Synthesis,Proton Conduction And Fluorescence Sensing Properties Of Metal-Organic Frameworks Constructed By Mixed Ligands

Metal-organic frameworks（MOFs）have broad application prospects in gas storage and separation,fluorescence sensing,catalysis and drug release due to their rich structure,high porosity and large specific surface area.Compared with MOFs constructed by single ligands,metal-organic skeletons constructed by both carboxylic acid and nitrogen-containing ligands can introduce a variety of functional groups and show more abundant properties.MOFs with designable structures provide an ideal platform for developing new proton conducting materials.Through reasonable molecular design or post-modification strategy,different types of proton sources are introduced into the pores to improve the proton transport capacity of MOFs materials.MOFs can also be used as a fluorescent sensing material.Luminous MOFs can detect metal ions,organic pollutants and drug molecules by using their specific shape and size of the pore and specific functional sites on the pore wall.At present,the development of multifunctional MOFs has become one of the new research hotspots.Although MOFs have excellent performance in proton conduction and fluorescence sensing,MOFs with both proton conduction and fluorescence sensing performance are very scarce.In this paper,we selected carboxylic acids and nitrogen heterocyclic ligands（4,4′-oxydibenzoic acid）,H₂oba;5-Nitrophthalic acid,H₂nip;5-Amino-1H tetrazole,Hatz;Aminotriazole,Hata,has been used to construct three new multifunctional MOFs with different metal ions,and their proton conduction and fluorescence sensing properties have been studied.The main content is as follows:（1）Three new MOFs were prepared by hydrothermal synthesis using mixed ligands[Co₆（oba）₄（Hatz）（atz）（H₂O）₂（μ₃-OH）₂（μ₂-OH）]·H₂O（1）,Co（nip）（Hata）·0.5H₂O（2）,Zn₂（nip）（ata）₂·2H₂O（3）.The structure of 1-3 was characterized by instruments.（2）The proton conduction properties of 1-3 are studied.Because the single crystal size of1 is large,it is suitable for the single crystal conductivity test.We have tested the proton conductivity of 1 using both powder and single crystal samples.The maximum proton conductivity of 1 powder sample is 5.6×10^-6 S cm^-1（55°C and 95%RH）.The single crystal conductivity of 1 has obvious anisotropic characteristics.Under the same conditions（55°C and 95%RH）,the conductivity along the c-axis reaches 1.1×10^-3 S cm^-1,the conductivity along the b axis is 9.1×10^-6 S cm^-1.At 50°C and 95%RH,the proton conductivity of 2 and 3reached the maximum value,respectively 4×10^-6 S cm^-1、1.4×10^-5 S cm^-1.In order to improve the proton conductivity of 2 and 3,2 and 3 are immersed in sulfuric acid solution to obtain composite materials H₂SO₄@2 and H₂SO₄@3.The proton conductivity of H₂SO₄@2 and H₂SO₄@3 are 6.0×10^-4 S cm^-1 and 1.3×10^-4 S cm^-1,1-2 orders of magnitude higher than 2 and3.At the same time,the proton conduction mechanism and conduction path of 1-3 are analyzed.In view of the excellent water stability and fluorescence performance of MOFs 1-3 in aqueous solution,the fluorescence sensing properties of 1-3 to contaminated ions in water are studied in this paper.The experiment shows that 1-3 can detect Fe³⁺and Cr₂O₇^2-ions in water by fluorescence quenching,showing high selectivity and sensitivity,and strong anti-interference ability.The detection limits for 1-3 of Fe³⁺are 2.98μM、18μM and 2μM respectively.The detection limits for Cr₂O₇^2-ion are 4.5μM、5.6μM and 30μM respectively.At the same time,the fluorescence quenching mechanism of 1-3 to Fe³⁺and Cr₂O₇^2-ions was studied.