Concentration Enrichment, Separation, and Cation Exchange in Nanoliter-Scale Water-in-Oil Droplets.

Droplet-based techniques have had a profound impact in chemistry, owing to their ability to perform rapid and massively parallel reactions in minute fluid volumes. In many applications, concentration enrichment is required to increase the speed of reactions or the sensitivity of assays; but in-droplet concentration enrichment remains challenging. Here, we interface electrokinetic concentration polarization with droplet microfluidics to accomplish in-droplet demixing. This result is significant because the concentration of any charged species in the droplet can be enriched and the approach can be readily integrated into existing droplet workflows. Further, we show that such electrokinetic enrichment is rapid, on the order of seconds, and is robust, occurring over a wide parametric space. We further demonstrate electrokinetic separation of two anionic fluorophores within the droplet. Such a capability potentiates the droplet-templated synthesis of particles with gradient composition and the development of mobility-shift assays, which rely on discrimination of multiple species tagged with a single color fluorophore. Finally, by using a calcium-binding dye as an indicator, we demonstrate in-droplet cation exchange. This demonstration of cation exchange in droplets is significant because of its broad applicability to strategies for synthesis and bioassays. These results lay the foundation for new advanced droplet techniques with transformative applications.