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

<p>Abstract</p> <p>Bifunctional magnetic&#8211;fluorescent composite nanoparticles (MPQDs) with Fe₃O₄MPs and Mn:ZnS/ZnS core&#8211;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 Fe₃O₄MPs and Mn:ZnS/ZnS core&#8211;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 Fe₃O₄nanoparticles, such as the ratio of water to surfactant (<it>R</it>), 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 Fe₃O₄could effectively suppress the interaction between the Fe₃O₄and 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.</p