Bean knowledge: there is gold in the pregnancy test.

what are the two bars that can have an important impact on life? The answer is probably the result of the pregnancy test.

the pregnancy test paper is accompanied by many women who have spent days and nights of excitement, excitement, or trepidation. Today, we don't talk about contraception or pregnancy, let's talk about the test paper itself.

the pregnancy test strip tests a substance called human chorionic gonadotropin (hCG), which occurs in the urine of early pregnant women. And how does the red bar on the test paper show? The answer is antibody + gold.

Yes, there is gold in the pregnancy test, but it is very different from the gold necklace and gold ring. If you look at the instructions of these test strips, you will see the word "colloidal gold". The gold mentioned here exists in the form of colloidal gold particles.

Colloidal gold is a kind of gold that exists in the form of small particles on a nanometer scale. it is generally made from gold salts such as chloroauric acid (HAuCl4) under the action of a reducing agent. The small gold particles made in this way have a negative charge on the surface, which can be dispersed to form colloids and can also be absorbed by Static and combined with various biological macromolecules.

one of the most special properties of colloidal gold is its distinct color from bulk gold.

the following bottles contain several kinds of colloidal gold with different particle sizes. With the change of particle size, these colloidal gold can show a variety of beautiful colors:

. In the pregnancy test paper, the specific binding of hormones to be tested depends on antibodies, and to show red is inseparable from the colloidal gold bound to it.

this color property of colloidal gold has been used by people since ancient times. The ancient Romans used a similar method to make (The Lycurgus Cup) glasses that could change color. The glass is green when seen directly from the outside, but if the white light is placed in the glass and the light is allowed to pass through the wall of the glass, the glass looks red.

scientists have found that the wine glass can change color because it contains silver alloy nanoparticles, which scatter and absorb green light. If you see mainly scattered light, you will see green, and if you see through the reflected light, it is the complementary color left after the green light is absorbed-red.

now, this property of colloidal gold has many applications in other fields.

so why does colloidal gold show various colors? This is the excitation phenomenon of "localized surface plasma" (localized surface plasmons). This is more difficult to understand. In short, it is related to the resonance of the free electrons on the nanoparticles with the incident visible light (electromagnetic wave).

PS: can't help thinking: is there any gold in mountain red?