Ion Transport through Atomically Thin Cap74illaries

I propose to study the size effect in ion transport through capillaries with principal dimensions of few angstroms (Å). Ion sieving is of extreme importance in many natural systems (sub-nm ion channels perform important functions in cellular membranes) and in many technologies including desalination, chemical separation, dialysis, bio-analytics, etc. It has so far been only a distant goal to create artificial channels of this size, tune their properties as required and investigate their functioning. Traditionally, zeolites and porous polymer membranes are used for ionic and molecular sieving but the large size distribution and quest for smart membranes has driven the research in this area. Despite all the progress during the last decades, including the use of nanotubes and advanced nanolithography techniques, this goal could not be even approached, with device dimensions rarely reaching the true nanoscale in a limited number of geometries and with a limited number of materials. This is a formidable challenge, but also a central reason to engage in this fascinating area of research and I want to address this challenge by the use of 2D-atomic crystals. 2D-atomic crystals are highly fascinating and offer a route to the fabrication of "devices-by-design" through van der Waals heterostructure assembly with their properties tuned via chosen materials. If individual atomic planes were removed from a bulk crystal leaving behind flat voids of a chosen height; the tiny empty space has so much to offer in terms of manipulation of fluids, liquids, gases, particles and ions.

Not only is this a groundbreaking technological advancement of the field of nanofluidics but also importantly the proposed capillaries offer a platform for studying fundamental scientific phenomenon of ionic transport in ultimately confined spaces. The key aims of this proposal are (1) investigation of in-depth intrinsic ion transport through these capillaries, including the role of steric effects, ion entry-exit effects especially when the size of ion is comparable to the capillary size, effect of 'quantum' confinement on the hydration shells surrounding the ions inside capillaries, etc. Such in-depth analysis is possible only because the proposed capillaries are atomically clean and involve little surface charge, unlike the previously studied experimental systems (e.g., nanotubes) dominated by the latter. (2) Gaining insights from the fundamentals of ion transport through these slits, smart capillaries will be constructed where the ions can be manipulated by a perpendicular electric field.

The project will be executed at the University of Manchester (UoM) in condensed matter physics group, school of physics which has pioneered graphene/2D-materials research and National Graphene Institute. At the UoM, the graphene group is spread across many schools in the faculty of physics, chemistry, computer science, materials and life sciences, widening the scope of the possible target applications of the smart capillaries and making the project truly interdisciplinary.

Our fabrication approach of angstrom-scale capillaries offers a great flexibility, reproducibility and possibility for design and sophisticated engineering, as described in the proposal. In particular, our fabrication procedures provide a new direction for the already exciting large field of nanofluidics but are not limited to only one area. By tackling a core issue i.e., understanding the intrinsic ion transport, alongside overcoming the primary obstacle to exploiting Å-scale confined spaces for size-selective ion separation, my research will impact across a broad range of fields and technologies including desalination, paving the way to future applications of far-reaching social and economic importance.

Atomic scale capillaries are a true revolution which have thus far never been demonstrated in nanofluidics research. Long-term impact of the research is that these can be employed in wide variety of applications where the nanofluidics is playing a role such as filtration, membrane based separations, desalination, nanoanalytics etc. This ambitious project will have immediate and lasting impact on the physics of ion transport through nanofluidic systems, cementing my position as a research leader and influencing the direction of multiple fields. A number of areas will significantly benefit from this research, including:

Nanofiltration- When we confine materials in such ultra-thin capillaries, their properties and transport at this truly atomic scale are expected to be quite different from those we are familiar with in our macroscopic world. The nanofiltration and molecular separation technology will benefit from the proposed study of ion permeation through Å-scale slits, which are free of defects, disorder and surface charge. These ultra-smooth capillaries will re-validate the so far reported theory and experiments related to the rejection or blocking of small ions through membranes at a high rate while allowing water, making it highly essential to develop state-of-the art Å-scale slits for ionic measurements. The question of whether the confinement could play a role in the filtration of such mixtures remains open, as do the potential implications for ion filtration in water (which would have both significant societal and economic impact). The flexibility in choosing channel wall materials offers a large range of parameters to explore. The insights gained here can be applied to the design of novel next generation nanofluidics-to make dense arrays of short (submicron) capillaries covering mm-size areas which can be of commercial interest for, e.g., nanofiltration, leading to impact across both academia and industry.

Nanocapillary technology-Miniaturization has driven a new industrial revolution and the benefits of it are also evident by the paradigm shift of the nanofluidics field which employs state-of-the-art microfabrication process for making nanocapillaries. The atomic scale capillaries proposed here consist of atomically-flat top and bottom 2D-crystal flakes such as graphene, hBN, MoS2, etc., that are separated by an array of spacers made from few-layer 2D-atomic crystal. My research programme addresses precisely addresses the bottlenecks of miniaturization for advancing the nanofluidics/nanocapillaries research, and as such should be of major benefit to the recently-established National Graphene Institute at Manchester, the industrial partners, and the extensive global community of 2D materials and nanotechnology researchers that exist both within and outside academia.

Chemical physics- It is of fundamental importance to study the charged molecule or metal ion transport through membranes, to understand biological processes. Gate-voltage controlled ionic transport of capillaries gives an opportunity to do this. Control on chemical and physical properties of the designed nanocapillaries (for example, hydrophilicity, surface smoothness) is possible by using different 2D-atomic crystals as channel walls. Such a degree of control on capillary dimensions and measurements was never achieved so far along with surface polarity and conductivity of the walls, as existing methods have one or the other restricted parameter. This topic will open up new academic collaborations with people working in field of nanofluidics for mimicking and understanding ionic processes in biological membranes within the UK as well as globally.
An important aspect of this grant is to encourage public awareness and understanding of cutting-edge nanotechnology, and support for further investment in both fundamental and applied research. Here, the impact will be on local communities within the North West, the wider public within the UK, and abroad