Synthetic Routes to Graphene for Applications in Barrier Materials and Energy Storage

New techniques for rapid graphite oxide reduction are illustrated. By exposing graphite oxide to a high intensity light, such as a camera flash, rapid deflagration and deoxygenation takes place. The resulting graphitic material is a conductor, with two orders of magnitude higher surface area than its insulating precursor. The technique has potential applications in micro patterning as well as distributed ignition. Flashed graphite oxide is also dispersable in a variety of organic solvents, making it compatible with polymer processing. Another synthetic route to graphene is through solvothermal reduction of graphite oxide. Refluxing dispersions of graphite oxide in N-Methyl-2-pyrrolidone yields charge stabilized colloidal dispersions of graphene. The mechanism of reduction is thermal in nature, while charge stabilization is accomplished through functionalization of graphene NMP moieties and surface energy matching of NMP to graphene sheets. Conductometric graphene/Pd(0) hydrogen sensors with increased sensitivity compared to pure graphene is demonstrated. Pd(0) nanoparticles on graphene's surface lower the adsorption energy barrier for H2 molecules and improve the surface chemisorption of H2.An inexpensive, solid-state method for producing graphene based electronic materials is presented. Utilizing an inexpensive LightScribe DVD drive to reduce graphene oxide to graphene, patterning any design on a variety of substrates is demonstrated. Highly reduced laser scribed graphene shows promise in applications such as supercapacitors, sensors and electrocatalysts. Metal nanoparticles can be grown directly on the graphene surfaces using metal salt precursors. Light initiated reactions enable formation of nanoparticles within seconds of laser exposure. Such a universal approach to nanoparticle formation is suitable for applications from supercapacitors to catalysis.An investigation into barrier properties of graphene and graphene oxide films illustrate excellent barrier characteristics of graphene oxide to all gases at STP conditions with rates of permeability being directly related to the kinetic diameter of the gas. Due to the hydrophilic nature of graphene oxide, it is highly susceptible to permeation of water through its layered structure. Barrier properties of graphene to water illustrate at least 60 times lower rates of permeability per unit of thickness

Pt/graphene nano-sheet based hydrogen gas sensor

In this paper, we present gas sensing properties of Pt/graphene-like nano-sheets towards hydrogen gas. The graphene-like nano-sheets were produced via the reduction of spray-coated graphite oxide deposited on SiC substrates by hydrazine vapor. Structural and morphological characterizations of the graphene sheets were analyzed by scanning electron and atomic force microscopy. Current-voltage and dynamic responses of the sensors were investigated towards different concentrations of hydrogen gas in a synthetic air mixture at 100°C. A voltage shift of 100 mV was recorded at 1 mA reverse bias current

Patterning and Electronic Tuning of Laser Scribed Graphene for Flexible All-Carbon Devices

Engineering a low-cost graphene-based electronic device has proven difficult to accomplish <i>via</i> a single-step fabrication process. Here we introduce a facile, inexpensive, solid-state method for generating, patterning, and electronic tuning of graphene-based materials. Laser scribed graphene (LSG) is shown to be successfully produced and selectively patterned from the direct laser irradiation of graphite oxide films under ambient conditions. Circuits and complex designs are directly patterned onto various flexible substrates without masks, templates, post-processing, transferring techniques, or metal catalysts. In addition, by varying the laser intensity and laser irradiation treatments, the electrical properties of LSG can be precisely tuned over 5 orders of magnitude of conductivity, a feature that has proven difficult with other methods. This inexpensive method for generating LSG on thin flexible substrates provides a mode for fabricating a low-cost graphene-based NO₂ gas sensor and enables its use as a heterogeneous scaffold for the selective growth of Pt nanoparticles. The LSG also shows exceptional electrochemical activity that surpasses other carbon-based electrodes in electron charge transfer rate as demonstrated using a ferro-/ferricyanide redox couple

Mass Spectrometry Reveals a Multifaceted Role of Glycosaminoglycan Chains in Factor Xa Inactivation by Antithrombin

Factor Xa (fXa) inhibition by antithrombin (AT) enabled by heparin or heparan sulfate is critical for controlling blood coagulation. AT activation by heparin has been investigated extensively, while interaction of heparin with trapped AT/fXa intermediates has received relatively little attention. We use native electrospray ionization mass spectrometry to study the role of heparin chains of varying length [hexa-, octa-, deca-, and eicosasaccharides (dp6, dp8, dp10, and dp20, respectively)] in AT/fXa complex assembly. Despite being critical promoters of AT/Xa binding, shorter heparin chains are excluded from the final products (trapped intermediates). However, replacement of short heparin segments with dp20 gives rise to a prominent ionic signal of ternary complexes. These species are also observed when the trapped intermediate is initially prepared in the presence of a short oligoheparin (dp6), followed by addition of a longer heparin chain (dp20), indicating that binding of heparin to AT/fXa complexes takes place after the inhibition event. The importance of the heparin chain length for its ability to associate with the trapped intermediate suggests that the binding likely occurs in a bidentate fashion (where two distinct segments of oligoheparin make contacts with the protein components, while the part of the chain separating these two segments is extended into solution to minimize electrostatic repulsion). This model is corroborated by both molecular dynamics simulations with an explicit solvent and ion mobility measurements in the gas phase. The observed post-inhibition binding of heparin to the trapped AT/fXa intermediates hints at the likely role played by heparan sulfate in their catabolism

Integration of molecular and enzymatic catalysts on graphene for biomimetic generation of antithrombotic species

The integration of multiple synergistic catalytic systems can enable the creation of biocompatible enzymatic mimics for cascading reactions under physiologically relevant conditions. Here we report the design of a graphene-haemin-glucose oxidase conjugate as a tandem catalyst, in which graphene functions as a unique support to integrate molecular catalyst haemin and enzymatic catalyst glucose oxidase for biomimetic generation of antithrombotic species. Monomeric haemin can be conjugated with graphene through π-π interactions to function as an effective catalyst for the oxidation of endogenous L-arginine by hydrogen peroxide. Furthermore, glucose oxidase can be covalently linked onto graphene for local generation of hydrogen peroxide through the oxidation of blood glucose. Thus, the integrated graphene-haemin-glucose oxidase catalysts can readily enable the continuous generation of nitroxyl, an antithrombotic species, from physiologically abundant glucose and L-arginine. Finally, we demonstrate that the conjugates can be embedded within polyurethane to create a long-lasting antithrombotic coating for blood-contacting biomedical devices