Fig. (2) Synthesis of cationic colloidal silica-coated ferromagnetic nanoparticles (NPs). Top: Fe₃O₄ (magnetit) nanoparticle synthesis. Bottom: 3-Aminopropyltriethoxysilane (APTES)-hydrolysis and condensation  at hydrated magnetit surface resulting in a positively charged NP surface (modified after Howarter and  Youngblood, 2006) [11].