Modulation of Electrokinetic Potentials Using Graphene-Based Surfaces and Variable Substrate Charge Density.

Enhanced electrokinetic phenomena, manifested through the observation of a large streaming potential (Vs), were obtained in microchannels with single-layer graphene (SLG)-coated and few-layer graphene (FLG)-coated surfaces. In comparison to silicon microchannels, the Vs obtained for a given pressure difference along the channel (ΔP) was higher by 75% for the graphene-based channels, with larger values in the SLG case. Computational modeling was used to correlate the surface charge density, tuned through plasma processing, and related zeta potential to measured Vs. The implications related to deploying lower dimensional material surfaces for modulating electrokinetic flows were investigated.

Surface Composites Synthesized through the Incorporation of Atomic Layer Deposited AlOx into Nanoporous Fuzzy Tungsten.

The incorporation of energetic helium gaseous species into materials such as tungsten (W) imparts intrinsic surface fragility, yielding fuzzy tungsten. To enhance the robustness of the surface layers, aluminum oxide (AlOx) was deposited by atomic layer deposition into the fuzzy W. The conformally deposited ceramic yields a new class of surface composites. Structural characterization of the fuzzy W-AlOx composites through nanoindentation testing indicated enhanced indentation modulus (Eind) and hardness (Hind) and was modeled through various rules of mixtures approaches. The distribution of AlOx in fuzzy W was explored and a systematic study of the extent of incorporation of the AlOx into the fuzzy W was carried out. The synthesized composites may be utilized for improved structural characteristics, e.g., in reducing crack initiation and fracture.

Toward the Ultimate Limit of Analyte Detection, in Graphene-Based Field-Effect Transistors.

The ultimate sensitivity of field-effect-transistor (FET)-based devices for ionic species detection is of great interest, given that such devices are capable of monitoring single-electron-level modulations. It is shown here, from both theoretical and experimental perspectives, that for such ultimate limits to be approached the thermodynamic as well as kinetic characteristics of the (FET surface)-(linker)-(ion-receptor) ensemble must be considered. The sensitivity was probed in terms of optimal packing of the ensemble, through a minimal charge state/capacitance point of view and atomic force microscopy. Through the fine-tuning of the linker and receptor interaction with the sensing surface, a record limit of detection as well as specificity in the femtomolar range, orders of magnitude better than previously obtained and in excellent accord with prediction, was observed.

Graphene and Two-Dimensional Materials for Biomolecule Sensing.

An ideal biosensor material at room temperature, with an extremely large surface area per unit mass combined with the possibility of harnessing quantum mechanical attributes, would be comprised of graphene and other two-dimensional (2D) materials. The sensing of a variety of sizes and types of biomolecules involves modulation of the electrical charge density of (current through) the 2D material and manifests through specific components of the capacitance (resistance). While sensitive detection at the single-molecule level, i.e., at zeptomolar concentrations, may be achieved, specificity in a complex mixture is more demanding. Attention should be paid to the influence of inevitably present defects in the 2D materials on the sensing, as well as calibration of obtained results with acceptable standards. The consequent establishment of a roadmap for the widespread deployment of 2D material-based biosensors in point-of-care platforms has the potential to revolutionize health care.

Enhanced graphene surface plasmonics through incorporation into metallic nanostructures.

A methodology for enhancing the surface plasmon polariton (SPP) resonance associated with graphene, through nanoscale metal-dielectric-metal (MDM) gaps, is proposed. The modulation of the resonances, in the range of 0.7 µm to 1 µm was done through tuning the carrier density in graphene and has been shown to be of potential utility for surface analyte sensing. It was shown, from finite element simulations in the frequency domain, that the related hybrid SPP modes could be clearly delineated in far field spectroscopy.

Possibility of Obtaining Two Orders of Magnitude Larger Electrokinetic Streaming Potentials, through Liquid Infiltrated Surfaces.

It is shown that the magnitude of the streaming potential (Vs) can be significantly enhanced from ∼0.02 V to as much as ∼1.6 V, in electrokinetic flows through microchannels. This was done through flows on liquid-filled surfaces, where the grooves were filled with oils of viscosity in the range 30-3000 mPa·s. The presence of immiscible oils and the improved slip are both factors that could significantly increase the Vs. The analytical relationship between streaming potential and filled liquid viscosity was derived and verified through corresponding experimental results. The work yields novel insights into complex electrolyte flows and indicates avenues for more efficient energy harvesting.

Direct DNA Methylation Profiling with an Electric Biosensor.

DNA methylation is one of the principal epigenetic mechanisms that control gene expression in humans, and its profiling provides critical information about health and disease. Current profiling methods require chemical modification of bases followed by sequencing, which is expensive and time-consuming. Here, we report a direct and rapid determination of DNA methylation using an electric biosensor. The device consists of a DNA-tweezer probe integrated on a graphene field-effect transistor for label-free, highly sensitive, and specific methylation profiling. The device performance was evaluated with a target DNA that harbors a sequence of the methylguanine-DNA methyltransferase, a promoter of glioblastoma multiforme, a lethal brain tumor. The results show that we successfully profiled the methylated and nonmethylated forms at picomolar concentrations. Further, fluorescence kinetics and molecular dynamics simulations revealed that the position of the methylation site(s), their proximity, and accessibility to the toe-hold region of the tweezer probe are the primary determinants of the device performance.

Iron redox pathway revealed in ferritin via electron transfer analysis.

Ferritin protein is involved in biological tissues in the storage and management of iron - an essential micro-nutrient in the majority of living systems. While there are extensive studies on iron-loaded ferritin, its functionality in iron delivery is not completely clear. Here, for the first time, differential pulse voltammetry (DPV) has been successfully adapted to address the challenge of resolving a cascade of fast and co-occurring redox steps in enzymatic systems such as ferritin. Using DPV, comparative analysis of ferritins from two evolutionary-distant organisms has allowed us to propose a stepwise resolution for the complex mix of concurrent redox steps that is inherent to ferritins and to fine-tune the structure-function relationship of each redox step. Indeed, the cyclic conversion between Fe3+ and Fe2+ as well as the different oxidative steps of the various ferroxidase centers already known in ferritins were successfully discriminated, bringing new evidence that both the 3-fold and 4-fold channels can be functional in ferritin.

Interaction and hybridization of orthogonal Fabry-Pérot like surface plasmon modes in metal-dielectric grating structures.

The interaction of specific surface plasmon modes in metal-dielectric-metal arrangements is investigated, motivated by their relevance to device-based configurations. The absorption spectra of the relevant nanostructures considering geometrical variation, such as the width and height of the metal or dielectric, are probed considering such interactions. Frequency domain simulations are used to study related multiple surface plasmon polariton resonance modes. It is indicated that the resonant energy level interaction due to the coupling between modes in a horizontal dielectric layer and those in a vertical groove can be engineered and understood in terms of energy level hybridization.

Modulation of the Streaming Potential and Slip Characteristics in Electrolyte Flow over Liquid-Filled Surfaces.

A significant enhancement in the streaming potential ( Vs) was obtained in experiments considering the flow of electrolyte over liquid-filled surfaces (LFSs), where the grooves in patterned substrates are filled with electrolyte immiscible oils. Such LFSs yield larger Vs (by a factor of 1.5) compared to superhydrophobic surfaces, with air-filled grooves, and offer tunability of electrokinetic flow. It is shown that the density, viscosity, conductivity, as well as the dielectric constant of the filling oil, in the LFS, determine Vs. Relating a hydrodynamic slip length to the obtained Vs offers insight into flow characteristics, as modulated by the liquid interfaces in the LFS.

Enhanced Environmental Stability Coupled with a 12.5% Power Conversion Efficiency in an Aluminum Oxide-Encapsulated n-Graphene/p-Silicon Solar Cell.

A significant improvement in the power conversion efficiency (PCE) and the environmental stability of n-Graphene/p-Si solar cells is indicated through effective n-doping of graphene, using low work function oxide capping layers. AlO x, deposited through atomic layer deposition, is particularly effective for such doping and in addition serves as an antireflection coating and a cell encapsulating layer. It is shown that the related charge transfer doping and interfacial engineering was crucial to achieve a record PCE of 12.5%. The work indicates a path forward, through work function engineering, for further efficiency gains in Gr-based solar cells.

An approach towards a perfect thermal diffuser.

A method for the most efficient removal of heat, through an anisotropic composite, is proposed. It is shown that a rational placement of constituent materials, in the radial and the azimuthal directions, at a given point in the composite yields a uniform temperature distribution in spherical diffusers. Such arrangement is accompanied by a very significant reduction of the source temperature, in principle, to infinitesimally above the ambient temperature and forms the basis for the design of a perfect thermal diffuser with maximal heat dissipation. Orders of magnitude enhanced performance, compared to that obtained through the use of a diffuser constituted from a single material with isotropic thermal conductivity has been observed and the analytical principles underlying the design were validated through extensive computational simulations.

Enhanced Power Conversion Efficiency of Graphene/Silicon Heterojunction Solar Cells Through NiO Induced Doping.

We report a doping strategy, where nickel oxide (NiO) nanoparticle film coating is employed for graphene/Si heterojunction solar cells to improve the power conversion efficiency (PCE). NiO doping has been shown to improve the short circuit current (J(SC)) by 12%, open circuit voltage (V(OC)) by 25% and fill factor (FF) by 145% of the cells, in turn increasing the PCE from 1.37% to 4.91%. Furthermore, NiO doped graphene/Si solar cells don't show any significant performance degradation over 10 days revealing that NiO doping can be a promising approach for practical applications of graphene in solar cells.

Photoresponse of a Single Y-Junction Carbon Nanotube.

We report investigation of optical response in a single strand of a branched carbon nanotube (CNT), a Y-junction CNT composed of multiwalled CNTs. The experiment was performed by connecting a pair of branches while grounding the remaining one. Of the three branch combinations, only one combination is optically active which also shows a nonlinear semiconductor-like I-V curve, while the other two branch combinations are optically inactive and show linear ohmic I-V curves. The photoresponse includes a zero-bias photocurrent from the active branch combination. Responsivity of ≈1.6 mA/W has been observed from a single Y-CNT at a moderate bias of 150 mV with an illumination of wavelength 488 nm. The photoresponse experiment allows us to understand the nature of internal connections in the Y-CNT. Analysis of data locates the region of photoactivity at the junction of only two branches and only the combination of these two branches (and not individual branches) exhibits photoresponse upon illumination. A model calculation based on back-to-back Schottky-type junctions at the branch connection explains the I-V data in the dark and shows that under illumination the barriers at the contacts become lowered due to the presence of photogenerated carriers.

High-performance flexible hydrogen sensor made of WS2 nanosheet-Pd nanoparticle composite film.

We report a flexible hydrogen sensor, composed of WS2 nanosheet-Pd nanoparticle composite film, fabricated on a flexible polyimide substrate. The sensor offers the advantages of light-weight, mechanical durability, room temperature operation, and high sensitivity. The WS2-Pd composite film exhibits sensitivity (R 1/R 2, the ratio of the initial resistance to final resistance of the sensor) of 7.8 to 50,000 ppm hydrogen. Moreover, the WS2-Pd composite film distinctly outperforms the graphene-Pd composite, whose sensitivity is only 1.14. Furthermore, the ease of fabrication holds great potential for scalable and low-cost manufacturing of hydrogen sensors.

Bioinspired superhydrophobic surfaces, fabricated through simple and scalable roll-to-roll processing.

A simple, scalable, non-lithographic, technique for fabricating durable superhydrophobic (SH) surfaces, based on the fingering instabilities associated with non-Newtonian flow and shear tearing, has been developed. The high viscosity of the nanotube/elastomer paste has been exploited for the fabrication. The fabricated SH surfaces had the appearance of bristled shark skin and were robust with respect to mechanical forces. While flow instability is regarded as adverse to roll-coating processes for fabricating uniform films, we especially use the effect to create the SH surface. Along with their durability and self-cleaning capabilities, we have demonstrated drag reduction effects of the fabricated films through dynamic flow measurements.

Solution-Processed CoFe2O4 Nanoparticles on 3D Carbon Fiber Papers for Durable Oxygen Evolution Reaction.

We report CoFe2O4 nanoparticles (NPs) synthesized using a facile hydrothermal growth and their attachment on 3D carbon fiber papers (CFPs) for efficient and durable oxygen evolution reaction (OER). The CFPs covered with CoFe2O4 NPs show orders of magnitude higher OER performance than bare CFP due to high activity of CoFe2O4 NPs, leading to a small overpotential of 378 mV to get a current density of 10 mA/cm(2). Significantly, the CoFe2O4 NPs-on-CFP electrodes exhibit remarkably long stability evaluated by continuous cycling (over 15 h) and operation with a high current density at a fixed potential (over 40 h) without any morphological change and with preservation of all materials within the electrode. Furthermore, the CoFe2O4 NPs-on-CFP electrodes also exhibit hydrogen evolution reaction (HER) performance, which is considerably higher than that of bare CFP, acting as a bifunctional electrocatalyst. The achieved results show promising potential for efficient, cost-effective, and durable hydrogen generation at large scales using earth-abundant materials and cheap fabrication processes.

Ultra-high optical absorption efficiency from the ultraviolet to the infrared using multi-walled carbon nanotube ensembles.

The optical absorption efficiencies of vertically aligned multi-walled (MW)-carbon nanotube (CNT) ensembles are characterized in the 350-7000 nm wavelength range where CNT site densities > 1 × 10(11) /cm(2) are achieved directly on metallic substrates. The site density directly impacts the optical absorption characteristics, and while high-density arrays of CNTs on electrically insulating and non-metallic substrates have been commonly reported, achieving high site-densities on metals has been challenging and remains an area of active research. These absorber ensembles are ultra-thin (<10 µm) and yet they still exhibit a reflectance as low as ∼0.02%, which is 100 times lower than the reference; these characteristics make them potentially attractive for high-sensitivity and high-speed thermal detectors. In addition, the use of a plasma-enhanced chemical vapor deposition process for the synthesis of the absorbers increases the portfolio of materials that can be integrated with such absorbers due to the potential for reduced synthesis temperatures. The remarkable ruggedness of the absorbers is also demonstrated as they are exposed to high temperatures in an oxidizing ambient environment, making them well-suited for extreme thermal environments encountered in the field, potentially for solar cell applications. Finally, a phenomenological model enables the determinatiom of the extinction coefficients in these nanostructures and the results compare well with experiment.

Elastic response of carbon nanotube forests to aerodynamic stresses.

The ability to determine static and (hydro)dynamic properties of carbon nanotubes (CNTs) is crucial for many applications. While their static properties (e.g., solubility and wettability) are fairly well understood, their mechanical responses (e.g., deflection under shear) to ambient fluid flow are to a large extent unknown. We analyze the elastic response of single-walled CNT forests, attached to the bottom wall of a channel, to the aerodynamic loading exerted by both laminar and turbulent flows. Our analysis yields analytical expressions for velocity distributions, the drag coefficient, and bending profiles of individual CNTs. This enables us to determine flexural rigidity of CNTs in wind-tunnel experiments. The model predictions agree with laboratory experiments for a large range of channel velocities.

Toxicity issues in the application of carbon nanotubes to biological systems.

Carbon nanotubes (CNTs) have recently emerged as a new option for possible use in methodologies of cancer treatment, bioengineering, and gene therapy. This review analyzes the potential, through possible toxicologic implications, of CNTs in nanomedicine. Generally, proven success in other fields may not translate to the use of CNTs in medicine for reasons including inconsistent data on cytotoxicity and limited control over functionalized-CNT behavior, both of which restrict predictability. Additionally, the lack of a centralized toxicity database limits comparison between research results. To better understand these problems, we seek insight from currently published toxicity studies, with data suggesting postexposure regeneration, resistance, and mechanisms of injury in cells, due to CNTs. FROM THE CLINICAL EDITOR: Carbon nanotubes (CNTs) have recently emerged as a new option for cancer treatment, bioengineering, and gene therapy. Inconsistent data on cytotoxicity and limited control over functionalized-CNT behavior currently restrict predictability of such applications.