Episode 19: Sweet spot found for ligand-stripping oleylamine-coated Fe3O4 nanoparticles

In this podcast episode, MRS Bulletin’s Laura Leay interviews Yaroslava Yingling and Joseph Tracy from North Carolina State University about their study on iron oxide colloidal nanoparticles (NPs) coated in oleylamine ligands. By combining experimental work with molecular simulations, their research group determined how to optimize ethanol solvent-mediated ligand stripping in order to control the functionality of the NPs. This work was published in a recent issue of Advanced Materials Interfaces.

Transcript

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on hot topics in materials research. My name is Laura Leay. When combined with experimental work, molecular simulations can provide an impressive level of detail, generating insight that experiments alone often cannot achieve. Colloidal nanoparticles, typically comprised of an inorganic core surrounded by a monolayer of organic ligands, often have the organic component stripped using a poor solvent to make them suitable for a given application. New research using large-scale molecular dynamics simulations shows that ligand stripping doesn’t always increase with poor solvent concentration, challenging a widely held belief. The research involved iron oxide coated in oleylamine ligands. These form stable dispersions in hexane. Ethanol can be added to strip the ligands and experimental work using thermogravimetric analysis and dynamic light scattering indicated different behaviors depending on the method of addition. Professor Joseph Tracy from NC State University explains.

JOSEPH TRACY: If we slowly add the ethanol to the nanoparticles and hexanes, we strip more ligands than if we add the nanoparticles to ethanol because then – if we slowly add the nanoparticles slowly to methanol – they’re immediately seeing a very high ethanol concentration, so that would kind of be like dumping all the ethanol in at once. And so, in that case, where we would quickly introduce the ethanol there’s less ligand stripping because there’s this protective effect; the particles agglomerate to some extent before the ligands are removed. This agglomeration is a faster process, or at least part of it is a faster process. Really it’s a matter that there are these processes – the ligand stripping and agglomeration – going on at different timescales.

LAURA LEAY: The nanoparticles began to agglomerate above 33% ethanol concentration and this agglomeration inhibited ligand stripping. Simulations meant that this behavior could be observed in detail and showed that the ligands began to interdigitate, preventing solvent molecules from accessing the surface of the nanoparticles. Larger nanoparticles, which have a greater ligand surface density, exhibited a greater degree of interdigitation which enhanced the protective effect of agglomeration.

YAROSLAVA YINGLING: A lot of my research centers around goldilocks rules: not too hot and not too cold. It was very interesting to see that we both agree experimentally and in simulations that instead of “more solvent equals more stripping” we have this sweet spot. So it’s interesting to see that nonlinear behavior and then being able to explain it in detail.

LAURA LEAY: That was Professor Yaroslava Yingling from NC State University. The 33% threshold was initially observed in the simulations, and then validated by experiment. Significant effort was required to develop a simulation that accurately predicted the experimental observation with a startling level of detail.

YAROSLAVA YINGLING: There is dynamic ligand rearrangement: it being able to be replaced from the solvent, stripped non-evenly where one side is exposed and the other is not but it’s protected. Showing all of these dynamic properties was not easy to get to. We had to go back to basics and develop a new force field potential for the surface of a nanoparticle, new potentials between the ligands and the particles so we can see all of these effects and still have the correct assumptions of the solubilities and the rates and everything else.

LAURA LEAY: The experimental systems were made small enough to be comparable to the simulations, which often featured just eight nanoparticles in solution and a simulation box close to 20 nm across. Different solvents were also tested to try to understand whether there are equivalent rules for different solvents and oleylamine was chosen as the ligand because it has neither a strong or weak binding affinity; sitting in the middle means that extrapolation could be made in either direction. Now that the model is validated by experimental results, it could be used to model other systems, offering further insights into nanoparticle processing. These outcomes are made possible as a result of graduate students working closely together, as well as funding to support this kind of fundamental science. Professor Yingling and Professor Tracy both agree. 

YAROSLAVA YINGLING: Excellent students are driven and interested in going deep into the fundamental research, both experimentally and in simulation, because the grad students had to understand both systems.

JOSEPH TRACY: We’re grateful for the funding, without which this work would not be possible. We’re grateful for the students, without which this work would not be possible.

LAURA LEAY: This work was published in a recent issue of Advanced Materials Interfaces. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on X, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.