Comparative assessment of the phase transfer behaviour of InP/ZnS and CuInS/ZnS quantum dots and CdSe/ZnS quantum dots under varying environmental conditions

It is vital to assess the behavior of quantum dots in different environmental media to determine their environmental impacts and potential health risks. There is a limited number of studies that explore the fate and transport of QDs and these studies have so far focused on cadmium-based QDs. With the increasing interest and transition to cadmium-free QDs, there is a need to examine their environmental performance compared to cadmium based QDs. In this study, the organic-to-water phase transfer behaviors of three types of quantum dots (QDs) that include CdSe/ZnS, InP/ZnS, and CuInS/ZnS coated with three different types of ligands (oleic acid (OA), oleylamine (OAM), octadecylamine (ODA)) were compared under different environmental conditions including humic acid (HA) content, pH and ionic strength. Phase transfer rates, aggregation and ion release processes were compared for these nine combinations with a multitude of characterization methods including UV-vis, TEM, ICP-MS, measurements using a Zetasizer and FTIR. Among the three types of coating ligands studied, QDs coated with OA were determined to be transferred more rapidly than QDs coated with OAM and ODA for CdSe/ZnS and InP/ZnS QDs whereas QDs coated with OAM were transferred more rapidly to the aqueous medium compared to those coated with OA and ODA for CuInS/ZnS QDs. For ligands with an amine functional group, the change in the valence of amine and carboxyl functional groups with a change in the pH level of the medium significantly influences phase transfer behavior making pH an important variable among the properties of the aqueous medium that affect phase transfer. The transfer rate increased with a decrease in pH and in the presence of ions and HA in the medium. The maximum ion release was from the indium-based QDs. Further fate and transport studies are required to investigate phase transfer behavior focusing on other phases as well as studies that focus on other types of QDs for environmental impact assessment of QD emissions.