How do self-tests work?

The idea is very simple and I’ll try to describe it with the help of some illustrations: the cotton swab that goes into your nose allows you to collect all the droppings in which the nasty virus aka the antigen would be entangled. Because yes: the test you do at the pharmacy and the one you do at home are exactly the same, the difference is that the pharmacist get the boogers higher up in the nose. By the way “Ag” on top of the test stands for … “Antigen” !

Then, these boogers are mixed with a transparent liquid which has no other purpose than to dilute them. You then put a few drops of the diluted boogers on a piece of filter paper. By capillary action, the liquid will be carried to a first area where there are gold nanoparticles decorated with antibodies.

Nanoparticles means that they are tiny, tiny, tiny spheres. At first sight I would say a little less than 100 nm, to give you an idea of how small it is, it is like cutting a hair 100 000 times (I know, it’s not easy to imagine). I’ll come back later on to the role of these nanoparticles. What you need to know is that on these nanoparticles, you find antibodies (red spheres with the purple mark in the sketch) which are molecules capable of specifically “recognising” the virus. When I say “recognise” it is not a mission entrusted to a secret agent, I refer to chemical affinities between the antibodies and proteins on the surface of the virus (the one called spike in this case). So if the virus is in the boogers, we end up with a virus on which many antibodies are stuck and the antibodies are in turn grafted onto nanoparticles. If there is no virus, the antibodies stuck to the nanoparticles remain free.

Meanwhile, the liquid continues to climb in the test and it leaves some kind of red trails: THESE ARE THE NANOPARTICLES!!! This liquid loaded with nanoparticles decorated with antibodies with or without virus will meet two zones:

  • the first one, noted T for “test”, also contains antibodies against the nasty virus. If, by any chance, a nanoparticle grafted with antibodies that have latched on to the virus passes by, it will be stopped in its tracks and a band will appear .
  • On the second area, marked C for “clear”, there are again antibodies but these are a bit different, they don’t recognise the virus but the antibodies grafted onto the nanoparticles. If the test works properly, this line should always appear.

There you have it! You know all about self-tests and other antigenic tests performed in pharmacies. Well, no, you don’t know everything. Because there is still one question to be answered: why do these nanoparticles appear red-dish? Gold at the nanoscale is not golden at all, it is red! The first thing to consider is that the light you shine on this self-test is multicoloured. Have you ever seen a rainbow? Well, all those colours are contained in the sunlight. When the light is going to tickle metal nanoparticles, and gold nanoparticles in particular, blue-green light is used to make the electrons oscillate. You have to imagine the blue light as a finger scratching a guitar string. This oscillation of electrons is called a “plasmon” and as it is my speciality, there is a good chance that I will talk about it again on this blog! Anyway, let’s get back to our plasmons. Since the blue-green is used by the nanoparticle, only the red-pink tones remain in the light. And that is what reaches your eyes!

Gold, when it is golden, means that it is present in large quantities and the plasmon is no longer excited in quite the same way. In certain configurations, gold can also appear blue, but I’ll explain another time. Finally, you should know that this story of red gold is as old as glassmaking. The incorporation of gold nanoparticles in glass is described in a 16th century treatise and the greatest glassmakers have used it to make luxurious works.

P.S. Many paper tests work in this way, such as pregnancy tests, which do not react to the presence of an antigen but to a hormone in the urine. The urine is already full of water so there is no need to dilute it and you just have to pee on the test.

That’s all for the booger and pee post, I hope you enjoyed it!