Enhanced Gating Effects in Responsive Sub-nanofluidic Ion Channels

Abstract

Conspectus The 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 = I Open/I Closed, where I Open and I Closed 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 gatin...

Ion transport in nanofluidics under external fields

“…The design and synthesis of functional smart molecules that can respond to external stimuli and their introduction into nanofluidic systems can realize single or multi-response ion regulation mechanisms. 25,120 Photoisomeric molecules such as azobenzene (Azo) and spiropyran (SP) have been used to design functionalized photoisomerized nanofluidic systems to achieve ion gating originating from the change of channel size and wettability. 19,121 Thermo-responsive nanochannels rely on the collapsing and swelling properties of temperature-responsive polymers to control the pore size for ion gating.…”

“…Despite the rapid development of nanofluidics, other interesting ion gating features (tunable ion selectivity ratio), not limited to high r on–off , have also been developed a lot. 120 By implanting Azo as the light-switchable linker to bridge the flexible building blocks, smart covalent organic network membranes with on–off–on light-switchable pores that can dynamically tune solvent permeance and dye rejection can be achieved. 119 Xu's group reported a sub-2 nm COF membrane with high monovalent cation permeation rates and extremely low multivalent cation permeability, leading to a high K + /Mg 2+ selectivity of about 765, realizing ultrahigh monovalent ion sieving.…”

“…10c). 120,122 More importantly, these sub-nanoscale nanofluidic systems exhibit better ion transport performance than nanoscale channels as the sub-1 nm confined space allows for more precise control of ion transport under external fields. The selectivity of K + /Mg 2+ increased from 10 2 to 10 3 with the increase of pH.…”

Mechanoresponsive Flexible Crystals

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Ultrahigh Lithium Selective Transport in Two-Dimensional Confined Ice

“…Selective ion transport within nanoconfined spaces is widely observed in nature, and it is critical for energy conversion and signal transduction in both biological and artificial systems. , Ion channels in biological systems, characterized by ion permselectivity, gating, and rectification effects, − facilitate selective transmembrane transport of specific ions. A notable instance is the potassium ion channel (KcsA), which exhibits a strong preference for K + over Na + , with a selectivity ratio exceeding 1000. ,− This natural phenomenon has inspired the development of artificial ion-selective channels to sieve monovalent cations, , addressing the growing demand for Li + in sectors such as batteries, − glass fibers, and polymers. , However, these artificial channels face challenges in selectively sieving monovalent cations due to their similar valence and affinity, alongside minimal differences in the radius of hydrated and dehydrated ions. − …”