Nitrogen-doped carbon quantum dots from biomass: Fast microwave synthesis and dual application in biomaterials and bioimaging in vitro

Nitrogen-doped carbon quantum dots (N-CQDs) can be readily synthesized from an extensive range of biomass-derived precursors such as fruit peels, seeds, proteins and even food waste. Biomass naturally contains carbon-rich and nitrogen-containing compounds, which can serve as both the carbon skeleton and the nitrogen dopant. However, when these sources are endemic to specific regions, reproducibility and scalability might be limited in other parts of the world. To ensure broader accessibility and reproducibility, it is advantageous to use widely available biomass resources from CQD production. Moreover, doping CQDs with nitrogen can significantly enhance their optical properties, quantum yield, and surface functionality. Thus, in this work, carbon quantum dots were synthesized from biomass (Morus alba) via microwave irradiation to obtain a nitrogen-doped carbon quantum dots (N-CQDs) with a high quantum yield in solution (Φ_F = 0.98). The material was structurally and morphologically characterized by AFM obtaining an average size of 4.81 nm in one of its dimensions. Its photophysical properties were evaluated showing a maximum emission at 449 nm. Raman spectra of N-CQDs showed D and G bands with an ID/IG ratio of 1.04, indicating a high degree of disorder. Transmission electron microscopy (TEM) showed N-CQDs with controlled morphology, relatively narrow size distribution (particle diameter of 4.74 ± 2.20 nm), and spatially uniform dispersion. This material was used as a dye for silk fibroin by immersion method obtaining a uniform staining. In addition, bioimages of Human Umbilical Vein Endothelial Cells (HUVEC) were obtained and their cytotoxicity was evaluated obtaining a cell viability higher than 95 %. This material allows observing the biomaterial and cell staining under 2 types of lasers which, together with its excellent cell viability, highlight its potential biomedical applications.