A general strategy to develop cell permeable and fluorogenic probes for multicolour nanoscopy.

Live-cell fluorescence nanoscopy is a powerful tool to study cellular biology on a molecular scale, yet its use is held back by the paucity of suitable fluorescent probes. Fluorescent probes based on regular fluorophores usually suffer from a low cell permeability and an unspecific background signal. Here we report a general strategy to transform regular fluorophores into fluorogenic probes with an excellent cell permeability and a low unspecific background signal. Conversion of a carboxyl group found in rhodamines and related fluorophores into an electron-deficient amide does not affect the spectroscopic properties of the fluorophore, but allows us to rationally tune the dynamic equilibrium between two different forms: a fluorescent zwitterion and a non-fluorescent, cell-permeable spirolactam. Furthermore, the equilibrium generally shifts towards the fluorescent form when the probe binds to its cellular targets. The resulting increase in fluorescence can be up to 1,000-fold. Using this simple design principle, we created fluorogenic probes in various colours for different cellular targets for wash-free, multicolour, live-cell nanoscopy.