Autonomic composite hydrogels by reactive printing: materials and oscillatory response

Autonomic materials are those that automatically respond to a change in environmental conditions, such as temperature or chemical composition. While such materials hold incredible potential for a wide range of uses, their implementation is limited by the small number of fully-developed material systems. To broaden the number of available systems, we have developed a post-functionalization technique where a reactive Ru catalyst ink is printed onto a non-responsive polymer substrate. Using a succinimide-amine coupling reaction, patterns are printed onto co-polymer or biomacromolecular films containing primary amine functionality, such as polyacrylamide (PAAm) or poly-N-isopropyl acrylamide (PNIPAAm) copolymerized with poly-N-(3-Aminopropyl)methacrylamide (PAPMAAm). When the films are placed in the Belousov-Zhabotinsky (BZ) solution medium, the reaction takes place only inside the printed nodes. In comparison to alternative BZ systems, where Ru-containing monomers are copolymerized with base monomers, reactive printing provides facile tuning of a range of hydrogel compositions, as well as enabling the formation of mechanically robust composite monoliths. The autonomic response of the printed nodes is similar for all matrices in the BZ solution concentrations examined, where the period of oscillation decreases in response to increasing sodium bromate or nitric acid concentration. A temperature increase reduces the period of oscillations and temperature gradients are shown to function as pace-makers, dictating the direction of the autonomic response (chemical waves).

Plasmon-induced transparency in the visible region via self-assembled gold nanorod heterodimers

The phenomenon of plasmon-induced transparency holds immense potential for high sensitivity sensors and optical information processing due to the extreme dispersion and slowing of light within a narrow spectral window. Unfortunately plasmonic metamaterials demonstrating this effect has been restricted to infrared and greater wavelengths due to requisite precision in structure fabrication. Here we report a novel metamaterial synthesized by bottom-up self-assembly of gold nanorods. The small dimensions (≤ 50/20 nm, length/diameter), atomically smooth surfaces, and nanometer resolution enable the first demonstration of plasmon-induced transparency at visible wavelengths. The slow-down factors within the reduced symmetry heterodimer cluster are comparable to longer wavelength counterparts. The inherent spectral tunability and facile large-scale integration afforded by self-assembled metamaterials will open a new paradigm for physically realizable on-chip photonic device designs.

Large scale solution assembly of quantum dot-gold nanorod architectures with plasmon enhanced fluorescence

Tailoring the efficiency of fluorescent emission via plasmon-exciton coupling requires structure control on a nanometer length scale using a high-yield fabrication route not achievable with current lithographic techniques. These systems can be fabricated using a bottom-up approach if problems of colloidal stability and low yield can be addressed. We report progress on this pathway with the assembly of quantum dots (emitter) on gold nanorods (plasmonic units) with precisely controlled spacing, quantum dot/nanorod ratio, and long-term colloidal stability, which enables the purification and encapsulation of the assembled architecture in a protective silica shell. Overall, such controllability with nanometer precision allows one to synthesize stable, complex architectures at large volume in a rational and controllable manner. The assembled architectures demonstrate photoluminescent enhancement (5×) useful for applications ranging from biological sensing to advanced optical communication.

Surface assembly and plasmonic properties in strongly coupled segmented gold nanorods

An assembly strategy is reported such that segmented nanorods fabricated through template-assisted methods can be robustly transferred and tethered to a pre-functionalized substrate with excellent uniformity over large surface areas. After embedding the rods, sacrificial nickel segments were selectively etched leaving behind strongly coupled segmented gold nanorods with gaps between rods below 40 nm and as small as 2 nm. Hyper-spectral imaging is utilized to measure Rayleigh scattering spectra from individual and coupled nanorod elements in contrast to common bulk measurements. This approach discerns the effects of not only changing segment and gap size but also the presence of characteristic defects on the plasmonic coupling between closely spaced nanorods. Polarized hyper-spectral measurements are conducted to provide direct observation of the anisotropic plasmonic resonance modes in individual and coupled nanorods, which are close to those predicted by computer simulations for nanorods with ideal shapes. Some common deviations from ideal shape such as non-flat facets and asymmetric tails are demonstrated to result in the appearance of characteristic plasmon resonances, which have not been considered before. The large-scale assembly of coupled noble nanostructures with fine control over geometry and high uniformity provides means to strongly tune the scattering, absorption, and near-field plasmonic properties through the geometric arrangement of precisely controlled nanorod segments.

Nonisotropic Self-Organization of Single-Component Hairy Nanoparticle Assemblies

Solvent-free assemblies of hairy nanoparticles (HNPs) are providing avenues to avoid issues of mixing, agglomeration, and limited inorganic content that plague nanocompositses based on polymer-nanoparticle blending. Here we demonstrate that the order within, and the elongational characteristics of, the neat HNP assembly (aHNP) evolve as the architecture of the polymeric corona in solution transitions from the concentrated (CPB) to semidilute (SDPB) polymer brush regimes (silica nanoparticle: radius r₀ = 8 nm with 120 kDa polystyrene grafts at σ = 0.01-0.1 chains/nm²). Specifically, local HNP packing adopts a nonisotropic local arrangement at intermediate graft densities where the transition from CPB-to-SDPB in solution is approximately r₀. In concert, the neat HNP assembly responds to viscoelastic elongational deformation in a manner analogous to semicrystalline elastomers. Domain orientation under load and subsequent buckling upon recovery lead to the appearance of two- and four-point small-angle X-ray patterns. The correlation between the corona architecture of the HNP and the physical characteristics of the solvent-free aHNP provides a framework akin to block-copolymers to tune mechanical, optical, and electrical properties of fibers and films via ordered mesoscale morphology.

Dielectric breakdown in silica-amorphous polymer nanocomposite films: the role of the polymer matrix

The ultimate energy storage performance of an electrostatic capacitor is determined by the dielectric characteristics of the material separating its conductive electrodes. Polymers are commonly employed due to their processability and high breakdown strength; however, demands for higher energy storage have encouraged investigations of ceramic-polymer composites. Maintaining dielectric strength, and thus minimizing flaw size and heterogeneities, has focused development toward nanocomposite (NC) films; but results lack consistency, potentially due to variations in polymer purity, nanoparticle surface treatments, nanoparticle size, and film morphology. To experimentally establish the dominant factors in broad structure-performance relationships, we compare the dielectric properties for four high-purity amorphous polymer films (polymethyl methacrylate, polystyrene, polyimide, and poly-4-vinylpyridine) incorporating uniformly dispersed silica colloids (up to 45% v/v). Factors known to contribute to premature breakdown-field exclusion and agglomeration-have been mitigated in this experiment to focus on what impact the polymer and polymer-nanoparticle interactions have on breakdown. Our findings indicate that adding colloidal silica to higher breakdown strength amorphous polymers (polymethyl methacrylate and polyimide) causes a reduction in dielectric strength as compared to the neat polymer. Alternatively, low breakdown strength amorphous polymers (poly-4-vinylpyridine and especially polystyrene) with comparable silica dispersion show similar or even improved breakdown strength for 7.5-15% v/v silica. At ∼15% v/v or greater silica content, all the polymer NC films exhibit breakdown at similar electric fields, implying that at these loadings failure becomes independent of polymer matrix and is dominated by silica.

Ultralow thermal conductivity in organoclay nanolaminates synthesized via simple self-assembly

Because interfaces impede phonon transport of thermal energy, nanostructuring can transform fully dense solids into ultralow thermal conductivity materials. Here we report a simple self-assembly approach to synthesizing organoclay nanolaminates with cross-planar thermal conductivities below 0.10 W m(-1) K(-1)-a 5-fold decrease compared to unmodified clay. These organoclays are produced via alkylammonium cation exchange with colloidally dispersed montmorillonite clay sheets followed by solvent casting. Time-domain thermoreflectance (TDTR) is used to evaluate the thermal conductivity of the organoclay nanolaminates. Variations in both organic layer thickness and cation chemistry are investigated. At these interface densities (1.0-1.5 interfaces/nm), we demonstrate that thermal conductivity is relatively independent of nanolaminate spacing. A simple series resistance model describes the behavior and gives an interfacial thermal conductance value of ≈150 MW m(-2) K(-1) for the organic/clay interface, consistent with similar organic-inorganic interfaces. The wide range of compositional substitutions and structural variations possible in these materials, make organoclays a versatile new platform for investigating the underlying physics of nanolaminate structures.

Orientation Sensing with Color Using Plasmonic Gold Nanorods and Assemblies

Colorimetric analysis of broadband illumination scattered from isolated gold nanorods and reduced symmetry Dolmen structures provide a visible measure of the local nanoscale orientation of the nanostructures relative to the laboratory frame of reference. Polarized dark-field scattering microscopy correlated with scanning electron microscopy of low and high aspect ratio gold nanorods demonstrated accuracies of 2.3 degrees, which is a 5-fold improvement over photothermal and defocused imaging methods. By assigning the three color channels of the imaging detector (red, green, and blue) to the plasmon resonance wavelengths of the nanostructure, the quantitative display of orientation improved by 200%. The reduced symmetry of a gold nanorod Dolmen structure further improved the sensitivity of colorimetric orientation by a factor of 2 due to the comparative intensities of the resonances. Thus the simplicity, high accuracy, and sensitivity of visual colorimetric sensing of local nanoscale orientation holds promise for high throughput, inexpensive structure and dynamics studies in biology and material science.

Control over position, orientation, and spacing of arrays of gold nanorods using chemically nanopatterned surfaces and tailored particle-particle-surface interactions

The synergy of self- and directed-assembly processes and lithography provides intriguing avenues to fabricate translationally ordered nanoparticle arrangements, but currently lacks the robustness necessary to deliver complex spatial organization. Here, we demonstrate that interparticle spacing and local orientation of gold nanorods (AuNR) can be tuned by controlling the Debye length of AuNR in solution and the dimensions of a chemical contrast pattern. Electrostatic and hydrophobic selectivity for AuNR to absorb to patterned regions of poly(2-vinylpyridine) (P2VP) and polystyrene brushes and mats was demonstrated for AuNR functionalized with mercaptopropane sulfonate (MS) and poly(ethylene glycol), respectively. For P2VP patterns of stripes with widths comparable to the length of the AuNR, single- and double-column arrangements of AuNR oriented parallel and perpendicular to the P2VP line were obtained for MS-AuNR. Furthermore, the spacing of the assembled AuNR was uniform along the stripe and related to the ionic strength of the AuNR dispersion. The different AuNR arrangements are consistent with predictions based on maximization of packing of AuNR within the confined strip.

High-yield assembly of soluble and stable gold nanorod pairs for high-temperature plasmonics

Colloidal synthetic approaches to discrete, soluble plasmonic architectures, such as nanorod pairs, offer numerous advantages relative to lithographic techniques, including compositionally asymmetric structures, atomically smooth surfaces, and continuous fabrication. Density-driven colloidal assembly, such as by solvent evaporation, produces some intriguing structures, e.g., particle chains; however, controllability and post-processibility of the final architecture is inadequate. Also the limited quantity of product nominally comprises a broad distribution of assembly size and type. Herein, the high-yield formation of soluble, stable, and compositionally discrete gold nanorod (Au NR) architectures by inducing-then arresting-flocculation is demonstrated using bifunctional nanorods and reversible modulation of solvent quality to deplete and reassemble an electrostatic stabilization layer, thereby eliminating the need for an additional encapsulant. Analogous to dimer formation during step-growth polymerization, the initial yield of Au nanorod side-by-side pairs can be greater than 50%. The high solubility and stability of the assembly enable purification, scale-up of nanomolarity solutions, and subsequent chemical modification of the assembled product. As an example, in situ silica deposition via Stöber synthesis onto the assembled pair produces highly processable nanostructures with a single pair of embedded Au NRs at their center, which exhibit thermal stability at temperatures in excess of 700 °C.

Nanolaminates: increasing dielectric breakdown strength of composites

Processable, low-cost, high-performance hybrid dielectrics are enablers for a vast array of green technologies, including high-temperature electrical insulation and pulsed power capacitors for all-electric transportation vehicles. Maximizing the dielectric breakdown field (E(BD)), in conjunction with minimization of leakage current, directly impacts system performance because of the field's quadratic relationship with electrostatic energy storage density. On the basis of the extreme internal interfacial area and ultrafine morphology, polymer-inorganic nanocomposites (PNCs) have demonstrated modest increases in E(BD) at very low inorganic loadings, but because of insufficient control of the hierarchal morphology of the blend, have yielded a precipitous decline in E(BD) at intermediate and high inorganic volume fractions. Here in, we demonstrate that E(BD) can be increased up to these intermediate inorganic volume fractions by creating uniform one-dimensional nanocomposites (nanolaminates) rather than blends of spherical inorganic nanoparticles and polymers. Free standing nanolaminates of highly aligned and dispersed montmorillonite in polyvinyl butyral exhibited enhancements in E(BD) up to 30 vol % inorganic (70 wt % organically modified montmorillonite). These relative enhancements extend up to five times the inorganic fraction observed for random nanoparticle dispersions, and are anywhere from two to four times greater than observed at comparable volume fraction of nanoparticles. The breakdown characteristics of this model system suggested a trade-off between increased path tortuosity and polymer-deficient structural defects. This implies that an idealized PNC morphology to retard the breakdown cascade perpendicular to the electrodes will occur at intermediate volume fractions and resemble a discotic nematic phase where highly aligned, high-aspect ratio nanometer thick plates are uniformly surrounded by nanoscopic regions of polymer.

Does shape matter? Bioeffects of gold nanomaterials in a human skin cell model

Gold nanomaterials (AuNMs) have distinctive electronic and optical properties, making them ideal candidates for biological, medical, and defense applications. Therefore, it is imperative to evaluate the potential biological impact of AuNMs before employing them in any application. This study investigates two AuNMs with different aspect ratios (AR) on mediation of biological responses in the human keratinocyte cell line (HaCaT) to model potential skin exposure to these AuNMs. The cellular responses were evaluated by cell viability, reactive oxygen species (ROS) generation, alteration in gene and protein expression, and inflammatory response. Gold nanospheres, nominally 20 nm in diameter and coated with mercaptopropane sulfonate (AuNS-MPS), formed agglomerates when dispersed in cell culture media, had a large fractal dimension (D(f) = 2.57 ± 0.4) (i.e., tightly bound and densely packed) and were found to be nontoxic even at the highest dose of 100 μg/mL. Highly uniform, 16.7 nm diameter, and 43.8 nm long polyethylene glycol-capped gold nanorods (AuNR-PEG) also formed agglomerates when dispersed into the cell culture media. However, the agglomerates had a smaller fractal dimension (D(f) = 1.28 ± 0.08) (i.e., loosely bound) and were found to be cytotoxic to the HaCaT cells, with a significant decrease in cell viability occurring at 25 μg/mL and higher. Moreover, AuNR-PEG caused significant ROS production and up-regulated several genes involved in cellular stress and toxicity. These results, combined with increased levels of inflammatory and apoptotic proteins, demonstrated that the AuNR-PEG induced apoptosis. Exposure to AuNS-MPS, however, did not show any of the detrimental effects observed from the AuNR-PEG. Therefore, we conclude that shape appears to play a key role in mediating the cellular response to AuNMs.

Swelling-induced delamination causes folding of surface-tethered polymer gels

When a polymer film that is weakly attached to a rigid substrate is exposed to solvent, swelling-induced compressive stress nucleates buckle delamination of the film from the substrate. Surprisingly, the buckles do not have a sinusoidal profile, instead, the film near the delamination buckles slides toward the buckles causing growth of sharp folds of high aspect ratio. These folds do not result from a wrinkle-to-fold transition; instead, the film goes directly from a flat state to a folded state. The folds persist even after the solvent evaporates. We propose that patterned delamination and folding may be exploited to realize high-aspect ratio topological features on surfaces through control of a set of boundary constraints arising from the interrelation of film-surface adhesion, film thickness and degree of swellabilty.

Holographically directed assembly of polymer nanocomposites

Layered polymer/nanoparticle composites have been created through the one-step two-beam interference lithographic exposure of a dispersion of 25 and 50 nm silica particles within a photopolymerizable mixture at a wavelength of 532 nm. The polymerizable mixture is composed of pentaerythritol triacrylate (monomer), 1-vinyl-2-pyrrolidinone (monomer), and photoinitiator. In the areas of constructive interference, the monomer begins to polymerize via a free-radical process and concurrently the nanoparticles move into the regions of destructive interference. The effects of exposure time, power density, nanoparticle size, and periodicity on the final nanocomposite structure were measured with transmission electron microscopy to determine the mechanism for particle segregation. Diffraction from the sample was monitored as well, though its magnitude was not a good predictor of nanostructure in this relatively low index contrast system. Exposure time did not have a strong effect on the final structure. The best nanoparticle sequestration was observed at reduced laser power density, smaller interferogram periodicity, and decreased nanoparticle size, indicating that particle segregation is dominated by diffusion-limited nanoparticle transport directed by a matrix containing a gradient of polymerization kinetics.

Canopy dynamics in nanoscale ionic materials

Nanoscale ionic materials (NIMS) are organic-inorganic hybrids in which a core nanostructure is functionalized with a covalently attached corona and an ionically tethered organic canopy. NIMS are engineered to be liquids under ambient conditions in the absence of solvent and are of interest for a variety of applications. We have used nuclear magnetic resonance (NMR) relaxation and pulse-field gradient (PFG) diffusion experiments to measure the canopy dynamics of NIMS prepared from 18-nm silica cores modified by an alkylsilane monolayer possessing terminal sulfonic acid functionality, paired with an amine-terminated ethylene oxide/propylene oxide block copolymer canopy. Carbon NMR studies show that the block copolymer canopy is mobile both in the bulk and in the NIMS and that the fast (ns) dynamics are insensitive to the presence of the silica nanoparticles. Canopy diffusion in the NIMS is slowed relative to the neat canopy, but not to the degree predicted from the diffusion of hard-sphere particles. Canopy diffusion is not restricted to the surface of the nanoparticles and shows unexpected behavior upon addition of excess canopy. Taken together, these data indicate that the liquid-like behavior in NIMS is due to rapid exchange of the block copolymer canopy between the ionically modified nanoparticles.

Role of solvent selectivity in the equilibrium surface composition of monolayers formed from a solution containing mixtures of organic thiols

We have developed a simple model to quantify the effect of solvent selectivity on the surface composition of two-component self-assembled monolayers formed from solutions containing mixtures of organic thiols. The coarse-grained molecular model incorporates the relevant intermolecular interactions in the solution and monolayer to yield an expression for the free energy of monolayer formation. Minimization of the free energy results in a simple and analytically tractable expression for the monolayer composition as a function of solvent selectivity (defined as the difference in the Flory-type interaction parameters of the two organic thiols in the solution) and the degree of incompatibility between the adsorbate molecules. A comparison of our theory to experiments on the formation of two-component self-assembled monolayers from solution indicates that the coarse-grained molecular model captures the trends in the experimental data quite well.

Bioassembled layered silicate-metal nanoparticle hybrids

Here we report on the bioenabled assembly of layered nanohybrids using peptides identified with regard to their affinity to the nanoparticle surface. A dodecamer peptide termed M1, determined from a phage peptide display library, was found to bind to the surface of a layered aluminosilicate (montmorillonite, MMT). Fusion of a metal binding domain to the M1 peptide or the M1 peptide by itself was able to direct the growth of metal nanoparticles, such as gold and cobalt-platinum, respectively, on the MMT. This method of producing hybrid nanoclay materials will have utility in catalytic, optical, biomedical, and composite materials applications.

Depletion-induced shape and size selection of gold nanoparticles

For nanoparticle-based technologies, efficient and rapid approaches that yield particles of high purity with a specific shape and size are critical to optimize the nanostructure-dependent optical, electrical, and magnetic properties, and not bias conclusions due to the existence of impurities. Notwithstanding the continual improvement of chemical methods for shaped nanoparticle synthesis, byproducts are inevitable. Separation of these impurities may be achieved, albeit inefficiently, through repeated centrifugation steps only when the sedimentation coefficient of the species shows sufficient contrast. We demonstrate a robust and efficient procedure of shape and size selection of Au nanoparticles (NPs) through the formation of reversible flocculates by surfactant micelle induced depletion interaction. Au NP flocculates form at a critical surfactant micelle molar concentration, C(m)* where the number of surfactant micelles is sufficient to induce an attractive potential energy between the Au NPs. Since the magnitude of this potential depends on the interparticle contact area of Au NPs, separation is achieved even for the NPs of the same mass with different shape by tuning the surfactant concentration and extracting flocculates from the sediment by centrifugation or gravitational sedimentation. The refined NPs are redispersed by subsequently decreasing the surfactant concentration to reduce the effective attractive potential. These concepts provide a robust method to improve the quality of large scale synthetic approaches of a diverse array of NPs, as well as fine-tune interparticle interactions for directed assembly, both crucial challenges to the continual realization of the broad technological potential of monodispersed NPs.

Tuning gold nanoparticle-poly(2-hydroxyethyl methacrylate) brush interactions: from reversible swelling to capture and release

Tailoring the interaction between surfaces and nanoparticles (NPs) affords great opportunities for a range of applications, including sensors, information storage, medical diagnostics, and filtration membranes. In addition to controlling local ordering and microscale patterning of the NPs, manipulating the temporal factors determining the strength of the interaction between NP and surface enables dynamic modulation of these structural characteristics. In this contribution we demonstrate robust polymer brush-NP hybrids that exhibit both reversible swelling and reversible NP adsorption/desorption. Polymer brush functionality is tailored through post-functionalization of poly(2-hydroxyethyl methacrylate) (PHEMA) brushes on flat solid substrates with alpha-amine conjugates ranging from perfluoro alkanes to poly(ethylene glycol) of varying molecular weights. The type of functionality controls NP affinity for the surfaces. In the case of poly(ethylene glycol) (PEG), the molecular weight (MW) of the PEG dictates adsorption and desorption phenomena. Higher MW PEG chains possess increased binding affinity toward NPs, which leads to higher relative Au-NP densities on the PHEMA-g-PEG brushes and concurrent sluggish desorption of NPs by thermal stimulus. Adsorption and desorption phenomena are further modulated by NP size yielding a system where adsorption and desorption are controlled by a delicate balance between the competitive energetics of polymer brush chelation versus solvation.

Direct measurement of the percolation probability in carbon nanofiber-polyimide nanocomposites

We present the first experimental measurement of the geometric critical exponent beta associated with the percolation probability, the probability a metallic filler belongs to the conducting network, of an electrical composite. The technique employs conducting-tip atomic force microscopy to obtain a conducting areal density, and is demonstrated on polyimide nanocomposites containing different concentrations of carbon nanofibers. We find beta approximately 1 and t (the exponent for bulk conductivity) approximately 3. These values are consistent with the predictions for the Bethe lattice and larger than the values predicted in the 3D lattice percolation model. Hence, this electrical composite likely belongs to the same universality class as the Bethe lattice. The ability to measure geometric and transport critical exponents on the same material is critical to drawing this conclusion.

Asymmetrically charged carbon nanotubes by controlled functionalization

Surface modification of carbon nanotubes (CNTs) has been widely studied for some years. However, the asymmetric modification of individual CNTs with different molecular species/nanoparticles at the two end-tips or along the nanotube length is only a recent development. As far as we are aware, no attempt has so far been made to asymmetrically functionalize individual CNTs with moieties of opposite charges. In this paper, we have demonstrated a simple, but effective, asymmetric modification of the sidewall of CNTs with oppositely charged moieties by plasma treatment and pi-pi stacking interaction. The as-prepared asymmetrically sidewall-functionalized CNTs can be used as a platform for bottom-up self-assembly of complex structures or can be charge-selectively self-assembled onto and/or between electrodes with specific biases under an appropriate applied voltage for potential device applications.

How rigid are viruses

Viruses have traditionally been studied as pathogens, but in recent years they have been adapted for applications ranging from drug delivery and gene therapy to nanotechnology, photonics, and electronics. Although the structures of many viruses are known, most of their biophysical properties remain largely unexplored. Using Brillouin light scattering, we analyzed the mechanical rigidity, intervirion coupling, and vibrational eigenmodes of Wiseana iridovirus (WIV). We identified phonon modes propagating through the viral assemblies as well as the localized vibrational eigenmode of individual viruses. The measurements indicate a Young's modulus of approximately 7 GPa for single virus particles and their assemblies, surprisingly high for "soft" materials. Mechanical modeling confirms that the DNA core dominates the WIV rigidity. The results also indicate a peculiar mechanical coupling during self-assembly of WIV particles.