Lighting Up NIR-II Fluorescence in Vivo: An Activable Probe for Noninvasive Hydroxyl Radical Imaging

The detection of hydroxyl radical (·OH) in vivo faces challenges as ·OH has short lifetime and low concentration in the body. Fluorescence imaging within the second near-infrared window (NIR-II, 1000–1700 nm) is a promising approach to in vivo organ and tissue imaging, but ·OH fluorescent probes emitting at this region have not been reported up to now because of the difficulty of probe design. Herein, we report the strategy to fabricate the first NIR-II probe for ·OH by directly breaking/recovering the conjugated system and rigid planar structure of an organic fluorophore, which could regulate the fluorescence intensity regardless of emission wavelength. This activable probe, Hydro-1080, emitted in 1000–1400 nm after responding to ·OH. Hydro-1080 exhibited excellent sensitivity (LOD = 0.5 nM) and selectivity to ·OH. It was able to track subtle variation of [·OH] in liver induced by external stimuli and offered clear images with high contrast. This work also indicates that this simple and straightforward strategy can be extended to develop NIR-II fluorescent probes efficiently.