•Solid-state fluorescence from Zn-doped CQDs@bioplastics nanocomposite is reported.•Zn-doped CQDs were synthesized from Brassicca juncea flower extract.•UVC-tube down-conversion was demonstrated with bioplastic nanocomposite.•Zn-doped bioplastic nanocomposite emits more brightly than the unintentional one.•CIE chromatic coordinates of (0.39, 0.56 vs. 0.29, 0.55) were obtained. The enhancement of solid-state fluorescence from nanocomposite bioplastics containing intentionally Zn-doped carbon quantum dots (CQDs) has been reported. For the synthesis of CQDs from Brassica juncea flower extract, an economical, ecologically sustainable, and one-step hydrothermal method was used. The morphological, structural, and physicochemical properties were examined. The quasi-circular shaped CQDs, with an average diameter of 4.7 nm, were discovered to emit yellow-greenish fluorescence upon UV stimulation, to have exceptional photostability, and to be highly water soluble, with a quantum yield of up to 18%. Furthermore, a commercial UVC-tube wrapped with bioplastic nanocomposite emitted a greenish-yellow colour where intentionally Zn-doped CQDs@bioplastic nanocomposite emits more intensely than an unintentional one with CIE chromatic coordinates (0.39, 0.56 vs. 0.29, 0.55).