Graphene quantum dots (GQDs) as multifunctional nanozymes and its prospective applications: A comprehensive review

The emergence of nanozymes, which are nanomaterials that possess an enzyme-like catalytic activity, has revolutionized ways in which we understand biosensing, environmental remediation, and nanomedicine. Graphene quantum dots (GQDs), a unique type of zero-dimensional carbon-based nanostructure gained a lot of research interest due to their tunable physicochemical properties, quantum confinement effects, edge-state-mediated reactivity, and ability to functionalize surfaces in many ways. This review distinguishes GQDs as a distinct catalytic category, contrasting their structural and electronic characteristics with those of carbon dots, graphene oxide, and traditional inorganic nanozymes like Fe 3 O 4 , CeO 2 , MnO 2 , and noble-metal nanoparticles, in contrast to broader reviews on carbon-based or graphene nanozymes. We extensively combine current advancements in GQD synthesis, including hydrothermal, electrochemical, microwave-assisted, and bottom-up molecular assembly methods, focusing on environmentally friendly and scalable production. A focused mechanistic section demonstrates how quantum confinement, edge defect density, heteroatom doping (N, S, B, P), and electronic band structure all work together to control catalytic output, which includes activities that are equivalent to peroxidase, oxidase, catalase, and superoxide dismutase. Standardized Michaelis-Menten parameters (Km, Vmax, and kcat/Km) were used to compare the kinetics of natural enzymes, classical nanozymes, metal-free GQDs, and metal-enhanced GQD hybrids. The biomedical effectiveness of GQD nanozymes is thoroughly assessed with respect to real-time biosensing, targeted drug delivery, photodynamic/chemodynamic cancer therapy, and antimicrobial strategies with a detailed examination of ROS-associated off-target toxicity, immune responses, and long-term in vivo biodistribution. Applications related to food safety and the environment, like antioxidant evaluation, heavy metal detection, and pollutant degradation, are extensively investigated. Along with suggestions for standardized characterisation, reproducibility issues like batch-to-batch variability, defect density control, and doping uniformity are addressed. Future research priorities are outlined to move GQD nanozymes from laboratory research to deployable environmental and biomedical devices.