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Self-assembly in aqueous solution of wheel-shaped Mo154 oxide clusters into vesicles


Surfactants and membrane lipids readily assemble into complex structures1 such as micelles, liposomes or hollow vesicles owing to their amphiphilic character—the fact that part of their structure is attracted to polar environments while another part is attracted to non-polar environments. The self-assembly of complex structures also occurs in polyoxometallate chemistry, as exemplified by the molybdenum blue solutions known for centuries. But while the presence of nanometre-sized metal oxide aggregates in these solutions has long been recognized, unravelling the composition and formation process of these aggregates proved difficult. Recent work has indicated that discrete, wheel-shaped mixed-valence polyoxomolybdate clusters of the type {Mo154} (refs 2–4) assemble into well-defined nanometre-sized aggregates, including spherical structures5. Here we report light-scattering data and transmission electron microscopy images of hollow spherical structures with an average, almost monodisperse radius of about 45 nm and composed of approximately 1,165 {Mo154} wheel-shaped clusters. The clusters appear to lie flat and homogeneously distributed on the vesicle surface. Unlike conventional lipid vesicles, the structures we observe are not stabilized by hydrophobic interactions. Instead, we believe the polyoxomolybdate-based vesicles form owing to a subtle interplay between short-range van der Waals attraction and long-range electrostatic repulsion, with important further stabilization arising from hydrogen bonding involving water molecules encapsulated between the wheel-shaped clusters and in the vesicles’ interior.

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We thank M. Schmidtmann for the generation of Fig. 1d. T.L. acknowledges support of this work by the US Department of Energy, Division of Materials Science, and A.M. correspondingly, the Deutsche Forschungsgemeinschaft, the European Union, the Fonds der Chemischen Industrie, and the Volkswagen-Stiftung. H.L. acknowledges the support of the LDRD fund from BNL.

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Correspondence to Tianbo Liu or Achim Müller.

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Further reading

Figure 1: Structure of the 3.6 mm size {Mo154}-type nanowheel with a hydrophilic surface and nanosized central cavity.
Figure 2: Zimm plot based on SLS measurements.
Figure 3: Transmission electron microscopy studies of wheel-type vesicles on carbon film.


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