Wetting of nanopores probed with pressure

Nanopores are both a tool to study single-molecule biophysics and nanoscale ion transport, but also a promising material for desalination or osmotic power generation. Understanding the physics underlying ion transport through nano-sized pores allows better design of porous membrane materials. Material surfaces can present hydrophobicity, a property which can make them prone to formation of surface nanobubbles. Nanobubbles can influence the electrical transport properties of such devices. We demonstrate an approach which uses hydraulic pressure to probe the electrical transport properties of solid state nanopores. We show how pressure can be used to wet pores, and how it allows control over bubbles or other contaminants in the nanometer scale range normally unachievable using only an electrical driving force. Molybdenum disulfide is then used as a typical example of a 2D material on which we demonstrate wetting and bubble induced nonlinear and linear conductance in the regimes typically used with these experiments. We show that by using pressure one can identify and evade wetting artifacts.

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

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

Electrochemical and morphological engineering of 2D materials for nanopore sensing

Martina Lihter

EPFL2020