Silica Coating on Colloidal Maghemite Particles

Core-Shell Magnetic Particles: Tailored Synthesis and Applications

Core-shell magnetic particles consisting of magnetic core and functional shells have aroused widespread attention in multidisciplinary fields spanning chemistry, materials science, physics, biomedicine, and bioengineering due to their distinctive magnetic properties, tunable interface features, and elaborately designed compositions. In recent decades, various surface engineering strategies have been developed to endow them desired properties (e.g., surface hydrophilicity, roughness, acidity, target recognition) for efficient applications in catalysis, optical modulation, environmental remediation, biomedicine, etc. Moreover, precise control over the shell structure features like thickness, porosity, crystallinity and compositions including metal oxides, carbon, silica, polymers, and metal-organic frameworks (MOFs) has been developed as the major method to exploit new functional materials. In this review, we highlight the synthesis methods, regulating strategies, interface engineering, and applications of core-shell magnetic particles over the past half-century. The fundamental methodologies for controllable synthesis of core-shell magnetic materials with diverse organic, inorganic, or hybrid compositions, surface morphology, and interface property are thoroughly elucidated and summarized. In addition, the influences of the synthesis conditions on the physicochemical properties (e.g., dispersibility, stability, stimulus-responsiveness, and surface functionality) are also discussed to provide constructive insight and guidelines for designing core-shell magnetic particles in specific applications. The brand-new concept of "core-shell assembly chemistry" holds great application potential in bioimaging, diagnosis, micro/nanorobots, and smart catalysis. Finally, the remaining challenges, future research directions and new applications for the core-shell magnetic particles are predicted and proposed.

From Low to High Saturation Magnetization in Magnetite Nanoparticles: The Crucial Role of the Molar Ratios Between the Chemicals

In this study, a comprehensive characterization of iron oxide nanoparticles synthesized by using a simple one-pot thermal decomposition route is presented. In order to obtain monodisperse magnetite nanoparticles with high saturation magnetization, close to the bulk material, the molar ratios between the starting materials (solvents, reducing agents, and surfactants) were varied. Two out of nine conditions investigated in this study resulted in monodisperse iron oxide nanoparticles with high saturation magnetization (90 and 93% of bulk magnetite). The X-ray diffraction analyses along with the inspection of the lattice structure through transmission electron micrographs revealed that the main cause of the reduced magnetization in the other seven samples is likely due to the presence of distortion and microstrain in the particles. Although the thermogravimetric analysis, Raman and Fourier transform infrared spectroscopies confirmed the presence of covalently bonded oleic acid on the surface of all the samples, the particles with higher polydispersity and the lowest surface coating molecules showed the lowest saturation magnetization. Based on the observed results, it could be speculated that the changes in the kinetics of the reactions, induced by varying the molar ratio of the starting chemicals, can lead to the production of the particles with higher polydispersity and/or lattice deformation in their crystal structures. Finally, it was concluded that the experimental conditions for obtaining high-quality iron oxide nanoparticles, particularly the molar ratios and the heating profile, should not be chosen independently; for any specific molar ratio, there may exist a specific heating profile or vice versa. Because this synthetic consideration has rarely been reported in the literature, our results can give insights into the design of iron oxide nanoparticles with high saturation magnetization for different applications.

Smart magnetic markers use in hydraulic fracturing

One of the main challenges and unknowns during shale gas exploration is to assess the range and efficiency of hydraulic fracturing. It is also essential to assess the distribution of proppant, which keeps the fracture pathways open. Solving these problems may considerably increase the efficiency of the shale gas extraction. Because of that, the idea of smart magnetic marker, which can be detected when added to fracturing fluid, has been considered for a long time. This study provides overview of the possibilities of magnetic marker application for shale gas extraction. The imaging methods using electromagnetic markers, are considered or developed in two directions. The first possibility is the markers' electromagnetic activity throughout the whole volume of the fracturing fluid. Thus, it can be assumed that the whole fracturing fluid is the marker. Among these type of hydraulic fracturing solutions, ferrofluid could be considered. The second possibility is marker, which is just one of many components of the fracturing fluid. In this case feedstock magnetic materials, ferrites and nanomaterials could be considered. Magnetic properties of magnetite could be too low and ferrofluids' or nanomaterials' price is unacceptably high. Because of that, ferrites, especially ZnMn ferrites seems to be the best material for magnetic marker. Because of the numerous applications in electronics, it is cheap and easily available, although the price is higher, then that of magnetite. The disadvantage of using ferrite, could be too small mechanical strength. It creates an essential need for combining magnetic marker with proppant into magnetic-ceramic composite.

Synergistic effect of magnetite and gold nanoparticles onto the response of a label-free impedimetric hepatitis B virus DNA biosensor

A magnetite and gold nanoparticle modified carbon paste electrode (CPE) was prepared for the immobilization of a thiol modified Hepatitis B virus (HBV) probe DNA and determination trace amount of target HBV DNA. Indeed, the sensing platform integrated two nanoparticles that had previously been employed individually in the DNA biosensors. The proposed DNA biosensor could measure target HBV DNA virus concentration with a low detection limit of 3.1 (±0.1)×10-(13)M, which was greatly lower than the detection limit reported with gold or magnetite nanoparticles alone. The change of interfacial charge transfer resistance (RCT) was confirmed the hybrid formation between probe and target HBV DNA. The RCT difference (before and after hybridization with the target HBV DNA) was in a linear relationship with the logarithm of complementary oligonucleotide concentrations in the range of 8.3 (±0.1)×10(-13) to 6.4 (±0.2)×10(-7)M. In addition, the novel methodology for specific DNA sequence detection was highly selective, repeatable, and reproducible. Finally, this work was successfully utilized for the sensitive and label free impedimetric determination of HBV target DNA in the urine and blood plasma samples.

Highly shape-selective synthesis, silica coating, self-assembly, and magnetic hydrogen sensing of hematite nanoparticles

The open forced hydrolysis method and controllable silica growth based on bound water to polyvinylpyrrolidone molecules have been developed for the highly shape (including rhombohedra, semispheres, and rods) selective synthesis, self-assembly, and uniform silica coating (in the unprecedented range of 5-200 nm) of hematite nanoparticles. The open system realizes the direct short-range self-assembly of hematite semispheres in their growth process. The bound water method has been extended to coat gold nanoparticles with tunable silica shell and directly assemble the cores into one-dimensional, dimer, and trimer nanostructures during the coating process. The silica coating increases the particle stability and monodispersity even as hematite is modified into ferromagnetic Fe(3)O(4). The hematite@silica core-shell spheres are assembled into long-range ordered structures with considerable photonic bandgap for the first time due to their high monodispersity. By exploiting the hematite antiferromagnetism caused by the superexchange interaction via intervening oxygen ions that are sensitive to hydrogen, a novel hydrogen sensing based on magnetization variations is achieved in the hematite assemblies. Weakening the antiferromagnetism by reducing the hematite size and/or covering the hematite surface by silica coating suppresses the sensitivity to hydrogen, showing that the antiferromagnetic spin variations on the hematite surface are responsible for the gas sensing.

Synthesis and cytotoxicity assessment of superparamagnetic iron-gold core-shell nanoparticles coated with polyglycerol

Core-shell iron-gold (Fe@Au) nanoparticles were synthesized by a facile reverse micelle procedure and the effect of water to surfactant molar ratio (w) on the size, size distribution and magnetic properties of the nanoparticles was studied. MTT assay was utilized to evaluate the cell toxicity of the nanoparticles. To functionalize the particles for MRI imaging and targeted drug delivery, the particles were coated by polyglycerol through capping with thiol followed by polymerization of glycidol. The characteristics of the particles were examined by X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), UV-visible spectroscopy, and Fourier transform infrared spectroscopy (FT-IR). It was found that the size and size distribution of the nanoparticles increase by increasing the water to surfactant molar ratio (w). The particles were spherical in shape with a thin layer of gold. Complementary growth of the gold shell on the iron core was noticed. Meanwhile, two types of agglomeration including magnetic beads and magnetic colloidal nanocrystals clusters were observed dependent on the w-value. The magnetic measurement studies revealed the superparamagnetic behavior of the nanoparticles. MTT assay result indicated the synthesized nanoparticles are nontoxic that will be useful for biomedical applications.

Synthesis of CeO2@SiO2 core–shell nanoparticles by water-in-oil microemulsion. Preparation of functional thin film

Synthesis of nanoparticles under restricted environment offered by water-in-oil (W/O) microemulsions provides excellent control over particle size and shape, and inter-particle spacing. Such an environment has been involved to synthesize silica nanoparticles with a CeO2 core, so-called CeO2@SiO2. Aqueous fluids made up of ceria nanoparticles with a size close to 5 nm have been used as the water phase component. The starting CeO2 sols and obtained CeO2@SiO2 nanoparticles have been characterized by dynamic light scattering (DLS), X-ray diffraction, scanning and transmission electron microscopy, and specific surface area measurements. The microemulsion process has been characterized by DLS. Preliminary results on CeO2@SiO2 thin films are presented.

Preparation of silica encapsulated quantum dot encoded beads for multiplex assay and its properties

Novel -COOH modified polystyrene beads were prepared by sulfonation grafting, and the surface area and pore volume are greatly improved in comparison with the swelling-treated beads. The optimization coating time is 4 h, and the corresponding -COOH content is approximately 2.1 mmol/g. The scanning electron microscope results show that the silica particles deposited on the beads and formed a silica shell that decreases the leakage of quantum dots (QDs) preferably and improves the bar code stability greatly. The anti-photobleaching of silica-coated beads was studied systemically, and the results show that the half-decay time (t1/2) of the coated beads increases to 537 s--seven times longer than that of the uncoated ones. Further DNA probe hybridization experiments indicated that the coding signal and target signal can be detected simultaneously and that the assays based on these probe-conjugated silica/QD/polystyrene beads have good specificity and sensitivity that can detect a concentration as low as 0.01 microg/ml target DNA in denatured calf thymus DNA solution, indicating that it is feasible to use this kind of bead for multiplex analysis.

Surface modification of magnetic nanoparticles capped by oleic acids: characterization and colloidal stability in polar solvents

The lyophobic surface of monodisperse magnetic nanoparticles capped by oleic acid was made to be more lyophilic by ozonolysis to increase the stability of the suspension in polar solvents like ethanol. The ozone oxidatively cleaved the double bond of oleic acid to form carbonyl and carboxyl groups on the surface of the nanoparticles. Additionally, interfacial ligand exchange of the capping molecules was applied to make the hydrophobic particle surface more hydrophilic. The magnetic particles showed enhanced miscibility and short-term stability in water after interfacial ligand exchange. The structure changes of the capping molecules on the nanoparticle surfaces were investigated using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). From these spectroscopy studies, the cleavage of the oleic acid and the formations of the carboxyl and carbonyl groups on the particle surface were confirmed. The shape and the magnetic properties of the nanoparticles were maintained after the surface modification. Ozonolysis is an effective method in modifying the lyophobic surface of the magnetic nanoparticles.

Preparation of agglomeration-free hematite particles coated with silica and their reduction behavior in hydrogen

To prepare silica-coated hematite particles without agglomeration, the effects of solid fraction, ion content in solution, and designed layer thickness on agglomeration and dispersion behavior after silica coating were examined. Since the ion concentration remained high in suspension after the hematite particles were prepared, these particles formed aggregates by the compression of an electric double layer on the hematite and silica layer produced a solid bridge between primary hematite particles. Silica bridge formation and agglomeration were almost completely prevented by decreasing the ion concentration and solid fraction of the hematite particles. Furthermore, the effects of the silica-layer thickness and structure on the reduction of hematite to iron under hydrogen gas flow and the iron core stability under air were discussed. When the solid fraction was low in suspension to prevent agglomeration during coating, a densely packed structure of nanoparticles formed by heterogeneous nucleation was observed on the silica-layer surface. Since this structure could not completely prevent oxide diffusion, the layer thickness was increased to 40 nm to obtain a stable iron core under air. Although a dense uniform layer was produced at a high solid fraction during coating, its thickness was reduced to 20 nm to completely reduce hematite to iron.

Use of water-dispersible Fe(2)O(3) nanoparticles with narrow size distributions in isolating avidin

Synthetic approaches that vigorously control the microstructures of water-dispersible gamma-Fe(2)O(3) nanoparticles such as size and size uniformity are of importance to the potential biological applications of these nanomaterials. In the present paper, water-dispersible gamma-Fe(2)O(3) nanocrystals with narrow size distributions (bipy-Fe(2)O(3)) were prepared via a site-exchange reaction. These particular materials are superparamagnetic and stable within a wide range of pH. Introduction of the biotin functionality onto the surfaces of bipy-Fe(2)O(3) enabled the affinity isolation of the protein avidin from its incubation solution magnetically with 96% efficiency.

Synthesis of alkyl sulfonate/alcohol-protected gamma-Fe(2)O(3) nanocrystals with narrow size distributions

Highly crystalline gamma-Fe(2)O(3) nanoparticles with narrow size distributions that are coated with 1-undecanesulfonic acid were synthesized via two distinct approaches using oxidation and site-exchange reactions. However, similar nanocrystals protected with 1-octanol could only be achieved via the site-exchange method, while the oxidation approach led to Fe(2)O(3) nanoparticles of poor crystallinity and size uniformity. Our magnetization measurements confirmed the superparamagnetic nature of our Fe(2)O(3) nanoparticle products and the effects of the coating materials on magnetization properties.