Molecular and dimensional profiling of highly purified extracellular vesicles by fluorescence fluctuation spectroscopy. 
Cells secrete extracellular vesicles (EVs) into their microenvironment that act as mediators of intercellular communication under physiological conditions and in this context also actively participate in spreading various diseases.  Large efforts are currently made to produce reliable EV samples and to develop, improve, and standardize techniques allowing their biophysical characterization.  Here, we used ultrafiltration and size-exclusion chromatography for the isolation and a model-free fluorescence fluctuation analysis for the investigation of the physical and biological properties of EVs secreted by mammalian cells.  Our purification strategy produced enriched samples of morphologically intact EVs free of extravesicular proteins and allowed labeling of marker molecules on the vesicle surface for single-vesicle analysis with single-molecule sensitivity.  This novel approach provides information on the distribution profile of both EV size and relative expression level of a specific exosomal marker, deciphering the overall heterogeneity of EV preparations.