Enhanced Gating Effects in Responsive Sub-nanofluidic Ion Channels

ConspectusThe smart regulation of ion flow in biological ion channels (BICs) is vital to life. In general, intelligent BICs possess three main functions: (i) to selectively transfer specific ions, (ii) to quickly conduct specific ions, and (iii) to responsively control the flow of ions. Since the early exploration of potassium (K+) and sodium (Na+) channels began in the 1950s, the gating behaviors of BICs have been investigated for more than 70 years. Taking the first reported voltage-gated ion transport process as an example, a gate, which acts as the voltage sensor in BICs, detects variation in the membrane voltage, triggering the opening and closing of the ion channels. A gating ratio (GR) can describe the gating effect of a BIC, GR = IOpen/IClosed, where IOpen and IClosed are measured ion currents of the channel at open and closed states, respectively. BICs usually have strong gating effects with an extraordinarily high gating ratio, which can be up to infinity for channels with zero-current closed states. Inspired by nature, artificial ion channels (AICs) have been constructed to control ion permeation intelligently. Since 2004, a wide range of AICs have been developed to regulate the flow of ions via external stimulation (i.e., light, voltage, pH, magnetic field, and temperature). These ion nanochannels, usually constructed with intrinsic or guest functionalities that are responsive to environmental simulation, drive the opening and closing of the channels. However, the gating performances of such nanoscale ion channels (i.e., gating ratios usually between 1 and 30) are far below those of BICs, due to the relatively larger nanopores in AICs, which cannot entirely block ion transport in the off states. Over the past decade, emerging advanced materials (i.e., 1D nanotubes, 2D nanosheets, and 1D-3D sub-nanoporous frameworks) with intrinsic sub-nanometer pores and stimuli-responsive properties have provided promising tools to fabricate responsive sub-nanofluidic channels with efficient gating performance. These AICs are remarkably comparable to their biological counterparts, because their more confined spaces enable a more effective closed state of the channels. Our team has developed a series of responsive sub-nanofluidic channels based on metal–organic frameworks, covalent organic frameworks, and 2D nanosheets. These sub-nanofluidic channels exhibit much higher on–off gating ratios than nanofluidic channels do, and the gating effects can be maintained over a wide range of ionic concentrations. Moreover, sub-nanofluidic channels also show stimuli-tunable ion selectivity and ion blockage effects. Therefore, this Account first summarizes recent progress in fabrication and functionalization methods for constructing artificial responsive sub-nanoscale ion channels and then compare the gating principles of sub-nanochannels and nanochannels, before discussing the unique gating effects of sub-nanofluidic channels (i.e., large ion blockage effect, high gating ratio, stimuli-tunable ion selectivity, and wide gating applicable ionic concentration range). Next, the applications of sub-nanofluidic channels/membranes for sensing ions, energy harvesting, ion adsorption, and ion separation are presented. Finally, we offer a perspective on the future development of artificial responsive sub-nanofluidic channels that further improve gating performance and have applications in real-world devices.