Reversibly photoswitchable dual-color fluorescent nanoparticles as new tools for live-cell imaging

Optically switchable dual-color fluorescent nanoparticles that incorporate two classes of dyes into the polymeric chains have been synthesized using an emulsion polymerization method. The nanoparticles consist of an organic photoisomerizable dye, spiropyran, as an optically responsive component and another fluorescent dye, perylene diimide, as a high-energy emitter. Under UV irradiation, the colorless spiropyran undergoes photoisomerization to yield merocyanine, which absorbs at 588 nm and fluoresces strongly at 670 nm. The absorption band of the merocyanine matches well the fluorescence bands of perylene diimide (at 535 and 575 nm), and therefore fluorescence resonance energy transfer (FRET) converts the high-energy green emission of the perylene into low-energy red emission when the merocyanine form is present. Upon exposure to alternating UV ( 420 nm) light, the nanoparticles cycle between red and green fluorescence as the spiro and mero forms of the optically responsive component...