Fabrication and practical applications of molybdenum disulfide nanopores

Among the different developed solid-state nanopores, nanopores constructed in a monolayer of molybdenum disulfide (MoS2) stand out as powerful devices for single-molecule analysis or osmotic power generation. Because the ionic current through a nanopore is inversely proportional to the thickness of the pore, ultrathin membranes have the advantage of providing relatively high ionic currents at very small pore sizes. This increases the signal generated during translocation of biomolecules and improves the nanopores’ efficiency when used for desalination or reverse electrodialysis applications. The atomic thickness of MoS2 nanopores approaches the inter-base distance of DNA, creating a potential candidate for DNA sequencing. In terms of geometry, MoS2 nanopores have a well-defined vertical profile due to their atomic thickness, which eliminates any unwanted effects associated with uneven pore profiles observed in other materials. This protocol details all the necessary procedures for the fabrication of solid-state devices. We discuss different methods for transfer of monolayer MoS2, different approaches for the creation of nanopores, their applicability in detecting DNA translocations and the analysis of translocation data through open-source programming packages. We present anticipated results through the application of our nanopores in DNA translocations and osmotic power generation. The procedure comprises four parts: fabrication of devices (2–3 d), transfer of MoS2 and cleaning procedure (24 h), the creation of nanopores within MoS2 (30 min) and performing DNA translocations (2–3 h). We anticipate that our protocol will enable large-scale manufacturing of single-molecule-analysis devices as well as next-generation DNA sequencing.

Electrochemical and morphological engineering of 2D materials for nanopore sensing

Martina Lihter

Nanopores are nanometer-sized holes that were initially proposed for DNA sequencing. Several years ago sequencing was made possible with biological nanopores. However, solid-state nanopores have plenty of advantages to offer compared to their biological counterparts. This thesis focuses on nanopores made in 2D materials, and their sensing capabilities. They provide an outstanding spatial resolution and a good signal-to-noise ratio (SNR) for sensing. As a 2D semiconductor, molybdenum disulfide (MoS2) exhibits unique (opto)electronic and electrochemical properties. The electronic band structure of a semiconducting MoS2 results in highly sensitivity to its chemical environment and external electric field. Therefore, monitoring the transverse current through the MoS2 during analyte translocation can provide a powerful sensing mechanism. As an n-type semiconductor, MoS2 exhibits a characteristic electrochemical behavior: it shows electrochemical activity when polarized cathodically, while in the anodic region becomes electrochemically inactive. This thesis covers five topics in which I demonstrate how the properties of 2D materials can be used for designing advanced sensing platforms for biosensing. Fabrication of MoS2 Nanopores. Here, we give an overview of a reliable fabrication process optimized for the production of high-quality devices. We discuss the main issues, how to solve and avoid them. In the end, we demonstrate applications of MoS2 nanopores for sensing and for osmotic power generation. Geometrical Effect in 2D Nanopores. We compare two different pore geometries: triangular pores in hexagonal boron-nitride (h-BN) vs. approximately circular pores in MoS2. In h-BN nanopores, we observe a lower conductance drop caused by DNA translocation, than expected from a conventional conductance model. As an explanation, we propose a reduced ion-concentration and ion-mobility inside the pore, supported by molecular dynamics simulations. Transverse Detection of DNA in MoS2 Nanopore. This chapter presents the realization of a hybrid nanopore-FET device, consisting of an MoS2 ribbon and a nanopore drilled in it. Such a device acts as a field-effect transistor (FET), where the transverse current through the MoS2 is modulated by the translocating molecule and the ionic voltage applied across the pore. We show that the transverse current is more sensitive to DNA translocation than the ionic current. Passivation of Electrodes Contacting MoS2. The method is based on the electrochemical deposition of a polymer, which blocks the electron transfer from the surface of the electrodes to ions in solution. To avoid the deposition on MoS2, we used poly(phenylene oxide) (PPO) that polymerizes in the potential window where MoS2 is inactive. Deposition is compatible with MoS2 and highly area-selective, even at the nanoscale, as we demonstrate on the nanoelectrodes of a nanopore-FET device. Electrochemical Modification of MoS2. By polarizing MoS2 cathodically, it is possible to electrochemically deposit a thin layer of aryl-diazonium compounds on MoS2 surface. This approach allows to introduce specific chemical groups on MoS2 and nanopore rim, which could be used for specific recognition of analytes as they translocate through the pore. Furthermore, since the deposition occurs only on the MoS2 to which the voltage was applied, this could enable specific functionalization of adjacent nanoribbons in nanopore-FET devices.

EPFL2020

Detecting the translocation of DNA through a nanopore using graphene nanoribbons

Mohamed Malik Benameur, Sergey Khlybov, Andras Kis, Daria Krasnozhon, Ke Liu, Aleksandra Radenovic, Camille Alice Raillon, Floriano Traversi

Solid-state nanopores can act as single-molecule sensors and could potentially be used to rapidly sequence DNA molecules. However, nanopores are typically fabricated in insulating membranes that are as thick as 15 bases, which makes it difficult for the devices to read individual bases. Graphene is only 0.335 nm thick (equivalent to the spacing between two bases in a DNA chain) and could therefore provide a suitable membrane for sequencing applications. Here, we show that a solid-state nanopore can be integrated with a graphene nanoribbon transistor to create a sensor for DNA translocation. As DNA molecules move through the pore, the device can simultaneously measure drops in ionic current and changes in local voltage in the transistor, which can both be used to detect the molecules. We examine the correlation between these two signals and use the ionic current measurements as a real-time control of the graphene-based sensing device.

Nature Publishing Group2013

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 and morphological engineering of 2D materials for nanopore sensing

Martina Lihter

EPFL2020