Customized NIR-II Dual-Phase Carbon Dots: Elucidating the Emission Mechanism for Smart Photonic Applications

Carbon dots (CDs) exhibiting dual-phase photoluminescence (PL) in the second near-infrared window (NIR-II, 900-1700 nm) region have shown considerable potential for applications in information encryption, lighting, and bioimaging. However, significant challenges remain owing to a lack of reliable design strategies. In this study, high-brightness dual-phase NIR-II PL CDs (NIR-CDs) were successfully prepared for the first time employing innovative strategies. NIR-CDs possess a distinctive core-shell structure, and the donor-acceptor interactions between the carbon core and strong electron-donating groups on the shell effectively induce a charge-transfer state. The NIR-CDs exhibited a fluorescence quantum yield (QY) of up to 5.91% at 920 nm in conjunction with excellent stability. Notably, the rich functional groups on the shell of NIR-CDs provided hydrogen bonding sites, enabling a 960 nm fluorescence emission and high absolute QY of 3.82% in the solid state. The dual-phase PL of NIR-CDs enables their use in NIR-II multilevel information encryption and fingerprint authentication. The NIR-II flexible film based on NIR-CDs exhibits excellent fluorescence stability under various external stimuli. Furthermore, the quick response code based on NIR-CDs remains clearly identifiable beneath 3 mm porcine tissue, which highlights their potential for anti-counterfeiting, encryption, and bioimaging sensing.