Nanoparticle sensors

One of the primary techniques in the field of nanoparticles is to harness the optical properties of gold. Matthew Millyard from the University of Cambridge uses gold nanoparticles in his work on sensors and he spoke to Kat Arney. 

Kat - So, tell me a bit about gold nanoparticles. They have quite a rich and to be honest, colourful history.

Matt - Yes, so gold nanoparticles have been used in stained glass for examples for almost 2000 years. They were actually, first used by the Romans, there's a rather fantastic example in the British Museum called The Lycurgus Cup which looks differently depending on whether you look at light that's reflected on it or light that's transmitted through it. This is because it contains various different gold and silver nanoparticles that absorb and react with light in different ways.

Kat - So, tell me about these gold nanoparticles. What sort of sizes are we talking about and what are some of the peculiar properties of them?

Matt - So, these gold nanoparticles sit around anywhere between say, 20 nanometres and a couple of hundred nano metres. They are so small that they trap the electrons that usually transport electricity in a bulk gold. And so, when you shine a light onto a nanoparticle, these electrons are moved from side to side of the nanoparticle and this means that each side of the nanoparticle becomes charged. Because of this, the nanoparticle isn't too dissimilar to a magnet. And so, if you bring two nanoparticles close together, there is an enhancement of the electric field between them. If you hold two magnets together you can feel this attractive force. That's because the magnetic field is attracted between the two different charges on each side of the magnet. It's similar with a nanoparticle and this can be used as a kind of magnifying lens for light. So, light gets focused into this gap and it allows us to see what's inside it.

Kat - So presumably then this is where the sensors bit comes in because if you've got 2 nanoparticles, you've got a gap in between that you can see through. You can then detect something else that gets in there.

Matt - Precisely. So, the idea is that if you shine light on to nanoparticles, they give off a certain colour, but if you put something into the gap, that will change the colour that you see. And so, as a sensor, you can have a look at changes in colour that will result in telling you what is exactly inside your gap.

Kat - So, you need to know kind of precisely what effect on the colour this particular chemical has and then you can say, "Right, this is the sensor that detects X molecule."

Matt - Yes, precisely. So, it's a very, very slight change that you see in this change of the light that you see. So, you have to have very, very sensitive detectors in order to be able to see what exactly you have in there. But this is a signature of whatever molecule you have in there. So for example, benzene ring which is a ring of 6 carbon atoms gives off a very distinct signature to any other kind of atom.

Kat - What kind of things could you detect with these kind of sensors and what sensitivity are we talking about? I mean, there are already some sensors out there that are quite efficient and can go down to quite low levels. Would these be significantly better?

Matt - Well potentially, and people have actually achieved single molecular detection with this kind of sensors. So, you'll be looking at just one molecule inside a gap between 2 nanoparticles and you'd be able to know what it is. But practically, what you can do with this is say, looking for contaminants in food. So, there's a research group in Singapore who have recently been looking at rather nasty molecules that can cause all kinds of birth defects in children in orange juice and milk. If they put a piece of contaminated milk onto a sensor made up of these gold nano structures, you can get a detection level significantly better than what Food and Drug Administrations would say is a safe level. So, it becomes a fantastically sensitive way of seeing how many molecules, potentially how many harmful molecules are actually in a solution.

Kat - It does sound really exciting, but is it all very expensive when you go to your funders and say, "Yeah, we need to buy loads of gold, sorry."?

Matt - When you're making these sensors, you're talking about gold nano particles. They're so minute that you're only using really, really tiny tiny amounts. You're looking less than gold dust in a 20 millilitre solution of gold nanoparticles. So, we really don't have to worry about expense gold. It's more the expense of the manufacturing process.

Kat - Where do you want the future of this kind of technology to go? Where do you think briefly, the most exciting things are?

Matt - So, the most exciting thing would be because you're only detecting the light that you're given off by these sensors, you could potentially use this within even mobile phone cameras if you had a sensor in front of a camera. You could hold it up to the sun after breathing on it say, and being able to tell if there was anything funny in what you're breathing out. So, any viruses or anything like that. That's probably a bit fantastical, but really, because you're only sensing using light as opposed to anything else, you can get to see some really exciting ideas and applications.