Templated Nanotubes and Nanofiltration

Classical filtration membranes and in particular anodized alumina membranes [199] and nanosieves [200] may also be transformed to nanotubular structures by deposition of appropriate materials in the filtration pores or channels. They will enable new ways of separating and detecting analytes for applications in chiral separations and single-molecule sensing. See Figure 66.

Nanotubule membranes are easy to make and each pore in the membrane essentially is a nanobeaker in which chemistry can occur. Metals can be deposited inside the pores, either electrochemically or through so-called electro-less plating by using a reducing agent to plate the metal from the solution. Nanotubules of inorganic materials such as silica or titania can be prepared through sol-gel chemistry, and carbon nanotubules can be made through chemical vapor deposition of ethylene inside the pores. See Figure 67.

The nanotubules come out embedded in a membrane, aligned, and monodisperse—that is, of uniform diameter and of a uniform length equal to the thickness of the membrane. Depending on deposition reaction times, thin- or thick-wall tubules are formed. The tubules can be capped on both ends to create a cluster of individual confined

NCA template Cobalt catalyst Carban nanotubes Al anodization Co deposition C2H2 pyrolysis


Figure 65. Highly ordered carbon nanotube arrays. Top left: Process scheme of fabrication. Bottom left: SEM image of the resulting hexag-onally ordered array of carbon nanotubes fabricated using the method in (a). Top right: SEM image showing oblique view of periodic carbon nanotube array. The inset at the lower left is an enlarged view of the tubes. The inset at the lower right is a histogram of the nanotube diameter showing a narrow size distribution around 47 nm. Top view SEM image, bottom right: carbon nanotubes showing hexagonal close-packed geometry. The hexagonal cells have sides approximately 57 nm long and the intercell spacing is 98 nm. The slight splitting of the tube ends and the apparent increase in tube wall thickness is an artifact of the non-specialized ion-milling apparatus that was used in the experiments. The inset shows a close-up view of a typical open-ended carbon nanotube in its hexagonal cell. Reprinted with permission from [198], J. Li et al., Appl. Phys. Lett. 75, 367 (1999). © 1999, American Institute of Physics.

NCA template Cobalt catalyst Carban nanotubes Al anodization Co deposition C2H2 pyrolysis

Figure 66. SEM micrograph of ordered nanochannels in anodic porous alumina. The anodizing of Al was conducted under a constant voltage condition in an oxalic acid solution. The hole interval of anodic porous alumina, in other words, the cell size, was determined by the applied voltage used for anodization. It was reported that the cell size has a good linear relationship with the applied voltage, where the proportionality constant of cell size for a specific applied voltage is approximately 2.5 nm/V. In the case of the sample shown, anodizing was conducted under a constant voltage of 40 V after pretexturing of a 100 nm period. Ideally ordered hole development was observed only in a pretextured area (left), while random development of holes took place in the untreated area (right). The hole interval corresponded to that of the pretextured concaves of Al. From this result, it was concluded that concave features formed by indentation could act as an initiation point and guide the growth of channels. Reprinted with permission form [201], H. Asoh et al., J. Electrochem. Soc. 148, 152 (2001). © 2001, The Electrochemical Society.

spaces, or the membrane can be removed to release individual nanotubules. The generality of this template-based synthetic methodology broadens the scope of nanomaterials that can be prepared. One of the ealiest separation applications to be explored was selective-ion transport. A simple experiment with a gold nanotubule membrane in a U-tube cell demonstrates this phenomenon [202]. On one side of the membrane is a feed solution containing KCl, which is colorless, and the cationic dye methylene blue. On the other side is a receiver solution of KCl. After a while, the colorless receiver solution turns blue, indicating transport of the cationic dye across the membrane. But when the feed contains permanganate anion, which is red, the receiver membrane disc containing nanochannels _


I plating of gold inside pore/channel | forming nanotubules

Figure 67. Schematic overview of the formation of nanotubules.

Figure 67. Schematic overview of the formation of nanotubules.

remains colorless. The anion, although smaller than the cationic dye, cannot cross the membrane. Only cations pass because the nanotubule walls have excess negative charge due to adsorbed chloride ions. The excess charge can also be controlled by applying a potential to the membrane provided that the membrane material is electrically conductive. If a positive potential is applied, the nanotubule wall will have excess positive charge and the nanotubule membrane will reject cations and transport only anions. At negative applied potentials, the membrane will transport cations and reject anions. At the specific potential where the wall is electrically neutral, the membrane will be nonselective. Thus, the nanotubules make up a switchable ion-exchange membrane [203]. Having shown these membranes to have charge-and size-based selectivities, attempts were made to engender chemical selectivity by modifying the chemistry of the nano-tubule walls in which alkyl and other groups are attached to the nanotubule walls through gold-sulfur bonds [204, 205]. Cysteine is a thiol as well as an amino acid. So it has both amino and carboxyl groups. At low pH, it will be positively charged; at high pH, it will be negatively charged. Lee and Martin have shown [206] that the cysteine-decorated membrane rejects cations at low pH and anions at high pH.

Chiral separation is important in the pharmaceutical industry, as it has become clear that the enantiomers of drugs produced as racemic mixtures could have very different properties. Lakshmi et al. [207] used a "sandwich" assembly to separate D-phenylalanine from L-phenylalanine. Kobayashi et al. [208] achieved the highly sensitive detection at 10-11 M with unadorned gold nanotubule membranes through which an ionic current passes.

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