Microscopy and Nanoparticle Tracking

It is recommended and encouraged to characterize exosomes by two indispensable and complementary techniques, which usually are EM and nanoparticle tracking analysis (NTA) (19). The former provides the image of exosome morphology, and the latter gives information on the size distribution and concentration of exosomes (17). It is necessary to obtain a clear and high-resolution image of exosomes to ensure that the intact structure of exosomes is maintained, which requires hours of microscopy operation and observation (64). Cellular experiments like the uptake of exosomes in the absence of cell–cell contact are sometimes used to demonstrate the bioactivity of purified exosomes. NTA detects the Brownian movement of total nanoparticles of a sample but is unable to distinguish the phenotype or the origin of exosomes (65).