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We report a fast and highly reproducible chemical synthesis method for colloidal gold nanoparticles which are negatively charged in nonpolar solvents and coated with hydrophobic organic molecules. If a hexane droplet containing charged gold nanoparticles is mixed with a larger toluene droplet, nanoparticles immediately float to the air-toluene interface and form a close-packed monolayer film. After evaporation of the solvent molecules, the monolayer film of nanoparticles can be deposited to any substrate without any limit in size. The synthesis does not require a postsynthesis cleaning step, since the two immiscible liquid phases separate the reaction byproducts from gold nanoparticles and a minimal amount of coating molecules is used.
Martin et al. (Wed,) studied this question.