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Quantum dots offer secure solutions

23 Jan 2004

Tiny light-emitting crystals with unique emission signatures could protect personal documents from counterfeiting.

Fluorescent inks containing quantum dots could be the key to creating identification codes that are invisible to the naked eye and very hard to counterfeit. The “Info-ink” codes developed at the Institute for National Measurement Standards in Ottawa, Canada could be ideal for use on passports or ID cards. (Optics Express 12 143).

Fluorescent quantum dots, light-emitting semiconductor nanocrystals, are already widely used in biology to tag different molecules. “In principle, the quantum dots we use are the same, however, some special issues, such as long-term stability and adhesion to the objects, must be taken into consideration,” said Shoude Chang, a member of the research team.

Chang’s info-inks, composed of a polymer, a solvent and a mixture of quantum-dots, can be painted or printed onto the surface of a document or object. By adjusting the number and emission wavelength of the quantum dots in the ink it is possible to create a digital fluorescence code that is unique to that object. Calculations suggest that the use of six different wavelengths and ten intensity values could create one million distinct codes.

To date, Chang and colleagues have made Info-inks containing CdSe nanocrystals (quantum dots), polystyrene and toluene. Experiments with five different emission wavelengths (535, 560, 585, 610 and 640 nm) have allowed the creation of inks containing a 3-digit code (see diagram).

The codes are read out by illuminating the ink with light from an ultraviolet (370 nm) LED to excite fluorescence from the dots. This fluorescence is captured by an optical fibre bundle and fed to a spectrometer connected to a PC. Analysis of the fluorescence spectrum reveals the code and thus the authenticity or identity of the item.

The approach overcomes many of the limitations of conventional identification technology such as barcodes. Although widely used, barcodes occupy a significant amount of space on the object and can only be read from a certain orientation.

In contrast, the quantum-dot code can be tiny and is easy to read-out. “Compared with barcode readers, the information retrieval of this technology is simpler,” said Chang. “It just collects the fluorescence emitted from the labelled object, and analyses the spectrum. No considerations need to be given to the changes of object's rotation, scale and position”.

Chang and colleagues have now made a prototype read-out system which they say is capable of retrieving a quantum-dot code that is encoded on the surface of a passport page, an ID card or the nail of a human finger.

The researchers are currently looking for industrial partners to help take this technology into practical devices. However, before this technology can be commercialized, the noise in the detection process and the durability of the quantum dots needs to be investigated further.

For more information, Shoude Chang can be contacted at shoude.chang@nrc-cnrc.gc.ca

By Siân Harris and Oliver Graydon

Siân Harris is features editor of Opto & Laser Europe magazine.

Oliver Graydon is editor of Opto & Laser Europe magazine.

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