Mapping out the Aqueous Surface Chemistry of Metal Oxide Nanocrystals: Carboxylate, Phosphonate, and Catecholate Ligands

Abstract

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.

Mechanistic Insight into the Precursor Chemistry of ZrO2and HfO2Nanocrystals; towards Size-Tunable Syntheses

“…Consequently, synthetic procedures to produce colloidally stable group 4 oxide nanocrystals are actively researched. 18 …”

Solution-Processed Inorganic Thermoelectric Materials: Opportunities and Challenges

“…However, this technique cannot provide all the necessary information needed to deduce the surface structure and does not work for most bound small molecules. Nuclear magnetic resonance (NMR) can be used to identify and quantify both bound organic ligands and solvated species. ,,− Solid-state NMR, on the other hand, can provide information on the atomic structures of NP surfaces . DLS combined with ζ-potential measurements allows the charge of colloids to be determined, which helps to elucidate the structure of the electrical double layer in charged NPs .…”

“…Nuclear magnetic resonance (NMR) can be used to identify and quantify both bound organic ligands and solvated species. ,,− Solid-state NMR, on the other hand, can provide information on the atomic structures of NP surfaces . DLS combined with ζ-potential measurements allows the charge of colloids to be determined, which helps to elucidate the structure of the electrical double layer in charged NPs . Grazing incidence X-ray absorption and photoelectron spectroscopies can also be used to disclose how NPs are on the surface …”

Photoinduced charge transfer from quantum dots measured by cyclic voltammetry

“…26 In contrast, FcCOOH does not undergo similar exchange with MEEAA-capped CdS QDs. The lack of exchange is rationalized by the lower pKa of MEEAA (pKa = 3.61) 41 compared to oleic acid (pKa= 9.85) 42 .…”