Preparation And Application Of Covalent Organic Polymer Nanosheets-based Mesoporous Organosilica Materials

The excellent properties and unique two-dimensional monolayer structure of graphene have attracted wide attention of researchers in two-dimensional materials.With further study and the expansion of cognition,some new materials with graphene-like structure,such as transition metal disulfide nanosheets,metal-organic frame nanosheets,two-dimensional polymer nanosheets and other two-dimensional single-layer structural materials are also emerging.Two-dimensional polymer nanosheets,also known as covalent organic polymer nanosheets,have many advantages,such as light weight,thin thickness,large transverse size,multiple surface active sites,good flexibility and adjustable structure,etc.They have attracted more and more attention in the fields of gas storage,adsorption,catalysis,sensing,energy storage and conversion.Layered covalent organic materials are often exfoliated by top-down method to prepare single-layer or small-layer nanosheets.The exfoliation methods mainly include mechanical exfoliation,ultrasonic exfoliation and chemical exfoliation.Mesoporous organosilicon nanoparticles(MONs)are a kind of organic-inorganic hybrid mesoporous materials,which organic groups are uniformly distributed on the pore wall in the form of covalent bonds,providing a rigid skeleton for organic groups and giving the material more active sites.Mesoporous organosilicon materials have a wide range of applications due to their superior properties,including adsorption,catalysis,chemical sensing,biological imaging,and drug delivery.The fluorescent mesoporous organosilicon materials bridge the organic fluorophore groups to the mesoporous material skeleton.The stability of the organic fluorophore was improved and the fluorescence quenching caused by the accumulation of fluorescent molecules was weakened due to the protection of the pore wall.This paper focuses on the preparation,properties and application of MONs material based on covalent organic polymer nanosheets.Firstly,the covalent organic polymer nanosheet TDSi was prepared by chemical exfoliation.Then using cetyltrimethyl ammonium bromide(CTAB)as structure guide agent,the covalent organic polymer nanosheet based mesoporous organosilicon nanoparticles(TD-MONs)with different doping contents of TD-Si were prepared by hydrolyzing and polycondensation of TD-Si and tetraethyl orthosilicate(TEOS).The prepared materials have a highly sensitive and selective fluorescence response to Cu2+and are expected to be a che1ical sensor for the detection of Cu2+.Because of the excitation wavelength-dependent property of TD-MONs,they have a potential application value in the fields of security labeling and biological imaging.The main contents of the study are as follows:(1)In Chapter 1,firstly,1,3,5-triformylphloroglucinol(Tp)and 3,3’-diaminobenzidine(DAB)were reacted at the ratio of 1:2 to prepare a covalent organic polymer(TpDAB)with a benzimidazole unit containing terminal amino groups.The structure and morphology of TpDAB were characterized by various experimental methods,which proving that TpDAB is a bulk,stacked polymer with particles size of approximately 1-2 μm.We exfoliated TpDAB into much smaller,dispersed nanosheets by two exfoliation methods.The first method uses strong acid exfoliation.TpDAB,which contains imidazole rings,is protonated in trifluoroacetic acid,which enhances the repulsive force between polymer layers,thus is easy to exfoliate into nanosheets(TD-H+)under ultrasonic.The size distribution of nanosheets prepared by this method is about 10-70 nm,and the thickness is about 1.5 to 8 nm.The second method uses chemical exfoliation.A novel organosiloxane precursor nanosheet(TD-Si)was synthesized by the reaction of TpDAB with propyl triethoxysilane isocyanate(IPTES).TD-Si has a particle size distribution of about 10-30 nm and a thickness of about 0.7-3.7 nm.The introduction of IPTES results in steric hindrance,which effectively inhibits π-π stacking between TpDAB layers.Thus,the polymer is easily chemically exfoliated into nanosheets in DMF solution.The optical properties of TD-Si prepared by chemical exfoliation method were studied.The changes of optical properties before and after exfoliation were studied by UV-visible absorption spectra and fluorescence emission spectra.Compared with the properties of TpDAB,the UV absorption and fluorescence properties of TD-Si were significantly improved.TD-Si has an excitation wavelength dependent(Ex-De)property.The position of emission wavelength gradually redshifts with the increase of the excitation wavelength.In DMSO solution,the maximum fluorescence emission intensity is found at 519 nm when the excitation wavelength is 400 nm.The selectivity of TD-Si to metal ions is studied.The addition of Cu2+caused obvious quenching of fluorescence,and it has good selectivity to Cu2+.(3)The research in Chapter 2 shows that the successful introduction of IPTES not only realizes the exfoliation of bulk materials into nanosheets,but also endows the silylation of TDSi,which can be introduced into mesoporous silicon skeleton as a precursor of organosilicon through hydrolytic condensation.In addition,by comparing the nanosheets prepared by two exfoliating methods,TD-Si nanosheets prepared by chemical exfoliation have smaller size,thinner thickness,and are more suitable for embedding into the mesoporous silica framework.Thus,in Chapter 3,TD-Si was used as the organosilicon precursor and TEOS used as inorganic silicon precursor as the mixed silicon source,and CTAB was used as the structure guide agent to prepare the bridging TD-MONs material.The structural and optical properties of the TDMONs was tested by small angle X-ray diffraction,29Si solid state nuclear magnetic resonance,scanning electron microscopy,high-resolution transmission electron microscopy,nitrogen adsorption-desorption isotherm,UV-vis absorption spectrum and fluorescence spectrum.Moreover,the material has obvious two-dimensional hexagonal mesoporous structure,good porosity,thermal stability and excellent Ex-De fluorescence emission performance.In addition,TD-MONs has good anti-interference and sensitive fluorescence response to Cu2+,and the detection limit is 3.77×10-7 M.TD-MONs can be used for highly selectivity detecting Cu2+.The coordination interaction between TD-MONs and copper ions was studied by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy.