The interaction between light and biological tissues in the second near-infrared (NIR-II) window is weak, which can effectively reduce the scattering and absorption of incoming light by biological tissues and enhance the resolution and sensing ability of in vivo photoacoustic (PA) imaging. In particular, tissues that carry blood and water produce the lowest PA background in the wavelength range of 1050 to 1150 nm. However, the development of the NIR-II PA probe for the above window faces great challenges. To tackle this challenge, the reduction-reoxidation of an organic dye was used to develop a PA imaging probe (Hydro-1048) as the first NIR-II PA probe of a hydroxy radical (·OH) for molecular imaging in deep tissue. The ·OH oxidized the C-N single bond in Hydro-1048 to double bonds, which formed Et-1065. This conversion extended the conjugate system of the molecule and shifted the absorption peak from 520 to 1065 nm, which resulted in a strong PA signal after irradiation with a 1065 nm laser. At a detection limit of 0.6 nM, a good linear relationship within the range of 5-1000 nM was obtained for the PA signal intensity versus the concentration of ·OH. The developed NIR-II PA probe can be used for the noninvasive high-resolution imaging of ·OH in deep tissue, and the PA imaging of ·OH can also be used to visually monitor in situ pathological processes related to hepatitis.

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