Colloids coated with long DNA strands spontaneously form a crystalline two-dimensional carpet that hangs several micrometers above a surface, say UK scientists. 

Erika Eiser and Nienke Geerts, at the University of Cambridge, were attempting to build a 3D photonic crystal, and were extremely surprised to discover the colloids floating above the surface. This had not been seen before, and only been suggested in theory, she explains. 

Colloids that are coated with very short DNA strands bind strongly to the surface forming an amorphous substrate explains Eiser. But here they assembled into a two dimensional 'carpet' due to the weaker adsorption of the longer DNA molecules onto the surface which allows the formation of dense 2D colloidal crystals. Eiser says that 'DNA is a beautiful, monodisperse polymer, and in this system we used it to interact with the surface and allow our colloids to aggregate.' 

'This is a real new finding,' comments Martien Cohen-Stuart, an expert in colloids from Wageningen University, The Netherlands. He says it is particularly interesting that the researchers were able to confine the particles in a plane by a very neat interaction with the surface. 

Eiser says that these colloids could be used as defect free photonic crystals, which have very high reflectivity and can be used as perfect mirrors. She also suggests that this sort of technique could have the potential to be used for building very structured devices like electronic switches. 

In the future, the group aim to expand the scientific understanding of how these crystals grow, and will try mixing different types of colloids to make patterns that interact differently with visible light. 

Holly Sheahan 

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