Invisible fluorescent security inks have been developed using single wall carbon nanotubes by researchers of Rice University. These inks contain multiple levels of information. The shapes and or words contain one level of information and CNT marker compositions within the inks are contained in the second levels of spectral information.

Nanoinks

Such invisible inks have found wide applications in business and commerce as a means of protection against fraud, counterfeiting, and theft. These inks are invisible when printed but can be made visible through various methods to decode the written information . These inks are used to prove the authenticity of document which is commercially valuable such as stock certificates, bearer bonds, checks, lottery tickets, vouchers and as security tags to protect against the misuse of personal identification documents. use of this ink can protect documents such as social security cards, passports, visas, identification cards, credit/debit cards, automobile licenses, and vital business and health records.

When monochromatic light of specific wavelength is made to fall on the inked surface, the letters printed with carbon nanotube ink are seen to fluoresce, emitting radiation in the NIR. This emission can be detected using a camera with suitable a detection range when exposed to multiple video frames of glowing ink made of carbon nanotubes that would otherwise be invisible. These inks after printing are colorless and transparent making them undetectable under normal light conditions but are made visible under UV light. Though conventional fluorescent inks are invisible under normal light, these inks can usually be seen when illuminated by a UV or other suitable wavelengths.

Researchers have used carbon nanotubes as the improved highly-specific security tags, as nanotubes can be grown in a variety of size with characteristic absorption and emission wavelengths. For example, an aqueous suspension of single-walled carbon nanotubes can be applied to paper or cloth to make invisible nanotag, but fluoresces when illuminated under a specific wavelength in the near-IR using appropriate InGaAs camera equipment.

Spectral filtering can distinguish different nanotube species in the tag, because each will show distinct absorption and emission wavelengths. When partially or fully structure-separated nanotube samples are used, the corresponding tags will have distinct wavelengths of excitation and emission. Other fluorescent inks do not offer a variety of wavelength-specific forms that can provide this added security feature. Also, there is virtually no background emission in the near-IR, thus only tiny quantities of nanotubes are required for marking.

This discovery will lead to nanoelectronics and biomedical contrast agents for non-invasive imaging of particular types of cells. Nanotube tags, comprised of nanotubes of different diameters, could selectively inscribe different denominations of currency. Nanotube tags might also be used as “spectral bar codes” for non-contact identification of items such as clothing.