Signs of Life
Applying Nanotechnology to the Search for Extraterrestrial Life
A century ago some astronomers believed that they could see colour changes in the Martian continents through their telescopes and speculated that this may be due to seasonal changes in vegetation. Back then, there were no space probes or orbiting telescopes far above the Earth’s turbulent atmosphere, so this slender evidence was as advanced as the search for extraterrestrial life got. Of course, in the modern world we have sophisticated probes able to actually visit the surface of mars and do some much more rigorous science. But how exactly do you look for life on another planet with robotic probes? Clearly if the images the probe sends back show little green men peering into the camera, you know you’ve found life. But what about the rather more realistic scenario of life existing in the form of bacteria or other microbes.
There are a number of ways to test for this using wet chemistry. Samples of soil can be mixed with water and nutrients and gas detectors can look for a tell tale “burp” indicating metabolism. The problem is that in spite of clever tricks like marking key molecules in the nutrient with radioactive isotopes it’s very hard to eliminate the possibility of an unknown “dead” chemical reaction mimicking the effects of living organisms.
Another method that may side step this problem centres on a compound known as Dipicolinic Acid or DPA. DPA is a key component in the spores of bacteria and fungi and is often used as an indicator of life. Almost all cells on Earth contain DPA, so given that the rest of the solar system formed from the same basic ingredients, it would be a fair bet that extraterrestrial life would have evolved this way too.
One of the simplest ways to detect DPA is by its interaction with Terbium. Under UV light DPA becomes photo excited and then in turn excites any neighbouring terbium atoms by electron transfer. The up shot of which is that if you pulse terbium with UV it will give a characteristically different fluorescence when DPA is also present.
In order to test a sample of Martian soil for life, it could be dissolved in solvent then deposited on a terbium surface. Once the solvent had evaporated the fluorescence signature could in principle tell you whether DPA was present in the residue. One problem with this technique is that the signal is likely to be quite weak, especially if the quantity of spores or cellular debris in the residue is small.
Simply smearing samples over a flat terbium surface isn’t a particularly efficient way to perform this test, since only the molecules on the very surface are in direct contact with the terbium and so are the only ones to be able to participate in electron transfer. One improvement would be to vastly increase the surface area of the terbium by making the surface highly convoluted. And one of the most convoluted and high surface to volume structures in existence is what’s known as nanowool.
Nanowool is a material developed by a team of scientists at ANU lead by Professor Rob Elliman. (See ScienceWise Vol.5 No.1). It’s a series of incredibly long silicon dioxide nano-filaments that grow into an interwoven mat on the surface of a silicon wafer when exposed to particular temperatures and atmospheres. In spite of only being a few nanometres across the huge length of these fibres and their large number create a mat that’s easy to see and handle. When peeled off the wafer it looks like a little square of blotting paper. This provides the extraordinary surface area needed for interaction with DPA but to work as a detector, the system also needs terbium.
To achieve this, a solution containing terbium can be added to the nanowool mat and by capillary action will instantly cover the exterior of all the fibres. The matt can then be heated to boil off the solvents and anneal the terbium onto the outside of each fibre like the cladding on an electrical cable. This process can be repeated a number of times to either increase the thickness of the coating or even create stripes of different materials as desired.
You now have a sensor that by virtue of it’s enormous surface area, is capable of identifying the smallest traces of DPA. It also has another potential advantage over wet chemistry. Once a measurement has been made it is possible to heat the detector to high temperature to burn off the residue and refresh the sensing capabilities. Since the nanowool is formed at high temperatures, it’s relatively immune to heating damage.
This feature may be of particular value on remote space exploration vehicles where it may be desirable to perform hundreds of tests using the minimum possible in-flight weight and space.
But the potential of nanowool DPA sensing isn’t limited to space exploration. Back on Earth security forces are frequently confronted with situations where unidentified and potentially bio-hazardous substances are found in public places. Quick decisions have to be made as to the best course of action. If you do nothing you risk spreading a potentially deadly infection to the wider population but you can’t quarantine hundreds of people somewhere like a shopping mall for the several hours or even days it takes for a full lab analysis.
In answer to this problem, several companies make on the spot detection kits for pathogens such as anthrax. One of the problems with current detection kits is that their sensitivity in certain areas is limited and they are single use only. Fluorescence based nanowool detectors may offer superior performance in this role and be able to provide more quantitative information about the nature of spores present in a sample and their concentration.