Gael Nguyen

DNA-modified polymer pores allow pH- and voltage-gated control of channel flux.

Biological channels embedded in cell membranes regulate ionic transport by responding to external stimuli such as pH, voltage, and molecular binding. Mimicking the gating properties of these biological structures would be instrumental in the preparation of smart membranes used in biosensing, drug delivery, and ionic circuit construction. Here we present a new concept for building synthetic nanopores that can simultaneously respond to pH and transmembrane potential changes. DNA oligomers containing protonatable A and C bases are attached at the narrow opening of an asymmetric nanopore. Lowering the pH to 5.5 causes the positively charged DNA molecules to bind to other strands with negative backbones, thereby creating an electrostatic mesh that closes the pore to unprecedentedly high resistances of several tens of gigaohms. At neutral pH values, voltage switching causes the isolated DNA strands to undergo nanomechanical movement, as seen by a reversible current modulation. We provide evidence that the pH-dependent reversible closing mechanism is robust and applicable for nanopores with opening diameters of up to 14 nm. The concept of creating an electrostatic mesh may also be applied to different organic polymers.

Comparison of bipolar and unipolar ionic diodes

Nanoporous ionic diodes, as well as devices for manipulating ions and molecules in a solution, have attracted a great deal of interest from researchers in various fields from the fundamental point of view. Ionic diodes allow the ions to be transported in one direction and block the transport in the other. There are two types of diodes that have been realized experimentally. A bipolar diode contains a junction between two zones of the pore walls with positive and negative surface charges. A unipolar diode contains a zone that is neutral and a zone that is charged. In this paper we discuss differences in operation of the diodes with a special emphasis on the sensitivity of their performance to the lengths of the charged and neutral zones. We also show that a bipolar diode offers more asymmetric current-voltage curves than a unipolar diode.

Rectification properties of conically shaped nanopores: consequences of miniaturization

Nanopores attracted a great deal of scientific interest as templates for biological sensors as well as model systems to understand transport phenomena at the nanoscale. The experimental and theoretical analysis of nanopores has been so far focused on understanding the effect of the pore opening diameter on ionic transport. In this article we present systematic studies on the dependence of ion transport properties on the pore length. Particular attention was given to the effect of ion current rectification exhibited in conically shaped nanopores with homogeneous surface charges. We found that reducing the length of conically shaped nanopores significantly lowered their ability to rectify ion current. However, rectification properties of short pores can be enhanced by tailoring the surface charge and the shape of the narrow opening. Furthermore we analyzed the relationship of the rectification behavior and ion selectivity for different pore lengths. All simulations were performed using MsSimPore, a software package for solving the Poisson-Nernst-Planck (PNP) equations. It is based on a novel finite element solver and allows for simulations up to surface charge densities of -2 e per nm(2). MsSimPore is based on 1D reduction of the PNP model, but allows for a direct treatment of the pore with bulk electrolyte reservoirs, a feature which was previously used in higher dimensional models only. MsSimPore includes these reservoirs in the calculations, a property especially important for short pores, where the ionic concentrations and the electric potential vary strongly inside the pore as well as in the regions next to the pore entrance.

DNA-Modified Polymer Pores Enable Ph- and Voltage-Gated Control of Channel Flux

Biological channels embedded in cell membranes regulate ionic transport by responding to external stimuli such as pH, voltage, and molecular binding. Mimicking the gating properties of these biological structures would be instrumental in the preparation of smart membranes used in biosensing, drug delivery, and ionic circuit construction. Here we present a new concept for building synthetic nanopores that can simultaneously respond to pH and transmembrane potential changes. These pores allow for the complete switching off of the channel flux as well as the ability to tune the preferred direction of ion current flow. DNA oligomers containing protonatable A and C bases are attached at the narrow opening of an asymmetrical track etched polymer nanopore. Lowering the pH to 5.5 causes the positively charged DNA molecules to bind to the negative backbone of other nearby strands. This creates an electrostatic mesh that closes the pore to unprecedentedly high resistances of several tens of gigaohms. In contrast, pores modified with DNA oligomers containing G and T bases did not show strong pH sensitivity. At neutral pH values, voltage switching causes the isolated DNA strands to undergo nanomechanical movement, as seen by a reversible current modulation. We provide evidence that the pH-dependent reversible closing mechanism is robust and applicable for nanopores with opening diameters of up to 14 nm. The concept of creating an electrostatic mesh should not be unique to DNA and may be applied to different organic polymers.

Ionic Liquids Transport through Single Nanopores

Room temperature ionic liquids are media consisting only of ions and are liquids at temperatures below 100 C. Constituent ions of ionic liquids are bulky and reach a size of 1 nm and larger. The finite size of the ions, which often is comparable to the diameter of the nanopores through which they get transported, makes application of classical electrochemical and electrostatic theories questionable. It is because the existing continuum theories for simplicity treat ions as point charges. A similar situation of a tight fit between transporting ions and pore diameter exists in the biological channels of a cell membrane.Using single nanopores prepared in polyethylene terephthalate (PET) by the track-etching technique, we investigated how ion current through nanopores and screening of surface charges on the pore walls were influenced by the size of the transported ions. Experiments were performed with aqueous solutions of KCl and the ionic liquids 1-Butyl-3-methylimidazolium methyl sulfate and 1-butyl-3-methylimidazolium chloride.The screening was evaluated using the rectification properties of homogeneously charged nanopores, as well as pores with a surface charge pattern that enables them to function as ionic diodes. Interactions of ions with surface charges were also described using reversal potential measurements in conjunction with Goldman Katz theory. Our experiments indicate that aqueous solution of ionic liquids screen surface charges over smaller distances than KCl.