Water-Stable, Magnetic Silica-Cobalt/Cobalt Oxide-Silica Multishell Submicrometer Spheres

Abstract

A method to produce monodisperse magnetic composite spheres with diameters from less than 100 nm to more than 1 μm in water solution is reported. The spheres consist of a dielectric silica core and a cobalt/cobalt oxide shell which can be protected from further oxidation with an outer shell of silica or, alternatively, they can be covered with the polymer polyvinylpyrrolidone as a stabilizer. The formation of a uniform magnetic shell proceeds with the adsorption of metallic cobalt seeds, produced by the reduction of cobalt chloride with sodium borohydride, on a self‐assembled layer of polyelectrolytes on the silica core. In the second step, an outer silica shell can be formed by the hydrolysis and condensation of (3‐aminopropyl)trimethoxysilane and tetraethoxysilane. The double‐shell composite spheres show excellent sphericity, monodispersity, and a magnetic hysteresis loop at room temperature.

Composite Silica Spheres with Magnetic and Luminescent Functionalities

“…An adsorption time of 20 min was then allowed, and excess QDs were removed by three repeated centrifugation/wash cycles. Silica-Coated Magnetic/Luminescent Particles: QD-patterned magnetic silica spheres were encapsulated with an outer shell of silica in a mixture of water/ethanol (1:4) containing 3-aminopropyl trimethoxysilane (Aldrich) (APS, 15 lL) and TEOS (15 lL) as a result of the hydrolysis and condensation of the silanes on the surface of the composite spheres, as described elsewhere [34]. These conditions produced an outer silica shell with an average thickness of 20 nm.…”

Increasing the Complexity of Magnetic Core/Shell Structured Nanocomposites for Biological Applications

“…[114,115] In this case, the thiol ligands that stabilized the QDs in solution may have been converted to disulfides under light irradiation and during the LbL process, yielding unprotected CdTe QDs on the surface of the magnetic silica spheres, which would explain the spectral shift toward shorter wavelengths, but keeping the feature of no influence between the different functionalities in the final composite. To increase the colloidal and chemical stability, the composite magnetic luminescent spheres were additionally coated with an outer layer of silica (nm average thickness 20 ), [116] as indicated in the top scheme of Figure 11. To elucidate how the silica coating affects the photoluminescence, spectra of the composite particles were measured before and after the deposition of the outer silica shell, again observing a blue-shift of the emission spectra (10 nm).…”

Synthesis of monodisperse spherical core-shell SiO2-SrAl2Si2O8: Eu2+phosphors by hydrothermal homogeneous precipitation method

“…These particles are frequently used in core-shell materials either as cores or as shells [8,14,15]. If the silica spheres could be coated with phosphor layers then a core-shell phosphor material with spherical morphology would be obtained and the size of the phosphor particles could be controlled by the silica cores.…”

“…Core-shell composites can be monodisperse spherical particles with good optical performance for applications. There are numerous methods of preparing core-shell-structured materials including layer-by-layer self-assembly [1,4], coprecipitation [5], sol-gel process [6,7], surface reaction [8] and metalorganic vapor phase epitaxy (MOCVD) [9]. These wet chemical techniques can offer the possibilities of controlling homogeneity, phase purity, size distribution, surface area and microstructure uniformity of the powder.…”