Self-assembly of nanocrystals into higher-order structures produces superparticles that have enhanced optical, catalytic, and electrical properties when compared to solid particles with similar diameters. These improved properties are due to the enhanced coupling among nanocrystals and the ability to tailor superparticle composition for targeted applications. Typically, superparticles are assembled from emulsions (e.g., oil-in-water), and the superparticle size depends on the initial size of the droplet in that emulsion. To improve the monodispersity of the droplets and, therefore, the size distribution of the superparticles, we use microfluidic devices to generate oil-in-water droplets to control superparticle self-assembly. These droplets contain a homogeneous concentration of Fe<sub>3</sub>O<sub>4</sub> nanocrystals (16 nm in diameter) coated with oleic acid ligands and suspended in anhydrous toluene (dispersed phase). The continuous phase is water with sodium dodecyl sulfate (SDS) which stabilizes the emulsions. The microchannels are etched to different cross-sectional areas to form uniform droplets with diameters of 10, 25, 35, and 50 mm and relative standard deviations (RSDs) of &lt; 1%. To control superparticle size, we varied the droplet diameter and nanocrystal concentration to produce well-defined superparticles with average diameters from 350 nm to 1.1 mm and RSDs &lt; 30%.

Generation of monodisperse, submicrometer, self-assembled superparticles with microfluidic devices

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