Shifi Kababya

Molecular control of quantum-dot internal electric field and its application to CdSe-based solar cells

Inorganic nanocrystals are attractive materials for solar-cell applications. However, the performance of such devices is often limited by an insufficient alignment of energy levels in the nanocrystals. Here, we report that by attaching two different molecules to a single quantum dot or nanocrystal one can induce electric fields large enough to significantly alter the electronic and optoelectronic properties of the quantum dot. This electric field is created within the nanocrystals owing to a mixture of amine- and thiol-anchor-group ligands. Examining the steady state as well as temporal evolution of the optical properties and the nuclear magnetic resonances of the nanocrystals we found that the first excitonic peak shifts as a function of the capping-layer composition. We also demonstrate that the use of a mixed-ligand-induced electric field markedly enhances the charge generation efficiency in layer-by-layer CdSe-nanocrystal-based solar cells, thus improving the overall cell efficiency.

In situ molecular NMR picture of bioavailable calcium stabilized as amorphous CaCO3 biomineral in crayfish gastroliths

Bioavailable calcium is maintained by some crustaceans, in particular freshwater crayfish, by stabilizing amorphous calcium carbonate (ACC) within reservoir organs—gastroliths, readily providing the Ca2+ needed to build a new exoskeleton. Despite the key scientific and biomedical importance of the in situ molecular-level picture of biogenic ACC and its stabilization in a bioavailable form, its description has eluded efforts to date. Herein, using multinuclear NMR, we accomplish in situ molecular-level characterization of ACC within intact gastroliths of the crayfish Cherax quadricarinatus. In addition to the known CaCO3, chitin scaffold and inorganic phosphate (Pi), we identify within the gastrolith two primary metabolites, citrate and phosphoenolpyruvate (PEP) and quantify their abundance by applying solution NMR techniques to the gastrolith “soluble matrix.” The long-standing question on the physico-chemical state of ACC stabilizing, P-bearing moieties within the gastrolith is answered directly by the application of solid state rotational-echo double-resonance (REDOR) and transferred-echo double-resonance (TEDOR) NMR to the intact gastroliths: Pi and PEP are found molecularly dispersed throughout the ACC as a solid solution. Citrate carboxylates are found < 5 Å from a phosphate (intermolecular C⋯P distance), an interaction that must be mediated by Ca2+. The high abundance and extensive interactions of these molecules with the ACC matrix identify them as the central constituents stabilizing the bioavailable form of calcium. This study further emphasizes that it is imperative to characterize the intact biogenic CaCO3. Solid state NMR spectroscopy is shown to be a robust and accessible means of determining composition, internal structure, and molecular functionality in situ.

Molecular Level Characterization of the Inorganic−Bioorganic Interface by Solid State NMR: Alanine on a Silica Surface, a Case Study

The molecular interface between bioorganics and inorganics plays a key role in diverse scientific and technological research areas including nanoelectronics, biomimetics, biomineralization, and medical applications such as drug delivery systems and implant coatings. However, the physical/chemical basis of recognition of inorganic surfaces by biomolecules remains unclear. The molecular level elucidation of specific interfacial interactions and the structural and dynamical state of the surface bound molecules is of prime scientific importance. In this study, we demonstrate the ability of solid state NMR methods to accomplish these goals. L-[1-(13)C,(15)N]Alanine loaded onto SBA-15 mesoporous silica with a high surface area served as a model system. The interacting alanine moiety was identified as the -NH(3)(+) functional group by (15)N{(1)H}SLF NMR. (29)Si{(15)N} and (15)N{(29)Si}REDOR NMR revealed intermolecular interactions between the alanine -NH(3)(+) and three to four surface Si species, predominantly Q(3), with similar internuclear N...Si distances of 4.0-4.2 A. Distinct dynamic states of the adsorbed biomolecules were identified by (15)N{(13)C}REDOR NMR, indicating both bound and free alanine populations, depending on hydration level and temperature. In the bound populations, the -NH(3)(+) group is surface anchored while the free carboxylate end undergoes librations, implying the carboxylate has small or no contributions to surface binding. When surface water clusters grow bigger with increased hydration, the libration amplitude of the carboxyl end amplifies, until onset of dissolution occurs. Our measurements provide the first direct, comprehensive, molecular-level identification of the bioorganic-inorganic interface, showing binding functional groups, geometric constraints, stoichiometry, and dynamics, both for the adsorbed amino acid and the silica surface.

A dynamic magic angle spinning NMR study of the local mobility of alanine in an aqueous environment at the inner surface of mesoporous materials.

Dynamic deuterium magic angle spinning NMR has been applied to study the slow motion of small molecules close to a silica surface. In particular, alanine-d(3) molecules dissolved in an aqueous solution were loaded into the pores of the mesoporous materials SBA-15 and MCM-41. Deuterium spectra were measured as a function of the water content of these materials and the temperature. From the analysis of these spectra and the corresponding proton spectra, using a simple molecular exchange model, relatively slow desorption rates of the binding of alanine to the inner pore surface were obtained and were correlated with the low proton concentrations at the pore surfaces.

Phosphate–Water Interplay Tunes Amorphous Calcium Carbonate Metastability: Spontaneous Phase Separation and Crystallization vs Stabilization Viewed by Solid State NMR

Organisms tune the metastability of amorphous calcium carbonates (ACC), often by incorporation of additives such as phosphate ions and water molecules, to serve diverse functions, such as modulating the availability of calcium reserves or constructing complex skeletal scaffolds. Although the effect of additive distribution on ACC was noted for several biogenic and synthetic systems, the molecular mechanisms by which additives govern ACC stability are not well understood. By precipitating ACC in the presence of different PO4(3-) concentrations and regulating the initial water content, we identify conditions yielding either kinetically locked or spontaneously transforming coprecipitates. Solid state NMR, supported by FTIR, XRD, and electron microscopy, define the interactions of phosphate and water within the initial amorphous matrix, showing that initially the coprecipitates are homogeneous molecular dispersions of structural water and phosphate in ACC, and a small fraction of P-rich phases. Monitoring the transformations of the homogeneous phase shows that PO4(3-) and waters are extracted first, and they phase separate, leading to solid-solid transformation of ACC to calcite; small part of ACC forms vaterite that subsequently converts to calcite. The simultaneous water-PO4(3-) extraction is the key for the subsequent water-mediated accumulation and crystallization of hydroxyapatite (HAp) and carbonated hydroxyapatite. The thermodynamic driving force for the transformations is calcite crystallization, yet it is gated by specific combinations of water-phosphate levels in the initial amorphous coprecipitates. The molecular details of the spontaneously transforming ACC and of the stabilized ACC modulated by phosphate and water at ambient conditions, provide insight into biogenic and biomimetic pathways.

Novel semi-IPN through vinyl silane polymerization and crosslinking within PVC films

Novel semi-IPN (interpenetrating polymer networks) were synthesized through vinyl silane modification of unplasticized poly(vinyl chloride) (PVC) films using relatively low temperatures, relatively high vinyl silane contents, and several different processing routes. A free-radical initiator was used to promote reaction of the vinyl groups, and an aqueous acetic acid solution was used to promote the methoxysilane hydrolysis and condensation (HC) reactions for siloxane crosslink formation. A gel consisting of silane alone was formed prior to the HC process, indicating the formation of a semi-IPN. The gel content following the HC process far exceeded the silane content, indicating a significant amount of PVC was entrapped by the silane network. This conclusion is supported by the homogeneous molecular structure and morphology of the films.

In situ observation of the internal structure and composition of biomineralized Emiliania huxleyi calcite by solid-state NMR Spectroscopy.

Biomineralization, particularly the formation of calcium carbonate structures by organisms under ambient conditions, is of vast fundamental and applied interest. Organisms finely control all aspects of the formation of the biomaterials: composition, polymorph, morphology, and macroscopic properties. While in situ molecular-level characterization of the resulting biominerals is a formidable task, solid-state magic angle spinning NMR is one of the most powerful analytical techniques for this purpose. It is employed in this study to elucidate the structure and composition of biogenic calcite formed by Emiliania huxleyi, a unicellular alga distinguished by its exquisitely sculptured calcite cell coverings known as coccoliths. Strain 371 (CCMP) was grown and harvested from (15)N- and (13)C-enriched growth medium, with biosynthetic labeling to enhance the sensitivity of the NMR measurements. Crystalline and interfacial calcite environments were selectively probed using direct and indirect (cross-polarized) (13)C excitation, respectively. Different crystalline environments, in particular structural defect sites at concentrations of up to 1.4% with P and N moieties incorporated, were identified using (13)C rotational-echo double-resonance (REDOR) NMR. REDOR-derived geometrical constraints show that the P and N atoms at the defect sites are 3.2 and 2.3 (+/-0.2) A apart from a crystalline carbon carbonate. The phosphorus and nitrogen moieties within the biogenic calcite are identified as small, non-protonated moieties, attributed to inorganic ions such as PO4(3-) and NO3(-). The carbonates adjacent to these defects are chemically indistinguishable from bulk crystalline carbonates, yet their immediate environments experience reduced rigidity, as reflected by substantial T1((13)CO3(2-)) shortening. Interfacial carbonates, on the other hand, reside in structurally/chemically perturbed environments, as reflected by heterogeneous line broadening. This study is the first to directly unravel evidence on the incorporation of P/N moieties as structural defects within E. huxleyi biogenic calcite, and on the state of the adjacent crystalline carbonates.

Celecoxib Encapsulation in β-Casein Micelles: Structure, Interactions, and Conformation.

β-Casein is a 24 kDa natural protein that has an open conformation and almost no folded or secondary structure, and thus is classified as an intrinsically unstructured protein. At neutral pH, β-casein has an amphiphilic character. Therefore, in contrast to most unstructured proteins that remain monomeric in solution, β-casein self-assembles into well-defined core-shell micelles. We recently developed these micelles as potential carriers for oral administration of poorly water-soluble pharmaceuticals, using celecoxib as a model drug. Herein we present deep and precise insight into the physicochemical characteristics of the protein-drug formulation, both in bulk solution and in dry form, emphasizing drug conformation, packing properties and aggregation state. In addition, the formulation is extensively studied in terms of structure and morphology, protein/drug interactions and physical stability. Particularly, NMR measurements indicated strong drug-protein interactions and noncrystalline drug conformation, which is expected to improve drug solubility and bioavailability. Small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM) were combined for nanostructural characterization, proving that drug-protein interactions lead to well-defined spheroidal micelles that become puffier and denser upon drug loading. Dynamice light scattering (DLS), turbidity measurements, and visual observations complemented the analysis for determining formulation structure, interactions, and stability. Additionally, it was shown that the loaded micelles retain their properties through freeze-drying and rehydration, providing long-term physical and chemical stability. Altogether, the formulation seems greatly promising for oral drug delivery.