Establishing processing-structure-property relationships for monolayer materials is crucial for a... more Establishing processing-structure-property relationships for monolayer materials is crucial for a range of applications spanning optics, catalysis, electronics and energy. Presently, for molybdenum disulfide, a promising catalyst for artificial photosynthesis, considerable debate surrounds the structure/property relationships of its various allotropes. Here we unambiguously solve the structure of molybdenum disulfide monolayers using high-resolution transmission electron microscopy supported by density functional theory and show lithium intercalation to direct a preferential transformation of the basal plane from 2H (trigonal prismatic) to 1T' (clustered Mo). These changes alter the energetics of molybdenum disulfide interactions with hydrogen (ΔGH), and, with respect to catalysis, the 1T' transformation renders the normally inert basal plane amenable towards hydrogen adsorption and hydrogen evolution. Indeed, we show basal plane activation of 1T' molybdenum disulfide an...
CO 2 captured from coal-fired power plants represents three-quarters of the total cost of an enti... more CO 2 captured from coal-fired power plants represents three-quarters of the total cost of an entire carbon sequestration process. Conventional amine absorption or cryogenic separation requires high capital investment and is very energy intensive. Our novel membrane process is energy efficient with great potential for economical CO 2 capture. Three classes of microporous sol-gel derived silica-based membranes were developed for selective CO 2 removal under simulated flue gas conditions (SFG), e.g. feed of 10% vol. CO 2 in N 2 , 1 atm total pressure, T = 50-60ºC, RH>50%, SO 2 >10 ppm. A novel class of amine-functional microporous silica membranes was prepared using an amine-derivatized alkoxysilane precursor, exhibiting enhanced (>70) CO 2 :N 2 selectivity in the presence of H 2 O vapor, but its CO 2 permeance was lagging (<1 MPU). Pure siliceous membranes showed higher CO 2 permeance (1.5-2 MPU) but subsequent densification occurred under prolonged SFG conditions. We incorporated NiO in the microporous network up to a loading of Ni:Si = 0.2 to retard densification and achieved CO 2 permeance of 0.5 MPU and CO 2 :N 2 selectivity of 50 after 163 h exposure to SFG conditions. However, CO 2 permeance should reach greater than 2.0 MPU in order to achieve the cost of electricity (COE) goal set by DOE. We introduced the atomic layer deposition (ALD), a molecular deposition technique that substantially reduces membrane thickness with intent to improve permeance and selectivity. The deposition technique also allows the incorporation of Ni or Ag cations by proper selection of metallorganic precursors. In addition, preliminary economic analysis provides a sensitivity study on the performance and cost of the proposed membranes for CO 2 capture. Significant progress has been made toward the practical applications for CO 2 capture. (1 MPU = 1.0 cm 3 (STP)•cm -2 •min - 1 •atm -1 ) KEYWORDS Inorganic membranes, ceramic membranes, non metallic membranes, carbon dioxide capture, carbon dioxide sequestration, gas separation, high temperature gas separation, flue gas CO 2 capture, sol-gel, global warming
Proceedings of the 2005 IEEE International Frequency Control Symposium and Exposition, 2005., 2005
Recent progress in nanotechnology has lead to development of novel materials with very unique sup... more Recent progress in nanotechnology has lead to development of novel materials with very unique superhydrophobic (SH) properties. In this paper we study the Thickness-Shear Mode (TSM) sensing technique as a potential measurement tool for characterization of SH properties of materials. The TSM sensors, coated with different SH thin films were capable to differentiate between a range of the SH properties of those films though the contact angle of those films, measured with optical techniques, have shown similar values. Also, the TSM sensors, coated with some of those SH films, exhibited almost zero attenuation and very small frequency shift when loaded with deionized water, which indicated a presence of significant slip in the mechanical interfacial boundary conditions. The observed sensitivity of the TSM technique to nanoscale-level structural features of SH films suggests that TSM sensing technology may provide more sensitive means for measurements and characterization of superhydrophobicity in comparison to the existing SH measurement techniques. I.
Various protocell models have been constructed de novo with the bottom-up approach. Here we descr... more Various protocell models have been constructed de novo with the bottom-up approach. Here we describe a silica-based protocell composed of a nanoporous amorphous silica core encapsulated within a lipid bilayer built by self-assembly that provides for independent definition of cell interior and the surface membrane. In this review, we will first describe the essential features of this architecture and then summarize the current development of silica-based protocells at both micro-and nanoscale with diverse functionalities. As the structure of the silica is relatively static, silica-core protocells do not have the ability to change shape, but their interior structure provides a highly crowded and, in some cases, authentic scaffold upon which biomolecular components and systems could be reconstituted. In basic research, the larger protocells based on precise silica replicas of cells could be developed into geometrically realistic bioreactor platforms to enable cellular functions like coupled biochemical reactions, while in translational research smaller protocells based on mesoporous silica nanoparticles are being developed for targeted nanomedicine. Ultimately we see two different motivations for protocell research and
Our discovery that the introduction of living cells (Saccharomyces cerevisiae) alters dramaticall... more Our discovery that the introduction of living cells (Saccharomyces cerevisiae) alters dramatically the evaporation driven self-assembly of lipid-silica nanostructures suggested the formation of novel bio/nano interfaces useful for cellular interrogation at the nanoscale. This one year "out of the box" LDRD focused on the localization of metallic and semi-conducting nanocrystals at the fluid, lipid-rich interface between S. cerevisiae and the surrounding phospholipid-templated silica nanostructure with the primary goal of creating Surface Enhanced Raman Spectroscopy (SERS)-active nanostructures and platforms for cellular integration into electrode arrays. Such structures are of interest for probing cellular responses to the onset of disease, understanding of cell-cell communication, and the development of cell-based bio-sensors. As SERS is known to be sensitive to the size and shape of metallic (principally gold and silver) nanocrystals, various sizes and shapes of nanocrystals were synthesized, functionalized and localized at the cellular surface by our 'cell-directed assembly' approach. Laser scanning confocal microscopy, SEM, and in situ grazing incidence small angle x-ray scattering (GISAXS) experiments were performed to study metallic nanocrystal localization.
This is the final report for the Presidential Early Career Award for Science and Engineering -PEC... more This is the final report for the Presidential Early Career Award for Science and Engineering -PECASE (LDRD projects 93369 and 118841) awarded to Professor
Novel and exciting phenomena which have the potential to revolutionize science, materials, nextge... more Novel and exciting phenomena which have the potential to revolutionize science, materials, nextgeneration production methods are manifested as structural dimensions approach the nanometer level. However, advances in lithography are insufficient to achieve this feature scale. As a result, molecular self-assembly has attracted a great deal of interest, since this provides a possible spontaneous mechanism by which nanometer sized arrays can be formed without mechanical intervention. Although these "low tech" processes are highly dependent on system/material specifics, we propose that new techniques and the science which underlies them can be developed in a manner that allows us to extend the natural order of self-assembling systems. The approach we will employ is based on previous work in which we fabricated templates of unprecedented size and regularity. These templates will then be applied to form nano-arrays based on a wide range of materials with tunable properties and nano-scale selective area overgrowth patterns for improving the materials quality of thin films.
This project set out to scientifically-tailor "smart" interfacial films and 3-D composite nanostr... more This project set out to scientifically-tailor "smart" interfacial films and 3-D composite nanostructures to exhibit photochromic responses to specific, highly-localized chemical and/or mechanical stimuli, and to integrate them into optical microsystems. The project involved the design of functionalized chromophoric self-assembled materials that possessed intense and environmentally-sensitive optical properties (absorbance, fluorescence) enabling their use as detectors of specific stimuli and transducers when interfaced with optical probes. The conjugated polymer polydiacetylene (PDA) proved to be the most promising material in many respects, although it had some drawbacks concerning reversibility. Throughout his work we used multi-task scanning probes (AFM, NSOM), offering simultaneous optical and interfacial force capabilities, to actuate and characterize the PDA with localized and specific interactions for detailed characterization of physical mechanisms and parameters. In addition to forming high quality mono-, bi-, and tri-layers of PDA via Langmuir-Blodgett deposition, we were successful in using the diacetylene monomer precursor as a surfactant that directed the selfassembly of an ordered, mesostructured inorganic host matrix. Remarkably, the diacetylene was polymerized in the matrix, thus providing a PDA-silica composite. The inorganic matrix serves as a perm-selective barrier to chemical and biological agents and provides structural support for improved material durability in microsystems. Our original goal was to use the composite films as a direct interface with microscale devices as optical elements (e.g., intracavity mirrors, diffraction gratings), taking advantage of the very high sensitivity of device performance to realtime dielectric changes in the films. However, our optical physics colleagues (M. Crawford and S. Kemme) were unsuccessful in these efforts, mainly due to the poor optical quality of the composite films. 1. Summary of accomplishments 2. High molecular orientation in mono-and tri-layer polydiacetylene films imaged by atomic force microscopy (SAND2000-0569J) published in J. Colloid Interface Sci. 229, 490 (2000). 3. First observation of mechanochromism at the nanometer scale (SAND99-1728J) published in Langmuir 16, 1270 (2000). 4. Shear-induced mechanochromism in polydiacetylene monolayers (SAND2000-2057J) published Tribology Letters 10, 89 (2001). 5. Large friction anisotropy of a polydiacetylene monolayer (SAND99-1211J) published in Tribology Letters 7, 79 (1999). 6. Spectroscopic ellipsometry and fluorescence study of thermochromism in an ultrathin polydiacetylene film: reversibility and transition kinetics (SAND2000-0158J) published in Langmuir 16, 4639 (2000). 7. Self-assembly of mesoscopically ordered chromatic polydiacetylene/silica nanocomposites (SAND2001-1285J) published in Nature 410, 913 (2001)
The sol-gel process uses metal alkoxides of network forming cations, such as Si, B, or A1, in alc... more The sol-gel process uses metal alkoxides of network forming cations, such as Si, B, or A1, in alcohol/water solutions to form glass-like, polymeric networks in liquid solution. Thin films are formed by depositing the solution on a substrate by spinning, dipping or spraying. When the film is then heated to moderate temperatures (400-500 o C), dense glass films or stable porous films are obtained. The use of sol-gel thin films in four solar energy materials applications is discussed: 1. Encapsulation of black chrome solar selective coatings improved the high temperature thermal stability by a factor of 2.7. 2. Formation of porous, antireflection coatings on glass envelopes used in solar thermal collectors increased the solar transmittance of the glass from 0.91 to greater than 0.96. 3. Double-layer, antireflection coatings of SiO 2 and TiO 2 on silicon solar cells reduced the solar reflectance of the cells from 0.36 to 0.04 and thereby improved cell efficiencies by 50%. 4. Protective coatings applied to silvered stainless steel substrates were used to form structural solar mirrors. Solar averaged specular reflectance values of 0.90 to 0.91 were obtained.
Recent work has demonstrated the feasibility of employing three dimensional (3D) protein hydrogel... more Recent work has demonstrated the feasibility of employing three dimensional (3D) protein hydrogels, fabricated using multiphoton-induced photochemistry, as chemically responsive microactuators in ''lab on a chip'' devices. In addition, these materials show great promise as cell capture/incubation devices, allowing single bacterial cells to reproduce into multicellular constructs with ''user-defined'' 3D geometries. However, to date, the mechanical properties of these materials, critical for these applications, have not been quantitatively characterized. In this work, we develop and apply a method to measure the elastic modulus of microfabricated protein hydrogels in situ under dynamic physical and chemical environments. We fabricated protein microcantilevers using a wide range of protein building blocks (albumin, lysozyme, avidin) and probed their mechanical properties using atomic force microscopy (AFM). The length dependence of the spring constant displayed by protein cantilevers followed the predicted cantilever model, yielding the elastic modulus of the material. By varying laser dwell time, the modulus of protein cantilevers could be tuned over 2 orders of magnitude (from 0.03 to 3 MPa for albumin), a range that encompasses modulus values for a number of biological tissues (e.g., cartilage, basement membrane). Further, the modulus was shown to vary strongly over a range of pH values (pH 2-12). Distinct profiles of pH vs. modulus for albumin, lysozyme and avidin cantilevers were observed, which correlate to structural transitions of the incorporated protein. Modification of protein cantilevers via ligand binding (biotin to avidin), increased cantilever stiffness. Finally, using the modulus of a hydrogel microchamber calculated in situ, we determined the pressure generated by a replicating bacterial colony entrapped in the microchamber to be 2.7 AE 1.3 kPa. This work demonstrates an ability to quantify mechanical properties under both chemically and biologically dynamic microenvironments and will enable the development of a robust platform to investigate cell/ microenvironmental interactions with high spatial resolution, in three dimensions, using mechanically tunable biological materials.
Neutron reflectivity (NR) is used to probe the solid, liquid, vapor interface of a porous superhy... more Neutron reflectivity (NR) is used to probe the solid, liquid, vapor interface of a porous superhydrophobic (SH) surface submerged in water. A low-temperature, low-pressure technique was used to prepare a rough, highly porous organosilica aerogel-like film. UV/ozone treatments were used to control the surface coverage of hydrophobic organic ligands on the silica framework, allowing the contact angle with water to be continuously varied over the range of 160°(superhydrophobic) to <10°(hydrophilic). NR shows that the superhydrophobic nature of the surface prevents infiltration of water into the porous film. Atomic force microscopy and density functional theory simulations are used in combination to interpret the NR results and help establish the location, width, and nature of the SH film-water interface.
Oriented mesostructured surfactant-silica nanocomposite thin films with a 2D hexagonal mesostruct... more Oriented mesostructured surfactant-silica nanocomposite thin films with a 2D hexagonal mesostructure of cylindrical micelles were prepared by evaporation-induced self-assembly using two different nonionic Brij surfactants and studied by small-angle X-ray scattering in symmetric reflection (SRSAXS) and grazing incidence (GISAXS) geometries. A novel SRSAXS evaluation approach was applied that allowed a good fitting of the SRSAXS data over almost the whole range of scattering vectors. Aside from the cylinder radius and the lattice parameter, the approach provided accurate values for the polydispersity of the micelles, lattice distortions, and preferred orientation. These analyses revealed a significant rise of the micelle radius and accordingly the lattice parameter upon an increase in the ratio surfactant/SiO 2, attributable to a decrease in the solubilization of the poly(ethylene oxide) (PEO) chains by water, in agreement with Monte Carlo simulations. Furthermore, the SRSAXS analysis was successfully applied to the corresponding mesoporous films for the determination of pore sizes.
Single bilayer membranes of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were formed o... more Single bilayer membranes of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were formed on ordered nanocomposite and nanoporous silica thin films by fusion of small unilamellar vesicles. The structure of these membranes was investigated using neutron reflectivity. The underlying thin films were formed by evaporation induced self-assembly to obtain periodic arrangements of silica and surfactant molecules in the nanocomposite thin films, followed by photocalcination to oxidatively remove the organics and render the films nanoporous. We show that this platform affords homogeneous and continuous bilayer membranes that have promising applications as model membranes and sensors.
It is now recognized that self-assembly is a powerful synthetic approach to the fabrication of na... more It is now recognized that self-assembly is a powerful synthetic approach to the fabrication of nanostructures with feature sizes smaller than achievable with state of the art lithography and with a complexity approaching that of biological systems. For example, recent research has shown that silica/ surfactant self-assembly combined with evaporation (so-called evaporation induced self-assembly EISA) can direct the formation of porous and composite thin-film mesostructures characterized by precise periodic arrangements of inorganic and organic constituents on the 1-50-nm scale. Despite the potential utility of these films for a diverse range of applications such as sensors, membranes, catalysts, waveguides, lasers, nano-fluidic systems, and low dielectric constant (so-called low k) insulators, the mechanism of EISA is not yet completely understood. Here, using time-resolved grazing incidence small-angle X-ray scattering (GISAXS) combined with gravimetric analysis and infrared spectroscopy, we structurally and compositionally characterize in situ the evaporation induced self-assembly of a homogeneous silica/surfactant/solvent solution into a highly ordered surfactant-templated mesostructure. Using CTAB (cetyltrimethylammonium bromide) as the structure-directing surfactant, a two-dimensional (2-D) hexagonal thin-film mesophase (p6mm) with cylinder axes oriented parallel to the substrate surface forms from an incipient lamellar mesophase through a correlated micellar intermediate. Comparison with the corresponding CTAB/water/ alcohol system (prepared without silica) shows that, for acidic conditions in which the siloxane condensation rate is minimized, the hydrophilic and nonvolatile silicic acid components replace water maintaining a fluidlike state that avoids kinetic barriers to self-assembly. tion, 21 or selective dewetting 18 drives silica/surfactant selfassembly 1 into uniform or spatially patterned 18 thin-film mesophases. Through subsequent heating or exposure to catalysts or light, 9,22 these films can be processed into porous or composite mesostructures of potential utility for a diverse range of applications such as sensors, 15 membranes, 16 catalysts,
We have developed structure/toxicity relationships for amorphous silica nanoparticles (NPs) synth... more We have developed structure/toxicity relationships for amorphous silica nanoparticles (NPs) synthesized through low temperature, colloidal (e.g. Stöber silica) or high temperature pyrolysis (e.g. fumed silica) routes. Through combined spectroscopic and physical analyses, we have determined the state of aggregation, hydroxyl concentration, relative proportion of strained and unstrained siloxane rings, and potential to generate hydroxyl radicals for Stöber and fumed silica NPs with comparable primary particle sizes (16-nm in diameter). Based on erythrocyte hemolytic assays and assessment of the viability and ATP levels in epithelial and macrophage cells, we discovered for fumed silica an important toxicity relationship to post-synthesis thermal annealing or environmental exposure, whereas colloidal silicas were essentially non-toxic under identical
Sol-gel thin film formation by dipping (or spinning) in volves gravitational (or centrifugal) dra... more Sol-gel thin film formation by dipping (or spinning) in volves gravitational (or centrifugal) draining accompa nied by vigorous solvent evaporation. During dipping a steady-state film thickness profile develops, the shape of which is established by the evaporation rates of the fluid components and possible surface tension gradient driven flows. The entrained inorganic species are con centrated by the accompanying evaporation of solvent and compacted by capillary pressure developed at the final stage of the deposition process as liquid-vapor menisci recede into the gel interior. The important microstructural properties of the deposited film (% porosity, surface area, and pore size) depend on the size and structure of the inorganic species, the relative rates of condensation and evaporation during deposi tion, the magnitude of the capillary pressure, and the accompanying shear stress.
Sol-gel thin films are formed by gravitational or centrifugal draining accompanied by vigorous dr... more Sol-gel thin films are formed by gravitational or centrifugal draining accompanied by vigorous drying. Drying largely establishes the shape of the fluid profile, the timescale of the deposition process, and the magnitude of the forces exerted on the solid phase. The combination of coating theory and experiment should define coating protocols to tailor the deposition process to specific applications.
In an effort to understand the various phenomena which occur during aging of silica gels at diffe... more In an effort to understand the various phenomena which occur during aging of silica gels at different pH, silica particles prepared by three routes distinguished principally by their silicon coordination (Qn distributions) (St6ber silica spheres, Ludox and Cab-O-Sil) were employed. In situ techniques were used including proton spin-lattice relaxation, SAXS and zeta potential for the wet state and 29Si MAS-NMR, SEM, TEM and N 2 sorption for dry samples. In solution, the surface area increased in the following order after aging at pH 10 as compared with that at pH 7 and the initial dry surface area: Cab-O-Sil < Ludox << St6ber which is inversely proportional to the degree of silicon coordination. This same trend was observed for the surface charge measurements. Surface areas calculated from 29Si MAS-NMR for four different St6ber sphere sizes were of order 1000 ma/g as compared with nitrogen/BET surface areas which varied from 9 to 370 m2/g, indicating significant internal surface which is inaccessible to nitrogen. The 29Si NMR results for the St6ber spheres were consistent with a close packed aggregate of ~ 3 nm primary particles. In addition to particulate silica, aging at varying pH of a two-step acid/base-catalyzed silica gel was studied. At all pH values, the surface area was higher in the wet gel as compared with the xerogel. Aging at higher pH was found to yield lower surface area, larger pore volume and a narrower pore size distribution depending upon the pH, aging time and surface tension of the final pore fluid.
Establishing processing-structure-property relationships for monolayer materials is crucial for a... more Establishing processing-structure-property relationships for monolayer materials is crucial for a range of applications spanning optics, catalysis, electronics and energy. Presently, for molybdenum disulfide, a promising catalyst for artificial photosynthesis, considerable debate surrounds the structure/property relationships of its various allotropes. Here we unambiguously solve the structure of molybdenum disulfide monolayers using high-resolution transmission electron microscopy supported by density functional theory and show lithium intercalation to direct a preferential transformation of the basal plane from 2H (trigonal prismatic) to 1T' (clustered Mo). These changes alter the energetics of molybdenum disulfide interactions with hydrogen (ΔGH), and, with respect to catalysis, the 1T' transformation renders the normally inert basal plane amenable towards hydrogen adsorption and hydrogen evolution. Indeed, we show basal plane activation of 1T' molybdenum disulfide an...
CO 2 captured from coal-fired power plants represents three-quarters of the total cost of an enti... more CO 2 captured from coal-fired power plants represents three-quarters of the total cost of an entire carbon sequestration process. Conventional amine absorption or cryogenic separation requires high capital investment and is very energy intensive. Our novel membrane process is energy efficient with great potential for economical CO 2 capture. Three classes of microporous sol-gel derived silica-based membranes were developed for selective CO 2 removal under simulated flue gas conditions (SFG), e.g. feed of 10% vol. CO 2 in N 2 , 1 atm total pressure, T = 50-60ºC, RH>50%, SO 2 >10 ppm. A novel class of amine-functional microporous silica membranes was prepared using an amine-derivatized alkoxysilane precursor, exhibiting enhanced (>70) CO 2 :N 2 selectivity in the presence of H 2 O vapor, but its CO 2 permeance was lagging (<1 MPU). Pure siliceous membranes showed higher CO 2 permeance (1.5-2 MPU) but subsequent densification occurred under prolonged SFG conditions. We incorporated NiO in the microporous network up to a loading of Ni:Si = 0.2 to retard densification and achieved CO 2 permeance of 0.5 MPU and CO 2 :N 2 selectivity of 50 after 163 h exposure to SFG conditions. However, CO 2 permeance should reach greater than 2.0 MPU in order to achieve the cost of electricity (COE) goal set by DOE. We introduced the atomic layer deposition (ALD), a molecular deposition technique that substantially reduces membrane thickness with intent to improve permeance and selectivity. The deposition technique also allows the incorporation of Ni or Ag cations by proper selection of metallorganic precursors. In addition, preliminary economic analysis provides a sensitivity study on the performance and cost of the proposed membranes for CO 2 capture. Significant progress has been made toward the practical applications for CO 2 capture. (1 MPU = 1.0 cm 3 (STP)•cm -2 •min - 1 •atm -1 ) KEYWORDS Inorganic membranes, ceramic membranes, non metallic membranes, carbon dioxide capture, carbon dioxide sequestration, gas separation, high temperature gas separation, flue gas CO 2 capture, sol-gel, global warming
Proceedings of the 2005 IEEE International Frequency Control Symposium and Exposition, 2005., 2005
Recent progress in nanotechnology has lead to development of novel materials with very unique sup... more Recent progress in nanotechnology has lead to development of novel materials with very unique superhydrophobic (SH) properties. In this paper we study the Thickness-Shear Mode (TSM) sensing technique as a potential measurement tool for characterization of SH properties of materials. The TSM sensors, coated with different SH thin films were capable to differentiate between a range of the SH properties of those films though the contact angle of those films, measured with optical techniques, have shown similar values. Also, the TSM sensors, coated with some of those SH films, exhibited almost zero attenuation and very small frequency shift when loaded with deionized water, which indicated a presence of significant slip in the mechanical interfacial boundary conditions. The observed sensitivity of the TSM technique to nanoscale-level structural features of SH films suggests that TSM sensing technology may provide more sensitive means for measurements and characterization of superhydrophobicity in comparison to the existing SH measurement techniques. I.
Various protocell models have been constructed de novo with the bottom-up approach. Here we descr... more Various protocell models have been constructed de novo with the bottom-up approach. Here we describe a silica-based protocell composed of a nanoporous amorphous silica core encapsulated within a lipid bilayer built by self-assembly that provides for independent definition of cell interior and the surface membrane. In this review, we will first describe the essential features of this architecture and then summarize the current development of silica-based protocells at both micro-and nanoscale with diverse functionalities. As the structure of the silica is relatively static, silica-core protocells do not have the ability to change shape, but their interior structure provides a highly crowded and, in some cases, authentic scaffold upon which biomolecular components and systems could be reconstituted. In basic research, the larger protocells based on precise silica replicas of cells could be developed into geometrically realistic bioreactor platforms to enable cellular functions like coupled biochemical reactions, while in translational research smaller protocells based on mesoporous silica nanoparticles are being developed for targeted nanomedicine. Ultimately we see two different motivations for protocell research and
Our discovery that the introduction of living cells (Saccharomyces cerevisiae) alters dramaticall... more Our discovery that the introduction of living cells (Saccharomyces cerevisiae) alters dramatically the evaporation driven self-assembly of lipid-silica nanostructures suggested the formation of novel bio/nano interfaces useful for cellular interrogation at the nanoscale. This one year "out of the box" LDRD focused on the localization of metallic and semi-conducting nanocrystals at the fluid, lipid-rich interface between S. cerevisiae and the surrounding phospholipid-templated silica nanostructure with the primary goal of creating Surface Enhanced Raman Spectroscopy (SERS)-active nanostructures and platforms for cellular integration into electrode arrays. Such structures are of interest for probing cellular responses to the onset of disease, understanding of cell-cell communication, and the development of cell-based bio-sensors. As SERS is known to be sensitive to the size and shape of metallic (principally gold and silver) nanocrystals, various sizes and shapes of nanocrystals were synthesized, functionalized and localized at the cellular surface by our 'cell-directed assembly' approach. Laser scanning confocal microscopy, SEM, and in situ grazing incidence small angle x-ray scattering (GISAXS) experiments were performed to study metallic nanocrystal localization.
This is the final report for the Presidential Early Career Award for Science and Engineering -PEC... more This is the final report for the Presidential Early Career Award for Science and Engineering -PECASE (LDRD projects 93369 and 118841) awarded to Professor
Novel and exciting phenomena which have the potential to revolutionize science, materials, nextge... more Novel and exciting phenomena which have the potential to revolutionize science, materials, nextgeneration production methods are manifested as structural dimensions approach the nanometer level. However, advances in lithography are insufficient to achieve this feature scale. As a result, molecular self-assembly has attracted a great deal of interest, since this provides a possible spontaneous mechanism by which nanometer sized arrays can be formed without mechanical intervention. Although these "low tech" processes are highly dependent on system/material specifics, we propose that new techniques and the science which underlies them can be developed in a manner that allows us to extend the natural order of self-assembling systems. The approach we will employ is based on previous work in which we fabricated templates of unprecedented size and regularity. These templates will then be applied to form nano-arrays based on a wide range of materials with tunable properties and nano-scale selective area overgrowth patterns for improving the materials quality of thin films.
This project set out to scientifically-tailor "smart" interfacial films and 3-D composite nanostr... more This project set out to scientifically-tailor "smart" interfacial films and 3-D composite nanostructures to exhibit photochromic responses to specific, highly-localized chemical and/or mechanical stimuli, and to integrate them into optical microsystems. The project involved the design of functionalized chromophoric self-assembled materials that possessed intense and environmentally-sensitive optical properties (absorbance, fluorescence) enabling their use as detectors of specific stimuli and transducers when interfaced with optical probes. The conjugated polymer polydiacetylene (PDA) proved to be the most promising material in many respects, although it had some drawbacks concerning reversibility. Throughout his work we used multi-task scanning probes (AFM, NSOM), offering simultaneous optical and interfacial force capabilities, to actuate and characterize the PDA with localized and specific interactions for detailed characterization of physical mechanisms and parameters. In addition to forming high quality mono-, bi-, and tri-layers of PDA via Langmuir-Blodgett deposition, we were successful in using the diacetylene monomer precursor as a surfactant that directed the selfassembly of an ordered, mesostructured inorganic host matrix. Remarkably, the diacetylene was polymerized in the matrix, thus providing a PDA-silica composite. The inorganic matrix serves as a perm-selective barrier to chemical and biological agents and provides structural support for improved material durability in microsystems. Our original goal was to use the composite films as a direct interface with microscale devices as optical elements (e.g., intracavity mirrors, diffraction gratings), taking advantage of the very high sensitivity of device performance to realtime dielectric changes in the films. However, our optical physics colleagues (M. Crawford and S. Kemme) were unsuccessful in these efforts, mainly due to the poor optical quality of the composite films. 1. Summary of accomplishments 2. High molecular orientation in mono-and tri-layer polydiacetylene films imaged by atomic force microscopy (SAND2000-0569J) published in J. Colloid Interface Sci. 229, 490 (2000). 3. First observation of mechanochromism at the nanometer scale (SAND99-1728J) published in Langmuir 16, 1270 (2000). 4. Shear-induced mechanochromism in polydiacetylene monolayers (SAND2000-2057J) published Tribology Letters 10, 89 (2001). 5. Large friction anisotropy of a polydiacetylene monolayer (SAND99-1211J) published in Tribology Letters 7, 79 (1999). 6. Spectroscopic ellipsometry and fluorescence study of thermochromism in an ultrathin polydiacetylene film: reversibility and transition kinetics (SAND2000-0158J) published in Langmuir 16, 4639 (2000). 7. Self-assembly of mesoscopically ordered chromatic polydiacetylene/silica nanocomposites (SAND2001-1285J) published in Nature 410, 913 (2001)
The sol-gel process uses metal alkoxides of network forming cations, such as Si, B, or A1, in alc... more The sol-gel process uses metal alkoxides of network forming cations, such as Si, B, or A1, in alcohol/water solutions to form glass-like, polymeric networks in liquid solution. Thin films are formed by depositing the solution on a substrate by spinning, dipping or spraying. When the film is then heated to moderate temperatures (400-500 o C), dense glass films or stable porous films are obtained. The use of sol-gel thin films in four solar energy materials applications is discussed: 1. Encapsulation of black chrome solar selective coatings improved the high temperature thermal stability by a factor of 2.7. 2. Formation of porous, antireflection coatings on glass envelopes used in solar thermal collectors increased the solar transmittance of the glass from 0.91 to greater than 0.96. 3. Double-layer, antireflection coatings of SiO 2 and TiO 2 on silicon solar cells reduced the solar reflectance of the cells from 0.36 to 0.04 and thereby improved cell efficiencies by 50%. 4. Protective coatings applied to silvered stainless steel substrates were used to form structural solar mirrors. Solar averaged specular reflectance values of 0.90 to 0.91 were obtained.
Recent work has demonstrated the feasibility of employing three dimensional (3D) protein hydrogel... more Recent work has demonstrated the feasibility of employing three dimensional (3D) protein hydrogels, fabricated using multiphoton-induced photochemistry, as chemically responsive microactuators in ''lab on a chip'' devices. In addition, these materials show great promise as cell capture/incubation devices, allowing single bacterial cells to reproduce into multicellular constructs with ''user-defined'' 3D geometries. However, to date, the mechanical properties of these materials, critical for these applications, have not been quantitatively characterized. In this work, we develop and apply a method to measure the elastic modulus of microfabricated protein hydrogels in situ under dynamic physical and chemical environments. We fabricated protein microcantilevers using a wide range of protein building blocks (albumin, lysozyme, avidin) and probed their mechanical properties using atomic force microscopy (AFM). The length dependence of the spring constant displayed by protein cantilevers followed the predicted cantilever model, yielding the elastic modulus of the material. By varying laser dwell time, the modulus of protein cantilevers could be tuned over 2 orders of magnitude (from 0.03 to 3 MPa for albumin), a range that encompasses modulus values for a number of biological tissues (e.g., cartilage, basement membrane). Further, the modulus was shown to vary strongly over a range of pH values (pH 2-12). Distinct profiles of pH vs. modulus for albumin, lysozyme and avidin cantilevers were observed, which correlate to structural transitions of the incorporated protein. Modification of protein cantilevers via ligand binding (biotin to avidin), increased cantilever stiffness. Finally, using the modulus of a hydrogel microchamber calculated in situ, we determined the pressure generated by a replicating bacterial colony entrapped in the microchamber to be 2.7 AE 1.3 kPa. This work demonstrates an ability to quantify mechanical properties under both chemically and biologically dynamic microenvironments and will enable the development of a robust platform to investigate cell/ microenvironmental interactions with high spatial resolution, in three dimensions, using mechanically tunable biological materials.
Neutron reflectivity (NR) is used to probe the solid, liquid, vapor interface of a porous superhy... more Neutron reflectivity (NR) is used to probe the solid, liquid, vapor interface of a porous superhydrophobic (SH) surface submerged in water. A low-temperature, low-pressure technique was used to prepare a rough, highly porous organosilica aerogel-like film. UV/ozone treatments were used to control the surface coverage of hydrophobic organic ligands on the silica framework, allowing the contact angle with water to be continuously varied over the range of 160°(superhydrophobic) to <10°(hydrophilic). NR shows that the superhydrophobic nature of the surface prevents infiltration of water into the porous film. Atomic force microscopy and density functional theory simulations are used in combination to interpret the NR results and help establish the location, width, and nature of the SH film-water interface.
Oriented mesostructured surfactant-silica nanocomposite thin films with a 2D hexagonal mesostruct... more Oriented mesostructured surfactant-silica nanocomposite thin films with a 2D hexagonal mesostructure of cylindrical micelles were prepared by evaporation-induced self-assembly using two different nonionic Brij surfactants and studied by small-angle X-ray scattering in symmetric reflection (SRSAXS) and grazing incidence (GISAXS) geometries. A novel SRSAXS evaluation approach was applied that allowed a good fitting of the SRSAXS data over almost the whole range of scattering vectors. Aside from the cylinder radius and the lattice parameter, the approach provided accurate values for the polydispersity of the micelles, lattice distortions, and preferred orientation. These analyses revealed a significant rise of the micelle radius and accordingly the lattice parameter upon an increase in the ratio surfactant/SiO 2, attributable to a decrease in the solubilization of the poly(ethylene oxide) (PEO) chains by water, in agreement with Monte Carlo simulations. Furthermore, the SRSAXS analysis was successfully applied to the corresponding mesoporous films for the determination of pore sizes.
Single bilayer membranes of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were formed o... more Single bilayer membranes of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were formed on ordered nanocomposite and nanoporous silica thin films by fusion of small unilamellar vesicles. The structure of these membranes was investigated using neutron reflectivity. The underlying thin films were formed by evaporation induced self-assembly to obtain periodic arrangements of silica and surfactant molecules in the nanocomposite thin films, followed by photocalcination to oxidatively remove the organics and render the films nanoporous. We show that this platform affords homogeneous and continuous bilayer membranes that have promising applications as model membranes and sensors.
It is now recognized that self-assembly is a powerful synthetic approach to the fabrication of na... more It is now recognized that self-assembly is a powerful synthetic approach to the fabrication of nanostructures with feature sizes smaller than achievable with state of the art lithography and with a complexity approaching that of biological systems. For example, recent research has shown that silica/ surfactant self-assembly combined with evaporation (so-called evaporation induced self-assembly EISA) can direct the formation of porous and composite thin-film mesostructures characterized by precise periodic arrangements of inorganic and organic constituents on the 1-50-nm scale. Despite the potential utility of these films for a diverse range of applications such as sensors, membranes, catalysts, waveguides, lasers, nano-fluidic systems, and low dielectric constant (so-called low k) insulators, the mechanism of EISA is not yet completely understood. Here, using time-resolved grazing incidence small-angle X-ray scattering (GISAXS) combined with gravimetric analysis and infrared spectroscopy, we structurally and compositionally characterize in situ the evaporation induced self-assembly of a homogeneous silica/surfactant/solvent solution into a highly ordered surfactant-templated mesostructure. Using CTAB (cetyltrimethylammonium bromide) as the structure-directing surfactant, a two-dimensional (2-D) hexagonal thin-film mesophase (p6mm) with cylinder axes oriented parallel to the substrate surface forms from an incipient lamellar mesophase through a correlated micellar intermediate. Comparison with the corresponding CTAB/water/ alcohol system (prepared without silica) shows that, for acidic conditions in which the siloxane condensation rate is minimized, the hydrophilic and nonvolatile silicic acid components replace water maintaining a fluidlike state that avoids kinetic barriers to self-assembly. tion, 21 or selective dewetting 18 drives silica/surfactant selfassembly 1 into uniform or spatially patterned 18 thin-film mesophases. Through subsequent heating or exposure to catalysts or light, 9,22 these films can be processed into porous or composite mesostructures of potential utility for a diverse range of applications such as sensors, 15 membranes, 16 catalysts,
We have developed structure/toxicity relationships for amorphous silica nanoparticles (NPs) synth... more We have developed structure/toxicity relationships for amorphous silica nanoparticles (NPs) synthesized through low temperature, colloidal (e.g. Stöber silica) or high temperature pyrolysis (e.g. fumed silica) routes. Through combined spectroscopic and physical analyses, we have determined the state of aggregation, hydroxyl concentration, relative proportion of strained and unstrained siloxane rings, and potential to generate hydroxyl radicals for Stöber and fumed silica NPs with comparable primary particle sizes (16-nm in diameter). Based on erythrocyte hemolytic assays and assessment of the viability and ATP levels in epithelial and macrophage cells, we discovered for fumed silica an important toxicity relationship to post-synthesis thermal annealing or environmental exposure, whereas colloidal silicas were essentially non-toxic under identical
Sol-gel thin film formation by dipping (or spinning) in volves gravitational (or centrifugal) dra... more Sol-gel thin film formation by dipping (or spinning) in volves gravitational (or centrifugal) draining accompa nied by vigorous solvent evaporation. During dipping a steady-state film thickness profile develops, the shape of which is established by the evaporation rates of the fluid components and possible surface tension gradient driven flows. The entrained inorganic species are con centrated by the accompanying evaporation of solvent and compacted by capillary pressure developed at the final stage of the deposition process as liquid-vapor menisci recede into the gel interior. The important microstructural properties of the deposited film (% porosity, surface area, and pore size) depend on the size and structure of the inorganic species, the relative rates of condensation and evaporation during deposi tion, the magnitude of the capillary pressure, and the accompanying shear stress.
Sol-gel thin films are formed by gravitational or centrifugal draining accompanied by vigorous dr... more Sol-gel thin films are formed by gravitational or centrifugal draining accompanied by vigorous drying. Drying largely establishes the shape of the fluid profile, the timescale of the deposition process, and the magnitude of the forces exerted on the solid phase. The combination of coating theory and experiment should define coating protocols to tailor the deposition process to specific applications.
In an effort to understand the various phenomena which occur during aging of silica gels at diffe... more In an effort to understand the various phenomena which occur during aging of silica gels at different pH, silica particles prepared by three routes distinguished principally by their silicon coordination (Qn distributions) (St6ber silica spheres, Ludox and Cab-O-Sil) were employed. In situ techniques were used including proton spin-lattice relaxation, SAXS and zeta potential for the wet state and 29Si MAS-NMR, SEM, TEM and N 2 sorption for dry samples. In solution, the surface area increased in the following order after aging at pH 10 as compared with that at pH 7 and the initial dry surface area: Cab-O-Sil < Ludox << St6ber which is inversely proportional to the degree of silicon coordination. This same trend was observed for the surface charge measurements. Surface areas calculated from 29Si MAS-NMR for four different St6ber sphere sizes were of order 1000 ma/g as compared with nitrogen/BET surface areas which varied from 9 to 370 m2/g, indicating significant internal surface which is inaccessible to nitrogen. The 29Si NMR results for the St6ber spheres were consistent with a close packed aggregate of ~ 3 nm primary particles. In addition to particulate silica, aging at varying pH of a two-step acid/base-catalyzed silica gel was studied. At all pH values, the surface area was higher in the wet gel as compared with the xerogel. Aging at higher pH was found to yield lower surface area, larger pore volume and a narrower pore size distribution depending upon the pH, aging time and surface tension of the final pore fluid.
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Papers by C. Brinker