Nanostructure characterization of polymer-stabilized gold nanoparticles and nanofilms derived from green synthesis

The fabrication and characterization of gold (Au) nanostructured materials draws significant attention because of their distinctive properties and their technological applications. The first objective of this study is to fabricate polymer-stabilized Au nanoparticles and nanofilms (PAN) through a cost effective and green synthetic methodology. In this study, the gold trication (Au³⁺) can be spontaneously converted into metallic gold atom using a non-toxic reductant (ascorbic acid). The ultrafine Au clusters were formed and stabilized through metallic bonds in the colloidal suspension, which was then deposited on a micro-glass or polymer-bead substrate to prepare thin films. It was found that ascorbic acid was the best reducing agent due to its rapid rate, spontaneity of reaction, and its non-toxic nature. In order to prevent aggregation of the nanoparticles, a dispersing agent (gum Arabic) was used. The second objective of this study was to analyze the PAN using a number of state-of-the-art instrumentation techniques and analytical approaches, such as X-ray powder diffraction (XRD), atomic force microscopy (AFM), scanning and transmission electron microscopy (SEM and TEM), ultraviolet–visible (UV–Vis) spectroscopy, and ZetaPALS. These techniques were applied to evaluate specific properties of the PAN, such as characterization of its crystalline phase, surface topology, characteristic plasmon, particle size distribution, and stability. From this study, it can be concluded that the ultrafine Au nanoparticles and uniform films were obtained using the green chemistry method. The ultrafine Au particles are highly stabilized and monodispersed as demonstrated by their high absolute value of zeta potential.     

Sintering of aluminum nanoparticles: A molecular dynamics study

Molecular-dynamics simulations have been used to study the consolidation of nanopartictes of aluminum. We have studied two- and three-particle sintering as it is influenced by temperature, particle size and crystallographic orientation. We have elucidated the dominant transport mechanisms by measuring the shear stresses that develop in the contact regions and the mobilities of the surface and core atoms during sintering. Our studies demonstrate that increasing the particles size slows down the sintering kinetics. The extent of interpenetration of the particles is independent of the particle size and is afunction of the temperature of the particles.     

Mechanosynthesis of polymer-stabilized lead bromide perovskites: insight into the formation and phase conversion of nanoparticles

The application of polymers to replace oleylamine(OLA)and oleic acid(OA)as ligands for perovskite nanocrystals is an effective strategy to improve their stability and durability especially for the solution-based processing.Herein,we report a mechanosynthesis of lead bromide perovskite nanoparticles(NPs)stabilized by partially hydrolyzed poly(methyl methacrylate)(h-PMMA)and highmolecular-weight highly-branched poly(ethylenimine)(PEI-25K).The as-synthesized NP solutions exhibited green emission centered at 516 nm,possessing a narrow full-width at half-maximum of 17 nm and as high photoluminescence quantum yield(PL QY)as 85%,while showing excellent durability and resistance to polar solvents,e.g.,methanol.The colloids of polymer-stabilized NPs were directly processable toform stable and strongly-emitting thin films and solids,making them attractive as gain media.Furthermore,the roles of h-PMMA and PEI-25K in the grinding process were studied in depth.The h-PMMA can form micelles in the grinding solvent of dichloromethane to act as size-regulating templates for the growth of NPs.The PEI-25K with large amounts of amino groups induced significant enrichment of PbBr₂in the reaction mixture,which in turn caused the formation of CsPb₂Br₅-mPbBr₂and CsPbBr₃-Cs₄PbBr₆-nCsBr NPs.The presence of CsPbBr₃-Cs₄PbBr₆-nCsBr NPs was responsible for the high PL QY,as the Cs₄PbBr₆phase with a wide energy bandgap can passivate the surface defects of the CsPbBr₃phase.This work describes a direct and facile mechanosynthesis of polymer-coordinated perovskite NPs and promotes in-depth understanding of the formation and phase conversion for perovskite NPs in the grinding process.     

Direct observation of Brownian dynamics of hard colloidal nanorods

We synthesize monodisperse selenium (Se) colloidal rods, and the suspensions exhibit the smectic phase at a particle volume fraction (phi) of 0.28. Side-by-side rod clustering occurs at phi > 0.04. Cluster-size distributions and persistence times are determined for various phi. In dense suspensions (phi > 0.1), individual rods reveal characteristic fundamental motions, e.g., reptation and synchronized rotation. Mean-square displacements of the rods suggest a cage trapping and escape. Estimated translational and rotational diffusion coefficients show a large difference from predictions by computer simulations.     

Brownian dynamics simulation for modeling ion permeation across bionanotubes

The principles underlying Brownian dynamics (BD), its statistical consistency, and algorithms for practical implementation are outlined here. The ability to compute current flow across ion channels confers a distinct advantage to BD simulations compared to other simulation techniques. Thus, two obvious applications of BD ion channels are in calculation of the current-voltage and current-concentration curves, which can be directly compared to the physiological measurements to assess the reliability of the model and predictive power of the method. We illustrate how BD simulations are used to unravel the permeation dynamics in two biological ion channels-the KcsA K/sup +/ channel and ClC Cl/sup -/ channel.     

Adaptive Brownian dynamics for shape estimation of sodium ion channels

Ion channels are protein macromolecules that form biological nanotubes across the membranes of living cells. Given many possible geometrical shapes of an ion channel, we propose a computational scheme of selecting the model that best replicates experimental observations, using adaptive Brownian dynamics simulations together with discrete optimization algorithms. Brownian dynamics simulations emulate the propagation of individual ions through the sodium channel nanotube at a femto time second time scale and Angstrom unit (10(-10) meter) spatial scale.     

Coalescence behaviour of amorphous and crystalline tantalum nanoparticles: a molecular dynamics study

Porous films of tantalum (Ta) and its oxides exhibit numerous properties suitable for high surface area applications, mainly in the semiconductor and bio-implant industries. Such films can be developed by Ta nanoparticle deposition using DC magnetron sputtering with gas aggregation. In order to engineer films of desirable properties, accurate control and in-depth understanding of the processes and parameters of nanoparticle growth, deposition and coalescence are crucial. Of utmost importance is to control the film’s porosity, since it determines many of the other physical properties. To this end, we performed a number of classical Molecular Dynamics simulations to study the coalescence of two or more Ta nanoparticles. Temperature, relative size and crystallographic orientation, defect content, degree of crystallinity and deposition rate effects were taken into account, and a mapping of the sintering processes was acquired. A broad range of possible interaction mechanisms were observed, from simple nanoparticle reorientation in order to achieve epitaxial configuration, to atomic adsorption, neck formation, twinning within the nanoparticles and full consolidation into a single, larger nanoparticle. The parameters studied are directly linked to experimental deposition parameters; therefore, fitting them accordingly can lead to growth of films with bespoke properties.     

Effectiveness of Brownian deposition of nanoparticles from a gas flow in a tube

A comparison of experimental data, analytical results, and numerical calculations of the Brownian deposition of spherical nanoparticles is carried out. It is shown that for nanoparticles smaller than 10 nm and larger than 5 nm the relative discrepancy between experimental data and analytical results does not exceed 15%. In other radius ranges of nanoparticles the discrepancy is substantially smaller.     

A molecular dynamics study of the structural dependence of boron oxide nanoparticles on shape

Molecular dynamics simulation is employed to study the effect of varying nanoparticle shape on the structure of boron oxide nanoparticles. Two nanoshapes are investigated and compared: a sphere of diameter 16 A and a cube of dimension 16 x 16 x 16 A. A many-body polarization model is employed within the simulation, accounting for dipole moments induced by local electric fields. The resulting network is described by a short-range structure consisting of planar BO(3) units, while the intermediate-range structure is described by six-membered planar boroxol rings. Both the fraction of boroxol rings and their locations differ between the two nanoshapes. All planar boroxol rings within the spherical simulation are located on the interior, while planar rings within the cubic simulation aggregate to the cube walls. In addition, structural differences appear between the two shapes at longer ranges, including the formation of "layers" aligned parallel to the walls of the cube, reminiscent of both the low-density crystalline phase and the high-density amorphous form of boron oxide.     

Brownian dynamics simulations of rigid rod-like macromolecular particles flowing in bounded channels

Brownian dynamics simulations have been carried out of the joint probability distribution functions (PDF), P(ξ,θ), for macromolecular rod-like particles in the limit of infinite dilution in a solution under hydrodynamic linear flow. These PDF are calculated as a function of the orientations of the rod-like particles, θ and of the positions, ξ, of their centres of mass measured from a solid surface boundary. These simulations are developed in the neighbourhood of a solid surface boundary and in a confined space bounded by two such boundaries. They are constructed for a wide range of key quantities depicting the ratio of the hydrodynamic shear rate to the rotational Brownian diffusion coefficient. The notion of restitution is introduced to develop an algorithm for the consequences of the Brownian and hydrodynamic collisions of these macromolecules with impenetrable solid surface boundaries, which approach applies to a wide range of surfaces and macromolecules. The simulation results for the PDF distributions are given for typically low and high hydrodynamic flow conditions, and their properties are discussed. We show, for example, for low shear rates that a phenomenon which we call Brownian restitution enables the macromolecular rods to pass through a channel that is narrower than the rod length.     

Flagellar dynamics of a connected chain of active,polar, Brownian particles

We show that active, self-propelled particles that are connected together to form a single chain that is anchored at one end can produce the graceful beating motions of flagella. Changing the boundary condition from a clamp to a pivot at the anchor leads to steadily rotating tight coils. Strong noise in the system disrupts the regularity of the oscillations. We use a combination of detailed numerical simulations, mean-field scaling analysis and first passage time theory to characterize the phase diagram as a function of the filament length, passive elasticity, propulsion force and noise. Our study suggests minimal experimental tests for the onset of oscillations in an active polar chain.     

Multiplexed sensing based on Brownian relaxation of magnetic nanoparticles using a compact AC susceptometer

A novel multiplexed sensing scheme based on the measurement of the magnetic susceptibility of the affinity captured target molecules on magnetic nanoparticles in liquid suspension is proposed. The AC magnetic susceptibility provides a measurement of Brownian relaxation behavior of biomolecules bound to magnetic nanoparticles (MNPs) that is related to its hydrodynamic size. A room temperature, compact AC susceptometer is designed and developed to measure complex AC magnetic susceptibility of such magnetic nanoparticles. The AC susceptometer exhibits high sensitivity in magnetic fields as low as 10 μT for 1 mg ml(-1) concentration and 5 μl volume, and is fully software programmable. The capability of biological sensing using the proposed scheme has been demonstrated in proof of principle using the binding of biotinylated horseradish peroxidase (HRP) to streptavidin-coated MNPs. The proposed technique and instrument are readily compatible with lab-on-chip applications for point-of-care medical applications.     

Variable optical attenuator with a polymer-stabilized dual-frequency liquid crystal

A transmission-type variable optical attenuator (VOA) based on a polymer-stabilized dual-frequency liquid crystal (PSDFLC) is demonstrated at gamma = 1.55 microm. The VOA is highly transparent in the voltage-off state but scatters light in the voltage-on state. By using a birefringent beam displacer incorporated with half-wave plates, we can obtain a VOA that is polarization independent and that exhibits a 31 dB dynamic range. The polymer networks and dual-frequency effect together reduce the response time (rise + decay) of a 16 microm PSDFLC cell to 30 ms at room temperature and at a voltage of 24 Vrms.     

Brownian relaxation of interacting magnetic nanoparticles in a colloid subjected to a pulsatile magnetic field

We have investigated and modeled the effect of interaction among magnetic particles and the magnitude and duration of external applied magnetic field on Brownian relaxation in a colloidal suspension. In the case of interacting magnetic particles, Brownian relaxation depends on the interparticle dipole-dipole interaction, which slows down the overall Brownian relaxation process of magnetic particles in the colloidal suspension. The individual magnetic particle experiences torque when a pulsatile magnetic field is applied. The torque due to the external field randomizes the particle rotation similar to that of the thermal energy. A faster Brownian relaxation is observed when individual magnetic particles are magnetized for a short duration. Magnetizing the magnetic particle for a longer duration suppress the rotational motion hence the effect of torque on Brownian relaxation.     

Brownian motion in a designer force field: dynamical effects of negative refraction on nanoparticles

Photonic crystals (PC) have demonstrated unique features that have renewed the fields of classical and quantum optics. Although holding great promises, associated mechanical effects have proven challenging to observe. We demonstrate for the first time that one of the most salient properties of PC, namely negative refraction, can induce specific forces on metal nanoparticles. By integrating a periodically patterned metal film in a fluidic cell, we show that near-field optical forces associated with negatively refracted surface plasmons are capable of controlling particle trajectories. Coupling particle motions to PC band structures draws new approaches and strategies for parallel and high resolution all-optical control of particle flows with applications for micro- and nanofluidic systems.     

Adaptive Brownian dynamics simulation for estimating potential mean force in ion channel permeation

Ion channels are biological nanotubes formed by large protein molecules in the cell membrane. This paper presents a novel multiparticle simulation methodology, which we call adaptive controlled Brownian dynamics, for estimating the force experienced by a permeating ion at each discrete position along the ion-conducting pathway. The profile of this force, commonly known as the potential of mean force, results from the electrostatic interactions between the ions in the conduit and all the charges carried by atoms forming the channel the protein, as well as the induced charges on the protein wall. The current across the channel is solely determined by the potential of mean force encountered by the permeant ions. The simulation algorithm yields consistent estimates of this profile. The algorithm operates on an angstrom unit spatial scale and femtosecond time scale. Numerical simulations on the gramicidin ion channel show that the algorithm yields the potential of mean force profile that accurately reproduces experimental observations.     

Hydrodechlorination of chlorobenzene over polymer-stabilized palladium-platinum bimetallic colloidal nanocatalysts

Poly(N-vinyl-2-pyrrolidone) (PVP)-stabilized Pd, Pt, Pd-Pt nanocatalysts were prepared and characterized by transmission electron microscopy (TEM). Hydrogenation of chlorobenzene was carried out over these colloidal nanocatalysts under ambient conditions. The catalytic properties for the hydrogenation of chlorobenzene depended on the composition of the bimetallic nanocatalysts. The conversion of chlorobenzene over PVP-Pd (83.64%) was higher than that of PVP-Pt (66.67%), which indicated that the activity of Pd was higher than that of Pt. In 10 hrs. the conversions of all the bimetallic nanocatalysts were higher than that of PVP-Pt (66.67%) monometallic nanocatalysts, and the maximum conversion of chlorobenzene (95.34%) was achieved using PVP-Pd/Pt = 1/1 catalytic system, which was much higher than that of the physical mixture of monometallic nanocatalysts (PVP-Pd and PVP-Pt) at the same Pd/Pt ratio as the PVP-Pd/Pt bimetallic nanocatalysts used. The selectivity to benzene and cyclohexane of the bimetallic nanocatalysts (with < or = 40 mol% Pt) was similar to that of PVP-Pd monometallic nanocatalysts, and nearly approximately 100% selectivity to benzene could be obtained, the selectivity to cyclohexane increased slowly with increasing of platinum content in bimetallic nanocatalysts.     

On the dynamics of vortex structure in ferroelectric nanoparticles

The dynamics of vortex structure for polarizations in free-standing ferroelectric nanoparticles has been numerically investigated based on a thermodynamics-based continuum phase field approach under open-circuit boundary conditions. Both size effect and surface effect have been considered in this work: different assumptions for the extrapolation length have been made for the electric boundary condition and therefore accounting for the intrinsic size effect; the surface effect is studied by introducing the intrinsic surface stress, which causes volume mechanical balancing stress in the nanoparticles below free surfaces. The computed results are summarized in this article for square-shaped nanodots. It has been noticed that the particle size and intrinsic surface stress together play significant roles in the dynamics of vortex structure for polarizations. They affect both polarization configuration and existence conditions in ferroelectric nanoparticles.     

Brownian motion and the drift of charged nanoparticles in laminar gas flow in a plane channel

Using two limiting cases (of adsorbing and reflecting channel walls), the influence of Brownian motion on the motion of nanoparticles in laminar gas flow under the action of an external electric field has been considered. Similarity criteria making it possible to classify experimental situations have been found. Numerical modeling of deposition from the flow has been carried out with the example of the motion of a monodisperse ensemble of spherical nanoparticles with a radius of 3 nm. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 82, No. 2, pp. 215–220, March–April, 2009.