Preparation and Characterization of Silica-Coated Magnetic-Fluorescent Bifunctional Microspheres

Bifunctional magnetic-fluorescent composite nanoparticles (MPQDs) with Fe(3)O(4) MPs and Mn:ZnS/ZnS core-shell quantum dots (QDs) encapsulated in silica spheres were synthesized through reverse microemulsion method and characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, vibration sample magnetometer, and photoluminescence (PL) spectra. Our strategy could offer the following features: (1) the formation of Mn:ZnS/ZnS core/shell QDs resulted in enhancement of the PL intensity with respect to that of bare Mn:ZnS nanocrystals due to the effective elimination of the surface defects; (2) the magnetic nanoparticles were coated with silica, in order to reduce any detrimental effects on the QD PL by the magnetic cores; and (3) both Fe(3)O(4) MPs and Mn:ZnS/ZnS core-shell QDs were encapsulated in silica spheres, and the obtained MPQDs became water soluble. The experimental conditions for the silica coating on the surface of Fe(3)O(4) nanoparticles, such as the ratio of water to surfactant (R), the amount of ammonia, and the amount of tetraethoxysilane, on the photoluminescence properties of MPQDs were studied. It was found that the silica coating on the surface of Fe(3)O(4) could effectively suppress the interaction between the Fe(3)O(4) and the QDs under the most optimal parameters, and the emission intensity of MPQDs showed a maximum. The bifunctional MPQDs prepared under the most optimal parameters have a typical diameter of 35 nm and a saturation magnetization of 4.35 emu/g at room temperature and exhibit strong photoluminescence intensity.