Tunable ion transport through ultimately small channels

Biological membranes allow not only fast and selective ion permeation but also tunable ion transport, passively or actively on demands, in response to external stimuli such as light, voltage, temperature, etc. At the core of the membrane is the ultimate small ion channels approaching the dimension of a single ion and water molecule in the angstrom scale. To mimic and better understand the functioning of biological ion channels, artificial systems of similar sizes are developed recently. As novel platforms, these systems provide insights into many important problems such as mechanisms of non-continuum ion transport that are difficult to be investigated in larger structures. The unique couplings among ions, channels and various external stimuli at this spatial scale further inspire potential technologies where efficient and tunable ion transport is required. We review the main concepts of creating angstrom-scale channels and focus on discussing the tunable ion transport behavior inside. Permeability and selectivity of ion permeation controlled by light, electric field and solution environment variation are highlighted.