Solvation and Hydration Shells around Iron Oxide Nanoparticles
In many everyday applications, nanoparticles can be found in a dispersed state, i.e. in sun lotions or as drug vehicles. Nanoparticles also leak from those products into our environment, where they interact with water.
Interfaces are of particular interest in various fields including colloid science, catalysis and biomedicine to just name a few. Restructuring within the adjacent materials is taking place and results in modified interfacial properties.
In a liquid, the arrangement of solvent molecules is determined by their mutual interaction. At an interface, for instance in a pore or around a colloidal particle, the arrangement of solvent molecules changes in comparison to bulk due to the interaction with the surface. Alcohols and water for example can build hydrogen bonds to surface species. The restructuring extends about 3-5 molecular layers into the bulk.
Only recently, we showed that such solvation shells around nanoparticles (NPs) can be studied with high-energy X-ray diffraction and pair distribution function (PDF) analysis.1,2
In the Figure, the restructuring of ethanol molecules around a NP decorated with an organic stabilizer and hydroxy groups is depicted. Restructured ethanol molecules exhbit a sinusoidal oscillation of the electron density profile along the surface normal (green curve).
From the oscillatory signal, important conclusions about the extent and magnitude of solvent-particle interactions can be drawn. We can also access the internal atomistic and molecular arrangement of the solvent molecules at the interface. Solvent-NP interactions are specific for each solvent-NP pair and dependent on many different factors like particle size, composition, crystallinity, surface decoration etc. Therefore, a systematic study of the influences of solvent restructuring around colloidal NPs is crucial.
We investigate solvation and hydration shells around iron oxide NPs, since they are of great interest for many different applications (e.g. in biomedicine) due to their magnetism. Besides the parametric studies of particle size and crystallinity, we want to tackle two important questions:
Do the solvent-particle interactions influence the resultant mesocrystal structure in a self-assembly process of iron oxide nanocubes? Do the free-functional groups of stabilizing molecules, which are exposed to the water-interface in an aqeous iron oxide NP suspension, influence the ordering within the hydration shell?
These free functional groups are the anchors for bioconjugation with antibodies or cancer cell receptor molecules and therefore play an important role for biomedical applications.
Thomä, S. L. J., Krauss, S. W., Eckardt, M., Chater, P., Zobel, M., Atomic insight into hydration shells around facetted nanoparticles, Nature Communications (2019)10:995, DOI: 10.1038/s41467-019-09007-1.
Zobel, M., Neder, R. B., Kimber, S. A. J., Universal solvent restructuring induced by colloidal nanoparticles, Science 347 (2015), 6219, 292-294.
Zobel, M., Observing structural reorientations at solvent-nanoparticle interfaces by X-ray diffraction - putting water in the spotlight, Acta Cryst. A72 (2016), 621-631