Ionic Current Rectification by Laminated Bipolar Silica Isoporous Membrane

Ionic current rectification (ICR) is one of interesting characteristics displayed by nanochannels with asymmetric geometry, ionic concentration or charge distribution, which has been utilized for the development of chemical sensors and biosensors. Herein we report the ICR phenomenon observed with ultrathin silica isoporous membrane (SIM), which was prepared by laminating two layers of SIM with opposite charges and different pore diameters, designated as bipolar SIM (bp-SIM). The negatively charged layer, called as n-SIM, was 86 nm-thick and consisted of channels with a diameter of 2-3 nm. The positively charged layer with a thickness of 59 nm, termed as p-SIM, was comprised of channels of 4.5-5.5 nm in diameter. They were primarily grown on the solid surface using the StOber-solution and biphasic-stratification growth approaches, respectively, and then exfoliated to obtain perforated structures by the polymer-protected chemical etching and transfer method. The negative charges of n-SIM and positive ones of p-SIM were generated by the deprotonation of pristine surface silanol and postmodified ammonium groups, respectively. Neither n-SIM nor p-SIM alone displays the ICR characteristic, because of their symmetric structure and uniform charge distribution. When laminating two of them, an apparent ICR characteristic was observed for the bp-SIM with a typical diode-like current voltage response. This behavior was rationalized to arise from the asymmetric charge distribution on two layers by finite element simulations. Considering the facile preparation and diverse surface functionalities, as well as its uniform and highly porous structure, the bp-SIM provides an attractive platform for designing ICR-based sensors.