We report the first successful encapsulation of pristine fullerene C 70 in polymer nanoparticles with very low size polydispersity. We obtained water-dispersed polystyrene (PS) nanoparticles with diameters from 60 nm to 190 nm using a miniemulsion polymerization technique. Contrarily to pristine fullerenes, which are insoluble in most solvents and materials, the nanoparticles containing fullerene C 70 (PS-C 70) are stable in water and can be easily incorporated in different materials. When blended with polyacrylonitrile (PAN), a virtually oxygen-impermeable polymer, the PS-C 70 nanoparticles show a strong temperature dependence of the thermally activated delayed fluorescence (TADF) intensity and lifetimes, even when exposed to air. This is the first fluorescence temperature sensor based on TADF that can operate in the presence of oxygen. Unlike other fluorescence temperature sensors, our sensor material is insensitive to oxygen, has emission lifetimes in the millisecond range, and shows a strong emission intensity increase when the temperature increases. This sensor exhibits a very broad sensitivity in a working range from À75 C to at least 105 C (based on fluorescence intensity), surpassing the performance of other temperature fluorescence sensors at high temperatures.