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

Eu 3+ doped CaF 2 and SrF 2 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 Eu 3+ ions enter into the fluorite CaF 2 and SrF 2 structure as substitutional defects on the metal site. Laser site selective spectroscopy demonstrates that the Eu 3+ 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 Eu 3+ sites are prevalent for bigger nanoparticles. Eu 3+ ions in high symmetry sites present lifetimes of the 5 D 0 level around 27 ms, among the longest lifetimes found in the literature for Eu 3+ activated materials. As a proof of concept of possible use of the Eu 3+ 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 Eu 3+ 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.