Nanoparticles in a colloid are purified, with the colloid including a fluid, unwanted matter, and the nanoparticles to be purified. An electric field is applied that is substantially spatially uniform over a distance that is at least equal to a characteristic dimension of the nanoparticles, so that at least some of the nanoparticles move towards at least one collection surface as a result of the force arising between their electrical charge and the electric field, whereupon nanoparticles are collected on said at least one collection surface. The collection surface(s) may be one or more electrodes to which a voltage potential is applied. The collected nanoparticles are then removed from the collection surface, e.g., by dispersing them into another fluid.