When things get small–like millionths-of-an-inch small–they get very interesting. The ordinary rules of physics we’re used to fade back as the oddness of quantum physics looms large. Engineers have taken advantage of this fact by fashioning tiny bits of matter, known as quantum dots, that behave in all sorts of useful ways. For example, quantum dots made from cadmium telluride will respond to ultraviolet light by giving off a flash of visible light–the color depending on their size. If you attach certain molecules to cadmium telluride quantum dots, they will latch onto certain targets, making it possible to detect trace amounts of substances ranging from pesticides to cancer cells.
As versatile as cadmium telluride quantum dots are, however, they’re not easy to make. It’s especially tedious to fashion them so that they’re not toxic to living cells, since both cadmium and tellurium are nasty metals. In the latest issue of Nature Nanotechnology, a group of scientists at Kings College London offer a remarkably easy way to make them.
The scientists started with some dirt, into which they mixed cadmium chloride and sodium tellurite. Then they dropped earthworms into this polluted soil. The earthworms did what earthworms do: they sucked the dirt into their mouth and pushed it down the length of their bodies, digesting the nutrients and excreting waste out their back ends.
After eleven days, the worms were still happily grazing through the dirt, despite its Superfund-scale pollution. The scientists cut the animals open and searched for metals inside their bodies. They found that the cadmium and telluride the worms had eaten had broken away from their original molecular partners and had combined into cadmium telluride. In other words, the worms had manufacture quantum dots.
When the scientists flashed the dots with ultraviolet light, they gave off a green glow. The worm-fashioned quantum dots played nicely with living cells, the scientists found. They could use the dots to make cancer cells shine amidst a background of ordinary tissue.
There are many things to marvel at in such a study. We can imagine engineers harvesting quantum dots from giant earthworm farms and not be considered mad. But what I marvel at most of all is the fact that the earthworms were naturally so well prepared for the challenge. They have evolved to be underground alchemists. After all, when you make a living eating dirt, you have to be prepared for all sorts of unexpected nastiness.
Earthworms can sense metals in their meals. They immediately respond by making special enzymes. Exposing a worm to cadmium, for example, causes it to produce enzymes called metallothionein in its gut. The metallothionein grabs hold of the cadmium and stores it away in special cavities inside the cells, where it undergoes chemical reactions to make it less dangerous to the worm. Then immune cells attack the cells and engulf them. The worm eventually excretes them safely out of its body.
When scientists began decipering the chemistry that the worms use, their first idea was to enlist worms to clean up heavy metal pollution. That turned out to be a failure of the imagination. It may be that in the realm of nanotechnology, earthworm may truly shine.
[Worm engraving: Florida Center for Instructional Technology]