Putting a bee under the microscope
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.