Nature provides a huge range of biological materials, just as ion channels, with various smart functions over millions of years of evolution, and which serve as a big source of bio-inspiration for biomimetic materials. In this critical review, a strategy for the design and synthesis of biomimetic smart nanopores and nanochannels is presented and put into context with recent progress in this rapidly growing field from biological, inorganic, organic to composite nanopore and nanochannel materials, which can respond to single/multiple external stimuli, e.g., pH, temperature, light, and so on. This review is intended to utilize a specific responsive behavior for regulating ionic transport properties inside the single nanopore or nanochannel as an example to demonstrate the feasibility of the design strategy, and provide an overview of this fascinating research field (109 references).

Living organisms make extensive use of micro-and nanometer-sized pores as gatekeepers for controlling the movement of fluids, vapors and solids between complex environments. The ability of such pores to coordinate multiphase transport, in a highly selective and subtly triggered fashion and without clogging, has inspired interest in synthetic gated pores for applications ranging from fluid processing to 3D printing and labon-chip systems 1,2,3,4,5,6,7,8,9,10 . But although specific gating and transport behaviors have been realized by precisely tailoring pore surface chemistries and pore geometries 6,11-17 , a single system capable of selectively handling and controlling complex multiphase transport has remained a distant prospect, and fouling is nearly inevitable 11,12 . Here, we introduce a gating mechanism that uses a capillary-stabilized fluid to seal pores in the closed state, and reversibly and rapidly reconfigures it under pressure to create a non-fouling, fluid-lined pore in the open state. Theoretical modeling and experiments demonstrate that for each transport substance, the gating threshold -the pressure needed to open pores -can be rationally tuned over a wide pressure range. This allows us to realize in one system differential response profiles for a variety of liquids and gases, even letting liquids flow through the pore while preventing gas escape. These capabilities allow us to dynamically modulate gas/liquid sorting in a microfluidic flow and to separate a three-phase air/water/oil mixture, with the fluid lining ensuring sustained antifouling behavior. Because the liquid gating strategy enables efficient long-term operation and can be applied to a variety of pore structures, membrane materials and micro-as well as macro-scale fluid systems, we expect it to prove useful in a wide range of applications.Our hypothesis that a liquid-filled pore could provide a unified gating strategy derives from the idea that a liquid stabilized inside a micropore offers a unique combination of dynamic and interfacial behaviors, and is inspired by nature's use of fluids as reconfigurable gates. Microscale stomata and xylem control air, water, and microbe exchange in plants by using fluid to mechanically reconfigure the pore 18 . The nuclear pore is directly lined with disordered fluidlike proteins that have been proposed not only to regulate differential transport of a wide range of cargos, but also to completely prevent fouling 19 . Most interestingly, micropores between air sacs in the lung are filled with liquid that has been proposed to reversibly reconfigure into an open, fluid-lined pore in response to pressure gradients 20 . Figure 1 contrasts the gating mechanisms in a traditional and in a liquid-filled pore. In the case of traditional nano/micropores (Fig.1a), gases will flow through passively regardless of 2 pore shape and surface chemistry, while liquids will enter the pore once the applied pressure reaches a critical value dictated by the balance of surface interactions, pore geometry and surface tension....

Switchable ion channels that are made of membrane proteins play different roles in cellular circuits. Since gating nanopore channels made of proteins can only work in the environment of lipid membrane, they are not fully compatible to the application requirement as a component of those nanodevice systems in which lipid membranes are hard to establish. Here we report a synthetic nanopore-DNA system where single solid-state conical nanopores can be reversibly gated by switching DNA motors immobilized inside the nanopores. High- (on-state) and low- (off-state) conductance states were found within this nanopore-DNA system corresponding to the single-stranded and i-motif structures of the attached DNA motors. The highest gating efficiency indicated as current ratio of on-state versus off-state was found when the length of the attached DNA molecule matched the tip diameter of the nanopore well. This novel nanopore-DNA system, which was gated by collective folding of structured DNA molecules responding to the external stimulus, provided an artificial counterpart of switchable protein-made nanopore channels. The concept of this DNA motor-driven nanopore switch can be used to build novel, biologically inspired nanopore machines with more precisely controlled functions in the near future by replacing the DNA molecules with other functional biomolecules, such as polypeptides or protein enzymes.