Visible and near infrared upconversion emission from Tm3+, Yb3+ ‎doped SrF2 nanoparticles

Tm3+, Yb3+-codoped SrF2 nanoparticles were synthesized through a facile hydrothermal ‎technique. Citrate ions were introduced as the capping agent into the reaction. Upconversion ‎nanoparticles were characterized by field emission scanning electron microscopy (FESEM), ‎Energy dispersive x-ray spectroscopy (EDS), x-ray diffraction (XRD), Dynamic light scattering ‎‎(DLS), Zeta potential, Fourier transform Infrared spectroscopy (Ft-IR), and the 980 nm laser induced ‎photoluminescence spectroscopy. Rare-earth ions (Na+), which are the cations of citrate salts, ‎are incorporated into the structure to  act as charge compensators. Upconversion emission in the ‎visible and NIR region was observed by the 980 nm irradiation. Nanoparticles with a narrow size ‎distribution and a uniform morphology were directly dispersible in water, forming a quite transparent ‎suspension. Nanoparticles size was approximately 10 nm. High penetration of the  Near-Infrared light into ‎the body tissue makes these nanoparticles appropriate for tumor targeting in the deeper tissues for ‎the purpose of bioimaging and photodynamic therapy‎.