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Carbon quantum dots (CQDs)-based composite as a luminescent down-conversion materials are gaining importance nowadays because of many favorable factors including stable fluorescence, ease of functionalization, tuning of emission in visible range etc. In this work, we report an economical, sustainable and green synthesis of CQDs derived from mustard (Brassica juncea) flower extract. The as-synthesized CQDs were unintentionally doped with nitrogen and calcium without the inclusion of any doping element precursors. However, phosphorus was intentionally doped to improve the luminescent characteristics of as-synthesized CQDs. The morphological, structural, and physicochemical properties of P-doped CQDs were examined. The quasi-circular or polygonal-shaped CQDs, with an average diameter of 3.4 nm, emerged to produce yellow-greenish fluorescence upon UV stimulation, demonstrate outstanding photostability, and had a quantum yield of up to 18.2%. Furthermore, the ecologically friendly and biodegradable bioplastic is attractive as a solid phase dispersion matrix of CQDs. Hence, this work revealed the effect of phosphorus doping on solid-state fluorescence in the down-conversion of LEDs and UV tubes by producing a composite constructed through inserting CQDs into the solid-state matrix of corn-starch-based bioplastic. [Display omitted] • Effect of intentional P-doped in N,Ca-doped CQDs@bioplastic composite is reported. • Intentional P-doped CQDs were synthesized from Brassica juncea flower extract. • Down-conversion of LEDs and UV tubes have been demonstrated. • Influence of pH on fluorescence properties have also been investigated. [ABSTRACT FROM AUTHOR]