| Environmental protection has become a hot topic in today's society,timely monitoring of the content of pollutants in the environmental system is essential.Traditional methods for the detection of organic pollutants and heavy metal ions in the environment have the disadvantages of complicated pretreatment,complex operation and high cost.Therefore,it is important to find a sensitive and convenient method to detect pollutants in the environment.Fluorescence sensing has become the most popular detection method because of its simple operation,high sensitivity and low cost.Fluorescence sensing materials mainly include carbon quantum dots,fluorescent dyes,metal oxides and fluorescent coordination polymers.Coordination polymers(CPs)have attracted extensive attention due to their advantages of good crystallinity,specific structure,adjustable pore size,high functionalization,large specific surface area and high repeatability.At present,sensors based on fluorescent CPs have been widely used to sense pollutants in the environment,such as heavy metal ions,antibiotics,pesticides and some biological small molecules.However,the uncontrollability of CPs self-assembly and the improvement of fluorescence sensitivity and accuracy are still the difficulties in the application of fluorescent CPs in practical detection.Therefore,it is necessary to reasonably design and construct fluorescent CPs and improve the sensitivity and accuracy of fluorescent CPs by the in situ and post modification methods.To sum up,under water/solvothermal conditions,seven new Zn/Cd CPs were constructed by the reaction of transition metal ions Zn2+/Cd2+with d10 structure and p-terphenyl-2,2'',5'',5'''-tetracarboxylate acid(H4tptc),3,5-di(2',5'-dicarboxyphenyl)pyridine(H4DDPP),2,4-di(2',5'-dicarboxylphenyl)benzoic acid(H5L),ortho–bis(imidazol-1-ylmethyl)benzene(o-bimb),meta-bis(imidazol-1-ylmethyl)benzene(m-bimb)2,2'-bipyridine(bpy),3-(pyrid-2-yl)pyrazole(pyp)ligands by one pot method.Meanwhile,their structures were characterized by powder X-ray diffraction(PXRD),thermogravimetry(TG),fourier transform infrared spectroscopy(FTIR)and scanning electron microscopy(SEM).In addition,the fluorescence sensing properties of CPs and post-modified compounds for antibiotics,carbaryl and Fe3+ion were further studied.The specific research contents and results in this work are as follows:(1)Cd-CP 1 is a dual interpenetrating three-dimensional(3D)structure of a 2-node(4,4)-connected{64.82}{66}2-bbf network.Cd-CP 2 is a 6-connected{36.46.53}-hxl layer structure,and the final 3D supramolecular structure is formed by hydrogen bonding between adjacent layers.The structures of Zn-CPs 3 and 4 are isostructural,they are 2D layers of 6-connected{44.62}-sql network,which are further expanded into a 3D supermolecular structure by hydrogen bonding interaction.Zn-CP 5 is a three-dimensional porous structure with the pore size of 10.8(?),which can be simplified as a 2-node 4,8-connected{412.612.84}{46}2 network structure.Cd-CP 6 is a porous 3D framework with two channels,with pore sizes of 14.9(?)and10.8(?),respectively.The structure can be simplified as a 3-node 4,4,8-connected{44.62}6{48.64.812.104}network structure.Cd-CP 7 is a layered structure with 2-node 3,5-connected{3.4.5}{32.4.5.62.74}network,and the adjacent 2D layers are further expanded into3D supramolecular structure through hydrogen bond interaction.(2)Fluorescence experiments showed that Cd-CPs 1 and 2 can be used as fluorescence sensors in DMF detection system,and have good sensing capability for common nitro antibiotics.In addition,the mechanism of fluorescence quenching shows that the combination of photoinduced electron transfer(PET)and fluorescence resonance energy transfer(FRET)processes together to dominate the fluorescence quenching of two Cd-CPs toward the nitro antibiotics.Meanwhile,the fluorescence lifetime of samples decreases slightly after fluorescence sensing,indicating that the fluorescence quenching process is more static.(3)In the actual detection,the residues of antibiotics mostly exist in the water system,so two kinds of Zn-CPs with strong water stability were constructed.Zn-CPs 3 and 4 showed high selectivity and sensitivity to nitrofurantoline(NFT)in aqueous solution.The quenching constants(Ksv)and detection limits(DLs)are 6.27×103 M-1 and 1.53×10-6 M,3.6×103 M-1 and2.66×10-6 M,respectively.In addition,the mechanism of fluorescence quenching is mainly attributed to the competitive absorption and PET processes between NFT and Zn-CPs 3 and 4.(4)To improve the selectivity of fluorescence sensing,microporous Zn-CP 5 with unique?3-OH-was constructed.Unique?3-OH-makes Zn-CP 5 have good adsorption properties for N2 and CO2,and improves the selectivity and sensitivity of fluorescence sensing by the hydrogen bond with NFT.The selectivity of Zn-CP 5 for the CO2/CH4(0.5:05)and CO2/CH4(0.05:0.95)mixtures are 79.3 and 25.4 at 298 K and 1 bar,respectively,which are calculated by the ideal absorbed solution theory(IAST)theory.The above results are mainly attributed to large polarizability,significant quadrupole moment,small dynamic radius and high Qst value of CO2.The vapour sorption experiments showed that Zn-CP 5 exhibits extremely high H2O vapor selectivity with a selectivity coefficient of 254.6(H2O/CH3OH=0.1:0.9)at 298 K and1 bar.Furthermore,based on Grand Canonical Monte Carlo(GCMC)simulation,the good selectivity of Zn-CP 5 for CO2 and H2O vapor is ascribed to?3-OH-and coordinated H2O molecules in the channel of Zn-CP 5 providing adsorption sites for the above molecules.Moreover,the fluorescence studies indicated that Zn-CP 5 has high sensing sensitivity and selectivity for NFT with the Ksv and DL values of 9.60×105 M-1 and 5.3×10-8 M in aqueous solution or real water samples.Additionally,the fluorescence quenching mechanisms of NFT for Zn-CP 5 are mainly ascribed to the competitive absorption between the absorption spectrum of NFT and the emission spectrum of Zn-CP 5,the weak interactions between?3-OH-of Zn-CP5 and NFT and electron transfer from the lowest unoccupied molecular orbital(LUMO)energy level of Zn-CP 5 to the LUMO energy level of NFT.(5)To improve the accuracy of fluorescence sensing of CPs,a stable MB@Cd-CP 6composite with dual-emitting characteristic was constructed by the in-situ encapsulation of methylene blue(MB)into the channel of Cd-CP 6.Compared with Cd-CP,MB@Cd-CP 6exhibits high accurate and sensitive sensing for carbaryl with the Ksv and DL values of 7.9×104 M-1 and 3.4×10-8 M,which are attributed to the fluorescence enhancement effect and dual-emitting characteristics of MB@Cd-CP 6.Finally,the fluorescence enhancement mechanisms indicates that the excellent fluorescence properties of MB@Cd-CP 6 as a self-calibrated sensor is mainly ascribed to energy transfer from carbaryl to MB@Cd-CP 6 and the PPET from carbaryl to H4DDPP.At the same time,the relative standard deviation of MB@Cd-CP 6 for carbaryl in real samples is less than 4.44%,indicating that MB@Cd-CP 6 has excellent sensing accuracy for carbaryl.(6)In order to improve the sensing sensitivity and convenience of 2D material,a new paper sensor(Cd-CP 7@PVDF)was constructed by loading Cd-CP 7 on the polyvinylidene fluoride(PVDF).The fluorescence experiments show that the sensor has high sensitivity to Fe3+ions,the values of Ksv and DL are 5.5×104 M-1 and 4.0×10-8 M,respectively.The sensing mechanisms of Cd-CP 7@PVDF for Fe3+ions are attributed to the competitive absorption and interaction between Cd-CP 7 and Fe3+ions.In addition,the binding energies of Fe3+with the framework of Cd-CP 7 were calculated by density functional theory(DFT),which further proved that there was an obvious interaction between Fe3+and the uncoordinated O atom in Cd-CP 7. |
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