Fig. 4: The relationship between the length of the etched tips and time, and the morphological changes of the UME tip during the etching process.

a Relationship between the length of the etched tips and processing time: SEM images of the UME tips etched by microplasma with the processing time of a-I 0 s (untreated), a-II 1 s, a-III 5 s, a-IV 10 s. b Magnified SEM image of the UME surface in the red box in a. c SEM image of the processed UME tip with submicron exposed length. Microplasma jet parameters: He flow rate: 20 sccm, O2 flow rate: 2 sccm, peak to peak applied voltage: 13 kV