An aerogel-like composite material was synthesized by casting a conformal 4–6 nm diisocyanate-derived polymer coating on the bird-nest like skeletal framework of mesoporous vanadia consisting of entangled 100–200 nm long, 30–40 nm thick worm-like objects. The new material does not fail even under high strain compression (>90%) and maintains a highly unusual ductility at cryogenic temperatures (−196 °C). By comparison, nanoparticulate silica crosslinked with the same polymer at the same bulk density (∼0.45 g cm−3) behaves as a typical polymer and metal, showing brittle behavior as the temperature decreases. The high strength of nanoencapsulated vanadia is attributed to interlocking of the skeletal nanoworms, and the high ductility at cryogenic temperatures to sintering-like melting and fusion of their polymer coating under compression.

A strong lightweight material (X-VOx) was formulated by nanocasting a conformal 4 nm thin layer of an isocyanate-derived polymer on the entangled worm-like skeletal framework of typical vanadia aerogels. The mechanical properties were characterized under both quasi-static loading conditions (dynamic mechanical analysis, compression and flexural bending testing) as well as high strain rate loading conditions using a split Hopkinson pressure bar (SHPB). The effects of mass density, moisture concentration and low temperature on the mechanical properties were determined and evaluated. Digital image correlation was used to measure the surface strains through analysis of images acquired by ultra-high speed photography, indicating nearly uniform compression at all stages of deformation during compression. The energy absorption of X-VOx was plotted as a function of the density, strain rate and temperature, and compared with that of plastic foams. X-VOx remains ductile even at −180 °C, a characteristic not found in most materials. This unusual ductility is derived from interlocking and sintering-like fusion of nanoworms during compression. X-VOx emerges as an ideal material for force protection under impact.

A new class of strong, lightweight, porous materials has been invented as an outgrowth of an effort to develop reinforced silica aerogels. The new material, called X-Aerogel is less hygroscopic, but no less porous and of similar density to the corresponding unmodified aerogels. However, the property that sets X-Aerogels apart is their mechanical strength, which can be as much as two and a half orders of magnitude stronger that the unmodified aerogels. X-Aerogels are envisioned to be useful for making extremely lightweight, thermally insulating, structural components, but they may also have applications as electrical insulators, components of laminates, catalyst supports, templates for electrode materials, fuel-cell components, and filter membranes.

Rare earth (RE) aerogels combine the typical high porosity of aerogels with useful electrical, magnetic, optical and catalytic properties of the skeletal framework. RE aerogels were prepared by supercritical fluid CO2 drying of wet gels, which in turn were obtained via a modification of literature procedures involving epichlorohydrine-induced gelation of ethanolic solutions of the hydrated chlorides. But even more so than their silica counterparts, RE aerogels are fragile materials. This problem is addressed by using the innate hydroxyl functionality of the mesoporous surfaces as the focal point for casting a conformal polyurethane/polyurea layer over their entire inorganic framework, thus preserving most of the mesoporosity of the native network (70% v/v after vs. 94% v/v before applying the polymer layer) and a significant portion of the mesoporous surface area (156 ± 19 m2 g−1 after vs. 368 ± 14 m2 g−1 before casting the polymer). Detailed chemical analysis shows that RE aerogels are far from pure oxides. For example, the RE metal content (Pr to Lu) is in the range of 58.0 ± 2.3% w/w, vs. 85.4–87.9% in the pure oxides. RE aerogels contain also carbonate, chloride and organic products from the gelation process. Despite their chemical complexity, however, both native and polymer encapsulated RE sol–gel materials are stoichiometrically similar, and by using the magnetic susceptibility as a probe, it is found that physical properties depending on the atomic number (AN) of the RE core element vary linearly with those of pure RE compounds. Therefore, from an applications design perspective RE sol–gel materials themselves can be treated as pure compounds. By analogy, similar types of core–shell structures and the associated benefits should be possible for all sol–gel materials.

Polymerization of a di-isocyanate with the amine-modified surface of a sol–gel derived mesoporous silica network crosslinks the nanoparticles of the silica skeleton, and reinforces the otherwise fragile framework. Systematically adjusting the processing variables affecting density produce aerogels whose macroscopic properties could be controlled, and are attributed to changing nanoscale morphology. Aerogels crosslinked using the smallest amount of silica studied exhibit as much as a 40-fold increase in strength over the corresponding non-crosslinked framework, and are flexible.

We describe a three-dimensional core–shell structure where the core is the assembly of nanoparticles that comprises the skeletal framework of a typical silica aerogel, and the shell is polystyrene. Specifically, the mesoporous surfaces of silica were first modified with amines by co-gelation of tetramethylorthosilicate (TMOS) and 3-aminopropyltriethoxysilane (APTES). Next, styrene moieties were attached to the amines by reaction with p-chloromethylstyrene. Finally, dangling styrene moieties were crosslinked by a free-radical polymerization process initiated by AIBN and styrene, p-chloromethylstyrene or 2,3,4,5-pentafluorostyerene introduced in the mesopores. Polystyrene crosslinked aerogels are mechanically strong, lightweight (0.41–0.77 g cm−3), highly porous materials (they consist of ca. 63% empty space, with a BET surface areas in the range of 213–393 m2 g−1). Their thermal conductivity (0.041 W m−1 K−1) is comparable to that of glass wool. Hydrophobicity, however, is the property that sets the new material apart from analogous polyurea and epoxy crosslinked aerogels. The contact angles of water droplets on disks cut from larger monoliths are >120°. (By comparison, the contact angle with polyurea crosslinked aerogels is only ca. 60°.) Polystyrene crosslinked aerogel monoliths float on water indefinitely, while their polyurea counterparts absorb water and sink within minutes.

The mesoporous surfaces of TMOS-derived silica aerogels have been modified with amines by copolymerization of TMOS with APTES. The amine sites have become anchors for cross-linking the nanoparticles of the skeletal backbone of the aerogel by attachment of di-, tri-, and tetra-functional epoxies. The resulting conformal coatings increase the density of the native aerogels by a factor of 2-3 but the strength of the resulting materials may increase by more than 2 orders of magnitude. Processing variables such as the amount of APTES used to make the gels, the epoxy type and concentration used for crosslinking. and the cross-linking temperature and time were varied according to a multivariable design-of-experiments (DOE) model. It was found that while elastic modulus follows a similar trend with density, maximum strength is attained neither at the maximum density nor at the highest concentration of -NH2 groups, suggesting surface saturation effects. Aerogels cross-linked with the trifunctional epoxide always show improved strength compared with aerogels cross-linked with the other two epoxides under identical conditions. Solid C-13 NMR studies show residual unreacted epoxides, which condense with one another by heating cross-linked aerogels at 150degreesC.

2,7-Di(4-cyanophenyl)-3,8-di(4-methylphenyl)-1,6-dioxapyrene (CN-diox), a symmetrically substituted 2,3,7,8-tetraaryldioxapyrene, was synthesized in seven steps from 1,5-dihydroxynaphthalene. The synthetic methodology incorporated a base-catalyzed ring closure process followed by dehydration to introduce the first tetraaryl-1,6-dioxapyrene. Crystal structure and electrochemical analysis were performed to directly compare the properties of CN-diox to previously reported dioxapyrene derivatives, specifically 1,6-dioxapyrene (Diox) and 3,8-diethyl-5,10-dimethyl-1,6-dioxapyrene (Alkyl-diox). Optical spectroscopy studies were performed to evaluate the potential of the 1,6-dioxapyrenes as fluorescent probes. CN-diox revealed a broad absorption centered near 450nm (ɛ=31,900M−1cm−1) in THF with a corresponding fluorescence at 619nm (Φf=0.011). This was in sharp contrast to both Diox and Alkyl-diox which displayed broad absorption bands near 400nm (ɛ∼5000–10,000M−1cm−1) in THF with corresponding fluorescence near 500nm (Φf=0.059 and 0.082 for Diox and Alkyl-diox, respectively). The luminescence of CN-diox was found to be solvatochromic (λmax=619–644nm) with single exponential lifetimes of less than 1.3ns and an excited state dipole moment of ∼22.81D. Neither Diox nor Alkyl-diox showed solvatochromic properties.

We describe a new mechanically strong lightweight porous composite material obtained by encapsulating the skeletal framework of amine-modified silica aerogels with polyurea. The conformal polymer coating preserves the mesoporous structure of the underlying silica framework and the thermal conductivity remains low at 0.041 +/- 0.001 W m(-1) K-1. The potential of the new cross-linked silica aerogels for load-carrying applications was determined through characterization of their mechanical behavior under compression, three-point bending, and dynamic mechanical analysis (DMA). A primary glass transition temperature of 130 degrees C was identified through DMA. At room temperature, results indicate a hyperfoam behavior where in compression cross-linked aerogels are linearly elastic under small strains ( < 4%) and then exhibit yield behavior (until 40% strain), followed by densification and inelastic hardening. At room temperature the compressive Young's modulus and the Poisson's ratio were determined to be 129 +/- 8 MPa and 0. 18, respectively, while the strain at ultimate failure is 77% and the average specific compressive stress at ultimate failure is 3.89 x 10(5) N in kg(-1). The specific flexural strength is 2.16 x 10(4) N in kg(-1). Effects on the compressive behavior of strain rate and low temperature were also evaluated.

Polymerization of di- and tri-isocyanates can be templated onto the mesoporous surface of a preformed network of sol–gel-derived silica nanoparticles, resulting in a conformal ‘crosslinked’ coating that renders the interparticle neck zone wider. Upon drying, these crosslinked networks yield aerogels which are up to ∼3× more dense than native aerogels based on the underlying silica framework, but also up to 10× less hygroscopic and they may take more than 300× the force to break. These results have been obtained with one-step based-catalyzed sol–gel silica networks, as well as with gels derived through a two-step process involving an acid-catalyzed sol and a based-catalyzed gel. Furthermore, it has been also found that crosslinking increases the dielectric constant only by ∼35% relative to values reported in the literature for native silica aerogels of about the same porosity. Chemical investigations into the polymerization reaction have shown that the process of crosslinking involves reaction of the isocyanate with: (a) OH groups at the surface of silica to form carbamate; and (b) adsorbed water, to form an amine and carbon dioxide. This amine then reacts with additional isocyanates resulting in polymer chain extension and bridging of particles with urethane-terminated polyurea.

We report the redox properties of four star systems incorporating the 4-benzoyl-N-alkylpyridinium cation; the redox potential varies along the branches but remains constant at fixed radii. Bulk electrolysis shows that at a semi-infinite time scale all redox centers are electrochemically accessible. However, voltammetric analysis (cyclic voltammetry and differential pulse voltammetry) shows that only two of the three redox-active centers in the perimeter are electrochemically accessible during potential sweeps as slow as 20 mV s-1 and as fast as 10 V s-1. On the contrary, both redox centers along branches are accessible electrochemically within the same time frame. These results are explained in terms of slow through-space charge transfer and the globular 3-D folding of the molecules and are discussed in terms of their implications on the design of efficient redox functional dendrimers.

NASA is interested in the development of strong lightweight materials for the dual role of thermal insulator and structural component for space vehicles; freeing more weight for useful payloads. Aerogels are very-low density materials (0.010 to 0.5 g/cc) that, due to high porosity (meso- and microporosity), can be, depending on the chemical nature of the network, ideal thermal insulators (thermal conductivity approx. 15 mW/mK). However, aerogels are extremely fragile. For practical application of aerogels, one must increase strength without compromising the physical properties attributed to low density. This has been achieved by templated growth of an epoxy polymer layer that crosslinks the pearl necklace network of nanoparticles: the framework of a typical silica aerogel. The requirement for conformal accumulation of the epoxy crosslinker is reaction both with the surface of silica and with itself. After cross-linking, the strength of a typical aerogel monolith increases by a factor of 200, in the expense of only a 2-fold increase in density. Strength is increased further by coupling residual unreacted epoxides with diamine.

In the search for materials with better mechanical, thermal, and electrical properties, it is becoming evident that oftentimes dispersing ceramic nanoparticles in plastics improves performance. Along these lines, chemical bonding (both covalent and noncovalent) between a filler and a polymer improves their compatibility, and thus enhances certain properties of the polymeric matrix above and beyond what is accomplished by simple doping with the filler. When a similarly sized dopant and matrix are used, elementary building blocks may also have certain distinct advantages (e.g., in catalysis). In this context, researchers at the NASA Glenn Research Center reasoned that in the extreme case, where the dopant and the matrix (e.g., a filler and a polymer) are not only sized similarly, but their relative amounts are comparable, the relative roles of the dopant and matrix can be reversed. Then, if the "filler," or a certain form thereof, possesses desirable properties of its own, such properties could be magnified by cross-linking with a polymer. We at Glenn have identified silica as such a filler in its lowest-density form, namely the silica aerogel.

Electrochemical methods, including cyclic voltammetry and scanning electrochemical microscopy, as well as surface techniques, including conductive atomic force microscopy (AFM) and X-ray photoelectron spectroscopy, were utilized to evaluate and characterize the extent of an oxide layer on platinum silicide (PtSi) surfaces that were pretreated by a variety of approaches; piranha solution (1:4 H2O2+H2SO4), hydrofluoric acid (HF), chemical reduction in NaBH4 and after mechanically polishing the surface. Electrochemical methods showed that in the presence of an oxide layer, the rate of electron transfer depended upon the charge of the redox couple: the more negative the charge, the slower is the rate of electron transfer. Additionally, the current levels observed in the presence of an extensive oxide layer were considerably lower than those observed after the oxide layer was removed either with HF acid or by mechanically polishing. Surface analysis and depth profiles obtained using Auger electron spectroscopy demonstrated that PtSi surfaces pretreated with piranha contained the largest amounts of surface oxides. AFM topographic scans along with localized surface conductivity showed that in the presence of this oxide layer, electron transfer occurred at nanoscale domains located between the PtSi grains with the rest of the surface, which most likely contains an oxide layer, being non-conductive. The surface oxide layer was used to attach the electrogenerated chemiluminescent (ECL) label, Ru(bpy)32+ covalently, either directly or via single-stranded DNA. Emission during oxidation in the presence of the co-reactant tri-n-propylamine was observed, illustrating the possible use of PtSi as a platform for ECL-based bioassays.

INTRODUCTION Since completion of the human genome project, many additional organisms have been sequenced and ~1.8 million human genetic Single Nucleotide Polymorphisms (SNPs) have been identified and documented by the SNP consortium, http://snp.cshl.org. While the technology workhorses for these efforts have been gel-based and capillary electrophoresis DNA sequencers, there is increased interest in the development of SNP technologies that utilize mass spectrometry (MS) for SNP analysis (e.g. http://www.sequenom.com). Thus, methods to amplify, manipulate and sequence DNA segments that are less than 20 to 30 bases in length and are compatible with MS systems are being developed. In free solution, polyelectrolytes that are much longer than their persistence length have an electrophoretic mobility that is independent of fragment length [1]. This characteristic makes it difficult to separate long DNA in free solution and is the reason that both gel and capillary electrophoresis systems use additional polymers to provide a molecular sieve that allows separation of these molecules based on length. Short DNA fragments, however, show an increase in mobility with length which becomes constant for lengths more than several hundred bases [2]. Sieving matrices are therefore unnecessary in separations used for genetic identification of short DNA segments. Use of fluorescence detection does, however, require the attachment of dye molecules that can affect the mobility. Thus, electrophoretic separations of oligonucleotide-dye complexes actually utilize a strategy known as end-labeled free solution electrophoresis or ELFSE, which has been shown to be quite useful for intermediate-length DNA fragments [3]. Another trend in electrophoresis technology development is the utilization of microfluidic channels on a chip. There are a number of practical advantages to using microfluidic chips in lieu of standard capillaries. First is size. The chips used in this experiment are only 1.6 cm wide and 9.5 cm long. Because of the size, the buffer and sample volumes are reduced and it is possible to place many sample flow paths on a single chip. As an example of this multiplexed approach, the research group of Mathies has developed a number of assays utilizing helped to pioneer chip-based capillary electrophoresis systems [4]. The use of small channels facilitates the application of higher field strengths yielding good separation performance. Since the channels on a microfluidic chip can be manufactured with dimensions smaller than those found on most standard capillaries, the surface to volume ratio is higher resulting in greater heat dissipation and less joule heating at high applied fields. The ability to apply higher field strengths results in separation completion on the order of minutes instead of hours. In the present work, free-solution electrophoresis of 15-mers of a single base and the corresponding, hybridized double strands are presented. This work was motivated by previous studies that showed multiple-peak separations for various multi-base ssDNA sequences. Sequence dependent conformation changes have been observed in other systems and our goal was to assess the effects of sequence on the formation of multiple peaks in high-field free-solution electrophoresis of short ssDNA.

By using high concentrations of buffer, electroosmotic flow within uncoated channels of a microfluidic chip was minimized, allowing the free solution electrophoretic separation of DNA. More importantly, because of the ability to efficiently dissipate heat within these channels, field strengths as high as 600 V/cm could be applied with minimal Joule heating (<2 degrees C). As a result of the higher field strengths, separations within an 8-cm-long channel were achieved within a few minutes. However, when the electrophoretic separation of single-stranded DNA (ssDNA) less than 22 bases in length was performed, containing the fluorophore Texas Red as an end label, more than the expected single peak was observed at this high electric field. On the other hand, the free solution electrophoresis of a double-stranded DNA (dsDNA) consisting of a random sequence did exhibit the expected single peak. The appearance of these multiple peaks for ssDNA is shown to be dependent upon the base content and sequence of the ssDNA as well as on the chemical structure of the fluorophore used to tag the DNA for detection. Specifically, the peaks can be attributed to different secondary structures that result either from hydrophobic interactions between the DNA bases and an uncharged fluorescent dye or from G-quadruplexes within guanine-rich strands.