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Forget the lemon juice and hot irons, there is a new way to write and read invisible messages – and it can be used again and again.
The approach, developed by researchers in China, involves the use of water to print messages on paper coated with chemicals containing manganese. The message, invisible to the naked eye, can be read by projecting UV light onto the paper.
By using hot air from a hair dryer, the message can be erased and the paper recycled.
Although digital communications and data security are becoming more sophisticated, the study authors said that finding ways to send paper messages safely was still important.
"Data leaks have become a worldwide problem with terrible consequences, especially in times of war and serious economic and social problems," they said in the study.
"Although electronic media have become indispensable in our daily lives, paper remains the most common medium for storing information, and many important documents are still based on paper."
In the journal Matter, the team said that the use of fluorescent inks is visible only under ultraviolet rays. However, they say that these have disadvantages, especially that they can not be erased and the paper reused, and that they are predictable – which means that messages can be easily deciphered.
The new approach is to cover the filter paper with a polymer containing manganese-containing chemicals, able to absorb UV rays and emit green light, then print messages on this paper with water pure using an ink jet printer. The team said the materials had very low toxicity.
When pure water interacts with chemicals containing manganese, it disrupts their structure, hence their photoluminescent properties. The result is that even though the message can not be seen with the naked eye, it appears in the form of a dark drawing under short-wave UV light.
If the paper is heated under a hair dryer for about 30 seconds, the message disappears. Until now, researchers have managed to reuse the paper up to 30 times before it breaks down – each printing costing fractions of a hundred.
"If we can find a good substrate to replace the filter paper, we think that cycle numbers will be further improved," said Dr. Yun Ma, co-author of the study, University of California. Nanjing stations and telecommunications.
The authors stated that the message did not disappear simply with the evaporation of water in the absence of hair dryers and that it could last more than three months under ambient conditions.
One possible explanation, they say, is that it might be necessary to melt the polymer coating to allow the movement required for the structure (and hence the photoluminescence) of the manganese-containing chemicals to be restored after the first time. evaporation of water.
The team also took a different approach, using two inks containing manganese to print a message and a background on a different type of coated filter paper.
The message can not be seen with the naked eye or, especially, under a UV light, because the similarity of the inks means both that the background emits a light of a length d & # 39; Wave and of a very similar intensity under UV illumination. However, the message emits light over a longer period than the background, which means that it can be read using time-based imaging techniques .
Continuing along this path, the team has developed an even more sophisticated approach of printing a series of numbers on paper using several inks containing manganese, each of which emits light on a different period of time after UV illumination.
By following the intensity of the luminous emission in time, a new order of the numbers is displayed, which makes it possible to decipher the code. The paper can then be washed in a special solution and the same string of digits reprinted with a different order of the inks to encode a different message.
"This method of decrypting information has an extremely high level of security and could be promising in the military and economic fields," said the team.
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