Carbon Quantum Dots: From Core-Shell Structure and Tunable Optical Properties to Top-Down and Bottom-Up Synthesis, Characterization and Emerging Applications

Carbon Quantum Dots (CQDs) are an emerging class of carbon-based nanomaterials known for their exceptional optical behavior, chemical versatility, and biocompatibility. Their core–shell architecture, enriched with surface functional groups such as hydroxyl, carboxyl and carbonyl groups, enables strong fluorescence, excellent water dispersibility and tunable electronic properties. CQDs exhibit excitation-dependent and up-conversion photoluminescence, broad UV–Vis absorption and size-dependent emission, all of which make them highly suitable for imaging, sensing and catalytic applications. CQDs can be synthesized through both top-down and bottom-up methods, including laser ablation, chemical oxidation, electrochemical carbonization, hydrothermal treatment, microwave irradiation, and pyrolysis. These methods offer flexibility in controlling particle size, structure and surface chemistry, although achieving uniformity and mechanistic clarity remains a challenge. Comprehensive characterization using HRTEM, FTIR, UV–Vis spectroscopy, PL analysis and XRD confirms their nanoscale morphology, surface functionalities, elemental composition and fluorescence behavior. Because of their excellent biocompatibility, strong and stable fluorescence and easy surface functionalization, CQDs have demonstrated wide applications in bioimaging, biosensing, drug delivery, photocatalysis and environmental remediation. They can detect biomolecules, metal ions, toxins, and disease markers with high sensitivity and selectivity, while also enhancing hydrogen generation and pollutant removal in environmental systems. Despite their immense potential, several challenges continue to limit the full-scale advancement of CQDs. These include the lack of a standardized classification system, an incomplete and often debated understanding of their photoluminescence mechanisms, and the difficulty of achieving consistent, reproducible synthesis with uniform structural features. To overcome these limitations, future research is expected to emphasize precise surface functionalization, the design of multifunctional CQDs capable of simultaneous sensing and imaging and the integration of CQD-based systems into portable, real-time and intelligent diagnostic platforms. With continued progress in these areas, CQDs stand out as a highly promising and adaptable nanomaterial with the capacity to transform biomedical, environmental and forensic technologies.