Hypoxia-Activated NIR-II Fluorescent Probes for Breast Tumor Imaging via N‑Oxide BF₂ Formazanate Scaffolds

Tumor hypoxia critically influences cancer progression and therapy resistance, yet its precise and selective visualization remains a major challenge. In this study, we report a class of small-molecule NIR-II fluorescent probes (BFO1-BFO3) based on N-oxide-functionalized boron difluoride formazanates, which are capable of specifically responding to oxygen-deficient environments without interference from nitroreductase activity. These probes undergo bioreduction via cytochrome P450-mediated deoxygenation under hypoxic conditions, leading to a pronounced "turn-on" fluorescence emission beyond 1000 nm. Among them, BFO1 demonstrated outstanding selectivity, sensitivity, and deep-tissue imaging performance, achieving high-resolution hypoxia detection up to a depth of 5 mm in a 1% intralipid model. Cellular and in vivo studies in 4T1 breast tumor models confirmed the robust and rapid hypoxia-activated NIR-II signal enhancement. Collectively, this work presents an N-oxide-based scaffold for the noninvasive NIR-II imaging of hypoxia, offering a powerful platform for early tumor diagnosis and monitoring of hypoxia-associated pathologies.