68 69 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 Hownanoparticles influence their liquid environment Related publication: Thomä S. L. J., Krauss S.W., Eckardt M., Chater P. & Zobel M. Atomic insight into hydration shells around facetted nanoparticles . Nat. Commun. 10 , 995 (2019). DOI: 10.1038/s41467-019-09007-1 Publication keywords: Pair distribution function (PDF); Hydration shell; Iron oxide nanoparticle; Liquid layering T here are many applications for nanoparticles in liquid suspensions, including cosmetic products, industrial catalysts and the contrast agents used in some medical imaging techniques. Previous research has shown that in these suspensions, liquid molecules group themselves aroundananoparticle like a shell.These“solvation shells”(or "hydration shells")were known tohave fromthree tofive layers, butwerenotfullyunderstood.AresearchteamfromtheUniversityofBayreuthinGermanyusedDiamond’sI15-1X-rayPairDistributionFunction (XPDF) beamline to take a closer look at the atomic andmolecular structures of the layers. Their research focused on magnetic nanoparticles, widely used for targeted drug release and magnetic resonance imaging (MRI). Using the pair distribution function (PDF) technique, they were able to precisely determine the relationships between magnetic nanoparticles and the surrounding liquid, down to the atomic level. Their results show that the crystalline structure of the nanoparticle has a significant influence on how nearby water molecules realign themselves. It has become clear that water molecules adhere to nanoparticles through dissociative bonds in some cases andmolecular adsorption in others. The knowledge gained could help to improve the self-assembly of nanoparticles and drive the understanding of nanoparticles with the environment. Crystallography Group Beamline I15-1 Nanoparticles in solution interact with their surroundings via hydration shells. Although the structure of these shells is used to explain nanoscopic properties, experimental structural insight is still missing. Water molecules are known to adsorb molecularly and dissociatively at surfaces of iron oxides. Now, for the first time, we could show with pair distribution function (PDF) experiments on aqueous dispersions of iron oxide nanoparticles that the water molecules adsorb and reorient at the particle surface. To our surprise, hydration shells around 7 and 15 nm sized magnetic iron oxide nanoparticles are comparable to shells on planar interfaces. The small organic capping agents used for particle stabilisation do not show a major impact on the water restructuring, whereas the interfacial structure is mainly dominated by the crystallinity of the particles. Individual interatomic distances of molecularly and dissociatively adsorbed water molecules directly at the particle surface could be identified, as predicted by theory 1,2 . DuringtheexperimentatbeamlineI15-1,wecollectedX-raydiffractiondataof various sets of functionalised iron oxide nanoparticles, bulk water and a dispersion of the nanoparticles at ca. 0.3–0.5 weight %.The nanoparticles were synthesised via coprecipitation in basic solution at elevated temperature and functionalised with 13 different small organic capping agents at the end of the synthesis. The cappingagentscompriseddibutanoicacids,benzoicacids,basicaminoacids,small α-hydroxyacidsandsomeotherbifunctionalmolecules.Stabilityofthedispersions has been confirmed with small angle X-ray scattering and zeta potential (surface charge) measurements. The zeta potential reflects the electrostatic repulsion between particles in the dispersion and the higher the modulus of its value, the better the colloidal stability. The nanoparticle size and faceting were determined with transmission electron microscopy. Thermogravimetric analysis showed that atleasthalfoftheparticlesurfacewasnotcoveredandthus interactingwithwater. In order to access the hydration shell signal, the scattering contribution from the bulk water and the dry nanoparticle powder were subtracted from the dispersion signal, see Fig. 1. The resulting double-difference PDF (dd-PDF) thus only contains information about the interfacial water structure which is different from the bulk water. The looser bound water layers at distances >3 Å create a sinusoidal oscillation with a wavelength of ca. 3 Å, in agreement with molecular dynamics simulations of water around hematite nanoparticles 3 . For distances <3 Å, three distinct peaks could be identified at 1.48, 1.95 and 2.39 Å, which correspond to different conformations of molecularly and dissociatively adsorbed water molecules, as predicted by density functional theory for water at magnetite facets 1,2 . Fig. 2 illustrates this first adsorbed layer and amore detailed discussion of the allocation of interatomic distances is found in the original publication. In conclusion, this study provides a method of investigating the interfacial hydration structure around colloidal nanoparticles. The effects of edges and cornersareexpectedtobesignificantonlyforevensmallerparticlediametersthan the 7 nm investigated here. The size of the organic capping agents had minimal impact on the hydration shell structure, while the impact of concentration remains to be determined. This X-ray scattering based method of elucidating hydration shell structures bridges the structural gap between spectroscopy and theory. References 1. Zhou C. et al. Density functional theory study of water dissociative chemisorption on the Fe 3 O 4 (111) surface. J. Phys. Chem. C 114 , 21405 (2010). DOI: 10.1021/jp105040v 2. Li X et al . Adsorption of water on the Fe 3 O 4 (111) surface: structures, stabilities, and vibrational properties studied by density functional theory. J. Phys. Chem. C 120 , 1056 (2016). DOI: 10.1021/acs.jpcc.5b10560 3. Spagnoli D. et al. Prediction of the effects of size andmorphology on the structure of water around hematite. Geochim. Cosmochim. Acta 73 , 4023 (2009). DOI: 10.1016/j.gca.2009.04.005 Funding acknowledgement: Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) via SFB 840, scholarship of the Bavarian Academy of Sciences and Humanities; Bavarian Polymer Institute (BPI) for access to the keylabs‘Electron and Optical Microscopy’ and‘Mesoscale Characterization: ScatteringTechniques’, Diamond Light Source for beamtime (EE17345–1). Corresponding author: Prof MirijamZobel, University of Bayreuth,
[email protected] Figure 1: Extraction of hydration shell signal from experimental data: the PDF of the arginine-capped nanoparticle powder (blue) is scaled to the difference PDF (d-PDF, red) of the dispersion and bulk water in the range of 15–30 Å as only iron oxide nanoparticle peaks are present there. The hydration shell (black, in offset) remains and contains the structural information of the molecular arrangement of the water molecules around the iron oxide nanoparticles. Figure 2: Schematic of two facets of iron oxide nanoparticle with dissociatively and molecularly adsorbed water molecules, together with experimental signal from hydration shell. Oxygen atoms are red, hydrogen white, Fe oct dark blue and Fe tet light blue. Bonds resulting in the peaks at 1.48, 1.95 and 2.39 Å are highlighted with solid lines in green, purple and yellow, respectively.