Distance-based thread analytical devices coupled with dispersive liquid-liquid microextraction for melatonin detection

This article reports the first distance-based thread analytical devices (dTADs) coupled with a dispersive liquid-liquid microextraction (DLLME) method for quantitative monitoring of trace melatonin levels in human samples and dietary supplements. Detection is based on colorimetric reaction between melatonin and 2,3-naphthalenedialdehyde (2,3-Nda) carried out on a thread substrate. Herein, melatonin is preconcentrated and extracted to the organic phase of an extraction solvent utilizing the DLLME method. A little droplet of the extraction solvent was then introduced into the dTAD. The yellow-orange colored length is measured, which is proportional to the amount of melatonin level. We obtained a linear range from 15.0 to 45.0 pg mL−1 (R2 = 0.9949) and a detection limit (LOD) of 2.50 pg mL−1, resulting in the clinically relevant ranges for melatonin monitoring in human samples. This technique also exhibited remarkable reproducibility and selectivity. The proposed method is simple, inexpensive, and exhibits more sensitivity than other methods that request expensive reagents and instrumentation. Additionally, this assay constituted an accurate and precise device for concentration quantification in human control samples with recovery percentage ranging from 98.42% to 102.52%. Likewise, there was no significant difference in measured melatonin levels obtained from our method against the nutrition label obtained from melatonin dietary supplements by t-test validation. The combination of an economical dTAD with a simple microextraction method, the proposed device therefore has the advantage of simplicity, instrument-free convenience, and affordability for trace melatonin detection, making it suitable for point-of-care medical diagnostics and food analysis.