Europium(III) Nanoparticles are fabricated for sensing and imaging of physiological temperatures (see image). The material shows visible-light excitation, line-like emission, inertness to external perturbers (such as oxygen in air), and a dynamic range that covers temperatures encountered in medicine and (cellular) biology. The resolution is +/- 0.3 degrees C. The nanoparticles may also be incorporated into a (conceivably sprayable) sensor film.