Carbon dots have gained tremendous interest attributable to their unique features. Two approaches are involved in the fabrication of quantum dots (Top-down and Bottom-up). Most of the synthesis methods are usually multistep, required harsh conditions, and costly carbon sources that may have a toxic effect, therefore green synthesis is more preferable. Herein, the current review presents the green synthesis of carbon quantum dots (CQDs) and graphene quantum dots (GQDs) that having a wide range of potential applications in bio-sensing, cellular imaging, and drug delivery. However, some drawbacks and limitations are still unclear. Other biomedical and biotechnological applications are also highlighted.

The benign preparation of cobalt oxide nanoparticles (Co3O4-NPs) was performed using marine red algae extract (Grateloupia sparsa) as a simple, cost-effective, scalable, and one-pot hydrothermal technique. The nominated extract was employed as an environmental reductant and stabilizing agent. The resultant product showed the typical peak of Co3O4-NPs around 400 nm wavelength as ascertained by UV–vis spectroscopy. Size and morphological techniques combined with X-ray diffraction (XRD) showed the small size of Co3O4-NPs deformed in a spherical shape. The activated carbon (AC) electrode and Co3O4-NP electrode delivered a specific capacitance (Csp) of 125 and 182 F g–1 at 1 A g–1, respectively. The energy density of the AC and AC/Co3O4 electrodes with a power density of 543.44 and 585 W kg–1 was equal to 17.36 and 25.27 Wh kg–1, respectively. The capacitance retention of designed electrodes was 99.2 and 99.5% after 3000 cycles. Additionally, a symmetric AC/Co3O4//AC/Co3O4 supercapacitor device had a specific capacitance (Csp) of 125 F g–1 and a high energy density of 55 Wh kg–1 at a power density of 650 W kg–1. Meanwhile, the symmetric device exhibited superior cyclic stability after 8000 cycles, with a capacitance retention of 93.75%. Overall, the adopted circular criteria, employed to use green technology to avoid noxious chemicals, make the AC/Co3O4 nanocomposite an easily accessible electrode for energy storage applications.

Graphene quantum dots (GQD) is an efficient nanomaterial composed of one or more layers of graphene with unique properties that combine both graphene and carbon dots (CDs). It can be synthesized using carbon-rich materials as precursors, such as graphite, macromolecules polysaccharides, and fullerene. This contribution emphasizes the utilization of GQD-based materials in the fields of sensing, bioimaging, energy storage, and corrosion inhibitors. Inspired by these numerous applications, various synthetic approaches have been developed to design and fabricate GQD, particularly bottom-up and top-down processes. In this context, the prime goal of this review is to emphasize possible eco-friendly and sustainable methodologies that have been successfully employed in the fabrication of GQDs. Furthermore, the fundamental and experimental aspects associated with GQDs such as possible mechanisms, the impact of size, surface alteration, and doping with other elements, together with their technological and industrial applications have been envisaged. Till now, understanding simple photo luminance (PL) operations in GQDs is very critical as well as there are various methods derived from the optical properties of manufactured GQDs can differ. Lack of determining exact size and morphology is highly required without loss of their optical features. Finally, GQDs are promising candidates in the after-mentioned application fields.