Preparation of a biocompatible magnetic film from an aqueous ferrofluid

Very promising nanoparticles for biomedical applications or in medical drug targeting are superparamagnetic nanoparticles based on a core consisting of iron oxides (SPION) that can be targeted through external magnets. Polyvinyl alcohol (PVA) is a unique synthetic biocompatible polymer that can be chemically cross-linked to form a gel. Biotechnology applications of magnetic gels include biosensors, targeted drug delivery, artificial muscles and magnetic buckles. These gels are produced by incorporating magnetic materials in the polymer composites. In this paper we report the synthesis of an aqueous ferrofluid and the preparation of a biocompatible magnetic gel with polyvinyl alcohol and glutharaldehyde (GTA). HClO₄ was used to induce the peptization since this kind of ferrofluid does not have surfactant. The magnetic gel was dried to generate a biocompatible film.     

Synthesis of core–shell nanoparticles with a Pt nanoparticle core and a silica shell

A method to prepare a core–shell structure consisting of a Pt metal core coated with a silica shell (Pt(in)SiO₂) is described herein. A silica shell was grown on poly(vinylpyrrolidone) (PVP)-stabilized Pt nanoparticles 2–3 nm in size through hydrolysis and condensation reactions of tetraethyl orthosilicate (TEOS) in a water/ethanol mixture with ammonia as a catalyst. This process requires precise control of the reaction conditions to avoid the formation of silica particles containing multiple Pt cores and core-free silica. The length of PVP molecules, water content, concentration of ammonia and Pt nanoparticles in solution were found to significantly influence the core–shell structure. By optimizing these parameters, it was possible to prepare core–shell particles each containing a single Pt nanoparticle with a silica layer coating approximately 10 nm thick.     

Preparation of Photoluminescent Porous Silicon Nanoparticles by High‐Pressure Microfluidization

The use of high‐shear microfluidization as a rapid, reproducible, and high‐yield method to prepare nanoparticles of porous silicon (pSi) with a narrow size distribution is described. Porous films prepared by electrochemical etch of a single‐crystal silicon wafer are removed from the substrate, fragmented, dispersed in an aqueous solution, and then processed with a microfluidizer, which generates high yields (57%) of pSi nanoparticles of narrow size distribution (PDI = 0.263) without a filtration step. Preparation of pSi nanoparticles via microfluidization improves yields (by 2.4‐fold) and particle size uniformity (by 1.8‐fold), and it lowers the total processing time (by 36‐fold) over standard ultrasonication or ball milling methods. The average diameter of the nanoparticles can be adjusted over the range 150–350 nm by appropriate adjustment of processing steps. If the fluid carrier in the microfluidizer contains an oxidant for Si, the resulting pSi particles are prepared with a core–shell structure, in which an elemental Si core is encased in a silicon oxide shell. When an aqueous sodium tetraborate processing solution is used, microfluidization generates photoluminescent core–shell pSi particles with a quantum yield of 19% in a single step in less than 20 min.     

Magnetic behavior of iron oxide nanoparticle–biomolecule assembly

Iron oxide nanoparticles of 8–20 nm in size were investigated as an assembly with biomolecules synthesized in an aqueous solution. The magnetic behavior of the biomolecule–nanoparticles assembly depends sensitively on the morphology and hence the distribution of the nanoparticles, where the dipole coupling between the nanoparticles governs the overall magnetic behavior. In assemblies of iron oxide nanoparticles with trypsin, we observe a formation of unusual self-alignment of nanoparticles within trypsin molecules. In such an assembly structure, the magnetic particles tend to exhibit a lower spin-glass transition temperature than as-synthesized bare iron oxide nanoparticles probably due to reduced interparticle couplings within the molecular matrix. The observed self-alignment of nanoparticles in biomolecules may be a useful approach for directed nanoparticles assembly.     

Synthesis and characterization of covalent diphenylalanine nanotube-folic acid conjugates

Traditionally, organosilica nanoparticles have been prepared inside micelles with an external silica shell for mechanical support. Here, we compare these hybrid core–shell particles with organosilica particles that are robust enough to be produced both inside micelles and alone in a sol–gel process. These particles form from octadecyltrimethoxy silane as silica source either in microemulsions, resulting in water-dispersible particles with a hydrophobic core, or precipitate from an aqueous mixture to form particles with both hydrophobic core and surface. We examine size and morphology of the particles by dynamic light scattering and transmission electron microscopy and show that the particles consist of Si–O–Si networks pervaded by alkyl chains using nuclear magnetic resonance, infrared spectroscopy, and thermogravimetric analysis.     

In situ preparation of high relaxivity iron oxide nanoparticles by coating with chitosan: A potential MRI contrast agent useful for cell tracking

Iron oxide nanocrystals are of considerable interest in nanoscience and nanotechnology because of their nanoscale dimensions, nontoxic nature, and superior magnetic properties. Colloidal solutions of magnetic nanoparticles (ferrofluids) with a high magnetite content are highly desirable for most molecular imaging applications. In this paper, we present a method for in situ coating of superparamagnetic iron oxide (SPIO) with chitosan in order to increase the content of magnetite. Iron chloride salts (Fe³⁺ and Fe²⁺) were directly coprecipitated inside a porous matrix of chitosan by Co-60 γ-ray irradiation in an aqueous solution of acetic acid. Following sonication, iron oxide nanoparticles were formed inside the chitosan matrix at a pH value of 9.5 and a temperature of 50 °C. The [Fe³⁺]:[Fe²⁺]:[NH₄OH] molar ratio was 1.6:1:15.8. The final ferrofluid was formed with a pH adjustment to approximately 2.0/3.0, alongside with the addition of mannitol and lactic acid. We subsequently characterized the particle size, the zeta potential, the iron concentration, the magnetic contrast, and the cellular uptake of our ferrofluid. Results showed a z-average diameter of 87.2 nm, a polydispersity index (PDI) of 0.251, a zeta potential of 47.9 mV, and an iron concentration of 10.4 mg Fe/mL. The MRI parameters included an R1 value of 22.0 mM⁻¹ s⁻¹, an R2 value of 202.6 mM⁻¹ s⁻¹, and a R2/R1 ratio of 9.2. An uptake of the ferrofluid by mouse macrophages was observed. Altogether, our data show that Co-60 γ-ray radiation on solid chitosan may improve chitosan coating of iron oxide nanoparticles and tackle its aqueous solubility at pH 7. Additionally, our methodology allowed to obtain a ferrofluid with a higher content of magnetite and a fairly unimodal distribution of monodisperse clusters. Finally, MRI and cell experiments demonstrated the potential usefulness of this product as a potential MRI contrast agent that might be used for cell tracking.     

Nanostructures of gold coated iron core-shell nanoparticles and the nanobands assembled under magnetic field

Pure metal iron nanoparticles are unstable in the air. By a coating iron on nanoparticle surface with a stable noble metal, these air-stable nanoparticles are protected from the oxidation and retain most of the favorable magnetic properties, which possess the potential application in high density memory device by forming self-assembling nanoarrays. Gold-coated iron core-shell structure nanoparticles (Fe/Au) synthesized using reverse micelles were characterized by transmission electron microscopy (TEM). The average nanoparticle size of the core-shell structure is about 8 nm, with about 6 nm diameter core and 1∼2 nm shell. Since the gold shell is not epitaxial growth related to the iron core, the morié pattern can be seen from the overlapping of iron core and gold shell. However, the gold shell lattice can be seen by changing the defocus of TEM. An energy dispersive X-ray spectrum (EDS) also shows the nanoparticles are air-stable. The magnetic measurement of the nanoparticles also proved successful synthesis of gold coated iron core-shell structure. The nanoparticles were then assembled under 0.5 T magnetic field and formed parallel nanobands with about 10 μm long. Assembling two dimensional ordered nanoarrays are still under going. Received 29 November 2000     

Experimental investigation for enhanced ferrofluid heat transfer under magnetic field effect

This paper reports an experimental work on the convective heat transfer of ferrofluid flowing through a heated copper tube in the laminar regime in the presence of magnetic field. Significant enhancement on the heat transfer of ferrofluid by applying various orders of magnetic field is observed in this experiment. Also in this experiment, the effect of magnetic nanoparticles concentrations and magnet position have been investigated. The main reason for the enhancement of heat transfer coefficient could be caused due to remarkable changes in thermophysical properties of ferrofluid under the influence of applied magnetic field.     

Magnetic Nanoparticle Thermometry via Controlled Diffusion

The process of magnetic nanoparticle heating releases enormous amounts of thermal energy. Through typical calorimetric analyses, the total thermal energy released can be easily quantified; however, knowledge of nanoscale temperature is necessary. Herein, a novel method of nanoscale thermometry by analyzing intra-particle diffusion in core–shell nanoparticles is proposed. Heating the iron cores with an alternating magnetic field in a saline suspension encourages the diffusion of sodium ions into the silica shells of the particles, which is modeled numerically; however, experimental measurements are needed in order to provide accurate diffusivity estimations. After determining the diffusion characteristics from X-ray photoelectron spectroscopy) depth profiling of silica films, energy dispersive analysis with high-resolution transmission electron microscopy measures the sodium ion gradient within single particles before and after heating. When compared directly to the numerical simulations, the results indicate that the temperature gradient between particles and saline suspension reaches significantly higher temperatures than the macro-scale temperature of the solution. By accurately knowing the thermal gradient between nanoparticles and the surrounding medium, nanoparticles can be engineered to limit surface resistances as much as possible and promote high rates of thermal energy transfer.     

Comparative analysis of the 1H NMR relaxation enhancement produced by iron oxide and core-shell iron-iron oxide nanoparticles

Physicochemical and magnetorelaxometric characterization of the colloidal suspensions consisting of Fe-based nanoparticles coated with dextran have been carried out. Iron oxide and iron core/iron oxide shell nanoparticles were obtained by laser-induced pyrolysis of Fe(CO)₅ vapours. Under different magnetic field strengths, the colloidal suspension formed by iron oxide nanoparticles showed longitudinal (R₁) and transverse (R₂) nuclear magnetic relaxation suspension (NMRD) profiles, similar to those previously reported for other commercial magnetic resonance imaging (MRI) contrast agents. However, colloidal suspension formed by ferromagnetic iron-core nanoparticles showed a strong increase of the R₁ values at low applied magnetic fields and a strong increase of the R₂ measured at high applied magnetic field. This behaviour was explained considering the larger magnetic aggregate size and saturation magnetization values measured for this sample, 92 nm and 31 emu/g Fe, respectively, with respect to those measured for the colloidal suspensions of iron oxide nanoparticles (61 nm and 23 emu/g Fe). This suspension can be used both as T₁ and T₂ contrast agent.     

Design and synthesis of plasmonic magnetic nanoparticles

Core–shell nanoparticles containing both iron oxide and gold are proposed for bioseparation applications. The surface plasmon resonance of gold makes it possible to track the positions of individual particles, even when they are smaller than the optical diffraction limit. The synthesis of water-dispersible iron oxide-gold nanoparticles is described. Absorption spectra show the plasmon peaks for Au shells on silica particles, suggesting that thin shells may be sufficient to impart a strong surface plasmon resonance to iron oxide-gold nanoparticles. Dark field optical microscopy illustrates the feasibility of single-particle detection. Calculations of magnetophoretic and drag forces for particles of different sizes reveal design requirements for effective separation of these small particles.     

Colloidal Flower‐Shaped Iron Oxide Nanoparticles: Synthesis Strategies and Coatings

The assembly of magnetic cores into regular structures may notably influence the properties displayed by a magnetic colloid. Here, key synthesis parameters driving the self‐assembly process capable of organizing colloidal magnetic cores into highly regular and reproducible multi‐core nanoparticles are determined. In addition, a self‐consistent picture that explains the collective magnetic properties exhibited by these complex assemblies is achieved through structural, colloidal, and magnetic means. For this purpose, different strategies to obtain flower‐shaped iron oxide assemblies in the size range 25–100 nm are examined. The routes are based on the partial oxidation of Fe(OH)₂, polyol‐mediated synthesis or the reduction of iron acetylacetonate. The nanoparticles are functionalized either with dextran, citric acid, or alternatively embedded in polystyrene and their long‐term stability is assessed. The core size is measured, calculated, and modeled using both structural and magnetic means, while the Debye model and multi‐core extended model are used to study interparticle interactions. This is the first step toward standardized protocols of synthesis and characterization of flower‐shaped nanoparticles.     

Core-shell structured iron nanoparticles well dispersed on montmorillonite

Iron nanoparticles have been successfully synthesized using sodium borohydride solution reduction of ferric trichloride hexahydrate in the presence of montmorillonite as an effective protective reagent and support as well. A combination of characterizations reveals the well disperse of these obtained iron nanoparticles supported on the external surface of clay with roughly spherical morphology and mean diameter of 55 nm. The particles are oxidation resistant well with iron core-iron oxide shell structure. The shell thickness of 3 nm remains almost invariable under ambient conditions. Discernable hysteresis loop reveals ferromagnetic behavior of the iron nanoparticles, which make them easy for magnetic separation and potential in some practical applications.     

Flow Synthesis of Plasmonic Gold Nanoshells via a Microreactor

Gold nanoshells with tunable surface plasmon resonances are a promising material for optical and biomedical applications. They are produced through seed‐mediated growth, in which gold nanoparticles (AuNPs) are seeded on the core particle surface followed by growth of the gold seeds into a shell. However, synthetic gold nanoshell production is typically a multistep, time‐consuming batch‐type process, and a simple and scalable process remains a challenge. In the present study, a continuous flow process for the seed‐mediated growth of silica–gold nanoshells is established by exploiting the excellent mixing performance of a microreactor. In the AuNP‐seeding step, the reduction of gold ions in the presence of core particles in the microreactor enables the one‐step flow synthesis of gold‐decorated silica particles through heterogeneous nucleation. Flow shell growth is also realized using the microreactor by selecting an appropriate reducing agent. Because self‐nucleation in the bulk solution phase is suppressed in the microreactor system, no washing is needed after each step, thus enabling the connection of the microreactors for the seeding and shell growth steps into a sequential flow process to synthesize gold nanoshells. The established system is simple and robust, thus making it a promising technology for producing gold nanoshells in an industrial setting.     

Irradiation effect of low-energy ion on polyurethane nanocoating containing metal oxide nanoparticles

Irradiation effect of low-energy ion beam has been investigated on nanocoating developed with silica, titania and silica–titania core–shell nanoparticles embedded in an organic binder for nanopaint application. In this work, we have taken polyurethane as a model organic binder. Silica nanoparticles have been prepared through sol–gel synthesis with a particle size of 85?nm. Titania and core–shell nanoparticles have been prepared through both sol–gel and peptization process. Particle sizes obtained were 107?nm for titania and 240?nm for core–shell nanoparticles prepared through sol–gel process and 75?nm for TiO₂ and 144?nm for core–shell nanoparticles prepared through peptization process. The coating formulations were developed with the above nanoparticles individually and nanoparticle concentration was varied from 1 to 6?wt% and the best performance in terms of hydrophobicity was obtained with 4?wt % of the nanoparticles in polyurethane coating formulation. All the coating formulations prepared were applied on a glass substrate and dried at 100°C. The dry film thickness obtained was around 100?µm in each case. These films dried on glass substrate were irradiated by nitrogen and argon ion beam with energy of 26?keV at fluences of 10¹⁴ to 10¹⁶?ions/cm². The anti-algal property of the irradiated samples was improved and hydrophobicity was reduced.     

Enhancing effects of nanoparticles on polymer-OLED performances

We have previously reported that the silane coating of magnetic nanoparticles (MNPs) of maghemite phase could be used to protect iron oxide cores during plasma heat treatment, and even help to reduce their phase to magnetite with higher magnetization. In this work, an additional layer of an electrically conductive polypyrrole was added on top of the silane-coated MNPs, producing core?Cshell particles with sizes ranging from 150 to 500?nm. A microwave plasma heat treatment was used to convert the amorphous, already-conductive polypyrrole coatings into a more electrically conductive graphitic structure, while simultaneously reducing the iron oxide phase to magnetite. The treatment produced core?Cshell particles with better microwave absorption properties over the frequency of 1?C18?GHz, with a maximum reflection loss (absorption) of these MNPs at ?37?dB at 10.3?GHz for samples containing 70?wt% of plasma-treated core?Cshell nanoparticles embedded in wax. By comparison, the maximum absorption for the same amount of untreated nanoparticles was only ?18?dB at 7.5?GHz. The improved electromagnetic wave absorption properties were due to higher electrical conductivity of the more ordered, graphitic-like polypyrrole shell structures. This relatively simple protocol could thus be used to synthesize highly magnetic and conductive nanocomposites for electromagnetic interference shielding applications, particularly at the high frequency range.     

Nucleation and growth of silver nanoshells through copper vapor laser irradiation

Silica core–silver shell, silver nanoshells (NSs), have been synthesized by an innovative laser-based approach. The NSs’ nucleation and growth progressed upon the pulse strikes of a copper vapor laser on a colloidal solution containing silver and silica nanoparticles (NPs). The silver NPs were separately synthesized by ablation of a silver target in deionized water by a 1064 nm Q-switched Nd:YAG laser. The dependence of silver NSs’ growth on the laser exposure time has been systematically studied by UV–VIS absorption spectroscopy technique. Transmission electron microscopy was exploited as well to visually confirm the NSs’ evolution through the process.     

Influence of dextran coating on the magnetic behaviour of iron oxide nanoparticles

Magnetic iron oxide nanoparticles with mean diameters in the range from 10 to 30 nm were prepared by modified chemical precipitation routes. The particles were suspended in an aqueous solution by coating of the particles with carboxymethyldextran. A stability against agglomeration was achieved over a period of more than 7 days. In the present investigation, the structural and the magnetic properties of the nanoparticles were investigated. The influence of the dextran shell on the strength of the dipole–dipole interactions between the neighbouring particles was determined by investigation of the remanence behaviour (Henkel plot) of coated as well as of uncoated particles.     

Iron/iron oxides core-shell nanoparticles by laser pyrolysis: Structural characterization and enhanced particle dispersion

Well-dispersed nanoparticles with iron/iron carbide core and iron oxide shell structures may constitute an excellent magnetic material for different applications as magnetic nanofluids, contrast agents in magnetic resonance imaging, sensors and catalysts. Based on the ability of the CO₂ laser pyrolysis technique to synthesize nanoparticles of the Fe/Fe₂O₃ core-shell type, we further improve the powder dispersion by first collecting the nanoparticles in a toluene bubbler, positioned downstream and prior to the collection filter. Structural characterisation of the samples by electron microscopy and X-ray diffraction was performed. Conditions in which clusters contain a reduced number of nanoparticles (around 50) are evidenced. Mean core-shell particle sizes of 15 nm were estimated. Finally, preliminary results on the morphology of iron/iron oxide core-shell nanoparticles as hydrocarbon-based magnetic nanofluids are presented.     

Silica‐Coating of Hematite Nanoparticles Using Reactive Water‐Soluble Polyalkoxysiloxanes

In this article, we report on a new one‐step synthetic route to obtain multi‐functional silica‐coated hematite particles using a water‐based surfactant‐free technology. The synthesis and properties of uniform silica‐coated hematite particles with adjustable size, morphology, and silica shell thickness are discussed in detail. The developed method allows simultaneous formation of the silica shell around hematite core and incorporation of reactive groups on the surface of core–shell nanoparticles. Vinyl groups are introduced to the silica surface at once by pre‐functionalization of a water‐soluble hyperbranched polyalkoxysiloxanes with active double bonds. The reactivity of these surface‐immobilized vinyl groups is demonstrated by covalent attachment of rhodamine B using a thiol‐en click reaction.