Rod Aspect Ratio Research Articles

Extremely sensitive dual imaging system in solid phantoms.

Herein we describe promising results from the combination of fluorescent lifetime imaging microscopy (FLIM) and diffusion reflection (DR) medical imaging techniques. Three different geometries of gold nanoparticles (GNPs) were prepared: spheres of 20nm diameter, rods (GNRs) of aspect ratio (AR) 2.5, and GNRs of AR 3.3. Each GNP geometry was then conjugated using PEG linkers estimated to be 10nm in length to each of 3 different fluorescent dyes: Fluorescein, Rhodamine B, and Sulforhodamine B. DR provided deep-volume measurements (up to 1cm) from within solid, tissue-imitating phantoms, indicating GNR presence corresponding to the light used by recording light scattered from the GNPs with increasing distance to a photodetector. FLIM imaged solutions as well as phantom surfaces, recording both the fluorescence lifetimes as well as the fluorescence intensities. Fluorescence quenching was observed for Fluorescein, while metal-enhanced fluorescence (MEF) was observed in Rhodamine B and Sulforhodamine B - the dyes with an absorption peak at a slightly longer wavelength than the GNP plasmon resonance peak. Our system is highly sensitive due to the increased intensity provided by MEF, and also because of the inherent sensitivity of both FLIM and DR. Together, these two modalities and MEF can provide a lot of meaningful information for molecular and functional imaging of biological samples.

Phase diagrams of charged colloidal rods: Can a uniaxial charge distribution break chiral symmetry?

We construct phase diagrams for charged rodlike colloids within the second-virial approximation as a function of rod concentration, salt concentration, and colloidal charge. Besides the expected isotropic-nematic transition, we also find parameter regimes with a coexistence between a nematic and a second, more highly aligned nematic phase including an isotropic-nematic-nematic triple point and a nematic-nematic critical point, which can all be explained in terms of the twisting effect. We compute the Frank elastic constants to see if the twist elastic constant can become negative, which would indicate the possibility of a cholesteric phase spontaneously forming. Although the twisting effect reduces the twist elastic constant, we find that it always remains positive. In addition, we find that for finite aspect-ratio rods the twist elastic constant is also always positive, such that there is no evidence of chiral symmetry breaking due to a uniaxial charge distribution.

In situ monitoring, separation, and characterization of gold nanorod transformation during seed-mediated synthesis.

The control of gold nanorod (GNR) solution-based syntheses has been hindered in part by the inability to examine and control the conversion of precursor seed populations to anisotropic materials, which have resulted in low yields of desired products and limited their commercial viability. The advantages offered by tandem separation and characterization methods utilizing asymmetric-flow field flow fractionation (A4F) are principally achieved as a result of their non-disruptive nature (minimizing artefacts), fast throughput, and in-situ analysis. With hyphenated A4F methods, resolved populations of seeds and secondary products, up to long aspect ratio rods, have been achieved and exemplify progress towards elucidating mechanistic aspects of formation and thus rational design. While there have been previously reported studies on A4F separation of GNRs, to our knowledge, this is the first published investigation of in situ GNR growth, separation, and characterization based on A4F, where its utilization in this capacity goes beyond traditional separation analysis. By using hydroquinone as the reducing agent, the conversion of the initial seed population to a distribution of products, including the GNRs, could be monitored in real time using A4F hyphenated with a diode array detector. Transmission electron microscopy confirms that the number of peaks observed during fractionation corresponds with size and shape dispersity. This proof-of-principle study introduces A4F as a technique that establishes a foundation for future mechanistic studies on the growth of GNRs from gold seeds, including conversion of the seed population to initial products, a topic highly relevant to advancing progress in nanomanufacturing.

Effect of kerf filler on the electromechanical coupling coefficient of 1–3 piezoelectric composites

A unified formula has been derived to quantitatively describe the electromechanical coupling factor for the thickness mode of 1–3 piezoelectric composites with rectangular rod. The effect of different kerf filler on the electromechanical coupling factor is analyzed by varying the aspect ratio of piezoelectric rod and the elastic stiffness of kerf filler, respectively. The results show that the coupling factor decreases with the elastic stiffness increase of the kerf filler. In addition, we have also compared the current rectangular rod model with previous cylinder model for 1–3 piezoelectric composites. It was found that the circular cross-section piezoelectric rod tends to provide higher electromechanical coupling factor than the square rod when the aspect ratio is between 0.3 and 4 in the 1–3 composites with the same volume fraction.

Influence of Oxidative Etching of Au Nanostructures by KMnO4 on its Surface Morphology, Electro-kinetic Properties and Improved Catalytic Activity

This paper reports the impact of oxidative etching of Au nanospheres and nanorods by KMnO4 on their surface morphology, electro-kinetic properties and catalytic activity. A significant blue-shift of the surface plasmon bands for Au nanospheres (536 to 527nm) and Au nanorods (679 to 532nm) were observed, due to their size and shape alterations after oxidative dissolution. TEM analysis also revealed the formation of various irregular Au nano-morphologies such as spheres (∼4-7nm), low aspect ratio rods (2.6) and spheroids (∼13nm) of narrow size distribution after KMnO4 etching. As a result, the hydrodynamic diameter of Au nanospheres (∼41nm) and Au nanorods (∼109nm) were reduced to ∼4nm and ∼34nm, respectively. The oxidative dissolution of Au0 by KMnO4 occurred via its oxidation to Au3+ ions as confirmed by the measured electrode potential, E0(Au0/Au3+)=-0.90V by cyclic voltammetry with significant increase in the zeta potential and conductance values. The etched Au nanoparticles being smaller in size and of higher surface to volume ratio resulted in ∼ 2 fold higher catalytic activities for the reduction of p-nitrophenol and p-nitrobenzoic acid as compared to bare unetched Au nanostructures.

Isotropic-nematic phase transition of polydisperse clay rods.

Rod-like colloidal particles are known to display an isotropic-nematic phase transition on increase of concentration, as predicted already by Onsager. Both natural clay particles and synthetic rods tend to be polydisperse, however, and the question arises how to allow for this in comparing experimental observations with theory. Experimental data for a wide range of samples (both from the literature and the new results) have been collated, with aspect ratios ranging from 14 to 35. As a characteristic, the concentration is taken where half of the sample volume is nematic. Experimental data agree well with predictions for monodisperse finite aspect ratio rods. However, compared to these predictions, the width of the transition (taken as the ratio of isotropic and nematic limiting concentrations) is noticeably broadened. Still, in most cases, the transition can be characterised by a linear increase of the nematic phase volume with sample concentration. The transition width is in broad agreement with theoretical predictions for infinitely thin rods.

Analysis of the mechanical response of biomimetic materials with highly oriented microstructures through 3D printing, mechanical testing and modeling

Many biomineralized organisms have evolved highly oriented nanostructures to perform specific functions. One key example is the abrasion-resistant rod-like microstructure found in the radular teeth of Chitons (Cryptochiton stelleri), a large mollusk. The teeth consist of a soft core and a hard shell that is abrasion resistant under extreme mechanical loads with which they are subjected during the scraping process. Such remarkable mechanical properties are achieved through a hierarchical arrangement of nanostructured magnetite rods surrounded with α-chitin. We present a combined biomimetic approach in which designs were analyzed with additive manufacturing, experiments, analytical and computational models to gain insights into the abrasion resistance and toughness of rod-like microstructures. Staggered configurations of hard hexagonal rods surrounded by thin weak interfacial material were printed, and mechanically characterized with a cube-corner indenter. Experimental results demonstrate a higher contact resistance and stiffness for the staggered alignments compared to randomly distributed fibrous materials. Moreover, we reveal an optimal rod aspect ratio that lead to an increase in the site-specific properties measured by indentation. Anisotropy has a significant effect (up to 50%) on the Young’s modulus in directions parallel and perpendicular to the longitudinal axis of the rods, and 30% on hardness and fracture toughness. Optical microscopy suggests that energy is dissipated in the form of median cracks when the load is parallel to the rods and lateral cracks when the load is perpendicular to the rods. Computational models suggest that inelastic deformation of the rods at early stages of indentation can vary the resistance to penetration. As such, we found that the mechanical behavior of the system is influenced by interfacial shear strain which influences the lateral load transfer and therefore the spread of damage. This new methodology can help to elucidate the evolutionary designs of biomineralized microstructures and understand the tolerance to fracture and damage of chiton radular teeth.

Influence of through-thickness reinforcement aspect ratio on mode I delamination fracture resistance

Interlaminar reinforcement may be achieved in a composite laminate by the insertion and bonding of stiff rods through the thickness. It is experimentally shown in the paper that increased aspect ratio (AR) of through-thickness rods significantly elevates mode I maximum delamination fracture resistance of a laminate. This effect is explained through the analysis of rod/laminate interfacial stress transfer. Lower interfacial shear stresses occur for interlaminar rods of greater AR for equivalent bridging traction against opening delamination. Thereby, a larger AR allows the rod to develop a greater closure force prior to the rod debonding from the laminate. Further, these results show that this approach for interlaminar reinforcement is best applied to laminates of thickness-to-rod diameter ratio sufficient to allow for insertion of high aspect ratio reinforcement rods.

Parallel Operation of Multiple Closely Spaced Small Aspect Ratio Rod Pinches

A series of simulations and experiments to resolve questions about the operation of arrays of closely spaced small aspect ratio rod pinches has been performed. Design and postshot analysis of the experimental results are supported by 3-D particle-in-cell simulations. Both simulations and experiments support these conclusions. Penetration of current to the interior of the array appears to be efficient, as the current on the center rods is essentially equal to the current on the outer rods. Current loss in the feed due to the formation of magnetic nulls was avoided in these experiments by design of the feed surface of the cathode and control of the gap to keep the electric fields on the cathode below the emission threshold. Some asymmetry in the electron flow to the rod was observed, but the flow appeared to symmetrize as it reached the end of the rod. Interaction between the rod pinches can be controlled to allow the stable and consistent operation of arrays of rod pinches.

Sphere to rod transitions in self assembled systems probed using direct force measurement.

The influence of nanoparticle shape, in particular the sphere to rod transition, on surface forces and consequently the properties of colloidal fluids is an interesting but not well investigated phenomenon. Here, the surface force behaviour of concentrated surfactant solutions containing cetyltrimethylammonium bromide and sodium salicylate with micelle shapes varying from slightly prolate to high aspect ratio rods was measured. Atomic force microscopy (AFM) with both rigid particle and soft droplet probes was used with comparisons and analysis made using the Chan-Dagastine-White model. It is observed that small changes to the micelle shape result in no discernable differences to the surface force behaviour, however, once the micelles are elongated significantly, the long range forces adjust in nature from oscillatory to that of a single attractive force well. This highlights the importance that nanocolloid shape has on the behaviour and properties of emulsions and other colloidal fluids, specifically for emulsion flocculation and handling in systems of rod and worm like micelles.

Magnetic confinement and coupling in narrow-diameter Au–Ni nanowires

Abstract Here we examine magnetic coupling in layered magnetic/nonmagnetic nanowires created using electrochemical deposition into nanoporous templates. By utilizing reproducible and tunable deposition methods, various aspect ratios and spacing between magnetic domains were created. Low aspect ratio disks were then coupled to high aspect ratio rods to control the mechanism for spin flip in the disk component of the system. The orthogonal relationship between the magnetic easy axis of the disk and rod geometries creates a multistate system with both in-plane and out-of-plane easy axes by balancing magnetic shape anisotropy with dipole coupling between the two layers.

High Temperature Nanoplasmonics: The Key Role of Nonlinear Effects

In this work we show the effect of high temperature on the plasmonic properties of metallic nanorods. Within this context, the dielectric function of metal has been modified to keep into account both the electron–electron and electron–phonon temperature dependent scattering mechanisms. In fact, the mentioned damping processes are very sensible to temperature variation. It is found that the damping modifications due to temperature change substantially modify both the near and far field optical response of metallic nanorods. Furthermore, the response alteration can be very different depending on the rod aspect ratio, suggesting the need of accurate investigations when metallic nanostructures are to be employed within high-temperature environments or under high intensity irradiation. In this regard, by taking into account different nanorods aspect ratio and incident powers, we provide detailed calculations showing the error committed in evaluating absorption, scattering efficiencies, and near-field enhanceme...

Below melting point photothermal reshaping of single gold nanorods driven by surface diffusion.

Plasmonic gold nanorod instability and reshaping behavior below melting points are important for many future applications but are yet to be fully understood, with existing nanoparticle melting theories unable to explain the observations. Here, we have systematically studied the photothermal reshaping behavior of gold nanorods irradiated with femtosecond laser pulses to report that the instability is driven by curvature-induced surface diffusion rather than a threshold melting process, and that the stability dramatically decreases with increasing aspect ratio. We successfully utilized the surface diffusion model to explain the observations and found that the activation energy for surface diffusion was dependent on the aspect ratio of the rods, from 0.6 eV for aspect ratio of 5 to 1.5 eV for aspect ratio less than 3. This result indicates that the surface atoms are much easier to diffuse around in larger aspect ratio rods than in shorter rods and can induce reshaping at any given temperature. Current plasmonics and nanorod applications with the sharp geometric features used for greater field enhancement will therefore need to consider surface diffusion driven shape change even at low temperatures.

Nanoscale dispersion crystal bundles of palygorskite by associated modification with phytic acid and high-pressure homogenization for enhanced colloidal properties

The nanoscale dispersion of rod-like crystal bundles or aggregates of natural palygorskite (PAL) is not only significant in practical application but is also a challenge. In this paper, phytic acid (PA) was introduced during high-pressure homogenization (HPH) process to simultaneously disperse PAL crystal bundles and restrain the re-aggregation of the dispersed nanorods. SEM, TEM, XRD, FTIR and N2 adsorption–desorption analyses confirmed that the crystal bundles or aggregates of PAL were highly disaggregated and dispersed to individual nanorod with no disruption to the aspect ratio of rods after being homogenized at 30MPa, and PA molecules are favorable to dispersion and restrain the re-aggregation of nanorods. The nanoscale dispersion of PAL rods increased the BET specific surface area and Zeta potentials, and effectively improved the colloidal properties. The colloidal viscosity of modified PAL was sharply increased by 110.4% (from 1728mPa·s to 3636mPa·s) at the optimal dosage of PA (0.1wt.%) and the homogenization pressure of 30MPa, and the suspension stability was clearly enhanced by 71%. This simple disaggregation process provides a new industrial approach to produce nanoscale PAL and extend its application in modern industry.

Facile Synthesis of Anisotropic Au Nanostructures by Laser Irradiation and Study of Their Optical and Electrokinetic Properties

This work describes a cost-effective single step technique for the preparation of Au nanostructures of many anisotropic shapes from Au nanorods by irradiating them with different laser energies (2.71, 2.54, and 2.41 eV). A variety of many irregular Au nanomorphologies such as multifaceted polygonal spheres (10–20 nm), low aspect ratio rods, oval egg shapes, squares, pentagons, and nanocapsules of varying sizes were formed after laser exposure. The small irregular shaped particles with new surface exposed atoms having cutting edges and sharp corners affect the optical, electrokinetic, and catalytic properties of fragmented Au nanoparticles. The percentage decomposition of Au nanorods is found to increase as 59% > 69% > 72% with increased energy of laser exposure; 2.41 eV (514.7 nm) > 2.50 eV (488 nm) > 2.71 eV (457.9 nm), as determined from the TEM images. The formation of quantum sized paticles upon laser irradiation results in the increase of zeta potential from +50.0 to +57.5 mV and conductance values. The co-catalytic activity of photofragmented Au nanorods is ∼20–25% higher than that of bare nanorods for benzaldehyde photooxidation.

Effect of Rod Material on the Impedance Behavior of Small Aspect Ratio Rod Pinches

A series of experiments on small aspect ratio rod pinches using solid tungsten, solid graphite, and thin-wall tantalum anode rods driven by a nominally 375-kV linear transformer driver, powered inductive voltage adder has characterized the impedance behavior over a range of aspect ratios and materials. For the tungsten and tantalum anodes, the impedance behavior is well described by the pinched current for self-magnetically insulated flow in a cylindrical geometry with ions present and gap closure velocities of 1-2 cm/μs. The graphite anodes always exhibited strong localized pinching of the electron beam onto the anode rod, with substantially higher operating impedance, as well as a delay in the formation of the plasma at the anode. The graphite anodes also had a longer radiation delay time, consistent with the longer anode plasma formation time.

Optimized combination of intrinsic and form birefringence in oriented LaPO4 nanorod assemblies

The birefringence of colloidal nematic suspensions and thin films of aligned LaPO4 nanorods is quantitatively studied. Single crystalline hexagonal hydrated-LaPO4 nanorods are synthesized with the c-axis along the rod long dimension. Films are prepared on a glass substrate by blade coating deposition from concentrated colloidal suspensions of rods, allowing for their alignment through the effect of shear stress. The obtained material exhibits a high degree of rod alignment and a resulting high birefringence (Δn ≈ 0.13). This remarkable value is shown to result from the optimized combination of both the intrinsic birefringence originating from the rod crystalline anisotropy and the form birefringence due to the high rod aspect ratio. The birefringence is measured over the spectral range between 250 nm and 2000 nm and is found to be almost constant while the material transmittance is close to unity. In the highly oriented soft state of colloidal rod suspension, the form birefringence, although reduced by a weakened refractive index contrast between the rods and the surrounding medium (solvent), still provides a significant contribution to the overall birefringence which can be exploited in electro-optical devices.

Improved surface properties and catalytic activity of anisotropic shapes of photoetched Au nanostructures formed by variable energy laser exposure

Anisotropic Au nanostructures with versatile shapes have been prepared in a single step through cost effective laser irradiation approach with different laser energies (≈2.41, 2.50 and 2.71eV) and investigated the changes in optoelectronic and catalytic properties. A variety of irregular Au nano-morphologies such as spheres (5–20nm), low aspect ratio rods, nano-capsules, oval egg shaped particles, ice cubes (∼7nm), truncated triangles (∼7–8nm), pentagonal (∼12nm), square shaped particles (∼8nm) and polygonal nanospheres (∼8–12nm) were formed after laser exposure. The modified surface morphology of photoetched Au nanoparticles was monitored by variable energy laser beam, follows an exponential (A=A0e−kt) disintegration rate as a function of particle geometric size and shape. The percentage decomposition of Au nanorods (aspect ratio=2.8) is found to increase as 59%>69%>72% with increased laser energy exposure; 2.41eV (514.7nm)>2.50eV (488nm)>2.71eV (457.9nm) as determined from the decrease in surface plasmon absorption band during laser illumination. The formation of small clusters on laser irradiation has resulted in the decrease of zeta potential from +49.79 (unetched particles) to +12.71mV (etched particles) and also conductance values. Catalytic reduction of p-nitrophenol to p-aminophenol and m-dinitrobenzene to m-phenylenediamine with photofragmented Au nanostructures is found to be ∼15–20% higher than non-etched Au particles.

Zinc oxide nano- and micro-crystals synthesised by co-precipitation and hydrothermal process

Micro-sized zinc oxide (ZnO) crystals with different shapes (spindle-like, flower-like and rod-like) were successfully synthesised by a simple co-precipitation and hydrothermal reaction using zinc chloride, Pluronic F127 (PF127) and NH3.H2O as raw materials. The results from X-ray diffraction and scanning electron microscopy show that the morphology and size of the ZnO were strongly dependent on the content of NH3.H2O, and the aspect ratio of ZnO crystals is also controlled by PF127. The presence of enough hydroxyl ions (OH−) can polish and modify the morphology of ZnO in the hydrothermal system, but the hexagonal cylinder rods disappear in a strong basic environment. As the content of PF127 increases, the aspect ratio of the ZnO rods also increases. If the amount of PF127 is excessive, the average aspect ratio is reduced. Systematic reactions in the presence of different environments were conducted to control the formation of various well shaped ZnO crystals.

Percolation in polydisperse systems of aligned rods: a lattice-based analysis.

A model is developed for percolation in polydisperse systems of oriented cylinders that integrates excluded volume arguments with an analogy to site percolation on a modified Bethe lattice. Results from this treatment are presented for the volume fraction at the percolation threshold (denoted ϕc) as a function of the degree of polydispersity, mixture composition, and degree of orientational ordering. For monodisperse systems, ϕc is found to be a monotonically increasing function of the traditional orientational order parameter that quantifies degree of alignment. The presence of a fraction of isotropically oriented rods of small aspect ratio is shown to lower the percolation threshold for systems in which the longer rods are strongly aligned.