Surge-protected mechanical amplifier of ion transport devised using a bipolar nanopore with broken symmetry.

Abstract

Electrokinetic ion transport under coupled external electric field and pressure drops has recently exhibited giant, but monotonous, mechanical amplifications of ionic current in some nanochannels. While such effects can be exploited to develop novel pressure-sensitive ionic circuits, they can also yield large undesirable current surges if the control parameter, i.e., the pressure drop, overshoots. To overcome this critical issue, we design a nanopore that displays a similar mechanical activation of ionic current, but up to a limited magnitude of pressure, after which current ceases to grow. We integrate a bipolar surface charge distribution with an asymmetric channel geometry in a nanopore to alter the effect of an applied pressure drop on the quasistatic distribution of ions in the pore, which directs how the ionic conductance varies with pressure, in a way that yields the desired current-pressure response. Via such steps, analyzed using numerical simulations, we devise a funnel-shaped nanopore with a negatively charged conical mouth and a positively charged elongated cylindrical stem, that acts as the intended "surge-protected" mechanical amplifier of ion transport. It yields a substantial pressure-driven growth in ionic current (by factors of ∼4) up to a limited intensity of pressure drop exerted (along a constant external electric field) toward the stem, beyond which current begins to fall. Besides exhibiting this hitherto undetected electrokinetic behavior, our study implements and paves the way to an innovative "function to design" approach in the field of electrokinetics that can help build improved nanofluidic devices with desired advanced ion transport functionalities.