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!