Rory Stine

Real-Time DNA Detection Using Reduced Graphene Oxide Field Effect Transistors

www.MaterialsViews.com C O M M Real-Time DNA Detection Using Reduced Graphene Oxide Field Effect Transistors U N IC A By Rory Stine , Jeremy T. Robinson , Paul E. Sheehan , and Cy R. Tamanaha * IO N Rapidly detecting and identifying biomolecules in solution is a pressing need in areas as diverse as medical diagnostics, food safety, and national defense. While traditional laboratory techniques using secondary labels such as fl uorophores or enzymes offer the greatest sensitivity, they cannot monitor in real time probe/target interactions and increase signifi cantly the cost and complexity of screening for biological agents in the fi eld. As such, numerous studies have been undertaken to develop labelfree sensors that directly detect the binding of a target. Multiple transduction approaches have been examined. Surface plasmon resonance (SPR) has long been considered the gold-standard for label-free biological detection, [ 1 ] and has achieved sensitivities in the low nanomolar range [ 2 , 3 ] for direct, non-amplifi ed DNA detection. Recently, sensors based on silicon nanowire [ 4 , 5 ] and carbon nanotube [ 6 ] fi eld effect transistors (FETs) have shown higher sensitivities. These transistors, which are gated by the adsorption of charged biomolecules, have shown DNA detection limits from low n M [ 7 , 8 ] to high p M [ 9 , 10 ] in solutions at or near physiological salt concentrations. Detection limits as low as f M [ 11 , 12 ] have also been shown but only for salt-free solutions that decrease the Debye screening. [ 13 ]

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.

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.

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.

Aminated graphene for DNA attachment produced via plasma functionalization

We demonstrate the use of a unique plasma source to controllably functionalize graphene with nitrogen and primary amines, thereby tuning the chemical, structural, and electrical properties. Critically, even highly aminated graphene remains electronically conductive, making it an ideal transduction material for biosensing. Proof-of-concept testing of aminated graphene as a bio-attachment platform in a biologically active field-effect transistor used for DNA detection is demonstrated.

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.

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.

Robust reduction of graphene fluoride using an electrostatically biased scanning probe

AbstractWe report a novel and easily accessible method to chemically reduce graphene fluoride (GF) sheets with nanoscopic precision using high electrostatic fields generated between an atomic force microscope (AFM) tip and the GF substrate. Reduction of fluorine by the electric field produces graphene nanoribbons (GNR) with a width of 105-1,800 nm with sheet resistivity drastically decreased from >1 TΩ·sq.−1 (GF) down to 46 kΩ·sq.−1 (GNR). Fluorine reduction also changes the topography, friction, and work function of the GF. Kelvin probe force microscopy measurements indicate that the work function of GF is 180–280 meV greater than that of graphene. The reduction process was optimized by varying the AFM probe velocity between 1.2 μm·s−1 and 12 μm·s−1 and the bias voltage applied to the sample between −8 and −12 V. The electrostatic field required to remove fluorine from carbon is ∼1.6 V·nm−1. Reduction of the fluorine may be due to the softening of the C-F bond in this intense field or to the accumulation and hydrolysis of adventitious water into a meniscus.

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.

Activation of radical addition to graphene by chemical hydrogenation

We report several methods of chemical dehydrogenation of hydrogenated graphene (HG), characterizing the results using Raman, X-ray photoelectron spectroscopy, and electrical conductivity measurements. Notably, the hydrogen–graphene bonds appear to activate the graphene toward subsequent reaction such that, in several cases, the addition of the dehydrogenating agent to the graphene accompanies the removal of hydrogen. We compare the uptake of chemical groups on HG to those on pristine graphene and find that HG reacts more readily than pristine graphene with radical generators such as chlorine and AIBN.