Concentrated Colloids Research Articles

γ-Irradiation-induced preparation of Ag and Au nanoparticles and their characterizations

Using γ-irradiation-induced reduction in the field of a 60Co γ-ray source, colloidal silver and gold nanoparticles were prepared from their corresponding metal salts in aqueous solution and compared with those by chemical reduction. The radiation-based method provided silver nanoparticles with higher concentration and narrower size distribution than those obtained by chemical reduction method while there was no significant difference between the two strategies for the preparation of gold nanoparticles. γ-Irradiation of 1.0 × 10 −3 M AgNO 3 solution resulted in nearly 100 times more highly concentrated silver colloids than those by citrate reduction. Furthermore, the radiation method could lead to more highly concentrated silver colloids by simply increasing the concentration of AgNO 3 solution up to 2.0 × 10 −2 M. The two metal nanoparticles prepared by the two different methods were characterized by UV–vis spectrophotometry and transmission electron microscopy (TEM), and compared regarding to their respective attainable concentrations and size distribution.

Reproducible Surface-Enhanced Raman Scattering Spectra of Bacteria on Aggregated Silver Nanoparticles

Surface-enhanced Raman scattering (SERS) is proven to be a powerful technique for rapid identification and discrimination of microorganisms. However, due to the heterogeneous nature of the samples, the acquisition of reproducible spectra hinders the further development of the technique. In this study, we demonstrate the influence of the experimental conditions on SERS spectra. Then, we report a simple sample preparation method coupled with a light microscope attached to a Raman spectrometer to find a proper spot on the sample to acquire reproducible SERS spectra. This method utilizes the excited surface plasmons of the aggregated silver nanoparticles to visualize the spots on the sample. The samples are prepared using the concentrated silver colloidal solutions. The collection time for one spectrum is 10 s and each spectrum is a very good representative of the other spectra acquired from the same sample. The nature of the surface charge of the silver nanoparticles influences the spectral features by determining the strength of the interactions between nanoparticles and bacteria and the aggregation properties of the nanoparticles. Although increasing the colloid concentration in the sample resulted in reproducible spectra from arbitrary points on the sample, a great variation from sample to sample prepared with the different colloidal solution concentrations is observed.

X-Ray Absorption of Gold Nanoparticles with Thin Silica Shell

Silica-coated gold (Au) nanoparticles were prepared and their morphological and X-ray absorption properties were investigated. These core-shell type nanoparticles are very stable in aqueous media and may be suitable for an X-ray contrast agent in biological systems. Transmission electron micrographs confirmed well-separated and relatively homogeneous morphology of the nanoparticles in highly concentrated colloids. Peak position for Au plasmon resonance was red-shifted with increasing shell thickness. X-ray absorption by the colloids of silica-coated Au nanoparticles was stronger than that by those of silica-coated Agl nanoparticles, a recently investigated X-ray contrast agent, at similar experimental conditions.

X-Ray Absorption of Gold Nanoparticles with Thin Silica Shell

Silica-coated gold (Au) nanoparticles were prepared and their morphological and X-ray absorption properties were investigated. These core–shell type nanoparticles are very stable in aqueous media and may be suitable for an X-ray contrast agent in biological systems. Transmission electron micrographs confirmed well-separated and relatively homogeneous morphology of the nanoparticles in highly concentrated colloids. Peak position for Au plasmon resonance was red-shifted with increasing shell thickness. X-ray absorption by the colloids of silica-coated Au nanoparticles was stronger than that by those of silica-coated AgI nanoparticles, a recently investigated X-ray contrast agent, at similar experimental conditions.

Aqueous routes to lanthanide-doped oxide nanophosphors

Besides well-known quantum dots, lanthanide-doped oxide nanocrystals form a new promising class of nanophosphors. Lanthanide vanadate and phosphate nanoparticles are prepared as well-dispersed and highly concentrated colloids by a very simple aqueous route, using competition between precipitation and complexation reactions. Under UV excitation, the three basic colors are furnished by YVO4 : Eu (red), LaPO4 : Ce·0.7H2O (blue–violet) and LaPO4 : CeTb·0.7H2O (green). In the case of cerium-doped phosphate nanoparticles, the epitaxial growth of a LaPO4·0.7H2O shell leads to “core–shell” nanocrystals presenting improved stability against cerium oxidation which is detrimental for the luminescence. Nanocrystals exhibiting passivation and chemical or biological functionalization are easily prepared using the silane strategy. The optimized nanophosphors are either dispersed in transparent sol–gel films or used as isolated particles for biological labelling. The luminescence properties of nanoparticles differ from corresponding bulk materials in several ways : (i) spectroscopic changes, as a systematic broadening of the luminescence bands in sub-20 nm nanocrystals, resulting from a structural disorder intrinsic to the small size; (ii) a lower luminescence quantum yield related to the presence of chemical species adsorbed at the surface, especially the OH groups; (iii) a shift towards higher values of the optimum doping concentration, due to the alteration of energy transfers either in the vanadate or cerium sub-networks.

Submicron patterning of Co colloid catalyst for growth of vertically aligned carbonnanotubes

Applications of carbon nanotubes such as field emission or microelectrode sensor arraysrequire a patterning of vertically aligned carbon nanotubes over large areas. A highlypurified and concentrated monodisperse cobalt colloid was produced for use as a catalystfor growth of carbon nanotubes. Nanocontact printing was employed to deposit the cobaltnanoparticles in regular patterns with feature sizes at the 100 nm scale onto silicon wafersat low cost over large areas. Vertically aligned carbon nanotubes were grown by directcurrent plasma enhanced chemical vapour deposition at temperatures ranging from 300 to640 °C.

Critical fouling conditions induced by colloidal surface interaction: from causes to consequences

Critical fouling conditions (CFC) are defined as the process operating conditions leading to the formation of multilayer irreversible fouling at the membrane surface. This irreversible fouling is the result of a phase transition in the accumulated matter from a dispersed phase (concentration polarisation) to a condensed phase (deposit or gel formation): the spinodal decomposition. Properties of concentrated colloid dispersions and their related phase transitions are integrated into a classical filtration mass balance via colloidal osmotic pressure, II. This then allows us to define CFC for both cross-flow and dead-end filtration. These CFC are expressed in terms of critical pairs of operating conditions: the set permeate flux/boundary layer thickness (directly linked to cross-flow velocity) in cross flow and the critical set permeate flux/filtered volume in dead end.

One-step preparation of highly concentrated well-stable gold colloids by direct mix of polyelectrolyte and HAuCl 4 aqueous solutions at room temperature

Highly concentrated, well-stable gold colloids can be prepared directly from an amine-bearing polyelectrolyte–HAuCl 4 aqueous solution at room temperature. It is found that increasing molar ratio of polyelectrolyte to gold leads to increasing particle size. UV–vis spectra, transmission electron microscopy (TEM), and X-ray photoelectron spectra (XPS) were used to characterize the products thus formed.

Surface-Enhanced Raman Spectra of Calf Thymus DNA Adsorbed on Concentrated Silver Colloid

Raman and surface-enhanced Raman scattering (SERS) spectra of calf thymus DNA were investigated. We have carried out improvements to the silver colloid preparation method of Lee and Meisel in two respects. In one method, the silver sol was boiled with rapid stirring for over two hours. In the second method, the silver sol was concentrated by centrifugation before adding it to the DNA solution. The resulting hydrosol could be stored for 15 months because of its high stabilization. Structural information with respect to the phosphate backbone, deoxyribose, and four bases of DNA could be obtained before and after the DNA solutions were added to the concentrated Ag colloid substrate. The intensities of almost all characteristic bands assigned to various groups of the components of DNA were enhanced to a remarkable degree. The enhancement effect of the DNA solution at neutral pH 7.0 was obviously much better than that at acidic pH 3.4 or at alkaline pH 8.5. Intensity increases of the SERS bands of the DNA solution with time were observed. The SERS signals obtained 16 hours after the interaction of the Ag colloid with the DNA solution were much better than the SERS signals obtained just after the mixed liquid was prepared. This method can be widely used to store the Ag colloid for long times and to obtain the SERS spectra of DNA molecules, and it can further be used to study the adsorption behavior of solute biomacromolecules in different solvents.

Optical properties of neodymium oxides at the nanometer scale

The polyol method has been recently developed for preparing concentrated and stable colloids of highly crystallized colloids. Already applied to europium oxide it is used here for synthesizing particles of neodymium oxide as the main constituent or as a 10 mol% doping element in RE 2O 3 (RE=Y, Gd). High-resolution transmission electron microscopy has identified particles with an average grain diameter in the range 3–5 nm crystallized in the cubic phase. The absorption spectra were recorded, from which the Judd–Ofelt analysis was performed. The calculated radiative lifetime of the 4F 3/2 Nd 3+ energy level was found to be 144 μs in the Nd 2O 3 nanoparticles, that is to say much longer than the measured lifetime (70 ns), indicating a strong luminescence quenching attributed to nonradiative losses.

Structure of hard-sphere colloid observed in real space by spin-echo small-angle neutron scattering

Spin-echo small-angle neutron scattering (SESANS) opens a wide range of potential applications. One of them is the study of colloids. Using SESANS we measured the structure of hard-sphere colloidal suspensions at two concentrations. We show that SESANS measures real-space correlations due to the particle shape as well as interactions between them. The dilute suspension showed no correlations exceeding the particle diameter. For a concentrated colloid the effect of the excluded volume is observed as a minimum in correlation function. Experimental correlation functions are compared with theoretical ones for noninteracting spheres and a hard-sphere liquid. The scattering density, volume fraction and the radius of particles are also determined from SESANS correlation function.

Emission properties and applications of nanostructured luminescent oxide nanoparticles

Abstract Rare earth doped oxide materials are well known for their numerous applications in light emitting devices. An interesting issue is to study the emission properties of nanoparticles, with the aim to understand the influence of small size and surface effects on the emission processes. These particles could furthermore be used in new applications such as the elaboration of transparent emitting devices or new biological labels. The work presented here concerns highly luminescent rare earth doped yttrium vanadates (YVO 4 :Eu) and lanthanum phosphate LaPO 4 :Ce,Tb· x H 2 O nanoparticles. Simple aqueous colloidal syntheses are used for the elaboration of concentrated colloids based on the progressive decomposition of polymeric precursors at moderate temperature (60–90 °C). Both types of particles exhibit strong emission (quantum yields of 25% and 45% for vanadates and phosphates, respectively), but significantly lower than that for the equivalent bulk materials. The alteration of the emission processes is discussed in terms of surface quenching effects. Improvements are obtained through the elaboration of core/shell nanostructures. Surface derivatization has been achieved through the controlled growth of an organically modified silica shell using a functionalized silane precursor. Two examples are given concerning the applications of those particles. The first one is the elaboration of transparent and highly luminescent thin films, obtained by the dispersion of the functionalized particles in a sol–gel silica matrix. The other one is the use of guanidine functionalized particles as biological labels for the single particle detection of sodium channels in cardiac cells.

Equilibrium cluster formation in concentrated protein solutions and colloids

Controlling interparticle interactions, aggregation and cluster formation is of central importance in a number of areas, ranging from cluster formation in various disease processes to protein crystallography and the production of photonic crystals. Recent developments in the description of the interaction of colloidal particles with short-range attractive potentials have led to interesting findings including metastable liquid-liquid phase separation and the formation of dynamically arrested states (such as the existence of attractive and repulsive glasses, and transient gels). The emerging glass paradigm has been successfully applied to complex soft-matter systems, such as colloid-polymer systems and concentrated protein solutions. However, intriguing problems like the frequent occurrence of cluster phases remain. Here we report small-angle scattering and confocal microscopy investigations of two model systems: protein solutions and colloid-polymer mixtures. We demonstrate that in both systems, a combination of short-range attraction and long-range repulsion results in the formation of small equilibrium clusters. We discuss the relevance of this finding for nucleation processes during protein crystallization, protein or DNA self-assembly and the previously observed formation of cluster and gel phases in colloidal suspensions.

Continuous shear thickening transitions in model concentrated colloids—The role of interparticle forces

The role of interactions between close particles in the shear thickening of concentrated colloids is examined by using a Stokesian dynamics simulation of model systems. The interactions are repulsive thermodynamic forces and lubrication forces. Three different models are contrasted in their thickening behavior: Brownian spheres, polymer coated spheres, and Hookian spheres. Respectively, they show: a “mild” continuous thickening, a “strong” continuous thickening, and a strain thickening with loss of steady state. The relationship of order-disorder transitions and thickening is examined. Depending on the volume fraction and range of repulsive forces, thickening can be observed with or without an order-disorder transition at its onset. The different thickening responses arise from the dependence of the relaxation time of close particle contacts on interparticle gap. A time-scale based criterion for strong thickening is proposed and supported by the simulations. A simple theoretical model based the motion of a pair of particles leads to this criterion, but also predicts the mild thickening of Brownian spheres. It gives a simple fitting of flow curves which includes the details of the interparticle interactions.

“Contact networks” in continuously shear thickening colloids

Many body effects that occur in continuous shear thickening of concentrated colloid suspensions are examined by using a Stokesian dynamics simulation of model systems of polymer coated particles. The shear thickening state is probed in a number of ways: computed scattering intensities, imaging of density variation using Voronoi constructions, examination of the distribution of interparticle forces, and computation of the fabric of contacts. The shear thickening transition in these systems is found to be associated with the growth of a network of close particle contacts and shear induced density variations. This paper focuses on the network of contacts. The distributions of force magnitude are found to be exponential. The network directly relates to the normal stress differences. Both the data and simple physical argument suggest that thickening can be viewed as an approach to “jamming.”

Electroacoustic theory for concentrated colloids with overlapped DLs at arbitrary kappa alpha. I. Application to nanocolloids and nonaqueous colloids.

Existing theories of electroacoustic phenomena in concentrated colloids neglect the possibility of double layer overlap and are valid mostly for the "thin double layer," when the double layer thickness is much less than the particle size. In this paper we present a new electroacoustic theory which removes this restriction. This would make this new theory applicable to characterizing a variety of aqueous nanocolloids and of nonaqueous dispersions. There are two versions of the theory leading to the analytical solutions. The first version corresponds to strongly overlapped diffuse layers (so-called quasi-homogeneous model). It yields a simple analytical formula for colloid vibration current (CVI), which is valid for arbitrary ultrasound frequency, but for restricted kappa alpha range. This version of the theory, as well the Smoluchowski theory for microelectrophoresis, is independent of particle shape and polydispersity. This makes it very attractive for practical use, with the hope that it might be as useful as classical Smoluchowski theory. In order to determine the kappa alpha range of the quasi-homogeneous model validity we develop the second version that limits ultrasound frequency, but applies no restriction on kappa alpha. The ultrasound frequency should substantially exceed the Maxwell-Wagner relaxation frequency. This limitation makes active conductivity related current negligible compared to the passive dielectric displacement current. It is possible to derive an expression for CVI in the concentrated dispersion as formulae inhering definite integrals with integrands depending on equilibrium potential distribution. This second version allowed us to estimate the ranges of the applicability of the first, quasi-homogeneous version. It turns out that the quasi-homogeneous model works for kappa alpha values up to almost 1. For instance, at volume fraction 30%, the highest kappa alpha limit of the quasi-homogeneous model is 0.65. Therefore, this version of the electroacoustic theory is valid for almost all nonaqueous dispersions and a wide variety of nanocolloids, especially with sizes under 100 nm.

Electroosmosis in concentrated colloids and the structure of the double electric layer

Abstract The existence of a non-solving layer of bound water (NBL) on the hydrophilic surface of SiO 2 particles, containing no counterions of the double electric layer (DEL), was demonstrated using the methods of electroosmotic transfer in a model system of monodisperse, nano-sized spherical particles and the method of osmotic pressure based on the equation of state. The thickness of the NBL was also determined. The nature of the NBL led to further consideration of the structure of the DEL. It is proposed that this layer contains a water interlayer between the surface itself and the inner part of the diffuse layer, where the concentration of counterions is very small or equal to zero.

A new formula for the dynamic mobility in a concentrated colloid

Abstract The dynamic mobility μ D is the analogue of the electrophoretic mobility in an alternating electric field. It can be determined for suspensions of arbitrary concentration at MHz frequencies using electroacoustic measurements. A considerable amount of effort has been expended during the last decade in the derivation of theories for the dynamic mobility in concentrated suspensions. The objective has been to come up with a formula that can be used to calculate particle size and zeta potential from dynamic mobility measurements in concentrated suspensions. Some of these theories are based on cell models, others are based on coupled phase models and another involves the use of experimentally determined parameters. In this paper we present a new approach to this problem. We derive a formula that does not involve any ad hoc boundary conditions or adjustable parameters. The formula is compared with experimental measurements, and is used to determine size and zeta potential for a number of concentrated colloids.

Colloid flow during thickening--a particle level understanding for core-shell particles.

The flow curve of sheared concentrated colloids can show a region of shear thickening. The underlying mechanisms involved in this effect has long been an issue. Recently the author and co-workers have used Stokesian dynamics approximated for high concentrations to simulate models of polymer coated particles in the thickening regime. This work continues the search for a particle level of understanding these systems in the thickening regime. Previous work found that shear thickening is associated with the formation of a network of contacts between polymer coats. Previous work by the author has examined the fabric (geometry) and texture (density distribution) of the contact network. However despite this many-body effect, it was also found that a very simple mean-field type argument at the level of a pair of particles in contact relates the particle interaction laws to the thickening part of the flow curve up to an unknown factor. In the argument this factor is determined by aspects of the destruction of particle contacts. In this paper several new results are reported. The first concerns the definition of the network and its coordination. The paper contrasts systems with different thickness of polymer coats and gives tentative evidence that the network formed sits close to what is known as the iso-static coordination. The second set of issues concerns the analysis of the lifetimes of particle contacts--in effect an examination of the unknown factor and assumptions of the pair argument. A surprising result is that some particle pairs have relatively long lifetimes. Finally, data is reported for the stress fluctuations in the thickening regime.

An ultrasound velocity and attenuation scanner for viewing the temporal evolution of a dispersed phase in fluids

Ultrasound scanning using the group velocity of sound to determine the concentration of a liquid or solid phase dispersed in a fluid has been used for many years to characterize dispersions with regard to their long-term stability. The technique has the twin advantages of speed and operation in concentrated, optically opaque dispersions. In this work, the group velocity technique is combined in a single instrument with phase velocity and attenuation spectroscopy measurements to give valuable additional information about particle size and the microscopic particle distribution, related to important destabilization phenomena such as particle flocculation. This provides earlier evidence of the processes that finally lead to gravitational destabilization and reduced shelf life of fluid dispersions such as emulsions. A further advantage is the ability to compare the measured spatial and temporal variation with computer models. The technique works in optically opaque materials and in concentrated colloids, giving a quantitative picture of the macroscopic spatial distribution of the dispersed phase and a semiquantitative picture of microscopic particle aggregation processes. Since these microscopic particle rearrangements are often responsible for the ultimate gravitational destabilization of colloidal systems, the Acoustiscan, as we have called the ultrasonic scanner described herein, may indicate product instability long in advance of visual evidence. New data are presented for protein containing sunflower oil-in-water emulsions, destabilized with Tween 20, in order to exemplify the use of the Acoustiscan for the characterization of food emulsions. The Acoustiscan instrument provides quantitative information about the destabilization of emulsions, dispersions, and colloidal systems in a rapid and informative manner. It simultaneously measures changes in the dispersed phase and follows microscopic changes in the arrangement of particles. The instrument has many other uses, for example, for characterizing crude oil, pharmaceuticals, cosmetics, and agrochemicals. It can also be used to follow crystallization processes. It does all this in materials over an extremely wide concentration range, from a few percent up to the highest concentrations obtainable. Moreover, when data are compared with computer models, it is possible to infer the presence of gels whose yield stress is far lower than any measurable by contemporary rheological equipment. This makes the Acoustiscan ideal for the study of the new soft solid materials currently in development.