Fabrication of stable single nanochannels with controllable ionic rectification.

The single conical nanochannel investigated here was produced in polymer films using the ion track-etching technique. [S1] Before the chemical etching process, the samples of PET were exposed to the UV light for 1 h from each side. To produce a conical nanochannel, etching was performed only from one side, the other side of the cell contains a solution that is able to neutralize the etchant as soon as the channel opens, thus slowing down the further etching process. To additionally stop the etching, the voltage (1 V) used to monitor the etching process was applied in such a way that the negatively charged ions of the etchant were drawn out of the channel tip. The following are the etching and stopping solutions for the etching of PET: NaOH (9 M) for etching, KCl (1 M) + HCOOH (1 M) for stopping. The large opening of the conical nanochannel was called base, while the small opening was called tip. The diameter of the base and tip were estimated from the multitrack membranes etch rate measured in the parallel etching experiments by AFM (see Fig. S1), due to the difficulty [S2] of locating both the tip and base opening in AFM. In this work, its base is usually controlled from ~400 to ~2500 nm, and its tip is from ~2 to ~700 nm.