Effects of Counterions on Nano-confined DNA Thin Films

DNA molecules spin-coated on amorphous quartz substrates are shown to form stable films. Electron density profiles (EDPs) along the film depth show that film prepared from aqueous solution of DNA exhibits layering of the molecules in three stacks parallel to the substrate whereas film prepared from counterion added solution does not have layering but have smaller thickness and enhanced surface roughness, although both films have 'liquid-like' height-height correlations. We explain these results by a model of film of a 'liquid' comprising of rod-like molecules where the counterion concentration in the liquid determines the rod length.