Hydrogels formed by l-histidine derivatives with highly selective release for charged dyes

Hydrogels formed by UIPCA with CA could selectively release anionic dyes, chrome azurol S (A) and cationic dyes, methyl violet (B), showing excellent separating ability for differently charged dyes.     

AFM and electron microscopy study of the unusual aggregation behavior of metallosurfactants based on iron(II) complexes with bipyridine ligands

The aggregation behavior in water-rich solutions of five iron(II) complexes with alkylated derivatives of 2,2'-bipyridine was studied by electron microscopy (cryo-SEM, SEM, and TEM) and AFM. The results obtained by cryo-SEM on frozen colloidal solutions show that the morphology of the aggregates strongly depends on the length of the alkyl chains in the bipyridine ligands, with shorter alkyl chains forming rod-like structures, whereas for compounds with longer alkyl chains, only spherical structures were detected. The self-aggregates were further characterized by SEM and TEM. The results show that their overall morphology depends only on the length of the alkyl chain of the bipyridine ligands and that the samples show a broad size distribution. In addition, TEM and SEM were used to study the stability of the self-aggregates in solution, the effect of addition of methanol, and the temperature used in the preparation of the colloidal solutions. AFM studies of the aggregates either dried in ambient conditions or dehydrated by long drying under vacuum showed partially collapsed self-aggregates in the latter case, showing that the aggregates contain water in their core, indicating that the self-aggregation leads to vesicle-type structures.     

Self-assembled monolayers based on phenanthroline-gold(111) bonding

The self-assembly of long-alkyl-chain substituted phenanthroline derivatives on highly oriented pyrolitic graphite (HOPG) and gold(111) is compared. Whereas the adsorption on HOPG is controlled by the affinity of alkyl chains for the substrate, which leads to flat-lying adsorbed molecules, alignments of upright-oriented molecules are formed on gold(111). This situation is explained by the bonding of chelating species with gold(111) surfaces and by the pi-stacking interaction between conjugated moieties. This intermediate situation between strong thiol-like chemical bonding and the weak n-alkane-like physical adsorption opens the route toward laterally organized functional molecular assemblies.     

Synthesis and Mesophase Behavior of Phenylthiophene Based Amphiphilic Molecules

Novel amphiphilic molecules consisting of a rigid 2‐phenylthiophene core, with a polar flexible tri(oxylethylene) moiety attached to the phenyl ring and one or two alkyl chains attached to the thiophene ring at the other side have been synthesized by using Ni(II) and Pd(0) catalyzed coupling reaction as key steps. The tri(oxylethylene) moieties were terminated with hydroxyl group, sodium carboxylate group and lithium carboxylate group respectively. The thermotropic and solvent induced liquid crystalline behavior of these substances was investigated by polarized optical microscopy, differential scanning calorimetry and X‐ray diffraction. Thereby the influence of the terminal groups attached to the tri(oxylethylene) moities as well as the influence of the length and the number of the alkyl chains on the mesophase behavior were investigated. The single alkyl chain Na‐carboxylate termianted derivatives show smectic A phases, double alkyl chain Na‐carboxylate terminated derivatives show a thermo tropic hexagonal columnar mesophase, while columnar mesophases are found in both single and double alkyl chain Li‐carbonate terminated derivatives. The model for molecular organization in the hexagonal columnar mesophase is established.     

Synthesis and self-assembly of novel oxacalix[2]arene[2]triazine amphiphiles

Heteracalixaromatics are an emerging generation of macrocyclic host molecules in supramolecular chemistry. As a typical example of heteracalixaromatics, oxacalix[2]arene[2]triazine adopts a shape-persistent 1,3-alternate conformation and can be easily functionalized. Taking it as a platform, a series of oxacalix[2]arene[2]triazine-based amphiphiles bearing long alkyl chains were synthesized through post-macrocyclization functionalization or 3+1 fragment coupling protocols. The self-assembly behavior of these amphiphiles in a mixture of tetrahydrofuran (THF) and water was investigated. Dynamic light scattering (DLS) measurements revealed that the size of the self-assembled aggregates is dependent on the structure of the amphiphiles. The long alkyl chain substituents and/or intermolecular hydrogen bonds were found to promote the self-assembly.     

Self-assembly and molecular recognition of adenine- and thymine-functionalized nucleolipids in the mixed monolayers and thymine-functionalized nucleolipids on aqueous melamine at the air-water interface

Self-assembly and molecular recognition of the monolayers composed of an equimolar mixture of adenine- and thymine-functionalized nucleolipids at the air-water interface have been investigated in detail using surface pressure-molecular area isotherms and in situ infrared reflection absorption spectroscopy (IRRAS). Prior to molecular recognition, the adenine moieties in the monolayer were almost oriented on an end-on mode through π-stacking and hydrogen bonding interactions, and the C-C-C planes of the alkyl chains were preferentially oriented perpendicular to the water surface, while the thymine moieties in the monolayer were involved in hydrogen bonding almost with a flat-on orientation. On aqueous subphases containing complementary bases, no significant molecular recognition was observed for the monolayers of individual nucleolipids. In the monolayer of equimolar mixture, molecular recognition occurred between the adenine and thymine moieties through hydrogen bonding probably with the development of cyclic structures of adenine-thymine-adenine-thymine quartets. Although molecular recognition between the monolayer of thymine-functionalized nucleolipids and aqueous melamine took place through triple hydrogen bonds, no melamine binding to the monolayer of equimolar mixture was observed, which reflects the formation of the quartets in the mixed monolayers at the air-water interface. FTIR and small-angle X-ray diffraction (XRD) results of the corresponding Langmuir-Blodgett films support the hydrogen bonding recognition and molecular orientation.     

Molecular recognition of cytosine- and guanine-functionalized nucleolipids in the mixed monolayers at the air-water interface and Langmuir-Blodgett films

Molecular recognition of mixed nucleolipids of 1-(2-octadecyloxycarbonylethyl)cytosine and 7-(2-octadecyloxycarbonylethyl)guanine in the monolayers at the air-water interface and Langmuir-Blodgett (LB) films has been investigated in detail using surface pressure/potential-area isotherms, infrared reflection-absorption spectroscopy (IRRAS), and Fourier transform infrared (FTIR) transmission spectroscopy, respectively. Prior to molecular recognition, the cytosine moieties in the monolayer were hydrogen bonded with an almost flat-on orientation, the alkyl chains were uniaxially oriented with respect to the film normal, the guanine moieties in the monolayer were stacked probably through pi-pi interaction with an end-on orientation, and the C-C-C planes of the alkyl chains were preferentially oriented parallel to the water surface. In the monolayer of equimolar mixture, molecular recognition between the cytosine and guanine moieties occurred together with the ring planes of base pairing and the C-C-C planes of the alkyl chains favorably oriented parallel to the water surface. The guanine moieties underwent an orientation change from an end-on mode before molecular recognition to a flat-on one after molecular recognition. The base pairing between the cytosine and guanine moieties in the monolayers was achieved since the N7-substituted guanine derivatives suppressed the formation of guanine tetramers. Both the IRRAS spectra of the monolayers and the FTIR spectra of the LB films presented the exact sites in the cytosine and guanine moieties for the formation of triple hydrogen bonds. The base pairing resulted in a change in molecular orientation and interaction, and the corresponding LB film exhibited a different phase transition behavior from a typical crystal transition for the cytosine-functionalized nucleolipids and an analogous glass transition for the guanine-functionalized nucleolipids. The thermal stability of the mixed LB film was improved in comparison to the LB films of pure components.     

Nonionic urea surfactants: formation of inverse hexagonal lyotropic liquid crystalline phases by introducing hydrocarbon chain unsaturation

The homo-interaction between urea moieties residing in close proximity to each other generally results in very strong intermolecular hydrogen bonding. The bifurcated hydrogen bonding exhibited by n-alkyl substituted ureas means that for those urea surfactants possessing medium and long hydrocarbon chain substituents the crystal to isotropic liquid melting point is high and the solubility in water is very low, compared to other similar chain length nonionic surfactants. In addition, saturated n-alkyl urea surfactants do not form lyotropic liquid crystalline phases in water. In this work the strong intermolecular hydrogen bonding of the urea headgroup has been ameliorated through the introduction of unsaturated hydrocarbon chains, viz., oleyl (cis-octadec-9-enyl), linoleyl (cis, cis-octadec-9,12-dienyl), and linolenyl (cis, cis, cis-octadec-9,12,15-trienyl) with one, two, and three carbon double bonds, respectively. Unsaturation in the C18 urea surfactants lowers the melting point and promotes an inverse hexagonal phase, in oleyl urea-water and linoleyl urea-water systems, which is thermodynamically stable in excess water. As the degree of unsaturation is increased to three in linolenyl urea, there is a tendency for autoxidation/polymerization. The occurrence of an inverse hexagonal phase in the nonionic urea surfactant-water systems has been rationalized in terms of both local molecular and global self-assembled aggregate packing constraints.     

Hierarchical organization of a robust porphyrin cage self-assembled by hydrogen bonds

Porphyrins appended with four rigid hydrogen bonding motifs on the meso positions were synthesized and self-assembled into a cofacial cage with four complementary bis(decyl)melamine units in dry solvents. The hydrocarbon chains on the melamine mediate the formation of nanofilms on surfaces as the solvent slowly evaporates.     

Preparation and characterization of self-assembled nanoparticles of heparin-deoxycholic acid conjugates

Various deoxycholic acid (DOCA) bearing heparin (HD) amphiphilic conjugates with different degrees of substitution (DS) with DOCA were synthesized using heparin as a hydrophilic segment and DOCA as a hydrophobic segment. Structural characteristics of these HD conjugates were investigated using 1H NMR, dynamic light scattering, zeta potential, transmission electron microscopy (TEM), and fluorescence spectroscopy. HD conjugates provided monodispersed self-aggregates in water, with mean diameters decreasing with increasing DOCA DS in the range of 120-200 nm. HD aggregates were covered with negatively charged heparin shells, exhibiting xi potentials near -56 mV. The critical aggregation concentration (cac) of the HD (0.02-0.003 mg/mL) depended upon DOCA DS. TEM images demonstrated that the shape of the self-aggregates was spherical. Partition equilibrium constants, Kv, for pyrene in the self-aggregates in water indicated that increasing DS enhanced the hydrophobicity of the self-aggregate inner core. The mean aggregation number of DOCA per hydrophobic microdomain, estimated by the fluorescence quenching methods using cetylpyridinium chloride, indicated that five to nine of HD chains comprised a hydrophobic domain in the conjugates.     

Nanocomposites and macroscopic materials: assembly of chemically modified graphene sheets

Self-assembly of chemically modified graphenes (CMGs), including graphene oxide (GO), reduced graphene oxide (RGO) and their derivatives, has emerged as one of the most appealing strategies to develop unprecedented graphene-based functional materials. With the assistance of various non-covalent forces such as hydrogen bonding, ionic, amphiphilic and π-π interactions, CMGs decorated with multiple functional groups are favorable for assembly with different organic and inorganic components which can result in hierarchical composites possessing unique structures and functions. In this review, we will summarize the state-of-the-art self-assembly strategies that have been established to construct CMG based nanomaterials, including nanoparticles, nanospheres, nanofibers, nanorods, nanosheets, and macroscopic thin films, fibers and porous networks. The driving forces involved in the self-assembly process will be elucidated in the context. Further, we will also highlight several representative examples of applications regarding the self-assembled CMG based materials.     

Self-assembled discotic nematic liquid crystals formed by simple hydrogen bonding between phenol and pyridine moieties

New discotic nematic liquid crystals have been prepared through intermolecular hydrogen bonding between the core of 1,3,5-trihydroxybenzene (phloroglucinol, PG) or 1,3,5-tris(4-hydroxyphenyl)benzene (THPB) and the peripheral molecules of stilbazole derivatives. The various nematic phases formed by new hydrogen bonding building blocks were investigated by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. The first discotic complexes of PG and trans-4-alkoxy-4′-stilbazoles exhibited nematic columnar (N_C) and hexagonal columnar phases depending on the length of alkyl chains, which were considered as the basic discotic structure. Several structural variations on the building blocks were attempted to examine their effects on the liquid crystalline properties of discotic complexes. The nematic lateral phase (N_L) with enhanced intercolumnar order was observed for the complexes of PG and trans-4-cyanoalkoxy-4′ stilbazoles due probably to the strong dipole interactions between cyano groups at the end of alkoxy chains. By introducing the nonlinear structure in three arms of supramolecular discotic mesogen, a discotic nematic phase (N_D) was observed for the complex of THPB and trans-4-octyloxy-4 -stilbazole. The single hydrogen bonding between phenol and pyridine moieties in this study provides a simple and effective method for preparing the rarely found discotic nematic liquid crystals.     

The self-assembly of a lipophilic guanosine nucleoside into polymeric columnar aggregates: the nucleoside strucutre contains sufficient information to drive the process towards a strikingly regular polymer

Lipophilic guanosine derivatives act as self-assembled ionophores. In the presence of alkali metal ions in organic solvents, these G derivatives can form tubular polymeric structures. The molecular aggregates formed by 3',5'-didecanoyl-2'-deoxyguanosine (1) have been characterised by SANS and NMR spectroscopy. The polymer is structured as a pile of stacked G quartets held together by the alkali metal ions that occupy the column's central channel. The deoxyribose moieties, with their alkyl substituents, surround the stacked G quartets, and the nucleoside's long-chain alkyl tails are in intimate contact with the organic solvent. In this polymeric structure, there is an amazing regularity in the rotamers around the glycosidic bond within each G quartet and in the repeat sequence of the G quartets along the columns. In hydrocarbon solvents, these columnar aggregates form lyomesophases of the cholesteric and hexagonal types.     

Self-assembled micelles based on hydrophobically modified quaternized cellulose for drug delivery

Novel amphiphilic cationic cellulose (HMQC) derivatives carrying long chain alkyl groups as hydrophobic moieties and quaternary ammonium groups as hydrophilic moieties were synthesized. Structure and properties of the amphiphilic cellulose derivatives were characterized by elemental analysis, FT-IR, (1)H NMR, ζ-potential measurement, dynamic light scattering (DLS), fluorescence spectroscopy and transmission electron microscopy (TEM). The results revealed that HMQCs can be self-assembled into cationic micelles in distilled water with the average hydrodynamic radius of 320-430 nm. The cytotoxicity study showed that the HMQC exhibited low cytotoxicity. Prednisone acetate, a water insoluble anti-inflammation drug, was chosen as a model drug to investigate the utilization of self-assembled HMQC micelles as a delivery carrier for poorly water-soluble drugs. The study indicated that the prednisone acetate could be incorporated effectively in the self-assembled HMQC micelles and be controlled released.     

Synthesis of thermoplastic curdlan alkyl carbamates having hydrogen bonding ability

Rigid and little moldable curdlan, a linear β-1,3-glucan having intra- and interchain hydrogen bonds, was reacted with several alkyl isocyanates, which gave thermoplastic curdlan alkyl carbamates (CrdC) with degree of substitution about 2. The alkyl carbamation at hydroxy groups in the glucan skeleton effectively broke the interchain hydrogen bonds of curdlan and increased flexibility of CrdC, while the newly formed carbamate moieties could moderately keep the hydrogen bonding ability in CrdC. Elongating the alkyl groups in the carbamate side chains increased solubility in organic solvents and thermoplasticity of CrdC, which enabled to make homogeneous and free-standing films by both methods of solution-casting and hot-pressing.     

Functionalized multiwall carbon nanotube/gold nanoparticle composites

Multiwall carbon nanotubes (MWCNTs) were chemically oxidized in a mixture of sulfuric acid and nitric acid (3:1) while being ultrasonicated. The effect of oxidative ultrasonication at room temperature on development of functional groups on the carbon nanotubes was investigated. The dispersability and the carboxylic acid group concentration of functionalized MWCNTs (fMWNTs) varied with reaction time. The concentration of carboxylic acid groups on fMWNTs increased from 4 x 10(-4) mol/g of fMWNTs to 1.1 x 10(-3) mol/g by doubling the treatment period from 4 to 8 h. The colloidal stability of aqueous fMWCNTs dispersions was enhanced through elongated oxidation. fMWCNTs that were reacted longer than 4 h did not precipitate in aqueous media for at least 24 h. The layer-by-layer self-assembly of polyelectrolytes on fMWCNTs was characterized by zeta potential measurements. The zeta potential of fMWCNTs changed from negative charge to positive charge when cationic polyelectrolytes were self-assembled on their surface. With addition of anionic polyelectrolytes, cationic polyelectrolyte coated fMWCNTs showed the expected charge reversal as expected for multilayer self-assembly. Complex formation of positively charged gold nanoparticles and negatively charged fMWCNTs was achieved with and without polyelectrolyte coatings by electrostatic interaction. The complex formation was characterized by high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy. The here found complex formation of positively charged colloidal gold and defect sites on fMWNTs indicates the location of functional groups on carbon nanotubes. It is suggested that positively charged colloids such as gold nanoparticles could be used for detection of defect sites on carbon nanotubes.     

Amphiphilic cholesteryl-bearing carboxymethylcellulose derivatives: self-assembly and rheological behaviour in aqueous solution

The self-assembly of amphiphilic cholesteryl-bearing carboxymethylcellulose derivatives (CCMCs) from individual molecules to self-aggregates, in addition to their rheological behaviour in aqueous solution were investigated. The conformations of the individual CCMC individual molecules and self-aggregates, and the microstructures of CCMC self-aggregates were characterized using the static and dynamic light scattering analyses, and the steady fluorescence technology. The results showed that the individual CCMC chains became compact, and aqueous NaCl solution changed from an approximate θ solvent to a bad one when the number of hydrophobic cholesteryl groups increased. The CCMC self-aggregates exhibited as compact spheres. The microstructural characterization indicated that one CCMC self-aggregate consisted of multiple individual CCMC molecules and hydrophobic microdomains, and multiple cholesteryl groups self-associated into one hydrophobic microdomain. The atomic force microscopy images of CCMC self-aggregates in phase mode showed that the multiple interior hydrophobic phase separation regions were surrounded by hydrophilic polysaccharide chains, providing an evidence for multiple hydrophobic microdomains in one self-aggregate. The rheological analysis proved that the CCMC self-aggregates were a microgel, in which the associations of cholesteryl groups provided multiple cross-linking points in the polymer network. The microgel solutions displayed a unique thixotropy and a shear thinning behaviour.     

Reorganization of hydrogen-bonded block copolymer complexes

Poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) copolymers and poly(acrylic acid) (PAA) have been mixed in organic solvents. Complexation via hydrogen bonding occurs between the P4VP and PAA blocks. Those insoluble complexes aggregate to form the core of micelles surrounded by a corona of PS chains. Reorganization of these structures occurs upon addition of acidic or basic water, which results in the breaking of the hydrogen bonds between the P4VP and PAA blocks. After transfer of the initial complexes in acidic water, micelles consisting of a PS core and a protonated P4VP corona are observed. In basic water, well-defined nanoparticles formed by the PS-b-P4VP copolymers are obtained. It is demonstrated that these nanoparticles are stabilized by the negatively charged PAA chains. Finally, thermally induced disintegration of the micelles is investigated in organic solvents.     

Hydrogen-bonded benzylidenebenzohydrazide macrocycles and oligomers: testing the robust capacity of an amide chain in promoting the formation of vesicles

This Letter describes 2-(2-(dioctylamino)-2-oxoethyl-amino)-2-oxoethoxyl (DOAOE)-tuned self-assembly of vesicles from rigid macrocycles and foldamer-like oligomers. The molecules are prepared through the formation of reversible hydrazone bonds from aldehyde and benzo-hydrazide precursors, which are further facilitated by intramolecular N?H-O hydrogen bonding. SEM, AFM, and fluorescent encapsulation studies reveal that the molecules all self-assemble into vesicular structures in methanol, while similar molecules bearing the triethylene glycol or n-decyl chains do not. The results illustrate that DOAOE is robust in promoting the formation of vesicles for aromatic systems in polar solvents.     

Self-assembly of surfactant-like peptides and their applications

Numerous peptides derived from naturally occurring proteins or de novo designed have been found to self-assemble into various nanostructures.These well-defined nanostructures have shown great potential for a variety of biomedical and biotechnological applications.In particular,surfactant-like peptides(SLPs)have distinctive advantages in their length,aggregating ability,and water solubility.In this article,we report recent advances in the mechanistic understanding of the self-assembly principles of SLPs and in their applications,most of which have been made in our laboratory.Hydrogen bonding between peptide backbones,hydrophobic interaction between hydrophobic side chains,and electrostatic repulsion between charged head groups all have roles in mediating the self-assembly of SLPs;the final self-assembled nanostructures are therefore dependent on their interplay.SLPs have shown diverse applications ranging from membrane protein stabilization and antimicrobial/anticancer agents to nanofabrication and biomineralization.Future advances in the self-assembly of SLPs will hinge on their large-scale production,the design of new functional SLPs with targeted properties,and the exploitation of new or improved applications.