NaBiF4:Yb3+,Tm3+ submicron particles as luminescent probes for in vitro imaging of cells.

Upconversion materials have attracted considerable research interest for their application in bioimaging due to their unique optical properties. NaREF4 (RE = Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) based host lattice, which is widely used for upconversion, requires expensive rare-earth elements and tedious reaction conditions. Hence there is a need to develop environmentally friendly and cost effective materials for upconversion. In this study, we propose NaBiF4 as a host material for upconversion which is based on environmentally friendly and cost-effective bismuth. NaBiF4 has not been explored as an imaging probe before. We report efficient Yb3+/Tm3+ doped NaBiF4 based upconversion submicron particles which exhibit a photostable, wide upconversion emission range (NIR-to-NIR and Vis) under NIR (980 nm) excitation, and in-vitro non-cytotoxic uptake by mammalian cancer cell lines as well as bacterial cells with a high signal to background ratio. The synthesis of the chosen host material co-doped with Yb3+/Tm3+ has not been reported earlier through such a non-aqueous quaternary reverse micelle route. Here, we functionally validate these submicron particles as viable alternatives to currently available upconversion nanomaterials and highlight their potential as luminescent probes for bioimaging.