Development of a Juglone/Mesoporous Carbon Modified Glassy Carbon Electrode for Ultrasensitive Determination of Melatonin in Dietary Supplements

A novel electrochemical sensing platform based on a glassy carbon electrode (GCE) modified with a juglone/mesoporous carbon composite (JUG-MC/GCE) was developed for the ultrasensitive determination of melatonin (MEL) in pharmaceutical formulations and dietary supplements. Incorporation of juglone and mesoporous carbon within the coating deposited on the glassy carbon electrode, resulted in enhanced charge-transfer characteristics at the electrode–electrolyte interface. Under optimized differential pulse voltammetry (DPV) conditions in a 0.1 mol L−1 McIlvaine buffer (pH 2.8), the proposed sensor demonstrated an exceptional electrocatalytic response for melatonin oxidation via a two-electron, one-proton irreversible pathway under kinetic control. The analytical performance reveal multiple linear calibration intervals with an ultra-low limit of detection (LOD) of 0.21 µg L−1 (0.9 nM). The sensor manifested robust operational repeatability (RSD ≤ 1.8%), long-term storage stability, and satisfactory selectivity toward melatonin in the presence of common tablet excipients and selected potential interferents. The practical utility of the JUG-MC/GCE platform was successfully validated through the direct quantification of melatonin in commercial tablets, capsules, and complex jelly matrices, confirming its suitability for melatonin determination in real samples.