Near‐Infrared Upconversion Luminescence and Bioimaging In Vivo Based on Quantum Dots

: Recently, upconversion luminescence (UCL) has been widely applied in bioimaging due to its low autofluorescence and high contrast. However, a relatively high power density is still needed in conventional UCL bioimaging. In the present study, an ultralow power density light, as low as 0.06 mW cm-2, is applied as an excitation source for UCL bioimaging with PbS/CdS/ZnS quantum dots (UCL-QDs) as probes. The speculated UCL mechanism is a phonon-assisted single-photon process, and the relative quantum yield is up to 4.6%. As determined by continuous irradiation with a 980 nm laser, the UCL-QDs show excellent photostability. Furthermore, UCL-QDs-based probe is applied in tumor, blood vessel, and lymph node bioimaging excited with an eye-safe low-power light-emitting diode light in a nude mouse with few heat effects.