(503a) Single-Molecule Translocation through Asymmetric Conic Nanopores: The Effects of Van Der Waal Absorption and Electro-Osmotic Flow

We examine the pertinent electrokinetics of single-molecule translocation through conic nanopores both experimentally and theoretically.  Due to the ion-selectivity of the nanopore and the conductivity gradients introduced by the spatial asymmetry, anomalous ion current phenomena such as rectification, transient hysteresis and water-splitting have been observed.  In our past work, we have derived asymptotic expressions for the rectification factor, the hysteresis window and the water-breaking cyclic voltammetry signatures with a perturbation analysis of the area-averaged ion transport equation.  We use optimum designs from such theories to deliver short (20 bases) DNAs to a conic nanopore and to delay their translocation time, two major obstacles for nanopore rapid sequencing technologies.  Using non-equilibrium concentration and charge polarization endowed by the asymmetry, we are able to produce an external electro-osmotic flow that linearizes, convects and focuses the DNA molecules into the nanopore.  At the pore tip, van der Waal forces between the pore and the translocating molecule are shown to be stronger than Coulombic repulsion after surface treatment with a high-permittivity material. Curiously, short 20b single-stranded DNAs show more absorption affinity than their double-stranded counterparts of the same length. This is attributed to the exposed ring structures of ssDNA with delocalized electrons that enhance the London dispersion attraction to the tip surface. We present Poisson distributions of both ssDNA and dsDNA with average translocation times that are separated by two orders of magnitude. We also show ion-current data and super-microscopy images that suggest a stick-slip dynamics for the translocating ssDNA.  

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