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Dendron is the Greek word for tree. Dendrons are tree-shaped branched molecules. Kim and Chiyoung Park selected a molecular “tree” with four long hydrocarbon chains as “branches”. At the end of the “trunk” they attached a pyrene group, a system made of four aromatic carbon rings. In solution, these dendrons come together “branch to branch” to form vesicles, or tiny bubbles. If the researchers add cyclodextrins, which are ring-shaped closed chains of glucose rings, each of these settles around a pyrene group like a cuff. This makes it more favorable for the dendrons to group themselves into long nanoscopic tubes whose surfaces are coated with the cyclodextrin “cuffs”. What makes this concept into a truly universal construction set is that the cyclodextrins can easily be equipped with a large variety of functional groups, which then dangle out into the solution from the surfaces of the tubes. The team was thus able to attach special groups that like to bind gold nanoparticles. Nanotubes that are densely covered in metal particles could have interesting applications in nanoelectronics. The pyrene groups on the nanotubes have another special advantage: they fluoresce. This property allows them to be used in the design of biosensors. To demonstrate this concept, the researchers constructed a specific test for the protein avidin. They equipped the surfaces of the nanotubes with biotin, a biomolecule that specifically binds the proteins avidin and streptavidin. If streptavidin bound to gold nanoparticles is added, these bind to the nanotubes by way of the biotin anchors. This brings the gold particles into the vicinity of the pyrene groups, which causes them to interact electronically, “switching off” the fluorescence. If the protein avidin and the gold-bound streptavidin are added, biotin anchors on the surface of the tube preferentially bind avidin. Pyrene groups in the vicinity of avidin fluoresce. The fluorescence quencher gold-strepavidin can only bind to the binding sites not occupied by avidin. The intensity of the fluorescence therefore depends on the avidin concentration.
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