A general E-E/C mechanism for the counter-peak in luminol electrochemiluminescence

The curves of ECL intensity (IECL) versus potential (E) (IECL–E curves) were obtained when the electrochemiluminescence (ECL) of luminol was induced by cyclic voltammetry (CV). In the IECL–E curves, if there was an ECL peak during initial scan, a corresponding ECL peak called as a counter-peak was usually observed around the similar potential during the reversal scan. In the present work, a couple of strong and well-resolved ECL peaks were found in a model luminol solution consisted of 0.1 mol/L glycine–NaOH buffer system at pH 11.0 and 0.1 mol/L NaNO3 at a gold electrode. The effects of various factors on this couple of ECL peaks, including electrolyte, buffer solution, electrode material, pH, N2/O2/air atmosphere, scan rate, and electrochemical technique, were studied. On this basis, a continuous electron transfer coupled with a competitive chemical reaction (E-E/C) mechanism involved in an accumulation and consumption model has been proposed for the formation of the ECL counter-peak. Electrochemical digital simulation method was used to simulate the formation of the ECL counter-peak according to the proposed E-E/C mechanism. The results also supported the mechanism. Subsequently, it was further validated that ECL peak width, reversal potential and coexisted electroactive species were three important factors affecting the emergence and intensity of the ECL counter-peak. The proposed E-E/C mechanism is considered to be the general explanation of usual counter-peak phenomenon in the luminol ECL under CV conditions since the mechanism is involved in most pathways of the luminol ECL.