Understanding and control of ion transport in a fluidic channel is of crucial importance for iontronics. The present study reports on quasi-stable ionic current characteristics in a SiNx nanopore under a salinity gradient. An intriguing interplay between electro-osmotic flow and local ion density distributions in a solid-state pore is found to induce highly asymmetric ion transport to negative differential resistance behavior under a 100-fold difference in the cross-membrane salt concentrations. Meanwhile, a subtle change in the salinity gradient profile led to observations of resistive switching. This peculiar characteristic was suggested to stem from quasi-stable local ion density around the channel that can be switched between two distinct states via the electro-osmotic flow under voltage control. The present findings may be useful for neuromorphic devices based on micro- and nanofluidic channels.

Fundamental understanding of ion transport in a fluidic channel is of critical importance for realizing iontronics. Here we report on asymmetric ion transport in a low thickness-to-diameter aspect ratio nanopore. Under uniform salt concentration conditions, the cross-pore ionic current showed ohmic characteristics with no bias polarity dependence. In stark contrast, despite the weak ion selectivity expected for the relatively large nanopores employed, we observed diode-like behavior when a salt gradient was imposed across the thin membrane. This unexpected result was attributed to the electroosmotic flow that served to modulate the access resistance through dragging the condensed ions into or out of the nanopore orifices. The simple mechanism was also revealed to be effective in fluidic channels of various size from micro- to nanoscale enabling rectification of the property engineering by the pore geometries. The present findings allow for novel designs of artificial ion channel building blocks.

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