Surprising Chemistry of 6-Azidotetrazolo[5,1-a]phthalazine: What a Purported Natural Product Reveals about the Polymorphism of Explosives.

6-Azidotetrazolo[5,1-a]phthalazine (ATPH) is a nitrogen-rich compound of surprisingly broad interest. It is purported to be a natural product, yet it is closely related to substances developed as explosives and is highly polymorphic despite having a nearly planar structure with little flexibility. Seven solid forms of ATPH have been characterized by single-crystal X-ray diffraction. The structures show diverse patterns of molecular organization, including both stacked sheets and herringbone packing. In all cases, N···N and C-H···N interactions play key roles in ensuring molecular cohesion. The high polymorphism of ATPH appears to arise in part from the ability of virtually every atom of nitrogen and hydrogen in the molecule to take part in close N···N and C-H···N contacts. As a result, adjacent molecules can adopt many different relative orientations that are energetically similar, thereby generating a polymorphic landscape with an unusually high density of potential structures. This landscape has been explored in detail by the computational prediction of crystal structures. Studying ATPH has provided insights into the field of energetic materials, where access to multiple polymorphs can be used to improve performance and clarify how it depends on molecular packing. In addition, our work with ATPH shows how valuable insights into molecular crystallization, often gleaned from statistical analyses of structural databases, can also come from in-depth empirical and theoretical studies of single compounds that show distinctive behavior.

Influence of the reaction stoichiometry on the mechanical and thermal properties of SWCNT-modified epoxy composites.

Previous studies suggest that carbon nanotubes (CNTs) have a considerable influence on the curing behavior and crosslink density of epoxy resins. This invariably has an important effect on different thermal and mechanical properties of the epoxy network. This work focuses on the important role of the epoxy/hardener mixing ratio on the mechanical and thermal properties of a high temperature aerospace-grade epoxy (MY0510 Araldite as an epoxy and 4,4'-diaminodiphenylsulfone as an aromatic hardener) modified with single-walled carbon nanotubes (SWCNTs). The effects of three different stoichiometries (stoichiometric and off-stoichiometric) on various mechanical and thermal properties (fracture toughness, tensile properties, glass transition temperature) of the epoxy resin and its SWCNT-modified composites were obtained. The results were also supported by Raman spectroscopy and scanning electron microscopy (SEM). For the neat resin, it was found that an epoxy/hardener molar ratio of 1:0.8 provides the best overall properties. In contrast, the pattern in property changes with the reaction stoichiometry was considerably different for composites reinforced with unfunctionalized SWCNTs and reduced SWCNTs. A comparison among composites suggests that a 1:1 molar ratio considerably outperforms the other two ratios examined in this work (1:0.8 and 1:1.1). This composition at 0.2 wt% SWCNT loading provides the highest overall mechanical properties by improving fracture toughness, ultimate tensile strength and ultimate tensile strain of the epoxy resin by 40%, 34%, 54%, respectively.

Total synthesis of pactamycin and pactamycate: a detailed account.

This article describes synthetic studies that culminated in the first total synthesis of pactamycin and pactamycate and, in parallel, the two known congeners, de-6-MSA-pactamycin and de-6-MSA-pactamycate, lacking the 6-methylsalicylyl moiety. Starting with L-threonine as a chiron, a series of stereocontrolled condensations led to a key cyclopentenone harboring a spirocyclic oxazoline. A series of systematic functionalizations led initially to the incorrect cyclopentanone epoxide, which was "inverted" under solvolytic conditions. Installation of the remaining groups and manipulation of the oxazoline eventually led to pactamycin, pactamycate, and their desalicylyl analogues.

Reactive fillers based on SWCNTs functionalized with matrix-based moieties for the production of epoxy composites with superior and tunable properties.

Composite materials based on epoxy matrix and single-walled carbon nanotubes (SWCNTs) are able to exhibit outstanding improvements in physical properties when using a tailored covalent functionalization with matrix-based moieties containing terminal amines or epoxide rings. The proper choice of grafted moiety and integration protocol makes it feasible to tune the composite physical properties. At 0.5 wt% SWCNT loading, these composites exhibit up to 65% improvement in storage modulus, 91% improvement in tensile strength, and 65% improvement in toughness. A 15 °C increase in the glass transition temperature relative to the parent matrix was also achieved. This suggests that a highly improved interfacial bonding between matrix and filler, coupled to improved dispersion, are achieved. The degradation temperatures show an upshift in the range of 40-60 °C, which indicates superior thermal performance. Electrical conductivity ranges from ~10(-13) to ~10(-3) S cm(-1), which also shows the possibility of tuning the insulating or conductive behaviour of the composites. The chemical affinity of the functionalization moieties with the matrix and the unchanged molecular structure at the SWCNT/matrix interface are responsible for such improvements.

Determination of moisture content of single-wall carbon nanotubes.

Several techniques were evaluated for the establishment of reliable water/moisture content of single-wall carbon nanotubes. Karl Fischer titration (KF) provides a direct measure of the water content and was used for benchmarking against results obtained by conventional oven drying, desiccation over anhydrous magnesium perchlorate as well as by thermogravimetry and prompt gamma-ray activation analysis. Agreement amongst results was satisfactory with the exception of thermogravimetry, although care must be taken with oven drying as it is possible to register mass gain after an initial moisture loss if prolonged drying time or elevated temperatures (120 °C) are used. Thermogravimetric data were precise but a bias was evident that could be accounted for by considering the non-selective loss of mass as volatile carbonaceous components. Simple drying over anhydrous magnesium perchlorate for a minimum period of 8-10 days is recommended if KF is not available for this measurement.

Glass Fiber-Epoxy Composites with Boron Nitride Nanotubes for Enhancing Interlaminar Properties in Structures.

Hybrid composite materials are a class of materials where more than one type of reinforcement is integrated into a matrix to achieve superior properties. This typically involves nanoparticle fillers employed within traditional advanced composites with fiber reinforcements such as carbon or glass. The current study builds on previous investigations of boron nitride nanotube (BNNT) hybrid composites, specifically glass fiber (GF)-epoxy/BNNT composite laminates. GF is an effective and affordable primary reinforcement fiber in many applications, and boron nitride nanotubes (BNNTs) exhibit impressive mechanical properties comparable to carbon nanotubes (CNTs) with distinct functional properties, such as electrical insulation, which is desirable in manufacturing insulating composites when combined with GF. GF-epoxy/BNNT composite laminates, incorporating BNNT materials with different loadings (1 and 2 wt %) and purity, were manufactured using a hand layup technique and prepared for three-point bending, modified Charpy, dynamic mechanical analysis (DMA), and fracture toughness (mode I and mode II) measurements. A comprehensive microscopy study was also performed using scanning electron microscopy (SEM) showing prominent failure mechanism, nanotube dispersion, and their mode of reinforcement in different loading scenarios. Enhanced properties, including a 43% increase in mode II fracture toughness, were observed in hybrid composites with 1 wt % BNNT compared to the GF composites with neat epoxy, and the reinforcement mechanisms were discussed.

Linear and nonlinear optical properties of carbon nanotube-coated single-mode optical fiber gratings.

Single-wall carbon nanotube deposition on the cladding of optical fibers has been carried out to fabricate an all-fiber nonlinear device. Two different nanotube deposition techniques were studied. The first consisted of repeatedly immersing the optical fiber into a nanotube supension, increasing the thickness of the coating in each step. The second deposition involved wrapping a thin film of nanotubes around the optical fiber. For both cases, interaction of transmitted light through the fiber core with the external coating was assisted by the cladding mode resonances of a tilted fiber Bragg grating. Ultrafast nonlinear effects of the nanotube-coated fiber were measured by means of a pump-probe pulses experiment.

Characterization of single-walled carbon nanotubes by scanning transmission X-ray spectromicroscopy: purification, order and dodecyl functionalization.

The C 1s X-ray absorption spectra of several isolated bundles of single-walled carbon nanotubes (SWCNT) have been measured using scanning transmission X-ray microscopy. First the C 1s and O 1s spectra of a purified but unfunctionalized SWCNT were measured. The C 1s --> pi* transition at 285 eV exhibited almost as strong a dichroic effect (spectral dependence on orientation) as that found in multiwalled carbon nanotubes (Najafi; et al. Small 2008, 7, 2279-2285). Second, purified SWCNT were functionalized with dodecyl and then investigated by STXM. Spectral evidence for the dodecyl functionalization is presented and discussed in comparison to the X-ray absorption spectra of aliphatic hydrocarbons. Both orientation and functionalization mapping of an individual SWCNT bundle is demonstrated.

Nanobeads highly loaded with superparamagnetic nanoparticles prepared by emulsification and seeded-emulsion polymerization.

Functional superparamagnetic colloids possessing high saturation magnetization are prepared by emulsification of superparamagnetic nanoparticles (SPM NPs) and heterogeneous polymerization. The colloids consist of a core of densely packed NPs encapsulated within a thin polymer shell. The cores are made by emulsifying SPM NPs and toluene into an aqueous surfactant solution, and subsequently condensing the emulsion droplets by removal of the solvent generating clusters of SPM NPs. By tuning the emulsification condition, this approach allows for control over the size of the clusters from approximately 40 to 200 nm. The polymer shells encapsulating the clusters are made by using seeded-emulsion polymerization concepts. Control over the thickness of the shell and the incorporation of functional groups to the colloid is achieved. Characterization by thermogravimetric analysis (TGA) and magnetometry shows that these colloids have 66 wt % of magnetic material and saturation magnetization of 47 emu/g, confirming that this route generates colloids with a high loading of SPM NPs and high saturation magnetizations.

Coupled thermogravimetry, mass spectrometry, and infrared spectroscopy for quantification of surface functionality on single-walled carbon nanotubes.

We have successfully applied coupled thermogravimetry, mass spectrometry, and infrared spectroscopy to the quantification of surface functional groups on single-walled carbon nanotubes. A high-purity single-walled carbon nanotube sample was subjected to a rapid functionalization reaction that attached butyric acid moieties to the nanotube sidewalls. This sample was then subjected to thermal analysis under inert desorption conditions. Resultant infrared and mass spectrometric data were easily utilized to identify the desorption of the butyric acid groups across a narrow temperature range and we were able to calculate the degree of substitution of the attached acid groups within the nanotube backbone as 1.7 carbon atoms per hundred, in very good agreement with independent analytical measurements made by inductively coupled plasma optical emission spectrometry (ICP-OES). The thermal analysis technique was also able to discern the presence of secondary functional moieties on the nanotube samples that were not accessible by ICP-OES. This work demonstrates the potential of this technique for assessing the presence of multiple and diverse functional addends on the nanotube sidewalls, beyond just the principal groups targeted by the specific functionalization reaction.

Novel method to produce single-walled carbon nanotube films and their thermal and electrical properties.

Single-walled carbon nanotube films are promising candidates for applications requiring transparent conductors due to their low sheet resistance and high transparency in the visible region. Vacuum filtration is a common and easy to implement technique to produce such films but it is complicated by the need to transfer the films to desired substrates. Here we report conditions under which single-walled carbon nanotube films produced by vacuum filtration detach from the filter membrane upon submersion into water, providing a facile method to transfer filtration-produced nanotube films to desired substrates. Sheet resistance and transparency measurements show that these films are competitive with other high conductivity films made through more cumbersome procedures. Films post-treated with nitric acid or made with acid pre-treated nanotubes have superior performance to those made with high-purity nanotubes without any acid treatment. Thermal imaging by scanning thermal microscopy indicates that heat dissipation by the film is comparable to that of a glass substrate.

Conformational Order in Aggregated rra-P3HT as an Indicator of Quality of Boron Nitride Nanotubes.

We report on investigation, by correlated polarized excitation fluorescence microscopy (PEFM) and atomic force microscopy (AFM) imaging, of the conformational order of regiorandom poly(3-hexyl-thiophene) (rra-P3HT) aggregated on two boron nitride nanotube (BNNT) materials (BNNT-2 and BNNT-3) processed by different purification methods. rra-P3HT photoluminescence excited by linearly polarized light shows polarization direction-dependent intensity with a modulation depth, M, generally >0.5 for rra-P3HT on nanotubes and <0.5 for rra-P3HT on nontubular impurities. PEFM-measured modulation depth value distributions can be decomposed into two components, one corresponding to ordered rra-P3HT on nanotubes and the other to disordered rra-P3HT on impurities. The nanotube component peaks at M = 0.64 and 0.70 and comprises 60% and 78% of the normalized distribution for rra-P3HT on BNNT-2 and BNNT-3, respectively, indicating higher quality and higher fraction of nanotubes in the latter material. The method can be integrated in a material development platform to monitor production and purification progress.

Single-domain antibody-nanoparticles: promising architectures for increased staphylococcus aureus detection specificity and sensitivity.

Because antibodies are highly target-specific and nanoparticles possess diverse, material-dependent properties that can be exploited in order to label and potentially identify biomolecules, the development of antibody-nanoparticle conjugates (nanoconjugates) has huge potential in biodiagnostics. Here, we describe a novel superparamagnetic nanoconjugate, one whose recognition component is a single-domain antibody. It is highly active toward its target Staphylococcus aureus, displays long shelf life, lacks cross-reactivity inherent to traditional homologue whole antibodies, and captures a few dozen S. aureus cells in a mixed cell population with ~100% efficiency and specificity. We ascribe the excellent performance of our nanoconjugate to its single-domain antibody component and recommend it as a general purpose recognition element.

About the solubility of reduced SWCNT in DMSO.

Single-walled carbon nanotubes (SWCNT) have been reduced with sodium naphthalide in THF. The reduced SWCNT are not only soluble in dimethylsulfoxide (DMSO) to form a stable solution/suspension, but also react spontaneously at room temperature with DMSO to evolve hydrocarbon gases and are converted into functionalized SWCNT. The degree of functionalization is about 2C% and the addends are mainly methyl and small oxygen-containing hydrocarbons. The functionalized SWCNT are apparently more soluble and stable in DMSO solution. It may open a new era for further processing and applications.

Reductive Fragmentation of Tetrazoles: Mechanistic Insights and Applications toward the Stereocontrolled Synthesis of 2,6-Polysubstituted Morpholines.

A new methodology to synthesize 2,6-di-, and 2,2',6-trisubstituted morpholines via the reduction of oxabicyclic tetrazoles under mild conditions is described. The reaction proved successful for a wide range of tetrazoles, including sterically encumbered ones harboring gem-substituents on tertiary carbon centers. The mechanism for the decades-old reduction of tetrazoles to secondary amines is elucidated.

Carbon Isotopic Measurements of Nanotubes to Differentiate Carbon Sources.

Stable carbon isotope (δ(13C)) analysis can provide information concerning the starting materials and the production process of a material. Carbon nanotubes (CNTs) are produced using a variety of starting materials, catalysts, and production methods. The use of δ(13C) as a tool to infer the nature of starting materials to gain insight into the mechanics of CNT growth was evaluated. The production process of NRC's SWCNT-1 was traced via the δ(13C) measurement of the available starting materials, intermediate products, and the final product. As isotopic fractionation is likely negligible at high temperatures, the δ(13C) value of the starting materials was reflected in the δ(13C) value of the final CNT product. For commercially available CNTs, the estimated δ(13C) values of identified starting materials were related to the δ(13C) signatures of CNTs. Using this information and the δ(13C) values of CNTs, the nature of unknown carbon sources was inferred for some samples. The use of δ(13C) analysis may be used as a tracer to differentiate between those processes that use relatively 13C-depleted carbon source(s) such as carbon monoxide, methane, or natural gas, and those that do not.

Assessing size-dependent cytotoxicity of boron nitride nanotubes using a novel cardiomyocyte AFM assay.

As boron nitride nanotubes (BNNTs) find increased use in numerous applications, potential adverse health effects of BNNT exposure are a growing concern. Current in vitro cytotoxicity studies on BNNTs are inconsistent and even contradictory, likely due to the lack of reference materials, standardized characterization methods and measurement protocols. New approaches, particularly with the potential to reliably relate in vitro to in vivo studies, are critically needed. This work introduces a novel atomic force microscopy (AFM)-based cardiomyocyte assay that reliably assesses the cytotoxicity of a well-characterized boron nitride nanotube reference material, code named BNNT-1. High energy probe sonication was used to modify and control the length of BNNT-1. The polymer polyethylenimine (PEI) was used concurrently with sonication to produce stable, aqueous dispersions of BNNT-1. These dispersions were used to perform a systematic analysis on both the length and height of BNNT-1 via a correlated characterization approach of dynamic light scattering (DLS) and AFM. Cytotoxicity studies using the novel cardiomyocyte AFM model were in agreement with traditional colorimetric cell metabolic assays, both revealing a correlation between tube length and cytotoxicity with longer tubes having higher cytotoxicity. In addition to the size-dependent cytotoxicity, it was found that BNNT-1 exhibits concentration and cell-line dependent cytotoxic effects.

Proximity-assisted cycloaddition reactions--facile Lewis acid-mediated synthesis of diversely functionalized bicyclic tetrazoles.

Aliphatic azidonitriles separated by three or four carbon atoms undergo facile Lewis acid-induced cycloadditions to give bicyclic tetrazoles, even at 0 degrees C. Extension to 3-azido-2-aryl-1,3-dioxolanes and the corresponding 1,3-dioxanes in the presence of TMSCN and BF3.OEt2 leads to a series of diversely functionalized novel oxabicyclic tetrazoles. The reactions represent new aspects of proximity-assisted dipolar cycloadditions that afford thermodynamically controlled enantiopure products proceeding through discrete oxocarbenium ion intermediates.

Role of Hydrogen in High-Yield Growth of Boron Nitride Nanotubes at Atmospheric Pressure by Induction Thermal Plasma.

We recently demonstrated scalable manufacturing of boron nitride nanotubes (BNNTs) directly from hexagonal BN (hBN) powder by using induction thermal plasma, with a high-yield rate approaching 20 g/h. The main finding was that the presence of hydrogen is crucial for the high-yield growth of BNNTs. Here we investigate the detailed role of hydrogen by numerical modeling and in situ optical emission spectroscopy (OES) and reveal that both the thermofluidic fields and chemical pathways are significantly altered by hydrogen in favor of rapid growth of BNNTs. The numerical simulation indicated improved particle heating and quenching rates (∼105 K/s) due to the high thermal conductivity of hydrogen over the temperature range of 3500-4000 K. These are crucial for the complete vaporization of the hBN feedstock and rapid formation of nanosized B droplets for the subsequent BNNT growth. Hydrogen is also found to extend the active BNNT growth zone toward the reactor downstream, maintaining the gas temperature above the B solidification limit (∼2300 K) by releasing the recombination heat of H atoms, which starts at 3800 K. The OES study revealed that H radicals also stabilize B or N radicals from dissociation of the feedstock as BH and NH radicals while suppressing the formation of N2 or N2+ species. Our density functional theory calculations showed that such radicals can provide faster chemical pathways for the formation of BN compared with relatively inert N2.

Carbon Nanotubes and Related Nanomaterials: Critical Advances and Challenges for Synthesis toward Mainstream Commercial Applications.

Advances in the synthesis and scalable manufacturing of single-walled carbon nanotubes (SWCNTs) remain critical to realizing many important commercial applications. Here we review recent breakthroughs in the synthesis of SWCNTs and highlight key ongoing research areas and challenges. A few key applications that capitalize on the properties of SWCNTs are also reviewed with respect to the recent synthesis breakthroughs and ways in which synthesis science can enable advances in these applications. While the primary focus of this review is on the science framework of SWCNT growth, we draw connections to mechanisms underlying the synthesis of other 1D and 2D materials such as boron nitride nanotubes and graphene.