Nanopores in Self-Assembled Monolayer-to-Multilayer MXene Films—From Fabrication to Application

In the last two decades, nanopore technology has revolutionized single-molecule studies of biomolecules by providing researchers with an inexpensive, rapid, and high-throughput tool for sensing, manipulating, and sequencing individual biopolymers (DNA, RNA, and proteins). In this technique, as a biomolecule is electrophoretically pulled through a nanometer-sized pore (either a biological or a synthetic nanopore), occlusion of the biomolecule within the nanopore is detected as a characteristic transient disruption in the ionic current. A hallmark feature of the nanopore technique is the dependence of the sensing resolution on the geometry of the nanopore (diameter and thickness in solid-state nanopores). Therefore, theoretically, two-dimensional (2D) materials afford the highest resolution possible, owing to their single to a few-atomic-layer thickness. We report on a 2D material-based nanopore platform based on MXenes, a new family of 2D transition metal carbides, nitrides, or carbonitrides with a unique combination of electrical, electrochemical, and mechanical properties with facile synthesis and processing in water due to their hydrophilicity¹. MXenes have shown outstanding performance in energy storage, electromagnetic interference shielding, and sensing applications, currently moving toward commercialization and industrial use. Nonetheless, while most 2D materials can be grown on a substrate and then transferred to the device carrier of choice, a major hurdle in MXene implementation into nanoscale actuators and sensors is the lack of a large-scale assembly and fabrication method of MXene films with a controlled number of layers. We have developed and refined a rapid liquid-liquid interfacial self-assembly method to fabricate wafer-scale MXene films with control over nominal film thickness (monolayer to multilayer), film density, and lateral dimension without the need for any specialized instrumentation. In this method, MXene flakes rapidly self-assemble to form a floating mosaic structure film with an arbitrary size at the interface of water and an organic solvent in the presence of co-solvent². Reactive force-field (ReaxFF) molecular dynamics simulations demonstrated the importance of the co-solvent in the ternary mixture, which renders the aqueous/organic interface energetically favorable for MXene flakes to reside while other factors such as surface energy and electrostatic repulsion collectively promote MXene film formation. After transferring the assembled film onto the substrate, we adopted monolayer to multilayer free-standing MXene films for single-molecule biomolecular sensing using nanopores directly fabricated through the membrane. Our results show that ultrathin MXene membranes afford high mechanical robustness, long-time stability, and low-noise ionic current recordings suitable for single-molecule detection³. Moreover, reversible intercalation of cations through the interlayer space of ultrathin free-standing MXene membranes couples a nanopore system to mechanical actuation. This unique coupled nanopore-actuator system could be further enhanced to a new class of nanopore readers with an in-plane cation reservoir to overcome some challenges that other solid-state and biological nanopores face, such as access resistance, which limits the sensing resolution⁴.<br/>Vahid Mohammadi <i>et al</i>., Science 372, 1165 (2021).<br/>Mojtabavi <i>et al</i>., ACS Nano, 15, 625–636 (2021).<br/>Mojtabavi <i>et al</i>., ACS Nano 13, 3042−3053 (2019).<br/>Comer <i>et al</i>., Nanoscale 8, 9600−9613 (2016).