Synthesis of NiAu alloy and core–shell nanoparticles in water-in-oil microemulsions

NiAu alloy nanoparticles with various Ni/Au molar ratios were synthesized by the hydrazine reduction of nickel chloride and hydrogen tetrachloroaurate in the microemulsion system. They had a face-centered cubic structure and a mean diameter of 6–13 nm, decreasing with increasing Au content. As Au nanoparticles did, they showed a characteristic absorption peak at about 520 nm but the intensity decreased with increasing Ni content. Also, they were nearly superparamagnetic, although the magnetization decreased significantly with increasing Au content. Under an external magnetic field, they could be self-organized into the parallel lines. In addition, the core–shell nanoparticles, Ni₃Au₁@Au, were prepared by the Au coating on the surface of Ni₃Au₁ alloy nanoparticles. By increasing the hydrogen tetrachloroaurate concentration for Au coating, the thickness of Au shells could be raised and led to an enhanced and red-shifted surface plasmon absorption.     

Sono and electrochemical synthesis and characterization of copper core–silver shell nanoparticles

Cu–Ag core–shell nanopowders have been prepared by ultrasound-assisted electrochemistry followed by a displacement reaction. The composition of the particles has been determined by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX). The XRD patterns versus time displacement show that higher are the silver peaks intensities, weaker are the copper ones. That exhibits the progressive recovering of copper by silver. EDX results and quartz crystal microbalance results indicate that various reaction mechanisms are implied in this process. Transmission electron microscopy (TEM) points out variable nanometric diameter grain and some small agglomerates. Elemental mapping obtained by electron energy-loss spectroscopy (EELS) underlines the core–shell structure.     

Broadband tunable plasmonic substrate using self-assembled gold–silver alloy nanoparticles

In the present work, a facile approach of Au–Ag alloy NPs fabrication is proposed for plasmonic inclusion applicative studies. These hybrid NPs were prepared by the sequential deposition of Ag, Au layers on a glass substrate via RF-sputtering followed by high-temperature annealing. Compositional and size controllability of the NPs was attained by changing the ratio of nominal thicknesses of the respective Au, and Ag layers during the deposition. LSPR red (blue) peak shift occurs with increasing layer thickness Au (Ag) signifying the peak sensitivity. The influence of geometrical, compositional, and background media for achieving LSPR-sensitivity is examined and validated by FDTD simulation studies. With the agreeable peak shift tendencies, the proposed study of broadband tunable LSPR-sensitivity is expected to provide a framework for a wide range of plasmonic applications.     

Size-dependent thermal stresses in the core–shell nanoparticles

The thermal stress in a magnetic core–shell nanoparticle during a thermal process is an important parameter to be known and controlled in the magnetization process of the core–shell system. In this paper we analyze the stress that appears in a core–shell nanoparticle subjected to a cooling process. The external surface temperature of the system, considered in equilibrium at room temperature, is instantly reduced to a target temperature. The thermal evolution of the system in time and the induced stress are studied using an analytical model based on a time-dependent heat conduction equation and a differential displacement equation in the formalism of elastic displacements. The source of internal stress is the difference in contraction between core and shell materials due to the temperature change. The thermal stress decreases in time and is minimized when the system reaches the thermal equilibrium. The radial and azimuthal stress components depend on system geometry, material properties, and initial and final temperatures. The magnitude of the stress changes the magnetic state of the core–shell system. For some materials, the values of the thermal stresses are larger than their specific elastic limits and the materials begin to deform plastically in the cooling process. The presence of the induced anisotropy due to the plastic deformation modifies the magnetic domain structure and the magnetic behavior of the system.     

Optical properties of Au–Ag core–shell nanorods on glass and ITO substrates

Gold-silver core–shell nanorods were deposited on glass and ITO plates. SEM observations indicated that some of these nanorods were standing on the ITO plate. The extinction spectra of the plates were measured by varying the angles of incidence of p-polarized monitor light. Deconvolution of these spectra gave six bands in the visible region. The dependence of the peak intensities on the incident angles indicated that the bands at 390 and 420 nm originated from surface plasmon bands in the transverse direction of the nanorods.     

Synthesis and characterization of Co@Ag core–shell nanoparticles

Journal of Nanoparticle Research - A micellar method has been used to prepare silver-coated cobalt (Co@Ag) nanoparticles. The synthesized particles have been deeply characterized by several...     

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.     

High quality and tuneable silica shell–magnetic core nanoparticles

Obtaining small (<50 nm), monodispersed, well-separated, single iron oxide core–silica (SiO₂) shell nanoparticles for biomedical applications is still a challenge. Preferably, they are synthesised by inverse microemulsion method. However, substantial amount of aggregated and multicore core–shell nanoparticles is the undesired outcome of the method. In this study, we report on the production of less than 50 nm overall size, monodispersed, free of necking, single core iron oxide–SiO₂ shell nanoparticles with tuneable shell thickness by a carefully optimized inverse microemulsion method. The high degree of control over the process is achieved by understanding the mechanism of core–shell nanoparticles formation. By varying the reaction time and precursor concentration, the thickness of silica layer on the core nanoparticles can be finely adjusted from 5 to 13 nm. Residual reactions during the workup were inhibited by a combination of pH control with shock freezing and ultracentrifuging. These high-quality tuneable core–shell nanocomposite particles exhibit superparamagnetic character and sufficiently high magnetization with great potential for biomedical applications (e.g. MRI, cell separation and magnetically driven drug delivery systems) either as-prepared or by additional surface modification for improved biocompatibility.     

Novel fabrication and catalytic application of poly(ethylenimine)-stabilized gold–silver alloy nanoparticles

Novel synthesis of amine-stabilized Au–Ag alloy nanoparticles with controlled composition has been devised using poly(ethylenimine) (PEI) as a reducing and a stabilizing agent simultaneously. The composition of Au–Ag alloy nanoparticles was readily controlled by varying the initial relative amount of HAuCl₄ and AgNO₃. Due to the presence of abundant amine functional groups in PEI, which could act as the dissolving ligand for AgCl, the precipitation problem of Ag⁺ in the presence of Cl⁻ from the gold salt was avoided. On this basis, the relatively high concentrations of HAuCl₄ and AgNO₃ salts were used for the fabrication of Au–Ag alloy nanoparticles. The PEI thus plays triple roles in this study that include the co-reducing agents for HAuCl₄ and AgNO₃, the stabilizing agents for Au–Ag alloy nanoparticles, and even the dissolving agents for AgCl. As a novel material for use in catalysis, the Au–Ag alloy nanoparticles including pure Au and Ag samples were exploited as catalysts for the reduction of 4-nitrophenol in the presence of NaBH₄. As the Au content was increased in the Au–Ag alloy nanoparticles, the rate constant of the reduction was exponentially increased from pure Ag to pure Au.     

Comparative study of core–shell iron/iron oxide gold covered magnetic nanoparticles obtained in different conditions

In this study, we report the synthesis and characterization of the core–shell Fe covered with Au shells nanoparticles with mean diameters between 5 and 8 nm. The inverse micelles method was utilized to produce the samples. X-ray diffraction studies show that both core–shell systems have the expected crystalline structure. High resolution transmission electron microscopy and atomic emission spectroscopy techniques give additional information concerning the structure and composition of nanoparticles. An intermediate shell of amorphous oxidized iron was found between the magnetic Fe core and the external gold shell. The magnetic behavior of different core–shell samples shows no hysteresis loop indicating the superparamagnetic behavior of Fe@Au systems. The superparamagnetic behavior is also evidenced from FC and ZFC dependences of the magnetization versus temperature. By using the temperature dependence of the thermoremanent magnetization combined with magnetization versus applied magnetic field, the effective anisotropy constant was determined. The Fe/Au interface contribution to the effective anisotropy constant was calculated and discussed in relation with the combined shape and stress anisotropies.     

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.     

Tunable near-infrared optical properties of three-layered gold–silica–gold nanoparticles

The absorption spectra of gold–silica–gold nanoshells have been investigated by using Mie theory with variation of the geometry. With an increase in core radius the plasmon resonance for the lower energy mode ω ₋⁻ shows a distinct redshift while for the high-energy mode ω ₋⁺ shows a blueshift. It is surprising that with increasing middle layer thickness the resonance of ω ₋⁻ mode blueshifts first and then redshifts. In addition, an increase of the dielectric constant of the middle layer is found to reduce the resonance energies of the particle. The tunable near-infrared optical properties are discussed in terms of plasmon hybridization theory.     

One-pot synthesis and characterization of rhodamine derivative-loaded magnetic core–shell nanoparticles

A new method to produce elaborate nanostructure with magnetic and fluorescent properties in one entity is reported in this article. Magnetite (Fe₃O₄) coated with fluorescent silica (SiO₂) shell was produced through the one-pot reaction, in which one reactor was utilized to realize the synthesis of superparamagnetic core of Fe₃O₄, the formation of SiO₂ coating through the condensation and polymerization of tetraethylorthosilicate (TEOS), and the encapsulation of tetramethyl rhodamine isothiocyanate-dextran (TRITC-dextran) within silica shell. Transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis, and X-ray diffraction (XRD) were carried out to investigate the core–shell structure. The magnetic core of the core–shell nanoparticles is 60 ± 10 nm in diameter. The thickness of the fluorescent SiO₂ shell is estimated at 15 ± 5 nm. In addition, the fluorescent signal of the SiO₂ shell has been detected by the laser confocal scanning microscopy (LCSM) with emission wavelength (λ_em) at 566 nm. In addition, the magnetic properties of TRITC-dextran loaded silica-coating iron oxide nanoparticles (Fe₃O₄@SiO₂ NPs) were studied. The hysteresis loop of the core–shell NPs measured at room temperature shows that the saturation magnetization (M _s) is not reached even at the field of 70 kOe (7T). Meanwhile, the very low coercivity (H _c) and remanent magnetization (M _r) are 0.375 kOe and 6.6 emu/g, respectively, at room temperature. It indicates that the core–shell particles have the superparamagnetic properties. The measured blocking temperature (T _B) of the TRITC-dextran loaded Fe₃O₄@SiO₂ NPs is about 122.5 K. It is expected that the multifunctional core–shell nanoparticles can be used in bio-imaging.     

Sensitive detection of DNA based on the optical properties of core–shell gold nanorods

In this article, a type of core–shell nanostructure, Au₂S/AuAgS/Ag₃AuS₂-coated gold nanorods (GNRs) with unique optical properties was used as a sensing platform to detect fish sperm DNA (fsDNA). The prepared core–shell nanorods are positively charged due to the adsorption of the positively charged cetyltrimethylammonium bromide (CTAB) cations on their surface. fsDNA can form ternary fsDNA–CTAB–nanorod complexes together with CTAB and nanorod, which provides a useful platform to detect fsDNA through absorption spectra and resonance light scattering (RLS) spectroscopy. In this sensitive core–shell nanorod sensor, CTAB concentration and the nanoparticle dosage play important roles and have been investigated. Moreover, the fsDNA–CTAB–nanorod complexes induce a great enhancement of RLS intensity of the core–shell GNRs and directly proportional to the concentration of fsDNA, reaching a detection limit of about 10⁻⁹ mg/mL. This study will be significant for as-prepared core–shell GNRs for future application in biological systems.     

Encapsulation and controlled release from core–shell nanoparticles fabricated by plasma polymerization

Core–shell nanostructures have been synthesized by plasma deposition in radio-frequency plasma reactor. Silica and KCl nanoparticles were encapsulated by deposition of isopropanol-based films of amorphous hydrogenated carbon. Through control of the deposition time, under constant deposition rate of 1 nm/min, particles are encapsulated in a layer of plasma polymer with thickness between 15 and 100 nm. Films are robust, chemically inert, thermally stable up to 250°C. The permeability of the shells is determined by depositing films of various thickness onto KCl nanoparticles and monitoring the dissolution of the core in aqueous solution. The dissolution profile is characterized by an initial rapid release, followed by a slow release that lasts up to 30 days for the thickest films. The profile is analyzed by Fickian diffusion through a spherical matrix. We find that this model captures very accurately the entire release profile except for the first 12 hours during which, the dissolution rate is higher than that predicted by the model. The overall diffusion coefficient for the dissolution of KCl is 3 × 10⁻²¹ m²/s.     

Optical properties of phosphate glasses co-doped with Yb~（3＋） and silver nanoparticles

We report the fabrication and spectroscopic characterization of Yb^3＋-doped phosphate glass, also containing silver nitrate. Scanning electron microscopy（SEM） provides the evidence of the formation of silver nano-particles（SNPs）, which are formed as a consequence of melting and thermal decomposition of Ag NO3. Absorption spectra of the samples in the visible-to-near-infrared spectral range reveal the presence of bands centered at 410 nm associated with the SNP-plasmon resonance, and at 976 nm due to the Yb^3＋. Under 916-nm laser-diode pumping, the effect of the SNP reflects that： i） the fluorescence in the 950-nm–1150-nm spectral range is strongly enhanced（～ 30 times）, while the fluorescence decay time associated with the ^2F5/2→^2F7/2transition of Yb^3＋ increases 25%, and ii） the basic lasing properties（saturation pumping intensity, the emission and absorption cross sections） are substantially improved.     

CuGaS2 hollow spheres from Ga–CuS core–shell nanoparticles

A liquid gallium emulsion was prepared as a starting material using ultrasound treatment in ethylene glycol. Core–shell particles of Ga@CuS were successfully synthesized by deposition of a CuS layer on gallium droplets through sonochemical deposition of copper ions and thiourea in an alcohol media. The core and shell of Ga@CuS products were composed of amorphous gallium metal and covellite phase CuS, which transformed into chalcopyrite CuGaS₂ hollow spheres after sulfurization at 450 °C, which was the lowest crystallization temperature. The formation of hollow nanostructures was ascribed to the Kirkendall mechanism, in which liquid gallium particles play an important role as reactive templates. In conclusion, we obtained CuGaS₂ hollow spheres with a 430 nm outer diameter and 120 nm shell thickness that had the same crystal structure and electrical properties as bulk CuGaS₂.     

Synthesis and characterisation of highly fluorescent core–shell nanoparticles based on Alexa dyes

Current and future developments in the emerging field of nanobiotechnology are closely linked to the rational design of novel fluorescent nanomaterials, e.g. for biosensing and imaging applications. Here, the synthesis of bright near infrared (NIR)-emissive nanoparticles based on the grafting of silica nanoparticles (SNPs) with 3-aminopropyl triethoxysilane (APTES) followed by covalent attachment of Alexa dyes and their subsequent shielding by an additional silica shell are presented. These nanoparticles were investigated by dynamic light scattering (DLS), transmission electron microscopy (TEM) and fluorescence spectroscopy. TEM studies revealed the monodispersity of the initially prepared and fluorophore-labelled silica particles and the subsequent formation of raspberry-like structures after addition of a silica precursor. Measurements of absolute fluorescence quantum yields of these scattering particle suspensions with an integrating sphere setup demonstrated the influence of dye labelling density-dependent fluorophore aggregation on the signaling behaviour of such nanoparticles.