SOMETIMES a copy can surpass the original. Imitation atoms made of microscopic polystyrene spheres have bonded with each other using the same three-dimensional geometries found in real molecules. These surrogate atoms could one day be used to build novel materials such as semiconductors that carry light rather than electricity.
The way real atoms link up to form molecules is governed by quantum mechanics, which dictates how an atom's outermost electrons arrange themselves. This arrangement determines the number of bonds the atom forms - and their relative geometry. For example, carbon, with four outer electrons, can bond to four hydrogen atoms to form tetrahedral methane, or to two oxygen atoms, forming linear carbon dioxide.
Simulating this process is tough, because surrogate atoms must stick together using only certain geometries. Previously the feat had been done in two dimensions, when researchers coated particles in strands of complementary DNA and got them to bind into layers like those inside a crystal.
Now a team led by David Pine of New York University has achieved it in 3D. For their atoms, the team used polystyrene microspheres - either 540 or 850 nanometres across, more than 2000 times bigger than real atoms - coated in a substance that binds to DNA. The researchers forced these to stick together, forming clusters of between two and seven spheres (see photos). Next, they filled in the gaps between the microspheres with liquid styrene, which was not coated in the DNA binder. The styrene swelled and solidified into balls of various sizes, each with bumpy hills - the exposed portions of the microspheres. When DNA was added, it bound only to the hills, turning them into regions that could bind to microspheres coated in complementary DNA.
The placing of these regions depends on the number of microspheres, giving the resulting objects the same bonding geometry as real atoms. For example, balls made with four microspheres acted like the carbon atom in methane, with four patches arranged tetrahedrally (see diagram), whereas those with two acted like the carbon in carbon dioxide.
Sure enough, microspheres with complementary DNA were able to bind to the hills on the fake carbons to form a tetrahedral "methane" and a straight "carbon dioxide" (Nature, DOI: 10.1038/nature11564). The surrogate atoms were so large and slow to clump together compared with real atoms that the team could watch them reacting in real time (see video above).
It's not clear what, if anything, this can teach us about real molecules, but Pine already has an application in mind: linking up several surrogate carbon atoms to create a "semiconductor" for light. Ordinary semiconductors can act as either a conductor of electrons or an insulator. In principle, there should be an equivalent for controlling the flow of photons. Such photonic crystals would be useful in ultra-fast optical computers, but have never been made in 3D.
Pine reckons a crystal built out of surrogate carbons could do the trick. The fakes may be better than real atoms at steering light because their size matches light's wavelengths (400 to 800 nm). "The rules of quantum mechanics that govern the way atoms bind are fairly restrictive," says Pine. "We don't have those kinds of restrictions."