B. Sartowska

Effect of nanopore geometry on ion current rectification

We present the results of systematic studies of ion current rectification performed on artificial asymmetric nanopores with different geometries and dimensions. The nanopores are fabricated by the ion track etching method using surfactant-doped alkaline solutions. By varying the alkali concentration in the etchant and the etching time, control over the pore profile and dimensions is achieved. The pore geometry is characterized in detail using field-emission scanning electron microscopy. The dependence of the ion current rectification ratio on the pore length, tip diameter, and the degree of pore taper is analysed. The experimental data are compared to the calculations based on the Poisson-Nernst-Planck equations. A strong effect of the tip geometry on the diode-like behaviour is confirmed.

Fabrication of nanopores in polymer foils with surfactant-controlled longitudinal profiles

We present a surfactant-controlled etching method which allows the production of asymmetric track-etched nanopore membranes with diode-like ionic conductivity. The asymmetry of the pores is provided by self-assembly of amphiphilic molecules at the pore entrances on one side of the membrane during chemical etching while this process is excluded on the other side. By varying the alkali concentration in the etchant, control over the pore profile is achieved. The pore geometry is characterized in detail using field-emission scanning electron microscopy. The method is equally applicable to membranes with many and with single pores and may constitute an alternative to existing methods of production of resistive-pulse sensors.

Asymmetric ion track nanopores for sensor technology. Reconstruction of pore profile from conductometric measurements.

We reconstruct the profile of asymmetric ion track nanopores from an algorithm developed for conductometric measurements of symmetric nanopores. The validity of the reconstruction is supported by FESEM observations. Our analysis reveals that asymmetric pores fabricated by one-sided etching are funnel-like and not conical. The analysis provides the constriction diameter and the pore profile as a function of etching time. The reconstruction of the pore profile defines the starting conditions of asymmetric nanopores at breakthrough. The deviation from the conical shape is most pronounced at the pore tip. This critical zone dominates transport properties relevant to ion conductance, selectivity, current rectification, resistive pulse sensing and biosensors. The classical cone approximation used until now underestimates the tip diameter by a factor of two. As transport processes in nanopores depend in a highly nonlinear way on the constriction diameter the presented reconstruction must be taken into account when studying ionic and molecular transport processes in asymmetric pores.

Preparation of porous polymer samples for SEM: combination of photo oxidation degradation with a freeze fracture technique

A new method for the preparation of porous polymer samples and investigations of their structure by scanning electron microscopy (SEM) is described. The technique for cleavage preparation is complemented by a preliminary treatment of a polymer with photo oxidation degradation to render it brittle. The advantages of this technique have been demonstrated with polyethylene tere-phthalate (PET) track membranes. The true size and shape of pores in the bulk sample can be seen in the cleavage plane. The degradation procedure does not change the membrane morphology. At the same time it allows one to reveal the characteristic features of the porous structure without its modification. Clear pictures of cross-sections of track membranes with cylindrical and bow-tie-like pore channels are presented.

Analysis of channel shapes in track membranes by scanning electron microscopy

Control over pore geometry opens the way to a number of new applications of track‐etch membranes (TMs). A special method of etching was developed to produce TMs with non‐cylindrical pore profile. The direct observation of channel shape on fractures of track membranes was performed with a scanning electron microscope (SEM). The SEM images of the surface and cross‐section of TMs with different pore morphology are shown. The channel diameter as a function of the depth below surface was measured and quantitative analysis was realized.

New methods of track membrane treatment in the preparation of samples for further observation with scanning electron microscopy

Track membranes are porous systems consisting of a polymer foil with thin channels (i.e. pores) piercing it from surface to surface. The creation of non‐cylindrical pores in a track membrane is important for the optimization of membrane characteristics, i.e. the highest productivity at the required selectivity. A new method of cleavage preparation (the irradiation of track membrane samples with accelerated electrons) for the observation of channel shapes directly in the membrane cross‐sections is presented. Diagrams showing the tensile and burst strengths as a function of the irradiation dose, and images of surfaces and cleavages of track membrane samples are presented in this work. The changes in the pore sizes and shapes along the channel were clearly seen. These results can be used for the optimization of track membrane production.

Asymmetrical nanopores in track membranes: Fabrication, the effect of nanopore shape and electric charge of pore walls, promising applications

The properties of asymmetrical nanopores prepared by chemical etching of tracks of accelerated heavy ions are studied. Procedures are developed for controlling the size and shape of pores within wide limits. The presence of charged functional groups on pore walls is an intrinsic property of track membranes, which makes them a convenient object for studying electrokinetic phenomena in nanocapillaries. In electrolyte solutions, the asymmetrical “track” membranes demonstrate the diode effect. Two methods for fabricating asymmetrical nanopores in polyethylene terephthalate films are proposed and introduced into practice. Specific features of both methods, their advantages and drawbacks are considered. In addition to the brief survey of available information on diode-like track membranes, the new results on the mechanism of pore formation and the peculiarities of their geometry and electrokinetic properties are discussed. The emerging and potential applications of track membranes with asymmetrical pores are discussed briefly.