Multilayered Particles Research Articles

Regime of validity of a spatially homogenised reaction model for multilayered particles

Numerical investigation of self-propagating reaction fronts in a network of Ni/Al nanolayered particles is conducted using the generalised reduced continuum model developed by Alawieh, Weihs and Knio [Combustion and Flame, Vol. 160, No. 9 (2013), pp. 1857–1869]. However, due to the high dimensionality and the high computational costs associated with simulating such systems, a further reduction of the model is sought through identifying regimes under which spatial homogenisation at the particle level would be valid. The limiting case of a single chain of particles with no porosity is considered, and comparisons between the computational results of the heterogeneous and the homogeneous reduced model descriptions are carried out. These enable us to determine a narrow region where the homogeneous particle approximation is valid, based on a criterion composed of the non-dimensional ratio of the particle's internal thermal resistance to its thermal contact resistance. We demonstrate that the validity of the homogeneous approximation may hold even for layered particles whose size is almost an order of magnitude larger than the thermal width of a reaction front steadily propagating in a uniform foil. The computations also show that the velocity of the front in the particle compact exhibits large variations as the thermal contact resistance is varied, which would offer the possibility of controlling reaction speed and ignition properties.

Monoclonal antibody-functionalized multilayered particles: targeting cancer cells in the presence of protein coronas.

Engineered particles adsorb biomolecules (e.g., proteins) when introduced in a biological medium to form a layer called a "corona". Coronas, in particular the protein corona, play an important role in determining the surface properties of particles and their targeting abilities. This study examines the influence of protein coronas on the targeting ability of layer-by-layer (LbL)-assembled polymer capsules and core-shell particles functionalized with monoclonal antibodies. Upon exposure of humanized A33 monoclonal antibody (huA33 mAb)-functionalized poly(methacrylic acid) (PMA) capsules or huA33 mAb-PMA particles to human serum, a total of 83 or 65 proteins were identified in the protein coronas, respectively. Human serum of varying concentrations altered the composition of the protein corona. The antibody-driven specific cell membrane binding was qualitatively and quantitatively assessed by flow cytometry and fluorescence microscopy in both the absence and presence of a protein corona. The findings show that although different protein coronas formed in human serum (at different concentrations), the targeting ability of both the huA33 mAb-functionalized PMA capsules and particles toward human colon cancer cells was retained, demonstrating no significant difference compared with capsules and particles in the absence of protein coronas: ∼70% and ∼90% A33-expressing cells were targeted by the huA33 mAb-PMA capsules and particles, respectively, in a mixed cell population. This result demonstrates that the formation of protein coronas did not significantly influence the targeting ability of antibody-functionalized LbL-polymer carriers, indicating that the surface functionality of engineered particles in the presence of protein coronas can be preserved.

Durable and flexible graphene composites based on artists’ paint for conductive paper applications

An acrylic emulsion artists’ paint containing chlorinated copper phthalocyanine pigment was modified with variable-size multilayer graphene (exfoliated graphite) to induce low electrical resistance; composite films were spray-cast on common printing paper, heat-cured, and subsequently polished under mild compression, to produce highly conductive paper. The mechanically robust conductive paint showed excellent adhesion to the underlying paper, as determined by Taber abrasion and tape peel tests, which displayed no adhesive failure under the test conditions studied. The conductivity of the paper substrates were tuned by changing the concentration and the size of the multilayer graphene particles. Detailed conductivity measurements showed stable Ohmic current–voltage behavior. The optimum graphene-in-paint formulations resulted in sheet resistances of the order of 10Ω/sq. Standard electrostatic force microscopy measurements showed uniform surface electric field gradient distribution strongly correlating with the surface topography. Similarly, scanning Kelvin probe microscopy measurements indicated stable work functions close to 5eV, comparable to highly-ordered pyrolytic graphite. Furthermore, Kelvin probe measurements were more sensitive to surface charges related to copper phthalocyanine domains, which are known to have semiconducting properties. Finally, the conductive papers were also tested in the 0.50–0.75 terahertz frequency range for electromagnetic interference shielding (EMI) characterization and displayed quasi-metallic shielding performance.

Cu(III)-independent oxidation and sensing of glucose on multi-layer stacked copper nanoparticles

A highly sensitive electrode for sensing glucose has been fabricated by electropolymerization of 2-amino-5-mercapto-1,3,4-thiadiazole on a solid carbon paste substrate, and subsequent electrodeposition of multi-layer stacked copper particles as an outer surface. The individual copper particles are characterized by a large number of edges and corners of crystallites. Their preferred orientation {111} is parallel to the electrode surface. The conductive polymer interlayer results in an increase of the particle nucleation density and a further decrease of the polarization overpotential for direct (enzyme-free) oxidation of glucose in 0.1 M NaOH solution. A well-shaped voltammetric peak can be observed at around 0.3–0.5 V (vs. SCE, depending on scan rate) that is due to glucose oxidation. This potential is much lower than the one required for Cu(III) formation. A bulk electrolysis experiment using a thin-layer electrochemical cell confirmed the assumption that that glucose undergoes 2-electron oxidation. The mechanism of glucose oxidation in the absence of Cu(III) is discussed. The electrode exhibits a very high sensitivity (slope) of 3.31 mA cm−2 mM−1, and the detection limit is 2 μM (at an SNR of 3). Features of the new sensor include the ease of fabrication, its high stability and good selectivity.

Recent developments in multilayered polymeric particles - from fabrication techniques to therapeutic formulations.

Multilayered particles are emerging as a powerful platform in pharmaceutics, especially for targeted, triggered and sustained drug delivery. These novel delivery systems exhibit advantages over single-layered particles, such as mitigating burst release of drugs. In this article, an overview of recent developments in the fabrication of multilayered polymeric particles, towards their utilization in therapeutic applications, will be provided. Fabrication techniques that aid in engineering multilayered particles will first be discussed. Towards the end, a critical outlook on the key issues associated with this particulate delivery system will be addressed.

Elastic Properties of a Polymer/Silicate Composite with Platelike Multilayer Filler Particles

A variant of stepwise calculations of the elastic constants of a composite containing platelike multilayer filler particles is presented. First, the effective characteristics of unexfoliated particles in the form of multilayer stacks with interlaminar galleries enlarged as a result of intercalation of polymer are determined. Then, the independent elastic constants of the transversely isotropic representative structural elements of a composite in which multilayer particles are located strictly coplanarly, i.e., parallel to each other, are calculated. Finally, the elastic constants of a composite with randomly oriented particles are obtained by using the method of orientational averaging. Data on the influence of the number of monolayers in the unexfoliated silicate filler particles on their reinforcing efficiency are reported and discussed.

Nonlinear Dynamics in Spin Vortex Pairs With Strong Core–Core Coupling

We investigate the dynamics of spin vortex pairs in magnetic multilayer particles, with the vortices closely spaced vertically and therefore with strong core-core coupling. We focus on the spin-dynamic behavior of the system beyond the linear small-signal regime, and on the state with antiparallel vortex chiralities and parallel cores, in which the vortex cores are strongly dipole coupled. The data show a clear transition from the dominant single rotational resonance at 2-3 GHz for small excitation field amplitudes to a dominant gyrational resonance at high excitation fields. The concomitant changes in the microwave spectra, seen as satellite resonances near the rotational peak as well as a pronounced low-frequency resonance, are interpreted as arising from the nonlinearities of the main rotational mode, which also mediate microwave power transfer from the high- to the low-frequency mode.

Sensitivity of aerosol radiative effects to different mixing assumptions in the AEROPT 1.0 submodel of the EMAC atmospheric-chemistry–climate model

Abstract. The modelling of aerosol radiative forcing is a major cause of uncertainty in the assessment of global and regional atmospheric energy budgets and climate change. One reason is the strong dependence of the aerosol optical properties on the mixing state of aerosol components, such as absorbing black carbon and, predominantly scattering sulfates. Using a new column version of the aerosol optical properties and radiative-transfer code of the ECHAM/MESSy atmospheric-chemistry–climate model (EMAC), we study the radiative transfer applying various mixing states. The aerosol optics code builds on the AEROPT (AERosol OPTical properties) submodel, which assumes homogeneous internal mixing utilising the volume average refractive index mixing rule. We have extended the submodel to additionally account for external mixing, partial external mixing and multilayered particles. Furthermore, we have implemented the volume average dielectric constant and Maxwell Garnett mixing rule. We performed regional case studies considering columns over China, India and Africa, corroborating much stronger absorption by internal than external mixtures. Well-mixed aerosol is a good approximation for particles with a black-carbon core, whereas particles with black carbon at the surface absorb significantly less. Based on a model simulation for the year 2005, we calculate that the global aerosol direct radiative forcing for homogeneous internal mixing differs from that for external mixing by about 0.5 W m−2.

Utilization of electrochemical impedance spectroscopy for experimental characterization of the diode features of charge recombination in a dye sensitized solar cell

The anode-electrolyte interface is a primary location for charge recombination in a dye sensitized solar cell (DSSC), and hence acts as a major efficiency-limiting factor of the device. The electrical signature of this recombination effect is similar to that of a semiconductor diode, and at the same time serves as an indicator of the recombination mechanisms. The present work focuses on certain detailed aspects of the experimental detection and analysis of this diode-like behavior of a DSSC. The recombination process is activated in a forward biased dark cell containing a photo-anode of multilayered TiO2 particles. The resulting reaction kinetics are probed with a combination of D.C. voltammetry and strategically designed electrochemical impedance spectroscopy (EIS). The voltage dependent impedance parameters of the multi-component solar cell are obtained from complex nonlinear least square analysis of the EIS data. Among these parameters, the characteristic frequency and the recombination resistance of charge transfer at the TiO2-electrolyte interface follow the same diode-like voltage dependence as that of the cell's D.C. current. The experimental considerations for analyzing these effects in a quantitative approach are discussed. The ideality factor of the DSSC diode is dictated by charge recombination in the mesoporous TiO2 photo-anode, and emerges with mutually agreeing values from independent D.C. and A.C. measurements. The results illustrate how the component-resolved analytical capability of EIS can be utilized for a detailed evaluation of the electrochemical performance of a DSSC.

Energy-redistribution signatures in transmission microscopy of Rayleigh and Mie particles.

We describe the transmission characteristics for the interaction of an arbitrary beam with (possibly multilayered) spherical particles of arbitrary size and electric permeability. Within the generalized Lorenz-Mie theory, expressions that generalize the total cross sections to their fractional counterparts are presented, which allow for an analytic quantification of transmission signals, both on-axis and off-axis. For Gaussian (Davis) beams, the relative angular domain of collection as compared to the beam's angle of divergence determines sensitively the shape and magnitude of the interference signal. Depending on the particle's position within the beam, the transmission signatures related to a pure energy redistribution as well as to accompanying absorption are discussed for Rayleigh particles in terms of their complex-valued polarizability. Implications for positioning, temperature control, spectroscopy, and optimized extinction measurements are discussed.

Impact of core dielectric properties on the localized surface plasmonic spectra of gold-coated magnetic core-shell nanoparticles.

Gold-coated iron oxide core-shell nanoparticles (IO-Au NPs) are of interest for use in numerous biomedical applications because of their unique combined magnetic-plasmonic properties. Although the effects of the core-dielectric constant on the localized surface plasmon resonance (LSPR) peak position of Au-shell particles have been previously investigated, the impact that light-absorbing core materials with complex dielectric functions have on the LSPR peak is not well established. In this study, we use extended Mie theory for multilayer particles to examine the individual effects of the real and imaginary components of core refractive indices on Au-shell NP plasmonic peaks. We find that the imaginary component dampens the intensity of the cavity plasmon and results in a decrease of surface plasmon coupling. For core materials with large imaginary refractive indices, the coupled mode LSPR peak disappears, and only the anticoupled mode remains. Our findings show that the addition of a nonabsorbing polymer layer to the core surface decreases the dampening of the cavity plasmon and increases LSPR spectral intensity. Additionally, we address apparent discrepancies in the literature regarding the effects of Au-shell thickness on LSPR peak shifts.

Monodispersed composite particles prepared via emulsifier‐free emulsion polymerization using waterborne polyurethane as microreactors

ABSTRACTMonodispersed polystyrene particles in submicrometer size were intriguingly prepared through emulsifier‐free batch‐seeded emulsion polymerization using nonmonodispersed waterborne polyurethane (WBPU) beads as microreactors. Different feed ratios of styrene (St)/WBPU for the preparation of composite particles were investigated, and the size–growth course was experimentally followed. The morphology and dispersity of the particles were characterized by scanning electron microscopy together with dynamic laser scattering particle size analyzer. Their inside structure was further characterized by transmission electron microscopy with ultramicrotomy combined with X‐ray photoelectron spectroscopy for the composite particles' surface analysis. The probable grafting polymerization of St from WBPU was verified by Fourier transform infrared spectroscopy and nuclear magnetic resonance instrument. The obtained composite particles were again employed as the seeds in the emulsion copolymerization of methyl methacrylate. As a result, the formed multilayered composite particles with reverse core–shell structure were also monodispersed and spherical. The mechanism of the formation of the monodispersed particles was proposed. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40985.

Fabrication of multilayered superparamagnetic particles based on sequential thermal deposition method

A simple method for the fabrication of superparamagnetic particles was developed. Polystyrene spheres were used as templates, and their surfaces were coated with a magnetic element (Ni) by thermal deposition, controlling their thicknesses strictly. Magnetic properties of fabricated particles depended on the Ni layer thickness; the fabricated particles were typically superparamagnetic for a Ni layer thinner than 3 nm and ferromagnetic for a Ni layer thicker than 4 nm. For the improvement of the force generated on a particle in a magnetic field, the formation of multiple Ni layers on a particle was examined by sequential depositions of Ni and SiO2, isolating two Ni layers with a SiO2 layer. The SiO2 layer thickness should be larger than 10 nm for a sufficient isolation of two Ni layers to maintain the superparamagnetic properties of the particle, and the magnetic charge of the particle increased proportionally with the number of Ni layers on a particle. These results indicate that enhanced superparamagnetic particles with various diameters can easily be fabricated by the suggested method.

Electrodynamics of soft multilayered particles dispersions: dielectric permittivity and dynamic mobility.

We report a theory for the evaluation of the electrodynamics of dispersions of spherical soft multilayered (bio)particles, with microorganisms and polyelectrolyte multilayers-coated particles as illustrative paradigms. These particles generally consist of a hard (ion- and water-impermeable) core component supporting a succession of step-function or diffuse-like concentric soft (permeable) polymeric layers defined by distinct electrostatic, hydrodynamic and structural properties. The formalism is based on a rigorous numerical resolution of the coupled Navier-Stokes-Brinkman equation, continuity equations for the flow and for the ionic species present in solution, and the non-linear Poisson equation corrected for the multilayered nature of the soft interphase. The frequency-dependent dynamic mobility and dielectric permittivity of such soft particles suspensions are discussed as a function of the key electrohydrodynamic features of the constituting particulate peripheral layers and solution salinity. It is shown that the frequency dependent permittivity is mostly affected by the total charge carried by the overall soft interphase. In contrast, the dynamic mobility is mainly determined by the charge and friction characteristics of the layers located within an electrokinetically-active outer particle region whose extension is defined by the electric double layer thickness and the Brinkman length. Results highlight that under particular electrolyte concentration and layer-to-layer thickness ratio conditions, the dynamic mobility may reflect the physico-chemical and structural properties of the only innermost layers of the soft particle coating.

Comparative thermodynamic study of functional polymeric latex particles with different morphologies

In this work, the thermodynamic behavior of three series of functionalized polymeric particles with different morphologies and different functional group concentrations were compared. Partial volumes and partial adiabatic compressibilities of particles at infinite dilution were calculated from density and sound speed data. These properties were interpreted in terms of the atomic, free volume and hydration contributions. In addition, this interpretation was extended to the particle components (polar and non-polar groups), developing a new thermodynamic methodology. For homogeneous particles without forced gradient, functional groups are located from the surface to the interior and the voidness effect plays an important role in the swelling process. In multilayer particles, functional groups are located at the boundary of each layer and they are completely hydrated while non-polar groups behave as a permeable membrane at high functional groups concentrations. Homogeneous particles with forced gradient behave as homogeneous particles without forced gradient at low functional groups concentrations and as a multilayer particle at higher concentrations of functional groups. In conclusion the shown thermodynamic tool allowed understanding the role played by each particle component and its interactions inside the particle.

Shell Structure of Natural Rubber Particles: Evidence of Chemical Stratification by Electrokinetics and Cryo-TEM

The interfacial structure of natural rubber (NR) colloids is investigated by means of cryogenic transmission electron microscopy (cryo-TEM) and electrokinetics over a broad range of KNO3 electrolyte concentrations (4-300 mM) and pH values (1-8). The asymptotic plateau value reached by NR electrophoretic mobility (μ) in the thin double layer limit supports the presence of a soft (ion- and water-permeable) polyelectrolytic type of layer located at the periphery of the NR particles. This property is confirmed by the analysis of the electron density profile obtained from cryo-TEM that evidences a ∼2-4 nm thick corona surrounding the NR polyisoprene core. The dependence of μ on pH and salt concentration is further marked by a dramatic decrease of the point of zero electrophoretic mobility (PZM) from 3.6 to 0.8 with increasing electrolyte concentration in the range 4-300 mM. Using a recent theory for electrohydrodynamics of soft multilayered particles, this "anomalous" dependence of the PZM on electrolyte concentration is shown to be consistent with a radial organization of anionic and cationic groups across the peripheral NR structure. The NR electrokinetic response in the pH range 1-8 is indeed found to be equivalent to that of particles surrounded by a positively charged ∼3.5 nm thick layer (mean dissociation pK ∼ 4.2) supporting a thin and negatively charged outermost layer (0.6 nm in thickness, pK ∼ 0.7). Altogether, the strong dependence of the PZM on electrolyte concentration suggests that the electrostatic properties of the outer peripheral region of the NR shell are mediated by lipidic residues protruding from a shell containing a significant amount of protein-like charges. This proposed NR shell interfacial structure questions previously reported NR representations according to which the shell consists of either a fully mixed lipid-protein layer, or a layer of phospholipids residing exclusively beneath an outer proteic film.

Synthesis and characterization of a novel silane-based vinylic monomer and its application in the multilayer core-shell latex

A novel monomer methacrylamidophenoxy dimethylsiloxy phenylphthalimide was obtained by a reaction of 4,4′-bis-aminophenoxy dimethylsilane, phthalic anhydride and methacryloyl chloride. Then it has been used in the synthesis of phase-separated polymer latex with a multilayer core-shell morphology by surface cross-linking emulsion polymerization. Poly(butyl acrylate) was used for the seed and the core of the latex, the inner shell was poly(butyl acrylate/styrene) cross-linked with divinylbenzene to avoid phase inversion, and the poly(methyl methacrylate/butyl acrylate/methacrylamidophenoxy dimethylsiloxy phenylphthalimide) was the outer shell. The structural elucidation of monomer was carried out by elemental analysis, FTIR, 1H NMR, and 13C NMR spectroscopic techniques. The morphology and glass transition temperatures of the synthesized product were investigated by transmission electron microscopy and differential scanning calorimetry, respectively. The multilayer core-shell structure was clearly shown in TEM micrographs, and the three-phase separation was confirmed by DSC analysis. The obtained results demonstrated that the average particle size is 81.8, 108 and 132 nm for the core, core-shell and multilayer core-shell particles, which agrees with the TEM micrograph measurement of 75, 103, and 131 nm, respectively.

The design of a Li-ion full cell battery using a nano silicon and nano multi-layer graphene composite anode

In this study, a Si–graphene composite, which is composed of nano Si particles and nano-sized multi-layer graphene particles, and micro-sized multi-layer graphene plate conductor, was used as the anode for Li-ion battery. The Si–graphene electrode showed the high capacity and stable cyclability at charge/discharge rate of C/2 in half cell tests. Nickel cobalt aluminum material (NCA) was used as a cathode in the full cell to evaluate the practicality of the new Si–graphene material. Although the Si–graphene anode has more capacity than the NCA cathode in this designed full cell, the Si–graphene anode had a greater effect on the full-cell performance due to its large initial irreversible capacity loss and continuous SEI formation during cycling. When fluoro-ethylene carbonate was added to the electrolyte, the cyclability of the full cell was much improved due to less SEI formation, which was confirmed by the decreases in the 1st irreversible capacity loss, overpotential for the 1st lithiation, and the resistance of the SEI.

Biocompatible Shaped Particles from Dried Multilayer Polymer Capsules

We demonstrated a simple and facile approach to fabricate biocompatible monodisperse hollow microparticles of controlled geometry. The hemispherical, spherical, and cubical microparticles are obtained by drying multilayer capsules of hydrogen-bonded poly(N-vinylpyrrolidone)/tannic acid (PVPON/TA)n. Drying spherical capsules results in hemispherical particles if 15 < n < 20. This shape transformation is controlled by capsule stiffness, which is regulated by the layer number, capsule diameter, and PVPON molecular weight. Cubical and spherical hollow particles maintaining their three-dimensional shapes in the dry state are obtained if n ≥ 25.5. A 17-fold stiffness increase is required to lead from totally collapsed (PVPON/TA)5.5 to dried self-supporting (PVPON/TA)25.5 particles of 2 μm in dimensions. All hollow particles could be further resuspended in aqueous solutions while retaining their shapes upon rehydration. The cell growth and viability studies using human cancer cells revealed noncytotoxic properties of the (PVPON/TA) multilayer particles. Both spherical and hemispherical capsules were internalized by macrophages with the uptake of the hemispherical particles per cell two times more efficient. The method presented here allows for a robust preparation of biocompatible shaped particles whose shape and dimensions can be easily tuned by controlling capsule size and wall thickness. The reported structures can be potentially useful for biomedical applications such as shape-controlled cellular uptake and flow dynamics.

Hollow latex particles functionalized with chitosan for the removal of formaldehyde from indoor air

Chitosan and polyethyleneimine (PEI) functionalized hollow latex (HL) particles were conveniently fabricated by coating poly(methyl methacrylate-co-divinyl benzene-co-acrylic acid) (P(MMA/DVB/AA)) HL particles with 5wt% chitosan or 14wt% PEI. The materials were used as formaldehyde adsorbent, where their adsorbent activity was examined by Fourier Transform Infrared (FTIR) spectroscopy. The nucleophilic addition of amines to carbonyls generated a carbinolamine intermediate with a characteristic band at 1020cm−1 and Schiff base product at 1650cm−1, whose intensity increased with prolonged formaldehyde exposure times. The major products observed in HL-chitosan were carbinolamine and Schiff base, whereas a small amount of Schiff base was obtained in HL-PEI particles, confirming a chemical bond formation without re-emission of formaldehyde. Compared to HL-PEI, HL-chitosan possesses higher formaldehyde adsorption efficiency. Besides providing opacity and whiteness, the multilayer HL-chitosan particles can effectively remove indoor air pollutants, i.e., formaldehyde gas, and, hence, would be useful in special coating applications.