Then, the polishing was stopped, the electrode tip was rinsed, dried under N2 flow, and the quality of the electrode tip was checked under a bright field optical microscope. The polishing of the C-UME tip was started with a lapping film of grit 14000 (1 µm). Once a flat electrode tip was observed under a bright field microscope, the C-UME was further polished with a lapping film of grit 60000 (0.3 µm), then rinsed with Millipore water, dried under a stream of N2, and placed in a closed petri dish for future use. (corresponding to S2O8 2-concentrations between 0.60 to 2.4 mM), the voltammetric current increases steadily, a consequence of the electrocatalytic homogeneous reduction of S2O8 2-by Ru(NH3)6 2+ (eq. 2), which regenerates Ru(NH3)6 3+ . Additionally, the dissociation of S2O8 3•-yields SO4 •-, which is also either directly reduced to SO4 2-at the electrode (eq. 5) or reduced through its homogeneous reaction with Ru(NH3)6 2+ to yield Ru(NH3)6 3+ (eq. 4) that is then reduced at the electrode (eq. 1). Thus, the combination of eqs. 1 and 4 (mediated reduction of SO4 •-pathway) is equivalent to eq. 5 (direct reduction of SO4 transferred from the electrode to yield SO4 2-. The overall reaction for the mediated reduction of S2O8 2-, eq. ( ), is obtained by combining eqs. 1, 2, 3, and 5 (direct reduction of SO4 •-pathway) or by combining eqs. 1 (twice), 2, 3, and 4 (indirect reduction of SO4 •-pathway).