Structural and Optical Properties of Gold/Silica “Mushroom” Particles Prepared by Interfacial Templating

Complex shaped nanoparticles featuring structural or surface chemical patchiness are of special interest in both fundamental and applied research areas. This study reports the preparation and optical properties of gold/silica “mushroom” nanoparticles, where a gold particle is only partially covered by the silica cap. The synthetic approach allows precise control over the particle structure. The interfacial preparation method relies on partially embedding the gold particles in a polystyrene layer that masks the immersed part of the gold particle during silica shell growth from an aqueous solution. By adjusting sacrificial polystyrene film thickness and silica growth time, precise control over the coverage and cap thickness can be achieved. Correlative electron microscopy and single particle scattering spectroscopy measurements underline the high precision and reproducibility of the method. The good agreement between the measured and simulated single particle spectra supported by near‐field calculations indicates that the observed changes in the dipolar plasmon resonance are influenced by the extent of coverage of the gold core by the silica cap. The straightforward methods readily available for gold and silica surface modification using range of different (bio)molecules make these well‐defined nanoscale objects excellent candidates to study fundamental processes of programmed self‐assembly or application as theranostic agents.     

Optimal design of gold nanoshells for optical imaging and photothermal therapy

Gold nanoshells are of great interest in optical imaging based on their light scattering properties and photothermal therapy due to their light absorption properties. Strong light scattering is essential for optical imaging, while effective photothermal therapy requires high light absorption. In this article, the optimal core radii and shell thicknesses of silica–gold and hollow gold nanoshells, possessing maximal light scattering and absorption at wavelengths between 700 and 1100 nm, are obtained using the Mie theory of a coated sphere. The results show that large-sized gold nanoshells of high aspect ratios (the aspect ratio is defined as the ratio of core radius to shell thickness) are the efficient contrast agents for optical imaging, while smaller gold nanoshells of high aspect ratios are the ideal therapeutic agents for photothermal therapy. From the comparison of the numerical results for silica–gold and hollow gold nanoshells, the latter are seen to offer a little superior light scattering and absorption at smaller particle size. Fitting expressions for the optimal core radii and shell thicknesses are also obtained, which can provide design guidelines for experimentalists to optimize the synthetic process of gold nanoshells.     

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.     

Reflectance spectroscopy of gold nanoshells: computational predictions and experimental measurements

Gold nanoshells are concentric spherical constructs that possess highly desirable optical responses in the near infrared. Gold nanoshells consist of a thin outer gold shell and a silica core and can be used for both diagnostic and therapeutic purposes by tuning the optical response through changing the core–shell ratio as well as the overall size. Although optical properties of gold nanoshells have already been well documented, the reflectance characteristics are not well understood and have not yet been elucidated by experimental measurements. Yet, in order to use gold nanoshells as an optical contrast agent for scattering-based optical methods such as reflectance spectroscopy, it is critical to characterize the reflectance behavior. With this in mind, we used a fiber-optic-based spectrometer to measure diffuse reflectance of gold nanoshell suspensions from 500 nm to 900 nm. Experimental results show that gold nanoshells cause a significant increase in the measured reflectance. Spectral features associated with scattering from large angles (~180°) were observed at low nanoshell concentrations. Monte Carlo modeling of gold nanoshells reflectance demonstrated the efficacy of using such methods to predict diffuse reflectance. Our studies suggest that gold nanoshells are an excellent candidate as optical contrast agents and that Monte Carlo methods are a useful tool for optimizing nanoshells best suited for scattering-based optical methods.     

金纳米球壳光学吸收的Mie理论分析

基于Mie散射理论研究了金壳厚度变化、内核尺寸变化及内核介质变化下金纳米球壳的吸收光谱.研究发现,随着金壳厚度的增加,颗粒光学吸收增加到最大值后逐渐降低;随着内核尺寸逐渐增加,金壳颗粒的光学吸收最大值逐渐减小.此外,还发现随着内核介电常数的增大,颗粒的光学吸收逐渐减弱,当内核为空心时,吸收最强.利用等离激元杂化理论及自由电子和振荡电子变化的竞争机制对上述现象进行了理论分析. 关键词： 金纳米球壳 等离激元共振 吸收光谱     

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.     

Silica encapsulation of luminescent silicon nanoparticles: stable and biocompatible nanohybrids

This article presents a process for surface coating and functionalization of luminescent silicon nanoparticles. The particles were coated with silica using a microemulsion process that was adapted to the fragile silicon nanoparticles. The as-produced core–shell particles have a mean diameter of 35 nm and exhibit the intrinsic photoluminescence of the silicon core. The silica layer protects the core from aqueous oxidation for several days, thus allowing the use of the nanoparticles for biological applications. The nanoparticles were further coated with amines and functionalized with polyethylene glycol chains and the toxicity of the particles has been evaluated at the different stages of the process. The core–shell nanoparticles exhibit no acute toxicity towards lung cells, which is promising for further development.     

Formation mechanisms of colloidal silica via sodium silicate

Colloidal silica is formed by titrating active silicic acid into a heated KOH with seed solution. The colloidal silica formation mechanisms are investigated by sampling the heated solution during titration. In the initial stage, the added seeds were dissolved. This might due to the dilution of seed concentration, the addition of potassium hydroxide (KOH) and the heating at 100°C. Homogenous nucleation and surface growth occur simultaneously in the second stage of colloidal silica formation. Homogenous nucleation is more important when the seed concentration is relatively low. On the other hand, surface growth plays an important role when the seed concentration is increased. In the middle seed concentration, the seed particles grow up and some new small particles are born by the homogenous nucleation process to form a bimodal size distribution product. As the titrating volume of active silicic acid exceeds a specific value in the last stage the particle size increases rapidly and the particle number decreases, which may be caused by the aggregation of particles. The intervals between each stage were varied with the seed concentration. Increasing the seed concentration led to the formation of uniform particle size colloidal silica.     

Colloidal Core–Satellite Supraparticles via Preprogramed Burst of Nanostructured Micro‐Raspberry Particles

Colloidal molecules, or more general supraparticles, i.e., particles which are themselves assembled of smaller nanoparticles in a defined way, are known to be synthesizable via bottom‐up assembly techniques in colloidal dispersion. The amount of synthesizable particles is mostly limited to milligrams. Herein, a bottom‐up‐programed, triggerable top‐down process is reported to obtain core–satellite supraparticles, i.e., particles composed of a larger core particle surrounded by smaller satellite particles. The key is to prepare a nanostructured, microparticulate powder into which defined burst behavior is preprogramed. Once the system is mechanically triggered, it bursts into well‐defined nanosized core–satellite supraparticles. Scale‐up is easily feasible and several hundred grams per batch can be demonstrated. The product is a ready‐to‐use and flexibly processible powder. Upon simple mixing with a polymer, it disintegrates into the preprogramed core–satellite supraparticles, thus forming a highly sophisticated nanocomposite with the polymer matrix. A pure silica nanoparticle system and a silica–iron oxide nanoparticle hybrid system are presented to demonstrate the versatility of the approach. Enhanced mechanical and unexpected magneto‐optical properties with the particle system are found. The disintegration of the microparticles into individual core–satellite colloidal supraparticles is confirmed via in situ liquid cell transmission electron microscopy.     

Spectra of resonance light scattering of gold nanoshells: Effects of polydispersity and limited electron free path

The effects of the polydispersity of the structure of gold nanoshells and of the limited electron free path in a thin metal layer on the spectra of resonance light scattering of a suspension of two-layer nanoparticles are studied theoretically and experimentally for the first time. It is shown theoretically that both factors lead to a broadening of the plasmon resonance in light scattering and to a change in its magnitude. To experimentally test the calculations, two samples of nanoshells based on gold and silicon dioxide (silica) were synthesized. Nanoshells of sample 1 have a diameter of the core of 90 nm and a broad thickness distribution of shells (with an average value of 30 nm), whereas nanoshells of sample 2 have a diameter of the core of 70 nm and a narrow thickness distribution of shells (with an average value of 12 nm). The core diameter, the shell thickness, and the polydispersity of the structure of nanoparticles are estimated by dynamic light scattering. It is shown that the simulation of the optical properties of nanoparticles with their parameters estimated from the dynamic light scattering data makes it possible to obtain good agreement between experimental and theoretical spectra of light scattering. For nanoshells of sample 1, the inhomogeneous broadening of the scattering spectrum is completely determined by the polydispersity; therefore, the bulk constants of gold can be used in simulation of the spectra of such nanoshells. The main mechanism of the broadening for nanoshells of sample 2 is connected with the limitation of the free path length of electrons, whereas the contribution from the thickness distribution of shells can be neglected.     

Preparation of composite with silica-coated nanoparticles of iron oxide spinels for applications based on magnetically induced hyperthermia

It is reported a novel method to prepare magnetic core (iron oxide spinels)–shell (silica) composites containing well-dispersed magnetic nanoparticles in aqueous solution. The synthetic process consists of two steps. In a first step, iron oxide nanoparticles obtained through co-precipitation are dispersed in an aqueous solution containing tetramethylammonium hydroxide; in a second step, particles of this sample are coated with silica, through hydrolyzation of tetraethyl orthosilicate. The intrinsic atomic structure and essential properties of the core–shell system were assessed with powder X-ray diffraction, Fourier transform infrared spectrometry, Mössbauer spectroscopy and transmission electron microscopy. The heat released by this ferrofluid under an AC-generated magnetic field was evaluated by following the temperature evolution under increasing magnetic field strengths. Results strongly indicate that this ferrofluid based on silica-coated iron oxide spinels is technologically a very promising material to be used in medical practices, in oncology.     

Bimetallic Nanoparticles with Exotic Facet Structures via Iodide‐Assisted Reduction of Palladium

Iodide is arguably the most challenging halide to control as a shape‐directing additive in metal nanoparticle synthesis and the addition of iodide during bimetallic nanoparticle growth often leads to inhomogeneously stellated products. Through judicious control of low micromolar concentrations of iodide ions in solution in a seed‐mediated approach, alloyed gold–palladium tetradecapod nanoparticles have been synthesized with a mixture of both well‐defined convex and concave surfaces. Notably, these particles are uniform and symmetrical, and this unusual combination of convex and concave features in a single nanostructure is not simply an artifact of intersecting spikes, as would be the case with stellated particles. Further, an important new role for iodide in catalyzing the reduction of palladium ions is identified, particularly at the edge sites of the growing gold nanoparticles. This differs from the commonly accepted theory that iodide slows metal ion reduction, and thus opens up promising new routes to the synthesis of other bimetallic nanoparticles with exotic shapes and surface structures.     

SHINERS and plasmonic properties of Au Core SiO2 shell nanoparticles with optimal core size and shell thickness

Shell‐isolated nanoparticles (NPs)‐enhanced Raman spectroscopy (SHINERS) can be potentially applied to virtually any substrate type and morphology. How to take a step forward to prepare SHINERS NPs (SHINs) with superior performance is critical for the practical applications of surface‐enhanced Raman scattering (SERS) in the breadth and depth. Here, we present a method to obtain 120 nm diameter gold NPs coated with ultrathin silica shells (1–4 nm). The silica shell can be controlled growth through carefully tuning a series of parameters, such as amount of 3‐aminopropyl triethoxysilane used, pH, reaction time, and reaction temperature. We compare the enhancement factor of the obtained 120 nm Au with a 4 nm silica shell NPs to the 55 nm Au with a 4 nm silica shell NPs, and the activity of a 120 nm SHINs is nearly 24 times that the 55 nm SHIN from a single particle view. We also compare the enhancement factor of 1 nm silica shell Au@SiO₂ NPs with the bare Au NPs. The enhancement factor of 1 nm silica shell Au@SiO₂ NPs was found to be about twice that of the bare particles. For a deeper understanding of the source of the giant enhanced electrical field of the 1 nm silica shell Au@SiO₂ NPs, we study the plasmonic property of single 1 nm silica shell Au@SiO₂ NP on a gold film substrate through correlation of the structure of single NP using SEM with its SPR spectroscopy. We find that the multipolar interaction between the single Au@SiO₂ NP and gold film substrate is important for the SERS. Our studies on the performance of 120 nm SHINs and the plasmonic property of these particles can significantly expand the applications of SHINERS technique and improve the understanding of physical nature of SHINs. Copyright © 2013 John Wiley & Sons, Ltd.     

The effect of pH value on the formation of gold nanoshells

The effect of the pH value of gold ion plating (PCG) solution on the growth of gold nanoshells with about 100-nm silica cores and 15-nm gold shells was systematically investigated by dynamic light scattering, UV–Vis spectroscopy, scanning electron microscopy, and transmission electron microscopy. The results showed that in PCG solutions, the rapid formation of monomers which intensively influenced by the reducibility of formaldehyde contributed to the growth of gold nanoshells. The reducibility of formaldehyde increased following the increasing pH value from 4.70 to 11.71. Thus at high pH value, it was easy to fabricate the complete gold nanoshells (except of the Cannizzaro reaction of formaldehyde at high pH value). Furthermore, we found that chloride ions also influenced the surface morphology of gold nanoshells by Ostwald ripening process through the Au–Cl complex.     

Bioconjugated Core–Shell Microparticles for High‐Force Optical Trapping

Due to their high spatial resolution and precise application of force, optical traps are widely used to study the mechanics of biomolecules and biopolymers at the single‐molecule level. Recently, core–shell particles with optical properties that enhance their trapping ability represent promising candidates for high‐force experiments. To fully harness their properties, methods for functionalizing these particles with biocompatible handles are required. Here, a straightforward synthesis is provided for producing functional titania core–shell microparticles with proteins and nucleic acids by adding a silane–thiol chemical group to the shell surface. These particles display higher trap stiffness compared to conventional plastic beads featured in optical tweezers experiments. These core–shell microparticles are also utilized in biophysical assays such as amyloid fiber pulling and actin rupturing to demonstrate their high‐force applications. It is anticipated that the functionalized core–shells can be used to probe the mechanics of stable proteins structures that are inaccessible using current trapping techniques.     

Tunable Fano resonances in silver–silica–silver multilayer nanoshells

We study the plasmonic properties of silver–silica–silver multilayer nanoshells using finite-difference time-domain methods. Silver is a weakly dissipating metal and is able to support higher order resonances compared to strongly dissipating metals like gold. We show that Fano resonances occur even in symmetric cases. Symmetry breaking via the introduction of core offset further enhances these Fano resonance peaks and leads to the appearance of higher order resonances. The optical properties of the multilayer nanoshells are explained using the plasmon hybridization theory and the results are compared to similar multilayer nanoshells with gold core and outer shell.     

Preparation and characterization of core/shell particles with siloxane in the shell

The core/shell particles consisting of polystyrene core and 3-(methacryloxypropyl)-trimethoxysilane (MPS) shell were prepared in the present study by successive seeding polymerization under kinetically controlled conditions and were characterized by particle size analyser, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The TEM image indicated that the particles containing organic siloxane presented an evident core/shell structure. Additionally, the study of XPS also revealed that MPS could be grafted onto the surface of polystyrene microspheres and the atomic ratio of C/Si on the surface of the core/shell particles (MPS-40) was very close to the ratio of C/Si in the molecule of MPS. The surface properties of the films produced from the core/shell particles were also investigated by the static contact angle method. Compared with the homopolymer of PS, the core/shell particles were more effective to create hydrophobic surface, so, the introduction of MPS was capable of obvious increase in water repellency.     

Spectroturbidimetric determination of the size,concentration, and refractive index of silica nanoparticles

A method for determining the refractive index, size, and concentration of silica nanoparticles used as cores in synthesis of gold nanoshells is described. The average refractive index of silica nanoparticles, n = 1.475 ± 0.005, is determined by a modified immersion method, which involves spectroturbidimetry data in immersion media (dimethyl sulfoxide + ethanol). Working calibrations are obtained, which allow one to determine the size and concentration of silica particles and, correspondingly, the concentration of gold nanoshells in final preparations from measured values of the wavelength exponent and optical density.     

Structural and optical tunability of metallodielectric composites with gradual shell growth

Metallodielectric (gold@silica) composites were prepared by seed and grow method. The dielectric microspheres (core material) of an average size of 400 nm were synthesized by sol–gel method and gold nanoparticles (AuNPs) were prepared by reducing the chloroauric solution. Shell growth around silica (SiO₂) microspheres was carried out in a multistep layer-by-layer process. The synthesized composites were characterized using techniques such as field emission-scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and UV–Visible (UV–Vis) spectroscopy. FE-SEM and FTIR analyses have confirmed the functionalization of SiO₂ surfaces with the amine terminal group along with the gold shell growth. XRD analysis has given an average crystallite size of 12.3 nm for metallodielectric composites. Absorption spectra have demonstrated the dependence of surface plasmon resonance (SPR) peak on the successive shell growth by exhibiting a red shift.     

Five-Level Structural Hierarchy: Microfluidically Supported Synthesis of Core–Shell Microparticles Containing Nested Set of Dispersed Metal and Polymer Micro and Nanoparticles

This study presents the development of a hierarchical design concept for the synthesis of multi-scale polymer particles with up to five levels of organization. The synthesis of core–shell microparticles containing nested sets of dispersed metal and polymer micro- and nanoparticles is achieved through in situ photopolymerization using a double co-axial capillaries microfluidic device. The flow rates of the carrier, shell, and core phases are optimized to control particle size and result in stable core–shell particles with well-dispersed three-level composites in the shell matrix. The robustness and reversibility of these core–shell particles are demonstrated through five cycles of drying and re-swelling, showing that the size and structure of core–shell particles remain unchanged. Additionally, the permeability and mobility of dye molecules within the shell matrix are tested and showed that different molecular weight dyes have different penetration times. This study highlights the potential of microfluidics as a powerful tool for the controlled and precise synthesis of complex structured materials and demonstrates the versatility and potential of these core–shell particles for sensing applications as particle-based surface-enhanced Raman scattering (SERS).