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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]

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]

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]