Lock‐In Thermography to Analyze Plasmonic Nanoparticle Dispersions

The physicochemical properties of nanoparticles (NPs) strongly rely on their colloidal stability, and any given dispersion can display remarkably different features, depending on whether it contains single particles or clusters. Thus, developing efficient experimental methods that are able to provide accurate and reproducible measures of the NP properties is a considerable challenge for both research and industrial development. By analyzing different NPs, through size and concentration, it is demonstrated that lock‐in thermography, based on light absorption and heat generation, is able to detect and differentiate the distinct aggregation and re‐dispersion behavior of plasmonic NPs, e.g., gold and silver. Most importantly, the approach is nonintrusive and potentially highly cost‐effective compared to standard analytical techniques.     

Comparison of reversible and irreversible dipolar assemblies in a ferrofluid

Zero-field aggregation of magnetic nanoparticles in a ferrofluid can either be irreversible or result from a dynamic equilibrium; the two cases can be distinguished by measurements of the complex magnetic susceptibility and by cryogenic transmission electron microscopy (cryo-TEM). We demonstrate this by comparing two colloidal systems that show dipolar structure formation in zero field. A dispersion of magnetic iron nanoparticles is gradually oxidized to decrease the magnetic moments, and despite the vanishing dipolar attractions, thermal motion does not break up the dipolar structures into single particles. Instead, the dipolar structures become chemically fixed during the oxidation process, an example of irreversible aggregation. In contrast, the zero-field dipolar structures in a chemically stable magnetite dispersion are found to disintegrate upon dilution, indicating that the structures are reversible and result from a dynamic equilibrium.     

The influence of long range interparticle correlations on the magnetically induced optical anisotropy in magnetic colloids

In this paper we develop a theoretical model for the magnetically induced optical anisotropy in dense magnetic colloids made of spherical and un-aggregated magnetic monodomain nanoparticles. Both dipole-field and dipole-dipole magnetic and electric interactions between the magnetic monodomain particles are taken into account in the Hamiltonian of the system. Using the pair correlation function in a colloidal suspension of magnetic nanoparticles developed by Ivanov and Kuznetsova (2001) [11], the complex dielectric constant of a magnetic colloid is modeled as a function of the light polarization direction, the magnetic field intensity and magnetic particle concentration and diameter. The two main features of the model are that, on the one hand, it predicts the possibility of magnetically induced optical anisotropy in dense magnetic colloids made of spherical and un-aggregated monodomain nanoparticles, and on the other hand, unlike the existing models for diluted samples, it predicts a non-linear dependence of dichroism and birefringence on magnetic particle concentration.     

Ageing of Alkylthiol‐Stabilized Gold Nanoparticles

The ageing of spherical gold nanoparticles having 6‐nm‐diameter cores and a ligand shell of dodecanethiol is investigated under different storage conditions. Losses caused by agglomeration and changes in optical particle properties are quantified. Changes in colloidal stability are probed by analytical centrifugation in a polar solvent mixture. Chemical changes are detected by elementary analysis of particles and solvent. Fractionation occurs under all storage conditions. Ageing is not uniform but broadens the property distributions of the particles. Small‐number statistics in the ligand shell density and the morphological heterogeneity of particles are possible explanations. Washing steps exacerbate ageing, a process that could not be fully reversed by excess ligands. Dry storage is not preferable to storage in solvent. Storage under inert argon atmosphere reduces losses more than all other conditions but could not prevent it entirely.     

Janus Multicolored Photonic Crystal Beads for Real‐Time Magnetic Responsive Patterns

Photonic Janus particles that can change colors in different stimuli have shown great promise in various applications, such as optical probes, catalyst supports, sensors, and display materials. However, it remains a challenge to produce Janus structural colored particles with a simple method. Inspired from ecsenius bicolors, a facile preparation of Janus structural colored beads (JSCBs) using sub‐micrometer‐sized colloidal particles and magnetic nanoparticles composed of Fe₃O₄ is established. A mixed emulsion is prepared by monodispersed colloidal particles and magnetic nanoparticles and initiator of PDMS precursor. Subsequent self‐assembly of the emulsion in the room temperature environment provides two different structural colors, thanks to the gravity deposition of colloidal particles and Fe₃O₄ nanoparticles. The photonic bandgap of JSCBs can be precisely controlled with varying size of monodispersed colloidal nanoparticles, and the high optical quality and mechanical strength of the structural colored face are attributed to the existence of PDMS. In the presence of a magnet, the JSCBs can change their orientation simultaneously between two different structural colors. Moreover, the JSCBs are capable of encoding and angle‐independent displaying, which are crucial to their applications in anticounterfeiting, information coding, and pattern display.     

Monte Carlo simulation of magnetic multi-core nanoparticles

In this paper, a Monte Carlo simulation is carried out to evaluate the equilibrium magnetization of magnetic multi-core nanoparticles in a liquid and subjected to a static magnetic field. The particles contain a magnetic multi-core consisting of a cluster of magnetic single-domains of magnetite. We show that the magnetization of multi-core nanoparticles cannot be fully described by a Langevin model. Inter-domain dipolar interactions and domain magnetic anisotropy contribute to decrease the magnetization of the particles, whereas the single-domain size distribution yields an increase in magnetization. Also, we show that the interactions affect the effective magnetic moment of the multi-core nanoparticles.     

Phase separation and structure in a concentrated colloidal dispersion of uniform plates

The structure and phase behaviour of a colloidal dispersion of plate-like particles are described. The plates are nickel (II) hydroxide and have short-range, repulsive interactions and a low polydispersity. As the concentration of the plates is increased, an equilibrium phase separation between a columnar phase and a less ordered phase is observed. Complementary measurements using small-angle neutron and small-angle X-ray scattering have been used to distinguish the columnar phase from other possible ordered structures. Previously isotropic-nematic phase transitions have been observed [#!ref1!#], however this dispersion forms the more highly ordered columnar phase, due to the aspect ratio and the low polydispersity of the plate-like particles. The concentration at which phase separation occurs, increases as the range of the particle interactions is reduced. This system provides an interesting model for comparison with theory and calculations of structures in liquid crystal and mesophase in which the particle interactions can be altered. Received 24 February 1999     

Stability and magnetorheological behaviour of magnetic fluids based on ionic liquids

This paper reports the preparation of magnetic fluids consisting of magnetite nanoparticles dispersed in an ionic liquid. Different additives were used in order to stabilize the fluids. Colloidal stability was checked by magnetic sedimentation, centrifugation and direct observation. The results of these tests showed that a true ferrofluid was only obtained when the nanoparticles were coated with a layer of surfactant compatible with the ionic liquid. These experiments also showed that stability could not be reached just by electrostatic repulsion. The conclusions of the stability tests were confirmed by calculations of the interparticle energies of interaction. The rheological behaviour of the magnetic fluids upon magnetic field application was also investigated. The experimental magnetoviscous response was fitted by a microstructural model. The model considered that the fluids consisted of two populations of particles, one with a magnetic core diameter of 9?nm, and another with a larger diameter. Upon field application chain-like structures are supposed to be induced. According to estimations particles of 9?nm are too small to aggregate upon field application. The results of the calculations showed that the intensity of the magnetoviscous response depends on the concentration and size of the large particles, and on the thickness of the surfactant layers.     

Effects of particle size and laser wavelength on heating of silver nanoparticles under laser irradiation in liquid

Laser energy absorption results in significant heating of metallic nanoparticles and controlling the heating of nanoparticles is one of the essential stages of selective cell targeting. It is necessary to note that the laser action should be done by laser pulses with a wavelength that is strongly absorbed by the particles and it is important to select wavelengths that are not absorbed by the medium. Laser pulse duration must be chosen sufficiently short to minimize heat flow emitted from absorbing particles. Numerical calculations based on Mie theory were used to obtain the effect of laser wavelength and particle size on absorption factor for colloidal silver nanoparticles with radii between 5 and 50 nm. Calculations for acquiring temperatures under irradiations of pulsed KrF laser and pulsed Nd:YAG laser were performed. We showed that for low wavelengths of the laser, smaller nanoparticles have larger absorption efficiency compared to larger nanoparticles and in high wavelengths, temperature of all particles increased in the same way.     

Negative viscosity due to magnetic properties of a non-dilute suspension composed of ferromagnetic rod-like particles with magnetic moment normal to the particle axis

The negative viscosity of a colloidal dispersion composed of ferromagnetic rod-like particles, which have a magnetic moment normal to the particle axis, have been investigated. A simple shear flow problem has been treated to clarify the particle orientational distribution and rheological properties of such a semi-dense dispersion, under circumstances of an external magnetic field applied in the direction normal to the shear plane of a simple shear flow. The results obtained here are summarized as follows. For the cases of a very strong magnetic field and magnetic interactions between particles, the magnetic moment of the rod-like particles is significantly restricted in the magnetic field direction, so that the particle approximately aligns in the shear flow direction. Also, the particle can easily rotate around the axis of the cluster almost freely even in a simple shear flow. Characteristic orientational properties of the particle cause negative viscosity, as in the previous study for a dilute dispersion. However, magnetic particle-particle interactions have a function to make such negative viscosity decrease.     

A Comprehensive Brownian Dynamics‐Based Forward Model for Analytical (Ultra)Centrifugation

In this study, it is demonstrated how Brownian dynamics (BD) simulations can be used as a comprehensive forward model for analytical ultracentrifugation (AUC) and analytical centrifugation experiments. BD simulations are a versatile alternative to the numerical solution of Lamm's equation. Time dependent concentration profiles can be accurately predicted by means of the BD simulations even for colloidal systems with multiple species. It is shown that in combination with Mie's theory for light scattering, BD simulations can be extended to generate data mimicking a multiwavelength detector for AUC. The forward model can be utilized as a comprehensive tool for developing and testing sophisticated data analyzing tools that can enhance the capabilities of using centrifugation‐based techniques for characterizing nanoparticles. The model is validated using common analysis tools developed for the evaluation of sedimentation velocity data generated by a single wavelength extinction detector. The accuracy of the code is also proved using the recently developed and highly versatile High Dynamic Range‐MULTIwavelength FITting tool for accurately predicting particle size distributions via gravitational sweep experiments performed in AUCs equipped with multiwavelength detectors.     

Numerical micromagnetics of interacting superparamagnetic nanoparticles assembled in clusters with different dimensionalities

We analyze here the equilibrium magnetization state of densely packed interacting superparamagnetic nanoparticles assembled in clusters of various sizes and dimensionalities by comparison with the non-interacting case. We demonstrate that the average magnetization of individual particles is strongly increased in linear chains aligned parallel with the external magnetic field. Two-dimensional (2D) distributions of superparamagnetic nanoparticles present weaker increases of their average magnetization with respect to the non-interacting approximation whereas volume distributions (3D) are almost equivalent with the non-interacting case. A large number of nanoparticles densely packed in 2D superparamagnetic clusters present almost the same magnetic moment as infinite superparamagnetic chains. The effect of mutual interactions on the total magnetic moment of 3D surfaces (spheroids with various aspect ratios) uniformly covered with densely packed monolayers of superparamagnetic nanoparticles is also investigated.     

Quasi-2D Monte Carlo simulations of a colloidal dispersion composed of magnetic plate-like particles with magnetic moment normal to the particle axis

We have investigated aggregation phenomena of a colloidal dispersion composed of magnetic plate-like particles by means of Monte Carlo simulations. Such plate-like particles have been modelled as disk-like particles with magnetic moment normal to the particle axis at the particle centre, with the section shape of a spherocylinder. The main objective of the present study is to clarify the influences of the magnetic field strength and magnetic interactions between particles on particle aggregation phenomena. We have concentrated our attention on a quasi-2D system from an application point of view such as the development of surface quality changing technology using such magnetic plate-like particles. A magnetic field is applied along the direction perpendicular to the plane of the monolayer. Internal structures of particle aggregates are discussed quantitatively in terms of radial distribution and orientational pair correlation functions. For the case of strong magnetic interactions between particles, particles form long column-like clusters with their magnetic moments alternating in direction between the neighbouring particles. These tendencies appear under circumstances of a weak applied magnetic field. However, as the magnetic field strength increases, particles incline towards the magnetic field direction, so that particles do not form such clusters.     

Magnetorheology of colloidal dispersion containing Fe nanoparticles synthesized by the arc-plasma method

Spherical crystalline Fe nanoparticles, ∼100 nm in diameter, were synthesized under Ar-50% H₂ arc-plasma. These nanoparticles were dispersed in silicone oil after silane treatment on as-grown thin oxide layer (∼2 nm) to make their surfaces hydrophobic. The resulting Fe nanoparticles exhibited a high saturation magnetization of ∼190 emu/g at room temperature. The static magnetorheological behavior was measured for the colloidal dispersion (solid concentration: 15 vol%) at room temperature under magnetic flux densities of 0-0.3 T, using a parallel-plate-type commercial rheometer. The yield stress continuously increased with magnetic flux density, demonstrating the Bingham plastic behavior. Moreover, subjecting the sample to a magnetic flux density of 0.3 T increased the yield stress by ∼10². Additionally, the colloidal dispersion exhibited good stability against sedimentation.     

Polarization effects in nanoaggregates of silver caused by local and nonlocal nonlinear-optical responses

A theoretical and experimental study is made into the combined manifestation of local and nonlocal optical responses in a cubic nonlinear isotropic medium such as an aggregated colloidal silver solution. The phenomenological treatment of polarization effects is performed for the general case with due regard for the frequency dispersion of both local and nonlocal nonlinearities and for the noncollinear propagation of pump and probe light waves. The inverse Faraday effect, the optical Kerr effect, and the self-rotation of the polarization ellipse in a fractal-disordered nonlinear medium are observed for the first time. The tensor components of the local and nonlocal cubic nonlinearities of colloidal silver solutions are measured for different degrees of aggregation. It is demonstrated that, as the size of silver aggregate increases, the nonlocal nonlinear response increases much more strongly than the local one. An inference is made that the mechanical motion of metal nanoparticles because of their dynamic interaction with the light wave field can contribute to the nonlinear polarization effects.     

Steering trajectories in magnetically actuated colloidal propellers

Microscale colloidal doublets composed of DNA-linked paramagnetic particles and floating close to a surface are able to propel in viscous fluids when subjected to external precessing magnetic fields. We show here that for certain values of the precession angle, the composite particles can be steered into tilted rather than linear trajectories characterized by a non-vanishing lateral velocity during motion. We extend the original model developed in Phys. Rev. Lett. 101, 218304 (2008) in order to explain this phenomenon, by including high-order corrections in the expansion of the director field and demonstrate the validity of this approach by comparing the analytical results with the experimental data.     

Noise-enhanced performance of ratchet cellular automata

We present the first experimental realization of a ratchet cellular automaton (RCA) which has recently been suggested as an alternative approach for performing logical operations with interacting (quasi)particles. Our study was performed with interacting colloidal particles which serve as a model system for other dissipative systems, i.e., magnetic vortices on a superconductor or ions in dissipative optical arrays. We demonstrate that noise can enhance the efficiency of information transport in RCA and consequently enables their optimal operation at finite temperatures.     

Time-resolved microfocused small-angle X-ray scattering investigation of the microfluidic concentration of charged nanoparticles

We describe the concentration process of a dispersion of silica nanoparticles undergoing evaporation in a dedicated microfluidic device. Using microfocused small-angle X-ray scattering, we measure in time and space both the concentration field of the dispersion and its structure factor. We show that the electrostatic interactions affect the concentration rate by strongly enhancing the collective diffusion coefficient of the nanoparticle dispersion. En route towards high concentrations, the nanoparticles eventually undergo a liquid-solid phase transition in which we evidence crystallites of micron size.     

Optical properties of graded colloidal nanocrystallines with tunable structure

We propose a class of graded colloidal crystalline materials which consist of polydisperse metallodielectric nanoshells stacked in layers. We take the Lekner-Lishchuk summation method to treat the graded systems which are not tractable by conventional approach such as Ewald-Kornfeld methods. It is demonstrated that this kind of graded materials exhibit a series of sharp peaks, which merge in a broadened resonant absorption band in the optical region, in contrast to colloidal crystal containing monodisperse nanoshells or nanoparticles. Effects of various gradient profiles of the ratio of the inner/outer radii in the nanoshells and lattice geometries on the optical properties are discussed. These materials are not hard to fabricate by contemporary nanofabrication techniques and they shall be useful in the engineering of optical nanomaterials.     

Physical fabrication of colloidal ZnO nanoparticles combining wet-grinding and laser fragmentation

Combination of wet-grinding and laser fragmentation is a promising approach to advance both methods: Laser fragmentation will be more efficient when combined with mechanical treatment and wet-grinding may take advance of the abrasion-free laser process to achieve fabrication of smaller particles. By mechanical pre-treatment of zinc oxide microparticles in a stirred-media mill, the starting material is activated by generation of crystallographic defects, which strongly enhance the efficiency of subsequent laser fragmentation. Picosecond-laser irradiation of mechanically treated and untreated microparticles suspended in water yielded in colloidal zinc oxide nanoparticles. Furthermore, nanoparticle productivity and properties can be controlled by variation of anionic surfactant concentration.