Facile green synthesis of nitrogen-doped carbon dots using Chionanthus retusus fruit extract and investigation of their suitability for metal ion sensing and biological applications

Nitrogen-doped carbon dots (N-CDs) were synthesized from Chionanthus retusus (C. retusus) fruit extract using a simple hydrothermal-carbonization method. Their ability to sense metal ions, and their biological activity in terms of cell viability and bioimaging applications were evaluated. The resulting N-CDs were characterized by various physicochemical techniques such as high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Raman spectroscopy. The optical properties were characterized by ultraviolet visible (UV-vis) fluorescence spectroscopy techniques. The average size of the N-CDs was approximately 5 +/- 2 nm with an interlayer distance of 0.21 nm, as calculated from the HRTEM images. The presence of phytoconstituent functionalities and the percentages of components in the N-CDs were confirmed by XPS studies, and a nitrogen content of 5.3% was detected. The N-CDs demonstrated highly durable fluorescence properties and low cytotoxicity with a quantum yield of 9%. The synthesized N-CDs were then used as probes for the detection of metal ions. The N-CDs exhibited high sensitivity and selectivity towards Fe3+, with a linear relationship between 0 and 2 mu M and a detection limit of 70 mu M. The synthesized N-CDs are anticipated to have diverse biomedical applications, particularly for bioimaging, given their high fluorescence, excellent water solubility, good cell permeability, and negligible cytotoxicity. Finally, the potential of N-CDs as biological probes was investigated using fungal (Candida albicans and Cryptococcus neoformans) strains via fluorescent microscopy. We found that N-CDs were suitable candidates for differential staining applications in yeast cells with good cell permeability, localization with negligible cytotoxicity. Hence, N-CDs may find dual utility as probes for the detection of cellular pools of metal ions (Fe3+) and also for early detection of opportunistic yeast infections in biological samples. (C) 2017 Elsevier B.V. All rights reserved.