Dynamic intracellular tracking nanoparticles via pH-evoked 'off–on' fluorescence

Dynamic tracking of nanoparticles in cells faces a big challenge due to fluorescence quenching. Here, the pH-switchable probe spiropyran (SP) was introduced into poly(ethylene glycol) (PEG) via atom transfer radical polymerization (ATRP) to construct expected "off-on" fluorescent nanoparticles. Strong red fluorescence was emitted to efficiently track nanoparticles in cells due to the isomerization of spiropyran to merocyanine in the endosome or lysosome.