Advances in Nitroreductase-Activated Fluorescent Probes Based on Photophysical Response Mechanisms

Nitroreductase (NTR) is a flavin mononucleotide-dependent oxidoreductase that selectively reduces aromatic nitro compounds to hydroxylamine or amino derivatives in the presence of NAD(P)H. Tumor hypoxia is often accompanied by upregulation of NTR levels, making it an ideal biomarker for hypoxia assessment. This article briefly reviews recent advances in NTR-activated fluorescent probes based on photophysical response mechanisms. According to their design principles, these probes are mainly classified into three categories: probes based on photoinduced electron transfer (PET), which utilize the nitro group to quench fluorescence and achieve a turn-on signal upon reduction; probes based on intramolecular charge transfer (ICT), which incorporate the nitro group into a donor-acceptor electronic system and produce spectral responses upon changes in electronic properties following reduction; and ratiometric probes based on Förster resonance energy transfer (FRET), which employ an internal reference channel to eliminate environmental interference. In addition, reversibly binding probes that block electron transfer by forming hydrogen bonds with the reduced cofactor represent a new mode of non-reactive detection. Finally, this review summarizes the core challenges faced by current probes and outlines future directions.