Tag Archives: invisible ink

Blank paper on ninja history wins top grades — it was written in invisible ink

A Japanese student of ninja history handed in a blank paper and still passed with flying colors. Her secret? Invisible ninja ink.

Eimi Haga.
Image via BBC.

Eimi Haga, a student at the Mie University in Japan, wrote her paper using the ninja technique of “aburidashi.” Apart from the time she used to research and write her assignment, Haga also spent hours soaking and crushing soybeans to make the ink, she told the BBC. Luckily, her professor eventually heated the paper over a gas stove to reveal the text, otherwise, her grades might not have reflected her commitment to the task.

Hidden in plain sight

“It is something I learned through a book when I was little,” Ms Haga told the BBC. “I just hoped that no-one would come up with the same idea.”

Her interest in ninjas, the infamous spies and assassins of medieval Japan, first sprouted during her childhood from watching animated TV shows. For her class on ninja history, the first-year student was asked to write about a visit to the Ninja Museum of Igaryu.

“When the professor said in class that he would give a high mark for creativity, I decided that I would make my essay stand out from others,” she said.

“I gave a thought for a while, and hit upon the idea of aburidashi.”

She created the ink by soaking soybeans overnight, crushing them, and squeezing them through a cloth. She then mixed the soybean extract with water, spending two hours to get the concentration just right to create the ink. The final step was to write her essay with a fine brush on “washi” (thin Japanese paper).

The essay, showing the heated and unheated sections.
Image via BBC.

After the ink dried, it became invisible. To make sure she wouldn’t be dismissed out of hand, she left a note (with normal ink) for her professor to “heat the paper”. The professor, Yuji Yamada, told the BBC he was “surprised” when he saw the essay.

“I had seen such reports written in code, but never seen one done in aburidashi,” he said. “To tell the truth, I had a little doubt that the words would come out clearly. But when I actually heated the paper over the gas stove in my house, the words appeared very clearly and I thought ‘Well done!’

He added that although he didn’t read the full paper — “I thought I should leave some part of the paper unheated, in case the media would somehow find this and take a picture” — he didn’t hesitate to give it full marks.

The art of the ninja is a tradition going back hundreds of years to Japan’s feudal era. Ninjas didn’t serve in battles directly, but they did operate akin to the special forces of today, gathering intel, taking out key individuals, and even helping to shape strategy. Techniques such as aburidashi allowed them to share information covertly and protect it from prying eyes.

Schematic of encryption and decryption of the perovskite NCs-MOF platform. Credit: Nature.

New printable invisible ink is no lame party trick

Every kid used to play spy by writing ‘secret’ messages with lemon juice. As odd it may sound, invisible ink is actually highly sought after even to this day, especially for securing information or preventing counterfeiting. Chinese researchers recently reported a new take on invisible ink, and their work is no party trick.

Schematic of encryption and decryption of the perovskite NCs-MOF platform. Credit: Nature.

Schematic of encryption and decryption of the perovskite NCs-MOF platform. Credit: Nature.

Liang Li and colleagues at Shanghai Jiao Tong University were initially looking to synthesize chemical compounds that glow at the nanoscale. They accidentally made a lead compound that was invisible to the human eye instead.

When halide salt is added to lead-based metal-organic frameworks (MOFs), the material is converted into luminescent perovskite nanocrystals (NCs) and the text becomes visible again.

Other invisible inks can easily be detected. Sometimes, a person just needs to hold the piece of paper etched with the hidden message over a lightbulb to read its contents. The MOFs-based ink, however, is undetectable unless you sprinkle salt over it. What’s more, the ink can be printed with just about any printer, with some small modifications, making it relatively easy to create and distribute secured letters.

The MOFs-based ink could also prove useful in anti-counterfeiting measures. Many currencies already employ variations of invisible ink to hide text or pictures from the naked eye.

NCs-MOFs patterns can be printed on transparent PET foils as well, with promising applications on multi-integrated light sources or other optoelectronic devices.

There are also some downsides. Lead can be highly toxic to organisms if exposure time is long enough. If you’re that kind of spy, maybe you really want the recipient to get harmed by the letter. The team is working on tin-based alternatives, in any event.

Scientific reference: Conversion of invisible metal-organic frameworks to luminescent perovskite nanocrystals for confidential information encryption and decryption, Nature Communications, DOI: 10.1038/s41467-017-01248-2.