Self-assembled State Research Articles

ChemInform Abstract: Characterization of Supramolecular Polymers

AbstractReview: 70 refs.

Emerging π-stacked dynamic nanostructured library

We describe a general strategy to control the state of molecular self-assembly under thermodynamic control in which gelation facilitates formation of a predominating π-stacked nanostructured product.

Characterization of supramolecular polymers

Supramolecular polymers are made of monomers that are held together by noncovalent interactions. This is the reason for the wide range of novel properties, such as reversibility and responses to stimuli, exhibited by supramolecular polymers. A range of supramolecular polymerization methods have been developed leading to a number of novel supramolecular materials. However, standard techniques for the characterization of supramolecular polymers have yet to be established. The dynamic nature of supramolecular polymers makes them difficult to be fully characterized using conventional polymer techniques. This tutorial review summarizes various methods for characterizing supramolecular polymers, including theoretical estimation, size exclusion chromatography, viscometry, light scattering, vapor pressure osmometry, mass spectrometry, NMR spectroscopy, scanning probe microscopy, electron microscopy, and atomic force microscopy-based single molecule force spectroscopy. Each of these methods has its own particular advantages and disadvantages. Most of the methods are used to characterize the supramolecular polymer chain itself. However, some of the methods can be used to study the self-assembled state formed by supramolecular polymers. The characterization of a supramolecular polymer cannot be realized with a single method; a convincing conclusion relies on the combination of several different techniques.

Comparative self-assembly studies and self-sorting of two structurally isomeric naphthalene-diimide (NDI)-gelators

We have reported here a comparative self-assembly and gelation studies of two isomeric bis-amide functionalized NDI-derivatives. In one case (NDI-1) the two amide groups were placed symmetrically on either side of the chromophore while for the other system (NDI-2) they were located on same side. In non-polar solvent both isomers formed self-assembled structures by synergistic effect of π-stacking and hydrogen-bonding. The propensity for self-assembly of NDI-1 was greater due to symmetrical placement of two amide groups on either arms of this chromophore which allowed π-stacking in tandem with hydrogen-bonding, while NDI-2 formed thermally more stable self-assembled fibres possibly due to location of two amide groups in close proximity along single arm of this chromophore. The structural difference in these two isomers lead to distinctly different morphology of their respective self-assembled structures which was further reflected on their gelation properties. Morphology of the self-assembled array of NDI-1 showed organized and regular entangled bundles of nanorods which imparted better gelation ability to this chromophore while the self-assembled fibres of NDI-2 showed less ordered and irregular fibres. We also probed self- assembly of these two chromophores in their mixture which revealed orthogonal assembly of the individual chromophores and no molecular mixing was noticed. Hydrogen bonding mediated self-assembly and gelation of two isomeric naphthalene-diimide (NDI) gelators is reported. NDI-1 showed precisely defined self-assembly resulting in stronger gelation due to symmetric placement of the two amide groups. Contrastingly, weak gelation was noticed for unsymmetrical NDI-2 due to irregular morphology even though thermal stability of self-assembled state in solution was found to be higher in this case due to close proximity of the two amide groups.

Enhanced Mechanical Rigidity of Hydrogels Formed from Enantiomeric Peptide Assemblies

Chirality can be used as a design tool to control the mechanical rigidity of hydrogels formed from self-assembling peptides. Hydrogels prepared from enantiomeric mixtures of self-assembling β-hairpins show nonadditive, synergistic, enhancement in material rigidity compared to gels prepared from either pure enantiomer, with the racemic hydrogel showing the greatest effect. CD spectroscopy, TEM, and AFM indicate that this enhancement is defined by nanoscale interactions between enantiomers in the self-assembled state.

Parallel versus Perpendicular Lamellar-within-Lamellar Self-Assembly of A-b-(B-b-A)n-b-C Ternary Multiblock Copolymer Melts

Different types of lamellar-within-lamellar structure formations in A-b-(B-b-A)(n)-b-C terpolymer melts, with a volume fraction of components A, B, and C in the ratio of 1:1:2, are analyzed in the strong segregation limit using a simple theoretical approach. We consider the lamellar, parallel lamellar-within-lamellar, and perpendicular lamellar-within-lamellar self-assembled states. The influence of the copolymer chain length N, the value of the Flory-Huggins interaction parameters chi(AB), chi(AC), and chi(BC), and the number of blocks n in the AB multiblock chain on the phase behavior is discussed. We show that in the limiting case of n >> 1, the perpendicular lamellar-within-lamellar state becomes stable when the interaction parameters satisfy the relation 0 < chi(BC) < 0.22 chi(AC).

Structural characterization of dual-metal containing diamond-like carbon nanocomposite films by pulsed laser deposition

Two metal dopants were simultaneously added into a diamond-like carbon (DLC) matrix using a KrF pulsed laser system at room temperature with no post-processing. The nanometer thin films were fabricated from carbon source targets containing the two metals of interest, Ti and Ni, in atomic percentages 2.5%, 5%, 7.5% and 10% each. Films from carbon targets containing only 5% Ni or 5% Ti were also deposited for comparison against the dual-metal containing films. Microstructure analysis shows that each individual metal reacted independently and uniquely with carbon as confirmed by XPS and surface analysis shows the presence of TiC bonds and Ni 0. Therefore, there was no reaction between Ti and Ni as metals confirmed by XPS. Through this independent interaction, a superposition of microstructural properties was obtained as if the metals were doped separately into DLC. The separate interactions of the two metals with carbon were important as they were able to play separate and different roles in enhancing the properties of DLC. In addition, TEM analysis confirmed a unique self-assembly state where the nickel ions converge into nanosized clusters of ∼ 5 nm in diameter and predominantly oriented in a (200) direction. The resultant films were also extremely smooth with RMS roughness of about 0.1 nm, thus retaining the inherent smoothness of DLC films. The combined Ti/Ni films could be used as substrates to grow carbon nanotubes with controlled density which could be used as cold electron emitters. Thus, it is interesting to study the growth mechanism and microstructure of the composite films.

Self-Assembly of Supramolecular Triblock Copolymer Complexes

Four different poly(tert-butoxystyrene)-b-polystyrene-b-poly(4-vinylpyridine) (PtBOS-b-PS-b-P4VP) linear triblock copolymers, with the P4VP weight fraction varying from 0.08 to 0.39, were synthesized via sequential anionic polymerization. The values of the unknown interaction parameters between styrene and tert-butoxystyrene and between tert-butoxystyrene and 4-vinylpyridine were determined from random copolymer blend miscibility studies and found to satisfy 0.031 < χS,tBOS < 0.034 and 0.39 < χ4VP,tBOS <0.43, the latter being slightly larger than the known 0.30 < χS,4VP ≤ 0.35 value range. All triblock copolymers synthesized adopted a P4VP/PS core/shell cylindrical self-assembled morphology. From these four triblock copolymers supramolecular complexes were prepared by hydrogen bonding a stoichiometric amount of pentadecylphenol (PDP) to the P4VP blocks. Three of these complexes formed a triple lamellar ordered state with additional short length scale ordering inside the P4VP(PDP) layers. The self-assembled state of the supramolecular complex based on the triblock copolymer with the largest fraction of P4VP consisted of alternating layers of PtBOS and P4VP(PDP) layers with PS cylinders inside the latter layers. The difference in morphology between the triblock copolymers and the supramolecular complexes is due to two effects: (i) a change in effective composition and, (ii) a reduction in interfacial tension between the PS and P4VP containing domains. The small angle X-ray scattering patterns of the supramolecules systems are very temperature sensitive. A striking feature is the disappearance of the first order scattering peak of the triple lamellar state in certain temperature intervals, while the higher order peaks (including the third order) remain. This is argued to be due to the thermal sensitivity of the hydrogen bonding and thus directly related to the very nature of these systems.

Desymmetrization of 3,3′‐Bis(acylamino)‐2,2′‐bipyridine‐Based Discotics: The High Fidelity of Their Self‐Assembly Behavior in the Liquid‐Crystalline State and in Solution

Two novel nonsymmetrical disc-shaped molecules 1 and 2 based on 3,3'-bis(acylamino)-2,2'-bipyridine units were synthesized by means of a statistical approach. Discotic 1 possesses six chiral dihydrocitronellyl tails and one peripheral phenyl group, whereas discotic 2 possesses six linear dodecyloxy tails and one peripheral pyridyl group. Preorganization by strong intramolecular hydrogen bonding and subsequent aromatic interactions induce self-assembly of the discotics. Liquid crystallinity of 1 and 2 was determined with the aid of polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction. Two columnar rectangular mesophases (Col(r)) have been identified, whereas for C(3)-symmetrical derivatives only one Col(r) mesophase has been found. In solution, the molecularly dissolved state in chloroform was studied with (1)H NMR spectroscopy, whereas the self-assembled state in apolar solution was examined with optical spectroscopy. Remarkably, these desymmetrized discotics, which lack one aliphatic wedge, behave similar to the symmetric parent compound. To prove that the stacking behavior of discotics 1 and 2 is similar to that of reported C(3)-symmetrical derivatives, a mixing experiment of chiral 1 with C(3)-symmetrical 13 has been undertaken; it has shown that they indeed belong to one type of self-assembly. This helical J-type self-assembly was further confirmed with UV/Vis and photoluminescence (PL) spectroscopy. Eventually, disc 2, functionalized with a hydrogen-bonding acceptor moiety, might perform secondary interactions with molecules such as acids.

Observing Self-Assembled Lipid Nanoparticles Building Order and Complexity through Low-Energy Transformation Processes

Future nanoscale soft matter design will be driven by the biological paradigms of hierarchical self-assembly and long-lived nonequilibrium states. To reproducibly control the low-energy self-assembly of nanomaterials for the future, we must first learn the lessons of biology. Many cellular organelles exhibit highly ordered cubic membrane structures. Determining the mechanistic origins of such lipid organelle complexity has been elusive. We report the first observation of the complete sequence of major transformations in the conversion from a 1D lamellar membrane to 3D inverse bicontinuous cubic nanostructure. Characterization was enabled by adding a steric stabilizer to dispersions of lipid nanoparticles which increased the lifetime of very short-lived nonequilibrium intermediate structures. By using synchrotron small-angle X-ray scattering and cryo-transmission electron microscopy we observed and characterized initial lipid bilayer contacts and stalk formation, followed by membrane pore development, pore evolution into 2D hexagonally packed lattices, and finally creation of 3D bicontinuous cubic structures.

Dependence of Self-Assembled Peptide Hydrogel Network Structure on Local Fibril Nanostructure.

Physically cross-linked, fibrillar hydrogel networks are formed by the self-assembly of β-hairpin peptide molecules with varying degrees of strand asymmetry. The peptide registry in the self-assembled state can be used as a design element to generate fibrils with twisting, nontwisting, or laminated morphology. The mass density of the networks varies significantly, and can be directly related to the local fibrillar morphology as evidenced by small angle neutron scattering (SANS) and in situ substantiation using cryogenic transmission electron microscopy (cryo-TEM) under identical concentrations and conditions. Similarly, the density of the network is dependent on changes in the peptide concentration. Bulk rheological properties of the hydrogels can be correlated to the fibrillar nanostructure, with the stiffer, laminated fibrils forming networks with a higher G' as compared to the flexible, singular fibrillar networks.

Cellular Morphogenesis In Silico

We describe a model that simulates spherical cells of different types that can migrate and interact either attractively or repulsively. We find that both expected morphologies and previously unreported patterns spontaneously self-assemble. Among the newly discovered patterns are a segmented state of alternating discs, and a “shish-kebab” state, in which one cell type forms a ring around a second type. We show that these unique states result from cellular attraction that increases with distance (e.g., as membranes stretch viscoelastically), and would not be seen in traditional, e.g., molecular, potentials that diminish with distance. Most of the states found computationally have been observed in vitro, and it remains to be established what role these self-assembled states may play in in vivo morphogenesis.

Reversible Self-Assembly: A Key Feature for a New Class of Autodelivering Therapeutic Peptides

Effective delivery is a critical issue in the use of conventional free drugs. Studies on the structure-function relationship of a therapeutic antibody-derived candidacidal decapeptide (killer peptide, KP) revealed its ability to spontaneously and reversibly self-assemble in an organized network of fibril-like structures. This process is catalyzed by 1,3-beta-glucans. While the self-assembled state may provide protection against proteases and the slow kinetic of dissociation assures a release of the active dimeric form over time, the beta-glucan affinity is responsible for targeted delivery. Thus, KP represents a novel paradigm of targeted autodelivering drugs.

Direct functionalization of self-assembled nanotubes overcomes unfavorable self-assembling processes

Diamides containing alkyne and azido were self-assembled into nanotubes and were reacted under their self-assembled state with small molecules by "click chemistry"; the resulting compounds remain self-assembled into new nanotubes that cannot be formed by simple self-assembly of the constituting molecules.

Concomitant [2 + 2] cycloaddition solid state reactions from co-crystals self-assembled via mechanochemistry

The combination of mechanochemical self-assembly and solid state reactivity have been explored to achieve unprecedented examples of concomitant topochemical [2 + 2] cycloaddition reactions into binary co-crystals.

Lamellar-in-lamellar self-assembled C–b–(B–b–A)m–b–B–b–C multiblock copolymers: Alexander–de Gennes approach and dissipative particle dynamics simulations

A simple theoretical analysis of the lamellar-in-lamellar self-assembled state of ternary C–b–(B–b–A)m–b–B–b–C multiblock copolymer melts in the strong segregation limit is presented using the Alexander–de Gennes approximation. For a given value of m, the influence of the chain length of the various blocks and the value of the Flory–Huggins χAB and χBC interaction parameters on the number k of internal domains is discussed in detail. The theoretically predicted tendencies are corroborated by computer simulations using the dissipative particle dynamics technique.

Lamellar-in-lamellar structure of binary linear multiblock copolymers

A theoretical description of the lamellar-in-lamellar self-assembly of binary A-b-(B-b-A)(m)-b-B-b-A multiblock copolymers in the strong segregation limit is presented. The essential difference between this binary multiblock system and the previously considered C-b-(B-b-A)(m)-b-B-b-C ternary multiblock copolymer system is discussed. Considering the situation with long end blocks, the free energy of the lamellar-in-lamellar self-assembled state is analyzed as a function of the number k of "thin" internal lamellar domains for different numbers m of repeating (B-b-A) units and different values of the Flory-Huggins chi(AB) interaction parameter. The theoretical predictions are in excellent agreement with the available experimental data.

Organogelators from self-assembling peptide based dendrimers: structural and morphological features

Two peptide based dendrimers containing l-aspartic acid as the branching unit and succinic acid/terephthalic acid as the core unit were synthesized, characterized, and studied. These dendritic peptides form gels in various organic solvents including n-hexanol, benzene, toluene, chlorobenzene, 1,2-dichlorobenzene, o-xylene, tetralin, and nitrobenzene. Gels were characterized by freeze fracture transmission electron microscopic (FF-TEM), field emission scanning electron microscopic (SEM), transmission electron microscopic (TEM), wide angle X-ray powder diffraction (WAXPD), and variable temperature FTIR (VT-FTIR) studies. The VT-FTIR study indicates that amides and ester groups are involved in intermolecular hydrogen bonding in the gel state. Two transitions have been observed for both the dendrimer gels upon heating: the first one corresponds to the gel to sol transition and corresponds to the breaking of hydrogen bonds between esters and amides; the second one corresponds to the breaking of hydrogen bonds between amides. In the case of dendrimer 1 structural reorganization occurs in the sol state after the first transition, which is absent in the dendrimer 2 in the sol state. FF-TEM observations showed that both dendritic peptides form a platelet structure in gel state. SEM images of these dried gels indicate different geometry in different solvents in their self-assembled gel state.

Partial characterization of a hydrophobin protein Po.HYD1 purified from the oyster mushroom Pleurotus ostreatus

Hydrophobins fulfill various physiological functions in fungal development and growth, based on their mechanism of self-assembly at hydrophilic–hydrophobic interfaces into nano-scale, amphipathic membranes. One hydrophobin with an approximate molecular weight of 15 kDa, designated Po.HYD1, was purified from aerial hyphae of Pleurotus ostreatus strain Pm039. Ultrastructures of self-assembled films formed by Po.HYD1 on hydrophobic and hydrophilic substrates were revealed by atomic force microscopy (AFM). A monomolecular adsorption layer, thickness ranging from 3.2 to 3.8 nm, was observed on the surface of highly oriented pyrolytic graphite (HOPG), while a typical rodlet layer with uniform thickness of 4.2 ± 0.1 nm formed on the mica surface. Comparison of CD spectra showed significant secondary structural changes between monomeric and self-assembled states. The spectrum of monomeric Po.HYD1 had a maximum ellipticity at 190 nm and a minimum at 209 nm, and that of assemblage showed the maximum at 195 nm and the minimum shifted to 215–218 nm. Po.HYD1 showed high surface activity, based on the dramatic drop of surface tension through self-assembly at the water–air interface. Moreover, Po.HYD1 is capable of stabilizing the emulsion consisting of water and hexane.

A theoretical model for digestive ripening

Recently, gram quantities of monodisperse gold or silver nanoparticles were reported to be produced through a digestive ripening process, in which colloidal particles of size from 2 to 40 nm are transformed to nearly monodisperse particles of 4–5 nm diameter. Digestive ripening, an example for inverse Ostwald ripening, is a puzzling phenomenon since it appears to avoid the usual capillary consequence, i.e., reduction of interfacial free energy. This paper presents a theoretical model which accounts for the self-assembled monodisperse state of such nanoparticles by considering the effect of charges, and therefore electrostatic energy, in the coarsening behavior of the particles. An appropriate Gibbs–Thomson equation is first derived, and then particle growth rates are calculated. The results show that success of a monodisperse state depends on, among other things, the initial particle distribution, and the size distribution at equilibrium follows some of the thermodynamic principles observed in binary phase diagrams of alloy systems.