After that time, the pH value of the reaction mixture was
again modified to 1.0 using HNO3 (63%); this modification
of pH produces a reversible flocculation of the nanoparticles,
allowing us to easily separate them from the subproducts of
the reaction by centrifugation at 6000 rpm. The supernatant
was discarded and the pellet was redispersed using deionized
water and, if necessary, the pH was adjusted to 7.0.