Optimizing design and exploring the dual potential of carbon quantum dots in environmental sensing and biomedical theranostics

Carbon quantum dots (CQDs) have emerged as next-generation nanomaterials owing to their tunable photoluminescence, chemical stability, biocompatibility, and facile synthesis from renewable precursors. This review critically examines recent advances in the rational design and optimization of CQDs, with particular focus on sustainable synthesis strategies notably the hydrothermal and microwave-assisted bottom-up routes, which currently dominate green fabrication approaches alongside electrochemical and sonochemical techniques utilizing biomass and waste-derived feedstocks. We highlight how controlled surface functionalization, heteroatom doping, and co-doping approaches enhance fluorescence quantum yield, emission tunability, and selectivity toward specific analytes. Mechanistic insights into fluorescence modulation including complexation, Förster resonance energy transfer (FRET), photoinduced electron transfer (PET), and inner filter effects (IFE) are systematically analyzed to clarify the origins of CQD sensing performance. Special emphasis is given to their application in detecting toxic heavy metals (Hg²⁺, Pb²⁺, Cr⁶⁺, Cu²⁺, Fe³⁺), where sensitivity and selectivity are directly linked to rational structural engineering. Beyond environmental monitoring, we discuss the expanding biomedical landscape of CQDs, encompassing bioimaging, biosensing, antimicrobial platforms, drug delivery, and photodynamic therapy, establishing them as promising theranostic agents. Finally, the review identifies key challenges including scalability, reproducibility, long-term biosafety, and regulatory barriers and outlines future opportunities in device integration, smart sensing, and clinical translation. By bridging mechanistic understanding with application-driven optimization, this work provides strategic insights to accelerate the deployment of CQDs in sustainable environmental and biomedical technologies.