Mo' Better Blues

For two centuries, chemists have puzzled over the origin of the beautiful colour obtained of so-called "blue water", a familiar phenomenon to Native Americans for millennia. The puzzle attracted the attention of famous names such as Sheele and Berzelius who, suspecting that the Tyndall effect was at play, tried to work out the exact composition of the particles giving rise to the effect.

In the 1990s, Achim Mueller of the University of Bielefeld and his colleagues finally determined the structure of the molybdenum particles in blue water as containing enormous wheel-shaped clusters of molybdenum and have been pursuing these molecules and their chemical cousins ever since. Now, they and Tianbo Liu of Brookhaven National Laboratory have homed in on the nature of the particles formed by these molybdenum wheels and discovered that they form enormous hollow aggregate particles, known as vesicles.

Soapy surfactant molecules and water-repelling cell membrane compounds, such as cholesterol, readily assemble into complex hollow spheres if added to water. These vesicles and liposomes form because one end of the molecule repels water and so they pack together forming effectively sub-microscopic bubbles.

Mueller and his colleagues have now used light-scattering measurements and transmission electron microscopy to produce images of the related hollow spheres formed by molybdenum wheel clusters that contain 154 molybdenum atoms. The clusters appear to lie flat and homogeneously distributed on the vesicle surface, explains Mueller. Moreover, unlike conventional liposomes and vesicles, the molybdenum clusters are stabilized by electrostatic forces, rather than the water-repellent effect that makes liposomes form.

"Our research is leading to new insights in the field of soft-matter physics and nanoscience," Mueller told Reactive Reports. "These molybdenum wheels have possible applications due to the fact that they are structurally well defined. The materials can be made functional in different ways," he adds. For instance, magnetic centres might be introduced and so unusual giant magnetic vesicles, or soft materials, could be formed.

Nature, 2003, 426, 59-62; http://dx.doi.org/10.1038/nature02036.