Surface-Enhanced Raman Spectroscopy of Fluid-Supported Lipid Bilayers.

Supported lipid bilayers are essential model systems for studying biological membranes and for membrane-based sensor development. Surface-enhanced Raman spectroscopy (SERS) stands to add considerably to our understanding of the dynamics and interactions of these systems through direct chemical information. Despite this potential, SERS of lipid bilayers is not routinely achieved. Here, we carried out the first measurements of a solid-supported lipid bilayer on a SERS-active substrate and characterized the bilayer using SERS, atomic force microscopy, surface plasmon resonance spectroscopy, ellipsometry, and fluorescence recovery after photobleaching (FRAP). The creation of a fluid, SERS-active supported lipid bilayer was accomplished through use of a novel silica-coated silver film-over-nanosphere substrate. These substrates offer a powerful new platform to couple common surface techniques that are challenging on the nanoscale, for example, ellipsometry and FRAP, with SERS for studying biological membranes and their dynamics.

Aqueous electromigration of single-walled carbon nanotubes and co-electromigration with copper ions.

The electromigration behaviour of raw and acid purified single walled carbon nanotubes (SWCNTs) in dilute aqueous systems (0.0034 mg mL-1), in the absence of surfactant, with the addition of either 0.85 M acetic acid or 0.1 M CuSO4, was evaluated using a 2-inch copper cathode and either a 2-inch copper or 0.5-inch platinum anode. The results showed that the electromigration of raw SWCNTs (with a high catalyst residue) in the presence of CuSO4 resulted in the formation of a Cu-SWCNT composite material at the cathode. In contrast, acid purified SWCNTs were observed to diffuse to a copper anode, creating fibrillated agglomerates with "rice-grain"-like morphologies. Upon acidification with acetic acid (or addition of CuSO4) the direction of electromigration reversed towards the cathode as a result of coordination of Cu2+ to the functional groups on the SWCNT overcoming the inherent negative charge of the acid purified SWCNTs. The result was the co-deposition of SWCNTs and Cu metal on the cathode. Addition of 0.005 M EDTA sequesters some of the Cu2+ and resulted in the separation of metal decorated SWCNTs to the cathode and un-decorated SWCNTs to the anode. The resulting SWCNT and Cu/SWCNT deposits were characterized by Raman spectroscopy, XPS, SEM, EDS, and TEM.

Overcoming Catalyst Residue Inhibition of the Functionalization of Single-Walled Carbon Nanotubes via the Billups-Birch Reduction.

The Billups-Birch Reduction chemistry has been shown to functionalize single-walled carbon nanotubes (SWCNTs) without damaging the sidewalls, but has challenges in scalability. Currently published work uses a large mole ratio of Li to carbon atoms in the SWCNT (Li:C) to account for lithium amide formation, however this increases the cost and hazard of the reaction. We report here the systematic understanding of the effect of various parameters on the extent of functionalization using resonant Raman spectroscopy. Addition of 1-iodododecane yielded alkyl-functionalized SWCNTs, which were isolated by solvent extraction and evaporation, and purified by a hydrocarbon wash. The presence of SWCNT growth catalyst residue (Fe) was shown to have a strong adverse effect on SWCNT functionalization. Chlorination-based SWCNT purification reduced the amount of residual Fe, and achieve a maximum ID/IG ratio using a Li:C ratio of 6:1 in a reaction time of 30 min. This result is consistent with published literature requiring 20-fold mole equivalents of Li per mole SWCNT with a reaction time of over 12 h. This new understanding of the factors influencing the functionalization chemistry will help cut down material and process costs, and also increase the selectivity of the reaction toward the desired product.

Structural Studies of Hydrographenes.

As a result of the unique physical and electrical properties, graphene continues to attract the interest of a large segment of the scientific community. Since graphene does not occur naturally, the ability to exfoliate and isolate individual layers of graphene from graphite is an important and challenging process. The interlayer cohesive energy of graphite that results from van der Waals attractions has been determined experimentally to be 61 meV per carbon atom (61 meV/C atom). This requires the development of a method to overcome the strong attractive forces associated with graphite. The exfoliation process that we, and others, have investigated involves electron transfer into bulk graphite from intercalated lithium to yield lithium graphenide. The resulting graphenide can be reacted with various reagents to yield functionalized graphene. As a part of our interest in the functionalization of graphene, we have explored the Birch reduction as a route to hydrographenes. The addition of hydrogen transforms graphene into an insulator, leading to the prediction that important applications will emerge. This Account focuses mainly on the characterization of the hydrographenes that are obtained from different types of graphite including synthetic graphite powder, natural flake graphite, and annealed graphite powder. Analysis by solid state 13C NMR spectroscopy proved to be important since it was shown that the hydrographenes are composed of interior, isolated aromatic (predominantly fully substituted benzene) rings surrounded by saturated rings. The expected clusters of benzene rings were not found. NMR spectroscopy also offers strong evidence for the presence of tert-butyl alcohol and ethanol (workup solvent) that could not be removed in vacuo from the samples. These compounds could be observed to move freely within the layers of the hydrographene. High-resolution transmission electron microscopy images revealed a remarkable change in morphology that results when hydrogen is added to the graphenide. The appearance of edge and circular dislocations and increased distances between graphitic layers are most visible in the case of the hydrographenes that are formed from annealed graphite. The repetitive hydrogenation of synthetic graphite powder leads to an increase in the distances between the graphitic layers in the (002) direction from 3.4 Å for the initial graphite to 4.11 Å after the first reduction and to 4.29 Å after a third reduction of the same material. Defect-free graphite is formed when the hydrographenes are heated. The distance between carbon layers decreases from 4.11 to 3.44 Å after heating the samples to 1200 °C. This trend toward the spacing of graphite confirms the reversibility of the functionalization process. The C-H bonds have been broken yielding hydrogen, and the exposed carbon orbitals are in close enough proximity to have reverted to graphite. This Account introduces only a narrow area of materials chemistry, and many applications of graphene and its derivatives can be expected as researchers exploit this burgeoning field.

Laser Desorption/Ionization Mass Spectrometric Imaging of Endogenous Lipids from Rat Brain Tissue Implanted with Silver Nanoparticles.

Mass spectrometry imaging (MSI) of tissue implanted with silver nanoparticulate (AgNP) matrix generates reproducible imaging of lipids in rodent models of disease and injury. Gas-phase production and acceleration of size-selected 8 nm AgNP is followed by controlled ion beam rastering and soft landing implantation of 500 eV AgNP into tissue. Focused 337 nm laser desorption produces high quality images for most lipid classes in rat brain tissue (in positive mode: galactoceramides, diacylglycerols, ceramides, phosphatidylcholines, cholesteryl ester, and cholesterol, and in negative ion mode: phosphatidylethanolamides, sulfatides, phosphatidylinositol, and sphingomyelins). Image reproducibility in serial sections of brain tissue is achieved within <10% tolerance by selecting argentated instead of alkali cationized ions. The imaging of brain tissues spotted with pure standards was used to demonstrate that Ag cationized ceramide and diacylglycerol ions are from intact, endogenous species. In contrast, almost all Ag cationized fatty acid ions are a result of fragmentations of numerous lipid types having the fatty acid as a subunit. Almost no argentated intact fatty acid ions come from the pure fatty acid standard on tissue. Graphical Abstract ᅟ.

Teslaphoresis of Carbon Nanotubes.

This paper introduces Teslaphoresis, the directed motion and self-assembly of matter by a Tesla coil, and studies this electrokinetic phenomenon using single-walled carbon nanotubes (CNTs). Conventional directed self-assembly of matter using electric fields has been restricted to small scale structures, but with Teslaphoresis, we exceed this limitation by using the Tesla coil's antenna to create a gradient high-voltage force field that projects into free space. CNTs placed within the Teslaphoretic (TEP) field polarize and self-assemble into wires that span from the nanoscale to the macroscale, the longest thus far being 15 cm. We show that the TEP field not only directs the self-assembly of long nanotube wires at remote distances (>30 cm) but can also wirelessly power nanotube-based LED circuits. Furthermore, individualized CNTs self-organize to form long parallel arrays with high fidelity alignment to the TEP field. Thus, Teslaphoresis is effective for directed self-assembly from the bottom-up to the macroscale.

Structural Characteristics and Properties of a New Graphitic-Based Material.

The hydrogenation of commercial graphite using lithium/ammonia as the reducing agent and tert-butyl alcohol as a proton source was investigated. Characterization of the products after successive reductions of the same material by high-resolution transmission electron microscopy revealed a new material that was replete with edge and circular dislocations. Analysis by solid-state (13)C NMR spectroscopy indicates that after three reductions, the remaining aromatic rings appear to be interior benzene rings. NMR spectroscopy also offers strong evidence for the presence of small amounts of tert-butyl alcohol and ethanol (workup solvent) that could not be removed in vacuo from the samples. These compounds could be observed to move freely between the layers of the hydrographene.

Impurity-induced plasmon damping in individual cobalt-doped hollow Au nanoshells.

The optical properties of plasmonic nanoparticles in the size range corresponding to the electrostatic, or dipole, limit have the potential to reveal effects otherwise masked by phase retardation. Here we examine the optical properties of individual, sub-50 nm hollow Au nanoshells (Co-HGNS), where Co is the initial sacrificial core nanoparticle, using single particle total internal reflection scattering (TIRS) spectroscopy. The residual Co present in the metallic shell induces a substantial broadening of the homogeneous plasmon resonance line width of the Co-HGNS, where the full width at half-maximum (fwhm) broadens proportionately with increasing Co content. This doping-induced line broadening provides a strategy for controlling plasmon line width independent of nanoparticle size, and has the potential to substantially modify the relative decay channels for localized nanoparticle surface plasmons.

Nanoshells made easy: improving Au layer growth on nanoparticle surfaces.

The growth of a continuous, uniform Au layer on a dielectric nanoparticle is the critical step in the synthesis of nanoparticles such as nanoshells or nanorice, giving rise to their unique geometry-dependent plasmon resonant properties. Here, we report a novel, streamlined method for Au layer metallization on prepared nanoparticle surfaces using carbon monoxide as the reducing agent. This approach consistently yields plasmonic nanoparticles with highly regular shell layers and is immune to variations in precursor or reagent preparation. Single particle spectroscopy combined with scanning electron microscopy reveal that thinner, more uniform shell layers with correspondingly red-shifted optical resonances are achievable with this approach.

Near-infrared excited Raman optical activity.

Measurements of near-infrared scattered circular polarization Raman optical activity (SCP-ROA) are presented using laser excitation at 780 nm for samples of S-(-)-alpha-pinene and L-alanyl-L-alanine. These are the first measurements of ROA outside the blue-to-green visible region between 488 and 532 nm. Comparison of Raman and ROA intensities measured with excitation at 532 and 780 nm demonstrate that the expected frequency to the fourth-power dependence for Raman scattering and the corresponding fifth-power dependence for ROA are observed. It can be concluded that, to within this frequency dependence, the same level of efficiency of Raman and ROA measurements using commercial instrumentation with 532 nm excitation is maintained with the change to near-infrared excitation at 780 nm.

Functionalization and extraction of large fullerenes and carbon-coated metal formed during the synthesis of single wall carbon nanotubes by laser oven, direct current arc, and high-pressure carbon monoxide production methods.

Large fullerenes and carbon-coated metal nanoparticles that are formed during the synthesis of carbon nanotubes have been functionalized by the addition of alkyl radicals and isolated by extraction into chloroform. The soluble, functionalized fullerenes have been isolated from raw single-wall carbon nanotube (SWNT) material prepared by laser oven, direct current arc, and high-pressure carbon monoxide production methods. Analyses of the extracted large fullerenes were carried out by thermogravimetric analysis, UV-vis-near-IR, laser desorption ionization mass spectrometry, and high-resolution transmission electron microscopy.