Earthworms: the future of nanotechnology?
Earthworms are one of the world’s unsung heroes. They’re a gardener’s best friend, constantly burrowing through soil, keeping it loose and fertilised. However, they also have a role to play in scientific research. Nancy Wilkinson has been learning about a group of scientists who are using earthworms to synthesise electricity-conducting nanoparticles.
Earthworms digest soil as they burrow through it, filtering out unwanted particles such as heavy metals in their gut. Large amounts of these metals collect in liver-like cells surrounding the worm’s gut before they are excreted. The worms can store up to about 200 times more of some heavy metals – as a proportion of their body weight – than would usually be found in humans. They can amass up to one thousandth of their body weight of cadmium, for example. Researchers based at King’s College London, with support from the Wellcome Trust EPSRC Centre of Excellence in Medical Engineering, have been able use the earthworms’ process of digestion to simultaneously synthesise small luminescent particles that can conduct electricity, called quantum dots.
The researchers showed that when the earthworms were put in soil that had been spiked with cadmium (Cd) and tellurium (Te), the two elements collected in the liver-like cells as expected. But then they reacted spontaneously inside the worm to produce cadmium telluride (CdTe) quantum dots. It’s an incredible example of bio-nanotechnology: using biological systems to synthesise useful particles on a nanometer (one billionth of a metre) scale.
Bulk CdTe is a dark grey crystal, but when CdTe is made into tiny particles just a few nanometres in diameter, they become luminescent and have electrical properties similar to semiconductors. One of their most exciting features is that the light they emit can be tuned by varying their size. The smaller the particle, the closer the light gets to the blue end of the visible spectrum, and the larger the crystal, the closer it gets to the red end. This makes them potentially useful for lights, displays and, in biology, cell imaging.
However, for a while researchers working in this field struggled to make the quantum dots water-soluble, which is essential for them to be used within animal cells. There are now a few methods known for making them water-compatible, but they tend to involve altering the surfaces of the quantum dots, which can change their optical properties and be very tricky. Making quantum dots in an earthworm means they are naturally water soluble and stable in animal cells.
The possible applications of quantum dots are extensive, as Dr Mark Haworth, University of Oxford, explained to me. Dr Haworth and his team have been researching biological uses for quantum dots, with funding from the Wellcome Trust, for several years. “They are extremely useful for all sorts of challenges in cell biology. In our lab, we have used quantum dots almost like molecular rulers – to measure distances between interacting molecules. It can be hard to see how molecules interact by microscopy alone, but the size of a quantum dot can be tuned precisely and can then be used to analyse the shape and movements of cells.”
At the moment, the quantum dots made by the earthworms are not as bright as the researchers would hope, something that would need to be improved before they were commercially viable. But the paper does outline a few examples of CdTe quantum dots being used as labels for human cells, such as ovarian cancer cells.
There is still a long way to go with this research, but it shows an interesting, novel way of creating this type of nanoparticle. And it gives us one more reason to celebrate the common earthworm. Who knew they could be so useful in so many ways?
Biosynthesis of luminescent quantum dots in an earthworm, M. Green, 2012, 10.1038/NNANO.2012.232