Reverse micelle-assisted dispersive liquid-liquid microextraction for trace determination of methylene blue in aqueous samples

Abstract

This study presents a sensitive and eco-friendly method for detecting trace-level methylene blue (MB) in water by combining reverse micelle-assisted dispersive liquid-liquid microextraction (RM-DLLME) with spectrophotometry. Mechanistic studies using dynamic light scattering (DLS) and transmission electron microscopy (TEM) revealed elliptical reverse micelles (110–140 nm) that expanded to 230–400 nm after MB extraction, visually confirming encapsulation. Spectral analysis indicated that MB displaced water molecules at sodium dodecyl sulfate (SDS) binding sites, with the remarkable enrichment efficiency attributed to molecular interactions, the spacious internal cavities and large contact surfaces of the reverse micelles. The method achieved excellent performance: a wide linear range (0.6–200 ng/mL, R² = 0.9987), a low detection limit of 0.18 ng/mL, a high preconcentration factor of 64.08, and good precision (intra- and inter-day relative standard deviations (RSDs) ≤ 3.45%). It demonstrated high accuracy (recoveries of 91.49–107.46%) across five water matrices and strong interference resistance toward common ions and competing dyes. With an Eco-Scale score of 83, the method offers an excellent balance of high sensitivity, operational simplicity, and environmental friendliness, overcoming key limitations of conventional spectrophotometry and providing a robust alternative for monitoring hydrophilic dyes in environmental waters.