The fabrication and functionalization of synthetic nanochannels has in the last years become of increasing interest for basic research as well as for applications. Nanochannels have an especially high surface-to-volume ratio which makes them ideal for handling very small amounts of liquid in nanofluidic devices or for designing highly sensitive sensing elements. In contrast to their biological counterparts like e.g. α-hemolysin, synthetic channels are more robust towards external conditions. At GSI Materials Research, nanochannels are fabricated by wet chemical etching of latent tracks in thin polymer membranes induced by heavy-ion irradiation. This technique permits the production of various channel geometries and facile control over the resulting channel diameter. Surfactant-assisted etching Nanochannels with conical longitudinal profiles in polyethyleneterephthalate (PET) are usually etched in an electrochemical cell, where an etching solution is applied on one side and a neutralizing solution on the other side of the membrane. The diameter of the small aperture of these channels can be as narrow as 2 nm. However, this etching technique is inhomogeneous and slow (etching times range between 1 and 2 h) due to insufficient wetting of the membrane and the need to employ a neutralizing medium. A way to solve these problems is provided by adding small amounts of a surfactant (in this case Dowfax 2A1 from Dow Chemicals) to the NaOH etching solution, into which the ion-track membranes are immersed. For achieving asymmetrical channel profiles, the membranes are sensitized on one side for 30 h with UV light. The resulting channels have bullet-shaped profiles (Fig. 1a) and rather large diameters which are usually not smaller than ≈ 20 nm. Surprisingly, these large channels exhibit very pronounced current rectification, which is probably due to a highly tapered geometry at the channel tip. In order to obtain narrower channel openings and thereby sustain th
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