Molecular Engineering of 2′, 7′-Dichlorofluorescein to Unlock Efficient Superoxide Anion NIR-II Fluorescent Imaging and Tumor Photothermal Therapy

Although classical fluorescent dyes feature advantages of high quantum yield, tunable “OFF-ON” fluorescence, and modifiable chemical structures, etc., their bio-applications in deep tissue remains challenging due to their excessively short emission wavelength (that may lead to superficial tissue penetration depth). Therefore, there is a pressing need for pushing the wavelength of classical dyes from visible region to NIR-II window. As a representative classical dye, the 2′,7′-Dichlorofluorescein (DCF), a derivative of Fluorescein, is selected and rationally engineered to develop a novel NIR-II platform, CR-OH, which exhibits a substantial red-shift in the wavelength from the visible region to the NIR-II region. This achievement is attributed to molecular modification strategies that include extending π-conjugation, enhancing molecular rigidity, and incorporating strong electron-withdrawing groups. Furthermore, based on this developed NIR-II platform, a NIR-II fluorescence probe and a photothermal nanoagent are successfully constructed to unlock its bio-application in the NIR-II fluorescence imaging of endogenous O₂^·– fluctuations in a CIRI model for the first time, as well as effective photothermal therapy for 4T1 tumors with a high photothermal conversion efficiency (44.0%). Significantly, this work overcomes the wavelength limitation of classical dyes, effectively unlocking their applications for the diagnosis and treatment of early disease in the NIR-II window.