R. Wang, L. Lei, P. Wang, A. Levine, G. Popescu, ” Dispersion-relation Fluorescence Spectroscopy” ,Phys. Rev. Lett.

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Because of its ability to study specifically labeled structures, fluorescence microscopy is the most
widely used technique for investigating live cell dynamics and function. Fluorescence correlation
spectroscopy is an established method for studying molecular transport and diffusion coefficients at a
fixed spatial scale. We propose a new approach, dispersion-relation fluorescence spectroscopy (DFS), to
study the transport dynamics over a broad range of spatial and temporal scales. The molecules of interest
are labeled with a fluorophore whose motion gives rise to spontaneous fluorescence intensity fluctuations
that are analyzed to quantify the governing mass transport dynamics. These data are characterized by the
effective dispersion relation. We report on experiments demonstrating that DFS can distinguish diffusive
from advection motion in a model system, where we obtain quantitatively accurate values of both
diffusivities and advection velocities. Because of its spatially resolved information, DFS can distinguish
between directed and diffusive transport in living cells. Our data indicate that the fluorescently labeled
actin cytoskeleton exhibits active transport motion along a direction parallel to the fibers and diffusive in
the perpendicular direction..