Tailoring carbon-based nanozymes without metal cofactors: Design principles and applications across therapeutics and sensing.

Metal-free nanozymes (MFNs), composed of heteroatom-doped carbon frameworks, conductive polymers, and π-conjugated networks, have emerged as versatile enzyme mimics that obviate the toxicity and instability associated with metal-based counterparts. Through the strategic incorporation of nitrogen (N), oxygen (O), sulfur (S), phosphorus (P), iodine (I), and selenium (Se) dopants, alongside the deliberate defect engineering of sp² domains and vacancy sites, these materials replicates superoxide dismutase, glutathione peroxidase, catalase, oxidase, and peroxidase activities under physiological and environmental conditions. This review not only outlines the structural design principles and catalytic mechanisms of MFNs but also presents a comparative analysis with traditional metal-based nanozymes, highlighting differences in catalytic pathways, stability, and biocompatibility. Recent studies are systematically categorized into four major application domains-antioxidant therapeutics, tumor-targeted catalytic therapy, diagnostic and biosensing platforms, antimicrobial systems, and environmental remediation-showcasing representative examples such as biomass-derived carbon dots, polyphenol-functionalized nanoparticles, single-atom catalysts, conductive polyaniline constructs, and photocatalytic carbon nitride. Representative case studies are discussed to elucidate key structure-function relationships and experimentally supported outcomes, encompassing validated in vitro and in vivo findings across antioxidant, therapeutic, and diagnostic systems. This comparative and integrative perspective establishes a cohesive framework for the rational design and translational development of next-generation MFNs.