Exploring poly-L-lysine-based particle capture for atomic force microscopy studies of extracellular vesicles

We herein investigate the effects of varying the main experimental variables in one of the most used protocols for extracellular vesicle (EV) immobilisation on substrates for subsequent atomic force microscopy (AFM) quantitative morphometry and nanoindentation performed in liquid. We introduce the parameter Q as a quantitative measure of total adsorbed material and show how it can be used as an estimator of relative sample concentrations across multiple AFM imaging experiments. We show how Q is logarithmically dependent on substrate charge density, whereas the EV contact angle (CA) surprisingly does not follow the same dependence. Finally, we propose an optimised protocol for AFM quantitative morphometry in air that yields the same EV size distributions obtained in liquid.