Near-infrared I/II emission and absorption carbon dots via constructing localized excited/charge transfer state for multiphoton imaging and photothermal therapy

Carbon dots (CDs) with absorption and emission properties in near-infrared (NIR) bio-windows highlight their application in the integration of diagnosis and treatment. In this work, CDs with NIR-I emission and NIR-II absorption have been demonstrated, by constructing localized excited state and charge-transfer state. NIR fluorescence together with photothermal effect was observed in the CDs under NIR light illumination. The electron donors (D) and acceptors (A) groups around the carbon core of the CDs lead to D-π-A charge-transfer state, producing a strong electronic push–pull effect and yielding the desired NIR-II absorption with a maximum absorption peak at 1033 nm. Localized excited state from large conjugated rigid sp2 domain leads to NIR-I emission of 670 nm. Multiphoton-induced NIR-I emission (quantum yield is 29%) and NIR-II induced heat with photothermal conversion efficiency of 41.19% have been realized. In addition, multiphoton cellular imaging and cancer photothermal therapy using the CDs as imaging and photothermal agents have been demonstrated. The results provide a route to CDs with NIR absorption/emission characters and prospect their potential application in multiple-photon bioimaging and photothermal therapy of cancers.