Upcycling eucalyptus kraft lignin into fluorescent graphene quantum dots for reproductive cell imaging in aquaculture biotechnology.

The Kraft pulping industry produces significant amounts of lignin-rich black liquor, a largely underutilized renewable resource. We report a scalable method to synthesize lignin-derived graphene quantum dots from raw eucalyptus kraft black liquor (EL-GQDs) through sequential acid oxidation and hydrothermal carbonization. Structural and surface analyses confirm the formation of graphitic nanostructures with oxygen- and nitrogen-rich functional groups, exhibiting strong visible fluorescence, good aqueous dispersibility, and high photostability, with uniform morphology and lattice fringes characteristic of crystalline GQDs. The bioimaging potential of EL-GQDs was tested using Nile tilapia testicular cells in vitro and in vivo, achieving efficient cell labeling with low toxicity. Labeled cells remained viable and localized within the gonads up to 30 days post-transplantation. These results demonstrate that EL-GQDs are biocompatible, photostable, and sustainable nanoprobes suitable for long-term cell tracking in aquatic reproductive research. Further studies on uptake pathways, cross-species application, and long-term biosafety are needed to advance this technology.