Enzyme-Driven Biodegradation of Ti3C2 MXene: Unveiling Peroxidase-Mediated Pathways and Enhanced Bioaccumulation Risks in Aquatic Systems

2D MXene nanosheets are increasingly attracting interest due to their promising applications in materials science and biomedicine. However, the environmental fate of MXenes, particularly their biotransformation, is poorly understood. Here, the biodegradability of Ti3C2 MXene nanosheets was investigated using a plant horseradish peroxidase (HRP)-mediated reaction. The degradation rates of Ti3C2 MXene nanosheets were determined to be first-order in both HRP and H2O2 dosage. Material characterizations and product identifications revealed that peroxidase-catalyzed the oxidation of Ti3C2 MXene in the presence of H2O2, resulting in the formation of holes on its basal plane, the generation of titanium dioxide (TiO2) nanoparticles, and the release of CO2. The assessment using Daphnia magna revealed visible toxicity and enrichment of Ti3C2 MXene to aquatic organisms, with LC50 values of pristine Ti3C2 MXene to Daphnia as 55.41, 7.24, and 2.97 mg L-1 at 48, 72, and 96 h, respectively. Substantial accumulation (74.30 μg Ti mg-1 of dry tissue) of Ti3C2 MXene by Daphnia was observed after 48 h of exposure. Furthermore, the biological effects of HRP-degraded Ti3C2 MXene products on Daphnia were examined. Although the toxicity to Daphnia was reduced, a substantial increase in the bioaccumulation of Ti3C2 MXene biodegradation products (137.36 μg Ti mg-1 of dry tissue) was observed. These findings reveal that enzymatic degradation alters the size and surface chemistry of Ti3C2 MXene, potentially changing its toxicity and altering its environmental compatibility.