Size photometry and fluorescence imaging of immobilized immersed extracellular vesicles

Abstract

Immunofluorescence analysis of individual extracellular vesicles (EVs) in common fluorescence microscopes is gaining popularity due to its accessibility and high fluorescence sensitivity, however, EV number and size are only measurable using fluorescent stains requiring extensive sample manipulations. Here we introduce highly sensitive label-free EV size photometry (SP) based on interferometric scattering (iSCAT) imaging of immersed EVs immobilized on a glass coverslip. We implement SP on a common inverted epifluorescence microscope with LED illumination and a simple 50:50 beamsplitter, permitting seamless integration of SP with fluorescence imaging (SPFI). We present a high-throughput SPFI workflow recording >10,000 EVs in 7 min over multiple fields of view, pre- and post-incubation imaging to suppress background, along with automated image alignment, aberration correction, spot detection, and EV sizing. We identify the upper sizing limit of SP with 440 nm illumination and extend EVs sizing from ∼35 nm in diameter to >200 nm with dual 440 nm and 740 nm illumination. We benchmark SP to flow cytometry using calibrated silica nanoparticles and demonstrate superior, label-free sensitivity. We showcase SPFI’s potential for EV analysis by experimentally distinguishing surface and volumetric EV dyes, observing the deformation of EVs adsorbed to a surface, and by uncovering distinct subpopulations in <100 nm-in-diameter EVs with fluorescently tagged membrane proteins.