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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]

[Display omitted] • We report a simple, cost-effective, eco-sustainable synthesis of fluorescent carbon quantum dots derived from precursor Cissus quadrangularis. • The as-synthesized nitrogen doped-CQDs demonstrated a homogeneous size distribution (4–9 nm), green fluorescent nature, great photo-stability, excellent water solubility, and a quantum yield of around 5 %. • A biodegradable wheat-starch-based bioplastic was employed to fabricate an N-doped CQDs@bioplastic composite to demonstrate its potential applications in quantum dot-based optical displays and down-conversion light emitting diodes. • The influence of different CQD concentrations and pH sensitivity were investigated further utilizing fabricated optical displays and LEDs. Carbon quantum dots (CQDs)-based composites as luminous down-conversion materials are becoming more popular due to several advantages such as steady fluorescence, ease of functionalization, tailoring of emission in the visible range, and so on. We report an inexpensive and environmentally sustainable synthesis of fluorescent nitrogen doped-CQDs produced from Cissus quadrangularis , a low-cost plant precursor with therapeutic value. The morphological, structural, and physicochemical features of the material were carefully investigated. Under UV stimulation (365 nm), almost spherical shaped N-CQDs with an average diameter of 5.1 nm were discovered to generate yellow-green fluorescence, have excellent photostability, and strong water solubility, with a quantum yield of up to 5 %. Furthermore, as a solid-phase dispersion matrix for CQDs, ecologically friendly and biodegradable bioplastic is appealing. The down-conversion of solid-state fluorescence of LEDs and UVC tubes was demonstrated by creating a nanocomposite by inserting N-CQDs into the solid matrix of a wheat starch-based bioplastic. Furthermore, employing constructed quantum dot-based optical displays, down-converted LEDs, and UVC tubes, the impacts of varied CQD concentrations and pH sensitivity were examined. [ABSTRACT FROM AUTHOR]