Dye-doped biodegradable nanoparticle SiO coating on zinc- and iron-oxide nanoparticles to improve biocompatibility and for in vivo imaging studies.

In vivo imaging and therapy represent one of the most promising areas in nanomedicine. Particularly, the identification and localization of nanomaterials within cells and tissues are key issues to understand their interaction with biological components, namely their cell internalization route, intracellular destination, therapeutic activity and possible cytotoxicity. Here, we show the development of multifunctional nanoparticles (NPs) by providing luminescent functionality to zinc and iron oxide NPs. We describe simple synthesis methods based on modified Stöber procedures to incorporate fluorescent molecules on the surface of oxide NPs. These procedures involve the successful coating of NPs with size-controlled amorphous silica (SiO) shells incorporating standard chromophores like fluorescein, rhodamine B or rhodamine B isothiocyanate. Specifically, spherical FeO NPs with an average size of 10 nm and commercial ZnO NPs (ca. 130 nm), both coated with an amorphous SiO shell of ca. 15 and 24 nm thickness, respectively, are presented. The magnetic nanoparticles, with a major presence of magnetite, show negligible coercitivity. Hence, interactions (dipolar) are very weak and the cores are in the superparamagnetic regime. Spectroscopic measurements confirm the presence of fluorescent molecules within the SiO shell, making these hybrid NPs suitable for bioimaging. Thus, our coating procedures improve NP dispersibility in physiological media and allow the identification and localization of intracellular ZnO and FeO NPs using confocal microscopy imaging preserving the fluorescence of the NP. We demonstrate how both FeO and ZnO NPs coated with luminescent SiO are internalized and accumulated in the cell cytoplasm after 24 hours. Besides, the SiO shell provides a platform for further functionalization that enables the design of targeted therapeutic strategies. Finally, we studied the degradation of the shell in different physiological environments, pointing out that the SiO coating is stable enough to reach the target cells maintaining its original structure. Degradation took place only 24 hours after exposure to different media.