Listening to the Electrical Noise for Nanofluidic Sensing – LENNS

We propose to transfer the concept of listening to the electrical noise from condensed matter physics, where it has been very successful, to nanofluidics and nano-electrochemistry. As technology develops toward smaller scales, applications such as nano-sensors become limited by the ability to interpret the measured signal. The aim is to better describe solid-liquid interfaces on scales smaller than 100 nm and retrieve the information on microscopic interfacial processes encoded in current fluctuations through nanotubes, for single molecule detection, and charge fluctuations of an electrode, that underlie Electrochemical Correlation Spectroscopy in nanofluidic devices.

Going down to the nanoscale poses two challenges for modelling: (i) the ultimate limit of the molecular nature of matter, which implies the breakdown of continuous theories such as macroscopic hydrodynamics or electrostatics (ii) the predominance of interfacial processes, with phenomena covering and sometime coupling several length and time scales, beyond the reach of molecular descriptions. We shall consider systems where the smallest dimension is much larger than the molecular one. Using continuous solvent models, we will focus (i) on key aspects induced by the large surface to volume ratio and leading to electrical fluctuations (small number of charge carriers; coupling between hydrodynamics, electrostatics and adsorption/desorption); (ii) on the corresponding challenges for modelling (overlap between electrical double layers, coupled length and time scales, electrokinetic effects, ...).

We will develop the theoretical framework and the simulation tools necessary to understand and model charge, current and potential fluctuations in systems involving fluids confined to the nanoscale. In order to go beyond mean-field approaches at the Poisson-Nernst-Planck level, we will use mesoscale Brownian Dynamics and Lattice-Boltzmann Electrokinetics (LBE) simulations. While the former allows to better account for ionic correlations and naturally includes thermal fluctuations, the latter accounts for the coupling of the ions with the solvent flow. We will extend these tools to impose a constant potential for a metallic wall, and consider the effect of sorption/desorption at the surface. We will also introduce thermal fluctuations into LBE simulations: This carries a larger risk but would provide a breakthrough in the simulation of complex fluids in general.

We will then apply these tools for single molecule detection and nano-electrochemistry. In a first step, simulations will be performed in a number of geometries and conditions representative of the corresponding application, in order to predict the expected signal (direct problem). Analysing these results will allow us to assess the possibility of extracting information on microscopic processes from the corresponding experiment in the standard design and conditions (inverse problem). Finally, we will explore new strategies and optimised setups, in order to push the detection limits. This will enable experimentalists and engineers to exploit their tools beyond the current state-of-the-art, e.g. at low ionic strength, and to anticipate future technological developments, e.g. working at higher frequencies or with smaller devices.

On the longer term, our research will have implications, beyond the scope of the proposal, for the development of nanofluidic sensors and more generally nanofluidics for the Factory of the Future, including other components not considered here such as nanoISFETs (Nanoscale Ion Sensitive Field Effect transistors). The methodologies and simulation tools developed during this project will be transferrable to other scientific and technological contexts, including biological sensors or the production of "Blue energy" from salinity gradients (e.g. between salty sea water and fresh river water) and sea water desalination.