Determination of melatonin by potentiometric stripping analysis using sensor based on boron-doped diamond electrodes

A three-electrode screen-printed sensor with heavily doped microcrystalline boron-doped diamond electrodes grown by chemical vapor deposition on alumina substrates was used to determine the concentration of melatonin by constant current potentiometric stripping analysis. This paper provides a detailed examination of the irreversible oxidation behavior of melatonin by cyclic voltammetry at a boron-doped diamond electrode. The relationship between the current response and the square root of the scan rate confirmed a diffusion-controlled oxidation process. The research highlights the strong effect of electrode pretreatment on electrode properties, demonstrating its influence on electrochemical activity and electron-transfer kinetics. After the electrodes were cathodically pretreated, the melatonin responses increased more than four times compared to the anodically pretreated electrodes. Optimum results were obtained in constant current potentiometric stripping analysis at pH value of 3.0 using the Britton-Robinson buffer solution as an electrolyte. Further optimization of the accumulation potential/time, and stripping current led to improved melatonin detection sensitivity, achieving a significantly lower limit of detection of 14.6 μg L^-1 compared to square wave voltammetry (110 μg L^-1). Interference studies assessed the impact of selected organic compounds on melatonin determination. While some compounds like dopamine, l-ascorbic acid, and uric acid showed minimal interference, l-tryptophan and serotonin significantly influenced melatonin responses. The sensor was successfully applied to determine of melatonin in a commercial pharmaceutical supplement. A satisfactory result was obtained by constant current potentiometric stripping analysis as the recovery reached 100.75 % compared to the reference value obtained by the LC-HRMS method. The proposed method, due to the excellent properties of the boron-doped diamond electrode and constant current potentiometric stripping analysis, provides a fast measurement procedure of melatonin with high sensitivity (compared to square wave voltammetry) at a stable background signal. Moreover, the method is insensitive to dissolved oxygen in the electrolyte and only the stripping current of the potentiometric stripping analysis needs to be optimized.