Eric W. Kaler

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.

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.

Structured metallic films for optical and spectroscopic applications via colloidal crystal templating

áeñ = 2.22 in the infiltrated one (reducing the contrast at the same time). After inversion the mean dielectric constant decreases to áeñ = 1.41 and, accordingly, the L pseudogap energy shifts upwards (see Fig. 5). The peak width is a function of both the dielectric contrast and the filling factor of the structure. Bare opals present a contrast eSiO2/eair = 2.1 that shifts to epolymer/eSiO2 = 1.24 when infiltration takes place. So, the pseudogap width is largely decreased, as is observed in both the experiment and band structure calculation. When inversion occurs, the dielectric contrast is increased up to epolymer/eair = 2.6. It has to be noticed that, although bare and inverse opal have similar values of the refractive index contrast, inverse opals show a much broader pseudogap than the direct opal structure which reflects the fact that inverse structures are more powerful scatterers (see Fig. 5A). When the sample is tilted with respect to normal incidence, the k vector ceases to be collinear with C±L. For a given direction (tilt angle), at some point of the energy scan, k crosses the Bragg plane and a reflection is obtained. Since L is the closest (to C) point of the Bragg plane, tilting increases both the wavevector length and the energy for which reflection occurs. This pseudogap energy position, can be followed along the L±U (or L±K or L±W) line in the Brillouin zone. In Figure 5C experimental data are superimposed on the band structure diagram by using Snells law with an average refractive index for calculating the internal angle (with respect to the C±L direction). The theory gives a good account of the behavior of the pseudogap position. In summary, we have obtained and optically analyzed polymer inverse opals with a long-range order. Their photonic crystal behavior has been studied both experimentally and theoretically, and a good agreement between band-structure calculations and experiments was found. From a fundamental point of view, they can be regarded as model systems, where studying the effect of topology and dielectric contrast is possible. Regarding their potential applications, they can be used to modify the emission properties of luminescent species, such as dyes, that can easily be incorporated into the polymer. Polymer inverse opals offer, in turn, the interesting possibility of being used as matrices to obtain new spherical colloidal particles, whose shape cannot be controlled otherwise, from different materials.

Why is the osmotic second virial coefficient related to protein crystallization

Abstract A molecular basis is presented for characterizing the osmotic second virial coefficient, B 22 , of dilute protein solutions, which provides a measure of the nature of protein–protein interactions and has been shown to be correlated with crystallization behavior. Experimental measurements of the second virial coefficient of lysozyme and bovine α -chymotrypsinogen A were performed by static light scattering, as a function of pH and electrolyte concentration. Although some of the trends can be explained qualitatively by simple colloidal models of protein interactions, a more realistic interpretation based on protein crystallographic structures suggests a different explanation of experimental trends. The interactions accounted for are solute–solute excluded volume (steric), electrostatic and short-range (mainly van der Waals) interactions. The interactions depend strongly on orientation, and this profoundly affects calculated second virial coefficients. We find that molecular configurations in which complementary surfaces are apposed contribute disproportionately to the second virial coefficient, mainly through short-range interactions; electrostatic interactions play a secondary role in many of these configurations. Thus molecular recognition events can play a role in determining the solution thermodynamic properties of proteins, and this provides a plausible basis for explaining the observed relationship with crystallization behavior.

Polymerization of and within self-organized media

Self-organized surfactant solutions, such as microemulsions, vesicular solutions or dispersions, or lyotropic mesophases can serve as templates for the structure directed synthesis of organic polymers. Recent developments of templating within these equilibrium nanostructured fluids are reviewed. Depending on the template structure and the reaction conditions, the outcomes may be polyampholytes, amphiphiles, nanoparticles, hollow spheres, or mesoporous polymers. For each structure and morphology, the final product materials reflect a delicate balance between phase behavior and the reaction and mass transfer parameters that set structure. Experimental and theoretical aspects of reaction kinetics and thermodynamics such as monomer partitioning, swelling behavior and polymerization-induced phase separation are discussed.

Patterns of protein–protein interactions in salt solutions and implications for protein crystallization

The second osmotic virial coefficients of seven proteins—ovalbumin, ribonuclease A, bovine serum albumin, α‐lactalbumin, myoglobin, cytochrome c, and catalase—were measured in salt solutions. Comparison of the interaction trends in terms of the dimensionless second virial coefficient b2 shows that, at low salt concentrations, protein–protein interactions can be either attractive or repulsive, possibly due to the anisotropy of the protein charge distribution. At high salt concentrations, the behavior depends on the salt: In sodium chloride, protein interactions generally show little salt dependence up to very high salt concentrations, whereas in ammonium sulfate, proteins show a sharp drop in b2 with increasing salt concentration beyond a particular threshold. The experimental phase behavior of the proteins corroborates these observations in that precipitation always follows the drop in b2. When the proteins crystallize, they do so at slightly lower salt concentrations than seen for precipitation. The b2 measurements were extended to other salts for ovalbumin and catalase. The trends follow the Hofmeister series, and the effect of the salt can be interpreted as a water‐mediated effect between the protein and salt molecules. The b2 trends quantify protein–protein interactions and provide some understanding of the corresponding phase behavior. The results explain both why ammonium sulfate is among the best crystallization agents, as well as some of the difficulties that can be encountered in protein crystallization.

Small angle neutron scattering near Lifshitz lines: Transition from weakly structured mixtures to microemulsions

We have studied the phase behavior, wetting transitions, and small angle neutron scattering (SANS) of water, n‐alkane, and n‐alkyl polyglycol ether (CiEj) systems in order to locate the transition between weakly structured mixtures and microemulsions, and to provide a measure for the transition. We first determined the wetting transition by macroscopic measurements and then measured the location of the Lifshitz lines by SANS. Starting with well‐structured mixtures (exhibiting nonwetting middle phases and well‐expressed scattering peaks, features that qualify them as microemulsions) the wetting transition was induced by increasing the chain length of the alkane or by changing the oil/water volume ratio, and then the Lifshitz line was crossed. Further, starting with systems past the disorder line (weakly structured mixtures that display wetting middle phases and no scattering peaks), local structure was induced by either increasing the surfactant concentration or decreasing the oil/water volume ratio or the...

Dielectrophoretic assembly of oriented and switchable two-dimensional photonic crystals

We show that one- and two-dimensional crystals can be assembled from suspensions of latex or silica microspheres subjected to an alternating electric field in a gap between planar electrodes on a surface. These crystals, with areas above 25 mm2, are specifically oriented without the need for micropatterned templates. The order–disorder transitions take place within seconds and can be repeated tens of times by switching the field on and off. The particles accumulate on the surface between the electrodes due to the field gradient, align into rows along the field direction, and then crystallize into hexagonal arrays. The lattice spacings can be controlled via the electrostatic repulsion.

Toward understanding microemulsion microstructure: A small‐angle x‐ray scattering study

The microemulsion phases formed in solutions of octane, commercial surfactant, and alcohol with various brines are examined with small‐angle x‐ray scattering (SAXS), electrical conductivity, and viscosity techniques. Models based on monodisperse populations of swollen micelles or microemulsion ‘‘droplets’’ adequately represent the SAXS data at low volume fractions of brine. Introduction of hard‐sphere interactions with the Percus–Yevick approximation allows us to model the composition dependence of the radius of gyration and isothermal compressibility up to volume fractions of brine near a percolation threshold for electrical conductivity. For brine volume fractions above the percolation threshold, a mean field attractive interaction term is needed to model the variation of isothermal compressibility; however, the same theory fails to model the composition dependence of the apparent radius of gyration. But predictions from a model for a bicontinuous microemulsion structure that is geometrically irregular yet topologically ordered and that evolves continuously into swollen (inverted) micellar solutions at low volume fraction of water (oil) are in good agreement with the SAXS and electrical conductivity data over a wide range of brine volume fractions.