The
researchers began with light-harvesting reaction centers from a purple
bacterium. Then they added some proteins and lipids for structure, and
carbon nanotubes to conduct the resulting electricity.
These ingredients were added to a water-filled dialysis bag — the
kind used to filter the blood of someone whose kidneys don’t work —
which has a membrane that only small molecules can pass through. The
soupy solution also contained sodium cholate, a surfactant to keep all
the ingredients from sticking together.
When the team filtered the surfactant out of the mix, the ingredients
self-assembled into a unit, capturing light and generating an electric
current.
The spontaneous assembly occurs thanks to the chemical properties of
the ingredients and their tendency to combine in the most energetically
comfortable positions. The scaffolding protein wraps around the lipid,
forming a little disc with the photosynthetic reaction center perched on
top. These discs line up along the carbon nanotube, which has pores
that electrons from the reaction center can pass through.
Adding the sodium cholate back into the mix disassembles the
complexes. But filtering it out again brings them right back together.
“The idea that it happens reversibly and at will is quite amazing,”
says Strano. “It approaches what happens in biology — forming a huge
amount of order with the flip of a switch. It’s kind of like taking
puzzle pieces and throwing them up in the air and them coming down
assembled.”
The complexes eventually lose power, but they are easily revived,
says Strano. The research team disassembled the units and replenished
the photosynthetic reaction centers. Four such replacements over the
course of a week kept keeping the complexes humming along.
“This is very nice work — the procedure they’ve got, the control they
have over the system,” says biochemist Mike Jones of the University of
Bristol in England. “It’s simple, it’s very nice.”
The units can’t compete with silicon-based solar cells in use today.
But silicon-based solar cells reached their current level of efficiency
only after decades of research and development, says Jones. Similar
investment in this new technology could yield a system that’s highly
efficient, can self-repair and works well under low light conditions, he
says.
What’s more, the main ingredients for these solar cells might one day
be easily extracted from plant material, says Strano, perhaps even from
garbage biomass. “We could turn waste into an organized product,” he
says.