Sonication of microliter volumes of aqueous silica, gold, or silica/gold colloids in toluene results in solidification within dissolving water droplets and the facile preparation of nanoparticle-based microspheres. At 0 and 10 °C, smooth spheres with mean diameters of 0.5 and 0.75 μm, respectively, are produced for samples prepared from gold or silica nanoparticles. In contrast, nanocomposite spheroids prepared from 2:1 silica/gold mixtures exhibit unusual surface textures due to higher-order aggregation between microspheres undergoing solidification. At 0 °C, binary collisions between similar sized droplets produce fused pairs that solidify and subsequently fragment into 0.5-μm-sized silica/gold microspheres with a pitted olive morphology consisting of a single circular indentation. Larger spheroids with multiple surface indentations and holes are produced for droplets of disparate size, for example at 10−20 °C or at 0−5 °C when the sonication time or water content are reduced to <3 min and <20 μL/100 mL, respectively. A wide range of nanoparticle-containing microspheres with complex morphology and multi-functionality for use in catalysis, separation science, and bioengineering should be accessible by development of the procedures described.
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Kulak et al. (2003) studied this question.