Small things are difficult to see. That’s why we use microscopes to look at ants, bacteria and human cells. The smaller the object, the more powerful and complicated a microscope must be – but the laws of physics mean we cannot keep going forever. Because light is a wave, it has a certain wavelength, and things smaller than this wavelength cannot be observed.
Many interesting biological molecules like DNA and proteins are significantly smaller than the wavelength of visible light. In fact, a DNA strand’s diameter is less than a hundredth the size of light’s wavelength, and therefore invisible under an optical microscope. Scientists have devised a number of techniques to nonetheless observe these small molecules. Physicists, for example, learn about their structure by prodding them with needles only a single atom wide. However, none of these techniques are quite as simple as peering through a microscope to see what’s there.
For my PhD, I’m working on a method that uses nanopores to “see what’s there” without having to worry about the wavelength of light. Nanopores are tiny holes with diameters of only tens of nanometres. These holes can be made in glass, very thin materials like graphene or even from interwoven strands of DNA with a technique called DNA origami. When these nanopores are immersed in salt solution and a voltage is applied across them, ions start to pass through the hole, much like electrons in a wire. Because many small biological molecules are charged, they too start passing through the nanopore. But as they do, they block the ions from going through at the same time, creating a drop in the current. These drops in current differ depending on the type of molecule that passes through the nanopore. Therefore, by analysing the number and shape of drops in the current, we can “see what’s there”: which molecules are present and how many there are in a solution.
Nanopores can be made quite easily, thus they promise to be a simple and cheap way of detecting molecules – molecules so small they are invisible to the light our eyes use to see the world.
Niklas Ermann
NanoDTC PhD Associate 2016
Image from WeClipart