Combination of Efficient Red Fluorescence and High Photothermal Conversion in the Second Near‐Infrared Window from Carbon Dots through Photoinduced Sodium‐Doping Approach

Abstract Carbon dots (CDs) are highly desired in biological applications. However, achieving a balance between photoluminescence (PL) efficiency and photothermal conversion efficiency (PTCE) is challenging. In this study, an unprecedent combination of efficient red fluorescence and high PTCE in the second near‐infrared (NIR‐II) window in a sodium‐doped CDs system is reported. Upon 808 nm laser irradiation, photo‐induced oxidation–reduction reactions happened on the surface of sodium cation‐functionalized CDs (Na‐CDs), leading to the partial reduction of surface‐functionalized sodium (Na) ions. The photo‐reduced Na atoms coordinated with sp² C domains in the core, resulting in Na‐doped CDs (ir‐Na‐CDs) with an enhanced absorption band in the NIR‐II window and a high PTCE of 43% under 1064 nm laser irradiation (1 W cm⁻²). Composing the ir‐Na‐CDs with bovine serum albumin (BSA) enhanced the PLQY of the red emission to 31% in water without diminishing the PTCE in the NIR‐II region. Transient absorption spectra revealed that no energy transfer occurred between the red emission center and the NIR‐II absorption band, revealing a novel CDs system with independent Janus photophysical processes. Moreover, ir‐Na‐CDs@BSA exhibited negligible to low cytotoxicity and demonstrated tumor accumulation capacity after intravenous injection, enabling effective tumor photothermal therapy in the NIR‐II region.