Structural-Mechanical Characterization of Nanoparticle Exosomes in Human Saliva, Using Correlative AFM, FESEM, and Force Spectroscopy

All living systems contain naturally occurring nanoparticles with unique structural, biochemical, and mechanical characteristics. Specifically, human saliva exosomes secreted by normal cells into saliva via exocytosis are novel biomarkers showing tumor-antigen enrichment during oral cancer. Here we show the substructure of single human saliva exosomes, using a new ultrasensitive low force atomic force microscopy (AFM) exhibiting substructural organization unresolvable in electron microscopy. We correlate the data with field emission scanning electron microscopy (FESEM) and AFM images to interpret the nanoscale structures of exosomes under varying forces. Single exosomes reveal reversible mechanical deformation displaying distinct elastic, 70-100 nm trilobed membrane with substructures carrying specific transmembrane receptors. Further, we imaged and investigated, using force spectroscopy with antiCD63 IgG functionalized AFM tips, highly specific and sensitive detection of antigenCD63, potentially useful cancer markers on individual exosomes. The quantitative nanoscale morphological, biomechanical, and surface biomolecular properties of single saliva exosomes are critical for the applications of exosomes for cancer diagnosis and as a model for developing new cell delivery systems.