Green fatty-acid-based hydrophobic NADES with ultrasonic-assisted dispersive liquid-liquid micro extraction for fluorometric detection of rhodamine 6G in cosmetic products.

The objective of this study is to develop an environmentally friendly, rapid, and innovative analytical approach for the sensitive and selective determination of rhodamine 6G (Rh6G) in cosmetic products. To this end, an ultrasonic-assisted dispersive liquid-liquid microextraction (UA-DLLME) method was designed using a hydrophobic natural deep eutectic solvent (HNADES) synthesized from long-chain fatty acids (dodecanoic acid - C12 and decanoic acid - C10) and combined with fluorescence spectroscopy. Optimal extraction performance was achieved with a 1 : 2 molar ratio, pH 6, a solvent volume of 150 µL, a sonication time of 2 minutes, and a temperature of 45 °C, which were optimised as the most suitable conditions. Fluorescence measurements were performed at wavelengths of 532 nm (ex) and 553 nm (em). The method exhibits a wide linear range (0.93-500 µg L-1), high correlation coefficient (R2 > 0.9995), low limit of detection (LOD: 0.28 µg L-1), and high enrichment factor (EF = 38.5). Extraction efficiency ranged from 91.8% to 98.2%, with relative standard deviation values below 4%. The selectivity tests in the presence of various ions and dyes indicated no significant matrix interference. On application to actual cosmetic samples (lipstick, lip cream, nail enamel and nail polish), recovery values ranging from 91.0% to 101.6% were obtained. Furthermore, evaluations employing the AGREE (0.79), AGREEprep (0.80), and BAGI (77.5) metrics confirmed high environmental sustainability and practical applicability. The proposed UA-DLLME-HNADES-based fluorescence method offers a robust alternative for the determination of Rh6G in cosmetic matrices as a fast, sensitive, and reliable analytical strategy that minimises the use of toxic organic solvents.