Pyridinic-N-Doped Graphene Paper from Perforated Graphene Oxide for Efficient Oxygen Reduction.

We report a simple approach to fabricate a pyridinic-N-doped graphene film (N-pGF) without high-temperature heat treatment from perforated graphene oxide (pGO). pGO is produced by a short etching treatment with hydrogen peroxide. GO perforation predominated in a short etching time (∼1 h), inducing larger holes and defects compared to pristine GO. The pGO is advantageous to the formation of a pyridinic N-doped graphene because of strong NH3 adsorption on vacancies with oxygen functional groups during the nitrogen-doping process, and the pyridinic-N-doped graphene exhibits good electrocatalytic activity for oxygen reduction reaction (ORR). Using rotating-disk electrode measurements, we confirm that N-pGF undergoes a four-electron-transfer process during the ORR in alkaline and acidic media by possessing sufficient diffusion pathways and readily available ORR active sites for efficient mass transport. A comparison between Pt/N-pGF and commercial Pt/C shows that Pt/N-pGF has superior performance, based on its more positive onset potential and higher limiting diffusion current at -0.5 V.

Antibacterial Activities of Graphene Oxide-Molybdenum Disulfide Nanocomposite Films.

Two-dimensional (2D) nanomaterials, such as graphene-based materials and transition metal dichalcogenide (TMD) nanosheets, are promising materials for biomedical applications owing to their remarkable cytocompatibility and physicochemical properties. On the basis of their potent antibacterial properties, 2D materials have potential as antibacterial films, wherein the 2D nanosheets are immobilized on the surface and the bacteria may contact with the basal planes of 2D nanosheets dominantly rather than contact with the sharp edges of nanosheets. To address these points, in this study, we prepared an effective antibacterial surface consisting of representative 2D materials, i.e., graphene oxide (GO) and molybdenum disulfide (MoS2), formed into nanosheets on a transparent substrate for real device applications. The antimicrobial properties of the GO-MoS2 nanocomposite surface toward the Gram-negative bacteria Escherichia coli were investigated, and the GO-MoS2 nanocomposite exhibited enhanced antimicrobial effects with increased glutathione oxidation capacity and partial conductivity. Furthermore, direct imaging of continuous morphological destruction in the individual bacterial cells having contacts with the GO-MoS2 nanocomposite surface was characterized by holotomographic (HT) microscopy, which could be used to detect the refractive index (RI) distribution of each voxel in bacterial cell and reconstruct the three-dimensional (3D) mapping images of bacteria. In this regard, the decreases in both the volume (67.2%) and the dry mass (78.8%) of bacterial cells that came in contact with the surface for 80 min were quantitatively measured, and releasing of intracellular components mediated by membrane and oxidative stress was observed. Our findings provided new insights into the antibacterial properties of 2D nanocomposite film with label-free tracing of bacterial cell which improve our understanding of antimicrobial activities and opened a window for the 2D nanocomposite as a practical antibacterial film in biomedical applications.

Friction and conductance imaging of sp(2)- and sp(3)-hybridized subdomains on single-layer graphene oxide.

We investigated the subdomain structures of single-layer graphene oxide (GO) by characterizing local friction and conductance using conductive atomic force microscopy. Friction and conductance mapping showed that a single-layer GO flake has subdomains several tens to a few hundreds of nanometers in lateral size. The GO subdomains exhibited low friction (high conductance) in the sp(2)-rich phase and high friction (low conductance) in the sp(3)-rich phase. Current-voltage spectroscopy revealed that the local current flow in single-layer GO depends on the quantity of hydroxyl and carboxyl groups, and epoxy bridges within the 2-dimensional carbon layer. The presence of subdomains with different sp(2)/sp(3) carbon ratios on a GO flake was also confirmed by chemical mapping using scanning transmission X-ray microscopy. These results suggest that spatial mapping of the friction and conductance can be used to rapidly identify the composition of heterogeneous single-layer GO at nanometer scale, which is essential for understanding charge transport in nanoelectronic devices.

DNA-Assisted Exfoliation of Tungsten Dichalcogenides and Their Antibacterial Effect.

This study reports a method for the facile and high-yield exfoliation of WX2 (X = S, Se) by sonication under aqueous conditions using single-stranded DNA (abbreviated as ssDNA) of high molecular weight. The ssDNA provided a high degree of stabilization and prevented reaggregation, and it enhanced the exfoliation efficiency of WX2 nanosheets due to adsorption on the WX2 surface and the electrostatic repulsion of sugars in the ssDNA backbone. The exfoliation yield was higher with ssDNA (80%-90%) than without (2%-4%); the yield with ssDNA was also higher than the value previously reported for aqueous exfoliation (∼10%). Given that two-dimensional nanomaterials have potential health and environmental applications, we investigated antibacterial activity of exfoliated WX2-ssDNA nanosheets, relative to graphene oxide (GO), and found that WSe2-ssDNA nanosheets had higher antibacterial activity against Escherichia coli K-12 MG1655 cells than GO. Our method enables large-scale exfoliation in an aqueous environment in a single step with a short reaction time and under ambient conditions, and it can be used to produce surface-active or catalytic materials that have broad applications in biomedicine and other areas.

Effective liquid-phase exfoliation and sodium ion battery application of MoS2 nanosheets.

Two-dimensional (2D) molybdenum disulfide (MoS2) has been taken much attention for various applications, such as catalyst, energy storage, and electronics. However, the lack of effective exfoliation methods for obtaining 2D materials in a large quantity has been one of the technical barriers for the real applications. We report a facile liquid-phase exfoliation method to improve the exfoliation efficiency for single-layer MoS2 sheets in 1-methyl-2-pyrrolidinone (NMP) with a sodium hydroxide (NaOH) assistant. The concentration of the exfoliated MoS2 nanosheets was greatly improved compared to that achieved with conventional liquid-phase exfoliation methods using NMP solvent. We demonstrate stable operation of sodium-ion battery by using the exfoliated MoS2 and MoS2-rGO composite as anode materials.

Rational design of modular allosteric aptamer sensor for label-free protein detection.

An aptamer can be redesigned to new functional molecules by conjugating with other oligonucleotides. However, it requires experimental trials to optimize the conjugating module with the sensitivity and selectivity toward a target. To reduce these efforts, we report rationally-designed modular allosteric aptamer sensor (MAAS), which is composed of coupled two aptamers and the regulator. For label-free protein detection, the protein-aptamer was conjugated with the malachite green (MG) aptamer for signaling. The MAAS additionally has the regulator domain which is designed to hybridize to a protein binding domain. The regulator makes MAAS to be inactive by destructing the original structure of the two aptamers. However, its conformation becomes active by dissociating the hybridization from the protein recognition signal, thereby inducing the binding of MG emitting the enhanced fluorescence. The design of regulator is based on the thermodynamic energy difference by the RNA conformational change and protein-aptamer affinity. Here we first demonstrated the MAAS for hepatitis C helicase and replicase. The target proteins were detected up to 250nM with minimized blank signals and displayed high specificities 10-fold greater than in non-specific proteins. The MAAS provides valuable tools that can be adapted to a wide range of configurations in bioanalytical applications.

A photoswitchable methylene-spaced fluorinated aryl azobenzene monolayer grafted on silicon.

Aryldiazonium-terminated methylene-spaced trifluoromethylazobenzene derivatives have been synthesized. Their self-assembled monolayers (SAMs) on a silicon surface allow for a stable and reversible molecular photoswitch.

Adsorption of azothiophene dye having an N-bridging bidentate tail group on gold.

The novel azothiophene derivatives [AT-di(CnSAc) (n=6 and 12)], an azo linkage (N=N) that bridges one phenyl ring with an N-bridging dialkylthioacetate tail and one thiophene ring, are synthesized and characterized. The azothiophene derivatives substituted with electron-withdrawing groups are blue in color, exhibit a bathochromic shift of a longer wavelength, and are electrochemically active. The formation and characterization of AT-di(CnSAc) SAMs with a bidentate tail group of the N-bridging dialkylthioacetate have been studied by surface sensitive techniques such as grazing angle Fourier transform infrared (FT-IR), a quartz crystal microbalance (QCM), spectroscopic ellipsometry, X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV). It has been shown that the adsorption reaction of the thioacetate group is almost spontaneous and the N-bridging dialkylthioacetate SAMs with longer methylene length have a packing density higher than that with a shorter methylene length. However, the sulfur tethers of the N-bridging dialkylthioacetate show incomplete binding of sulfur atoms in AT-di(CnSAc) SAMs.

High-fidelity formation of a molecular-junction device using a thickness-controlled bilayer architecture.

The device yield of molecular junctions has become a major issue for the practical application of molecular electronics based on a crossbar system of a metal-molecule-metal (MMM) junction. As the thickness of self-assembled monolayers (SAMs) is typically 1-2 nm, it is difficult to avoid electrical shorts due to the penetration of top metal particles into the SAMs. A simple and effective strategy for the creation of a reliable molecular junction using a thickness-controlled bilayer with a bifunctional heterostructure is presented. In the MMM device, the Au adlayer on the molecular layer is spontaneously formed with deposition of the top Au metals and the sandwiched molecular layer maintains the quality of the SAMs. This method greatly reduces electrical shorts by preventing the diffusion of the top metal electrode and offsetting the surface roughness of the bottom metal electrode, resulting in a device yield of more than 90%.

Molecular conductance switch-on of single ruthenium complex molecules.

Thiol-tethered Ru(II) terpyridine complexes were synthesized for a voltage-driven molecular switch and used to understand the switch-on mechanism of the molecular switches of single metal complexes in the solid-state molecular junction in a vacuum. Molecularly resolved scanning tunneling microscopy (STM) images revealed well-defined single Ru(II) complexes isolated in the highly ordered dielectric monolayer. When a negative sample-bias was applied, the threshold voltage to the high conductance state in the molecular junctions of the Ru(II) complex was consistent with the electronic energy gap between the Fermi level of the gold substrate and the lowest ligand-centered redox state of the metal complex molecule. As an active redox center leading to conductance switching in the molecule, the lowest ligand-centered redox state of Ru(II) complexes was suggested to trap an electron injected from the gold substrate. Our suggestions for a single-molecule switch-on mechanism in the solid state can provide guidance in a design that improves the charge-trapping efficiency of the ligands with different metal substrates.

Rose bengal dye on thiol-terminated bilayer for molecular devices.

Recently, it has become increasingly important to control molecular layers, especially with regard to the formation of bilayers, in order to avoid electrical shorts in molecular electronics. In this paper, we report on the characterization of an in situ thiol-terminated bilayer that is formed by hydrogen bonding between the amine group of an aminoalkanethiol monolayer on a gold surface and the free amine group of aminoalkanethiolates in a bulk solution. We also report on the use of a rose bengal (RB) monolayer on a thiol-terminated bilayer for the purpose of application in a molecular memory device. Using surface-sensitive techniques such as grazing angle Fourier transform infrared (FT-IR) spectroscopy, quartz crystal microbalance (QCM) measurement, ellipsometry, X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV), we characterized a thiol-terminated bilayer (TUA-AUT) and an RB functionalized monolayer on a bilayered surface (RB-TUA-AUT). For a control experiment, we prepared a single RB monolayer attached by an ethanethiol group to a gold surface. In order to assess the feasibility of the present approach with respect to application in molecular electronics, we tested the switching property of the self-assembled monolayers (SAMs) using conducting-probe atomic force microscopy (CP-AFM). The RB monolayer on the bilayered surface exhibited hysteresis, while a single RB monolayer gave an electrical short.

A novel electrochemical detection method for aptamer biosensors.

A beacon aptamer-based biosensor for the detection of thrombin was developed using electrochemical transduction method. Gold surface was modified with a beacon aptamer covalently linked at 5'-terminus with a linker containing a primary aliphatic amine. Methylene blue (MB) was intercalated into the beacon sequence, and used as an electrochemical marker. When the beacon aptamer immobilized on gold surface encounters thrombin, the hairpin forming beacon aptamer is conformationally changed to release the intercalated MB, resulting a decrease in electrical current intensity in voltamogram. The peak signal of the MB is clearly decreased by the binding of thrombin onto the beacon aptamer. The linear range of the signal was observed between 0 and 50.8 nM of thrombin with 0.999 correlation factor. This method was able to linearly and selectively detect thrombin with a detection limit of 11 nM.