Nuclear Track-etched Membranes Research Articles

Polydopamine Induced in-Situ Formation of Metallic Nanoparticles in Confined Microchannels of Porous Membrane as Flexible Catalytic Reactor.

Oxidant-regulated polymerization of dopamine was exploited, for the first time, for effective surface engineering of the well-defined cylindrical pores of nuclear track-etched membranes (NTEMs) to develop novel catalytic membrane reactor. First, in the presence of a strong oxidant, controlled synthesis of polydopamine (PDA) with tunable particle size was achieved, allowing a homogeneous deposition to the confined pore channels of NTEMs. The PDA interfaces rich in catechol and amine groups provided enhanced hydrophilicity to promote mass transport across the membrane and abundant nucleation sites for formation and stabilization of metallic nanoparticles (NPs). In-situ reductive growth of multiple metallic NPs, including Pd, Ag, and Au, was then achieved inside the cylindrical pores of NTEMs. Using the functionalized membrane as a catalytic reactor, efficient reduction of 4-nitrophenol (4-NP) was demonstrated in a flow-through mode. Moreover, after dissolution removal of the NTEMs, self-sustained one-dimensional (1D) PDA/M (M = Pd, Ag, or Au) hybrid nanotubes (NTs), with determined aspect ratio and a length reaching up to 10 μm, were obtained for catalysis of 4-NP in a batch reaction mode. This study established a facile and versatile method, by rational tuning of the polymerization behavior of dopamine, for effective modification of confined microscale/nanoscale cavities with different surface characteristics. The integration of PDA chemistry with NTEMs would provide more opportunities for development of novel catalytic membrane reactors as well as for the tailored synthesis of functional 1D nanotubes for broadened applications.

Potential use of nuclear track membranes in ophthalmology

Potential use of a nuclear track-etched membrane as a semipermeable membrane for fluid transport and maintenance of the cornea in a slightly dehydrated state for its transparency has been studied. The track membrane TOMTREK based on polyethylene terephthalate (PET) with a pore diameter of 0.4 μm and a density of 5 × 106 pore/cm2 has been used in the experiments. The pores have been formed by bombarding the polymer PET with 41.5-MeV 40Ar8+ ions. After bombardment, the membrane was subjected to chemical treatment in an alkali solution. Contact angle θ on the track membrane surface has been measured using the sessile drop method. Disks of a 10 mm diameter have been cut from plasma-treated membrane samples. An appropriate shape has been imparted to the model by incising and sealing the disk edges.

Confocal pore size measurement based on super-resolution image restoration.

A confocal pore size measurement based on super-resolution image restoration is proposed to obtain a fast and accurate measurement for submicrometer pore size of nuclear track-etched membranes (NTEMs). This method facilitates the online inspection of the pore size evolution during etching. Combining confocal microscopy with super-resolution image restoration significantly improves the lateral resolution of the NTEM image, yields a reasonable circle edge-setting criterion of 0.2408, and achieves precise pore edge detection. Theoretical analysis shows that the minimum measuring diameter can reach 0.19μm, and the root mean square of the residuals is only 1.4nm. Edge response simulation and experiment reveal that the edge response of the proposed method is better than 80nm. The NTEM pore size measurement results obtained by the proposed method agree well with that obtained by scanning electron microscopy.

Molecular sieving using nanofilters: Past, present and future

Filtration of molecules by nanometer-sized structures is ubiquitous in our everyday life, but our understanding of such molecular filtration processes is far less than desired. Until recently, one of the main reasons was the lack of experimental methods that can help provide detailed, microscopic pictures of molecule-nanostructure interactions. Several innovations in experimental methods, such as nuclear track-etched membranes developed in the 70s, and more recent development of nanofluidic molecular filters, played pivotal roles in advancing our understanding. With the ability to make truly molecular-scale filters and pores with well-defined sizes, shapes, and surface properties, now we are well positioned to engineer better functionality in molecular sieving, separation and other membrane applications. Reviewing past theoretical developments (often scattered across different fields) and connecting them to the most recent advances in the field would be essential to get a full, unified view on this important engineering question.

A study of spatially coupled bipolar electrochemistry on the sub-micrometer scale: colloidal particles on surfaces and cylinders in nuclear-track etched membranes

In the present study we explore the feasibility of applying spatially coupled bipolar electrochemistry to the sub-micrometer regime. This is a technique where electrically isolated objects can be interconnected by the application of electric fields and has previously been demonstrated on the millimeter and micrometer scales. Three experimental designs were explored: annealed Au and Ag films on glass, annealed Ag films on silicon nitride membranes and Ag tubes immobilized within polycarbonate nuclear-track etched membranes. In the evaporated and annealed film part of this study, particles on the order of 20–50 nm were exposed to electric fields up to 2 kV cm −1 for periods up to 180 s, in a mixture of toluene and acetonitrile. Plasmon resonance measurements and scanning electron microscopy (SEM) were used to characterize the changes following field application. At all field intensities and times studied, the gold particle sub-monolayers did not appear to be discernibly affected. The plasmon resonance absorption of the silver sub-monolayers displayed significant peak broadening after field application. Further experiments using transmission electron microscopy (TEM) analysis of annealed silver films on silicon nitride membranes demonstrated particle agglomeration without evidence for particle interconnection or morphological change. This result suggests that, under these experimental conditions, physical movement of the Ag particles occurs instead of electrochemical processes. In order to prevent particle movement, polycarbonate membranes were used to anchor silver cylinders with diameters of 1 μm, 400nm and 200 nm. Results from these experiments demonstrated that for Ag in 1:1 toluene–acetonitrile, spatially coupled bipolar electrochemistry (SCBE) reaches a practical limit for structures between 200 and 400 nm since the width of the deposit approaches the size of the metal particles. At 200 nm the result is electrochemical migration of the particles, where a commensurate amount of Ag is deposited on one side and dissolved on the other. This size limitation is specific only to SCBE, where structures are required to electro-dissolve, not to the application of bipolar electrochemistry to structures below 200 nm.

Characterization of polycarbonate nuclear track-etched membranes by means of the gas permeation method

The gas permeation technique has been applied to analyse flat polycarbonate nuclear track-etched membranes (TEMs), with pore size smaller than 100 nm. The samples analysed were the commercial membranes Nuclepore and the ones which are being developed at IPEN using the IEA-R1 research reactor (2 MW). A linear trend has been obtained for the flux versus transmembrane pressure for various gases, when measured at a constant temperature of 293 K, indicating a constant permeability for this kind of membranes. Furthermore, the permeability has shown a tendency of increasing when the molecular weight of the gases decreased following the Knudsen model predictions. An estimation of the pore radius for all TEMs were realized using the Knudsen equation and the results obtained were in good agreement with those determined by the SEM technique.

Electric field induced permeability modulation in pure and mixed Langmuir-Blodgett multilayers of hemicyanine dyes and octadecanoic acid on nanoporous solid supports

Composite structures have been prepared, in which nanoporous (nuclear track-etched) membranes are coated with supported Langmuir-Blodgett (LB) barrier layers. Permeability in these structures is a strong function of membrane composition and applied dc and ac electric fields. Absolute permeabilities fall in the range 3×10 −11 cm 2 s −1 ≤ P≤3×10 −9 cm 2 s −1, depending on composition of the barrier layer, identity (charge state) of the probe, and presence of a supporting electrolyte. Zero-field permeabilities showed a definite dependence on composition, with membranes possessing barrier layers on both sides performing better than single-sided membranes, barrier layers with LB multilayers performing better than those with just the support layer, and LB layers composed of mixed stilbazolium amphiphiles and octadecanoic acid performing better than those composed purely of stilbazolium amphiphile. All types of barrier layers studied exhibit permeability changes in the presence of applied electric fields. The magnitude of the effect is a strong function of composition of the barrier layer and the presence of supporting electrolyte. The results support electroporation over iontophoresis as the dominant mechanism for field-mediated increases in permeability. Details of the field-induced permeability changes in phosphate buffer and deionized water suggest that at least two effects are important in determining the transport behavior in these structures: a field-induced structural change in the barrier layer which mediates the electroporation and a field-mediated alteration in transport through nanopores of the nuclear track-etched membrane.

The anti-fouling action of polymers preadsorbed on ultrafiltration and microfiltration membranes

Preadsorption of hydrophilic polymers on hydrophobic ultrafiltration membranes can reduce the susceptibility of the modified membranes to protein fouling. The mechanisms of this anti-fouling action were investigated. Polysulfone and nuclear track-etched membranes with different average pore diameters were hydrophilized by preadsorption of two water-soluble polymers. The fouling of the unmodified and modified membranes due to filtration of a whey protein solution or to adsorption of the whey protein at the membrane surface was characterized by flux measurements and by electron microscopy. Adsorption of protein at the pore walls of ultrafiltration membranes, resulting in the narrowing of pores, is prevented by partly sealing off the pore entrances by polymer molecules presorbed at the external membrane surface. The observed blockage of pores of microfiltration membranes cannot be averted by the preadsorption technique.