Iron oxide magnetic aggregates: Aspects of synthesis, computational approaches and applications

Superparamagnetic magnetite nanoparticles have been central to numerous investigations in the past few decades for their use in many applications, such as drug delivery, medical diagnostics, magnetic separation, and material science. However, the properties of single magnetic nanoparticles are sometimes not sufficient to accomplish tasks where a strong magnetic response is required. In light of this, aggregated magnetite nanoparticles have been proposed as an alternative advanced material, which may expand and combine some of the advantages of single magnetic nanoparticles, including superparamagnetism, with an enhanced magnetic moment and increased colloidal stability. This review comprehensively discusses the current literature on aggregates made of magnetic iron oxide nanoparticles. This review is divided into three sections. First, the current synthetic strategies for magnetite nanoparticle aggregates are discussed, together with the influence of different stabilizers on the primary crystals and the final aggregate size and morphology. The second section is dedicated to computational approaches, such as density functional methods (which permit accurate predictions of electronic and magnetic properties and shed light on the behavior of surfactant molecules on iron oxide surfaces) and molecular dynamics simulations (which provide additional insight into the influence of ligands on the surface chemistry of iron oxide nanocrystals). The last section discusses current and possible future applications of iron oxide magnetic aggregates, including wastewater treatment, water purification, medical applications, and magnetic aggregates for materials displaying structural colors.

Fabrication of a Magnetic Fluorinated Covalent Organic Framework for the Selective Capture of Benzoylurea Insecticide Residue in Beverages

Efficient capture of benzoylurea insecticide (BU) residue in food is a vital procedure for food safe monitoring. Herein, a core-shell structured magnetic fluorinated covalent organic framework with good magnetic responsiveness and abundant fluorine affinity sites was successfully synthesized, suitable for magnetic solid-phase extraction (MSPE) of BUs. Using a room-temperature synthesis strategy, the magnetic fluorinated covalent organic framework was fabricated by in situ polymerization of 1,3,5-tris(4-aminophenyl) triazine (TAPT) and 2,3,5,6-tetrafluoroterephthaldehyde (TFTA) on the surface of carboxylated Fe₃O₄ nanoparticles. The competitive adsorption experiment and molecular simulation verified that this magnetic fluorinated covalent organic framework possesses favorable adsorption affinity for BUs. This magnetic fluorinated covalent organic framework could be easily regenerated and reused at least eight times with no reduction of enrichment performance. Combining this magnetic fluorinated covalent organic framework-based MSPE with high-performance liquid chromatography-tandem mass spectrometry, a novel sensitive method for the analysis of BUs was developed. In yellow wine and fruit juice samples, good linear correlations were obtained for BUs in the range of 10-2000 and 20-4000 ng·L^-1, respectively. The limit of quantitation of the BUs ranged from 1.4 to 13.3 ng·L^-1 in the two beverage matrices. Desirable precision was achieved, with intraday and interday relative standard deviations lower than 11%.

Amphiphilic copolymer self-assembly of magnetic nanoparticles for construction of magnetically responsive photonic crystals based on steric hindrance

Herein, a facile, simple and rapid self-assembly of magnetic colloidal nanoparticles (MCNPs) to build magnetically responsive photonic crystals (MRPCs) was devolved. A nonionic amphiphilic random copolymer poly(styrene-co-vinylpyrrolidone) P(St-co-VP) with the monomer molar ratio of 1 : 9 was used not only as an emulsifier for miniemulsion self-assembly of Fe₃O₄ magnetic nanoclusters, but also as the coating material on the magnetic nanoclusters through itself assembly. The self-assembly of the magnetic nanocluster and the polymer coating were completed simultaneously without another polymerization process. The characterization of the MCNPs and the optical properties of the MRPCs were investigated in details. TEM showed that the MCNPs had regular spherical structures with an average diameter of 104.6 nm (RSD = 13.9%, n = 100). P(St-co-VP) self-assembly coating was confirmed by IR and XPS, and thermogravimetric analysis showed that the magnetite content was 76.15%. The large content of magnetite and the thin coating of the copolymer gave MCNPs the high saturated magnetization (M_s) of 52.60 emu g⁻¹. Under an external magnetic field, the MCNPs could assemble MRPCs instantaneously and reversibly. The structural color covered entire visible spectrum by tuning the strength of the external magnetic field. On basis of the steric hindrance from neighboring PVP stretching chains, rather than electrostatic repulsion or solvation layer to counterbalance magnetic attraction, the MRPCs could tolerate the electrolyte as high as 0.10 mol L⁻¹ and the variance of pH from 2.0–12.0. The stability of P(St-co-VP) self-assembly coating was testified through the invariability of the structural color of MRPCs after repeated washing, as well as the recovery of structural color after removing the electrolytes.

One-step self-assembly of magnetic nanoparticles with amphiphilic copolymer for construction of magnetically responsive photonic crystals based on steric hindrance.

Synthesis of monodisperse ferromagnetic CoxFe3−xO4 colloidal particles with magnetically tunable optical properties

Monodisperse Co_xFe_3–xO₄ colloidal particles with uniform size and tunable composition have been prepared using a one-step hydrothermal method.

Magnetically Responsive Assemblies of Polymer-Brush-Decorated Nanoparticle Clusters That Exhibit Structural Color

The development of new magnetic materials for applications such as magnetic-driven drug delivery, next-generation display materials, and magnetic resonance imaging is an important objective. To that end, we synthesized monodispersed, magnetically responsive particles grafted with well-defined polymer brushes and investigated the formation of their ordered arrays in organic solvents in response to a magnetic field. To achieve this, we prepared monodispersed magnetic nanoparticle clusters (MNCs) composed of large numbers of superparamagnetic ferrite ZnFe₂O₄ nanoparticles. The MNCs were subsequently coated with thin silica layers through the hydrolysis of tetraethoxysilane. The colloidal particles were surface-modified with initiating groups for atom transfer radical polymerization (ATRP) using a triethoxysilane derivative with an ATRP initiation site. To demonstrate the ability of the synthesized particles to produce well-defined polymer brushes on their surfaces, the ATRP-initiator-functionalized silica-coated MNCs were subjected to surface-initiated ATRP with methyl methacrylate. This polymerization proceeded in a living fashion to produce graft polymers with targeted molar masses and narrow molar mass distributions. The average graft density was determined to be 0.65 chains/nm², which confirms the formation of concentrated polymer brushes on the MNCs. The hybrid particles were analyzed by dynamic light scattering and transmission electron microscopy techniques, which revealed excellent uniformity and solvent dispersibility. A suspension of the polymer-brush-decorated MNCs in acetone quickly developed intense structural color in response to approaching a magnet that depended on the strength of the magnetic field.

Autoclave-free facile approach to the synthesis of highly tunable nanocrystal clusters for magnetic responsive photonic crystals

Art work conducted in normal glass containers: an alternative facile method for state-of-the-art magnetite nanocrystal clusters have been introduced.

Achieving enhanced NIR light-induced toxicity via novel hybrid magnetic nanoparticles

Novel zinc-doped magnetic nanoparticles have been rationally designed and applied as promising candidates for phototherapies of cancer in vitro.

Surface modified multifunctional ZnFe2O4 nanoparticles for hydrophobic and hydrophilic anti-cancer drug molecule loading

By appropriate surface functionalization, multifunctional ZnFe₂O₄ nanoparticles exhibiting RT ferromagnetism and green emission has been loaded with a hydrophobic drug molecule-curcumin and a hydrophilic drug molecule-daunorubicin.

Recyclable graphene oxide-supported titanium dioxide photocatalysts with tunable properties

A modular synthesis technique was developed for producing graphene-supported titanium dioxide photocatalysts. The modular synthesis allowed for simple tuning of the ratio of particle loading on the graphene oxide (GO) surface as well as good photocatalytic activity of the composite and quick, efficient magnetic separability. GO flakes were used as a support for titanium dioxide nanoparticles and SiO2 insulated nano-sized magnetite aggregates. Different composition ratios were tested, resulting in a catalyst formulation with photocatalytic activity exceeding that of a commercial photocatalyst by a factor of 1.2 as well as excellent recyclability, with the capability to degrade 3 mg/L methylene blue in aqueous solution over 10 consecutive trials with minimal loss in photocatalytic efficiency. Recovery of the catalyst was achieved by simply exposing the nanocomposite to a magnetic field for ∼1 minute. Furthermore, it was found that the catalyst could be regenerated to its initial efficiency through simple UV treatment to provide additional re-use. To highlight the importance of the nanocomposite to the current water treatment industry, we showed rapid degradation of pharmaceutical compounds caffeine and carbamazepine within 60 min. The nanocomposite shows activity exceeding that of commercial photocatalyst P25 with the added benefit of being fully recoverable, reusable, and easy to produce. Overall, a simple technique for producing and tuning an effective magnetically recyclable nanocomposite was developed which should allow easy scalability and industrial production, a factor critical for the implementation of nano-based water treatment techniques.

Uniform iron oxide hollow spheres for high-performance delivery of insoluble anticancer drugs

As an intrinsic characteristic of many anticancer drugs, low solubility in physiological conditions limits the usage of these active ingredients in clinics. To overcome this bottleneck, we attempt to design and construct a high-performance magnetic-targeted delivery system based on uniform iron oxide hollow spheres. Via a facile one-pot solvothermal route, well-defined iron oxide hollow spheres were prepared with inexpensive inhesion. Compared with previously reported mesoporous Fe3O4 nanoparticles, our iron oxide hollow spheres have a larger void space giving the structures a higher storage capacity for guest molecules. In our present work, camptothecin (CPT) was selected as a model insoluble anticancer drug to confirm the efficiency of drug-loading and chemotherapy in vitro. Detailed anticancer efficacy was further investigated by using MTT assays and microscope imaging methods, indicating that these iron oxide hollow spheres are promising for insoluble drug delivery.

One-pot solvothermal synthesis of highly water-dispersible size-tunable functionalized magnetite nanocrystal clusters for lipase immobilization

A facile one-pot synthesis of highly water-dispersible size-tunable magnetite (Fe3O4) nanocrystal clusters (MNCs) end-functionalized with amino or carboxyl groups by a modified solvothermal reduction reaction has been developed. Dopamine and 3,4-dihydroxyhydroxycinnamic acid were used for the first time as both a surfactant and interparticle linker in a polylol process for economical and environment-friendly purposes. Morphology, chemical composition, and magnetic properties of the prepared particles were investigated by several methods, including FESEM, TEM, XRD, XPS, Raman, FTIR, TGA, zeta potential, and VSM. The sizes of the particles could be easily tuned over a wide range from 175 to 500 nm by varying the surfactant concentration. Moreover, ethylene glycol/diethylene glycol (EG/DEG) solvent mixtures with different ratios could be used as reductants to obtain the particles with smaller sizes. The XRD data demonstrated that the surfactants restrained the crystal growth of the grains. The nanoparticles showed superior magnetic properties and high colloidal stability in water. The cytotoxicity results indicated the feasibility of using the synthesized nanocrystals in biology-related fields. To estimate the applicability of the obtained MNCs in biotechnology, Candida rugosa lipase was selected for the enzyme immobilization process. The immobilized lipase exhibited excellent thermal stability and reusability in comparison with the free enzyme. This novel strategy would simplify the reaction protocol and improve the efficiency of materials functionalization, thus offering new potential applications in biotechnology and organocatalysis.

Synthesis of magnetic rattle-type nanostructures for use in water treatment

A hydrothermal technique to simultaneously remove a SiO2 template and crystallize a TiO2 outer layer was used to create magnetically separable, hollow rattle-type nanoparticles consisting of a magnetic Fe3O4 core contained within a hollow TiO2 shell. Fe3O4 cores approximately 240 nm in diameter were synthesized, subsequently coated by SiO2 and finally coated with TiO2. This was followed by a hydrothermal treatment to selectively etch the silica, resulting in rattle-type particles with a final outer shell diameter of approximately 390 nm. The product of hydrothermal treatment were rattle-type particles with increased crystallinity and a 68% increase in surface area. Characterization confirmed the ability to etch a hard SiO2 template through use of a simple and benign thermal treatment with pure water, while simultaneously introducing a crystalline phase into the TiO2 active layer. The potential of the particles to be employed as a catalyst in UV induced advanced water treatment for removal of organic contaminants was evaluated through a colorimetric photocatalytic degradation assay using methylene blue as a model contaminant. The ability of the particles to be magnetically separated from solution after treatment and recycled for consecutive treatment cycles was then demonstrated. This technique for selectively removing a hard SiO2 template while simultaneously crystallizing a TiO2 shell avoids the use of hazardous chemical etchants or complex processing, rendering the synthesis of hierarchical, multimaterial, hollow, porous rattle-type particles a simple, attractive, and environmentally friendly "one-pot" technique for potential industrial application.

Monodisperse inorganic supraparticles: formation mechanism, properties and applications

Monodisperse inorganic supraparticles (SPs) are an emerging and hot research topic in the chemistry, physics and materials science communities in the past several years. Monodisperse inorganic SPs exhibit unique physiochemical properties due to their well-defined shape and distinctive topological structure. This review summarizes recent progress in the study of formation mechanism, properties and applications of inorganic monodisperse SPs. The future developments in this research area are also discussed.

Synthesis of carbon-encapsulated superparamagnetic colloidal nanoparticles with magnetic-responsive photonic crystal property

The core/shell structure of magnetite/carbon colloidal nanoparticles (CNPs) with average size about 190 nm has been prepared via a one-step solvothermal process using ferrocene as a single reactant. The composition, phase, and morphology of the nanostructure have been characterized by X-ray diffraction, and transmission electron microscopy. Magnetic measurements reveal the superparamagnetic nature of the material with a magnetization saturation of 40.2 emu/g at room temperature. Under the induction of an external magnetic field, strong diffraction in the visible light spectrum can be observed in a suspension of CNPs in ethanol and the diffraction wavelength varies with the strength of the external magnetic field. After being stored for eight months in an ethanol solution, these CNPs can still diffract visible lights when a magnetic field was applied, which is attributed to carbon coating and creating carboxyl groups on the surface of carbon shells introducing both a steric hindrance and electrostatic repulsions between magnetite nanoparticles.