Orlin D. Velev

Porous silica via colloidal crystallization

Microstructured porous silicas have potential applications in catalysis, separations, coatings, microelectronics and electro-optics, but methods for producing materials with uniform submicrometre pores have not been available. We have now developed a method in which modified colloidal crystals are used as templates for silica polymerization. This method yields products with highly uniform and structured pores of tuneable size in the submicrometre region.

Structured porous materials via colloidal crystal templating: from inorganic oxides to metals

The formation of nanostructured materials by using colloidal crystals as templates is a relatively new but rapidly growing area of materials science. Colloid crystalline templates are three-dimensional close-packed crystals of submicrometer spheres, whose long-ranged ordered structure is replicated in a solid matrix, to yield materials with ordered pores. These materials hold promise for use as photonic crystals, advanced catalysts, and in a variety of other applications. Here we review the wide range of materials that have been made following the original synthesis of structured porous silica. This method has been recently modified to produce porous metals.

Colloidal crystals as templates for porous materials

Close-packed colloidal crystals are promising precursors for novel materials, but only after appropriate methods are developed to fix their structure. A wide range of advanced materials has recently been synthesized by replicating the structure of colloidal crystals into durable solid matrices. Such materials with structured pores have promise as photonic crystals, catalysts, and membranes, and in a variety of other applications. This paper reviews the methods used in the formation of these materials and likely future trends in the field.

Materials: A class of porous metallic nanostructures

Colloidal crystals are ordered arrays of particles in the nanometre-to-micrometre size range. Useful microstructured materials can be created by replicating colloidal crystals in a durable matrix that preserves their key feature of long-range periodic structure. For example, colloidal crystals have been used to fabricate structures from inorganic oxides, polymers, diamond and glassy carbon, and semiconductor quantum dots, and some structures have photonic properties or are patterned on different hierarchical length scales. By using colloidal crystals as templates, we have synthesized a new class of metallic materials with long-range nano-scale ordering and hierarchical porosity.

PROTEIN INTERACTIONS IN SOLUTION CHARACTERIZED BY LIGHT AND NEUTRON SCATTERING : COMPARISON OF LYSOZYME AND CHYMOTRYPSINOGEN

The effects of pH and electrolyte concentration on protein-protein interactions in lysozyme and chymotrypsinogen solutions were investigated by static light scattering (SLS) and small-angle neutron scattering (SANS). Very good agreement between the values of the virial coefficients measured by SLS and SANS was obtained without use of adjustable parameters. At low electrolyte concentration, the virial coefficients depend strongly on pH and change from positive to negative as the pH increases. All coefficients at high salt concentration are slightly negative and depend weakly on pH. For lysozyme, the coefficients always decrease with increasing electrolyte concentration. However, for chymotrypsinogen there is a cross-over point around pH 5.2, above which the virial coefficients decrease with increasing ionic strength, indicating the presence of attractive electrostatic interactions. The data are in agreement with Derjaguin-Landau-Verwey-Overbeek (DLVO)-type modeling, accounting for the repulsive and attractive electrostatic, van der Waals, and excluded volume interactions of equivalent colloid spheres. This model, however, is unable to resolve the complex short-ranged orientational interactions. The results of protein precipitation and crystallization experiments are in qualitative correlation with the patterns of the virial coefficients and demonstrate that interaction mapping could help outline new crystallization regions.

Induced-charge electrophoresis of metallodielectric particles.

The application of ac electric fields in aqueous suspensions of anisotropic particles leads to unbalanced liquid flows and nonlinear, induced-charge electrophoretic motion. We report experimental observations of the motion of Janus microparticles with one dielectric and one metal-coated hemisphere induced by uniform fields of frequency 100 Hz-10 kHz in NaCl solutions. The motion is perpendicular to the field axis and persists after particles are attracted to a glass wall. This phenomenon may find applications in microactuators, microsensors, and microfluidic devices.

On-chip micromanipulation and assembly of colloidal particles by electric fields

We overview the ways in which electric fields can be used for on-chip manipulation and assembly of colloidal particles. Particles suspended in water readily respond to alternating (AC) or direct current (DC) electric fields. Charged particles in DC fields are moved towards oppositely charged electrodes by electrophoresis. Dielectrophoresis, particle mobility in AC fields, allows precise manipulation of particles through a range of parameters including field strength and frequency and electrode geometry. Simultaneously, DC or AC electrokinetics may drive liquid flows inside the experimental cells, which also leads to transport and redistribution of the suspended particles. Examples of dielectrophoretic manipulation and assembly of nanoparticles and microparticles by planar on-chip electrodes are presented. The structures assembled include conductive microwires from metallic nanoparticles and switchable two-dimensional crystals from polymer microspheres. We also discuss how dielectrophoresis and AC electrokinetics can be used in droplet-based microfluidic chips, biosensors, and devices for collection of particles from diluted suspensions.

Fabrication of asymmetrically coated colloid particles by microcontact printing techniques

We developed a novel method for preparation of asymmetrically coated colloid particles by using a microcontact printing technique. Films of water-insoluble ionic surfactants deposited on PDMS stamps were printed onto latex particle monolayers of opposite surface charge in order to produce spherical latex particles of dipolar surface charge distribution. We studied the effects of salt on the aggregation of such dipolar particles in aqueous suspensions. Upon addition of salt, dipolar colloid particles were found to give “linear” aggregates. We then extended this microcontact printing technique to the directed assembly of colloidal particles. Microcontact printing of one colloidal monolayer over a latex particle monolayer of opposite charge was used to fabricate particles of complex internal structure. We demonstrated that if the two colloid monolayers consist of particles of comparable sizes, this method allows fabrication of particle doublets. When the particle monolayer was stamped with another colloid monolayer of much smaller particle size complex structures as half-coated “raspberry”-like particles were obtained. Possible applications of these asymmetrically coated colloids include photonic crystals with novel symmetries, colloidal substitutes for liquid crystals and water-based electrorheological fluids.

Reconfigurable responsive structures assembled from magnetic Janus particles

Magnetic Janus particles are assembled into novel staggered chain structures under the action of magnetic and electric fields. The magnetic assembly can result in permanent structures, which could be disassembled on demand by remote demagnetization.

Long-Term Stabilization of Foams and Emulsions with In-Situ Formed Microparticles from Hydrophobic Cellulose

We report a simple method to produce foams and emulsions of extraordinary stability by using hydrophobic cellulose microparticles, which are formed in situ by a liquid-liquid dispersion technique. The hydrophobic cellulose derivative, hypromellose phthalate (HP), was initially dissolved in water-miscible solvents such as acetone and ethanol/water mixtures. As these HP stock solutions were sheared in aqueous media, micron sized cellulose particles formed by the solvent attrition. We also designed and investigated an effective and simple process for making HP particles without any organic solvents, where both the solvent and antisolvent were aqueous buffer solutions at different pH. Consequently, the HP particles adsorbed onto the water/air or water/oil interfaces created during shear blending, resulting in highly stable foams or foam/emulsions. The formation of HP particles and their ability for short-term and long-term stabilization of interfaces strongly depended on the HP concentration in stock solutions, as well as the solvent chemistry of both stock solutions and continuous phase media. Some foams and emulsion samples formed in the presence of ca. 1 wt% HP were stable for months. This new class of nontoxic inexpensive cellulose-based particle stabilizers has the potential to substitute conventional synthetic surfactants, especially in edible, pharmaceutical and biodegradable products.