Combating Fraud a Matter of Connecting the Dots


By Kevin Ritchart

A staggering $650 billion per year is lost to counterfeiting worldwide, according to the International Chamber of Commerce, but advancements in the realm of invisible ink could put a dent in that number in the years to come.

A team of Chinese scientists has developed a new form of invisible ink based on carbon nitride quantum dots. Information written with this ink is not visible under ambient or ultraviolet (UV) light, but it can be viewed under a fluorescence microplate reader. Anything written with this type of ink can be further encrypted or decrypted by quenching or recovering fluorescence with different types of reagents.

Quantum dots have optoelectronic properties that can be controlled by changing the size of the dots. The Chinese scientists used quantum dots made from graphitic carbon nitride in the development of this new type of invisible ink. Graphitic carbon nitride consists of ring systems of carbon and nitrogen atoms that are linked in two-dimensional molecular layers. The makeup of the dots is similar to graphite, which is one of the forms of pure carbon, but it also has the properties of a semiconductor.

The primary function of fluorescing security inks is to ensure products and documents – such as stock certificates, transport documents, currency notes and identity cards – are secure. Counterfeiting of these types of documents can cost a company money in lost profits or damage to its reputation. When it comes to fraud involving pharmaceuticals or machine parts, human lives could be endangered.

Let There Be Light

Counterfeiters have figured out how to imitate UV tags, but copying security ink that’s not visible under UV light is much more difficult to do. Researchers also have had success in developing an inexpensive form of invisible ink that is based on water-soluble quantum dots, which are essentially nanoscopic “heaps” of semiconducting material. Anything written with this new type of ink is unable to be viewed in ambient or UV light because it’s nearly transparent in the visible light range and emits fluorescence with a peak in the UV range. The ink is only visible under a microplate reader like those used in biological fluorescence tests.

Writing using the ink can be further encrypted or decrypted using oxalic acid, which makes the ink invisible even under the influence of the microplate reader. Sodium bicarbonate can be used to reverse the process, making the ink visible once again.

Any change to ink’s color or composition also makes the act of counterfeiting more difficult. Without specific knowledge of every ingredient and the precise amounts used, counterfeiters would be unable to replicate ink. Both the chemical makeup and various proprietary encryption processes make it impossible for even the ink’s creators to re-create or reverse-engineer the end product.

Primary Colors

Scientists at Northwestern University formulated the ink by mixing a simple sugar (cyclodextrin) and a competitive binding agent with an active ingredient (a molecule called heterorotaxane) whose fluorescent color changes along the spectrum from red to yellow to green depending on how the components come together. An infinite number of color combinations can be easily defined. The sugar itself is colorless, but it encapsulates some of the other components of the ink selectively. This makes it difficult to predict and difficult for counterfeiters to replicate.

While the ink itself can change colors based on its own composition, the type of paper the ink is applied to can also impact where the ink falls on the color spectrum. For example, scientists found that an ink that appears orange on standard copy paper appears green when applied to newsprint.

Advancements in the field of invisible ink essentially allow individuals and organizations to create their own unique security codes by manually adjusting the parameters of an ink’s key components. Without knowing the parameters of a particular security in “recipe,” it would be virtually impossible for counterfeiters to break the code.


  • What documents and products do you think would benefit from security inks? Do you have any documents or products that use security inks?
  • What applications other than security could invisible inks have?


  • Fluorescence 
  • Quantum 
  • UV Light