Rare Earth-Doped Nanoparticles for Advanced In Vivo Near Infrared Imaging

Rare earth-doped nanoparticles (RENPs) with emissions in near infrared (NIR) region hold great promise for in vivo optical bioimaging, as they possess distinguished advantages of having tunable narrow-band emissions in the biological windows (700–950 nm, NIR-I widow; 1000–1700 nm, NIR-II window), presenting long-lived luminescence lifetimes (up to milliseconds), allowing probing centimeter-deep tissues, and enabling imaging with absence of autofluorescence. In this chapter, we present two types of NIR-emitting RENPs (upconversion and downshifting) towards this regard. Alongside steady-state bioimaging, we also describe the use of time-gated technique to empower autofluorescence-free imaging and multiplexed imaging in the time domain. The existing problems and future thrusts of RENPs for in vivo optical imaging are also discussed.