Rapid antigenic tests for SARS-CoV-2 with swab are an extremely valuable resource in detecting the presence or absence of the virus in the event of a suspected infection and are one of the tools, although not the most accurate ever, which have made it possible to save lives during the ongoing health emergency by monitoring infections.
The technology behind these tests is now well established and is the same used for "do it yourself" pregnancy tests and is based on gold nanoparticles, water capillarity and antibodies. The technical name of the method is "immunochromatography in colloidal gold”, but let's not be frightened by the high-sounding name and let's go in order.
The technique is based on antibodies. With the SARS-CoV-2 pandemic, the word "antibody" has entered our everyday life, but what are antibodies?? They are very effective particles in catching another particle in a very specific way. Precisely because they are linked to something and not something else, the cells of the immune system make specific antibodies that bind to things that shouldn't be in our bodies, such as SARS-CoV-2, thus signaling the virus to immune system cells to remove it. Anything that the antibody binds to is called ANTIGEN. For the most curious: antibodies are proteins and have a characteristic Y shape, with two "heads" (pictured as PAC-MAN in this image) each able to bind the antigen.
If a part of the antibody serves to bind to something in a specific way, the rest of the antibody can be attached to something else, such as gold spheres large a few billionths of a meter, or nanometers. These gold nanoparticles, precisely because of their size, appear a red / purplish color.
We can therefore create a specific antibody for the SARS-CoV-2 virus that appears colored, but how to use it to detect if there is a virus in our nose? With a sandwich, a so-called "antibody sandwich", with the virus as a side dish. Now let me explain better.
In order to understand whether the virus is present in a nasal or oropharyngeal swab or not, we use 4 antibodies and a nitrocellulose tablet (a porous material with a high affinity for water) on which our sample will flow by capillarity. The 4 antibodies are:
– Antibody for SARS-CoV-2 with gold nanoparticles (PAC-MAN yellow): it will be carried by the sample and if the virus is present it will bind to it
– Control antibody with gold nanoparticles (PAC-MAN blu) : sample will be transported and will bind to the control antibody immobilized on the tablet, for which it is specific. This is to verify that the test is working
– Immobilized antibody to SARS-CoV-2 (PAC-MAN verde): it is fixed to the tablet in a precise point and is specific for the virus
– Immobilized control antibody (PAC-MAN fucsia): it is fixed to the tablet in a precise point and will bind to the control antibody with gold nanoparticles.
So what happens while we do the test?
1) To do the test, you have to swab the nasal cavities and then dip the swab into the diluent vial, a saline solution.
2) Subsequently, a few drops of diluent solution must be deposited in a special circular window on the tablet.
3) Hence the sample, by capillarity of the water, it will spread throughout the tablet. The sample content will first meet the antibodies with gold particles for SARS-CoV-2 and control and will transport them to the other antibodies.
4) If the virus is present, the antibody with gold particles for SARS-CoV-2 will bind to it.
5) When the virus-antibody complex with gold particles finds the immobilized antibodies to SARS-CoV-2, a "sandwich" will form, as the immobilized antibody will "catch" the virus and with it the antibody and the gold particles, creating a colored band (T)
6) The second colored bar, that of control (C) it will form when the control antibody with gold nanoparticles meets the immobilized control antibody. The two antibodies were selected to bind to each other.
If the virus is not there, there is nothing that holds the anti SARS-CoV-2 antibodies with gold nanoparticles on the tablet, therefore they will flow past without forming a colored band, but the control band (C) will still appear.
The pregnancy test works practically identical, but instead of SARS-CoV-2 what causes the test to be positive and ends up between the two antibodies is human chorionic gonadotropin (hCG), produced by the placenta in pregnant women and present in the urine.