98	1.96	1.48	±0.48
  9	772.57	569.75	3.73	2.51	±1.22
  10	772.57	615.56	2.67	1.93	±0.74
  a Zr-MOF/DMF＝2.9 mL, 0.001 mg/mL; sample＝0.1 mL; λex＝275 nm; bF0/F＝1+KSV[Fe3+]; c y＝1.02613—105x+0.0054.

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  Table 2.  Detection of Fe³⁺ concentration in water samples by fluorescence of Zr-MOF^a

  Entry	F0	F	[Fe3+]/(μmol•L－1)		Error
  			F0/Fb	yc
  1	815.93	570.07	4.52	2.88	±1.64
  2	815.93	399.74	10.91	4.92	±5.99
  3	815.93	692.14	1.87	1.43	±0.44
  4	815.93	653.98	2.60	1.88	±0.72
  5	815.93	726.91	1.28	1.02	±3.26
  6	815.93	525.49	5.79	3.42	±2.37
  7	815.93	558.47	4.83	3.02	±1.81
  8	815.93	725.21	1.31	1.03	±0.28
  9	815.93	653.84	2.60	1.88	±0.72
  10	815.93	572.65	4.45	2.85	±1.60
  a Zr-MOF/DMF＝2.9 mL, 0.001 mg/mL; sample＝0.1 mL; λex＝275 nm. b F0/F＝1+KSV[Fe3+]. c y＝1.02613×105x+0.0054.

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  Table 3.  Analog detection of acetone, CCl₄, and xylene by Zr-MOF^a

  Sample	[Fe3+]			Error
  	F0/Fb	Y	Standard value
  Acetone	12 nL	29 nLc	10 nL	2~19 nL
  CCl4	31 nL	49 nLd	50 nL	1~19 nL
  Xylene	0.4 μL	0.7 μLe	0.50 μL	0.1~0.2 μL
  a Zr-MOF/DMF＝2.9 mL, 0.001 mg/mL; mixed solution＝0.1 mL; λex＝275 nm. b F0/F＝1+KSV[Fe3+]. cy＝2177.8x+0.0069. d y＝461.55x+0.1055. e y＝784.9x+0.0275.

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