Microfluidics-enabled rational design of ZnO micro-/nanoparticles with enhanced photocatalysis, cytotoxicity, and piezoelectric properties

Microfluidics-based reactors enable controllable synthesis of micro-/nanostructures for a broad spectrum of applications from materials science, bioengineering to medicine. In this study, we first develop a facile and straightforward flow synthesis strategy to control zinc oxide (ZnO) of different shapes (sphere, ellipsoid, short rod, long rod, cube, urchin, and platelet) on a few seconds time scale, based on the 1.5-run spiral-shaped microfluidic reactor with a relative short microchannel length of ca. 92 mm. The formation of ZnO is realized simply by mixing reactants through two inlet flows, one containing zinc nitrate and the other sodium hydroxide. The structures of ZnO are tuned by choosing appropriate flow rates and reactant concentrations of two inlet fluids. The formation mechanism behind microfluidics is proposed. The photocatalysis, cytotoxicity, and piezoelectric capabilities of as-synthesized ZnO from microreactors are further examined, and the structure-dependent efficacy is observed, where higher surface area ZnO structures generally behave better performance. These results bring new insights not only in the rational design of functional micro-/nanoparticles from microfluidics, but also for deeper understanding of the structure-efficacy relationship when translating micro-/nanomaterials into practical applications.