a Schematic of the nanofluidic device with 16 funnel-shaped, parallel nanochannels connecting two microchannels. Pressure differences ΔP between the in- and outlets continuously drive buffers through the microchannels. Different salt concentrations CN and CW in the two microchannels at the narrow and wide ends of the nanochannels, respectively, establish a salt gradient across the nanochannels. Nanoparticles (red dots) are loaded in the microchannel with low salinity and some get trapped in the nanochannels. b, c Schematic top and side view of the funnel-shaped nanochannels, respectively. d Schematic of nanoparticle trapping in a funnel-shaped nanochannel by diffusioosmosis and diffusiophoresis. The gradient induces a diffusioosmotic fluid flow velocity νos and a diffusiophoretic particle velocity νph. Nanoparticles (red dots) get trapped around the position x0 where the fluid and particle velocities balance each other. The concentration of nanoparticles in the nanochannel is denoted Cp(x). e Composite fluorescence microscope image of trapped exosomes in the same nanochannel (outlined with yellow) for three different salinity gradients, that is, different ln(CN/CW) values. Images were averaged over 10 s. f Fluorescence intensity along the nanochannels for the data shown in e (blue points). Full, red lines are independent fits to Cp(x) in Eq. (7). Fit parameters are the diameter d and the zeta potential ζ. g Exosome diameters and zeta potentials from fits shown in f. Red, dotted lines are the weighted averages. Error bars are the standard errors on the means for measurements in three different nanochannels (n = 3).