Ionophore-Based Biphasic Chemical Sensing in Droplet Microfluidics

Droplet microfluidics is an enabling platform for high-throughput screens, single-cell studies, low-volume chemical diagnostics, and microscale material syntheses. Analytical methods for real-time and in situ detection of chemicals in the droplets will benefit these applications, but they remain limited. Reported herein is a novel heterogeneous chemical sensing strategy based on functionalization of the oil phase with rationally combined sensing reagents. Sub-nanoliter oil segments containing pH-sensitive fluorophores, ionophores, and ion-exchangers enable highly selective and rapid fluorescence detection of physiologically important electrolytes (K+, Na+, and Cl-) and polyions (protamine) in sub-nanoliter aqueous droplets. Electrolyte analysis in whole blood is demonstrated without suffering from optical interference from the sample matrix. Moreover, an oil phase doped with an aza-BODIPY dye allows indication of H2O2 in the aqueous droplets, exemplifying sensing of targets beyond ionic species.