Direct Observation of C₂O₄ ^•- and CO₂ ^•- by Oxidation of Oxalate within Nanogap of Scanning Electrochemical Microscope

Oxalate oxidation in the presence of different oxidized luminophores leads to the emission of light and has been studied extensively in electrogenerated chemiluminescence (ECL). The proposed mechanism involves the initial formation of the oxalate radical anion, C₂O₄ ^•-. The ensuing decomposition of C₂O₄ ^•- produces a very strong reductant, CO₂ ^•-, which reacts with the oxidized luminophores to generate excited states that emit light. Although the mechanism has been proposed for decades, the experimental demonstration is still lacking, because of the complexity of the system and the short lifetimes of both radical anions. To address these issues, we studied oxalate oxidation at platinum ultramicroelectrodes (UMEs) in anhydrous N,N-dimethylformamide (DMF) solution by nanoscale scanning electrochemical microscopy (SECM) with the tip generation/substrate collection (TG/SC) mode. A Pt nanoelectrode was utilized as the SECM generator for oxalate oxidation, while another Pt UME served as the SECM collector and was used to capture the generated intermediates. We studied the influence of the gap distance, d, on the substrate current (i_s). The results indicate that, when 73 nm < d < 500 nm, the species captured by the substrate were primarily CO₂ ^•-, while C₂O₄ ^•- was the predominant intermediate measured when d was below 73 nm. A half-life of 1.3 μs for C₂O₄ ^•- was obtained, which indicates a stepwise mechanism for oxalate oxidation. The relevance of these observations to the use of oxalate as the coreactant in ECL systems is also discussed.