Preparation Of TiO₂/SiO₂ Composite Materials And Their Application To The Separation Of Casein Phosphopeptides

In recent years,because of excellent chemical stability,wide pH application range and amphoteric character,TiO₂ has attracted extensive attention.However,the application of TiO₂ in solid phase extraction and separation analysis was restricted by its small specific surface area and pore size.Meanwhile,SiO₂ as traditional solid phase extraction and chromatographic packing materials possesses relative large specific surface area and pore diameter,but irreversible adsorption for alkali compounds is unfavorable for the application of SiO₂.Fabrication of composite materials combining merits of TiO₂ and SiO₂ could provide powerful tools for separation analysis.In the present study,layer-by-layer self-assembly method and hydrothermal approach were used for the construction of TiO₂/SiO₂ core-shell composite materials.Adsorption performance of the composite materials for casein phosphopeptides was also investigated.Our results could provide reliable reference for the preparation and application of TiO₂/SiO₂ composite materials in chromatographic analysis.This thesis consisted of three parts:（1）TiO₂/SiO₂ core-shell composite materials were prepared by layer-by-layer selfassembly method.TiO₂ sol was prepared using titanium isopropoxide as the titanium source.Mesoporous SiO₂ particles were added into the prepared TiO₂ sol,and small TiO₂ particles were adsorbed on the surface of SiO₂ by self-assembly.The addition amount of titanium isopropoxide used for preparation of TiO₂ sol,number of lays of self-assembly,ratios of TiO₂ to SiO₂,aging time and calcination temperature were investigated to synthesize TiO₂/SiO₂ core-shell composite materials with high performance.Characterizations of the prepared TiO₂/SiO₂ core-shell composite materials were performed on optical microscope,scanning electron microscope（SEM）,energy dispersive spectrometer（EDS）,X-ray diffractometer and specific surface area/pore size analyzer to optimize the preparation conditions.Obvious coreshell structure were observed in the TiO₂/SiO₂ composite materials.The specific surface area and average pore diameter of composite materials are 116.26 m2/g and 8.42 nm,respectively.These results indicated the composite materials could be good packing for solid phase extraction.However,this approach was time-consuming.The adsorption of TiO₂ on the surface of SiO₂ was week,and the distribution of TiO₂ was uneven.This is due to the weak electrostatic interaction between TiO₂ and SiO₂ during the self-assembly process.TiO₂ coating should be repeated several times to achieve the desired results,and the coated shell was easy to fall off.Thus,hydrothermal method was carried out to overcome these problems.Uniform TiO₂/SiO₂ core-shell composite materials with desired TiO₂ shell thickness were obtained.（2）TiO₂/SiO₂ core-shell composite materials were prepared by hydrothermal method.Mesoporous SiO₂ particles were added into the mix solution containing titanium isopropoxide.TiO₂/SiO₂ composite materials with large specific surface area and obvious core-shell structure were prepared by hydrothermal reaction in high-pressure reactor.The addition amount of titanium isopropoxide and number of bonding reaction were investigated to synthesize stable composite materials with SiO₂ as core and TiO₂ as shell.Characterizations of the TiO₂/SiO₂ core-shell composite materials were performed on optical microscope,scanning electron microscope（SEM）,energy dispersive spectrometer（EDS）,Xray diffractometer,FT-IR Spectrometer and specific surface area/pore size analyzer.Results indicated the TiO₂ shell with desired thickness distributed uniformly around the core.The size,specific surface area and average pore diameter of the TiO₂/SiO₂ composite materials were 11.48 μm,122.83 m2/g and 8.39 nm,respectively.These results indicated the composite materials could be good packing for solid phase extraction.（3）The adsorption performance of casein phosphopeptides by the TiO₂/SiO₂ core-shell composite material prepared by hydrothermal method was also studied.Influence of pH value on the adsorption capacity of casein phosphopeptides was investigated through static adsorption experiment,meanwhile,the saturation adsorption capacity of casein phosphopeptides was determined.The results showed that the maximum adsorption capacity of TiO₂/SiO₂ core-shell composites for casein phosphopeptides was under pH value 3 with saturation adsorption capacity 48.62 mg/g.Then solid-phase extraction columns were packed with TiO₂/SiO₂ core-shell composites prepared by hydrothermal method and used for dynamic adsorption of casein phosphopeptides.Influence of pH value and flow rate on the adsorption rate of casein phosphopeptides was also investigated,and the optimal flow rate and pH value were 0.4 mL /min and 3,respectively.Under these optimal conditions,the adsorption rate could reach 86.12%.The influence of ammonia concentration on the desorption rate of casein peptide phosphate was also investigated.The optimum concentration of ammonia was 0.9 mol/L,and the desorption rate could reach 81.23%.