All electrochemical measurements were performed in 6 M LiCl supporting electrolyte aqueous solution, which minimized the evaporation of the liquid. Dispensation and manipulation of the microdroplets on the surface was controlled using a pneumatic microinjector and a micromanipulator reported previously. The potential (E) was applied to the graphite electrode using a PGSTAT302N potentiostat (Metrohm Autolab) and a scan rate of 100 mV s -1 was used in order to achieve a steady-state ultramicroelectrode response without thin-layer cell diffusional effects (Supporting Fig. ). The standard heterogeneous electron transfer rate constant (k 0 ), limiting current density (j lim ), and transfer coefficient (α or 1-α) were determined by fitting the voltammograms with an empirical function using equations (10), (11) and (13) in ref. , derived based on the Butler-Volmer model of electrochemical kinetics. The diffusion coefficients of 2.5 (2.2) × 10 -6 cm 2 s -1 for the oxidized (reduced) form of [Ru(NH 3 ) 6 ] 3+/2+ and 2.4 (2.1) × 10 -6 cm 2 s - 1 for the oxidized (reduced) form of [IrCl 6 ] 2-/3-, were determined as averages from cyclic voltammetry and chronoamperometry at a Pt disk electrode (1.1 mm radius) using the Randles-Ševčík and Cottrell equations, respectively. The formal potentials (E 0' ) of [Ru(NH 3 ) 6 ] 3+/2+ and [IrCl 6 ] 2-/3-reduction/oxidation were determined from cyclic voltammetry using the same electrode. The potential, measured against an Ag/AgCl reference electrode, was converted to the standard hydrogen electrode (SHE) scale. A large-area Pt counter electrode was used to complete the three-electrode configuration. All measurements were carried out at ambient temperature (20-25 ºC).