Enhanced electrogenerated chemiluminescence of Ru(bpy)32+/TPrA (bpy=2,2׳-bipyridine; TPrA=tri-n-propylamine) via oxygen quenching using melatonin

The effect on the electrogenerated chemiluminescence (ECL) of the Ru(bpy)32+/TPrA (bpy=2,2׳-bipyridine; TPra=tri-n-propylamine) system when melatonin (N-acetyl-5-methoxytryptamine; MLT) and structurally related compounds (e.g., d- and l-Tryptophan (TRY), 7-Azatryptophan (7-AZA) and Serotonin (SER)) are present in aqueous buffered solution is reported. MLT, d- and l-TRY, SER and 7-AZA display weak intrinsic ECL when TPrA is used as an oxidative-reductive coreactant. However, micromolar concentrations of melatonin result in up to 2.5-fold enhancement of Ru(bpy)32+/TPrA ECL while the other analytes attenuate ECL between 2- and 1000-fold. Photoluminescence (PL) emission efficiencies do not change in the presence of melatonin unless the melatonin solution has undergone electrochemical bulk oxidation, at which point PL and ECL are nearly indistinguishable. Spectroscopic, electrochemical and spectroelectrochemical studies indicate that the mechanism involves oxygen scavenging by melatonin oxidation products. This scavenging prevents the quenching of the *Ru(bpy)32+ excited states by dissolved oxygen in solution. Melatonin can be coupled with 30% by volume 2,2,2-trifluorethanol (TFE) or with the nonionic surfactant Triton X-100 (polyethylene glycol tert-octylphenyl ether) for even greater enhancement of Ru(bpy)32+/TPrA ECL.