For more detailed discussion related to the stripping analysis using bismuth electrode, see previous report [15,17,18]. Ionic liquids including 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF6) were also used to test the direct electrochemistry of targeted metal oxides, and no stripping signals can be observed at room temperature.