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[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]