Correlation of Fabrication Methods and Enhanced Wear Performance in Nanoporous Anodic Aluminum Oxide with Incorporated Molybdenum Disulfide (MoS2) Nanomaterials.

Wear performance is integral to component longevity, minimizing industrial waste and excess energy costs in a wide variety of applications. Anodized aluminum oxide (AAO) has many beneficial properties leading to its wide use across industries as a surface treatment for many aluminum components, but the wear properties of the coating could be improved significantly. Here, we used an electrochemical method to incorporate molybdenum disulfide (MoS2), a nanomaterial used as a dry lubricant, to modify alloys of aluminum during AAO preparation. Using Raman spectroscopy and tribological scratch measurements, we thoroughly characterized the structure and wear behavior of the films. The MoS2 deposition procedure was optimal on aluminum 5052 anodized in higher acid concentrations, with friction coefficients at around 0.05 (~10× better than unmodified AAO). Changing anodization conditions to produce harder films with smaller pores led to worsened wear properties, likely because of lower MoS2 content. Studying a commercial MoS2/AAO film of a different Al alloy (7075) showed that a heat treatment step intended to fully convert all deposited MoSx species to MoS2 can adversely affect wear in some alloys. While Al 6061 and 1100 produced films with worse wear performance compared to Al 5052 or 7075, our results show evidence that acid cleaning after initial anodization likely removes residual alloying elements, affecting MoS2 incorporation. This study demonstrates a nanomaterial modified AAO film with superior wear characteristics to unmodified AAO and relates fabrication procedure, film structure, and practical performance.

Electrodeposition and analysis of thick bismuth films.

Due to its unique physical and chemical properties, bismuth is an attractive candidate for a wide range of applications such as battery anodes, radiation shielding, and semiconductors, to name a few. This work presents the electrodeposition of mechanically stable and homogenous bismuth films at micron-scale thicknesses. A simple one-step electrodeposition process using either a pulse/reverse or direct current source yielded thick, homogenous, and mechanically stable bismuth films. Morphology, electrochemical behavior, adhesion, and mechanical stability of bismuth coatings plated with varying parameters were characterized via optical profilometry, cyclic voltammetry, electron microscopy, and tribology. Scratch testing on thick electroplated coatings (> 100 µm) revealed similar wear resistance properties between the pulse/reverse plated and direct current electroplated films. This study presents a versatile bismuth electroplating process with the possibility to replace lead in radiation shields with an inexpensive, non-toxic metal, or to make industrially relevant electrocatalytic devices.

Visualizing Grain Statistics in MOCVD WSe2 through Four-Dimensional Scanning Transmission Electron Microscopy.

Using four-dimensional scanning transmission electron microscopy, we demonstrate a method to visualize grains and grain boundaries in WSe2 grown by metal organic chemical vapor deposition (MOCVD) directly onto silicon dioxide. Despite the chemical purity and uniform thickness and texture of the MOCVD-grown WSe2, we observe a high density of small grains that corresponds with the overall selenium deficiency we measure through ion beam analysis. Moreover, reconstruction of grain information permits the creation of orientation maps that demonstrate the nucleation mechanism for new layers-triangular domains with the same orientation as the layer underneath induces a tensile strain increasing the lattice parameter at these sites.

Recyclable Graphene Sheets as a Growth Template for Crystalline ZnO Nanowires.

Recent advances in nanoscience have opened ways of recycling substrates for nanomaterial growth. Novel materials, such as atomically thin materials, are highly desirable for the recycling substrates. In this work, we report recycling of monolayer graphene as a growth template for synthesis of single crystalline ZnO nanowires. Selective nucleation of ZnO nanowires on graphene was elucidated by scanning electron microscopy and density functional theory calculation. Growth and subsequent separation of ZnO nanowires was repeated up to seven times on the same monolayer graphene film. Raman analyses were also performed to investigate the quality of graphene structure along the recycling processes. The chemical robustness of graphene enables the repetitive ZnO nanowire growth without noticeable degradation of the graphene quality. This work presents a route for graphene as a multifunctional growth template for diverse nanomaterials such as nanocrystals, aligned nanowires, other two-dimensional materials, and semiconductor thin films.

Monolithic Nanoporous Gold Foams with Catalytic Activity for Chemical Vapor Deposition Growth of Carbon Nanostructures.

While bulk gold is generally considered to be a catalytically inactive material, nanostructured forms of gold can in fact be highly catalytically active. However, few methods exist for preparing high-purity macroscopic forms of catalytically active gold. In this work, we describe the synthesis of catalytically active macroscopic nanoporous gold foams via combustion synthesis of gold bis(tetrazolato)amine complexes. The resulting metallically pure porous gold nanoarchitectures exhibit bulk densities of <0.1 g/cm3 and Brunauer-Emmett-Teller (BET) surface areas as high as 10.9 m2/g, making them among the lowest-density and highest-surface-area monolithic forms of gold produced to date. Thanks to the presence of a highly nanostructured gold surface, such gold nanofoams have also been found to be highly catalytically active toward thermal chemical vapor deposition (CVD) growth of carbon nanotubes, providing a novel method for direct synthesis of carbon nanostructures on macroscopic gold substrates. In contrast, analogous copper nanofoams were found to be catalytically inactive toward the growth of graphitic nanostructures under the same synthesis conditions, highlighting the unusually high catalytic propensity of this form factor of gold. The combustion synthesis process described herein represents a never-wet approach for directly synthesizing macroscopic catalytically active gold. Unlike sol-gel and dealloying approaches, combustion synthesis eliminates the time-consuming diffusion-mediated steps associated with previous methods and offers multiple degrees of freedom for tuning morphology, electrical conductivity, and mechanical properties.

Raman spectroscopy of bottom-up synthesized graphene quantum dots: size and structure dependence.

Graphene quantum dots (GQDs) have attracted significant interest as synthetically tunable optoelectronic and photonic materials that can also serve as model systems for understanding size-dependent behaviors of related graphene structures such as nanoribbons. We present a Raman spectroscopy study of bottom-up synthesized GQDs with lateral dimensions between 0.97 to 1.62 nm, well-defined (armchair) edge type, and fully benzenoid structures. For a better understanding of observed size-dependent trends, the study is extended to larger graphene structures including nano-graphene platelets (>25 nm) and large-area graphene. Raman spectra of GQDs reveal the presence of D and G bands, as well as higher order modes (2D, D + G, and 2G). The D and G band frequencies and intensity were found to increase as GQD size increases, while higher order modes (2D, D + G, and 2G) also increased in intensity and became more well-defined. The integrated intensity ratios of D and G bands (ID/IG) increase as the size of the GQDs approaches 2 nm and rapidly decrease for larger graphene structures. We present a quantitative comparison of ID/IG ratios for the GQDs and for defects introduced into large area graphenes through ion bombardment, for which inter-defect distances are comparable to the sizes of GQDs studied here. Close agreement suggests the ID/IG ratio as a size diagnostic for other nanographenes. Finally, we show that Raman spectroscopy is also a good diagnostic tool for monitoring the formation of bottom-up synthesized GQDs.

Size-Dependent Electronic Properties of Uniform Ensembles of Strongly Confined Graphene Quantum Dots.

The electronic structure of a series of bottom-up synthesized graphene quantum dots (GQDs) smaller than 2 nm was investigated by spectroelectrochemistry, yielding insights not previously available from ensemble-level studies. The results show that for the strongly confined GQDs the dependence of the band gap on the GQD size deviates from the prediction of the standard Dirac Fermion model but agrees well with the models explicitly accounting for the electron-electron and electron-hole interactions. The HOMO/LUMO energy levels are found to be distributed nearly symmetrically around the 0 V value versus normal hydrogen electrode (NHE), becoming more positive/negative, respectively, with increasing GQD size. The exciton binding energies are found to follow power dependence on the number of carbon atoms per GQD, with the experimental values falling within the range of ∼0.1 to ∼0.6 eV. Given the broad accessibility of the described experimental tools and methods, our work opens a path to a more systematic examination of quantum confinement effects in GQDs.

Ultra-thin and strong formvar-based membranes with controlled porosity for micro- and nano-scale systems.

We present a methodology for developing ultra-thin and strong formvar-based membranes with controlled morphologies. Formvar is a thin hydrophilic and oleophilic polymer inert to most chemicals and resistant to radiation. The formvar-based membranes are viable materials as support structures in micro- and macro-scale systems depending on thinness and porosity control. Tunable sub-micron thick porous membranes with 20%-65% porosity were synthesized by controlling the ratios of formvar, glycerol, and chloroform. This synthesis process does not require complex separation or handling methods and allows for the production of strong, thin, and porous formvar-based membranes. An expansive array of these membrane characterizations including chemical compatibility, mechanical responses, wettability, as well as the mathematical simulations as a function of porosity has been presented. The wide range of chemical compatibility allows for membrane applications in various environments, where other polymers would not be suitable. Our formvar-based membranes were found to have an elastic modulus of 7.8 GPa, a surface free energy of 50 mN m-1 and an average thickness of 125 nm. Stochastic model simulations indicate that formvar with the porosity of ∼50% is the optimal membrane formulation, allowing the most material transfer across the membrane while also withstanding the highest simulated pressure loadings before tearing. Development of novel, resilient and versatile membranes with controlled porosity offers a wide range of exciting applications in the fields of nanoscience, microfluidics, and MEMS.

Photoacoustic in vitro flow cytometry for nanomaterial research.

Conventional flow cytometry is a versatile tool for drug research and cell characterization. However, it is poorly suited for quantification of non-fluorescent proteins and artificial nanomaterials without the use of additional labeling. The rapid growth of biomedical applications for small non-fluorescent nanoparticles (NPs) for drug delivery and contrast and therapy enhancement, as well as research focused on natural cell pigments and chromophores, demands high-throughput quantification methods for the non-fluorescent components. In this work, we present a novel photoacoustic (PA) fluorescence flow cytometry (PAFFC) platform that combines NP quantification though PA detection with conventional in vitro flow cytometry sample characterization using fluorescence labeling. PAFFC simplifies high-throughput analysis of cell-NP interactions, optimization of targeted nanodrugs, and NP toxicity assessment, providing a direct correlation between NP uptake and characterization of toxicity markers for every cell.

Comparative study of plant responses to carbon-based nanomaterials with different morphologies.

The relationship between the morphology of carbon-based nanomaterials (CBNs) and the specific response of plants exposed to CBNs has not been studied systematically. Here, we prove that CBNs with different morphologies can activate cell growth, germination, and plant growth. A tobacco cell culture growth was found to increase by 22%-46% when CBNs such as helical multi-wall carbon nanotubes (MWCNTs), few-layered graphene, long MWCNTs, and short MWCNTs were added to the growth medium at a concentration of 50 µg ml(-1). The germination of exposed tomato seeds, as well as the growth of exposed tomato seedlings, were significantly enhanced by the addition of all tested CBNs. The presence of CBNs inside exposed seeds was confirmed by transmission electron microscopy and Raman spectroscopy. The effects of helical MWCNTs on gene expression in tomato seeds and seedlings were investigated by microarray technology and real time-PCR. Helical MWCNTs affected a number of genes involved in cellular and metabolic processes and response to stress factors. It was shown that the expression of the tomato water channel gene in tomato seeds exposed to helical MWCNTs was upregulated. These established findings demonstrate that CBNs with different morphologies can cause the same biological effects and share similar mechanisms in planta.

Graphene supports in vitro proliferation and osteogenic differentiation of goat adult mesenchymal stem cells: potential for bone tissue engineering.

Current treatments for bone loss injuries involve autologous and allogenic bone grafts, metal alloys and ceramics. Although these therapies have proved useful, they suffer from inherent challenges, and hence, an adequate bone replacement therapy has not yet been found. We hypothesize that graphene may be a useful nanoscaffold for mesenchymal stem cells and will promote proliferation and differentiation into bone progenitor cells. In this study, we evaluate graphene, a biocompatible inert nanomaterial, for its effect on in vitro growth and differentiation of goat adult mesenchymal stem cells. Cell proliferation and differentiation are compared between polystyrene-coated tissue culture plates and graphene-coated plates. Graphitic materials are cytocompatible and support cell adhesion and proliferation. Importantly, cells seeded on to oxidized graphene films undergo osteogenic differentiation in fetal bovine serum-containing medium without the addition of any glucocorticoid or specific growth factors. These findings support graphene's potential to act as an osteoinducer and a vehicle to deliver mesenchymal stem cells, and suggest that the combination of graphene and goat mesenchymal stem cells provides a promising construct for bone tissue engineering.

Single-walled carbon nanotube and graphene nanodelivery of gambogic acid increases its cytotoxicity in breast and pancreatic cancer cells.

Graphene and single-walled carbon nanotubes were used to deliver the natural low-toxicity drug gambogic acid (GA) to breast and pancreatic cancer cells in vitro, and the effectiveness of this complex in suppressing cellular integrity was assessed. Cytotoxicity was assessed by measuring lactate dehydrogenase release, mitochondria dehydrogenase activity, mitochondrial membrane depolarization, DNA fragmentation, intracellular lipid content, and membrane permeability/caspase activity. The nanomaterials showed no toxicity at the concentrations used, and the antiproliferative effects of GA were significantly enhanced by nanodelivery. The results suggest that these complexes inhibit human breast and pancreatic cancer cells grown in vitro. This analysis represents a first step toward assessing their effectiveness in more complex, targeted, nanodelivery systems.

Applications of surface-enhanced Raman scattering in advanced bio-medical technologies and diagnostics.

In this review of the literature on surface-enhanced Raman scattering (SERS), we describe recent developments of this technique in the medical field. SERS has developed rapidly in the last few years as a result of the fascinating advancements in instrumentation and the ability to interpret complex Raman data using high-processional, computer-aided programs. This technique, has many advantages over ordinary spectroscopic analytical techniques - such as extremely high sensitivity, molecular selectivity, intense signal and great precision - that can be leveraged to address complex medical diagnostics problems. This review focuses on the SERS-active substrate, as well as major advances in cancer and bacteria detection and imaging. Finally, we present a perspective on anticipated future advancements in SERS techniques to address some of the most critical challenges in the areas of diagnostics, detection, and sensing.

Super-resolution nonlinear photothermal microscopy.

Super-resolution fluorescence microscopy enables imaging of fluorescent structures beyond the diffraction limit. However, this technique cannot be applied to weakly fluorescent cellular components or labels. As an alternative, photothermal microscopy based on nonradiative transformation of absorbed energy into heat has demonstrated imaging of nonfluorescent structures including single molecules and ~1-nm gold nanoparticles. However, previously photothermal imaging has been performed with a diffraction-limited resolution only. Herein, super-resolution, far-field photothermal microscopy based on nonlinear signal dependence on the laser energy is introduced. Among various nonlinear phenomena, including absorption saturation, multiphoton absorption, and signal temperature dependence, signal amplification by laser-induced nanobubbles around overheated nano-objects is explored. A Gaussian laser beam profile is used to demonstrate the image spatial sharpening for calibrated 260-nm metal strips, resolving of a plasmonic nanoassembly, visualization of 10-nm gold nanoparticles in graphene, and hemoglobin nanoclusters in live erythrocytes with resolution down to 50 nm. These nonlinear phenomena can be used for 3D imaging with improved lateral and axial resolution in most photothermal methods, including photoacoustic microscopy.

Impact of carbon nanotube exposure to seeds of valuable crops.

Multiwalled carbon nanotubes (MWCNTs) affected seed germination, growth, and the development of three important crops (barley, soybean, corn). Early seed germination and activation of growth in exposed seedlings was observed when MWCNTs were added to sterile agar medium. Similarly, seed germination was activated for all tested crop species when MWCNTs were deposited on seed surfaces. The ability of MWCNTs to penetrate the seed coats of corn, barley, and soybean was proven by detection of nanotube agglomerates inside MWCNT-exposed seeds using Raman spectroscopy and transmission electron microscopy (TEM). Reverse transcription polymerase chain reaction (RT-PCR) analysis revealed that the expression of genes encoding several types of water channel proteins was increased in soybean, corn, and barley seeds coated with MWCNTs compared with uncoated control seeds. Our results indicate that the positive effect of MWCNTs on the germination and growth of seedlings is reproducible between crop species and can be observed for different methods of delivering carbon nanotubes. Such studies could prove the significant potential of carbon nanotubes as regulators of germination and plant growth.

Few-layer graphene sheets with embedded gold nanoparticles for electrochemical analysis of adenine.

This work describes the synthesis of few-layer graphene sheets embedded with various amounts of gold nanoparticles (Gr-Au-x) over an Aux/MgO catalytic system (where × = 1, 2, or 3 wt%). The sheet-like morphology of the Gr-Au-x nanostructures was confirmed by transmission electron microscopy and high resolution transmission electron microscopy, which also demonstrated that the number of layers within the sheets varied from two to seven. The sample with the highest percentage of gold nanoparticles embedded within the graphitic layers (Gr-Au-3) showed the highest degree of crystallinity. This distinct feature, along with the large number of edge-planes seen in high resolution transmission electron microscopic images, has a crucial effect on the electrocatalytic properties of this material. The reaction yields (40%-50%) and the final purity (96%-98%) of the Gr-Au-x composites were obtained by thermogravimetric analysis. The Gr-Au-x composites were used to modify platinum substrates and subsequently to detect adenine, one of the DNA bases. For the bare electrode, no oxidation signal was recorded. In contrast, all of the modified electrodes showed a strong electrocatalytic effect, and a clear peak for adenine oxidation was recorded at approximately +1.05 V. The highest increase in the electrochemical signal was obtained using a platinum/Gr-Au-3-modified electrode. In addition, this modified electrode had an exchange current density (I(0), obtained from the Tafel plot) one order of magnitude higher than that of the bare platinum electrode, which also confirmed that the transfer of electrons took place more readily at the Gr-Au-3-modified electrode.

Carbon nanotubes as plant growth regulators: effects on tomato growth, reproductive system, and soil microbial community.

Multi-walled carbon nanotubes (CNTs) can affect plant phenotype and the composition of soil microbiota. Tomato plants grown in soil supplemented with CNTs produce two times more flowers and fruit compared to plants grown in control soil. The effect of carbon nanotubes on microbial community of CNT-treated soil is determined by denaturing gradient gel electrophoresis and pyrosequencing analysis. Phylogenetic analysis indicates that Proteobacteria and Bacteroidetes are the most dominant groups in the microbial community of soil. The relative abundances of Bacteroidetes and Firmicutes are found to increase, whereas Proteobacteria and Verrucomicorbia decrease with increasing concentration of CNTs. The results of comparing diversity indices and species level phylotypes (OTUs) between samples showed that there is not a significant affect on bacterial diversity.

Role of carbonaceous nanomaterials in stimulating osteogenesis in mammalian bone cells.

In this work, we demonstrate that graphitic nanomaterials-carboxylated multi-walled carbon nanotubes (MWCNTs) and carboxylated graphenes (Gn)-have the ability to stimulate the process of osteogenesis in mammalian bone cells and significantly increase the level of bone mineralization. Exposure of MC3T3-E1 bone cells to carboxylated MWCNTs-nano-sized (nano-Gn) and micro-sized (micro-Gn) in concentrations of 1-10 µg ml-1-resulted in the enhancement of mineralization in a time-dependent manner for the cells exposed to the nanomaterials, as compared to unexposed cells. However, the graphitic nanomaterials did not show significant toxicity in the concentration levels that were studied. Gene expression analysis revealed that the MWCNTs activated expression of the mid-stage osteogenic marker, Col I, on the 12th day of cell incubation. The gene expression of the earliest osteogenic marker, Cbfa-1, and the downstream effector of BMP signaling, SMAD1, were significantly increased in bone cells exposed to both materials (MWCNTs and nano-Gn) as compared to unexposed control cells. Our data clearly demonstrate the ability of graphitic nano-materials to penetrate bone cells and regulate deposition of minerals in an in vitro model system. Our findings highlight the potential use of such materials in regenerative nanomedicine.

Raman spectroscopy analysis and mapping the biodistribution of inhaled carbon nanotubes in the lungs and blood of mice.

Because of their small size, robust structure and unique characteristics, carbon nanotubes (CNTs) are increasingly being used in a variety of biomedical applications, materials and products. As their use increases, so does the probability of their unintended release and human exposure. Therefore, it is important to establish their potential biodistribution and biopersistence to better understand the potential effects of their exposure to humans. This study examines the distribution of CNTs in CD-1 mice after exposure by inhalation of single-walled carbon nanotubes (SWCNTs) and investigates the possibility that inhaled nanoparticles could enter the circulatory system via the lungs. Raman spectroscopy was employed for the detection of CNTs in lung tissue and blood based on their unique spectroscopic signatures. These studies have important implications concerning the potential effects of exposure to SWCNTs and their use as potential transport vehicles in nanomedicine.

Novel multifunctional graphene sheets with encased Au/Ag nanoparticles for advanced electrochemical analysis of organic compounds.

This work is the first presentation of the synthesis of few-layer graphene decorated with gold and silver nanoparticles (Gr-Au-Ag) by chemical vapor deposition over a catalytic system formed of bimetallic Au-Ag nanoclusters supported on MgO and with methane used as the source of carbon. The sheetlike morphology of the graphene nanostructures, with mean sizes in the range of hundreds of nanometers, was observed by high-resolution electron microscopy. The distinctive feature found in all the samples was the regular rectangular or square shapes. This multi-component organic-inorganic nanomaterial was used to modify a platinum substrate and subsequently employed for the detection of carbamazepine, an anti-convulsion drug. UV/Vis spectroscopy revealed that a strong hypochromism occurred over time, after mixing solutions of graphene-Au-Ag with carbamazepine. This can be attributed to π-π stacking between the aromatic groups of the two compounds. Linear sweep voltammetry (LCV) provided evidence that the modified platinum substrate presented a significant electrocatalytic reaction toward the oxidation of carbamazepine. The intensity of the current was found to increase by up to 2.5 times, and the oxidation potential shifted from +1.5 to +1.35 V(Ag/AgCl) in comparison with the unmodified electrode. Electrochemical impedance spectroscopy (EIS) was further used to thoroughly assess the activity of the platinum electrode that was modified by the deposition of the Gr-Au-Ag composites in the presence of various concentrations of carbamazepine. The experimental EIS records were used for the generation of an equivalent electrical circuit, based on the charge-transfer resistance (R(ct)), Warburg impedance (Z(D)), solution resistance (R(s)), and a constant phase element (CPE) that characterizes the non-ideal interface capacitive responses.