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Transparent golden nano-sphere

Gilded prison for dye

 

 In emerging nanotechnologies, the discovery of transparent gold nano-spheres is timely. These spheres can protect dyes from destruction by light and oxygen.

 Microscopic hollow balls with gold walls were produced during collaboration between research teams from the Ecole normale superieure de Lyon and the universities of Grenoble and Lyon. Although metallic, these nano-spheres – with a diameter close to a ten thousandth of a millimeter – are transparent to light. How can they be made? What can be seen inside? Why can we envisage using such a discovery for laser protection?

 

Representation of the light passing through the transparent wall of a hollow nano-gold sphere containing a red dye. The closer the dye is to the wall, the more it shines.

Sphère dor

 

The preparation of these nano-spheres is based on a series of transformations that have long been known, but that were performed for the first time, using a new set up. Gold may indeed be oxidized to produce a salt which, in turn, can be transformed back into gold. In this research, this last transformation was achieved in a strongly whipped up mixture of water, oil and few additives, a kind of vinaigrette called an emulsion, studded with mini droplets around which gold assembles and takes its spherical shape. If dye is added to the emulsion, it will be found inside the nano-gold sphere and, as the latter is transparent, the dye can be lit up and observed from outside.

The comparison of the dye properties, inside or outside of the nano-spheres, showed a surprising behavior that was confirmed by theoretical calculations. For example, the dye retains its usual property of absorbing light and, after a short time, re-emitting it all around. This property, called fluorescence, is characterized by the delay between absorption and emission. It has been shown that this delay is five times shorter in the nano-sphere than outside it. Even more incredible, this delay is further reduced when the dye approaches the gold wall of the sphere. This decrease explains why the dye glows differently depending on its location in the nano-sphere (see figure).

This + X has a significant impact. The gold cage prevents the dye from being destroyed by oxygen and light. This destruction, called a photobleaching, can occur only during the short delay between the absorption of light and its re-emission. Therefore, reducing the delay by having the dye in the nano-sphere reduces photobleaching. The dye becomes protected from a high power laser beam It is also protected againts the active oxygen that could be created as a cancer cell killer.

 

Reference : Transparent plasmonic nano-containers protect organic fluorophores against photobleaching. Nanoletters 2011, 11 (5), pp 2043–2047.
Corresponding author: patrice baldeck ; stephane parola
Rédacteur de la note : jflm@ens-lyon.fr