Iron oxide and hafnium oxide nanocrystals are two of the few successful examples of inorganic nanocrystals used in a clinical setting. Although crucial to their application, their aqueous surface chemistry is not fully understood. The literature contains conflicting reports regarding the optimum binding group. To alleviate these inconsistencies, we set out to systematically investigate the interaction of carboxylic acids, phosphonic acids, and catechols to metal oxide nanocrystals in polar media. Using nuclear magnetic resonance spectroscopy and dynamic light scattering, we map out the pHdependent binding affinity of the ligands toward hafnium oxide nanocrystals (an NMR-compatible model system). Carboxylic acids easily desorb in water from the surface and only provide limited colloidal stability from pH 2 to pH 6. Phosphonic acids, on the other hand, provide colloidal stability over a broader pH range but also feature a pH-dependent desorption from the surface. They are most suited for acidic to neutral environments (pH <8). Finally, nitrocatechol derivatives provide a tightly bound ligand shell and colloidal stability at physiological and basic pH (6−10). Whereas dynamically bound ligands (carboxylates and phosphonates) do not provide colloidal stability in phosphate-buffered saline, the tightly bound nitrocatechols provide long-term stability. We thus shed light on the complex ligand binding dynamics on metal oxide nanocrystals in aqueous environments. Finally, we provide a practical colloidal stability map, guiding researchers to rationally design ligands for their desired application.

Metal oxo clusters of the type M6O4(OH)4(OOCR)12 (M = Zr or Hf) are valuable building blocks for materials science.

Vascular corrosion casting is a method used to visualize the three dimensional anatomy and branching pattern of blood vessels, guiding insight into health and cardiovascular disease pathogenesis and progression. A polymer resin is injected in the vascular system and, after curing, the surrounding tissue is removed. This corrosion process often deforms or even fractures the fragile cast, resulting in an overall loss of information. Here, we propose a method that does not require corrosion of the tissue, based on in-situ high-resolution computed tomography (micro-CT) scans. Since there is a lack of CT contrast between the polymer cast and the animals’ surrounding soft tissue, we introduce hafnium oxide nanocrystals (HfO2 NCs) as CT contrast agents into the resin. The NCs dramatically improve the overall CT contrast of the cast and allow for straightforward segmentation in the CT scans. We designed the NC surface chemistry to ensure colloidal stability of the NCs in the casting resin, resulting in a homogeneous dispersion that remains stable during casting and curing. Using only 5 m% of HfO2 NCs, high-quality casts of both zebrafish and mouse models could be segmented using CT imaging software, allowing us to differentiate even μm scale details, without having to alter the resin injection method or affecting the resin’s mechanical properties. Our new method of virtual dissection by visualizing casts in-situ using contrast enhanced CT imaging greatly expands the application potential of the technique.