Back Cover: Ultra‐Small Air‐Stable Triplet‐Triplet Annihilation Upconversion Nanoparticles for Anti‐Stokes Time‐Resolved Imaging

Anti-Stokes fluorescence lifetime imaging in living cells was demonstrated by Rachel C. Evans et al. in their Research Article (e202308602). This was achieved through the design of ultra-small nanoparticle probes based on triplet–triplet annihilation upconversion (TTA-UC) chromophores encapsulated in an organic–inorganic ureasil host. Fluctuations of the UC lifetime in the stained cells suggest that the NPs can be used to map local oxygen diffusion across the subcellular structure.