A Noncovalent Fluorescence Turn-on Strategy for Hypoxia Imaging.

Hypoxia plays crucial roles in many diseases and is a central target for them. Present hypoxia imaging is restricted to the covalent approach, which needs tedious synthesis. In this work, a new supramolecular host-guest approach, based on the complexation of a hypoxia-responsive macrocycle with a commercial dye, is proposed. To exemplify the strategy, a carboxyl-modified azocalix[4]arene (CAC4A) was designed that binds to rhodamine 123 (Rho123) and quenches its fluorescence. The azo groups of CAC4A were selectively reduced under hypoxia, leading to the release of Rho123 and recovery of its fluorescence. The noncovalent strategy was validated through hypoxia imaging in living cells treated with the CAC4A-Rho123 reporter pair.