In recent years, there has been an increasing demand in using water-in-oil (W/O) microdroplet compartmentalization for biochemical reactions. Each W/O droplet holds pico to nanoliter volume of aqueous solution and can represent a separate micro-reactor. The use of W/O reduces the cost of sample consumption and increases signal sensitivity due to the large number of droplets in small volume.
Fig. 1. (a) W/O droplets containing one or two GFP-expressing E. coli in Brain Heart Infusion liquid medium immediately after encapsulation. Red arrows indicate GFP-expressing E. coli. (b) W/O droplets 20 h after encapsulation before (left) and after (right) sorting on On-chip Sort.
Flow cytometry can be used to analyze and sort W/O droplets after carrying out reactions inside them. However, conventional cell sorters are incompatible with non-aqueous suspensions, hence it is necessary to encapsulate W/O droplets in another aqueous phase, forming water-in-oil-in-water (W/O/W) double emulsions. In comparison to single emulsion droplets, double emulsions have poor monodispersity and low encapsulation efficiency, thus the selection of ‘active’ double droplets during sorting becomes stringent. On-chip Sort enables efficient sorting of W/O droplets using oil as the sheath fluid.
W/O droplets (40 µm in size) were generated to trap one or two GFP-expressing E. coli in Brain Heart Infusion liquid medium. Approximately 50% of the emulsion droplets in the sample contained E. coli. The emulsion droplets generated were left to stand at room temperature for 20 h. After 20 h, droplets containing E. coli with high GFP intensity were isolated on On-chip Sort using mineral oil as the sheath fluid (Fig. 1). After sorting, almost 100% of the isolated emulsion droplets contained E. coli. Therefore, these results suggest that On-chip Sort efficiently isolated E. coli enclosed water-in-oil emulsion droplets using oil as the sheath fluid.