Research on the Affinity between Ligands, Solvents, and Membranes and Its Effect on the Filtration Process of Colloidal Quantum Dot Dispersions

We report the possibility of classification into single nanosized colloidal quantum dots (CQDs) by membrane filtration by considering the affinity balance between ligands, solvents, and membranes. CQDs are expected to have a wide variety of applications in the near future. To realize the social implementation, efficient process technologies for mass production of CQDs have been eagerly awaited. CQDs are easily agglomerated due to the nanosized colloidal particle, so the surface is usually preserved by ligands. On the other hand, size classification is a matter for CQDs because the optical property is closely related to the size of the particle. Recently, various separation and classification methods using porous membranes have been attempted. However, there are few reports about the membrane filtration of nanosized particles in organic solvents such as CQDs. Therefore, the influence of ligands in organic solvents on membrane filtration has not been researched enough. Lack of basic knowledge has been a hurdle to the widespread use of membrane filtration processes in organic-solvent-based dispersions. In this article, the dispersions of CQDs capped by organic and inorganic ligands in various solvents were examined in detail. In addition, the effects of the ligands and solvents on membrane filtration were also investigated. As a result, the CQDs can exist as clusters in the solvent, which has been conventionally known as a good solvent. Moreover, the size of the clusters changed, depending on the affinity between ligands and solvents. By organizing these results based on the idea of the Hansen solubility parameter, we successfully explained agglomeration in the dispersion or adsorption on the membrane. Furthermore, the Hansen solubility parameter of iodide-ligand CQDs was estimated based on the dispersing test of various solvents, and a new mixed solvent for iodide-capped CQDs was discovered for the first time. The experimental results indicate the possibility of membrane filtration of single nanosized CQDs. Based on this study, we suggest optimal filtration conditions for CQD dispersion.