Smartphone Imaging-Based Luminol Electrochemiluminescence at the Cobalt Electrode for Sinapic Acid Determination

Developing simple and portable luminol electrochemiluminescence (ECL) systems without externally added co-reactants is important for on-site analysis. Cobalt is a low-cost and redox-active transition metal with favorable electrocatalytic properties toward water-oxidation-related interfacial reactions in alkaline media, which may facilitate luminol oxidation and the generation or activation of reactive oxygen intermediates during ECL. In this work, for the first time, a bare cobalt electrode was introduced into a co-reactant-free luminol ECL system and used to construct a simple direct-current two-electrode platform, with the ECL signal recorded by smartphone imaging. Compared to a carbon-fiber electrode, the cobalt electrode generated a much stronger luminol ECL signal, with an enhancement of more than 80-fold in the smartphone imaging system, indicating its strong promotion of the electrode reactions associated with luminol ECL. After optimization of the imaging parameters and experimental conditions, a portable analytical method for sinapic acid determination was established. Under the optimized conditions, the proposed system showed a linear range of 1–100 μM and a limit of detection of 0.26 μM. The method also showed acceptable anti-interference capability under the tested conditions and satisfactory recoveries in spiked white mustard seed samples. These results demonstrate a simple, low-cost, and portable strategy for co-reactant-free luminol ECL analysis and highlight the potential of cobalt electrodes for proof-of-concept on-site sensing.