2-Dimensional Clay/Reduced Graphene Oxide Ordered Heterostructures Dispersible in Water via a One-Step Hydrothermal Route.

A facile and green method for trapping the hydrophobic reduced graphene oxide between the hydrophilic Kunipia F layers in order to attain stable aqueous dispersions of reduced graphene oxide is described. Initially stable aqueous dispersions of hydrophilic clay intercalated with hydrophilic graphene oxide sheets were formed providing well-organized heterostructures, as it was revealed by scanning electron microscopy images. These structures were preserved in the product obtained after hydrothermal treatment where the hydrophilic graphene oxide was converted to hydrophobic reduced graphene oxide. Ultraviolet measurements revealed the aforementioned conversion which was accompanied by a characteristic change in color from yellow-brown to black in the corresponding aqueous dispersions of these hybrids before and after hydrothermal treatment. The stability of these homogeneous dispersions was confirmed by Zeta Potential measurements implying interactions both in cases of graphene oxide and reduced graphene oxide with clay sheets that made feasible the effective interstratification of graphene-clay layered materials. In these stable dispersions chemistry in aqueous environment could be fully utilized making possible their incorporation e.g., as fillers to hydrophilic polymeric matrices extending thus the limits of application.

Reducing the layer number of AB stacked multilayer graphene grown on nickel by annealing at low temperature.

Controlling the number of layers of graphene grown by chemical vapor deposition is crucial for large scale graphene application. We propose here an etching process of graphene which can be applied immediately after growth to control the number of layers. We use nickel (Ni) foil at high temperature (T = 900 °C) to produce multilayer-AB-stacked-graphene (MLG). The etching process is based on annealing the samples in a hydrogen/argon atmosphere at a relatively low temperature (T = 450 °C) inside the growth chamber. The extent of etching is mainly controlled by the annealing process duration. Using Raman spectroscopy we demonstrate that the number of layers was reduced, changing from MLG to few-layer-AB-stacked-graphene and in some cases to randomly oriented few layer graphene near the substrate. Furthermore, our method offers the significant advantage that it does not introduce defects in the samples, maintaining their original high quality. This fact and the low temperature our method uses make it a good candidate for controlling the layer number of already grown graphene in processes with a low thermal budget.

A new approach for the one-step synthesis of bioactive PS vs. PMMA silica hybrid microspheres as potential drug delivery systems.

In this work, hybrid microspheres were prepared in a two-step process combining the emulsifier free-emulsion polymerization and the sol-gel coating method. In the first step, polystyrene (St) and poly(methyl methacrylate) (PMMA) microspheres were prepared as sacrificial template and in the second step a silanol shell was fabricated. The functionalized surface of the hybrid microspheres by silane analogs (APTES, TEOS) resulted in enhanced effects. The hollow microspheres were resulted either in an additional step by template dissolution and/or during the coating process. The microspheres' surface interactions and the size distribution were optimized by treatment in simulated body fluids, which resulted in the in vitro prediction of bioactivity. The bioassay test indicated that the induced hydroxyapatite resembled in structure to naturally occurring bone apatite. The drug doxorubicin (DOX) was used as a model entity for the evaluation of drug loading and release. The drug release study was performed in two different pH conditions, at acidic (pH=4.5) close to cancer cell environment and at slightly basic pH (pH=7.4) resembling the orthopedic environment. The results of the present study indicated promising hybrid microspheres for the potential application as drug delivery vehicles, for dual orthopedic functionalities in bone defects, bone inflammation, bone cancer and bone repair.

Nanostructuring the surface of dual responsive hollow polymer microspheres for versatile utilization in nanomedicine-related applications.

The design and fabrication of hollow polymer microspheres responsive to various stimuli comprises a promising approach for the development of multifunctional and efficient systems for various nanomedicine-related applications. In this paper, we present the preparation of poly(methacrylic acid-co-N,N'-methylenebis(acrylamide)-co-poly(ethylene glycol) methyl ether methacrylate-co-N,N'-bis(acryloyl)cystamine) (PMAA(S-S)) hollow microspheres following a two-stage distillation precipitation polymerization procedure. Magnetic and silver nanocrystals were chemically grown on the surface of the hollow polymer microspheres, resulting in a composite system with interesting properties. We evaluated the performance of the composite hollow microspheres as magnetic hyperthermia mediators and their surface-enhanced Raman spectroscopy activity. Assessment of Daunorubicin-loaded PMAA(S-S) hollow microspheres performance as effective drug carriers was carried out through drug release experiments upon application of different pH and reducing conditions. pH and redox responsiveness as well as basic mechanisms of release profiles are discussed. Furthermore, in vitro cytotoxicity of empty and drug-loaded PMAA(S-S) hollow microspheres against MCF-7 cancer cells was investigated in order to evaluate their performance as drug carriers.

Comparative study of LbL and crosslinked pH sensitive PEGylated LbL microspheres: synthesis, characterization and biological evaluation.

In this report, the fabrication of hollow, pH-sensitive microspheres using the layer by layer method is being demonstrated. The process is based on the coating of colloidal silica templates with polyelectrolyte layers, followed by dissolution of the core using a buffer system of hydrofluoric acid and ammonium fluoride. With this buffer system, the template can be dissolved in mild pH conditions, where the polymeric layers are still stable. The resulting microspheres show pH dependent properties, where they swell and dissolve at a certain pH value. In order to improve the nanocontainers properties, such as solubility and cytotoxicity a second specimen of microspheres was fabricated by coating the fabricated microspheres with one layer of poly(acrylic acid) and one layer of methoxy-polyethylene glycol amine, and then these additional layers were crosslinked by carbodiimide chemistry. Loading and release properties of the fabricated microspheres have been studied in order to investigate their behavior as a function of pH and ionic strength. These novel microspheres have potential application in the fields of drug delivery, diagnostics and life sciences.

Microspheres as therapeutic delivery agents: synthesis and biological evaluation of pH responsiveness.

A soft template method was used for the synthesis of pH-responsive microcontainers with an inner cavity. Poly(glycidyl methacrylate) (PGMA) microspheres of narrow size distribution were synthesized by soap-free radical emulsion polymerization and the coating of the microspheres was carried out by the same procedure. The procedure consists of two steps. In the first step the sacrificial template is synthesized and in the second step the shell is formed. Acrylic acid was used as a coating monomer, with the aim of introducing pH sensitivity in the synthesized microcontainers. A loading and release study of the anthracycline drug doxorubicin (DOX) was also carried out. The toxicity evaluation of the drug was carried out using the MTT assay, and the necrotic effect was studied using trypan blue.

Sacrificial template-directed fabrication of superparamagnetic polymer microcontainers for pH-activated controlled release of Daunorubicin.

Magnetic pH-sensitive microcontainers were produced by a four-step process. The first step involves the synthesis of citrate-modified magnetic nanoparticles via the coprecipitation method. The second step consists of the encapsulation of magnetic nanoparticles in non-cross-linked poly(methacrylic acid) (PMAA) microspheres through distillation precipitation polymerization, resulting in a core/shell structure. The third step concerns the formation of a poly(N,N'-methylenebis(acrylamide)-co-mathacrylic acid) (P(MBAAm-co-MAA)) layer on the surface of magnetic PMAA microspheres by second distillation precipitation polymerization in order to produce a trilayer hybrid microsphere. The last step deals with the removal of PMAA layer in ethanol and formation of a stable P(MBAAm-co-MAA) microcontainer with magnetic nanoparticles entrapped inside the formed cavity. This process is simple and leads to the formation of superparamagnetic pH-sensitive microcontainers. The structure and properties of the magnetic microcontainers were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), vibrating sample magnetometry (VSM), and dynamic light scattering (DLS) to determine the functionalities of the hybrid structure. The magnetic pH-sensitive microcontainers were loaded with Daunorubicin and tested with respect to release rate at different pH values in order to evaluate their functionality as controlled release system.

Synthesis and magnetic properties of Fe3O4 nanoparticles coated with biocompatible double hydrophilic block copolymer.

Fe3O4 nanoparticles coated with double hydrophilic biocompatible poly(sodium(2-sulfamate-3-carboxylate)isoprene)-b-poly(ethylene oxide) block copolymer were prepared by a one step precipitation method. The magnetic nanoparticles have 15 nm mean diameter (TEM), 68 nm hydrodynamic diameter, -30.10 mV zeta-potential and form very stable dispersion in aqueous media. Structural characterization using powder XRD and Mössbauer spectroscopy establish the magnetite phase, while thermogravimetric analysis and FT-IR spectroscopy confirm the presence of the block copolymer on the nanoparticles surface. The magnetic properties were determined using a vibrating sample magnetometer (VSM) at room temperature and reveal superparamagnetic behavior while the composite materials shows high saturation magnetization up to 67.7 emu/gr.

Zinc oxide nanorod based photonic devices: recent progress in growth, light emitting diodes and lasers.

Zinc oxide (ZnO), with its excellent luminescent properties and the ease of growth of its nanostructures, holds promise for the development of photonic devices. The recent advances in growth of ZnO nanorods are discussed. Results from both low temperature and high temperature growth approaches are presented. The techniques which are presented include metal-organic chemical vapour deposition (MOCVD), vapour phase epitaxy (VPE), pulse laser deposition (PLD), vapour-liquid-solid (VLS), aqueous chemical growth (ACG) and finally the electrodeposition technique as an example of a selective growth approach. Results from structural as well as optical properties of a variety of ZnO nanorods are shown and analysed using different techniques, including high resolution transmission electron microscopy (HR-TEM), scanning electron microscopy (SEM), photoluminescence (PL) and cathodoluminescence (CL), for both room temperature and for low temperature performance. These results indicate that the grown ZnO nanorods possess reproducible and interesting optical properties. Results on obtaining p-type doping in ZnO micro- and nanorods are also demonstrated using PLD. Three independent indications were found for p-type conducting, phosphorus-doped ZnO nanorods: first, acceptor-related CL peaks, second, opposite transfer characteristics of back-gate field effect transistors using undoped and phosphorus doped wire channels, and finally, rectifying I-V characteristics of ZnO:P nanowire/ZnO:Ga p-n junctions. Then light emitting diodes (LEDs) based on n-ZnO nanorods combined with different technologies (hybrid technologies) are suggested and the recent electrical, as well as electro-optical, characteristics of these LEDs are shown and discussed. The hybrid LEDs reviewed and discussed here are mainly presented for two groups: those based on n-ZnO nanorods and p-type crystalline substrates, and those based on n-ZnO nanorods and p-type amorphous substrates. Promising electroluminescence characteristics aimed at the development of white LEDs are demonstrated. Although some of the presented LEDs show visible emission for applied biases in excess of 10 V, optimized structures are expected to provide the same emission at much lower voltage. Finally, lasing from ZnO nanorods is briefly reviewed. An example of a recent whispering gallery mode (WGM) lasing from ZnO is demonstrated as a way to enhance the stimulated emission from small size structures.

Homogeneous core/shell ZnO/ZnMgO quantum well heterostructures on vertical ZnO nanowires.

Low-area density ZnO nanowire arrays, growing perpendicularly to the substrate, are synthesized with high-pressure pulsed laser deposition. The introduction of a ZnO buffer layer enables us to fabricate individual nanowires several micrometres apart (area density<0.1 nanowire microm(-2)), suppressing any shadowing effect by neighbouring nanowires during subsequent growth. These low density ZnO nanowires, whose c-axis is perpendicular to the substrate surface, are then used as templates to grow ZnO/ZnMgO core-shell nanowire heterostructures with conventional low-pressure pulsed laser deposition. Cathodoluminescence spectroscopy as well as transmission electron microscopy show that a sharp interface forms between the ZnO core and the ZnMgO shell. Based on these findings, we have grown a series of radial ZnO/ZnMgO quantum wells with different thicknesses that exhibit quantum confinement effects, with thicker quantum wells emitting at lower energies. Spatially resolved cathodoluminescence confirms the homogeneity of the quantum well structure along the full nanowire length of about 3 microm.

Removal of Reactive Red 195 from aqueous solutions by adsorption on the surface of TiO2 nanoparticles.

Nanoparticles of TiO2 were synthesized and characterized by XRD, BET, TG/DTA and TEM measurements. The commercial azo dye Reactive Red 195 (RR195) was selected as a model dye in order to examine the adsorption capacity of TiO2 at room temperature, under dark conditions. It was demonstrated that RR195 could be efficiently adsorbed in aqueous suspension of TiO2. A study on the effects of various parameters like initial pH, concentration of dye and concentration of adsorbent has been carried out in order to find optimum adsorption conditions. The optimum pH of sorption was 3. Substantial reduction of COD, besides removal of colour, was also achieved. The experimental data were analyzed by the Langmuir and Freundlich adsorption models. Equilibrium data fitted very well with the Langmuir model signifying the energetic homogeneity of TiO2 surface adsorption sites. At the temperature of 30 degrees C, the maximum monolayer adsorption capacity obtained from the Langmuir model is approximately 87 mg/g (pH 3.0). Kinetic studies were carried out and showed a rapid sorption of dye in the first 30 min while equilibrium was reached at 1h. Three kinetic adsorption models were used to describe the kinetics data, the pseudo-first-order model, the pseudo-second-order model and the intraparticle diffusion model. The sorption kinetics of dye was best described by the pseudo-second-order kinetic model.

A general chemical route for the synthesis of capped nanocrystalline materials.

Noble metals, magnetic and semiconducting nanocrystalline materials have been synthesized via the thermolytic decomposition of inorganic metal salts, at high temperature, in commercial oleyl amine. The oleyl amine acts as high boiling point coordinating solvent, capping agent and, when required, as reducing agent. The crystal structure and morphology of the nanostructured materials have been studied with powder X-ray analysis (XRD) and transmission electron microscopy (TEM). The particles are well dispersed in non polar solvents such as hexane, toluene and chloroform and have uniform morphology.

Ultraviolet femtosecond, picosecond and nanosecond laser microstructuring of silicon: structural and optical properties.

The effect of laser pulse duration on the morphology, composition, crystallinity and optical properties of self-organized Si microcones fabricated using 248 nm laser pulses (500 fs, 5 ps and 15 ns) in an SF(6) atmosphere, is presented in this paper. Despite distinct differences in the morphology, the Si cones show similar structure and composition independently of the laser pulse duration used: a core of single-crystalline Si, covered by a few hundred nanometer thick, sulfur-doped nanocrystalline Si layer, where no amorphous Si is present. The obtained features exhibit strong below-bandgap absorptance, making them excellent candidates for Si based photodetectors with improved spectral response.

Spatial fluctuations of optical emission from single ZnO/MgZnO nanowire quantum wells.

MgZnO/ZnO quantum wells on top of ZnO nanowires were grown by pulsed laser deposition. Ensembles of spatially fluctuating and narrow cathodoluminescence peaks with single widths down to 1 meV were found at the spectral position of the quantum well emission at 4 K. In addition, the number of these narrow QW peaks increases with increasing excitation power in micro-photoluminescence, thus pointing to quantum-dot-like emission centers. Indeed, laterally strained areas of about 5 nm diameter were identified at the quantum well positions on top of the nanowires by high-resolution transmission electron microscopy.

Engineering of FePt nanoparticles by e-beam co-evaporation.

Fe(50)Pt(50) nanoparticles were deposited on thermally oxidized Si substrates by electron-beam co-evaporation of Fe and Pt, at substrate temperatures T(s) between 300 and 700 degrees C. The co-deposition led to the formation of drop-like, coalesced nanoparticles, chain-like structures or continuous films, the morphology being dependent on T(s) or the nominal thickness of the layer, f. The nanoparticles have a mean diameter D(p) between 3 and 45 nm, which increases with increasing f. The degree of crystallization in the ordered face centred tetragonal (fct) phase of the samples depends strongly on the growth conditions and increases with increasing T(s) and f. Nanoparticles with a higher proportion of the fct phase exhibit higher coercivity, with a maximum value of approximately 10.3 kOe (for the specimens prepared at 600 degrees C with f = 8.5 nm). Conversely, samples with a high proportion of the cubic phase are either superparamagnetic or ferromagnetically soft. The thermal annealing performed on selected samples resulted in structural transformation as well as magnetic hardening that depended on f and D(p).

Large-scale synthesis, size control, and anisotropic growth of gamma-Fe2O3 nanoparticles: organosols and hydrosols.

Monodispersed, spherical gamma-Fe2O3 nanoparticles with controllable size in large-scale were prepared by thermolytic decomposition of FeCl3.6H2O in aliphatic amines. The nanoparticles gave very stable colloidal solution in organic solvents and can be easily converted to water-soluble by a very simple route. Their characterisation was based on TEM microscopy, XRD, Mössbauer, and magnetic measurements. Furthermore, a small amount of Pt can lead to the formation of anisotropic gamma-Fe2O3 nanostructures.

In vitro studies on ultrasmall superparamagnetic iron oxide nanoparticles coated with gummic acid for T2 MRI contrast agent.

Ultrasmall superparamagnetic iron oxide nanoparticles coated with gummic acid have been investigated as possible constituents of aqueous ferrofluids for biomedical applications and especially for MRI contrast agent. The structural characteristics and the size of the nanoparticles have been analyzed as well as the magnetic properties. In order to evaluate any possible capabilities as a contrast agent, the relaxation time, T2, of hydrogen protons in the colloidal solutions of nanoparticles have been measured in order to gain information on the relaxation behavior compared to other MRI contrast agents. The in vitro cytotoxicity of the obtained magnetic nanoparticles of iron oxide coated with gummic acid was investigated by two separate methods (MTT and FACS analysis) and by using three different normal and transformed cell lines. Our results showed that the synthesized nanoparticles had no toxic effect on any of the cell lines used.

The effect of Mn doping in FePt nanoparticles on the magnetic properties of the L1(0) phase.

FePtMn nanoparticles with a narrow size distribution and an average diameter of 3 nm were synthesized by the chemical reduction of Fe(acac)(3) and Pt(acac)(2) by NaBH(4) and the thermal decomposition of Mn(2)(CO)(10) in phenyl ether. The as-made nanoparticles have a disordered face-centred cubic (fcc) structure, which transformed after thermal treatment at 650 °C to an ordered face-centred tetragonal (fct) structure, possessing coercivity values up to 13.7 kOe at room temperature. The coercivity of the annealed samples depends on the amount of Mn added to the reaction mixture, with the coercive field increasing significantly with the partial substitution of Pt by Mn, while the partial substitution of Fe by Mn does not affect the magnetic properties strongly.

Structural study of very thin anodic alumina films on silicon by anodization in citric acid aqueous solution.

The formation of thin alumina films on a silicon substrate by anodization in a mild acid, specifically in 1% wt citric acid aqueous solution, is investigated by transmission electron microscopy (TEM). We present a comparative study between two cases of starting material: pure aluminum and an alloy of aluminum with 1% silicon. In both cases the thickness of the Al layer was less than 50 nm. It was observed that under exactly the same conditions, in the first case the anodization was stopping before anodizing the whole film and a remaining non-anodized Al layer was always present, while in the second case, the Al layer was fully anodized, resulting in an alumina matrix with a very high density of silicon nanocrystals of uniform sizes embedded in it. In both cases the alumina film was compact and amorphous.