The structural origin of photoluminescence in graphene oxide revealed by ¹³C stable isotopic labeling

Graphene oxide (GO) possesses a chemically tunable structure and diverse surface functional groups, endowing it with exceptional photoluminescence efficiency and biocompatibility, which are highly advantageous for applications in bioimaging, biosensing, and biomedical diagnostics. However, the structural heterogeneity and complexity of GO surfaces result in an elusive fluorescence mechanism, hindering precise modulation of its optical properties and limiting broader practical utilization. Stable isotope labeling provides a powerful approach for probing the fine surface structure of graphene and elucidating the origin of its photoluminescence. In this study, we successfully synthesized ¹³C-labeled reduced graphene oxide via hydrothermal reduction followed by high-temperature annealing. The surface structural composition was quantitatively characterized using ¹³C solid-state nuclear magnetic resonance (¹³C-SSNMR) and X-ray photoelectron spectroscopy (XPS). By correlating steady-state fluorescence spectral fit peak area with structural data, we established definitive relationships between photoluminescence behavior, microstructure, and surface oxygen-containing functional groups. The research results indicate that the photoluminescence of graphene oxide is caused by the local π-π electron cloud density perturbation resulting from the oxygen-containing groups on the surface. Among them, the less stable groups (such as C–O–C, C–OH and C[dbnd]O) play a major role in the fluorescence emission at around 650 nm; the stable groups (such as O–C–O) contribute to the fluorescence emission at around 450 nm; the O–C[dbnd]O group has no effect on the fluorescence emission. This work provides critical insights into the structural origins of graphene oxide's optical properties, offering a foundation for targeted material design.