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We have been exploring the transport and electrochemical properties of nanotube membranes prepared by the template method, a general approach for preparing nanomaterials. This method entails synthesis or deposition of the desired material within the cylindrical and monodisperse pores of a nanopore membrane or other solid. We have used polycarbonate filters, prepared via the “track-etch” method, and nanopore aluminas, electrochemically prepared from Al foil, as our template materials. Cylindrical nanostructures with monodisperse diameters and lengths are obtained, and depending on the membrane and synthetic method used, these may be solid nanowires or hollow nanotubes. We and others have used this method to prepare nanowires and tubes composed of metals, polymers, semiconductors, carbons, and Li+ intercalation materials. It is also possible to prepare composite nanostructures, both concentric tubular composites, where an outer tube of one material surrounds an inner tube of another, and segmented composite nanowires.
One application for these nanotube membranes is in electroanalytical chemistry where the membrane is used to sense analyte species. In that work, membranes containing gold nanotubes with inside diameters that approached molecular dimensions (1–4 nm) were used. The Au nanotube membrane was placed between two salt solutions and a constant transmembrane potential was applied. The resulting transmembrane current, associated with migration of ions through the nanotubes, was measured. When an analyte molecule whose diameter was comparable to the inside diameter of the nanotubes was added to one salt solution, this molecule partitioned into the nanotubes and partially occluded the pathway for ion transport. This resulted in a decrease in the transmembrane ion current, and the magnitude of the drop in current was found to be proportional to the concentration of the analyte.
In the experiment discussed above, a baseline transmembrane ion current was established, and the analyte molecule, in essence, turned off this current. It occurred to us that there might be an advantage in doing the opposite, i.e., starting with an ideally zero current situation and having the analyte molecule switch on the ion current. In other words, we would like to make a synthetic membrane that mimics the function of a ligand-gated ion channel. An example is the acetylcholine-gated ion channel, which is closed (“off” state) in the absence of acetylcholine but opens (and supports an ion current, “on” state) when acetylcholine binds to the channel. To accomplish this, the off state was obtained by making gold and alumina membranes hydrophobic, and the on state was obtained by introducing ions and electrolyte into the membrane. Ions were introduced by either partitioning a hydrophobic ionic species (e.g., a drug or a surfactant) into the membrane.