Fluorescence Polarization Anisotropy in Microdroplets.

Chemical reactions can be greatly accelerated in microdroplets, but the factors that lead to acceleration are still being elucidated. Using rhodamine 6G (R6G) as a model compound, we studied the density distribution and fluorescence polarization anisotropy of this dye in water-in-oil microdroplets. We found the density of R6G is higher on the surface of the microdroplets, and the ratio of the surface density to that of the center grows with increasing microdroplet radius or with decreasing R6G concentration. The measured fluorescence polarization anisotropy at the surface is almost the same for droplets of different sizes but becomes larger when the concentration is lowered. We also performed three-dimensional simulations by treating R6G and its associated anion as a dipole of fixed length and magnitude. The simulation results match quite well the experimental measurements, showing that the density distribution and fluorescence polarization anisotropy can be largely explained by a simple electrostatic model.