Sticking molecules together
Scanning probe microscope links dendritic polymer molecules
The idea of simply grabbing molecules with a pair of tweezers and sticking them together just as you wish is not nearly as absurd as it sounds. Berlin researchers headed by Jürgen P. Rabe and A. Dieter Schlüter have pulled off just such a trick. Their "construction with molecules" is an important step toward molecular nanostructures for nanotechnolgy.
The research team, consisting of chemists and physicists from the Free University and Humboldt University, works with dendritic polymers. These are long molecular chains made of bulky, branched building blocks. The branched building blocks are equipped with azide groups at their "tips". Azide groups are functional groups that -- once they are activated by UV-light, for example -- are highly reactive. When they are deposited onto a special support, the molecular chains look like cylindrical strands under the scanning probe microscope. However, this device allows for the manipulation as well as the observation of molecules. In scanning probe microscopy, a very fine tip scans a surface. The force emanating from this tip is strong enough to "grasp" the tiniest of objects -- such as the polymer strands -- like a pair of tweezers and move them around very precisely on the support. The researchers use these "tweezers" to bring two of their polymer strands into contact. These are then irradiated with UV light, which activates the azide groups, causing them to react to form a strong chemical bond between the two strands. Depending on where the strands are connected, different structures can be built, in the shape of an X, Y, O, or 8, for example. The researchers also showed that the bond holds fast by firmly trying to pull the strands apart.
This method is not limited to bonds between dendritic polymer strands. Instead of using long, chainlike molecules, it is possible to attach azide groups to highly branched spherical molecules for use as building blocks. This "molecular glue" should allow all sorts of macromolecules to be attached to each other. Even hybrid structures made of completely different types of nano-objects, such as DNA and carbon nanotubes, should be accessible in this way.
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