Engineering less toxic nanomaterials that maintain valuable functional properties is crucial to the sustainability of the nanotech industry. In this study, a safer formulation concept for flame-generated nanomaterials was demonstrated. It is based on the encapsulation of potentially toxic nanomaterials by a biologically inert nanothin amorphous SiO2 layer. The core-shell particles maintain their intrinsic optical, magnetic or plasmonic properties of the core material but exhibit surface properties of their SiO2 shell. The SiO2-coating process was performed using the recently developed flame spray pyrolysis (FSP)-based Versatile Engineered Nanomaterial Generation System (VENGES). A coating reactor was added in which core ENMs are coated in-flight by the swirl injection of hexamethyldisoloxane (HMDSO) vapor. The HMDSO vapor concentration can be adjusted to precisely control the thickness of the coating. We first demonstrate the versatility of the proposed SiO2-coating process by applying it to several ENMs (CeO2, Fe2O3, ZnO, Ag) marked by their prevalence in consumer products as well as their range in toxicity. We quantified the thickness of the SiO2-coating (TEM, XPS), evaluated its efficiency (Chemisorption) and its effect on the core material structure, composition and morphology (XRD, BET, and TEM). Furthermore, we examined the mobility and aggregation potential of SiO2-coated and uncoated ENMs in DI-water and biological media using dynamic light scattering (DLS), and compared to those of pure silica nanoparticles. Finally, we provide valuable in-vitro toxicological evidence for the safety of this novel formulation concept by evaluating the relative toxicity of SiO2-coated vs. uncoated ENMs using a number of cellular assays (MTT, LDH, fluorescence Live/Dead) and several cell-lines (A549 cancer alveolar epithelial cells and THP-1 macrophages). Our results show that the proposed method can be used to effectively and uniformly coat flame generated ENMs with a nanothin layer of amorphous SiO2 that significantly reduces their toxicological effects. This scalable method can be applied in nanomanufacturing of nanomaterials developing safer by design nanoparticles.
S. Gass, J. Cohen, G. Pyrgiotakis, G.A. Sotiriou, S.E. Pratsinis, P. Demokritou, “Safer Formulation Concept for Flame-Generated Engineered Nanomaterials”, ACS Sustainable Chem. Eng. in press, doi: 10.1021/sc300152f (2013). link