An improvement of solid-state fluorescence from a luminous nanocomposite made from corn-starch-based bioplastic by adding intentionally Cu-doped carbon quantum dots (CQDs) has been reported. An affordable, environmentally sustainable, one-step hydrothermal technique was employed to synthesize CQDs from Brassica juncea flower extract. The morphological, structural, and physicochemical characteristics of the material were investigated. The quasi-circular shaped CQDs have been found to have a size distribution of 2–5 nm, to have yellow-greenish fluorescence upon UV stimulation, to have excellent photostability, and to be highly water soluble, with a quantum yield of up to 18.4%. Interestingly, the fluorescence spectra of intentionally Cu-doped CQDs (hereafter N, Ca, Cu-doped CQDs) were found to be brighter after 280 nm illumination than unintentional one. Furthermore, as a solid-phase dispersion matrix of CQDs, the environmentally benign and biodegradable bioplastic is appealing. As a result, this work demonstrated the effect of Cu-doping on solid-state fluorescence in the down-conversion of UVC tubes via a composite constructed by inserting CQDs into the solid-state matrix of corn-starch-based bioplastic nanocomposite, which demonstrated vivid greenish-yellow emission in the electroluminescent spectra with CIE chromatic coordinates (0.33, 0.58 vs. 0.31, 0.67).