A ratiometric fluorescent biosensor based on conjugated polymers for sensitive detection of nitroreductase and hypoxia diagnosis in tumor cells

The over-expressed nitroreductase (NTR) is a common biomarker of tumor hypoxia. Thus, achieving sensitive and accurate detection of NTR is significant for the guidance of tumor therapy. In this work, a novel ratiometric fluorescent biosensor based on water-soluble conjugated polymers PFP-NA has been established for NTR sensing and hypoxia diagnosis. The 1,8-naphthalimide (NA) modified with p-nitrobenzene is linked to the side chain of PFP through covalent bonds, which leads to the simultaneous occurrence of fluorescence resonance energy transfer (FRET) and photoinduced electron transfer (PET). Due to the fluorescence of NA being quenched by PET, the emission ratio of NA to PFP backbone is quite low (around 0.09). When NTR exists, the nitro group is reduced to an amino group, leading to PET being blocked. Thus the ratio increases greatly (> 54-fold), bringing the calculated detection limit as low as 19.7 ng/mL. Importantly, the fluorescence of PFP backbone remains unchanged, which enables it being used as an internal reference. Moreover, our ratiometric probe exhibits high selectivity and low cytotoxicity, and can achieve intracellular detection of NTR and hypoxia diagnosis in tumor cells. Briefly, this work provides a promising ratiometric biosensor for sensitive and accurate nitroreductase detection and hypoxia imaging.