Ionic current through nano-channels

Membranes (natural or artificial) are ubiquitous. They ensure compartmentalization, in living organisms at all scales (nucleus, mitochondria, cell, organ, organism), but also in all artificial electrochemical devices (batteries, power cells). Basically, the role of this compartmentalization is to slow down the chemical reactions between constituents of each compartment. But these chemical reactions are nevertheless necessary for the device to operate. A membrane plays thus a double and contradictory role : it must separate, but not too much. For this, membranes are generally equipped with pores (holes with length and diameter of the same order of magnitude) or channels (length much greater than diameter) allowing the transport of ions or molecules from one compartment to the other. We are interested to these latter.

The properties of a liquid, for example an electrolyte, inside a nanoscale channel are quite different from those commonly observed in a much larger space. Confinement disturbes the mutual organization of molecules (for instance the transient network formed by water molecules) and also gives a particular importance to phenomena that occur on the surface.

These two kinds of effect are expected to impact the transmembrane transport. We have studied them by the mean of measurements of electric transport through nanochannels inside artificial membranes. Beyond certain technical aspects which have been addressed, the main experimental facts reported are as follows.

Concerning the first kind of effect, we showed that depending on the electrolyte the transport was facilitated or hampered compared to what it is in a large space, and we have linked this effect to a noise analysis of the electric current, that is to say an analysis of time correlations of the conductance.

As for the second kind of effect, we expect in particular an important role of the shape of the channel. When the transit is slow enough for the ions to have time to explore the channel transversely by Brownian motion, longitudinal variations of its cross-section produce an entropy gradient to which corresponds a force which is all the greater as the ions are large. So that, when the channel displays an asymmetrical shape (e.g. conical like that of a funnel), this effect can induce a non-return mechanism similar to a ratchet wheel or the valve of a pump operating at a molecular level. These systems are known as “Brownian ratchets” because they are mainly driven by thermal agitation and entropy.

Ionic transport through a conical nanochannel (scale not respected). The shape produces an entropic component in the potential of charge carriers giving it a ratchet profil.

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