Synthesis of highly crystalline and monodisperse maghemite nanocrystallites without a size-selection process

Magnetic nanoparticles with functional silanes: evolution of well-defined shells from anhydride containing silane

Modification of iron oxide nanoparticles (NPs) synthesized by high temperature solvothermal routes is carried out using two silanes: (i) N-(6-aminohexyl)-aminopropyltrimethoxysilane (AHAPS) where only one end of the molecule reacts with the surface Fe-OH groups and (ii) 3-(triethoxysilyl)propylsuccinic anhydride (SSA) where both ends are reactive with Fe-OH. Depending on the NP synthesis protocol, the amount of surface OH groups on the NPs may differ, however, for all the cases presented here, the comparatively low OH group density prevents a high density of AHAPS coverage, yielding NP aggregates instead of single particles in aqueous solutions. Alternatively, use of SSA containing two terminal functionalities, anhydride and siloxy, which are both reactive towards the NP surface, results in the formation of discrete dense polymeric shells, providing stability of individual NPs in water. The mechanism of the SSA shell formation is discussed. The evolution of the chemical transformations leads to shells of different thickness and density, yet this evolution can be halted by hydrolysis, after which the NPs are water soluble, negatively charged and exhibit excellent stability in aqueous media.

Magnetic removal of dyes from aqueous solution using multi-walled carbon nanotubes filled with Fe2O3 particles

The Fe2O3 nanoparticles have been introduced into the multi-walled carbon nanotubes (MWCNTs) via wet chemical method. The resulting products are characterized by TEM, EDX, XRD and VSM. The magnetic MWCNTs have been employed as adsorbent for the magnetic separation of dye contaminants from water. The adsorption test of dyes (Methylene Blue and Neutral Red) demonstrates that it only takes 60min to attain equilibrium and the adsorption capacities for Methylene Blue and Neutral Red in the concentration range studied are 42.3 and 77.5mg/g, respectively. The magnetic MWCNTs can be easily manipulated in magnetic field for desired separation, leading to the removal of dyes from polluted water. The integration of MWCNTs with Fe2O3 nanoparticles has great potential application to remove organic dyes from polluted water.

A novel method to prepare water-dispersible magnetic nanoparticles and their biomedical applications: magnetic capture probe and specific cellular uptake

A novel and simple method to form water-dispersed magnetic nanoparticles was successfully developed through glucosaminic acid-surface modification of iron oxide nanoparticles. The resultant glucosaminic acid-modified magnetic nanoparticles (GA-MNPs) had not only good uniformity in spherical shape with diameter of about 10-13 nm, but also possessed excellent water-dispersity and stability. In cell culture experiments, the internalization of GA-MNPs into different kinds of cells was observed over a 5-day period. The results indicated that the internalization of GA-MNPs into mouse macrophage cells and mouse embryonic fibroblast cells was not observed after 40 h of culturing. However, the GA-MNPs were internalized quickly into cancer cells after just 24 h of culturing. TEM images of the GA-MNPs uptake in ECA-109 cells were used to study the internalization mechanisms of GA-MNPs and their distribution in ECA-109 cells. Additionally, a water-dispersed magnetic capture probe was prepared by immobilization of oligonucleotides onto GA-MNPs, and the probe was used for detection and separation of their complementary oligonucleotides sequence.

Transmission electron microscope-induced structural evolution in amorphous Fe, Co, and Ni oxide nanoparticles

The high-energy electron beams in transmission electron microscopes (TEM) are known to cause structural changes and damage in some materials. In this paper, we describe unique and reproducible TEM-induced changes to the morphology of amorphous metal oxide (Fe, Co, and Ni) nanoparticles. The studied particles were synthesized via literature methods and fully characterized by X-ray powder diffraction and time-resolved, low-dose TEM. As a result of electron beam irradiation, we observe these particles to transform from initially solid spheres to core/void/shell structures and eventually to hollow nanoparticles. The rate of these transformations depends on the size and composition of the particles but is not unique to the Fe oxide we previously reported. These data suggest that structural analysis of nanoparticles by TEM must consider the impact of the high-energy electron beam and use low-dose imaging.

Colloidal chemical synthesis and formation kinetics of uniformly sized nanocrystals of metals, oxides, and chalcogenides

Nanocrystals exhibit interesting electrical, optical, magnetic, and chemical properties not achieved by their bulk counterparts. Consequently, to fully exploit the potential of nanocrystals, the synthesis of nanocrystals must focus on producing materials with uniform size and shape. Top-down physical processes can produce large quantities of nanocrystals, but controlling the size is difficult with these methods. On the other hand, colloidal chemical synthetic methods can produce uniform nanocrystals with a controlled particle size. In this Account, we present our synthesis of uniform nanocrystals of various shapes and materials, and we discuss the kinetics of nanocrystal formation. We employed four different synthetic approaches including thermal decomposition, nonhydrolytic sol-gel reactions, thermal reduction, and use of reactive chalcogen reagents. We synthesized uniform oxide nanocrystals via heat-up methods. This method involved slowly heat-up reaction mixtures composed of metal precursors, surfactants, and solvents from room temperature to high temperature. We then held reaction mixtures at an aging temperature for a few minutes to a few hours. Kinetics studies revealed a three-step mechanism for the synthesis of nanocrystals through the heat-up method with size distribution control. First, as metal precursors thermally decompose, monomers accumulate. At the aging temperature, burst nucleation occurs rapidly; at the end of this second phase, nucleation stops, but continued diffusion-controlled growth leads to size focusing to produce uniform nanocrystals. We used nonhydrolytic sol-gel reactions to synthesize various transition metal oxide nanocrystals. We employed ester elimination reactions for the synthesis of ZnO and TiO(2) nanocrystals. Uniform Pd nanoparticles were synthesized via a thermal reduction reaction induced by heating up a mixture of Pd(acac)(2), tri-n-octylphosphine, and oleylamine to the aging temperature. Similarly, we synthesized nanoparticles of copper and nickel using metal(II) acetylacetonates. Ni/Pd core/shell nanoparticles were synthesized by simply heating the reaction mixture composed of acetylacetonates of nickel and palladium. Using alternative chalcogen reagents, we synthesized uniform nanocrystals of various metal chalcogenides. Uniform nanocrystals of PbS, ZnS, CdS, and MnS were obtained by heating reaction mixtures composed of metal chlorides and sulfur dissolved in oleylamine. In the future, a detailed understanding of nanocrystal formation kinetics and synthetic chemistry will lead to the synthesis of uniform nanocrystals with controlled size, shape, and composition. In particular, the synthesis of uniform nanocrystals of doped materials, core/shell materials, and multicomponent materials is still a challenge. We expect that these uniformly sized nanocrystals will find important applications in areas including information technology, biomedicine, and energy/environmental technology.

Microwave-Assisted Synthesis of Titania Nanocubes, Nanospheres and Nanorods for Photocatalytic Dye Degradation

TiO(2) nanostructures with fascinating morphologies like cubes, spheres, and rods were synthesized by a simple microwave irradiation technique. Tuning of different morphologies was achieved by changing the pH and the nature of the medium or the precipitating agent. As-synthesized titania nanostructures were characterized by X-ray diffraction (XRD), UV-visible spectroscopy, infrared spectroscopy (IR), BET surface area, photoluminescence (PL), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and atomic force microscopy (AFM) techniques. Photocatalytic dye degradation studies were conducted using methylene blue under ultraviolet light irradiation. Dye degradation ability for nanocubes was found to be superior to the spheres and the rods and can be attributed to the observed high surface area of nanocubes. As-synthesized titania nanostructures have shown higher photocatalytic activity than the commercial photocatalyst Degussa P25 TiO(2).

Targeted folic acid-PEG nanoparticles for noninvasive imaging of folate receptor by MRI

The surface of superparamagnetic iron oxide nanoparticles (SPIO) with different molecular weight of poly(ethylene glycol) (PEG) and folic acid (FA) were synthesized. The SPIO-PEG-FA nanoparticles are well-dispersed and have good stability in various pH solutions. The lack of hysterestis and remanence at ambient temperatures is characteristic of superparamagnetic materials for SPIO-PEG-FA. The uptake by macrophage for SPIO-PEG-FA is lower than that of Feridex I.V. even at higher concentration. Internalization of SPIO-PEG-FA in targeted cells (KB cells) was observed by flow-cytometric analysis and in vitro MR imaging. The intensity change of positive KB cell tumor (-20 to 25%) is significantly lower than that of negative HT-1080 cell tumor from precontrast to postcontrast images of the tumor by in vivo MR imaging. These preliminary results demonstrated that SPIO-PEG-FA have the ability to target folate receptor.

Magnetic colloids as drug vehicles

This review article is a description of the present status of magnetic drug delivery systems (DDS). These are colloidal dispersions of composite nanoparticles consisting of a (polymeric or inorganic) biocompatible matrix and magnetic units, and designed to load and release therapeutic drugs. The matrix, together perhaps with adsorbed polymers or polyelectrolytes, provides the DDS with additional colloidal stability and eventually control of the immune response, and the magnetic inclusions have the goal of providing magnetic guidance. The techniques used in the production of the particles are described. The large surface/volume ratio of the particles brings about a superlative importance of the interface aspects, which are depicted in some detail. Attention is also paid to the possibilities that magnetic DDS offer to be guided by magnetic fields, and to their fate upon entering in contact with the blood proteins and the tumor cells. A description of in vitro and in vivo biodistribution experiments helps in this description. The number of animal experiments performed using magnetic DDS is rather large, but results in humans are far from being sufficient in number, something easily understood. The hopes for improvement and the challenges that must be overcome are described in the closing section.

Some physicochemical aspects of nanoparticulate magnetic iron oxide colloids in neat water and in the presence of poly(vinyl alcohol)

The preparation of magnetic iron oxide colloids directly from the coprecipitation of Fe (2+) and Fe (3+) species at different temperatures may lead to crystallites of higher size as the temperature of the reaction increases. On the other hand, dynamic light scattering investigations and dielectric measurements rather point to the similar colloidal size of the entities existing in their aqueous or solid-state dispersions, irrespective of the size of the primary nanocrystallites. Significant enhancement of the stability of the colloids, even in the presence of high electrolyte concentrations, is furnished after the addition of relatively small amounts of poly(vinyl alcohol), and the stabilization mechanism is discussed in terms of the various forces participating in the system. The experimental results suggest that the increased colloidal stability is triggered from the particles' decrease of velocity rather than from steric (entropic) effects originating from polymer absorption.

Multifunctional and stimuli-sensitive pharmaceutical nanocarriers

Currently used pharmaceutical nanocarriers, such as liposomes, micelles, and polymeric nanoparticles, demonstrate a broad variety of useful properties, such as longevity in the body; specific targeting to certain disease sites; enhanced intracellular penetration; contrast properties allowing for direct carrier visualization in vivo; stimuli-sensitivity, and others. Some of those pharmaceutical carriers have already made their way into clinic, while others are still under preclinical development. In certain cases, the pharmaceutical nanocarriers combine several of the listed properties. Long-circulating immunoliposomes capable of prolonged residence in the blood and specific target recognition represent one of the examples of this kind. The engineering of multifunctional pharmaceutical nanocarriers combining several useful properties in one particle can significantly enhance the efficacy of many therapeutic and diagnostic protocols. This paper considers the current status and possible future directions in the emerging area of multifunctional nanocarriers with primary attention on the combination of such properties as longevity, targetability, intracellular penetration, contrast loading, and stimuli-sensitivity.

Magnetic iron oxide nanoparticles: synthesis and surface functionalization strategies

Surface functionalized magnetic iron oxide nanoparticles (NPs) are a kind of novel functional materials, which have been widely used in the biotechnology and catalysis. This review focuses on the recent development and various strategies in preparation, structure, and magnetic properties of naked and surface functionalized iron oxide NPs and their corresponding application briefly. In order to implement the practical application, the particles must have combined properties of high magnetic saturation, stability, biocompatibility, and interactive functions at the surface. Moreover, the surface of iron oxide NPs could be modified by organic materials or inorganic materials, such as polymers, biomolecules, silica, metals, etc. The problems and major challenges, along with the directions for the synthesis and surface functionalization of iron oxide NPs, are considered. Finally, some future trends and prospective in these research areas are also discussed.

Highly water-dispersible PEG surface modified ultra small superparamagnetic iron oxide nanoparticles useful for target-specific biomedical applications

For the application of superparamagnetic iron oxide nanoparticles in biomedical fields for target-specific purposes, they should be ultra small in diameter. We developed a simple one-step synthesis of surface modified ultra small superparamagnetic iron oxide nanoparticles (USPIONs) with an average particle diameter of 1.7 nm in a polar organic solvent. Polyethylene glycol diacid (PEG) surface modified USPIONs synthesized in triethylene glycol were nearly monodisperse in diameter and highly water-dispersible. The PEG surface modified USPIONs were tested for use as magnetic resonance (MR) contrast agents. They had a low r(2)/r(1) relaxivity ratio of 3.4 (r(1) = 4.46 and r(2) = 15.01 mM(-1) s(-1)) and showed clear dose-dependent T(1) and T(2) map images, indicating that they will be useful as both target-specific T(1) and T(2) MR contrast agents due to their ultra small size.

2-pyrrolidone - capped Mn3O4 nanocrystals

Water-soluble Mn3O4 nanocrystals have been prepared through thermal decomposition in a high temperature boiling solvent, 2-pyrrolidone. The final product was characterized with XRD, SEM, TEM, FTIR and Zeta Potential measurements. Average crystallite size was calculated as ∼15 nm using XRD peak broadening. TEM analysis revealed spherical nanoparticles with an average diameter of 14±0.4 nm. FTIR analysis indicated that 2-pyrrolidone coordinates with the Mn3O4 nanocrystals only via O from the carbonyl group, thus confining their growth and protecting their surfaces from interaction with neighboring particles.

Sonochemically assisted synthesis of zinc-doped maghemite

Nanoparticles of zinc-doped maghemite were prepared using ultrasonic radiation. As a precursor, a suspension of maghemite in an alkaline aqueous solution of zinc nitrate at pH 9 was sonicated. The zinc-doped maghemite nanoparticles were investigated by X-ray diffraction, Mössbauer spectroscopy, high-resolution electron microscopy (HREM) and SQUID magnetometry. The Mössbauer measurements, which cover the temperature range 4.2 K to room temperature, were acquired in zero field and an applied field of 5 T. The results show that by using ultrasound radiation, zinc Zn2+ can substitute for Fe3+ up to a composition close to zinc ferrite (ZnFe2O4), which has a random distribution of Fe3+ ions over both A and B sublattices in the spinel structure with an inversity parameter of delta=0.322. This leads to a maximum saturation magnetization (Ms) of 64.1 emu/g at 300 K and 73.5 emu/g at 2 K.

The one-pot synthesis of dextran-based nanoparticles and their application in in-situ fabrication of dextran-magnetite nanocomposites

The dextran-based nanoparticles containing carboxyl groups were synthesized by a one-pot approach, without using any organic solvents and surfactants. The resultant dextran-based nanoparticles was used as a host for the growing and organization of Fe(3)O(4) nanoparticles. The approach consists of the mixture of ferrous/ferric ions aqueous solution and host nanoparticles and subsequent coprecipitation of ferrous/ferric ions in basic medium. The magnetic nanocomposite material obtained was characterized using transmission electron microscopy (TEM), dynamic light scattering (DLS), thermogravimetric analysis (TGA), X-ray diffraction techniques (XRD) and vibrating sample magnetometry (VSM). The data demonstrate that the carboxyls which can capture cationic ferrous/ferric by electronic interaction in the dextran-based hosts plays a crucial role in fabricating nanocomposites with a homogeneous spatial distribution of magnetite nanoparticles. The magnetic nanocomposites exhibit comparable saturation magnetizations to that of reported Fe(3)O(4) nanoparticles, and therefore display great potential in a large scope of biomedical fields.

Multifunctional Particles: Magnetic Nanocrystals and Gold Nanorods Coated with Fluorescent Dye-Doped Silica Shells

Multifunctional colloidal core-shell nanoparticles of magnetic nanocrystals (of iron oxide or FePt) or gold nanorods encapsulated in silica shells doped with the fluorescent dye, Tris(2,2'-bipyridyl)dichlororuthenium(II) hexahydrate (Rubpy) were synthesized. The as-prepared magnetic nanocrystals are initially hydrophobic and were coated with silica using a microemulsion approach, while the as-prepared gold nanorods are hydrophilic and were coated with silica using a Stöber-type of process. Each approach yielded monodisperse nanoparticles with uniform fluorescent dye-doped silica shells. These colloidal heterostructures have the potential to be used as dual-purpose tags-exhibiting a fluorescent signal that could be combined with either dark-field optical contrast (in the case of the gold nanorods), or enhanced contrast in magnetic resonance images (in the case of magnetic nanocrystal cores). The optical and magnetic properties of the fluorescent silica-coated gold nanorods and magnetic nanocrystals are reported.

Amphoteric CdSe nanocrystalline quantum dots

The nanocrystal quantum dot (NQD) charge states strongly influence their electrical transport properties in photovoltaic and electroluminescent devices, optical gains in NQD lasers, and the stability of the dots in thin films. We report a unique electrostatic nature of CdSe NQDs, studied by electrophoretic methods. When we submerged a pair of metal electrodes, in a parallel plate capacitor configuration, into a dilute solution of CdSe NQDs in hexane, and applied a DC voltage across the pair, thin films of CdSe NQDs were deposited on both the positive and the negative electrodes. Extensive characterizations including scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared (FTIR) and Raman studies revealed that the films on both the positive and the negative electrodes were identical in every respect, clearly indicating that: (1) a fraction (<1%) of the CdSe NQDs in free form in hexane solution are charged and, more importantly, (2) there are equal numbers of positive and negative CdSe NQDs in the hexane solution. Experiments also show that the number of deposited dots is at least an order of magnitude higher than the number of initially charged dots, indicating regeneration. We used simple thermodynamics to explain such amphoteric nature and the charging/regeneration of the CdSe NQDs.

Synthesis of carbohydrate-conjugated nanoparticles and quantum dots

Nanoparticle-based probes are emerging as alternatives to molecular probes due to their various advantages, such as bright and tunable optical property, enhanced chemical and photochemical stability, and ease of introduction of multifunctionality. This work presents a simple and general approach for functionalizing various nanoparticle systems for use as glycobiological probes. Silica-coated nanoparticles of Ag, Fe3O4, and ZnS-CdSe were synthesized and functionalized with dextran. The resulting 10-40-nm-sized particles were robust, water-soluble, colloidally stable, and biochemically active.

A facile interfacial reaction route to prepare magnetic hollow spheres with tunable shell thickness

Magnetic Fe3O4 hollow spheres were successfully synthesized with a water in oil in water (W/O/W) emulsion. During the facile procedure, no high pressure, high temperature, or other complex reaction conditions were required. Transmission electric microscope (TEM) images showed that all the hollow structural products have a good spherical morphology with an average diameter of 160 nm. The average size and the size distribution were further determined with dynamic light scattering (DLS), which reveals that the hollow nanospheres have a narrow size distribution. The average size from DLS was about 180 nm, which approximated that from TEM data. X-ray diffraction (XRD) demonstrates that the products were all Fe3O4 phase without any impurity. By increasing or decreasing the dosage of precipitate and precipitant sources, we controlled the shell thickness successfully in the tens of nanometers range. The formation mechanism of those hollow magnetic nanospheres was discussed by using the "reverse micelle transport" mechanism.

Synthesis of superparamagnetic nanotubes as MRI contrast agents and for cell labeling

AIMS

Magnetic nanoparticles have been studied widely as MRI contrast agents to increase the sensitivity of this technique. This work describes the synthesis and characterization of magnetic nanotubes (MNTs) as a novel MRI contrast agent.

METHODS

MNTs with high saturation magnetization were fabricated by the synthesis of superparamagnetic iron oxide nanoparticles (SPIONs) directly in the pores of silica nanotubes (SNTs). The MNTs were characterized by electron microscopy, superconducting quantum interference device and MRI. Preliminary studies on in vitro cytotoxicity and cell labeling were carried out.

RESULTS

The MNTs retained the superparamagnetic characteristics in bulk solutions with a considerably high saturation magnetization of 95 emu/gFe. The nuclear magnetic resonance (NMR) relaxivities for MNTs of 500 nm in length and of 60 nm in diameter were r(1) = 1.6 +/- 0.3 mM(-1)s(-1) and r(2) = 264 +/- 56 mM(-1)s(-1) and, for the MNTs of 2 microm in length and 70 nm in diameter, the r(1) and r(2) were 3.0 +/- 1.3 and 358 +/- 65 mM(-1)s(-1), respectively. In vitro cell labeling showed promising results with excellent labeling efficiency. No cellular toxicity was observed in vitro.

CONCLUSIONS

The integration of SPIONs with SNTs imparts the superparamagnetic characteristics of SPIONs onto the SNTs, creating unique magnetic nanoparticles with multifunctionality. The MNTs showed promising results as a MRI contrast agent with high NMR relaxivities, little cytotoxicity and high cell-labeling efficiency.

Poly(n-isopropylacrylamide)-based hydrogel coatings on magnetite nanoparticles via atom transfer radical polymerization

Core magnetite (Fe(3)O(4)) nanoparticles have been functionalized with a model intelligent hydrogel system based on the temperature responsive polymer poly(n-isopropyl acrylamide) (PNIPAAm) to obtain magnetically responsive core-shell nanocomposites. Fe(3)O(4) nanoparticles were obtained from a one-pot co-precipitation method which provided either oleic acid (hydrophobic) or citric acid (hydrophilic) coated nanoparticles. Subsequent ligand exchange of these coatings with various bromine alkyl halides and a bromo silane provided initiating sites for functionalization with NIPAAm using atom transfer radical polymerization (ATRP). The bromine alkyl halides that were used were 2-bromo-2-methyl propionic acid (BMPA) and 2-bromopropionyl bromide (BPB). The bromo silane that was used was 3-bromopropyl trimethoxysilane (BPTS). The intelligent polymeric shell consists of NIPAAm crosslinked with poly(ethylene glycol) 400 dimethacrylate (PEG400DMA). Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and transmission electron microscopy (TEM) were used to confirm the presence of the polymeric shell. Dynamic light scattering (DLS) was used to characterize the nanocomposites for particle size changes with temperature. Their magnetic and temperature responsiveness show great promise for further biomedical applications. This platform for functionalizing magnetic nanoparticles with intelligent hydrogels promises to impact a wide range of medical and biological applications of magnetic nanoparticles.