Ferromagnetic Cobalt Nanoparticles Research Articles

Hydrazine metal complexes as a single source in the mechanochemical preparation of metallic magnetic nanoparticles and investigation of their magnetic hyperthermia properties

Ferromagnetic metallic cobalt and nickel nanoparticles were synthesized by intramolecular metal-hydrazine ligand oxidation reduction reaction in basic media and the solid state. Nickel nanoparticles Ni1 and Ni2 are obtained from two complexes, Ni(N2H4)2(C2O4) and Ni(N2H4)2(HCO2)2, respectively. X-ray powder diffraction (XRD) and energy-dispersive X-ray (EDX) data show pure fcc crystal structures of nanoparticles. Scanning electron microscopy (SEM) images revealed both Ni1 and Ni2 nanoparticles have agglomerated sphere morphologies. The crystallite size estimated by using the Scherer method for Ni1 and Ni2 were 18.7 and 6.2 nm. Vibrating sample magnetometer (VSM) data show the coercivity of metallic nickel samples Ni1 and Ni2 are 50 and 153 Oe, respectively. Both nanoparticles cause an increase in the temperature of water under applied alternative magnetic field and showed specific loss power (SLP) of 70 W/g and 104.8 W/g, respectively. The same method is used in preparation of cobalt nanoparticles from two complexes, Co(N2H4)2(C2O4) and Co(N2H4)2(HCO2)2. XRD analysis data show both nanoparticles have hcp crystal structure. SEM images show nanosheet structure with sheet thickness of 10 to 30 nm. Coercivity of Co1 and Co2 nanoparticles obtained by VSM and Hc are 182 and 171 Oe, respectively. In spite of high coercivity in both Co1 and Co2 nanoparticles, the magnetic hyperthermia heating effects are not observed in practice and specific loss power (SLP) is zero.

Triboelectric Enhancement of Polyvinylidene Fluoride Membrane Using Magnetic Nanoparticle for Water-Based Energy Harvesting.

Produced by magnetic material dispersed in a viscous environment for the purpose of collecting and converting energy, magnetic rheological compounds greatly strengthen the development of skin-attachable and wearable electrical equipment. Given that magnetic nanomaterial anisotropy has a substantial influence on the interface polarizing of polyvinylidene fluoride (PVDF), it is critical to explore the function of magnetic polymer compounds in the triboelectric layer of triboelectric nanogenerator (TENG) output power. In this study, ferromagnetic cobalt ferrite, CoFe2O4 (CFO), nanoparticles, and PVDF were employed to create a triboelectric composite membrane to improve TENG energy output. The content of β phase in PVDF increased significantly from 51.2% of pure PVDF membrane to 77.7% of 5 wt% CFO nanoparticles in the PVDF matrix, which further increase the dielectric constant and negative charge of the membrane. As a consequence, the energy output of CFO/PVDF-5 TENG increased significantly with a voltage of 17.2 V, a current of 2.27 μA, and a power density of 90.3 mW/m2, which is 2.4 times the performance of pure PVDF TENG. Finally, the proposal for TENG hopes that its extraordinary stability and durability will provide additional views on hydrodynamic power generation in the future.

Hyperthermia response of PEGylated magnetic graphene nanocomposites for heating applications and accelerate antibacterial activity using magnetic fluid hyperthermia

In this research work, graphene/cobalt nanocomposites are functionalized with polyethylene glycol (PEG) to be a platform for theranostics application and antibacterial activity. The non-covalent functionalization of PEG on the surfaces of nanocomposites improved their stability and diminished their cytotoxicity. The PEGylated nanocomposites are demonstrated to allow simultaneous administration of two cancer therapy methods such as magnetic fluids hyperthermia (MFH) which is carried out by converting magnetic energy into heat through ferromagnetic cobalt nanoparticles and heat generation through near-infrared optical absorption by the reduced graphene oxide. A concise simulation is carried out to approximate in vivo conditions and provide a glimpse of practical PEGylated nanocomposites performance. Furthermore, we demonstrate the antimicrobial activity of the PEGylated nanocomposites against Gram-negative Escherichia coli bacteria under alternative current magnetic field (AMF). The graphene/cobalt/PEG (rGO/Co/PEG) nanocomposites show antibacterial activity toward E. coli bacteria of about 100%, when the bacteria intact with nanocomposites are placed under AMF for 15 min. For comparison, the antimicrobial activities of graphene/PEG sheets and cobalt/PEG nanoparticles are also checked. This study concentrates on the design of novel non-toxic PEGylated nanocomposites to be used for dual hyperthermia therapy and bacteria killing by MFH treatments.

Synthesis of Polystyrene-Coated Superparamagnetic and Ferromagnetic Cobalt Nanoparticles.

Polystyrene-coated cobalt nanoparticles (NPs) were synthesized through a dual-stage thermolysis of cobalt carbonyl (Co2(CO)8). The amine end-functionalized polystyrene surfactants with varying molecular weight were prepared via atom-transfer radical polymerization technique. By changing the concentration of these polymeric surfactants, Co NPs with different size, size distribution, and magnetic properties were obtained. Transmission electron microscopy characterization showed that the size of Co NPs stabilized with lower molecular weight polystyrene surfactants (Mn = 2300 g/mol) varied from 12–22 nm, while the size of Co NPs coated with polystyrene of middle (Mn = 4500 g/mol) and higher molecular weight (Mn = 10,500 g/mol) showed little change around 20 nm. Magnetic measurements revealed that the small cobalt particles were superparamagnetic, while larger particles were ferromagnetic and self-assembled into 1-D chain structures. Thermogravimetric analysis revealed that the grafting density of polystyrene with lower molecular weight is high. To the best of our knowledge, this is the first study to obtain both superparamagnetic and ferromagnetic Co NPs by changing the molecular weight and concentration of polystyrene through the dual-stage decomposition method.

Nondestructive in-line sub-picomolar detection of magnetic nanoparticles in flowing complex fluids

Over the last decades, the use of magnetic nanoparticles in research and commercial applications has increased dramatically. However, direct detection of trace quantities remains a challenge in terms of equipment cost, operating conditions and data acquisition times, especially in flowing conditions within complex media. Here we present the in-line, non-destructive detection of magnetic nanoparticles using high performance atomic magnetometers at ambient conditions in flowing media. We achieve sub-picomolar sensitivities measuring ~30 nm ferromagnetic iron and cobalt nanoparticles that are suitable for biomedical and industrial applications, under flowing conditions in water and whole blood. Additionally, we demonstrate real-time surveillance of the magnetic separation of nanoparticles from water and whole blood. Overall our system has the merit of in-line direct measurement of trace quantities of ferromagnetic nanoparticles with so far unreached sensitivities and could be applied in the biomedical field (diagnostics and therapeutics) but also in the industrial sector.

Dilution effects on combined magnetic and electric dipole interactions: A study of ferromagnetic cobalt nanoparticles with tuneable interactions.

Improved understanding of complex interactions between nanoparticles will facilitate the control over the ensuing self-assembled structures. In this work, we consider the dynamic changes occurring upon dilution in the self-assembly of a system of ferromagnetic cobalt nanoparticles that combine magnetic, electric, and steric interactions. The systems examined here vary in the strength of the magnetic dipole interactions and the amount of point charges per particle. Scattering techniques are employed for the characterization of the self-assembly aggregates, and zeta-potential measurements are employed for the estimation of surface charges. Our experiments show that for particles with relatively small initial number of surface electric dipoles, an increase in particle concentration results in an increase in diffusion coefficients; whereas for particles with relatively high number of surface dipoles, no effect is observed upon concentration changes. We attribute these changes to a shift in the adsorption/desorption equilibrium of the tri-n-octylphosphine oxide (TOPO) molecules on the particle surface. We put forward an explanation, based on the combination of two theoretical models. One predicts that the growing concentration of electric dipoles, stemming from the addition of tri-n-octylphosphine oxide (TOPO) as co-surfactant during particle synthesis, on the surface of the particles results in the overall repulsive interaction. Secondly, using density functional theory, we explain that the observed behaviour of the diffusion coefficient can be treated as a result of the concentration dependent nanoparticle self-assembly: additional repulsion leads to the reduction in self-assembled aggregate size despite the shorter average interparticle distances, and as such provides the growth of the diffusion coefficient.

Control of the crystal habit and magnetic properties of Co nanoparticles through the stirring rate

Ferromagnetic hcp cobalt nanoparticles (NPs) are prepared by the polyol process using long-chain carboxylates as shape-directing agents, applying different stirring rates during the synthesis. Particles prepared with a stirring rate lower than ca. 150 rpm are very well-defined anisotropic crystals with smooth lateral facets and low stacking fault densities. The growth proceeds along the c axis of the hcp structure which corresponds to the long axis of the anisotropic particles. Increasing the stirring speed leads to a lowering of the mean aspect ratio, an increase in structural disorder as well as to a strong modification of the particle habit. The resulting Co NPs then display a marked surface roughness. We propose here a mechanism to explain the different morphologies observed. A high stacking fault density triggers the local disorganisation of the adsorbed capping agent layer which in turn favours the diffusion of Co monomers through the organic layer, resulting in the formation of excrescences perpendicular to the long axis of the particles. The magnetic properties are strongly related to the morphological and micro-structural properties of the particles. The stirring rate appears, so far, as the driving force to control the magnetic properties of the NPs from hard magnetic behaviour at low stirring rates to an almost soft magnetic one under high stirring conditions.

A simple procedure for the production of large ferromagnetic cobalt nanoparticles

Epsilon cobalt (ε-Co) nanoparticles in a number of octahedral morphologies have been synthesised. The particles are polycrystalline, with sizes in the order of 30 nm. Magnetic studies reveal the particles are ferromagnetic, with a room temperature saturation magnetisation of 131 emu g(-1). Unlike other large cubic ε-Co syntheses, we have not added an additional co-surfactant. Instead, we have modified the heating regime and reaction agitation. This alternative method highlights the complex chemistry associated with the formation of cobalt nanoparticles by thermal decomposition.

Spin wave free spectrum and magnetic field gradient of nanopatterned planes of ferromagnetic cobalt nanoparticles: key properties for magnetic resonance based quantum computing

We present a study by ferromagnetic resonance at microwave Q band of two sheets of cobalt nanoparticles obtained by annealing SiO2 layers implanted with cobalt ions. This ex- perimental study is performed as a function of the applied magnetic field orientation, tempera- ture, and dose of implanted cobalt ions. We demonstrate that each of those magnetic sheet of cobalt nanoparticles can be well modelled by a nearly two dimensional ferromagnetic sheet hav- ing a reduced effective saturation magnetization, compared to a regular thin film of cobalt. The nanoparticles are found superparamagnetic above around 210 K and ferromagnetic below this blocking temperature. Magnetostatic calculations show that a strong magnetic field gradient of around 0.1 G/nm could be produced by a ferromagnetic nanostripe patterned in such magnetic sheet of cobalt nanoparticles. Such a strong magnetic field gradient combined with electron para- magnetic resonance may be relevant for implementing an intermediate scale quantum computer based on arrays of coupled electron spins, as previously reported (Eur. Phys. J. B (2014) 87, 183). However, this new approach only works if no additional spin decoherence is introduced by the spin waves exitations of the ferromagnetic nanostructure. We thus suggest theoretically some possible magnetic anisotropy engineering of cobalt nanoparticles that could allow to suppress the spin qubit decoherence induced by the unwanted collective excitation of their spins.

Colloidal Polymers via Dipolar Assembly of Magnetic Nanoparticle Monomers

In this Spotlight on Applications, we describe our recent progress in the preparation of hierarchical one-dimensional (1-D) materials constructed from polymer-coated ferromagnetic cobalt nanoparticles. We begin with a general discussion of nanoparticles capable of 1-D self-organization to form 1-D assemblies, which we term colloidal polymers. The need for efficient, highly directional interactions prompted our investigation with polymer-coated ferromagnetic nanoparticles, which spontaneously form linear assemblies through coupling of north and south magnetic poles present in these single-domain ferromagnetic nanoparticles. These highly directional N-S interactions and the resulting formation of 1-D assemblies can be understood in the context of traditional polymer-forming reactions. The dipolar assembly of these ferromagnetic nanoparticles into chains and binary assemblies while dispersed in organic media has been investigated as a key foundation to form novel magnetic materials and heterostructured nanocomposites. These studies enabled the fabrication of magnetic nanoactuating systems resembling "artificial cilia and flagella". We then discuss our recent efforts to prepare cobalt oxide nanowires using various nanoparticle conversion reactions through a process termed colloidal polymerization. A series of novel functional "colloidal monomers" based on dipolar cobalt nanoparticles were also prepared, incorporating noble metal or semiconductor nanoinclusions to form heterostructured cobalt oxide nanocomposites.

Synthesis of functionalized, dispersible carbon-coated cobalt nanoparticles for potential biomedical applications.

Ferromagnetic carbon-coated cobalt nanoparticles have been ligated by a covalent or non-covalent strategy with dendrons or polymers bearing amino-, hydroxy- or polyether-functionalities, aiming at synthesizing materials that are dispersible in aqueous solutions, being a prerequisite for applications in biomedicine. Most efficiently for this purpose proved to be a covalently anchored polyethyleneimine (PEI) with a loading of approximately 10 mmol of amino groups per gram of hybrid nanomaterial, resulting in dispersions that are stable in water for several days.

Morphology of Ferromagnetic Cobalt Nanoparticles Grown on Al<SUB>2</SUB>O<SUB>3</SUB> (0001) by Pulsed Laser Deposition

Morphology of Ferromagnetic Cobalt Nanoparticles Grown on Al<SUB>2</SUB>O<SUB>3</SUB> (0001) by Pulsed Laser Deposition

Rapid prototyping of magnetic valve based on nanocomposite Co/PDMS membrane

In this study, a new type of active membrane based on magnetic elastomer composite is manufactured, characterized and integrated into a simple valve. The simple and low-cost fabrication process combined with large displacement capability of the membrane is favorable for use in disposable fluidic devices. Passivated ferromagnetic cobalt nanoparticles (~37 nm) synthesized by the chemical route were embedded in polydimethylsiloxane (PDMS) to fabricate nano-composite flexible membranes. Magneto-mechanical and mechanical properties of the PDMS composite elastomeric membrane loaded with various concentrations of cobalt (Co) nanoparticles (between 15 and 75 % by weight) were studied. Dynamic mechanical analysis (DMA) measurements of the nano-composite membranes were conducted as a function of the applied frequency (between 0.1 and 56 Hz). With higher concentration (50-wt%) of Co nanoparticles in PDMS, the elastic modulus was increased by 3---4 times as compared with that of membranes with lower concentrations of nanoparticles. Shore hardness was maximum for the nano-composite membrane loaded with 50-wt% of Co nanoparticles. A fluidic actuator with 400 μm thick PDMS membrane of 18 mm free diameter loaded with 50-wt% Co nanoparticles was manufactured and tested under external magnetic field. In the region where the magnetic field gradient is highest, high deflection of the membrane could be obtained (0.68 mm for 1 Tesla). However some hysteresis of the membrane deflection could be observed, even at very low frequency. Loading of PDMS with Co nanoparticles allowed a wider range of control of the wetting properties of PDMS surfaces under oxygen plasma treatment, from hydrophobic to hydrophilic to super-hydrophilic. Tunability in hydrophilicity could be achieved by varying the process parameters as verified by contact angles and Fourier transforms infrared (FTIR) spectra before and after plasma treatment. Under certain conditions, 50 % Cobalt-PDMS membrane surfaces exhibited a super-hydrophilic behavior (contact angle ~5°).

In vivo visualization of single native pancreatic islets in the mouse

The purpose of this study was to investigate the potential of a novel targeted contrast agent (CA) for the in vivo visualization of single native pancreatic islets, the sites of insulin production, in the pancreas of mice using magnetic resonance imaging (MRI). The CA for intravenous administration was composed of the β-cell-specific single-chain antibody fragment, SCA B1, and ferromagnetic carbon-coated cobalt nanoparticles. MRI experiments were performed at 7, 9.4 and 16.4 T in excised organs (pancreas, liver, kidney, spleen), at 7 T in mice fixed in formalin and at 9.4 and 16.4 T in living mice. Image contrast in untreated control animals was compared with images from mice treated with unspecific and specific CA. For the validation of MRI results, selected pancreases were subjected to immunohistochemical staining and numerical contrast simulations were performed. Ex vivo results and the outcome of immunohistochemistry suggest that islets are marked only by the CA containing SCA B1. Strong accumulation of particles was found also in other investigated organs owing to the uptake by the reticuloendothelial system, but the contrast in the MR images is clearly distinguishable from the islet specific contrast in pancreases and numerical predictions. In vivo experiments based on averaged dynamic sampling with 66 × 66 × 100 µm³ and triggered acquisition with 90 × 90 × 200 µm³ nominal resolution resulted in similar particle contrast to in in vitro measurements. The newly developed CA and MRI strategies have the potential to be used for studying mouse diabetes models by visualizing single native pancreatic islets.

Synthesis and characterisation of ferromagnetic hexagonal cobalt nanoparticles

Ferromagnetic hexagonal cobalt nanoparticles are synthesised via reduction of cobalt acetate tetrahydrate by using sodium borohydride as a reducing agent. The morphology, crystal structure and magnetic properties of as synthesised particles are characterised by using transmission electron microscopy (TEM), powder X-ray diffraction (XRD) and vibratory sample magnetometer (VSM). The results show that the particles are spherical in shape with a size of 5 nm. These particles show ferromagnetic behaviour with magnetic coercivity of 583 Oe at room temperature. Oleic acid acts as the surfactant and its bonding nature on the cobalt nanoparticles is studied by Fourier transform infrared spectrometry.

Controlling length and areal density of artificial cilia through the dipolar assembly of ferromagnetic nanoparticles

Artificial cilia have been explored for use in microrobotics, MEMS, and lab-on-a-chip devices for applications ranging from micromixers, microfluidic pumps, locomotion, acoustic detection, and heat transfer. We have previously demonstrated the ability to assemble dense brushes of magnetically actuated artificial cilia through the dipolar assembly of 24 nm ferromagnetic cobalt nanoparticles. With little more than a strategically placed permanent magnet, the nanoparticles assemble at room temperature in less than 60 s. Despite areal densities exceeding 1 cilium/μm2, diameters below 25 nm, aspect ratios exceeding 400, and flexural rigidities below 3 × 10−28 N m2, these seemingly delicate structures resist collapse upon each other or the underlying substrate. The current study investigates the ability to rationally control their average length and areal density by changing the nanoparticle concentration and the dimensions of the rectangular capillary tube containing the dispersion. We find that the length and areal density obey a simple conservation of mass relationship with concentration and capillary height such that the product of the former is directly proportional to the product of the latter. Detailed statistical analysis supports a mechanism in which the role of the external field is to align pre-existing chains with the external field, assist stacking of chains along the axis of the field, and then draw them towards the ends of the permanent magnets, where the magnetic field gradient is steepest.

CCVD Synthesis of Carbon‐Encapsulated Cobalt Nanoparticles for Biomedical Applications

AbstractCarbon‐encapsulated ferromagnetic Cobalt nanoparticles (Co@C) have been synthesized by catalytic chemical vapour deposition (CCVD). The nanoparticles, mainly ranging between 10 and 15 nm, are tightly encapsulated by 2–3 concentric graphitic carbon shells and protected from oxidation. Because of their magnetic properties (saturation magnetization of 106 emu/g and a coercivity HC of 250 Oe), Co@C nanoparticles have been investigated for hyperthermia application. Although the observed values of the specific absorption rate (28.7 W/gCo@C at 30 kA/m and 215.4 W/gCo@C at 70 kA/m) are slightly lower than required in actual hyperthermia therapies, the observed strong heating effect provides a very promising starting point for future clinical application. It is also demonstrated that these nanoparticles can at the same time be used for magnetic resonance imaging (MRI) with an efficiency comparable to commercially available T2 contrast agents.

Colloidal Polymerization of Polymer-Coated Ferromagnetic Cobalt Nanoparticles into Pt-Co3O4Nanowires

In this report, functional one-dimensional (1-D) Pt-Co3O4 heterostruc-tures with enhanced electrochemical properties were synthesized via colloidal polymerization of polymer-coated ferromagnetic cobalt nanoparticles (PS-CoNPs). Colloidal polymerization of dipolar nanoparticles into hollow metal−semiconductor nanowires was achieved via a consecutive galvanic replacement reaction between Co0 and Pt2+ precursors, followed by a nanoscale Kirkendall oxidation reaction and a calcination treatment. X-ray diffraction (XRD), transmission electron microscopy (TEM), high-angle annular dark field scanning TEM (HAADF-STEM), and field-emission scanning electron microscopy (FESEM) revealed the structural and morphological evolution of the hollow cobalt oxide nanowires (D = 40 nm) with platinum nanoparticles (PtNPs; D ∼ 2 nm) entrapped within the growing oxide shell. Various calcination conditions were investigated via X-ray photoelectron spectroscopy (XPS) to obtain the optimal surface composition of the metallic Pt and...

Dipolar organization and magnetic actuation of flagella-like nanoparticle assemblies

Modeled after the design of eukaryotic protozoa, we fabricated artificial microscopic swimmers through the dipolar assembly of a bidisperse mixture of 250 nm superparamagnetic magnetite colloids and 24 nm ferromagnetic cobalt nanoparticles. The cobalt nanoparticles self-assemble into long, 1-D chains measuring approximately 24 nm × 5 µm. These chains then co-assemble with the magnetite beads to form “head” + “tail” structures. These types of asymmetric “flagella-like” colloidal assemblies were formed and maintained solely through dipolar interactions and is the first demonstration using randomly mixed dispersions of disparate magnetic colloids. When actuated by a pair of orthogonal static and sinusoidal magnetic fields, they undergo an asymmetric undulation that is the essential condition for locomotion at low Reynolds numbers. Based upon their shape, size, and articulation, these assemblies are potentially among the smallest structures capable of overcoming Brownian motion to perform useful locomotion. In addition to the head and tail structure, a variety of irregular structures formed that were incapable of swimming. A design of experiments (DOE) study was therefore implemented to optimize the production of artificial swimmers within a large parameter space that included concentration, the amount of sonication, and magnetic field strength. The artificial swimmers were most prevalent for intermediate concentrations of Co and magnetite particles. Statistical analysis suggested that the permanent dipole of the Co nanoparticles stimulated the assembly of the bidisperse mixture into complex, heterogeneous structures. Demonstration of in situ imaging of the magnetic actuation of these dipolar NP assemblies was conducted by optical microscopy.

Synthesis of ferromagnetic polymer coated nanoparticles on multi-gram scale with tunable particle size

The synthesis of ferromagnetic cobalt nanoparticles with polymeric ligands with tunable particle size is reported on multi-gram scales. End-functional polystyrene ligands were prepared via atom transfer radical polymerization. Mechanistic studies on this system were conducted as well as direct comparisons of polymer vs. small molecule ligand effects on nanoparticle formation.