Self-assembly of nanoparticles at fluid interfaces has emerged as a simple yet efficient way to create two-dimensional membranes with tunable properties. In these membranes, inorganic nanoparticles are coated with a shell of organic ligands that interlock as spacers and provide tensile strength. Although curvature due to gradients in lipid-bilayer composition and protein scaffolding is a key feature of many biological membranes, creating gradients in nanoparticle

membranes has been difficult.

We demonstrated by X-ray scattering that nanoparticle membranes formed at air/water interfaces exhibit a small but significant 6Å difference in average ligand-shell thickness between their two sides. This affects surface-enhanced Raman scattering and can be used to fold detached free-standing membranes into tubes by exposure to electron beams. Molecular dynamics simulations were used to elucidate the roles of ligand coverage and mobility in producing and maintaining this asymmetry.

Understanding this Janus-like membrane asymmetry opens up new avenues for designing nanoparticle superstructures.

1. •Zhang Jiang, Jinbo He, Sanket A. Deshmukh, Pongsakorn Kanjanaboos, Ganesh Kamath, Yifan Wang, Subramanian K.R.S. Sankaranarayanan, Jin Wang, Heinrich M. Jaeger, and Xiao-Min Lin, “Janus-like Nanoparticle Monolayer Membranes from Subnanometer Ligand Shell Asymmetry”, Nature Materials, publ. online 6-8-2015; link
   

This was a close collaboration with colleagues at Argonne National Lab, in particular Xiao-Min Lin, who spearheaded the study and the groups of J. Wang (Advanced Photon Source) and S. Sankaranarayanan (computer simulations).