Unique molecular structure offers insight into nanoscale self-assembly, solution chemistry

Scientists at the U.S. Department of Energy’s Brookhaven National Laboratory and the University of Bielefeld, Germany, have discovered a new type of hollow spherical vesicles formed by large-scale, wheel-shaped inorganic molecules. These vesicles, described in the November 6, 2003, issue of Nature, represent a new kind of self-assembly in nature with implications for the emerging field of nanoscience as well the solution behavior of other types of particles or systems previously thought to be unrelated.

“These vesicles are totally different from the common vesicles formed by other types of molecules, such as the biolipids of cell membranes and surfactants used in soaps,” said Brookhaven physicist Tianbo Liu, lead author on the paper. In those cases, he explains, the molecules have both hydrophilic (“water-loving”) and hydrophobic (“water-hating”) parts. The water-hating portions all line up facing one another, leaving the water-loving parts exposed to the surface so the entire vesicle can exist in an aqueous environment.

But the molecules described by Liu and his co-authors — giant wheel-shaped polyoxomolybdate (POM) molecules, composed of hundreds or even thousands of molybdenum and oxygen atoms — have no hydrophobic parts. Each wheel-shaped molecule also carries some negative charge, which should make the wheels repel one another. Yet, by using light scattering and transmission electron microscope (TEM) techniques, Liu and his coworkers found that, in dilute solution, more than 1,000 of these wheel-shaped POMs associate and evenly distribute onto the surface of 90-nanometer-wide hollow spheres. The TEM measurements were performed by Brookhaven biologist Huilin Li.

The study helps elucidates how these spheres form. It turns out that hydrogen bonds formed between water molecules play an important role. “In the nanometer-size spaces between the wheel molecules, the viscosity of water could increase by several orders of magnitude,” says Liu. This happens, he explains, because the water molecules are confined in the tiny spaces, so hydrogen bonds readily form between adjacent water molecules. “The properties of this heavily hydrogen-bonded water are more like those of ice than liquid water,” he adds. “So the water between the wheel-shaped molecules acts like a glue that overcomes the repulsive electrostatic forces and ’freezes’ the wheels in place.”

The electrically charged POM molecules can be thought of as large, single, inorganic ions, but also as polyelectrolytes — substances made of repeating subunits that carry an overall electric charge (like proteins or DNA). They can also behave in ways similar to colloidal suspensions, where large particles such as nanoparticles, dust, or aerosols are dispersed but not truly dissolved in another substance like a liquid or air. With these three simultaneous identities, the POMs can serve as a perfect model system for studying how these other substances behave in solution, which, prior to the discovery of this “missing link,” were all independent fields, Liu says.

In the fields of nanoscience and nanotechnology, the POM giant molecules may offer another “dual-personality” benefit: They possess the advantages of single molecules, such as well-defined structures and uniform size and mass, as well as those of nanoparticles, such as complex and variable electronic, magnetic, and colloidal properties. This combination of properties, especially the molecules’ monodispersed nature and adjustable chemical and physical properties, could help to develop more diverse nanomaterials than were previously thought possible.

This work builds on more than 200 years of curiosity about molybdenum solutions, which often have a distinctive blue color. Before anyone knew the element molybdenum at valence state +5 (MoV) was responsible for the blue color, Native Americans gave the name “Blue Waters” to certain fountains near today’s Idaho Springs and the Valley of the Ten Thousand Smokes. Even after the secret of the color was revealed some 200 years ago, the detailed molecular structures of the solutes remained unclear. Then, in the last decade, a series of nanoscale, wheel-shaped, blue color, POM molecules were identified by a German group led by Achim Müller, a co-author of the current paper. This progress introduced the more fascinating puzzle of how these giant molecules dissolve in water. The current study offers an explanation for the mechanism of vesicle formation, and opens a new avenue of exploration for scientists interested in what happens as inorganic molecules reach the nanometer scale.