A strategy for synthesis of polymer-protective bimetallic colloids

A strategy for the synthesis of polymer-protective bimetallic sols is proposed which has the advantage of the strong protective effect of the polymer agent on one of the composites to stabilize the bimetallic colloidal particles. This strategy is certified to be useful to the predication of the formation of stable bimetallic colloids protected by polymer and to the control of the particle size to a certain extent. A series of new PVP-protective bimetallic colloidal dispersions containing noble metal element (Pt, Rh, Pd), light transition metal element (Co, Fe), and boron are obtained from the corresponding salts by the methods of reduction by stage and coreduction, using the NaBH4 as reductant. The TEM, XRD and EPMA measurements indicate the formation of bimetallic colloids. An interesting kind of coil aggregation is observed in the systems of PVP-Pt-Co and PVP-Rh-Co prepared by the method of reduction by stage.     

Angular-ratiometric plasmon-resonance based light scattering for bioaffinity sensing

We describe an exciting opportunity for affinity biosensing using a ratiometric approach to the angular-dependent light scattering from bioactivated and subsequently aggregated noble metal colloids. This new model sensing platform utilizes the changes in particle scattering from very small colloids, which scatter light according to traditional Rayleigh theory, as compared to the changes in scattering observed by much larger colloidal aggregates, formed due to a bioaffinity reaction. These larger aggregates no longer scatter incident light in a Cos(2) theta dependence, as is the case for Rayleigh scattering, but instead scatter light in an increased forward direction as compared to the incident geometry. By subsequently taking the ratio of the scattered intensity at two angles, namely 90 degrees and 140 degrees , relative to the incident light, we can follow the association of biotinylated bovine serum albumin-coated 20 nm gold colloids, cross-linked by additions of streptavidin. This new model system can be potentially applied to many other nanoparticle assays and has many advantages over traditional fluorescence sensing and indeed light-scattering approaches. For example, a single nanoparticle can have the equivalent scattered intensity as 10(5) fluorescing fluorescein molecules substantially increasing detection; the angular distribution of scattered light from noble metal colloids is substantially easier to predict as compared to fluorescence; the scattered light is not quenched by biospecies; the ratiometric measurements described here are not dependent on colloid concentration as are other scattering techniques; and finally, the noble metal colloids are not prone to photodestruction, as is the case with organic fluorophores.     

Synthesis and characterization of colloidal gold particles as labels for antibodies as used in lateral flow devices

Based on well established citrate reduction protocols for the synthesis of colloidal gold particles, this work focuses on the characterization of these colloids for further use as color labels in lateral flow devices. A reproducible production method has been developed for the synthesis of well characterized colloidal gold particles to be employed in Lateral Flow Devices (LFDs). It has been demonstrated that when undertaking chemical reduction of gold salts with sodium citrate, the amount of reducing agent employed could be used to directly control the size of the resultant particles. A protocol was thereby developed for the synthesis of colloidal gold particles of pre-defined diameters in the range of 15 to 60 nm and of consistent size distribution. The absorption maxima (λ(max)) of the reaction solutions were analyzed by UV/VIS measurements to determine approximate particle sizes, which were confirmed with transmission electron microscopy (TEM) measurements. Colloidal gold particles of about 40 nm in diameter were synthesized and used for labeling monoclonal anti-mycotoxin antibodies (e.g. zearalenone). To deduce the extent of antibody coupling to these particles, smaller colloids with 15 nm diameter were labeled with anti-species specific antibodies. Both solutions were mixed and then scanned by TEM to obtain information about the success of coupling.     

Preparation and Ultrafast Optical Characterization of Metal and Semiconductor Colloidal Nano-Particles

The ultrafast dynamics of photoinduced electrons in several metal and semiconductor colloidal nanoparticle systems are characterized using femtosecond laser spectroscopy. Various preparation methods are used and, in several cases, modified for making particles with long-term stability and narrow and controllable size distributions. The particle size and size distribution are determined using transmission electron microscopy and electronic absorption spectroscopy. For aqueous gold and silver colloids, spatial size confinement is found to cause substantially slower electronic relaxation due to reduction of non-equilibrium electron transport and weaker electron-phonon coupling. In gold colloids, photoejection of electrons into the liquid is observed, which is attributed to a two-photon enhanced ionization process. The effect of surfactant on the electron dynamics in CdS colloids is examined and found to be significant, substantiating the notion that electrons are dominantly trapped at the liquid-solid interface. In Ru³⁺-doped TiO₂ colloids, the electronic decay is found to be as fast as or even faster than in undoped TiO₂ and other semiconductor colloids such as CdS, suggesting that ion doping of large bandgap semiconductor colloids is not necessarily effective in lengthening the electron lifetime. In almost all cases studied, the majority of the photoinduced electrons are found to decay within a few tens of picoseconds due to non-radiative relaxation. The results are discussed in the context of the potential applications of metal and semiconductor nano-particles in areas including photocatalysis and photoelectrochemistry.     

Gram-scale synthesis of monodisperse gold colloids by the solvated metal atom dispersion method and digestive ripening and their organization into two- and three-dimensional structures

We describe a synthetic procedure for preparation of large quantities of monodisperse thiol-stabilized gold colloids in toluene solution. The method is based on the solvated metal atom dispersion technique (SMAD), which is very suitable for preparation of large amounts of metal colloidal solutions, as well as of metal sulfide, metal oxide, and other types of dispersed compounds in different solvents. A combination of two different solvents like acetone and toluene is used for the preparation of the gold colloids. The necessity of initially carrying out the SMAD reaction in acetone comes from its high degree of solvation of gold particles. Acetone acts as a preliminary stabilizing agent. After its removal from the system, the particles are stabilized by dodecanethiol molecules, which enable their very good dispersion in toluene solution. A digestive ripening procedure is carried out with the gold-toluene colloid, and for this purpose pure toluene as solvent is necessary. This has a dramatic effect on the narrowing of particle size distribution and almost monodisperse colloids are obtained (some discussion of the probable mechanism of this remarkable digestive ripening step is given). These colloidal solutions have a great tendency to organize in two- and three-dimensional structures (nanocrystal superlattices, NCSs). We believe that this procedure provides a real opportunity to synthesize large amounts of gold nanocrystals as well as NCSs.     

Size fractionation and characterization of natural colloids by flow-field flow fractionation coupled to multi-angle laser light scattering

Flow-field flow fractionation (FlFFF) coupled to multi-angle laser light scattering (MALLS) was evaluated for size and shape determination of standard spherical and arbitrarily shaped natural colloids. Different fitting methods for light scattering data retrieved from MALLS were evaluated to determine the particle size of spherical standards and natural colloids. In addition, FlFFF was optimized for best fractionation in connection to MALLS, minimal colloids-membrane interaction, and minimal sample losses. FlFFF, calibrated with standard particles, was used to determine hydrodynamic diameter, or radius (D(h) or R(h)), of the fractionated colloids, whereas the MALLS was used to determine root mean square radius of gyration (R(g)) for fractionated colloids. Combining both results, by calculating the R(g)/R(h) ratio, allows an estimation of colloid deviation from the shape of homogeneous sphere. Accordingly, this study demonstrates that, FlFFF-MALLS is a valuable technique for characterizing heterogeneous and arbitrarily shaped natural colloidal particles in terms of size and shape. To check the usefulness of FlFFF-MALLS in natural colloid studies, the technique was used to investigate the sedimentation behavior of extracted soil colloidal particles. Results illustrate that, in a silty till sample, carbonates function as cement between the colloidal particles, and consequently, change their sedimentation behavior. On the other hand, carbonate dissolution generates a more homogeneous colloidal sample.     

Micellization of amphiphilic block copolymers and use of their micelles as nanosized reaction vessels

Amphiphilic block copolymers made by a variety of techniques form in selective solvents micelles of well defined size and shape. We report on the underlying principles of micelle formation, as revealed by light scattering experiments. In addition, we will delineate some applications of these block-copolymer micelles where we focus on two technological relevant cases, namely the stabilization of polymer particles in low cohesion energy environments as well as the stabilization of metal and semiconductor colloids inside specially functionalized block copolymer micelles. In the latter case, stable hybride materials are obtained where the properties both of the polymer and the inorganic are added in a synergistic way. First data on special catalytic properties, the generation of superparamagnetic materials, and special colloids with optical functionality are presented.     

Sol-Gel Preparation of Coating Films Containing Noble Metal Colloids

Coating films containing Au, Ag, Pt and Pd metal colloids have been prepared by sol-gel processing. It is shown that for oxide films the temperature where the metal particles are precipitated by heating in air depends on metal species: 200°C for Au, 600°C for Ag, 800°C for Pt and 1000°C for Pd. The use of reducing atmosphere lowers the temperature for formation of noble metal colloids. This procedure can be used for direct formation of metal colloids from metal ions in the film as well as reduction of oxide particles to metal particles in the film. For an organic-inorganic matrix, noble metal colloids are precipitated by thermal reduction or photo-reduction. Thermal reduction occurs as a result of reduction by decomposing organic matter. Photo-reduction occurs as a result of UV irradiation.     

A Magnetooptical Effect in a Rotating Field in the Case of a Polymorphic Colloid

Magnetooptical methods are used to study particle polymorphism in colloids and suspensions. Electrooptical methods of studying disperse systems make it possible to determine the average value of the shape factor of colloidal particles approximated by ellipsoids of revolution. However, they do not enable one to solve the problem of the particle shape distribution in real disperse systems, because the orientation of particles in an electric field always depends on their size as well as shape. The use of a magnetic field makes it possible to create the conditions under which the orientation of particles is determined by their shape irrespective of the size.     

Colloid-polymer mixtures in solution with refractive index matched acrylate colloids

Colloid-polymer (CP) mixtures extend between two limiting cases, the colloid limit with the polymer coil size small compared to the colloid radius Rcol and the protein limit with the colloidal particles much smaller in size than the radius of gyration of the polymer chains Rg. In the present work, model systems are developed for the protein limit. The colloid-solvent pairs are optimized in terms of their isorefractivity in order to facilitate the characterization of large polystyrene chains in suspensions of small colloids. The degree of isorefractivity of colloidal particles was successfully evaluated in terms of a reduced scattering intensity. Two polystyrene samples with radii of gyration of Rg = 96 nm and Rg = 78 nm, respectively, are used. The radii of the colloidal particles are close to Rcol = 12 nm, leading to size ratios of Rg/Rcol = 8 and Rg/Rcol = 6.5. Four colloid solvent systems were found to be suitable for polymer characterization by light scattering, one based on silica particles and three systems with acrylate particles. The present investigation is focused on the three acrylate systems: poly(methyl methacrylate) in ethyl benzoate (ETB) at 7 degrees C, poly(ethyl methacrylate) in toluene at 7 degrees C and poly(ethyl methacrylate) in ETB at 40 degrees C. Characterization of PS chains is for the first time performed in colloid concentrations up to 2.5% by weight. In all cases, the size and shape of the polymer chains remain unchanged. A slight mismatch of the colloid scattering or a limited colloid solubility prevented investigation of PS chains at higher colloid concentration.     

Covalently Immobilized Colloidal Metal Particles as Selective Catalysts for Olefin Hydrogenation

Abstract

Homogeneous colloidal dispersions of ultrafine noble metal particles have been prepared by the reduction of the corresponding metal ions in the presence of protective polymers. These colloidal metal particles show effective and selective catalyses in hydrogenation of olefins’. However, separation of these homogeneous catalysts from reaction mixtures for the repeated use is difficult. Thus, immobilization of these colloidal metal particles to supports is required. This paper reports immobilization of colloidal rhodium particles onto polymer support by use of covalent bonding between the protective polymer and the support. Activities and selectivities of the resultant immobilized catalysts for hydrogenation of olefins are shown.     

Colloids based on gold nanoparticles dispersed in castor oil: Synthesis parameters and the effect of the free fatty acid content

Herein, we present our results related to the synthesis of colloidal solutions of gold nanoparticles (AuNPs) dispersed in castor oil. These colloids were prepared via a wet chemistry process by mixing specific amounts of castor oil, ethanol, and aqueous solutions of tetrachloroauric(III) acid and sodium hydroxide. The size and shape of the AuNPs obtained could be modulated by the amount of gold source added and the Au/OH^– molar ratio used. In this study, we observed that the free fatty acid content in the reaction medium was an important parameter to be considered in the syntheses of the colloidal solutions and the respective form and shape of the AuNPs produced. Thus, we evaluated the effect of oil acidity by adding different amounts of myristic acid (MA) in the reaction medium. The colloids were characterized by UV–vis spectroscopy, and the size and shape of the AuNPs produced were characterized by transmission electron microscopy (TEM).     

Preparation of gold colloid using pyrrole-2-carboxylic acid and characterization of its particle growth

Gold colloid was prepared by reduction of tetrachloroaurate using pyrrole-2-carboxylic acid in the presence of α-methoxy-ω-mercaptoethyl-poly(ethylene glycol). The shape and size of the obtained gold colloids were monitored by absorption spectroscopy and transmission electron microscopy during the reduction reaction. In the early stage of the reducing reaction, large fluffy particles with diameters >100 μm were observed. Some of these large particles had rectangular parallelepiped shapes. However, the mean particle size decreased with increasing time. Eventually, these large, non-round-shaped particles disappeared almost completely, and uniform round-shaped particles with a mean diameter of 10.5 nm became dominant.     

Mechanism of Formation of Monodispersed Colloids by Aggregation of Nanosize Precursors

It has been experimentally established in numerous cases that precipitation of monodispersed colloids from homogeneous solutions is a complex process. Specifically, it was found that in many systems nuclei, produced rapidly in a supersaturated solution, grow to nanosize primary particles (singlets), which then coagulate to form much larger final colloids in a process dominated by irreversible capture of these singlets. This paper describes a kinetic model that explains the formation of dispersions of narrow size distribution in such systems. Numerical simulations of the kinetic equations, with experimental model parameter values, are reported. The model was tested for a system involving formation of uniform spherical gold particles by reduction of auric chloride in aqueous solutions. The calculated average size, the width of the particle size distribution, and the time scale of the process agreed reasonably well with the experimental values. Copyright 1999 Academic Press.     

A general method to synthesis of amphiphilic colloidal nanoparticles of CdS and noble metals

Amphiphilic colloids of CdS and noble metal nanoparticles, which can be dispersed both in water and organic solvents such as ethanol, N,N-dimethylformamide, chloroform, and toluene, are studied. The amphiphilic colloidal nanoparticles are synthesized by grafting the amphiphilic and thermoresponsive polymer of thiol-terminated poly(N-isopropylacrylamide) to CdS and noble metal nanoparticles. The size and morphology of the PNIPAM-grafted colloidal nanoparticles of CdS@PNIPAM can be tuned by changing the molar ratio of PNIPAM/CdS. The size of CdS@PNIPAM nanoparticles slightly decreases first from 5.5 to 4.4 nm then slightly increases from 4.4 to 6.1 nm with the decrease in the molar ratio from 1/1 to 1/10. Spherical nanoparticles of CdS@PNIPAM are synthesized at a higher molar ratio and worm-like nanoparticles are obtained at a lower molar ratio. The resultant PNIPAM-grafted colloidal nanoparticles of CdS@PNIPAM, Au@PNIPAM, Pd@PNIPAM, and Ag@PNIPAM are thermoresponsive in water and show a cloud-point temperature at about 32.5 degrees C.     

Chemico‐Physical Synthesis of Surfactant‐ and Ligand‐Free Gold Nanoparticles and Their Anti‐Galvanic Reduction Property

Galvanic reduction (GR) is a classic reaction. In simple terms, metals can reduce less reactive (or more noble) metal ions, while the opposite—metals reduce more reactive (or less noble) metal ions—should not occur. However, recently we found that anti‐galvanic reduction (AGR) occurred to thiolated gold and silver nanoparticles. However, the essential issue whether the occurrence of AGR requires the assistance of reductive thiolate ligands or not still remained unanswered. In this work, by using a novel protocol (chemical reduction and physical ablation), we synthesized surfactant‐ and ligand‐free gold nanoparticles. We found that these as‐prepared nanoparticles can reduce silver ions and copper ions, thus illustrating that AGR is not dependent on reductive ligands. Further experiments demonstrated that AGR is applicable to other metal (such as Pt and Pd) nanoparticles and that the AGR process is size‐dependent. Finally, it was found that the Raman scattering signals of Rhodamine 6G are distinctly enhanced on the gold nanoparticles that had been reacted with silver ions, which indicates the use of AGR for tuning the property of nanoparticles.     

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.     

Optical Effects Associated with Aggregates of Clusters

The effect of aggregation on the optical properties of nanometer-sized particles is studied. It is shown that for small noble metal clusters as well as for pigments of Fe₂O₃, TiN, or ZrN, the aggregation leads to changes in the color of the colloidal systems which are caused by electromagnetic coupling among the clusters in the aggregates. The model of interacting particles is shown to be helpful also for interpretation of optical properties of organic dyes with incorporated metal clusters and for interpretation of the reflectance of magnetooptical cluster systems. For soot particles it is shown that scattering and absorption are enhanced over the whole visible spectral region compared to isolated carbonaceous clusters. Finally, it is shown that the model of interacting clusters can also be applied for data interpretation in scanning near-field optical microscopy.     

Synthesis, characterization and gold loading of polystyrene-poly(pyridyl methacrylate) core-shell latex systems

In this research, novel 3-(2-pyridyl)propyl methacrylate and 3-(3-pyridyloxy)propyl methacrylate monomers were synthesized and emulsion polymerized on colloidal polystyrene seeds, resulting in core-shell latex systems. The cores and the core-shell particles were characterized by static light scattering and scanning electron microscopy. Transmission electron microscopy was used to study the morphology of the core-shell particles. Monodisperse beads with a regular core-shell internal structure were found. The pyridine-functional shells were loaded with HAuCl₄ and irradiated with UV light to reduce the salts to metallic gold. FTIR, UV-Vis, TEM and XPS were employed to monitor the metal loading and reduction processes. Core-shell systems, decorated with gold nanoparticles, were obtained.     

Asymmetric dimers can be formed by dewetting half-shells of gold deposited on the surfaces of spherical oxide colloids

Asymmetric dimers consisting of gold microcrystals and spherical silica colloids have been fabricated by depositing thin films of gold onto the spherical colloids to form half-shells, followed by annealing at elevated temperatures. The capability and feasibility of this procedure have been demonstrated with silica and titania beads of 0.2-2 mum in diameter and gamma-Fe2O3/polystyrene@SiO2 core-shell particles 0.5 mum in size. The dimensions of gold microcrystals could be conveniently varied in the range of 100-650 nm by controlling the thickness of gold films and/or the diameter of the spherical colloids. This method provides another route to asymmetric dimers made of colloidal particles that could be different in size, chemical composition, surface functionality, density or sign of surface charge, bulk property, or a combination of these properties.