Ivan Vlassiouk

Tuning transport properties of nanofluidic devices with local charge inversion.

Nanotubes can selectively conduct ions across membranes to make ionic devices with transport characteristics similar to biological ion channels and semiconductor electron devices. Depending on the surface charge profile of the nanopore, ohmic resistors, rectifiers, and diodes can be made. Here we show that a uniformly charged conical nanopore can have all these transport properties by changing the ion species and their concentrations on each side of the membrane. Moreover, the cation versus anion selectivity of the pores can be changed. We find that polyvalent cations like Ca(2+) and the trivalent cobalt sepulchrate produce localized charge inversion to change the effective pore surface charge profile from negative to positive. These effects are reversible so that the transport and selectivity characteristics of ionic devices can be tuned, much as the gate voltage tunes the properties of a semiconductor.

Control of ionic transport through gated single conical nanopores

AbstractControl of ionic transport through nanoporous systems is a topic of scientific interest for the ability to create new devices that are applicable for ions and molecules in water solutions. We show the preparation of an ionic transistor based on single conical nanopores in polymer films with an insulated gold thin film “gate.” By changing the electric potential applied to the “gate,” the current through the device can be changed from the rectifying behavior of a typical conical nanopore to the almost linear behavior seen in cylindrical nanopores. The mechanism for this change in transport behavior is thought to be the enhancement of concentration polarization induced by the gate.n Figurexa0

Nanoprecipitation-assisted ion current oscillations

Nanoscale pores exhibit transport properties that are not seen in micrometre-scale pores, such as increased ionic concentrations inside the pore relative to the bulk solution, ionic selectivity and ionic rectification. These nanoscale effects are all caused by the presence of permanent surface charges on the walls of the pore. Here we report a new phenomenon in which the addition of small amounts of divalent cations to a buffered monovalent ionic solution results in an oscillating ionic current through a conical nanopore. This behaviour is caused by the transient formation and redissolution of nanoprecipitates, which temporarily block the ionic current through the pore. The frequency and character of ionic current instabilities are regulated by the potential across the membrane and the chemistry of the precipitate. We discuss how oscillating nanopores could be used as model systems for studying nonlinear electrochemical processes and the early stages of crystallization in sub-femtolitre volumes. Such nanopore systems might also form the basis for a stochastic sensor.

Hydrothermally shrunk alumina nanopores and their application to DNA sensing

Hydrothermal treatment of anodized alumina membranes has been known for years and is believed to seal the pores by transforming aluminium oxide into lower density hydroxides. We demonstrate that, at least for 60 nm diameter pores grown from anodization in oxalic acid at 40 V, the hydrothermal treatment significantly shrinks but does not fully seal the nanopores. The pores shrink to a neck of less than 10 nm in diameter and 2-4 microm in length, in which the diffusion coefficient of ions is five orders of magnitude smaller than in the bulk. Because of a high electrolyte resistance through hydrothermally treated shrunken nanopores, they can be used for electrical sensing applications, as demonstrated using the example of DNA sensing. Hybridization of target DNA with a complementary ssDNA covalently immobilized inside the nanopores causes an increase in impedance by more than 50% while a noncomplementary ssDNA has no measurable effect.