Helical ribbon aggregate composed of a crown-appended cholesterol derivative which acts as an amphiphilic gelator of organic solvents and as a template for chiral silica transcription

By what means should nanoscaled materials be constructed: molecule, medium, or human?

There is great potential in nanoscale science and technology, and construction of macrosized materials and systems possessing nanoscale structural features is a crucial factor in the everyday application of nanoscience and nanotechnology. Because nanoscale substances are often constructed through self-assembly of unit molecules and nanomaterials, control of the self-assembly process is required. In order to establish general guidelines for the fabrication of materials with nanoscale structural characteristics, i.e., nanoscaled materials, we introduce here examples of recent research in related fields categorised as: (i) self-assembled structures with forms generally determined by intrinsic interactions between molecules and/or unit nanomaterials, (ii) self-assemblies influenced by their surrounding media, especially interfacial environments, (iii) modulation of self-assembly by artificial operation or external stimuli. Examples are not limited to organic molecules, which are often regarded as the archetypal species in self-assembly chemistry, and many examples of inorganic assemblies and hybrid structures are included in this review.

Metal-driven hierarchical self-assembled one-dimensional nanohelices

Sophisticated helical structure has been an attractive subject due to its significance in understanding of biological self-assembly and appealing application in nanoscience. In this work, a facile route toward one-dimensional helical nanostructure is presented based on metal-cholate supramolecular self-assembly. Well-defined right-handed helical nanoribbons in calcium-cholate systems are systematically investigated and a series of metal ions are exploited to drive metal-cholate supramolecular helix. It is anticipated that the incorporation of metal ions may endow versatile functionalities and merits to the self-assembled nanohelices. Particularly helical inorganic nanomaterials (i.e., SiO(2) and ZnS) have been prepared based on metal-cholate supramolecular nanohelix via two distinct templating strategies.

Strategy to control the chromism and fluorescence emission of a perylene dye in composite organogel phases

Composite organogels based on 1,3,5-tris(4-dodecyloxybenzoylamino)phenylbenzene (DBAPB), a known gelator, and N,N'-di(octadecyl)-perylene-3,4,9,10-tetracarboxylic diimide (C18PTCDI), a nongelator dye, have been achieved, leading to controllable color and emitting color changes. SEM images and XRD patterns revealed that the packing of the DBAPB-based gelator could almost be maintained in the composite gels. The temperature-dependent UV-vis absorption and temperature-dependent fluorescence emission spectra illustrated that the color and emitting color of the composite gels could be controlled by the content of C18PTCDI as well as the temperature in the gel phases. When the content of C18PTCDI was 1 mol %, C18PTCDI could be isolated as unimolecules in the composite gel, which was yellow and gave bright greenish-yellow emission under 365 nm light. For the mixed systems containing 2-10 mol % C18PTCDI, the fresh gels, which were obtained after cooling the hot solutions for a short time, were yellow and produced greenish-yellow emission under 365 nm illumination. However, the corresponding stable composite gels, which were obtained via prolonging the cooling time, were red and emitted weak red emission excited by UV light as a result of the formation of C18PTCDI aggregates. The reversible color and emitting color changes could be realized in the gel phases over a narrow temperature range. Moreover, the excitation energy of DBAPB could be transferred to C18PTCDI in the composite gels, leading to obvious emission quenching of the former.

Metathesis within self-assembled gels: transcribing nanostructured soft materials into a more robust form

This article reports the covalent capture of self-assembled gel-phase materials using alkene metathesis. Gels assembled from a gelator functionalized with peripheral alkene groups were reacted with Grubbs' second generation catalyst, added as a solution to the top of the gel and allowed to diffuse into the material for 24 h. Using this approach, the fibrillar self-assembled network was covalently captured, yielding a large amount of insoluble material that was robust, thermally stable, and highly swellable in solvents compatible with the gelator. Scanning electron microscopy demonstrated that the insoluble metathesized material contained nanoscale fibers, which were aligned into rigid fiber bundles on drying. When the gelator was assembled in the presence of a second non-cross-linkable gelator, self-sorting took place, giving rise to two independent gelator networks. Metathesis then generated an insoluble material in which the individual gel fibers of the cross-linkable gelator were captured, whereas the nonreactive gelator could be washed away. Intriguingly, using this approach appeared to hinder the alignment of gel fibers into rigid fiber bundles. Instead, individual, well-defined, robust gelator nanofibers were visualized in the dried materials. In addition, the material synthesized this way appeared to be even more highly porous and swellable on the addition of solvent. In summary, this article demonstrates that metathesis is an effective way to capture nanostructured gel-phase materials covalently, with the judicious choice of additives helping to control the morphology and behavior of the materials generated. This approach to nanofabrication could ultimately give rise to nanostructured polymeric materials with a wide range of applications.

Organogelation and hydrogelation of low-molecular-weight amphiphilic dipeptides: pH responsiveness in phase-selective gelation and dye removal

The search for efficient low-molecular-weight gelators (LMWGs) with possible structure-activity correlation is on the rise. The present work reports a novel set of amphiphilic dipeptide-based carboxylic acids capable of efficiently gelating organic solvents. More interestingly, their sodium salts showed enhanced efficiency in organogelation with the additional ability to gelate water. Electrostatic interactions present in the aggregation of the sodium carboxylates of amphiphilic dipeptides seem to be important because some of the nongelator carboxylic acids turned out to be excellent gelators upon salt formation. The combinations and sequence of the amino acids in the dipeptide moiety were systematically altered to understand the collective importance of the nonpolar aliphatic/aromatic substitution in amino acids in the self-assembling behavior of amphiphiles. Almost a 20-fold enhancement in the gelation ability was observed on reversing the sequence of the amino acid residues, and in some cases, nongelators were transformed to efficient gelators. Spectroscopic and microscopic studies of these thermoreversible organo/hydrogels revealed that balanced participation of the noncovalent interactions including hydrogen bonding and van der Waals interactions are crucial for organo/hydrogelation. These dipeptides selectively gelate organic solvents from their mixtures with water, and the xerogels prepared from these organogels showed time-dependent adsorption of dyes such as crystal violet. The most remarkable feature of these gelators is the pH responsiveness, which was aptly utilized for the pH-dependent phase-selective gelation of either solvent in a biphasic mixture of oil and water. The dissimilar gelation ability of the acid and its salt originating from the pH responsiveness of the amphiphilic dipeptide was employed in the instant removal of large amounts of dyes for wastewater treatment.

Polycondensation and stabilization of chirally ordered molecular organogels derived from alkoxysilyl group- containing L-glutamide lipid

A lipophilic L-glutamide-derived lipid with a triethoxysilyl headgroup (Si-lipid) was newly synthesized as a self-assembling organogelator to stabilize the chirally ordered state of the aggregates. The Si-lipid formed nanofibrous network structures in various organic solvents such as benzene, cyclohexane, and dimethylformamide and entrapped them to form gels. The gels were transformed to sols by heat, and this gel-to-sol transition was thermally reversible. Polycondensation of the triethoxysilyl groups was carried out by acid-catalyzed hydrolysis and condensation in a benzene gel and confirmed by 29Si CP/MAS NMR and FT-IR measurements. After polycondensation, a gel state was maintained, and the thermal and mechanical stabilities of the aggregates increased markedly. Interestingly, polycondensation in chloroform and acetonitrile induced gelation, whereas no gelation was observed before polycondensation. Xerogel, which was prepared by freeze drying organogels, had fibrous network structures similar to those of the original gels. A strong CD signal was observed around the amide bonds in a cyclohexane gel at 20 degrees C, indicating that the gel contained chirally oriented structures based on intermolecular hydrogen bonds. An enhanced CD signal was observed even after polycondensation of the ethoxysilyl group of Si-lipid (poly(Si-lipid)) and was maintained at 70 degrees C, which is above the temperature of the gel-to-sol phase transition of the original gel. These results indicate that the formation of siloxane network structure among the fibrous aggregates stabilizes the chiral orientation of lipid aggregates.

Two-component organogelators based on two L-amino acids: effect of combination of L-lysine with various L-amino acids on organogelation behavior

A two-component organogelation system, consisting of Nepsilon-dodecyl-L-lysine esters (amine components) and N-dodecyl-L-amino acids (valine, phenylalanine, alanine, glycine, L-lysine) as acid components, was proposed, and its organogelation properties were investigated. The mixture of amine and acid components produced an organic salt compound through acid-base interactions, and it created a three-dimensional network by entangling self-assembled nanofibers through hydrogen bonding and van der Waals interactions, thus leading to the formation of organogels. The combination of Lys/Phe yielded rigid dodecane gel, and the gelators of Lys/ Lys yielded a thermally stable dodecane gel. The organogelation properties significantly depended on the combination of amine and acid components, and they could be controlled by the selection of suitable components.

Cholesterol-appended aromatic imine organogelators: a case study of gelation-driven component selection

This letter describes a novel approach for developing organogelators through the formation of reversible imine bonds from two molecular components and the enriching behavior of the gelating imines. Cholesterol-appended aniline 1 and 4-substituted benzaldehydes 2a-d did not gelate any solvents. Their condensation products, imines 3a-d, however, could gelate alcohols because of the enhanced stacking interaction of the imine unit. For a further component selectivity test, the reactions of the mixture of 1, 2b-d, and cholesterol-free aniline 7 (1:1:1) in different solvents were performed. The resulting imines were reduced to the corresponding amines and analyzed with 1H NMR. It was revealed that, for the reactions resulting in no formation of the gel phase, imines 8a-c formed from 2b-d and 7 were obtained as the major product (64-76%) and all of the reactions that led to the formation of the gel phase gave rise to 3b-d as the major product (55-61%).

Supramolecular chiral transcription and recognition by mesoporous silica prepared by chiral imprinting of a helical micelle

Let's twist again: The chirality of a helical propeller-like micelle has been memorized by functional groups on the mesopore surface of chiral mesoporous silicas. Such imprinted supramolecular chirality can be transcripted to poly(propiolic acid) sodium salt and tetraphenylporphine tetrasulfonic acid (see picture), and recognized by B-DNA.

Carbazole-based organogel as a scaffold to construct energy transfer arrays with controllable fluorescence emission

A diaryldiketopyrrolopyrrole derivative functionalized with phenothiazine moieties (DPPP) was synthesized and introduced into the ordered 4-(3,6-di-tert-butyl-9H-carbazol-9-yl)benzamide (TBCB) organogel system. It was found that TBCB-based gel became a scaffold to make DPPP molecules line up along the gel fibers, resulting in new self-assembled arrays, whose XRD patterns were quite different from those of the neat TBCB gel and DPPP crystal. In the composite gel, the occurrence of a partial energy transfer from the excited light-harvesting antenna of TBCB to the DPPP acceptor was confirmed on the basis of time-dependent and time-resolved fluorescence investigations. Remarkably, the composite gel could emit intense red light or purplish white light by tuning the excitation wavelength. Such ordered soft materials with color-tunable emission may possess potential applications in sensor and photonic devices.

Individualized silica nanohelices and nanotubes: tuning inorganic nanostructures using lipidic self-assemblies

Diverse chiral nanometric ribbons and tubules formed by self-assembly of organic amphiphilic molecules could be transcribed to inorganic nanostructures using a novel sol-gel transcription protocol with tetraethoxysilane (TEOS) in the absence of catalyst or cosolvent. By controlling parameters such as temperature or the concentration of the different reactants, we could finely tune the morphology of the inorganic nanostructures formed from organic templates. This fine-tuning has also been achieved upon controlling the kinetics of both organic assembly formation and inorganic polycondensation. The results presented herein show that the dynamic and versatile nature of the organic gels considerably enhances the tunability of inorganic materials with rich polymorphisms.

Pentapeptide based organogels: the role of adjacently located phenylalanine residues in gel formation

A terminally protected self-assembling pentapeptide Boc-Leu(1)-Val(2)-Phe(3)-Phe(4)-Ala(5)-OMe 1 bearing sequence similarity with Aβ17-21 (the fragment 17-21 of the amyloid β-peptide Aβ42) forms thermoreversible transparent gels in various organic solvents including benzene, toluene, m-xylene and 1,2-dichlorobenzene. A series of its variants have been synthesized in order to study the role of adjacently located phenylalanine residues and the protecting groups for gelation in different organic solvents. Replacement of any of the Phe residues of the Phe-Phe segment with any other hydrophobic α-amino acid residue drastically changes the gel forming properties indicating that both Phe residues have an important role in gel formation. These gels are characterised using field emission scaning electron microscopy (FE-SEM), transmission electron microscopy (TEM), FT-IR and wide angle X-ray scattering (WAXS) studies. WAXS studies of the peptide 1-benzene gel indicate that π-π interaction is responsible for gel formation and it reveals the necessity of the Phe residues in gel formation. Transmission electron microscopy (TEM) of the gels reveals a nanofibrillar morphology, which is obtained from the self-assembled gelators in the gel phase. These gels bind with a physiological dye, Congo red, and show a green birefringence under cross polarizers, which is a characteristic feature of amyloid fibrils.

Unlocking the potential of bile acids in synthesis, supramolecular/materials chemistry and nanoscience

The Maitra group has explored a variety of chemistry with bile acids during the past 15 years and these experiments have covered a wide variety of chemistry-asymmetric synthesis, molecular recognition, ion receptors/sensors, dendrimers, low molecular mass organo and hydrogelators, gel-nanoparticle composites, etc. Some of what excites us in this field is highlighted in this perspective article.

Challenges and breakthroughs in recent research on self-assembly

The controlled fabrication of nanometer-scale objects is without doubt one of the central issues in current science and technology. However, existing fabrication techniques suffer from several disadvantages including size-restrictions and a general paucity of applicable materials. Because of this, the development of alternative approaches based on supramolecular self-assembly processes is anticipated as a breakthrough methodology. This review article aims to comprehensively summarize the salient aspects of self-assembly through the introduction of the recent challenges and breakthroughs in three categories: (i) types of self-assembly in bulk media; (ii) types of components for self-assembly in bulk media; and (iii) self-assembly at interfaces.

Preparation and characterization of a novel organic-inorganic nanohybrid "cerasome" formed with a liposomal membrane and silicate surface

A novel class of organic-inorganic hybrids, the so-called cerasomes, which have a bilayer vesicular structure and a silicate surface, has been synthesized by combination of sol-gel reaction and self-assembly of organoalkoxysilanes with a molecular structure analogous to lipids. We have synthesized two cerasome-forming organoalkoxysilanes, N-[N-(3-triethoxysilyl)propylsuccinamoyl]dihexadecylamine (1) and N,N-dihexadecyl-N (alpha)-[6-[(3-triethoxysilyl)propyldimethylammonio]hexanoyl]glycinamide bromide (2), and investigated the synthetic conditions of the cerasomes and their structural characteristics. For the proamphiphilic 1, the cerasome was obtained under restricted pH conditions where acid-catalyzed hydrolysis of the triethoxysilyl moiety proceeded without disturbing the vesicle formation. In contrast, the amphiphilic 2, additionally having a hydrophilic quaternary ammonium group, formed stable dispersions of the cerasome in a wide pH range. The hydrolysis behavior of the triethoxysilyl groups was monitored by (1)H NMR spectroscopy. Morphology of the cerasomes having the liposomal vesicular structure was confirmed by TEM observations. Extent of the development of siloxane networks through condensation among the silanol groups on the cerasome surface was evaluated by using MALDI-TOF-MS spectrometry. Formation of oligomers of the cerasome-forming lipids in the vesicle was clearly confirmed. Due to the siloxane network formation, the cerasome showed remarkably high morphological stability compared with a reference liposome, as evaluated by surfactant dissolution measurements.

Chiral Supramolecular Assemblies of a Squaraine Dye in Solution and Thin Films: Concentration-, Temperature-, and Solvent-Induced Chirality Inversion

We prepared novel cholesterol-appended squaraine dye 1 and model squaraine dye 2 and investigated their aggregation behavior in solution and thin films using photophysical, chiroptical, and microscopic techniques. Investigations on the dependence of aggregation on solvent composition (good/poor, CHCl3/CH3CN) demonstrated that squaraine dye 1 forms two novel H-type chiral supramolecular assemblies with opposite chirality at different good/poor solvent compositions. Model compound 2 formed J-type achiral assemblies under similar conditions. The supramolecular assembly of 1 observed at lower fractions of the poor solvent could be assigned to the thermodynamically stable form, while a kinetically controlled assembly is formed at higher fractions of the poor solvent. This assignment is evidenced by temperature- and concentration-dependent experiments. With increasing temperature, the chirality of the kinetically controlled aggregate was lost and, on cooling, the aggregate with the opposite chirality was formed. On further heating and cooling the aggregates thus formed resulted in no significant changes in chirality, that is they are thermodynamically stable. Similarly, at lower concentrations, the thermodynamically stable form exists, but at higher concentration aggregation was found to proceed with kinetic control. Based on these observations it can be assumed that formation of the kinetically controlled assembly might be largely dependent on the presence of the nonpolar cholesterol moiety as well as the amount of poor solvent present. However, under solvent-free conditions, structurally different aggregates were observed when drop cast from solutions containing monomer, whereas a left-handed CD signal corresponding to the thermodynamically controlled assemblies was observed from pre-aggregated solutions.