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Active nanofluidics towards ionic machines

Periodic Reporting for period 2 - SHADOKS (Active nanofluidics towards ionic machines)

Reporting period: 2020-01-01 to 2021-06-30

Filtering and water purification rely traditionally on the concept of passive sieving across properly decorated nanopores. Such basic separation principle contrasts with the highly advanced membrane processes existing in Nature, which harness the full subtleties of active transport across channels. This involves advanced functions like ionic pumps, ultra-high selective channels, or voltage-gated nanopores, which all play a key role in many vital needs and neuronal functions.

The Shadoks project is at the frontier of the field of nanofluidics, which explores the transport of fluids and ionic species at the nanometric scales. Over the last 10 years, the domain has undergone a quantum leap with the development of individual artificial channels with nanometric and even sub-nanometric size with manifold geometries. Many ‘exotic’ properties have been indeed unveiled over the recent years [Bocquet, Nature Materials 2020].

The Shadoks projects builds on this flourishing know-how to push the frontiers of nanofluidics in order to invent and develop the concept of artificial ionic machines based on active nanofluidic transport.

The Shadoks project is an experimental project involving a strong theoretical counterpart, essential to experimental advances and prototyping. We investigate a wealth of strongly nonequilibrium transport phenomena occurring at the nanoscales, taking advantage of our unique know-how in building nanofluidic heterostructures, in particular made of carbon and boron-nitride. We target ionic Coulomb blockade, stimulated transport and gated nanopore, ionic pumps, dynamical osmosis. These processes allow to tune ionic fluxes against the gradients and induce out-of-equilibrium charge separation, hereby conceiving active sieving as a novel route for separation and desalination. Those new building blocks will subsequently be assembled to create advanced bioinspired membrane functionalities. Furthermore, we use the active nanofluidics building blocks to mimic a basic machinery of neuronal processes toward primitive artificial ionic machines.
Since the beginning of the project, manifold tasks towards the development of active nanofluidics have been accomplished. However, several key milestones have been already achieved:
First, we have developed a detailed understanding of the ionic Coulomb blockade [Kavokin et al., Nature Nano 2019]. Based on an exact calculation of the statistical properties of ions in a 1D nanotube, we have shown that Coulomb blockade occurs in ionic systems due to a fractional Wien effect, associated with the dissociation of Bjerrum pairs with the gate charge. This mechanism leads unexpectedly to quantized ionic transport in a non-quantum system. Furthermore ionic Coulomb blockade can be harvested to design an efficient ionic pump thanks to out-of-phase gate oscillations. This active device paves the way to fabricate artificial ionic pumps.

Extending the phenomenology to two dimensions - in direct line with the experimental Angströslits systems – we have unveiled a similar Wien effect in the 2D ionic systems.
A groundbreaking outcome in 2D is that the non-linear ionic transport results in memory dependent conduction, due to the slow time dynamics of the 2D pairing. In other words, these 2D systems behave as memristors. Coupling two of these ionic 2D-memristors, in the spirit of Hodgkin-Huxley model of neuronal dynamics, we have shown thanks to a molecular simulation that this nanofluidic device highlight ionic spiking, in full analogy to real neurons. This serendipitous outcome is a quantum leap to develop artificial neural machines, in particular in view of the accessibility to fabricate such 2D systems in the lab.

Second, a strong experimental effort has been achieved to develop active nanofluidic devices. We target in particular the so-called “Single Digit Nanopores” (SDN), with a size below 10nm, which have highlighted many peculiar transport properties. This relies on our expertize to fabricate 1D nano-assemblies, now reaching sub-2nm nanotubes. We have also extended our know-how to fabricate 2D systems and heterostructures, in the footsteps of the recent advances achieved on 2D slits by the Manchester laboratory (Prof. A. Geim et al.). A key experimental result in both 1D and 2D SDNs is the demonstration of stimulated transport [Mouterde et al., Nature 2019; Marcotte et al., Nature Materials, 2020; Emmerich, under review 2021]. The ionic conduction is shown to be non-linearly modulated by the mechanical stimuli, here the applied pressure, akin mechanical transistor, furthermore strongly material dependent. We showed that the far-from-equilibrium response takes its root in the molecular friction of the fluid on the confining walls. These responses echo directly the behaviour of some biological channels, which exhibit activated responses under various stimuli. The mechanosensitive channels such as Piezos, which are involved in touch sensing and in hearing, do highlight a conduction depending on the applied force or pressure. The reported stimulated transport in sub-2 nm carbon nanotubes thus display mechanosensitive responses that resemble strongly those of biological channels [Marcotte et al., Nature Materials, 2020].
The general objective of the Shadoks projects is to unveil new nanofluidic behaviors, in relation to non-equilibrium and active transport, and harness these behaviors as building-blocks towards a ionic machinery. The next steps of the Shadok project follow these tracks which have been maped since the beginning of the project. We highlight in particular a few key projects among the targeted objectives. On the theoretical side, we aim at a better understanding of the ‘bizarre’ water-carbon interface. It emerges from our (and others) experimental reports in distinct systems that water behaves in a most peculiar way close to graphitic interfaces, with numerous examples showing how graphite ‘outperforms’, in some way or the other, alternative confining materials. This cries out for a proper theoretical understanding. Quantum effects are the usual suspect, but how these properties are related to hydrodynamics remains until now to be understood. We have accordingly setup a completely new framework based on a fully quantum treatment, and aiming at understanding the water-carbon interactions (and more generally with metallic and semi-conducting interfaces). A clear answer to these questions is expected in the remaining years of Shadoks, which would represent a groundbreaking step. Furthermore in the context of active nanofluidics, unveiling such couplings would allow for specific electronic engineering of fluid and ion transport, providing news leads to design ionic machines. A second highlight of our quest will be the experimental demonstration of ionic coulomb blockade in 1D systems and of the memristor behaviour in 2D systems. For these behaviors, we have already unveiled the general conditions under which such behaviors may be observed and this does prototype the experimental setups to be designed. The development of new nanofluidics structures will be a key asset in this quest. The ultimate objective to develop a primitive neuronal structure coupling several nanofluidic devices. This paves the way to mimicking elementary neuronal functionalities in an artificial device, which is one of our grand challenge.
shadoks pumping