Transverse Detection of DNA Using a MoS2 Nanopore

Classical nanopore sensing relies on the measurement of the ion current passing through a nanopore. Whenever a molecule electrophoretically translocates through the narrow constriction, it modulates the ion current. Although this approach allows one to measure single molecules, the access resistance limits the spatial resolution. This physical limitation could potentially be overcome by an alternative sensing scheme taking advantage of the current across the membrane material itself. Such an electronic readout would also allow better temporal resolution than the ionic current. In this work, we present the fabrication of an electrically contacted molybdenum disulfide (MoS2) nanoribbon integrated with a nanopore. DNA molecules are sensed by correlated signals from the ionic current through the nanopore and the transverse current through the nanoribbon. The resulting signal suggests a field-effect sensing scheme where the charge of the molecule is directly sensed by the nanoribbon. We discuss different sensing schemes such as local potential sensing and direct charge sensing. Furthermore, we show that the fabrication of freestanding MoS2 ribbons with metal contacts is reliable and discuss the challenges that arise in the fabrication and usage of these devices.

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

Michael Graf

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.

EPFL2019

Atomically Thin Molybdenum Disulfide Nanopores with High Sensitivity for DNA Translocation

Jiandong Feng, Andras Kis, Ke Liu, Aleksandra Radenovic

Atomically thin nanopore membranes are considered to be a promising approach to achieve single base resolution with the ultimate aim of rapid and cheap DNA sequencing. Molybdenum disulfide (MoS2) is newly emerging as a material complementary to graphene due to its semiconductive nature and other interesting physical properties that can enable a wide range of potential sensing and nanoelectronics applications. Here, we demonstrate that monolayer or few-layer thick exfoliated MoS2 with subnanometer thickness can be transferred and suspended on a predesigned location on the 20 nm thick SiNx. membranes. Nanopores in MoS2 are further sculpted with variable sizes using a transmission electron microscope (TEM) to drill through suspended portions of the MoS2 membrane. Various types of double-stranded (ds) DNA with different lengths and conformations are translocated through such a novel architecture, showing improved sensitivity (signal-to-noise ratio >10) compared to the conventional silicon nitride (SiNx) nanopores with tens of nanometers thickness. Unlike graphene nanopores, no special surface treatment is needed to avoid hydrophobic interaction between DNA and the surface. Our results imply that MoS2 membranes with nanopore can complement graphene nanopore membranes and offer potentially better performance in transverse detection.

Amer Chemical Soc2014

Electrochemical Reaction in Single Layer MoS2: nanopores opened atom by atom

Duncan Alexander, Roman Bulushev, Dumitru Dumcenco, Jiandong Feng, Michael Graf, Andras Kis, Daria Krasnozhon, Martina Lihter, Ke Liu, Aleksandra Radenovic

Ultrathin nanopore membranes based on 2D materials have demonstrated ultimate resolution toward DNA sequencing. Among them, molybdenum disulfide (MoS2) shows long-term stability as well as superior sensitivity enabling high throughput performance. The traditional method of fabricating nanopores with nanometer precision is based on the use of focused electron beams in transmission electron microscope (TEM). This nanopore fabrication process is time-consuming, expensive, not scalable, and hard to control below 1 nm. Here, we exploited the electrochemical activity of MoS2 and developed a convenient and scalable method to controllably make nanopores in single-layer MoS2 with subnanometer precision using electrochemical reaction (ECR). The electrochemical reaction on the surface of single-layer MoS2 is initiated at the location of defects or single atom vacancy, followed by the successive removals of individual atoms or unit cells from single-layer MoS2 lattice and finally formation of a nanopore. Step-like features in the ionic current through the growing nanopore provide direct feedback on the nanopore size inferred from a widely used conductance vs pore size model. Furthermore, DNA translocations can be detected in situ when as-fabricated MoS2 nanopores are used. The atomic resolution and accessibility of this approach paves the way for mass production of nanopores in 2D membranes for potential solid-state nanopore sequencing.

Amer Chemical Soc2015

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

Michael Graf

EPFL2019