DNA as a Molecular Wire: Distance and Sequence Dependence iconDNA as a Molecular Wire: Distance and Sequence Dependence

DNA as a Molecular Wire: Distance and Sequence Dependence

Dec 1, 2014 10:30am ‐ Dec 1, 2014 10:45am

Identification: A1.06

Charge transport (CT) through DNA has been extensively studied, and yet the mechanism of this process is still not yet fully understood. Besides the benefits of understanding charge transport through this fundamental molecule, further understanding of this process will elucidate the biological implications of DNA CT and advance sensing technology. Therefore, we have investigated the temperature and length dependence of DNA CT by measuring the electrochemical response of DNA monolayers modified with a redox-active probe. By using multiplexed electrodes, we are able to compare square wave voltammetry of distinct DNA sequences under identical experimental conditions. We vary the position of the probe, within a well matched DNA duplex, in order to investigate distance dependent kinetics. Through modeling analysis we are able to determine the charge transfer rates (k), transfer coefficients (α), and the redox active surface concentration (Γ*) of the DNA monolayer. The yield of transport is strongly connected to the stability of the duplex, linearly correlated to the melting temperature of the duplex. Additionally, the results show Arrhenius like behavior for multiple probe locations, with the transport rates following a 1/L length dependence, consistent with a hopping mechanism of transport. These results begin to clarify the significance of length and sequence on the stability of the duplex, which in turn, may be used to establish the guidelines for using DNA as a molecular wire in nanoscale electronics and sensing applications
Circular Interferometric Plasmonic Biosensor Arrays for High-Performance Label-Free Biomolecular Detection iconCircular Interferometric Plasmonic Biosensor Arrays for High-Performance Label-Free Biomolecular Detection

Circular Interferometric Plasmonic Biosensor Arrays for High-Performance Label-Free Biomolecular Detection

Dec 1, 2014 10:30am ‐ Dec 1, 2014 11:00am

Identification: D2.01

Plasmonic nanostructures have demonstrated unique capabilities for label-free biosensing through the excitation of localized surface plasmon resonance (LSPR) or the propagation surface plasmon polaritons (SPPs). Carefully designed nanoplasmonic biosensors convert small changes in the local refractive index caused by surface bio-molecular binding into spectral shifts. Here, we investigate a class of plasmonic interferometric biosensors that consist of arrays of circular aperture-groove nanostructures patterned on a gold film for phase-sensitive biomolecular detection. When the whole structure is illuminated by a collimated white light beam, the nanogrooves excite and focus the SPPs to the central aperture, where the SPPs interfere with the light that is directly transmitted through the aperture and modulate the far-field transmission. These biosensors achieve superior performance within a microscale footprint by combining SPR interactions with sensitive interferometric techniques. The phase and amplitude of interfering SPPs in the proposed device can be effectively engineered by structural tuning, providing a flexible and efficient control over the plasmon line shape observed through SPP interference. By careful structural tuning, spectral fringes with high contrast, narrow linewidth, and large amplitude have been experimentally measured and permit sensitive detection of protein surface coverage as low as 0.4 pg/mm2. This sensor resolution compares favorably with commercial prism-based surface plasmon resonance systems (0.1 pg/mm2), but is achieved here using a significantly simpler collinear transmission geometry, a miniaturized sensor footprint (150-150-m2), and a low-cost compact spectrometer, showing great promise to develop fast, inexpensive, compact biomedical devices for personal healthcare. The circular plasmonic interferometric biosensors were also operated in the intensity interrogation mode for high-throughput sensing applications, achieving a record high sensing figure-of-merit (FOM*) of 146 in the visible region, surpassing previous plasmonic biosensors and facilitating ultrasensitive high-throughput detection. Efforts were also made to differentiate surface analyte binding events in complex solutions from bulk refractive index variations due to changes in temperature and concentration of non-specific components in multi-component solutions.
New Ideas for Advancing Thermoelectric Performance iconNew Ideas for Advancing Thermoelectric Performance

New Ideas for Advancing Thermoelectric Performance

Dec 1, 2014 10:45am ‐ Dec 1, 2014 11:15am

Identification: CC1.07

A series of new ideas has been proposed during the last two decades for enhancing the figure of merit of thermoelectric materials, but the interdependences of the Seebeck coefficient, the electrical conductivity, and the thermal conductivity make the problem very difficult to solve. A new idea, proposed by Shuang Tang, which suggests an approach to achieving a more convergent path towards enhancing the thermoelectric figure of merit will be discussed. Recent progress made with pursuing this approach will be illustrated and discussed.
Structure-Specific Growth of Single-Walled Carbon Nanotubes with Tungsten-Based Alloy Catalysts iconStructure-Specific Growth of Single-Walled Carbon Nanotubes with Tungsten-Based Alloy Catalysts

Structure-Specific Growth of Single-Walled Carbon Nanotubes with Tungsten-Based Alloy Catalysts

Dec 1, 2014 10:45am ‐ Dec 1, 2014 11:00am

Identification: MM1.04

Single-walled carbon nanotubes (SWNTs) have shown great potentials in various fields attributing to their unique structure-dependent properties, therefore, the structure-controlled preparation of SWNTs is a crucial issue for their advanced applications (e.g. carbon-based nanoelectronics) and has been a great challenge for about two decades. To fully utilize the outstanding performance of SWNTs, it is preferred to control the structure of SWNTs during growth rather than in post-synthesis treatments. However, up to date, the direct synthesis of SWNT samples with a single dominating chirality > 60% have never been realized. Here we report a strategy to produce SWNTs with specific chirality by using a new family of catalysts, tungsten-based bimetallic alloy nanoparticles of non-cubic symmetry, which have high melting points and consequently are able to maintain their crystal structure during the chemical vapor deposition (CVD) process, to regulate the chirality of the grown SWNTs. The (12,6) SWNTs are directly synthesized at an abundance of > 92% by using W-Co catalysts. Experimental evidence and theoretical simulation reveal that the (0 0 12) planes of W6Co7 nanocrystals extremely match the structure of (12,6) SWNTs, therefore facilitate the preferential growth of (12,6) tubes. This method is also valid for other tungsten-based alloy nanocatalysts to grow SWNTs of various designed chirality. Employing alloy nanocrystals with unique structure as catalysts paves a way for the ultimate chirality control in SWNT growth and thus may promote the development in SWNT applications.

References:

[1] F. Yang, X. Wang, D. Zhang, J. Yang, D. Luo, Z. Xu, J. Wei, J.-Q. Wang, Z. Xu, F. Peng, X. Li, R. Li, Y. Li, M. Li, X. Bai, F. Ding, Y. Li*, 'Chirality-specific growth of single-walled carbon nanotubes on solid alloy catalysts', Nature, DOI 10.1038/nature13434 (2014).
[2] Y. Li*, R. Cui, L. Ding, Y. Liu, W. Zhou, Y. Zhang, Z. Jin, F. Peng, J. Liu*, 'How Catalysts affect the Growth of Single-Walled Carbon Nanotubes on Substrates', Advanced Materials, 22, 1508-1515 (2010).
Path Information Theory Methods for Coarse-Graining and Uncertainty Quantification of Molecular Systems iconPath Information Theory Methods for Coarse-Graining and Uncertainty Quantification of Molecular Systems

Path Information Theory Methods for Coarse-Graining and Uncertainty Quantification of Molecular Systems

Dec 1, 2014 10:45am ‐ Dec 1, 2014 11:15am

Identification: NN1.07

In this talk we discuss path-space information theory-based sensitivity analysis and parameter identification methods for complex high-dimensional dynamics, as well as information-theoretic tools for parameterized coarse-graining of non-equilibrium extended systems. Furthermore, we relate such information-theoretic methods with observables and goal-oriented approaches through the derivation of path-space Cramer-Rao-type inequalities, which also allow us to address transferability questions in coarse-graining.
The combination of proposed methodologies is capable to tackle molecular-level models with a very large number of parameters, as well as non-equilibrium processes, typically associated with coupled physicochemical mechanisms, boundary conditions, etc. (such as reaction-diffusion and/or driven systems), and where even steady states are unknown altogether, e.g. do not have a Gibbs structure. Finally, the path-wise information theory tools yield a surprisingly simple, tractable and easy-to-implement approach to quantify and rank parameter sensitivities, as well as provide reliable molecular model parameterizations based on fine-scale data through suitable path-space (dynamics-based) information criteria.

The proposed methods are tested against a wide range of high-dimensional stochastic processes, ranging from complex biochemical reaction networks with hundreds of parameters, to spatially extended Kinetic Monte Carlo models in catalysis and Langevin dynamics of interacting molecules with internal degrees of freedom.

Label-Free Mid-IR Photothermal Spectroscopy for Biological Imaging Using a Robust Fiber Laser Probe iconLabel-Free Mid-IR Photothermal Spectroscopy for Biological Imaging Using a Robust Fiber Laser Probe

Label-Free Mid-IR Photothermal Spectroscopy for Biological Imaging Using a Robust Fiber Laser Probe

Dec 1, 2014 11:00am ‐ Dec 1, 2014 11:15am

Identification: D2.02

We demonstrate the use of mid-infrared (mid-IR) photothermal spectroscopy as a sensitive and label-free technique for hyperspectral chemical imaging. In photothermal spectroscopy, a pump beam is absorbed by the sample causing a temperature-induced change in the local refractive index. The scattered probe beam then is detected in a heterodyne measurement. Here, a pulsed mid-IR quantum cascade laser pump (tunable around 6µm) and an erbium-doped fiber laser probe at eye-safe wavelengths (around 1.55µm) are used. Extending photothermal spectroscopy into the mid-IR is attractive due to the presence of a large number of characteristic infrared-active normal vibrational modes, particularly in the molecular fingerprint region, allowing bond-specific imaging of biological and chemical samples.

Photothermal studies of a liquid crystal, 4′-octyl-4-biphenylcarbonitrile (8CB), targeting the C-H scissoring band at 1607cm-1 were conducted. Spectral scans of 8CB were demonstrated with a good signal-to-noise ratio, an increase of over one order of magnitude from previous studies that used a titanium:sapphire probe laser. Contributions to the signal-to-noise ratio from several different probe laser configurations, including continuous-wave and mode-locked lasers, are investigated. Histopathological bird brain sections were studied and high contrast mid-IR photothermal images were obtained without the use of stains. Images of the bird brain sections were acquired by targeting the amide-I band centered at 1670cm-1 and raster-scanning the sample.

Photothermal spectroscopy with a fiber laser probe offers many advantages for imaging in the mid-IR. Fiber lasers can be used with commercially available and highly sensitive photodetectors that operate at room temperature, in contrast to other mid-IR spectroscopy techniques such as Fourier Transform Infrared Spectroscopy (FTIR) which commonly uses cryogenically cooled mid-IR detectors. Turn-key operation of compact and stable fiber lasers makes the photothermal system more robust and user-friendly. These studies show that high-contrast images of biological samples with good sensitivity can be obtained using mid-IR photothermal spectroscopy with a fiber laser probe.
Unprecedented Solid-State Viscoelasticity Discovered in Exfoliated Graphite iconUnprecedented Solid-State Viscoelasticity Discovered in Exfoliated Graphite

Unprecedented Solid-State Viscoelasticity Discovered in Exfoliated Graphite

Dec 1, 2014 11:00am ‐ Dec 1, 2014 11:15am

Identification: K2.02

Viscoelastic solids are valuable for passive vibration damping. Unprecedented solid-state viscoelasticity has been discovered in exfoliated graphite. The highly viscous behavior is due to the friction at the interface between the carbon layers in the graphite. The interfacial mechanism of mechanical energy dissipation is in contrast to the bulk viscous deformation mechanism, which is the case for polymers such as rubber. The loss tangent of the graphite reaches 35, compared to 0.7 for rubber. The incorporation of this graphite in a cement matrix results in microscale constrained-layer damping, with the loss modulus reaching the exceptionally high value of 7.5 GPa. In order for exfoliation to occur, the graphite layers that make up the wall of an intercalate island must be able to stretch greatly. The stretching of the wall enables an intercalate island to expand like a balloon. A wall consists of multiple layers of graphite, such that each layer does not necessarily extend all the way across the length of an island. There are about 60 graphite layers (on the average) in the cell wall of the exfoliated graphite used. The stretching of a wall is made possible by the sliding of the graphite layers with respect to one another within the wall. This sliding requires the overcoming of the van der Waals’ forces between the graphite layers. The vapor-related driving force for exfoliation is adequate for overcoming these forces. For an irreversibly exfoliated graphite, the tremendous sliding of the graphite layers has already occurred during the completed exfoliation, so no further tremendous sliding occurs upon subsequent vibration. Nevertheless, the exfoliation process has irreversibly loosened the binding of the graphite layers and, as a consequence, a degree of sliding between the layers can easily occur upon subsequent vibration. This looseness is consistent with a very low modulus (~110 kPa) in the direction perpendicular to the wall, as shown by nanoindentation testing. The smoothness of the load vs. displacement curve during nanoindentation indicates that the indentation mechanism involves the stretching of the cell walls rather than the breakthrough of the walls. The deformation is mostly reversible upon unloading. With the displacement attributed to the sliding between the graphite layers in a cell wall (width ~20 nm), the maximum shear strain in the cell wall is ~39. This indicates elastomeric deformation, which has been previously reported in polymers only. The reversibility of the sliding is probably made possible by the ease of sliding of the graphite layers and the cellular structure, in which the extremities of a cell serve as pinning points that effectively link the graphite layers. Such links are akin to the crosslinks in rubber. Without exfoliation, the sliding is relatively difficult, due to the strong binding between the layers.
Optical Metamaterials as Antireflection Coatings for Solar Cells iconOptical Metamaterials as Antireflection Coatings for Solar Cells

Optical Metamaterials as Antireflection Coatings for Solar Cells

Dec 1, 2014 11:00am ‐ Dec 1, 2014 11:15am

Identification: L1.07

One of the factors limiting solar cell efficiency is light reflection at the surface and at the multiple interfaces of the device stack. Suppressing reflections increases the amount of light transmitted into the actual absorber layer. In the recent years, many different approaches have been proposed to employ antireflection coatings that can be adapted to any material. Unfortunately, exotic materials, metals, or a fine tuning of nanostructures size and shape are usually required. As a consequence, large scale application is not easy to achieve yet. In this work, we propose the use of optical metamaterials as antireflection coatings. Solar cells will be one of the possible applications.

We develop the 1D and 2D Maxwell-Garnett theory to design effective layers with arbitrary refractive index. We use the transfer matrix method and full-field simulations to explore the optical behavior of different types of metamaterial designs. In particular, we obtain complete suppression of light reflection from a high index material by tuning the filling fraction and thickness of our layer. Moreover, we explore the practical realization of such metamaterials by using Focused Ion Beam to fabricate sub-wavelength linear grating on a silicon wafer. We measure optical reflectivity over the entire visible spectrum from our samples, demonstrating good antireflection properties. The effect of light polarization is also investigated. In fact, the effective refractive index is strongly dependent on the incident polarization. We propose a double layer linear grating in order to achieve zero reflectivity at both polarizations.

In addition, we discuss the transition between deep sub-wavelength and resonant regime. If resonant modes are supported, the effective refractive index deviates from the Maxwell-Garnett theory. We elucidate the role of light trapping in the efficiency of the antireflection properties.

Finally, we extend our approach to a realistic aSi solar cell. We propose the patterning of the transparent conductive oxide (TCO) layer atop the absorbing material, while keeping the rest of the fabrication process flow unaltered. We show an increase of the solar cell efficiency by more than 10%, reaching the single-pass absorption limit of the TCO.
Ultra-Small-Angle X-Ray Scattering - X-Ray Photon Correlation Spectroscopy for Material Dynamics at the Mesoscale iconUltra-Small-Angle X-Ray Scattering - X-Ray Photon Correlation Spectroscopy for Material Dynamics at the Mesoscale

Ultra-Small-Angle X-Ray Scattering - X-Ray Photon Correlation Spectroscopy for Material Dynamics at the Mesoscale

Dec 1, 2014 11:00am ‐ Dec 1, 2014 11:15am

Identification: TT1.05

X-ray Photon Correlation Spectroscopy (XPCS) offers unprecedented sensitivity to the dynamics of structural changes in materials. However, XPCS facilities have generally been limited to microstructure length scales smaller than ≈ 50 nm, thus eliminating large classes of materials that are of major technological importance. In recent years, we have been able to extend the range of this technique dramatically (into the micrometer scale regime) by combining XPCS speckle measurements with Bonse-Hart ultrasmall-angle scattering (USAXS) studies at the Advanced Photon Source. [1-4] While USAXS characterizes microstructures over the nanometer-to-micrometer scale range, use of a small entrance slit allows the coherence of the undulator X-ray beam to be exploited to give XPCS measurements of the internal microstructure dynamics. At the large end of the scale range, the slower material dynamics are well matched to the time resolution offered by USAXS-based XPCS. Using a point-counting configuration at selected Q values, we have established that phenomena previously observed for nanoparticle dispersions, including de Gennes narrowing, extend to these coarser length scales. [4] This is important because the slower relaxation times at mesoscale lengths in aqueous colloidal suspensions can be followed directly using USAXS-XPCS. Thus, phenomena such as bimodal interparticle interactions or suspension liquid phase transformations can be studied at the mesoscale while retaining relevance for nanoscale phenomena, where much shorter relaxation times make direct studies difficult.

USAXS-XPCS can also be configured to make repeated, short USAXS scans to detect incipient (precursor) microstructure changes under non-equilibrium conditions by following associated changes in the observed speckles. [3] We have applied this approach to study amorphous-to-crystalline phase transformations in dental composites. [3]

Finally, we have explored the feasibility of conducting simultaneous multiple USAXS-XPCS measurements using a nanofabricated slit array with each partially coherent X-ray beam paired to a group of pixels on a position-sensitive detector. This would allow rapid measurement of the dynamics in a heterogeneous material or could be used to follow a reaction front advancing across the sample.

References:

[1] F. Zhang, A.J. Allen, L.E. Levine, J. Ilavsky, G.G. Long A.R. Sandy; J. Appl. Cryst., 44, 200-212 (2011).

[2] F. Zhang, A.J. Allen, L.E. Levine, J. Ilavsky, G.G. Long; Metall. Mater. Trans. A,43, 1445-1453 (2012).

[3] F. Zhang, A.J. Allen, L.E. Levine, L. Espinal, J.M. Antonucci, D. Skrtic, J.N.R. O’Donnell, J. Ilavsky; J. Biomed. Mater. Res. A, 100, 1293-1306 (2012).

[4] F. Zhang, A.J. Allen, L.E. Levine, J. Ilavsky, G.G. Long; Langmuir 29, 1379-1387 (2013).

Scalable, Transparent, Flexible, Thin-Film Pressure Sensors Utilizing Ultrasoft Elastomers iconScalable, Transparent, Flexible, Thin-Film Pressure Sensors Utilizing Ultrasoft Elastomers

Scalable, Transparent, Flexible, Thin-Film Pressure Sensors Utilizing Ultrasoft Elastomers

Dec 1, 2014 11:15am ‐ Dec 1, 2014 11:30am

Identification: B2.05

Thin-film pressure sensors based on elastomers have required either foams or microstructuring. Solid films of elastomer have not been used due to low sensitivities mostly due to relatively high moduli on the order of megapascals. However, solid elastomeric films are more easily incorporated into electronics, are more scalable and can be transparent. Here, we demonstrate elastomers with Young’s moduli on the order of 1 kPa while maintaining elastic recovery. Typically, materials with kPa moduli are viscoelastic. Within a pressure sensor, a viscoelastic material will have a substantial hysteresis and will recover from pressure slowly and incompletely. Off-stoichiometry thiol-click chemistry with thiol-functionalized polydimethylsilane (PDMS) and vinyl-terminated PDMS oligomers is used to form elastomers with low crosslink densities and long, flexible side chains. These side chains plasticize the elastomer, leading to a much smaller Young’s modulus and helping to maintain the elastic behavior. These elastomers are transparent, ultrasoft, and have little to no hysteresis in compression cycles, indicating mostly elastic behavior.

Pressure sensors using these elastomers are made by depositing electrodes onto films with thicknesses below 100 nanometers. The thin films are made by diluting the PDMS oligomers into a good solvent and rod coating the solutions onto glass. The resulting pressure sensors act as variable resistors. With the application of pressure and voltage, the film is compressed and current flows between the electrodes. For 2 mm x 2 mm pressure sensing pixels and an applied voltage of 1V, the current increases from a picoamp baseline to 10’s of picoamps to nanoamps depending on the electrode design and the amount of applied pressure from 1 g to 1 kg of force. This range of sensitivity matches that of finger pressure, which makes these sensors suitable for pressure-sensitive touchscreens. These pressure sensors are incorporated into a matrix-addressable array by using a transistor array with the variable resistors in series with the drain and the drain current is measured. When using transparent electrodes, these arrays are transparent, low-power, highly sensitive and have high 2-dimensional spatial resolution. This allows these pressure sensor arrays to be used in many applications including pressure-sensitive, multi-touch touchscreens.