“…Due to a wealth of electronic transitions within the 4f electron shells of lanthanide ions, nanostructures doped with lanthanide ions can form a unique class of functional optical devices 6,7 . Among them, spherical upconversion nanoparticles (UCNPs) have been created with many unique optical properties, including tuneable colors 8 , multiplexed lifetimes 9,10 , long-distance energy migration 11 , ampli ed stimulated emissions [12][13][14][15][16] and their responses to external elds of temperature 17 and mechanical force 18 , which enables many novel applications, including full-colour displays 8 , solar energy harvesting 19 , security inks 9 , biomolecular sensing 20 , force sensing 21 , nanothermometry 22,23 , uorescence microscopy 12 , optical multiplexing 24 , deep-tissue optogenetics 25 , multimodal bio-imaging 26,27 and lighttriggered drug delivery 28,29 .…”