Self-assembled nanostructures of oligopyridine molecules

The high potential of self-assembly processes of molecular building blocks is reflected in the vast variety of different functional nanostructures reported in the literature. The constituting units must fulfill several requirements like synthetic accessibility, presence of functional groups for appropriate intermolecular interactions and depending on the type of self-assembly processsignificant chemical and thermal stability. It is shown that oligopyridines are versatile building blocks for two- and three-dimensional (2D and 3D) self-assembly. They can be employed for building up different architectures like gridlike metal complexes in solution. By the appropriate tailoring of the heterocycles, further metal coordinating and/or hydrogen bonding capabilities to the heteroaromatic molecules can be added. Thus, the above-mentioned architectures can be extended in one-step processes to larger entities, or in a hierarchical fashion to infinite assemblies in the solid state, respectively. Besides the organizational properties of small molecules in solution, 2D assemblies on surfaces offer certain advantages over 3D arrays. By precise tailoring of the molecular structures, the intermolecular interactions can be fine-tuned expressed by a large variety of resulting 2D patterns. Oligopyridines prove to be ideal candidates for 2D assemblies on graphite and metal sufaces, respectively, expressing highly ordered structures. A slight structural variation in the periphery of the molecules leads to strongly changed 2D packing motifs based on weak hydrogen bonding interactions. Such 2D assemblies can be exploited for building up host-guest networks which are attractive candidates for manipulation experiments on the single-molecule level. Thus, "erasing" and "writing" processes by the scanning tunneling microscopy (STM) tip at the liquid/solid interface are shown. The 2D networks are also employed for performing coordination chemistry experiments at surfaces.     

Comparison of the thermal behavior and conformational changes in partially and fully hydrated dipalmitoylphosphatidylcholine systems

The temperature dependence of conformational changes for partially and fully hydrated DPPC systems through two physicochemical techniques, namely DSC and Raman spectroscopy, is studied. DSC experiments have shown a different thermal behavior between the two considered systems, indicating the effective role of water in the thermal behavior. A temperature resolution of inter- and intramolecular interactions during the main melting phase transition was achieved by using three different Raman intensity ratios, which confirm that the main phase transition represents a two-stage transition. Van’t Hoff plots for the C–C, C–H, C=O and C₄N⁺ stretching modes, in a temperature range just below the main transition temperature, have been used to compare the thermodynamic parameters extracted by the two physicochemical techniques. The significance of these results can be summarized as follows: (a) DSC and Raman spectroscopy have shown complementary results indicating that DPPC exists in partially or fully hydrated states; (b) thermodynamic parameters ΔΗ and ΔS calculated in both techniques for the two different hydration states of DPPC were in harmony; (c) water more significantly affects the thermal and dynamic properties of fully hydrated DPPC bilayers than of the partially hydrated DPPC; and (d) water disturbs the head-group packing, the alkyl chains interactions and the mesophase region. It appears that the amount of water plays a vital role in the bilayer structure. As more and more extensive studies appear in the literature on biomolecules or drug membrane interactions, this information will be valuable in understanding the role of water in these interactions.     

Self-assembled template-directed synthesis of one-dimensional silica and titania nanostructures

Mineralized biological materials such as shells, skeleton, and teeth experience biomineralization. Biomimetic materials exploit the biomineralization process to form functional organic-inorganic hybrid nanostructures. In this work, we mimicked the biomineralization process by the de novo design of an amyloid-like peptide that self-assembles into nanofibers. Chemically active groups enhancing the affinity for metal ions were used to accumulate silicon and titanium precursors on the organic template. The self-assembly process and template effect were characterized by CD, FT-IR, UV-vis, fluorescence, rheology, TGA, SEM, and TEM. The self-assembled organic nanostructures were exploited as a template to form high-aspect-ratio 1-D silica and titania nanostructures by the addition of appropriate precursors. Herein, a new bottom-up approach was demonstrated to form silica and titania nanostructures that can yield wide opportunities to produce high-aspect-ratio inorganic nanostructures with high surface areas. The materials developed in this work have vast potential in the fields of catalysis and electronic materials.     

Self-assembled helical nanostructures from an asymmetrical perylene diimide

An asymmetrical perylene diimide 3, N-（4-methoxyphenyl）-N＇-（4-nitrophenyl）-perylene-3,4,9,10-tetracarboxylic diimide, was synthesized, and its self-assembly and dissociation behaviors in chloroform was studied in detail by UV-vis and fluorescence spectroscopies. The resulting unique helical nanostructures from 3 were proposed to be self-assembled via the cooperative actions of π-π stacking, steric hindrance and electrophile-nucleophile type pairing.     

Self-assembled lamellar nanostructures of wholly aromatic rod-rod-type block molecules

Wholly aromatic rod-rod type di- and triblock molecules, oligo(ether sulfone)-b-oligo(ether ketone)s (OES-OEK), were synthesized to study a solid-state self-assembled nanostructure. The OES and OEK segments in the block molecules form segregated crystalline domains. The energy-filtering transmission electron microscopy images revealed that the di- and triblock OES-OEK co-oligomers formed lamellar nanostructures with a periodicity of approximately 9 and 13 nm, respectively. [structure: see text].     

Self-assembled nanostructures of a biomimetic glycopolymer–polypeptide triblock copolymer

The self-assembling behavior of a biomimetic glycopolymer–polypeptide triblock copolymer in aqueous solution was described and characterized by employing the hydrophobic dye solubilization method and transmission electron microscopy. The large spherical micelles can be easily generated from the dissolution of triblock copolymer in water. The morphology changes from sphere to lamellae, then to worm-like micelle, can be conveniently transformed by initial copolymer concentration. The multivalent interaction of lectins with lactose-installed polymeric aggregates was preliminarily investigated by UV-Vis spectra. Notably, this kind of aggregates may be useful as artificial polyvalent ligands in the investigation of carbohydrate–protein recognition and for the design of site-specific drug delivery systems.     

Self-assembled nanostructures with tridentate cyclometalated platinum(II) complexes

Self-assembly of positively charged and charge-neutral tridentate cyclometalated platinum(II) complexes leads to nanowires and the drop-cast film containing these nanowires behaves as a n-type semiconductor.     

Self-assembled organic nanostructures: effect of substituents on the morphology.

Three perylene-3,4;9,10-tetracarboxydiimide (PTCDI) compounds with two dodecyloxy or thiododecyl chains attached at the bay positions of the perylene ring, PTCDIs 1-3, were fabricated into nanoassemblies by a solution injection method. The morphologies of these self-assembled nanostructures were determined by transmission electronic microscopy (TEM), scanning electronic microscopy (SEM), and atomic force microscopy (AFM). PTCDI compound 1, with two dodecyloxy groups, forms long, flexible nanowires with an aspect ratio of over 200, while analogue 3, with two thiododecyl groups, self-assembles into spherical particles. In line with these results, PTCDI 2, with one dodecyloxy group and one thiododecyl group, forms nanorods with an aspect ratio of around 20. Electronic absorption and fluorescence spectroscopy results reveal the formation of H-aggregates in the nanostructures of these PTCDI compounds owing to the pi-pi interaction between the substituted perylene molecules and also suggest a decreasing pi-pi interaction in the order 1>2>3, which corresponds well with the morphology of the corresponding nanoassemblies. On the basis of DFT calculations, the effect of different substituents at the bay positions of the perylene ring on the pi-pi interaction between substituted perylene molecules and the morphology of self-assembled nanostructures is rationalized by the differing degree of twisting of the conjugated perylene system caused by the different substituents and the different bending of the alkoxy and thioalkyl groups with respect to the plane of the naphthalene.     

Self-assembled nanostructures on VSe2 surfaces induced by Cu deposition.

Analytical transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have been applied for the characterization of evolution, lateral arrangements, orientations, and the microscopic nature of nanostructures formed during the early stages of ultrahigh vacuum electron beam evaporation of Cu onto surfaces of VSe2 layered crystals. Linear nanostructure of relatively large lateral dimension (100-500 nm) and networks of smaller nanostructures (lateral dimension: 15-30 nm; mesh sizes: 500-2000 nm) are subsequently formed on the substrate surfaces. Both types of nanostructures are not Cu nanowires but are composed of two strands of crystalline substrate material elevating above the substrate surface. For the large nanostructures a symmetric roof structure with an inclination angle of approximately 30 degrees with respect to the substrate surface could be deduced from detailed diffraction contrast experiments. In addition to the nanostructure networks a thin layer of a Cu-VSe2 intercalation phase of 3R polytype is observed at the substrate surface. A dense network of interface dislocations indicates that the phase formation is accompanied by in-plane strain. We present a model that explains the formation of large and small nanostructures as consequences of compressive layer strains that are relaxed by the formation of rooflike nanostructures, finally evolving into the observed networks with increasing deposition time. The dominating contributions to the compressive layer strains are considered to be an electronic charge transfer from the Cu adsorbate to the substrate and the formation of a Cu-VSe2 intercalation compound in a thin surface layer.     

Self-assembled, pH-sensitive retinoate nanostructures ionically complexed with PEG-grafted cationic polyelectrolytes

All-trans-retinoic acids (ATRA) are ionically complexed with cationic polyelectrolytes containing tertiary amines and self-assembled into nanoscale colloidal structures. Poly(2-(dimethylamino)ethyl methacrylate) grafted with polyethylene glycol, poly(DMAEMA-g-PEG), is used as a double hydrophilic, cationic polyelectrolyte. The polyion/ATRA complexes are formed by adding ATRA in dimethyl sulfoxide into aqueous solution of poly(DMAEMA-g-PEG). This complexation effectively suppresses the formation of undesirable drug crystallites and produces stable colloidal nanostructures having a hydrodynamic diameter of about 15?nm at a neutral pH. However, as the pH decreases below about 6, they undergo dramatic structural change into large aggregates of about 250?nm in diameter presumably due to the dissociation of ATRA from the polyelectrolyte. We expect that this pH-sensitive response of the polyion/ATRA complexes is useful for intracellular translocation at a neutral pH followed by the endosomal escape of ATRA in an acidic condition.     

Thermal behaviour of hydrated dodecatungstosilicic acid

The thermal behaviour of H₄SiW₁₂O₄₀·24.8H₂O (SiW₁₂) was investigated by using DTA, TG and FTIR. Endothermic effects were observed at 40, 98 and 217°C, corresponding to the fusion of SiW₁₂ in its own crystallization water, boiling of the solution and decomposition of the remaining tetrahydrate into anhydrous SiW₁₂, respectively. The mass of the sample remained constant on heating from about 250 to 400°C. Subsequently, it slowly decreased and reached a constant value at about 500°C. At 526°C a DTA peak appeared. There was an abrupt change in the FTIR spectrum of the sample heated to 550°C. The typical spectrum of the Keggin unit vanished and new bands at 807.5 and 1030 cm^?1 indicated the presence of free WO₃ and SiO₂, respectively.     

Self-assembled peptide nanostructures: the design of molecular building blocks and their technological utilization

In this tutorial review the process and applications of peptide self-assembly into nanotubes, nanospheres, nanofibrils, nanotapes, and other ordered structures at the nano-scale are discussed. The formation of well-ordered nanostructures by a process of self-association represents the essence of modern nanotechnology. Such self-assembled structures can be formed by a variety of building blocks, both organic and inorganic. Of the organic building blocks, peptides are among the most useful ones. Peptides possess the biocompatibility and chemical diversity that are found in proteins, yet they are much more stable and robust and can be readily synthesized on a large scale. Short peptides can spontaneously associate to form nanotubes, nanospheres, nanofibrils, nanotapes, and other ordered structures at the nano-scale. Peptides can also form macroscopic assemblies such as hydrogels with nano-scale order. The application of peptide building blocks in biosensors, tissue engineering, and the development of antibacterial agents has already been demonstrated.     

Molecular characterization of gel and liquid-crystalline structures of fully hydrated POPC and POPE bilayers

Molecular dynamics simulations were used for a comprehensive study of the structural properties of monounsaturated POPC and POPE bilayers in the gel and liquid-crystalline state at a number of temperatures, ranging from 250 to 330 K. Though the chemical structures of POPC and POPE are largely similar (choline versus ethanolamine headgroup), their transformation processes from a gel to a liquid-crystalline state are contrasting. In the similarities, the lipid tails for both systems are tilted below the phase transition and become more random above the phase transition temperature. The average area per lipid and bilayer thickness were found less sensitive to phase transition changes as the unsaturated tails are able to buffer reordering of the bilayer structure, as observed from hysteresis loops in annealing simulations. For POPC, changes in the structural properties such as the lipid tail order parameter, hydrocarbon trans-gauche isomerization, lipid tail tilt-angle, and level of interdigitation identified a phase transition at about 270 K. For POPE, three temperature ranges were identified, in which the lower one (270-280 K) was associated with a pre-transition state and the higher (290-300 K) with the post-transition state. In the pre-transition state, there was a significant increase in the number of gauche arrangements formed along the lipid tails. Near the main transition (280-290 K), there was a lowering of the lipid order parameters and a disappearance of the tilted lipid arrangement. In the post-transition state, the carbon atoms along the lipid tails became less hindered as their density profiles showed uniform distributions. This study also demonstrates that atomistic simulations of current lipid force fields are capable of capturing the phase transition behavior of lipid bilayers, providing a rich set of molecular and structural information at and near the main transition state.     

Equilibrated stereostructures of hydrated geraniin and mallotusinic acid

Partial revision of the hydrated structure, and determination of the absolute configurations of geraniin and mallotusinic acid lead to structures I-A and II-A, respectively.     

Polyvalent nucleic acid nanostructures

Polyvalent oligonucleotide-nanoparticle conjugates possess several unique emergent properties, including enhanced cellular uptake, high antisense bioactivity, and nuclease resistance, which hypothetically originate from the dense packing and orientation of oligonucleotides on the surface of the nanoparticle. In this Communication, we describe a new class of polyvalent nucleic acid nanostructures (PNANs), which are comprised of only cross-linked and oriented nucleic acids. We demonstrate that these particles are capable of effecting high cellular uptake and gene regulation without the need of a cationic polymer co-carrier. The PNANs also exhibit cooperative binding behavior and nuclease resistance properties.     

Dielectric spectroscopy in the GHz region on fully hydrated zwitterionic amino acids

The complex dielectric permittivity of eight different amino acids in water solutions was determined in the frequency range from 0.2 to 20 GHz at room temperature, trying to span the whole range of solubility in each case. Two relaxations were observed at room temperature in this frequency range, which can be mainly assigned to the rotation of amino acids in the aqueous environment, and the reorientational motion of water molecules, respectively. Although the amino acids have a charged (zwitterionic) nature with huge dipole moments, the tendency towards dipolar alignment seems to be very weak, over the investigated concentration ranges. For these small bio-molecules, water screens solute-solute interactions and amino acids remain typically as isolated hydrated monomers. The dielectric results were used to estimate the number of water molecules restrained by each solute molecule. Finally, the comparison between the amino acid relaxation times made it possible to discuss the relationship between rotational dynamics and the structure and hydrodynamic coupling of the amino acid studied.     

Self-assembled monolayers of alkylphosphonic acid on GaN substrates

In this paper we describe the formation and characterization of self-assembled monolayers of octadecylphosphonic acid (ODPA) on epitaxial (0001) GaN films on sapphire. By immersing the substrate in its toluene solution, ODPA strongly adsorbed onto UV/O 3-treated GaN to give a hydrophobic surface. Spectroscopic ellipsometry verified the formation of a well-packed monolayer of ODPA on the GaN substrate. In contrast, adsorption of other primarily substituted hydrocarbons (C n H 2 n+1 X; n = 16-18; X = -COOH, -NH 2, -SH, and -OH) offered less hydrophobic surfaces, reflecting their weaker interaction with the GaN substrate surfaces. A UV/O 3-treated N-polar GaN had a high affinity to the -COOH group in addition to ODPA, possibly reflecting the basic properties of the surface. These observations suggested that the molecular adsorption was primarily based on hydrogen bond interactions between the surface oxide layer on the GaN substrate and the polar functional groups of the molecules. The as-prepared ODPA monolayers were desorbed from the GaN substrates by soaking in an aqueous solution, particularly in a basic solution. However, ODPA monolayers heated at 160 degrees C exhibited suppressed desorption in acidic and neutral aqueous solution maybe due to covalent bond formation between ODPA and the surface. X-ray photoelectron spectroscopy provided insight into the effect of the UV/O 3 treatment on the surface composition of the GaN substrate and also the ODPA monolayer formation. These results demonstrate that the surface of a GaN substrate can be tailored with organic molecules having an alkylphosphonic acid moiety for future sensor and device applications.