A New Phenolate-Ion-Type Two-Photon Near Infrared Fluorophore-Based Biosensor for High-Performance Detection of HNO

Although (E)-4-(2-(4-(dicyanomethylene)-4H-chromen-2-yl)vinyl)phenolate anion (DCPO^- ) has recently emerged as a potential near infrared (NIR) biosensor signaling unit, the pK_a value of its conjugate acid is relatively high (∼9); this will lead to relatively low concentrations of DCPO^- under physiological conditions and, hence, unsatisfactory sensitivity of DCPO^- -based bio-probes. By difluoro-substitution on DCPO^- , we have exploited a new fluorophore of o-FDCPO^- whose conjugate acid has a much lower pK_a value of 7.42. Meanwhile, o-FDCPO^- is NIR emissive with λ_em =693 nm and has a 0.76-fold higher fluorescence efficiency than DCPO^- . The significant superiority of o-FDCPO^- over DCPO^- in sensitivity for NIR biosensor applications was confirmed by comparative studies on two HNO probes, namely o-FDCPO-P and DCPO-P, which bear signaling units of o-FDCPO^- and DCPO^- , respectively. Moreover, o-FDCPO-P has been demonstrated to be a high-performance HNO probe with high selectivity, high sensitivity (detection limit: 50 nm), and a rapid response, together with a two-photon NIR-excitation imaging capability.