Monodisperse, colloidal and luminescent calcium fluoride nanoparticles via a citrate-assisted hydrothermal route

Luminescent calcium fluoride (CaF₂) nanoparticles, because of their excellent biocompatibility, excellent photostability and strong fluorescence, have received increasing attention as drug carriers and bioprobes in cell imaging. Inspired by the role of citrate in the growth of apatite crystals during natural bio-mineralization, uniform and nearly monodisperse Eu³⁺-doped CaF₂ nanoparticles with excellent colloidal stability and high fluorescence in aqueous media have been successfully synthesized in the presence of sodium citrate using a hydrothermal method. X-ray diffraction and transmission electron microscopy show that CaF₂ nanoparticles grown in the presence of sodium citrate are cubes of relatively uniform size (15 nm), and that the Eu³⁺ doping level has little effect on size and morphology. Zeta potentials and dynamic light scattering demonstrate that in the synthesis with sodium citrate, the colloidal stability of CaF₂ nanoparticles is greatly improved upon the increase of Eu³⁺ doping level. Moreover, aqueous dispersions of these nanoparticles are colloidally stable and can be maintained over a wide range of pH from 5.0 to 11.0 for more than a month. Fluorescence spectra demonstrate that the doped CaF₂ nanoparticles display strong red fluorescence. Fourier transform infrared spectra and thermogravimetric analyses demonstrate the adsorption of substantial quantities of sodium citrate on the surfaces of the CaF₂ nanoparticles. Taken together, such colloidal behavior should be related to strong crystal inhibition of citrate ions and Eu³⁺ doping induced promotion thermal-decomplexing between citrate ions and calcium ions. The luminescent CaF₂ nanoparticles obtained using this protocol should be promising candidates for use in many bio-related applications.