CdS/CTS QDs exhibit good photophysical properties. Thus, they can be used to detect heavy metal ions. The dispersed solution of CdS/CTS QDs shows yellow color (Figure ). The color of CdS/CTS QDs after adding metal ions such as Cu 2+ , Ca 2+ , Zn 2+ , Mg 2+ , Ni 2+ , Fe 2+ , Co 2+ , Na + , and K + was evaluated in Figure . Except for Cu 2+ , there are insignificant changes in the solution's color of CdS/CTS QDs (Figure ). The change in the yellow color of CdS/CTS QDs upon interaction with Cu 2+ ions indicates the potential of our probe to detect Cu 2+ ions using naked eyes (Figure ). These observations suggest the high selectivity of CdS/CTS QDs toward Cu 2+ ions compared to other metal ions (Figure ). The absorption peak of the colored metal salts is also observed (Figure ). The change in the optical properties of CdS/CTS upon interaction with different can be recorded using UV-Vis spectroscopy (Figure ). The color change upon interaction with Cu 2+ ions is speedy and can occur once the complete addition for the solution of Cu The optical properties of CdS/CTS before and after interaction with color metal salts, i.e., Ni 2+ , Co 2+ , and Cu 2+ , are evaluated using DRS (Figure ) of CdS/CTS was shifted to 3.2 eV, indicating the strong interaction of the CdS and Cu 2+ ions (Figure ). This observation can be confirmed by the color change in the solution of CdS/CTS. Other metal ions such as Co 2+ and Ni 2+ show minimal change indicating that they can only interact with the functional groups of CTS. Thus, we can claim that CdS/CTS shows selective interaction with Cu 2+ ions.