Track-etch membranes

Pores with a very regular, linear shape can be produced by the track-etch method (Quinn et al. 1972). Here a thin layer of a material is bombarded with highly energetic particles from a radioactive source. The track left behind in the material is much more sensitive to an etchant in the direction of the track axis than perpendicular to it. So etching the material results in straight pores of uniform shape and size with pore diameters ranging between 6 nm and 1200 nm. To avoid overlap of pores only 2-5% of the surface can be occupied by the pores. This process has been applied on polymers (e.g. Nuclepore membranes) and on some inorganic systems like mica. Membranes so obtained are attractive as model systems for fundamental studies. 

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Track-etched membranes are made by exposing thin films (mica, polycarbonate, etc) to fission fragments from a radiation source. The high energy particles chemically alter material in their path. The material is then dissolved by suitable reagents, leaving nearly cylindrical holes (19). 

Within the scope of thermoelectric nanostructures, Sima et al. [161] prepared nanorod (fibril) and microtube (tubule) arrays of PbSei. , Tej by potentiostatic electrodeposition from nitric acid solutions of Pb(N03)2, H2Se03, and Te02, using a 30 fim thick polycarbonate track-etch membrane, with pores 100-2,000 nm in diameter, as template (Cu supported). After electrodeposition the polymer membrane was dissolved in CH2CI2. Solid rods were obtained in membranes with small pores, and hollow tubes in those with large pores. The formation of microtubes rather than nanorods in the larger pores was attributed to the higher deposition current. 

Sima M, Enculescu I, Visan T, Spohr R, Trautmann C (2004) Electrochemical deposition of PbSei j Tej nanorod arrays using ion track etched membranes as template. Mol Cryst Liq Cryst 418 749-755... 

Track-Etched Track-etched membranes (Fig. 20-66) are now made by exposing a thin polymer film to a collimated beam of radiation strong enough to break chemical bonds in the polymer chains. The film is then etched in a bath which selectively attacks the damaged polymer. The technique produces a film with photogenic pores. 

There has been extensive recent use of track-etched membranes as templates. As will be discussed in detail below, these membranes are ideal for producing parallel arrays of metal nanowires or nanotubules. This is usually done via electroless metal deposition [25], but many metals have also been deposited electrochemically [26]. For example, several groups have used track-etched templates for deposition of nanowires and segmented nanowires, which they then examined for giant magnetoresistance [27-29]. Other materials templated in the pores of track etch membranes include conducting polymers [30] and polymer-metal composites [31]. 

In this section a short introduction will be given on the synthesis of porous ceramic membranes by sol-gel techniques and anodization, carbon membranes, glass membranes and track-etch membranes. An extensive discussion will be given in Sections 2.3-2.S. 

An example of the track-etch membrane was given in Section 2.2 (Quinn et al. 1972). Booman and Delmastro (1974) have also described the layer deposition method to produce a microporous membrane by a track-etch method. 

The track-etch membrane can be used in reverse osmosis and electrodialysis separation processes where it consists of a thin metal layer with a thin layer of insulator material on each side. The membrane pore diameters were in the range 0.5-10 nm. 

Quinn, J. A., J. L. Anderson, W. S. Ho, and W. J. Petzny, Model pores of molecular dimension - Preparation and characterization of track-etched membranes , Biophysical Journal, 12, 990-1007 (1972). 

The transport velocity of Li+ is faster than that of Na+ and K+ due to the size of the cation. The data are consistent with a hopping transport mechanism of the cations accompanied by a non-specific co-transport of the anions. The transport rates for N03 > Cl- > C104 are related to the adjacent hydrate shell and not yet fully understood. Anyway, a path in the center of the supramolecular tubes, where the crown ethers assemble, must exist and allow for the co-transport of the anions. By forming the membranes in the pores of track-etched membranes, the transport rates could be improved by an order of magnitude due to the orientation of the channels perpendicular to the membrane surface. 

Track-etch membranes were developed by the General Electric Corporation Schenectady Laboratory [3], The two-step preparation process is illustrated in Figure 3.4. First, a thin polymer film is irradiated with fission particles from a nuclear reactor or other radiation source. The massive particles pass through the film, breaking polymer chains and leaving behind a sensitized track of damaged polymer molecules. These tracks are much more susceptible to chemical attack than the base polymer material. So when the film is passed through a solution... 

Mitchell, B. C., and Deen, W. M. (1986). Effect of concentration on the rejection coefficients of rigid macromolecules in track-etch membranes, J. Coll. Inter. Sci. 113, 132. 

Pandey, A.K., Gautam, M.M., Shukla, J.P., and Iyer, R.H., Effect of pore characteristics on carrier-facihtated transport of Am(III) across track-etched membranes. J. Membr. Sci., 2001, 190 9-20. 

Nanowires of 60-200 nm anatase Ti02 were fabricated using liquid phase deposition in a track etched membrane [47]. The acidic precursor solution used was (NH4)2Tip6 (0.2 M) and H3BO3 (0.1 M) and was adjusted to a pH of around 1.4 with 1 1 ammonia solution. 

Camphor sulfonic acid CV cyclic voltammogram CYP2D6 — cytochrome P450-2D6 DBSA dodecyl benzenesulfonic acid EAQ — Eastman AQ polymersTM FTIR — Fourier Transform infrared spectroscopy HRP — Horseradish peroxidase NMP N-methylpyrollidone PTM particle track-etched membrane PANIs... 

Liposome sizing filters (pore sizes 100 and 200 nm) (Nuclepore Track-Etch Membrane, Whatman pic, UK). 

K. J. Kim, P.V. Stevens and A.G. Fane, Porosity dependence of pore entry shape in track-etched membranes by image analysis. /. Membrane Sci., 93 (1994) 79. 

Track-etched membranes, from Nucleopore, different pore diameters (in nm) +3 mV at pH 3 negative potential at pH 4. 

Kim, K.J. et al., Chemical and elechical characterization of virgin and protein-fouled polycarbonate track-etched membranes by FTIR and sheaming-potential measurements,/. Membr. Sci., 134, 199, 1997. 

Nuclear track-etch membranes (capillary-pore type). 

To date, none of these methods has produced submicron capillary-pore membranes at a reasonable cost in the large areas suitable for industrial applications except the "track-etch" membrane produced by Nudepore Corp. 

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