We aimed to quantify the interaction of water-soluble-functionalized CdS quantum dots (QDs) with metal cations from their composition and physical properties. From the diameter of thioglycerol-capped nanoparticles (TG-CdS QDs) measured by electronic microscopy ( D = 12.3 ± 0.3 nm), we calculated the molecular mass of the individual particle MAQD = (3 ± 0.5) × 106 g·mol-1 and its molar absorption coefficient ε450 = 21 × 106 M-1·cm-1. We built a three-dimensional model of the TG-CdS QDs in agreement with the structural data, which allowed us to quantify the number of thioglycerol grafted chains to ∼2000 per QD. This value fully matches the saturation binding curve of Al3+ cations interacting with TG-CdS QDs. The reaction occurred with a slow association rate ( kon = 2.1 × 103 M-1·s-1), as expected for heavy QDs. The photophysical properties of the functionalized QDs were studied using an absolute QD concentration of 7 nM, which allowed us to investigate the interaction with 14 metallic cations in water. The fluorescence intensity of TG-CdS QDs could be quenched only in the presence of Al3+ ions in the range 0.2-10 μM but not with other cations and was not observed with other kinds of grafting chains.