Ultrasmall, water dispersible, TWEEN80 modified Yb:Er:NaGd(WO₄)₂ nanoparticles with record upconversion ratiometric thermal sensitivity and their internalization by mesenchymal stem cells

This work presents the synthesis by coprecipitation of diamond shaped Yb:Er:NaGd(WO₄)₂ crystalline nanoparticles (NPs) with diagonal dimensions in the 5–7 nm × 10–12 nm range which have been modified with TWEEN80 for their dispersion in water, and their interaction with mesenchymal stem cells (MSCs) proposed as cellular NP vehicles. These NPs belong to a large family of tetragonal Yb:Er:NaT(XO₄)₂ (T = Y, La, Gd, Lu; X = Mo, W) compounds with green (²H_11/2 + ⁴S_3/2 → ⁴I_15/2) Er-related upconversion (UC) efficiency comparable to that of Yb:Er: β -NaYF₄ reference compound, but with a ratiometric thermal sensitivity ( S ) 2.5–3.5 times larger than that of the fluoride. At the temperature range of interest for biomedical applications (∼293–317 K/20–44 °C) S  = 108–118 × 10⁻⁴ K⁻¹ for 20 at%Yb:5 at%Er:NaGd(WO₄)₂ NPs, being the largest values so far reported using the ²H_11/2/⁴S_3/2 Er intensity ratiometric method. Cultured MSCs, incubated with these water NP emulsions, internalize and accumulate the NPs enclosed in endosomes/lysosomes. Incubations with up to 10 μ g of NPs per ml of culture medium maintain cellular metabolism at 72 h. A thermal assisted excitation path is discussed as responsible for the UC behavior of Yb:Er:NaT(XO₄)₂ compounds. (paper)