Is the "silver needle test for poison" in TV dramas real? Just for entertainment, don't take it seriously!

In ancient times, there was a common method of testing poison with silver needles. After a silver needle or a silver hairpin came into contact with food, if the needle turned black, the food was considered poisonous. This method has a history of about a thousand years and is a very traditional method of testing poison.

Source: Stills from the TV series "The Legend of Zhen Huan"

So, is the silver needle test for poison a panacea? Can it detect all poisons? To answer this question, we need to understand two things:

①What is the principle of testing poison with silver needles?

②What are the differences between poisons in ancient and modern times?

Silver needle to detect poison? An unexpected result

In many costume TV dramas and movies, we often see scenes of using silver needles to test for poison in food, and even coroners use silver needles to test for poison as one of the means of examining the cause of death of corpses. For example, in Stephen Chow's movie "Justice, My Foot", the coroner used silver needles to test for poison and proved that the thirteen members of the Qi family, including a dog, died of poisoning.

Image source: Stills from the movie "Justice, My Foot"

Of course, there are some elements of artistic processing here, but it was understandable at the time. Because the use of poisons in ancient times was relatively limited, the commonly used poison was arsenic (arsenic trioxide). There is another kind of red realgar, whose main component is also arsenic trioxide. It looks red, similar to the color of the top of the red-crowned crane's head, and is also called Crane's Crown Red. Whether it is arsenic or Crane's Crown Red, the toxic component is arsenic trioxide. Therefore, the silver needle test for poison is mainly to see whether there is arsenic trioxide, but this process is a bit of a coincidence.

In fact, arsenic trioxide and silver do not react. The reason why the silver needle turns black is mainly because the purification technology in ancient times was limited, and the poison was always mixed with some sulfur or sulfide. After the sulfur element comes into contact with the silver needle, it reacts with the assistance of oxygen and quickly generates black silver sulfide (Ag + S --> Ag2S). Therefore, the silver needle actually only detects impurities in the impure poison.

There are also some rumors that silver jewelry will turn black after being worn for a period of time, which means that the body has toxins. This is not true. In fact, the salt and sulfur elements in human sweat react with silver jewelry after long-term contact, resulting in a layer of black silver scale (silver sulfide and a small amount of silver oxide) on the surface of silver jewelry. From this, we can see that the silver needle test for poison actually reacts to the sulfur element and produces black chemical products. When we understand the principle, we have to sigh that this method of testing poison that only knows the result but not the reason has caused many false and wrongful convictions. There were only a few commonly used poisons in ancient times, but there are many more in modern times.

What are the toxic substances now?

There are many kinds of modern poisons, and the properties of the poisons vary significantly, and the mechanisms of toxicity are different. For example, the "Catalogue of Hazardous Chemicals" formulated by the State Administration of Work Safety in conjunction with the Ministry of Industry and Information Technology, the Ministry of Public Security, the Ministry of Environmental Protection, the Ministry of Transport, the Ministry of Agriculture, the National Health and Family Planning Commission, the General Administration of Quality Supervision, Inspection and Quarantine, the Railway Bureau, and the Civil Aviation Administration lists 2,828 types of chemical substances, and their acute and chronic toxicities are very different.

In addition to chemicals, many biological toxins produced by microbial metabolism are also highly toxic, such as the well-known pufferfish toxin and aflatoxin. There are also some highly toxic substances in life, such as mercury (mercury, which is contained in ordinary thermometers). Many chemical poisons or biological toxins cannot produce color reactions with silver elements, so it is impossible to detect poisons by turning the silver needle black.

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Since the silver needle test is no longer a panacea, what are the current methods of drug testing? In fact, modern drug testing is more based on instrumental analysis and testing, similar to when we go to the hospital for a physical examination, each indicator corresponds to a chemical/biological analysis. However, these high-end equipment are complicated to operate, occupy a large area, and are not very convenient to use in many cases. Therefore, in addition to these high-end equipment, test strips/test cards are also common in life. Below, we will briefly introduce a type of drug testing technology related to "gold".

Modern drug testing can use gold

Of course, the "gold" test we are talking about here does not mean directly taking out a gold needle to poke around. Now we often use colloidal gold test paper technology. The following is a specific introduction to the color development mechanism, detection principle and detection object of colloidal gold.

Colloidal gold is usually a spherical gold nanoparticle (AuNPs) with a diameter between 5 and 100 nanometers (1 nanometer = 1*10-9 meters) and good dispersion. When gold nanoparticles aggregate, the resonance absorption wavelength generated is red-shifted, and the solution appears purple or purple-red. By directly observing the color change with the naked eye, it can be determined whether the gold nanoparticles have aggregated [1]. If we can perform a little processing on the gold nanoparticles so that they aggregate and change color when they come into contact with specific toxic substances, we can make a simple and intuitive drug detection tool.

For example, as shown in the figure below, after the surface of gold nanoparticles is modified with azide benzene and active ester, if it comes into contact with hydrogen sulfide (a toxic gas), hydrogen sulfide can reduce azide benzene to aniline, which then reacts with the active ester to form an amide compound, inducing the aggregation of gold nanoparticles and causing them to change color [2]. Thus, we know that hydrogen sulfide may exist nearby. Similarly, by modifying the surface of gold nanoparticles with different groups with similar functions, they can be used to detect many different substances.

Schematic diagram of gold nanoparticles detecting hydrogen sulfide

The new crown antigen test turns out to be this

Since the outbreak of the COVID-19 pandemic, we have become very familiar with various testing methods. Among them, there is a new coronavirus antigen test kit that we can currently test at home, which uses the principle mentioned above.

The commonly used detection method at present is the double antibody sandwich method, that is, before detecting poisons, the surface of gold nanoparticles needs to be modified with antibodies, usually using biological molecules such as proteins to achieve functionalization. These surface antibody functional molecules selectively interact with the target poisons, resulting in the enrichment of poisons on the surface of gold nanoparticles.

At this time, the test paper also embeds poison antibodies and control antibodies in two areas. The gold nanoparticles bound to the poison will form an "antibody-poison-antibody" sandwich complex in a specific area and deposit and appear red. This area is called the detection zone (also called the T line). The gold nanoparticles that are not bound to the poison will deposit and color in another area, which is called the control zone (also called the C line).

Gold nanoparticles are too small and have high surface chemical activity, so they tend to aggregate spontaneously. If the color-developed area is purple or black, the test strip is invalid. If no control band is formed, the result is recorded as invalid. Only when the control band is normal and there is color on the test band, the poison is detected.

Test card result determination

According to the detection principle, as long as the corresponding poison recognition molecules are modified on the surface of gold nanoparticles, theoretically any poison can be recognized. The existing colloidal gold modification molecules are mainly divided into two types: chelating agents and antibodies. Therefore, the test paper has a good detection ability for the corresponding heavy metal ions, anions, viruses and other poisons. For example, the new coronavirus antigen test card mentioned above.

Looking back at the previous article, the transformation of poison detection from "silver to gold", from the initial accidental success where we knew the results but not the reasons, to now being able to customize special detection reagents for different poisons at will because we have figured out the principles, is the result of our increasingly in-depth understanding of chemistry.

References

[1] Z. Yuan, C.-C. Hu, H.-T. Chang, C. Lu, Analyst 2016, 141, 1611-1626.

[2] Z. Yuan, F. Lu, M. Peng, C.-W. Wang, Y.-T. Tseng, Y. Du, N. Cai, C.-W. Lien, H.-T. Chang, Y. He, ES Yeung, Anal. Chem. 2015, 87, 7267-7273.

Planning and production

Author: Yuan Zhiqin, Associate Professor of Beijing University of Chemical Technology

Reviewer: Wei Rui, Professor of the School of Chemistry, Beijing Normal University

Planning丨Ding Zong

Editor: Yinuo

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