Pavel Apel

Track etching technique in membrane technology

Abstract Track membrane (TM) technology is an example of industrial application of track etching technique. Track-etch membranes offer distinct advantages over conventional membranes due to their precisely determined structure. Their pore size, shape and density can be varied in a controllable manner so that a membrane with the required transport and retention characteristics can be produced. The use of heavy ion accelerators made it possible to vary LET of track-forming particles, angle distribution of pore channels and pore lengths. So far the track formation and etching process has been studied in much detail for several polymeric materials. Today we understand determining factors and have numerous empirical data enabling us to manufacture any particular product based on polyethylene terephthalate (PET) or polycarbonate (PC) films. Pore shape can be made cylindrical, conical, funnel-like, or cigar-like at will. A number of modification methods has been developed for creating TMs with special properties and functions. Applications of “conventional” track membranes can be categorized into three groups: process filtration, cell culture, and laboratory filtration. The use in biology stands out among other areas. Nuclear track pores find diverse applications as model systems and as templates for the synthesis of micro- and nanostructures.

Preparation of synthetic nanopores with transport properties analogous to biological channels

Abstract Conically shaped pores have been prepared in polyethylene terephthalate (PET) and polyimide foils by applying the track-etching technique. For this purpose, a thin polymer foil was penetrated by a single heavy ion (e.g. Au, Bi, U) of total kinetic energy of several hundred MeV to some GeV, followed by preferential chemical etching of the ion track. Asymmetric etching conditions allowed the preparation of charged pores of conical shape, similar to biological voltage-sensitive channels. The nanopores in PET and polyimide behave as ion current rectifiers where the preferential direction of the cation flow is from the narrow entrance towards the wide aperture of the pore. The PET pore shows voltage-dependent ion current fluctuations with opening and closing kinetics similar to voltage-gated biological ion channels. In contrast to PET, the polyimide nanopore exhibits a stable ion current signal. We discuss the possibility of using the synthetic nanopores as model for voltage-gated biochannels.

Pore structure and function of synthetic nanopores with fixed charges: tip shape and rectification properties.

We present a complete theoretical study of the relationship between the structure (tip shape and dimensions) and function (selectivity and rectification) of asymmetric nanopores on the basis of previous experimental studies. The theoretical model uses a continuum approach based on the Nernst-Planck equations. According to our results, the nanopore transport properties, such as current-voltage (I-V) characteristics, conductance, rectification ratio, and selectivity, are dictated mainly by the shape of the pore tip (we have distinguished bullet-like, conical, trumpet-like, and hybrid shapes) and the concentration of pore surface charges. As a consequence, the nanopore performance in practical applications will depend not only on the base and tip openings but also on the pore shape. In particular, we show that the pore opening dimensions estimated from the pore conductance can be very different, depending on the pore shape assumed. The results obtained can also be of practical relevance for the design of nanopores, nanopipettes, and nanoelectrodes, where the electrical interactions between the charges attached to the nanostructure and the mobile charges confined in the reduced volume of the inside solution dictate the device performance in practical applications. Because single tracks are the elementary building blocks for nanoporous membranes, the understanding and control of their individual properties should also be crucial in protein separation, water desalination, and bio-molecule detection using arrays of identical nanopores.

Ion transport through asymmetric nanopores prepared by ion track etching

Transport properties of single asymmetric nanopores in polyethylene terephthalate (PET) and polyimide (Kapton) membranes are investigated. The pores are produced by the track-etching technique based on irradiation of the polymer with heavy ions and subsequent chemical etching. Electrolytic conductivity measurements show that asymmetric pores in both polymeric materials rectify the ionic current. The PET and Kapton pores differ however significantly in their transient transport properties. The ion current through the PET nanopore fluctuates with the amplitudes reaching even 100% of the mean current, whereas nanopores in Kapton exhibit a stable current signal. We show that the transient properties of the pores depend on the chemical structure of the polymer as well as on the irradiation and etching procedures used in this work.

Versatile ultrathin nanoporous silicon nitride membranes

Single- and multiple-nanopore membranes are both highly interesting for biosensing and separation processes, as well as their ability to mimic biological membranes. The density of pores, their shape, and their surface chemistry are the key factors that determine membrane transport and separation capabilities. Here, we report silicon nitride (SiN) membranes with fully controlled porosity, pore geometry, and pore surface chemistry. An ultrathin freestanding SiN platform is described with conical or double-conical nanopores of diameters as small as several nanometers, prepared by the track-etching technique. This technique allows the membrane porosity to be tuned from one to billions of pores per square centimeter. We demonstrate the separation capabilities of these membranes by discrimination of dye and protein molecules based on their charge and size. This separation process is based on an electrostatic mechanism and operates in physiological electrolyte conditions. As we have also shown, the separation capabilities can be tuned by chemically modifying the pore walls. Compared with typical membranes with cylindrical pores, the conical and double-conical pores reported here allow for higher fluxes, a critical advantage in separation applications. In addition, the conical pore shape results in a shorter effective length, which gives advantages for single biomolecule detection applications such as nanopore-based DNA analysis.

Track size and track structure in polymer irradiated by heavy ions

The structure of latent tracks in polyethylene terephthalate (PET) was studied using chemical etching combined with a conductometric technique. Polymer samples were irradiated with Ar, Kr, Xe, Au, and U ions with energies in the range of 1 to 11.6 MeV/u. The etching kinetics of the tracks was investigated in the radii range 0–100 nm. The highly damaged track core manifests itself on the etching curves as a zone where the etch rate changes dramatically and reaches its minimum at a radius of a few nm. It was found that the track core radius is approximately proportional to (dE/dx)0.55. The track core is surrounded by a halo. In the track halo the etching proceeds at a rate that slowly increases approaching a constant value. Cross linking of macromolecules causes reduction of the etch rate in the halo which extends up to distances exceeding 100 nm in the case of the heaviest ions. Measurable change of the etch rate at such large radii could not be predicted from the shape of the calculated spatial distributions of energy dissipated in tracks. Obviously, formation of the extended track halo is influenced by the diffusion of active intermediates from the track core to the polymer bulk.

Swift ion effects in polymers: industrial applications

Abstract This paper is a review about methods of polymeric material modification based on the irradiation with accelerated heavy ions in the 1–10 MeV/u energy range. Chemical etching of ion tracks in polymers is a method which is widely used in the fabrication of micro- and nanostructures with pre-determined characteristics. Micro- and ultrafiltration membranes produced in this way and known as “track-etch membranes” have found several niches in the market since the seventies. This is an example of mature technology based on irradiation with swift ions. Apart from the membrane technology, the ion track pores find diverse applications as templates for the synthesis of micro- and nanowires and tubes, textured surfaces and bodies with special optical properties. Some recent achievements and promising ideas utilizing swift ion beams are presented.

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.

Tracks of very heavy ions in polymers

Abstract A comparative study of latent and etched track parameters in various polymers is performed with the emphasis on the tracks of very heavy particles such as 238U ions in the energy range of 1–11.6 MeV/u. Samples of polyethylene terephthalate (PET), polypropylene (PP) and polysulphone (PSU) films were irradiated with heavy ions of various masses. The etching kinetics of the tracks in PET were investigated by a conductometric technique. The sizes of a highly damaged “track core” and a cross-linked “halo” were derived from the kinetics of the etching curves for the ions of various masses. The ratio of track to bulk etch rate, V, was determined as a function of the ion energy loss, d E d x . In the case of PP a distinct maximum of the V( d E d x ) function at d E d x = 7 keV/nm was observed. No profound increase of V was observed when passing from Xe to U tracks in both PET and PSU. Obviously, at very high energy losses the destruction and construction processes coexist and the relative role of construction increases at a certain level of d E d x . The experimental data seem to indicate that accelerated ions of moderate masses are favoured for the production of structures with a high aspect ratio.