Self-Photooxidation-Restructuring Enables NIR-II Absorption of Carbon Dots for Cancer Phototherapy
Carbon dots (CDs) with absorption in the second near-infrared window (NIR-II, 900-1700 nm) hold promise for tumor theranostics, yet existing synthesis methods often involve complex procedures, harsh conditions, or lack precise control. Here we report a 'self-photooxidation-restructuring' strategy that enables structural reorganization of the carbon core in CDs, achieving a significant redshift of absorption into the NIR-II region. Under ultraviolet (UV) light irradiation, the precursor (B-CDs, absorption in UV region) generates singlet oxygen, which self-oxidizes aldehyde groups and the carbon skeleton of B-CDs to stronger electron-withdrawing carboxyl groups and carbon radicals, respectively. These processes facilitate the formation of new C=C bonds between isolated aromatic domains, thereby transforming B-CDs into novel CDs (N-CDs) characterized by enhanced donor-acceptor interactions and a redshift in absorption toward the NIR-II window. Various experimental data, including high-resolution XPS, FTIR, NMR, EPR, have proved the proposed formation mechanism. The novel N-CDs afforded a high photothermal conversion efficiency of up to 71.33%, which enabled 1064 nm laser-activated photoacoustic imaging (PAI)-guided photothermal therapy (PTT) in tumors. This work opens a new avenue for the synthesis and modulation of CDs in the NIR-II region.