Automated temperature-responsive deep eutectic solvent microextraction for rapid determination of synthetic dyes in food samples

A automated microextraction method based on temperature-responsive deep eutectic solvents (TRDESs) was developed for the determination of synthetic dyes in food samples. The system employs a homogeneous aqueous TRDES solution, composed of lidocaine and heptanoic acid (1:1 mol/mol), which acts both as extraction medium and as a thermally induced phase-separating agent. Upon heating above the lower critical solution temperature, spontaneous phase separation occurs, allowing direct collection of the enriched TRDES phase without centrifugation or manual handling. The entire process from sample introduction to extract collection is integrated into a programmable flow-based platform. The method was optimized and validated for four banned synthetic dyes: Rhodamine 6G, Sudan I, II, and III. Good linearity (0.025–100 mg L⁻¹), low limits of detection (0.001–0.008 mg L⁻¹), and high intra-/inter-day precision (RSDs: 2–7 %) were achieved. Real beverage samples were successfully analyzed, and spiked recoveries showed good agreement with a reference DLLME method (relative bias: 1–5 %). All steps were performed without the use of toxic organic solvents or centrifugation. Molecular dynamics simulations confirmed that π–π stacking and van der Waals interactions between rhodamine and lidocaine govern the extraction mechanism. Additionally, the green performance of the method was assessed using AGREEprep and ComplexGAPI, demonstrating high environmental sustainability. Overall, the proposed method offers a robust, automated, and environmentally friendly solution for trace-level screening of banned dyes in complex food matrices.