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Nature’s nanoantenna

In the majority of his research, Northeastern associate professor of electrical and computer engineering Hossein Mosallaei tries to develop new materials that aren’t available in nature. But in some recent work with colleagues at Harvard University’s Department of Chemistry and Chemical Biology, Mosallaei is looking to nature herself for the inspiration.

The materials Mosallaei typically works with are called “metamaterials,” which are basically carefully arranged stacks of different materials. Think of a polymer sheet aligned with tiny squares of gold or copper and then a series of those sheets all stacked on top of each other. The resulting cube is a new material, one with very different properties than the gold or the polymer alone.

What those properties are depends on how the various components are arranged. Mosallaei’s team does extensive computational work to design specifically arranged metamaterials, which the researchers hope will open doors to entirely new fields: optical computing, for instance. Or nanoantennae that work similarly to the rabbit ears on your grandparents’ television but fit on a tiny corner of your fingernail.

Turns out there’s already a structure just like that in nature. It’s called a chlorosome, and it’s critical to the energy harvesting process of a photosynthetic bacteria species called Chlorobium tepidum. Also called green sulfur bacteria, these guys are thought to be one of the first photosynthetic organisms ever to have evolved since they can produce energy without needing oxygen. They do it with their chlorosomes.

The chloro­some is a crit­ical ele­ment in the pho­to­syn­thetic system of green sulfur bac­teria. Image cour­tesy of Hos­sein Mosallaei.

The chloro­some is a crit­ical ele­ment in the pho­to­syn­thetic system of green sulfur bac­teria. Image cour­tesy of Hos­sein Mosallaei.

This tiny structure looks a lot like a naturally-occurring metamaterial. It’s only a couple hundred nanometers across (that’s about 1,000 times thinner than a strand of hair), but it consists of tens of thousands of light-absorbing molecules called bacteriochlorphylls all bundled together in a cylinder. These molecules, Mosallaei told me, act like tiny charged wires. One side is positively charged, the other negatively charged. Together they do the heavy lifting of capturing units of energy and moving them from the chlorosome to the other parts of the bacteria’s photosynthetic machinery. “That’s the whole point of a nanoantenna,” Mosallaei said. “It receives and transmits energy.”

In ongoing work, Mosallaei and his colleagues are examining how chlorosomes work. They want to understand how chlorosomes capture light and how they transmit it to neighboring structures. In a paper released earlier this year, the team showed that it’s not an ordered process at all. Here’s the thing: there isn’t just one kind of bacteriochlorophyll. There are several types and they can be arranged, it seems, in a variety of configurations within the chlorosome.

“The things we see are random, not ordered,” Mosallaei said. “But antenna are always ordered. We have to arrange them so precisely so they can omit coherently.” Not so in this naturally-occurring structure.

This may be because there are just so very many molecules in the mix. If efficiency of the whole unit isn’t terribly high, that doesn’t mean it won’t do a good job at photosynthesis.

Mosallaei hopes the work will allow people in his field to create synthetic nanoantenna based on these natural ones and that work even better than what we’ve got so far. That’s not going to be easy, and they’re still in the early stages. But a tiny bacterium is—at least to my mind—a cool place to go for inspiration on an engineering project!