Scheme 2. Optical properties of UCNPs and upconversion optogenetics designing. a) UCNPs composed of a crystalline host and lanthanide dopant ions embedded in the host lattice. Typical energy level diagram and emission spectra under 980 nm laser excitation. UC multicolor fine-tuning through the use of lanthanide-doped NaYF4 nanoparticles with varied dopant ratios (Yb/Tm/Er). Schematic diagrams of the nanoparticle and the energy transfer are reproduced with permission.[18] Copyright 2010, Royal Society of Chemistry. Emission spectra and luminescent photos are reprinted with permission.[19] Copyright 2008, American Chemical Society. b) Nd3+-sensitized UCNPs with multicolor UC luminescence upon 800 nm excitation. Reproduced with permission.[20] Copyright 2013, American Chemical Society. c) Upconversion optogenetics for NIR manipulation of membrane ion channels.