Luminescence of Eu³⁺ Activated CaF₂ and SrF₂ Nanoparticles: Effect of the Particle Size and Codoping with Alkaline Ions

Eu³⁺ doped CaF₂ and SrF₂ nanoparticles were synthesized through a facile hydrothermal technique, using citrate ions as capping agents and Na⁺ or K⁺ as charge compensator ions. A proper tuning of the reaction time can modulate the nanoparticle size, from few to several tens of nanometers. Analysis of EXAFS spectra indicate that the Eu³⁺ ions enter into the fluorite CaF₂ and SrF₂ structure as substitutional defects on the metal site. Laser site selective spectroscopy demonstrates that the Eu³⁺ ions are mainly accommodated in two sites with different symmetries. The relative site distribution for lanthanide ions depends on the nanoparticle size, and higher symmetry Eu³⁺ sites are prevalent for bigger nanoparticles. Eu³⁺ ions in high symmetry sites present lifetimes of the ⁵D₀ level around 27 ms, among the longest lifetimes found in the literature for Eu³⁺ activated materials. As a proof of concept of possible use of the Eu³⁺ activated alkaline-earth fluoride nanoparticles in nanomedicine, the red luminescence generated by two-photon absorption using pulsed laser excitation at 790 nm (in the first biological window) has been detected. The long Eu³⁺ lifetimes suggest that the present nanomaterials can be interesting as luminescent probes in time-resolved fluorescence techniques in biomedical imaging (e.g., FLIM) where fast autofluorescence is a drawback to avoid