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.