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 size, by direct anodization of Co foil with charge balanced by the microplasma at the flowing-helium/pure-ethanol interface under ambient condition. The anodic oxidation of cobalt in ethanol was analysed after characterising the solution using nuclear magnetic resonance (NMR), light absorption, photoluminescence (PL), and the solid product using X-ray photoemission spectroscopy (XPS), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Using the microplasma with high voltage under ambient condition, ethanol was oxidised to acetate as the charge carrier and the size of Co₃O₄ quantum dots can be controlled by the limiting current. The quantum dots from this method are well dispersed in ethanol and a dense coating for light absorption and a rectified diode can be processed directly from the suspension. These results reveals that microplasma-assisted anodisation in ethanol is an efficient and green route capable of manufacturing quantum dots at low-temperature and avoiding the use of extraneous ionic salts in electrolyte.