Engineering Surface Patterning of Colloidal Rings through Plateau–Rayleigh Instability

Plateau–Rayleigh (P‐R) instability occurring on Brownian colloidal particles is presented. This instability can be used for the surface patterning of Brownian colloidal rings. This idea was realized by employing polystyrene(PS)/SiO₂ core/shell rings, for which PS layer was selectively grown onto the interior surface of SiO₂ rings. The P‐R instability was initiated in the ring's dispersion by adding a good solvent of PS. By using both experiments and theory, it is shown that the number of patches is tunable and that it is linearly related to a function of two variables, namely, solvent quantity and contact angle. In particular, one‐patch Janus rings and patchy disks were also synthesized at high yields. The patch size of all particles is tunable by step‐by‐step polymerization and the patches can be functionalized, for example by ATRP grafting with pH‐sensitive polymers. This approach can be adapted for the synthesis of other patchy colloids with designated complexity.     

Crystallization,Vitrification, and Gelation of Patchy Colloidal Particles

We present the phase diagrams for neutral patchy colloidal particles whose surface is decorated by different number of identical patches, where each patch serves as an associating site. The hard-core Lennard-Jones (LJ) potential and associating interaction are incorpo-rated into the free energies of patchy particles in phases of the fluid (F), random close packing (RCP), and face-centered-cubic (FCC) crystal. A rich phase structure of patchy particles with F-F, F-RCP, and F-FCC transitions can be observed. Meanwhile, the sol-gel transition (SGT) characterizing the connectivity of patchy particles is also investigated. It is shown that, depending on the number of patches and associating energy, the F-F transition might be metastable or stable with respect to the F-RCP and F-FCC transitions. Meanwhile, the critical temperatures, critical densities, triple points, and SGT can be significantly regulated by these factors.     

Some Aspects of Seedless Synthesis of Gold Nanorods

Seedless synthesis of gold nanorods with the use of sodium borohydride and hydroquinone as reductants of metal ions has been systematically studied. The effect of reaction system composition on the morphology and optical characteristics of the formed particles has been determined. It has been found that the position of the band of the longitudinal surface plasmon resonance of the nanorods varies nonmonotonically with variations in the concentration of hydroquinone or silver nitrate. The seedless synthesis has been shown to yield high-quality gold nanorods, with the tunable position of their longitudinal surface plasmon resonance in a wide spectral range (from 700 to ~1050 nm). Therewith, the conversion of metal ions is no lower than 78 wt %.     

Computational Study of the Surface‐Enhanced Raman Scattering from Silica‐Coated Silver Nanowires

Surface‐enhanced Raman scattering (SERS) is a popular vibrational spectroscopic technique that can have several applications in chemical and biological sensing. Within the last decade or so, our ability to chemically synthesize nanostructures has improved to the point that the rational design of a variety of SERS substrates is now viable. In this report, we describe a computational study using the finite element method (FEM) to investigate the effects of patchy silica coatings on silver nanowires. We found that varying the degree of silica coating on silver nanowires impacts the enhancement and may be explained through two processes. The first process is a consequence of changes in the dielectric environment surrounding the nanowire due to the silica. As additional layers of silica coat the nanowire, the localized surface plasmon resonance of the nanowire redshifts. The second process is a result of silica distorting the local electric field around the nanowire surface. Anisotropic silica coating can influence anticipated enhancement depending on its spatial localization with respect to excited plasmon modes in the nanowire. We propose that the design of nanostructures with patchy silica coatings can be a viable tool for increasing surface enhancement.     

Micrometer scale adhesion on nanometer-scale patchy surfaces: adhesion rates, adhesion thresholds, and curvature-based selectivity

Using a model system based on electrostatics, we probe interactions between spherical particles (negative silica) and planar surfaces that present randomly placed discrete attractive regions, 10 nm in size, in a repulsive background (silica flats carrying cationic surface constructs). Experiments measure the adhesion rates of particles onto the patchy collecting surfaces from flowing dispersions, as a function of the surface loading of the attractive patches, for different particle sizes (0.5 and 1 mum diameter spheres) and different ionic strengths. Surfaces densely populated with patches, such that they present net electrostatic attractions to approaching particles, capture particles at the transport-limited (maximum) rate. Surfaces sparsely loaded with attractive patches (which present a repulsive mean field to approaching particles) are usually still adhesive, but the particle adhesion rate depends on particle size, ionic strength, and patch loading. Most significant is an adhesion threshold, a critical density of patches needed to capture particles. This threshold, which occurs at average patch spacings of 30 nm and larger and which can be tuned through ionic strength, comprises the ability of the patchy surfaces to selectively distinguish particles of different sizes or objects of different local curvature or roughness. The observation of such an adhesion threshold implicates spatial fluctuations in patch arrangement. In addition to experiments, this paper develops arguments for lengthscales that govern adhesion rate behavior, comparing particle geometry and fluctuation lengthscales, and then demonstrating qualitative consistency with the localized colloidal potentials involved.     

Preparation, characterization, and optical properties of gold, silver, and gold-silver alloy nanoshells having silica cores

This report describes the structural and optical properties of a series of spherical shell/core nanoparticles in which the shell is comprised of a thin layer of gold, silver, or gold-silver alloy, and the core is comprised of a monodispersed silica nanoparticle. The silica core particles were prepared using the St?ber method, functionalized with terminal amine groups, and then seeded with small gold nanoparticles (approximately 2 nm in diameter). The gold-seeded silica particles were coated with a layer of gold, silver, or gold-silver alloy via solution-phase reduction of an appropriate metal ion or mixture of metal ions. The size, morphology, and elemental composition of the composite nanoparticles were characterized by field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, thermal gravimetric analysis (TGA), dynamic light scattering (DLS), and transmission electron microscopy (TEM). The optical properties of the nanoparticles were analyzed by UV-vis spectroscopy, which showed strong absorptions ranging from 400 nm into the near-IR region, where the position of the plasmon band reflected not only the thickness of the metal shell, but also the nature of the metal comprising the shell. Importantly, the results demonstrate a new strategy for tuning the position of the plasmon resonance without having to vary the core diameter or the shell thickness.     

Shape-Shifting Patchy Particles

A facile method to synthesize shape-shifting patchy particles on the colloidal scale is described. The design is based on the solvent-induced shifting of the patch shape between concave and convex features. The initial concave patchy particles were synthesized in a water suspension by a swelling-induced buckling process. Upon exposure to different solvents, the patches were tuned reversibly to be either concave or convex. These particles can be assembled into chained, branched, zigzag, and cyclic colloidal superstructures in a highly site-specific manner by surface–liquid capillary bridging. The biphasic nature of the particles also enables site-selective surface functionalization.     

Preparation and properties of silica–gold core–shell particles

Silica-metal core–shell particles, as for instance those having siliceous core and nanostructured gold shell, attracted a lot of attention because of their unique properties resulting from combination of mechanical and thermal stability of silica and magnetic, electric, optical and catalytic properties of metal nanocrystals such as gold, silver, platinum and palladium. Often, the shell of the core–shell particles consists of a large number of metal nanoparticles deposited on the surface of relatively large silica particles, which is the case considered in this work. Namely, silica particles having size of about 600 nm were subjected to surface modification with 3-aminopropyltrimethoxysilane. This modification altered the surface properties of silica particles, which was demonstrated by low pressure nitrogen adsorption at ?196 °C. Next, gold nanoparticles were deposited on the surface of aminopropyl-modified silica particles using two strategies: (i) direct deposition of gold nanoparticles having size of about 10 nm, and (ii) formation of gold nanoparticles by adsorption of tetrachloroauric acid on aminopropyl groups followed by its reduction with formaldehyde.The overall morphology of silica–gold particles and the distribution of gold nanoparticles on the surface of modified silica colloids were characterized by scanning electron microscopy. It was shown that direct deposition of colloidal gold on the surface of large silica particles gives more regular distribution of gold nanopartciles than that obtained by reduction of tetrachloroauric acid. In the latter case the gold layer consists of larger nanoparticles (size of about 50 nm) and is less regular. Note that both deposition strategies afforded silica–gold particles having siliceous cores covered with shells consisting of gold nanoparticles of tunable concentration.     

Shape‐Shifting Patchy Particles

A facile method to synthesize shape‐shifting patchy particles on the colloidal scale is described. The design is based on the solvent‐induced shifting of the patch shape between concave and convex features. The initial concave patchy particles were synthesized in a water suspension by a swelling‐induced buckling process. Upon exposure to different solvents, the patches were tuned reversibly to be either concave or convex. These particles can be assembled into chained, branched, zigzag, and cyclic colloidal superstructures in a highly site‐specific manner by surface–liquid capillary bridging. The biphasic nature of the particles also enables site‐selective surface functionalization.     

银离子在二氧化硅粒子表面上的成核反应

在含有乙醇的SiO₂溶胶中,Ag⁺不管在碱性、中性或在弱碱、弱酸情况下都能在SiO₂粒子表面上还原成核,形成金属Ag颗粒.研究表明,银的等离子体共振吸收峰的移动是尺寸效应和表面效应共同作用的结果.     

Facile synthesis of silver core - silica shell composite nanoparticles

Combining metal nanoparticles and dielectrics (e.g. silica) to produce composite materials with high dielectric constant is motivated by application in energy storage. Control over dielectric properties and their uniformity throughout the composite material is best accomplished if the composite is comprised of metal core - dielectric shell structured nanoparticles with tunable dimensions. We have synthesized silver nanoparticles in the range of 40-100nm average size using low concentration of saccharide simultaneously as the reducing agent and electrostatic stabilizer. Coating these silver particles with silica from tetraalkoxysilanes has different outcomes depending on the alcoholic solvent and the silver particle concentration. A common issue in solution-based synthesis of core-shell particles is heterogeneous nucleation whereupon two populations are formed: the desired core-shell particles and undesired coreless particles of the shell material. We report the formation of Ag@SiO(2) core-shell particles without coreless silica particles as the byproduct in 2-propanol. In ethanol, it depends on the silver surface area available whether homogeneous nucleation of silica on silver is achieved. In methanol and 1-butanol, core-shell particles did not form. This demonstrates the significance of controlling the tetraalkoxysilane hydrolysis rate when growing silica shells on silver nanoparticles.     

Synthesis of gold functionalized poly-bisvinylethyleneurea/silica hybrid materials

Novel gold/poly-(1,3-divinyl-imidazolid-2-one)/silica [poly-bisvinylethyleneurea (poly-BVU)/silica)] hybrid particles have been produced by adsorption and spontaneously occurring in situ reduction of Au(3+) cations on the surface of poly-BVU/silica hybrid particles. The successful functionalization of the poly-BVU/silica particles with gold nanoclusters has been evidenced by UV/vis and XPS spectroscopy as well as scanning electron microscopy. The size of the resulting gold clusters, estimated by means of the Mie-Drude theory on the full peak width at half-maximum of the surface plasmon UV/vis absorbance, correlates with the polymer content of the poly-BVU/silica hybrid particles used for the modification. Therefore, it is possible to control the size of the gold clusters simply by adjusting the monomer/silica ratio in the polymerization process, which corresponds with the polymer content of the hybrids.     

New multifunctional nanoparticles with mesoporous cores and silver shells

The feasibility of preparing continuous silver shells of different thicknesses on mesoporous silica particles is demonstrated for the first time. The resulting composite nanoparticles with a tuned localized surface plasmon resonance can simultaneously be used as containers for a number of compounds, e.g., drugs for combined therapy of tumors.     

Enzyme-encapsulated silica monolayers for rapid functionalization of a gold surface

We report a simple and rapid method for the deposition of amorphous silica onto a gold surface. The method is based on the ability of lysozyme to mediate the formation of silica nanoparticles. A monolayer of lysozyme is deposited via non-specific binding to gold. The lysozyme then mediates the self-assembled formation of a silica monolayer. The silica formation described herein occurs on a surface plasmon resonance (SPR) gold surface and is characterized by SPR spectroscopy. The silica layer significantly increases the surface area compared to the gold substrate and is directly compatible with a detection system. The maximum surface concentration of lysozyme was found to be a monolayer of 2.6 ng/mm(2) which allowed the deposition of a silica layer of a further 2 ng/mm(2). For additional surface functionalization, the silica was also demonstrated to be a suitable matrix for immobilization of biomolecules. The encapsulation of organophosphate hydrolase (OPH) was demonstrated as a model system. The silica forms at ambient conditions in a reaction that allows the encapsulation of enzymes directly during silica formation. OPH was successfully encapsulated within the silica particles and a detection limit for the substrate, paraoxon, using the surface-encapsulated enzyme was found to be 20 microM.     

Colloidal synthesis of new silver-based nanostructures with tailored localized surface plasmon resonance

Silver-based nanostructures with tailored localized surface plasmon resonance are of interest for a number of practical applications. They can conventionally be divided into three main groups: (1) anisotropic silver particles, (2) particles of alloys of silver with other metals, and (3) composite particles with dielectric or magnetic cores and silver shells. Fine “tuning” of plasmon resonance of these particles is ensured by changes in their shapes, composition, and/or structure. Procedures for the colloidal synthesis of nanostructures of all these groups and some fields of their application are described, with the main attention focused on core/shell composite particles.     

帽状铝纳米粒子的制备及表面等离子共振特性

金属纳米材料具有许多独特的物理和化学性质,其中一个重要的光学性质就是表面等离子共振,然而在大多数情况下,金属纳米粒子表面等离子共振所产生的吸收峰被限制在相对狭小的范围内,很难进行调谐。近年来,以电介质为核金属为壳的核壳结构复合纳米材料成功的解决了这一问题,通过设计和剪裁内核的直径与外壳层厚度的比值,可以实现光学性质可调的特性[1～5]。此类复合材料可被广泛应用于光催化、传感器、光信息存储、生物光子学、生物医学等领域[6～11]。美国莱斯大学及德州的研究人员利用这类核壳结构纳米材料成功地实现了对体外乳腺肿瘤的杀灭实验[12]。在这种类型的材料中,对称性降低的即不完全包裹的纳米粒子如杯状[13]、帽状[13,14]、半球壳状[15]、月牙状[16]等核壳结构复合粒子由     

Synthesis and plasmonic properties of silver and gold nanoshells on polystyrene cores of different size and of gold–silver core–shell nanostructures

Simple methods of preparing silver and gold nanoshells on the surfaces of monodispersed polystyrene microspheres of different sizes as well as of silver nanoshells on free-standing gold nanoparticles are presented. The plasmon resonance absorption spectra of these materials are presented and compared to predictions of extended Mie scattering theory. Both silver and gold nanoshells were grown on polystyrene microspheres with diameters ranging from 188 to 543 nm. The commercially available, initially carboxylate-terminated polystyrene spheres were reacted with 2-aminoethanethiol hydrochloride (AET) to yield thiol-terminated microspheres to which gold nanoparticles were then attached. Reduction of silver nitrate or gold hydroxide onto these gold-decorated microspheres resulted in increasing coverage of silver or gold on the polystyrene core. The nanoshells were characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and UV–vis spectroscopy. By varying the core size of the polystyrene particles and the amount of metal (silver or gold) reduced onto them, the surface plasmon resonance of the nanoshell could be tuned across the visible and the near-infrared regions of the electromagnetic spectrum. Necklace-like chain aggregate structures of gold core–silver shell nanoparticles were formed by reducing silver nitrate onto free citrate-gold nanoparticles. The plasmon resonance absorption of these nanoparticles could also be systematically tuned across the visible spectrum.     

Reverse microemulsion-mediated synthesis of silica-coated gold and silver nanoparticles

A reverse microemulsion method is reported for preparing monodispersed silica-coated gold (or silver) nanoparticles without the use of a silane coupling agent or polymer as the surface primer. This method enables a fine control of the silica shell thickness with nanometer precision. As compared to the St?ber method reported for direct silica coating, which can only coat large gold particles ( approximately 50 nm in diameter) at low concentrations (<1.5 x 10(10) particles/mL), this new approach is capable of coating gold particles of a wide range of sizes (from 10 to 50 nm) at a much higher concentration ( approximately 1.5 x 10(13) particles/mL). Moreover, it enables straightforward surface functionalization via co-condensation between tetraethyl orthosilicate and another silane with the desired functional groups. The functional groups introduced by this method are readily accessible and thus useful for various applications.     

Electroless gold island thin films: photoluminescence and thermal transformation to nanoparticle ensembles

Electroless gold island thin films are formed by galvanic replacement of silver reduced onto a tin-sensitized silica surface. A novel approach to create nanoparticle ensembles with tunable particle dimensions, densities, and distributions by thermal transformation of these electroless gold island thin films is presented. Deposition time is adjusted to produce monomodal ensembles of nanoparticles from 9.5 +/- 4.0 to 266 +/- 22 nm at densities from 2.6 x 1011 to 4.3 x 108 particles cm-2. Scanning electron microscopy and atomic force microscopy reveal electroless gold island film structures as well as nanoparticle dimensions, densities, and distributions obtained by watershed analysis. Transmission UV-vis spectroscopy reveals photoluminescent features that suggest ultrathin EL films may be smoother than sputtered Au films. X-ray diffraction shows Au films have predominantly (111) orientation.