Upconverting nanoparticles for pre-clinical diffuse optical imaging, microscopy and sensing: Current trends and future challenges

Upconverting nanoparticles (UCNPs) are a class of recently developed luminescent biomarkers that - in several aspects - are superior to organic dyes and quantum dots. UCNPs can emit spectrally narrow anti-Stokes shifted light with quantum yields which greatly exceed those of two-photon dyes for fluence rates relevant for deep tissue imaging. Compared with conventionally used Stokes-shifting fluorophores, UCNP-based imaging systems can acquire completely autofluorescence-free data with superb contrast. For diffuse optical imaging, the multi-photon process involved in the upconversion process can be used to obtain images with unprecedented resolution. These unique properties make UCNPs extremely attractive in the field of biophotonics. UCNPs have already been applied in microscopy, small-animal imaging, multi-modal imaging, highly sensitive bioassays, temperature sensing and photodynamic therapy. In this review, the current state-of-the-art UCNPs and their applications for diffuse imaging, microscopy and sensing targeted towards solving essential biological issues are discussed.