Design And Synthesis Of Nonlinear Optical Metal-organic Frameworks And Their Application In Sensing And Imaging

Metal-Organic Frameworks?MOFs?are formed of organic ligands and metal ions/clusters through coordination bonds.Due to superior structural designability,adjustable pore size,good thermal/chemical stability and biocompatibility,MOFs has been extensively studied in the fields of display lighting,optical sensing,biological imaging,gas/pollutant adsorption or separation,catalysis and biotherapy.Among them,the excellent optical properties of MOFs make it have broad research prospects and application potential in the field of optical sensing and imaging.At present,there still exist some unsolved problems in this field,such as poor detection limits,poor resolution,shallow penetration depth,small Stokes displacement of the light signal in biological cells and tissues.Therefore,in this paper,starting from the characteristics of MOFs such as electrical properties,ligand sites,and channels,a method for improving the detection limit is proposed by preparing new MOFs;and the optical signals of MOFs are extended from traditional linear optics to nonlinear optics.The systematic research on the relationship between the nonlinear optics of MOFs,especially the second-order nonlinear optical performance and its structure had been studied;further we applied MOFs'second-order nonlinear optics to excite UV-Vis dyes,and combined third-order nonlinear optics to develop dual-emission sensing materials.To a certain extent,it solves the problems of low sensitivity and homogeneous optical signal of MOFs in the field of optical sensing and imaging,and broadens their application scope.Zn-TPTC with nitrogen-containing ligand was designed and synthesized,which has excellent light-emitting properties from the ligand.Compared with the pure ligand,the quantum efficiency of Zn-TPTC has been significantly improved.It was found that Hg²⁺can produce significant fluorescence quenching of Zn-TPTC.Within 0¹×10^-4 M,the concentration of Hg²⁺and the fluorescence intensity of Zn-TPTC show a good linear relationship,and its detection limit is 3.7 nM.The reasons for low detection limit are as follows:Zn-TPTC contains multiple N sites to facilitate the binding of N-Hg;The anionic framework can increase the attraction to cations;The channel can provide an enrichment site for Hg²⁺;At the same time,the soft acid nature of Hg²⁺is beneficial to enhance the sensing effect and selectivity.Zn-TPTC is an excellent Hg²⁺sensing material with high selectivity and low detection limit.The relationship between the structural characteristics of MOFs and their second-order nonlinear optical properties have been explored.Lanthanide metals and organic ligand H₆HCOO were used to synthesize 13 kinds of MOFs Ln-HCOO.It was found that La^Tb-HCOO possess asymmetric structure and Dy^Yb-HCOO is centrally symmetric.The second harmonic generation?SHG?performance of La^Tb-HCOO is studied and it is found that La^Tb-HCOO has excellent SHG performance.Further research shows that MOFs with a crystal size of 170-300 meshes have the best performance because they are beneficial to the collection of SHG signals,and the intensity of SHG gradually increases with the decrease of the average coordination bond length.This phenomenon is well explained by polarization theory.Aiming at the current difficulties of organic dyes in the field of biological imaging,dyes NR,BB17,BO14,applying in the ultraviolet-visible light range,were encapsulated in frameworks of Gd-HCOO,which possess excellent SHG properties.Gd-HCOO?dyes have better performance in stability,fluorescence and quantum efficiency than individual dyes.Based on this,a method has been developed that utilizes the SHG performance of Gd-HCOO to enable UV-Vis dyes to produce fluorescence under the pumping of near-infrared light,which expands the application in the field of UV-Vis dye bioimaging.Furthermore,Nonlinear confocal microscope image of PC12cells co-cultured with Gd-HCOO?NR was obtained,which shows that the Gd-HCOO?NR composite can generate SHG and fluorescence signals simultaneously,and the fluorescent signal remains stable in the biological environment.In order to obtain stable MOFs and realize its accurate temperature sensing in physiological environment,nano MOFs Zn-TCOMA with excellent chemical stability was synthesized by choosing flexible ligand H₄TCOMA and zinc nitrate.The material was able to maintain the structure stability in boiling water with pH of 1-11.Its excellent chemical stability attributed to two-penetration frameworks,smaller pore surface area,ionic frameworks,hydrophobicity,and the oxygen atom on the ether bond in the ligand acts as a hydrogen bond acceptor.Zn-TCOMA has excellent SHG performance which utilized as reference signal in sensing.On this basis,two-photon fluorescence is introduced as the temperature sensing signal.The fluorescence performance,quantum efficiency and stability of two-photon fluorescence dye are improved after encapsolation.The intensity ratio of SHG signal to the two-photon fluorescence signal of Zn-TCOMA?dpee show good linear relationship with temperature.With good reproducibility and biocompatibility,Zn-TCOMA?dpee provide the possibility for its practical application in biological tissues.