Track-etched Polycarbonate Membranes Research Articles

Removal of Actinides from Acidic Solution via Carrier-Mediated Facilitated Transport Across Mesoporous Substrates with Nanoengineered Surfaces: Thiol Self-Assembled Monolayers with D(tBu)ΦD(iBu)CMPO Ligands

:The surface engineering of polycarbonate track-etched (PCTE) membranes was performed to create a permselective membrane for the removal of trace levels of fission products and transuranic metals from acidic streams. Electroless gold deposition was used to place a gold coating on the PCTE substrates. The gold coating was subsequently functionalized through the formation of a self-assembled monolayer of dodecanethiol, which was further functionalized with di-(4-t-butylphenyl)-N, N-diisobutylcarbamoylmethylphosphine oxide. Transport characteristics of metal ions, 137Cs, 85Sr, 241Am, and 239Pu, were obtained using a two-compartment diffusion apparatus. Transport studies showed that the functionalized membranes enhance the transport rates of 241 Am and 239Pu and inhibit the transport of 137Cs and 85Sr. Diffusion studies over an extended time revealed that the facilitated transport characteristics of 239Pu through the ligand-coated membranes are dependent on nitrate anion concentrations in the feed and sink cells.

Electrocatalytic H 2O 2 amperometric detection using gold nanotube electrode ensembles

Arrays of nanoscopic gold tubes were prepared by electroless plating of the metal within the pores of nanoporous polycarbonate track-etched membranes. A procedure for fabricating an ensemble of enzyme-modified nanoelectrodes has been developed based on the efficient immobilization of horseradish peroxidase (HRP) to the gold nanotubes array using self-assembled monolayers (mercaptoethylamine or mercaptopropionic acid) as anchoring layers. Hydrogen peroxide (H 2O 2) was determined electrochemically by using gold nanoelectrode ensembles (NEE) functionalized or not in phosphate buffer solution (PB) with or without a mediator (hydroquinone, H 2Q). Bare NEE displays a remarkable sensitivity (14 μA mM −1 in H 2Q at −0.1 V versus Ag/AgCl) compared to a classical gold macroelectrode (0.41 μA mM −1). The gold nanoparticles that form the tubular structure act as excellent catalytic surfaces towards the oxidation and the reduction of H 2O 2. The HRP modified NEE presents a slightly lower sensitivity (9.5 μA mM −1) than bare NEE. However, this system presents an enhanced limit of detection (up to 4 × 10 −6 M) and a higher selectivity towards the detection of H 2O 2 over a wide range of potentials. The lifetime, fabrication reproducibility and measurement repeatability of the HRP enzyme electrode were evaluated with satisfactory results.

Bienzyme HRP–GOx-modified gold nanoelectrodes for the sensitive amperometric detection of glucose at low overpotentials

Gold nanotubular electrode ensembles were prepared by using electroless deposition of the metal within the pores of polycarbonate track-etched membranes. Mono-enzyme (GOx) and monolayer/bilayer bienzyme (GOx/HRP) bioelectrodes were prepared by immobilizing the enzymes onto gold nanotubes surfaces modified with mercaptoethylamine. Batch amperometric responses to glucose for the different bioelectrodes were determined and compared. The response of the two geometries (monolayer and bilayer) of the bienzyme electrodes was shown to vary with regard to sensitivity at detection potentials above 0 V. On the contrary, at detection potentials below 0 V, no noticeable influence of the configuration of the bienzyme on the response intensity was observed. The mono-enzyme (650 μA mM −1 in benzoquinone (BQ) at −0.8 V versus Ag/AgCl) and the two bienzyme bioelectrodes (±400 μA mM −1 in hydroquinone (H 2Q) at −0.2 V versus Ag/AgCl) display remarkable sensitivities compared to a classical GOx-modified gold macroelectrode (13 μA mM −1 in BQ at −0.8 V versus Ag/AgCl). A remarkable feature of the bienzyme electrodes is the possibility to detect glucose at very low applied potentials where the noise level and interferences from other electro-oxidizable compounds are minimal. Another important characteristic of the monolayer bienzyme electrode is the possible existence of a direct electronic communication between HRP and the transducer surface.

Permeability and Separation Characteristics of Polypeptide-Functionalized Polycarbonate Track-Etched Membranes

The use of polypeptide-functionalized membranes with known porosity and uniform pore sizes allows a better understanding of the separation characteristics in nanodomains. A stimuli-responsive polypeptide, poly-l-glutamic acid (PLGA), was immobilized on a polycarbonate track-etched (PCTE) membrane. First, PCTE membrane was gold-coated under convective conditions, and a thiol, 3-mercapto 1,2-propanediol (MPD), was chemisorbed on the modified surface. Second, the MPD molecule was oxidized with sodium periodate to obtain an aldehyde functionality, which was further reacted with the amino group on the PLGA molecule, making possible a single point polypeptide attachment. The morphology of the modified membrane was analyzed by electron microscopy (SEM, TEM) imaging, confirming uniform structure. Modified membrane performances, with starting diameters of 30 and 100 nm, were evaluated in terms of solute (ionic and neutral) rejections and water permeability. Both solute and water transport through membrane were rev...

Microtubule sensors and sensor array based on polyaniline synthesized in the presence of poly(styrene sulfonate).

Microtubule sensors for glucose, urea, and triglyceride were fabricated based on poly(styrene sulfonate)-polyaniline (PSS-PANI) composites synthesized within the pores of track-etched polycarbonate membranes. The synthesis of a sufficiently thick and conducting PSS-PANI film at pH 5 provided the advantage of immobilizing enzymes during polymerization. This resulted in the improvement of sensor response for urea and triglyceride by a factor of approximately 10(2) with a significant increase in the linear region of response compared to polyaniline-based sensors, where the enzymes were immobilized by physical adsorption after the polymerization. The sensors based on urea and triglyceride were found to have a higher linear range of response, better sensitivity, improved multiple use capability, and faster response time compared to the potentiometric and amperometric sensors based on polyaniline. A microtubule sensor array for glucose, urea, and triglyceride based on PSS-PANI was fabricated by immobilization of three different sets of enzymes on three closely spaced devices and its response was found to be free from cross-interference when a sample containing a mixture of the above analytes was analyzed in a single measurement.

The Pressure-Dependence of the Size of Extruded Vesicles

Variations in the size of vesicles formed by extrusion through small pores are discussed in terms of a simple model. Our model predicts that the radius should decrease as the square root of the applied pressure, consistent with data for vesicles extruded under various conditions. The model also predicts dependencies on the pore size used and on the lysis tension of the vesicles being extruded that are consistent with our data. The pore size was varied by using track-etched polycarbonate membranes with average pore diameters ranging from 50 to 200 nm. To vary the lysis tension, vesicles made from POPC (1-palmitoyl-2-oleoyl- sn-glycero-3-phosphatidylcholine), mixtures of POPC and cholesterol, and mixtures of POPC and C 16-ceramide were studied. The lysis tension, as measured by an extrusion-based technique, of POPC:cholesterol vesicles is higher than that of pure POPC vesicles whereas POPC:ceramide vesicles have lower lysis tensions than POPC vesicles.

Electrochemical Growth of Single-Crystal Metal Nanowires via a Two-Dimensional Nucleation and Growth Mechanism

Metallic nanowires (Au, Ag, Cu, Ni, Co, and Rh) with an average diameter of 40 nm and a length of 3-5 μm have been fabricated by electrodeposition in the pores of track-etched polycarbonate membranes. Structural characterizations by transmission electron microscopy (TEM) and electron diffraction showed that nanowires of Au, Ag, and Cu are single-crystalline with a preferred [111] orientation, whereas Ni, Co, and Rh wires are polycrystalline. Possible mechanisms responsible for nucleation and growth for single-crystal noble metals versus polycrystalline group VIII-B metals are discussed.

Gateable nanofluidic interconnects for multilayered microfluidic separation systems.

The extension of microfluidic devices to include three-dimensional fluidic networks allows complex fluidic and chemical manipulations but requires innovative methods to interface fluidic layers. Externally controllable interconnects, employing nuclear track-etched polycarbonate membranes containing nanometer-diameter capillaries, are described that produce hybrid three-dimensional fluidic architectures. Controllable nanofluidic transfer is achieved by controlling applied bias, polarity, and density of the immobile nanopore surface charge and the impedance of the nanocapillary array relative to the microfluidic channels. Analyte transport between vertically separated microchannels has three stable transfer levels, corresponding to zero, reverse, and forward bias. The transfer can even depend on the properties of the analyte being transferred such as the molecular size, illustrating the flexible character of the analyte transfer. In a specific analysis implementation, nanochannel array gating is applied to capillary electrophoresis separations, allowing selected separated components to be isolated for further manipulation, thereby opening the way for preparative separations at attomole analyte mass levels.

Template Synthesis of Au/Co Multilayered Nanowires by Electrochemical Deposition

AbstractThe electrochemical deposition of Au/Co multilayers into 20 μm‐thick ion track etched polycarbonate membranes with pore diameters of 110–150 nm was studied in a single electrolyte containing cobalt sulfate, gold cyanide, and citric acid. Cyclic voltammetry, chronoamperometry, and pulse‐potential experiments were used to determine the deposition conditions for pure Au and 98 wt.‐% Co layers. The Co‐rich metallic nanowires were deposited at –1100 mV and the Au nanowires at –490 mV vs. Ag/AgCl. Under open‐circuit conditions an Au‐displacement reaction occurred. Open‐circuit conditions were applied in combination with a steep ramp between the Co and Au potential pulses to avoid the otherwise severe Co dissolution when the potential was switched to less negative values. Scanning and transmission electron microscopy of the deposited multilayered nanowires revealed dense and well‐defined layer interfaces. X‐ray diffraction investigations of 20 μm long Au and Co nanowires showed that the Au deposits exhibit a face‐centered cubic (fcc) <111> texture, while the Co deposits exhibit a hexagonal close‐packed (hcp) (002) structure. The magnetic measurements of 12 nm Co/4 nm Au nanowires for fields applied parallel and perpendicular to the film plane indicate a low remnant magnetization.

Electrical resistivity of polypyrrole nanotube measured by conductive scanning probe microscope: The role of contact force

Polypyrrole (PPy) nanotubes were synthesized using the pores of track-etched polycarbonate membrane as a template. Its size depends on the pore diameter of template, range from 50 to 200 nm. Direct I–V measurements of PPy nanotube (diameter of 120 nm) deposited on Au were done using a metal-coated tapping-mode atomic-force-microscope tip. Linear I–V characteristics are observed, and the resistance is decreased as the contact force is increased. Using the Hertz model, the elastic modulus E and electrical resistivity ρ are estimated to be E∼1 GPa and ρ∼1 Ωcm. These values are consistent with those obtained in bulk PPy film.

Effect of pore morphology on fluid flow and particle deposition on a track-etched polycarbonate membrane

Three-dimensional particles deposition on a filter pore of a track-etched polycarbonate membrane was studied numerically by analyzing forces acting on the depositing particles. The simulation was applied to examine how the morphology of the filter pore affects the deposition of particles on a membrane pore at the initial stage of filtration. The model of membrane structure is a 3D porous surface with circular holes of finite thickness. Both of the effects of constriction and expansion of inner pore geometries on fluid flow and particle deposition are taken into consideration in numerical calculation, and are compared with that of capillary pore. The disturbance of flow field and particle trajectory due to the presence of particles was taken into consideration during each simulation. Simulated results show that the effect of filtration rate, i.e., operating pressure, has a significant influence on the packing morphology of particles on a filter pore.

Preparation and magnetic behavior of arrays of electrodeposited Co nanowires

Cobalt nanowires have been synthesized by electrodeposition into porous track-etched polycarbonate membranes with a quoted pore diameter of D p≈200–400 nm and a thickness of L≈7 μm. Magnetization curves and torque experiments of arrays of Co wires confirm that when the lengths of the wires are increased a crossover takes place from a parallel easy direction of magnetization towards an easy direction perpendicular to the axis of the wire. This change in the easy direction of magnetization is analyzed considering the competition between demagnetizing field, magnetocrystalline anisotropy, and dipolar interaction among wires.

Template Growth of Photoconductive Metal−CdSe−Metal Nanowires

Metal−CdSe−metal (metal = Au, Ni) nanowires were grown by electrochemical replication of porous aluminum oxide and polycarbonate track etch membranes with pore diameters of 350 and 70 nm, respectively. The lengths of the individual segments of the nanowires were controlled by varying the amount of charge that was passed. The composition of the CdSe segments was characterized by energy-dispersive X-ray spectroscopy. A 1:1 ratio could be obtained, and Cd- and Se-rich stoichiometries were also made by adjusting the concentrations of Cd2+ and SeO2 in the aqueous plating solutions. X-ray powder diffraction showed the presence of both zinc blende and wurzite phases, and grain sizes on the order of 10 nm were observed by TEM. The nanowires were resistive in the dark but showed pronounced visible light photoconductivity.

Protein Transport in Nanoporous Membranes Modified with Self-Assembled Monolayers of Functionalized Thiols

Control of external pH and ionic strength is used to separate proteins with surface-modified, nanoporous polycarbonate track etched (PCTE) membranes. The porous PCTE membranes were modified with monolayers of self-assembled thiols (HSC10H20COOH) on electroless gold. The hydraulic radius of the pores in the surface-modified membranes was 8.7 nm. Two proteins of nearly identical molecular weight, bovine serum albumin (BSA) and bovine hemoglobin (BHb), were used as the permeants. The fluxes of BSA and BHb through the membranes show maximum values at the isoelectric points (pI) of the proteins. At pH values above and below the pI, charge interactions between the proteins, their counterions, and the pore surface leads to a decrease in flux. The imposition of a difference in ionic strength across the membrane causes osmotic flow and leads to a significant increase in the protein fluxes and an enhancement of the selectivity of BSA over BHb. In protein separation experiments, the BSA and BHb fluxes are nearly 3 times larger than those observed with no ionic strength difference.

Template synthesis of metal nanowires containing monolayer molecular junctions.

Metal nanowires containing in-wire monolayer junctions of 16-mercaptohexanoic acid were made by replication of the pores of 70 nm diameter polycarbonate track etch membranes. Au was electrochemically deposited halfway through the 6 microm long pores and a self-assembled monolayer (SAM) of 16-mercaptohexadecanoic acid was adsorbed on top. A thin layer of Au was then electrolessly grown to form a metal cap separated from the bottom part of the wire by the SAM. Electron micrographs showed that the bottom and top metal segments were separated by an approximately 2 nm thick organic monolayer. Current-voltage measurements of individual nanowires confirmed that the organic monolayer could be contacted electrically on the top and bottom by the metal nanowire segments without introducing electrical short circuits that penetrate the monolayer. The values of the electrical properties for zero-bias resistance, current density, and breakdown field strength were within the ranges expected for a well-ordered alkanethiol SAM of this thickness.

Electrochemical synthesis of Ag/Co multilayered nanowires in porous polycarbonate membranes

The electrodeposition of Ag/Co multilayers into 20 μm thick ion track etched polycarbonate membranes with pore diameter of 120 nm were studied using a single bath containing cobalt sulphate, silver cyanide and potassium pyrophosphate. Cyclic voltammetry and transmission electron microscopy (TEM) was used to determine deposition conditions for pure Ag and 97 wt.% Co layers. The Co rich metallic nanowires were deposited at −1100 mV and the Ag nanowires were deposited at −600 mV vs. Ag/AgCl. By using Faraday's law and linear relation between the charges passed during the time interval for each metal layers and bilayer thickness, a current efficiency of 58% was determined for pulsed deposition of Co. This implies that hydrogen or initial layer re-dissolution of Co during Ag deposition consumes 31% of the charge passed during Co deposition. Multilayered nanowires with 8 and 15 nm thick Ag and Co rich layers, respectively, exhibit well-defined layers in TEM. X-ray diffraction investigations of 20 μm long, Ag, Co and Ag/Co multilayered nanowires show that the Ag deposits exhibited a f.c.c. 〈111〉 texturing while the Co deposit exhibited basal plane diffraction of h.c.p. (002) and f.c.c. (111). Magnetic measurements on Ag/Co multilayered nanowires have been made with the external field parallel and perpendicular to the film plane on two series of multilayers: thickness of Co (50 and 15 nm)/Ag (8 nm). In the case of longer cylinder (t Co=50 nm) the axis along the wire starts to become the easy axis (lower saturation field) as expected in the limit where t Ag=0 (case of single Co nanowires), due to the shape anisotropy.

Electrochemical deposition of Co nanowire arrays; quantitative consideration of concentration profiles

Electrodeposition of Co into ion track etched polycarbonate membranes by a chronoamperometric method has been studied. The concentration variation and diffusion limiting current during nanoprocessing for growth of Co in nanoelectrode applications are described for two regimes; in the vicinity of the electrode and at the mouth of pores. The behaviour of the as-deposited nanoelectrodes can be modelled as a recessed microelectrode. A diffusion controlled limiting current of the nanodes is studied by the Cottrell equation at short times, i.e. current αt −1/2. At longer times, a steady-state current is obtained due to gradually increasing radii, r, of the spherical diffusion zones from each recessed nanode, i.e. current α(1/( r+ L)), where L is the membrane thickness. The experimental value of diffusion coefficient D for Co ions was found: D=2.5×10 –5 cm 2 s −1. XRD and TEM measurements on 250 nm diameter and 20 μm long Co nanowires showed a hexagonal closed packed phase with a 〈100〉 texturing. The nanowires exhibited an enhanced magnetic coercivity in comparison to bulk Co. The difference of saturation fields between the parallel and perpendicular orientation fields corresponds well to the expected demagnetisation field of 2 πM=8796 Oe value due to the shape anisotropy in case of an infinite thin Co cylinder.

Fabrication and temperature dependence of the resistance of single-crystalline Bi nanowires

Single-crystalline Bi nanowires with diameters ranging from 20 to 70 nm were prepared by electrodeposition using nanoporous aluminum oxide membranes rather than the more usual track-etched polycarbonate membranes. X-ray diffraction and selected area electron diffraction investigations revealed that the nanowires are essentially single crystalline and highly oriented. The temperature dependence of zero-field resistance of different diameter nanowires indicated that these Bi nanowires undergo a semimetal–semiconductor transition due to two-dimensional quantum confinement effects. The resistance maximum was observed at 50 K in zero magnetic field for 20 nm Bi nanowires, while the resistance minimum at 258 K for 50 nm Bi nanowires, due to the quantum size effect.

Characterization of microporous membranes using confocal scanning laser microscopy in fluorescence mode

Confocal Scanning Laser Microscopy (CSLM) in fluorescence mode was used to characterize microporous membranes. Two microfiltration membranes were investigated: a mixed ester (cellulose nitrate/cellulose acetate) 1.2 μ m-rated membrane and a polycarbonate track-etched membrane with cylindrical pores of 2 μ m diameter. Optical sections of the membranes stained with rhodamine and mounted in glycerol were performed at 1 μ m intervals, from 0 to 10 μ m. CSLM was found useful for microporous membrane characterization, as it gives some insight into bulk membrane morphology.

A Combined Pore Blockage and Cake Filtration Model for Protein Fouling during Microfiltration

Previous studies of protein fouling during microfiltration have shown significant discrepancies between filtrate flux data and predictions of the classical pore blockage, pore constriction, and cake filtration models. A new mathematical model was developed for the filtrate flux which accounts for initial fouling due to pore blockage and subsequent fouling due to the growth of a protein cake or deposit over these initially blocked regions. The model explicitly accounts for the inhomogeneity in the cake layer thickness over different regions of the membrane arising from the time-dependent blockage of the pore surface. The model was shown to be in excellent agreement with experimental data obtained during the stirred cell filtration of bovine serum albumin solutions through polycarbonate track-etched microfiltration membranes over the entire course of the filtration. The model provides a smooth transition from the pore blockage to cake filtration regimes, eliminating the need to use different mathematical formulations to describe these two phenomena. In addition, the model provides the first quantitative explanation for some of the unusual observations reported previously in investigations of protein microfiltration. The results provide important insights into the underlying mechanisms of protein fouling during microfiltration.