Abstract Fluorescent Security Tag Using Carbon Nanotubes Technology ID
As a result of advanced printing ink technology, invisible inks have found a new venue in business and commerce as a means of protection against fraud, counterfeiting, and theft. The most useful inks are those that are invisible after printing, but which can be made visible through various means to prove the authenticity of a particular document. These products are being used to authenticate a broad range of valuable commercial documents, such as stock certificates, bearer bonds, checks, lottery tickets, and vouchers. Also, particularly in light of recent terrorist activity, security tags in general are being applied to protect against the misuse of personal identification documents, such as Social Security cards, passports, visas, identification cards, credit/debit cards, automobile licenses, and vital business and health records. Counterfeiting, estimated to account for some 5% of world trade with record losses increasing each year, affects every part of our lives from currency to clothing to medicine. Commerically-available security tags, such as inks used in documents as a covert security feature, are colorless and transparent after printing, making them undetectable under normal light conditions. Applied by themselves or incorporated into another security ink, they become visible under UV light. To further foil counterfeiters, the UV-fluorescent effect is impossible to photocopy or digitally recreate. Fluorescent and phosphorescent inks requiring UV light have been used in many applications. The new US $20 bill has a photochromic thread that glows green under UV light and essentially is invisible to a photocopier. However, though conventional fluorescent inks are invisible under normal light, these inks can usually be seen when illuminated by a UV or �blacklight� emitting long- or short-wave UV (350-400nm); thus limiting their security or concealment, in that they are visible under a range of wavelengths. Professors in Rice�s Chemistry Department have pioneered the use of carbon nanotubes as the embodiment of improved, highly-specific security tags. Nanotubes can be grown in a variety of diameters, and each size has its own characteristic absorption and emission wavelengths. For example, an aqueous suspension of single-walled carbon nanotubes can be applied to paper or cloth. When allowed a few minutes to dry, this �nanotag� is virtually invisible, but fluoresces when illuminated under a specific wavelength. The resulting images can be visualized 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. The discovery that carbon nanotubes emit near-IR light after absorbing light at visible or UV wavelengths could have nanoelectronics and biomedical applications in addition to those of conventional fluorescent inks. 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. Professor Bruce Weisman, one of the inventors, �expect[s] that fluorescence will provide a very sensitive and selective method for detecting nanotubes in environmental and biological systems. It may lead to the use of nanotubes as biomedical contrast agents for non-invasive imaging of particular types of cells.� In addition, the nanotubes can be used in non-ink form in a greater range of applications.
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