Potential-dependent electrochemiluminescence of luminol in alkaline solution at a gold electrode

The behavior of luminol electrochemiluminescence (ECL) at a polycrystalline gold electrode was studied under conventional cyclic voltammetric (CV) conditions. At least six ECL peaks were observed at 0.28 (ECL-1), 0.56 (ECL-2), 0.95 (ECL-3), 1.37 (ECL-4), −0.43 (ECL-5) and 1.00 (ECL-6, a broad wave after the reverse scan from +1.66) V (vs. SCE), respectively, on the curve of ECL intensity versus the potential. These ECL peaks were found to depend on the presence of O2 and N2, the pH of the solution, KCl concentration, scan rate, and potential scan ranges. The emitter of all ECL peaks was identified as 3-aminophthalate by analyzing the CL spectra. It is believed that ECL-1 at 0.28 V was correlated to luminol radicals produced by the electro-oxidation of luminol anion and ECL-2 at 0.56 V was caused by the reaction of luminol radical anions with gold oxide formed on the electrode surface. ECL-1 and ECL-2 could be strongly enhanced by O2 and O2−. ECL-3 at 0.95 V was likely to be due to the reaction of luminol radical anions with O2 oxidized by OH−. ECL-4 at 1.37 V suggested that OH− was electro-oxidized to HO2− at this potential and then to O2−, which reacted with luminol radical anions to produce light emission. ECL-5 at −0.43 V seems to be due to the reaction of luminol with ClO− electrogenerated at higher positive potential and HO2− electrogenerated at negative potential. ECL-6 was attributed to the reaction of luminol radical anions and ClO− electrogenerated at higher positive potential. The results indicated that luminol ECL can be readily initiated by various oxygen-containing species electrogenerated at different potentials, leading to multi-channel light emissions. Furthermore, the present work also reveals that ECL-2 is a predominant ECL reaction route at a gold electrode with higher potential scan rates under CV conditions.