High-efficiency single-molecule detection within trapped aqueous microdroplets.

Aqueous droplets were used as a tool to confine a molecular population and enable highly efficient detection at the single-molecule level. Picoliter-sized aqueous droplets were generated using classical multiphase microfluidics with an aqueous stream containing the analyte under investigation and an oil carrier phase. The droplets were then localized and isolated in specially designed trapping areas within the microfluidic channel to provide a static environment for detection of the encapsulated molecules. We show that by continuously flowing the carrier oil phase while holding the aqueous stationary, we can significantly improve on measuring repeat single-molecule events. Further, we find that the flowing oil stream induces a circulation within the trapped droplets which is proportional to the volumetric flow velocity. This circulation phenomenon allows a given molecule to be detected multiple times during an experiment and can therefore be used for performing time-dependent single-molecule analysis.