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

Abstract

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 alkalineearth 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.

Particle size-related limitations of persistent phosphors based on the doped Y3Al2Ga3O12 system

“…The main factor that can influence the obtained data are the appearance of afterglow for the co-doped crystal, which significantly increases the background. The effective DT 57 was calculated to compare the obtained curve shapes and further calculate the ET coefficient. Based on the DT, the ET efficiencies from Ce 3+ to Pr 3+ and Cr 3+ were calculated for each annealing temperature (see S.I.…”

Multisite formation in gadolinium doped SrF2 nanoparticles

“…The recent advances in nanotechnology have stimulated in depth investigations of nanofluoride systems important for various technological applications. In particular, studies of singly and doubly doped SrF2 nanoparticles show promising results for designing novel luminescent materials [1][2][3][4], thermoluminescent dosimeters [5], infrared radiation up-converters [6,7] and bioimaging nano-probes [8,9]. Another field of growing interest is transparent glass-ceramic composites, where SrF2 nanocrystals can be precipitated in a chemically and thermally stable glass matrix [10][11][12][13][14][15][16][17].…”

Upconverting SrF2 nanoparticles doped with Yb3+/Ho3+, Yb3+/Er3+ and Yb3+/Tm3+ ions – optimisation of synthesis method, structural, spectroscopic and cytotoxicity studies

“…Hydrothermal method is very common for synthesis of upconverting SrF 2 what is reflected by many of published articles 12,14,15,23,25,26 . One of the first papers was published by J.…”