Labeling and long-term tracking of bone marrow mesenchymal stem cells in vitro using NaYF₄:Yb³⁺,Er³⁺ upconversion nanoparticles

Mesenchymal stem cells (MSCs) hold great promise as cell therapy candidate in clinics. However, the underlying mechanisms remain elusive due to the lack of effective cell tracking approaches during therapeutic processes. In this study, we successfully synthesized and utilized NaYF₄:Yb³⁺,Er³⁺ upconversion nanoparticles (UCNPs) to label and track rabbit bone marrow mesenchymal stem cells (rBMSCs) during the osteogenic differentiation in vitro. To improve their biocompatibility and cellular uptake, we modified the UCNPs with negatively-charged poly(acrylic acid) and positively-charged poly(allylamine hydrochloride) in turns (i.e., PAH-PAA-UCNPs). The effect of cellular uptake of UCNPs on the osteogenic differentiation of rBMSCs was systematically evaluated, and no significant difference was found between rBMSCs labeled with UCNPs (concentration range of 0–50 μg/mL) and UCNPs-free rBMSCs in terms of cell viability, ALP activity, osteogenic protein expressions and production of mineralized nodules. Moreover, the PAH-PAA-UCNPs at a concentration of 50 μg/mL exhibited the highest biocompatibility and stability, which could well track rBMSCs during the osteogenesis process. These results would provide a positive reference for the application of these lanthanide-doped UCNPs as fluorescent nanoprobes for stem cell tracking to further understand the mechanism of stem cell fate in tissue engineering and stem cell therapy. Statement of Significance Upconversion nanoparticles (UCNPs) have attracted increasing attention as alternative probes for tracking various types of cells including stem cells. The reported fluorapatite-based UCNPs with the needle-like morphology showed a little poor performance on stem cell tracking, which was possibly attributed to the low upconversion efficiency and cell labeling efficiency potentially due to nanomaterial composition, crystal structure and shape. Here, we synthesized the positively-charged NaYF₄:Yb³⁺,Er³⁺ UCNPs with hexagonal phase and sphere-like morphology to enhance their upconversion efficiency, biocompatibility and cellular uptake, leading to a successful tracking of rBMSCs in osteogenesis process without impairing cell viability and differentiation capacity. This study provided a necessary reference for the application of UCNPs in stem cell tracking to better understand the mechanism of stem cell fate in tissue engineering, stem cell therapy, etc.