Molecular-level insights into the impact of different dissolved organic matter on the aggregation, dissolution and sedimentation of Zn-doped CdTe quantum dots

Abstract

This study investigated the aggregation, dissolution, and sedimentation of Zn-doped CdTe quantum dots (CdTe: Zn²⁺ QDs) incubated with or without three different dissolved organic matter (DOM) species in artificial freshwater and seawater as well as the underlying mechanisms. Results revealed that water composition and the type of DOM species significantly affected the above behaviors. Compared with those in freshwater, bare QDs and QDs–DOM in seawater formed larger aggregates, released higher Cd²⁺ contents and sedimented faster. Under most circumstances, DOM inhibited QDs aggregation through enhanced electrosteric stabilization. Meanwhile, it accelerated QDs dissolution via promoting particle contact. Bovine serum albumin (BSA), humic acid (HA) and fulvic acid (FA) formed the largest, medium and smallest aggregates with QDs in freshwater, respectively, which agreed well with the molecular weight of DOM and the stoichiometry of QDs–DOM interactions. QDs–FA released the largest content of Cd²⁺ in freshwater since FA has the strongest capacity for complexing with QDs. In both artificial waters, HA and FA adsorption accelerated the sedimentation of QDs, while BSA impeded the sedimentation process. Isothermal titration calorimetric experiments revealed that the major driving forces of all QDs–DOM interactions are hydrophobic forces. The binding stoichiometries of QDs–HA, QDs–FA and QDs–BSA are 6.3, 4.7 and 9.4, respectively. Results from multi-spectroscopic experiments suggested that the nano-form of QDs mainly interacted with the aromatic skeletons in HA rather than with Cd²⁺. This study could shed light on predicting the fate and assessing the risk of QDs in aquatic environments.