Plasma at the gas/liquid interface can promote a complex mixture of reactions in solution and microplasma-assisted direct-current anodic oxidation is an efficient and green process in synthesising nanoscale materials for various applications. In this study, we demonstrated the direct synthesis of crystalline Co3O4 quantum dots, ca. 2–5 nm in diameter, by the direct anodization of Co foil with charge balanced by the microplasma at the flowing-helium/pure-ethanol interface under ambient conditions.(Green Chemistry 20, 2018, 2101).