Condensed Matter Seminar features DNA in Nanopores

Solid-state nanopores are single molecule sensors that measure changes in ionic current as charged polymers such as DNA pass through. A rich phenomenology of DNA translocation behaviour has been observed but there is still a limited understanding of the underlying transport mechanism. Here, we will show that glass nanopores with mean opening diameters of down to 15 nm, fabricated from quartz glass capillaries, are ideal model systems to investigate the physical principles of polymer translocation. We present comprehensive experiments on the length, voltage and salt dependence of DNA translocation. Using optical tweezers we extract the force acting on single DNA molecules and quantify the role of electrokinetic effects. For DNA tethered to a bead held in an optical trap we prove that a simple scaling argument based on a mean field theory of the hydrodynamic interactions between multiple DNA strands explains the forces between DNA strands. For free DNA translocations we observe an entropic barrier limited, length dependent translocation rate at 4M LiCl salt concentration and a drift- dominated, length independent translocation rate at 1M KCl salt concentration. These observations are described by a unifying convection-diffusion equation which includes the contribution of an entropic barrier for polymer entry. Finally, we will introduce DNA carriers that have specific protein binding sites. We measure the ionic current signal of these translocating DNA carriers and show detection down to the single protein level. Our assay allows for identifying a single protein species within a protein mixture.