From understanding and harnessing colloidal stability to scalable post-synthetic separation in mixtures of ultrasmall colloidal nanoparticles

Colloidal nanoparticles smaller than 10 nm represent a critical class of materials with exceptional properties that are increasingly leveraged in catalysis, energy conversion, biomedicine, and optoelectronics. Their small size and high surface‑to‑volume ratio, however, result in dominant interfacial effects, which complicate their stabilization, handling, and post‑synthesis processing, particularly in multicomponent systems. This dissertation presents a systematic, experimentally validated framework for understanding and manipulating the colloidal stability and post‑synthesis separation in multicomponent nanoparticle dispersions of such ultrasmall nanoparticles. Focusing on binary mixtures of colloidal gold nanoparticles (AuNPs) and semiconductor quantum dots (QDs), such as ZnS QDs and InP/ZnS core‑shell QDs, the research explores how surface chemistry, solvent environment, and residual surface-active species (i.e., excess ligands, counterions, and weakly adsorbed molecules that may be released into the continuous phase) shape the dispersion behavior in interface‑governed colloidal systems. A reproducible protocol for the synthesis, purification, and surface functionalization of AuNPs was developed to serve as a model platform for investigating colloidal interactions. By employing UV‑visible spectroscopy, zeta potential measurements, and dynamic light scattering, the influence of ligand identity and purification methods on nanoparticle stability was systematically analyzed. Establishing this reproducible protocol was essential for ensuring consistent properties of the synthesized nanoparticles throughout the study. In particular, the purification of AuNPs, a step that remains underreported and poorly standardized in the literature, was investigated in detail, including the effects of various ligands, and provided a critical foundation for the subsequent analyses. The influence of surface chemistry and synthesis residues on dispersion behavior under varying solvent compositions was explored, with particular focus on water‑ethanol systems. Building upon the established protocols, mixed dispersions of AuNPs and ZnS QDs were designed to investigate interparticle interactions in these binary mixtures. A key achievement was the introduction of a spectroscopy‑based strategy that enabled independent monitoring of the colloidal stability of each nanoparticle component. This dual‑component‑tracking approach proved essential for disentangling the complex destabilization mechanisms influenced by solvent composition, surface ligand identity, and residual surface-active species. To explore agglomeration mechanisms in more detail, the AuNP component was systematically varied by introducing additional surface chemistries while keeping the ZnS QDs unchanged. This included ligand‑free AuNPs, citrate‑stabilized AuNPs, tannic acid‑citrate‑stabilized AuNPs, and AuNPs which were functionalized with bis(p‑sulfonatophenyl) phenyl phosphine. The influence of ZnS QD synthesis residues (e.g., Thioglycerol, and Zinc acetate) on the stability of AuNPs in these binary mixtures was evaluated, confirming the ligand‑specific nature of interparticle interactions. Experimental evidence validated a hypothesized mechanism of destabilization based on surface ligand compatibility and the chosen purification strategy. Insights gained from these studies were translated into the development of a post‑synthetic separation technique based on size‑selective agglomeration. A single‑step size‑selective agglomeration was applied to QDs produced via continuous flow synthesis to separate the ZnS QDs, formed by homogeneous nucleation as an undesirable by‑product, from the desired InP/ZnS core‑shell QDs, enabling efficient isolation of the target product. Validation was provided through UV‑visible and photoluminescence spectroscopy, high‑resolution scanning transmission electron microscopy, and energy‑dispersive X‑ray spectroscopy. This strategy was further extended into a multi‑step separation protocol capable of high‑resolution classification of QD mixtures by size and composition. Applying the multi‑step separation to a single feed QD dispersion resulted in 13 fractions, which were subsequently characterized in depth using the high‑resolution toolbox from above complemented by total reflection X‑ray fluorescence, and analytical ultracentrifugation. Systematic trends in optical and compositional properties revealed the extent of sample dispersity and enabled categorization of fractions enriched in InP/ZnS core‑shell QDs or ZnS by‑product QDs. This detailed profiling offered valuable feedback for optimizing the synthesis, particularly in terms of photoluminescence quantum yield. In summary, this dissertation establishes a reproducible model system of AuNPs and ZnS QDs, introduces a systematic approach to disentangle colloidal stability in binary mixtures, and demonstrates a scalable agglomeration‑based protocol for high‑resolution fractionation. These contributions provide a foundation for rational nanoparticle formulation, scalable post‑synthetic purification, and future integration into industrially relevant nanomaterials processing.