Science Interviews

Nano-Sensors

Michael - We work on nano materials: primarily silicon or iron oxide-based materials. These are devices or materials that are so small they often can’t been seen by the human eye or they just look like little specs. They have structures inside them that are built to the nano scale.

Ben - When we say nanoscale we know this is quite a cliché now but how big would these be in comparison to a human hair?

Michael - About a thousand times smaller than a human hair. The real challenge of nanotechnology is to build a very sophisticated structure in a small space. The reason you want to make the small space for environmental sensing is because those smaller spaces allow you to do things you can’t do with bigger things. For example, we make sensors that can go on the body that can be small enough to fit on a needle. For environmental sensing you really don’t need something that small and typically the materials that we work with at the environmental sensing area are large enough to see, maybe the size of a coin. They contain a nano structure. For example, we make these chips that have very small nano pores in them. These very small pores kind of suck up molecules very effectively. It’s a phenomenon known as microcapillary condensation. If you have a very small pore, typically on the order of about a nanometre the vapours will spontaneously condense in those pores. It’s a means of concentrating the gas that you’re trying to sense.

Ben - Once you have been able to concentrate it how can you tell what you have there?

Michael - These are little silicon-based chips. Each one of these is about the size of a coin and you can see that they have a very pretty colour to them: intense green-blue or red colours. The colours derive from a nanostructure. What’s more is that colour will change. I’ve got a little bottle of ethanol here. If I put a drop on this chip you’ll see the colour will change from green to red.

Ben - It has immediately changed. This is just what looks like  a glass microscope slide but it immediately changed in response to ethanol. Is it possible to get rid of the ethanol and use that slide again?

Michael - If you look at it you’ll see that ethanol will evaporate and the colour will come back. So it’s a reversible sensor and the really cool thing about this is that the material is changing from green to red and so the gas or the toxin when it gets into the chip is giving you a red colour (red means stop and green means go). It has a very simple mnemonic  feature to it. Why is that? The reason it has a colour is because of its nanostructure. If we didn’t have that nanostructure there everything here would just be clear. You wouldn’t be able to tell that there was a chemical there. That’s really one of the advantages of having a nano structure, building the nano structure this way as it allows us to get a much more high-fidelity measurement. If this ethanol were spilled on the table maybe you’d be able to smell it but lets’ say you were at a distance and didn’t want to be smelling it, how would you tell it’s ethanol or water? If you put a water drop on that chip it won’t change colour at all. The chemistry inside this chip has an ability to distinguish between those two molecules.

Ben - And it gives you an immediate answer as well. It tells you immediately the ethanol’s present. In fact the ethanol must have evaporated by now because that’s gone back to being a vivid green. What sort of limitations do we have? What can we actually use these sensors to detect?

Michael - We do a lot of work in biological sensing and chemical sensing. What tricks can you play with this nano structure to cause the colour to change when it only sees something like sarin gas? That’s the gas used in the Tokyo subway bombings: a kind of terrorist weapon of the day. So how do you detect a nerve agent or how do you detect whether it’s anthrax or it’s an Ebola virus? What you need to use is some kind of chemistry or biochemistry to get you that specificity. For example, in the case of these chips to make a biosensor out of them one of the common tricks we’ll play is we’ll place an antibody inside the pores. This antibody may be very specific for certain kinds of molecules. We’ve got a system where we’re looking for cholera toxin. Where we can detect cholera in drinking water based on putting an antibody in the pores that specifically binds just to that toxin and to nothing else. If we’re looking for gas space molecules we play other tricks. For example the sarin detector that I just mentioned to detect that kind of chemical we place a specific catalyst in the pores, a copper-based catalyst, that will react specifically with the sarin and create products that react very uniquely with the structure and cause a colour change that we can detect.

November 2008