Novel synthesis of highly ordered mesoporous Fe2O3/SiO2 nanocomposites for a room temperature VOC sensor

The controlled synthesis of mesoporous silica and metal oxide nanocomposites with a highly ordered porous structure and large specific surface area for specific applications has been an attractive topic in the field of porous materials. Herein, we introduce a novel method for the fabrication of highly ordered mesoporous structured and large specific surface area Fe₂O₃/SiO₂ nanocomposites, and consider their application in room temperature gas sensors. The mesoporous Fe₂O₃/SiO₂ nanocomposites were synthesised by a two-step method, which combines the hydrothermal growth of Fe₂O₃ nanoparticles and the microemulsion phase of Brij 56 (C₁₆EO₁₀) surfactant as templates in instantly direct-templating synthesis. This synthesis method enables the fabrication of mesoporous Fe₂O₃/SiO₂ nanocomposites without distortion of the ordered porous structure after calcination at high temperature. The synthesised materials were found to be efficient in a room temperature VOC sensor application, with good recovery.     

Engineering the Internal Structure of Magnetic Silica Nanoparticles by Thermal Control

Calcination of hydrated iron salts in the pores of both spherical and rod‐shaped mesoporous silica nanoparticles (NPs) changes the internal structure from an ordered 2D hexagonal structure into a smaller number of large voids in the particles with sizes ranging from large hollow cores down to ten nanometer voids. The voids only form when the heating rate is rapid at a rate of 30 °C min^?1. The sizes of the voids are controlled reproducibly by the final calcination temperature; as the temperature is decreased the number of voids decreases as their size increases. The phase of the iron oxide NPs is α‐Fe₂O₃ when annealed at 500 °C, and Fe₃O₄ when annealed at lower temperatures. The water molecules in the hydrated iron (III) chloride precursor salts appear to play important roles by hydrolyzing Si? O? Si bonding, and the resulting silanol is mobile enough to affect the reconstruction into the framed hollow structures at high temperature. Along with hexahydrates, trivalent Fe³⁺ ions are assumed to contribute to the structure disruption of mesoporous silica by replacing tetrahedral Si⁴⁺ ions and making Fe? O? Si bonding. Volume fraction tomography images generated from transmission electron microscopy (TEM) images enable precise visualization of the structures. These results provide a controllable method of engineering the internal shapes in silica matrices containing superparamagnetic NPs.     

Photonic band gap thin films from mesoporous silica spheres acting as receptacles for species yielding added functionality

We demonstrate that mesoporous silica particles capable of acting as receptacles for functional nanoparticles may be readily synthesized and then assembled into photonic crystal thin films displaying well-defined Bragg diffraction features in the visible region. We show that it is possible to tailor not only the particle diameter, but also the pore diameters within the particles. As an example of the possible application of such particles in photonic crystal fabrication we show that it is possible to deposit Fe₂O₃ nanoparticles on the exterior of and inside the pores of hexagonal phase mesoporous silica spheres.     

The synthesis of clusters of iron oxides in mesopores of monodisperse spherical silica particles

The method of obtaining nanoclusters α-Fe₂O₃ in the pores of monodisperse spherical particles of mesoporous silica (mSiO₂) by a single impregnation of the pores with a melt of crystalline hydrate of ferric nitrate and its subsequent thermal destruction has been proposed. Fe₃O₄ nanoclusters are synthesized from α-Fe₂O₃ in the pores by reducing in thermodynamically equilibrium conditions. Then particles containing Fe₃O₄ were annealed in oxygen for the conversion of Fe₃O₄ back to α-Fe₂O₃. In the result, the particles with the structure of the core-shell mSiO₂/Fe₃O₄@mSiO₂/α-Fe₂O₃ are obtained. The composition and structure of synthesized materials as well as the field dependence of the magnetic moment on the magnetic field strength have been investigated.

     

Biosynthesis and magnetic properties of mesoporous Fe3O4 composites

Magnetic Fe₃O₄ materials with mesoporous structure are synthesized by co-precipitation method using yeast cells as a template. The X-ray diffraction (XRD) pattern indicates that the as-synthesized mesoporous hybrid Fe₃O₄ is well crystallized. The Barrett-Joyner-Halenda (BJH) models reveal the existence of mesostructure in the dried sample which has a specific surface area of 96.31 m²/g and a pore size distribution of 8-14 nm. Transmission electron microscopy (TEM) measurements confirm the wormhole-like structure of the resulting samples. The composition and chemical bonds of the Fe₃O₄/cells composites are studied by Fourier transform infrared (FT-IR) spectroscopy. Preliminary magnetic properties of the mesoporous hybrid Fe₃O₄ are characterized by a vibrating sample magnetometer (VSM). The magnetic Fe₃O₄/cells composites with mesoporous structure have potential applications in biomedical areas, such as drug delivery.     

Ultrasonic-assisted ultra-rapid synthesis of monodisperse meso-SiO2@Fe3O4 microspheres with enhanced mesoporous structure

A core–shell-type of meso-SiO₂@Fe₃O₄ microsphere was synthesized via an ultrasonic-assisted surfactant-templating process using solvothermal synthesized Fe₃O₄ as core, tetraethoxysilane (TEOS) as silica source, and cetyltrimethyl ammonium bromide (CTAB) as templates. The samples were characterized by FT-IR, XRD, TEM, N₂ adsorption–desorption technology, and vibrating sample magnetometer (VSM). The results show that as-prepared meso-SiO₂@Fe₃O₄(E) and meso-SiO₂@Fe₃O₄(C) microspheres, treated by acetone extraction and high temperature calcination, respectively, still maintain uniform core–shell structure with desirable mesoporous silica shell. Therein, the meso-SiO₂@Fe₃O₄(E) microspheres possess a distinct pore size distribution in 1.8–3.0 nm with large specific surface area (468.6 m²/g) and pore volume (0.35 cm³/g). Noteworthily, the coating period of this ultrasonic-assisted method (40 min) is much shorter than that of the conventional method (12–24 h). The morphology of microspheres and the mesoporous structure of silica shell are significantly influenced by initial concentration of CTAB (C_CTAB), ultrasonic irradiation power (P) and ultrasonic irradiation time (t). The acceleration roles of ultrasonic irradiation take effect during the whole coating process of mesoporous silica shell, including hydrolysis-condensation process of TEOS, co-assembly of hydrolyzed precursors and CTAB, and deposition of silica oligomers. In addition, the use of ultrasonic irradiation is favorable for improving the homogeneity of silica shell and the monodispersity of meso-SiO₂@Fe₃O₄ microspheres.     

Multifunctional core–shell nanoparticles: superparamagnetic,mesoporous, and thermosensitive

Multifunctional core–shell composite nanoparticles (NPs) have been developed by the combination of three functionalities into one entity, which is composed of a single Fe₃O₄ NP as the magnetic core, mesoporous silica (mSiO₂) with cavities as the sandwiched layer, and thermosensitive poly(N-isopropylacrylamide-co-acrylamide) (P(NIPAAm-co-AAm)) copolymer as the outer shell. The mSiO₂-coated Fe₃O₄ NPs (Fe₃O₄@mSiO₂) are monodisperse and the particle sizes were varied from 25 to 95 nm by precisely controlling the thickness of mSiO₂-coating layer. The P(NIPAAm-co-AAm) were then grown onto surface-initiator-modified Fe₃O₄@mSiO₂ NPs through free radical polymerization. These core–shell composite NPs (designated as Fe₃O₄@mSiO₂@P(NIPAAm-co-AAm)) were found to be superparamagnetic with high r ₂ relaxivity. To manipulate the phase transition behavior of these thermosensitive polymer-coated NPs for future in vivo applications, the characteristic lower critical solution temperature (LCST) was subtly tuned by adjusting the composition of the monomers to be around the human body temperature (i.e. 37 °C), from ca. 34 to ca. 42 °C. The thermal response of the core–shell composite NPs to the external magnetic field was also demonstrated. Owing to their multiple functionality characteristics, these porous superparamagnetic and thermosensitive NPs may prove valuable for simultaneous magnetic resonance imaging (MRI), temperature-controlled drug release, and temperature-programed magnetic targeting and separation applications.     

Fabrication and application of highly ordered mesoporous Co3O4, NiO, and their metals

Highly ordered mesoporous Co₃O₄, NiO, and their metals were synthesized by nanocasting method using there corresponding mesoporous SBA-15 silica as a template. The obtained porous metal oxides have high surface areas, large pore volume, and a narrow pore size distribution. The N₂-adsorption data for mesoporous metal oxides have provided the BET area of 257.7 m² g⁻¹ and the total pore volume of 0.46 cm³ g⁻¹. The mesoporous metals were employed as a catalyst in the synthesis of (S)-3-pyrrolidinol from chiral (S)-4-chloro-3-hydroxybutyronitrile, and a high yield to (S)-3-pyrrolidinol-salt was obtained on the mesoporous Co metal catalyst.     

Preparation and characterization of silica-coated Fe3O4 nanoparticles used as precursor of ferrofluids

Fe₃O₄ magnetic nanoparticles (MNPs) were synthesized by the co-precipitation of Fe³⁺ and Fe²⁺ with ammonium hydroxide. The sodium citrate-modified Fe₃O₄ MNPs were prepared under Ar protection and were characterized by Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and vibrating sample magnetometer (VSM). To improve the oxidation resistance of Fe₃O₄ MNPs, a silica layer was coated onto the modified and unmodified MNPs by the hydrolysis of tetraethoxysilane (TEOS) at 50 °C and pH 9. Afterwards, the silica-coated Fe₃O₄ core/shell MNPs were modified by oleic acid (OA) and were tested by IR and VSM. IR results revealed that the OA was successfully grafted onto the silica shell. The Fe₃O₄/SiO₂ core/shell MNPs modified by OA were used to prepare water-based ferrofluids (FFs) using PEG as the second layer of surfactants. The properties of FFs were characterized using a UV-vis spectrophotometer, a Gouy magnetic balance, a laser particle size analyzer and a Brookfield LVDV-III+ rheometer.     

A novel approach to fabrication of superparamagnetite hollow silica/magnetic composite spheres

We described a method for synthesizing hollow silica/magnetic composite spheres using sulfonic acid functionalized hollow silica spheres (SAFHSS) as templates. The Fe₃O₄ nanoparticles were deposited on or imbedded in the hollow silica shell by a precipitation reaction. The morphologies, composition and properties of the hollow composite spheres were characterized by transmission electron microscopy, Fourier transform infrared analysis, X-ray diffraction measurement and vibrating-sample magnetometry measurement. The results indicated crystal sizes and amount of the Fe₃O₄ nanoparticles on the SAFHSS. The magnetic properties of the hollow composite spheres were controlled by adjusting the proportion between Fe²⁺ and Fe³⁺ and iron ion total concentration. When appropriate loading species were added into the system, superparamagnetite hollow composite spheres were obtained. The method also could be applicable to prepare other superparamagnetite hollow silica/ferrite composite spheres.     

The Study of Aggregation Processes in Colloidal Solutions of Magnetite–Silica Nanoparticles by NMR Relaxometry,AFM, and UV–Vis-Spectroscopy

Colloidal solutions of magnetic nanoparticles were studied as a promising magnetic resonance imaging (MRI) contrast agent. The problem of aggregative stability of solutions is considered. Sol-gel synthesis of magnetite colloidal solutions stabilized by silica is described. Transmittance spectra were measured to analyze sedimentation of nanoparticles in magnetite–silica solutions of different compositions and concentrations. It is shown that the synthesized nanoparticles can be used as MRI contrast agents. The surface morphology and particle size of Fe₃O₄/SiO₂ layers were estimated by atomic force mictroscopy (AFM) technique. The mechanism of magnetic-field-induced aggregation of Fe₃O₄/SiO₂ nanoparticles into chain-like and fractal structures is described.     

Magnetic nanocomposite spinel and FeCo core-shell and mesoporous systems

The fabrication of condensed silica and mesoporous silica coated spinel CoFe₂O₄ and FeCo alloy magnetic nanocomposites are reported. The encapsulation of well-defined 5 nm thick uniform silica layer on CoFe₂O₄ magnetic nanoparticles was performed. The formation of mesopores in the shell was a consequence of removal of organic group of the precursor through annealing. The NiO nanoparticles were loaded into the mesoporous silica. The mesoporous silica shells leads to a larger coercivity than that of pure CoFe₂O₄ magnetic nanoparticles due to the decrease of interparticle interactions and magneto-elastic anisotropy. In addition, the FeCo nanoparticles were coated by condensed and mesoporous silica. The condensed silica can protect the reactive FeCo alloy from oxidation up to 300 °C. However, saturation magnetization of FeCo nanoparticles coated by silica after 400 °C annealing is dramatically decreased due to the oxidation of the FeCo core. The mesoporous silica coated magnetic nanostructure loaded with NiO as a final product could be used in the field of biomedical applications.     

New Synthesis of Ferrite–Silica Nanocomposites by a Sol–Gel Auto-Combustion

A sol–gel autocombustion method was used to synthesize nanometric metal-oxide powders, and was extended for the first time to prepare ferrite–silica nanocomposites. The gels obtained by mixing suitable amounts of citric acid, metal nitrates, ammonia (pure phases) and tetraethylortosilicate (nanocomposites) were converted directly to ferrite (either γ-Fe₂O₃ or CoFe₂O₄) or ferrite–silica composites through a rapid autocombustion reaction. The combustion involves a thermally induced autocatalytic oxidation–reduction reaction between the nitrate and the citrate ions. The sample characterization by X-ray diffraction, transmission electron microscopy and N₂ physisorption measurements revealed nanosized pure phase powders and nanocomposites in which small spherical nanoparticles (mean size 3.5 and 5.0nm, respectively for the γ-Fe₂O₃and CoFe₂O₄) are homogeneously dispersed over a mesoporous silica matrix.     

One-pot synthesis of hybrid nanospheres with multistructure for selective adsorption of Hg<Superscript>2+</Superscript>

Novel complex nanospheres with core/shell structure for selective adsorption of Hg²⁺ have been prepared by a simple one-pot method. Scanning electron microscopy (SEM) and transmission electron microscope (TEM) images showed the nanospheres had perpendicularly thiol-functionalized mesoporous SiO₂ hybrid shell and Fe₃O₄@SiO₂ core (Fe₃O₄@nSiO₂@mSiO₂–SH). XRD patterns of as-synthesized nanospheres confirmed the observation of the SEM and TEM. The size of the nanospheres is about 100 nm. Based on the analysis of N₂ sorption–desorption isotherm, the surface area and pore volume of the adsorbent are 861 m²/g and 0.48 cm³/g, respectively. The saturation magnetization value for Fe₃O₄@nSiO₂@mSiO₂–SH is as high as 6.87 emu g⁻¹. The nanospheres showed more accessible active sites and high dispersibility in water, exhibited excellent performance for selective Hg²⁺ adsorption, had a stable structure, and could be recycled easily with magnet.     

NMR Relaxation and Diffusion Measurements on Iron(III)-Doped Kaolin Clay

Kaolin clay samples were mixed with various amounts of Fe₂O₃ powder. The influence of this magnetic impurity on NMR relaxation and diffusion measurements on the water in this porous material was investigated. The NMR relaxation measurements showed a nearly mono-exponential decay, leading to the conclusion that the pore size distribution of the clay samples is either narrow and/or that the pores are interconnected very well. Both the longitudinal and the transverse relaxation rate depend linearly on the concentration of the Fe₂O₃ impurity. The NMR diffusion measurements revealed that the Fe₂O₃ causes internal magnetic field gradients that largely exceed the maximum external gradient that could be applied by our NMR apparatus (0.3 T/m). Additional SQUID measurements yielded the magnetization and magnetic susceptibility of the samples at the magnetic field strength used in the NMR measurements (0.8 T). A theoretical estimate of the internal magnetic field gradients leads to the conclusion that the water in the porous clay samples cannot be described by the commonly observed motional averaging regime. Probably an intermediate or a localization regime is induced by the large internal gradients, which are estimated to be on the order of 1 to 10 T/m in the pore volume and may exceed 1000 T/m at the pore surface.     

Single-step preparation,characterization and photocatalytic mechanism of mesoporous Fe-doped sulfated titania

Mesoporous Fe-doped sulfated titania photocatalysts were prepared by one-step thermal hydrolysis of industrial titanyl sulfate and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and N₂ adsorption–desorption techniques. The effects of the m(Fe)/m(TiO₂) on the structures of the titania photocatalysts were investigated. The photocatalytic activity of the mesoporous Fe-doped sulfated titania catalysts was evaluated using the photooxidation of methylene blue in aqueous solutions under UV light irradiation. The results indicated that Fe³⁺ substitutes Ti⁴⁺ in titania lattice, which induced the formation of oxygen vacancies. The oxygen vacancies are favorable to the dissociation adsorption H₂O and formation of surface hydroxyl group. Fe³⁺ captures the photoinduced electrons or holes that are conductive to the efficient separation of the photogenerated carriers, but too many doping Fe³⁺ will promote recombination of the photogenerated carrier. Sulfur species in the form of sulfate are incorporated into the network of TiOTi and coordinated to titania in bidentate model, resulting in the strong inductive effect, large specific surface area, and mesoporous structure. All these are beneficial to improve the photocatalytic activities of the mesoporous Fe-doped sulfated titania photocatalysts.     

Superparamagnetic silica nanoparticles with immobilized metal affinity ligands for protein adsorption

Superparamagnetic silica-coated magnetite (Fe₃O₄) nanoparticles with immobilized metal affinity ligands were prepared for protein adsorption. First, magnetite nanoparticles were synthesized by co-precipitating Fe²⁺ and Fe³⁺ in an ammonia solution. Then silica was coated on the Fe₃O₄ nanoparticles using a sol–gel method to obtain magnetic silica nanoparticles. The condensation product of 3-Glycidoxypropyltrimethoxysilane (GLYMO) and iminodiacetic acid (IDA) was immobilized on them and after charged with Cu²⁺, the magnetic silica nanoparticles with immobilized Cu²⁺ were applied for the adsorption of bovine serum albumin (BSA). Scanning electron micrograph showed that the magnetic silica nanoparticles with an average size of 190 nm were well dispersed without aggregation. X-ray diffraction showed the spinel structure for the magnetite particles coated with silica. Magnetic measurement revealed the magnetic silica nanoparticles were superparamagnetic and the saturation magnetization was about 15.0 emu/g. Protein adsorption results showed that the nanoparticles had high adsorption capacity for BSA (73 mg/g) and low nonspecific adsorption. The regeneration of these nanoparticles was also studied.     

Growth and characterization of Fe3O4 nanoparticles in silica matrix

Nano-magnetic Fe₃O₄ particles coated with silica are synthesized. The study of structural and magnetic properties was carried out using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and vibrating sample magnetometer (VSM) techniques. The VSM results show that these kinds of composite particles exhibit superparamagnetic behavior with zero coercivity and remanence. The magnetic spheroid alumina carriers containing these magnetic composite particles were prepared by an internal gelation process. The SiO₂ coatings prevent the reaction between Fe₃O₄ and Al₂O₃ during the sintering process and maintain the superparamagnetic behavior of the catalyst carriers.     

Synthesis and characterization of mesoporous and hollow-mesoporous M<Subscript>x</Subscript>Fe<Subscript>3-x</Subscript>O<Subscript>4</Subscript> (M=Mg,Mn, Fe,Co, Ni,Cu, Zn) microspheres for microwave-triggered controllable drug delivery

Spinel ferrites can be used in magnetic targeting and microwave heating and can therefore be used for targeted and controllable drug delivery. We used the cetyltrimethylammonium bromide-assisted solvothermal method to synthesize a series of spinel ferrites (M_xFe_3-xO₄, M=Mg, Mn, Fe, Co, Ni, Cu, Zn) with a mesoporous or hollow-mesoporous structure suitable for direct drug loading and the particle diameters ranging from 200 to 350 nm. We investigated the effects of M²⁺ cation on the morphology and properties of these products by analyzing their transmission electron microscopy images, mesoporous properties, magnetic properties, and microwave responses. We chose hollow-mesoporous M_xFe_3-xO₄ (M=Fe, Co, Zn) nanoparticles, which had better overall properties, for the drug VP16 (etoposide) loading and microwave-controlled release. The Co_xFe_3-xO₄ and Fe₃O₄ particles trapped 61.5 and 64.8%, respectively, of the VP16, which were higher than that (60.4%) of Zn_xFe_3-xO₄. Controllable drug release by these simple magnetic nanocarriers can be achieved by microwave irradiation, and VP16-loaded Co_xFe_3-xO₄ released the most VP16 molecules (more than 50% after 1 h and 69.1% after 6 h) under microwave irradiation. Our results confirm the favorable drug loading and microwave-controlled delivery by these ferrites, and lay a theoretical foundation to promote clinical application of the targeted controllable drug delivery system.

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Graphical abstract In the present study, we prepared mesoporous or hollow-mesoporous spinel ferrites (M_xFe_3-xO₄, M=Mg, Mn, Fe, Co, Ni, Cu, Zn) by CTAB-assisted solvothermal method and solved the problem of Cu and Ni impurities in Cu_xFe_3-xO₄ and Ni_xFe_3-xO₄ products by means of magnetic separation and additional redox reactions, respectively. We investigated the effects of the M²⁺ cation on the morphology, mesoporous properties, magnetic properties, and microwave responses of these ferrites. Then, the drug loading and microwave-controlled drug release of hollow-mesoporous M_xFe_3-xO₄ (M?=?Fe, Co, Zn) nanoparticles with better overall properties were also studied. Co_xFe_3-xO₄ has the best overall performances for microwave-controlled drug release.

     

Templated synthesis of monodisperse mesoporous maghemite/silica microspheres for magnetic separation of genomic DNA

A novel method is described for the preparation of superparamagnetic mesoporous maghemite (γ-Fe₂O₃)/silica (SiO₂) composite microspheres to allow rapid magnetic separation of DNA from biological samples. With magnetite (Fe₃O₄) and silica nanoparticles as starting materials, such microspheres were synthesized by the following two consecutive steps: (1) formation of monodispersed organic/inorganic hybrid microspheres through urea-formaldedyde (UF) polymerization and (2) removal of the organic template and phase transformation of Fe₃O₄ to γ-Fe₂O₃ by calcination at elevated temperatures. The as-synthesized particles obtained by heating at temperature 300 °C feature spherical shape and uniform particle size (d_particle=1.72 μm), high saturation magnetization (M_s=17.22 emu/g), superparamagnetism (M_r/M_s=0.023), high surface area (S_BET=240 m²/g), and mesoporosity (d_pore=6.62 nm). The composite microsphere consists of interlocked amorphous SiO₂ nanoparticles, in which cubic γ-Fe₂O₃ nanocrystals are homogeneously dispersed and thermally stable against γ- to α-phase transformation at temperatures up to 600 °C. With the exposed iron oxide nanoparticles coated with a thin layer of silica shell, the magnetic microspheres were used as a solid-phase adsorbent for rapid extraction of genomic DNA from plant samples. The results show that the DNA templates isolated from pea and green pepper displayed single bands with molecular weights greater than 8 kb and A₂₆₀/A₂₈₀ values of 1.60-1.72. The PCR amplification of a fragment encoding the endogenous chloroplast ndhB gene confirmed that the DNA templates obtained were inhibitor-free and amenable to sensitive amplification-based DNA technologies.