NMR Characterization of Nanoscale Surface Patterning in Mixed Ligand Nanoparticles

Abstract

Spontaneous phase separation in binary mixed ligand shells is a proposed strategy to create patchy nanoparticles. The surface anisotropy, providing directionality along with interfacial properties emerging from both ligands, is highly desirable for targeted drug delivery, catalysis, and other applications. However, characterization of phase separation on the nanoscale remains quite challenging. Here we have adapted solid-state 1 H spin diffusion NMR experiments designed to detect and quantify spatial heterogeneity in polymeric materials to nanoparticles (NPs) functionalized with mixed short ligands. Janus NPs and physical mixtures of homoligand 3.5 nm diameter ZrO 2 NPs, with aromatic (phenylphosphonic acid, PPA) and aliphatic (oleic acid, OA) ligands, were used to calibrate the 1 H spin diffusion experiments. The Janus NPs, prepared by a facile wax/water Pickering emulsion method, and mixed ligand NPs, produced by ligand exchange, both with 1:1 PPA:OA ligand compositions, display strikingly different solvent and particle−particle interactions. 1 H spin diffusion NMR experiments are most consistent with a lamellar surface pattern for the mixed ligand ZrO 2 NPs. Solid-state 1 H spin diffusion NMR is shown to be a valuable additional characterization tool for mixed ligand NPs, as it not only detects the presence of nanoscale phase separation but also allows measurement of the domain sizes and geometries of the surface phase separation.

Nanocrystal Assemblies: Current Advances and Open Problems

“…Dynamic self-assembly using solvent gradients and localized solvent addition has also been explored, – leading to the formation of patterned NC films . To further advance in NC assembly, it is crucial to understand the role of surface ligands, especially in those systems where a single NC can host a multitude of ligands, as well as balancing attractive van der Waals interactions and steric repulsion.…”

Nanoimaging of Water Bending and Carboxylate Stretching Modes in Colloidal Nanocrystal Films

“…Consequently, the resulting solid films typically comprise not just the NCs but also the aggregates of excess surface ligands and trapped solvent molecules, such as water. To investigate the role and properties of surface ligands, multiple analytical techniques have been utilized, including attenuated total reflectance infrared (ATR-IR) spectroscopy, sum-frequency generation, − X-ray photoelectron spectroscopy, , and solid-state nuclear magnetic resonance. , These methods, however, are limited in their spatial resolution, which hampers our ability to distinguish whether the observed signals emanate from molecules directly adsorbed onto the NC surface or from distant aggregates. While SERS shows promise in detecting molecules in close proximity to the NC surface, , it cannot offer a complete characterization of heterogeneous films that might contain both NC aggregates and excess ligands.…”

Direct Readout of Homo- vs Heterochiral Ligand Shell of Quantum Dots

“…Indeed, QDs are an important research area in nanoscience and nanotechnology and while the research in the field is constantly moving forward, there are a number of challenges . For example, detailed characterization of the ligand shell composition is required to optimize NC properties and surface interactions for a vast array of applications. − Transferring chirality from enantiomeric molecules to colloidal QDs represents another level of tailorability and has attracted immense attention across chemistry, materials, and biomedical science. , Chiroptical properties of NCs arouse interest at the level of basic research, , as well as in the context of various applications, including photonics, catalysis, sensing, and biomedicine. − For instance, nanomaterials fabricated using chiral ligands have aroused substantial interest due to the special chirality-dependent biological effects, and a recent seminal study demonstrated that nanoscale chirality can be exploited to modulate immunological responses and various types of chiral nanoparticles have been explored in cancer therapy . Hence, the development of chiroptically active colloidal QDs and understanding of their optical activity have been a vital issue of nanoresearch over the past decade , following the first observation of chiroptical activity in CdS QDs prepared by microwave-assisted synthesis in the presence of l - or d -penicillamine by Moloney et al in 2007 .…”