Electrogenerated chemiluminescence at droplet-modified electrodes: towards biphasic pKa measurement via proton-coupled electron transfer at liquid|liquid interfaces

This preliminary work reports, for the first time, electrogenerated chemiluminescence (ECL) at droplet-modified electrodes. The system involves oxidation of aqueous tris(2,2'-bipyridyl) ruthenium(II) ions at a glassy carbon electrode on which a random array of microdroplets of a highly hydrophobic tertiary amine (trioctylamine) are immobilised. Luminescence, produced via electron transfer over the cupola surface of the droplets from the oil to aqueous tris(2,2'-bipyridyl) ruthenium(III) ions synthesised via intra muros electrochemical oxidation at the uncovered parts of the electrode surface, is demonstrated. The extent of ECL production is shown to be dependent on the degree of interfacial protonation, with a proton-coupled biphasic electron transfer reaction occurring when the liquid vertical bar liquid interface is fully protonated, changing to a biphasic outer-sphere electron transfer mechanism when the interface is fully deprotonated. The competition and gradual dominance of one of these extreme mechanisms under intermediate interfacial protonation conditions thence enables the sensitive, kinetic estimation of the biphasic pK(a).