<p>In the present study, the synthesis and characterization of fluorescent carbon quantum dots (CQDs) sourced from the marine cyanobacterium <i>Leptolyngbya</i> sp. nov. KMBMA-1, obtained from the coastal waters of South Andaman, Port Blair, was executed utilizing an environment-friendly, sustainable hydrothermal methodology. The carbon quantum dots exhibited dimensions ranging from 2 to 8&#xa0;nm, thereby affirming their classification as carbon quantum dots. Transmission electron microscopy imaging substantiated the spherical morphology of the CQDs, with an average dimension of 2–8&#xa0;nm and a homogeneous size distribution. The carbon quantum dots demonstrated pronounced blue fluorescence, with emission maxima at 520&#xa0;nm and broad emission peaks in the vicinity of 440–460&#xa0;nm. Selected area electron diffraction analysis elucidated the crystalline characteristics of the carbon quantum dots, as evidenced by their diffuse ring patterns devoid of distinct periodicity. The UV–Vis absorption spectrum exhibited a prominent peak at approximately 260&#xa0;nm, which is ascribed to the π–π* transitions of aromatic C = C bonds. X-ray diffraction analysis of the carbon quantum dots revealed distinct sharp peaks that signify crystalline structures, with a significant peak observed at 31° (2θ), implying the presence of graphitic or sp²-hybridized carbon. Fourier transform infrared spectroscopy identified absorption peaks at 3292&#xa0;cm⁻¹, which point to hydroxyl (OH) groups, as well as additional peaks signifying the presence of functional groups such as alkenes, alcohols, and hydroxyl entities. These observations afford significant insights into the structural properties of carbon quantum dots, which are imperative for their prospective applications in bioimaging and sensing.</p> Graphical Abstract <p>Synthesis and characterization of CQDs</p> <p></p>