Encryption at the flick of a light switch

Scientists have designed a grid of light responsive colloidal particles to function as pixels that could be used to create barcodes for cryptographic data storage.

Photochromic dyes are used in films to respond to light, for example in self-dimming sunglasses. These dyes have two isomers, one forms in visible light and is transparent, the other forms in UV light and absorbs light, darkening the sunglasses. If a photochromic dye is placed in a film with a fluorescent dye, and the wavelength of the fluorescence is matched to that absorbed by the photochromic dye, the photochromic dye can be used to switch the fluorescence off and on when exposed to UV or visible light.

Clemens Weiß and his colleagues at the Max-Planck Institute for Polymer Research in Germany, have devised a way to use this kind of light triggered dye switch to store data. Encapsulating the photochromic/fluorescent dye pair inside polymer colloids traps the molecules together prolonging the lifetime of the ‘on’ or ‘off’ state for several days. Assembling these functional colloids within a monolayer of larger colloids creates a grid of fluorescent ‘colloidal pixels’. Shining UV light on chosen areas of the grid turns the pixels’ fluorescence off creating dark areas on the grid whilst leaving others fluorescent.

The fluorescence can be reversibly switched on and off

They propose that a focussed laser could be used to switch individual colloidal pixels off to create an intricate pattern that could be read like a bar code. The pattern would be erased as soon as it was read with visible light, providing a cheap and simple way to encrypt information.

Jianguo Huang, an expert in functional self-assembly from Zhejiang University in China, says this research is more than ‘just fun for the researchers’ – it has real potential for practical data storage – but optimisation of the grids’ mechanical stability will be important. Weiß comments that a nice feature of this grid system is that the substrate can be changed, for example the grid could be embedded in silica to give it better durability.

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