Colloidal stabilization of nanoparticles in concentrated suspensions

The stabilization of nanoparticles in concentrated aqueous suspensions is required in many manufacturing technologies and industrial products. Nanoparticles are commonly stabilized through the adsorption of a dispersant layer around the particle surface. The formation of a dispersant layer (adlayer) of appropriate thickness is crucial for the stabilization of suspensions containing high nanoparticle concentrations. Thick adlayers result in an excessive excluded volume around the particles, whereas thin adlayers lead to particle agglomeration. Both effects reduce the maximum concentration of nanoparticles in the suspension. However, conventional dispersants do not allow for a systematic control of the adlayer thickness on the particle surface. In this study, we synthesized dispersants with a molecular architecture that enables better control over the particle adlayer thickness. By tailoring the chemistry and length of these novel dispersants, we were able to prepare fluid suspensions (viscosity < 1 Pa.s at 100 s-1) with more than 40 vol % of 65-nm alumina particles in water, as opposed to the 30 vol % achieved with a state-of-the-art dispersing agent. This remarkably high concentration facilitates the fabrication of a wide range of products and intermediates in materials technology, cosmetics, pharmacy, and in all other areas where concentrated nanoparticle suspensions are required. On the basis of the proposed molecular architecture, one can also envisage other similar molecules that could be successfully applied for the functionalization of surfaces for biosensing, chromatography, medical imaging, drug delivery, and aqueous lubrication, among others.     

Stabilization of foams with inorganic colloidal particles

Wet foams are used in many important technologies either as end or intermediate products. However, the thermodynamic instability of wet foams leads to undesired bubble coarsening over time. Foam stability can be drastically improved by using particles instead of surfactants as foam stabilizers, since particles tend to adsorb irreversibly at the air-water interface. Recently, we presented a novel method for the preparation of high-volume particle-stabilized foams which show neither bubble growth nor drainage over more than 4 days. The method is based on the in-situ hydrophobization of initially hydrophilic particles to enable their adsorption on the surface of air bubbles. In-situ hydrophobization is accomplished through the adsorption of short-chain amphiphiles on the particle surface. In this work, we illustrate how this novel method can be applied to particles with various surface chemistries. For that purpose, the functional group of the amphiphilic molecule was tailored according to the surface chemistry of the particles to be used as foam stabilizers. Short-chain carboxylic acids, alkyl gallates, and alkylamines were shown to be appropriate amphiphiles to in-situ hydrophobize the surface of different inorganic particles. Ultrastable wet foams of various chemical compositions were prepared using these amphiphiles. The simplicity and versatility of this approach is expected to aid the formulation of stable wet foams for a variety of applications in materials manufacturing, food, cosmetics, and oil recovery, among others.     

Influence of stoichiometry on the luminescent properties of InAs quantum dots grown on a heterostructure

We report photoluminescence (PL) spectra of InP/In_xGa_1-xAs/InAs/InP dot-in-a-well structures grown by MOVPE, with different compositions of the ternary layer. Measurements with atomic force microscopy showed that the largest quantum dot (QD) height is obtained when the InAs QDs are grown on the In_xGa_1-xAs layer with a mismatch of 1000 ppm, and the height decreases as the mismatch departs from this value. PL spectra of the QDs showed an asymmetric band, which involves transitions between dot energy levels and can be deconvoluted into two peaks. The highest energy PL peak of this band was observed for the sample with the QDs grown on top of the lattice-matched In_xGa_1-xAs layer and it shifted to lower energies for strained samples as the degree of mismatch increased. Theoretical calculations of the energy levels of the entire structure were used to interpret the obtained PL spectra and determine the possible detection tunability range.     

Hierarchical porous materials made by drying complex suspensions

Porous structures containing pores at different length scales are often encountered in nature and are important in many applications. While several processing routes have been demonstrated to create such hierarchical porous materials, most methods either require chemical gelation reactions or do not allow for the desired control of pore sizes over multiple length scales. We describe a versatile and simple approach to produce tailor-made hierarchical porous materials that relies solely on the process of drying. Our results show that simple drying of a complex suspension can lead to the self-assembly of droplets, colloidal particles and molecular species into unique 3D hierarchical porous structures. Using a microfluidic device to produce monodisperse templating droplets of tunable size, we prepared materials with up to three levels of hierarchy exhibiting monodisperse pores ranging from 10 nm to 800 μm. While the size of macropores obtained after drying is determined by the size of initial droplets, the interconnectivity between macropores is strongly affected by the type of droplet stabilizer (surfactants or particles). This simple route can be used to prepare porous materials of many chemical compositions and has great potential for creating artificial porous structures that capture some of the exquisite hierarchical features of porous biological materials.     

Optical properties of -doped semiconductors: Plasmon–phonon coupling and Raman spectra

Collective excitations and their coupling to optical phonons have been studied for a two-dimensional electron gas in -doped polar semiconductors within the random-phase approximation. The inelastic light scattering spectrum due to the coupled plasmon–phonon modes are calculated for the multisubband two-dimensional electron systems. Our calculation shows that, due to the high electron density in these systems, both intrasubband and intersubband plasmons are strongly coupled to the optical-phonons. On the other hand, due to the high impurity concentration, level broadening modifies the inelastic light scattering spectrum significantly.     

Change of zeta potential of biocompatible colloidal oxide particles upon adsorption of bovine serum albumin and lysozyme

The amounts of negatively charged bovine serum albumin and positively charged lysozyme adsorbed on alumina, silica, titania, and zirconia particles (diameters 73 to 271 nm) in aqueous suspensions are measured. The adsorbed proteins change the zeta potentials and the isoelectric points (IEP) of the oxide particles. The added to adsorbed protein ratios at pH 7.5 are compared with the protein treated particle zeta potentials. It is found that the amounts of adsorbed proteins on the alumina, silica, and titania (but not on the zirconia) particle surfaces are highly correlated with the zeta potential. For the slightly less hydrophilic zirconia particles high amounts of protein adsorption are observed even under repulsive electrostatic conditions. One reason could be that the hydrophobic effect plays a more important role for zirconia than electrostatic interaction.     

Conservation of spin current as a consequence of a 2D Fermi surface

The possible existence of a previously unseen conservation in the theory of Fermi liquids is proposed for a two-dimensional geometry. If the ground state can be described by a smooth curve in momentum space then one expects spin current to be conserved, as it is not the case in three dimensions. Some immediate consequences that can be checked experimentally are pointed out.     

Thermal characterization of dental composites by TG/DTG and DSC

Dental composites can be improved by heat treatment, as a possible way to increase mechanical properties due to additional cure (post-cure). Direct dental composites are essentially similar to the indirect ones, supposing they have the same indication. Therefore, to establish a heat treatment protocol for direct composites, using as indirect (photoactivated by continuous and pulse-delay techniques), a characterization (TG/DTG and DSC) is necessary to determine parameters, such as mass loss by thermal decomposition, heat of reaction and glass transition temperature (T _g). By the results of this study, a heat treatment could be carried out above 160 °C (above T _g, and even higher than the endset exothermic event) and under 180 °C (temperature of significant initial mass loss).