Synthesis of Silica-coated Iron Oxide Nanoparticles: Preventing Aggregation without Using Additives or Seed Pretreatment

The Stober process is frequently used to prepare silica-coated iron oxide nanoparticles. This is usually achieved by seeding a reaction mixture consisting of water, ethanol and a catalyst with iron oxide particles and adding a silica precursor. The hydrolysis and condensation of precursor monomers results in the deposition of a silica layer on iron oxide particles. However, this process is accompanied by an increase in the ionic strength of the medium which promotes the rapid aggregation of iron oxide particles. A number of methods have been developed to prevent seed aggregation during the coating process. The majority of these methods include a pretreatment step in which the surface of iron oxide particles is modified in a manner that increases their stability in aqueous solutions. Here we suggest that by decreasing the initial concentration of the catalyst for a short period to minimize nucleation by reducing precursor hydrolysis rate and then gradually increasing the concentration to the optimum level to allow silica formation to proceed normally it may be possible to prevent aggregation without surface modification. The properties of the resulting nanoparticles as analyzed by transmission electron microscopy and magnetometry as well as their efficiency at extracting genomic DNA from different bacterial strains compared to that of a commercial extraction kit are also reported.