Red-Emissive Azure A-Derived Carbon Dots with Semi-Ordered sp² Domains for Photodynamic Therapy

Red-emissive carbon dots (CDs) with efficient reactive oxygen species (ROS) generation capability are promising candidates for photodynamic therapy (PDT), yet the rational development of long-wavelength-responsive CDs remains challenging. Herein, we report the synthesis of Azure A-derived carbon dots (AzA-CDs) via a simple hydrothermal process using Azure A chloride as a molecular precursor. The intrinsic nitrogen- and sulfur-containing heterocyclic structure and extended π-conjugation of the precursor facilitate bandgap modulation, resulting in red-shifted optical absorption and emission. Structural characterization by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS) confirms the formation of heteroatom-doped carbon nanostructures. Under light irradiation (λ = 530 nm), AzA-CDs efficiently generate singlet oxygen (¹O₂) as verified by DPBF assays. In vitro studies using HepG2 cells demonstrate negligible cytotoxicity at concentrations up to 50 μg/mL and pronounced light-triggered photodynamic effects. Confocal fluorescence imaging further confirms effective cellular internalization of AzA-CDs. Collectively, these results demonstrate that dye-derived red carbon dots can serve as biocompatible and efficient photosensitizers, highlighting their potential for biomedical applications in photodynamic therapy.