ABSTRACT
Electrogenerated chemiluminescence (ECL) can be generated by annihilation reactions between oxidized and reduced species produced at a single electrode by using an alternating potential, or at two separate electrodes in close proximity to each other by holding one electrode at a reductive potential and the other at an oxidative potential [1]. Because annihilation reactions are very energetic (typically 2-3 eV) and potential windows for aqueous solutions are generally too narrow to allow convenient electrolytic generation of both the oxidized and reduced ECL precursors, most annihilation ECL processes have been investigated in organic solvents or partially organic solutions [2]. By adding certain species, called coreactants, into solutions containing luminophore species, ECL can also be generated with a single potential step or one directional potential scanning at an electrode; this permits ECL to be observed in aqueous solutions [1]. Depending on the polarity of the applied potential, both the luminophore and the coreactant species can first be oxidized or reduced at the electrode to form radicals, and intermediates formed from the coreactant then decompose to produce a powerful reducing or oxidizing species that reacts with the oxidized or reduced luminophore to produce the excited states that emit light. Because highly reducing intermediate species are generated after an electrochemical oxidation of a coreactant, or highly oxidizing ones are produced after an electrochemical reduction, the corresponding ECL reactions are often referred to as “oxidative reduction” ECL and “reductive oxidation” ECL, respectively [2,3]. Thus, a coreactant is a species that, upon electrochemical oxidation or reduction, immediately undergoes chemical decomposition to form a strong reducing or oxidizing intermediate that can react with an oxidized or reduced ECL luminophore to generate excited states. Common
coreactants for oxidative reduction ECL are oxalate (See Section II.A) and tertiary amines (See Section II.C). Upon oxidation, the oxalate ion loses CO2, producing CO2•, a strongly reducing agent, whereas tertiary amines deprotonate to yield strongly reducing radical species. A typical example of coreactants used for reductive oxidation ECL would be peroxydisulfate (Section II.B), which, upon reduction, forms SO4•, a strong oxidant. Clearly, unlike annihilation ECL, where all starting species can be regenerated after light emission, coreactant ECL can regenerate luminophore species only while the coreactant is consumed via the ECE [1] reactions.
