Carbon Quantum Dots Prepared by Hydrothermal Method for Drug Delivery, Photocatalysis, and Bioimaging Applications

Carbon quantum dots (CQDs) have gained significant attention due to their unique optical properties, biocompatibility, and ease of synthesis, making them highly valuable in various scientific and industrial applications. This review explores the synthesis, properties, and applications of CQDs, with a particular focus on the hydrothermal synthesis technique. Hydrothermal synthesis is a widely used method due to its simplicity, cost-effectiveness, and environmental sustainability. Unlike conventional chemical synthesis routes, hydrothermal techniques use mild reaction conditions and eco-friendly precursors, making them ideal for large-scale CQD production. Additionally, hydrothermal synthesis allows fine control over CQD size, morphology, and surface functionalities, enabling tunable fluorescence properties. This review compares hydrothermal synthesis with other synthesis techniques, such as chemical oxidation, microwave-assisted, electrochemical, and solvothermal methods. While alternative methods provide advantages in terms of reaction time or specific functionalization, hydrothermal synthesis remains a preferred route due to its versatility and scalability. The role of precursor selection, temperature, pressure, and reaction time in determining the physicochemical properties of CQDs is also discussed. Recent advances in green synthesis approaches, including the use of biomass-derived precursors, are highlighted as an important step toward sustainable nanotechnology. CQDs have been extensively investigated for their applications in drug delivery, bioimaging, photocatalysis, and environmental remediation. In drug delivery, CQDs serve as excellent nanocarriers due to their high surface area, functionalization capabilities, and biocompatibility. They have been employed in targeted drug delivery systems to improve therapeutic efficiency and reduce systemic toxicity. Their fluorescence properties enable real-time imaging and tracking of drug release within biological systems. In bioimaging, CQDs provide an alternative to traditional organic dyes and semiconductor quantum dots due to their low toxicity, photostability, and tunable emission wavelengths. Their applications extend to fluorescence-guided surgery and real-time cell imaging. CQDs have also demonstrated significant potential in photocatalysis, where they act as effective electron donors and acceptors in photochemical reactions. Their role in enhancing photocatalytic degradation of organic pollutants, hydrogen production, and CO₂ reduction is discussed. The review further examines recent advancements in modifying CQDs to enhance their catalytic efficiency, including heteroatom doping, surface passivation, and hybrid nanostructure formation. In environmental science, CQDs have been employed in sensors for detecting heavy metal ions, organic pollutants, and biomolecules due to their high sensitivity and selectivity. Despite these promising applications, challenges remain in optimizing CQD synthesis for large-scale production while maintaining consistent quality and performance. Key challenges include controlling the uniformity of CQD size distribution, improving quantum yield, and enhancing their stability under physiological and environmental conditions. This review identifies future research directions, emphasizing the need for more efficient synthesis routes, better functionalization strategies, and improved understanding of CQD interactions with biological and environmental systems. Advances in these areas will further expand the potential applications of CQDs in nanomedicine, energy storage, and environmental remediation. In conclusion, hydrothermally synthesized CQDs offer a sustainable and efficient approach for developing next-generation nanomaterials with diverse applications. Their unique properties and multifunctionality make them an exciting area of research, with continued advancements expected to drive innovations in nanotechnology, biomedicine, and environmental science.