A general method for entrapment of hydrophobically coated nanocrystals in micrometer and submicrometer composite silica spheres, nano@micro, was developed. The method employs two starting solutionsshydrophobic solvent containing the sol-gel precursor, a polymer, and the nanocrystals, and an emulsifying hydrophilic phase which catalyzes the sol-gel process. The use of a hydrophobic polymer, polystyrene, serves to encapsulate the nanocrystals inside the spheres while maintaining many of their original properties. The obtained nano@micro spheres were characterized structurally by transmission electron microscopy and scanning electron microscopy, chemically by energy dispersive X-ray spectroscopy, and optically by ensemble and single-particle fluorescence spectroscopy. It is possible to control the size of the microspheres from the 100 nm scale to the micrometer scale, with good monodispersivity and with good separation between the microspheres. The method is demonstrated for encapsulating a wide variety of nanocrystals, primarily semiconductors covering different spectral bands, and of different shapes including spheres and rods. The semiconductor nanocrystals impart widely tunable emission to the microspheres. A similar encapsulation technique was also applied to thiol-coated Au particles. The technique is generally applicable to other hydrophobic nanocrystal systems of magnetic, oxide, and other materials.

Polyethylene (PE) and silica are perhaps the simplest and most common organic and inorganic polymers, respectively. We describe, for the first time, a physically interpenetrating nanocomposite between these two elementary polymers. While polymer-silica composites are well known, the nanometric physical blending of PE and silica has remained a challenge. A method for the preparation of such materials, which is based on the entrapment of dissolved PE in a polymerizing tetraethoxysilane (TEOS) system, has been developed. Specifically, the preparation of submicron particles of low-density PE@silica and high-density PE@silica is detailed, which is based on carrying out a silica sol-gel polycondensation process within emulsion droplets of TEOS dissolved PE, at elevated temperatures. The key to the successful preparation of this new composite has been the identification of a surfactant, PE-b-PEG, that is capable of stabilizing the emulsion and promoting the dissolution of the PE. A mechanism for the formation of the particles as well as their inner structure are proposed, based on a large battery of analyses, including transmission electron microscopy (TEM) and scanning electron microscopies (SEM), surface area and porosity analyses, various thermal analyses including thermal gravimetric analysis (TGA/DTA) and differential scanning calorimetry (DSC) measurements, small-angle X-ray scattering (SAXS) measurements and solid-state NMR spectroscopy.

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