Digestive biotransformation of graphene oxide and reduced graphene oxide in the amphibian Xenopus laevis: structural, chemical, and toxicological changes

Abstract

Graphene oxide (GO) and reduced graphene oxide (rGO) are carbon-based nanomaterials increasingly used in industrial applications, yet their environmental fate remains poorly understood. Once released into aquatic ecosystems, their interactions with biological systems can alter their physicochemical properties, with implications for their bioavailability and potential toxicity. This study investigates the digestive biotransformation of GO and rGO following ingestion by Xenopus laevis larvae, an established aquatic model organism. After 24 hours of exposure, feces containing the digested nanomaterials (dGO and drGO) were collected and analysed using transmission electron microscopy (TEM), Raman spectroscopy, and infrared spectroscopy (FTIR) to assess morphological, structural, and chemical changes. TEM imaging revealed particle alterations, edge erosion, and agglomeration in the digested samples. Raman analysis showed shifts in D* and D″ band positions and decreased defect-related intensity ratios, consistent with partial reduction of GO and rGO. IR spectroscopy confirmed a substantial loss of oxygenated functional groups, with a marked decrease in the oxygen-to-carbon signal ratio in dGO. Notably, carbonyl groups were more strongly reduced than C–O functional groups (epoxy, hydroxyl, alkoxy), suggesting preferential degradation at the sheet edges. rGO, being less oxidized initially, appeared less affected. In parallel, their toxic potential was assessed by measuring cellular viability of bacteria (Escherichia coli) and mammalian cell lines (IEC-6 and TR146) exposed to the materials. The results demonstrated that digested GO exhibited lower toxicity towards IEC-6 cells, while retaining antibacterial activity at low concentrations. However, antibacterial effects are lost at higher doses, likely due to agglomeration and reduced bioavailability, whereas rGO and drGO exhibited minimal toxicity across all conditions. These findings highlight the transformative role of digestive processes on graphene-based nanomaterials and underscore the need to consider such biotransformations in environmental risk assessments.