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Biocompatibility, low toxicity, ease of functionalization, ecologically sustainable synthesis, and a diverse variety of applications have all contributed to the current popularity of fluorescent carbon quantum dots (CQDs), a relatively new rising star among zero-dimensional carbon nanomaterials. However, expensive precursors and production, as well as a time-consuming procedure, frequently restrict the economic design that must be addressed. In this work, we report an economical, sustainable and green synthesis of fluorescent CQDs co-doped with N and Ca from mustard (Brassica) flower extract without the need of additional doping element precursors. The quasi-circular or polygonal shaped CQDs were revealed to be yellow-greenish fluorescent in nature with outstanding photo-stability and excellent water solubility, with an average diameter of 4.38 nm and a relative quantum yield of about 15%. The compositional analysis, crystallinity, and functional groups contained in as-synthesized N,Ca-co-doped CQDs were determined by XPS, XRD, and FTIR analysis, respectively. The presence of dominant sp3 hybridized carbons over sp2 was confirmed by 1H NMR and 13C NMR spectroscopy. The plausible fluorescence mechanisms are also proposed. Additionally, a biodegradable bioplastic derived from corn starch was used to develop an N, Ca-co-doped CQDs@bioplastic composite in order to demonstrate how it may be used to produce down-conversion light emitting diodes and optical displays. The impact of CQD concentrations and pH sensitivity were investigated further. [Display omitted] [ABSTRACT FROM AUTHOR]

This study demonstrated an environmentally responsible, inexpensive and simple one-step hydrothermal method to synthesize biocompatible fluorescent carbon quantum dots (CQDs) from Aegle marmelos fruit extract. The near-spherical CQDs were discovered to have a naturally distributed size of 3–6 nm, were unintentionally doped with heteroatoms (Ca, K, and N) without the need for doping element precursors due to mineral contents in extract, had a quantum yield of about 4.06%, strong photostability, and excellent water solubility. Several morphological, compositional, physicochemical, and optical properties were studied. Other variables influencing CQD photoluminescence, such as hydrothermal process parameters and pH sensitivity, were also examined. Zeta potential analysis showed a negative surface charge with a zeta potential value of −5.29 mV and good dispersibility in aqueous solution. Furthermore, as an alternative to organic or synthetic probes, there is a huge need for the development of simple, affordable, and non-destructive fluorescent biomarkers. As-synthesized Ca, K, N-doped CQDs have demonstrated good fluorescent staining abilities and may be employed as a helpful probe for bio-imaging of cancer cells and other organisms. Cell survival and cytotoxicity were found to be satisfactory, with 90% of SH-SY5Y and HCT-116 cells surviving, compared to 63–77% for the other organisms, even at higher dosages (200 µg/mL) for up to 4 days. [Display omitted] • A facile heteroatom doped CQDs have been synthesized from Aegle marmelos. • Hydrothermal process dependent and pH responsive PL were examined. • Bio-imaging of cancer cells and other living organisms are demonstrated. • Cytotoxicity were found over 90% of SH-SY5Y and HCT-116 cancer cells. [ABSTRACT FROM AUTHOR]