Detailed Description ot the Research Projects
 
Orlin Velev - 1999

Novel materials and devices by colloidally engineered assembly of microstructures.

Protein-protein and protein-surfactant interactions in solution. Protein crystallization, precipitation and separation.

 

Colloidal crystals as templates for the formation of novel microstructured porous materials. We have introduced a novel approach for the formation of structured materials by using colloidal crystals as templates (Velev et al., Nature, 389, 447, 1997; Chem. Mater., 10, 3597, 1998). The first application yielded highly structured silica materials in which the pore size, shape and ordering can be precisely controlled in a wide region (100 to 1000 nm) that had previously been unattainable. In this method the colloidal crystals of submicrometer latex spheres a are functionalized with surfactant and infused with silica solution, which polymerizes in the cavities. Silica with highly uniform and ordered pores, representing a negative replica of the original colloidal crystal, is obtained after the latex particles are removed by calcination. The method has been extended by others to a whole range of new materials, from oxides through polymers to carbons. Recently we modified our method to obtain a unique new material nanostructured porous gold (Velev et al., Nature, 401, Oct. 7, 1999). The metallic structure is assembled from nanometer size gold particles that are templated by the colloidal crystal to yield a metallic nanostructure with 3D organization of ordered pores. We developed two alternative pathways for the formation of the porous gold, one of which leads to a structure that is simply macroporous, while the other results in a hierarchical macroporous-mesoporous structure with precisely tunable porosity on two different length scales. The material has unique optical and photonic properties and holds promise for advanced applications in electro-optics, microelectronics, or catalysis. Our present efforts are directed to extending the method to the formation of structured porous coatings, which may have reflective, energy-harvesting, or self-lubricating properties.

Microporous particles of defined size and shape by droplet templating. The colloidal crystals can potentially provide extraordinary optical, electro-optical and structural properties in advanced materials, but only after appropriate methods are developed for the templated formation of objects of defined size and shape. We have invented and are studying now a novel method to form materials in which colloidal particles self-assemble into spherical or globular crystalline structures (microballs). The method can be applied to the controlled assembly of a variety of nano- and microstructured, composite and anisotropic particles, with applications in catalysis, chromatography, chemical spill detection and control, drug delivery and electro-optical devices. Details on the method and the novel materials obtained will be posted after publication.

Microscopic biosensors by interfacing colloidal assemblies with electronic chips. There is a growing interest in replacing large and complex biosensors based on SPR, TIRF, or piezoelectric oscillators, by miniature and disposable on-chip devices. We have developed a new method in which microscopic electronically readable biosensors are assembled in situ from the widely available latex particles used in traditional agglutination assays (Velev and Kaler, Langmuir, 15, 3693, 1999). The micrometer sized active areas of the sensors are generated by dielectrophoretic collection of suspended particles in gaps between on-chip electrodes. The key step in the assembly scheme is to decrease the repulsive electrostatic or by steric interactions to the point where the van der Waals and hydrophobic attractive forces coagulate the particles. The model target molecule for the sensors was human immunoglobulin (IgG). Detection was carried out by tagging the IgG molecules with colloidal gold conjugated to a secondary antibody and fusing the gold particles via silver enhancer. The electronic readout is carried out by simply measuring the resistance between the microscopic pairs of electrodes. The experimentally estimated LOD of the sensors was on the order of the commercial IgG agglutination assays and immunosensors available. Thus we assembled functional devices, which even at this exploratory stage are of micrometer size and possess sensitivity comparable to that of clinical assays. The theoretical sensitivity of the microscopic active elements is very high, and in principle approaches the lowest imaginable limit of a few tens or hundreds of molecules. Arrays of different sensors can be assembled on the same "chip" by addressing different gaps. We believe that further research and improvement of the method can lead to small, simple and disposable sensors with simple electronic readout.

Protein-surfactant interactions related to protein separations and crystallization. The equilibria in systems containing ionic surfactants and proteins have been investigated extensively, but the molecular mechanisms of the interactions involved are still not well understood. We have obtained complementary light scattering, neutron scattering and solution equilibrium data on lysozyme interactions mediated by the presence of anionic surfactants of varying molecular weight. The data can not be explained in terms of DLVO concepts and indicate the existence of highly dynamic equilibria based on structural interactions. The ionic surfactants modify the crystallization of lysozyme and lead to the formation of phases with morphologies different from those seen in the absence of surfactant, some of which display structured patterns on the micrometer scale. By using fluorescent probes it was demonstrated that the surfactants are incorporated in the growing crystals or can penetrate the crystals and adsorb afterwards. A pattern of protein re-crystallization with time was established. The X-ray diffraction quality and resolution (up to 1.8 Å) of the lysozyme crystals obtained are comparable to those of the best crystals ever obtained in the absence of surfactants. A quantitative model to explain the protein-surfactant precipitation equilibria is under development.

Protein crystals as novel biomaterials: Characterization and photo-manipulation. The protein crystals are naturally organized biological matrixes, which can serve as hosts for smaller molecules. We carried out quantitative microscopic observations of the infusion of lysozyme crystals with pyrene-based fluorescent surfactants to characterize the surfactant mass transfer and energy of adsorption inside the protein matrix. The fluorescence intensity data were fitted by a mass transfer model that gives the effective diffusion coefficient of the fluorophore in the crystals. The diffusion coefficients obtained range from 2 to 30x10-10 cm2/s and depend on the type and size of the surfactant. The slow infusion is a consequence of the strong surfactant adsorption on the protein lattice. FRAP experiments with saturated crystals showed that surfactant self-diffusion in these crystals is negligible, which is yet another indication of the strong adsorption. The adsorbed pyrenes can serve as light energy converters, which can denature the host lysozyme molecules. Based on this, we discovered a method for light-manipulated etching and drilling of surfactant-infused crystals. It was also found out, that collimated light beams can directionally deform fluorescent crystals and deposit protein phases (Velev et al., Adv. Mater., in press, 1999). The data suggest ways of creating, studying and photo-manipulating new types of materials based on crystalline protein matrices, which will be explored and refined further.

Interactions and structure formation in protein systems related to precipitation, gelation and crystallization. The general objective of the project is to develop experimental and theoretical tools for predicting the performance of protein separation and purification processes, based on the molecular structure and protein-protein interactions. To verify the models and provide appropriate parameters for the calculations, the intra- and intermolecular structure of proteins in solution are measured as a function of pH, ionic strength and concentration of protein and additives (Velev et. al., Biophys. J., 75, 2682, 1998). The experimental methods include static and dynamic light scattering, small-angle neutron scattering (SANS), X-ray diffraction, quantitative fluorescence microscopy, AFM and others. Particular attention is focused on the development of highly efficient crystallization and separation schemes based on control and manipulation of interactions. A research project sponsored by a major company is directed to understanding and enhancing gelation in protein systems.

 


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Last updated Oct. 05, 1999. Please note that this material is for fair use and information purposes only.