Mechanical Nanomanipulation of Water Confined in a Naturally Occurring Water-Hybrid 2D System

Water is the matrix of life and its confinement in nanocavities and flow through nanocapillaries are central topics from geophysics to nanotribology. Water in nanoscale media can show distinct elastic and viscosity behaviors, presenting exquisite properties such as low dielectric constant [1]. A more recent development in the emergence of nanofluidics is the ability to fabricate artificial nanocapillaries for water transport through van der Waals (vdW) assembly of atomically-thin two-dimensional (2D) materials, such as hexagonal boron nitride and graphene [2]. Although the engineering of these nanocapillaries can be beneficial in controlling specific properties, there is plenty of natural water-based heterostructures barely explored so far. These water-hybrid 2D systems intercalate water layers in the lamellar structure of naturally occurring vdW materials that can show unprecedented and highly tunable properties for nanotechnology applications.<br/>Hydrous minerals act as natural nanocavities for water transport on Earth, which is essential to biochemical processes with cycling of nutrients and elements. In this scenario, phyllosilicate minerals emerge as an abundant class of hydrous minerals that are wide band gap insulators. They can be exfoliated down to monolayers due to their lamellar structure that stacks silicon oxide tetrahedral layers with (Al,Mg)-octahedral layers with O/OH at the vertices [3]. Some specimens of phyllosilicates can be pillared by ions, such as clays, or further octahedral layers, such as chlorites. They present the unique ability to absorb water by confining water layers in the interlamellar space, being water-hybrid 2D systems of natural occurrence.<br/>Theoretical studies suggest that the electrostatic charge distribution due to the presence of substitutional ions in the structure of phyllosilicates modulates how water confines between its layers [4]. Within the barely explored group of chlorite phyllosilicates, we can identify clinochlore as the specimen that favors higher incorporation of interlamellar water due to its more complex structure. In a previous works, we observed a contribution about 14%wt attributed to the presence of water/hydroxyl ions in clinochlore by quantitative analysis [5]. This expressive water-related amount suggests clinochlore as one of the most suitable natural platforms for water confinement in 2D systems among phyllosilicate minerals. However, the hydration of phyllosilicates by interlamellar water intercalation is not a well understood process besides its importance in several fields.<br/>In the forefront of advancing our understanding of nanoconfined water in 2D systems, we will present the nanoimaging of interlamellar water in ultrathin exfoliated clinochlore by infrared scattering-type scanning near-field optical microscopy. We observed a striation pattern for the nanoconfined water capillaries in clinochlore that changes the overall dielectric properties of the system and, consequently, its surface potential probed by Kelvin probe force microscopy. As a unique result, we performed the nanomanipulation of confined interlamellar water in clinochlore by atomic force microscopy in contact mode. By applying a constant local force between the tip and the sample during the scanning, it was possible to drain practically all the fluid from the analyzed area in a controlled manner, changing the clinochlore surface potential. This pioneering result allows the multifunctionalization of these natural water-hybrid 2D system for catalytic applications, sensing, microfluidic devices, and patterning of biomolecules in 2D systems.<br/><b>Acknowledgements</b><br/>We thank our collaborators, funding agencies (CNPq, CAPES, FAPESP and FAPEMIG), Neaspec, ALS, UFMG and CNPEM.<br/><b>References (DOI)</b><br/>[1] 10.1126/science.aat4191<br/>[2] 10.1038/nature19363<br/>[3] 10.1038/s41699-020-00172-2<br/>[4] 10.1346/CCMN.1990.0380510<br/>[5] 10.1016/j.apsusc.2022.153959