One-step green synthesis of in–situ functionalized carbon quantum dots from Tagetes patula flowers: Applications as a fluorescent probe for detecting Fe3+ ions and as an antifungal agent

[Display omitted] •Green Synthesis of fluorescent carbon quantum dots (CQDs) from Tagetes patula flowers with a quantum yield of 29.88%.•It exhibits fluorescence quenching behavior towards Fe3+ ions with a limit of detection of 0.32 μM.•Investigation of antifungal activity of CQDs on the growth of pathogenic fungus (Magnaporthe oryzae).•Study of the effectiveness of the CQDs on the growth of rice plant (Oryza sativa). In this research, for the first time, fluorescent carbon quantum dots (CQDs) synthesized by using a green, simple, and efficient one-step hydrothermal carbonization technique from Tagetes patula flowers as carbon precursors without using any toxic chemicals are reported. The as-prepared CQDs have been characterized by Transmission electron microscope (TEM), Field emission scanning electron microscopy with Energy dispersive X-ray spectroscopy (FESEM-EDX), UV–Visible spectroscopy, Raman spectroscopy, X-ray Diffraction (XRD), zeta potential analysis, Fourier-transform infrared spectroscopy (FTIR), fluorescence spectroscopy, and X-ray photoelectron spectroscopy (XPS). As analyzed by TEM and XRD analysis, it is observed that the as-synthesized CQDs are spherical having an average particle diameter of about 5.15 nm. The quantum yield (QY) of the CQDs was calculated and found to be 29.88%. The as-synthesized CQDs show an excellent fluorescence quenching behavior towards Fe3+ ions as compared to other metal ions. Therefore, CQDs were used as a sensing probe for detecting Fe3+ ions in a linear range of 0–4 μM and a limit of detection (LOD) of 0.32 μM. A comprehensive study has been conducted to show the effectiveness of the CQDs on the growth of rice plants (Oryza sativa) and the antifungal activities of CQDs on the mycelial growth of Magnaporthe oryzae (a plant pathogenic fungus).