Optical nanoprobes for biomedical applications: shining a light on upconverting and near-infrared emitting nanoparticles for imaging, thermal sensing, and photodynamic therapy

Lanthanide (Ln(3+))-based nanoparticles (NPs) with light emission in the ultraviolet (UV), visible, and near-infrared (NIR) region under NIR excitation are emerging multifunctional players in the biomedical field. Excitation and emission wavelengths matching the NIR biological transparency windows (700-1870 nm) set the basis for deeper penetration depth into biological tissues due to less absorption, higher resolution as a result of reduced scattering, and improved imaging contrast due to tissue autofluorescence minimization; all being inevitable requirements for optical in vivo bioimaging. Moreover, frequency-conversion in Ln(3+)-based NPs is sensitive to their thermal environment, thus allowing for application as optical temperature sensors. Additional heating capability opens the pathway to self-monitored optical agents with potential for photothermal therapies (PTTs). UV-visible emitting upconverting nanoparticles (UCNPs) are of particular interest for applications in photodynamic therapies (PDTs) owing to their capability to trigger various photochemical processes upon NIR excitation. It is the aim of this review to highlight the most recent achievements and advances in the field of Ln(3+)-based NPs as frequency-converting materials for biomedical applications and to identify current trends as well as remaining challenges. Special focus will be set on the most recently presented Ln(3+)-based systems whose potential has been evaluated in vitro and in vivo as NIR-to-NIR bioimaging probes, NIR-operating nanothermometers and-heaters as well as photochemical agents.