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 x 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 (H-2(11/2) + S-4(3/2) -> I-4(15/2)) Er-related upconversion (UC) efficiency comparable to that of Yb:Er:beta-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 (similar to 293-317 K/20-44 degrees C) S = 108-118 x 10(-4)K(-1) for 20 at%Yb:5 at%Er:NaGd(WO4)(2) NPs, being the largest values so far reported using the H-2(11/2)/S-4(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 mu 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.