Observation of FRET in collision of droplets

Förster Resonance Energy Transfer (FRET) is a radiationless distance-dependent transfer of energy from an excited donor fluorophore to an acceptor fluorophore. This radiationless interaction of a donor-acceptor pair through resonance is observed by an increase/decrease in the acceptor/donor fluorescence intensity, respectively. Here we present preliminary results on the fluorescence spectra of optically levitated micro-droplets doped with two different dyes that works as FRET pair. The laser light used for levitation (λ=660 nm) passes through a telecentric system of lenses to form a controllable double optical trap system. Micrometer sized droplets are produced using two on-demand piezo-driven dispensers. This allows independent trapping of differently dyed droplets in two traps where a collision between the droplets can be induced by moving the trap positions. The dye molecules mix when two droplets collide and coalesce. The emission spectrum obtained when the droplets are illuminated with laser having a wavelength of 532 nm is observed with a spectrometer which can record up to 26,000 spectra per second. We compare the results with the spectra taken from the same solutions in a cuvette. The results indicate that we are able to observe the FRET effect in single droplets with an exposure time as short as 100 µs. This spectroscopic investigation is an ongoing research project with the long-term goal to investigate environmental effects of aerosols in the atmosphere.