Strong Concentration Enhancement of Molecules at the Interface of Aqueous Microdroplets
Hanqing Xiong, Jae Kyoo Lee, Richard N. Zare, Wei Min
Abstract
Water is arguably the most common and yet least understood material on Earth. The interface between water and a hydrophobic medium, such as air, oil, or lipids, plays a fundamental role in chemistry and biology. However, the behavior of molecules at interface of micron-sized water droplets (microdroplets) in such media is poorly characterized. Herein we employed two-photon fluorescence microscopy and Förster resonant energy transfer imaging to study the probe localization in water-oil microdroplets with high contrast and resolution. We found that there exists a general effect of surface enrichment and orientation alignment for water-soluble probes. Remarkably, probes are concentrated into a ∼10 nm thin layer at the microdroplet water-oil interface by up to 10 000-fold compared to the bulk counterpart. We suggest that the strong enrichment and alignment of water-soluble molecules, likely to be induced in part by a local electric field at the interface, could be a major factor accounting for orders of magnitude faster reaction rates observed in aqueous microdroplets compared to their bulk counterparts.