Characterization of Dynamics at the Solid-Liquid Interface via Dynamic Nuclear Polarization

• Applicant: Dr. Tomas Orlando
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2021 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 455993474

Solid/liquid interfaces are usually difficult to investigate spectroscopically at the molecular level under ambient pressure and temperature conditions. Nuclear magnetic resonance (NMR) is a versatile tool for assessing the liquid dynamics but, when dealing with molecules at the interfaces, struggles with low sensitivity and line broadening. Dynamic nuclear polarization (DNP) denotes a class of techniques that are capable of increasing NMR signals of several orders of magnitude. DNP in the solid-state at cryogenic temperatures enabled the characterization of surfaces in catalytic media, porous materials, and metal-organic frameworks that would have been impossible with standard NMR. However, the dynamics of the liquid at the interface remained inaccessible. DNP in the liquid state at room temperature proved to be a valuable tool to both enhance the NMR signal and to investigate liquid dynamics and molecular interactions on the picoseconds timescale. However, this approach is still limited to low magnetic fields (0.34 T) and 1H as a target nucleus. Recent discoveries showed a much wider potential of the technique: high enhancements (~10^2-10^3) at fields up to 9.4 T were measured on 13C and 31P in small molecules doped with organic radicals as polarizing agents (PAs). Mechanistic studies revealed that fast molecular collisions and structural reorientations are driving efficient polarization transfer and could become more prominent once the mobility of the target molecule or the polarizing agent is constrained.With this project, we aim at expanding the capabilities of DNP in the liquid state towards a fast and sensitive characterization of heterogeneous materials and interfaces. We will perform a mechanistic study on the polarization transfer when either the target molecule or the PA is bound to the surface of a nanoparticle. This will give us the necessary understanding of DNP mechanisms in the slow-motion regime. DNP-NMR experiments will be performed up to 9.4 T, where the DNP enhancement can be coupled with chemical shift resolution. Proof-of-concept experiments will be performed on monolayer-coated gold nanoparticles that can selectively bind molecules at the surface. In this way, using the PA as a local probe on the surface, we aim to disentangle the dynamics of the liquid at the surface, revealing the binding mechanisms and the transient interactions. This project will be pivotal for demonstrating the capability of DNP-NMR in the liquid state for studying the solid/liquid interface. The outcome of the project will enable a new class of experiments to address specific problems in material science, catalysis, and sensing.

• DFG Programme: Research Grants