A novel fluorescent nanoparticle with reversible on‐off switching properties has been synthesized. Three different wavelengths of light are used for switching‐on light, switching‐off light and excitation light, respectively. Thus, when this particle is used as a fluorescent probe by irradiation of the excitation light, the on‐off status can be maintained. We also showed that the on‐off status of the fluorescent particle even embedded in hydrogels can be remotely controlled by using two different wavelengths of light. These results promise that this kind of fluorescent particles will introduce a new concept and it will possibly be applied as a novel fluorescent probe, a photo memory, and a switching devise for photonics.
