Nanozymes, which are nanomaterials that mimic enzyme activity, offer compelling advantages like low cost, high activity, long-term stability, and easy surface modification. Among these, graphene quantum dots (GQDs) are particularly promising for diverse sensing applications due to their unique optical and electronic properties. Their inherent small size and the chemistry of their functional groups further enhance their catalytic capabilities. Accurately and sensitively detecting NADPH, which is a high-energy electron carrier for various metabolic processes, is crucial for gaining a deeper understanding of fundamental cellular function.

In this study, we successfully synthesized a GQD-based nanozyme. The synthesis involved a hydrothermal process (200 ^oC for 12 hours) using a hydrophilic polyethyleneimine (PEI) precursor doped with hemin. Comprehensive characterization with UV-Vis, energy-dispersive spectroscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy confirmed the successful formation of the nanomaterial. These analyses revealed an average diameter of 7.1 ± 1.5 nm for the synthesized GQDs, with a distinct excitation peak at 360 nm and an emission peak at 460 nm. Furthermore, elemental analysis confirmed the successful incorporation of both iron and nitrogen into the GQD structure, which is indicated by the presence of carbon, nitrogen, oxygen, and iron. The observed fluorescence quenching of this GQD nanozyme upon its interaction with NADPH clearly demonstrates its promising potential for sensitive and accurate NADPH detection, with a strong potential for targeted applications in detecting oxidative stress, which is essential for research in neurovascular health and metabolic diseases.