Mica is a naturally abundant crystal and its layered structure means that it is strongly insulating. Mica consists of 1 nm thick layers of silicate ionically bonded by a group I cation (usually K+). Cleaving open a fresh surface requires that half the K+ ions stay on one surface and half on the other. These ions can be exchanged with other ions by immersing the surface in salt solutions but exchange times can last hours and it is difficult to see how the ions are distributed. This is because surface science techniques such as X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) in their standard formats typically lack adequate spatial resolution and surface sensitivity.

Freshly cleaved mica is a commonly used support surface for imaging biological molecules because it provides an atomically flat uncontaminated surface. To bind DNA strongly, the surface exchanges with divalent cations, in this case, nickel ions. While imaging DNA under aqueous solutions, we noticed that sections of the DNA were still moving, which should not be the case if the DNA molecules were strongly bound to the nickel. This observation led us to carry out the detailed study described in this work using a range of DNA fragment lengths from 200 base pairs long upwards and quantify how ions are distributed along different distances on the mica. Our findings might be used to improve how DNA binds to inroganic surfaces in general with a view to patterning high-density arrays.

More information about the research can be found in the journal Nanotechnology 25 025704.

Further reading

Mica pairs up with graphene (Oct 2010)
Tip measures nanochannel kinetics in thin water films (Apr 2011)