“Until now these types of vesicles were only observed due to hydrophobic effects,” Achim Müller told nanotechweb.org. “Here it is just the reverse, as extremely pronounced hydrophilic surfaces are responsible.”
The polyoxomolybdate nanowheels making up the vesicle were about 3.6 nm in diameter. They had a similar composition to natural molybdenum blue - the mineral ilsemannite that has the formula Mo3O8•nH2O and was responsible for Native Americans naming the Blue Waters fountains near Idaho Springs. (A few years ago, Müller and colleagues discovered that molybdenum-oxide nanoclusters containing up to 200 metal atoms are responsible for the blue colour.)
The negatively charged nanowheels, in this case containing 152 or 154 molybdenum atoms, self-assembled into hollow spherical vesicles with an average radius of about 45 nm. Calculations showed that the vesicles contained about 1165 individual nanowheels. If the nanowheels were distributed on the surface in a hexagonal closest packing arrangement, they would be roughly 4.9 nm apart. This indicates that the nanowheels do not touch - the scientists believe that water plays an important role in filling the gaps between them and holding the vesicle together.
According to Tianbo Liu of Brookhaven National Laboratory, hydrogen bonds readily form between the water molecules confined in the spaces between the nanowheels. “The properties of this heavily hydrogen-bonded water are more like those of ice than liquid water,” he said. “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 researchers found that they could tune the size of the vesicles by adjusting the pH of the solution or by adding electrolytes. Using a solvent with a lower polarity also caused larger vesicles to form.
“Structurally well-defined molybdenum-oxide based nanowheels, which are the abundant species/clusters in solution, can be functionalized in different ways, for example by introducing magnetic centres,” said Müller. “This allows the generation of unusual giant magnetic vesicles/soft materials. On the other hand it will be possible to get more information about water structures in general (a tremendous problem), as we can structurally characterize the water clusters between the nanoclusters forming the vesicles.”
The researchers reported their work in Nature.