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[Display omitted] • Biocompatible fluorescent N,K,Ca-doped CQDs derived from Chenopodium album. • Fluorescence quantum yield up to 48.12 % was achieved. • Bioimaging of SH-SY5Y human neuroblastoma cells, bacteria, fungal, and plant cells. • The cytotoxicity and cell viability have also been examined. This study demonstrated an economical, ecologically sustainable, and simple method for producing biocompatible fluorescent carbon quantum dots (CQDs) co-doped with N, K, and Ca from bathua (Chenopodium album), a popular medicinal herb leaf extract and nutritious vegetable. The nearly spherical shaped CQDs were discovered to be 3–5 nm in size in nature, with a quantum yield of around 48.12%, robust photo-stability, and excellent water solubility. Various morphological, compositional, physicochemical and optical characteristics were investigated. Additional factors affecting the photoluminescence of CQDs, such as hydrothermal process parameters, pH, and solvent sensitivity, were investigated further. Moreover, the newly synthesized N, K, Ca-doped CQDs were demonstrated to be a useful fluorescent nanoprobe for in vitro cellular imaging of SH-SY5Y human neuroblastoma cells, as well as other living organisms such as bacteria, fungal, and plant cells, as a cost-effective alternative to organic or synthetic fluorescent probes. Cell survival and cytotoxicity were found to be acceptable, with 95% of SH-SY5Y and HCT-116 cells surviving and other organisms able to withstand higher doses (10.9 mg/mL) for up to 4 days. [ABSTRACT FROM AUTHOR]

This study demonstrated an environmentally friendly, cost-effective, and easy method for manufacturing biocompatible fluorescent carbon quantum dots (CQDs) from the leaf extract of the medicinal herb Cissus quadrangularis. As-synthesized nitrogen doped-CQDs had fluorescence quantum yields of up to 5%. The presence of pyrrolic/pyridinic-N, quaternary-N (N–C 3), and O C–N, as verified by X-ray photoelectron spectroscopy, reveals that nitrogen atoms were efficiently doped in CQDs. The size distribution of as-synthesized CQDs was found to be within 4–9 nm using a high-resolution transmission electron microscope. Meanwhile, nuclear magnetic resonance studies showed the presence of a significant proportion of sp2 and sp3 carbon groups. The zeta potential of as-synthesized CQDs was −7.8 mV, demonstrating the presence of a negatively charged surface with good stability in aqueous solution. Furthermore, the development of easy, affordable, and non-destructive fluorescence-based staining agents as an alternative to organic or synthetic dyes is widely sought. In this regard, as-synthesized N-doped CQDs shown remarkable fluorescent staining capabilities in this study and might be employed as a useful probe for optical and bio-imaging of bacteria, fungal, and plant cells. On the other hand, the cytotoxicity and cell viability of pine leaf, mushrooms, algae, fungus, and bacteria have also been examined. Even at higher doses (10.9 mg/ml), cell survival was demonstrated to be rather excellent and cytotoxicity was minimal for up to 4 days. [Display omitted] • A biocompatible fluorescent carbon quantum dots (CQDs) from Cissus quadrangularis leaf extract has been explored. • Fluorescence quantum yields of up to 5% were observed in as-synthesized nitrogen doped-CQDs. • The as-synthesized CQDs have a high water suspension capacity. • N-doped CQDs have been demonstrated as a fluorescent staining agent in bio-labelling of bacteria, fungi, and plant cells. • The cytotoxicity and cell viability of pine leaf, mushrooms, algae, fungus, and bacteria have been examined. [ABSTRACT FROM AUTHOR]