Smart temperature-responsive deep eutectic solvent for multimechanistic homogeneous microextraction: Determination of toxic metals and food colorants

Smart deep eutectic solvents provide external control over analytical operations through stimulus-dependent changes in their physicochemical and phase behavior. In this work, a smart temperature-responsive deep eutectic solvent composed of lidocaine and heptanoic acid (1:1 M ratio) was investigated as a multimechanistic extraction medium for preconcentration and determination of inorganic (toxic metals) and organic (food colorants) analytes in water-soluble food products. Addition of 300 μL of the solvent to 10.0 mL of aqueous sample solution produced a homogeneous system, whereas heating at 80 °C for 10 min triggered phase separation and generated approximately 200 μL of an analyte-enriched upper phase without centrifugation. Hg(II), Pb(II), and Cd(II) were transferred through carboxylate-mediated reactive extraction. The sulfonated colorant Brilliant Blue FCF was retained predominantly through ion association with protonated lidocaine, whereas curcumin (E100) was extracted by hydrophobic partitioning. A common pH of 5.0 provided balanced extraction of all analytes. The linear ranges were 0.3-6.0, 0.6-6.0, and 0.9-7.0 μg kg^-1 for Hg(II), Pb(II), and Cd(II), respectively, 0.3-10 mg kg^-1 for Brilliant Blue FCF and 0.4-12 mg kg^-1 for curcumin in honey, sugar, and lozenge samples. Spike recoveries in real samples were 94-102 % for metals and 91-104 % for colorants. The results demonstrate that stimulus-controlled phase behavior can be combined with complementary analyte-dependent interactions within the same smart deep eutectic solvent-based extraction platform.