Lanthanide-doped LaOBr nanocrystals: controlled synthesis, optical spectroscopy and bioimaging

Lanthanide (Ln³⁺)-doped nanocrystals (NCs) have shown great promise in diverse bioapplications. Exploring new host materials to realize efficient downshifting (DS) and upconversion (UC) luminescence is a goal of general concern. Ln³⁺-doped oxybromides, which bring together the advantages of high chemical/thermal stability of oxides and low phonon energy of bromides, have been rarely reported so far. Herein, we report the synthesis of monodisperse tetragonal-phase LaOBr:Ln³⁺ NCs via a modified thermal decomposition approach. Ln³⁺ dopants in LaOBr NCs occupying surface and lattice sites were verified based on high-resolution photoluminescence spectra of Eu³⁺ at 10 K. Intense DS emissions were observed in Ce³⁺ and Ce³⁺/Tb³⁺ doped LaOBr NCs upon excitation at 353.0 nm, which is far from the deep-UV absorption band of proteins. Moreover, UC properties of LaOBr:Yb³⁺/Ho³⁺ and Yb³⁺/Er³⁺ NCs were comprehensively surveyed upon near-infrared excitation at 980 nm. Particularly, the red-to-green ratio can be markedly increased from 0.1 to ∼300 in LaOBr:Yb³⁺/Er³⁺ NCs, resulting in high-chromatic-purity single-band red UC emission. Furthermore, we have employed the LaOBr:Ln³⁺ NCs for cancer cell imaging by virtue of their superior optical properties, showing great potential of LaOBr:Ln³⁺ nanoprobes in bioimaging.