New Research in Invisible Inks

It’s a lot more chemistry than I understand:

Invisible inks based on “smart” fluorescent materials have been shining brightly (if only you could see them) in the data-encryption/decryption arena lately…. But some of the materials are costly or difficult to prepare, and many of these inks remain somewhat visible when illuminated with ambient or ultraviolet light. Liang Li and coworkers at Shanghai Jiao Tong University may have come up with a way to get around those problems. The team prepared a colorless solution of an inexpensive lead-based metal-organic framework (MOF) compound and used it in an ink-jet printer to create completely invisible patterns on paper. Then they exposed the paper to a methylammonium bromide decryption solution…revealing the pattern…. They rendered the pattern invisible again by briefly treating the paper with a polar solvent….

Full paper.

Posted on November 10, 2017 at 6:06 AM22 Comments


layman noboby November 10, 2017 7:30 AM

Allow me to add some generalized basics to your chemistry understanding then.

Lead: bad for you.
Almost anything called Methyl-whatever is bad.
Brom/Bromides: not healthy either.

All together a toxic scenario.
With Lead and Brom both being elements you have a ‘hard’ toxicity problem, whereas you can safely do something about methyls and ammonium (incinerate). Incinerating something with lead usually leaves you with Lead oxide, which is highly undesirable. You want to avoid bromides as well. Think Clive and and his insights about ways to poison folks.

Makes handling a hazard. And you would want to avoid burning the papar where you might get exposed to the smoke/fumes and ash/soot later on.

I wonder though, how much it will show up on something like a airport xray scan, due to the lead content. Perhaps not immediately readable, but maybe enough to get more attention.
The ink cartridge would obviously stand out like a sore thumb.

225 November 10, 2017 7:58 AM

@layman nobody I read the thickness of dried inject print is 0.04 micrometers, so even if it was a layer of pure lead, you still would not see it on an airport x-ray machine.

I’m not even too worried about the poison, just clean your hands before lunch.

Andre Maciel de Amorim November 10, 2017 8:56 AM

Almost anything called Methyl-whatever is bad.

LoL, Some how it reminds me a digital version of Heisenberg Labs, (Breaking Bad)
Nice Tv Show,
Happy squid friday all.

nobody November 10, 2017 10:37 AM

Everyone is terrified of Lead, and not exactly without cause.

But just to put this into perspective, as a child I used to crimp lead weights onto fishing line with my teeth. Tasted sweet. In hindsight, not the wisest thing to do. But I seem to be faring okay.

albert November 10, 2017 11:06 AM

Methylammonium bromide is classified as an “irritant”: skin, eyes, lungs(wiki). It’s the only thing the ‘reader’ of the document needs to worry about. The maker of the ink has more serious concerns, but ink manufacture can be automated safely.

The whole issue is silly anyway. Methylammonium bromide isn’t something you find in your cupboard (like lemon juice:), or your medicine cabinet. So just arrest anyone who possesses methylammonium bromide, and you’re good to go.

. .. . .. — ….

hmm November 10, 2017 11:53 AM

“You want to avoid bromides as well. Think Clive”

I knew that guy was into something bad, but bromides?

milkshaken November 10, 2017 12:36 PM

this is a reactive pigment that turns fluorescent if sprayed with the right chemical. It is not ideal – you still have to spray the paper with a non-volatile chemical that can later damage the document. It is like that simple iodine marker pen, used to test for cheap homemade banknote forgeries (printer paper unlike real banknote paper contains starch)

Also, his pigment can react with other chemicals in the environment – i.e. hydrogen sulfide from a pile of rotting garbage in a Chinese kitchen -which can ruin its properties. Spray of briny water/mist of salt near sea cost will probably affect it also.

In this case, much better / simpler alternative is to use infrared inks. Obviously anyone with IR camera and IR flashlight can detect it

hmm November 10, 2017 2:32 PM

Uh, so why does the “rotting garbage” have to be in a specifically “Chinese kitchen” there?

Are you being casually racist or am I missing some specifically Chinese reference there?

You probably could have omitted that without hurting your actual points. Food for rotten thought.

Mike Barno November 10, 2017 3:38 PM

@ hmm, @ Clive Robinson:

“You want to avoid bromides as well. Think Clive”

I knew that guy was into something bad, but bromides?

Maybe his bromides, Wiktionary: bromide definition 4: “A platitude” are more dangerous than his bromides, definition 1: “(inorganic chemistry) A binary compound of bromine and some other element or radical”.

Wael November 10, 2017 10:05 PM

Tried to read the paper – gave me a headache. Is this Cryptography or Steganography? Sounds like both?

Anura November 10, 2017 10:49 PM


They say encryption and decryption but it doesn’t sound like there is a key, so I’d say it’s really just steganography.

Keyed invisible ink would be pretty cool if you can work out a mechanism. I’d guess you would need a lot of tiny dots of different inks to obscure any potential pattern, which the message can only be made out when coated with the right proportions of chemicals.

Wael November 11, 2017 2:42 AM

Tried to read it again. Poorly written paper… The main innovation is the ability to hide / unhide the pattern. Since Cryptography is the science of hiding the meaning of the message, whereas Steganography is the science (and art) of hiding the existence of the message, it follows that this is “Steganography”, perhaps with a modern / chemical twist that enables something lemon juice can’t.

Then they exposed the paper to a methylammonium bromide decryption solution

Ahem, de-steganographizing solution…
Writing an academic research paper: the art of spinning and gift-wrapping the message! Not impressed.

Clive Robinson November 11, 2017 12:50 PM

@ Mike Barno,

Maybe his bromides

Hmm I shall try not to get into a “brown study” over that 😉

@ Wael,

Tried to read the paper – gave me a headache. Is this Cryptography or Steganography? Sounds like both?

If it relies on “being out of sight” to the uninformed then it’s steganography.

@ ALL,

There is a problem with “secret inks” and it’s been known since before the second world war.

An ink has to be visable at the surface of the paper. Which by the laws of physics means it either has to “soak in” to the paper like old fashioned fountain pen ink, or it has to “adhere to” the surface of the paper in some way.

Either way the surface of the paper where the secret ink is, is different to other parts of the paper. That is a real physical diffrence not just chemical.

In the case of “soaking in” the fibers in the paper would have swollen prior to the ink drying, and this leaves a permanent physical mark on the fibers which can be seen via various quite simple techniques. In the case of adhering to the surface of the paper it to will cause a physical change that again can be seen by the same or similar simple techniques…

The simplest test is realy quite simple you need a bright light source, a darkish room and a slightly curved surface. If you arange the light to come at near right angles in a thin “sheet beam” from a slit or similar. You then put the sheet of paper on the curved surface such that it just touches into the beam and rock it slowly backwards and forwards so you get the beam to just kiss the surface of the paper… Any physical changes in the surface of the paper casts shadows on the surface. Think of it as the same process of dawn breaking across a mountain range that photographas love to put into photo competitions because of the contrasts etc.

But… There is something else all substances have frequency responses in the various light spectra from far infra-red through to the far ultra-violet. Which means they stand out under spectroscopic analysis as the ink will have a different frequency response to that of the underlying paper finish (often a calcium compound these days).

Lastly there is the method David Khan mentioned in his crypto book that was allegedly developed at the FBI laboratories (though there are mentions of others doing the same thing in Europe at the same time and earlier). It’s the iodine vapour test and the modern equivalent the “super glue” vapour test. You hang the paper in a fish tank size vapour chamber and by the use of an electrically heated dish you form a thin vapour of iodine or super glue, which then with a little static magic adhears to the surface of the paper at varying thicknesses. Which with the appropriate light source highlights the ink or paper at a different level to the other. Certainly enough for it to be photographed.

The real way to do secret writing these days is to apply a very thin uniform coating to the paper, such there are no physical differences. In this film you use an appropriate halide emulsion much as Silver bromide and silver chloride were used in gelatin for photographic materials. These chemicals form electron traps when exposed to light but as they are in effect embeded in the emulsion they don’t show up to the simple tests. As with photogrphic film you have to fix and later develop the image.

However promising modern chemicals are those that also need a magnetic field as well as light, though they do tend to be quite poisonous… Similar chemicals are in the likes of rewritable CDs / DVDs etc. Oh and those that use UV etc like those on your children’s teeth to protect them from modern diets.

tz November 11, 2017 5:06 PM

Pilot FriXion erasable pens.
Around for a while. I have several, especially or color diagrams.
heat activates a “bleach” removing color, but putting them in the freezer can recover the text. It seems to work many times but I haven’t tested it.

David Pearce November 12, 2017 6:54 PM

The article mentioned organic metal lead compounds, which are extremely toxic like the tetraethyl lead that was used in petrol

helix November 13, 2017 9:38 AM

It would be best to use a system like RNA which is capable of digital information processing rather than the analog nature of most chemical systems.

The ink would contain RNA with a particular sequence, which can be read using an anti-sense probe. The rest of the paper could be covered with RNA with different sequences, possibly including false messages.

Unfortunately I don’t think RNA on paper is stable for long enough to be of use.

Peter November 13, 2017 2:00 PM

@David Pearce

This is a complex of an inorganic lead salt. It has more in common with lead paint than with tetra-ethyl lead (ie much less dangerous, but still bad).

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