Direct large-scale synthesis of water-soluble and biocompatible upconversion nanoparticles for in vivo imaging.

Deep tissues can be optically imaged using near-infrared windows without radiation hazard. This work proposes a straightforward one-pot method for directly synthesizing water-soluble and biocompatible upconversion nanoparticles on a large scale for in vivo imaging. Safety assessment, coupled with luminescence imaging in mice, demonstrates the excellent stability and promising biological applications of the upconversion nanoparticles.