Optical Control of Förster Resonance Energy Transfer in a Single Ionic-Liquid Microdroplet.

Controlling intermolecular distance in microscale environments is an effective strategy for regulating photophysical processes such as Förster resonance energy transfer (FRET). However, direct control of the local molecular concentration required for this purpose remains challenging due to limited approaches. Here, we demonstrate that laser-generated ionic-liquid (IL) microdroplets provide a platform for optically controlling molecular concentration. Optical tweezers induce local liquid-liquid phase separation in an aqueous solution of tributyl-n-octylphosphonium bromide, producing a single microdroplet. Fluorescence-energy donor and acceptor molecules are rapidly coextracted and concentrated within the droplet, and their close proximity induces FRET. Notably, fluorescence lifetime analysis reveals two donor populations, corresponding to FRET-inactive and FRET-active species, with the fraction of the latter increasing during droplet growth. These results demonstrate that FRET enhancement arises from IL-mediated nanoscale colocalization rather than homogeneous concentration effects.