Preparation Of Monodisperse Fe₃O₄ Nanocomposite Microspheres And Its Application In Colloidal Array Assembly

Fe₃O₄ nanospheres are widely applied in magnetic separation,targeted drug delivery,catalysis,nuclear magnetic resonance and other fields,due to their good paramagnetism,biocompatibility,magnetic responsiveness and surface modifiability.At the same time,Fe₃O₄ nanospheres have attracted more and more attentions in the field of colloid array assembly and photonic crystals due to their high refractive index,black properties and magnetic responsiveness.In this paper,Fe₃O₄ nanospheres were prepared by solvothermal method,and then Si O₂ layer was coated on the Fe₃O₄nanosphere surface via St?ber method to obtain Fe₃O₄@Si O₂ nanocomposite microspheres with relatively uniform size.On this basis,amorphous colloid array（ACA）patterns were quickly assembled on paper by infiltration-driven colloidal assembly method.Moreover,colloid array hydrogel films were also prepared by magnetic field induced assembly and subsequent polymerization triggered by ultraviolet light.Specific research contents are as follows:Sodium citrate（SC）modified Fe₃O₄ nanospheres were prepared by solvothermal reaction using SC as stabilizer.However,these microspheres tend to agglomerate in the process of Si O₂ coating,leading to formation of peanut or popcorn-shape microspheres.Therefore,polyacrylic acid（PAA）modified Fe₃O₄ nanospheres were further synthesized by one-step solvothermal method using PAA as stabilizer.The effects of solvothermal reaction system parameters（Fe Cl₃?6H₂O,Na Ac,PAA concentration）on the size and distribution of the microspheres were investigated in detail.On this basis,Fe₃O₄ microspheres with sizes of 172 and 133 nm were synthesized.Thereafter,the Si O₂ layer was coated on Fe₃O₄ nanosphere surface by St?ber method,and the shell thickness was adjusted by the amount of ethyl orthosilicate（TEOS）.Finally,Fe₃O₄@Si O₂ nanocomposite microspheres with relatively uniform sizes of 252,208and 173 nm were successfully prepared.The aqueous dispersion of magnetic microspheres can produce gorgeous structural color under magnetic field induction,and blue shift occurs with increasing magnetic field intensity.Fe₃O₄@Si O₂ microspheres with a size of 252,208 or 173 nm were assembled on paper materials by infiltration-driven colloidal assembly method,and red,yellow or green ACA pattern was obtained,respectively.Two-dimensional Fourier transform（2D-FFT）images show that the microspheres are arranged in a short-range order,and therefore the resulting array pattern displays angle-independent structural colors.On this basis,the mechanical properties of ACA patterns consisted of Fe₃O₄@Si O₂microspheres and polyacrylate（PA）nanoparticles were studied in detail.Results show that the mechanical stability of the array can be improved after PA nanoparticles are embedded in the microsphere gaps;in this situation,the peak value of the reflection spectrum exhibited a red shift to a certain extent.When the content of PA nanoparticles reached 1/5,the array pattern showed satisfactory friction resistance.During ultrasonic washing,the paper array pattern was easy to be damaged.However,when the PA ratio reached 1/1,the array pattern could remain intact shape,although the structure color exhibited a red shift.After introduction of fluorine-containing components into PA particles,the ultrasonic washing resistance of the ACA pattern could not be improved significantly.Fe₃O₄@Si O₂ microspheres were dispersed into ethylene glycol（EG）solution,and monomer,crosslinking agent and photoinitiator were then added and completely dissolved.The resulting aqueous dispersion showed bright color after magnetic field induction assembly.After photopolymerization,colloid array hydrogel films were successfully prepared.Effect of the magnetic field intensity during assembly process on the structure color of the colloid array film was studied in detail.It was found that,with increasing magnetic field intensity,the structure color of the hydrogel film exhibited a blue-shift and the reflected spectral peak decreased from 640 nm to 530 nm.The influence of ambient humidity on the colloid array film was also investigated.With the increase of ambient humidity,the hydrogel absorbed water and the structure color exhibited a red shift.When the humidity increased from 45%to 75%,the peak of reflectance spectrum shifted from 520 nm to 620 nm.Furthermore,when the colloid array film was stretched,the structure color exhibited a blue shift.