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(low currents at positive potentials) and an on state (high currents at negative potentials) (Figure 9.25). We propose the rectification observed is due to electrophoretic movement of the DNA chains into (off state, Figure 9.25C) and out of (on state, Figure 9.25B) the nano-tube mouth. The movement of the DNA chains into the nanotube mouth results in occlusion of the nanotube orifice, resulting in a higher ionic resistance. In Figure 9.25, the effect of chain length on rectification can be clearly observed. That is to say, as DNA chain length increases, the extent of rectification increases. It was found that an optimal length of DNA induces rectification based on the diameter of the small end of the nanotube. This work demonstrated the first example of a simple chemical (DNA chain length) or physical (nanotube pore size) method to control the extent of rectification of an artificial ion channel.

Studies of nanotubes and conical nanotubes that function as artificial ion channels are a relatively new endeavor in bioanalytical chemistry. We expect future applications of nanotube membranes to include highly sensitive and selective chemical sensors based on the design principles of mother nature.

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