Microfluidic formation of surface nanodroplets using green deep eutectic solvents for liquid–liquid nanoextraction and controlled precipitation

Hypothesis: Surface nanodroplets have recently been employed for in situ chemical analysis leveraging their low volume, e.g. O(10⁻¹⁵ L), that enables rapid analyte extraction and pre-concentration. So far, most surface nanodroplets have been formed using single organic solvents such as 1-octanol, toluene, among others. Designing multicomponent surface nanodroplet with controllable composition is highly desirable for extending their application as extractant. Experiment: Here, we formed surface nanodroplets using green deep eutectic solvent (gDES) composed of thymol and decanoic acid, both of which are naturally occurring chemicals. The influence of parameters such as flowrate and the composition of deep eutectic solvent on the surface nanodroplet formation were studied. As proof-of-concept, the gDES surface nanodroplets were further used to extract and detect trace amounts of fluorescent rhodamine 6G dye and copper ions from water. Findings: The formation of gDES surface nanodroplets follows the theoretical model which states that the final droplet volume (V_f) scales with the Peclét number (Pe) of the flow during formation by the solvent exchange process, that is V_f ∼ Pe^3/4, and the nanodroplets demonstrate excellent ability as extractant for rhodamine 6G and copper ions from water. Surprisingly, the confined volume of gDES surface nanodroplets enables fast and controlled formation of Cu (II)-decanoate crystal.