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.
A three-dimensional flow-through electrode is shown to avoid loss of analytes during their preconcentration by electrokinetic focusing. An additional bed of insulating microbeads enhances focusing and minimizes unwanted electroconvective mixing.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.