2D nanopores: fabrication, energy harvesting and field-effect sensing

Solid-state nanopores are man-made, nano-sized openings in membranes separating two chambers containing an electrolyte solution. When applying an electric field across the membrane, the nanopore provides the only path for mobile ions to pass from one side of the membrane to the other. This current of ions is highly dependent on the pore size, membrane thickness, and surface charge. Small modulations in the system can lead to a large current modulation. We take advantage of this by translocating biomolecules through the nanopore. Typically, molecules such as DNA have an intrinsic charge in solution and will be attracted by the electric field generated around the nanopore. If the size of the pore allows it, they will thread into the pore and translocate to the opposite chamber. Generally, the thinner the membrane, the more ion current is generated and therefore the larger is the recorded signal. The isolation of mono-atomic crystals of carbon, also known as graphene, at the beginning of the 21st century, sparked much interest in using 2D materials for nanopore sensors. The thickness of these materials is approaching the distance between two bases in a DNA molecule, which raised the hopes of sequencing DNA when it passes through the orifice.

First, I will introduce the reader to 2D nanopores made in MoS2. In the last few years, I gained a lot of insights into 2D-nanopore fabrication. Therefore, I will first detail how MoS2-nanopore devices can be fabricated reliably and I will discuss potential pitfalls that researchers might encounter when manufacturing these devices.

We realized that nanopores in atomically thin MoS2 not only provide a system with low resistance to ionic current but also exhibit excellent ion-selectivity. Therefore, we developed an energy-harvesting system based on the osmotic pressure generated when concentration gradients are applied across the membrane. Converting this osmotic energy through reverse-electrodialysis relies on ion-selective membranes. By exploiting the photo-excitability of MoS2 membranes, I will show that we can raise the ion selectivity of the membrane by a factor of 5 while staying at a neutral pH and conclude that the observed effect is due to a change in the surface charge caused by light-induced charge generation.

Although ionic sensing with nanopores allows the precise measurements of single molecules, the spatial resolution in ionic sensing is limited by the access resistance. This limitation can potentially be overcome by an alternative sensing scheme independent from the ionic current. I will present a supplementary sensing scheme taking advantage of the semiconducting properties of MoS2. I fabricated freestanding nanoribbons of monolayer MoS2 in which a nanopore is drilled. The nanoribbons are then contacted through metal leads, which allow measuring the current through the material itself. The ionic current and the transverse current are recorded simultaneously and show correlated current modulations when DNA molecules translocate through the nanopore. The precise sensing mechanism of these devices is currently not well understood but is believed to originate from the charged molecules themselves or from local potential changes near the nanopore. I will discuss the challenges in fabricating such devices and propose possible explanations for the observed current traces.