Gold Nanotubule Membranes with Electrochemically Switchable Ion-Transport Selectivity

GOLD NANOTUBULE MEMBRANES WITH ELECTROCHEMICALLY SWITCHABLE ION-TRANSPORT SELECTIVITY 

The pores in a commercially available polycarbonate filtration membrane (Poretics) were used as templates to form the nanotubules (pore diameter = 50 nm pore density = 6 X 10 pores cm thickness = 6 pm). As before, the electrolessly plated Au deposits both on the pore walls and the membrane faces [71]. The gold surface layers on the membrane faces allow us to make electrical contact to the Au nanotubules within the pores. The thickness of the gold layers deposited on the pore walls can be controlled 

Marcel Dekker, Inc. 270 Madison Avenue, New York, New York 10016 

We describe the results of three sets of experiments that demonstrate that these Au nanotubule membranes can show selective ion transport. All of these experiments involve a U-tube cell in which the membrane to be studied separates two aqueous solutions. The simplest experiment entails using a feed solution of a colored anionic or cationic species on one side of the membrane and a permeate solution that is initially devoid of the colored species on the other side of the membrane. A membrane that contained 2.5-nm-radius [86,87] Au nanotubules was used for these experiments. 

When the feed solution is 1 mM in KCl and 0.5 mM in the cationic dye methylene blue and the receiver solution is 1 mM KCl, the initially colorless receiver solution turns blue due to transport of the cationic dye across the membrane [71]. In contrast, when the feed solution is 1 mM in KCl and 5 mM in KMn04 (Mn04 is red) and the receiver solution is 1 mM KCl, the receiver solution remains colorless [71]. These experiments provide simple visual evidence that this membrane transports a large cation but does not transport a much smaller anion. We have used potentiometric measurements to explore the nature of this cation permselectivity. 

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