[Show abstract][Hide abstract] ABSTRACT: We have explored the electromechanical properties of DNA on a nanometer-length scale using an electric field to force single molecules through synthetic nanopores in ultrathin silicon nitride membranes. At low electric fields, E < 200 mV/10 nm, we observed that single-stranded DNA can permeate pores with a diameter >/=1.0 nm, whereas double-stranded DNA only permeates pores with a diameter >/=3 nm. For pores <3.0 nm diameter, we find a threshold for permeation of double-stranded DNA that depends on the electric field and pH. For a 2 nm diameter pore, the electric field threshold is approximately 3.1 V/10 nm at pH = 8.5; the threshold decreases as pH becomes more acidic or the diameter increases. Molecular dynamics indicates that the field threshold originates from a stretching transition in DNA that occurs under the force gradient in a nanopore. Lowering pH destabilizes the double helix, facilitating DNA translocation at lower fields.
[Show abstract][Hide abstract] ABSTRACT: We have developed a novel strategy for producing nanopores in inorganic, CMOS-compatible membranes using a tightly focused, high energy electron beam. We are able to characterize the nanopores physically (TEM, AFM) as well as electrically (ionic conductivity and "wetting" curves). Subsequently, we used the nanopore as a molecular Coulter counter to detect the size of a DNA molecule. This is the first report of the use of an inorganic nanopore for discriminating DNA molecules.