In this study, carbon quantum dots/Cu2O nanocubes (CQDs/Cu2O NCBs) nanomaterial has been successfully synthesized using a simple and easy-to-implement method with a quick reaction time (90 min) at 80°C. In particular, carbon quantum dots (CQDs) were obtained by green and environmentally friendly synthesis methods, using abundant and naturally available raw materials from rice flour and applying hydrothermal approach. Besides, cuprous oxide nanocubes (Cu2O NCBs) were also synthesized through non-toxic compounds. The morphology and composition of CQDs/Cu2O NCBs and Cu2O NCBs were analyzed through UV-vis, FTIR, XRD, FE-SEM, TEM, and EDX spectroscopy. The results showed that Cu2O NCBs have a cubic shape with an average particle size of ~ 29–31 nm, and CQDs have spherical shape with a size of ~ 4–8 nm. In addition, CQDs/Cu2O NCBs nanomaterial was also employed as a “turn-on” fluorescence sensor” for application in metal ions detection. The obtained results showed that the CQDs/Cu2O NCBs are potential and promising in detecting Pb²⁺ ions in the wide range from 1.8 M to 10^− 9 M and extremely low limit of detection of 27.8 nM at the excitation wavelength of 340 nm. CQDs/Cu2O NCBs-based sensors are highly sensitive and selective for Pb²⁺ detection. Therefore, CQDs/Cu2O are promising as photocatalytic materials for various applications, including sensors, bio-probes biomedical, optoelectronics, etc.