Amal Kasry

Chemical doping of large-area stacked graphene films for use as transparent, conducting electrodes

Graphene is considered a leading candidate to replace conventional transparent conducting electrodes because of its high transparency and exceptional transport properties. The effect of chemical p-type doping on graphene stacks was studied in order to reduce the sheet resistance of graphene films to values approaching those of conventional transparent conducting oxides. In this report, we show that large-area, stacked graphene films are effectively p-doped with nitric acid. The doping decreases the sheet resistance by a factor of 3, yielding films comprising eight stacked layers with a sheet resistance of 90 Omega/(square) at a transmittance of 80%. The films were doped either after all of the layers were stacked (last-layer-doped) or after each layer was added (interlayer-doped). A theoretical model that accurately describes the stacked graphene film system as a resistor network was developed. The model defines a characteristic transfer length where all the channels in the graphene films actively contribute to electrical transport. The experimental data shows a linear increase in conductivity with the number of graphene layers, indicating that each layer provides an additional transport channel, in good agreement with the theoretical model.

Long range surface plasmon fluorescence spectroscopy

Surface plasmon modes, resonantly excited at the two sides of an ultrathin (noble) metal layer in contact with two (nearly) identical dielectric media interact via the overlap of their electromagnetic fields resulting in two new coupled modes, i.e., a short range and a long range surface plasmon (LRSP), respectively. The authors demonstrate that both the enhanced optical field of the LRSP wave at the metal/dielectric interface as well as its increased (evanescent) penetration depth reaching farther into the analyte solution can be used for significant enhancements when using LRSP optics in a fluorescence spectroscopic mode of operation. They demonstrate this for the detection of fluorescence intensities from chromophore labeled proteins bound to the sensor surface matrix.

New concepts with surface plasmons and nano-biointerfaces

In classical surface plasmon-based optical biosensors, a surface plasmon mode is resonantly excited on the metallic sensor surface to probe any analyte-binding-induced refractive index changes. The field of the surface plasmon mode evanescently decays from the metal into an adjacent analyte solution with a typical penetration depth of 200 nm. In order to maximize the sensitivity of SPR biosensors, interfacial polymer architectures with binding site densities that considerably exceed planar arrangements through the use of three-dimensional microstructures were introduced. For biosensors based on surface plasmon fluorescence spectroscopy, this type of matrix offers the additional advantage of preventing fluorescence quenching, which is caused by the proximity of the chromophore label to the acceptor states of the noble metal. By means of probing the binding events with long range surface plasmon modes of which field extend much farther into the analyte solution (up to the micrometer range), substantially thicker sensor matrix layers can be used. Into such matrices larger amounts of ligands can be loaded, which enables one to increase the surface density of binding sites and thus to enhance the sensitivity of the biosensor. We present results which show that functionalized hydrogels are very well suited for meeting the demands of these novel biosensor platforms.

Comparison of methods for generating planar DNA-modified surfaces for hybridization studies.

The surface conformation and accessibility of oligonucleotides within arrays are two key parameters that affect the utility of immobilized nucleic acids in sensor technologies. In this work, a novel combination of analytical techniques was used to compare two methods for DNA immobilization on glass. The aim of the study was to identify a method that generated a high surface density of hybridization-accessible oligonucleotides in a true planar monolayer. The first method based on direct coupling of silanized DNA to the glass surface showed a high immobilization density of 0.013 molecules/nm2 but low surface accessibility, as shown by the hybridization measurements (< or =15%). The second method, based on the biotin-streptavidin interaction, generated a high immobilization density (0.02 molecules/nm2) and high surface accessibility (90%). Atomic force microscopy and X-ray photoelectron spectroscopy indicated that both methods achieved uniform surfaces. Using the biotin-streptavidin system, the intermolecular distance between the hybridized molecules could be tightly controlled by titrating biotinylated complementary and noncomplementary oligonucleotides.

Surface Plasmon Fluorescence Techniques for Bioaffinity Studies

Among the various sensing principles proposed for bioaffinity studies, optical evanescent wave techniques have gained the lead in popularity. Next to evanescent ellipsometry [1] and the various optical waveguide platforms [2,3], surface plasmon resonance (SPR) spectroscopy [4–6], in particular, has ...

Electrical and optical properties of graphene mono- and multi-layers; towards graphene-based optoelectronics

Graphene, a newly discovered material with unusual electrical and optical properties, has attracted interest for a number of potential applications. One of the most actively pursued applications is using graphene as a transparent conducting electrode for use in solar cells, displays or touch screens. In this work, two studies are pursued in parallel to explore the electrical and optical properties of graphene. Graphene was prepared on copper by the standard chemical vapor deposition (CVD) method [1], the preparation procedure and conditions are described in [2]. The effect of chemical p-type doping on graphene stacks was studied in order to reduce the sheet resistance of graphene. The doping decreases the sheet resistance by a factor of 3, yielding films comprised of eight stacked layers with a sheet resistance of 90 Ω/ at a transmittance of 80% [2]. A theoretical model that accurately describes the stacked graphene film system as a resistor network was also developed [2]. The experimental data shows a linear increase in conductivity with the number of graphene layers, indicating that each layer provides an additional transport channel, in good agreement with the theoretical model (Fig. 1).

Doing More with Less: A Method for Low Total Mass, Affinity Measurement Using Variable-Length Nanotethers

Interactions between biomolecules are an important feature of biological systems and understanding these interactions is a key goal in biochemical studies. Using conventional techniques, such as surface plasmon resonance and isothermal titration calorimetry, the determination of the binding constants requires a significant amount of time and resources to produce and purify sufficient quantities of biomolecules in order to measure the affinity of biological interactions. Using DNA hybridization, we have demonstrated a new technique based on the use of nanotethers and time-resolved Forster resonance energy transfer (FRET) that significantly reduces the amount of material required to carry out quantitative binding assays. Test biomolecules were colocalized and attached to a surface using DNA tethers constructed from overlapping oligonucleotides. The length of the tethers defines the concentration of the tethered biomolecule. Effective end concentrations ranging from 56 nM to 3.8 μM were demonstrated. The use of variable length tethers may have wider applications in the quantitative measurement of affinity binding parameters.