Ionic Current Rectification Induced by Charge Polarity in Janus Graphene Channels

Abstract

Ionic current rectification (ICR) in nanofluidic diodes is determined by a combination of channel geometry and surface charge, where most of the previous works have focused on charged asymmetric channels. In this work, through a series of molecular dynamics simulations, we find a surprising ICR phenomenon in a Janus graphene channel, whose geometry is symmetric, while the surface charge is asymmetric by tuning the ratio of cationic and anionic surface modification. A key observation is that when the electric field changes from positive to negative direction, the ionic current exhibits a switchable on−off state depending on whether the dipole moment of the channel's inner surface is parallel to the electrical force. Strikingly, the ICR ratio shows a maximum behavior with the increase in cationic modification, attributed to the change in the range of electrostatic repulsion under the negative electric field. Furthermore, for a given modification ratio, with the increase in graphene layer distance, the ICR ratio also displays a maximum behavior because of the reduced range of the electric double layer (EDL). For small layer distance, the EDL overlap leads to a Coulomb blockade effect under the positive electric field, corresponding to a small ionic current. However, the large layer distance reduces the influence of EDL on ion transport owing to the weakened electrostatic interactions, which ultimately leads to the decrease in ICR ratio. Our results shed light on the essential role of charge polarity in ICR performance, which could open a new window for the design of novel nanofluidic diodes based on symmetric Janus channels.

Desalination Performance in Janus Graphene Oxide Channels: Geometric Asymmetry vs Charge Polarity

“…The number distributions of ions along the z -axis for the six systems are presented in Figure . The results show that the ions distribute mostly in the antielectrical region, similar to the case in electric fields, i.e., Na + is in the COO – -functionalized region and Cl – is in the NH 3 + -functionalized region. Furthermore, the ion distribution peaks locate at the position of residue functionalization, originating from localized ion retention induced by a robust electrostatic interaction, which forms a solid electrical double layer (EDL).…”

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

“…As seen in Figure , we construct bipolar Janus GO channels by attaching negatively (COO – ) and positively (NH 3 + ) charged residues onto the inner surface of the graphene channel, resembling the carboxylation and amination of GO. , Experimentally, the charge proportion of Janus channels is determined by measuring the etching depth via modulation of the functionalization time . Previous work has demonstrated that the Janus GO channels with a 1:1 ratio of positive to negative residues have the best rectification performance, and therefore, we focus on this proportion for the present work. The absolute density of surface charge in all investigated channels is fixed at 1.0 e/nm 2 , similar to previous experimental work. , The entire simulation system is shown in Figure S1 of the Supporting Information, where the Janus GO channel is embedded between two graphene sheets in an ionic solution.…”

“…36,37 Experimentally, the charge proportion of Janus channels is determined by measuring the etching depth via modulation of the functionalization time. 38 Previous work has demonstrated that the Janus GO channels with a 1:1 ratio of positive to negative residues have the best rectification performance, 15 and therefore, we focus on this proportion for the present work. The absolute density of surface charge in all investigated channels is fixed at 1.0 e/nm 2 , similar to previous experimental work.…”

“…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 .…”

Influence of the Dielectric Constant on the Ionic Current Rectification of Bipolar Nanopores

“…In particular, the latter simulations examined the impact of surface charge in bipolar cylindrical pores on the overall membrane’s performance, with a special emphasis on the relevance of electrokinetic phenomena . A different, recent approach relied on molecular dynamics (MD) simulations to study the ICR in Janus graphene channels . That report considered highly confined geometries with 1–2 nm pore heights and an asymmetric surface charge.…”