Characterizing and controlling the motion of ssDNA in a solid-state nanopore.

Sequencing DNA in a synthetic solid-state nanopore is potentially a low-cost and high-throughput method. Essential to the nanopore-based DNA sequencing method is the ability to control the motion of a single-stranded DNA (ssDNA) molecule at single-base resolution. Experimental studies showed that the average translocation speed of DNA driven by a biasing electric field can be affected by ionic concentration, solvent viscosity, or temperature. Even though it is possible to slow down the average translocation speed, instantaneous motion of DNA is too diffusive to allow each DNA base to stay in front of a sensor site for its measurement. Using extensive all-atom molecular dynamics simulations, we study the diffusion constant, friction coefficient, electrophoretic mobility, and effective charge of ssDNA in a solid-state nanopore. Simulation results show that the spatial fluctuation of ssDNA in 1 ns is comparable to the spacing between neighboring nucleotides in ssDNA, which makes the sensing of a DNA base very difficult. We demonstrate that the recently proposed DNA transistor could potentially solve this problem by electrically trapping ssDNA inside the DNA transistor and ratcheting ssDNA base-by-base in a biasing electric field. When increasing the biasing electric field, we observed that the translocation of ssDNA changes from ratcheting to steady-sliding. The simulated translocation of ssDNA in the DNA transistor was theoretically characterized using Fokker-Planck analysis.