Cyano-Mediated Donor-Acceptor Engineering: A Rational Strategy to Boost Charge Transfer in Carbon Dots for NIR-II Bioimaging.

Carbon dots (CDs) have emerged as an important class of probes for biomedical imaging. However, engineering CDs with near-infrared emission, especially in the second near-infrared window (NIR-II), remains a significant challenge. Herein, we propose a cyano-mediated donor-acceptor (D-A) engineering strategy to synthesize NIR-II emissive CDs (FC-CDs), using benzene-1,2,4,5-tetracarbonitrile, a molecule rich in electron-withdrawing cyano groups, as the precursor. Meanwhile, the in situ introduction of folic acid (FA) during the solvothermal process facilitates enhanced biocompatibility and tumor-targeting ability. The cyano groups on FC-CDs enable the formation of a unique D-A configuration, in which the carbon core acts as the electron donor and the electron-withdrawing-groups-functionalized surface serves as the electron acceptor. Experimental and theoretical evidence suggests that this D-A structure facilitates efficient intramolecular charge transfer and narrows the optical bandgap, both of which are key to achieving NIR-II emission. Thanks to the good biocompatibility and NIR-II characteristics, the performance of FC-CDs in in vivo NIR-II bioimaging (e.g., angiography, hepatic ischemia-reperfusion monitoring, and tumor-targeted imaging) has been verified. This cyano-mediated D-A engineering strategy thus provides insights into designing NIR-II emissive CDs for biomedical applications.