AIM AND TASK: This project concerns an experimental-computational approach to calculate and collect protons chemical shifts from water molecules and biomolecules in contact with metal oxides surfaces. Two different interfaces will be explored namely TiO2 and CeO2, both reference materials for bio-water-solid interactions. The chemical shift will be obtained from the electron density of snapshots of MD-DFT trajectories. The resulting accumulated proton shift histrograms will be directly compared with experimental NMR spectra and help decipher the latter and validate the former.
CONTEXT: Water-metal-oxide interfaces play decisive roles in a range of vital applications such as photoelectrochemical energy conversion, nanotoxicology, biosensing and energy storage (e.g. Li-ion batteries). To understand and tailor the structure and functionality of such interfaces is therefore of prime interest. One key aspect is the structure of water molecules in the proximity of nanosurfaces (say, within 1 nm) that mediate the interaction with the surrounding environment. Probing minute amounts of adsorbed surface water species in the presence of significant excess of a bulk water phase represents a serious challenge for experimental approaches, due to the dispersion of the interfacial signal with respect to the bulk liquid phase. Relatively few attempts to probe metal-oxide surfaces directly in aqueous environments have been reported so far. These include studies by inelastic and quasi-elastic neutron scattering, atomic force microscopy and sum frequency generation. Although these studies indicate the existence of a layer of constrained water molecules exhibiting limited mobility at the nanoparticle perimeter, no further information on structure and speciation within that layer could be elucidated. This is a very challenging field in strong development.
Here solid-state NMR spectroscopy represents a unique tool among experimental techniques applicable to probing nanointerfaces. With appropriate sample preparation and experimental strategy involved, solid-state NMR is capable to deliver atomic and molecular level information regardless of challenges associated with low species concentration and surface disorder.
The nuclear magnetic shielding and therefore NMR chemical shift is a physical property of an atom/molecule that can be readily computed from calculated electron density and compared directly with experiment:
Protons chemical shifts at the solid-water interface can be computed once the water structure in proximity of the solid surface is known and well sampled dynamically. Due to the intrinsic reactivity of metal-oxide surfaces towards water the electronic degrees of freedom of the system must be taken into account in the simulations, and this is done in the framework of the density functional theory (DFT), that is computationally very demanding.
This project is now funded by VR international post-doc granted by me for the period 2022-2025. Accordingly the combined computational-experimental approach will be extended to the chemical shift calculation of adsorbing amino acids on TiO2 in aqueous environment. The computational data will be collected from trajectories in the context of ab-initio metadynamics.
Such simulations form the project's core and the structures collected will be the basis of the subsequent chemical shift calculations.