Silica-coating as protective shell for the risk management of nanoparticles

Nanoparticles (NPs) surface functionalization with silica (SiO2) has attracted high attention due to hydrophilicity, biocompatibility, chemical and thermal stability of silica. The present work is addressed to the production and characterization of SiO2-coatings on titanium dioxide (TiO2) and silver (Ag) NPs dispersed in aqueous solutions (commercial nanosols) with the aim to manage the potential risk that such NPs could generate in occupational exposure scenarios. A colloidal approach, based on principles of heterocoagulation in which opposite charged NPs are forced to coagulate in hierarchical structures, imposed by their relative size and weight ratio, was followed. The results were compared with a chemical approach, based on nucleation of silica phase from silica precursor solutions on the surfaces of TiO2 or Ag NPs seeds. In order to increase the adhesion of silica on TiO2 and Ag surfaces, heterocoagulated sols were spray-dried and subsequently redispersed in water to check the feasibility of such approach at industrial level. Physicochemical properties such as zeta potential, electrical conductivity, particle-size distribution, specific surface area and morphology of the samples produced with different SiO2:TiO2 and SiO2:Ag weight ratios were collected and compared. Indirect evidences of silica coating were obtained.