418 research outputs found
Coarse-grained simulation of polymer translocation through an artificial nanopore
The translocation of a macromolecule through a nanometer-sized pore is an
interesting process with important applications in the development of
biosensors for single--molecule analysis and in drug delivery and gene therapy.
We have carried out a molecular dynamics simulation study of electrophoretic
translocation of a charged polymer through an artificial nanopore to explore
the feasibility of semiconductor--based nanopore devices for ultra--fast DNA
sequencing. The polymer is represented by a simple bead--spring model designed
to yield an appropriate coarse-grained description of the phosphate backbone of
DNA in salt--free aqueous solution. A detailed analysis of single translocation
event is presented to assess whether the passage of individual ions through the
pore can be detected by a nanoscale field--effect transistor by measuring
variations in electrostatic potential during polymer translocation. We find
that it is possible to identify single events corresponding to the passage of
counterions through the pore, but that discrimination of individual ions on the
polymer chain based on variations in electrostatic potential is problematic.
Several distinct stages in the translocation process are identified,
characterized by changes in polymer conformation and by variations in the
magnitude and direction of the internal electric field induced by the
fluctuating charge distribution. The dependence of the condensed fraction of
counterions on Bjerrum length leads to significant changes in polymer
conformation, which profoundly affect the dynamics of electrophoresis and
translocation.Comment: 37 pages Revtex, 11 postscript figure
Dielectric spectroscopy of ferroelectric nematic liquid crystals: Measuring the capacitance of insulating interfacial layers
Numerous measurements of the dielectric constant of the recently
discovered ferroelectric nematic () liquid crystal (LC) phase report
extraordinarily large values of (up to ~30,000). We show that
what is in fact being measured in such experiments is the high capacitance of
the non-ferroelectric, interfacial, insulating layers of nanoscale thickness
that bound the material in typical cells.
We analyze a parallel-plate cell filled with material of
high-polarization , oriented parallel to the plates at zero applied
voltage. Minimization of the dominant electrostatic energy renders
spatially uniform and orients it to make the electric field in the as
small as possible, a condition under which the voltage applied to the cell
appears almost entirely across the high-capacity interfacial layers. This
coupling of orientation and charge creates a combined polarization-external
capacitance (PCG) Goldstone reorientation mode requiring applied voltages
orders of magnitude smaller than that of the layer alone to effectively
transport charge across the layer. The layer acts as a low-value
resistor and the interfacial capacitors as reversible energy storage
reservoirs, lowering the restoring force (mass) of the PCG mode and producing
strong reactive dielectric behavior. Analysis of data from several experiments
on ferroelectric liquid crystals (chiral smectics C, bent-core smectics, and
the phase supports the PCG model, showing deriving that deriving
dielectric constants from electrical impedance measurements of
high-polarization ferroelectric LCs, without properly accounting for the
self-screening effects of polarization charge and the capacitive contributions
of interfacial layers, can result in overestimation of the
values of the LC by many orders of magnitude.Comment: 26 pages of text, 10 figures, 49 references (40 pages total
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