μPADs for a Safer Night's Sleep: An Alternative for Quality Control of Melatonin Pharmaceutical Formulations

Melatonin, known as the "sleep hormone," has its primary function in regulating the circadian cycle, a 24 h sleep-wake pattern that leads to significant physical, mental, and behavioral changes. The use of melatonin in pharmaceutical formulations gained popularity as a tool to combat sleep disorders such as insomnia and has been increasing in the last few decades. Melatonin-based pharmaceutical formulations are classified as dietary supplements, which leads to a different perspective on the necessity of strict quality control for these products. There are already extensive descriptions in the literature of the serious side effects associated with the administration of those formulations, including, for example, higher dosages than indicated on the label or contamination by other hormones. Still, there is a gap in devices that could make an easy and low-cost determination of melatonin. This work presents a paper-based sensor for the quality control of melatonin in pharmaceutical formulations, utilizing the modified Van Urk reaction. This reaction causes a color change from white to blue on the paper surface, correlating with the melatonin concentration. The constructed sensor performed well in quantifying melatonin in solid (0.22 mg/tablet) and liquid (0.20 mg/drop) pharmaceutical formulations, considering the maximum standard deviation when compared with results obtained by a gold-standard technique (liquid chromatography). The developed method demonstrated good linearity (R² = 0.9915) over a melatonin concentration range of 40-300 mg L^-1 and exhibited a specific response to the hormone when tested against the vehicles and excipients of the pharmaceutical formulations. Therefore, the proposed sensor demonstrates excellent potential as an inexpensive tool for quality control, introducing an innovative approach to the use of paper-based microfluidic devices in this field.