Track-etched Polyethylene Terephthalate Research Articles

Fabrication of Two-Layer Microfluidic Devices with Porous Electrodes Using Printed Sacrificial Layers.

Two-layer microfluidic devices with porous membranes have been widely used in bioapplications such as microphysiological systems (MPS). Porous electrodes, instead of membranes, have recently been incorporated into devices for electrochemical cell analysis. Generally, microfluidic channels are prepared using soft lithography and assembled into two-layer microfluidic devices. In addition to soft lithography, three-dimensional (3D) printing has been widely used for the direct fabrication of microfluidic devices because of its high flexibility. However, this technique has not yet been applied to the fabrication of two-layer microfluidic devices with porous electrodes. This paper proposes a novel fabrication process for this type of device. In brief, Pluronic F-127 ink was three-dimensionally printed in the form of sacrificial layers. A porous Au electrode, fabricated by sputtering Au on track-etched polyethylene terephthalate membranes, was placed between the top and bottom sacrificial layers. After covering with polydimethylsiloxane, the sacrificial layers were removed by flushing with a cold solution. To the best of our knowledge, this is the first report on the sacrificial approach-based fabrication of two-layer microfluidic devices with a porous electrode. Furthermore, the device was used for electrochemical assays of serotonin and could successfully measure concentrations up to 5 µM. In the future, this device can be used for MPS applications.

Modification of Track-Etched Polyethylene Terephthalate Membranes with Functionalized Silanes for Immobilizing Silver Nanoparticles

Modification of Track-Etched Polyethylene Terephthalate Membranes with Functionalized Silanes for Immobilizing Silver Nanoparticles

Bioinspired Dual-Responsive Nanofluidic Diodes by Poly-l-lysine Modification.

A smart nanofluidic device attracts attention as it enables to control the physicochemical properties and transportation phenomena, by using stimuli-responsive materials. This work reports a bioinspired modification of a conical ion track-etched polyethylene terephthalate nanopore surface by coating a layer of poly-l-lysine (PLL), which is a commonly used coating in biotechnology to achieve a dual-responsive nanofluidic channel by pH or temperature. The rectification of ionic transportation can be reversed by assembling PLL because of the change of surface bonds from the carboxyl to amine group. The PLL-modified nanopore becomes nonconductive as an “OFF” state at pH 11.5 and at a temperature of 70 °C in solution. The ionic transport in nanopores can be switched to the “ON” (conductive) state, by either decreasing pH or temperature. The transitions between “ON” and “OFF” states present excellent reversibility, which make the PLL-modified nanopores a promising smart nanofluidic device that can be used for drug delivery or biomimic ion/mass transport in future, besides the good biocompatibility and ease of use of PLL modification.

Selective-area ALD for positively and negatively charged layers into the ion-beam track-etched conical pores in polyethylene terephthalate

This paper presents the strategy of selective-area growing of a positively-charged layer of Al2O3 and the negatively-charged layer of HfO2 on ion beam track-etched polyethylene terephthalate (PET) nanotubes through the thermal atomic layer deposition (T-ALD) technique. We used the self-assembled monolayer of octadecyl trichlorosilane (OTS-SAMs) on the surface to serve as a passivation layer and then selectively deposited the Al2O3 and HfO2 in the nanotubes. The influence of the dipping time of the substrates in the OTS solution and the experimental conditions on the roughness and the thickness of the OTS monolayer have been investigated. X-ray photoelectron spectroscopy (XPS) was used to analyze the composition of the ALD Al2O3 and HfO2 films. Atomic force microscope (AFM) and scanning electron microscope (SEM) were employed to study the morphologies before and after the ALD of Al2O3 and HfO2. The I-V characteristics of the film confirmed the surface charge polarities in the nanotubes, i.e. the positively-charged Al2O3 and negatively-charged HfO2, in the electrically-neutral solution. The results will aid surface modification and functionalization of PET by nanotubes.

Microfiltration membrane characterization by gas-liquid displacement porometry: Matching experimental pore number distribution with liquid permeability and bulk porosity

The variable viscosity Poiseuille model (VVPM), developed recently to treat gas-liquid displacement (GLD) porometry data for track-etched polyethylene terephthalate (PET) membranes, is applied to polyethersulfone (PES) and polypropylene (PP) microfiltration membranes prepared by phase inversion. It is found that in general aspects the pore size distribution as estimated by the model, with the intrinsic assumption of isolated capillary pores, perfectly reproduces wet and dry fluxes during porometry measurements, but the estimated porosity appeared to be much higher than unity. To eliminate the discrepancy, two additional parameters, namely non-uniformity and tortuosity coefficient, have been introduced in the flux and porosity estimating equations. The apparent viscosity of the gas in porometry analysis is found to be several fold higher than that available in literature, and also the viscosity range appeared to be membrane type dependent. The pore size distribution curve shifts along the diameter axis depending on the gas flow direction applied in porometry data acquisition. However, because the membranes had narrow pore size disribution, the estimated parameters had been averaged within acceptable confidence range. With the inclusion of non-uniformity and tortuosity of the capillaries, the revised version of the VVPM made an advancement in porometry data analysis of membranes with both isolated and interconnected pore structure; it characterizes the assumed model capillaries in terms of Young-Laplace pore diameter, but also non-uniformity and tortuosity of pores. The procedure for data treatment has been illustrated in detail and the method could easily be adapted to gas-liquid displacement porometer software system for regular data treatment. Based on a set of readily available experimental data, including the typical output from standard GLD porometry, the here established extended version of the VVPM allows the step-by-step determination of all the parameters which characterize the pore structure of the membranes. Also the absolute pore number density distributions can be obtained, including estimates of additional pore characteristics such as non-uniformity and tortuosity.

Modification of Track-Etched PET Membranes by Graft Copolymerization of Acrylic Acid and N-Vinylimidazole

Photoinduced graft polymerization of N-vinylimidazole (VI) and graft copolymerization of N-vinylimidazole with acrylic acid (AA) onto track-etched polyethylene terephthalate membranes (PET TeMs) with a pore diameter of 450 ± 15 nm have been systematically studied. The effect of different parameters, such as irradiation time, monomer concentration, and solvent nature, has been examined. The samples have been characterized using the instrumental techniques of FTIR and XPS spectroscopies, thermogravimetry, and scanning electron microscopy. The copolymer composition has been determined spectrophotometrically using the Toluidine Blue and Acid Orange dyes. Optimal conditions of AA and VI copolymerization have been found. It has been shown that the surface of modified PET TeMs is reach in functional groups which makes them a promising material for the development of nanosensors and nanocatalysts.

Structure and phase composition of thin TiO2 films grown on the surface of metallized track-etched polyethylene terephthalate membranes by reactive magnetron sputtering

We have studied TiO2, Ag, Ag/TiO2, and Cu/TiO2 coatings grown on track-etched polyethylene terephthalate membranes. The metals and oxides were deposited by reactive vacuum sputtering using a planar magnetron. The microstructure of the samples were examined by scanning and transmission electron microscopy techniques. The elemental composition of the coatings were determined by energy dispersive X-ray microanalysis, and their phase composition was determined by X-ray diffraction at different temperatures and by transmission electron diffraction. Titanium dioxide can be present on the surface of track-etched membranes (TMs) in three forms: nanocrystals of tetragonal anatase with orthorhombic brookite and amorphous TiO2 impurities. The copper-metallized TM has been shown to contain cubic Cu2O. The optical properties of the composite membranes and films have been studied by absorption spectroscopy. The energies of direct and indirect allowed optical transitions have been evaluated from measured absorption spectra of the TiO2, Ag/TiO2, and Cu/TiO2 coatings.

Capillary pore membranes with grafted diblock copolymers showing reversibly changing ultrafiltration properties with independent response to ions and temperature

Track-etched polyethylene terephthalate membranes with a pore size of 110nm have been modified by a grafted diblock copolymer structure in order to obtain dual-responsive ultrafiltration membranes. The temperature-responsive poly-N-isopropylacrylamide (PNIPAAm) and the ion-responsive poly-N,N-dimethyl-N-methacryloyloxyethyl-N-(3-sulfopropyl) ammonium betaine (PSPE) were grafted from the pore walls by sequential surface-initiated atom transfer radical polymerization. To achieve a well controlled pore functionalization, the polymerization rate had been adjusted to a very slow chain growth. A successful grafting of PSPE as first block and of PNIPAAm as second block could be demonstrated. To study the temperature and ion responsivity, hydraulic permeability and dextran diffusion experiments at 25°C and 40°C with different salt solutions had been carried out. It could be shown that such membranes can change their barrier pore size from a more open to a more closed state in dependency of temperature as well as kind of ions and their concentration in the feed. The grafted layer thickness showed a particularly strong increase in the presence of KClO4. Like expected for a well defined polymer brush, an expanded or collapsed state for each of the individual blocks could be obtained. In presence of KClO4 at 25°C both blocks are expanded and the pores are in a more closed state. If one of these stimuli is changed, for example when temperature is increased to 40°C or ions are removed, the block copolymer brushes collapse partially what leads to more open pores. However, if both stimuli are applied together, i.e. if temperature is 40°C and no ions are in solution, diblock copolymers collapse fully what leads to fully opened pores. It had also been demonstrated that this dual responsivity and the magnitude of the effects depend very strongly on the absolute degree of grafting and the ratio between the two different responsive polymer blocks. Furthermore, the ability of changing effective membrane barrier pore size by ions and temperature has been investigated in more detail by dextran diffusion experiments, and very pronounced and reversible changes of molecular sieving curve and molecular weight cut-off could be obtained.

Anomalous Transit Time and Pulse Amplitude of Highly Charged Particles in Resistive Pulsing

With resistive pulse technology, three types of particles with similar size but different surface charge densities have been investigated in track-etched polyethylene terephthalate (PET) single micropores. We have shown that pores with axially undulating diameter constitute a sensitive tool for spherical particles’ detection and sizing. Shape of resistive pulses carries information on the shape and charge of passing particles, as well as shape and charge of pores. Passage of single particles causes a decrease of the recorded current whose amplitude is correlated with the particle volume. However, our experiments have revealed, the blockage current of the strongly charged particles can be significantly larger than those from the other two types of particles and this observation cannot be described by the classical resistive-pulse model. This finding is also in contrast to earlier reports showing that pulse amplitude created by a charged molecule/particle is smaller compared to an uncharged particle of the same volume. Our results are explained by the distortion of the electrical double layer adjacent to the charged particle, which create a region with depleted ionic concentrations. We have also found that the particle passage time results from an interplay of three effects: (i) ion condensation, (ii) formation of an asymmetric electrical double layer around the particle, and (iii) electroosmotic flow induced by the charges on the pore walls and the particle surfaces. Our results are important for applying resistive-pulse technique to determine surface charge density and zeta potential of the particles. This research has implications for separation between particles with different surface charge properties, particle detection and sizing, and for the fundamental understanding of fluid flow at the meso-scale.

Structural and physicochemical properties of titanium dioxide thin films obtained by reactive magnetron sputtering, on the surface of track-etched membranes

Titanium dioxide thin films have been prepared by reactive sputtering onto the surface of tracketched polyethylene terephthalate membranes using an inverted dc magnetron, and their structural and physicochemical properties have been examined. A silver film applied onto the membrane surface by physical vapor deposition has been used to protect the polymer from the photocatalytic action of titanium dioxide. The structural characteristics of the composite membranes have been studied by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, spectrophotometry, and atomic force microscopy. The effects of photoinduced surface hydrophilization and catalytic decomposition of the dye rhodamine 6G have been investigated.

Investigation of self-assembled protein dimers through an artificial ion channel for DNA sensing

Artificial nanopores have become promising tools for sensing DNA. Here, we report a new technique for sensing DNA through a conical-shaped nanopore embedded in track-etched polyethylene terephthalate (PET) membrane. Two different streptavidin-conjugated monovalent DNA probes were prepared that can bind to two distinct segments (at either end) of the target DNA. The size of target DNA-linked to the two streptavidin-conjugated monovalent DNA probes is double that of the individual probes. By precisely controlling the tip diameter of the conical nanopore embedded in the PET polymer, events due to the translocation of the streptavidin-conjugated monovalent DNA probes through the nanopore can be filtered and purposely undetected, whereas the current pulses due to the translocation of the target DNA-induced self-assembled complexes can be detected. The two streptavidin-conjugated DNA probes cannot be linked by multi-mismatched DNA. Therefore, multi-mismatched (non-specific) DNA will not induce any current pulse signatures. The current pulse signatures for the self-assembled complex can be used to confirm the presence of the target DNA. The size-dependent detection of self-assembled complexes on the molecular level shows strong promise for the detection of biomolecules without interference from the probes.

Magnetic Fe3O4 nanoparticle heaters in smart porous membrane valves.

The application of magnetic nanoparticles (NPs) in membrane technology is still very new and combinations of such NPs and separation membranes are essentially unexplored. By the integration of NPs into polymeric membranes it is possible to create new functionalities based on the synergies of the materials. Here nanoparticle polymer hybrid membranes were created by immobilizing (super)paramagnetic Fe3O4 NPs on the walls of track-etched polyethylene terephthalate pores and further functionalization with poly(N-isopropylacryl amide) (PNIPAAm). The hybrid system can be controlled via local heat generation by the NPs induced by an external high frequency electromagnetic field, i.e., effective membrane pore size can be switched by the magnetic field due to the synergy between the nanoparticle functionality and the temperature-responsive properties of PNIPAAm. Analytical characterizations showed a successful and stable NP integration and the feasibility of functionalization with grafted PNIPAAm in the presence of immobilized NPs on the membrane pore surface. The valve function of the nanoparticle polymer hybrid materials with an external control via a high frequency electromagnetic field was demonstrated in water permeability experiments, but such systems will have significant potential for other applications such as drug release or mass separation.

Antibody-Imprinted Membrane Adsorber via Two-Step Surface Grafting

In this work, a recently established, novel two-step imprinting strategy combining surface imprinting and scaffold imprinting was applied successfully to prepare a molecularly imprinted polymer (MIP) adsorber for immunoglobulin G (IgG). Track-etched polyethylene terephthalate (PET) membranes with previously introduced aliphatic C-Br groups as initiator on the pore surface were used to prepare first a functional polymer scaffold, grafted poly(methacrylic acid), via surface-initiated atom transfer radical polymerization (SI-ATRP). After template protein (IgG) binding to the scaffold, UV-initiated cross-linking copolymerization of acrylamide and methylenebisacrylamide (MBAA) as second step lead to a grafted MIP hydrogel layer. The influences of the three independent parameters, scaffold chain length by SI-ATRP time, degree of cross-linking of the MIP layer by MBAA content, and grafted MIP layer thickness by UV irradiation time, were studied to optimize protein binding capacity and selectivity. The results were also compared to previously obtained data for lysozyme imprinting using the same method, and significant effects of protein size on imprinting efficiency could be identified. The best IgG MIP membrane adsorber was then used to separate IgG from mixtures with human serum albumin (HSA), demonstrating IgG binding capacities and eluted IgG purities, which were almost independent of the excess of HSA. The results of this study are a significant extension of the scope of molecular imprinting toward large target bionanoparticles. The transfer of the approach from the model PET to other base membranes with higher specific surface area is straightforward, and the resulting affinity materials would, in principle, be suited for "capturing" of an antibody from a complex mixture.

Protein-selective adsorbers by molecular imprinting via a novel two-step surface grafting method.

Molecularly imprinted polymers (MIP) offer in principle a robust, cost-efficient alternative to antibodies, but it is still a challenge to develop such materials for protein recognition. Here, we report the molecular imprinting of a functional polymeric hydrogel layer with lysozyme as the template in a two-step grafting procedure by a novel initiation approach on track-etched polyethylene terephthalate membrane surface. This is based on surface functionalization with aliphatic C-Br groups which can be used as an initiator for surface-initiated atom transfer radical polymerization (SI-ATRP) and photo-initiated copolymerization. At first, the scaffold poly(methacrylic acid) (PMAA) was obtained through SI-ATRP of poly(tert-butyl methacrylate) and subsequent hydrolysis. Thereafter, it was assembled with the template to form a stable PMAA/lysozyme complex. In the second step, a polyacrylamide (PAAm) hydrogel was synthesized via UV-initiated surface grafting/crosslinking copolymerization around the scaffold/protein complex. Finally, the template was eluted to yield the grafted hydrogel layer with binding sites having complementary size, shape and appropriate arrangement of the functional groups to rebind lysozyme. The selectivity of lysozyme recognition, relative to cytochrome C with a similar size and isoelectric point, was increased by optimization of the scaffold chain length, UV grafting/crosslinking time and the chemical crosslinking degree of the PAAm-based hydrogel. The feasibility for the development of protein MIP in a straightforward way by independent optimization of crucial parameters - structures of scaffold with functional groups and of the crosslinked hydrogel matrix - have been demonstrated.

Polyethylene terephthalate membrane grafted with peptidomimetics: endothelial cell compatibility and retention under shear stress

The present work aimed to treat a polyethylene terephthalate (PET) surface to make the biomaterial more ‘attractive’ in terms of attachment and shear stress response to endothelial cells with a view to possible applications in vascular grafting. A surface wet-chemistry protocol was applied to graft track-etched PET membranes with RGD peptidomimetics based on the tyrosine template and active at the nano-level vs. isolated human αvβ3 receptor, which was monitored by X-ray photoelectron spectroscopy, contact angle measurement and atomic force microscopy for characterization. A primary culture of human saphenous vein endothelial cells was used before and after sterilization of the membranes (heat treatment or γ-ray irradiation) to test the benefit of grafting. The optimal surface concentrations of grafted molecules were around 50 pmol/cm². Compared to GRGDS, the peptidomimetics promoted cell attachment with similar or slightly better performances. Endothelialized grafted supports were further exposed to 2 h of shear stress mimicking arterial conditions. Cells were lost on non-grafted PET whereas cells on grafted polymers sterilized by γ-ray irradiation withstood forces with no significant difference in focal contacts. At the mRNA level, cells on functionalized PET were able to respond to shear stress with NFkB upregulation. Thus, grafting of peptidomimetics as ligands of the αvβ3 integrin could be a relevant strategy to improve the adhesion of human endothelial cells and to obtain an efficient endothelialized PET for the surgery of small-diameter vascular prostheses.

Polystyrene Beads as a Model System for Virus Particles Reveal Pore Substructure as they Translocate

Our research focuses on the extension of the resistive-pulse or Coulter counter technique to detect and analyze virus particles. Using polystyrene spheres as a model system, we show that particles can be distinguished not only by their size, but also by their charge, as they translocate through track-etched polyethylene terephthalate (PET) pores. In addition, we discovered that as a polystyrene sphere translocates, it reveals variations in the pore structure along its length as a series of peaks and valleys in the measured ionic current. This sequence of peaks and valleys is unique to a particular pore, but for that pore, we observed the same sequence for thousands of particle translocations, even particles of different size or with different charge. Also, particles translocating in the reverse direction through the pore give the reverse sequence of peaks and valleys in the ionic current.We analyzed this data to extract information about the particle velocity as it passes through the pore and examined how this effect could enhance virus sensing capabilities. For example, due to the unique pattern of peaks and valleys, it is possible to unambiguously detect when two particles are in the pore at the same time. In addition, we expect that this characteristic pattern will be modulated by the particle geometry. Therefore, it will likely be possible to distinguish between particles of the same size but different shapes. After these model system studies, we plan to test our system with actual virus particles.

Effect of Orientation Stretching on the Geometric Shape of Track-Etched Polyethylene Terephthalate Membranes

This article looks at the effect of orientation stretching of irradiated polyethylene terephthalate films on the shape of the pores formed during chemical pore etching. Shows that orientation stretching enables latent tracks in the polymer to be stretched, which gives rise to track membranes with elliptical pores. Examines the effect of deformation of irradiated polymer on the structure and physical/mechanical characteristics of the track membranes formed. It is assumed that the change in the polymer structure resulting from heavy-ion irradiation may be linked to local heating at the locations where the charged particles pass.

Correlation of membrane structure and transport activity using combined scanning electrochemical–atomic force microscopy

The iontophoretic transport of Fe ( CN ) 6 4 - across a synthetic track-etched polyethylene terephthalate membrane has been investigated using combined scanning electrochemical–atomic force microscopy (SECM–AFM). This approach allows local fluxes, measured by amperometrically detecting Fe ( CN ) 6 4 - emerging across the membrane, to be related to the corresponding membrane topography. By comparing AFM and electrochemical images, only a fraction of candidate pore sites – identified topographically – are found to be active to transport. The ability to characterise the dimensions of the active pores allows quantitative analysis of electrochemical data. In contrast, conventional SECM alone does not allow a structural interpretation of measured fluxes, and the spatial resolution is poorer, due to the need to employ greater tip to surface separations and often larger tips.

Fluctuation of Surface Charge in Membrane Pores

Surface charge in track-etched polyethylene terephthalate (PET) membranes with narrow pores has been probed with a fluorescent cationic dye (3,3′-diethyloxacarbocyanine iodide (diO-C 2-(3))) using confocal microscopy. Staining of negatively charged PET membranes with diO-C 2-(3) is a useful measure of surface charge for the following reasons: 1) the dye inhibits K + currents through the pores and reduces their selectivity for cations; 2) it inhibits [ 3H]-choline + transport and promotes 36Cl − transport across the membrane in a pH- and ionic-strength-dependent fashion; and 3) staining of pores by diO-C 2-(3) is reduced by low pH and by the presence of divalent cations such as Ca 2+ and Zn 2+. Measurement of the time dependence of cyanine staining of pores shows fluctuations of fluorescence intensity that occur on the same time scale as do fluctuations of ionic current in such pores. These data support our earlier proposal that fluctuations in ionic current across pores in synthetic and biological membranes reflect fluctuations in the surface charge of the pore walls in addition to molecular changes in pore proteins.

Diffusion through Narrow Pores: Movement of Ions, Water and Nonelectrolytes through Track-etched PETP Membranes

The rates at which ions (86Rb+, [3H]-choline, 36Cl), 3H2O and nonelectrolytes ([14C]-urea, [14C]-glycerol, and [14C]-sugars) equilibrate across track-etched polyethyleneterephthalate (PETP) membranes (isotopic diffusion) have been measured by a 'static' and a 'dynamic' technique under conditions where no net flow takes place; the two techniques give essentially the same results. All tracers diffuse faster the longer the membranes are etched, consistent with an increase in pore size. Water and neutral solutes diffuse at rates that are relatively independent of ionic strength, pH or the presence of divalent cations. Diffusion of cations is decreased by high ionic strength, by reducing pH or by addition of divalent cations; diffusion of chloride is increased by these procedures. Treatment of the membrane with diazomethane to reduce the negative fixed charge decreases diffusion of cations and increases that of anions; diffusion of water and neutral solutes is unaffected by methylation except in the membranes with the narrowest pores (i.e., those etched for the shortest time), in which case diffusion is reduced. We conclude (1) that the special features of flow near a charged surface apply to ions but not to water or nonelectrolytes and (2) that calculation of absolute rates of diffusion leads to values for the radii of pores through track-etched PETP membranes that are in remarkably good agreement with measured values.