Abstract
We present a method to photo-tag individual microfluidic droplets for latter selection by passive sorting. The use of a specific surfactant leads to the interfacial tension to be very sensitive to droplet pH. The photoexcitation of droplets containing a photoacid, pyranine, leads to a decrease in droplet pH. The concurrent increase in droplet interfacial tension enables the passive selection of irradiated droplets. The technique is used to select individual droplets within a droplet array as illuminated droplets remain in the wells while other droplets are eluted by the flow of the external oil. This method was used to select droplets in an array containing cells at a specific stage of apoptosis. The technique is also adaptable to continuous-flow sorting. By passing confined droplets over a microfabricated trench positioned diagonally in relation to the direction of flow, photo-tagged droplets were directed toward a different chip exit based on their lateral movement. The technique can be performed on a conventional fluorescence microscope and uncouples the observation and selection of droplets, thus enabling the selection on a large variety of signals, or based on qualitative user-defined features.
Highlights
Droplet microfluidics confines chemicals or cellular reagents in picoliter droplets transported in inert oil and is well suited for large-scale chemical or biological analysis [1,2]
We have recently developed a passive technology, Sorting by Interfacial Tension (SIFT), [31,32] that sorts droplets based on differences in interfacial tension that is linked to droplet pH
The technique is based on the observation that the interfacial tension of droplets can be very sensitive to droplet pH for a specific oil and surfactant combination
Summary
Droplet microfluidics confines chemicals or cellular reagents in picoliter droplets transported in inert oil and is well suited for large-scale chemical or biological analysis [1,2]. Fluorescence-activated droplet sorting (FADS) [10] uses electric fields for robust selection at rates as high as 30 kHz [15]. Powerful, these techniques are endpoint measurements that require the observation and active sorting to be performed close to each other in both time and space. Droplets are observed in series and the order of droplets must be maintained to ensure that the active sorting component selects the correct droplets. This limits the sorting criteria in two major ways. Kinetic measurements in droplets, such as fast kinetics [23] and Michaelis-Menton kinetic measurements [24], are difficult to integrate with current active droplet sorting technology
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