Synthesis of organic nanoparticles in a 3D flow focusing microreactor

In this paper, we present the synthesis of organic nanocrystals in a 3D hydrodynamic focusing device through a non-solvent crystallization process. This microreactor was designed in order to control the supersaturation level, while manipulating mixing conditions, and to avoid fouling and clogging within the main channel. Rubrene, the molecule of interest, was dissolved in a THF–ethanol mixture and was injected through a silica capillary. Two side flows containing the non-solvent and the surfactant, water and CTACl respectively, surround and interact with the capillary flow and mixing occurs through diffu- sion. First, we quantify the water diffusion process within the focused beam by confocal fluorescence microscopy in the presence of fluorescein. It is shown here that the efficiency and the quality of the mix- ing between the aqueous side flow and the focused organic solution are improved by increasing the side flow rate to the capillary flow rate ratio, i.e. the focusing ratio. Second, the synthesis of rubrene crystals were performed under a variety of flow rate conditions. We show that, by increasing the focusing ratio (from 5 to 40), the mean size of nanocrystals decreases (from 110 to 50 nm). The characterization of the mixing process allows for a better understanding of rubrene nanocrystal synthesis: a fast supersaturation induces the generation of numerous nuclei, which limits the crystal growth. Herein, we show that the 3D hydrodynamic focusing microreactor efficiently produces organic nanoparticles of controlled size, without the deposition of large, unwanted crystals on the microchannel walls.