Measuring Size and Zeta Potential of Nanoparticles with a Salt Gradient

Nanometer-sized lipid vesicles play an important role in the fields of drug delivery and diagnostics. Their size and zeta potential affect their adsorption on the cell membrane and subsequent absorption by the cell. Hence the size and zeta potential are key parameters for designing liposome-based drug delivery vehicles and for understanding the body's cell-to-cell communication with exosomes. Current nanoparticle characterization techniques require, however, several separate experiments and are challenged by heterogeneous samples. Here, we concentrate both exosomes and liposomes from dilute solutions and measure their size and zeta potential in a one-step measurement with a salt gradient in a funnel-shaped nanochannel. The salt gradient induces a diffusiophoretic particle migration towards higher salt concentration and an oppositely directed difussioosmotic fluid flow. Particles in the nanochannel are trapped where the diffusiophoretic particle velocity and the diffusioosmotic fluid velocity balance each other. As the diffusiophoretic particle velocity depends on the particles’ diameter and zeta potential, these parameters can be inferred from a single measurement of the spatial distribution of particles in the trap. We concentrate ensemble of particles to approximately 500 times their initial concentration and characterize them. We also demonstrate trapping and characterization of individual particles, which allows for resolving sample heterogeneity. The trapping position can in both cases be tuned to occur at physiological salinity. The method is implemented in a nanofluidic polymer device that can be mass-produced at low cost, and it is also applicable for other types of nanoparticles. As nanoparticles can be kept in the trap while reactants are introduced in the nanochannel, monitoring surface reaction on nanoparticles can be envisioned.