Evolving a Unique Red-Emitting Fluorophore with an Optically Tunable Hydroxy Group for Imaging Nitroreductase in Cells, in Tissues, and in Vivo.

Fluorescence molecular imaging has attracted increasing attention due to its various advantages. Lots of fluorophores have been developed to meet various molecular imaging needs. However, it is still inconvenient due to the lack of excellent fluorophores with an optically tunable group for biological molecular imaging. Here a new platform of a versatile long wavelength fluorophore with an optically tunable hydroxyl group was successfully developed by regulating molecular planarity and the twisted intramolecular charge transfer effect with a protected and deprotected hydroxyl group approach via "step by step" modifying strategy. As an excellent representative of this new type of fluorophore, LDOH-4 possesses good chemical and optical properties and shows a potential application prospect. As a proof-of-concept, a nitroreductase-activated TP fluorescent probe LDO-NTR was designed, which not only sensitively recognizes NTR with more than 310-fold response signal enhancement in vitro but also tracks NTR in a hypoxia tumor mouse model in vivo by using two-photon imaging. It is hopeful that the long wavelength fluorophore with the optically tunable hydroxyl group can serve as a useful platform to extend capable detection tools in biological chemistry and biomedical applications.