Nano@micro: General method for entrapment of nanocrystals in sol-gel-derived composite hydrophobic silica spheres

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 solutions-hydrophobic 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 An particles. The technique is generally applicable to other hydrophobic nanocrystal systems of magnetic, oxide, and other materials.