Extracellular
vesicles (EVs) are membranous particles released
by most cells in our body, which are involved in many cell-to-cell
signaling processes. Given the nanometer sizes and heterogeneity of
EVs, highly sensitive methods with single-molecule resolution are
fundamental to investigating their biophysical properties. Here, we
demonstrate the sizing of EVs using a fluorescence-based flow analyzer
with single-molecule sensitivity. Using a dye that selectively partitions
into the vesicle’s membrane, we show that the fluorescence
intensity of a vesicle is proportional to its diameter. We discuss
the constraints in sample preparation which are inherent to sizing
nanoscale vesicles with a fluorescent membrane dye and propose several
guidelines to improve data consistency. After optimizing staining
conditions, we were able to measure the size of vesicles in the range
∼35–300 nm, covering the spectrum of EV sizes. Lastly,
we developed a method to correct the signal intensity from each vesicle
based on its traveling speed inside the microfluidic channel, by operating
at a high sampling rate (10 kHz) and measuring the time required for
the particle to cross the laser beam. Using this correction, we obtained
a threefold greater accuracy in EV sizing, with a precision of ±15–25%.