A sustainable synthesis and applications of biomass waste-derived tunable fluorescent carbon dots: In vitro and in vivo fluorescent imaging

Different heteroatoms introduced to carbon dots (CDs) have been used extensively owing to their excellent optical properties, simple synthesis method, and multifunctional applications such as environmental detection, multicolor cell imaging, and gene therapy/delivery. Nitrogen and sulfur codoped carbon dots (PSNCDs) were prepared using persimmon fruit (Diospyros kaki) peel biomass waste and sodium thiosulphate by the straight route, showing excellent optical properties. The resulting PSNCDs exhibit consistent fluorescence emission at 440 nm under excitation at 360 nm, with a high quantum yield (approximately 16 %), making them suitable for a range of applications, including bioimaging, drug delivery, sensing, catalysts, and so on. The as-synthesized PSNCDs were subjected to various analytical methods to confirm their morphology and surface functionalization. High-resolution transmission electron microscopy displayed that the PSNCDs are predominately 2–4 nm with an average size of around 3 nm. Fourier-transform infrared and X-ray photoelectron spectroscopy analysis confirmed the amine and sulfur functional moieties on the outer surfaces and edges of the PSNCDs. The uniform particle size distribution and structural morphology were confirmed by transmission electron microscopy analysis. The Raman and X-ray diffraction investigation reveals that the as-synthesized PSNCDs appeared to be defective graphite-like structures with an intensity ratio of D to G is 0.65. Furthermore, PSNCDs were also utilized as fluorescent probes for cellular imaging. PSNCDs exhibited enhanced biocompatibility (cell viability 98 %) with regular fibroblast cells; in both in vitro and in vivo scenarios, they exhibited robust fluorescence signals upon fluorescent imaging of cultured fibroblast cells. These characteristics collectively validate the potential of PSNCDs for cell imaging without the need for supplementary modifications.