Bipolar Nanochannels: A Systematic Approach to Asymmetric Problems

Abstract

Nanofluidic diodes are capable of rectifying the electrical current by several orders of magnitude. In the current state of affairs, determining the rectification factor is not possible as it depends on many system parameters. In this work, we systematically scan the effects of geometry and excess counterion concentrations (i.e., surface charge effects). We show that the current−voltage response varies between the two extreme behaviors of unipolar and bipolar responses. The exact behavior depends on the geometry and surface charge properties of the system. Here, we have gone beyond the typical setup that only considers the dynamics within the nanochannel itself and we have included the effects of the adjoining microchannels. Systems that include both nanochannels and microchannels exhibit the classical signatures of concentration polarization, such as ionic depletion and enrichment. Here, where we have scanned a wide range of parameters, we show that bipolar and semi-bipolar systems exhibit a wider range of phenomena that are intrinsically more complicated. Our system characterization is for both, the much more investigated case of steady state and the less investigated, but equally interesting, time-transient case. For example, it is common to characterize the system by its steady-state result (current−voltage response, rectification factor, and transport number). Here, we demonstrate that the time-transient behavior of the fluxes can also be used to characterize the system, and that the time-dependent rectification factors and transport numbers are meaningful. The systematic approach taken in this work, and the results presented herein, can be used to further elucidate the complicated behavior of the current−voltage response of nanofluidic diodes and to rationalize experimental results. The insights of this work can be used to enhance and improve the design of all nanofluidic diodes.

Nanofluidic diodes based on asymmetric bio-inspired surface coatings in straight glass nanochannels

“…The differences between the two are related to the distribution of surface charge; a bipolar diode consists of a junction between positively and negatively charged zones of the pore/channel walls, while unipolar diodes have the same surface charge polarity on the walls and usually achieve rectification through geometric asymmetry. For unipolar systems, the current can reach a limiting value for both positive and negative voltages; 31 these limiting currents can only be considerably surpassed by the formation of an electroconvective instability. In contrast, bipolar diodes exhibit more asymmetric current–voltage characteristics than unipolar diodes and reach a limiting current only under reverse bias conditions, while the forward-bias current can increase without restriction.…”

Ionic Current Rectification Induced by Charge Polarity in Janus Graphene Channels

“…Electrolyte concentration and pH , deeply modulate ion enrichment and depletion inside the channel, which in turn impacts the rectification performance of devices or even leads to a reversal of rectification direction . Increasing the solution concentration decreases the Debye length , and alters the range of ion electrostatic shielding as well as the extent to which ions interact with the channel surface.…”

“…27 Stepwisely, varying the local filling position of the nanochannels yields to a better rectifying performance with the aim of amplifying ion concentration differences at opposite voltages. 28 Electrolyte concentration 29 and pH 30,31 deeply modulate ion enrichment and depletion inside the channel, which in turn impacts the rectification performance of devices or even leads to a reversal of rectification direction. 32 Increasing the solution concentration decreases the Debye length 26,33 and alters the range of ion electrostatic shielding as well as the extent to which ions interact with the channel surface.…”

Regulating Nonlinear Ion Transport through a Solid-State Pore by Partial Surface Coatings

“…The strength of ion selectivity depends on the channel size and Debye length, which are closely related to the ion concentration and mobility, and the physical conditions of the channel. , In addition, multicomponent and multivalent electrolyte solutions also have varying degrees of influence on ion transport , because of the ion-dependent surface charge regulation . Therefore, many approaches are used to modify the properties of ion transport, including but not limited to, molecule modification, − high-dimensional materials, − and gate potential. , Furthermore, bipolar channel − or Janus membrane are reported to achieve the function of an ionic diode, even a solid-state bipolar junction transistor (BJT) …”