Large, single crystals are extremely valuable. The natural order taken by the atoms within form lattices and patterns that give single crystals properties not found in any disordered material. So, as you would expect, science has figured out how to make most of them in the laboratory rather than having to dig them out of the mountainside. Forming perfect silicon crystals has led to the explosion of semiconductors and the tiny computers in each of your pockets while pushing carbon together in the proper formation has made it slightly less expensive to put a ring on that finger you like.
Now, researchers from Northwestern University have developed a way to build crystals out of things nature never intended. Using some of the same free energy principles and some fancy chemistry, they have, for the first time, gotten nanoparticles to form crystals using strands of DNA as their glue.
In nature, the strongest crystals are formed when atoms stick together thanks to the sharing of electrons. But chemical bonding doesn’t really work outside of the atomic world—a nanoparticle can’t link up with another nanoparticle via electron sharing. You can, however, attach other molecules to the surfaces of nanoparticles using such techniques. The only question, then, is what do you want to use for your glue?
In the new experiment, the researchers turned to DNA. As most of you are (hopefully) familiar, DNA forms in a double helix with one strand connecting to the other strand through a series of base pairs. What’s more, each individual base will stick only to its partner in biological crime: guanine with cytosine and adenine with thymine.
Using this quirk of nature, researchers created two types of glues, sort of like two sets of Velcro that will only work with each other and won’t interact. They then stuck these biological glue sticks onto the nanoparticles, heated them up, and let them cool slowly over the course of a couple of days.
As predicted, as the system slowly lost energy the nanoparticles self-arranged into a crystalline structure, as physics dictates they do. They formed near-perfect single 12-sided polyhedron made up of congruent rhombic faces (if you don’t know what that means, check out the image above… one of those guys).
“If you get the right ratio it makes a perfect crystal — isn’t that fun?” said Monica Olvera de la Cruz, who also is a professor of chemistry in the Weinberg College of Arts and Sciences. “That’s the fascinating thing, that you have to have the right ratio. We are learning so many rules for calculating things that other people cannot compute in atoms, in atomic crystals.”
So what can people do with large single crystals made out of nanoparticles? Hell if I know; I’m no expert. But seeing as how a pure crystal typically tends to pop up in extremely lucrative industrial practices—computing, manufacturing, cutting, lasers, etc.—I have no doubt that the technique will find a way to influence lives on a daily basis… eventually.
The paper, “DNA-mediated nanoparticle crystallization into Wulff polyhedra,” was published in the journal Nature by Olvera de la Cruz, nanoscientist Chad A. Mirkin, Evelyn Auyeung (first author), Ting I. N. G. Li, Andrew J. Senesi, Abrin L. Schmucker, and Bridget C. Pals, all from Northwestern.