pH-Reversed Ionic Current Rectification Displayed by Conically Shaped Nanopore Without Any Modification

Ion current through nascent nanopore 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 if had, much weaker rectification than that for nanopore with negative surface charge, since the surface charge on the membrane decrease to zero along with decreasing the pH value of the buffer solution down to the pKa 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 nanopores exhibited much distinct ion current rectification as expected for nanopores with positive surface charge, if voltages beyond ±2v range were scanned. We reasoned that the current rectification engendered by the positive surface charge of conical nanopore 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 nanopore technology and indicate that nanofluidic diode based on pH-reversed ion current rectification through conical nanopore can be achieved without any modification of the PET membrane.