Density of Grafted Chains in Thioglycerol-Capped CdS Quantum Dots Determines Their Interaction with Aluminum(III) in Water

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) × 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 (k_on = 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.