Ionic liquid-based compound droplet microfluidics for 'on-drop' separations and sensingPublished as part of a special issue dedicated to Emerging Investigators: Guest Editors: Aaron Wheeler and Amy Herr.Electronic supplementary information (ESI) available: Supplementary details, figures (Fig. S1-S4), table (Table ST1) and movies (Movie M1-M5). See DOI: 10.1039/c004853d

We present a new and general scheme for analytical applications of droplet-based microfluidics in which flowing droplets function not only as isolated reaction flasks, but are also capable of on - drop separation and sensing. To demonstrate this, we choose ionic liquids as designer fluids whose chemical and physical properties can be tailored in task-specific fashion. We create aqueous-ionic liquid compound droplets with tunable structures using an imidazolium-based ionic liquid, and present two analytical applications-separation of a binary aqueous mixture of organic dyes and dynamic pH sensing-to highlight the salient features of this scheme. By combining designer fluids with designer microfluidic emulsions, our work opens up a rich space of exploration for analytical microfluidics. A new and general scheme for analytical applications of droplet microfluidics, in which ionic liquid-based compound droplets function not only as isolated reaction flasks, but are also capable of 'on-drop' separation and sensing.