In an age before computers became our main form of communication, invisible ink was a favorite way to pass information without drawing unwanted attention. It may have always been more common in detective stories than real life, but it was easy enough to make at home. The idea may be less important in popular culture as information technology rises, but fluorescent inks that can only be read under ultraviolet light are in increasing demand for document security. Existing ones just don't work all that well.
Traditional home-made invisible inks may have been great for a hand-scrawled note telling people where to look, or who to look out for, but printing long documents is something else. For all the role computers play, “Paper still plays a dominant role in information storage,” a new study in the journal Matter notes.
By coating paper with a manganese complex researchers at Nanjing University have turned it into a sort of erasable whiteboard. The marking is done with water, about the cheapest and most environmentally friendly medium, applied with a jet printer. Anything printed is invisible under ordinary light, but reveals itself when illuminated in ultraviolet, specifically at 254 nanometers, a wavelength too short even for animals, like bees, that can see in the near UV.
Once the message has been read it is easy to prevent it falling into the wrong hands. Rather than burning the message (let alone having it self-destruct), blow-drying the paper for 15-30 seconds not only erases the message but leaves the paper ready to be printed on again.
"The rewritable feature significantly reduces the cost. The cost per print is estimated to be RMB0.014 ($0.002)," said senior author Dr Qiang Zhao in a statement. "Most fluorescent security inks on the market used to record confidential information are environmentally unfriendly and cannot be erased. The paper is only a disposable recording medium."
Environmental footprints may not be front of mind for those sending clandestine messages, but Zhao stresses it’s not just the ink that does well on this score. The manganese coating is also low in impact, including in toxicity for those handling the paper.
Like all security procedures, Zhao’s product has a weak point. If a paper printed with a secret message falls into the wrong hands, provided they recognize the apparently blank page may contain a message and are aware of the technology, they are likely to be able to shine an appropriate ultraviolet light on the material and reveal its secrets.
For these cases, Zhao has got a step further, combining a suitable coating with magnesium chloride and magnesium bromide inks that are invisible under both sunlight and UV. In these cases only photoluminescence lifetime imaging, whose availability is limited, will reveal the hidden text.