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Posted: Jun 17, 2015
Nanoscale asymmetry leads to Janus-like nanoparticle membranes
(Nanowerk News) Using grazing incidence small angle x-ray scattering (GISAXS) and surface-enhanced Raman scattering (SERS), very small differences in the distribution of coated-nanoparticle membranes were detected and found to be responsible for their folding into tubular structures. Molecular dynamics simulations show this is related to surface molecular packing density and mobility.
The self-assembly of nanoparticles at liquid-liquid and liquid-air interfaces has emerged as a simple and efficient way to create two-dimensional membranes with tunable electrical, optical, magnetic, and mechanical properties. In these membranes, inorganic nanoparticles are coated with a shell of ligand molecules that interlock as spacers and provide tensile strength. Here, researchers from the Center for Nanoscale Materials (CNM), Advanced Photon Source (APS), the University of Chicago, and the University of Missouri, discovered that a heterogeneous environment, such as an air-water interface, strongly influences the membrane properties ("Subnanometre ligand-shell asymmetry leads to Janus-like nanoparticle membranes").
(top) A membrane of 5.8 nm coated gold nanoparticles folds into a tube under an electron beam; (bottom left) High resolution SEM image of a single tube (bottom right) MD simulation shows two types of membranes with an asymmetric coating of about 0.6 nm. (background) Simulated GISAXS pattern that is used to compare with experiments.
When free-standing dodecanethiol-ligated gold nanoparticle membranes detach from the edge, they tend to fold into tubes upon exposure to electron beams, but always towards the water-facing side of the membranes. SERS measurements reveal this behavior originates from an asymmetry in the ligand shell developed during the formation of the membrane at the air-water interface. Highly quantitative surface X-ray scattering shows a difference of ~6Å in average ligand shell thickness between the two sides of the membrane, corresponding to ~30% of the extended ligand length.
Experiments and molecular dynamics simulations further elucidate the role and interplay of ligand coverage and mobility in producing and maintaining this asymmetry, even after the water is removed and the membranes become free-standing. Understanding this Janus-like membrane based solely on ligand distribution asymmetry opens up new avenues for designing nanoparticle superstructures.