WonHyoung Ryu at Yonsei University in Seoul, South Korea, working with overseas colleagues from Stanford, found an interesting way of hybridization between a green alga, Chlamydomonas, and gold nanowires, to directly collect the electrons produced through photosynthesis by the plant. “Theoretically we should be able to collect all photosynthetic electrons,” he says.
The researchers trapped the unicellular algae in tiny traps, attached an ultra-sharp gold electrode to an atomic force microscope and then inserted a 30 nm-wide gold tip into the chloroplasts of the alga. The other gold wire was inserted into the algae’s medium. What they got was a current of 1.2 picoamps, meaning that if applied to a square centimeter, they would get 0.6 mA. Further increasing the light intensity, the value raises to a maximum of 6 mA per square centimeter.
Some silicon solar cells have a current density of 35 mA/cm², so having 6 mA directly from a plant is not something you would neglect.
The team thinks the gold electrode managed to snaffle around 20 per cent of the total number of photosynthetic electrons from the alga. They calculated this by comparing the number of electrons flowing through the circuit to the theoretical number that an untapped cell would use to generate oxygen under the same conditions. Improvements to electrode design should boost that figure, says Ryu.
Green plants are naturally optimized to harvest light in certain spectrum bands, like blue or red, but less green (hence their color). So are inorganic solar cells, but the efforts to widen their absorption band are huge and have immense costs.
The team now struggles to find out the technology to keep the alga alive for as long as possible, although some researchers are skeptic about the experiment’s success in the long run. Even if somehow it won’t be a success, the experiment demonstrates some basic principles and helps understanding the way plants work.