Hyper-miniaturization of monodisperse alginate–TiO2 composite particles with densely packed TiO2 nanoparticles

We report a novel technique to fabricate alginate–TiO2 composite particles with densely packed TiO2 nanoparticles. Using a microfluidic device, monodisperse sodium alginate droplets containing low-density TiO2 nanoparticles (1 or 5 w/v%) were formed in the oil phase. The sodium alginate droplets formed in the oil phase were subsequently placed on a Ca2+-loaded agarose-gel plate to induce shrinkage by water removal (from the droplets to the Ca2+-loaded agarose-gel plate) and gelation by Ca2+ transport (from the Ca2+-loaded agarose-gel plate to the droplets). Thus, the produced alginate–TiO2 composite particles containing densely packed TiO2 nanoparticles were significantly smaller than the microchannel. We also investigated the optimal conditions to successfully produce spherical composite particles by varying the oil phases, surfactants, calcium concentrations and gel strength of the agarose-gel plate. Moreover, our method could decrease the probability of channel clogging that often occurs when a colloidal suspension (e.g., nanoparticles) is used as the dispersed phase. This method facilitates the stable production of monodisperse alginate–inorganic composite particles for a wide range of applications.