(168b) Compound Droplet Microfluidics for On-Drop Separations and Sensing

The ability to rapidly compartmentalize chemical and biological reactions into picoliter drops and perform automated analysis on each individual drop promises to revolutionize laboratory-based experimentation, enabling both time-resolved kinetic measurements and rapid exploration of large experimental parameter spaces. In this paper, we present a new and general droplet-based microfluidic scheme with biphasic compound droplets in which flowing droplets function not only as isolated reaction flasks, but are also capable of on-drop separation and sensing. To demonstrate this, ionic liquids (ILs) are chosen as designer liquids whose chemical and physical properties can be tailored in task-specific fashion. We create aqueous-ionic liquid compound droplets with tunable structures and present two analytical applications. As a first application example, we demonstrate separation of a binary aqueous mixture of organic dyes, one of which is highly soluble in the IL-phase ([EMIm][NTf₂]). Therefore, that particular dye is rapidly extracted into the IL compartment, while another one remains in the aqueous phase. The changes in the color of flowing droplets are tracked and extraction time obtained matches well with theoretical estimation. As for the second application, we demonstrate ?on-drop' sensing where the ionic liquid in the compound droplet serves as a non-invasive chemical sensor directly attached onto an aqueous droplet. A pH indicator-doped ionic liquid is used to measure the acidity of an aqueous phase, where the IL compartment gradually changes color as it becomes progressively acidic by mass transfer of the acid from the aqueous to IL phase. Steady-state operation enables us to calibrate original pH of the aqueous phase versus dynamic color change of the ionic liquid as the compound droplets translate along the microchannel. By combining designer fluids with designer microfluidic emulsions, our work paves the way for applications where a myriad of reactive and analytical processes occur and can be sensed concurrently within flowing microscale droplets.