Using UV light to switch fluorescent molecules on and off in aqueous environments could help scientists develop better fluorescent probes for biomedical research. The molecules first must be strategically trapped inside water-soluble particles, which isn’t new; water-soluble particles have been used in previous studies to bring organic molecules into water. What is new in this system, developed by scientists at the University of Miami, is the addition of the photoswitching mechanism. UV light now can switch fluorescent molecules on and off in aqueous environments. Here, live cells have been incubated with the polymer nanoparticles. The green color is the fluorescence coming from the molecules trapped within the nanoparticles. Images courtesy of Francisco Raymo, University of Miami. “Finding a way to switch fluorescence inside cells is one of the main challenges in the development of fluorescent probes for bioimaging applications,” said chemistry professor Francisco Raymo. “Our fluorescent switches can be operated in water efficiently, offering the opportunity to image biological samples with resolution at the nanometer level.” Because fluorescent molecules are not water soluble, Raymo and his team created their system by embedding them in synthetic water-soluble polymer nanoparticles, which serve as transport vehicles into living cells. Once inside cells, the fluorescence of the molecules trapped inside the nanoparticles can be turned on and off under optical control. “The polymers can preserve the properties of the fluorescent molecules and at the same time assist the transfer of the molecules into water,” Raymo said. “It’s a bit like having a fish in a bowl, so the fish can carry on with its activities in the bowl, and the whole bowl can be transferred into a different environment.” Francisco Raymo, a professor of chemistry at the University of Miami, has developed a way to switch fluorescent molecules on and off within aqueous environments. The technique could help scientists develop better fluorescent probes for biomedical research. The new system is faster and more stable than current methods. The fluorescent molecules glow when exposed simultaneously to UV and visible light and revert to their original nonluminous state in less than 10 µs after UV light is removed. Using synthetic molecules enables the system to overcome the natural wear that organic molecules endure from overexposure to UV light. “The system can be switched back and forth between the fluorescent and nonfluorescent states for hundreds of cycles, without sign of degradation,” Raymo said. The system’s surface can be customized to help it attach to specific molecules of interest, enabling researchers to visualize structures and activity within cells, in real time, with unprecedented resolution. The findings were reported online by Chemistry-A European Journal (doi: 10.1002/chem.201201184)