Self-assembly of nanoparticles at fluid interfaces has emerged as a simple yet efficient way to create two-dimensional membranes with tunable properties1,2,3,4,5,6. 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 scaffolding7,8 is a key feature of many biological membranes, creating gradients in nanoparticle membranes has been difficult. Here, we show by X-ray scattering that nanoparticle membranes formed at air/water interfaces exhibit a small but significant ∼6 Å 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 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.
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The authors thank S. McBride and E. Barry for many stimulating discussions. We also benefited from discussions with Y. Rabin of Ilan University, Israel, and R. Salvarezza of INIFTA, Argentina. This work was performed at the Center of Nanoscale Materials and 8-ID at the Advanced Photon Source, a US Department of Energy, Office of Science, Office of Basic Energy Sciences User Facility under Contract No. DE-AC02-06CH11357. The work at the University of Chicago was supported by the NSF through grant DMR-1207204 and through the Chicago MRSEC, under NSF DMR-1420709. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the US Department of Energy under Contract No. DE-AC02-05CH11231. This research also used resources of the Argonne Leadership Computing Facility, which is a DOE Office of Science User Facility supported under Contract DE-AC02-06CH11357.
The authors declare no competing financial interests.
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Jiang, Z., He, J., Deshmukh, S. et al. Subnanometre ligand-shell asymmetry leads to Janus-like nanoparticle membranes. Nature Mater 14, 912–917 (2015). https://doi.org/10.1038/nmat4321
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