A method for purifying a suspension containing colloid-seeded nanoparticles and excess colloids is provided that includes adding to the suspension a filter aid comprising a salt. The method further includes filtering the suspension with a filter of a pore size intermediate between the average colloid-seeded nanoparticle size and the average excess colloid size, so as to form a retentate that includes the majority of the colloid-seeded nanoparticles and a filtrate that includes the majority of the excess colloids. Still further, the method includes collecting the retentate. The method may be incorporated into a method of making metallized nanoparticles, such as nanoshells, by reduction of metal ions onto the purified colloid-seed nanoparticles so as to form the metallized nanoparticles.