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 MA(QD) = (3 +/- 0.5) x 10(6) g.mol(-1) and its molar absorption coefficient epsilon(450) = 21 x 10(6) 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 similar 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 (k(on) = 2.1 x 10(3) 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 mu M but not with other cations and was not observed with other kinds of grafting chains.