Earthworms roped into making quantum dots

IS IT time chemists abandoned their white coats in favour of trowels? Ordinary earthworms are proving to be a sophisticated chemistry lab: they can put together substances with unusual light-emitting properties. It is the first time organisms other than fungi and bacteria have been seen to do this.
The blind dot-maker
(Image: Jef Meul/Minden Pictures/FLPA)
Mark Green at King’s College London has spent his career making quantum dots – nanometre-sized chunks of semiconducting metals that fluoresce intensely because of the way their electrons behave. These quantum dots have been used to improve the efficiency of solar panels and build high-tech display screens. However, to be able to use them in the body – to track cancer cells, for example – intricate and expensive chemical processes are required that can reduce the dots’ luminescence.
Perhaps no longer, though: Green has found that earthworms, fed the right chemicals, can serve as a natural production line for biocompatible quantum dots.
The discovery came as the result of an educated guess. Green knew that placing thiols – sulphur-containing groups of atoms – on water-soluble cadmium telluride dots made them more luminescent. When he learned that earthworms produce thiols that bind to metal atoms, he wondered whether worm-made quantum dots were a possibility.
Green got together with colleagues studying how earthworms process cadmium in contaminated soil that they eat. The team exposed the worms to cadmium chloride and sodium tellurite, and found that they moved the metals to their chloragogen cells – their equivalent of the liver.
After 11 days, the researchers removed the chloragogen cells, put them in water and exposed the mixture to a UV lamp. They found that it glowed green. To Green, that characteristic behaviour meant only one thing: “It said to me it was cadmium telluride quantum dots,” he says. Observations with an electron microscope showed that he was right (Nature Nanotechnology, What’s more, the dots were able to light up cancer cells in an imaging experiment.
The earthworms make quantum dots because they want the metals out, says Green. “It’s a biodefence mechanism.”
Quantum dots had already been made in fungi and bacteria. The finding that organisms as complex as earthworms are capable of the feat opens up the prospect of using animals as natural factories. It is an intriguing idea, says quantum dot researcher Andrey Rogach at the City University of Hong Kong. But he notes that the worm-made dots do not glow for as long as ones produced synthetically.
Anil Suresh at City of Hope, a cancer research centre in Duarte, California, has made quantum dots using microorganisms. He, too, has reservations about using worms, saying they may not make the dots in practical quantities.
For Green, such concerns do not take the shine off the idea of using animals to tackle some chemical chores. “I just wanted to see if we could make luminescent nanoparticles in a living higher animal,” he says. He now hopes to improve the quality of the dots produced in this way.
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