Multiscale Characterization of a Wood-Based Biocrude as a Green Compatibilizing Agent for High-Impact Polystyrene/Halloysite Nanotube Nanocomposites.

This paper investigates merits of using a wood-based biocrude (WB) from aspen wood to improve the compatibility of halloysite nanotubes (HNTs) with high-impact polystyrene to develop nanocomposites with desirable thermomechanical properties. Morphological, thermal, and rheological properties of the resulting nanocomposite are used as indicators of the compatibility and dispersion of the modified HNT within the polymer matrix. Computational modeling using density functional theory is used along with laboratory experiments to provide a multiscale characterization of the above biocrude and nanocomposites. Studies performed through dispersion-corrected density functional theory calculations show that the active functional groups of WB molecules including carbonyl, hydroxyl, and carboxylic interact with the HNT surface, while their aromatic tails interact with the phenyl groups of the polystyrene. Furthermore, the studies reveal how WB molecules act as bridges between the hydrophobic polymer and the hydrophilic clay improving the compatibility. The latter was confirmed by Hansen solubility parameters and was evidenced in improved dispersion of clay within the polystyrene matrix observed by microscopy. Rheological and thermal analyses of the modified HNT and nanocomposites showed physical interactions of WB with HNT surface as well as interactions between the WB-modified HNT and the high-impact polystyrene. The WB was found to be a strong candidate as a green compatibilizing agent for HNT in high-impact polystyrene. The study results can provide insights for formulators and manufacturers looking for green compatibilizing agents in conventional nanocomposites for construction and manufacturing applications.

Compositional mapping of bitumen using local electrostatic force interactions in atomic force microscopy.

In recent years, many researchers have investigated bitumen surface morphology, especially the so-called bee-like structures, in an attempt to relate the chemical composition and molecular conformation to bitumen micromechanics and ultimately performance properties. Even though recent studies related surface morphology and its evolution to stiffness and stress localization, the complex chemical nature of bitumen and its time- and temperature-dependent properties still engender significant questions about the nature and origin of the observed morphological features and how they evolve due to exposure to various environmental and loading conditions. One such question is whether the observed surface features are formed from wax or from the coprecipitation of wax and asphaltene. Our prior work was mainly theoretical; it used density functional theory and showed that the coprecipitation theory may not stand, mainly because wax-asphaltene interactions are not thermodynamically favourable compared to wax-wax interactions. This paper presents a comprehensive approach based on experiments to study surface morphology of bitumen and conduct compositional mapping to shed light on the origin of the bee-like surface morphological features. We used Atomic Force Microscopy (AFM), with the main focus being on single-pass detection and mapping of local electric properties, as a novel approach to enhance existing compositional mapping techniques. This method was found to be highly effective in differentiating various domains with respect to their polarity. The results of our study favour the hypothesis that the bee-like features are mainly composed of wax, including a variety of alkanes.

Mechanobiology of Antimicrobial Resistant Escherichia coli and Listeria innocua.

A majority of antibiotic-resistant bacterial infections in the United States are associated with biofilms. Nanoscale biophysical measures are increasingly revealing that adhesive and viscoelastic properties of bacteria play essential roles across multiple stages of biofilm development. Atomic Force Microscopy (AFM) applied to strains with variation in antimicrobial resistance enables new opportunities for investigating the function of adhesive forces (stickiness) in biofilm formation. AFM force spectroscopy analysis of a field strain of Listeria innocua and the strain Escherichia coli K-12 MG1655 revealed differing adhesive forces between antimicrobial resistant and nonresistant strains. Significant increases in stickiness were found at the nanonewton level for strains of Listeria innocua and Escherichia coli in association with benzalkonium chloride and silver nanoparticle resistance respectively. This advancement in the usage of AFM provides for a fast and reliable avenue for analyzing antimicrobial resistant cells and the molecular dynamics of biofilm formation as a protective mechanism.

Switchable nanodumbbell probes for analyte detection.

Nanodumbbell gold nanoparticle (AuNP) dimers connected by DNA show significant change in interparticle distance in the presence of a specific analyte, ATP. The nanodumbbell begins in an extended state, but after the addition of the analyte, the DNA connecting the AuNPs forms a stable hairpin, which causes a large decrease in the interparticle distance.

Facile thermal treatment process for assembling vertically aligned semiconductor nanorods in solution.

Large-area films of vertically-aligned semiconductor nanorods have the potential to be useful, active materials for optoelectronic devices. We demonstrate here a highly facile thermal annealing approach for reversibly assembling 28 nm long CdSe nanorods into vertically aligned arrays in solution. Using temperature to control solvent strength, aggregated nanorods in a marginally poor solvent mixture were first dispersed at elevated temperatures and then reassembled into freely suspended, ordered sheets of aligned nanorods up to 24 µm in diameter upon slow cooling. The assembly method was tolerant of nanorod polydispersity and was effective over a wide range of solvents and nanorod concentrations. The pre-assembled nanorods could be directly drop-cast from solution onto a substrate and rapidly dried to obtain a film of vertically aligned nanorods.

Solvent-based assembly of CdSe nanorods in solution.

We demonstrate a purely solvent-based approach to assembling CdSe nanorods into vertically aligned, hexagonally packed monolayers in solution. Nanorods were dispersed in a mixture of good solvent with high vapor pressure and bad solvent with low vapor pressure, and preferential evaporation of the good solvent led to ordered assembly under conditions of continuously decreasing solvent quality. No applied external bias, extensive control of drying conditions, exceptionally monodisperse nanoparticles, or high concentrations of additives were required. This clean and facile method yielded ordered nanorod sheets of up to 7.5 µm wide with potential use as active materials in unique applications.

Facile One-Pot Synthesis of Polymer-Phospholipid Composite Microbubbles with Enhanced Drug Loading Capacity for Ultrasound-Triggered Therapy.

This paper reports the one-pot synthesis of perfluorocarbon microbubbles with crosslinked shells of poly(acrylic acid) and phospholipid that boast excellent ultrasound contrast enhancement, enhanced loading capacity, and the ability to retain or release their contents through variation in the level of ultrasound exposure.

Templated assembly of DNA origami gold nanoparticle arrays on lithographically patterned surfaces.

Artificial DNA nanostructures such as DNA origami have garnered significant interest as templates for sub-20 nm lithography because their rational design allows for the incorporation of binding sites to assemble nanocomponents with 6 nm resolution. In addition, their overall size of 100 nm is easily accessible by top-down lithographic methods. Combining the strengths of top-down lithography and bottom-up self-assembly using DNA nanostructures may provide a commercially viable route to fabricating electronic and photonic devices with nanometer-scale features. We have demonstrated just such a comprehensive process in which 5 nm gold nanoparticles are first assembled in high yield on DNA origami. The constructs are then organized, rinsed, and dried on patterned silicon substrates, yielding large area arrays of both origami and nanoparticles.

Recent advances in DNA-based directed assembly on surfaces.

In the last decade, "small" and "integrated" have been keywords in the field of device fabrication as the desire to exploit nanoscale phenomena and make electronic, photonic and magnetic arrays has grown. In an effort to improve resolution and control costs, much work has been dedicated to developing alternatives to conventional microfabrication technology. For this purpose, biomolecular assembly and DNA nanotechnology in particular are appealing owing to their inherent size and capacity for molecular recognition. Herein, we review recent achievements in DNA-based directed assembly on substrates. These include novel methods for patterning and depositing nanomaterials on DNA-modified surfaces as well as using synthetic DNA nanostructures such as DNA tiles and origami as templates to direct the assembly of nanoscale components. Particular attention is paid to integrating self-assembly with top-down lithography, and some possible directions for future work are discussed.

Site-specific patterning of highly ordered nanocrystal superlattices through biomolecular surface confinement.

With the increasing demand in recent years for high-performance devices for both energy and health applications, there has been extensive research to direct the assembly of nanoparticles into meso- or macroscale single two- and three-dimensional crystals of arbitrary configuration or orientation. Inorganic nanoparticle arrays can have intriguing physical properties that differ from either individual nanoparticles or bulk materials. For most device applications, it is necessary to fabricate two-dimensional nanoparticle superlattices at programmed sites on a surface. However, it has remained a significant challenge to generate patterned arrays with long-range positional order because most highly ordered close-packed nanocrystal arrays are typically obtained by kinetically driven evaporation processes. In this report, we demonstrate a method to generate patterned nanocrystal superlattices by confining nanoparticles to geometrically defined 2-D DNA sites on a surface and using associative biomolecular interparticle interactions to produce thermodynamically stable arrays of hexagonally packed nanocrystals with significant long-range order observed over 1-2 µm. We also demonstrate the role of chemical and geometrical confinement on particle packing and obtaining long-range order. Finally, we also demonstrate that the formation of DNA-mediated nanocrystal superlattices requires both interparticle DNA hybridization and solvent-less thermal annealing.

Large-area spatially ordered arrays of gold nanoparticles directed by lithographically confined DNA origami.

The development of nanoscale electronic and photonic devices will require a combination of the high throughput of lithographic patterning and the high resolution and chemical precision afforded by self-assembly. However, the incorporation of nanomaterials with dimensions of less than 10 nm into functional devices has been hindered by the disparity between their size and the 100 nm feature sizes that can be routinely generated by lithography. Biomolecules offer a bridge between the two size regimes, with sub-10 nm dimensions, synthetic flexibility and a capability for self-recognition. Here, we report the directed assembly of 5-nm gold particles into large-area, spatially ordered, two-dimensional arrays through the site-selective deposition of mesoscopic DNA origami onto lithographically patterned substrates and the precise binding of gold nanocrystals to each DNA structure. We show organization with registry both within an individual DNA template and between components on neighbouring DNA origami, expanding the generality of this method towards many types of patterns and sizes.

Placement and orientation of individual DNA shapes on lithographically patterned surfaces.

Artificial DNA nanostructures show promise for the organization of functional materials to create nanoelectronic or nano-optical devices. DNA origami, in which a long single strand of DNA is folded into a shape using shorter 'staple strands', can display 6-nm-resolution patterns of binding sites, in principle allowing complex arrangements of carbon nanotubes, silicon nanowires, or quantum dots. However, DNA origami are synthesized in solution and uncontrolled deposition results in random arrangements; this makes it difficult to measure the properties of attached nanodevices or to integrate them with conventionally fabricated microcircuitry. Here we describe the use of electron-beam lithography and dry oxidative etching to create DNA origami-shaped binding sites on technologically useful materials, such as SiO(2) and diamond-like carbon. In buffer with approximately 100 mM MgCl(2), DNA origami bind with high selectivity and good orientation: 70-95% of sites have individual origami aligned with an angular dispersion (+/-1 s.d.) as low as +/-10 degrees (on diamond-like carbon) or +/-20 degrees (on SiO(2)).

50 nm DNA nanoarrays generated from uniform oligonucleotide films.

One of the most challenging but potentially rewarding goals in nanoscience is the ability to direct the assembly of nanoscale materials into functional architectures with high yields, minimal steps, and inexpensive procedures. Despite their unique physical properties, the inherent difficulties of engineering wafer-level arrays of useful devices from nanoscale materials in a cost-effective manner have provided serious roadblocks toward technological impact. To address nanoscale features while still maintaining low fabrication costs, we demonstrate here an inexpensive printing method that enables repeated patterning of large-area arrays of nanoscale materials. DNA strands were patterned over 4 mm areas with 50 nm resolution by a soft-lithographic subtraction printing process, and DNA hybridization was used to direct the assembly of sub-20 nm materials to create highly ordered two-dimensional nanoparticle arrays. The entire printing and assembly process was accomplished in as few as three fabrication steps and required only a single lithographically templated silicon master that could be used repeatedly. The low-cost procedures developed to generate nanoscale DNA patterns can be easily extended toward roll-to-roll assembly of nanoscale materials with sub-50 nm resolution and fidelity.

Understanding factors affecting alignment of self-assembling nanofibers patterned by sonication-assisted solution embossing.

We recently reported simultaneous self-assembly, alignment, and patterning of peptide amphiphile (PA) nanofibers over large areas by a soft lithographic technique termed sonication-assisted solution embossing (SASE). The present work examines the effect of ultrasonication, channel width, and nanofiber persistence length lambda on the degree of alignment of nanofibers patterned by SASE. Polarized transmission and reflection infrared spectroscopy are used to establish a figure of merit for comparing nanofiber alignment based on a model of the supramolecular structures being composed of oriented beta-sheets. The aligned nanostructures show orientation parameters of up to 0.4, and estimates of the persistence length lambda from atomic force microscopy (AFM) images range from 2.0 to 11 mum depending on the chemical structure of molecules used. The data suggest that stiffer nanofibers, defined as those with longer persistence lengths, may actually align less well due to increased difficulty in cleaving them during the alignment process. Alignment can be enhanced with the addition of ultrasonic agitation and confinement of the self-assembled structures within channels around 400 nm in width.

Simultaneous self-assembly, orientation, and patterning of peptide-amphiphile nanofibers by soft lithography.

Self-assembled nanofibers of peptide-amphiphile molecules have been of great interest because of their bioactivity both in vitro and in vivo. In this work, we demonstrate the simultaneous self-assembly, alignment, and patterning of these nanofibers over large areas by a novel technique termed sonication-assisted solution embossing. In this soft lithographic technique, the nanostructures self-assemble by solvent evaporation while under the influence of ultrasonic agitation and confinement within the topographical features of an elastomeric stamp. The nanofibers orient parallel to the channels as they assemble out of solution, yielding bundles of aligned nanofibers on the substrate after the stamp is removed. Alignment is likely a result of steric confinement and possibly a transition to a lyotropic liquid crystalline phase as solvent evaporates. This technique is not limited to uniaxial alignment and is shown to be able to guide nanofibers around turns. Alignment of nanostructures by this method introduces the possibility of controlling macroscale cellular behavior or material properties by tuning the directionality of interactions at the nanoscale.