Facile approach to broaden UV-absorbing sun protection via structural transformation of edible bird nest into nitrogen-doped carbon dots

The structural transformation of edible bird’s nest (EBN) into carbon-based nanomaterials markedly enhanced its optical functionality, converting a naturally narrow UV absorber into a broad-spectrum UV absorber—a critical advancement for efficient sun protection. Nitrogen-doped carbon dots (NCDs) were successfully synthesized through a rapid microwave-assisted carbonization process using urea as the nitrogen precursor. Nitrogen incorporation not only tailored surface functionalities but also broadened the absorption by introducing additional n–π* transitions associated with C O and C N bonds. Structural characterizations (XRD, FTIR, TEM) confirmed the formation of predominantly amorphous carbon frameworks enriched with carboxyl, amide, and amine groups, and a particle size of approximately 14.93 nm. High-resolution spectra further verified successful nitrogen incorporation in the form of pyridinic and pyrrolic N, along with oxygen-containing groups such as carboxyl and C–O bonds. Optical studies exhibited strong absorption bands (225, 288, and 388 nm) and tunable photoluminescence modulated by nitrogen-induced defect states. Importantly, the optimized NCDs achieved an SPF of 46.46 and a critical wavelength of approximately 381 nm, satisfying the FDA board-spectrum threshold (≥ 370 nm). Under UV and heat irradiation, the NCDs maintain stable UV absorption with only slight changes, supporting their consistent SPF performance. The zeta potential of the EBN-derived NCDs is relatively low, indicating that the particles are prone to aggregation. Overall, nanomaterialization of EBN into NCDs significantly enhanced its UV response and demonstrated outstanding photoprotective performance, establishing it as a sustainable, high-efficiency candidate for next-generation photoprotective materials.