If the electrical currents that flow through our computers are due to electron movements, all the circuitry of living beings is based on the transport of ions (including sodium, chlorine, calcium, potassium ions). In biological systems, there is indeed an artillery of ion channels performing very varied functions, exploiting the often exotic behaviour of ion transport at molecular scales. Achieving such functionality in artificial channels remains a considerable technical challenge.
Researchers from the Micromégas team of the ENS Physics Laboratory (ENS, CNRS, SU, Paris-Diderot), in collaboration with the laboratory of Prof. André Geim of the University of Manchester, have successfully developed sub-nanometric ion channels and have studied their properties. This work was published in the journal Nature.
The so-called van der Waals assembly consists of stacking two-dimensional atomic layers in the manner of Lego bricks and makes it possible to build a battery of very large artificial channels on a molecular scale (0.1 µm) and yet of infinitesimal thickness (0.6 nm), all in a smooth way on an atomic scale. With this strong containment, water and ions are forced to organize themselves into a two-dimensional monolayer. The experiments then showed that a considerable ionic current is generated when a flow is induced in the channel by applying a pressure difference. But more surprisingly, this flow current can be modulated extremely sensitively by the joint application of an electric field: a voltage of 0.1 V multiplies the mobility by a factor of 20 (cf Fig. 1) ! This phenomenon, unprecedented for ion transport, which is similar to a transistor effect, can be explained by the differential friction of water and ions on the walls at these molecular scales. As the effect of the walls becomes important, it is even possible to probe the electronic properties of the confining material by studying this phenomenon.
Figure 1 : Flow current Istr per channel as a function of the pressure gradient ∆P/L for different voltages ∆V between -75 mV and 75 mV. Figures (a) et (b) show the results for graphite and boron nitride channels respectively (L being the channel lenght).
This coupling between voltage and pressure at minute scales is very similar to those observed in mechanically sensitive biological ion channels such as PIEZO1. Is it then possible to better understand the extreme confinement situations at work in living systems, and in the longer term to mimic elementary computational functions based on ion transport? The study of these artificial channels at the molecular level still conceals many surprises...