Reimar Spohr

Diode-like single-ion track membrane prepared by electro-stopping

The prepn. of an asym. membrane in poly(ethylene terephthalate) (PET) is described, using a combination of chem. and electro-stopping. For this purpose, a single-ion-irradiated PET film is inserted ...

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.

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.

Pore geometry of etched ion tracks in polyimide

Abstract Tracks of energetic heavy ions in the polyimide Kapton were etched in a NaOCl solution. It was found that the pH value of the etchant plays a crucial role for the selectivity of track etching. The bulk etching rate νb increased exponentially with pH. From the temperature dependence of the bulk etching rate an activation energy of Ea = 0.74 eV was deduced. In contrast to νb, only a slight and linear increase with pH value was observed for the track etching rate νt. As a consequence, the etch ratio ν t ν b can be adjusted over one order of magnitude by controlled varying the pH of the etchant. This is a new way to tailor the pore geometry from nearly cylindrical to funnel-shaped pores.

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.

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.

Template synthesis and magnetoresistance property of Ni and Co single nanowires electrodeposited into nanopores with a wide range of aspect ratios

Template synthesis and magnetoresistance property of Ni and Co single nanowires electrodeposited into nano-pores with wide range of aspect ratios

Thermal control of drug release by a responsive ion track membrane observed by radio tracer flow dialysis

The combination of a responsive hydrogel with a rigid porous supporting structure yield a membrane with high mechanical strength and high on-off-permeability ratio. A membrane consisting of an ion track filter with a thermally responsive lining was prepared by penetrating a 19 micron thick foil of poly(ethylene terephthalate) (PET) with swift heavy ions at a fluence of 5 x 10(5) ions/cm2, followed by etching of the ion tracks to generate an ion track filter with 2.9 micron pore diameter, onto which a thin layer of poly(N-isopropylacrylamide) (NIPAAm) hydrogel was grafted. It was revealed that the mass flow of various molecules (water, chloride-, choline+, insulin, and albumin) through the membrane could be thermally controlled. The on-off-permeability ratio ranged between 3 and 10 increasing with molecular weight. Over a storage time of 5 months the permeabilities varied up to a factor of 2.6, while the on-off-permeability ratio and temperature sensitivity remained practically constant.

Electrolyte transport in charged single ion track capillaries

Abstract The pH and concentration dependence of the electric current through etched single ion tracks in poly (ethyleneterephthalate) (PET) membranes is investigated using KCl solutions. The pH dependence of the current is attributed to the dissociation of the immobile carboxylic end groups attached to the PET leading to charged pore walls. As consequence a counterion layer near the pore walls is formed with increased conductivity. At low KCl concentrations surface conductivity can be several orders of magnitude above the regular volume conductivity.

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.