Aggregation Of Magnetic Nanoparticles Research Articles

Band Structure Dependence on the External Perpendicular Magnetic Field and Zn Concentration of Photonic Crystals Made of Co1–xZnxFe2O4Nanoparticles

Ferrofluids based on the Co1– x Zn x Fe2O4 magnetic nanoparticles are considered as soft magnetic materials with interesting technological applications in nanotechnology when the ferrofluids are placed in an external magnetic field. The morphological characterization of Co1– x Zn x Fe2O4 magnetic nanoparticles was obtained by means of high-resolution transmission electron microscopy studies. In addition, we observed self-assembles of Co1– x Zn x Fe2O4 magnetic nanoparticle aggregates forming columns in a hexagonal 2-D photonic crystal (PC) when the ferrofluid was encapsulated in a cell glass under the action of a tunable magnetic field applied perpendicular to the film. The dependence of the ferrofluid effective refractive index with the applied magnetic field strength was determined by laser interferometry measurements using a Mach–Zehnder interferometer. To determine the optical response of 2-D magnetocontrollable PCs based on ferrofluids under the action of a dc external magnetic field, the Maxwell–Garnett theory, experimentally calculated effective refractive index, and plane-wave expansion method were used. The photonic band structure (PBS) of 2-D PCs for these ferrofluids was simulated, and we found that the optical response of PC depends on the ferrofluid effective refractive index, area ratio, permittivity of magnetic columns, and composition of Co1– x Zn x Fe2O4 ferrofluids. The results showed that the PBS of ferrofluid with the less Zn concentration ( $x = 0.25$ ) presented a complete bandgap in the IR range ( $\lambda \cong 10.6~\mu \text{m}$ ) at low perpendicular applied magnetic field. Finally, the procedure used in this paper may be considered as an alternative method to characterize the optical response of nanostructured ferrofluids in the presence of external magnetic fields. These results can be considered for technological applications of ferrofluids as magnetocontrollable PCs.

Tuning Electron-Conduction and Spin Transport in Magnetic Iron Oxide Nanoparticle Assemblies via Tetrathiafulvalene-Fused Ligands.

We report a strategy to coat Fe3O4 nanoparticles (NPs) with tetrathiafulvalene-fused carboxylic ligands (TTF-COO-) and to control electron conduction and magnetoresistance (MR) within the NP assemblies. The TTF-COO-Fe3O4 NPs were prepared by replacing oleylamine (OA) from OA-coated 5.7 nm Fe3O4 NPs. In the TTF-COO-Fe3O4 NPs, the ligand binding density was controlled by the ligand size, and spin polarization on the Fe3O4 NPs was greatly improved. As a result, the interparticle spacing within the TTF-COO-Fe3O4 NP assemblies are readily controlled by the geometric length of TTF-based ligand. The shorter the distance and the better the conjugation between the TTF's HOMO and LUMO, the higher the conductivity and MR of the assembly. The TTF-coating further stabilized the Fe3O4 NPs against deep oxidation and allowed I2-doping to increase electron conduction, making it possible to measure MR of the NP assembly at low temperature (<100 K). The TTF-COO-coating provides a viable way for producing stable magnetic Fe3O4 NP assemblies with controlled electron transport and MR for spintronics applications.

Surface Enhanced Raman Spectroscopy Sensor Based on Magnetic Beads-induced Nanoparticles Aggregation for Detection of Bacterial Deoxyribonucleic Acid

Abstract A novel method was developed for the highly sensitive detection of bacterial DNA by surface-enhanced Raman spectroscopy (SERS) with 5,5′-dithio-bis(2-nitrobenzoic acid) (DTNB)-modified gold nanoparticles as reporter probes and on the basis of the separation and enrichment effect of magnetic beads and the high affinity of fully complementary hybridization of DNA strands. Capture probe was immobilized onto the surface of Streptavidin-enwrapped magnetic beads (SA-MB) through the force between biotin and avidin, which hybridized with the target bacterial DNA sequence partly, and the remaining connected with the report probe decorated AuNPs with DTNB and SH-DNA (AuNPs@DTNB@DNA). Compared with previous methods, this design shortened the distance between particles by the aggregation effect of the magnetic beads to nanoparticles, and produced the plasma resonance coupling effect, which increased the SERS signal significantly. The results showed that, under the optimized conditions, the concentration of DNA in the range from 5 pM to 5 nM performed a good linear relationship with Raman intensity. The limit of detection (LOD) for bacterial DNA was estimated to be about 5 pM, showing a certain selectivity. The method was simple in design with low cost, also it was used in the sensitive and selective detection of bacterial DNA.

Clusters of Magnetic Nanoparticles as Contrast Agents for MRI: Effect of Aggregation on Transverse Relaxivity

The effect of aggregation of magnetic nanoparticles on the transverse relaxivity ( $r_{2})$ is analyzed with respect to the size of clusters. The nanoparticles employed are based on La0.75Sr0.25MnO3 ferromagnetic phase with the mean size of crystallites $d_{\mathrm {XRD}} = 26$ nm synthetized via sol-gel route followed by thermal treatment and mechanical processing. The subsequent silica coating provides colloidally stable particles whose magnetic cores are mostly composed of compact clusters of manganite crystallites. The product has been subjected to repeated differential centrifugation and several size fractions, possessing the same $d_{\mathrm {XRD}}$ but differing in the effective size of magnetic cores, are isolated. Thorough analyses of their size distributions by transmission electron microscopy and dynamic light scattering measurements are carried out together with SQUID magnetometry. The concentration of particles in aqueous suspensions is accurately determined by atomic absorption spectroscopy and a detailed study of transverse relaxation at the magnetic field $B_{0} = 0.5$ T is performed. The highest $r_{\mathrm {2}}$ values are clearly observed for midsized clusters and the temperature dependence of $r_{2}$ resembles the evolution of magnetization with temperature. Supplemental samples with different thicknesses of the silica shell are also synthetized and thoroughly analyzed.

Acoustic spectroscopy of magnetic fluids based on transformer oil

The structural changes in transformer oil–based magnetic fluids with magnetite nanoparticles, FeO·Fe2O3, of different diameters upon the effect of an external magnetic field were studied by acoustic spectroscopy. The corresponding changes of the acoustic wave attenuation as a function of magnetic field were measured. Over the linear increasing magnetic field, the interaction between the magnetic field and the magnetic moments of the nanoparticles led to the aggregation of magnetic nanoparticles and following chain or cluster formation. These structures enlarged with the magnetic field and this process had the influence on the value of the acoustic attenuation. The remarkable changes in the acoustic attenuation were also observed in the magnetic fluid subjected to a jumped magnetic field observed for a given time. After the back change of magnetic field to zero, the attenuation drastically decreases because the lifetime of big clusters is in this case small. The anisotropy of the acoustic attenuation in external magnetic field was measured and analyzed at various temperatures, and the cluster radius, the number density of the colloidal nanoparticles, and some other parameters of magnetic nanoparticles could be determined from experimental results.

Assembly of magnetic nanoparticles at a liquid/liquid interface. Catalytic effect on ion transfer process

The adsorption of cobalt (Co) and cobalt–boron (CoxB) magnetic nanoparticles (NPs) at a liquid/liquid interface is examined. The Co NPs have a remanent magnetization while the CoxB is completely demagnetized in the absence of an external magnetic field. Cyclic voltammetry experiments reveal that the adsorption of these NPs at the interface shifts the positive potential limit toward lower values showing that a catalytic effect on the ion transfer process is occurring, while electrochemical impedance spectroscopy demonstrates that their mode of self-assembly is directed by their magnetic properties. CoxB NPs form a homogeneous film while Co segregates in macroscopic patches, leaving some areas of the interface uncovered.

Abstract 311: High-throughput spheroid formation in a 384-well format using magnetic 3D bioprinting

Abstract Technical limitations in the rapid and reproducible formation of multicellular tumor spheroids prevent their widespread use in high-throughput screening for cancer research and drug development. Existing systems for spheroid formation require lengthy processing times, and make simple tasks like media exchange and sample retention challenging. An ideal 3D cell culture system would print spheroids rapidly, while being easy to handle and process for high-throughput applications. To address this unmet need, we use magnetic 3D bioprinting for high-throughput spheroid formation. Magnetic 3D bioprinting is based on the principle of magnetizing cells using a magnetic nanoparticle assembly. Once magnetized, these cells can be directed and aggregated using magnetic forces; specifically, magnetized cells can be aggregated into spheroids in 384-well plates. These spheroids can be cultured long-term, and, as they are magnetic, the spheroids can be held down or transferred out of the well, avoiding the technical issues of other 3D systems to retain samples. Moreover, the magnetic nanoparticles have no effect on cell behavior and do not interfere with fluorescence. We demonstrated this method by printing spheroids of HepG2, HCT-116, A549, and PANC-1 cancer cells in 384-well plates. We printed spheroids rapidly (15 min) and reproducibly, with cell numbers as small as 100 cells. We assayed the spheroids by using their contraction over time as an endpoint, which we validated as a toxicity endpoint correlating with cell migration, proliferation, and viability. In running these experiments we also demonstrated that these spheroids are amenable to high-content testing, with multiple experiments being performed on the same spheroid to yield more data per test. Thus, this study introduces magnetic 3D bioprinting for high-throughput spheroid formation, where multicellular tumor spheroids that represent native tumor environments can be rapidly and easily printed for cancer research and compound screening. Citation Format: Hubert Tseng, Jacob A. Gage, William L. Haisler, Glauco R. Souza. High-throughput spheroid formation in a 384-well format using magnetic 3D bioprinting. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 311. doi:10.1158/1538-7445.AM2015-311

Simulations of super-structure domain walls in two dimensional assemblies of magnetic nanoparticles

We simulate the formation of domain walls in two-dimensional assemblies of magnetic nanoparticles. Particle parameters are chosen to match recent electron holography and Lorentz microscopy studies of almost monodisperse cobalt nanoparticles assembled into regular, elongated lattices. As the particles are small enough to consist of a single magnetic domain each, their magnetic interactions can be described by a spin model in which each particle is assigned a macroscopic “superspin.” Thus, the magnetic behaviour of these lattices may be compared to magnetic crystals with nanoparticle superspins taking the role of the atomic spins. The coupling is, however, different. The superspins interact only by dipolar interactions as exchange coupling between individual nanoparticles may be neglected due to interparticle spacing. We observe that it is energetically favorable to introduce domain walls oriented along the long dimension of nanoparticle assemblies rather than along the short dimension. This is unlike what is typically observed in continuous magnetic materials, where the exchange interaction introduces an energetic cost proportional to the area of the domain walls. Structural disorder, which will always be present in realistic assemblies, pins longitudinal domain walls when the external field is reversed, and makes a gradual reversal of the magnetization by migration of longitudinal domain walls possible, in agreement with previous experimental results.

Magnetically Active Carbon Nanotubes at Work.

Endohedral and exohedral assembly of magnetic nanoparticles (MNPs) and carbon nanotubes (CNTs) recently gave birth to a large body of new hybrid nanomaterials (MNPs-CNTs) featuring properties that are otherwise not in reach with only the graphitic or metallic cores themselves. These materials feature enhanced magnetically guided motions (rotation and translation), magnetic saturation and coercivity, large surface area, and thermal stability. By guiding the reader through the most significant examples in this Concept paper, we describe how researchers in the field engineered and exploited the synergistic combination of these two types of nanoparticles in a large variety of current and potential applications, such as magnetic fluid hyperthermia therapeutics and in magnetic resonance imaging to name a few.

Ferronematics Based on Paramagnetic Nitroxide Radical Liquid Crystal

We have prepared novel ferronematics based on a paramagnetic liquid crystalline (LC) material. Our ferronematics can disperse a higher volume fraction of magnetic nanoparticles compared to classical ferronematics because paramagnetic nature of the host LC material prevents the aggregation of magnetic nanoparticles. The interactions between the magnetic nanoparticles and the LC material enhance a magnetic anisotropy of ferronematics and improve the magnetic responsivity.

Structure of nanoparticles in transformer oil-based magnetic fluids, anisotropy of acoustic attenuation

The anisotropy of acoustic attenuation in transformer oil-based magnetic fluids upon the external magnetic field was studied to discover the structure of nanoparticles. When a magnetic field is increased, the interaction between the external magnetic field and the magnetic moments of the nanoparticles leads to the aggregation of magnetic nanoparticles and following clusters formation. However, the temperature of magnetic fluids and the concentration of nanoparticles also have very important influence on the structural changes. The measurement of the dependence of the acoustic attenuation on the angle between the magnetic field direction and acoustic wave vector (anisotropy) can give the useful information about the structure of magnetic nanoparticles formations. In the present, the results of anisotropy measurements of the transformer oil-based magnetic fluids are described and using appropriate theory the basic parameters of clusters are calculated. On the basis of the performed calculations, the proportion of the acoustic wave energy used for excitation of the translational and rotational degrees of freedom was also established.

Magnetic nanoparticle motion in external magnetic field

A set of equations describing the motion of a free magnetic nanoparticle in an external magnetic field in a vacuum, or in a medium with negligibly small friction forces is postulated. The conservation of the total particle momentum, i.e. the sum of the mechanical and the total spin momentum of the nanoparticle is taken into account explicitly. It is shown that for the motion of a nanoparticle in uniform magnetic field there are three different modes of precession of the unit magnetization vector and the director that is parallel the particle easy anisotropy axis. These modes differ significantly in the precession frequency. For the high-frequency mode the director points approximately along the external magnetic field, whereas the frequency and the characteristic relaxation time of the precession of the unit magnetization vector are close to the corresponding values for conventional ferromagnetic resonance. On the other hand, for the low-frequency modes the unit magnetization vector and the director are nearly parallel and rotate in unison around the external magnetic field. The characteristic relaxation time for the low-frequency modes is remarkably long. This means that in a rare assembly of magnetic nanoparticles there is a possibility of additional resonant absorption of the energy of alternating magnetic field at a frequency that is much smaller compared to conventional ferromagnetic resonance frequency. The scattering of a beam of magnetic nanoparticles in a vacuum in a non-uniform external magnetic field is also considered taking into account the precession of the unit magnetization vector and director.

Magnetic Field Induced Enhancement in Thermal Conductivity and Viscosity of Stabilized Vacuum Pump Oil (VPO)—Fe&lt;SUB&gt;3&lt;/SUB&gt;O&lt;SUB&gt;4&lt;/SUB&gt; Magnetic Nanofluids

In this paper, an experimental investigaitons were made to analyze the thermal conductivity and viscosity of non-polar magnetic nanofluids with and without influence of magnetic field. Magnetic Fe3O4 nanoparticles were synthesized by chemical coprecipitation method and stable nanofluids were prepared by dispersing them into vacuum pump oil using sonicator without adding any surfactant. Both thermal conductivity and viscosity experiments were conducted in the volume concentrations from 0.05% to 1.0% and in the temperatures from 20 � C to 60 � C. Based on the results, without magnetic field, thermal conductivity and viscosity enhancements are 6.20% and 66%, respectively and with magnetic field of 900 G, thermal conductivity enhancement is 69%, with magnetic field of 1300 G, viscosity enhancement is 256% was observed for 1.0% of nanofluid at a temperature of 20 � C. The viscosity enhancement is more compared to thermal conductivity enhancement at same particle concentration and temperature and magnetic field. The chain-like structure of magnetic nanoparticles aggregates under magnetic field caused the enhancement in both thermal conductivity and viscosity.

Self assembly of magnetic nanoparticles at silicon surfaces.

Neutron reflectometry was used to study the assembly of magnetite nanoparticles in a water-based ferrofluid close to a silicon surface. Under three conditions, static, under shear and with a magnetic field, the depth profile is extracted. The particles have an average diameter of 11 nm and a volume density of 5% in a D2O-H2O mixture. They are surrounded by a 4 nm thick bilayer of carboxylic acid for steric repulsion. The reflectivity data were fitted to a model using a least square routine based on the Parratt formalism. From the scattering length density depth profiles the following behavior is concluded: the fits indicate that excess carboxylic acid covers the silicon surface and almost eliminates the water in the densely packed wetting layer that forms close to the silicon surface. Under constant shear the wetting layer persists but a depletion layer forms between the wetting layer and the moving ferrofluid. Once the flow is stopped, the wetting layer becomes more pronounced with dense packing and is accompanied by a looser packed second layer. In the case of an applied magnetic field the prolate particles experience a torque and align with their long axes along the silicon surface which leads to a higher particle density.

Controlled assembly of magnetic nanoparticles on microbubbles for multimodal imaging

Magnetic microbubbles (MMBs) consisting of microbubbles (MBs) and magnetic nanoparticles (MNPs) were synthesized for use as novel markers for improving multifunctional biomedical imaging. The MMBs were fabricated by assembling MNPs in different concentrations on the surfaces of MBs. The relationships between the structure, magnetic properties, stability of the MMBs, and their use in magnetic resonance/ultrasound (MR/US) dual imaging applications were determined. The MNPs used were NPs of 3-aminopropyltriethoxysilane (APTS)-functionalized superparamagnetic iron oxide γ-Fe2O3 (SPIO). SPIO was assembled on the surfaces of polymer MBs using a "surface-coating" approach. An analysis of the underlying mechanism showed that the synergistic effects of covalent coupling, electrostatic adsorption, and aggregation of the MNPs allowed them to be unevenly assembled in large amounts on the surfaces of the MBs. With an increase in the MNP loading amount, the magnetic properties of the MMBs improved significantly; in this way, the shell structure and mechanical properties of the MMBs could be modified. For surface densities ranging from 2.45 × 10(-7) μg per MMB to 8.45 × 10(-7) μg per MMB, in vitro MR/US imaging experiments showed that, with an increase in the number of MNPs on the surfaces of the MBs, the MMBs exhibited better T2 MR imaging contrast, as well as an increase in the US contrast for longer durations. In vivo experiments also showed that, by optimizing the structure of the MMBs, enhanced MR/US dual-modality image signals could be obtained for mouse tumors. Therefore, by adjusting the shell composition of MBs through the assembly of MNPs in different concentrations, MMBs with good magnetic and acoustic properties for MR/US dual-modality imaging contrast agents could be obtained.

Biologically controlled synthesis and assembly of magnetite nanoparticles.

Magnetite nanoparticles have size- and shape-dependent magnetic properties. In addition, assemblies of magnetite nanoparticles forming one-dimensional nanostructures have magnetic properties distinct from zero-dimensional or non-organized materials due to strong uniaxial shape anisotropy. However, assemblies of free-standing magnetic nanoparticles tend to collapse and form closed-ring structures rather than chains in order to minimize their energy. Magnetotactic bacteria, ubiquitous microorganisms, have the capability to mineralize magnetite nanoparticles, the so-called magnetosomes, and to direct their assembly in stable chains via biological macromolecules. In this contribution, the synthesis and assembly of biological magnetite to obtain functional magnetic dipoles in magnetotactic bacteria are presented, with a focus on the assembly. We present tomographic reconstructions based on cryo-FIB sectioning and SEM imaging of a magnetotactic bacterium to exemplify that the magnetosome chain is indeed a paradigm of a 1D magnetic nanostructure, based on the assembly of several individual particles. We show that the biological forces are a major player in the formation of the magnetosome chain. Finally, we demonstrate by super resolution fluorescence microscopy that MamK, a protein of the actin family necessary to form the chain backbone in the bacteria, forms a bundle of filaments that are not only found in the vicinity of the magnetosome chain but are widespread within the cytoplasm, illustrating the dynamic localization of the protein within the cells. These very simple microorganisms have thus much to teach us with regards to controlling the design of functional 1D magnetic nanoassembly.

Concentrated assemblies of magnetic nanoparticles in ionic liquids.

Maghemite (γ-Fe2O3) nanoparticles (NPs) can be successfully dispersed in a protic ionic liquid, ethylammonium nitrate (EAN), by transfer from aqueous dispersions into EAN. As the aqueous systems are well controlled, several parameters can be tuned. Their crucial role towards the interparticle potential and the structure of the dispersions is evidenced: (i) the size of the NPs tunes the interparticle attraction monitoring dispersions to be either monophasic or gas-liquid-like phase separated; (ii) the nature of the initial counterion in water (here sodium, lithium or ethylammonium) and the amount of added water (<20 vol%) modulate the interparticle repulsion. Very concentrated dispersions with a volume fraction of around 25% are obtained thanks to the gas-liquid-like phase separations. Such conclusions are derived from a fine structural and dynamical study of the dispersions on a large range of spatial scales by coupling several techniques: chemical analyses, optical microscopy, dynamic light scattering, magneto-optic birefringence and small angle scattering.

Acoustic Investigation of Biocompatible Fluid Under Magnetic Field

The behavior of BCMF in an external magnetic field was studied by acoustic spectroscopy. The linear increase, decrease and jump change of the external magnetic field up to 200 mT was applied and corresponding changes of acoustic attenuation were studied. The change of acoustic attenuation was connected with the structural changes of magnetic nanoparticles in the water-based magnetic fluid when an external magnetic field is applied. Measurements were made also for several temperatures of the range of 15-30°C to know the influence of thermal motion to nanoparticle aggregations. The significant influence of both magnetic field and temperature on the change of acoustic attenuation was observed. The obtained results are very important for magnetic drug targeting (MDT) for example, for the using of optimal concentration of magnetic nanoparticles (MNPs) as it is impossible to use very concentrated BCMF due to the strong aggregation of MNPs.

Magnetic nanoparticle assembly arrays prepared by hierarchical self-assembly on a patterned surface.

Inverted pyramid hole arrays were fabricated by photolithography and used as templates to direct the growth of colloidal nanoparticle assemblies. Cobalt ferrite nanoparticles deposit in the holes to yield high quality pyramid magnetic nanoparticle assembly arrays by carefully controlling the evaporation of the carrier fluid. Magnetic measurements indicate that the pyramid magnetic nanoparticle assembly arrays preferentially magnetize perpendicular to the substrate.

Monodisperse magnetic nanoparticle assemblies prepared at scale by competitive stabiliser desorption.

We report a scalable and reproducible method to assemble magnetic nanoparticle clusters from oleic acid stabilised iron oxide nanoparticles. By controlling the surface coverage of oleic acid on the nanoparticle surface we have achieved controlled nanoparticle assembly following exposure of the suspension to a substrate layer of cyanopropyl-modified silica which competes for the ligand. The clusters can be formed reproducibly and their final size can be selected over a range covering almost two orders of magnitude. Most unusually, the relative monodispersity of the cluster suspension is improved compared to the starting nanoparticle suspension, and the yield is close to 100%. Interestingly, we find that the kinetics of assembly is not altered by scaling up, which is surprising for a complex process involving molecular transport. Kinetic studies provided mechanistic insight into the process, and suggested general requirements for controlled assembly of other nanoparticle types.