Fig. 5. (a) Optical image of water microdroplets loaded with 10 mM Ru(bpy)3Cl2 (luminophore) and 50 mM sodium oxalate (coreactant) adsorbed to an inverted 1.5 mm radius glassy carbon electrode. (b) ECL signal generated by the application of 1.3 V vs. Ag/AgCl to oxidize both Ru(bpy)32+ and oxalate, false-colored as red. The signal is confined to the water|electrode interface due to the insolubility of oxalate in the oil phase, producing a contrast at the water|1,2-dichloroethane interface and allowing a direct measurement of the contact radius. (c) Overlay of the optical and ECL images. (d) Schematic representation of the two-phase system where ECL is confined within the aqueous microdroplet. Tetrabutylammonium perchlorate ([TBA][ClO4]) acts as a supporting electrolyte and charge balance mediator by the transfer of ClO4− into the droplet upon oxidation of the ECL reagents. (e) Schematic representation of the direct ECL reaction pathway. (f) Plot of the microdroplet radius as a function of the measured contact radius revealing a linear trend. A silver chloride wire and platinum wire served as the reference and counter electrode, respectively. The exposure time was 10 s. Reproduced from ref. 64 with permission from the American Chemical Society, copyright [2020].
