Enhanced Gating Effects in Responsive Sub-nanofluidic Ion Channels

Conspectus The smartregulation of ion flow in biological ion channels (BICs)is vital to life. In general, intelligent BICs possess three mainfunctions: (i) to selectively transfer specific ions, (ii) to quicklyconduct specific ions, and (iii) to responsively control the flowof ions. Since the early exploration of potassium (K+)and sodium (Na+) channels began in the 1950s, the gatingbehaviors of BICs have been investigated for more than 70 years. Takingthe first reported voltage-gated ion transport process as an example,a gate, which acts as the voltage sensor in BICs, detects variationin the membrane voltage, triggering the opening and closing of theion channels. A gating ratio (GR) can describe the gating effect ofa BIC, GR = I (Open)/I (Closed), where I (Open) and I (Closed) are measured ion currents of the channelat open and closed states, respectively. BICs usually have stronggating effects with an extraordinarily high gating ratio, which canbe up to infinity for channels with zero-current closed states. Inspiredby nature, artificial ion channels (AICs) have been constructed tocontrol ion permeation intelligently. Since 2004, a wide range ofAICs have been developed to regulate the flow of ions via externalstimulation (i.e., light, voltage, pH, magnetic field, and temperature).These ion nanochannels, usually constructed with intrinsic or guestfunctionalities that are responsive to environmental simulation, drivethe opening and closing of the channels. However, the gating performancesof such nanoscale ion channels (i.e., gating ratios usually between1 and 30) are far below those of BICs, due to the relatively largernanopores in AICs, which cannot entirely block ion transport in theoff 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 propertieshave provided promising tools to fabricate responsive sub-nanofluidicchannels with efficient gating performance. These AICs are remarkablycomparable to their biological counterparts, because their more confinedspaces enable a more effective closed state of the channels. Our teamhas developed a series of responsive sub-nanofluidic channels basedon metal-organic frameworks, covalent organic frameworks, and2D nanosheets. These sub-nanofluidic channels exhibit much higheron-off gating ratios than nanofluidic channels do, and thegating effects can be maintained over a wide range of ionic concentrations.Moreover, sub-nanofluidic channels also show stimuli-tunable ion selectivityand ion blockage effects. Therefore, this Account first summarizesrecent progress in fabrication and functionalization methods for constructingartificial responsive sub-nanoscale ion channels and then comparethe gating principles of sub-nanochannels and nanochannels, beforediscussing the unique gating effects of sub-nanofluidic channels (i.e.,large ion blockage effect, high gating ratio, stimuli-tunable ionselectivity, and wide gating applicable ionic concentration range).Next, the applications of sub-nanofluidic channels/membranes for sensingions, energy harvesting, ion adsorption, and ion separation are presented.Finally, we offer a perspective on the future development of artificialresponsive sub-nanofluidic channels that further improve gating performanceand have applications in real-world devices.