A Spin Around the Electron

Recently the issue of microplastics has been in the news - they’re tiny bits of plastic from a few millimeters in diameter to even nanometers, that can be washed away from everyday objects including cosmetics and clothes, or can be from larger pieces of plastic breaking down over time. Because they’re so small, they aren’t easily filtered out by our current sewage systems, meaning they can end up in the sea, and can cause issues in the marine world. Now, scientists from the University of Adelaide in Australia have announced a new catalyst that they hope can speed up the breakdown of microplastics, in an environmentally friendly way. Ankita Anirban spoke to Ljiljana Fruk from the chemical engineering department at Cambridge, who wasn’t involved in the study, and took a look at this paper for us...

Ljiljana - So in this paper the authors have designed a new catalyst, so they speed up the reaction. So they made a catalyst which is a hybrid between carbon nanotubes and manganese compounds. And manganese is a chemical element which is known as a catalyst in many chemical reactions.

So by designing these catalyst they have shown that they can actually use it to degrade micro plastics. And when we think about the degradation that means not kind of cleaving it into the smaller pieces, but really degrading the chemical structure. This is of course an interesting approach.

Ankita - So you said it was carbon nanotubes - are these just really small tubes of carbon?

Ljiljana - If you imagine a very very very teeny layer of carbon, if you would bind this into the tube then you will get a very small nanotube. In their case it was several hundred nanometres in length, these carbon nanotubes.

Ankita - We can put a catalyst in to speed up a reaction, so does that mean that the reaction was going to happen anyway and we're just speeding it up?

Ljiljana - You know in general there is nothing in nature that stays the same. So eventually the plastics would degrade. It just takes thousands of years. So you would like to speed it up. So what they've shown in paper is exactly how the plastic structure changes over six to eight hours. Eventually they say that you can degrade the plastics into CO2, carbon dioxide, and then this carbon dioxide could be used by marine organisms in the photosynthesis for example if you have a plankton, to produce  bio materials.

So I think the plan with some of these new strategies is basically to use them in sewage water treatment plants. And if you have a combination where you have one reactor within this plant with microorganisms, then the products could be used to create a biomass, so you would have a circular system.

Ankita - So these catalysts are described as springs, why is the shape of that important?

Ljiljana - For catalysis, the surface is important. So if you have a spring-like surface you are introducing different curvature so that the molecules can fit in, but you are also increasing the surface amount that is available for catalysis.

So it's much better to have a curved surface than for example just a planar one.

Ankita - These springs are also magnetic.

Ljiljana - Yes, so having magnetic materials is of course very useful because you can imagine that you throw this catalyst into a mass of water. So how are you going to get it out? One way of getting it out is to use magnetic force. So you basically use a big magnet where you remove your catalyst when the reaction is done. So you first recycle your catalyst, and you ensure that a catalyst is not ending up in the drinking water.

Ankita - So do you think we'll be using these kind of catalysts in sewage treatment plants anytime soon?

Ljiljana - Although this is an interesting concept it will take a while until this is practically usable, because one issue is the production, scale up, of these materials. And the other thing is of course there needs to be a certain time which you invest into studying the bio-compatibility. You would not like anything to leak out into the water what is maybe more toxic than the plastic itself.

Ankita - For now, do you think we should just use less plastic?

Ljiljana - Well you know one of the biggest things is that dealing with plastic would require changes in our lifestyle. This whole hype about microplastics is relatively recent, but I think we first also need to focus on the other plastic waste. Because even if we deal with microplastics there will be new micro plastics produced from the plastic waste we have. So there needs to be changes in policy making but also in the personal relationship to plastics.

Us humans get a lot from our sense of smell. But other animals, such as dogs, rely on their noses so much more. Why might that be? And why did we evolve to have the noses we do? New research in Science Advances sheds some light on those questions and Darren Logan from the Waltham Centre for Pet Nutrition joined Katie Haylor in the studio to chat all about it...

Darren - Our sense of smell is essentially a chemical scent. So the volatiles that's coming off the freshly baked bread - into the atmosphere - is what we're sensing through our sense of smell. And every time you take a deep breath in, the volatiles will rush through your nose and hit these molecular receptors on the surface of your nose called olfactory receptors and they exist in olfactory sensory neurons.

In a human nose we have about 300 different types of these neurons and each one detects a different combination of small molecules and it's the combination and the pattern of those together that your brain interprets as the smell of - in this case freshly baked bread.

Katie - 300 doesn't sound like an awful lot so how do we end up with the incredible amount of smells that we're able to recognize?

Darren - So that is the sort of the real trick of your olfactory system. So it's due to something called combinatorial coding, so each receptor - each of the 300 receptors - can detect a combination of different molecules and each molecule can activate a combination of different receptors. So when you multiply those together we think that we can detect up to a trillion different orders.

Katie - Okay. So how did you analyse smell in this study? What were you interested in?

Darren - So we took advantage of a particular quirk of the olfactory system - which is that each sensory neuron in the nose expresses just one olfactory receptor. And so because we knew that, we were able to use something called RNA sequencing to quantify the RNA of each receptor and that allows us to tell us the number of each type of neuron the nose. You might expect that of the 300 neural types, they'd all be equally represented. And what we found out - that wasn't the case. There were a very small number, actually, about 10 or 15 are very very highly represented in the nose and the vast majority are relatively lowly represented.

Katie - So what does this mean then? What do we take away from this?

Darren - So we were really interested about what these receptors are - there are these neurons that are there in very high abundance. And so, what we did is, we looked at what those neurons are detecting and what we found in the case of humans is that they're detecting what we call key food orders. So these are the orders that are produced by our food. So - as you mentioned - the orders in freshly baked bread.

When we looked in other species - we looked in mice, rats, dogs and a number of primates - we found that wasn't the case. And likewise, when we looked in mice, we found that the neurons that were very abundant actually detect pheromones - so cues that the mice used to sexual communicate with each other. So what we think that means is that each mammal has evolved to to have a nose that is very specific to its niche.

Katie - Can we take from that that sourcing food is particularly important to us but there may be other equally pressing matters for other animals. How would you pick that apart?

Darren - Yeah that's our hypothesis. There was a theory that our senses of smells were essentially not under evolutionary pressure. They're just drifting around and they can detect anything that we happen to run into. What this research - we think - suggests that's not true, that actually our noses are tuned and over time have been tuned to the things that are important to us - to promote our reproduction and our survival. In the case of humans, our sense of smell is particularly important for detecting food and scavenging for food. And that's why we think our noses are tuned the way they are.

Katie - So more receptors equals better smelling ability. Is that pretty much right?

Darren - This is a bit of a mystery in the olfactory field. Species like dogs or mice or indeed elephants, who we think have the most receptors, may be able to smell more, but we actually think at the moment that it's likely that they don't smell more - they just discriminate better. So they can tell subtle differences between things that we as humans - who are not the best smellers in the world - probably couldn't.

Katie - Having said we're not the best smellers in the world, I've got to say, I’ve named myself the bloodhound of the Naked Scientist office because I feel like my sense of smell is really good. Why would that be? Why would I be better at smelling than say Izzie, for instance?

Darren - Well there are people who are better at smelling than others and it probably down to genetic variation. We know there is a lot of variation in the olfactory receptors, however, we also know that people who are often deemed or described as better smellers are often more verbal - so are able to describe the scent, the smells that they detect better and that appears that they’re therefore better but they're actually better explaining it.

Katie - I'm still going to take credit for that one. Very briefly... what's the next step then with this particular piece of work?

Darren - We are doing two things - I guess - one is that we are looking to spatially identify the position of the neurons in the nose, rather than just the abundance of them. And this is important because when you smell, the air rushes through the nose and depending on which parts of the nose it hits, we might think it works differently. And secondly, we are particularly interested in those abundant neurons and finding out exactly what they detect.

The word electron comes from the ancient Greek word for amber. If you rub a piece of amber, you can generate static electricity, a bit like rubbing a balloon on your head. Some Greek physicians even suggested putting electric eels on your head to cure migraines - an early version of electrotherapy. But although we have known about electricity for a while, the electron itself, wasn’t discovered until 1897. Ankita Anirban took a trip to the Cavendish Laboratory in Cambridge to speak with Katy Duncan, from the Department of History and Philosophy of Science in the University of Cambridge, to find out how this came about...

Ankita - We've come to the Cavendish Lab which is where J. J. Thompson discovered the electron. And we’re walking down this corridor, and there's lots of portraits of people in the Cavendish, and we’re slowly going back in time. Lots of men in suits in the 20s. And the first one is 1919 with Professor Sir J. J. Thompson right in the middle.

We walked through the museum until we came to a case which held a glass tube. It was about the length of my arm and maybe twice as wide, with large bubbles along it. This was the famous cathode ray tube used in the discovery of the electron. You might remember old TVs with huge boxes at the back. These held cathode ray tubes that were used to generate the images on the screen. And in fact even today we still use cathode ray tube to produce X-rays. I spoke to Katy Duncan from the Department of the History and Philosophy of Science at Cambridge about how they work.

Katy - So if you've ever seen a neon light this is very similar to what these gas discharge tubes are. They tend to be long glass tubes with an electrode at either end, one of them is positive and one of them is negative, and you connect them up to a battery. When you put a voltage across it, whatever is in that tube, they found that it would glow. And that's essentially what a cathode ray tube is.

Ankita - So how did cathode ray tubes come about? J. J. Thompson made his famous discovery of the electron in 1897. What did we know about electrons - or rather, I should say, electricity - before that?

Katy - The 1800s were a period where we see understandings of what we can do with electricity massively increase. So we could develop formulas and ways to manipulate it, and we could use it, such as the electric telegraph. But what electricity actually was? People didn't really know. There were understandings that there were two kinds of charge, but how that manifested wasn't really known. Whether that was an electrical fluid, two electrical fluids... it was all really quite up in the air. At the time lots of physicists were interested in finding out about the electrical conductivity of gases at low pressure, because usually in air electricity doesn't travel anywhere - otherwise we'd be electrocuting ourselves all the time as we walked through the atmosphere. If you had a low pressure tube that you filled with gas you could actually put a current across it. So this is what they were investigating; the electron just happened to be something, almost as an accident. 

Ankita - How do you discover the electron by accident?

Katy - When he was experimenting with this tube he took it to a very, very, very low pressure, which is something that other physicists at the time had been trying to attempt. And when he did this he found that there was a shadow that was being cast behind the anodes at the positive end, which indicated that something was travelling in a straight line from the negative end to the positive end. And the bits that weren’t hitting the anode were hitting the surrounding glass and glowing. So he investigated what this ray was by putting an electric field around it and a magnetic field around it, and also examining how much heat that this ray itself was generating. And he used a Lorentz force law to calculate a mass-to-charge ratio.

Ankita - Electricity and magnetism are two sides of the same coin. And the Lorentz force is a law which links the two. It states that if you place an electric particle in a magnetic field it will be deflected at an angle; the amount of deflection depends on the charge and the mass of the particle. By measuring this deflection, J. J. Thompson could measure the charge-to-mass ratio of the electron.

Katy - And in doing so he found that this mass was a thousand times smaller than the size of the smallest known atom at the time, which was hydrogen. So this started to make some interesting questions about whether there was something smaller than the atomic, whether there was something sub-atomic.

Ankita - So he effectively discovered that the atom - which we had previously thought of as the fundamental unit of matter, something that couldn't be broken down any further - was in fact made up of even smaller particles. How did that fit into the existing theories at the time?

Katy - Well there were lots of different theories at the time. J. J. Thompson didn't actually call what he found the electron, he called it a corpuscle. And he believed that this corpuscle was in fact the only sub-constituent of an atom. And he devised what we know now as a plum pudding model. So these were small, hard electrons that existed in a sea of positive charge, and they could move around freely, and then they could be made to leave this plum pudding model under the presence of something like a high electric charge. And that's what we saw in the cathode rays.

Ankita - But the usual picture of the electron today is not like a plum pudding. Most of our science textbooks now describe electrons as orbiting the nucleus of an atom, a bit like planets around the sun. So where did this theory come from?

Katy - That was only a few years after Thompson came up with his plum pudding model. Thompson had a student called Ernest Rutherford and he undertook a few groundbreaking experiments, the results of which didn't actually fit with any theory that the physicists at the time had. Rutherford demonstrated that atoms are actually the mostly empty space model that we know today.

Ankita - Rutherford did this by firing tiny positively-charged particles at a gold foil. He found that the vast majority of the particles made it through, and only a handful bounced back. And this led him to believe that most of the atom was in fact empty space with a small positive nucleus in the middle and a cloud of negative electrons circling it. And this is a model that's still taught in schools today.

Science is rarely simple and not everyone at the time accepted this model.

Katy - It took a good number of years for the scientists to come around to the idea that it was indeed a particle. So it went through this very fluid definition in the early 20th century before it became what we recognise as the electron today.

Today, electrons are an important tool in scientific research. Electron microscopes use beams of electrons, rather than of light, to take images with very high resolutions. This means that they can be used to look at things in huge amounts of detail that we can’t see by eye. They’ve allowed us to see how tiny viruses look, and can even be used to image individual atoms. We sent out a message on our social media to ask what you, our listeners, wanted to see using an electron microscope. Listener Hamish Symington replied suggesting we look at a bee... so Naked Scientists Adam Murphy and Ankita Anirban took a trip to the materials department in Cambridge to meet Hamish, along with researcher John Walmsley and technician Simon Griggs, and put a bee into a scanning electron microscope...

Hamish - I've brought some bees along. These are bees which I've been working with, which have died of natural causes after we finished the experiment. And we can have a look at some of the things which make the bees really cool.

Adam - But, before we could put the bee into the microscope, we needed to do some sample preparation. Because we're firing electrons into the sample, it's important for the sample to conduct electricity so that the electrons can pass through. If the sample is an insulator, the electrons would just get stuck in the sample. A bee doesn't conduct electricity very well - so John Wamsley, a senior technical officer in the Department of Materials, explains how to get around this challenge.

John - For the samples we're looking at today - they've been coated in a very very thin layer of metal to make them electrically conducting. Gold is the favorite because it's very nice and stable it doesn't tarnish and oxidize in the air. In this case we've used platinum, but it tends to be a heavy stable metal.

Adam - With the sample prepped and ready to go, we approached the microscope. It was a large box about the size of a dishwasher, connected to a series of tubes and pumps and warning labels with a small draw at the front. Simon Griggs a technician looking after the microscope opened up the drawer and placed our bee onto the stage.

Simon - Slide it onto the stage, so we can move it around in x and y and we can tilt the stage as well - which is good. It's in the right orientation for me to do so. So we got nice bees there - place it into the machine. The sound was obviously air going into vent it and then we'll pump it. So… press the pump button… and it’ll go through a pump sequence.

Adam - And this is just sucking all the air out of the thing?

Simon - Yeah, that’s right. It is sucking out the air of the machine. It will get down to a reasonable vacuum - probably in the ten to the minus five millibar range and then we're allowed to get the electrons going down the column. It takes about two or three minutes to do so - and then sit down and find out where things are.

Adam - As Katie mentioned earlier, air is not a good conductor of electrons, which is great to avoid constant electrocution - but a challenge if you want to actually send electrons onto a sample to image it. In order to make sure the electrons reached the sample we needed to pump down the chamber into a vacuum. While we waited for this to happen, we asked John a bit more about how the microscope actually works.

John - The principle of the electron microscope is we have, effectively, a small accelerator unit at the top of this electron column. So we generate electrons and we accelerate them up to fairly high energy  - say between five and 30 kilovolts - about five thousand thirty thousand volts. And they’re then focused through a set of electromagnetic lenses and they're quite analogous to the optical lenses in a conventional benchtop optical microscope.

Adam - The limitation of any kind of imaging technique is the resolution we can have. When we use light, we are limited to resolving features that are bigger than the wavelength of that light, which means we can really only see down to about micrometres scale - that's a hundredth of the width of a human hair - but electrons have a much shorter wavelength so we can see down to nanometres, which are thousands of times smaller than that.

John - The way we produce the image is to focus a pencil beam of electrons onto the sample and scan the beam across the sample. Now as we're scanning, we monitor the various signals that come off the sample

Adam - When electrons hit a sample with high energy, they send a ripple through the atoms on the surface of that sample. This shakes up electrons within the sample and sends some of them back up from the surface. It's a bit like firing a high powered hose into a swimming pool and watching the water splash back up.

John - By measuring the intensity of these generated electrons and the modulating - that's according to the position we are on on the sample - we get actually very very intuitively clear images of the sample we're looking at.

Adam - We waited for the chamber to pump down to a vacuum before we could fire up the electron beam. There was a small camera - so we could actually see inside the chamber while this happened. And as our two or three minute wait turned into 10, 20, 30 minutes… we noticed that the bee started to bubble! It turns out - our bee had recently drunk a lot of nectar which was still in its stomach. It was going to take a while to pump a bee full of nectar down to a vacuum, so we decided to take it out and chop its head off - which was professionally done by Hamish - and just have a look at the bee sting.  As we turned on the electron beam, a black and white image of a bee sting started to flicker onto the screen and Hamish explained what it was we were seeing.

Hamish - Yes he got the barbs brilliant. Okay. So those things there are backwards pointing barbs - and that's how it sticks in you. This is a worker honey bee. And worker honey bees when they've got their sting in you - it actually - when they fly away they leave the stinger there. So the bee will die after it's stung you. That's only the case with honeybees and what we can see here is the reason for that. We've got these little backwards pointing barbs on the end of her sting. It's a bit like a fish hook - you push it in very nice and easily but when you try and pull it out it won't come.