The changes in the functional groups of CTS for CdS/CTS QDs upon interactions with Cu 2+ ions (i.e., Cu_CdS/CTS) are characterized using FT-IR spectroscopy (Figure ). Like the XRD pattern, the FT-IR spectrum for Cu_CdS/CTS displays the main functional groups of pure CdS/CTS with an insignificant shift in the vibrational wavenumber (Table ). Moreover, extra peaks at a wavenumber of 1331 cm -1 and 655 cm -1 can be assigned corresponding to Cu-S. Several studies reported the high affinity of metal ions toward CTS's functional groups, i.e., N-H, O-H. An adsorption study reported that the metal ions of different cations could be adsorbed on a CTS film following the order of Cu 2+ ≫ Hg 2+ > Zn 2+ > Cd 2+ > Ni 2+ > Co 2+ ions . This observation could explain the high selectivity of CdS/CTS QDs toward Cu 2+ ions. XPS analysis of Cu_CdS/CTS is shown in Figure . Data analysis of CdS/CTS before and after interaction with Cu 2+ ions indicates significant changes in the peak fitting. The analysis of the C1s spectrum shows peaks at binding of 283.8 eV, 284.9 eV, 286.3 eV, 287.6 eV, and 288.6 eV (Figure ). The peaks at binding energies of 283.8 eV, 286.3 eV, and 288.6 eV correspond to C-C, C-O-C, and C(N)-C=O, respectively (Figure ). The extra peaks are due to the interactions with Cu 2+ ions that confirm the interaction between Cu 2+ ions and the functional groups of CTS, i.e., O-H and N-H. N 1s spectrum shows peaks at 405.2 eV, 406.1 eV, and 406.8 eV. No depth conclusion can be taken from the analysis of N1s, even the significant change in the data fitting, due to the overlap between N 1s and Cd 3d. The analysis of the O1s spectrum shows a peak at 531.0 eV, 531.9 eV, and 533.2 eV corresponding to C-O→Cd, C-O→Cu, and C=O, respectively (Figure ). Cd 3d5/2 shows peaks at 405.3 eV, 406.2 eV, 411.8 eV, and 412.8 eV. Cd3d5/2 shows peaks at 405.3 eV, and 412.8 eV corresponding to Cd 3d5/2 and Cd 3d3/2 (Figure ). The analysis of S 2p shows a peak at binding energies of . XPS analysis shows a peak at a binding energy of 933.2 eV corresponding to Cu 2p (Figure ). The XPS spectrum of Cu 2p shows peaks at binding energies of 932.2 eV and 952.1 eV corresponding to Cu 2p3/2 and Cu 2p1/2, respectively. We couldn't observe satellite peaks reported for CuO, indicating that the present form of Cu is pure CuS . The peaks fitting Cu 2p3/2 show three peaks at 932.2 eV, 933.8 eV, and 935.1 eV corresponding to Cu-S, Cu-O, and Cu-N, respectively. The chemical analysis of the elements in CdS/CTS and Cu_CdS/CTS (Figure ). The presence of Na + ions in CdS/CTS is due to the ion mineralization of This makes direct quantitative analysis a tedious task.