Chemiluminescent oxidation of 2-methylindol catalyzed by cobalt(II)

Chemiluminescence is the emission of light from a chemical reaction. Due to its intrinsic selectivity and sensitivity, chemiluminescent reactions are widely applied to detect biological and nonbiological analytes, including transition metal ions. This work presents the study and development of a chemiluminescent reaction mediated by cobalt(II). We found that cobalt(II) is an efficient catalyst for the oxidation of 2-methylindol (2-MI) by hydrogen peroxide (H2O2). The oxidation produced intense and long-lasting chemiluminescence. The selectivity to 2-MI was attested by comparing it with other indole derivatives. The light emission elicited by the oxidation of 2-MI was two orders of magnitude higher than the reaction using the isomer 3-methylindol (3-MI). A relationship between the exclusive formation of indoxyl dimers for the oxidation of 2-MI and not for 3-MI explained the specificity. The reaction was also selective regarding the metal ion used as a catalyst, as attested by comparing cobalt(II) with nine other transition metals, which were not able to promote light emission. The reaction was optimized regarding the pH, buffer constituents, and H2O2 concentration. The detection limit of Co2+ was 0.15 μM. Superoxide radical anion was generated in the reaction course and is involved in light emission efficiency. In conclusion, these findings open new perspectives for metal ion-catalyzed chemiluminescent reactions.