Hydrosomes: optically trapped water droplets as nano-containers

Abstract

We demonstrate a novel technique for creating, manipulating, and combining femtoliter to attoliter volume chemical containers. Possible uses include creating controlled chemical reactions involving small quantities of reagent, and studying the dynamics of single molecules. The containers, which we call hydrosomes, are surfactant stabilized aqueous droplets in a low index-of-refraction fluorocarbon medium. The index of refraction mismatch between the container and fluorocarbon is such that individual hydrosomes can be optically trapped by single focus laser beams, i.e. optical tweezers. Previous work on single molecules usually involved the tethering of the molecule to a surface, in order to interrogate the molecule for an extended period of time. The use of hydrosomes opens up the possibility for studying free molecules, away from any perturbing surface. We show that this is indeed true in the case of quantitative FRET with RNA. Furthermore, we demonstrate the controlled fusion of two hydrosomes for studying reactions, such as DNA binding kinetics, and single molecule dynamics under non-equilibrium conditions. We also show the applicability of our technique in analytical chemistry, such as for molecule identification and sorting.