Ion Current Rectification and Long-Range Interference in Conical Silicon Micropores

Abstract

Fluidic devices exhibiting ion current rectification (ICR), or ionic diodes, are of broad interest for applications including desalination, energy harvesting, and sensing, among others. For such applications a large conductance is desirable, which can be achieved by simultaneously using thin membranes and wide pores. In this paper we demonstrate ICR in micrometer sized conical channels in a thin silicon membrane with pore diameters comparable to the membrane thickness but both much larger than the electrolyte screening length. We show that for these pores the entrance resistance is key not only to Ohmic conductance around 0 V but also for understanding ICR, both of which we measure experimentally and capture within a single analytic theoretical framework. The only fit parameter in this theory is the membrane surface potential, for which we find that it is voltage dependent and its value is excessively large compared to the literature. From this we infer that surface charge outside the pore strongly contributes to the observed Ohmic conductance and rectification by a different extent. We experimentally verify this hypothesis in a small array of pores and find that ICR vanishes due to pore−pore interactions mediated through the membrane surface, while Ohmic conductance around 0 V remains unaffected. We find that the pore−pore interaction for ICR is set by a long-ranged decay of the concentration which explains the surprising finding that the ICR vanishes for even a sparsely populated array with a pore−pore spacing as large as 7 μm.

Iontronic neuromorphic signalling with conical microfluidic memristors

“…On the far side of the reservoir connected to the base we impose an electric potential V (t), while the far side of the other reservoir is grounded, which leads to an electric potential profile Ψ(x, r,t), an electro-osmotic fluid flow with velocity field u(x, r,t) and ionic fluxes j ± (x, r,t) with j + − j − the charge flux. A relatively low surface potential ψ 0 ensures a weak electro-osmotic flow Q(V ), which in turn allows us to tune the channel conductance over a wider voltage range [20,44,73]. We have Q(V )/V = −πR t R b εψ 0 /(ηL) ≈ 22.7 µm 3 s −1 V −1 for our standard parameter set [20].…”

“…[20] the stationary state version of the PNPS equations ( 1)-( 4) is solved for a static potential V , which gives rise to the stationary charge current I = g ∞ (V )V , where the static conductance g ∞ (V ) = g 0 L 0 ρ s (x)dx/(2ρ b L) depends on the Ohmic cone conductance g 0 = (πR t R b /L)(2ρ b e 2 D/k B T ) and the radially averaged salt concentration ρ s (x) = 2 R(x) 0 rρ s (x, r)dr/R(x) 2 , with ρ s = ρ + + ρ − , so g ∞ (V ) is determined by the laterally averaged salt concentration. For small potentials e|V |/k B T |w|(R b /R t ), with w = eDη/(k B T εψ 0 ) −9.5 the ratio of ionic to electro-osmotic mobility [73], the pore concentration equals the bulk concentration ρ s (x) = 2ρ b , yielding g ∞ (V ) = g 0 and the resulting current follows Ohm's law I = g 0 V . For large static potential drops the cone exhibits diodic behaviour [20] due to concentration polarisation, the dimensionless channel conductance in steady state being…”

Enhanced Rectification Performance in Bipolar Janus Graphene Oxide Channels by Lateral Electric Fields

“…Previous work showed that the geometric asymmetry improves the ICR ratio due to the enhanced difference in the EDL distribution . Actually, both the geometric asymmetry and charge polarization synergistically enhance the rectification performance . However, our simulation results demonstrate that at the nanoscale, the rectification performance of symmetric bipolar Janus GO channels is significantly outperformed over asymmetric ones.…”

“…10 Actually, both the geometric asymmetry and charge polarization synergistically enhance the rectification performance. 11 However, our simulation results demonstrate that at the nanoscale, the rectification performance of symmetric bipolar Janus GO channels is significantly outperformed over asymmetric ones. This improvement stems from the fact that the symmetric structure perfectly enlarges the differences in ion transport under opposite electric fields.…”

“…This phenomenon is generally caused by the disparities in the ionic transmembrane environment when subjected to opposing bias voltages. The ICR performance is determined by many factors, such as the channel geometry, − anisotropic interfacial materials, , and surface charge polarization, , all of which offer a synergistic contribution to the ion transport. Furthermore, the ICR also is significantly influenced by the overlap of ionic concentration polarization (ICP) regions , and the interface resistance induced by ICP .…”