The fabrication of magnetic composite core-shell particles and hollow spheres with tailored dimensions and compositions has been accomplished by a multistep (layer-by-layer) strategy. Composite particles were prepared by coating submicrometer-sized anionic polystyrene (PS) latices with magnetite (Fe 3 O 4 ) nanoparticle layers alternately adsorbed with polyelectrolyte from aqueous solution. The thickness of the deposited multilayers could be finely tuned with nanoscale precision, either by selection of the number of adsorption cycles performed or by the number of polyelectrolyte layers deposited between each nanoparticle layer (i.e., interlayer). As demonstrated by transmission electron microscopy, a marked improvement in the growth, uniformity, and regularity of the composite multilayers was achieved when the number of polyelectrolyte interlayers was increased from one [(poly(diallyldimethylammonium chloride) (PDADMAC)] to three [(PDADMAC/poly(styrenesulfonate) (PSS)/PDAD-MAC)]. Hollow, intact magnetic spheres were obtained by calcination of the core-shell particles at elevated temperature. Furthermore, composite hollow spheres were prepared by calcination of PS latices coated with multilayers of silica and Fe 3 O 4 nanoparticles. These nanoengineered colloidal particles may potentially find applications as delivery systems, or in diagnostics, where the particles can be directed by application of an external magnetic field.
A new class of highly fluorescent, photostable, and magnetic core/shell nanoparticles in the submicrometer size range has been synthesized from a modified Stöber method combined with the layer‐by‐layer (LbL) assembly technique. Luminescent magnetic nanoparticles are prepared via two main steps. The first step involves controlled addition of tetraethoxysilane to a dispersion of Fe3O4/γ‐Fe2O3 nanoparticles, which are thereby homogeneously incorporated as cores into monodisperse silica spheres. The second step involves the LbL assembly of polyelectrolytes and luminescent CdTe quantum dots onto the surfaces of the silica‐coated magnetite/maghemite particles, which are finally covered with an outer shell of silica. These spherical particles have a typical diameter of 220 ± 10 nm and a saturation magnetization of 1.34 emu g–1 at room temperature, and exhibit strong excitonic photoluminescence. Nanoparticles with such a core/shell architecture have the added benefit of providing a robust platform (the outer silica shell) for incorporating diverse functionalities into a single nanoparticle.
Core–shell particles with tailored optical properties are obtained via manipulation of the number of Au@SiO2 nanoparticle layers deposited onto larger polymer spheres. The surface characteristics of silica, which surrounds the gold nanoparticles, are exploited to achieve a dense and uniform nanoparticle coating. The layer‐by‐layer (LbL) self‐assembly technique yet again proves to be a worthy asset for the preparation of these novel metal‐based core–shell and hollow colloids (see Figure).
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