Mapping out the aqueous surface chemistry of metal oxide nanocrystals; carboxylate, phosphonate and catecholate ligands

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 pH-dependent binding affinity of the ligands towards 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 – 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). While 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.

Anisotropic nanocrystal shape and ligand design for co-assembly

The use of nanocrystal (NC) building blocks to create metamaterials is a powerful approach to access emergent materials. Given the immense library of materials choices, progress in this area for anisotropic NCs is limited by the lack of co-assembly design principles. Here, we use a rational design approach to guide the co-assembly of two such anisotropic systems. We modulate the removal of geometrical incompatibilities between NCs by tuning the ligand shell, taking advantage of the lock-and-key motifs between emergent shapes of the ligand coating to subvert phase separation. Using a combination of theory, simulation, and experiments, we use our strategy to achieve co-assembly of a binary system of cubes and triangular plates and a secondary system involving two two-dimensional (2D) nanoplates. This theory-guided approach to NC assembly has the potential to direct materials choices for targeted binary co-assembly.

Observation of ordered organic capping ligands on semiconducting quantum dots via powder X-ray diffraction

Powder X-ray diffraction is one of the key techniques used to characterize the inorganic structure of colloidal nanocrystals. The comparatively low scattering factor of nuclei of the organic capping ligands and their propensity to be disordered has led investigators to typically consider them effectively invisible to this technique. In this report, we demonstrate that a commonly observed powder X-ray diffraction peak around $$q=1.4{\AA}^{-1}$$ q = 1.4 Å − 1 observed in many small, colloidal quantum dots can be assigned to well-ordered aliphatic ligands bound to and capping the nanocrystals. This conclusion differs from a variety of explanations ascribed by previous sources, the majority of which propose an excess of organic material. Additionally, we demonstrate that the observed ligand peak is a sensitive probe of ligand shell ordering. Changes as a function of ligand length, geometry, and temperature can all be readily observed by X-ray diffraction and manipulated to achieve desired outcomes for the final colloidal system.

Snapshots into carbon dots formation through a combined spectroscopic approach

The design of novel carbon dots with ad hoc properties requires a comprehensive understanding of their formation mechanism, which is a complex task considering the number of variables involved, such as reaction time, structure of precursors or synthetic protocol employed. Herein, we systematically investigated the formation of carbon nanodots by tracking structural, chemical and photophysical features during the hydrothermal synthesis. We demonstrate that the formation of carbon nanodots consists of 4 consecutive steps: (i) aggregation of small organic molecules, (ii) formation of a dense core with an extended shell, (iii) collapse of the shell and (iv) aromatization of the core. In addition, we provide examples of routes towards tuning the core-shell design, synthesizing five novel carbon dots that all consist of an electron-dense core covered by an amine rich ligand shell.

The Effect of Surface Coating of Iron Oxide Nanoparticles on Magnetic Resonance Imaging Relaxivity

Iron oxide nanoparticles (IONPs) with acceptable biocompatibility and size-dependent magnetic properties can be used as efficient contrast agents in magnetic resonance imaging (MRI). Herein, we have investigated the impact of particle size and surface coating on the proton relaxivity of IONPs, as well as engineering of small IONPs' surface coating as a strategy for achieving gadolinium-free contrast agents. Accordingly, polymer coating using poly(isobutylene-alt-maleic anhydride) (PMA) with overcoating of the original ligands was applied for providing colloidal stability to originally oleic acid–capped IONPs in aqueous solution. In case of replacement of the original ligand shell, the polymer had been modified with dopamine. Furthermore, the colloidal stability of the polymer-coated IONPs was evaluated in NaCl and bovine serum albumin (BSA) solutions. The results indicate that the polymer-coated IONPs which involved replacement of the original ligands exhibited considerably better colloidal stability and higher proton relaxivity in comparison to polymer-coated IONPs with maintained ligand shell. The highest r2/r1 we obtained was around 300.

Electronic Structure Study of Divanadium Complexes with Rigid Covalent Coordination: Potential Molecular Qubits with Slow Spin Relaxation

The electronic structures of homovalent [V2(μ-S2)2(R2dtc)4] (R = Et, iBu) and mixed-valent [V2(μ-S2)2(R2dtc)4]+ are reported here. The soft-donor, eight-coordinate ligand shell combined with the fully delocalised ground state provides a...

Ligand Control of Low-Frequency Electron Paramagnetic Resonance Linewidth in Cr(III) Complexes

Understanding how the ligand shell controls low-frequency electron paramagnetic resonance (EPR) spectroscopic properties of metal ions is essential if they are to be used in EPR-based bioimaging schemes. In this...