Simple, common but functional: biocompatible and luminescent rare-earth doped magnesium and calcium hydroxides from miniemulsion

Abstract

Nanostructured (d ∼ 20–35 nm) and highly luminescent Ca(OH)₂:Ln and Mg(OH)₂:Ln (Ln = Eu^III, Sm^III, Tb^III, Mg(Ca)/Ln = 20 : 1 atomic) nanostructures were obtained in inverse (water in oil – w/o) miniemulsion (ME), by exploiting the nanosized compartments of the droplets to spatially confine the hydroxide precipitation in basic environment (NaOH). The functional nanostructures were prepared using different surfactants (Span80 (span) and a mixture of Igepal co-630 and Brij 52 (mix)) to optimise ME stability and hydroxide biocompatibility as well as tune the droplet sizes. X-Ray diffraction (XRD) analyses testify the achievement of a pure brucite-Mg(OH)₂-phase and pure portlandite-Ca(OH)₂-phase with a high degree of crystallinity. Besides structural characterisations, the products were thoroughly characterised by means of several and complementary techniques (dynamic light scattering (DLS), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), micro-Raman spectroscopy, inductively coupled plasma mass spectrometry (ICP-MS) and Fourier transform infrared spectroscopy (FT-IR)) to assess their chemico-physical properties as well as their morphological and microstructural features. The stoichiometry of the doped systems was confirmed using ICP-MS measurements. Finally, the cytotoxicity of the nanoparticles was assessed by in vitro tests using ES2 cells in order to provide preliminary data on the biocompatibility of this kind of nanoparticles. The luminescence of the Eu-doped and Tb-doped materials is clearly visible to the naked eye in the red and green regions, respectively, corroborating their employment as materials for imaging in the optical window of interest.