A network with crystalline bundles of filaments. Credit: Yuri S. Velichko.
A team from Northwestern University reports in the new issue of Science about the role that X-rays can play in crystal formation. The researchers say they accidentally discovered that X-rays can trigger the formation of a new type of crystal that is composed of charged cylindrical filaments. These crystals are ordered like a bundle of pencils experiencing repulsive forces.
They hope their work will expand the use of X-rays from not just an analytical tool but also a method to control the structure of materials.
In a NU press release, Samuel Stupp, one of the paper’s authors, describes what the group thinks is going on with the X-rays. “The filaments are charged so one would expect them to repel each other, not to organize into a crystal. Even though they are repelling each other, we believe the hundreds of thousands of filaments in the bundles are trapped within a network and form a crystal to become more stable,” says Stupp, who is a professor of chemistry, materials science and engineering and medicine.
The discovery happened when members of Stupp’s research team, working on a separate organic project, zapped a solution of peptide nanofibers with synchrotron X-ray radiation. Unexpectedly, the solution turned opaque. “There was a dramatic change in the way filaments scattered the radiation,” says coauthor Honggang Cui. “The X-rays turned a disordered structure into something ordered – a crystal.”
The group theorizes that the X-rays increase the charge of the material and causes a hexagonal stacking of filaments. They say that because of repulsive forces, the filaments are positioned far apart from each other, with as much as 320 angstroms separating the filaments.