Study of the bio-interaction of nitrogen-doped graphene quantum dots with human serum albumin aiming a quantification method

In recent years, carbon-based nanomaterials, such as graphene quantum dots (GQDs), have garnered significant attention due to their remarkable physicochemical and biological properties. This work presents a study evaluating the interaction between N-doped GQDs, produced from a mixture of citric acid and urea (referred here as GQDs-U), with human serum albumin (HSA). The GQDs-U were characterized using spectroscopic techniques, and their concentration was expressed in terms of total carbon content in aqueous dispersion. They present average diameter of about 8 nm, with average hydrodynamic diameter of around 12 nm, and a surface charge of -7 mV at pH 7.4. Assays revealed that a progressive increase in GQDs-U concentration led to the quenching of HSA fluorescence with quenching directly correlated with temperature, also indicating a mixed quenching mechanism with a predominance of the dynamic type, suggesting mutual conformational rearrangements of HSA/GQDs-U before reaching thermodynamic equilibrium. Additionally, it was shown that GQDs-U could be used as a probe to quantify HAS with the increase in photoluminescence of GQDs-U upon the addition of HSA demonstrating effective energy transfer, as GQDs-U were found to promote quenching in HSA photoluminescence. This study indicated sound evidence of stability of the nanomaterial, in aqueous dispersions, in terms of optical and morphological properties. The stability over time suggests the viability of GQDs-U as analytical probes for HSA even after a long period (30 days) of GQDs-U aging. The work also enhanced the understanding of the complex time-structure-activity relationship in this biomolecule/nanomaterial system.