van der Waals screening by single-layer graphene and molybdenum disulfide.

A sharp tip of atomic force microscope is employed to probe van der Waals forces of a silicon oxide substrate with adhered graphene. Experimental results obtained in the range of distances from 3 to 20 nm indicate that single-, double-, and triple-layer graphenes screen the van der Waals forces of the substrate. Fluorination of graphene, which makes it electrically insulating, lifts the screening in the single-layer graphene. The van der Waals force from graphene determined per layer decreases with the number of layers. In addition, increased hole doping of graphene increases the force. Finally, we also demonstrate screening of the van der Waals forces of the silicon oxide substrate by single- and double-layer molybdenum disulfide.

Graphene veils: A versatile surface chemistry for sensors.

Thin spun-coat films (~4 nm thick) of graphene oxide (GO) constitute a versatile surface chemistry compatible with a broad range of technologically important sensor materials. Countless publications are dedicated to the nuances of surface chemistries that have been developed for sensors, with almost every material having unique characteristics. There would be enormous value in a surface chemistry that could be applied generally with functionalization and passivation already optimized regardless of the sensor material it covered. Such a film would need to be thin, conformal, and allow for multiple routes toward covalent linkages. It is also vital that the film permit the underlying sensor to transduce. Here we show that GO films can be applied over a diverse set of sensor surfaces, can link biomolecules through multiple reaction pathways, and can support cell growth. Application of a graphene veil atop a magnetic sensor array is demonstrated with an immunoassay. We also present biosensing and material characterization data for these graphene veils.

Chemical stability of graphene fluoride produced by exposure to XeF2.

Fluorination can alter the electronic properties of graphene and activate sites for subsequent chemistry. Here, we show that graphene fluorination depends on several variables, including XeF2 exposure and the choice of substrate. After fluorination, fluorine content declines by 50-80% over several days before stabilizing. While highly fluorinated samples remain insulating, mildly fluorinated samples regain some conductivity over this period. Finally, this loss does not reduce reactivity with alkylamines, suggesting that only nonvolatile fluorine participates in these reactions.

Chemical gradients on graphene to drive droplet motion.

This work demonstrates the production of a well-controlled, chemical gradient on the surface of graphene. By inducing a gradient of oxygen functional groups, drops of water and dimethyl-methylphosphonate (a nerve agent simulant) are "pulled" in the direction of increasing oxygen content, while fluorine gradients "push" the droplet motion in the direction of decreasing fluorine content. The direction of motion is broadly attributed to increasing/decreasing hydrophilicity, which is correlated to high/low adhesion and binding energy. Such tunability in surface chemistry provides additional capabilities in device design for applications ranging from microfluidics to chemical sensing.

Engineering graphene mechanical systems.

We report a method to introduce direct bonding between graphene platelets that enables the transformation of a multilayer chemically modified graphene (CMG) film from a "paper mache-like" structure into a stiff, high strength material. On the basis of chemical/defect manipulation and recrystallization, this technique allows wide-range engineering of mechanical properties (stiffness, strength, density, and built-in stress) in ultrathin CMG films. A dramatic increase in the Young's modulus (up to 800 GPa) and enhanced strength (sustainable stress ≥1 GPa) due to cross-linking, in combination with high tensile stress, produced high-performance (quality factor of 31,000 at room temperature) radio frequency nanomechanical resonators. The ability to fine-tune intraplatelet mechanical properties through chemical modification and to locally activate direct carbon-carbon bonding within carbon-based nanomaterials will transform these systems into true "materials-by-design" for nanomechanics.

High-density amine-terminated monolayers formed on fluorinated CVD-grown graphene.

There has been considerable interest in chemically functionalizing graphene films to control their electronic properties, to enhance their binding to other molecules for sensing, and to strengthen their interfaces with matrices in a composite material. Most reports to date have largely focused on noncovalent methods or the use of graphene oxide. Here, we present a method to activate CVD-grown graphene sheets using fluorination followed by reaction with ethylenediamine (EDA) to form covalent bonds. Reacted graphene was characterized via X-ray photoelectron spectroscopy (XPS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and Raman spectroscopy as well as measurements of electrical properties. The functionalization results in stable, densely packed layers, and the unbound amine of EDA was shown to be active toward subsequent chemical reactions.

Chemically isolated graphene nanoribbons reversibly formed in fluorographene using polymer nanowire masks.

We demonstrated the fabrication of graphene nanoribbons (GNRs) as narrow as 35 nm created using scanning probe lithography to deposit a polymer mask(1-3) and then fluorinating the sample to isolate the masked graphene from the surrounding wide band gap fluorographene. The polymer protected the GNR from atmospheric adsorbates while the adjacent fluorographene stably p-doped the GNRs which had electron mobilities of ∼2700 cm2/(V·s). Chemical isolation of the GNR enabled resetting the device to nearly pristine graphene.

Self-assembled monolayers of alkanethiols on InAs.

We describe the deposition and properties of self-assembled monolayers (SAMs) of methyl-terminated alkanethiols on InAs(001) surface. For these model hydrophobic films, we used water contact angle measurements to survey the preparation of alkanethiol monolayers from base-activated ethanolic solutions as a function of the solution and deposition parameters, including chain length of alkanethiols, deposition time, and solution temperature and pH. We then used X-ray photoelectron spectroscopy (XPS), ellipsometry, and electrochemistry to characterize the composition and structure of octadecanethiol (ODT) monolayers deposited on InAs under optimized conditions. When applied to a thoroughly degreased InAs(001) wafer surface, the basic ODT solution removes the native oxide without excessively etching the underlying InAs(001) substrate. The resulting film contains approximately one monolayer of ODT molecules, attached to the InAs surface almost exclusively via thiolate bonds to In atoms, with organic chains extended away from the surface. These ODT monolayers are stable against degradation and oxidation in air, organic solvents, and aqueous buffers. The same base-activated ODT treatment can also be used to passivate exposed InAs/AlSb quantum well (QW) devices, preserving the unique electronic properties of InAs surfaces and allowing the operation of such passivated devices as continuous flow pH-sensors.

Formation of primary amines on silicon nitride surfaces: a direct, plasma-based pathway to functionalization.

Silicon nitride is the most commonly used passivation layer in biosensor applications where electronic components must be interfaced with ionic solutions. Unfortunately, the predominant method for functionalizing silicon nitride surfaces, silane chemistry, suffers from a lack of reproducibility. As an alternative, we have developed a silane-free pathway that allows for the direct functionalization of silicon nitride through the creation of primary amines formed by exposure to a radio frequency glow discharge plasma fed with humidified air. The aminated surfaces can then be further functionalized by a variety of methods; here we demonstrate using glutaraldehyde as a bifunctional linker to attach a robust NeutrAvidin (NA) protein layer. Optimal amine formation, based on plasma exposure time, was determined by labeling treated surfaces with an amine-specific fluorinated probe and characterizing the coverage using X-ray photoelectron spectroscopy (XPS). XPS and radiolabeling studies also reveal that plasma-modified surfaces, as compared with silane-modified surfaces, result in similar NA surface coverage, but notably better reproducibility.

Heat-stabilized phospholipid films: film characterization and the production of protein-resistant surfaces.

Phospholipid films have been shown in a number of studies to exhibit potential as nonfouling surfaces for biomaterial applications. However, the practical application of such films has been hindered by instability in aqueous solutions and significant detachment under mild shear stresses. Methods for stabilizing lipid films have been investigated, but to date require the presence of specific functional groups or chemical modification of the lipid molecule. In contrast to these methods, we present a process for heat-stabilization of lipid films. These heat-stabilized films have been shown to be able to withstand repeated rinsing without significant detachment. Phosphatidylcholine monolayers were formed on hydrophobic self-assembled monolayers using the liposome fusion method and stabilized at 80 degrees C. The films were characterized using Fourier transform infrared spectroscopy, ellipsometry, and atomic force microscopy and were shown to be defect free after repeated rinsing. Further experiments using a quartz crystal microbalance showed that the heat-stabilized lipid films were highly resistant to nonspecific protein adhesion and compared very favorably with poly(ethylene glycol)-coated surfaces under identical exposure conditions.

Comparison of glycosphingolipids and antibodies as receptor molecules for ricin detection.

Glycosphingolipids (GSLs) have been shown to undergo strong interactions with a number of protein toxins, including potential bioterrorism agents such as ricin and botulinum neurotoxin. Characterization of this interaction in recent years has led to a number of studies where GSLs were used as the recognition molecules for biosensing applications. Here, we offer a comparison of quartz crystal microbalance (QCM) sensors for the detection of ricin using antibodies and the GSLs GM1 and asialoGM1, which have been shown to undergo strong interactions with ricin. The presence, orientation, and activity of the GSL and antibody films were confirmed using ellipsometry, Fourier transform infrared spectroscopy (FT-IR), and QCM. It was found that the GSLs offered more sensitive detection limits when directly compared with antibodies. Both GSLs had lower detection limits at 5 microg/mL, approximately 5 times lower than were found for antibodies (25 microg/mL), and their linear detection range extended to the highest concentrations tested (100 microg/mL), almost an order of magnitude beyond the saturation point for the antibody sensors. Potential sites for nonspecific adsorption were blocked using serum albumin without sacrificing toxin specificity.

Heat-stabilized glycosphingolipid films for biosensing applications.

We have investigated a means of producing thin, oriented lipid monolayers which are stable under repeated washing and which may be useful in biosensing or surface-coating applications. Phosphatidylcholine and the glycosphingolipid GM1 were used as representative lipids for this work. Initially, a mixed self-assembled monolayer of octanethiol and hexadecanethiol was produced on a gold surface. This hydrophobic monolayer was then brought into contact with a thin lipid film that had been assembled at the liquid/air interface of a solution, allowing the lipid to deposit on the gold surface through hydrophobic interactions. The lipid layer was then heated to cause intermingling of the fatty acid and alkanethiol chains and cooled to form a highly stable film which withstood repeated rinsing and solution exposure. Presence and stability of the film were confirmed via ellipsometry, Fourier transform infrared spectroscopy, and quartz crystal microbalance (QCM), with an average overall film thickness of approximately 3.5 nm. This method was then utilized to produce GM1 layers on gold-coated QCM crystals for affinity sensing trials with cholera toxin. For these sensing elements, the lower detection limit of cholera toxin was found to be approximately 0.5 microg/mL, with a logarithmic relationship between toxin concentration and frequency response spanning over several orders of magnitude. Potential sites for nonspecific adsorption were blocked using serum albumin without sacrificing toxin specificity.