Molecular-level insight 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) incubating with or without three different dissolved organic matter (DOM) in artificial freshwater and seawater as well as the underlying mechanisms. Our results found that water composition and DOM species significantly affected the above behaviors. Compared with 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 enhancing electrosteric stabilization and accelerated QDs dissolution via promoting their contacts. Bovine serum albumin (BSA), humic acid (HA) and fulvic acid (FA) formed the largest, medium and smallest aggregates with QDs in freshwater, which were well agreed 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 of complexing QDs. In both artificial waters, HA and FA adsorption accelerated the sedimentation of QDs; while BSA impeded the sedimentation. Isothermal titration calorimetric experiments found that the major driving forces of all QDs-DOM interactions are hydrophobic forces. The stoichiometries for HA, FA and BSA binding to QDs are 6.3, 4.7 and 9.4, respectively. Results from multi-spectroscopic experiments suggested that the nano-form of QDs mainly interact with aromatic skeletons in HA rather than Cd2+. This study could shed light on predicting the fate and assessing the risk of QDs in aquatic environment.