Fig. 5

Upconversion nanoparticles (UCNPs). a Schematic illustration (left) of UCNPs composed of a crystalline host and lanthanide dopant ions embedded in the host lattice. High-resolution transmission electron microscope image (center) revealing the single-crystalline nature of the crystal. Enlarged view (right) shows lanthanide (Ln) and sodium host (Na) atomic columns. Reproduced with permission from Ref. [87]. Copyright (2011), Nature Publishing Group. b Embedded within polymeric films, UCNPs can serve as internally excitable light sources that convert NIR light into blue light, thus facilitating optogenetic activation of channelrhodopsin (ChR)-expressing neurons. Reproduced form Ref. [92] with permission from The Royal Society of Chemistry