Comment on 'Modeling the conductance and DNA blockade of solid-state nanopores'.

Recently, Kowalczyk et al (2011 Nanotechnology 22 315101) introduced a model for calculating the conductance of an hourglass-shaped nanopore, and the conductance change when DNA is introduced into such a pore. Here, we comment on their model in the context of other literature addressing the same general problem. Existing work includes widespread use of an alternative model, contradicts the authors' claim that the conductance change should be expected not to vary with pore size, and has frequently utilised access resistance.

Use of tunable nanopore blockade rates to investigate colloidal dispersions.

Tunable nanopores fabricated in elastomeric membranes have been used to study the dependence of ionic current blockade rate on the concentration and electrophoretic mobility of particles in aqueous suspensions. A range of nanoparticle sizes, materials and surface functionalities has been tested. Using pressure-driven flow through a pore, the blockade rate for 100 nm carboxylated polystyrene particles was found to be linearly proportional to both transmembrane pressure (between 0 and 1.8 kPa) and particle concentration (between 7 × 10(8) and 4.5 × 10(10) ml( - 1)). This result can be accurately modelled using Nernst-Planck transport theory, enabling measurement of particle concentrations. Using only an applied potential across a pore, the blockade rates for carboxylic acid and amine coated 500 and 200 nm silica particles were found to correspond to changes in their mobility as a function of the solution pH. Scanning electron microscopy and confocal microscopy have been used to visualize changes in the tunable nanopore geometry in three dimensions as a function of applied mechanical strain. The pores were conical in shape, and changes in pore size were consistent with ionic current measurements. A zone of inelastic deformation adjacent to the pore has been identified as important in the tuning process.

Reversible mechanical actuation of elastomeric nanopores.

Mechanical resizing of individual nanopores in a thermoplastic polyurethane elastomer has been characterized. Specimen nanopores were conical, with smaller hole dimensions of the order of tens to hundreds of nanometres. Electrophoretic current measurements show that the estimated nanopore radius can be reversibly actuated over an order of magnitude by stretching and relaxing the elastomer. Within a working range of stretching, current is proportional to specimen extension to the power of a constant, n, which ranges from 0.9 to 2.3 for different specimens. The data indicate that scaling of the effective pore radius is super-affine. At strains below the working range, the pore size is relatively unresponsive to stretching. Macroscopic elastomer extension has been related to local radial strain (50-250 µm from the pore) using optical microscopy. Scanning electron microscopy and atomic force microscopy have been used to observe membrane surface features.

Measurement of Newtonian fluid slip using a torsional ultrasonic oscillator.

The composite torsional ultrasonic oscillator, a versatile experimental system, can be used to investigate slip of a Newtonian fluid at a smooth surface. A rigorous analysis of slip-dependent damping for the oscillator is presented. Initially, the phenomenon of finite surface slip and the slip length are considered for a half space of Newtonian fluid in contact with a smooth, oscillating solid surface. Definitions are reconsidered and clarified in light of inconsistencies in the literature. We point out that, in general oscillating flows, Navier's slip length b is a complex number. An intuitive velocity discontinuity parameter of unrestricted phase is used to describe the effect of slip on measurement of viscous shear damping. The analysis is applied to the composite oscillator, and preliminary experimental work for a 40 kHz oscillator is presented. The nonslip boundary condition has been verified for a hydrophobic surface in water to within approximately 60 nm of |b|=0 nm . Experiments were carried out at shear rate amplitudes between 230 and 6800 s(-1), corresponding to linear displacement amplitudes between 3.2 and 96 nm.