Optical Properties of Ag and Au Nanoparticles Dispersed within the Pores of Monolithic Mesoporous Silica

The optical absorption of silver and gold nanoparticles dispersed within the pores of monolithic mesoporous silica upon annealing at elevated temperatures has been investigated. With decreasing particle size, the surface plasmon resonance position of the particles blue-shifts first and then red-shifts for silver/silica samples, but only red-shifts for gold/silica samples. This size evolution of the resonance position is completely different from that previously reported for fully embedded particles. We assume a local porosity at the particle/matrix interface, such that free surface of particles within the pores may be in contact with ambient air, and present a two-layer core/shell model to calculate the optical properties. These calculations also consider deviations from the optical constants of bulk matter to account for corresponding effects below about 10 nm particle size. From the good agreement between experimental results and model calculations, we conclude a peculiar particle/ambience interaction dominating the size evolution of the resonance. Because of the difference of core electron structure, the relative importance of the effects of local porosity and free surface, respectively, are different for silver and gold. For silver, the effect of the local porosity is stronger, but for gold the opposite is found.     

Gold nanoparticles enclosed in silica xerogels by high-pressure processing

The dispersion of gold nanoparticles (AuNPs) in solid matrices, maintaining their optical properties as in liquid medium, has been explored, enlarging the possibilities of handling and applications of these nanoparticles. In this study, AuNPs were successfully enclosed in silica using the sol–gel method and high-pressure processing at 7.7 GPa, at room temperature, obtaining compacts with densities as high as 85% of silica glass and Vickers microhardness of 50% of quartz. This densification was confirmed by the reduced values of specific surface area and closed pores. UV–Vis spectra showed that the AuNPs maintain their optical properties, in the monoliths and in the compacts, presenting an absorption band in the characteristic region of AuNPs smaller than 20 nm. The slight red-shifts of the absorption peaks cannot be explained in the same way as for AuNPs in solution, where it is associated to an increase in particle diameter. In solid samples, additional factors as matrix properties should be considered. Transmission electron microscopy (TEM) images showed that the AuNPs have spherical shape both in solution and in the monolith, with an average diameter not exceeding 10 nm. It was also observed that the high-pressure processing did not destroy or misshape the AuNPs.     

Diagnostics of aqueous colloids of noble metals by extinction modeling based on mie theory

Two-factor dependences of the maximum and half-width of a surface plasmon resonance band on both the average diameter of nanoparticles and the scatter in their particle-size distributions were defined for colloidal silver and gold aqueous solutions based on modeling the extinction effectiveness factor by Mie theory. The obtained three-dimensional surfaces determined the shape of calibration curves used to define the average particle diameters and the scatter in their particle-size distributions from measurements of the maximum and half-width of the surface plasmon resonance band in spectra of the silver and gold colloidal solutions. The calibration curves were correlated with experimental samples of aqueous ultradispersed media containing silver and gold nanoparticles.     

胶体金纳米颗粒的表面等离子体发射特性

利用电化学方法制备出粒径为20-80 nm的胶体金纳米颗粒。研究其荧光发射光谱特性,在485nm处观察到表面自由电子集体激发导致的表面等离子体共振发射峰,其位置不随激励光波长的变化而移动。当激励光波长为485 nm时,观察到最强的发射峰。在240和640 nm处,还观察到倍频发射峰和3/4分频发射峰。增加金纳米颗粒粒径,观察到发射谱的峰值增大而发射峰的位置只有很小的红移。     

Photoinduced formation and aggregation of silver nanoparticles at the surface of carboxymethylcellulose films

Formation and aggregation of photolytic silver nanoparticles at the surface of silver salt of carboxymethylcellulose films (CMCAg films) have been investigated. Detailed X-ray photoelectron spectroscopy (XPS) study and field emission type scanning electron microscopy (FE-SEM) observation have been carried out to characterize silver nanoparticles at the film surface. When the CMCAg films were irradiated with UV light in wet air at room temperature for 30–60 min, silver nanoparticles of ca. 10 nm size were formed at the irradiated surface. According to the FE-SEM observation, the growth of the particle diameter and aggregation of nanoparticles took place after prolonged irradiation, and finally, the irradiated side of the film surface was densely covered with the silver nanoparticles of ca. 35 nm size. Chemical composition analysis by the XPS measurements has confirmed the increase in the atomic concentration of silver with irradiation time. It is suggested that silver atoms and clusters can move in the film and precipitate at the irradiated surface.     

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.     

Solgel synthesis and structural characterization of silver-silica nanocomposites

The solgel process has been successfully used to prepare silver/silica nanocomposites. After drying in air at 50°C for 30 min, samples were heat treated in air, at 100, 200, 400 and 500°C for the formation of silver nanoparticles. Evolution of silver nano-particles in the amorphous SiO₂ matrix as a function of annealing temperature has been studied. Characterizations were made by X-ray diffraction, ultraviolet-visible, and infrared spectroscopy. Mechanisms of silver clusters formation in the densified silica matrix with respect to thermal treatment are discussed.     

A novel method of nanocrystal fabrication based on laser ablation in liquid environment

Metal nanoparticles can be prepared by a novel technique that consists of the laser ablation of a solid target immersed in a water solution of a metal salt. Silicon was chosen as the most adequate target to synthesize silver and gold nanoparticles from a water solution of either AgNO₃ or HAuCl₄. The influence of both the silver nitrate concentrations and the irradiation time of the Si target on the optical properties of the Au and Ag nanoparticles have been investigated. The crystalline nature of the metal nanoparticles has been determined by X-ray diffraction (XRD). Average size and particle size distribution have been measured by means of TEM. The absorbance spectra show the characteristic band of the surface resonant plasmon of silver and gold nanoparticles.     

Real-time SAXS and ultraviolet-visible spectral studies on size and shape evolution of gold nanoparticles in aqueous solution

Gold nanoparticles were synthesized with seed mediated wet growth method. A coexistent system with gold nanorods and nanospheres was prepared. In-situ real-time small angle X-ray scattering (SAXS) and ultraviolet-visible (UV-vis) absorption were used to monitor the growth process of Au nanoparticles in the aqueous solution. Combining SAXS measurement and UV-vis spectra, the contribution of gold nanorods was separated from that of gold nanospheres. The growth behavior of the gold nanorods and the gold nanospheres was studied. The size and shape evolution of both gold nanoparticles as well as their volume fractions were obtained. After adding the seed-solution into the growth solution to form a particle suspension, within the first 18 min, a mutual competitive growth occurred between the gold nanorods and nanospheres. And their average particle sizes always increased with growth time. 18-min later, the evolution of size and shape of gold nanoparticles was almost stopped. The results demonstrate that the aspect ratio of gold nanorods in the particle suspension follows an exponential decay change after the initial 5-min growth. The evolution process of size and shape of gold nanoparticles in the particle suspension has been discussed.     

Size characteristics of surface plasmons and their manifestation in scattering properties of metal particles

The change of the scattering properties of sodium, gold and silver spherical particles with size is discussed in the context of surface multipolar plasmon resonances. The presented surface plasmon size characteristics are abstracted from the quantity which is observed and deliver multipolar plasmon resonance frequencies and plasmon damping rates in the form of a continuous function of particle radius. The performed analysis of the plasmon dispersion relation is analogous to the problem of surface plasmon localized at a semi-infinite, flat metal/dielectric interface.Correlation between the multipolar plasmon resonance parameters, and the spectroscopic optical properties of conductive nanoparticles appearing as peaks in the measurable light intensities is analyzed. We discuss the fact, that such peaks arise from interference of all the electromagnetic fields contributing to the measured intensity, and not solely to the fields due to surface plasmon multipolar modes.We describe the results of light scattering experiment in orthogonal polarization geometries with use of spontaneously growing sodium droplets. The polarization geometry of the experiment allows for distinct separation of resonant contribution of dipole and quadrupole plasmon TM mode contributions to the measured intensities as a function of size.Predictions concerning size characteristics for dipole and quadrupole plasmons are compared with the results of light scattering experiments using spherical sodium droplets (our results) and gold and silver particles in suspension [other authors: Sönnichsen C, Franzl T, Wilk T, von Plessen G, Feldmann J. Plasmon resonances in large noble-metal clusters. New J Phys 2002; 4:93.1–8; Haiss W, Thanh NTK, Aveyard J, Fernig DG. Determination of size and concentration of gold nanoparticles from UV–vis spectra. Anal Chem 2007; 79:4215–21; Njoki PN, Lim I-IS, Mott D, Park H-Y, Khan B, Mishra S, et al. Size correlation of optical and spectroscopic properties for gold nanoparticles. J Phys Chem C 2007; 111:14664–9; Mock JJ, Barbic M, Smith DR, Schultz DA, Schultz S. Shape effects in plasmon resonance of individual colloidal silver nanoparticles. J Chem Phys 2002; 116:6755–9].     

Synthesis and characterization of silica-gold core-shell (SiO<Subscript>2</Subscript>@Au) nanoparticles

This paper reports a systematic investigation of the growth and attachment of small gold nanoparticles to the functionalized surface of larger silica nanoparticles by three different methods. Nearly monodispersed silica particles and gold nanoparticles were prepared by sol-gel method. The size of the particle could be altered by changing the concentration of reactants, temperature and the time for which they react. The nanocoreshell particles prepared by three different methods were studied using scanning electron microscopy (SEM), UV-vis spectroscopy and Fourier transform infrared spectroscopy. We have found that the third method (c), a combination of the first two methods (a) and (b), has given better results.      

Structural change of mesoporous silica with sonochemically prepared gold nanoparticles in its pores

Ultrasonic irradiation of mesoporous silica soaked in a mixture of chloroauric acid and isopropanol for 120 min in Ar atmosphere at room temperature yielded Au/SiO₂ mesoporous composite, which was characterized by high resolution transmission electron microscopy and optical absorption measurement. The structure of mesoporous silica after sonochemical preparation of gold (Au) nanoparticles within its pores was studied by nitrogen adsorption technique. It has been shown that the structural parameters, such as specific surface area (SSA), porosity (P), the mean pore diameter (l_p) were increased significantly after ultrasonic irradiation. It is suggested that the collision of Au nanoparticles with pore walls and localized erosion induced by the asymmetric implosive collapse of cavities on the extensive liquid–solid interface that are responsible for the structural change in the mesoporous solid.     

微乳液法制备纳米银粒子的结构及其荧光现象研究

采用微乳液法合成了不同粒径的纳米银粒子,考察了环己烷和甲苯作为油相对制备纳米银的影响.对纳米银粒子的尺寸与结构进行了表征,观察到近球形多晶粒子,并有孪晶结构存在,对晶体结构的分析表明银粒子存在不同程度的点阵畸变,晶面间距增大.不同粒径的纳米银粒子氯仿体系可呈现荧光光谱,而纳米银甲苯体系则无荧光发射.结合紫外—可见吸收光谱和电子自旋共振谱对该体系的荧光发射机理进行了分析 关键词： 微乳液 纳米银粒子 纳米晶结构 荧光     

Rapid synthesize of gold nanoparticles by microwave irradiation method and its application as an optical limiting material

We present rapid synthesis of gold nanoparticles by microwave irradiation method. Sample with average particle size 7.7 nm is obtained from TEM. Linear and nonlinear optical studies of the prepared samples are discussed. Reverse saturable absorption (RSA) at longitudinal surface plasmon resonance (SPR) in gold nanoparticles (Au NPs) have been observed using Z-scan and transient absorption techniques with 532 nm laser pulses. Such RSA behavior makes Au NPs an ideal candidate for optical limiting applications.     

Gold aggregates on silica templates and decorated silica arrays for SERS applications

In this work, we report the fabrication and characterization of size controllable gold nanoparticles (NPs) aggregates for their application in surface enhanced Raman scattering (SERS). Aggregates were prepared using two methodologies: (i) by using silica particles arrays as a template to agglomerate gold NPs between the inter-particle interstices, and (ii) by functionalizing silica particles to be used as support to graft gold nanoparticles and thus to form decorated silica particle arrays. These substrates were used in the detection of Rhodamine 6G producing an enhancement factor (EF) from 10⁴ to 10⁶ that is associated to the increment of hot spot (HS) sites, and the fact that plasmon resonance from aggregates and absorption wavelength of test molecules are closely in resonance with excitation wavelength. The EF was also reduced when the plasmon resonance was red-shifted as a result of the increment of aggregate size. In spite of this, the EF is high enough to make these SERS substrates excellent candidates for sensing applications.     

Ambient effect on optical absorption of Au nanoparticles dispersed within pores of mesoporous SiO2

A gold nanoparticles/monolithic mesoporous silica assembly was synthesized by ultrasonic irradiation of monolithic porous silica presoaked with precursor solution. Subsequent exposure to ambient air (ageing) and then drying at 120°C induce a new optical absorption peak around 470?nm (falling into the range from 460 to 475?nm) which is stable at room temperature, in addition to the normal surface plasmon resonance (SPR) of Au nanoparticles. Further drying results in the decline and disappearance of this peak, accompanied by increase of the normal SPR. If the sample, in which the new peak has disappeared due to long drying at 120°C, is exposed to the ambience once more, this peak will appear again after subsequent drying at 120°C, showing reversibility. Further experiments indicate that ambient ageing for a certain time plays a crucial role in the appearance of the new peak after subsequent drying at 120°C. Increased ageing time increases this peak. In addition, the ambient relative humidity and temperature during exposure are also important to the appearance of this peak. This peak may be associated with Au clusters with a size less than 1.5?nm. Based on the porous structure of the assembly and hydrophilicity of its pore wall, a nanodroplet formation and evaporation model is presented which can well explain all evolution behaviours of this peak. The model predicts the existence of the peak at 470?nm in the Au/silica assembly prepared by methods other than ultrasonic irradiation, which has also been confirmed by further experiments.     

后退火处理对铟锡氧化物表面等离激元共振特性的影响

近年来,表面等离激元光子学发展迅速,并取得了众多新成果.重掺杂半导体材料的表面等离激元共振性质的研究,也得到了人们越来越多的关注.本文通过纳米球刻印技术制备准三维二氧化硅纳米球阵列,在阵列上沉积铟锡氧化物薄膜,通过不同条件下的后退火处理改变铟锡氧化物薄膜的载流子浓度和载流子迁移率,并研究随着材料性质的改变其相应表面等离激元共振特性的变化规律.结果表明：退火处理均使铟锡氧化物薄膜的晶粒长大,光学透过率增加;在空气中退火会导致铟锡氧化物薄膜的载流子浓度减少,其表面等离激元共振峰红移;而真空退火则使铟锡氧化物薄膜的载流子浓度增加,共振峰蓝移.这些研究结果可为后续铟锡氧化物表面等离激元材料及器件的研究提供科学依据和实际指导.     

Size control synthesis of starch capped-gold nanoparticles

Metallic gold nanoparticles have been synthesized by the reduction of chloroaurate anions [AuCl₄]⁻ solution with hydrazine in the aqueous starch and ethylene glycol solution at room temperature and at atmospheric pressure. The characterization of synthesized gold nanoparticles by UV–vis spectroscopy, high resolution transmission electron microscopy (HRTEM), electron diffraction analysis, X-ray diffraction (XRD), and X-rays photoelectron spectroscopy (XPS) indicate that average size of pure gold nanoparticles is 3.5 nm, they are spherical in shape and are pure metallic gold. The concentration effects of [AuCl₄]⁻ anions, starch, ethylene glycol, and hydrazine, on particle size, were investigated, and the stabilization mechanism of Au nanoparticles by starch polymer molecules was also studied by FT-IR and thermogravimetric analysis (TGA). FT-IR and TGA analysis shows that hydroxyl groups of starch are responsible of capping and stabilizing gold nanoparticles. The UV–vis spectrum of these samples shows that there is blue shift in surface plasmon resonance peak with decrease in particle size due to the quantum confinement effect, a supporting evidence of formation of gold nanoparticles and this shift remains stable even after 3 months.     

Comparison of silver nanoparticles confined in nanoporous silica prepared by chemical synthesis and by ultra-short pulsed laser ablation in liquid

Hexagonally ordered mesoporous silica materials, MCM-41 and SBA-15, have been synthesized and loaded with Ag nanoparticles, utilizing both chemical synthesis and ultra-short pulsed laser ablation in liquid. In laser ablation, a silver target, immersed in aqueous suspension of ordered mesoporous silica SBA-15, was irradiated by ultra-short laser pulses to generate silver nanoparticles. For comparison, samples of similar silver contents were prepared either by incorporating silver into the SBA-15 during a hydrothermal synthesis or by introducing silver in MCM-41 by template ion-exchange. Samples were characterized by XRD, N₂ physisorption, TEM and UV–vis spectroscopy. All preparations contained significant amount of 5–50 nm size silver agglomerates on the outer surface of the silica particles. The laser ablation process did not cause significant destruction of the SBA-15 structure and metallic silver (Ag⁰) nanoparticles were mainly generated. It is demonstrated that by laser ablation in aqueous silica suspension smaller and more uniform metallic silver particles can be produced and loaded on the surface of the silica support than by synthesis procedures. Catalytic properties of the samples have been tested in the total oxidation of toluene. Because of its favorable Ag dispersity, the Ag/SBA-15 catalyst, generated by the laser ablation method, had better catalytic stability and, relative to its Ag load, higher activity than the conventional Ag/SBA-15 preparations.     

Concurrent Detection of Protein Adsorption on Mixed Nanoparticles by Differential Centrifugal Sedimentation

In mixtures of nanoparticles of various sizes or compositions, monitoring protein partitioning on their surfaces provides important information about particle–protein interactions during competitive adsorption. Utilizing the size‐resolving capability of differential centrifugal sedimentation, the adsorption of bovine serum albumin on multisize gold nanoparticles with diameters ranging from 20 to 100 nm or gold, silver, and silica nanoparticles with similar diameter can be concurrently observed. This method can be used to gain insight into nanoparticle–protein interactions based on analyses of curvature and relative abundance.