Electrochemiluminescence-based detection of ferrocene derivatives at monolayer-coated electrodes

Luminol/hydrogen peroxide chemiluminescence catalyzed by electrochemically oxidized ferrocene derivatives was investigated at gold electrodes coated with alkanethiolate monolayers. The monolayer serves not only to suppress the direct luminol electrooxidation and consequent background chemiluminescence, but also to promote oxidation of certain ferrocene derivatives, which then act to catalyze the luminescent reaction between luminol and hydrogen peroxide. Chemiluminescence was particularly strongly promoted in a designed monolayer/catalyst system whereby attractive electrostatic interactions between a positively-charged ferrocene catalyst and a negatively-charged monolayer promote facile ferrocene oxidation, whereas repulsive interactions between the monolayer and the negatively-charged luminol molecule inhibit direct luminol oxidation. The ferrocene catalyst was detected both voltammetrically and by light emission in this system; however, detection by light emission exhibited a higher signal to background ratio than detection by oxidative current at comparable ferrocene concentrations.