Quantum dots are nanoscale-sized pieces of semiconductor. Their small size ensures that quantum effects, like the Pauli exclusion principle, influence the behavior of electrons within them. This gives the dots properties that a bulk material with the same composition lacks, and it makes them appealing candidates for things like tiny lasers, photovoltaic materials, and LEDs.

Another area where they've shown promise is medical imaging. In terms of absorbing and emitting light, quantum dots behave much like the fluorescent molecules we can use to label cells of interest. But, since their fluorescent properties depend on the shape of the particles rather than the chemical structure of a molecule, they are much less prone to undergoing reactions that destroy their fluorescence. The problem is that most semiconductors aren't especially biocompatible, meaning additional chemical reactions need to be performed before the dots can attach to or enter cells.

Some researchers have started to look towards making the dots in biological systems, figuring that the output would necessarily be biocompatible. After some successes with bacteria and yeast, they've moved on to a larger target: the earthworm. And it appears to work very well.

The authors were interested in creating CdTe quantum dots. Although CdTe is primarily used in thin-film photovoltaics, it could be useful for imaging, since it absorbs UV light and fluoresces in the green area of the spectrum. Both of the individual elements on their own are toxic, so organisms will typically remove them from their interior spaces and concentrate them somewhere harmless.

For earthworms, that means an organ called the chloragogenous tissue, which surrounds the digestive tract (conveniently labeled in the paper in a figure entitled "Schematics of the earthworm used"). This appears to be the worm's rough equivalent of the liver. Previous studies had shown that cadmium was detoxified by concentration into particles there, and the chemistry of this process suggested it should be able to handle tellurium, as well. With the right chemical reactions, a CdTe salt should be able to form.

The authors of the paper therefore spiked some soil with CdCl 2 and Na 2 TeO 3 , and left earthworms in it for 11 days. At the end of this time, visible dots were present at the edges of the chloragogenous tissue. When checked, these particles glowed at the characteristic wavelengths associated with CdTe quantum dots.

The average particle was only 2.33nm across and appeared to be surrounded by a coating of organic chemicals, possibly proteins or amino acids from the earthworm. Without any further processing, these quantum dots were absorbed by cancer cells in a culture dish, after which the cells glowed green when exposed to UV light. Macrophages, white blood cells that swallow and digest foreign materials in the body, wouldn't directly take up the nanoparticles but would do so after the particles were given a further coating of polyethylene glycol (PEG).

The quantum dots only appear to be good for several days when stored in water, so the mix as isolated isn't entirely stable. Still, it's entirely possible that researchers could identify a storage solution that would keep the dots viable for much longer. But what the authors appear to be most interested in is trying out different salts in order to see what other materials could be concentrated into useful products by the guts of earthworms.

Nature Nanotechnology, 2012. DOI: 10.1038/NNANO.2012.232 (About DOIs).

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