pH-Reversed ionic current rectification displayed by conically shaped nanochannel without any modification

Ion current through a nascent nanochannel with conically shaped geometry in PET (polyethylene terephthalate) membrane sandwiched between two same buffer solutions at pH <= 3 was routinely considered to exhibit no rectification and, if any, much weaker rectification than that for a nanochannel with a negative surface charge, since the surface charge on the membrane decreases to zero along with decreasing the pH value of the buffer solution down to the pK(a) of carboxylic acid. However, in this study, we discovered that in the buffer solution with low ionic strength at pH values below 3, the conically shaped nanochannels exhibited distinct ion current rectification, as expected for nanochannels with a positive surface charge, if voltages beyond +/- 2V range were scanned. We reasoned that the current rectification engendered by the positive surface charge of a conical nanochannel was due to further protonation of the hydrogen bonded hydrogel layer or neutral carboxylic acid inside the nanochannel. Therefore, our results enrich the knowledge about nanochannel technology and indicate that a nanofluidic diode based on pH-reversed ion current rectification through a conical nanochannel can be achieved without any modification of the PET membrane.