Due to their fascinating properties, there is a rise in the critical consideration of carbon-based nanomaterials in a plethora of applications. Carbon nanomaterials, such as nanotubes, graphene, fullerenes, and nanodiamonds, have broad applicability and potential research prospects. In the past few years, the developments and consumption of still smaller nanomaterials, namely graphene quantum dots and carbon nanodots or carbon dots (CDs) have been explored. Since carbon as a component exhibits insignificant cytotoxicity and remarkable biocompatibility, CDs have found a wide scope of potential applications. Owing to their fascinating aspects, such as small size, biocompatibility, low toxic nature, environment-friendliness, costeffectiveness, ease of chemical functionalization, derivatization and surface modification, and photoluminescence tenability, CDs have been widely acknowledged. CDs have found major prospects in the areas of catalysis, sensors, and optical and bio-related applications. CDs are generally synthesized by employing techniques of pyrolysis, laser ablation, arc discharge, electrochemical method; hydrothermal and solvothermal techniques; and microwave and ultrasonic irradiations. This review article presents a brief account of the major properties of CDs, and applications, with particular emphasis on the green and environment-friendly synthesis methodologies. An overview of the microwave and ultrasound irradiation-induced syntheses for the preparation of CDs is presented in the light of green chemistry principles. In addition, some of the green and environmentally benign precursors for the production of CDs are outlined. The most recent work on CDs is included in this review article.

    Abstract
    1 Introduction
    2 Green synthesis of CDs
        2.1 Top-down and bottom-up approaches
        2.2 Green synthetic methods of CDs
    3 Significance of green synthetic methods for the preparation of CDs
        3.1 MW irradiation
        3.2 US irradiation
    4 Importance of green precursors for CDs synthesis
    5 Luminescence mechanism in CDs
    6 Application of green synthesized CDs
        6.1 Light-emitting diodes
        6.2 Solar panels
        6.3 Electrocatalysis
        6.4 Photocatalysis
        6.5 Photodynamic therapy
        6.6 Bioimaging
        6.7 Chemical sensors
        6.8 Corrosion protection
    7 Conclusions and outlook
    References

• Dheeraj Singh Chauhan(Modern National Chemicals, Second Industrial City, Center of Research Excellence in Corrosion, Research Institute, King Fahd University of Petroleum and Minerals)
• M. A. Quraishi(Interdisciplinary Research Center for Advanced Materials, King Fahd University of Petroleum and Minerals)
• Chandrabhan Verma(Interdisciplinary Research Center for Advanced Materials, King Fahd University of Petroleum and Minerals)