Sorting Short Fragments of Single-Stranded DNA with an Evolving Electric Double Layer

We demonstrate a new procedure for separation of single-stranded DNA (ssDNA) fragments that are anchored to the surface of a gold electrode by end hybridization. The new separation procedure takes advantage of the strong yet evolving nonuniform electric field near the gold surface in contact with a buffer solution gradually being diluted with deionized water. Separation of short ssDNA fragments is demonstrated by monitoring the DNA at the gold surface with <i>in situ</i> fluorescence measurement. The experimental results can be rationalized with a simple theoretical model of electric double layer that relates the strength of the surface pulling force to the ionic concentration of the changing buffer solution